THE PROPERTY OF, 7' * MEIDICAL SCHOOL i /' PRINCIPLES OF HUMAN PHYSIOLOGY, WITH THEIR CHIEF APPLICATIONS TO PATHOLOGY, HYGIENE, AND FORENSIC MEDICINE. ESPECIALLY DESIGNED FOR THE USE OF STUDENTS. BY WILLIAM B. jgARPENTER, M.D., F.R.S., FULLERIAN PROFESSOR OF PHYSIOLOGY IN THE ROYAL INSTITUTION OF GREAT BRITAIN; LECTURER ON NATURAL HISTORY AND COMPARATIVE ANATOMY AT ST. THOMAS' HOSPITAL, AND CORRESPONDING MEMBER OF THE NATIONAL INSTITUTE OF THE UNITED STATES. Second ^merfcnn, from tjje last Hontjon IJQftfon. WITH NOTES AND ADDITIONS. BY MEREDITH CLYMER, M. D., PHYSICIAN TO THE PHILADELPHIA HOSPITAL; FELLOW OF THE COLLEGE OF PHYSICIANS. ETC. WITH TWO HUNDRED AND SIXTEEN WOOD-CUT AN L E A JA N,i; 1845. ILLUSTRATIONS. . ENTERED according to Act of Congress, in the year 1845, by LEA AND BLANCHARD, In the Office of the Clerk of the District Court for the Eastern District of Pennsylvania. ' PRIMERS. '2 TO WILLIAM PULTENEY ALISON, M.D., F.R.S.E., &c. &c. PROFESSOR OF THE PRACTICE OF MEDICINE OF THE UNIVERSITY OF EDINBURGH. MY DEAR SIR, I take the liberty of inscribing the following Work to you, as an expression of my grateful remembrance of the value of your instructions, of my respect for those intellectual faculties which render you pre-emi- nent amongst the Medical Philosophers of our time, and of my admiration for those moral excellencies which call forth the warm regard of all who are acquainted with your character. In many parts of this Treatise you will find that doctrines, which you have long upheld in opposition to almost the whole Physiological world, are defended with such resources as I could command ; and that, in many instances, such convincing evidence of their truth has been afforded by recent observations, that further opposition to them would now seem vain. And if I have presumed to differ from you on some points, it has been in the spirit of that independence which you have uniformly encouraged in your pupils; yet with a distrust of my own judgment, wherever it came into collision \*ith YQ^fl^ That you may long be spared to be the ornam^B iBbur University, and the honour of your City, is the earnest wish ^^ Dear Sir, Your obliged Pupil, WILLIAM B. CARPENTER. Ripky, Surrey, Sept. 20 ; 1844. t.348. PREFACE OF THE AMERICAN EDITOR. "THE peculiar character of Dr. Carpenter's Human Physiology,' particularly adapts it to the wants of the medical student. The close connection between Physiology and the practical branches of medicine, has been throughout the work carefully borne in mind, and their inti- mate relations pointed out. The present Treatise will be found to be a concise, yet comprehensive exposition of the actual condition of physi- ological science, conceived and executed in an acute and philosophical spirit. With much that is profound and original, the Author has pre- sented all the received or probable facts of the Science of Life, in a well-digested and lucid manner, deducing from them legitimate infer- ences, and carefully abstaining from the discussion of controverted questions, and never hazarding any startling hypothesis." The above opinion was expressed by the Editor in the Preface to the First American Edition of this work. The universal favour with which it has been received, and the rapid sale of a very large impression, are proofs that his judgment was neither erroneous nor too partial. It is now generally conceded in this country and in England, that Dr. Car- penter's "Principles of Human Physiology" is one of the best digested, most complete and compendious expositions of the Science of Life in its actual state, that has yet been written, and admirably adapted to the purposes of the medical student. 9^^k In preparing the present edition, no pains have^^^BApared to render it complete in all respects. Besides other sources, 1f$e Editor has availed himself of much valuable matter contained in the excellent Annual Report on the Progress of Anatomy and Physiology in the British and Foreign Medical Review for January, 1845. He has also introduced about eighty additional wood-cut illustrations, which, being accom- panied with very copious references, will, he' trusts, materially add to the value of the work as a text-book. The new matter added by the American Editor is in smaller type, and is distinguished thus [ M. C.] The new cuts are included be- tween brackets, thus [ ]. No. 230 Spruce St., Philadelphia, April, 1845. X PREFACE TO THE SECOND EDITION. 4. Observations on the Nature and Destination of the Food of Animals ; principally based on Prof. Liebig's views, $ 430434. 5. Mr. Goodsir's researches on the absorbent Cells of the Intestinal Villi, 461, 462. 6. Researches of Mr. Addison and M. Bourgery on the structure of the Lungs, 525. 7. Researches of MM. Andral and Gavarret, and of Prof. Scharling, on Respiration, . 534 note, and 544. 8. The chapter on Nutrition, including the history of the Chyle, Blood and Tissues, almost wholly re-written; so as to include the results of the most recent observations on these subjects, and the views to which the Author has been led by them. 9. The doctrine of Secretion by Cells, as developed by Henle,Goodsir and others, 651. 10. The researches of Mr. Bowman on the structure and functions of the Malpighian bodies of the Kidney, 667, 668. 11. The account of the constitution of Urine lately given by Liebig, 673 678. "12. The inquiries of Dr. Lehmann, respecting the influence of diet on the constitution of the Urinary Secretion, 679, 680. 13. The researches of Dr. Evans on the structure of the Spleen, 708, 709. 14. The inquiries of Dr. Buchanan on the changes in the Blood produced by admixture of Chyle, 714 note, 715. 15. The investigations of Chossat on the effects of Inanition, 730. 16. The results of M. Raciborski's inquiries in regard to the relation between the periods of Conception and Menstruation, 742. 17. The researches of Dr. Ritchie, into the various conditions of the Ovaria, 744. Many other additions, not of sufficient importance to be thus specified, are scattered throughout the work ; and in many instances these are the result of the Author's own investigations. He has particularly directed, his attention to the settlement of points which appeared to him to have been left doubtful by others ; and hence will sometimes be found to have expressed his views with a degree of confidence which the evi- dence adduced by them may scarcely appear to warrant. Notwithstanding the attacks which have been made on Dr. Barry's views respecting the Reproductive process, by Prof. Bischoff and Mr. Wharton Jones, tl^Author has not felt it desirable to modify in any great degree, t^e^^ount he had previously given of that Function, chiefly on Dr. Barry's authority. It happens that he is well convinced, by his own observations, of Dr. B.'s accuracy on some of the leading points which have been most strongly objected to; and he feels the more confidence, therefore, in regard to the rest. He may also advert to the very remarkable confirmation, which has been afforded to Dr. B.'s statements regarding the early changes that take place within the germi- nal vesicle, by the parallel researches of Mr. H. S. Goodsir on the ova of Acephalocysts (Trans, of Roy. Soc. of Edinb., vol. xv.). He cannot but regret that the subject has been discussed with so much acrimony of feeling, and with so much more desire to find fault than to elicit Truth. In regard to certain subjects which have excited much attention since the appearance of the First Edition, the Author may be permitted to PREFACE TO THE SECOND EDITION. XI remark that he has found less occasion to introduce alterations or addi- tions than might have been expected ; for the general doctrine respecting the connection of the waste of the Organic structure with its functional activity, and the dependence of the excreting processes upon the disin- tegration of the bodily framework, which have been considered novel features in the Animal Chemistry of Prof. Liebig, had been very clearly set forth by himself. He cannot but think that the value of that able Chemist's actual contributions to Physiological Science have been con- siderably overrated ; though none can estimate more highly than he does, the importance of the path of inquiry which Prof. L. has opened up. The only point in which any considerable fault has been found w r ith this work, in the criticisms offered by the Medical Journals, has respect to the arrangement of its subjects. It seems to be the general opinion that the consideration of the Organic Functions should precede that of the Functions of Animal Life, on account of the greater complexity of the latter. The Author's reasons for the course he has adopted will be found in 82 '; and he would only add here that the reader who is so disposed will find no difficulty in passing at once from the General View of the Functions (Chap. II.) to the Second Division of the book, com- mencing with Chap. VIII.; deferring the consideration of the Nervous System and its operations until the last. It is the Author's own opinion, however, that this latter class of phenomena is, as now understood, the least complex of the two. He has only further to notice, that a considerable number of new Woodcuts, and a new Plate, have been added to the present Edition. Ripley, Surrey, Sept. 21th, 1844. LIST OF WOODCUT ILLUSTRATIONS. FIG. PAGE 1. Structure of the Star-fish, after Tiedemann - - 37 2. External aspect of Aplysia, after Rang - 38 3. Structure of Aplysia, after Cuvier - - 40 4. Section of Cockchafer, after Strauss-Durckheim .... 42 5. Comparative view of the base of the Skull of Man, and of the Orang Outan, after Owen - ... - 61 6. Comparative view of the Skeletons of Man and the Orang, after Owen - 62 7. Structure of Nerve-tubes, after Wagner ..... 91 8. Primitive fibres and globules of ganglia, after Wagner 92 9. Primitive fibres and ganglionic globules of Human brain, after Purkinje - 92 10. Nervous system of Aplysia, after Cuvier - ... 106 11. Nervous system of Larva of Sphinx Ligustri, after Newport - - 108 12. Portion of Ganglionic tract of Polydesmus maculatus, after Newport - 109 13. Parts of Nervous System of Articulata, after Newport - 113 14. Stomato-gastric system of Gryllotalpa vulgaris, after Brandt - 114 15. View of the Great Sympathetic Nerve - 120 16. Nervous centres in Frog, after Leuret ..... 123 17. Transverse sections of Spinal Cord at different points, after Solly - - 123 18. A transverse section of Spinal Marrow - - - 124 19. Structure of the Spinal Cord, according to Stilling - - - 125 20. A posterior superior view of the Pons Varolii, Cerebellum, &c. - - 127 21. A posterior view of ;he Medulla Oblongata ----- 127 22. An anterior view of the Medulla Oblongata ----- 128 23. Course of the Motor tract, after Sir C. Bell - - - 129 24. Course of the Sensory tract, after Sir C. Bell - - 130 25. A lateral view, of the Spinal Marrow, &c., of a new-born infant - - 131 26. A posterior view of the Medulla Spinalis ----- 131 27. Brains of Fox-Shark, Cod, and Pike, after Leuret - - - 159 28. Human Embryo, showing rudiments of Encephalon, after Wagner - - 160 29. Brain of Turtle, after Solly - ----- 161 30. Brain of Buzzard, after Leuret ..... 161 31. Brain of Human Embryo, at 12th week, after Tiedemann - 162 32. Brain of Rabbit, after Leuret - 162 33. Base of the Cerebrum and Cerebellum, with their nerves - - - 163 34. First pair, or Olfactory Nerves ..... 164 35. Portio Mollis of Seventh Pair, or Auditory Nerve - - - 167 36. Diagram of the distribution of the Fifth Pair - - 168 37. Distribution of the Fifth Pair .... - 168 '38. Diagram of the distribution of the Facial Nerve .... 170 39. Diagram of the distribution of the Eighth Pair .... 172 40. Distribution of the Glosso-Pharyngeal, Pneumogastric,and Spinal Accessory Nerves, or Eighth .Pair - - - - 172 41. Course and distribution of the Hypo-Glossal or Ninth Pair - - 181 42. View of the Third, Fourth, and Sixth Pair - - - - 183 43. Longitudinal Section of Eye - ... 242 44. Horizontal Section of Eye Ball - - - 243 45. Magnified view of outer surface of Retina of Frog, after Treviranus - 245 46. Do. of inner surface ..... 245 47. Portion of the Retina of an Infant, magnified 246 48. Imaginary Plan of the Cochlea - 257 49. Axis of Cochlea and Lamina Spiralis - 257 20 XXVI LIST OF WOODCUT ILLUSTRATIONS. FIG. PAGE 50. Section of the Cochlea, after Breschet - - 258 51. Papillae of Auditory nerve in Mouse, after Treviranus - 259 52. Auditory Nerve taken out of Cochlea - 259 53. Magnified view of Lamina Spiralis ------ 259 54. Soft parts of the Vestibule - - 260 55. Ampulla of the External Semi-circular Membranous Canal - 260 56. Labyrinth laid open, after Breschet - - 261 57. Labyrinth of the Left Side - - - 265 58. Left Ear in its natural state - ... 267 59. Anterior view of the External Ear, Meatus Auditorius, &c. - - 267 60. Fasciculus of fibres of Voluntary Muscle, after Baly - 271 61. Portion of Human Muscular Fibre, separating into disks, after Bowman - 272 62. Cleavage of Striped Elementary Fibres - - 272 63. Fibre of Human Muscle broken across, after Bowman - - 273 64. Transverse Section of Muscular fibres of Teal after Bowman - 274 65. Fragment of Muscular fibre from Heart of Ox, after Bowman - 275 66. Muscular fibre of Dytiscus, contracted in the centre, after Bowman - 276 67. Muscular fibre of Skate, in different stages of contraction, after Bowman - 277 68. Attachment of Tendon to Muscular fibre, after Bowman - - 278 69. Stages of the development of Striped Muscular Fibre, after Bowman - 278 70. Another view of the development of Muscular Fibre - 279 71. Terminating loops of Nerves in Muscles, after Burdach - - 282 72. Thyroid and Cricoid Cartilages - 297 73. Side-views of Larynx, after Willis - - 297 74. Posterior and Anterior views of the left Arytenoid Cartilage - 298 75. Bird's-eye view of Larynx from above, after Willis - 298 76. Vertical Section of the Larynx - - - - 298 77. Diagram of the direction of the muscular forces of the Larynx, after Willis 299 7' ( Artificial Larynx, after Willis - - 303 80. View of the Organs of Digestion in their whole length - 326 81. Muscles of the Tongue, Palate, Larynx and Pharynx - 328 82. Front view of the Stomach distended ----- 333 83. Interior of the Stomach - - - 333 84. Interior of the Stomach and Duodenum - - 334 85. Vessels of Intestinal Villus of Hare, after Dollinger - - - 346 86. Do. Do. of Man, after Krause - 346 87. Commencement of Lacteal in Villus, after Krause - - 346 88. Web of Frog's Foot, after Wagner - - 359 89. Do. more highly magnified, after Wagner - - 360 90. First appearance of blood-vessels in Germinal Membrane, after Wagner - 361 91. Diagram of Heart - - - - 368 92. Haemadynamometer of Poisseuille - - . - 371 93. Lung of Triton, slightly magnified, after Wagner - - 392 94. Portion of the same more highly magnified, after Wagner - - 392 95. Capillary circulation in Lung of living Triton, after Wagner - 392 96. Portion of Lung of Pig, after Wagner ... - 393 97. Development of Lungs, after Rathke ... 394 98. Larynx, Trachea and Bronchiae - - - 394 99. Bronchiae and Blood-vessels of the Lungs - - - - 395 100. Corpuscle of Human Blood, after Wagner - - - 428 101. Corpuscles of Frog's Blood, after Wagner - - - - 428 102. Production of Blood-corpuscles in Chick, after Wagner - - 432 103. Circulation of oval and colourless corpuscles, in venous trunk of Frog's foot, after Wagner - - 434 104. Shape of Fat Vesicles in close pressure ... . 468 105. Blood-vessels of Fat - - 468 106. Fat Vesicles from an emaciated subject - - 469 107. Examples of Cilia - - 472 108. Magnified view of Human Hair - - - 475 109. Magnified view of a Hair from the beard - - 475 110. Root of a Hair of the beard, magnified - - 476 1 1 1. Follicle of a Hair of the Beard, with Arteries - 476 112. Nutrient vessels of Cartilage, after Toynbee -^ 478 113. Minute structure of Bone, after Wilson ----- 481 LIST OF WOODCUT ILLUSTRATIONS. XXV11 FIG. PAGE 114. Transverse section of a long Bone - ... -482 115. Transverse section of a Tibia ..... 482 116. Haversian Canals in a long Bone ...... 483 117. Scapula of a Foetus, showing the process of ossification - - - 483 118. Vertical section of Cartilage - ... 484 119. Longitudinal section of an Incisor and Molar Tooth - - 488 120. Vertical section of an adult Bicuspid - - 488 121. Section of an imperfectly developed Incisor - - 489 122. Hexagonal Terminations of Fibres of Enamel - - - 489 ] 23. Fibres of Enamel viewed sideways - - '489 124. Vertical section of Bicuspid, highly magnified .... 489 125. Main Tubes of Dental Bone - ... - 490 126. Ramifications of the Tubes of Dental Bone - 490 127. Transverse Section of Crown of Bicuspid, highly magnified - - 490 128.. Position of the Main Tubes near the Root of Bicuspid - - 490 129. First stage of formation of Teeth, after Goodsir - - 492 130. Diagram illustrating subsequent stages of formation of Teeth, after Goodsir 492 131. Do. Do. Do. - 493 132. Elements of Areolar Tissue - ... 498 133. Development of Areolar Tissue, after Schwann - - 498 134. Inferior Surface of the Liver .... . 512 135. Three Coats of the Gall Bladder ... - - 513 136. Gall Bladder distended, with vessels injected .... 513 137. Nucleated Cells of Parenchyma of Liver - 514 138. Lobules of Liver, with branches of Hepatic vein, after Kiernan - - 515 139. Nucleated Cells forming Parenchyma of Liver, after Bowman - - 515 140. Horizontal section of Lobules, showing the arrangement of their vessels, after Kiernan - ' - 515 141. Do. Do. bile-ducts, after Kiernan 516 142. Lobules in a state of Anaemia, after Kiernan - 518 143. Do. in first stage of hepatic-venous congestion, after Kiernan - - 518 144. Do. in second stage of do. - .... 519 145. Do. in a state of Portal venous congestion, after Kiernan - - 519 146. Hepatic Cells loaded with Fat, after Bowman - 520 147. Right Kidney, with Renal Capsule ...... 525 148. Section of Kidney, after Wilson .... . 525 149. Half a Kidney, divided vertically ...... 525 150. Kidney divided vertically, with Arteries injected .... 525 151. Section of a Pyramid of Malpighi - - 527 152. Kidney of new-born Infant, after Wagner ----- 528 153. Extremity (1) of Tubulus Uriniferus, after Wagner - 528 154. Section of small portion of Kidney, after Wagner - - 529 155. Structure of Malpighian body, after Bowman - 530 156. Diagram of Circulation in the Kidney, after Bowman - 530 157. Corpora Wolffiana, after Muller - - 530 158. Mammary Gland - ... . . 541 159. Vertical Section of Mammary Gland .... 541 160. Distribution of Milk-ducts in Mammary Gland, after Sir A. Cooper - 541 161. Termination of Milk-ducts in Follicles, after Sir A. Cooper - 543 162. Lobule of Parotid Gland, after Wagner - ... 549 163. The Testicle injected with Mercury - 552 164. Minute structure of the Testis .... 552 165. Human Testis, injected with Mercury, after Lauth - - - 553 166. Diagram of the structure of the same ..... 553 167. Sudoriferous Gland, after Wagner ------ 555 168. Cutaneous Glands of external Meatus Auditorius, after Wagner - - 557 169. Mesocolon in connection with Mesentery - - 558 170. Gastric Glands in Human Stomach, after Wagner - 558 171. The same, from another part, after Wagner - 558 172. Entrances to Secreting Tubes, after Boyd - - 558 173. Villi and Follicles of LieberkQhn on surface of Ileum - 559 174. Mucous coat of Small Intestines, as altered in Fever, after Boehm - 560 175. One of the Glandulae Solitariae of Peyer, from the Large Intestine, after Boehm 560 176. Conglomerate Glands of Brunner, after Boehm - - 560 177. Glands of Peyer on Small Intestine - - 561 XXV111 LIST OF WOODCUT ILLUSTRATIONS. FIG. PAGE 178. Patch of aggregated Peyerian Glands, from Ileum, after Boehm - 561 179. Section of Thymus Gland, after Sir A. Cooper - 565 180. Section of Uterus, showing formation of Decidua Vera, after Wagner - 601 181. Do. "more advanced, after Wagner - - 601 182. Diagram illustrating position of Embryo in Ovum, after Wagner - - 613 183. Do. more advanced, the Amnion beginning to form - 613 184. Do. still more advanced, the Allantois beginning to appear, after Wagner - ... . 615 185. Do. at a later time, the Amnion fully formed, after Wagner 615 186. Section of Uterus, showing the Ovum, Membranes, &c., after Wagner - 616 187. Diagram illustrating the Foetal Circulation - - 618 188. Curve representing the relative Viability of Human Male and Female at different ages ... . _ g25 189. Do. Do. Heights and Weights of the Human Male and Female at different ages, after Quetelet - . 626 Also, Two Lithographic Plates with 27 Figures. FROM THE PREFACE TO THE FIRST EDITION. THE composition of such a Treatise as the following was a part of the original plan of the Author, when he first came before the Public as a writer on Physiology. Being desirous, however, of making his first essay in the path which had been previously the most incompletely explored, he deemed it better to await the verdict upon this before pro- ceeding further ; and he was not without hope that some Writer, more fully competent to the^task, might in the mean time take up the subject of Human Physiology in such a way as to leave nothing for the Student to desire. This, however, has not been accomplished. The previously- existing Treatises upon it, which have been every year becoming more antiquated, have not been replaced by any works that can be considered as at the same time sufficiently elevated in their character, to represent the present condition of Physiological Science, sufficiently compendi- ous in their bulk for the limited time at the disposal of most Students, and sufficiently practical in their tendency to lead their readers to the useful applications of the facts and principles they place before them. This is not the opinion of the Author alone, but that of numerous expe- rienced Teachers throughout the country; and he has been led to regard the present as a good time for carrying his purpose into execution. The plan and objects of his Treatise may be gathered from the pre- ceding statement of the reasons which have occasioned its production. In this, as in his previous work, it has been his object to place the Reader in the possession of the highest principles, that can be regarded as firmly established, in each department of the Science ; and to explain and illustrate these by the introduction of as many important facts as could be included within moderate limits. In every instance he has endeavoured to make his statements clear and precise, without being formal or dogmatical ; and definite enough to admit of practical appli- cation, without appearing to be unimprovable by further inquiry. Phy- siology is essentially a science of progress ; and it must happen that 2 XIV PREFACE. much of what is now regarded as established truth, will need great modification to be brought into accordance with the results of new inquiries. It is very desirable, therefore, that the Student should not be made to think so confidently of his acquirements as to be indisposed to receive new information, even though it should tend to diminish their value. The present Treatise is to be regarded as complete in itself, and as quite independent of the Author's "Principles of General and Compara- tive Physiology." That it may be so, he has inserted an introductory chapter on the "Place of Man in the Scale of Being," and numerous references to the Comparative Physiology of the lower Animals. Still he does not hesitate to express the opinion that, the greater the amount of the Student's previous general knowledge of the Science, the better will he be prepared to enter upon any department of it, especially that peculiarly complex and difficult branch, the Physiology of Man. On every topic it has been the Author's aim to present the latest and most satisfactory information within his reach ; and he believes that the Volume contains much that will be new to the Physiologist, whose reading has not been tolerably extensive. Its materials have been but little derived from other Systematic Treatises on the subject; and it will not be found to bear, as a whole, any considerable resemblance to those already before the public. The Author has rather endeavoured to bring together the valuable facts and principles, scattered through the best of the numerous Monographs that have been recently published on special divisions of Physiology and Medicine; and to reduce these disjecta membra to that systematic form, which they can only be rightly made to assume, when brought into relation with" each other, and shown to be subservient to principles of still higher generality. In regard to this, as to his former Treatise, the Author believes that he may claim a somewhat higher character than that of the mere Compiler ; and that even the well-read Physiologist will find in it many facts and deductions which have not been previously brought before him in the same form. In apportioning the amount of space to be devoted to each division of the subject, the Author has had in view its practical relations, much more than its merely scientific interest; and he has on this account bestowed a much larger share on the Organs of Animal life than some may think just, when compared with the narrow limits within which other important topics are discussed. But he has endeavoured to keep always in view, that he is writing for the guidance of the Student who is to become a Practitioner, rather than for him who makes the pursuit of Science his professed object; and that much that is of the highest interest to the latter, is comparatively valueless to the former. Hence PREFACE. XV many topics of great scientific interest are entirely passed over ; and it is hoped that such omissions will not be accounted as faults in the esti- mation of those, who dread lest the attention of the Student should be too much drawn off by the seducing novelties of Science, from his less attractive, but more important objetts. For a large part of his illustrations the Author is indebted to the valuable and beautiful Icones Physiologies of Prof. Wagner. The sources of all are indicated. In conclusion, the Author would repeat what he has already had occasion to state ; that in a work involving many details, it is not to be expected that no error should have crept in ; but that he has endea- voured to secure correctness, by relying only upon such authorities as appeared to him competent, and by comparing their statements with such general principles as he considers well established. For the truth of those principles he holds himself responsible ; for the correctness of the details, he must appeal to those from whom they are derived, and to whom he has generally referred. He hopes that he will not be found unwilling to modify either, when they have been proved to be errone- ous; nor indisposed to profit by criticism, when administered in' a friendly spirit. Bristol, Feb. 1, 1842. TABLE OF CONTENTS. INTRODUCTION. GENERAL VIEW OF THE CONNECTION OF PHYSIOLOGY WITH OTHER BRANCHES OF MEDICINE, 25 31. CHAPTER I. ON THE PLACE OF MAN IN THE SCALE OF BEING, 32 68. PAGE 1. Distinction between Animals and Plants - 32 2. General sub-divisions of the Animal Kingdom - 34 3. General characters of Radiata ...... 35 4. General characters of Mollusca - - 37 5. General characters of Articulata - 41 6. General characters of Vertebrata - 43 7. General characters of Fishes ...... 46 8. General characters of Reptiles 47 9. General characters of Birds - ... 50 10. General characters of Mammalia - ... 54 11. Chief sub-divisions of Mammalia ...... 57 12. Characteristics of Man ...... 60 CHAPTER II. GENERAL VIEW OF THE FUNCTIONS, 69 88. .1. Of Vital Actions, and their mutual dependence 69 2. Functions of Vegetative Life - 76 3. Functions of Animal Life ... . 85 CHAPTER III. FUNCTIONS OF THE NERVOUS SYSTEM, 88 224. 1. General Summary .... 88 2. Elementary Structure of the Nervous System 90 3. Elementary Functions of Nervous Structure - 93 4. Mode of determining the Functions of Nerves - - 96 5. Nature of the Changes in the Nervous System 98 6. Comparative Anatomy and Physiology of the Nervous System - 100 7. Nervous System of Vertebrata 118 8. Functions of the Spinal Cord - - 132 9. Respiratory Movements - 139 10. Deglutition and Defecation ..... 145 XV111 CONTENTS. PAGE 11. Protecting Agency of the Spinal Cord - -- . - 153 12. Other Functions of Spinal Cord ..... 155 13. Comparative Anatomy of the Encephalon - - - - 158 14. Functions of the Cephalic Nerves - .... 164 15. 'Motor Nerves of the Orbit ....... 182 16. Consensual Movements of the Eye - - - - 186 17. General Functions of the Encephalon - - . - 191 18. Functions of the Tubercula Quadrigemina, &c.; Emotional and Instinctive Actions 191 19. Functions of the Cerebellum .... jgg 20. Functions of the Cerebrum - ... 204 21. General Recapitulation and Pathological Applications - - - 216 CHAPTER IV. OF SENSATION AND THE ORGANS OF THE SENSES, 224 270. 1. Of Sensation in General - - ... 224 2. Sense of Touch .... 234 3. Sense of Taste . . 237 4. Sense of Smell - r 238 5. Sense of Vision ... . 240 6. Sense of Hearing - ... 255 CHAPTER V. OF MUSCULAR CONTRACTILITY, 270 296. 1. Of Contractility in General - ... 270 2. Muscles of Animal Life . - 271 3. Muscles of Organic Life - 279 4. Properties of Muscular Fibre - - 280 5. Energy and Rapidity of Muscular Contraction - 290 6. Applications of Muscular Power - 292 7. Sensibility of Muscles - 295 CHAPTER VI. OF THE VOICE AND SPEECH, 296 312. 1. The Larynx and its Actions - - - 296 2. Of Articulate Sounds 307 CHAPTER VII. INFLUENCE OF THE NERVOUS SYSTEM ON THE ORGANIC FUNCTIONS, 312 319. CHAPTER VIII. ON DIGESTION AND NUTRITIVE ABSORPTION, 320 357. 1. Nature and Destination of the Food of Animals - 320 2. Mastication and Deglutition ... - 327 3. Action of the Stomach - 329 4. Action of the Intestinal Tube - 335 5. Nature of Chymification - 336 6. Lacteal and Lymphatic Absorption - - 345 CONTENTS. XIX PAGE 7. Absorption by the General Surface - - 348 8. Supply of Food required by Man - - 353 CHAPTER IX. OF THE CIRCULATION, 358388. 1. Of the Circulation in General - 358 2. Action of the Heart - - 363 3. Causes influencing the Circulation in the Arteries and Capillaries - 373 4. Of the Venous Circulation - - - 384 5. Peculiarities of the Circulation in different parts - - 387 CHAPTER X. ON RESPIRATION, 388 413 1. Nature of the Function; and Provisions for its Performance - 388 2. Chemical Phenomena of Respiration - 400 3. Effects of Respiration on the Blood - - 403 4. Exhalation and Absorption by the Lungs > - - 411 CHAPTER XI. ON NUTRITION, 413507. 1. Organizable Principles .... . 413 2. Formation of Cells - - 418 3. Elaboration of Chyle and Lymph ... . 423 4. Physical and Vital Properties of the Blood - - 427 5. Pathological changes in the Blood - - 446 6. Origin of the Solid Tissues. Reparative processes ... 451 7. Varying Activity of the Nutritive processes - 457 8. Abnormal forms of the Nutritive process - - 460 9. Formation of the Tissues - - 463 CHAPTER XII. OF SECRETION, 507 567. 1. Of Secretion in General ... . 50? 2. The Liver. Secretion of Bile - 511 3. The Kidneys. Secretion of Urine - - 524 4. Mammary GJand. Secretion of Milk - 540 5. Salivary Glands and Pancreas ... . 549 6. Lachrymal Gland - - 551 7. The Testis. Spermatic Fluid - 551 8. Cutaneous and Mucous Follicles - - 554 9. The Spleen, and Supra-Renal Capsules - 562 10. Thymus and Thyroid Glands - 565 CHAPTER XIII.* GENERAL REVIEW OF THE NUTRITIVE PROCESSES, 567585. 1. Review of the Nutritive processes, with practical applications 567 2. Animal Heat - 573 XX CONTENTS. CHAPTER XIV. OF REPRODUCTION, 585627. PAGE 1. General character of the Function - - 585 2. Action of the Male - - 587 3. Action of the Female - 59 4. Development of the Embryo - - 610 tfv , y ,., WJMB^^ X \ d '$&$#$ *^.\ \ II^M.-^ * ,/ , V 22 '%<*< ff$ h "^ " ^ w 19 ^ 23 -> /?? ^fl^^S^V^ '^ ""^%, Cf H ; - .^i^.-r,.:^ Z> : ^ "b b . Pkuaa. EXPLANATION OF PLATES. PLATE I. The first 16 Figures in this Plate are from Dr. Barry's Embryological Researches in the Philosophical Transactions for 1837, 1839 and 1840. FIG. 1. A very early stage of the formation of the Ovum ; the vesicles, the largest of which measures only 1-1 125th of an inch, are seen in the midst of dark granules or globules ( 740). 2. A stage somewhat more advanced; the vesicles are surrounded by envelops of smaller vesicles, amongst which the granules are still seen ( 740). 3. A still later stage ; a central vesicle, a, is seen, with a spot, b, upon its walls, and surrounded with numerous granules; this has now evidently become the Germi- nal Vesicle ( 740). 4. Ovisacs from Human Ovum, l-200th of an inch, and upwards, in diameter; the largest exhibits the Germinal Vesicle, a, very distinctly ( 740). 5. Ovisac from Cat, showing its contents when near matarity; a, ovisac; b, its con- tained granules ; c, zona pellucida; d, granules of the yolk; e, germinal. vesicle ; /, germinal spot; magnified 440 diameters ( 739). 6. Ovum of Rabbit at the periphery of the Graafian follicle, with part of the membrana granulosa removed; g, g, membrana granulosa; ov, ovulum; r, retinacula ( 740, 744). 7. Ovum with its tunica granulosa and retinacula, removed from the Graafian follicle; a, germinal vesicle; b, germinal spot; c, zona pellucida; d, globules of the yolk; r, r, retinacula ; t g, tunica granulosa ( 740). 8. Graafian follicle discharging its Ovum, ov, to which the tunica granulosa, t g, and re- tinacula, r, r, remain attached ( 744). 9. Ovarium Ovum in preparation for fecundation ; a, germinal spot, beginning to resolve itself into cells at its margin ; b, germinal vesicle ; c, elliptical cells in the place of the yolk; d, zona pellucida. 100 Diameters ( 745). 10. Ovum nearly ready for fecundation: a, germinal spot more fully developed into cells, of which concentric layers occupy the germinal vesicle bf c, elliptical discs or cells; d, zona pellucida; e, outer layer of cells of yolk ( 745). 11. Fecundated Ovum of nine hours; the germinal vesicle, having returned to the centre of the ovum, is concealed by the large elliptical discs, which fill the cavity of the zona pellucida ( 745, 746). 12. Plan of one of these discs or cells: its nucleus, , has developed itself into con- centric rings of cells : and in the most fully developed of these, the nucleus, b, is seen to be commencing the same kind of evolution. In the centre of the original nucleus, a pellucid spot, the nucleolus of Schwann and Schleiden, is observed ( 745). 13. Ovum from the Uterus, measuring about l-68th of an iocb in diameter: a, pair of XX11 EXPLANATION OF PLATES. FIG. cells now occupying the greater part of the germinal vesicle b,- c, zona pellu- cida: d, chorion, a new envelop, separated from the last by the fluid it has ab- sorbed ( 746). 14. Ovum, of which the essential part, a, the pair of cells occupying the germinal vesicle, has advanced further than in the last case; the other contents of the germinal vesicle have undergone liquefaction. The chorion is here incipient: and the remains of the cells of which it is composed are seen at cho ( 747). 15. More advanced ovum; the cavity of the germinal vesicle filled with cells, a, that have originated in the two represented in the last figure; these cells have nuclei, b, which are undergoing a corresponding process of evolution into secondary cells; c and d as in Fig. 13 ( 758). 16. Ovum in a state rather more advanced; a, central cell of the germinal mass, now come to the surface, and showing the nucleus b with a pellucid centre, from which most of the embryonic structures are developed; c, cavity in the germinal mass, caused by the approach of its peripheral cells to the enclosing membrane, d ( 758). 17. Formation of the Membrana Decidua; a, a, a, villi of the mucous membrane^of the uterus; b, substance secreted between and upon these; c, uterine vessels pro- longed into the decidua and forming loops. After Baer. 18. Human Spermatozoa; a, seminal granules. After Wagner ( 735). 19. Cyst of evolution. After Wagner ( 735). 20. Capsular bundle of Spermatozoa, just previous to their separation. After Wagner ( 735). 21. Globules fiom the Chyle; a, ordinary globules; b, a globule (cytoblastl) surrounding itself with an envelop (a forming cell?); c, minute molecules of chyle; d,a co- lourless corpuscle from the blood. After Wagner ( 563). 22. Particles of Blood undergoing multiplication: a, b, c, d, e, successive stages. After Barry ( 576). 23. Extremity of one of the tufts of fetal vessels forming the Placenta ; this includes (like a branchial tuft) an artery and vein. After Reid ( 749). 24. Plan of the structure of the Placenta, according to Dr. J. Reid's view of it ; a, a, por- tion of substance of uterus ; b, b, b, b, section of uterine sinuses, some of them opening on the inner surface into the cavity of the placenta ; c, curling artery of uterus ; d, d, ramifications of foetal vessels, some of them sending down pro- longed tufts which dip into the uterine sinuses ( 749). PLATE II. 25. Uterine Ovum of Rabbit, showing the Area Pellucida, with the annular nucleus of the embryonic cell (Fig. 14, 6) now elongated. In the clear space enclosed by this is a well-marked dark groove, occupying the position in which the nervous centres are subsequently to be developed. The cephalic extremity of this is already rounded and the caudal extremity pointed. After Bischoff ( 760). 26. More advanced ovum, showing the incipient formation of the Vertebral column ; and the dilatation of the primitive groove at its anterior extremity. After Bischoff ( 760). 27. More advanced embryo, seen on its ventral side, and showing the first development of the Circulating apparatus. Around the Vascular Area is shown the terminal PL ATE 11. My. 27 EXPLANATION OF PLATES. XX111 sinus, a, a, a. The blood returns from this by two superior branches, b, 6, and two inferior, c, c, of the omphalo-raeseraic veins, to the heart, d; which , at this period, a tube curved on itself, and presenting the first indication of a division into cavities. The two aortic trunks appear, in the abnormal region, as the inferior vertebral arteries, e, e; from which are given off the omphalo- meseraic arteries, /,/, which form a net-work that distributes the blood over the vascular area. In the cephalic region is seen the anterior cerebral vesi- cles, with the two ocular vesicles, g. After Bischoff ( 762). INTRODUCTION. GENERAL VIEW OF THE CONNECTION OF PHYSIOLOGY WITH OTHER BRANCHES OF MEDICINE. 1. THE object of the Science of Physiology is to bring- together, in a sys- tematic form, the phenomena which normally present themselves during the existence of living beings ; and to classify and compare these, in such a man- ner as to deduce from them the general laws or principles, by which they are regulated. In order to attain a correct knowledge of the latter, a very exten- sive comparison is requisite. Principles, which might seem of paramount importance in regard to one group of living beings, are often found, on a more general review, to be quite subordinate. For example, the predominance of the Nervous System in the higher classes of Animals, and its evidently close connection with many of the functions of life, have led several physiologists to the opinion, that its influence is essential to the performance of the functions of Nutrition, Secretion, &c. : but, on turning our attention to the Vegetable kingdom, in which nothing analogous to a nervous system can be proved to exist, we find these functions going on with even greater activity than in Animals. It is clear, therefore, that they may be performed without it ; and on a closer examination of the phenomena presented by Animals, it is seen that they may be explained equally well, or even better, on the principle that the nervous system has a powerful influence on these actions, than on the idea that it affords a condition essential to them. This is only one out of many instances which it would be easy to adduce, in proof of the necessity of bring- ing together all the phenomena of the same kind, in whatever class of living beings they may be presented, before we erect any general principles in Physiology. 2. The object of the present work, however, is not to follow out such an investigation, but to show the detailed application of the principles, of which Physiological Science may now be said to consist, to the phenomena exhibited by the Human being, during what may be called his normal life. Every one knows the difficulty of defining the two conditions health and disease. The former may be said to be that state in which the various actions of life are normally or regularly performed ; and the latter to result from a disturbance or irregularity in these actions, constituting an abnormal state. But this is only substituting one term for another ; the difficulty remains the same. Many variations occur, within the limits of what must be called in some per- sons the normal state, which in others must be regarded as abnormal actions. Thus in most adults the pulse averages about 70;. but it is easily raised by exercise to 90, without any injurious consequences ; and such must be regarded as a normal or physiological state. But we occasionally meet with instances in which the usual pulse is not quicker than 40 ; and for this to rise to 90 might indicate a very alarming state of the system ; since there are individuals 3 26 CONNECTION OF PHYSIOLOGY WITH in whom such a pulse would be equivalent to one of 140, in a person whose circulation was in health of the average rapidity. Thus an abnormal state in any individual can frequently be ascertained only by comparison with his normal condition. This is a difficulty from which we can never hope to make a complete escape ; for it is the peculiar character of living beings, to exhibit such variations in their phenomena, resulting from the number of concurrent causes which are involved in the production of these ; a slight alteration in any one of which will most seriously affect the general result. Upon the distinction between normal and abnormal life, however, is founded that of the two sciences Physiology and Pathology. These are very closely related to each other ; and neither can be pursued with the prospect of complete success, except in connection \vith the other. 3. The relation between the sciences of Physiology and Pathology on the one hand, and the various departments of the Ars Medendi on the other, may be shown within a brief compass ; and it is perhaps desirable to point them out here, in order to place the importance of these sciences in its proper light. Science must strictly be said to consist of general principles, embodying the phenomena which present themselves to the observer, in any particular de- partment of observation ; and a science is perfect, in so far as its collection of facts is reducible to these general laws, the ascertainment of which enables us to extend our knowledge of similar facts. An Art is, properly speaking, the application of a science to practical purposes, consisting of a set of rules de- duced from its principles ; and its perfection will be exactly proportional to that of the science upon which it rests. The most perfect of all sciences is astronomy ; all its leading phenomena may be reduced to one general prin- ciple ; and phenomena, previously unobserved, have been predicted as the necessary results of that principle. The Art of Navigation is a collection of rules, framed by those who were profoundly conversant with the principles of the science ; but capable of being employed by those whose knowledge extends no further than to the mode of applying them. When, however, the science has not this degree of perfection, the art has not this independent character. In Chemistry, for example, many principles of high generality have been attained ; and yet unknown phenomena cannot be predicted from them with certainty (in a great variety of cases at least), owing to the number of other conditions by which they may be affected. Hence no art founded upon this science can be supplied with any other than very limited rules. In the case of dyeing, for example, great improvement has been effected by chemical knowledge ; but the greater part of its rules are empirical, that is to say, founded on a limited induction, not comprehended in more general prin- ciples, and therefore quite uncertain in their results. Thus, if a new Animal or Vegetable dye were discovered, the modes of fixing- and discharging it, and of varying its shades of colour, w r ould have to be determined by experiment, before it could be brought into advantageous employment. We can neverthe- less imagine, and (from the recent great advance in Organic Chemistry) in some degree anticipate, the period, when chemical science shall be so far advanced, that a simple analysis of the material (supplying data corresponding to the solar or lunar observations of the navigator) may enable the manufac- turer, by a reference to his code of rules, to avail himself to the fullest extent of its capabilities, without being himself aware of the principles upon which those rules are founded. 4. An Art, then, will be scientific or empirical (that is, its rules will be based upon general principles, or upon induction from a limited experience), in proportion to the comprehensiveness of the laws that have been attained in the Science on which it rests. This distinction, however, has nothing to do with the certainty or uncertainty of its application in particular instances. OTHER BRANCHES OF MEDICINE. 27 An art may be entirely empirical, and yet be perfect so far as it goes ; but no unknown cases are provided for, no contingencies foreseen. It is in its adap- tation to these, that the triumph of a scientific over an empirical art manifests itself; and in proportion as, from the nature of the subjects embraced by it, a greater or less variety of novel cases presents itself, in that proportion is its superiority more evident. It was well observed by Lord Bacon, that "it is the office and excellence of all sciences to shorten the long turnings and windings of experience." The deficiency of higher or more comprehensive laws should not prevent us from making cautious use of those we already possess ; and, where the demands of mankind require that an art should be practised even in its imperfect condition, we must be content with such means of satisfying them as lie within our reach. Contentment, however, by no means involves a tacit acquiescence in the infirmities of our condition ; and the man of noble and elevated mind will not only aim at the perfection of his science, from that abstract love of knowledge, which is, as Sir H. Davy has beautifully remarked, "in its ultimate and perfect development, the love of infinite wisdom and un- bounded power, or the love of God," but may also safely cherish the belief, that every contribution which he makes to general laws will ultimately have its practical bearing on the condition of humanity. 5. In no department of inquiry is it more necessary to keep these princi- ples in view, than in that which relates to the phenomena of Vitality. The changes which characterize living beings, and which in their totality constitute the Life of these, are as capable of being referred to general laws, expressive of their uniform conditions, as are phenomena of any other kind. But there are many causes which render the attainment of these laws so difficult, that at present we cannot assign to Biology (a term which may be advantageously employed for the science of Vital Action, including Physiology and Pathology), a high rank amongst the sciences. Hence the rules of any arts which are founded upon it, can be only in part regarded as possessing that certainty which it is desirable they should have. Some there are, which are derived from laws of such high generality, that we cannot imagine any cause which can interfere with their application. For example, it is one of these facts of universal application, that a large mixture of carbonic acid in the medium to which the circulating fluid is exposed for aeration, is prejudicial to life ; and an obvious rule thence follows. But it is not a fact of equal universality, that a dose of a purgative medicine will induce increased action of the bowels ; for there may be many conditions of the system in which this shall not occur. The physician, in directing the ventilation of a room, would be guided by a high scientific principle ; whilst in administering a medicine, he is working upon an induction derived from a comparatively limited experience. 6. The art which most directly springs out of the science of Physiology, is that of Hygiene, which may be defined as a system of rules for the preserva- tion of the body in health, deduced from the principles by which its actions are governed. Were the science of Physiology perfect, the art would require little skill for its practice; this, however, is far from being the case. Its rules are at present founded, in great part, upon too limited an induction to deserve the title of universal; and their operation is frequently interfered with by a number of causes, of whose mode of action we are almost entirely ignorant. Still much has been done, by calling public attention to those, of which the general importance is acknowledged, in preserving the body in health by removing the causes of disease ; as the increased value of human life, shown by statistical returns, abundantly testifies. And the Physiologist can readily point out many more, which have not yet received the attention that their importance deserves. The term Hygiene is sometimes used to include the art of restoring as well as preserving health, by the use of means not strictly 28 CONNECTION OF PHYSIOLOGY WITH to be regarded as medical, e. g., the regulation of diet, temperature, &c.; but this employment of it is not strictly correct, such treatment being properly a part of Therapeutics, an art which stands in the same relation to Hygiene, that Pathology bears to Physiology. In proportion as the science of Physio- logy is perfected, will the simplicity and certainty of its practical applications increase; and though we may not anticipate a return of patriarchal longevity, yet the experience of the last century has amply shown, that every general increase of attention to its simple and universally-acknowledged truths, is attended with a prolongation of life, and contributes to that not less important object its emancipation from disease. Hence the establishment of the rules of Hygiene may be considered as the most direct practical benefit, afforded by the pursuit of Physiological science. 7. In the assistance which it affords, however, in the establishment of the principles of Pathology, the importance of Physiology is by no means inferior; and it is surprising how much the relation of the two has been neglected. That the knowledge of the normal actions of a living system is essential to success, in the investigation of the causes and mode of cure of its irregulari- ties, seems almost a self-evident proposition. We should all think it absurd for a person to attempt to repair a watch or a steam-engine that might be acting wrongly, without being acquainted with the uses of the several parts of its structure, both singly and in combination with each other. He might have such an acquaintance with their form and mechanical arrangement, as might enable him to delineate them, or even to construct the counterpart of the whole machine, without being able to put it into successful operation. Just so it is with an anatomist, who regards the mere acquaintance with the structure of the human body as a sufficient guide in the treatment of disease. But this is really of little assistance in any thing but surgical operations ; that which we require to know, for the rectification of morbid phenomena, being the normal history of those phenomena, and the conditions on which they are dependent. The neglect of physiological science as an adjunct to the ars medendi, may probably be in part attributed to the facility with which striking curative effects may often be produced, by the application of empi- rical rules only. Thus a person usually healthy, who is suffering from head- ache, feverishness and constipation, the effects of an overloaded alimentary canal, may be pretty certainly relieved by a brisk purge ; or a stout child, w r ho is suffering from cough, tightness of the chest, and heat and dryness of skin, resulting from recent exposure to cold and damp, by a strong dose of an antimonial. These are results with which every tyro is well acquainted ; they are based upon ordinary experience, and, if applied without further con- sideration of their rationale, are strictly empirical. The case might be com- pared to that, in which a person unacquainted with the construction or prin- ciples of action of a watch or a steam-engine, alters its rate of movement, by shifting a lever or opening a cock. But, though usually successful, excep- tional cases will occur, in which unexpected results will follow ; and the merely empirical practitioner is baffled and confounded. For these, some- thing more is requisite ; and no treatment can be successful, otherwise than by an accidental coincidence, in which the causes of the derangement are not carefully inquired into, and their operation understood. And how can their operation, in producing a disturbance of the system, be comprehended, when its regular actions are not even known, far less, their principles ascertained? 8. The study of Physiology, being the inquiry into the phenomena of nor- mal life and the conditions of those phenomena, requires a knowledge of the two sets of causes which must be concerned in them, the organized struc- ture or mechanism, possessed of certain properties, and the agents or sti- muli, by whose operation on this mechanism its properties are made to de- OTHER BRANCHES OF MEDICINE. 29 velop themselves in the production of phenomena. These require to be separately considered ; just in the same manner as when, in examining the action of a steam-engine, we inquire into its mechanical structure, and the effects upon it of the agencies by which it is put in operation. Now in the study of Pathology, or the science of diseased action, we have to attend, in the same manner, to two sets of conditions. On the one hand, we have to make ourselves acquainted with the characters of all the external agents, which can produce a deleterious effect upon the living body, whether their operation be mechanical, chemical, or more directly vital ; as well as with the results of the suspension, partial or complete, of the conditions by which its healthy action is maintained. On the other side, we have to investigate the changes of structure which manifest themselves in the body itself, the causes by which these are produced, and the new results which they will themselves occasion. Now one of the chief difficulties in the pursuit of Pathological Science results from this, that we are at present so imperfectly acquainted with the conditions required for normal action, that we cannot ascertain what those changes are, in which the derangement primarily consists. Hence we are in constant danger of mistaking the more evident changes, which are often but secondary results of the morbid action, for the real source of the dis- ease. For example, we are not yet sufficiently acquainted with the condi- tions necessary for the transmission of nervous influence, to be able to state when those conditions are interfered with ; hence a great extent of morbid alteration not unfrequently presents itself, in the parts which we know to be concerned in this operation, without such symptoms as we should expect to correspond with it ; and, on the other hand, we frequently observe during life most decided deviations from the ordinary sequence of nervous phenomena, which we cannot attribute to any change of structure that we can discover after death. Here, then, is a case in which Pathology must necessarily be imperfect, until Physiology has greatly advanced ; and numbers of similar instances might be pointed out. Again, it would be easy to show the direct benefit which the Physician has derived from the Physiologist, by reference to the same class of phenomena ; but for this we may refer to the subsequent part of this Treatise, in which the chief practical applications of late discove- ries as to the Functions of the Nervous System, will be set forth. It will be scarcely questioned, then, that the Science of Pathology has so direct and immediate a dependence upon that of Physiology, that the former cannot be pursued with a fair prospect of success, without a knowledge both of the principles and of the chief phenomena of the latter. 9. Another illustration may be useful. Few Pathologists regard any mor- bid process as better understood than that of Inflammation ; and yet scarcely any two are agreed as to its real nature. By some, its essential condition is stated to be a contraction or a dilatation of the capillary vessels; and this alteration is supposed by one to result from an exalted and by another from a diminished degree of vitality in their walls. Others, again, regarding Inflammation as an affection of the sensory rather than of the organic func- tions, have imagined its seat to be in the nervous system. Now it may be stated with tolerable confidence, that no theoretical view of the nature of Inflammation has exerted any beneficial influence on its treatment ; and that all the rules of practice to which we trust for the cure, are founded on expe- rience alone. It is therefore evident, that there must be something very faulty in the mode of cultivation, since the fruits yielded by a domain so fertile of phenomena are thus useless ; and the Physiologist has not much difficulty in pointing out several sources of error thaf have resulted from the insufficient acquaintance possessed by most Pathologists, with those normal actions of which Inflammation is a disturbed form. Thus, he can show that Innamma- 30 CONNECTION OF PHYSIOLOGY WITH tion is not primarily a disorder of the function of Circulation, but rather of Nutrition, the vascular apparatus being only secondarily affected ; so that no observations on the state of the vessels, and on the movement of the blood through them, give us any real information as to the nature of the morbid action. Further, he is aware that no inferences can be valid, that are founded on experiments made on the cold-blooded Vertebrata, since in them the true Inflammatory state can with difficulty be induced : and also, that the nervous system cannot be an element in the primary phenomena of Inflam- mation, since these are manifested by beings that do not possess it. It will hereafter be pointed out, that, by attention to the principles of Physiology, our knowledge of the real character of this and of many other morbid processes, is now being rapidly increased ; and that it is at the same time acquiring a degree of definiteness, which cannot but lead to important improvements in practice. 10. As Hygiene, or the art of preserving health, arises out of the science of Physiology, so does the Therapeutic art depend upon the science of Patho- logy ; or, to use language rendered venerable by its antiquity, the ars medendi, to be perfect, must be guided by the ratio medendi. The term Therapeutics, however, is sometimes used to denote that division of the Science of Pathology, which concerns the principles of the application of curative agents to the treatment of disease. There is no real ground, however, for distinguishing this as any other than a section of Pathology ; or for considering the practical use of these principles as any thing but an Art. As Life, in the healthy condition, is known to be maintained by the operation of external agents upon organized tissues endowed with vital properties, so is it found that, in dis- eased states of the system, such a change takes place in the character of these actions, as adapts them to its altered circumstances ; thus, in a febrile con- dition, when any increase of stimulus would be injurious, there is no longer an appetite for food. Moreover, it is found that by the regulation of the natural actions, or the substitution of new ones, the diseased condition may frequently be controlled, and the normal action restored. Hence the inquiry into the curative influence of external agents upon the phenomena of disease, is as much a part of the Science of Pathology, as the study of the influence of the ordinary- vital stimuli, in producing the normal actions of the system, is a division (as it is universally allowed to be) of the Science of Physiology. If this inquiry had terminated in the discovery of general principles, all difficulty would be removed from the Therapeutic art, as soon as the perplexities of diagnosis had been over- come ; and, in proportion as such are approached, and our knowledge of the essential nature of diseased actions is extended, will be the facility and the success of our curative treatment. 11. In the mean time the practitioner must be content to follow a middle course. His aim must be to avoid, on the one hand, confiding too exclusively in general principles, however stable and comprehensive he may imagine them to be, until he is satisfied that he knows, not only the principle itself, but the subordinate laws which regulate or modify its application to individual cases. Long after the highest laws of motion had been established by New- ton, no astronomer could, on the faith of them, have predicted the situation of a planet, with more than an approximation to certainty ; the law of attraction had to be applied in numberless modes not contemplated by its discoverer, before perfect accuracy could be attained. There is great danger, then, in the present state of the science of Pathology, in trusting to principles which we may consider unassailable, as our sole guides in the practice of our art ; and hence it is not always the scientific practitioner, as he is emphatically termed, who is the most successful in his treatment. On the other hand, to apply a particular mode of treatment to a particular set of symptoms, without inquiring OTHER BRANCHES OF MEDICINE. into the cause of those symptoms, merely on account of its having been suc- cessful in some case that appeared analogous, is a mode of practice completely empirical. Yet such a plan is constantly being pursued. But as soon as we begin to inquire into the cause of the morbid action, and seek to remove or counteract this, by the application of remedial means, which experience has shown to be effectual for such an object, we are really acting, to a certain extent, on scientific principles. The recorded experience of ages, in its con- densed form, must of necessity assume the appearance of general rules of practice ; and it is in the application of these rules to individual cases, and in the distinction of those phenomena whose causes are subservient to them, from those which are beyond their pale and which require a mode of treat- ment altogether different, that the sagacity of the practitioner most displays itself. The rational empiricism which prevails in this country at the present day, is a mode of practice that may be regarded as best combining the advan- tages of scientific knowledge and of recorded experience. The value of facts as the only sure basis of general principles is duly appreciated; and yet there is no indisposition to make trial of such principles when announced, and to abide by them so far as they appear practically available. This is the only method in which the young practitioner can hope to succeed. The increased attention at present paid to diagnosis, will frequently enable him to determine the real nature of the malady with much more precision than a man of age and experience can do, who has not kept pace with the progress of medical science ; whilst the latter will have decidedly the advantage in the application of therapeutic means, especially in those obscure cases that most require that tact which can only be gained by long and attentive observation. 12. The numerical method, which is at present much valued by many, as a guide in the Study and Practice of Medicine, is simply a statistical arrange- ment of the phenomena presented by various diseases, with a view of deter- mining the frequency of their occurrence, their connections with each other, and the influence of various modes of treatment upon them. Its advantages in substituting an accurate and definite record of facts, for the vague state- ments which we so frequently meet with, are unquestionable. Yet we must be careful not to attach too much importance to the results afforded by it. They have a tendency to lead to the substitution of empirical rules for scientific principles ; and if too exclusively followed, therefore, will tend to the retardation of Pathology. If the practitioner is led to reason thus on every particular instance, " In nine-tenths of the cases exhibiting these symptoms, such-and- such a treatment is successful; therefore I shall adopt this treatment in the present one," he is acting on a most grossly empirical system. A general law admits of no exceptions ; and if such appear to present themselves, they must be due to some cause interfering with its operation. His object ought to be rather, therefore, to ascertain what plan of treatment is constantly successful in each form of disease; in other words, to determine that invariable sequence of cause and effect, on which alone general principles or laws can be erected ; and in order to do this, he must carefully analyze the unsuccessful cases, and ascertain in what their conditions differed from the rest, so as to be able to determine positively to which head he is to refer the case before him, and to be guided in his treatment accordingly. In this manner he will advance the Science ; whilst in the other he is reducing the Art to its lowest condition.* * For a lucid analysis of the value of the numerical method, Dr. Symonds's Retrospec- tive Address (at the Liverpool Meeting of the Provincial Medical Association) may be advantageously consulted. 32 CHAPTER I. ON THE PLACE OF MAN IN THE SCALE OF BEING. Distinction "between Animals and Plants. 13. IN entering upon the general survey of the Animal Kingdom, which it is desirable to take, before we consider in detail any particular member of it, the question naturally arises, how is the Animal distinguished from the Vegetable ? There is no difficulty in replying to this, if we keep in view merely the higher tribes of each division; no one, for example, would be in any danger of confounding a Whale with a Palnij or an Elephant with an Oak. It is when we descend to the opposite extremity of the scale, that we encounter the greatest difficulty ; from the circumstance, that the distinguish- ing characters of each kingdom disappear, one after another, until we are reduced to those which seem common to both. So completely is this the case, that there are many tribes, which cannot, in the present state of our know- ledge, be referred with certainty to either one division or the other. We are accustomed to think of Animals as beings, which not only grow and reproduce themselves, but also possess the power of spontaneously moving from place to place, and are conscious of impressions made upon them: and we usually regard Plants as beings which are entirely destitute of sensibility and of the power of spontaneous motion, going through all their processes of growth, reproduction, and decay, alike unconscious of pleasure and of pain, and devoid of all power of voluntarily changing their condition. Such a definition is pro- bably the most correct that we can employ ; but great difficulties lie in the way of its application. There are many tribes which possess a general structure more allied to that of beings known to be Animals, than to that of any Plants ; and which yet present no decided indications either of sensibility or of volun- tary power. Such is the Sponge, the fabric of which closely corresponds with that of many Alcyonian Polypes, whose animality is undoubted; and yet neither observation nor experiment has ever succeeded in proving that the Sponge feels or spontaneously moves. Yet there are no known Vegetables, to which it presents any near resemblance. On the other hand, there are many vegetables that perform evident movements, which, at first sight, appear to be spontaneous, as if they indicated sensibility on the part of the being that performs them. Such movements, however, can in some instances (as in that of the Sensitive-Plant, or of the Venus's Fly-trap), be referred to a sort of mechanism, the action of which does not involve sensibility, and which may be compared with the many movements (such as that of the heart) that are constantly taking place in the bodies of the highest animals, without their consciousness; and in other cases (as in the Oscillalorise) they are so rhythm- ical, as to impress the observer with the idea, that they are rather the result of some physical, than of any mental, influence. In this respect they corre- spond with the motions of the constantly-vibrating cilia, which cover the sur- face of the mucous membranes of Animals. 14. However difficult it may be for us, owing to our imperfect knowledge, DISTINCTION BETWEEN ANIMALS AND PLANTS. 33 to ~4raw the line in individual cases, it cannot be doubted that a boundary <'x1st; and in general a very simple mark will suffice to establish the distmchoiv. This mark is the presence or absence of a stomach or internal cavitji/flh* the reception of food. The possession of a stomach cannot be regarded, however, as in itself an essential distinction between the two king- doms (as some have represented it) ; for its presence is merely a result, so to speak, of tlje nature of the food of Animals, and of the mode in which it is obtained. Vegetables are dependent for their support, upon those materials only which they obtain from the surrounding elements ; carbonic acid, water, and ammonia, duly supplied to them, with a small quantity of certain mineral ingredients, afford all the conditions they require, for the production of the most massive fabrics, and the greatest variety of secretions. But these same elements, if supplied to Animals, could not be converted by them into the materials of organized structures ; for they can only employ as food substances which have bee.n already organized ; and they are consequently dependent, either directly or indirectly, upon the Vegetable kingdom, for their means of support. Now they cannot incorporate any. alimentary substance into their own tissues, until it has been reduced to the fluid form ; hence they need the means of effecting this reduction, which are supplied by the stomach. Again, they cannot be always in immediate relation with their food ; they have to go in search of it, and need a store-room in which it may be deposited during the intervals ; this purpose also is supplied by the stomach. It is evident,' moreover, that the powers of voluntary locomotion and sensation, which Ani- mals enjoy, are connected with the peculiar nature of the food they require ; for if they were fixed in the ground, like Plants, they would not be able to obtain that which they require for their support. It is true that there are some which seem almost rooted to one spot ; but these have the power of bringing their food within their reach, though they cannot go in search of it. Such is the case with many Polypes, which use their outspread tentacula for this purpose ; and with the lower Mollusca, which can create currents by means of ciliary action. 15. A distinction might probably be erected between the Animal and Vege- table kingdoms, upon the mode in which the first development of the germ takes place. The seed of the Plant, at the time of fertilization, principally consists of a store of nourishment prepared by the parent for the supply of the germ, Avhich is introduced into the midst of it. The same may be said of the egg of the Animal. In both instances, the first development of the germ is into a membranous expansion, which absorbs the alimentary materials with which it is in contact ; and it prepares these by assimilation, for the nourish- ment of the embryonic structure, the most important parts of which, in the higher classes of Animals and in Phanerogamic Plants, the only permanent parts are in its centre. Now in Plants, this membranous expansion (the single or double cotyledon) absorbs -by its outer surface, which is applied to the albumen of the seed, and takes it more or less completely into its own substance. In Animals, this expansion is developed in such a manner, that it surrounds the albumen, enclosing it in a sac, the inner surface only of which is 'concerned in absorption. The sac is, then, the temporary stomach of the embryonic structure ; it becomes the permanent stomach of the Radiata ; but in the higher classes, only a portion of it is retained in the fabric of the adult, the remainder being cast offj like the cotyledon of Plants, as soon as it has performed its function. Thus, then, the first nisus of Animal development is towards the formation of a stomach, for the internal reception and digestion of food ; whilst the first processes of Vegetable evolution tend to the production of a frond-like membrane, which, like the permanent frond of the lower classes of Plants, absorbs nourishment by its expanded surface only. 34 ON THE PLACE OF MAN IN THE SCALE OF BEING. 16. Some Physiologists have asserted, that the nature of the respiratory process affords a ground of distinction between Animals and Plants ; oxygen being absorbed, and carbonic acid evolved, by the former, and a converse change being effected in the surrounding air by the latter. It is not correct, however, to designate this converse change as a consequence of the respiratory process ; for in Plants, as in Animals, there is a continual absorption of oxy- gen and evolution of carbonic acid, which constitute the function of respira- tion ; but the effects of this change are masked (as it were) by those of the process of fixation of carbon from the atmosphere, which only takes place under the influence of sun-light, and which is much more analogous to the digestion of Animals. The most valid distinction, in doubtful cases, seems likely to be founded on the chemical constitution of the tissues themselves. In the Plant, the whole of the organized structure, when freed from the pro- ducts of secretion which are deposited in it, (many of these containing the same proportion of nitrogen as exists in animal flesh, 454,) is found to have the same composition with starch ; being formed of oxygen, hydrogen, and carbon only. In the Animal, on the other hand, the organized tissues all con- tain azote as part of their proper substance ; non-azotized compounds, such as fatty matter, being merely deposited in these, as products of secretion. Hence if the chemical composition of the organized tissues themselves can be cor- rectly determined, the Vegetable or Animal nature of a doubtful body may be ascertained. By this test the long-disputed question of the nature of the true Corallines has been set at rest ; their tissue, when freed from the lime depo- sited in it, being found to have the composition of that of Plants.* General Subdivisions of the Jlnimal Kingdom. 17. The Animal kingdom was formerly divided into two primary groups, the Vertebrated and the Invertebrated; the former comprising those which are distinguished by the possession of a jointed spinal column, consisting of a number of internal bones termed vertebrae ; and the latter including all those animals, which are destitute of this support. It was pointed out by Cuvier, however, that, among the Invertebrata, there are three types of organization, as distinct from each other, as any of them are from the Vertebrata ; and he accordingly distributed the whole under four primary divisions or sub-king- doms : of these, the VERTEBRATA rank highest ; next, the ARTIUULATA and the MOLLUSCA, which are both inferior in degree of organization to the Vertebrata, but are superior to the lowest group, the RADIATA, which contains those ani- mals that border most closely, both in external aspect, and in general character, upon the Vegetable kingdom. The members of these groups are readily sepa- rated from each other by the structure of their skeletons, or organs of support and protection ; as well as by many other characters. In the Vertebrata, the skeleton consists of a number of internal jointed bones, which are clothed by the muscles that are attached to them and move them ; these bones are tra- versed by blood-vessels and absorbents, and are to be regarded as in all re- spects analogous to the other living tissues of the body. In the Articulata, the soft parts are supported by a hard external envelope, which is of corresponding form on the two sides of the median line, and is divided into several pieces, jointed or articulated together by a membrane, in Such a manner as still to allow of free motion ; and the muscles, which are numerous and complex, are attached to the interior of these. In the Mollusca, the whole body is quite soft ; and many species exist, in which it has no external protection : in . a large proportion of the group, however, the surface has the power of exuding * See London Physiological Journal, vol. i., p. 29, and Comptes Rendus, July 3, 1843. GENERAL CHARACTERS OF RADIATA. 35 shelly matter, so as to form a protective habitation, within which the animal can withdraw its body, but which is by no means to be regarded as a part of it, and does not exhibit any definite type of form. In the Radiata, all the parts are arranged in a circular manner, the mouth being in the centre ; some of them are protected by firmly-jointed external skeletons, like those of the Arti- culata ; whilst others deposit calcareous matter in the centre of their soft fleshy structures, as if sketching out the internal skeleton of the Vertebrata. The skeletons of most of the Invertebrata differ, however, from those of Vertebrate animals, in this important character, that they are not permeated by vessels, and are formed only by a superficial deposition. Hence they are termed extra- vascular : and it is an obvious result of an arrangement of this kind, that parts once formed are never changed, except by the ordinary processes of decay, and that they can only be extended by addition to their exterior ; whilst in Verte- brata, the bones are subject to alterations of any kind, whether of removal or addition, throughout their entire substance. It is not correct to regard them, however, as destitute of vitality ; since they consist, in all instances, of a regu- larly-organized tissue, in which the mineral matter, where such exists, is deposited ; and in several cases they are traversed by tubes, which seem to convey a fluid destined for their nutrition, if not actual blood. Structures of this kind are on the same footing with the dentine and enamel of the teeth of Vertebrata, ( 633, 634) ; to which they sometimes bear a very strong resem- blance. A more detailed account of the general structure of these sub-king- doms will now be given, beginning with the lowest. General characters of Radiata. 18. The RADIATA possess many points of affinity with the Vegetable king- dom ; and of these, the circular arrangement of their parts is one of the most evident. Many species of Sea-Anemone, for instance, present an appearance so much resembling that of various composite blossoms, as to have been com- monly termed Animal-flowers, a designation to which they further seem entitled, from the small amount of sensibility they manifest, and the evident influence of light upon their opening and closing. But it is in the tendency to the production of compound fabrics, each containing a number of indivi- duals, which have the power of existing independently, but which are to a certain degree connected with one another, that we recognize the greatest affinity in structure between this group and the Vegetable kingdom. Every tree is made up of a large number of buds, which are composed of leaves ar- ranged round a common axis ; each bud has the power of preserving its own life, and of reproducing the original structure, when removed from the parent stem, if placed in circumstances favourable to its growth ; and yet all are con- nected in the growing tree, by a system of vessels, which forms a communica- tion between them. This is precisely the nature of those structures, which are formed by the animals of the class that may be regarded as the most charac- teristic of the group. Every mass of coral is the skeleton of a compound animal, consisting of a number of polypes, connected together by a soft flesh, in which vessels are channeled out ; these polypes are capable of existing separately, since each one, when removed from the rest, can in time produce a massive compound fabric, like that of its parent ; but they all contribute to the maintenance of the composite structure, so long as they are in connection with it. In some instances, the skeleton is stony, and is formed by the deposition of calcareous matter, either in the centre of each fleshy column, so as to form a solid stem, or on its exterior, so as to form a tube. In other cases, it is horny; and then it may be a flexible axis, or a delicate tube. Both the stony and horny corals frequently possess the form of plants or trees : and as their skele- 36 ON THE PLACE OF MAN IN THE SCALE OF BEING. tons are often found with no obvious traces of the animals to which they be- longed, they have been accounted Vegetable growths. This idea receives confirmation from examination of their intimate structure ; for they are com- posed of a tissue which bears more resemblance to the cellular tissue of Plants than to the areolar tissue of the higher Animals. There is not the least doubt, however, as to the Animal origin of the greatest part of these plant-like struc- tures ; and one group only, that of Corallines, remains a source of much perplexity to the Naturalist. 19. The affinity, however, between the lowest Radiata and Plants, in regard to the vital phenomena they exhibit, is still more close than that manifested by their structure. Although, in the higher groups, movements may be con- stantly witnessed, which evidently indicate consciousness and voluntary power, this js far from being the case in the lower. There are many tribes, whose reception of food, growth, and reproduction, are not known to be accompanied by any phenomena which distinctly indicate their animal character. The most violent lacerations produce no signs of sensibility ; and the movements they occasionally exhibit have not so much of a spontaneous character as those which are performed by many plants. This is the case, for example, with the Sponge tribe, and also with a number of microscopic species." So doubtful is the nature of these beings, that their Animal or Vegetable character is rather to be decided by their affinity with species known to belong to one or the other kingdom, than in any other way. 20. It is very different, however, in regard to the higher Radiata. Even among the Zoophytes (as the plant-like animals just alluded to are commonly termed) there are some species which are unattached during the whole period of their lives, and have a power of voluntarily moving from place to place, such as is never possessed by plants. And in the highest class, the Echino- dermata, including the Star-fish, Sea Urchin, &c., we meet with a considerable degree of complexity of structure, and a corresponding variety of actions. Still, except in those species which connect this group with others, the same cha- racter of radial or circular symmetry is maintained throughout ; and in no ani- mal is it more remarkable than in the common Star-fish. It is exhibited alike in its internal conformation and external aspect. The mouth, placed in the centre of the disk, leads to a stomach which occupies the greatest part of the cavity of the body ; and this sends prolongations into the (arms which are exactly alike in form), and occupy a precisely similar position in every one. Each arm is furnished, on its under side, with a curious apparatus for locomo- tion, consisting of a series of short elastic tubes, which are prolonged through apertures in the hard envelop, from a series of vesicles placed along the floor (as it may be termed) of the ray. The system of vessels for absorbing nutri- ment and conveying it through the system, is also disposed upon the same plan ; and the same may be said of the nervous system, and of the only organs of special sensation which this animal appears to possess, the rudimentary eyes, of which one is found at the extremity of each ray. 21. Amongst other results of the repetition of similar organs, so remarkable in this group, is this, that one or more of them may be removed without per- manent injury to the whole structure, and may even develop themselves into an entire fabric. Thus in the Star-fish, instances are known of the loss of one, two, three, and even four rays, which have been gradually reproduced ; the whole process appearing to be attended with little inconvenience to the animal. In some species of isolated Polypifera, such as the common Sea- Anemone, and Hydra (Fresh-water Polype), this power of reproduction is much greater. The Hydra may be cut into a large number of pieces (it is said as many as 40), of which every one shall be capable of developing itself in time into a per- fect polype. The Sea- Anemone, when divided either transversely or vertically, GENERAL CHARACTERS OF MOLLUSCA. Fig. 1. Asterias aurantiaca, with the upper side of the hard envelope removed: a, central stomach; b, coeca upon its upper surface, probably answering to the liver; c, e, coecal prolongations of stomach into rays; c'.c', the same empty ; d, the same opened; e, under surface, showing vesicles of feet;/, vesicles contracted, showing skeleton between them. still lives ; and each half produces the other, so as to re-form the perfect ani- mal. This is another character, which shows the affinity of the Radiata to the Vegetable kingdom ; and there is yet another, derived from their mode of re- production. In many Polypifera, we observe a propagation by buds, in all respects conformable to that which plants effect, and quite different from the regular multiplication by distinct germs. This gemmiparous reproduction, as it is called, takes place, not only in the compound Polypifera, whose plant- like structures are extended by it, but also in some isolated species, such as the Hydra ; from the body of which, one or more young polypes bud forth at the same time ; and these buds may themselves put forth another generation, previously to their separation from their parent. This kind of reproduction is not seen anywhere else, in the whole Animal kingdom, except in a few of the lowest Mollusca and Articulata, which border most closely on the Radiata. General characters of Mollusca. 22. The range of Animal forms comprehended in the sub-kingdom MOL- LUSCA is so great, that it would be difficult to include them in any positive definition, which should be applicable to all. They present few traces of the circular disposition of organs around the mouth, which is characteristic of the Radiated classes; and we seldom meet with any marked approach to the 4 38 ON THE PLACE OF MAN IN THE SCALE OF BEING. elongation of the body, still seldomer with any indication of that division into segments, which are the chief peculiarities of the Articulata. It is by the absence of these, and of any trace of the Vertebrated structure, that the Mollusca are most readily defined. The variety of form which they present is less surprising, when it is considered, that the bulk of their bodies is almost entirely made up by organs of nutrition ; and the organs of sensation and loco- motion which they possess, are subservient to the supply of these. We find, in the lowest tribes of this group, living beings which are fixed to one spot during all but the earliest period of their lives ; and which scarcely possess within themselves so much power of movement as that enjoyed by the in- dividual Polypes in a compound polypidom ; and yet these exhibit a complex and powerful digestive apparatus, a regular circulation of blood, and an active respiration. We never find, throughout the whole Animal kingdom, that the apparatus of organic life is arranged on any definite plan of its own ; its con- formation is adapted to the type which predominates in the structure of each group, and which is principally manifested in the disposition of the locomotive organs. Thus, the stomach of the Star-fish is circular, and sends a prolongation into each ray ; whilst the digestive cavity of the Articulata is prolonged into a tube. In the Mollusca, there is no such definite type, the apparatus of nutri- tion having the predominance over that of locomotion ; and the form of the body is, therefore, extremely variable. The relative places, even of the most important organs (such as the gills), are found to undergo complete changes, as we pass from one tribe to another ; although their general structure is but little altered. 23. The lower Mollusca may be characterized as consisting merely of a bag of viscera ; they have not even any prominence for the mouth, nor any organs of special sense, such as would distinguish a head; and they are entirely desti- tute of symmetry, the radiated arrangement of parts seen in the lower tribes being absent, as well as the bi-lateral correspondence which is characteristic of the higher. In the more elevated Mollusca, however, which possess not merely sensitive tentacula, but eyes and even organs of smell and hearing, we find these disposed in a symmetrical manner; so that the head, which is the part concerned peculiarly in animal life, does present a bi-lateral equality of parts, even when the remainder of the body wants it. Further, in the more active among the higher classes, we find this bi-lateral symmetry showing itself in the exterior of the whole body ; evidently bearing a pretty close rela- tion to its degree of locomotive power. It is most evident and complete in the Cephalopoda (Cuttle-fish tribe); many of which are adapted to lead the life of Fishes, and resemble them in the general form of the body, and in the structure of many of the individual organs. It is also manifested in many Fig. 2. Aplysia depilans ; a, branchiae or gills. GENERAL CHARACTERS OF MOLLUSCA. 39 of the shell-less Gasteropoda, such as the Slug, or the Aplysia (Sea-Hare) ; as will be seen by the accompanying representation of a species of the latter. But this symmetry does not extend to the arrangement of the internal organs ; and appears to be only designed to adapt the body for more convenient loco- motion. 24. As a group, however, the Mollusca are to be characterized rather by the absence than by the possession, of any definite form ; and there is a cor- responding absence of any regular organs of support, by which such a form could be maintained. The name they have received designates them as soft animals ; and this they are pre-eminently, as every one knows who has taken a Slug between his fingers. The shell, where it exists, is to be regarded rather in the light of an appendage, designed for the mere protection of the body, and deriving its shape from the latter, than as a skeleton, giving attachment to muscles, and regulating the form of the whole structure. It is in no in- stance a fixed point for the muscles of locomotion ; and it is only, indeed, where the body is uncovered by a shell or where a locomotive organ may be pro- jected beyond it, that any active movements can be executed. This locomotive organ, the foot as it is commonly termed, is nothing else than a fleshy mass, formed by the increased development of the muscular portion of one part of the general envelope of the body, termed the mantle in which the visceral mass is loosely included. The mantle is not essentially different from the skin of other animals ; but it is usually thicker, possessing a considerable amount of muscular fibre interwoven with it, and its surface having frequently a glan- dular character. This general muscular envelope is the only locomotive organ possessed by a large proportion of the Mollusca; but its contractile properties are usually greatest at some particular spot, where it is thickened into a sort of disk, by the alternate contraction and extension of which the animal can slowly propel itself; this is well seen, by causing a Snail or Slug to crawl over a piece of glass, so that the under surface of the disk may be seen whilst it is in operation. The general character of their locomotion, however, is well expressed by the term sluggish; and there are scarcely any among the typical Mollusca, whose activity is such as to demand for them any higher appellation. 25. The general development of their organs of Nutrition, however, is much higher than is met with among the Articulata; and, in proportion to that of the organs of Locomotion, it is much greater than will be elsewhere observed throughout the Animal kingdom. The justice of this statement will be made evident by a slight examination of the adjoined figure, in which the interior structure of the Jlplysia, showing the general character of that of the group, is displayed. The only set of muscles which this animal possesses, is that connected with the mouth, which it is able to push forwards or to draw back ; and which possesses considerable powers of mastication, and is furnished with large salivary glands. The nervous centres (of which more will be said here- after) are seen to be principally disposed around the oesophagus. The whole digestive apparatus is observed to be very complex and highly-developed ; the liver alone occupies a considerable part of the cavity. The heart has distinct muscular walls, and is divided into a separate auricle and ventricle ; and a large respiratory organ is developed for the aeration of the blood. The posi- tion of the gills, which are external to the cavity, but which are concealed in part by a fold of the mantle, and in part by the rudimentary shell, is seen at , Fig. 2. The generative apparatus, also, is highly developed. Yet with all this complex organization, the locomotive power of tfye animal is not much greater than that of the Slug ; no other means being provided for the purpose, than the contractility of the general envelope, which is greatest on the under side of the body. 26. The blood of the Mollusca is white, and the number of corpuscles in it is small. Their temperature is low, being seldom more than one or two 40 ON THE PLACE OF MAN IN THE SCALE OF BEING. degrees above that of the surrounding medium ; but many of them are capa- ble of being subject to extreme variations of heat and cold, without their vita- lity being thereby destroyed. Their respiration is for the most part aquatic ; and is performed by means of gills, over which a current of water is con- stantly being propelled, by the vibration of the cilia that cover their surface. Many of them are dependent on the same current for their supplies of food ; part of the water so introduced being taken into the stomach ; and a part flow- ing over the res^ratory surface. The higher tribes, however, go in search of their food, and have instruments of mastication for reducing it; but in these, as in the former, the anal orifice of the intestine opens into the passage, through which the current that has passed over the respiratory organs finds egress ; so that the faBcal matter from the former, and the fluid that has served Fig. 3. t- Aplysia cut open, showing the viscera; a, upper part of oesophagus; b, pen's; c. c, salivary glands; d, superior or cephalic ganglion ; e, e, inferior, or subcesophageal ganglia ; /, termination of oesophagus ; g. g. first stomach; h, third stomach; i, second stomach; k, intestine; Z, I, Z, liver: 771, posterior ganglion; n, aorta; o, hepatic artery; y, ventricle of heart; g, auricle; r, s, branchiae ; t } testis; w, lower part of intestine ; , ovary ; w, anus. GENERAL CHARACTERS OF ARTICULATA. 41 the purpose of the latter, are discharged together. Although very voracious, when supplies of food come in their way, most of the Mollusca are capable of fasting for long intervals, where none offer themselves, a fact which is readily explained by that general inertness of their vital processes, which has been stated to be the characteristic of the group. General characters of Articulata. 27. The members of the sub-kingdom ARTICULATA are distinguished for the most part, by characters which are exactly opposed tfj those just enume- rated. Their characteristic form is easily defined; and in no instance is there any wide departure from it. The body is more or less elongated, and pre- sents throughout a most exact bi-lateral symmetry. It is completely enclosed in an integument of greater density than the rest of the structure, which is divided into distinct rings or segments ; these, being held together by a flexi- ble membrane, allow considerable freedom of motion, whilst they firmly pro- tect the soft parts, and afford attachment to numerous muscles. It is in the Centipede, arid other such animals, that this division into segments is most distinctly and regularly marked. In the lower Articulata, such as the Leech and the Earth-worm, the integument is altogether so soft, that the intervals of the articulations are not very distinct from the rings themselves ; and in the highest Crustacea and Arachmda, the segments are so closely united together, as to be in some instances scarcely recognizable. In the former, the move- ments of the body are entirely effected by its own flexion ; whilst in the latter, they are committed to members developed for that special purpose. These members also have an articulated external skeleton. The bulk, of the body in the Articulata is made up of the muscles, by which the several segments, and their various appendages, are put in motion ; these muscles have their fixed points on the interior of the hard envelope, just as they are attached in vertebrated animals to the exterior of the bones ; and they form a system of great complexity. 28. The development of the organs of Nutrition in Articulata would seem to be altogether subservient to that of the Locomotive apparatus, their func- tion being chiefly to supply the muscles with the aliment necessary to main- tain their vigour. The power of these muscles is so great in proportion to their size, that in energy and rapidity of movement, some of the Articulated tribes surpass all other animals. Their movements are directed by organs of sensation, which, although not developed on so high a plan as those of some Mollusca, are evidently very acute in their powers. There are very few in- stances of Articulated animals being in any way restrained as to freedom of locomotion ; and these are found in a single group, the Cirrhopoda or Barnacle tribe, which connects this sub-kingdom with the last. In general, they roam freely abroad in search of food, and are supplied with prehensile organs for capturing their prey, and with a complex masticating apparatus for reducing it. Their actions are evidently directed almost solely by instinctive propensi- ties, which are adapted to meet every ordinary contingency, being of similar character in each individual of the same species, and presenting but little ap- pearance of ever being modified by intelligence. Hence these animals seem like machines, contrived to execute a certain set of operations; many of them producing immediate results, which even Man, by the highest efforts of his reason, has found it difficult to attain.* 29. All the Articulata, save a few of the very lowest species, possess a dis- tinct head at one end of the body, furnished with organs of special sensation, * Reference is here especially made to the celebrated problem of Miraldi ( 155). 4* ON THE PLACE OF MAN IN THE SCALE OF BEING. and with lateral jaws for the prehension and reduction of food; and their movements, being principally guided by the special senses, take place in this direction. The bi-lateral symmetry of the body is not confined to its exte- rior; for it prevails most completely in the whole muscular apparatus; and even the organs of nutrition present more distinct traces of it than are to be seen elsewhere. The compact heart of the Mollusca, for instance, is here re- placed by a long tube, the dorsal vessel, placed on the median line; and the respiratory organs, which are usually diffused through the whole system, are uniform on the two sides. Even the intestinal canal partakes of this sym- metry ; in some Species it runs straight from end to end of the body ; and even where it is otherwise disposed, its appendages are nearly equal on the two sides. The respiration of this group is for the most part aerial; and the appa- ratus for the purpose consists of a series of chambers or tubes, which are dis- persed or extended through the whole body. By this means, the air, the blood, and the tissue to be nourished, are all brought into contact at the same points ; and a much less vigorous circulation is required, therefore, than would otherwise be needed. The whole apparatus of nutrition is comprised within a comparatively small part of the body ; and the bulk of the organs which compose it, is never at all comparable with that which we ordinarily find in the Mollusca. Thus, the liver, which in the Oyster forms a large part of the whole substance, is often scarcely recognizable as such in the Insect ; and the intestinal tube seldom makes many convolutions in its course from one extre- mity to the other. The blood is usually white, as in the other Invertebrated classes ; but it contains a larger number of corpuscles than are seen in that of most of the Mollusca. The temperature varies to a certain degree with that of the atmosphere ; but there are many Insects that have the power of gene- rating a large amount of independent heat, which is strictly proportionable to the quantity of oxygen converted by them into carbonic acid in the respiratory process. All the actions of the Articulata are performed with great energy ; and, at the time of the most rapid increase of the body, the demand for food is so great, that a short suspension of the supply of aliment is fatal. They are capable, however, of being submitted to the influence of very extreme temperatures, with little permanent injury. 30. The adjoining figure, which displays the muscular apparatus of the Fig. 4. Section of the trunk of Melolontha vulgaris (after Strauss-Durckheim), showing the complexity of the muscular system. The first segment of the thorax (2) is chiefly occupied by the muscles of the head, and^ by those of the first pair of legs. The second and third segments (3 and 4) contain the very larg muscles of the wings, and those of the other two pairs of legs. The chief muscles of the abdomen are the long dorsal and abdominal recti, which move the several segments one upon another. GENERAL CHARACTERS OF VERTEBRATA. 43 interior of the body of a Cock-chafer, will give an idea of its complexity and variety, and of the large portion of the trunk which is occupied by it ; and will also show the division of the skeleton into segments, the number of which in Insects is limited to thirteen. These are nearly equal and similar to each other in the Larva ; but, in the perfect Insect, the three behind the head are united into the thorax, to which the legs and wings are attached ; and the remainder form the abdomen, which has little concern in locomotion. General characters of Vertebrata. 31. In none of the three preceding divisions of the Animal kingdom, does the Nervous System attain such a degree of development, as to give it that predominance in the whole fabric, which it evidently possesses in VERTE- BRATA. In the Radiata and Mollusca, its functions are obviously restricted^to the maintenance of the nutritive operations ; and to the guidance of the ani- mal, by means of its sensory endowments, in the choice of food, as well as (in some instances) in the search for an individual of the opposite sex : in the Articulata, its purpose appears similar, but is carried into effect in a different manner, the locomotive organs being the parts chiefly supplied by it. In the Vertebrata, on the other hand, the development of all the other organs appears to be subordinate to that of the Nervous System ; their object being solely to give to it the means of the exercise of its powers. This statement is not, of course, as applicable to the lower Vertebrata as it is to the higher ; but it is intended to express the general character of the group. The predominance of the nervous system is manifested, not only in the increased size of its cen- tres, but also in the special provision which we here find, for the protection of these from injury. In the invertebrated classes, wherever t&e nervous system is enclosed in any protective envelope, that envelope serves equally for the protection of the whole body. This is the case, for example, in regard to the spiny integument of the Star-Fish, the shell of the Mollusca, and firm jointed rings of the Insect. The only exceptions occur in a few tribes, in which the nervous system is much concentrated ; and in which the general organization approaches that of the Vertebrata.* In Vertebrated animals, we find that the skeleton essentially consists of a series of parts, which are destined to enclose the nervous centres, and to give attachment on their exterior to the muscles by which the body is moved: hence it may be termed the neuro-skeleton; in contra-distinction to the dermo-skeleton, which envelops the whole body .iff' many Invertebrata, being formed on the basis of their integument. The tis- sues, bone and cartilage, of which the former is composed, are more closely connected with the vascular system, than are the hard parts of Invertebrata ; and are consequently more capable of undergoing interstitial change. 32. In considering the essential character of the skeleton of Vertebrata, we should look' at its simplest forms, those in which it has the least number of superadded parts. We find these in the Serpent tribe among Reptiles, and in the Eel and its allies among Fish. If we examine their skeletons, we per- ceive that the Spinal Column, with the cranium at its anterior extremity, con- stitutes the essential part of the vertebrated framework ; and that the develop- ment of members is secondary to this. The Spinal Column usually consists of a number of distinct bones, the Vertebrae ; each of which is perforated by a large aperture, in such a manner that, when the whole is united, a continu- * Thus, in the highest Crustacea, there is an internal projection from the shell, on each side of the median line, which forms a sort of arch enclosing the ventral cord ; and in the naked Cephalopoda, the nervous centres are supported, and in part protected, by cartilaginous plates, which are evidently the rudiments of the internal skeleton of the Vertebrata. 44 ON THE PLACE OF MAN IN THE SCALE OF BEING. ous tube is formed for the lodgment of the spinal cord. The Cranium, which it bears at its upper end, is in reality formed of the same elements as the vertebras, instead of differing from them completely in structure, as we might be led to suppose by examination of its most developed forms only. The object of this enlargement is to enclose the brain, or mass of cephalic ganglia, which attains a greatly increased size in the Vertebrata ; and also to afford support and protection to the organs of special sense, which are far more highly developed among them than elsewhere. The true nature of the cra- nium is best seen in those animals,*in which the brain bears but a small pro- portion to the spinal cord, such as the lower Reptiles and Fishes ; and an examination of its structure in these satisfactorily proves the reality of this view, which is further borne out by the history of its development, and of that of its contained parts, in the higher Vertebrata. , 33. The Vertebral column, at its opposite extremity, is usually contracted instead of being dilated, forming a tail, or a rudiment of one, from which the nervous centres are entirely withdrawn ; the development of the tail is com- monly seen to be in an inverse proportion to that of the cranium. To this column, the ribs and extremities are merely apendages, which we find more or less developed in the various tribes, and often entirely absent ; whilst the vertebral column is never wanting, although reduced in some species to a very rudimentary state. It is interesting to compare its various conditions with those which have been noticed in the external skeleton of the Articulata. In the lowest animals of the group, locomotion is pricipally or even entirely per- formed by flexion of the body itself ; and here, as in the warm tribe, we find the skeleton extremely flexible, the whole being comparatively soft, and its divisions indistinct. This is the case, for example, in the Lamprey and other Cyclostome fishes ; in which there is no distinct division into vertebrae, the spinal column scarcely possessing even the density of cartilage. In propor- tion, however, as distinct members are developed, and the power of locomotion is committed to them, we find the firmness of the spinal column increasing, and its flexibility diminishing ; and in Birds, in which, as in Insects, the movements of the body through the air are effected by muscles which must have very firm points of support, the vertebral column is much consolidated by the union of its different parts, so as to form a solid frame-work. As a general rule, then, the mobility of the extremities, and the firmness of the ver- tebral column, vary in a like proportion. The number of these extremities in Vertebrata never exceeds four and two of them are not unfrequently absent. The power of locomotion is not developed to nearly the same proportional ex- tent as in the Articulata ; the swiftest Bird, for example, not passing through nearly so many times its own length in the same period, as a large proportion of the Insect tribes : but it is far greater than that which is characteristic of the Mollusca ; and there is no species that is fixed to one spot, without the power of changing its place. On the other hand, the highest Mollusca approach them very nearly in the development of organs of special sense, of which Verte- brata almost invariably possess all four kinds, sight, hearing, smell, and taste. 34. The perfection of the Articulate structure has been shown to consist in the development of those powers which enable the animal to perform actions denoting the highest instinctive faculties. That of the Vertebrata evidently tends to remove the animal from the dominion of undiscerning, uncontrollable instinct ; and to place all its operations under the dominion of an intelligent will. We no longer witness in these operations that uniformity, which has been mentioned as so remarkable a characteristic of instinctive actions. There is evidently, among the higher Vertebrata especially, a power of choice and of determination, guided by a perception of the nature of the object to be attained, and of the means to be employed, constituting the simplest form of the reason- GENERAL CHARACTERS OF VERTEBRATA. 45 ing faculty ; and the amount of this bears so close a relation with the deve- lopment of the brain, that it is scarcely possible to regard the two as uncon- nected. In Man, whose brain is far larger in proportion to his size, as well as more complex in its structure, than that of any other animal, the reasoning faculties attain the highest perfection that we know to be anywhere manifested by them in connection with a material instrument ; the instinctive propensities are placed under their subjection ; and all his acts, excepting those imme- diately required for the maintenance of his organic functions, are put under their control. It is to Man, therefore, that what was just now stated, of the predominance of the neryous system in Vertebrata, particularly applies ; but the same may be noticed, though in a less striking degree, throughout the group. Not only is the influence of the nervous system to be traced in the sensible movements which they perform, but also in various modifications of the organic functions, which take place under the influence of particular states of mind, and the occurrence of which there is no reason to suspect in the lower tribes of animals. These are even much more striking in Man than in the lower Vertebrata ; indeed, the comparative slightness of the influence of the mind upon the body, is one of the causes which render the lower Mammalia more able than Man is, to recover from the effects of severe injuries. The Mollusca seem to grow like plants ; their massive organs increasing by their own sepa- rate vitality, and being but little dependent upon each other. Even the act of respiration, which is in most animals performed by a series of distinct mus- cular contractions, is there principally effected through the medium of the cilia which clothe the respiratory surface. 'But in the Vertebrata, the nervous system possesses a distinct and independent rank ; its offices are those which more particularly constitute the active life of the animal ; the organic func- tions have for their chief object, the maintenance of the nervous and muscular apparatus in the conditions requisite for their activity, and, in consequence, all these different kinds of apparatus are so interwoven together, that their mutual dependence is very close. 35. The foregoing remarks will be found to have an important bearing on the details subsequently to be given, respecting the functions of the nervous system in Man ; and it is desirable to set out with clear ideas on this subject, since there is no department of Physiology, regarding which more error is prevalent. There is no valid reason for believing, that the organic functions in Animals, any more than the corresponding changes in Plants, are dependent on the nervous system for their performance ; but common observation shows, that they are much influenced by it in the higher animals ; and from such a comparison as that which has been just now briefly made, it would appear that the higher the general development of the nervous system, the closer is their relation with it. 36. This general character of the Vertebrata harmonizes well with what may be observed, on a cursory glance at the structure of their bodies, of the proportion between the organs of Nutritive and those of Animal life. The former, contained in the cavities of the trunk, are highly developed ; but, as in the Mollusca, they are for the most part unsymmetrically disposed. Of the latter, the nervous system and organs of the senses occupy the head ; whilst the muscles of locomotion are principally connected with the extremities : both are symmetrical, as in the Articulata; but, whilst that part of the nervous centres, which is the instrument of reason, is very largely developed, the por- tion which is specially destined to locomotion, together with the muscular system itself, bears much the same proportion to the whole bulk of the body, as it does in the Articulated series. Hence we observe that the Vertebrata unite the unsymmetrical apparatus of nutrition, characteristic of the Mollusca, with the symmetrical system of nerves and muscles of locomotion, which is 46 ON THE PLACE OF MAN IN THE SCALE OF BEING. the prominent characteristic of the Articulata ; both, however, being rendered subordinate to the great purpose to be attained in their fabric, the develop- ment of an organ, through which intelligence peculiarly manifests itself. For the operations of this, a degree of general perfection is required, which is not met with elsewhere. The higher Vertebrata have a power of constantly keeping the temperature of the body up to a point, which it can only attain occasionally, and under peculiar circumstances, in the Articulata, and which it never reaches in the Mollusca. This involves an energetic performance of the functions of respiration and circulation ; and these again require consider- able activity of digestion. All the Vertebrata have red blood, which is pro- pelled through the system by a distinct muscular heart ; and the number of corpuscles, which any given amount of the fluid contains, bears a nearly con- stant proportion to the ordinary temperature of the animal. They are further distinguished from Articulata by a character which seems of little importance, but which is very constant in each group. Whilst the mouth of the latter is furnished with two or three pairs of jaws which open sideways, that of the former has never more than one pair of jaws, which are placed one above or before the other; and these jaws are usually armed with teeth, which are very analogous in their structure to bone. General characters of Fishes. 37. The Vertebrata are subdivided into classes, principally according to their mode of performing the functions of respiration and reproduction. -Thus, FISHES are at once separated from all other groups, from the circumstance of their being adapted, like the aquatic Invertebrata, to aerate their blood by gills; and being hence enabled to inhabit water during their whole lives, without the necessity of coming to the surface to breathe. The low amount of their respiration prevents their bodies from ever attaining a temperature much above that of the surrounding medium; hence they are spoken of as cold-blooded. Further, they are oviparous ; an ovum or egg being deposited by the parent, from which, in due time, the young makes its way; or if, as sometimes happens, the ovum is retained within the body of the parent until it is hatched, the young animal, though produced alive, is not subse- quently dependent upon its parent for support. In many respects, the organ- ization of Fishes is not much advanced beyond that of the higher Mol- lusca. Their respiratory apparatus has the same character; and the organs by which the blood is depurated of its superfluous azote, rather correspond with the temporary Corpora Wolffiana of higher animals, than with their true kidneys ( 669, 670). The vertebral column itself is often very imper- fectly developed; in a large proportion of the group, the skeleton is cartila- ginous only; and in the lowest species, it does not even manifest a distinct division into vertebrae. Living habitually in an element which is nearly of the same specific gravity with their own bodies, Fishes have no weight to sup- port, and have only to propel themselves through the water. Accordingly we find their structure adapted rather for great freedom of motion than for firm- ness and solidity; and as progressive motion is chiefly effected by the lateral action of the spine, the vertebrae are so united as to move very readily upon one another. Instead of being articulated together by surfaces nearly flat, as in Mammalia, or by ball-and-socket joints, as in Serpents, they have both their surfaces concave ; and these glide over a bag of fluid (the representa- tive of the invertebral substance in the higher animals) which is interposed between each pair. The tail is flattened vertically; so as, by its lateral stroke, to propel the Fish through the water. By this character, true Fishes are distinguished from those aquatic Mammalia, which are adapted to GENERAL CHARACTERS OF REPTILES. 47 inhabit their element, and which commonly receive the same designation; for the latter, being air-breathing Animals, are obliged to come frequently to the surface to respire ; and their tail is flattened horizontally, to enable them to do this with facility. The lateral surface of the body of Fish is further extended above, by the projection of the dorsal fin, which is supported on prolongations of the spines of the vertebras ; and below, by the abdominal fin, which also is placed on the median line ; these will, of course, increase the power of the lateral stroke of the body, and can only be moved with the spine. The pec- toral and ventral fins on the other hand, the former of which answer to the superior extremities, and the latter to the inferior extremities of Man, serve, by their independent movements, rather as steering than as propelling organs ; and they also assist in raising and depressing the animal through the water. The scales with which the bodies of all Fishes are covered, are frequently of a bony hardness, and sometimes form a firmly-jointed casing, in which the trunk is completely enclosed ; this is especially the case, when the internal skeleton is imperfectly developed ; so that here we have an approach to the character of the Invertebrata. 38. The swimming-bladder, as it is commonly termed, of the Fish, is not an organ sui generis ; but is ascertained, by comparison with the pulmonary sacs of the lower Reptiles, to be a rudimentary lung. It does not, however, give any assistance in the aeration of the blood, except in a few instances ; but seems to be in general subservient to the elevation and depression of the body in its element. The heart of the Fish is extremely simple in its construction containing two cavities only; and the course of the circulation is equally simple. The blood, which returns from the body in a venous condition, is received into the single auricle or recipient cavity ; and from this it passes into the ventricle or propellent cavity. The latter forces it into a large trunk, which subdivides into branches that are distributed to the branchial arches on each side ; and in these it undergoes aeration. Being collected from the gills by returning ves- sels, the blood, now become arterial in its character, is transmitted to the large systemic trunk, the aorta, by which it is distributed through the system, returning again to the heart, when it has passed through the organs and tissues of the body. Hence it is evident that the whole of the blood passes through the gills before it goes a second time to the system ; by which the imperfection of the aerating process itself is in some degree compensated. There is a special provision, too, for renewing by muscular power the stratum of water in contact with the gills ; continual currents being sent over them from the pharynx, with which their cavity communicates. It is worth noticing, that whilst, in the Osseous Fishes, there is a single large external gill-opening on either side, with a valve-like operculum or gill-cover, there are, in the Cartila- ginous Fishes, several slits on each side of the neck, one corresponding with each branchial arch. Similar apertures in the neck may be seen in the em- bryo of Man and of other Mammalia, as well as of Birds and Reptiles, at the time that the circulation is in the condition of that of the Fish, the heart possessing only two cavities, and the blood being first propelled through a series of branchial arches. General characters of Reptiles. 39. The class of REPTILES is oviparous and cold-blooded, like that of Fishes ; but the animals belonging to it are formed to breathe air, and to inhabit the surface of the earth, the few which are adapted to make the water their dwelling, being. obliged to come to the surface to breathe. Although they breathe air, however, their respiration is not usually so energetic as that of Fishes; and their general activity is much less. The heart possesses three 48 ON THE PLACE OF MAN IN THE SCALE OF BEING. cavities, one of which receives the blood from the lungs, and another from the general system; the arterial and the venous blood contained in these two auricles respectively, are transmitted to the third or propelling cavity, the ventricle, where they are mixed; and the half-arterialized fluid is then trans- mitted to the system at large, a part being sent to the lungs. Thus only a portion of the blood expelled from the heart is exposed to the influence of the air ; and that which is transmitted to the body is very imperfectly arterialized. In some of the higher Reptiles, as the Crocodile, the ventricle is double, as in the superior Vertebrata ; and the course of the circulation is so arranged, that pure arterial blood shall go to the head, where it is most required, whilst a mixed fluid is sent to the rest of the body. This plan exactly cor- responds with the one which is adopted in the circulation of the Human foetus, from the time of the formation of the four cavities in its heart, and of the permanent system of vessels, up to the period of birth. The imperfect arterialization of the blood in Reptiles causes a great degree of general inert- ness in their functions. Their motions are principally confined to crawling and swimming ; their general habits are sluggish, and their sensations are obtuse ; and their nutritive functions are very slowly performed. Hence they can exist for a long time with a very feeble exercise of these functions, under cir- cumstances that would be fatal to animals in which they are performed with greater activity. In cold and temperate climates, they pass.the whole winter in a state of torpidity; and at other seasons, they may be kept during a long time from their due supplies of food and air, without appearing to suffer much inconvenience. 40. In regard to the structure of their skeleton, and the external form of the body, there is a considerable difference among the sev'eral orders of Reptiles. Thus, Tortoises, Lizards and Serpents, differ from each other so widely, that a common observer would separate them completely ; and yet they not only agree in all the foregoing characters, but pass into one another by links of transition so gradual, that it is even difficult to classify them. They differ, however, more in the configuration of the accessory parts than in the structure of the essential portion of the skeleton, the spinal column. This is characterized by the ball-and-socket articulation of the vertebrae, each vertebra liaving one surface convex, and the other concave, a structure which is more strongly marked in Serpents, whose movements are performed chiefly by the flexion of the spinal column itself, than it is in the other tribes. The chief characteristic of the Tortoise tribe, is the shell or case in which the body is contained. The upper arch of this shell, termed the carapace, is formed by a bony expansion from the edges of the ribs, which is covered by a set of horny plates, that are to be regarded (like smaller scales) as epidermic appendages. The under portion, termed the plastron, is composed of the sternum, which is in like manner extended laterally. In the land-tortoises, this usually forms a complete floor; but in the aquatic species, a part is commonly absent, the interval being filled up by cartilage and membrane. The skeleton of the Lizards is formed more upon the general plan of that of Mammalia, but may be readily distinguished from it. The sternum is usually prolonged over the front of the abdomen, and the ribs are continued through a much larger part of the spinal column ; of these abdominal ribs, the white lines across the recti muscles in the higher Vertebrata are evidently the rudiments. In the higher Lizards, the power of locomotion is almost entirely delegated to the extremities ; but in the less typi- cal species, the body and tail are much prolonged, so as to present a serpenti- form aspect ; and first one pair of feet, and then the other disappear, until the form is altogether that of the Serpent. Even in Serpents, however, rudiments of extremities are frequently to. be found; but their mode of progression is very different, and these rudiments are of no assistance to them. The most re- GENERAL CHARACTERS OF REPTILES. 49 markable feature in the Serpent's skeleton, besides the absence of legs, and the large number of ribs and vertebrae, is the deficiency of a sternum ; through the absence of this, the extremities of the ribs are free, and they become in fact the fixed points, on which the animal crawls, when advancing slowly for- wards, in a manner which bears a strong resemblance to the progression of the Centipede. 41. Although the configuration of the cranium varies much in the different orders of Reptiles, yet there is a remarkable agreement in certain general characters, and in the general degree of development. It consists of a much larger number of parts than are to be found in the cranium of adult Birds or Mammalia ; each principal bone being subdivided, as it were, into smaller ones. This condition exactly corresponds with that which may be observed during the process of ossification in higher Vertebrata ; for each of the larger bones of the cranium is formed from several centres of ossification ; so that, if the cranium of a foetus or young infant be macerated, it will fall into a number of pieces nearly corresponding with those of the Reptile's skull. The different orders of Reptiles have a close agreement in various other points ; especially in the degree of development of their several organs of nutrition. Thus, in all of them, the lungs, though commonly of large size, are so little subdivided, as really to expose but a small extent of surface. The glandular structures, too, are formed upon a much more simple type than is characteristic of the warm blooded Vertebrata. They all agree, moreover, in having the body covered with scales ; which, though generally small, are sometimes large flattened plates. 42. Between Fishes and true Reptiles, there is a group that remarkably combines the characters of both ; being composed of animals which come forth from the egg in the condition of Fishes, but which afterwards attain a form and structure closely corresponding with that of true Reptiles. This group, consisting of the Frog and its allies, is sometimes associated as an order (Ba- trachia) of the class of Reptiles ; and is sometimes made to rank as a distinct class, the Amphibia. The Tadpole or larva of the Frog is in every essential respect a Fish. Its respiration and circulation, its digestion and nutrition, its locomotion and sensation, are entirely accordant with those of Fishes. The body is destitute of members for progression, but is propelled through the water by the lateral undulations of the spinal column, which is articulated in the same manner as is that of Fishes. At a certain period, a metamorphosis commences, in which almost every organ in the body undergoes an essential change. Lungs are developed, which take the place (in regard to their" func- tion) of the gills ; and the latter are atrophied. The auricle of the heart is divided into two ; and the circulation is performed on the plan of that of the true Reptile. Two pairs of members are usually formed, to which, when they are fully developed, the power of progression is committed, the tail disappear- ing ; in some species, however, the tail remains, and the extremities are small. The digestive system undergoes a remarkable alteration, the intestinal canal, which was previously of enormous length in proportion to the body, being now considerably shortened, in accordance with the different kind of food on which the animal has to subsist. The mode of articulation of the spinal column, also, undergoes a change, which brings it to the type of that of Reptiles. On the whole, there scarcely appears sufficient reason for separating these animals, in their adult condition, from the class of Reptiles. The most important point of difference is the nakedness of the skin, by which the Batrachia may be at once distinguished, even when their external configuration approaches that of Rep- tiles in general. In this manner, the common Salamander or Water-Newt may be recognized as belonging to this group, though we should otherwise have placed it among the Lizards ; and the Ccecilia, which has the form of 5 50 ON THE PLACE OF MAN IN THE SCALE OF BEING. the Serpent, is in like manner known to be really allied to the Frog. An acquaintance with the history of these animals confirms such an arrangement, by showing that the Salamander and the Coecilia* undergo a metamorphosis ; breathing by gills, and having the general structure of Fishes, in the early part of their lives. 43. Besides those animals, however, which attain the condition of perfect Reptiles, this group contains several whose development is arrested, as it were, in an intermediate or transition state ; their adult form presenting a remarka- ble mixture of the characters of the two classes, which they thus connect. This is the case in the Proteus, Siren, and other less known species, which retain their gills through the whole of their lives, whilst their lungs are at the same 'time developed ; so that, as they can respire in either air or water, they are the only true amphibious animals. In their entire organization, they correspond with the Tadpole of the Frog at an advanced period of its metamor- phosis ; and it is a most interesting fact (which has been established by the experiments of Dr. W. F. Edwards) that if Tadpoles be kept in such a man- ner as to be freely supplied with food, and exposed to a constantly-renewed current of water, but be secluded from light and from the direct influence of the solar heat, they will continue to grow as Tadpoles ; their metamorphosis being checked. The metamorphosis of the Batrachia closely corresponds with that of Insects ; the young animal, in each case, at the time of its emersion from the egg, having a resemblance, in all essential particulars, to a class be- low that to which it is ultimately to belong. This kind of metamorphosis is by no means confined to them, however ; for the gradual extension of our knowledge of the early history of different tribes of animals, is constantly bringing to light new facts of the same kind. The Polypes and lower Mollus- ca, for instance, come forth from the egg, and swim about for some time, in a condition which can scarcely be termed animal; for there is not even a mouth leading to a digestive cavity, nor are there any other organs of locomotion than the cilia, the action of which is involuntary. And, in tracing the development of the Human embryo, we shall find that it undergoes a series of progressive changes equally remarkable; the principal difference being, that these changegf are not so arranged in harmony with each other, as to cause the embryo to present, at any one time, the combination of characters which belong to the Fish, Reptile, &c., or to enable it to sustain an independent existence. General characters of Birds. 44. From Reptiles to BIRDS, the transition would seem rather abrupt ; since the latter class is, in almost every respect, the opposite of the former. Never- theless it would seem to have been affected by the now extinct Pterodactylus, which combined in a most remarkable degree the characters of the two groups. Birds are, like Fishes and Reptiles, oviparous Vertebrata; but they differ essentially from both, in being themselves warm-blooded, and in the assistance which they afford by their own heat in the development of the ovum. Birds correspond with Mammalia, in possessing a heart with four cavities, and a complete double circulation ; by which the whole of the blood, that has circu- lated through the body, is exposed to the influence of the air, before being again transmitted to the system. This high amount of oxygenation of the blood is accompanied by great activity and energy of all the organic functions, acuteness of the senses, and rapid and powerful locomotion ; as well as by the evolution of a degree of heat, superior to that which we ordinarly meet with among the Mammalia. The temperature of Birds ranges from about 104 to * This fact, in regard to the Ccecilia, has only been recently substantiated. See Annals of Natural History, May 1841. GENERAL CHARACTERS OF BIRDS. 51 The lowest is in the aquatic species, whose general activity is much less than that of the tribes, which spend most of their time in the air ; the highest is among those distinguished for the rapidity and energy of their flight, such as the Swallow. 45. Birds have been denominated, and not inappropriately, the Insects of the Vertebrated series. As in the animals of that class, we find the whole struc- ture peculiarly adapted to motion, not in water, nor upon solid ground, but in the elastic and yielding air. It is impossible to conceive any more beautiful series of adaptions of structure to conditions of existence, than that which is exhibited in the conformation of the Bird, with reference to its intended mode of life. In order to adapt the Vertebrated animal to its aerial residence, its body must be rendered of as low specific gravity as possible. It is further necessary that the surface should be capable of being greatly extended ; and this by some kind of appendage that should be extremely light, and at the same time possessed of considerable resistance. The degree of muscular ' power required for support and propulsion in the air, involves the necessity of a very high amount of respiration ( 392), for which it has been seen that an express provision exists in Insects ; and as the general activity of the vital processes depends greatly upon the high temperature which this energetic respiration keeps up, a provision is required for keeping in this heat, and not allowing it to be carried away by the atmosphere through which the Bird is rapidly flying. 46. The first and third of these objects, the lightening of the body, and the extension of the respiratory surface, are beautifully fulfilled in a mode, which will be found to correspond with the plan adopted for the same purpose in Insects. The air which enters the body, is not restricted to a single pair of air-sacs or lungs placed near the throat ; but is transmitted from the true lungs, to a series of large air-cells, disposed in the abdomen and in various other parts of the body. Even the interior of the bones is made subservient to the same purpose ; being hollow, and lined with a delicate membrane, over which the blood-vessels are minutely distributed. In this manner, the respiratory surface is greatly extended ; whilst, by the large quantity of air introduced into the mass, its specific gravity is diminished. The subservience of the cavities in the bones to the respiratory function, is curiously shown by the fact, which has been ascertained both accidentally and by a designed experiment, that, if the trachea of a Bird be tied, and an aperture be made in one of the long bones, it will respire through this. 47. The other two objects, the extension of the surface, and the retention of the heat within the body, are also accomplished in combination, by a most beautiful and refined contrivance, the covering of feathers. Like hair or scales, feathers are to be regarded as appendages to the cutis ; the stem is formed from it by an apparatus, which may be likened to a hair-bulb on a very large scale ; but there are some additional parts for the production of the laminse, which form the vane of the feather, and which are joined to the stem during its development. These laminae, when perfectly formed, are connected by minute barbs at their edges, which hook into one another, and thus give the necessary means of resistance to the air. The substance of which feathers consists, is a very bad conductor of heat ; and when they are lying one over the other, small quantities of air are included, which still further obstruct its transmission by their non-conducting power. Thus the two chief objects are fulfilled ; power of resistance and slow conducting properties being obtained, in combination with lightness and elasticity. At the two extremes of the class, however, we meet with remarkable modifications in the typical structure^ of feathers. In the Penguin, those which cover the surface of the wings have a strong resemblance to scales ; and the wings are not employed to raise this 52 ON THE PLACE OF MAN IN THE SCALE OF BEING. Bird in the air, but only to propel it through water (as fins would do) by their action on the liquid. On the other hand, in the Ostrich tribe, the laminae of the feather are quite distinct from each other, and no longer form a con- tinuous surface ; so that the feathers more resemble branching hairs. Here the wings are almost or completely absent ; the birds of this tribe being con- stantly upon the ground, propelling themselves by running, and approaching the Mammalia in many points of their conformation. 48. The bony framework of Birds presents many remarkable adaptations to the same purposes. In the first place it is to be remarked, that the faculty of locomotion is here entirely delegated to the extremities ; and that the skeleton of the trunk must be consolidated, in proportion to the power with which they are to be endowed, in order to afford their muscles a firm attachment ( 33.) Just as the segments of the external skeleton of the Articulata, therefore, are consolidated in Insects, do we find that the vertebral column and its appen- dages are firmly knit together, in the upper part of the trunk of Birds. The vertebrae are closely united to each other ; and the ribs are connected with the sternum by bony prolongations of the latter, instead of by cartilages. This union is so arranged, that the state of expansion is natural to the thorax, whilst that of contraction is forced. Reptiles possess but a very imperfect mechan- ism for inflating their lungs. Being destitute of a diaphragm, they are obliged to force air into the chest, by a process resembling deglutition ; so that, strange as it may appear, a reptile may be suffocated by holding its mouth open. The diaphragm is absent among Birds, as among Reptiles ; except in a few species which most nearly approach the Mammalia. But its deficiency is compen- sated by this contrivance, which keeps the lungs and air-sacs always full, except when the Bird, by a muscular effort, expels the air from them, in order that they may be re-filled by a fresh supply. By this means, also, the specific gravity of the body is more constantly diminished than it could be if the lungs had been subjected to the constantly-alternating contractions and expansions which they perform in Mammalia. It is worthy of remark, that the air which enters the bones and the air-sacs, passes through the lungs both on its entrance and return ; so as to yield to their capillaries all the oxygen which they can take from it, and of which the blood that it has elsewhere met with has not deprived it. It is only in the lungs that it meets with purely venous blood ; for they alone receive the branches of the pulmonary artery ; the vessels which are distributed upon the respiratory surface of the air-sacs and bones being a part of the systemic circulating apparatus. Hence we may regard this curi- ous provision, as being partly designed for the aeration of the blood in its course through the system, (this, it will be remembered, being the sole mode in which the function is performed in Insects,) and partly for supplying the lungs with air as from a reservoir, during the violent actions of flight. 49. The articulation of the anterior extremity with the trunk exhibits a peculiar provision for strength and power which we find in no other Verte- brata. The two clavicles are united together on the central line, forming the furcula or merry-thought ; and the use of this is to keep the shoulders apart, notwithstanding the opposing force exerted by the pectoral muscles in the action of flight. It is generally firm, and its angle open in proportion to the power of the wings. Besides this bone, there is another connecting the ster- num with the scapula on each side ; this is the coracoid bone, which in Man and other Mammalia is scarcely developed, being merely a short process which dose not reach the sternum. The sternum of Birds usually exhibits a very remarkable development on the median line ; an elevated keel or ridge being seen on it, which serves for the attachment of the powerful muscles that de- press the wings. In the great development of the sternum, Birds have some analogy with the Turtle tribe (which they also resemble in the deficiency of GENERAL CHARACTERS OF BIRDS. teeth, and in the development of a horny covering to the jaws) ; but in these, the lateral elements of the sternum are the parts most developed, whilst in Birds it is the central portion which exhibits the peculiarity. From the depth of the keel of the sternum, a judgment may be formed of the thickness of the pectoral muscles, and thence of the powers of flight ; in the Ostrich tribe, where the wings are not sufficiently developed to raise the bird off the ground, the sternum is quite flat, as in the Mammalia. The want of flexibility in the trunk is counterbalanced by the length and flexibility of the neck ; the num- ber of cervical vertebrae is very considerable, varying from 12 to 23, the highest number being present in the Swan tribe. They are so articulated, that the head can be turned completely round, or moved in any direction. The anterior extremities of Birds being solely adapted to sustain them in flight, the posterior are necessarily modified for their support on the ground. They are usually placed rather far back ; but the spine has a position more inclined than horizontal, so that the weight may not be altogether thrown for- wards. The trunk is supported on the thighs by powerful muscles ; and there is another series, which passes from the lower part of the spine continu- ously to the toes, turning over the knee and heel, in such a manner that the flexion of these joints shall tighten the tendons ; by this contrivance, the simple weight of the body flexes the toes, and Birds are thus enabled to main- tain their position during sleep, without any active muscular effort. 50. Not only do Birds resemble Insects in their general structure and mode of life, but also in the peculiar development of the instinctive powers. Under the direction of these, the place for their nest appears to be selected; their materials collected ; the nests themselves built, and the young reared in them ; the migrations are performed; and many curious stratagems are employed to obtain food. It is sufficient to indicate these in general terms; since it is well known, that the habits of Birds have peculiarities restricted to each.species ; and that in all the individuals of each species, they are as precisely alike as their circumstances will admit. Nevertheless, there is observed in Birds a degree and kind of adaptation to varying conditions, which Insects do not possess, and which display an amount of intelligence far superior to what is found in that class ( 280). This is evinced also in their educability ; for no animal can be taught to perform actions which are not natural to it, unless it possesses in a considerable degree the powers of memory and association, at least, if not some of the higher mental faculties, such as the power of perceiving and comparing the relations of ideas. Moreover, in the domesticability of many tribes of Birds, \ve see this educability combined with a degree of that higher form of attachment to Man, which is so strikingly exhibited by certain spe- cies of Mammalia. The development of the senses of Birds varies in differ- ent tribes, according to the mode in which they are adapted to obtain their prey. The sight is almost always extremely acute, and is their chief means of seeking food ; and where this would be of comparatively little service, as in the nocturnal rapacious birds, it is compensated by a much higher deve- lopment of the faculty of hearing than is usual amongst other tribes. The senses of smell, taste, and touch, do not seem to be usually very acute in Birds ; but there are particular tribes, in which each of these is more deve- loped than in the rest. 51. As might be expected from their analogy with Insects, the development of the organs of nutrition (excepting that of the respiratory -organs) is much less striking in Birds than is that of the locomotive apparatus. The whole cavity of the trunk, especially in Birds distinguished for their powers of flight, is small in comparison with that of the body ; but what is wanting in the size of the organs, is made up in their energy of function. Hence the demand for food is more active in them than in any other class of animals. 5* 54 ON THE PLACE OF MAN IN THE SCALE OF BEING. It is interesting to observe, that there is more bi-lateral symmetry in the arrangement of the viscera than we usually find in the higher Vertebrata. This is evidently connected with their active locomotive powers ; as it is ob- viously necessary that the two sides of the body should be balanced with per- fect equality, and that their energy should be exactly correspondent. The lungs and air-sacs are precisely similar in size and situation on the two sides ; consequently the heart is placed on the median line ; and the mode of origin, from the aorta, of the trunks supplying the head and upper extremities, is alike on the two sides. The liver, also, is less symmetrical than we usually find it in the Mammalia. 52. It has been remarked, that the assistance afforded by the parent, in the development of the young, is greater in Birds than in the lower Vertebrata ; but is less than in Mammalia. Whilst Reptiles and Fishes show little or no concern for their eggs after they have deposited them, Birds sedulously tend them, affording them not only protection but warmth, by means of their powerful heat-producing apparatus. The yolk-bag of the Bird's egg is so suspended in the midst of the white albumen, that, when the egg is laid upon its side, it will always rise to the highest part of it; and the relative weight of the several parts is further adjusted in such a manner, that the cicatricula or germ-spot shall always be at the point nearest the shell, so as to come into the closest proximity with the source of heat, and also to be in the most imme- diate relation with the surrounding air. There are some birds, inhabiting the equatorial regions, which do not always incubate their eggs, trusting to the solar heat for their maturation. It is said that the Ostriches of the inter- tropical deserts are content with covering their eggs with a thin layer of sand, so as to admit the action of the sun by day, and to keep them warm at night ; but that those living under a less constantly elevated temperature, sit upon their eggs, if not constantly, at any rate when the solar heat is not suf- ficient. This statement has been disputed; but its truth seems to be con- firmed by a curious observation made by Mr. Knight, that a Fly-catcher, which built for several years in one of his hot-houses, sat upon its eggs when the temperature was below 72, but left them when it rose above that stand- ard. The degree of assistance afforded by the parent Birds to their young, after their emersion from the shell, varies much in different tribes ; in general it may be remarked, however, that it is most prolonged in those which ultimately attain the highest development, and especially in those whose intel- ligence is the greatest. Thus the Chicken and the Duckling, when just hatched, are able to shift for themselves ; but among the Raptorial and Inses- sorial birds, which rank far higher in the scale, the young are for a long time dependent upon the parent for food ; and in the Parrot tribe, which unques- tionably surpasses all others in intelligence, the parent not only supplies its young with food which it has obtained for them, but partly nourishes them by a milky secretion from the interior of the craw ; impregnating with this the aliment which it swallows, and which it afterwards disgorges for its offspring. General characters of Mammalia. 53. The MAMMALIA are universally regarded as the highest group in the Animal kingdom ; not only from being the one to which Man belongs (so far, at least, as his bodily structure is concerned), but also as possessing the most complex organization, adapted to perform the greatest number and variety of actions, and to execute these with the greatest intelligence. The contrast is here extremely strong between the reasoning and the instinctive powers ; even when we put Man out of view. When we compare, for example, the saga- city of a Dog, Monkey, or Elephant, and the great variety of circumstances GENERAL CHARACTERS OF MAMMALIA. 55 in which they will display an intelligent adaptation of means to ends, with the limited operations of Insects, over which the judgment and will seem to have no control, we cannot help being struck with the difference. The former are educable in the highest degree next to Man ; the latter could not be made to change their, habits, in any essential degree, by the most prolonged course of discipline ; still the difference, in this respect, between Man and the most intelligent of other Mammalia, is so strongly marked, that some Naturalists have proposed to exclude him altogether from the classification of this group, and even from the Animal kingdom. This is, however, by no means a phi- losophical plan; since the mind of Man is, in the present state of being, as closely connected with its material tenement, as we have reason to believe that of brutes to be ; and since there is scarcely any distinction of kind between their faculties, which we have a right to assume as characteristic, the difference being chiefly in degree ( 72). Man is like them actuated by instinctive pro- pensities 7 , w r hich have an immediate bearing on his corporeal wants ; an^they have, like him, the power of adapting their actions to gain certain feds, of which they are conscious. A Dog or an Elephant may show more real wis- dom, in controlling for a time its instinctive propensities, from the desire to accomplish some particular object, than is displayed by many Men, wfco give free scope to the exercise of their sensual passions, although warned by their reason of the injurious consequences of such indulgence. 54. This high development of the intelligence in Mammalia, is evidently connected with the greatly prolonged connection between the parent and the offspring, which we find to be a characteristic of this class. Mammalia are, like Birds, warm-blooded Vertebrata, possessing a complete double circula- tion ; and some of them are adapted to lead the life of Birds, passing a large part of their time in darting through the air on wings, in pursuit of Insect prey. But they differ from Birds in this essential particular, that they are not oviparous, but viviparous, producing their young alive, that is, in a con- dition in which they can perform spontaneous movements, and can appro- priate nourishment supplied to them from without. But they are not dis- tinguished from all other animals by this character alone ; for there are some species among Reptiles, Fishes, and even Insects, which produce their young alive, the egg being retained within the oviduct and hatched there. The real distinction is that which the name of the class imports, the subsequent nourishment of the young by suckling. There is another distinction, which is not, however, equally applicable to the whole class. In all, the yolk-bag is very small in proportion to its size in Birds ; and the contents of the ovum, instead of furnishing (as in that class) the materials necessary for the develop- ment of the young animal, up to the time when it can ingest food for itself, only serve for the earliest set of changes in which this process consists. In all the later stages of the evolution of the embryo, it is supplied with nutri- ment directly imbibed from its parent. This is at first accomplished by means oT a series of root-like tufts, which are prolonged from the surface of the ovum, and insinuate themselves among the maternal vessels, without, how- ever, uniting with them. These tufts absorb, from the maternal fluid, the ingredients necessary for the support of the embryo ; and also convey back to the parent its effete particles, which are received back into her blood, and cast out of her system, by the processes of secretion, respiration, &c. 55. The Mammalia may be divided into two sub-classes ; in one of which the structure just described is the greatest advance ever made, in the appara- tus by which the foetus is nourished ; whilst in the other, a more concentrated form is subsequently assumed by it. The ovum of the latter is delayed for a longer period, in a cavity formed by the union of the two oviducts, termed the uterus : which can be scarcely said to be developed in the Marsupialia and 56 ON THE PLACE OF MAN IN THE SCALE OF BEING. Monotremata, the two orders constituting the first sub-class. The vascular tufts proceeding from the chorion become especially developed at one point, and the vessels of the uterus are extremely enlarged in a corresponding situa- tion ; the tufts dip down, as it were, into a chamber formed by an extension of the inner lining of these vessels, and serve the combined purpose of the roots of plants and of the branchiae of aquatic animals, absorbing from the maternal blood the materials required for the nourishment of the embryo, and aerating that of the foetus by exposing it to the influence of the parents ( 749). The peculiar organ thus formed is termed the placenta ; and the two sub-classes of the Mammalia have thence received the appellations of placental and non- placental. The animals belonging to the latter present many points of affinity to Birds, in the structure of their internal organs. That of the brain is very nearly allied in these two groups ; and their amount of intelligence seems, as far as can be determined, to bear a close correspondence. The Ornithorhyn- cus, in particular, has so many marks of alliance to Birds, in its osteology, as well as in its horny bill, and in the spur on its hind leg (which resembles that of the Cock), that Naturalists have much debated, whether it could really be termed a Mammiferous animal. No positive evidence has yet been obtained, that its young are born alive ; but on the other hand, there is a strong reason to believe, that they come into the world unenclosed in the ovum, although in a very imperfect condition. Moreover, it has been satisfactorily ascertained, that the young are nourished, for some time after their birth, by a mammary secretion, which the organization of their mouth at that period enables them to obtain from the parent. In the Marsupialia, there is a remarkable compen- sation for the abrupt termination of the period of uterine gestation, the young being received into a pouch or marsupium, within which the nipple is situ- ated ; this is extremely prolonged, and the mouth of the foetus (for so the being must still be regarded) is adapted to receive and hold on by it ; so that the little creature, which looks at first more like an earth-worm than a Mam- miferous animal, is thus suspended within the protective pouch, until its development is so far advanced, that it can shift for itself in the same degree as other new-born animals can do. 56. The period of gestation in the higher class of Mammalia, is usually prolonged until the foetus is able, on its entrance into the world, to' execute regular movements ; some of these being merely indicative of its desire for food, and others evidently designed for the acquirement of it. In many species, the young animal seems to be from the first in the full possession of its senses, and has considerable power of active locomotion ; in general, however, it is very dependent upon its parent ; only being able to obtain food, when this is placed within its immediate grasp. Such is the case with the Human infant, which is more or less closely dependent upon its parent, during a larger pro- portion of its existence, than is the young of any other animal. Here, again, therefore, we perceive the application of the general law, that, the higher the grade of development a being is ultimately to assume, the more does it require to be assisted during the early stages of its progress. In the case of Man, the prolongation of this period has a most. important and evident influence upon the social condition of the race ; being, in fact, one of the chief means, by which the solitary are bound together in families. 57. The class Mammalia, taken as a whole, is not characterized so much by the possession of any one particular faculty, like that which has been seen in Birds, as by the perfect combination of the different powers, which renders the animals belonging to it susceptible of a much greater variety of actions than any others can perform. There are none that can compete with Birds in acuteness of sight ; but there are few that do not possess the senses of smell, taste, and touch, in a more elevated degree. There are none which can rival CHIEF SUB-DIVISIONS OF MAMMALIA. Birds in rapidity of locomotion ; but there are few that cannot perform several kinds of progression. Several of their movements require a considerable amount of flexibility in the spine; hence the vertebral column, and -the bony framework of the trunk, are never so much consolidated as they are in Birds. On the other hand, the neck is much less movable ; it never consists of more than seven vertebrae, and these are always present; so that they are sometimes of great length, as in the Giraffe, and sometimes extremely short, as in the Whale, which seems to have no neck at all. In the greatest number of Mam- malia, the body is supported upon all the four extremities, as in Reptiles; being adapted for progression along the surface of the earth. There are some species, however, in which the typical structure has undergone a metamor- phosis, by which it is made to resemble that of a Bird ; whilst in others it is modified, so as to conform to the character of the Fish. In the Bats, the power of motion is almost entirely delegated to the wings, which are com- posed of skin, stretched over a bony framework formed of the widely ex- tended hand; and the sternum has a projecting keel for the attachment of the pectoral muscles, as in Birds. And in the Whale tribe, the power of locomotion is almost completely taken from the extremities, and given back to the trunk, as in Fishes ; for the posterior extremities are entirely absent, and the anterior serve only for guidance; there is this important difference, how- ever, that the tail, which is flattened vertically in Fishes, is flattened horizon- tally in the Cetacea, which require the power of frequently coming to the surface to breathe. 58. The inferior energy of muscular movement in the Mammalia, is ac- companied by an inferior amount of respiration ; the type of the respiratory apparatus, however, is higher than in Birds, a large extent of surface being comprised within a smaller space. The lungs are confined to the cavity of the thorax; and there is a provision for the regular renewal of the air received into them, by the action of the diaphragm, which here completely separates that cavity from the abdomen. The diminished amount of respiration, again, involves the production of a lower degree of animal heat ; so that the tempera- ture of this class seldom rises above 104. There is, therefore, less need of means for effectually confining the caloric, especially, too, as their greater size causes their radiating surface to be much less, in proportion to their bulk, than is that of Birds ; and accordingly we find them provided only with a covering of hair or fur, which is much less warm than that of feathers, and which is thin and scanty in those which inhabit tropical climates. The chief exception to the last rule is in the case of the Sloths and of some Monkeys, which inhabit situations exposed to the most powerful rays of the sun, and which are covered with a long but thin and coarse hair, the purpose of which is evidently the protection of their skin from the external heat. The inferior energy of the respiration and circulation, involves a diminished activity of the other functions of nutrition, as compared with those of Birds ; and the demand for food appears to be somewhat less constant. Their various organs, however, are developed upon a higher plan ; as we have already observed in regard to those of respi- ration. Chief Sub-divisions of Mammalia. 59. In sub-dividing the truly Viviparous division of the class, so as to sepa- rate Man from the tribes with which he is associated in it, we may be advan- tageously guided, in the first place, by the conformation of the extremities ; since upon* the perfection of the organs of touch, will depend much of the address of an animal, in executing the actions to which it is prompted by its intelligence. The degree of this perfection is estimated, by the number and 58 ON THE PLACE OF MAN IN THE SCALE OF BEING. mobility of the fingers, and by the degree in which their extremities are en- veloped, by the nail, claw, or hoof, that terminates them. When the fingers are partly absent, or are consolidated together, and a hoof envelops all that portion which touches the ground, it is obvious that the sensibility must be blunted, whilst at the same time the extremity becomes incapable of prehen- sion. The opposite extreme is where (as in Man) a thin nail covers only one side of the extremity of the finger, leaving the other possessed of all its deli- cacy ; where several such fingers exist, of which one can be opposed to the rest, so as to render prehension more perfect, and to perform a great variety of actions ; and where the plane of the whole hand can be turned in any posi- tion, by the nature of its attachment to the forearm. Between these, there are many intermediate gradations. By these characters, the viviparous Mam- malia may be divided into the Unguiculated, which have separate fingers, terminated by distinct nails or claws ; and the Ungitlated, in which the fingers are more or less consolidated, and enclosed at their extremity in a hard hoof. Hoofed animals are necessarily Herbivorous, inasmuch as the conformation of their feet precludes the possibility of their seizing a living prey ; and they have flat-crowned grinding teeth for triturating their food. The summits of these teeth are usually not covered by a smooth coat of enamel, but present a .series of elevations and depressions ; these are occasioned by the peculiar structure of the teeth, which consist of alternating plates of enamel, ivory or dentine, and cementum or crusta petrosa ; these are of three different degrees of hardness ; and, as the softer portions will of course wear down first, the harder remain as projecting ridges. In order to give effect to these, there is usually a considerable power of lateral motion possessed by the lower jaw ; so that a regular grinding action may be performed, which is favourable to the complete reduction of the tough vegetable substances that serve as their food. 60. Animals with Unguiculated fingers are capable of more variety in the character of their food. In some it is almost exclusively vegetable, as in the Rodentia; and here the power of prehension possessed by the extremities is small, the forearm not being so constructed as to be capable of the motions of pronation and supination. In this order, the mouth is remarkably adapted for grinding down hard vegetable substances, the molar teeth being furnished with transverse ridges of enamel, and the jaws having a powerful movement backwards and forwards.* In others, again, there is an almost exclusive adap- tation to animal food. The toes are furnished with long and sharp claws ; and the forefoot may be placed in a variety of positions, by the rotation of the two bones composing the lower part of the leg. The grinding teeth are very narrow, and are formed with sharp points and edgqs, so as to be adapted for dividing animal flesh ; these are firmly set in short strong jaws, which are fitted together like the blades of a pair of scissors, having no action but a verti- cal one ; and the constant friction of the edges of the molar teeth against each other, keeps them sharp.! In this group, too, we find the greatest develop- ment of the canine teeth, which are commonly absent or but slightly developed * The action of trituration is chiefly performed by the external pteregoid muscles. When these are in operation together, they draw the whole of the lower jaw forwards, so as to make the lower teeth project beyond the upper; and the jaw being drawn back again by the digastric muscles, a rapid alternate movement may be thus effected, such as is seen in the Rodentia. When only the muscles of one side acts, the condyle of that side is thrown forwards; and by the alternating operation of the two, aided by other muscles, that rotatory motion is given which we see especially in Ruminating Quad- rupeds. f In Carnivorous animals, the muscles which elevate the lower jaw attain a very high degree of development. This is very remarkably seen in the internal pteregoid, which in Man is of subordinate size and importance, but which is a very powerful muscle in the Lion, Tiger, &c. CHIEF SUB-DIVISIONS OF MAMMALIA. 59 among herbivorous quadrupeds ; these are the most powerful weapons with which Carnivorous animals are furnished, serving both for the first attack of their prey, and for subsequently tearing it in pieces. It is evident that the whole structure of the body must undergo modification, in conformity with the nature of the food. The simple stomach and intestinal canal of the carnivo- rous animal, adapted only to the digestion of aliment consisting of materials similar to those of its body, would be totally useless to an animal prevented by its general organization from obtaining any other than vegetable food ; and on the other hand, the teeth and hoofs of the herbivorous quadruped would be of little assistance to an animal whose instincts and general conformation adapted it for the pursuit of animal prey. It will be presently seen that, in regard to his organization, Man holds an intermediate place, between the purely herbi- vorous and the purely carnivorous tribes ; being capable of subsisting exclu- sively upon either kind of diet, but being obviously intended by nature to employ both in combination. 61. The classification of the Mammalia by Linnaeus, although not strictly natural, affords us the readiest means of separating Man zoologically from all other animals. He arranged under his order Primates, all the unguiculated Mammalia, which have four incisor teeth and two canines in each jaw ; and thus Man, with the Monkeys and the Bats, was distinguished from the remainder of those Quadrupeds, which have separate fingers with distinct nails or claws. This group is now sub-divided into three orders, correspond- ing with the Linnaean genera, Homo, Simla and Vespertilio. The last of these orders, named Cheiroptera, includes the Bat tribe, which is easily sepa- rated from all others, by the peculiar conformation of the anterior extremities, from which its name is derived. The second, termed Quadrumana, compre- hends the Apes, Monkeys, and Baboons, which exhibit a regular series, the highest approaching Man in general conformation, and the lowest having much more of the general organization of the inferior carnivorous quadrupeds. They are distinguished from other viviparous Mammalia, by possessing an opposable thumb on all four extremities (whence they are termed four-handed), a character which is only found elsewhere in the Opossums. Although some of the higher members of this group are capable of maintaining the erect position without difficulty for some time, even whilst walking, it is cer- tainly not that which is natural to them. The posterior extremity, being formed on the plan of a hand, for prehension rather than for direct support, is destitute of the heel which is characteristic of Man : and although Apes can climb trees with facility, they cannot plant the foot firmly on the ground, so as to resist attempts to overthrow them ; since the foot rests rather upon the outer side than upon its sole, and the narrowness of the pelvis is unfavourable to an equilibrium. There are many points of striking resemblance to Man, however, in the details of the conformation of the Gtuadrumana, especially among the most elevated species ; the order being distinguished by the same characters from most others. The structure of their alimentary canal differs extremely little from his. The eyes are directed forwards, when the trunk is erect ; and the orbit is completely separated from the temporal fossae, by a bony partition. The mammae are situated on the thorax ; and the penis is pendant. Their coitus, however, is reverse, as in the lower Mammalia. The form of the brain in the higher species corresponds with that of Man, in this remarkable character, that it is divided into three lobes, of which the pos- terior is prolongea backwards so as to cover the cerebellum ; this is not the case in the hio-hest of the other Mammalia. 60 ON THE PLACE OF MAN IN THE SCALE OF BEING. Characteristics of Man. 62. We shall now review, somewhat in detail, the distinctive characters that separate Man from those animals which present the nearest approach to him in general structure and aspect. These may be advantageously classified according to their ohvious purposes ; and the first series we shall notice, con- sists of those by which Man is peculiarly adapted to the erect attitude. On examining his cranium, we remark that the condyles, by which it is articu- lated with the spinal column, are so placed, that a "perpendicular let fall from the centre of gravity of the head would nearly fall between them, so as to be within the base on whi&h it rests. The foramen magnum is not placed in the centre of the base of the skull, but just behind it, in order to compensate for the greater specific gravity of the posterior part of the head, which is entirely filled with solid matter, whilst the anterior part contains many cavities. There is, indeed, a little over-compensation, which gives a slight preponde- rance to the front of the head ; so that it drops forwards and downwards, when all the muscles are relaxed. But the muscles which are attached to the back of the head, are far larger and more numerous than those in front of the con- dyles ; so that they are evidently intended to counteract this disposition ; and we find, accordingly, that we can keep up the head for the whole day, with so slight and involuntary an effort, that no fatigue is produced by it. More- over, the surfaces of the condyles have a horizontal direction, when the head is upright ; and thus the weight of the skull is laid vertically by them, upon the top of the vertebral column. If these arrangements be compared with the position and direction of the occipital condyles in other Mammalia, it will be found that these are placed in the latter much nearer to the back of the head, and that their plane is more oblique. Thus, whilst the foramen magnum is situated, in Man, just behind the centre of the base of the skull, it is found, in the Chimpanzee and Orang Outan to occupy the middle of the posterior third ; and, as we descend through the scale of Mammalia, we observe that it gradu- ally approaches the back of the skull, and at last comes- nearly into the line of its longest diameter, as we see in the Horse. The obliquity of the condyles differs in a similar degree. In all Mammalia except Man, their plane is oblique ; so that, even if the head were equally balanced upon them, the force of gravity would tend to carry it forwards and downwards. In Man, the angle which they make with the horizontal is very small ; in the Orang Outan it is as much as 37 ; and in the Horse, their plane is vertical, making the angle 90. If, therefore, the natural posture of Man were horizontal, he would in this respect be circumstanced like the Horse ; for the plane of his condyles, which is nearly horizontal in the erect position, would then be vertical ; and the head, instead of being nearly balanced in the erect position, would hang at the end of the neck, so that its whole weight would have to be supported by some external and constantly-acting power. But for this, there is neither in the skeleton, nor in the muscular system of Man, any adequate provision. In other Mammalia, the head is maintained in such a position, by a strong and thick ligament (the ligamentum nuchae), which passes from the spines of the cervical and dorsal vertebrae to the most prominent part of the occiput ; but of this there is scarcely any trace in Man. In the horizontal position, therefore, he would have the heaviest head, with the least npwer of support- ing it. 63. The position of the face immediately beneath the brain, so that its front is nearly in the same plane as the forehead, is peculiarly characteristic of Man ; for the crania of the Chimpanzee and Orang, which approach nearest to that of Man, are entirely posterior to, and not above, the face. It should be CHARACTERISTICS OF MAN. 61 remarked that, in the young Ape, there is a much greater resemblance to Man in this respect, than there is in the adult. For at the time of the second dentition, the muzzle of the Ape undergoes a great elongation, so that it pro- jects much more beyond the forehead ; this is seen in Fig. 5. The whole Fig. 5. View of the base of Skull of Man, compared with that of the Orang Outan. cast of the features is altered at the same time, so that it approaches much more to that of the lower Q,uadrumana, than would be supposed from observa- tion of the young animal only.* This increased projection of the muzzle, is an evidence of want of perfect adaptation to the erect posture : whilst the absence of it in Man, shows that no other position is natural to him. Sup- posing that, with a head formed as at present, he were to move on all-fours, so that his face would be brought into the same plane with 'the ground, as painful an effort would be required to examine with the eyes an object placed in front of the body, as is now necessary to keep the eyes fixed on the zenith ; the nose would be unable to perceive any other odours than those which pro- ceeded from the earth or from the body itself; and the mouth could not touch the ground, 'without bringing the forehead and chin also into contact with it. The oblique position of the condyleg in the Quadrumana enables them, without much difficulty, to adapt the inclination of their heads to the horizontal or to the erect position of the body ; but the Natural position, in the highest among them, is unquestionably one in which &e spinal column is inclined, the body being partially thrown forwards, so as to rest upon the anterior extremities ; and in this position, the face is directed forwards without any effort, owing to the mode in which the head is articulated with the spine. 64. The vertebral column in Man, though not absolutely straight, has its curves so arranged, that, when the body is in an erect posture, a vertical line from its summit would fall exactly on the centre of its base. It increases considerably in size in the lumbar region, so as to be altogether somewhat pyramidal in form. The lumbar portion, in the Chimpanzee and Orang, is not of the same proportional strength ; and contains but four vertebrae instead of five. The processes for the attachment of the muscles of the back to this * None but young specimens of the Chimpanzee and Orang Outan have ever been brought arlive to this country ; and they have never survived the period of their second dentition. 6 62 ON THE PLACE OF MAN IN THE SCALE OF BEING. Fig. 6 Comparative view of the Skeleton of Man and that of the Orang Outan. CHARACTERISTICS OF MAN. 63 part, are peculiarly large and strong in Man ; and this arrangement is obviously adapted to overcome the tendency, which the weight of the viscera in front of the column would have, to draw it forwards and downwards. On the other hand, the spinous processes of the cervical and dorsal vertebrae, which grfe in other Mammalia large and strong, for the attachment of the ligamentum nuchae to support the head, have in Man but little prominence, his head being nearly balanced on the top of the column. The base of the human vertebral column is placed on a sacrum of greater proportional breadth than that of any other animal; this sacrum is fixed between two widely-expanded ilia ; and the whole pelvis is thus peculiarly broad. In this manner, the femoral articulations are thrown very far apart, so as to give a wide basis of support ; and by the oblique direction of the whole pelvis, the weight of the body is transmitted almost ver- tically, from the top of the sacrum to the upper part of the thigh bones. The pelvis of every other species of the class is very differently constructed ; as will be seen in the adjoining Figure (6), in which the skeleton of the Orang is placed in proximity with that of Man. It is much longer and narrower, having a far smaller space between the iliac bones and the lowest ribs ; the sacrum is lengthened and reduced in width ; the alae of the ilia are much less expanded ; and the whole pelvis is brought nearly into a line with the verte- bral column. The position of the human femur, in which it is most securely fixed in its deep acetabulum, is that which it has, when supporting the body in the erect attitude. In the Chimpanzee and Orang, its analogous position is at an oblique angle to the long axis of the pelvis, with the body supported obliquely in front of it ; in many Mammalia, as in the Elephant, it forms nearly a right angle ; and in several others, as the Horse, Ox, &c., it forms an acute angle with the axis of the pelvis and spinal column. 65. The lower extremities of Man are remarkable for their length, which is proportionably greater than that which we find in any other Mammalia, ex- cept the Kangaroo tribe. It is evident that there could be no greater obstacle to his progression in the horizontal posture, than this length of what would then be his hind legs. Either Man would be obliged to rest on his knees, with his thighs so bent towards the trunk, that the attempt to advance them would be inconvenient, his legs and feet being entirely useless ; or he must elevate his trunk upon the extremities of his toes, throwing his head downwards, and exerting himself violently at every attempt to bring forward the thighs by a rotatory motion at the hip-joint. In either case, the only useful joint would be that at the hip ; and the le^s would be scarcely superior to wooden or other rigid supports. The chief difference in their proportional length, between Man and the semi-erect Apes, is seen in the thigh ; and from the comparative shortness of his arms, his hands only reach the middle of the thighs ; whilst in the Chimpanzee they hang on a level with the knees, and in the Orang they descend to the ankles. The Human femur is distinguished by its form and position, as well as by its length. The obliquity and length of its neck still further increase the breadth of the hips ; whilst they cause the lower ex- tremities of these bones to be somewhat obliquely directed towards each other, so that the knees are brought more into the line of the axis of the body. This position is obviously of great use in walking, when the whole weight has to be alternately supported on each limb ; for, if the knees had been further apart, the whole body must have been swung from side to side at each step, so as to bring the centre of gravity over the top of each tibia ; and, as a matter of fact, it is noticed that the walk of women, in whom the pelvis is broader and the knees more separated, is less steady than that of man. There is a very marked contrast between the knee-joint of Man, and that even of the highest Apes. In the former, the opposed extremities of the femur and the tibia are expanded, so as to present a very broad articulating surface ; and the internal condyle of 64 ON THE PLACE OF MAN IN THE SCALE OF BEING. the femur is lengthened, so that the two are in the same horizontal plane, in the usual oblique position of the femur. In this manner, the whole weight of the body, in its erect posture, falls vertically on the top of the tibia, when the joint is in the firmest position in which it can be placed : and a comparison of the knee-joint of the Orang with that of Man, will make it at once evident, that the former is not intended to serve as more than a partial support. The weight of the body is transmitted through the tibia, to the upper convex sur- face of the astragalus, and thence to the other bones of the foot. The Human foot is, in proportion to the size of the whole body, larger, broader, and stronger, than that of any other Mammal, save the Kangaroo. The sole of the foot is concave, so that the weight of the body falls on the summit of an arch, of which the os calcis and the metatarsal bones form the two points of support. This arched form of the foot, and the natural contact of the os calcis with the ground, are peculiar to Man alone. All the Apes have the os calcis small, straight, and more or less raised from the ground ; which they touch, when standing erect, with the outer side only of the foot : whilst in animals more remote from Man, the os calcis is brought still more into the line of the tibia ; and the foot being more elongated and narrowed, only the extremities of- the toes come in contact with the ground. Hence Man is the only species of Mammalia, which can stand upon one leg. If we look at the conformation of the upper extremity of Man, we observe similar proofs that it is not intended as an organ of support being destitute of all these adaptations, and having a conformation obviously designed for other purposes, which could not be possi- bly answered, if it were not completely relieved from the necessity of bearing the weight of the body. This peculiar conformation will be subsequently considered. 66. The other parts of the Human body concerned in locomotion, are ex- actly adapted to the peculiar construction of the skeleton. The tibia is kept erect upon the foot by the very powerful muscles, which are attached to the heel and which form the calf of the leg, a prominence observed in no other animal, in nearly the same degree. The flexor longus pollicis pedis, which is attached in the Chimpanzee and Orang to the three middle toes, proceeds in Man exclusively to the great toe, on which the weight of the body is often supported. The extensors of the leg upon the thigh are much more powerful than the flexors, an arrangement seen in no other animal. The glutsei, by which .the pelvis is kept erect upon the thigh, are of far greater size than is elsewhere seen. The superior power of the muscles tending to draw the head and spine backwards, has been already referred to. In the general form of the trunk, there is a considerable difference between man and most other Mam- malia. His chest is large, but is flattened in front, and expanded laterally, so that its transverse diameter is greater than its antero-posterior ; a peculiarity in which only the most Man-like monkeys partake. His sternum ^s short and broad ; and there is a considerable distance between the lower ribs and the ilia, in consequence of the small number of ribs, and the length of the lumbar portion of the vertebral column. The viscera in this space, which in the hori- zontal position would be but insufficiently held up by the abdominal muscles, are, in the erect attitude, securely supported by the expanded pelvis. From all these facts it is an indisputable conclusion, that the erect attitude and biped progression are natural to man ; and we must regard as in great degree fabu- lous, all those histories of supposed wild men, who, it has been said, were found in woods, dumb, hairy, and crawling on all-fours. The most elaborate investigation* of the structure of the anthropoid Apes, and the fullest acquaint- * See especially Mr. Owen's paper on the Chimpanzee and the Orang Outan, in the Zoological Transactions, vol. i. CHARACTERISTICS OF MAN. 65 ance with their habits, concur in proving, that their movements are not easy or agile, unless they employ all their limbs for the support of their bodies. (>7. The name Bimana is the most appropriate that could be founcl, for an order constituted by the Human species only ; since Man alone is two-handed. " That," says Cuvier, " which constitutes the hand, properly so called, is the faculty of opposing the thumb to the other fingers, so as to seize the most minute objects, a faculty which is carried to its highest degree of perfection in Man, in whom the whole anterior extremity is free, and can be employed in prehension." Some naturalists refuse the term hand to the extremities of the Monkey tribe, preferring to call them graspers; for it is certainly true, that, although usually possessing an opposable thumb, they are destitute of the power of performing many of those actions which we regard as most charac- teristic of the hand. These actions are chiefly dependent on the size and power of the thumb ; which is much more developed in Man than it is even in the highest Apes. The thumb of the Human hand can be brought into exact opposition to the extremities of all the fingers, whether singly or in com- bination ; whilst in those Quadrumana which most nearly approach man, the thumb is so short and weak, and the fingers so long and slender, that their tips can scarcely be brought into opposition, and can never be opposed in near contact with each other, with any degree of force. Hence, although admirably adapted for clinging round bodies of a certain size, such as the small branches of trees, &c., the extremities of the Quadrumana can neither seize very mi- nute objects with such precision, nor support large ones with such firmness, as are essential to the dexterous performance of a variety of operations, for which the hand of Man is admirably adapted. Hence the possession of " four hands" is not, as might be supposed, a character which raises the animals that pos- sess it above two-handed Man ; for none of these four hands are adapted to the same variety of actions of prehension, of which his are capable ; and all of them are in some degree required for support. In this respect, their cha- racter approaches much nearer to that of the extremities of the lower Mam- malia ; and there are several among them, in which the opposable power of the thumb being deficient, there is no very marked distinction between the so- called hand, and the foot of some Carnivora. There is much truth, then, in Sir C.. Bell's remark, that "We ought to define the hand as belonging exclu- sively to Man." There is in him, what we observe in none of the Mammalia that approach him in other respects, a complete distinction in the functional character of the anterior and posterior extremities ; the former being adapted for prehension alone, and the latter for support alone. Thus each function is performed with a much higher degree of perfection than it can be where two such opposite purposes have to be united. The arm of the Ape has as wide a range of motion as in Man, so far as its articulations are concerned ; but it is" only when the animal is in the erect attitude that its arm can have free play. Thus the structure of the whole frame must conform to that of the hand, and must act with reference to it. But it cannot be said with truth (as some have maintained), that Man owes his superiority to his hand alone; for without the directing mind, the hand would be comparatively valueless. His elevated position is due to his mind and its instruments conjointly; for if destitute of either, mankind would be speedily extinguished altogether, or reduced to a very subordinate grade of existence. 08. Thus, then, although the order Bimana cannot be separated from the order duadrumana by any single obvious structural distinction, like that which characterizes the Cetacea or the Cheiroptera, it is really as far removed by the minuter, but not less important modifications which have been detailed. A few other distinctive characters will now be noticed. With one exception (the fossil genus Anoplotherium, which is allied to the Tapir tribe), Man is dis- 6* 66 ON THE PLACE OF MAN IN THE SCALE OF BEING. tinguished from all other animals, by the equality in the length of all his teeth, and by the equally close approximation of them all in each jaw. Even the anthropoid Apes have the canine teeth longer than the others, and an interval in the line of teeth in each side of the jaw, to receive the canine teeth of the opposite jaw. This is more evident in the adult than in the young animal. The vertical position of the Human teeth, on which one of the most charac- teristic features of the human face the prominent chin depends, is also quite peculiar; and is intimately connected both with his erect attitude, and with the perfection of the hands, by which the food is divided and conveyed to the mouth. He has no occasion for that protrusion of the muzzle and lips, which, in animals that seize their food with the mouth only, is required to pre- vent the face from coming into general contact with it. The absence of any weapons of offence, and of direct means of defence, are remarkable charac- teristics of Man, and distinguish him from other animals. On those to whom Nature has denied weapons of attack, she has bestowed the means either of passive defence, of concealment, or of flight. Yet Man, by his superior rea- son, has not only been enabled to resist the attacks of other animals, but even to bring them under subjection to himself. His intellect can scarcely suggest the mechanism which his hands cannot frame ; and he has devised and con- structed arms more powerful than those which any other creature wields, and defences so secure as to defy the assaults of all but his fellow-men. We find, on comparing the brain of Man with that of the lower Mammalia, that, as might have been anticipated, its proportional dimensions are much greater, and its structure more complex. The former part of this statement is easily veri- fied by an examination of the cranium alone, comparing tbe size of its cavity with that of the face. The amount of the facial angle, taken after the man- ner of Camper, affords a tolerably correct indication of the relative sizes of these parts. In Man, the facial angle is, in the average of Europeans, 80 ; in Negroes, it is about 70. In the adult Chimpanzee (which approaches in this respect nearest to Man), the facial angle is only 35 ; and in the Orang, it is no more than 30. In other animals it is still less, except when it is in- creased by the prominence of large frontal sinuses, or by the comparative shortness of the jaws. In regard to the structure of the brain, we shall here only remark generally, that the Encephalon of Man far exceeds that of the highest Quadrumana, in the size of the cerebral hemispheres, in the com- plexity and development of its internal parts, and in the depth and number of its convolutions. 69. Man cannot be regarded as distinguished from other Mammalia, how- ever, either by acuteness of sensibility, or by muscular power. His swiftness in running, and agility in leaping, are inferior to that of other animals of his size, the full-grown Orang, for example. The smallness of his face, compared with that of the cranium, shows that the portion of the nervous system distri- buted to the organs of sense, is less developed in him than it is in most other animals ; and the small proportional size of the ganglionic centres, with which these organs are immediately connected, is another indication of the same fact. Accordingly, he is surpassed by many in the acuteness of his sensibility to light, sound, &c., but he stands alone in the power of comparing his sensa- tions, and of drawing conclusions from them. Moreover, although none of his senses are very acute in his natural state, they are all moderately so, which is not the case in other animals ; and they are capable (as is also his swiftness of foot) of being much improved by practice, especially when circumstances strongly call for their exercise. This power of adaptation to varieties in external conditions, which makes him to a great extent independent of them, is manifested in other features of his structure and economy. He is capable of sustaining the lowest as well as the highest extremes of temperature and CHARACTERISTICS OF MAN. 67 of atmospheric pressure. In the former of these particulars, he is strikingly contrasted with the anthropoid Apes, such as the Chimpanzee, which is restricted to a few of the hottest parts of Africa, and the Orang Outan, which is only found in Borneo and Sumatra : these cannot be kept alive in temperate climates, without the assistance of artificial heat ; and even when this is afforded, they speedily become diseased and die. His diet is naturally of a mixed kind ; but he can support himself in health and strength, on either animal or vegetable food exclusively. It is by the demands which his peculiar condition makes upon the exercise of his ingenuity, that his mental powers are first called into active operation ; but, when once aroused, their develop- ment has no assignable limit. The slow growth of Man? and the length of time during which he remains in a state of dependence upon his parents, have been already mentioned as peculiarities, by which he is distinguished from all other animals. He is unable to seek his own food, during at least the three first years of his life ; and he does not attain to his full stature until he is more than twenty years of age. In proportion to his size, too, the whole sum of his life is greater than that of other Mammalia. The greatest age of the Horse, for example, which is an animal of much superior bulk, is between thirty and forty years. That of the Orang, which, when full grown, surpasses Man in stature, is about the same, so far as can be ascertained. The age to which the life of Man is frequently prolonged, is well known to be above a hundred years ; and instances of such longevity are to be found in all nations. 70. Still, however widely Man may be distinguished from other animals, by these and other peculiarities of his structure and economy, he is yet more distinguished by those mental endowments, and the habitudes of life and action thence resulting, which must be regarded as the essential characteristics of humanity. In the highest among brutes, the mere instinctive propensities (as already defined 28, 34,) are the frequent springs of action ; and although the intelligent will is called into exercise, to a 'certain extent, the character never rises beyond that of a child. In fact, the correspondence between the psychical endowments of the Chimpanzee, and those of the Human infant of between two and three years old, is very close. In Man, however, the instinctive propensities only manifest themselves strongly whilst the intellect is undeve- loped ; and nearly all the actions of adult life are performed under the direction of the intelligent will. From the intelligence of Man results this improva- bility ; and his improved condition impresses itself upon his organization. This capability of improvement in the bodily as well as the mental constitution of Man, is the cause of the comforts now enjoyed by civilized races, and of the means which they possess of still further elevation. In the process by which these are attained, we observe a remarkable difference between the character of Man and that of other animals. The arts of which these last are capable are limited, and peculiar to each species ; and there seems to be no general power of adapting these to any great variety of purposes, or of profiting by the experience of others. Where a particular adaptation of means to ends, of actions to circumstances, is made by an individual (as is frequently the case, when some amount of intelligence or rationality exists), the rest do not seem to profit by it ; so that there is no proof that any species or race among the lower animals ever makes an advance towards an improvement or alteration in its condition. That modifications in structure and instincts may be induced by circumstances, in some of the most improvable species, such as the Dog, has been shown by abundant evidence ; and these modifications, if connected with the original habits and instincts of the species, may be hereditarily trans- mitted. There is ample proof that the same is the case, in regard both to the corporeal structure and psychical endowments of Man. Under the influence of education, physical and mental, continued through successive generations, ON THE PLACE OF MAN IN THE SCALE OF BEING. the capabilities of his whole nature, and especially those of his brain, are called out ; so that the general character of the race is greatly improved. On the other hand, under the influence of a degraded condition, there is an equally certain retrogression ; so that, to bring up the New Holland Savage, or the African Bushman to the level of the European, would probably require centu- ries of civilization. One of the most important aids to the use and development of the human mind, is the power of producing articulate sounds, or language ; of which, as far as we know, Man is the only animal in possession. There is no doubt, that many other species have certain powers of communication between individuals ; but these are probably very limited, and of a kind very different from a veAal language. 71. Although, as we have stated, there is nothing in Man's present condi- tion which removes him from the pale of the Animal kingdom, and although his reasoning powers differ rather in degree than in kind from those of the inferior animals, he seems distinguished by one innate tendency, to which we have no reason to suppose that any thing analogous elsewhere exists, and which we might term an instinct, were it not that this designation is generally applied to propensities of a much lower character. The tendency here referred to, is that which seems universal in Man, to believe in some unseen Existence. This may take various forms, but is never entirely absent from any race or nation, although (like other innate tendencies) it may be defective in indi- viduals. Attempts have been made by some travelers to prove that particular nations are destitute of it ; but such assertions have been based only upon a limited acquaintance with their habits of thought, and with their outward observances. For there are probably none that do not possess the idea of some invisible Power external to themselves ; whose favour they seek, and whose anger they deprecate, by sacrifice and other religious observances. It requires a higher mental cultivation than is always to be met with, to conceive of this Power as having a Spiritual existence ; but wherever the idea of spirit- uality can be denned, it seems connected with it. The vulgar readiness to believe in demons, ghosts, &c., is only an irregular or depraved manifestation of the same tendency. Closely connected with it, is the desire to share in this spiritual existence ; which has been implanted by the Creator in the mind of Man ; and which, developed as it is by the mental cultivation that is almost necessary for the formation of the idea, has been regarded by philosophers in all ages, as one of the chief natural arguments for the immortality of the soul. By this Immortal Soul, the existence of which is thus guessed by Man, but of whose presence within him he derives the strongest assurance from Revelation, Man is connected with beings of a higher order, amongst whom intelligence exists, unrestrained in its exercise by the imperfections of that corporeal me- chanism through which it here operates; and to this state, a state of more intimate communion of mind with mind, and of creatures with their Creator, he is encouraged to aspire, as the reward of his improvement of the talents here committed to his charge. OF VITAL ACTIONS, AND THEIR MUTUAL DEPENDENCE. 69 CHAPTER II. GENERAL VIEW OF THE FUNCTIONS. SECTION I. Of Vital Actions, and their mutual dependence. 72. The idea of Life or Vital Action obviously involves that of change. We do not consider any being as alive, which is not undergoing some continual alteration perceptible to the senses. This alteration may be so trifling in its amount, as not to be recognized except by frequent comparison. The slow- growing Lichen, that forms the gray or yellow spots upon old walls, or the. Oyster that is lying motionless in its massive bed, may appear to perform no action ; and yet a sufficiently prolonged knowledge of the former would show, that it is gradually though slowly extending itself, and that it is multiplying its race by a humble yet effectual process of fructification ; whilst closer obser- vation of the latter would enable us to perceive, that its surfaces are covered with cilia which are in continual vibration, that food is being regularly taken into its stomach, undergoes digestion, and is converted into materials fit for the aliment of the body, that a constant circulation of blood is maintained, by the action of a powerful heart, that this circulation is subservient to the various processes of nutrition, secretion, and reproduction, that in due time a number of young Oysters are produced, which swim forth from between the valves of the parent shell, and locate themselves elsewhere, and lastly that, apathetic as the creature seems, it may be excited by some kinds of stimuli to a movement which seems to evince sensation, the closure of the shell being produced by any mechanical irritation of the contained animal, or even, when it lies undisturbed in its native haunts, by a shadow passing between it and the sun. Thus, then, change of some kind is essential to our idea of Life. It may be asked what is the condition of a seed, which remains unchanged during a period of many centuries, and at last vegetates, when placed in favourable circumstances, as if it had been ripened but the year before. The seed is not alive, but it is possessed of the^property of vitality, or the power of performing vital actions, when aroused to them by the necessary stimuli, such as warmth, moisture, oxygen, &c. Its condition is analogous to that of the human being in profound sleep ; he is not then a feeling thinking man ; but he is capable of feeling and thinking, when he is aroused from his slum- ber, and his mind is put into activity by the impressions of external objects. 73. As the activity of a living being, then, is dependent upon two sets of conditions, the organized structure which it possesses, and the stimuli to which this is exposed, we can scarcely separate from our notion of an organ- ized structure, that of the peculiar properties with which it is endowed; for we never see an organized structure remaining as such, unless it possesses some degree of vitality. It may be said that, when an Animal or Plant is killed by a strong electric shock, its organization is unaffected, yet its vital properties are destroyed. Yet no proof of such an assertion, which is con- trary to all analogy, has ever been afforded. In no other circumstances do we ever witness the departure of vitality, without some change of structure or of 70 GENERAL VIEW OF THE FUNCTIONS. composition, which can be made evident. In the ordinary death of an Ani- mal, we may commonly trace the action of the morbific cause upon some par- ticular organ, whose function is thereby either suspended or perverted ; and the cessation of the whole train of actions necessarily results, if this organ be one of those essentially concerned in them. Thus, to take a not uncommon case, a patient with tubercular deposition nearly filling both lungs, becomes the subject of an ulceration, which suddenly opens a passage from one of the bronchi to the pleural cavity on the same side ; death from this cause is fre- quently almost instantaneous, from the total incapacity of the other lung to maintain by itself those respiratory actions which are necessary to the con- tinuance of the circulation. Take again, for example, the influence of a nar- cotic poison ; it occasions torpidity, first of the brain, and then of the medulla oblongata. So long as its action is confined to the brain, the general train of vital operations is no more disturbed than it is in profound sleep ; but as soon as it affects the medulla oblongata, the respiratory movements become para- lyzed (from causes hereafter to be explained), and the circulation is soon brought to a stand ; and every organ in the body speedily loses its character- istic properties, by the commencement of chemical changes in its composition. But if the respiration be artificially sustained, the circulation will continue, and all the processes of nutrition, secretion, &c., to Avhich it is subservient, will be performed with little interruption. Hence the cessation of the whole train, which would otherwise ensue, and the loss of vitality of the general structure, are due to the local change produced by the morbific cause ; and the same may be traced, though not always so evidently, in a variety of other instances. 74. If we consider the actions exhibited by any living being, in which they are sufficiently complex and numerous to admit of being classified, we shall perceive that they may be associated into groups, termed Functions; of which every one, taken as a whole, has some positive and determinate purpose. Thus, one of the most universal of all the changes necessary to the existence of a living being, is the exposure of its nutritious fluid to the air ; by the action of which upon it, certain alterations are effected. For the performance of this aeration, simple as the change appears, many provisions are required. In the first place, there must be an aerating surface, consisting of a thin mem- brane, permeable to gases ; on the one side of which the blood may be spread out, whilst the air is in contact with the other. Then there must be a provi- sion for continually renewing the blood, which is brought to this surface ; in order that the whole mass of fluid may be equally benefited by the process. And, in like manner, the stratum of air must also be renewed, as frequently as its constituents have undergone any essential change. We include, therefore, in speaking of the Function of Respiration, not only the actual aerating pro- cess, but also the various changes which are necessary to carry this into effect, and which obviously have it for their ultimate purpose. 75. On further examining and comparing these Functions, we find that they are themselves capable of some degree of classification. Indeed, the distinc- tion between the groups into which they may be arranged, is one of essential importance in Animal Physiology. If we contemplate the history of the Life of a Plant, we perceive that it grows from a germ to a fabric of sometimes gigantic size, generates a large quantity of organized structure, and many organic compounds, which form the products of secretion, but do not undergo organization, multiplies its species, by the production of germs similar to that from which it originated; but that it performs all these complex operations, without (so far as we can perceive) either feeling or thinking, without con- sciousness or will. All the functions of which its Life is composed, are, there- fore, grouped together, under the general designation of Functions of Organic OF VITAL ACTIONS, AND THEIR MUTUAL DEPENDENCE. 71 or Vegetative life ; and they are subdivided into those concerned in the main- tenance of the structure of the individual, which are termed functions of Nu- trition, and those to which the Reproduction of the species is due. The functions of Nutrition in Plants may be thus generally described. The first step in the process, is the Absorption of nutriment from without. This is car- ried by Circulation to the parts of the structure distant from those at which it was absorbed. At some of these parts, the absorbed fluid is brought into relation with the air, by which certain changes are effected in its constitution ; these may be included under the general term deration, only a part of them being analogous to the Respiration of Animals. Having undergone these changes, and lost a considerable part of its superfluous water by the process of Exhalation, the alimentary fluid is prepared to be applied to its various purposes in the system ; and, being carried through the fabric by the Circu- lation, it becomes subservient to the Nutrition and extension of the fabric, and to the formation of various products of Secretion. It also affords the means, to the organs of Reproduction, of the performance of their functions ; since a new germ cannot be formed, any more than the parent structure can be ex- tended, without organizable materials prepared by the foregoing processes, and supplied to the parts where active changes are going on. 76. On analyzing the operations which take place in the Animal body, we find that a large number of them are essentially the same in character with the foregoing, and differ only in the conditions under which they are per- formed; and that we may, in fact, readily separate the Organic functions, which are directly concerned in the riiaintenance of the fabric, from those of Animal life, the chief purpose of which is entirely different. In commencing the survey of these, we must revert to what has been already said in regard to the nature of the food of Animals, and the means by which it is prepared to be applied to the wants of their system ( 14). Not being received (in general at least) in any but the solid form, it needs to be reduced to the fluid state, before it can be introduced by absorption into the system: this reduction,, which is termed Digestion, must be regarded as not merely a process of me- chanical separation or solution, but as one of chemical change. By a part of the same process, a certain degree of separation is effected, between that por- tion of the reduced aliment which is fit for absorption and that which is not adapted to serve any purpose in the economy ; and the latter, together with certain products of secretion, which it is equally desirable to get rid of com- pletely, is at once cast out of the system. The alimentary fluid is then taken up by Absorption, through the vessels spread out upon the walls of the stom- ach and intestinal tube, precisely in the same manner as it is received into Plants through their roots distributed' in the soil: hence the earth has been not unaptly designated as the common stomach of Plants ; and Animals have been said to carry their soil about with them. The absorbed fluid, having been introduced into the general current of the Circulation, is first carried to the Respiratory organs, where it is exposed to the action of the air ; and it is then transmitted to the system, for the purposes of Nutrition, Secretion, and Repro- duction. So far, then, the functions of the Animal system coincide with those of the Plant. The Organic functions of the former, however, have a purpose or object superadded to that which they perform in the latter, Avhere their only end seems to be the production and maintenance of the individual fabric, and the continuance of the race. They are made subservient in Animals to those functions by which they are peculiarly characterized, namely, Sensation and Voluntary Motion ; all the instruments of these operations being main- tained, like the rest of the organic structure, in a state fit for activity, by the processes of Nutrition, which are performed on the same plan in them as in other parts. 72 GENERAL VIEW OF THE FUNCTIONS. 77. Indeed it appears to be principally for the maintenance of these instru- ments in a state fit for activity, that by far the largest part of the food ingested by most Animals is required. * The duration of the existence of the Muscular and Nervous tissues, appears to depend entirely upon the use that is made of them ; being less as their functional activity is greater. Thus, when an ani- mal is very inactive, it requires but little nutrition ; if in moderate activity, there is a moderate demand for food ; but if its muscular energy be frequently and powerfully aroused, the supply must be increased, in order to maintain the vigour of the system. There would seem reason, then, to believe, that every animal movement, requiring the expenditure of a certain degree of mus- cular power, does really involve the death and re-formation of a certain amount of muscular tissue ; and this idea is confirmed by the fact, that the quantity of waste thrown off by the excreting processes, or, at least, of that part of it which results from the disintegration of the muscular structure, is propor- tional to the expenditure of muscular power, increasing (like the demand for food which is consequent upon it) with the general activity, and diminishing with rest. This doctrine, which was first pointedly stated by Liebig, though propounded in more general terms by previous writers, may probably be ex- tended from the Muscular system (in regard to which alone it has been urged by Liebig) to the Nervous, as well as to the various organs of Nutrition. Many circumstances lead to the belief, that the Nervous tissue, when in a state of functional activity, undergoes a rapid waste or disintegration, and a correspond- ing renewal. The very large quantity of blood with which the nervous cen- tres are supplied, and the immediate dependence of nervous power upon the maintenance of that supply ( 177), strongly indicate this; for we invariablj find, that parts which undergo little interstitial change, receive but a small supply of blood. Again, it is well known that, when the nervous system has been in unusual activity, there is a marked increase in the phosphatic depo- sits in the urine ; and, as the quantity of phosphorus in any others of the soft tissues is very inconsiderable, it is scarcely possible to attribute this liberation of phosphorus from the system to any other cause than the waste of nervous matter, that is, its decomposition, resulting from the discharge of its vital function. Again, the close chemical relation which subsists between nervous and adipose matter (the substance peculiar to nervous tissue being a fatty acid, containing a very small proportion of azote, but united with a considerable amount of phosphorus,) corresponds exactly with the old observation, that per- sons of " nervous temperament" are seldom fat ; whilst those of inert bodily and mental habits are much more subject to this deposit. Since nervous matter is chiefly formed out of the same elements as those which would other- wise be deposited as adipose tissue, it appears probable that the demand for these, occasioned by the continual use of the nervous system, prevents the de- posit of fat ; whilst its inactivity allows their accumulation in another form. It may probably be stated, then, as a general ,truth, that every act of Mind is inseparably connected, in our present state of being, with material changes in the Nervous System ; a doctrine not in the least inconsistent with the belief in the separate immaterial existence of the Mind itself. 78. The degree in which the operations of the mind are dependent upon its material instruments, is a question which cannot be regarded as conclusively determined by scientific evidence alone ; and it has little practical bearing on Physiological research. The doctrine usually regarded as having the best Scriptural basis, that the mind has an existence altogether distinct from that of the body, is attended with several difficulties, of which those arising dut of the phenomena of Insanity are perhaps the most important. On the other hand, the opinion held by some, that mental phenomena are the mere results of ma- terial changes, appears to involve difficulties at least equal ; amongst which \ OF VITAL ACTIONS, AND THEIR MUTUAL DEPENDENCE. 73 may be noticed, the consciousness of personal identity, preserved throughout the continual and rapid changes to which the Nervous structure is subject. The assertion, however, that psychical operations cannot be the result of mate- rial changes, is based on the assumption, that we know far more of the essential characters of both, than is admitted by the best metaphysicians to be the case regarding either. This is one of the questions, which scarcely comes within the boundaries of mere human knowledge. Neither hypothesis is inconsistent with the Revealed doctrine of the Immortality of the Soul ; though the second could not be made to conform to it, without the additional supposition that some refined form of matter, on which psychical operations essentially depend, has also an eternal existence ; and the upholders of this doctrine seek a confirma- tion of it in the expression " spiritual body," used by an authority which is all but supreme.* The certainty of a future existence, in which all that is cor- ruptible shall be done away, is the grand practical fact for the Christian ; on the mode of it the philosopher may speculate ; and, even though he may come to the conclusion that "Mind and Matter are logically distinct existences," yet he finds their operations so inextricably interwoven in the phenomena of Man's terrestrial life, that he cannot pursue either class by itself alone. The physi- ologist, therefore, will enter upon the inquiry with the best prospect of success, if untrammeled by any preformed opinions, and ready to form his deductions from the facts presented to his notice. 79. That a very close relation may be traced between the variety and im- portance of the psychical phenomena of different classes of animals, and the complexity and size of their material instruments, all must admit ; and it seems difficult, on the supposition of the completely distinct existence of Mind, to separate the phenomena to which organic changes are and must be essential, from such as do not require these for their production. For example, it is universally admitted, that the mind cannot become cognizant of any impression made by an object external to it, except through the medium of a material change, commencing in the orgaji of sense, and propagated to the central sensorium ; and yet of the absolute nature of this change we know nothing. Now the Sensation thus produced cannot give rise (as will.be shown hereafter, 288,) to a Perception, the formation of an elementary notion of the nature of the object causing the impression, without a series of changes, in which Memory, Association, Judgment, &c., are involved. Memory seems clearly the result of the permanency of the material change effected by the sensation ; for it is peculiarly liable to be affected by disorders or injuries of the brain, which do not impair that power of Comparison, and perception of Causation, by which the Reasoning faculty works upon the materials submitted to it. If Memory be thus connected with organic changes, the power of mental Concep- tion, which is dependent upon the reftewal of the state immediately produced by Sensation and Perception, is scarcely to be separated from them. Now it seems impossible to draw a distinct line between these operations, on the one hand, and the power of Imagination, which derives most or all of its materials * The writer is most happy to find himself supported in these views, by so high a theological authority as that of the propounder of the "Physical Theory of Another Life;" who, after pointing: out how completely the question, whether the human soul is ever actually and entirely separated from matter is passed over by St. Paul, as an in- quiry altogether irrelevant to religion, thus continues : " Let it then be distinctly kept in view, that although the essential independence of mind and matter, or the abstract possi- bility of the former existing apart from corporeal life, may well be considered as tacitly implied in the Christian scheme, yet that an actual incorporeal state of the human soul, at any period of its course, is not necessarily involved in the principles of our faith, any more than it is explicitly asserted. This doctrine, concerning what is called the immate- riality of the soul, should ever be treated simply as a philosophical speculation, and as unimportant to our Christian profession." 74 GENERAL VIEW OF 1*HE FUNCTIONS. from Conception, and the Reasoning Faculties, which are still more closely dependent upon Impressions made from without, on the other. For the phe- nomena of Insanity are continually presenting to us instances of the disorder of these powers, without any corresponding disorder of the operations, which intervene between them and the external world ; and such disorder is often (perhaps uniformly) coincident with some morbid condition of the brain. In regard to the Moral Feelings and Emotions, again, it would seem equally impossible to separate these by a distinct line, from the lower passions and instinctive propensities, which are still more closely connected with material changes ; and the daily experience, even of a person in ordinary health, reveals to him how strongly the emotional conditions of the mind are influenced by the state of the organic functions ; and how powerfully, on the other hand, the latter are reacted on by the former. These, being phenomena which strictly form a part of the Life of Man, evidently belong to the domain of the Physi- ologist ; and no speculative views can (or, at least, ought to) affect our reasoning from facts. 80. The operations of the Mind and of its instruments, taken collectively, constitute what are known as the Functions of Jlnimal Life. Those most ob- viously connected with the bodily fabric, are Sensation and spontaneous Motion; for these we find special instruments provided, the organs of sense and the muscular apparatus. Both these, with the nervous system itself are composed, like other parts of the fabric, of organized structure, which does not differ essen- tially from that of the apparatus of Vegetative life, either in the mode of its first production, or in that in which its integrity is maintained, and its activity preserved. The conditions requisite for these objects will be presently dis- cussed. But, although the functions of Animal life may be regarded as in themselves completely isolated from those of Organic life, the latter merely supplying the conditions of the former, by keeping (so to speak) their instru- ments in good order, yet there are certain links of connection between the two, which render the latter equally dependent on the former. Thus, in re- gard to the acquisition of food, the Animal has to make use of its senses, its psychical faculties, and its power of locomotion, to obtain that, which the Plant, from the different provision made for its support, can derive without any such assistance. Moreover, the propulsion of the food along the alimentary canal is effected by a series of operations, in which the Nervous and Muscular systems are together involved at the two extremes, whilst simple Muscular contractility is alone employed through the greater part of the intestinal canal. Thus, the change in the condition required for the ingestion of food by Ani- mals, has rendered necessary the introduction of an additional element in the apparatus, to which nothing comparable was to be found in Plants. Again, in the function of Respiration, as performed in the higher Animals, the Ner- vous and Muscular systems are alike involved ; for the movements by which the air in the lungs is being continually renewed, are dependent upon the action of both ; and those by which the blood is propelled through the respi- ratory organs, are chiefly occasioned by the contractility of a muscular organ, -the heart. But in regard to the simple contractility of muscular fibre, upon the direct application of a stimulus to it, which is the agent in the movements of the heart and of the alimentary canal, it may be remarked, that it does not differ in any essential degree from that which is Avitnessed in many Vege- tables ; and that it strictly belongs, therefore, to the functions of Organic life. And with respect to those concerned in the act of Respiration, as well as those which govern the two orifices of the alimentary tube, it will hereafter appear that they result, equally with the former, from the application of a stimulus ; and that they may be performed without any consciousness on the part of the individual (though- ordinarily accompanied by it) : the difference being, that OF VITAL ACTIONS, AND THEIR MUTUAL DEPENDENCE. 75 in the former the stimulus is applied to the contractile part itself, whilst in the latter it is applied to an organ with which this is connected by nerves only. Now we have, even in Vegetables, instances of the propagation of an irrita- tion from one part to another, so that a motion results in a part distant from that stimulated, as in the case of the Sensitive Plant, or Venus's Fly-trap. The only essential difference, therefore, between those movements of Animals, which are thus closely connected with the maintenance of the organic func- tions, and those of Plants, consists in the medium through which they are performed, this being in Animals the Nervous and Muscular apparatus, whilst in Plants it is only a peculiar modification of the ordinary structure. 81. From what has been said, then, it appears that all the functions of the Animal body are so completely bound up together, that none can be suspended without the cessation of the rest. The properties of all the tissues and organs are dependent upon their regular Nutrition by a due supply of perfectly ela- borated blood; this cannot be effected, unless the functions of Circulation, Respiration and Secretion, be performed with regularity, the first being necessary to convey the supply of nutritious fluid, and the tw r o latter to sepa- rate it from its impurities. The Respiration cannot be maintained without the integrity of a certain part of the nervous system ; and the due action of this, again, is dependent upon its regular nutrition. The materials necessary for the replacement of those, which are continually being separated from the blood, can only be derived by the Absorption of ingested aliment ; and this cannot be accomplished, without the preliminary process of Digestion. The intro- duction of food into the stomach, again, is dependent, like the actions of Re- spiration, upon the operations of the muscular apparatus and of a part of the nervous centres; and the previous acquirement of food necessarily involves the purely Animal powers. Now it will serve to show the distinction between these powers, and those which are merely subservient to Organic life, if we advert to the case, which is of no unfrequent occurrence, of a human being, deprived by some morbid condition of the brain, of all the powers of Animal life, Sensation, Thought, Volition, &c. ; and yet capable of maintaining a vegetative existence, all the organic functions going on as usual, the morbid condition not having affected the division of the nervous system that is con- cerned in the movements on which some of them depend. It is evident that we can assign no definite limits to such a state, so long as the necessary food is placed within the reach of the grasp of the muscles, that will convey it into the stomach : as a matter of fact, however, it is seldom of long continuance ; since the disordered state of the brain is sure to extend itself, sooner or later, to the rest of the nervous system. This condition may be experimentally imi- tated, however, by the removal of the brain, in many of the lower animals, whose bodies will sustain life for many months after such a mutilation ; but this can only take place, when that food is conveyed by external agency, within the pharynx, which they would, if in their natural condition, have obtained for themselves. A similar experiment is sometimes made by Nature for the physiologist, in the production of fo3tuses, as well of the human as of other species, in which the brain is absent ; these can breathe and suck and swal- low, and perform all their organic functions ; and there is no assignable limit to their existence, so long as they are duly supplied with food. Hence we may learn the exact nature of the dependence of the Organic functions upon those of purely Animal life ; and we perceive that, though less immediate than it is upon the simple organic operations of the nervous and muscular systems, it is not less complete. On the other hand, the functions of Animal life are even more closely dependent upon the Nutritive actions than are those of or- ganic life in general ; for many tissues will retain their several properties, and their power of growth and extension, for a much longer period after a general 76 GENERAL VIEW OF THE FUNCTIONS. interruption of the circulation, than will the Nervous structure, which is, in- deed, instantaneously affected by a cessation of the due supply of blood, or by the depravation of its quality. 82. It is of little consequence, then, with which group of functions we com- mence the detailed study of the phenomena which, in their totality, make up the life of Man. In viewing him merely as one of the widely-extended group of organized beings, it would be natural to commence with those phenomena which are common to all ; and to make, therefore, the Organic functions the first object of our consideration. On the other hand, regarding Man as a being in some degree isolated from all these by his peculiar characteristics, it seems right to inquire into the latter in the first instance ; more especially as, in a general view of his life, these occupy the most prominent place. It will be necessary, however, previously to entering upon them, to take a more detailed survey than we have hitherto done, of the vital operations performed by his several organs, and of their connections with each other. We shall commence with those of Vegetative Life. Functions of Vegetative Life. 83. It is one of the most peculiar characteristics of organized structure, that its elements have a constant tendency (under ordinary circumstances at least) to separate into more simple combinations ; and, although it has been ordina- rily considered that their living state prevents such a change, and that they have no. tendency to it except when dead, reason will hereafter be given for the belief that no such distinction exists ( 645, 646). The maintenance of the vital properties of all organized structure, then, requires either that this structure should be completely secluded from air, moisture, warmth, and other agents which tend to its decomposition ; or that it should be renewed as fast as it decays. Now the exclusion of these decomposing agents would prevent any vital actions from being called into operation ; since they are the ordinary stimuli, which are necessary to them. For instance, a seed which is buried so deep in the soil as to be excluded from the contact of air, and from the warmth of the sun, will not vegetate, although it may retain its power of germi- nating when placed in more favourable circumstances ; and it will not decay, because secluded from the air and warmth which are necessary to its decom- position. But as a certain decomposition appears to be a necessary condition of its vital activity, it is obviously necessary that a provision should be made, for removing from the organism all those particles, which are manifesting an incipient tendency to decay, and are thus losing their vital properties ; and for replacing these by newly-combined particles, which in their turn undergo the same process. Thus we find that, in the softest parts of the Animal frame- work, as in those of the Plant, there is much less permanency than there is in those harder and more solid portions, which often seem altogether to defy the lapse of time. Now it is in the former that the most active vital changes take place, those of the nervous system, for example ; whilst of the latter, the function is chiefly, if not entirely, that of giving mechanical support to the structure. The fact, which is easily proved, that the former organs are renewed many times, whilst the fabric of the latter is not once completely changed, shows a very interesting correspondence between the degree in which the action of any organized structure is removed from, or is similar to, that of a mere inorganic substance, and the amount of tendency to decomposition which that structure exhibits ; since this constant renewal can scarcely serve any other purpose than that of making up for the effects of decay. 84. One of the most important purposes of the supply of aliment, therefore, which all living beings continually require, is the replacement of the portions FUNCTIONS OF VEGETATIVE LIFE. 77 of the fabric that are thus lost. The effects of the process of decay, when uncompensated by that of renovation, are remarkably seen in cases of starva- tion ; for it is a very constant indication of this condition, that the body exhales a putrescent odour, even before death, and that it subsequently passes very rapidly into decomposition. This, it may be considered, is the reason why a constant supply of aliment is still required for the maintenance of every organic structure, though it may have arrived at its full growth ; and it also affords one source of explanation of the fact, that old people require less food than adults, since their tissues are more consolidated, and thus become at the same time unable to perform their usual actions with their pristine energy, whilst their tendency to decomposition is less. In the growing state, however, an additional important source of demand for food obviously exists, in the extension which the tissues themselves are constantly receiving ; yet this, perhaps, does not make so great a difference as it appears to do in the supply which is requisite. For if the addition which is made by growth to the body in any given time, be compared with the amount of exchange which has taken place in the same time, the latter being judged of by the quantity of matter excreted from the lungs, liver, kidneys, skin, &c., it will be found to bear but a very small proportion to it, except during fetal life, when the growth is very rapid, and a large proportion of the effete particles are brought to the maternal blood, to be excreted from it. The real cause of the increased demand for nutriment during the early part of life is rather this, that the tissues are far from having acquired that firmness and consolidation which they gain at adult age ; and that they are, therefore, more prone to decomposition, at the same time that their vital activity is greater, as is well known to be the case. The feeling of hunger or desire for food originates, we shall hereafter find reason to believe ( 430, 437), not so much in the stomach itself, as in the system at large ; of whose condition, in regard to the requirement of an increased supply of ali- ment, it may, during the state of health, be considered as a pretty faithful index. The same may be said of thirst. The feeling of hunger, then, is the stimulus to the mental operations which have for their object the acquisition qf food, whether these be of a voluntary or of a purely instinctive kind ; in Man they are obviously the former, during all but infant life.* 85. The food received into the mouth, and prepared there by the acts of mastication and insalivation (the movements concerned in which are dependent upon the brain, and can only be performed when it is in a condition of some activity), is brought by them within reach of the pharyngeal muscles, whose contraction cannot be effected by the will, but is purely instinctive, resulting merely from the impression made upon the fauces by the contact of the sub- stance swallowed, which impression is conveyed to the medulla oblongata and reflected back to the muscle ( 191). By these it is propelled down the oeso- phagus ; and, after their action has ceased, it is taken up (as it were) by the muscular coat of the oesophagus itself, and conveyed into the stomach. How far the movements of the lower parts of the oesophagus and of the stomach are in Man dependent upon reflex action, is uncertain; the facts which have been ascertained on this point, by experiment on animals, will be detailed in their proper place ( 194). In the stomach, the food is subjected to the gastric secretion ; the chemical action of which, aided by the constantly elevated tem- perature of the interior of the body, and by the continual agitation effected by the contractions of the parietes of the organ, effects a more or less complete solution of it. Reason will hereafter appear for the belief that, up to this point, no action peculiarly vital is immediately concerned in the reduction of the food ; and that, if the physical conditions of the process could be exactly imi- tated out of the body, the result would be precisely the same. The mixture of the biliary and pancreatic secretions with the chyme thus produced, occa- 7* 78 GENERAL VIEW OF THE FUNCTIONS. sions a separation of its elements into those adapted for nutrition, and those of which the character is excrementitious ; and this separation can scarcely be regarded in any other light than as a chemical precipitation. The nutritious portion is then taken up by the Absorbent vessels, or Lacteals, which are dis- tributed on the walls of the alimentary canal ; whilst the remainder is propelled along the intestinal tube by the simple contractility of its walls, undergoing at the same time some further change, by which the nutritive materials are still more completely extracted from it. And at last, the excrementitious matter, consisting not only of a portion of the food taken into the stomach, but also of part of the secretion of the liver, and of that of the mucous surface of the intestines, is voided from the opposite extremity of the canal, by a muscular exertion, which is partly reflex, like that of deglutition, but is partly voluntary, especially (as it would appear) in Man. 86. There seems no doubt that fluid, containing saline or other soluble matters, may be absorbed by the blood-vessels, with which the mucous mem- brane of the alimentary canal is so copiously supplied ; and this simple process of Imbibition probably takes place, according to the physical laws of Endos- mose. But the Selection and Absorption of the nutritious fluid appear to be performed, not by vessels, but by cells ; which are developed at the extremities of the villi, from germs previously existing there ; and which, after having filled themselves with the ingredients drawn from the cavity of the intestinal canal, deliver these to the lacteal absorbents, either by bursting, or by dissolv- ftig away, their own term of life being expired. The absorbed fluid, which now receives the name of Chyle, is propelled through the Lacteals by the contractility of their walls ; aided in part, perhaps, by a vis a tergo derived from the force of the absorption itself. With the reception of the nutritious fluid into the absorbent vessels, commences its real preparation for organiza- tion. Up to that period, it cannot be said to be in any degree vitalized ; the changes which it has undergone being only of a chemical and physical nature, and such as merely prepare it for subsequent assimilation. But in its passage through the long and tortuous system of absorbent vessels and glands, it un- dergoes changes which, Avith little chemical difference, manifest themselves by a decided alteration in its properties ; so that the chyle of the thoracic duct is evidently a very different fluid from the chyle of the lacteals, approaching much nearer to blood in its general characters. These characters are such as indi- cate that the process of organization and vitalization has commenced ; as may be known alike from the microscopic appearance of the fluid, and from the actions it performs when removed from the body. There is reason to believe that the changes, which the Chyle undergoes in its progress through the lac- teals, are due to the action of certain cells which are seen to be diffused through the liquid ; these, by their own independent powers of growth, are continually absorbing into themselves the fluid in which they float ; whilst, by bursting or liquefying, as soon as their term of life is completed, they give this back in an altered state. The Chyle thus modified is conveyed into the Sanguiferous system of vessels, and flows directly to the heart ; by which it is transmitted, with the mass of the blood, to the lungs. It there has the opportunity of ex- creting its superfluous carbonic acid, and of absorbing oxygen ; and probably acquires gradually the properties by which the blood previously formed is distinguished, thus becoming the pabulum vitas for the whole system. 87. The Circulation of the Blood through the tissues and organs which it is destined to support, is a process evidently necessary for the conveyance to them of the nutritious materials, which are provided for the repair of their waste ; and for the removal of those elements of their fabric, which are in a state of incipient decomposition. In the lowest classes of organized beings, every portion of the structure is in direct relation with its nutritive materials ; FUNCTIONS OF VEGETATIVE LIFE. 79 it can absorb for itself that which is required, and it can readily part with that of which it is desirable to get rid. Hence in such, no general circulation is necessary. In Man, on the other hand, the digestive cavity occupies so small a portion of the body, that the organs at a distance from it have no other means, than their vascular communication affords, of participating in the results of its operations ; and it is, moreover, necessary, that they should be continually furnished with the organizable materials, of which the occasional operation of the digestive process would otherwise afford only an intermitting supply. This is especially the case, as already mentioned, with the nervous system, which is so predominant a feature in the constitution of Man ; and we accordingly find both objects provided for, in the formation of a large quantity of a semi-organized product, which contains within itself the materials of all the tissues, and is constantly being carried into relation with them. Blood has been not unaptly termed chair coulante, or liquid flesh ; and although it has been heretofore much questioned, whether it could be regarded as either organized or endowed with vital properties, there now appears to be sufficient reason for admitting that this is the case to a very considerable extent. The propulsion of the blood through the large trunks, which subsequently divide into capillary vessels, is due to the contractions of a hollow muscular organ, the Heart ; but these, like the peristaltic movements of the alimentary canal, are quite independent of (though frequently influenced by) the agency of the nervous system ; and are therefore to be referred to the class of organic move^ ments, such as occur in Vegetables. 88. Upon the circulation of the blood through all parts of the fabric, depends in the first place the Nutrition of the tissues. Upon this subject, formerly involved in the greatest obscurity, much light has recently been thrown. In the lowest classes of Plants and Animals, the whole or the greatest part of the fabric is composed of vesicles or cells aggregated together, each of which has a certain degree of independent vitality, and can live to a great extent by itself alone, if duly supplied with nutriment. These cells differ but little from each other in structure and endowment ; and the whole mass approaches far more nearly, therefore, to the homogeneous character of inorganic bodies than does the fabric of beings more elevated in the scale. This is precisely the condi- tion of the embryonic structure of the highest Animals, at an early period of their existence. Now in such fabrics, there is no distinct vascular system. Every cell absorbs, either from the surrounding nutritious materials with which it may be itself in contact, or from other cells in nearer proximity to these, the aliment it requires for its own growth and reproduction ; and per- forms all its vital processes, with little direct influence from any general con- trolling power. The extension of the individual structure is partly effected by the enlargement of its original vesicles ; but principally by the generation of new ones within these ; and the latter, in their turn, go through the same pro- cesses. In the higher Plants, however, we find a greater variety of tissues ; but these all take their origin in cells. The straight tubes, for instance, which convey the sap from the roots to the leaves, were evidently at first a line of large cells, laid end to end, the partitions between which have broken down, so that their cavities coalesce ; and the network of anastomozing vessels, by which the descending or nutritious sap is conveyed through the tissues, may be traced to a corresponding origin. The circulation of the sap which thus regularly takes place, causes these Plants to receive the name of Vascular, whilst the others are designated as Cellular ; but still it is to be remembered, that the great bulk of the structure in the former, like the whole of the latter, is composed of cellular tissue ; and that the central part of the islands, so to speak, which are composed of this, in the interstices of the vascular network, cannot be nourished in any other way than by absorption from the cells which 80 GENERAL VIEW OF THE FUNCTIONS. surround them. In the higher Animals, the variety of tissues which present themselves in the adult structure, all formed by a metamorphosis from the original vesicles of the embryo, is very great ; but these are all nourished, in a more or less energetic manner, by the blood conveyed to them in the network of minute vessels which traverses them. Still between the reticulations of these vessels, there must necessarily be islands of solid tissue (as seen in Fig. 90), of no inconsiderable size ; and the central portions of these must derive their nourishment from the surrounding cells, exactly as in the humblest Cel- lular Plants. Moreover there are some tissues in which, in the healthy state at least, no very minute distribution of blood-vessels can be ascertained to exist ; and in these the cellular nutrition must go on to a considerable extent. The decay and renewal of such tissues, however, are by no means rapid ; and it is only in such as require little change from time to time, and whose actions are of a physical rather than of a vital character (such, for instance, as Car- tilage), that this mode of nutrition is sufficient. 89. In the nutrition of the tissues which are already completely formed, it seems probable that the fluid portion of the blood performs the chief part. It will be shown hereafter, that the particles of this substance, probably in virtue of the preparation it has undergone by the agency of the cells just described ( 86), which exist in blood as well as in chyle, and are known under the name of colourless corpuscles, have the power of so arranging themselves as to form a regular fibrous tissue. These are the only corpuscles existing in the circulating fluid of Invertebrated animals ; but in the blood of Vertebrata there are others, which contain a colouring fluid, and give to the whole mass its red or purple hue. * These red corpuscles appear to serve the important purpose of conveying oxygen from the lungs into the interior of the system, and of carrying away carbonic acid from the tissues ; since it is evidently in them, that the chief chemical changes effected by Respiration are produced ; and the heat regularly maintained in any class of animals, bears a very close proportion to the quantity of red particles in their blood. 90. The history of the changes by which one group of cells is transformed into bone, another into cartilage, another into nerve, another into muscle, and so on, is extremely interesting, and will be given hereafter in as much de- tail as the limits of this work permit. Of the reason why this variety of pro- ducts should spring up, when the cells in which they all originate appear to be so exactly alike, and have themselves a common origin, no account can be given ; and this is one of the most cufious problems that at present offers itself for investigation. The important^ discoveries, which are here briefly summed up, are not confined to healthy structures ; for it has been ascertained that diseased growths have a similar origin and mode of extension ; and that the malignant character, assigned to Cancer, Fungus Hcmnatodes, and other such productions, is to be traced to the fact, that they are composed of cells which undergo little metamorphosis, and retain their reproductive power ;~-so that from a single cell, as from that of a Vegetable Fungus, a large structure may rapidly spring up, the removal of which is by no means attended with any certainty that it will not speedily re-appear, from some germs left in the system. 91. The independent character of the cells in which all organized tissues originate, might be of itself a satisfactory proof that, in Animals, as in Plants, the actions of Nutrition are performed by the powers with which they are individually endowed ; and that, whatever influence the nervous system may have upon them, they are not in any way essentially dependent upon it. Moreover, there is an evident improbability in the idea, "that any one of the solid textures of the living body should have for its office, to give to any other the power of taking on any vital actions ;" and the improbability becomes FUNCTIONS OF VEGETATIVE LIFE. 81 an impossibility, when the fact is made known, that no formation of nervous matter takes place in the embryonic structure, until the processes of organic life have been for some time in active operation. The influence which the Nervous System is known to have upon the Function of Nutrition, is probably exerted, rather through the medium of its power of regulating the diameter of the arteries and capillaries, by which it controls in some degree the afflux of blood, and of affecting those preliminary actions on which the quantity and quality of the nutritious fluid depend, than in any more direct manner. At any rate, it may be safely asserted, that no such proof of its more direct influ- ence, as is required to counterbalance the manifest improbability which has been shown to attend it, has yet been given, all the facts which have been adduced in support of this hypothesis being equally explicable on the other, which, being in itself more probable, ought to be preferred. 92. The renewal which the various tissues of the body are continually undergoing, has for its chief object the counteraction of the decay, into which they would otherwise speedily pass ; and it is obviously required that a means should be provided for conveying away the waste, as well as for supplying the new material. This is partly effected by the venous circulation, which takes up a large part of the products of incipient decomposition, and conveys them to organs where they may be separated and cast forth from the body. The first product of the decay of all organized structures, is carbonic acid ; and this is the one which is most constantly and rapidly accumulating in the system, and the retention of which, therefore, within the body, is the most in- jurious. Accordingly we find two large organs the Lungs and the Liver adapted to remove it; and to both these Venous blood passes, before it is again sent through the system. The function of the Lungs is so important in warm- blooded animals, that a special heart is provided for propelling the blood through them, in addition to the one possessed by most of the lower animals, the function of which is the propulsion of the blood through the system. In these organs, the blood is subjected to the influence of the atmosphere, by which the carbonic acid with which it was charged, is removed and replaced by oxygen ; and this change takes place, through the delicate membrane that lines the air-cells of the lungs, according to the physical law of the mutual diffusion of gases. The introduction of oxygen into the blood seems necessary alike to maintain its general vivifying powers, and to remove the carbon set free in the tissues, by converting it into carbonic acid; which corresponds with the general fact, that carbonic *acid cannot be formed by decomposition, at least to any large amount, except when the decaying substance has oxygen within its reach. The continual formation of carbonic acid in the tissues, ap- pears to have a most important purpose in the vital economy, that of keeping up its temperature to a fixed standard ; for the union of carbon and oxygen in this situation may be compared to a process of slow combustion ; and it is well known that the more energetic this is the higher is the 'temperature. Thus in Birds, whose nutrition is so active, and whose respiration is so energetic, the temperature is constantly maintained at a point higher than that which other animals ever attain, in the healthy state at least ; whilst in Reptiles, which present a condition exactly the reverse of this, the temperature is scarcely above that of the surrounding medium. The function of the Liver is, like that of the lungs, two-fold : it separates from the blood a large quantity of the superfluous carbon which it acquires by circulating through the tissues ; and it combines that carbon with other elements, into a secretion, which, as we have seen, is of great importance in the digestive process. The hepatic circulation, however, is not kept up by a distinct impelling organ ; but the venous blood from ; the abdominal viscera (and, in the lower Vertebrata, that 82 GENERAL VIEW OF f HE FUNCTIONS. from the posterior part of the body) passes through the Liver on its return to the heart. 93. All animal substances have a tendency, during their decomposition, to throw off nitrogen, as well as carbonic acid ; and this nitrogen may take either the form of cyanogen, by going off in combination with carbon, or of ammonia, by uniting at the time of its liberation with hydrogen. The chief function of the Kidneys is evidently to separate the azotized products of decay from the circulating fluid ; for the secretion which is characteristic of them, namely, urea, contains a larger proportion of nitrogen than is found in any other or- ganic compound ; it is identical in its chemical nature with cyanide of ammonia, and may be considered as the result of the union of these two products of ani- mal decomposition. The action of the kidneys is equally essential to the con- tinued performance of the other vital functions, with that of the lungs and liver ; since death invariably follows its suspension, unless some other means be provided by Nature (as occasionally happens), for the separation of its characteristic excretion from the circulating blood. But death does not so speedily ensue, when the functional action of the liver and the kidneys is suspended, as when that of the lungs is checked ; and for this obvious reason, that only apart of the whole current of blood flows through the former organs, and that, although a disturbance of the usual course of the circulation must ensue from a stagnation of the flow through them, it is not from this cause brought to a stand; whilst, in the case of the lungs, the fact that the whole of the blood is sent to them, before it can be again impelled through the body, necessitates the immediate cessation of the systemic circulation, when the pul- monary has been checked. In the class of Reptiles, the lungs are on some- what of the same footing w r ith the liver and kidneys in warm-blooded Verte- brata, that is, only a part of the blood which has returned from the system is transmitted through them, before being again propelled through the body ; and, accordingly, the interruption of the pulmonary circulation does not in them involve immediate death. Indeed, in the naked-skinned Batrachia, the cutaneous surface has enough respiratory power, to effect that degree of al j ra- tion of the blood which is necessary, whilst the temperature is low, and the vital actions thereby diminished in energy. 94. There seems reason to believe, however, that, of the products of decom- position which are set free in the various tissues and organs of the body, only a part is destined to be immediately excreted ; and that it is this part which is taken up by the Veins, and conveyed, by the general vascular apparatus, to the several glands which are to separate it. The remainder, consisting of substances which are fit to be re-assimilated, appears to be taken up by a dis- tinct system of vessels, termed Lymphatics ; which may be considered as an extension of the Lacteal system through the fabric at large. There is good reason to believe, that the special functions of the Lymphatics is, like that of the Lacteals, to minister to Nutritive absorption, (although other substances may find their way into them, by the mere physical process of imbibition) ; the latter being especially destined to take up assimilable matter from the di- gestive cavity, whilst the former absorb the products of the secondary digestion which is continually going on in every part of the body. (See 404 467.) Of these, however, a portion may still be destined to immediate excretion. 95. The various Secretions which have not already been adverted to, appear for the most part to have for their object the performance of some special function in the system, rather than the conveyance out of it of any substances which it would be injurious to retain. This is the case, for example, in regard to the secretion of the Lachrymal, Salivary, and Mammary Glands, as well as with that of the Mucous and Serous Membranes. The Excretion of fluid from the cutaneous surface, however, appears to answer two important purposes, FUNCTIONS OF VEGETATIVE LIFE. 83 the removal from the body of a portion of its superfluous fluid, and the regu- lation of its temperature. Just as, by the action of the Lungs, the conditions are supplied, by which the temperature of the body is kept up to a certain standard, so, by that of the Skin, it is prevented from rising too high; for by the continual excretion from its surface, of fluid which has to be carried off by evaporation, a degree of cold is generated, which keeps the calorifip processes in check; and this excretion is augmented, in proportion to the elevation of the external temperature, which seems, in fact, the direct stimulus to the pro- cess. In all forms of true Secretion, the selection of the materials to be sepa- rated from the blood, is accomplished, like selective Absorption, by the agency of cells. These are developed in the interior of the secreting organ ; and when they are distended with the fluid they have imbibed, their term of life, appears to have expired, so that they burst or liquefy, yielding their contents to the ducts, by which the secreted product is conveyed away. In the case of Adi- pose tissue, we have an instance in which the secreted product (separated from the blood by the cells of which this tissue essentially consists) is not carried out of the body, but remains to form a constituent part of it. The regulation of the amount of fluid in the vessels, is provided in a kind of safety- valve structure, which has been lately shown to exist in the Ipdneys. This readily permits the escape of aqueous fluid from the capillary vessels, into the urinary canals, by a process altogether distinct from the secretion of the solid matter, which it is the office of the kidneys to separate from the circulating fluid. Hence, if the excretion of fluid from the skin be checked by cold, so that an accumulation would take place in the vessels, the increased pressure within them causes an increased escape of water through the kidneys. The relation between the true process of Secretion, which is performed by the selective power of cells, and that of simple Transudation, is the same as that which has been already pointed out between Selective Absorption, and simple Imbibition ( 86). 96. There is no sufficient reason to believe, that the Nervous System has any more direct influence on the process of Secretion than it has been stated to have on that of Nutrition. That almost every secretion in the body is affected by states of mind, which must operate through the nerves, daily experience teaches ; but the very remarkable degree of control which the nervous system possesses over the circulation, appears sufficient to explain any of these effects, whether they be local or general. The flow of the secreted fluids through their efferent ducts seems to be principally caused by the proper contractility of these, which (like that of the heart and alimentary canal) is directly stimu- lated by the contact of their contents ; but there is also evidence that this con- tractility may be affected (as it is in those two instances) by the nervous system; and thus we have an additional means of influence, by which the nervous system can operate on these processes, since its power is probably not confined to the large ducts, but extends to their ultimate ramifications. Where, as happens in the case of the urinary excretion, there is a reservoir -into which it is received as fast as it is formed, for the purpose of preventing the incon- venience which its constant passages from the body would otherwise occasion, the power of emptying this reservoir is usually placed in some degree under the dominion of the will, although chiefly governed by reflex action. It is obvious that such a provision is by no means essential to the function ; and that it has for its object the adaptation, merely, of that function, to the conditions of .Animal existence. 97. Thus we see that, when we enter, as it were, into the penetralia of the Animal system, and study those processes of which the Life of the material fabric essentially consists, we find them performed under conditions essentially the same as those which obtain in Plants ; and we observe that the operations 84 GENERAL VIEW OF THE FUNCTIONS. of the Nervous System have none but an indirect influence or control over them. ' It is, therefore, quite philosophical to distinguish these Organic Func- tions, or phenomena of Vegetative Life, from those concerned in the Life of Relation, or Animal Life. The distinction is, indeed, of great practical im- portance, and lies at the foundation of all Physiological Science ; yet it is seldom accurately made, and a very confused notion on the subject is generally pre- valent. It is commonly said, for example, that the function of Respiration is the connecting link between the two : the fact being, however, that the true process of Respiration is no more a function of Animal life than is any ordi- nary process of secretion; but that, in order to secure that constant interchange of air, which is necessary to its performance, the assistance of the nervous and muscular systems is called in, though not in a manner which necessarily in- volves either consciousness or will. 98. The process of Reproduction, like that of Nutrition, has been until re- cently involved in great obscurity ; and although it cannot be said to be yet fully elucidated, it has been brought, by late investigations, far more within our comprehension than was formerly deemed possible. The close connection between the Reproductive and Nutritive operations, both as regards their re- spective characters, and their dependence upon one another, has long been recognized ; and it is now rendered still more evident. Nutrition has been not unaptly designated "a perpetual reproduction;" and the expression is strictly correct. In the fully-formed organism, the supply of alimentary ma- terial to every part of the fabric enables it to produce a tissue resembling itself; thus we only find true bone produced in continuity with bone, nerve with nerve, muscle with muscle, and so on. Hence it would appear that, when a group of cells has once taken on a particular kind of development, it continues to reproduce itself on the same plan. But in the Reproductive process it is different. A single cell is generated by certain preliminary actions, from which single cell, all those which subsequently compose the embryonic struc- tures, take their origin ; and it is not until a later period, that any distinction of parts can be traced, in the mass of vesicles which spring from it. Hence the essential character of the process of Reproduction consists in the formation of a cell, which can give origin to others, from which again others spring ; and in the capability of these last to undergo several kinds of transformation, so as ultimately to produce a fabric, in which the number of different parts is equal to that of the functions to be performed, every separate part having a purpose distinct from that of the rest. Such a fabric is considered as a very heterogeneous one; and is eminently distinguished from those homogeneous organisms, in which every part is but a repetition of the rest. Of all Animals, Man possesses, as already shown, the greatest variety of endowments, the greatest number of distinct organs ; and yet Man, in common with the simplest Animal or Plant, takes his origin in a single cell. It is in the almost homo- geneous fabrics of the Cellular Plants, that we find the closest connection be- tween the function of Nutrition and that of Reproduction ; for every one of the vesicles which compose their fabric, is endowed with the power of generating others similar to itself; and these may either extend the parent structure, or separate into new and distinct organisms. Hence it is scarcely possible to draw a line, in these cases, between the Nutrition of the individual, and the Reproduction of the species. 99. But, it will be inquired, how and where in the Human body (and in the higher Animals in general) is this embryonic vesicle produced, and what are the relative offices of the two sexes in its formation? This is a question which must still be answered with some degree of doubt ; and yet observed phenomena, if explained by the aid of analogy, seem to lead to a very direct conclusion. The embryonic vesicle itself, like other cells, must arise from a FUNCTIONS OF ANIMAL LIFE. 85 germ ; and reasons will be hereafter given for the belief, that the germ is sup- plied by the male parent, and that the female supplies only the materials for its development. Here, as in the Nutritive processes, we find that the opera- tions immediately concerned in this function, namely, the act of fecundation, and the development of the ovum, are not directly influenced in any way by the nervous system ; and that the functions of Animal Life are called into play only in the preliminary and concluding steps of the process. In many of the lower Animals, there is no sexual congress, even where the concurrence of two sets of organs (as in the Phanerogamic Plants) is necessary for the pro- cess ; the ova are liberated by one, and the spermatozoa by the other; and the accidental meeting of the two produces the desired result. In many Animals higher in the scale, the impulse which brings the sexes together is of a purely instinctive kind. But in Man, it is of a very compound nature. The instinc- tive propensity, unless unduly strong, is controlled and guided by the will, and serves (like the feelings of hunger and thirst) as a stimulus to the reasoning processes, by which the means of gratifying it are obtained ; and a moral sen- timent or affection of a much higher kind is closely connected with it, which acts as an additional incitement. Those movements, however, which are most closely connected with the essential part of the process, are, like those of deglu- tition, respiration, &c., simply reflex and involuntary in their character; and thus we have another proof of the constancy of the principle, that, where the action of the apparatus of Animal Life is brought into near connection with the Organic functions, it is not such as requires the operation of the purely animal powers, sensation and volition. Thus, then, as it has been lucidly remarked, " the Nervous System lives and grows within an Animal, as a para- sitic Plant does in a Vegetable ; with its life and growth, certain sensations and mental acts, varying in the different classes of Animals, are connected by nature in a manner altogether inscrutable to man; but the objects of the existence of Animals require, that these mental acts should exert a powerful controlling influence over all the textures and organs of which they are com- posed." Functions of Animal Life. 100. The existence of consciousness, by which the individual (le moi, in the language of French physiologists) becomes sensible of impressions made upon its bodily structure, and the power of spontaneously exciting contractions in its tissues, by which evident motions are produced, have been already stated to be the peculiar attributes of the beings composing the Animal kingdom. The evident motions exhibited by some Plants, cannot be regarded as indicating the existence of any psychical endowments in the beings included in the Vege- table kingdom ; for they are usually to be referred without difficulty to the action, either direct or indirect, of an external stimulus, upon a contractile tissue ; and even where no such action evidently takes place, there is good reason to suppose its existence. To refer, therefore, the movements of Vege- tables to a nervous system, of which no traces can be found, still more to suppose them endowed with consciousness and will, as some have done, is to violate most grossly a well-known rule in philosophy, which cannot be too steadily kept in view in prosecuting physiological inquiries nonfingere hy- potheses. 101. There are in Animals, however, many movements which are equally dependent upon direct stimuli for their production. Such are (as we have seen), even in the highest, the actions of the heart and of the alimentary canal. These, in the lowest tribes, probably bear a much greater proportion to the 8 86 GENERAL VIEW OF THE FUNCTIONS. whole amount of those exhibited by the beings, than they do in the higher; whilst those which we may regard as specially dependent on a nervous sys- tem, appear to constitute but a small part of their general vital actions. The life of such beings, therefore, bears a much closer resemblance to that of the Vegetable than to that of the higher Animal. Their organic functions are performed with scarcely more of sensible movement than is seen in plants ; and of the motions which they do exhibit (nearly all of them immediately concerned in the maintenance of the organic functions), it is probable that many are the result of the simple contractility of their tissues, called into ac- tion by the. stimuli directly applied to them. It is scarcely possible to imagine that such beings can enjoy any of those higher mental powers, which Man recognizes by observation on himself, and of which he discerns the manifes- tations in those tribes, which, from their nearer relation to himself, he regards as more elevated in the scale of existence. If we direct our attention, on the other hand, to the psychical* operations of Man, as forming part of his gene- ral vital actions, we perceive that the proportion is completely reversed. So far from his organic life exhibiting a predominance, it appears entirely subor- dinate to his animal functions, and seems destined only to afford the conditions for their performance. If we could imagine his nervous and muscular systems to be isolated from the remainder of his corporeal structure, and endowed in themselves with the power of retaining their integrity and activity, we should have all that is essential to our idea of Man. But, as at present constituted, these organs are dependent, for the maintenance of their integrity and func- tional activity, upon the nutritive apparatus ; and the whole object of the latter appears to be the supply of those conditions which are necessary to the exer- cise of the peculiarly animal functions. That his mental activity should be thus made dependent upon the due supply of his bodily wants, is a part of the general scheme of his probationary existence ; and the first excitement of his intellectual powers is in a great degree dependent upon this arrangement. 102. The most simple or elementary function of the Nervous System is, as already observed, the establishment of a communication between a part which is susceptible of impressions, and another which can perform contractile move- ments ; so that a stimulus applied to one may immediately excite a respondent action in the other, however great may be its distance. Hence it may be said to have an internuncial function ; but this, so far as it is performed without the necessary participation of the consciousness or will of the individual, is not essentially higher in character than the corresponding function in Plants, although the latter is effected by a different apparatus. The ministration of the nervous system to purely Animal life, obviously consists in its rendering the mind cognizant of that which is taking place around, and in enabling it to act upon the material world, by the instruments with which the body is pro- vided for the purpose. It is curious to observe that every method at present known, by which mind can act upon mind, requires muscular contraction as its medium, and sensation as its recipient. This is the case, for example, not only in that communication which takes place by language, whether written or spoken ; but in that less evident but not less eloquent converse, by which two minds "attuned to nature's sweetest harmony," can read each other's thoughts. The look, the touch, the gesture, which are so frequently more expressive than any words can be, are all the result of muscular contractions excited in the nervous centres ; and thus we trace the limitation which, even in communication that appears so far removed from the material world, con- * Here and elsewhere this term will be employed in its most extended sense, to desig- nate all the mental operations, whether intellectual, emotional, or instinctive, of which Man's nervous system is the instrument. FUNCTIONS OF ANIMAL LIFE. stantly bounds the operations of the most powerful intellect, and the highest flights of the imagination. That in a future state of heing the communion of mind with mind will be more intimate, and that Man will be admitted into more immediate converse with his Maker, appear to be alike the teachings of the most comprehensive philosophical inquiries, and of the most direct Revelation of the Divinity. 103. The Organs of Sense are instruments which are adapted to enable particular nerves to receive impressions from without ; of a kind, and in a degree, of which they would not otherwise be sensible. Thus, although the simple contact of a hard body with the nerve may be readily conceived to pro- duce a material change in it, of such a kind as would be easily propagated to the central sensorium, it is evident that a nerve must be peculiarly modified to receive and conduct sonorouS impressions from the undulations of the air ; still more-^-to be susceptible of the impressions produced by the undulations of that ethereal medium, to the vibrations of which most Natural Philosophers now attribute the transmission of light. And, even when this difficulty has been provided for by some modification in the structure of the nerve itself, there is evidently another still remaining, that of understanding how distinct images of the form, colour, &c., of external objects can be communicated to the nerve of sight, or ideas of the direction, pitch, quality, &c., of sono- rous undulations, can be obtained through the auditory nerve. There is reason to believe that many among the lower Animals, which do not see objects around them, are conscious of the influence of light ; and thus the distinction between the mere reception of the impression, and the communication of the optical image, becomes evident. The former may take place through the intervention of nerves, whose sensory extremities offer no peculiarities : the latter can only be received through the medium of an instrumenf/which shall, from the mix- ture of rays falling equally upon every part of a surface, produce an optical image, and then impress it upon the expanded surface of the nerve ; so that each fibril may receive a distinct impression, the image presented to the mind being formed by the combination of the whole. That this is, in fact, the share which the organs of special sense bear in the general endowments of the whole apparatus, may be inferred especially from the conformation of the Eye, which is in every respect a merely optical instrument of the greatest beauty and per- fection, adapted to present to the nerve, in the most advantageous manner, the images of surrounding objects in all their variations; and we might conceive that, if it were possible for the interior of the living eye to be replaced by one constructed of inorganic materials by the hand of man, and for the retina to preserve its functional activity, the pow r er of sight would be but little impaired, except through the incapability, on the part of any piece of human mecha- nism, to imitate those wondrous contrivances of Infinite Skill, which have for their object the adaptation of the instrument to varieties of distance, of intensity of light, &?c. There can be little doubt that the structure of the Ear is arranged to do the same for the sonorous vibrations which the eye does for the rays of light; that is, through its means, the undulations which strike upon the external surface of the organ are separated and distinguished, those of a like kind being brought together upon one division of the nerve, and those of another order upon a different set of fibres; so that the different kinds of sound, and the peculiar quality and direction of each, may be discriminated ; whilst, by the concentration of all the impressions of the same character, a higher amount of force is given to them. Of the sense of Smell, no similar account can be given; since the medium by which odours are propagated is not known. If, as is generally believed, this is accomplished by the diffusion through space of minute particles of the odoriferous body itself (which supposition seems to derive support from the general fact, that the most volatile substances are 88 FUNCTIONS OF THE NERVOUS SYSTEM. usually most odoriferous*), smell may be regarded, as taste also is probably to be considered, in the light of a refined kind of touch. 104. Thus, the general rule holds good, here as elsewhere, that the pro- cesses by which the organism is immediately brought into relation with the external world, are performed in obedience to physical laws ; the living struc- ture only affording certain peculiar conditions which may be imitated in a great degree by other means. This is the case, for example, with regard to Digestion, which is in itself a simply Chemical process, that will- take place out of the body as well as in it, if the materials and the necessary solvent be submitted to the same circumstances, as those to which they are exposed in the stomach ; and in regard also to the act of Respiration, which depends upon the Physical tendency to mutual diffusion, inseparable from the existence of gases ; and we notice the prevalence of the same general fact in the Animal as in the Organic life. We cannot become cognizant of the changes, or even of the existence, of the external world, unless some material effect be produced by it on our organs of sense ; nor can we produce any alteration in its condi- tion, except by powers which act according to purely mechanical principles. 105. In regard to the Muscular System, it has already been sufficiently ex- plained that it forms a part of the apparatus of Animal life no otherwise than as the instrument by which nervous energy operates upon external objects. The contractility which it manifests on the application of a stimulus, is an endowment which it derives from its own structure, and not from the nervous system ; for it will be clearly proved in its appropriate place, that the presence of this contractility is connected with the healthy nutrition of the tissue, and its due supply of arterial blood ; and that the complete separation of any mus- cular part from all its nervdpB connections, has none but an indirect influence on its properties. CHAPTER III. FUNCTIONS OF THE NERVOUS SYSTEM. I. General Summary. 106. ALL our positive knowledge of the functions of the Nervous System, is derived from observation of the movements exhibited by animals, and from our own consciousness of what passes within ourselves. Except through the movements consequent upon them, we have no means of ascertaining whether or not particular changes in the nervous system are attended with sensation. The cries and struggles of the animal made the subject of experiment, are ordinarily considered as indications of sensations ; but it is not right so to re- gard them in every instance; nor are we justified in asserting that conscious- ness results from any external irritation, merely because movements evidently tending to get rid of this are performed in respondence to it. We know that the contractions of the heart and alimentary tube are ordinarily excited by a stimulus, without any sensation being involved ; and these movements, like * Some of the most strongly odoriferous substances, however, are solid; for instance, musk: and it has been experimentally proved, that the loss of weight, which follows the free exposure of a minute quantity of this perfume to an atmosphere constantly renewed during several years, is not appreciable by the finest balance. GENERAL SUMMARY. 89 all that are concerned in the maintenance of the organic functions, have an obvious design, when considered either in their immediate effects, or in their more remote consequences. The character of adaptiveness, then, in muscular movements excited by external stimuli, is no proof that they are performed in obedience to sensation ; much less, that they have a voluntary character. In no case is this adaptiveness more remarkable than in some of those purely instinctive actions which are not only performed without any effort of the will, but which the will cannot imitate. This is the case, for example, with the act of deglutition ; the muscles concerned in which cannot be thrown into contrac- tion by a voluntary impulse, being stimulated only by impressions conveyed from the mucous surface of the fauces to the medulla oblongata, and thence reflected along the motor nerves. No one can swallow, without producing an impression of some kind upon this surface, to which the muscular movements will immediately respond. Now it is impossible to conceive any movements more perfectly adapted to a given purpose than those of the parts in ques- tion ; and yet they are not only independent of volition, but of sensation, being still performed in cases in which consciousness is completely suspended, or entirely absent. 107. There is much difficulty, then, in ascertaining the really elementary functions of the Nervous system, by experiments upon animals ; and it is only when their results are corrected and explained by pathological observation on Man, the sole case in which we can obtain satisfactory evidence of the pre- sence or absence of sensation, that they have much value to the physiological inquirer. From these combined sources, however, a vast amount of know- ledge of the functions of the nervous system has recently been gained ; and the general purposes to which it is subservient may be advantageously stated in a systematic form, before we enter upon any detailed examination of them. I. The Nervous System receives impressions, which, being conveyed by its afferent fibres to the sensorium, are there communicated to the conscious mind. It is subservient in some way to the acts of that mind ; and, as the result of these acts, a motor impulse is transmitted along the efferent trunks to particular muscles, exciting them to contraction. This motor impulse, however, may be either of an emotional or a voluntary character. We shall hereafter see reason to believe that, to these functions, the Encephalon, and the nerves proceeding from it, are subservient. II. Certain parts of the Nervous System receive impressions which are propagated along afferent fibres, that terminate in ganglionic centres distinct from the sensorium; and in these a reflex motor impulse is thus excited, which, being conveyed along the efferent trunks proceeding from them, excites muscular contraction, without any necessary intervention of sensation or volition. Of this function (called by Dr. Hall, to whom the discovery of it is in great part due, the reflex function), we shall find that the portion of the Spinal, Cord of Vertebrata, which is not continuous with the fibrous structure of the brain, together with the portion of the nervous trunks which are con- nected with it alone, is the instrument. III. Another division of the Nervous System appears to have for its object, to combine and harmonize the muscular movements immediately connected with the maintenance of organic life, and to bring these into relation with certain conditions of the mind. There is reason to believe (though this is less certain) that it also influences, and brings into connection with each other, the processes of nutrition, secretion, &c. ; though these, like the muscular move- ments just mentioned, are essentially independent of it. 108. Now, in reference to the first class of operations, it is well to explain that, though the physiologist speaks of the intellectual powers, moral feelings, &c., as functions of the Nervous System, they are not so in the sense in which 8* 90 FUNCTIONS OF THE NERVOUS SYSTEM. the term is employed in regard to other operations of the hodily frame, tn general, by the function of an organ, we understand some change which may be made evident to the senses, as well in our own system as in the body of another. Sensation, Thought, Emotion and Volition, however, are changes im- perceptible to our senses, by any means of observation we at present possess. We are cognizant of them in ourselves, without the intervention of those pro- cesses by which we observe material changes external to our minds ; but we judge of them in others, only by inferences founded on the actions to which they give rise, when compared with our own. When we speak of sensation, thought, emotion, or volition, therefore, as functions of the Nervous System, we mean only that this system furnishes the conditions under which they take place in the living body; and we leave the question entirely open, whether the tyxn nas or nas not an existence independent of that of the mate- rial organism, by which it operates in Man as he is at present constituted. 109. In regard to the second class of actions, it may be remarked, that they are nearly all connected, more or less closely, with the organic functions, or with the protection of the body from danger. Thus the movements of the pharynx supply to the stomach the alimentary materials it prepares for the nutrition of the body; those of the muscles of the thorax, &c., maintain that constant interchange of air in the lungs, which is necessary for the aeration of the blood ; whilst those by which a limb is involuntarily retracted from any cause of pain or irritation, are obviously adapted to the latter of these two ends. II. Elementary Structure of the Nervous System. 110. Wherever a distinct Nervous System can be observed, it is found to consist of two kinds of structure ; the presence of both of which, therefore, may be regarded as essential to our idea of it as a whole. One of these is that which is designated the white or fibrous matter. This constitutes (with the neurilema or nerve-sheath, and the areolar tissue which it encloses), the whole of the nervous trunks, wherever they occur ; and forms a large part of the central masses with which they are connected. It consists of tubes of great minuteness, which are composed of an interlacement of extremely deli- cate fibres ; some of these passing in a longitudinal, and some in a transverse or spiral direction. When these tubes are examined immediately after death, their contents appear pellucid and homogeneous, and of a fluid consistence ; so that each tube or fibre 'looks like a cylinder of clear glass, with simple, well-defined, dark edges. But a kind of coagulation soon takes place in the contained substance, which makes it easily distinguishable from the tube itself; for the latter is then marked by a double line, as shown in Fig. 7. This last state of the nervous fibre has been regarded by Remak and others (but probably erroneously) as the natural one ; and the substance contained within the tube has been described as a band or axis, composed of several distinct filaments. It has even been asserted that, at the extremities of these nerves, the filaments diverge from one another, and form loops, the tubular envelop being lost ; but this is probably erroneous. The walls of the tube are not unfrequently seen to contain the nuclei of the cells, by the coa- lescence of which it was originally formed. The diameter of the cylindrical tubuli of the nerve-trunks is estimated to vary from about the l-2000th to the 1 -3000th of an inch. The fibres gradually decrease in size, however, as they approach the brain, either directly, or through the medium of the spinal cord ; and in the brain itself they continue to grow less as they pass through the medullary to the cortical part ; so that in the former they measure from 1 -7000th to l-8000th of an inch, and in the latter not more than l-14000th of an inch. The fibres of the olfactory and optic, and, in a less degree, those of ELEMENTARY STRUCTURE OF NERVOUS SYSTEM. 91 the auditory nerves, are equally small in every part of their course ; and thus resemble, in size as well as in structure, those portions of the other nervous fibres which are continued into the nervous centres. A different structure has been described by Ehrenberg, as composing the bulk of the medullary substance of the brain, under the name of varicose tubes ; and he states that these are also found largely in the spinal cord, and less abundantly in the nerves of special sense ; but that they are seldom to be met with in the other trunks. These tubes were so named, from their not being cylindrical, but presenting dilatations at intervals, so as to resemble a string of beads ; and the appearance of these dilatations has given rise to the opinion, that the brain is composed of globules. It is now, however, satisfactorily shown that they are the result of the pressure and other manipulations, to which the objects are subjected in preparation for the microscope ; and that, if the nervous fibres of the brain and other parts are examined in a recent state, they are cylindrical, like those of the nervous trunks in general. Still there must be some differ- ence in their structure, since they exhibit this tendency to become varicose, which, is elsewhere wanting ; this difference appears chiefly to consist in the greater thinness and delicacy of the walls of the tube itself, rendering it more liable to be distended at certain points, by the accumulation of the contained substance in little masses, when coagulating. Besides these tubular fibres, which constitute the white portions of the nervous matter, there are other fila- ments of a yellowish-gray colour, and of about half the usual diameter, with- out distinct cavities, which exist especially in the sympathetic nerves, but which may also be detected in others. These fibres may be termed organic; those existing in the sympathetic system of nerves may be traced to its gan- glipnic centres ; whilst those which are found in the cerebro-spinal nerves, are connected with the ganglia upon their posterior roots.* Fig. 7. Structure of nerve-tubes, magnified 350 Diam. A, cylindrical tubuli from nerve. B, varicose tubuli from brain, c, nerve-tubes, of which one exhibits the remains of nuclei in its walls. (After Wagner.) 111. The other elementary form of Nervous structure is termed the dneri- tious or gray matter. It seems to consist principally of a plexus of blood-ves- sels, into which the fibres of the former may be traced ; and amongst these lie a number of nucleated globules, which do not seem to have any definite ar- rangement. This substance is usually disposed in the interior of the larger * The distinctness of the sympathetic fibres from the cerebro-spinal, which has been denied by some Microscopists, appears to have been fully established by the recent laborious inquiries of Volkmann and Bidder. FUNCTIONS OF THE NERVOUS SYSTEM. Fig. 8. Primitive fibres and ganglionic globules. A, from sympathetic ganglion ; * a separate vesicle, showing its pellucid nucleus and nucleolus. B, from gray substance of human cerebellum ; a, 6, plexus of primitive fibres; c, nucleated globules: * a separate globule from human Gasserian ganglion. 350 Diam. (After Wagner.) masses, with which the nervous trunks are connected. It forms the centre of the ganglia, which are the centres of the nervous system in the Invertebrata : it occupies part of the interior of the spinal cord of Vertebrate d animals, which may be regarded as a chain of similar ganglia; and in the distinct ganglia which occur among the latter, it holds the same relative position. In the brain of Vertebrata, however, it is disposed externally, and forms a sort of coating to the mass beneath, which almost entirely consists of fibrous structure ; hence it has been called the cortical substance, whilst the fibrous portion has been termed medullary matter. The ganglion-globules (Figs. 8 and 9) are usually from l-300th to l-1250th of an inch in diameter ; possessing a spherical or oval form, more or less flattened ; and having a reddish colour, to which the characteristic hue of the ganglionic substance (whence it has received the ap- pellation cineritious) is chiefly owing. Each contains one or more nuclei with subordinate nucleoli; and is enclosed in a very fine filamentous investment, in Primitive fibres and ganglionic globules of human brain, after Purkinje. A, gangl'onic globules lying, amongst varicose nerve-tubes, and blood-vessels, in substance of optic thalamus; a, globule more enlarged; 6, vascular trunk. B, B, globules with variously-formed peduncles, from dark portion of crus cerebri. 350 Diara. (After Wagner.) ELEMENTARY FUNCTIONS OF NERVOUS STRUCTURE. 93 which it is commonly found to be somewhat loosely suspended. The sheaths of the several globules are connected with each other by prolonged filaments or peduncles, as seen at BB, Fig. 9 ; and these form a kind of network, which occupies the interstices of the fine vascular plexus, by which every part of the gray matter is traversed. On the surface of the cortical substance of the brain, another kind of structure is present, which is found, however, in smaller quantity in other parts of the gray matter: this is a finely granular substance, containing spherical or oval vesicles, with one or two dark granules in them. In a rather deeper layer, these vesicles, instead of being irregularly scattered through the granular substance, seem to have appropriated each to itself a portion of the latter for an independent covering; and from this condition there seems to be a regular gradation, till, in the yet depeer layers of the cortical substance, the vesicles with their granular coverings, are replaced by perfect ganglion-like globules with their filamentous sheaths. Hence it may be surmised, that this substance is an incipient state of the true ganglionic matter. 11*2. It appears uncertain, from the results of the most recent microscopic inquiries, whether the nervous fibres can be said to have any distinct termina- tions, either in the ganglionic centres, or in the organs to which they are distributed. In the gray matter of the brain of Vertebrata, they seem to form a kind of plexus of loops, like that represented in Fig. 8, B ; but the ultimate fibres never anastomoze. It Avould appear that, in the spinal cord of Verte- brata, and in the ganglionic cord of the Articulata, the fibres pass through the ganglionic substance, without any interruption ; but Mr. Newport has remarked that they evidently become softer, and that their diameter increases, whilst they are traversing it. (See 143 and Fig. 12.) The following appears to be the mode in which the nervous fibres are distributed to the peripheral organs. The trunks subdivide into small fasciculi, each of which consists of from two to six fibres ; and these form plexuses, whose arrangement bears a general resemblance to that of the elements of the tissue in which they are placed. The primitive fibres then separate; and each, after passing over several elementary parts of the containing tissue (as in the case of muscular fibre), or after forming a single narrow loop (as in the sensory papillae), returns to the same or to an adjoining plexus, and pursues* its way back to the nervous centre from which it set out. In other words, each fibre becomes continuous with another arising from the same or a neighbouring fasciculus. There is thus, strictly speaking, no more termination of nerves, than there is of blood- vessels; for both form circles. The characters of the fibres are scarcely altered in the substance of the organs receiving them ; their sheaths become finer, but they are npt lost; and there is no fusion of the nervous into the adjacent sub- stance. The loops of the sensory nerves are surrounded, when they enter the papillae, by a plexus of blood-vessels, including globules, which bear a close resemblance to those of the ganglionic substance ; and each of these papilla may probably be considered like the larger ganglia as a distinct originator of nervous power ( 116). III. Elementary Functions of Nervous Structure. 113. There can be little doubt that the functions of these two divisions of the nervous system are different. That of the fibrous structure, as it exists in the nervous trunks, is unquestionably to conduct or convey the influence of changes, which have taken place elsewhere. And in accordance with what has previously been stated, of the mode in which the mind is brought into relation with the external world, through this nervous apparatus, we find that there are (in the higher tribes of Animals, at least, if not in all) two sets of fibres : one of which has for its office to convey external impressions towards FUNCTIONS OR THE NERVOUS SYSTEM. the nervous centres ; whilst the other conveys the influence of these central organs to the structure at large, and especially to the muscular system. Hence it will be convenient to denominate the first afferent fibres, and the second effe- rent. These are to be regarded as general terms, expressing only the direc- tion in which they propagate the changes to which they are subservient. The nature of these changes will be a subject of future inquiry. Although, as just stated, the fibres can no where be said to have free extremities, yet the afferent fibres, which convey to the central organs the influence of changes taking place at the periphery, may be said to originate in the ganglionic matter dif- fused through the latter, and to terminate in the central ganglia: whilst the efferent fibres, which convey to the muscles the motor influence generated in these last organs, may be said to originate in the central ganglia, and to termi- nate in the muscular tissue. 114. Every fibre, there is reason to believe, runs a distinct course, between the central organ, in which it loses itself at one extremity, and the muscle or organ of sense in which it terminates at the other. Each Nervous Trunk is made up of several fasciculi of these fibres ; and each fasciculus is composed of a large number of the ultimate fibres themselves. Although the fasciculi occa- sionally intermix and exchange fibres with one another, (as occurs in what is termed a plexus,) the fibres themselves never inosculate. Each fibre would seem, therefore, to have its appropriate office, which it cannot share with ano- ther. The objects of a plexus are twofold. In some instances it serves to intermix fibres, which have endowments fundamentally different: for example, the spinal accessory nerve, at its origin, appears to be exclusively motor, and the roots of the par vagum are as exclusively sensory ; but by the early admix- ture of these, a large number of motor fibres are imparted to the par vagum, and are distributed, in variable proportion, with its different branches ; whilst few of its sensory filaments seem to enter the spinal accessory. In other in- stances the object of a plexus appears to be, to give a more advantageous dis- tribution to fibres, which all possess corresponding endowments. Thus the brachial plexus mixes together the fibres arising from five segments of the spinal cord, and sends off five principal trunks to supply the arm. Now if each of these trunks had arisen by itself, from a distinct segment of the spinal cord, so that the parts on which it is distributed had only a single connection with the nervous centres, they would have been much more liable to paralysis than at present. By means of the plexus, every part is supplied with fibres arising from each segment of the spinal cord ; and the functions of the whole must therefore be suspended, before complete paralysis of any part can occur, from a cause which operates above the plexus. Such a view is borne out by direct experiment ; for it has been ascertained by Fanizza that, in Frogs, whose crural plexus is much less complicated than that of Mammalia, section of the roots of one of the three nerves which enter into it, produces little effect on the general movements of the limb ; and that, even when two are divided, there is no paralysis of any of its actions, all being weakened in a nearly similar degree. 115. It is not unlikely also that, by this arrangement, a consent aneousness of action is in some degree favoured, as is supposed by Sir C. Bell; for com- parative anatomy shows that something resembling it may be traced, wherever a similar purpose has to be attained. "Thus, in the Hymenoptera, there is a similar interlacement between the nerves of the anterior and posterior pairs of wings, which act very powerfully together ; whilst in the Coleoptera, in which the anterior wings are converted into elytra, and are motionless during flight, the nerves supplying each pair run their course distinctly. In the Octopus, or Poulp, again, the trunks which radiate from the cephalic mass to the eight large anus surrounding the head, are connected by a circular band ; forming a ELEMENTARY FUNCTIONS OF THE NERVOUS STRUCTURE. 95 kind of plexus, which evidently contributes to the very powerful and harmo- nious movements of the arms of this Cephalopoda It is considered by Dr. Ali- son, that the origin of the trunks which supply the various muscles of the ex- tremities from several segments of the spinal cord, instead of one, has the further use of enabling the mind to vary, in greater degree than would otherwise be possible, the power with which the muscle shall be called into action; and this idea is certainly supported by the curious fact, that it is in the nerves of the extremities only that this plexiform arrangement prevails ; and that the nerves of the eyeball, in whose action there is an equal degree of consentaneousness, but far less variety of power, arise from single points of the cerebro-spinal axis. It is further considered by Dr. A., that the plexiform arrangement may enable the sensations proceeding from the muscles (which are important guides in their movement), to be more distinct, and consequently more easily discrimi- nated from one another, than they would otherwise be ; and there does not seem any reason why the same view should not be extended to the sensory impressions communicated from the general surface of the extremities. 116. There is no valid reason to believe that the nervous fibres undergo any change of function along their whole course ; and we may, probably, com- pare them, by way of analogy only, to the conducting wires of a galvanic apparatus. But it is evident that the special structure into which they pass, the plexus of blood-vessels and ganglionic globules, must have some particu- lar function ; and there seems no reasonable ground for doubt, that in this struc- ture, those changes originate which are conducted by the fibres to distant points. Following out the same analogy, then, we might compare this struc- ture with the galvanic combination, by which the electric influence is gene- rated, that is conveyed to some distant point by the connecting wires and there produces a decomposition or other similar change; and there seems to be as much reason for thus assigning the functions of the production and conduction of nervous power to the gray and white portions of the nervous system respectively, as there is for attributing the production of electric power to the galvanic trough, and its conduction to the connecting wires. Wherever we have reason to believe that new power is generated, there do we find gray matter ; and, on the other hand, there are few, if any, instances in which gray matter is present, without our being able to assign to it some obvious purpose of this kind. 117. The belief that all changes in the nervous system, whether they take place at the centre or at the periphery, originate at the points in which the fibres come into relation with the vascular plexus, derives confirmation from the well-known dependence of these changes upon the activity of the circu- lation through the part at which they occur. Thus, if the circulation of blood through the brain be suspended for an instant, insensibility supervenes. If the cause of suspension be local only, the remdnder of the nervous system may still be excited to action. This was the case in experiments made by Sir A. Cooper. After having tied both carotid arteries in a dog, he compressed the vertebral trunks, and immediate insensibility resulted, proving the inactive condition of the brain ; whilst convulsions also occurred, showing that the functions of the spinal cord were not suspended, but only deranged. But if, as in syncope, the circulation through the spinal cord also be weakened, its power of producing motions in respondence to impressions is diminished in like proportion. In the same manner the production of impressions on the peripheral origins of the afferent nerves, appears equally dependent upon the active influence of the vascular system. Every one knows that cold, which retards the circulation of blood through, the skin, diminishes also its sensi- bility ; and obstruction to the circulation by any other cause, such as pressure on the arterial trunks, produces the same effect. Moreover, it is always 96 FUNCTIONS OF THE NERVOUS SYSTEM. found, that an increase in local circulation is accompanied by an exaltation of the sensibility of the part. This may be especially noticed in the genital organs of animals during the period of heat ; and in those of man when in a state of venereal excitement. It may be remarked, also, in those affections so closely bordering upon inflammation, to which the term active congestion, or determination of blood, has been applied. The pain which usually accom- panies inflammation may be partly referred to this source ; but it seems prin- cipally dependent upon other causes. 118. It may be argued against this view of the respective functions of the granular and fibrous structures, that sensation may be produced by pinching an afferent trunk in its course, and that motion may be excited by irritating an efferent nerve ; so that the changes which have been spoken of as occurring at their points of origin in the vascular plexus, are not to be regarded as the means by which such influences are produced. But this argument will have little weight, when it is recollected that on the same ground, we might infer that neither the organs of sensation on the one hand, nor any part of the brain, or spinal cord, on the other, are the sources of the changes in question. The effects are obviously due to the fact that the artificial stimulus imitates the natural one ; and thus it is that if a sensory nerve be compressed, the sensa- tion produced is referred to the part of the surface, to which its branches are distributed. 119. Our simplest idea, then, of a nervous system, includes a Central organ, of which the gray matter, formed by the intermixture of nervous fibres, blood- vessels, and ganglionic globules, is the essential part ; and an afferent and efferent set of fibres connected with it, one conveying to it the impressions produced by external changes upon the periphery (where also the nervous structure comes into peculiar relation with the vascular system), and the other conducting from it the motor stimulus, originating in itself, to the contractile tissue. This is precisely what we find in the lowest of those animals in which a nervous apparatus can be distinguished, as will be hereafter explained. At present it will be desirable to consider some other questions, which early pre- sent themselves in the study of Neurology. IV. Mode of determining the Functions of Nerves. 120. Various methods of determining the functions of particular nerves present themselves to the physiological inquirer. One source of evidence is drawn from their anatomical distribution. For example, if a nervous trunk is found to lose itself entirely in the substance of muscles, it may be inferred to be chiefly, if not entirely, motor or efferent. In this manner, Willis long ago deter- mined that the third, fourth, sixth, portio dura of the seventh and ninth cranial nerves, are almost entirely subservient to muscular movement ; and the same had been observed of the fibres proceeding from the small root of the fifth pair, before Sir C. Bell experimentally determined the double function of that division of the nerve, into which alone it enters. Again, where a nerve passes through the muscles, with little' or no ramification among them, and proceeds to a cutaneous or mucous surface, on which its branches are minutely distributed, there is equal reason to believe that it is of a sensory, or rather of an afferent, character. In this manner Willis came to the conclusion, that the ijfth pair of cranial nerves differs from those previously mentioned, in being partly sensory. Further, where a nerve is entirely distributed upon a surface adapted to receive impressions of a special kind, as that of the Schneiderian membrane, the retina, or the membrane lining the internal ear, it may be inferred that it is not capable of transmitting any other kind of impressions ; for experiment has shown that the special sensory nerves, do not possess com- MODE OF DETERMINING THE FUNCTIONS OF NERVES. 97 mon sensibility. The case is different, however, in regard to the sense of taste, which originates in impressions not far removed from those of ordinary touch; and it is probable that the same nerves minister to both. Anatomical evidence of this kind is valuable also, not only in reference to the functions of a principal trunk, but even as to those of its several branches, which, in some instances, differ considerably. Thus, some of the branches of the par vagum are especially motor, and others almost exclusively afferent; and anatomical examination, carefully prosecuted, not only assigns the reasons for these func- tions, when ascertained, but is in itself nearly sufficient to determine them. Thus the superior laryngeal nerve is distributed almost entirely upon the mucous surface of the larynx, the only muscle it supplies being the crico- thyroid ; whilst the inferior laryngeal or recurrent is almost exclusively dis- tributed to the muscles. From this we should infer that the former is an afferent, and the latter a motor nerve ; and experimental inquiries (hereafter to be detailed) fully confirm this view. In like manner it may be shown, that the glosso-pharyngeal is chiefly an afferent nerve, since it is distributed to the surface of the tongue and *pharynx, and scarcely at all to the muscles of those parts ; whilst the pharyngeal branches of the par vagum are chiefly, if not entirely, motor. Lower down, however, the branches of the glosso- pharyngeal cease, and the oesophageal branches of the par vagum are dis- tributed both to the mucous surface and to the muscles ; from which it may be inferred that they are both afferent and motor a deduction which experi- ment confirms. 121. We perceive, therefore, that much knowledge of the function of a nerve may be obtained from the attentive study of its ultimate distribution : but it is necessary that this should be very carefully ascertained, before it is made to serve as the foundation for physiological inferences. As an example of former errors in this respect, may be mentioned the description of the pprtio dura of the seventh, at first given by Sir C. Bell : he stated it to be distributed to the skin as well as to the muscles of the face, and evidently regarded it as in part an afferent nerve, subservient to respiratory impressions as well as to motions. In the same manner, from inaccurate observation of the ultimate distribution of the superior laryngeal nerve, it was long regarded as that which stimulated to action the constrictors of the glottis. But the knowledge obtained by such anatomical examinations alone is of a very gene- ral kind; and requires to be made particular, to be corrected and modified, by other sources of information. One of these relates to the connection of the trunks with the central organs. The evidence derived from this source, however, is seldom of a very definite character ; and, in fact, the functions of particular divisions of the nervous centres have been hitherto rather judged of by those of the nerves with which they are connected, than affording aid in the determination of the latter. Still, this kind of examination is not without its use, when there is reason to believe that a particular tract of fibrous structure has a certain function, and when the office of a nerve whose roots terminate in it is doubtful. Here, again, however, very minute and accurate examina- tion is necessary, before any sound physiological inferences can be drawn from facts of this description ; and many instances might be adduced to show, that the real connections of nerves and nervous centres are often very different from their apparent ones. 122. Experimental inquiries into the functions of particular nerves are also liable to give fallacious results, unless they are prosecuted with a full know- ledge of all the precautions necessary to insure success. Some of these will be here explained. Suppose that, upon irritating the trunk of a nerve, whilst still in connection with its centre, muscular movements are excited ; it must not be hence concluded that the nerve is an efferent one, for it may have no 9 98 FUNCTIONS OF THE NERVOUS SYSTEM. directly motor powers. The next step would be to divide the trunk, and to irritate each of the cut extremities. If, upon irritating the end separated from the centre, muscular contractions are produced, it may be safely inferred that the nerve is, in part at least, of an efferent character. Should no such result fol- low, this would be doubtful. If, on the other hand, muscular movement should be produced by irritating the extremity in connection with the centre, it will then be evident, that it is occasioned by an impression conveyed towards the centre by this trunk, and propagated to the muscles by some other; in other words, to use the language of Dr. M. Hall, this nerve is an exciter of motion, not a direct motor nerve. The glosso-pharyngeal nerve has been satisfactorily determined to be chiefly, if not entirely, an efferent nerve; by experiments of this kind, performed by Dr. J. Reid. 123. It has been from the want of a proper mode of experimenting, that the functions of the posterior roots of the spinal nerves have been regarded as in any degree motor. If they be irritated, without division of either root, motions are often excited ; but if they be divided, and their separated trunks be then irritated, no motions ensue ; nor are any movements produced by irritation of the roots in connection with the spinal cord, if the anterior roots have been divided. Hence it appears that the motor powers of these fibres are not direct, but that they convey an impression to the centre, which is reflected to the muscles through the anterior roots. Another source of fallacy is to be guarded against, arising from the communication to a nerve, in its course, of properties it did not possess at its root, by inosculation with another nerve. Of this many instances will hereafter present themselves. 124. The same difficulties do not attend the determination of the sensory properties of nerves. If, when the trunk of a nerve be pricked or pinched, the animal exhibits signs of pain, it may be concluded that the nerve is sensi- ble to ordinary impressions at its peripheral extremity, feut not unfrequently this sensibility is derived by inosculation with another nerve ; as is the case with the portio dura, which is sensory after it has passed through the parotid gland, having received there a twig from the fifth pair. A similar inoscula- tion explains the apparent sensibility of the anterior roots of the spinal nerves. If these be irritated, the animal usually gives signs of uneasiness ; but if they be divided, and the cut ends nearest the centre be irritated, none such are ex- hibited; whilst they are still shown, when the farther ends are irritated, but not if the posterior roots are divided. This seems to indicate that, from the point of junction of the two roots, sensory fibres derived from the posterior root pass backwards (or towards the centre) in the anterior ; and thus its ap- parent sensory endowments are entirely dependent upon its connection with the posterior column of the spinal cord, through the posterior roots. 125. The fallacies to which all experiments upon the nerves are subject, arising from the partial loss of their powers of receiving and conveying im- pressions, and of exciting the muscles to action, after death, are too obvious to require particular mention here ; yet they are frequently overlooked. Of a similar description are those arising from severe disturbance of the system, in consequence of operations ; which also have not been enough regarded by experimenters. f" V. Nature of the Changes in the Nervous System. 126. Of the actual nature of the changes, by which impressions are received upon the peripheral origins of the afferent nerves, or are communicated to the central origins of the motor, and are conducted along each to their opposite extremities, physiologists have no certain knowledge. That they are electri- cal has been, and still continues to be, a favourite theory with some ; and that NATURE OF THE CHANGES IN THE NERVOUS SYSTEM. 99 there is a great analogy between the propagation of nervous and that of elec- trical influence, cannot be denied. But the reasons in favour of their identity are not greater than those which might be adduced to prove that nervous in- fluence is identical with other physical forces ; since mechanical and chemical stimulation will, equally with electricity, imitate, to a certain extent, the natural changes in this system. On the other hand, there are many valid reasons against such a supposition ; of which one of the most cogent is, that by putting a ligature round a trunk, its functions as a conductor of nervous influence are paralyzed, whilst it is still capable of conveying electricity. The various fibrils, too, are not as completely insulated from each other in regard to the passage of electricity, as we know them to be in respect to nervous agency. To the influence (whatever its nature may be) which the nerves convey, the term vis nervosa has been provisionally applied. ; and it is convenient to employ a term of this nature, when the laws according to which it operates are being speci- fied. It must be remembered, however, that nothing is really gained by the use of such a term, which resembles one of the unknown quadpies in algebra. It is quite possible that the changes in the afferent nerves may differ from those that take place in the efferent ; and that the changes which convey some kinds of impressions through the former, may differ from those concerned in others. No real progress is made, therefore, by attributing any phenomena of the ner- vous system to the vis nervosa ; any more than by referring the various mate- rial changes in the organism to the operation of the vital principle. The laws according to which these changes take place are, however, legitimate subjects for physiological investigation. Those regulating the propagation of nervous agency may be briefly stated as follows. They evidently result from the facts already mentioned, respecting the isolated character of each fibril, and the identity of its endowments through its whole course. They are here stated, with some modification, in the language of Mil Her. I. When the whole trunk of a sensory nerve is irritated, a sensation is pro- duced, which is referred by the mind to the parts to which its branches are ultimately distributed ; and if only part of the trunk be irritated, the sensation will be referred to those parts only which are supplied by the fibrils it con- tains. This is evidently caused by the production of a change in the senso- rium, corresponding with that which would have been transmitted from the peripheral origins of the nerves, had the impression been made upon them. Such a change only requires the integrity of the afferent trunk between the point irritated and the sensorium ; and is not at all dependent upon the state of the extremity to which the sensations are referred. This may have been paralyzed by the division of the nerve ; or altogether separated, as in ampu- tation ; or the relative position of its parts may have been changed. It results from the foregoing, that, when different parts of the thickness of the same trunk are separately subjected to irritation, the sensations are successively referred to the several parts supplied by these divisions. This may be easily shown by compressing the ulnar nerve, in different directions, where it passes at the inner side of the elbow-joint. II. The sensation produced by irritation of a branch of the nerve, is con- fined to the parts to which that branch is distributed, and does not affect the branches which come off from the nerve higher up. The rationale of this law is at once understood : but it should be mentioned that there are certain conditions, in which the irritation of a single nerve will give rise to sensations over a great extent of the body. This is due, however, to a particular state of the central organs, and not to any direct communication among the sensory fibres. X ' ,* ,,*< ,^ III.. The motor influent is?prcpagate< ohiy inte ceftf-riftig^Mirection, never in a retrograde course. Jtjnay originate in a spoiltaneeu^ change in the cen- 10U FUNCTIONS OF THE NERVOUS SYSTEM. tral organs : or it may be excited by an impression conveyed to them through afferent nerves; but in both cases its law is the same. IV. When the whole trunk of a motor nerve is irritated, all the muscles which it supplies are caused to contract ; but when only a part of the trunk or a branch is irritated, the contraction is confined to the muscles which receive their nervous fibres from it. This contraction evidently results from the simi- larity between the effect of an artificial stimulus applied to the trunk in its course, and that of the change in the central organs by which the vis nervosa is ordinarily propagated. In this instance, as in the other, there is no lateral communication between the fibrils. VI. Comparative Anatomy and Physiology of the Nervous System. 127. Although the structure and distribution of the Nervous System in the different classes of Animals have been, until recently, but little appealed to in the determi^^Hp- of its functions, they are capable of supplying evidence regarding some of these, not less important in its character than that which Comparative Anatomy affords to other departments of Physiology. Some of the principal of these contributions will now be pointed out 128. In the lowest tribes of the RADIATED division of the animal kingdom, no nervous system has yet been discovered. These have, therefore, been separated by some naturalists into a new primary group, to which the desig- nation of Jlcrita has been given, on account of the (supposed) " indistinct, diffused, or molecular character of their nervous system." This idea of a " diffused nervous system" seems to be regarded l>y many physiologists as well as naturalists as the necessary alternative, resulting from the want of any definite indications of its presence. It may be said, however, to be based on very erroneous notions, as to the true offices of the nervous apparatus. Its influence is not required to endow the tissues with contractility ; a property possessed in a high degree by the structures of many Plants, to which these beings present a much greater general resemblance than they bear to the higher Animals; and, even in the latter (as will be shown hereafter), this pro- perty is independent of "nervous agency," although generally called into ex- ercise by it. That a nervous system is not required by them for the performance of the functions of Nutrition and Reproduction, otherwise than to supply, by its locomotive actions, the conditions of those functions, would also appear from its absence in Plants. It is on the sensible movements of these beings that our belief in their possession of a nervous system must be founded, when we cannot render it cognizable by our senses. But we must be careful not to draw hasty inferences from such phenomena. Sensible movements are, as we have seen, performed by the Dionsea and Sensitive plant, in respondence to external stimuli acting on distant organs; and here the channel of communication is probably the vascular system. We observe, however, that even in Polypes, an impression made upon one part (one of the tentacula, for example), is propa- gated to distant parts, and excites respondent movements in them, more rapid- ly than we could imagine to occur, without such a channel of communication as a nervous system only is known to afford. Moreover, some of their actions appear to show a certain degree of voluntary power, and, therefore, of con- sciousness ; being independent, so far as can be ascertained, of the operation of external stimuli. These phenomena, then, would lead us to suspect the existence of a nervous system in the beings which exhibit them ; not, how- ever, in a " diffused" condition, but in the form* of connected filaments. For, what consentanepusness of-actio can he jb&le fpr*in a being whose nervous matter is incorporated -lii th<> state pf," isolated gltfbules with its tissues ? How should an impression tnatle' on one part be propagated by these to a distance ? COMPARATIVE ANATOMY AND PHYSIOLOGY. And how can that consciousness and will, which are one in each individual, exist in so many unconnected particles ? If, then, we allow any sensibility, consciousness, and voluntary power, to the beings of this group of Acrita to deny which would be in effect to exclude them from the Animal Kingdom we must regard these faculties as associated with nervous filaments, of such delicacy as to elude our means of research. When the general softness of the textures, and the laxity of structure that characterize the nervous fibres, in the lowest animals in which they can be traced, are kept in view, little dif- ficulty need be felt in accounting for their apparent absence. The case is very different from that of Vegetable structure, the greater consistency of which enables us to place much more reliance upon the negative evidence afforded by anatomical research. 129. The correctness of this view (which has been here dwelt on the longer, because it involves a fundamental question in Nervous Physiology), is borne out by the fact, that, in those members of the group whose size and consistency allow their structures to be sufficiently examined, a definite nervous system has been detected, in the position which it might, a priori, be expected to occupy, according to the type of the individual. Thus, in the large fleshy isolated polype, commonly known as the Sea-Anemone (Jlctinia), a nervous ring has been discovered, surrounding the mouth as in other Radiata, and sending off branches to the tentacula, with a minute ganglionic enlargement at the base of each. In the higher Radiata, as the Star-Fish, the nervous system has the same regular form as that which prevails through the other organs. The mouth is surrounded by a filamentous ring, which presents a regular series of gangliohic enlargements, one of them corresponding with each segment of the body. From every one of these, a branch is transmitted to the corresponding ray ; and two smaller ones proceed to the viscera in- cluded in the central disk. 180. The Polypiftra being the lowest of the Radiated classes in w r hich there is a regularly-organized digestive apparatus, and which perform move- ments of a character ascribable only to a nervous system, it will be desirable to inquire a little more particularly into the phenomena they exhibit, and the degree in which these necessarily involve the possession of the higher mental endowments. In this inquiry we shall refer principally to the little Hydra, or fresh-water Polype, the habits of which are better known than those of any other species. Although no nervous filaments have been detected in this, we have a right to infer their presence for the reasons already given ; and they probably form a ring around the mouth, as in the Actinia, sending filaments to the tentacula. This interesting little being may be regarded as essentially a stomach; and the orifice of this is provided with tentacula, which contract when irritated by the touch of any adjacent body, and endeavour to draw it towards the entrance. Now, the action in the Human body, to which this is most allied, is evidently that of the muscles of Deglutition ; which lay hold, as it were, of the food that has been conveyed to the fauces, and carry it into the stomach. These muscles are called into action, not by an effort of the will, but by the contact of the food with the lining membrane of the pharynx. This impression is propagated by the glosso-pharyngeal nerve to the medulla oblongata, where a respondent motor impulse is excited, which is transmitted through the pharyngeal branches of the par vagum to the muscles of degluti- tion, and causes their contraction. This phenomenon will be more fully ex- amined hereafter ; it is here adduced simply as an instance of the important class of reflex movements which are independent of the brain (though, to a certain extent, controlled by it), which are altogether involuntary, and which do not necessarily involve the production of sensation. There would appear to be little difference, in the character of this movement, between the simple 9* 102 FUNCTIONS OF THE NERVOUS SYSTEJVI. Hydra and the most perfect Vertebrated animal. In the latter, however, an- other set of muscles are superadded to these, for the purpose of preparing the aliment by mastication for the operation of the stomach, and of bringing it within reach of the pharyngeal constriction. But, it has been urged, the inac- tivity of the tentacula when the Hydra is gorged with food, proves that they are excited to action by the will of the animal. This inference, however, may be easily disproved. The muscles of deglutition in Man are not called into action with nearly the same readiness and energy, w T hen the stomach is dis- tended, as when it is empty ; a fact of which any one may convince himself, by observing the relative facility of swallowing at the commencement and the termination of a full meal. No one will assert that this variation is an effect of the will ; indeed, it is often opposed to it ; being one of those beautiful adaptations, by which the welfare of the economy is provided for, but which the indulgence of the sensual appetites opposes. Most of the movements of this animal, and of others of the class, appear to be equally the result of ex- ternal stimuli with that already described ; and it is only in a few instances, principally those of absolute locomotion or change of place, that any evidence of voluntary action can be discerned. It may be occasionally remarked, how- ever, that one or more of the tentacula are retracted or extended, without the slightest appreciable change in any of those external circumstances which seem ordinarily to affect the motions of the animal ; and this action we can scarcely regard as otherwise than voluntary. 131. Thus in the Nervous System of Radiated Animals, we have an instance of that community of function which is so remarkable in the organism of the lower tribes, when contrasted with the separation which is perceptible in those at the opposite extremity of the scale. The visceral nerves of the Asterias are not isolated at their central terminations from those which are connected with the sensorial and locomotive functions : nor are those which minister to the instinctive actions separable from those which convey the influence of the will. Every segment of the body appears equal in its character and endow- ments to the remainder ; each has a ganglion appropriated to it ; and, as the ganglia, like the segments, are all alike, neither of them can be regarded as having any presiding character. 132. From the Radiated we now pass to the MOLLUSCOUS glasses, the gene- ral character of which, as a natural group, is the remarkable predominance of the Nutritive system over that of Animal life. In fact, although the organs which minister to their vegetative functions attain a very high degree of deve- lopment, the animal powers of sensation and locomotion are, in general, so feebly manifested, as to show that they are entirely subservient to the exercise of tMe former. There is not in the Mollusca, as in theRadiata, any repetition of parts around a common centre ; and we do not, therefore, meet in them with a number of ganglia nearly or altogether alike in endowments. In some of the higher species, there is a conformity between the two sides of the body, or a lateral symmetry : which involves a subdivision of some of the ganglia, that are single in the inferior tribes, into two masses, which always remain in connection with each other. With this exception, it may be observed, that all the ganglia, to the number of four or five, which we meet with in the higher Mollusca, appear to have distinct functions ; as may be determined by tracing the .distribution of their nerves. Thus we find a pair of cephalic ganglia, situated above the oesophagus, connected with the organs of special sensation, and sending motor nerves (as we shall see reason to believe) to all parts of the body. This is obviously analogous to the brain of Vertebrata. Below the oesophagus there is generally a small ganglion, connected with the apparatus of deglutition, which may be called the stomato-gastric ganglion. In connec- tion with the gills we have always one ganglion, sometimes a pair, which may COMPARATIVE ANATOMY AND PHYSIOLOGY. 103 be termed the branchial ganglion. Another is found at the base of the foot, which may be called the pedal ganglion. And there is sometimes another, which especially supplies the mantle with nerves ; and this may be called the pallcal ganglion. The distribution of their nerves to the different organs would alone indicate their respective functions ; but these are placed beyond doubt by that very great variety in the disposition of these organs which is characteristic of the Mollusca. The development of the sensory organs, the situation of the gills, the structure and position of the foot, the conformation and uses of the mantle, are well known to differ, in the most obvious manner, in genera which are closely allied to each other. Hence the anatomist is able, by the discovery of corresponding changes in the nervous system, to satisfy himself of the particular functions of its different centres.* 133. It is only in the higher tribes, however, that this separation of function is evident ; and it may be especially noticed in the class GASTEROPODA ; which is so named from the presence of a kind of foot, or locomotive organ, on the under side of the body, this being formed by a thickening of the muscular part of the mantle in that situation. Of the animals belonging to this class, some form univalve shells, whilst others are entirely shell-less. They are much superior in general organization to the animals inhabiting bivalve shells, which are included in the class Conchifera; and this superiority manifests itself strongly in the development of the powers of locomotion and sensation. The Conchifera belong to the group of bicephalous, or headless Mollusca ; the mouth not being placed upon a prominent part of the body, nor guarded with organs of special sensation. The lowest form of this group consists of the class Tunicata ; composed of animals in which the whole body is enclosed in a tunic or bag, having but two orifices, through one of which the water is drawn in by ciliary action, whilst through the other it is expelled. This bag forms a large chamber, the lining of which is devoted to the respiratory function ; and at the bottom of it lies the mass of the viscera, on which is the true mouth or entrance to the stomach. A part of the water which is taken into the respiratory chamber flows into this, and passes through the in- testinal canal ; being discharged along with that which has only served the purpose of aerating the blood. These animals have no power of motion, but such as is effected by the general contraction of the respiratory sac ; that is effected by a single ganglion placed between its orifices, w r hich is there- fore chiefly a branchial ganglion, and is the only nervous centre they possess. Although none of the GASTEROPODA are able to execute very active move- ments, few are entirely fixed; all are more or less dependent upon the exer- cise of these powers for their supply of food ; and the higher tribes employ them also in the perpetuation of the race, since the connection of two individuals is in them an essential part of this function. Although the foot is the chief instrument of locomotion, some of the naked aquatic species have other means of propelling themselves. These move through the water by the undulations of their whole bodies, like the leech, or the vermiform fishes : and a few appear materially assisted by an expansion of the mantle on the ante- rior part of the body, which contains muscular fibres, and seems to act as a fin. In every division of the Animal Kingdom, we find the development of special sensory organs to bear a close relation with that of the locomotive appa- ratus. In the present instance, we observe an evident example of this general rule. The organs of vision, which, when existing at all among the Conchi- fera, were very imperfect, are here almost constant and more highly deve- loped; rudimentary organs of hearing maybe detected; the tentacula are more * See Mr. Garner on the Nervous System of the Mollusca, in the Linnasan Transac- tions, vol. xvii. 104 FUNCTIONS OF THE NERVOUS SYSTEM. sensitive, and are sometimes increased in number to six or eight ; and there is reason to believe that some of them occasionally minister to the sense of smell. These senses, as well as the locomotive powers of the animals, have an obvious relation with the supply of the digestive system ; which is not here, as in the inferior classes, dependent upon the miscellaneous aliment conveyed to the mouth by the movement of the surrounding fluid medium, but is more limited as to the character of the food to which it is adapted ; so that the animal re- quires the means of becoming acquainted with the proximity of what it can digest. 134. It is not a little curious, however, that, although the general surface appears highly susceptible of impressions which excite responsive movements adapted to fulfil some important office in the economy, it does not seem to be susceptible of painful impressions, in any thing like the same degree. This, which cannot but be regarded as a beneficent provision for the happiness of animals so incapable of offering any active resistance to injury, would appear from the observations of various experimenters, and especially from the testi- mony of M. Ferussac, who says, "I have seen the terrestrial Gasteropods allow their skin to be eaten by others, and, in spite of large wounds thus pro- duced, show no pain." This fact has an important bearing on our general views of the operations of the nervous system ; since it would seem to confirm an opinion founded upon other phenomena, that the impressions which pro- duce reflex actions through the nervous system do not always involve the production of sensation. ( 173 182.) 135. The nervous system of the Gasteropoda consists of at least three dis- tinct centres ; the relative position of which varies with that of the organs they supply. The anterior or cephalic ganglia are larger, in proportion to the rest, than in the Conchifera ; and they exhibit a tendency to gain a position anterior to the oesophagus, and to approximate towards each other, so as to meet and form a single ganglionic mass on the median line. The branchial ganglion is constantly to be met with, but its position is extremely variable. This centre, however, always bears a close relation with the gills, both in situa- tion and in degree of development; and even where conjoined, as it frequently is, with the pedal ganglion, it may be distinguished from it by the distribution of its nerves, as well as by its separate connection with the cephalic ganglia, which is always noticed in such cases. This may be observed in the Patella (limpet) and Limax (slug). Sometimes the functions of this ganglion are subdivided .between two; of which one is still appropriated to the branchiae; whilst the other is connected with the general surface of the mantle, and with the respiratory passages which are prolongations of it, and hence may be called the palleal ganglion. The position of the pedal ganglion (which is generally double in the Gasteropoda, though the foot is single) also varies, but in a less degree, since it is generally in the neighbourhood of the head. Besides these nervous centres, we find, in many of the Gasteropoda, a sepa- rate system connected with a very important set of organs, the gustatory and manducatory, which are but slightly shadowed out among the Conchifera. In these higher tribes, the oesophagus is dilated at its commencement into a mus- cular cavity, containing a curious rasp-like tongue, often supported upon car- tilages, which serves to reduce the food ; and sometimes furnished with horny maxillae. The nerves which supply these do not proceed directly from the cephalic ganglia, but from a distinct centre ; from which ramifications proceed along the oesophagus and stomach, and these are occasionally connected with the other nerves by inosculating filaments. This set of ganglia and nerves, which is even more important from its relative development in some other classes, and into the analogies of which, in the nervous system of Vertebrata, COMPARATIVE ANATOMY AND PHYSIOLOGY. 105 we shall hereafter inquire, may be called, from its distribution, the stomato- g as trie system. 130. The ganglia just described may be regarded as corresponding with those parts of the nervous centres in the Vertebrata, the distribution of whose ntrves is analogous. Thus the branchial ganglion obviously corresponds with that portion of the Medulla Oblongata which is the centre of the respiratory actions in Vertebrata. The pedal ganglion is analogous to that division of the Spinal Cord, from which the nerves of the anterior or posterior extremities pass off. It is well known that such portions of the spinal cord may be com- pletely isolated, without destroying the functions to which they minister. Thus, the brain and lower part of the spinal cord may be removed, that portion only of the cerebro-spinal axis being left which connects the principal respiratory nerves, in fact the respiratory ganglion, and yet the animal may continue to exist for some time. It is then reduced to a condition similar to that of the Tunicata, whose single ganglion, though combining, in some degree, the functions of those which exist separately in the higher tribes, has evidently the regulation of the respiratory movements for its chief object. In the same manner, the integrity of the segment of the cord, with which the nerves of the extremities are connected, will enable them to execute those movements of a reflex character which depend upon its power as their centre, even though it is isolated from every other part of the nervous apparatus. The cephalic ganglia must be regarded as analogous, not to any single portion of the Encephalon in Vertebrata, but in some degree to the whole. We find nerves of special sensation proceeding from them, certainly to eyes and an auditory apparatus, perhaps also to olfactive organs, as well as others of com- mon sensation, supplying the tentacula and mouth. Hence we must admit that they perform the functions of the optic ganglia of Vertebrata,. and perhaps also of the olfactory lobes, as well as of the portion of the medulla oblongata in which the sensory portion of the fifth pair terminates. Moreover, they certainly give origin also to motor nerves, and must thus perform the functions of the Medulla Oblongata, from which the corresponding nerves arise in Ver- tebrata, as well as, perhaps, of the Cerebellum. And if we regard these animals as enjoying the perceptive, reasoning and volitional faculties, in how- ever low a degree, we must attribute to their cephalic ganglia some portion of the attributes of the cerebral hemispheres in the highest classes. This com- bination of function will not appear so extraordinary, when it is recollected that all the central operations of the nervous system are performed in the Tunicata by one ganglion, and in the Radiata by a series, of which each is but a repetition of the rest ; and it is quite conformable to the general principle of the gradual specialization of function which may be observed in ascending the scale of organization. 137. It is obvious that the portion of the Nervous system of the Gasteropod Mollusca, into the analogies of which we have thus inquired, cannot in the least be compared as a whole with the Sympathetic system of the Vertebrata, which it was formerly imagined to resemble. The distribution of some of its nerves to the viscera, however, may indicate that it partly performs the func- tions of that system; with which it is structurally intermixed, even in Verte- brata, as the late inquiries of Miiller, Volkmann, and others (of which the results will hereafter be stated), have shown. But the stomato-gastric system may, perhaps, with more probability be considered as executing its offices. Into the peculiar character of that system we shall be more competent to inquire, when we have traced it through other classes of Invertebrata. 138. Having thus separately considered the nervous centres of the Gastero- poda, and determined their special functions by their structural relations, we shall inquire into the mode in which these functions are combined, so as to 108 FUNCTIONS OF THE NERVOUS SYSTEM. Fig. 10. Nervous system of Aplysia. The most anterior ganglion is the pharyngeal; and below this is seen the cephalic. The cephalic is connected, by three distinct cords on each side, with the lateral masses, which combine the functions of pedal and palleal ganglia; these are united with each other by two transverse bands, between which the aorta passes. From the lateral ganglia, a connecting cord passes backwards on each side to the branchial ganglion ; this cord is continuous \vith one of the three pro- ceeding from the cephalic ganglion. enable them to act in harmony. This is an inquiry of much interest, in re- ference to the determination of the offices of the different parts of the nervous centres in Vertebrated a^i- mals. If we examine the mode in which the different ganglia are united by connecting trunks, we are led to perceive the important fact that, while they have little or no communication with each other, they are all directly connected with the cephalic ganglia ; which seem thus to harmonize and control their individual actions. Fre- quently a communication with one another appears to exist, where there is really none. Thus, in the Aplysia, a cord passes from the branchial gan- glion, which is situated in the pos- terior part of the body, to the pedal ganglion. Where such is the case, the trunk is not united with that pro- ceeding from the ganglion through which it passes ; but the two remain distinct, though running in the same direction. Moreover, the double func- tion of a ganglion may be sometimes recognized, by its being connected with the cephalic mass by a double trunk. Thus, in the Aplysia, that which has been termed the pedal ganglion is really made up of a pedal and palleal ganglion, as it is proved by the distribution of its branches; and in conformity with this double function, we find it communicating with the cephalic mass by two cords, besides the one which has been just mentioned as passing through it, and which appears as a third. In the BulldRa, whose nervous system is dis- posed on the same general plan, the pedal and palleal ganglia are sepa- rately connected with the cephalic; the cord from the branchial ganglion passing through the palleal. 139. Further, a careful examination of these ganglia, and of their connecting cords, discloses this important fact, which is peculiarly evident in the case of the pedal ganglia that the cord does not lose itself in the gray matter of the ganglion, but divides itself into filaments, which mix with those proceeding from it, to form the nervous trunks which it distributes. We can scarcely, then, fail to infer that the pedal ganglion, with the nervous fibrils proceeding from itself, is the source of the reflex actions of this organ ; whilst the filaments which are continuous with those of the connecting trunk, and which are thus connected with the nucleus of the cephalic ganglia, are the channels of sensory impressions, and of the motor impulses of volition or instinct. This is well COMPARATIVE ANATOMY AND PHYSIOLOGY. 107 illustrated in the curious disposition of parts which we find in the arms of the Cuttle-fish. These are provided, it is well known, with a series of suckers, which are to the animal important instruments of locomotion and prehension. It has been observed by Dr. Sharpey, that the nerves which supply these arms are furnished with ganglionic enlargements, of which one corresponds with each sucker: and that each trunk consists of two tracts, in one of which the ganglionic enlargements exist ; whilst the other passes continuously over these, but sends off nervous filaments, which help to form the branch going to each sucker. It has been supposed that the white or fibrous tract is the motor por- tion, and the ganglionic the sensory ; but this is inconsistent with the facts known, regarding the influence of the nerves upon the movements of the suckers. When the animal wishes to embrace any object firmly with its arm, it brings all the suckers simultaneously to bear upon it. There can be little doubt that this action is occasioned by a motor impulse, propagated from the cephalic masses by the non-ganglionic portion of the cord, which supplies all the suckers alike. On the other hand, any individual sucker may be made to attach itself, by placing a substance in contact with it alone ; this action is independent of the cephalic ganglia, as is evident from the fact that it will take place when the arm is severed from the body, or even in a small piece of the arm, if recently separated ; and it can scarcely be doubted, that it is due to the reflection of the impression made upon the sucker, through the small ganglion in its neighbourhood, where it excites a motor impulse. The operation of these independent centres appears, in the entire living animal, to be con- trolled, directed and combined, by the cephalic ganglia, through the medium of the fibrous band which passes over them, and which mixes its branches with theirs. A very similar arrangement will be presently shown to exist in the double nervous column of the Articulata. 140. Upon reviewing all the anatomical facts hitherto stated, it will be per- ceived that ganglionic masses, characterized by nuclei, of gray matter, or of something equivalent to it, seem to exist, wherever it is desirable that impres- sions made upon the afferent nerves should excite motions ; and that, as we rise in the scale, there is an increase in the number of centres possessing a diversity of functions. We have seen that sometimes these centres are, for the sake of convenient disposition, united into one mass ; whilst, on the other hand, when the organs are multiplied, they also are repeated to a like extent ; especially when it is desirable that they should be able to act independently of one another, as in the case of the suckers of the Cuttle-fish. It may further be remarked, that, wherever the presence of special sensory organs, confined to one part of the body, gives to that part a predominance over the remainder (the entrance to the alimentary canal being always in this neighbourhood), we find the ganglia with which they are connected possessing a special relation with all the rest, which these do not possess with each other. It is obvious that, where visual organs are developed, the impressions made upon these will determine the movements of the animal more than those of any other kind; and it would seem to be chiefly owing to the information they communicate, that the cephalic ganglion has such an evident presiding influence over the rest, even when smaller than any of .them. This is, however, more the case in animals whose movements are rapid, and in which, therefore, the perception of distant objects is more important as in the Insect tribes. Except in the Cephalopoda, the subservience of the nervous system to the nutritive functions of the Mollusca is so great, that it might almost be regarded as an appendage to the digestive organs, destined for the selection and prehension of aliment. But in the more active members of that class, it derives a more elevated cha- racter, from the development of organs of special sensation and of active loco- motion. 108 FUNCTIONS OF THE NERVOUS SYSTEM. Fig. 11. Nervous System of Larva of SPHINX LIGUS- TRI, after Newport; A, cephalic ganglia; 1-11, ganglia of the trunk, disposed at nearly equal distances; the last is formed by the consolida- tion of the llth and !S2th. 141 . The animals composing the group ARTICULATA, all present, in a more or less evident degree, a division into segments, which have an obvious tendency to re- semble one another, as in the Radiata. In those in which these segments differ but little (as in the Centipede, or the Ca- terpillar of the Insect), the nervous system is a repetition of similar parts ; disposed, not in a circle, as in the Radiata, but in a continuous line. The most interior of the ganglia, however, has an evident pre- dominating influence over the rest, for the reason just specified; and this influ- ence will be found, by comparison in other classes, to diminish with the loss, and to increase with the development, of the faculties of special sensation, which have their seat there. The locomotive powers are just as predominant in the Articulated series, as are the nutritive functions among the Mollusca. Accordingly, we find the development of the Nervous System to bear a special reference to them ; and the sen sori -motor divisions of it can be more distinctly separated than in the Mollusca, from the portion which ministers to the organic functions. 142. The general arrangement of the Nervous System differs so little, except as to the degree of concentration of the gan- glia, in the different classes of this sub- kingdom, that it is of little consequence what example we select. It will be con- venient to take for illustration that of the Larva of the Sphinx Ligustri, or Privet Hawk-Moth, which has been minutely described by Mr. Newport. Here we ob- serve a chain of ganglia running from one extremity of the body to the other, along the ventral surface, and in the median line. These ganglia are connected by trunks, which, on close examination, are seen to consist of two cords closely united. The cephalic ganglion is bilobed; evi- dently consisting of two masses, which are united on the median line. These re- ceive the nerves of the eyes and antennae ; but they are still of small size, in accord- ance with the low development of the sen- sory organs. The ganglia of the longi- tudinal cord are nearly equal, from one extremity of the body to the other. Each sends off nerves to its respective segments ; and the branches proceeding from the COMPARATIVE ANATOMY AND PHYSIOLOGY. 109 different ganglia have little communication with each other. The highest of them, situated just beneath the oesophagus, is connected with the cephalic masses by two cords, between which that canal passes, encircled, as it were, in a ring. 143. The most detailed account of the conformation of the Nervous Centres in the Articulata, is that recently given by Mr. Newport, in regard to the Julus, and other animals of the class MYRIAPODA.* Their general arrange- ment corresponds with that which has been just described in the larva of the Sphinx Liguslri; but the number of ganglia is much greater. In each lateral half of the cord, two distinct tracts or layers of fibres can be detected ; of these, one known as the fibrous tract is continuous with the brain, and contains no gray matter ; whilst the other known as the ganglionic tract has gray matter deposited at intervals amongst its fibres, some of which are continuous with the brain, whilst others do not reach it. (Fig. 13, A.) Every nerve that is given off from this ventral column, is connected w r ith both tracts ; and thus it has two sets of roots, one proceeding to the brain, the other e^ter- ing the ganglion near which it arises. Of this last division, a part crosses to the oppo- Fig. 12. site side, forming the commissural fibres which unite together the lateral halves of the cord ; whilst another bundle of fibres runs along the side of the ganglionic tract, for a greater or less proportion of its length, and then emerges again, forming part of another nervous trunk. In Fig. 12 is seen Mr. N.'s representation of one of the ventral ganglia, and part of the cord, of Polydesmus maculalus ; showing the longitudinal and commissural fibres, together with those to which he has given the name of fibres of reinforcement. These lateral fibres, *r which do not pass on to the brain, but issue Portion of the ganglionic tract of Pf> . again from the ventral cord at a point a little lydesmus macuiatus; 6, imer-gangiionic distant from their entrance, Seem tO be more cord; c, anterior nerves; d, posterior numerous in the hinder part Of the bdldy of nerves; /, k, fibres of reinforcement; the Centipede tribe, than in its front portion : f- /J ' cora ssu ; al fi , bres ; '' lo s hudi " al , , i . , 1 11- c i i fibres, softened and enlarged, as they and thus it is, that the whole size of the cord pass lhrough ganglionic matter . remains nearly the same along its entire length; whilst that of the portion which passes backwards from the brain, must be con- tinually diminishing, as it gives off fibres to the nerves. 144. After what has been said of the offices which the ganglia perform in the Mollusca, and of the relation which they bear to the cephalic mass, we shall have little difficulty in understanding the character of the nervous appa- ratus in the Articulata, if our minds be unoccupied by any preconceived notion. When we examine into the actions of the ventral cord, we perceive that those of all its ganglia are similar to each other ; being related only to the move- ments of their respective segments, and of the members which belong to them. In fact, these ganglia may be regarded as so many repetitions of the pedal or locomotive ganglion of the Mollusca. It is easily proved, that the movements of each pair of feet may be produced by that ganglion alone with which it is connected ; since a single segment, isolated from the rest, will continue to perform these movements for some time, under favourable circumstances. If an Earth\vorm be cut in two, whilst crawling, each portion will continue to advance, though the anterior one only will permanently preserve its vitality. * Philosophical Transactions, 1843. 10 1 10 FUNCTIONS OF THE NERVOUS SYSTEM If a Centipede be divided into several portions under the same circumstances, each will execute motions of progression for some time. But it is evident that these must be placed, in the living animal, under some general control ; by which the consentaneousness of action, that is essential to regular locomotion, may be produced. This is easily proved by experiment. If in a Mantis, for example, the nervous cord be divided between the first and second thoracic ganglia, so as to isolate the ganglionic centres of the posterior legs, the limbs will continue to move energetically, but not with a combined object, and no progression will take place. We can scarcely account for the exercise of this general control, otherwise than by attributing it to the fibrous portion of the cord,* which connects each of the nervous trunks immediately with the cepha- lic ganglia, as in the Mollusca ; and this must, therefore, conduct to the sen- sorium (whose seat is probably in the latter) the impressions which there pro- duce sensations, and must convey downwards the locomotive impulse ; whilst the ganglion of each segment, with the filaments connected with its nucleus, will form the circle necessary for the simply reflex actions of its members. The independence of the segments of the Articulata, as far as their reflex actions are concerned, and their common subordination to one presiding centre of the will, are fully explained on this supposition. It is also quite conform- able to the analogy both of the Mollusca and of Vertebrata. 145. The number and variety of the reflex actions which take place in the Articulata after decapitation, is very remarkable ; and they seem to have a consentaneousness, proportioned to the closeness of the relation between the nervous centres in the respective species. Thus, in the Centipede, we find the ganglia of the several segments distinct, but connected by a commissural trunk. Here an impression made equally upon the afferent nerves of aU the ganglia, will produce a consentaneous action. Thus, if the respiratory orifices on one side of a decapitated Centipede be exposed to an irritating vapour, the body will be immediately flexed in the opposite direction ; and if the stigmata of the other side be then similarly irritated, a contrary movement will occur. But different actions may be excited in different parts of the cord, by the pro- per disposition of the irritating cause. In the higher classes, however, where the ganglia of the locomotive organs are much concentrated, the same irritation will produce consentaneous motions mseveral members, similar to those which the unmutilated animal performs. In the Mantis religiosa, for example, which ordinarily places itself in a very curious position, especially when threatened or attacked, resting upon its two posterior pair of legs, and elevat- ing its thorax and the anterior pair, which are armed with powerful claws,- if the anterior segment of the thorax, with its attached members, be removed, the posterior part of the body will still remain balanced upon the four legs which belong to it, resisting any attempts to overthrow it, recovering its posi- tion when disturbed, and performing the same agitated movements of the wings and elytra, as when the unmutilated animal is irritated : on the other hand, the detached portion of the thorax, which contains a 'ganglion, will, when sepa- rated from the head, set in motion its long arms, and impress their hooks on the fingers which hold it. These facts prove unequivocally, that the automa- * It is believed by Mr. Newport, that the fibrous portion of the ganglionic trad, which lies nearest the surface of the body, may be the channel by which sensory impressions are conveyed to the brain; whilst the fibrous tract itself may convey downwards the motor impulses which originate in the cephalic ganglia. The chief reason for this sup- position, is the correspondence in position, relatively to each other, and to the rest of the body, between the fibrous and ganglionic columns in Articulata, and the portions of the Spinal Cord of Vertebrata, from which the anterior or motor roots, and the posterior or sensory, respectively arise. But the fibres which are peculiar to the ganglionic tract, obviously form a distinct system. COMPARATIVE ANATOMY AND PHYSIOLOGY. Ill tic movements of these parts, which are performed in direct respondence to external impressions, are only dependent for their stimulation upon that gan- glionic centre with which the nerves that excite them are immediately con- nected. Another instance, related by Burmeister, is still more satisfactory in regard to the manner in which these movements are excited. A specimen of the Dytiscus sulcatus, from which the cephalic ganglia had been removed, and which remained in a motionless condition whilst lying with its abdomen on a dry hard surface, executed the usual swimming motions, when cast into water, with great energy and rapidity, striking all its comrades to one side by its violence, and persisting in this for half an hour. 146. These conclusions are also fully confirmed by the experiments of Mr. Newport, upon various Insects and Myriapoda ; the results of which have been recently made public.* The following, upon the Julus terrestris, is particularly interesting. " The cord was divided in the fourteenth and also the twentieth segment ; and the intervening portion was destroyed by break- ing it down with a needle. The animal exhibited, in the anterior part of its body, all the evidences of perfect volition. It moved actively along, turning itself back on either side repeatedly, as if to examine the anterior wounded portion, which it felt again and again with its antennae ; and when attempting to escape, frequently turned back as if in pain and aware of some hindrance to its movements ; but it seemed perfectly unconscious of the existence of the posterior part of its body, behind the first incision. In those segments in which the cord was destroyed, the legs were motionless ; while those of the posterior division, behind the second incision, were in constant but involuntary motion, the movements being similar to those of walking or running, uniformly continued, but without any consentaneous action with those of the anterior part, by which locomotion was performed, dragging the posterior divisions of the body after them. When the animal was held by the posterior segments, reflex actions were excited in the legs, and powerful contractions and gyra- tions of the whole animal were performed in those segments ; but these move- ments appeared to be entirely the result of reflex actions of the muscles, since exactly similar ones took place in the whole body of decapitated specimens. At the expiration of twelve hours, the most perfectly voluntary acts were per- formed by the head and anterior division of the body, such as locomotion for- wards or to either side, avoidance of any obstacle, touching it with the anten- nas, (which were in rapid action, as in an uninjured animal,) and attempting to reach and to climb up an object presented to it, but not in immediate con- tact with it. But reflex movements alone existed in the posterior division, in which the legs were very slowly moved, even when the animal was not pro- gressing. Brisk actions were now more easily excited in them than at first, either by contact with the segments, by irritation of one or two of the legs themselves, or by a sudden current of air. By these means, when the animal was lying still, actions were immediately excited in all the legs of the poste- rior parts of the body, anterior and posterior to those which were irritated ; and these actions were induced in those of both sides of the body, but appeared to commence on the opposite side, in the legs corresponding to those which were first irritated. In eighteen hours, the anterior part of the body was quite dead, so that no motions whatever could be excited in it, either voluntary or reflex ; but reflex actions were then readily excited in the posterior, and also slightly so by mechanical irritation, even at twenty-four hours." It would appear, then, that we may obtain more decided proof, in the Articulated series, of the real character of reflex actions, and of their dependence upon a distinct system of nerves, than we can draw from any other class of animals. In the Vertebrata, it is easy to distinguish the sensory from the motor, the afferent * Philos. Trans., 1843, p. 267. FUNCTIONS OF THE NERVOUS SYSTEM. from the efferent fibres ; but the distinctness of the excito-motor system from the sensori-volitional, is not so clearly made out. Here, however, the afferent and efferent fibres cannot be readily distinguished ; but it is obvious that the reflex actions, which manifest themselves when the communication with the cephalic ganglia is cut off, are to be attributed to those fibres which enter the cord under the afferent character, pass into the edge of the ganglion as the fibres of reinforce?nent, or cross it as commissural fibres, and then emerge again as efferent fibres, either in the nerves of the same segment, or in those of another more or less distant. By traversing the cord along a part of its length, and thus placing the several segments in communication with each other, the fibres of reinforcement thus constitute a part of the longitudinal filaments of the cord, the remainder consisting of the fibres continuous with the cephalic ganglia. 147. Without describing -in minute detail the forms which the nervous system presents in the higher classes of Articulata, or tracing that interesting series of changes which it undergoes during the metamorphosis of Insects, a few particulars may be stated on these subjects as having an immediate bear- ing on our present object. The nervous system of the Larva, like that of the Annelida, or Myriapoda, presents an obvious relation to the means and extent of locomotion possessed by the animal. Each segment is equally concerned in locomotion; and with each is associated a pedal ganglion. None of the movements of the animal are very energetic; simple and slow progression is all for which its structure is adapted ; and the uniformity in the actions of its legs would render it easy to combine them at the will of the animal, even though their respective centres remain much isolated from one another. But, in the perfect Insect, the whole locomotive apparatus is concentrated in the thorax. The six legs (which are now all that remain), and the single or double pairs of wings, are all developed from its three segments; and a much greater variety of action is required, as well as more complete consentaneous- ness, on account of the increased number and velocity of the movements of the animal. We accordingly find that the ganglionic matter of the ventral cord of perfect Insects, is more or legs concentrated in the thoracic region ; whilst the ganglia of the abdomen are usually few and small ; the nerves to its segments, however, being given off as before at regular intervals. In some of the Cole- optera and Hemiptera, the concentration of the thoracic ganglia takes place to such an extent, that they seem to form but one mass ; and this is the case also among some of the Crustacea, the different forms of whose nervous system are exactly parallel to those of their congeners among the inhabitants of the air and land. The nerves which supply the wings of Insects are found, in all stages of the development of these organs, to have a double origin. One root arises from the fibrous tract alone ; whilst the other takes its origin from both tracts at the point of enlargement. When the ganglionic centres which sup- ply the anterior and posterior pairs of wings remain distinct, there is a curious plexiform arrangement of their nerves ; more or less intricate, according as the wings are destined to act with greater or less consentaneous energy ; and absent when the anterior pair serve only as elytra, and do not assist in flight.* This would remind us of the circular filament, which connects the nerves of the arms in the naked Cephalopoda. Besides these nerves, the wings are supplied from the respiratory system next to be described, from which scarcely any branches go to the legs. This will be readily understood, when it is con- sidered that the wings are developed, as it were, out of an extension of the respiratory apparatus,! and that their movements are closely connected with its actions. * Newport, in Phil. Trans., 1834, p. 394, 5. j- See Principles of General and Comparative Physiology, 2J ed. 465. COMPARATIVE ANATOMY AND PHYSIOLOGY. 113 148. Hitherto we have spoken only of that division of the nervous system of the Articulata, which may be regarded as corresponding with the sensory and locomotive ganglia of the Mollusca ; namely, the cephalic, the pedal, and (in some instances) the palleal. We have next to inquire what we find corre- sponding with the branchial ganglion. It is to be recollected that the respira- tory apparatus of Insects is diffused throughout the whole body, so that its presiding system of nerves must be proportionally extended ; and we are, therefore, prepared to find the branchial ganglion of the Mollusca repeated, like the pedal, in each segment. Besides the nervous trunks proceeding from the ventral cord at its ganglionic enlargement, we find, in most of the Articu- lated classes, a series of smaller nerves, given off at intermediate points, with- out any apparent swelling at the points of divergence. The connections of these are most distinctly seen in the thoracic region, just as the Larva is pass- ing into the Pupa state ; for the cords of the ventral column then diverge, so that an additional tract may be seen, which occupies the central line. By a close scrutiny, this tract may be found in the perfect Insect, on the superior or visceral aspect of the cord ; and its nerves are given off from minute gangli- onic enlargements upon it. It seems to be quite unconnected along its whole course with the column upon which it lies. Its nerves, however, communi- cate with those of the sensori-motor system ; but they have a separate distri- bution, being transmitted especially to the trachea, on the parietes of which Fig. 13. Parts of Nervous System of Articulata, after Newport. A, single ganglion of Centipede, much enlarged, showing the distinctness of the purely fibrous tract, fc, from the ganglionic column, a. B, portion of the double cord from thorax of Pupa of Sphinx ligustri, showing the respiratory ganglia and nerves between the separated cords of the symmetrical system, c, view of two systems combined, showing their arrange- ment in the Larva; a, ganglion of ventral column; 6, fibrous tract passing over it; cc, respiratory system of nerves distinct from both. 10* 114 FUNCTIONS OF THE NERVOUS SYSTEM. Fig. 14. they ramif}?- minutely, and also to the muscles concerned in the respiratory movements. (The latter, however, being a part of the general locomotive apparatus, are also supplied from the principal ganglionic column.) These nerves, then, which are evidently analogous to those of the gills and siphonic apparatus in the Mollusca, may be regarded as corresponding with the pneu- monic portion of the par vagum in Vertebrata (which is in like manner distri- buted on the air-passages), and with its associated motor nerves. 149. In comparing the nervous system of Insects with that of the higher Mollusca, it will be seen that they differ more in the arrangement and in the relative proportions of their parts than in their essential character. In both there is a cephalic division of the ganglionic centres, in which sensibility and psychical power appear to reside more particularly, if not entirely. In both there is a division specially appropriated to the locomotive apparatus, differing only in the multiplication of the centres in Insects, conformably with the arrangement of the members they supply; and sometimes consolidated to nearly the same degree. In both, also, we find a division appropriated to the respiratory apparatus, in which there is a corresponding multiplicity of cen- tres in the Articulata, in harmony with the universal distribution of their tracheal system. And in both, as Ave shall now see, there is a separate system of nerves, distributed to the alimentary apparatus, and supplying the organs of mastication (with the salivary glands), of deglutition, and of digestion. 150. Of the stomato-gastric system, some traces may be found in nearly all the Articulated classes. Thus, in the Leech, we find a minute ganglion existing at the base of each of the three teeth which form the mouth; these ganglia are connected together, and, to the cephalic, by slender filaments ; and they seem also to be in connection with other filaments, which may be traced on the alimentary canal. As a specimen of its highly-developed form, we shall describe that of the Gryllotalpa vulgaris (common Mole-Cricket). Here we find it consisting of two divisions ; one placed on the median line, which may hence be called the median system; the other running on each side at some little distance, and heflce called the lateral sys- tem. The Median system appears to originate in a small ganglion, situated anteriorly and inferiorly to the cephalic mass, with which it communicates by a connecting branch on each side. From this ganglion, nerves proceed to the walls of the buccal cavity, the mandibles, &c. Its principal trunk, however, (the re- current of authors,) is sent backwards beneath the pharynx. The ramifications of this are distributed along the resophageal tube and dorsal vessel ; whilst the trunk passes downwards to the stomach, where its branches inosculate with those supplied by the lateral system, and seem to assist in forming a pair of small ganglia, from which most of the visceral nerves radi- ate. The ganglia of the Lateral system are two on each side, lying behind and beneath the cephalic masses . The anter i r pair are the largest, and meet on Stomato-gastric system of GRYLLOTALPA VULGABIS; A A, cephalic ganglia; a, anterior g ang ii a . with which they communicate. Posteriorly to these COMPARATIVE ANATOMY AND PHYSIOLOGY. 115 lie the second pair, which are in connection with them. Two cords pass backwards on each side, one derived from the anterior, the other from the posterior, of these ganglia. They run along the sides of the oesophagus and dorsal vessel; and, after inosculating with the branches of the central system, enter the two coeliac ganglia, from which branches radiate to the abdominal viscera. 151. This system of ganglia and nerves has an evident affinity with the Sympathetic system of Vertebrata, as well as with some parts of the Cerebro- spinal system, more especially with the Par Vagum. It is to be remembered, that the Pneumogastric nerve of Vertebrata is distributed to three separate systems the respiratory, the circulating, and the digestive. As we know that the ultimate fibrils of nerves never anastomoze, there can be no doubt that these branches might be separately traced backwards into their ganglionic centres ; and they may thus be regarded as functionally three distinct nerves, though bound up in a single trunk. There is no difficulty, then, in understanding that the respiratory system of Insects and other Invertebrata may be analogous with the pneumonic portion of the Par Vagum ; although it bears no relation with the cardiac and gastric divisions of the nerve. To the latter divisions the analogy of the recurrent nerve becomes sufficiently plain, when we look at its distribution upon the dorsal vessel, oesophagus, and stomach ;* but its commencement in the anterior ganglion, which also supplies the mouth and pharynx, might seem to place it on a different footing, until we have deter- mined the true analogy of this last centre. It may be inferred from its situa- tion, and from the distribution of its nerves, that this anterior ganglion is analogous both to the labial and pharyngeal ganglia of the higher Mollusca. These appear to form a division of the nervous system, by which the actions immediately concerned in the prehension of food are performed ; and these seem almost as independent of the cephalic ganglia as are those of respira- tion. There is evidently, however, a greater tendency towards the union of these centres with the cesophageal collar than of those presiding over the respiratory function, which is more independent of the will. 152. The division of the nervous system of Vertebrata with which this Stomato-Gastric system corresponds, is a question of more apparent difficulty ; but if we bring into comparison not only the highest but the lowest forms of the cerebro-spinal apparatus, the chief difficulties will be removed. The ana- logies drawn from the distribution of the nervous branches would lead us to infer, that the third division of the Fifth pair (including its sensory and motor origins), the Glosso-Pharyngeal, and the gastric portion of the Par Vagum, would most nearly represent it. Now, when the fifth pair is traced back to its true origin, it is found to be not a cerebral but a spinal nerve ; and it is then seen to arise from the medulla oblongata, in such close approximation with the Par Vagum and Glosso-Pharyngeal, as to show, that if this portion of the ner- vous centres were isolated from the rest, the nerves which proceed from it would form, anatomically, as well as functionally, a natural group. The fifth pair, like other spinal nerves, may act in a simply reflex character; although, in Man, it is usually under the dominion of the will. In the lower animals we find these reflex actions bearing a much larger proportion to the voluntary than in Man ; and even in him we not unfrequently meet with cases in which the functions of the cerebral hemispheres seem suspended, whilst those of the spinal cord are unimpaired ; so that the prehension of food by the lips may take place without any effort of the will. This has been observed in anen- cephalous foetuses, in puppies from which the brain has been removed, and in profound apoplexy. Further, the connection between the Fifth pair and * See Newport, in Phil. Trans., 1832, p. 386. 116 FUNCTIONS OF THE NERVOUS SYSTEM. Par Vagum is very intimate in Fishes, the class which approaches nearest in the character of its nervous system to the Invertebrata. We may reasonably infer, then, that the anterior ganglion is the principal centre of the reflex actions of the nerves which correspond to the third branch of the fifth pair, to the glosso-pharyngeal, and to the gastric portion of the Par Vagum, in Vertebrata ; whilst the branches which connect them with the cephalic ganglia, bring these nerves more or less under the influence of the latter. 153. The lateral ganglia seem more analogous to the centres of the Sympa- thetic system in Vertebrata; especially in the connection of their branches with all the other systems of nerves; and in the share which they have in the formation of the coeliac ganglia. This view of the relative functions of these two divisions of the stomato-gastric system, is strengthened by the fact, that the connection between the Sympathetic system of Fishes and the Par Vagum is much more intimate than in the higher Vertebrata; although, even in the latter, as will be shown hereafter, it is by no means so slight as it appears.* 154. Upon taking a general review of the facts which have been stated, and of the inferences which have been erected upon them, we perceive that between the strictly sensorial functions of the nervous system and those ope- rations in which its internuncial character only is employed, a tolerably distinct line of demarkation may generally be drawn. We have hitherto viewed this apparatus under two aspects:!. As the instrument of the mind, by which it acquires a knowledge of the external world through the medium of sensation, and operates upon it by an exercise of volition. 2. As the means by which various movements are excited in the bodily structure, that are immediately necessary to the performance of the organic functions, and to its protection from injury; these motions take place in direct respondence to external im- pressions, without the intervention of the will, without any designed adapta- tion to purpose on the part of the animal, and often without its consciousness being necessarily affected . The first of these objects appears to be answered, chiefly if not entirely, by the cephalic ganglia and the nerves proceeding from them. The second is carried into effect by the ganglia connected with each organ, or series of organs, whose movements are thus excited. We have seen, that, however small is the bulk of the cephalic ganglia compared with the sum of the other masses, they send nerves to every part of the body supplied by the latter; for the purpose, it would seem, of controlling, harmonizing, or antagonizing their actions. These nerves proceed as connecting trunks from the cephalic ganglia to the other centres ; and then divide into filaments, which unite with those proceeding from them to the several organs. Each organ, therefore, receives four sets of fibres; an afferent and efferent set, which con- nect it with the cephalic ganglia, and are the channels of sensation, and of the influence of the will; and an afferent and efferent set, which connect it with its own peculiar ganglion, as well as with other similar ganglia ; and which serve to convey the stimulus of impressions that produce motions by reflected influence. In proportion as the special sensory organs are developed, and the actions of the animal are less completely of a simply reflex character, we find the cephalic ganglia and system of nerves more predominant. 155. We observe among the Articulata the greatest perfection of instinctive movements any where exhibited. In these movements there is a most remark- * The view given above of the comparative structure and offices of the Nervous Sys- tem in the Invertebrated animals, is chiefly abridged from the Author's Prize Thesis on this subject, in which additional details will be found, as well as many other illustrative figures and references to authorities. He has there, also, discussed the physiological explanation which had been previously given of the double nervous cord of the Arti- culata; and having shown that it is neither consistent with itself, nor capable of being applied to the other Invertebrata, he has deemed it unnecessary to complicate the pre- sent sketch by introducing it. COMPARATIVE ANATOMY AND PHYSIOLOGY. 117 able adaptation of means to ends; as in the construction of habitations by various Insects, and especially by the social Hymenoptera. But few persons will maintain that this adaptation is performed by the mind of the animal ; since, on this supposition, every Bee solves a problem which has afforded scope for the laborious inquiries of the acutest human mathematician.* The adaptation is in the original construction of a nervous system, which should occasion particular movements to be performed under particular external condi- tions ; and the constancy with which these are performed by different indi- viduals of the same species, when placed in the same conditions, leads at once to the belief, that they must be independent of any operations so variable as those of judgment and voluntary exertion. On the other hand, in the Verte- brata, we find the purely instinctive movements forming a smaller proportion of the whole actions, and brought under a more complete subjection to the sensori-volitional system. This is evident from the greater variety which the actions exhibit ; from the mode in which they are adapted to peculiar circum- stances; from the degree in which they may be modified by education; and from various other indications of a superior kind of Intelligence. At last, in Man, those instinctive movements which are not immediately requisite (like those of respiration) for the maintenance of the organic functions, are placed under the control of the will. This is especially true of the locomotive organs, whose reflex actions are entirely guided by the will ; being only distinguish- able, when, from peculiar states of the system, the immediate influence of the brain upon them is suspended. 156. There is a third aspect, however, under which we are to consider the Nervous System ; and this becomes more important in the highest division of the Animal kingdom, on which we are now about to dwell. We have hitherto spoken only of its influence on the contractile properties of the tissues, to which it is distributed. It has, however, an important and direct connection with the purely organic functions of Nutrition and Secretion ; and we shall see reason to regard it as the means, not only of placing the animal in relation with the external world, but of harmonizing and controlling the organic changes taking place in its own structure, and of bringing these under the influence of particular mental conditions. The opinion is entertained by many, that all the organic functions are dependent upon the innervation supplied to them by the system of nerves, which has been termed Sympathetic or vsiceral. It is incumbent, however, on those who uphold the necessity of this nervous power, * The hexagonal form of the cell is the one in which the greatest strength, and the nearest approach to the cylindrical cavity required for containing the larva, are attained, with the least expenditure of material. But the instinct which directs the Bees in the construction of the partition that forms the bottom or end of the cell, is of a nature still more wonderful than that which governs its general shape. The bottom of each cell rests upon three partitions of cells upon the opposite side of the comb; so that it is rendered much stronger than if it merely divided the cavities of two cells opposed to one another. The partition is not a single plane surface ; but is formed by the union of three rhomboidal planes, uniting in the centre of each cell. The angles formed by the sides of these rhombs, were determined by the measurements of Maraldi to be 109 28' and 70 32'; and these have been shown by mathematical calculation to be precisely the angles at which the greatest strength and capacity can be attained with the least expen- diture of wax. The solution of the problem was first attempted by Koenig, a pupil of the celebrated Bernoulli; and as his result proved to differ from the observed angle by only two minutes of a degree, it was presumed that the discrepancy was due to an error of observation, which it was easy to account for by the smallness of the surfaces whose inclination had to be measured. The question has been since taken up, however, by Lord Brougham, (Appendix to his Illustrated edition of Paley's Natural Theology), who has worked it out afresh, and has shown that, when certain small quantities, neglected by Krenig, are properly introduced into the calculation, the result is exactly accordant with observation, the Bees being thus proved to be right, and the Mathematician wrong. 118 FUNCTIONS OF THE NERVOUS SYSTEM. to prove it definitely ; since all analogy leads to an opposite conclusion. We may regard the capability of separating a particular' secretion from the blood, as a peculiar property inherent in the glandular membrane, just as contractility is the inherent property of muscular fibre. As the peculiar arrangement of the excitable and contractile tissues in Animals requires a nervous system to act as a conductor between them, and to blend their actions, so may the com- plicated organic functions of Animals require to be harmonized and kept in sympathy with each other, by some mode of communication more direct and certain than that afforded by the circulating system, which is their bond of union in Plants. We have seen, in the foregoing sketch, that the visceral system does not exist in a distinct form in the lower classes of Invertebrated animals ; and also that the nervous system of these classes cannot, as a whole, be compared with it, although it may be regarded as containing some rudi- ments of it. As the divisions of this system become more evident, however, and the organic functions more complicated, some appearance of a separate Sympathetic system presents itself; but this is never so distinct as in Verte- brata. Hence it may fairly be inferred that, as the Sympathetic system is not developed in proportion to the predominant activity of the functions of organic life (which is so remarkable in the Mollusca when contrasted with the Articulata), but in proportion to the development of the higher divisions of the nervous system, its office is not to contribute to these functions any thing essential to tljeir performance ; but rather to exercise that general control over them, which becomes the more necessary as they become more independent of one another; and to bring them into relation with the system of Animal life. VII. Nervous System of Vertebrata. 157. When we direct our attention to the Nervous system of the Verte- brated classes, we are immediately struck by two remarkable differences which its condition presents, from that under which we have seen it to exist in the Invertebrata. In the latter it has seemed but a mere appendage to the rest of the organism, a mechanism superadded for the purpose of bringing its various parts into more advantageous relation. On the other hand, in the Vertebrata the whole structure appears subservient to it, and designed but to carry its purposes into operation. Again, in the Invertebrata, we do not find any special adaptation of the organs of support, for the protection of the Nervous System. It is either enclosed, with the other soft parts of the body, in one general hard tegumentary envelop, as in the Echinodermata and Articulata, or it receives a still more imperfect protection, as in the Mollusca. In the latter, the naked species are destitute of any means of passive resistance, and the Nervous Sys- tem shares the general exposed condition of the whole body ; and it is not a little remarkable that, in the testaceous kinds, the portion of the body contain- ing the nervous centres should be protruded beyond the shell, whilst the prin- cipal viscera are retained within it. Now, in the Vertebrata, we find a special and complex bony apparatus, adapted in the most perfect manner for the pro- tection of the Nervous system ; and it is, in fact, the possession of a jointed spinal column, and of its cranial expansion, which best characterizes the group. 158. When we look more particularly at the Nervous Centres themselves, we perceive that they combine the general characters of those of the Articu- lata with those of the Mollusca. In the former, the power of active locomotion seems the chief object to be attained ; and the predominant part of the appa- ratus is evidently the series of ganglia connected with the locomotive organs'. The sensory ganglia appear subservient to these both in size and func- tion. On the other hand, in the Mollusca, the sensory ganglia predominate ; and under their function, which is to direct these walking stomachs to their NERVOUS SYSTEM OF VERTEBRATA. 119 food, the control of the locomotive apparatus seems to be placed. Now, in the Vertebrata we have the locomotive powers of the Articulata (comparatively reduced, however, in activity"), united with the complex nutritive system of the Molmsca ; and we find this combination manifested, not only in the organs themselves, but in the Nervous System, which stands in so close a relation with them. The Spinal Cord of Vertebrata is evidently the analogue of the ventral columns of Articulata. It is a continuous ganglion, containing two por- tions as distinct as the two tracts in the Articulata; a fibrous structure, which is continuous between the Brain and the spinal nerves, and thus resembles the white tract in Insects; and a ganglionic portion, principally composed of gray matter. Into this gray matter, as in the ventral ganglia of Insects, a part of the roots of the spinal nerves may be traced ; whilst others pass on continu- ously to the brain. At the upper extremity of the Spinal Cord (commonly termed the Medulla Oblongafa), we find the ganglia and nerves of special sensation ; and the organs which these supply are placed in immediate proxi- mity with the entrance to the alimentary canal, holding the same relation to it as in the Mollusca. But in addition to these, we find two ganglionic masses in all Vertebrata, to which we have no distinct analogue in the lower classes the Cerebral Hemispheres, and the Cerebellum. With the development of the former of these, the perfection of the reasoning powers appears to hold a close relation ; that of the latter seems connected with the necessity which exists for the adjustment and combination of the locomotive powers, when the variety of movements performed by the animal is great, and the harmony required among them is more perfect. Upon these points, however, we shall hereafter dwell. 159. The Visceral system of nerves now assumes a more distinct form. It does not share the protection of the Spinal column ; but its ganglia lie for the most part in the general cavity of the trunk. The connections of the Cerebro-Spinal and Sympathetic systems may be best studied in the trunk ; since the regularity of the distribution of the spinal nerves prevents the exist- ence of that doubt regarding the nature of the communication which obscures the relation of the cranial nerves with the sympathetic. The great visceral ganglia the cardiac and semilunar, may probably be regarded as the true centres of the Nervous system of Organic life ; whilst the chain of ganglia, which lie along the spine, are intermediate between these and the cerebro- spinal system. When the filaments connecting these ganglia with the roots of the spinal nerves are closely examined, they are found to contain both kinds of fibres ; and they can no more, therefore, be regarded as the roots by which the sympathetic system arises from the cerebro-spinal (as they were formerly considered), than as the roots by which the latter originates from the former. The white tubular fibres which these filaments contain, are found, upon close examination, to be derived from both roots of the spinal nerves, and not from the posterior only, as some have supposed. The quantity of white fibres in the nerves proceeding from this lateral chain of ganglia, is much greater than that contained in the nerves of the solar plexus ; and it is confirmatory of the idea just stated (that the visceral ganglia are the true centres of the sympa- thetic system), to find that the nerves proceeding from them are almost entirely composed of the fibres characteristic of this system ( 110). A few visceral fibres may be found in almost all the Cerebro-spinal nerves; they are particu- larly abundant, however, in the first division of the Fifth pair. It would seem that only a part of these are derived immediately from the sympathetic nerve ; and that the remainder may be traced into the gray matter of the ganglia, formed on the posterior roots of the spinal nerves, and on the larger root of the fifth pair. As this gray matter consists of nucleated globules, like those which are found in the centres of the Sympathetic system, it may be surmised that 120 FUNCTIONS OF THE NERVOUS SYSTEM. Fig. 15. this series of ganglia also may be regarded as belonging to that sys- tem ; and as bearing the same re- lation to the gray fibres ' contained in the cerebro-spinal nerves, as that which the semilunar ganglia have to the nerves of the solar plexus, in other words, acting as their centres of power. 160. The branches proceeding from the Semilunar ganglia are distributed upon the abdominal viscera ; and those of the Cardiac ganglia upon the heart and the vessels proceeding from it. The latter seem to accompany the arte- rial trunks through their whole course, ramifying minutely upon their surface ; and can scarcely be doubted, that they exercise an im- portant influence over their func- tions. What the nature of that influence is, however, will be a subject for future inquiry. It is so evidently connected with the operations of nutrition, secretion, &c., that the designation of "nerv- ous system of organic life," as ap- plied to this system, does not seem objectionable, provided that we do not understand it as denoting the dependence of these functions upon it. Even in Vertebrata, however, we do not always find the visceral system distinct from the cerebro- spinal. In the Cyclostome Fishes, the par vagum supplies the intes- tinal canal along its whole length, as well as the heart ; and no ap- pearance of a distinct sympathetic can be discovered. In Serpents, again, the lower part of the ali- mentary canal is supplied from the Spinal Cord, and the upper A view of the Great Sympathetic Nerve. 1, the plexus on the carotid artery in the carotid foramen; 2, sixth nerve (motor externus) ; 3. first branch of the fifth or ophthalmic nerve; 4. a branch on the sep- lum narium going to the incisive foramen; 5, the recurrent branch or vidian nerve dividing into the caro- tid and petrosal branches; 6, posterior palatine branches: 7, the lingual nerve joined by the corda tympani; e. the portio dura of the seventh pair or the facial nerve; 9. the superior cervical ganglion; 10. the mid- dle cervical ganglion; 11, the inferior cervical ganglion; 12, the roots of the great splanchnic nerve arising from the dorsal ganglia; 13, the lesser splanchnic nerve; 14, the renal plexus; 15, the solar plexus; 16, the mesenteric plexus; 17, the lumbar ganglia: 18, the sacral ganglia; 19, the vesical plexus; 20. the rectal plexus; 21, the lumbar plexus (cerebro-spinal) ; 22, the rectum: 23. the bladder; 24, ihe pubis ; 2o. : the csest of the ileum; 26, the kidney; 27, the aorta; 23, the diaphragm; 29, the heart; 30, the larynx; 31 the sub-maxillary gland; 32, the incisor teeth; 33, nasal septum; 34, globe of the eye ; 35, 36, cavity of the cranium. 24 NERVOUS SYSTEM OF VERTEBRATA. 121 part by the par vagum ; and though the lateral cords of the sympathetic may be traced, they are almost destitute of ganglia. Even in the highest Verte- brata, some of the glands, of which the secretion is most directly influenced by the condition of the mind, are supplied with most of their nerves from the cerebro-spinal system ; thus, the lachrymal and sublingual glands receive large branches from the fifth pair, and the mammary glands from the intercostal nerves. It may therefore be regarded as not improbable, that the organic fibres contained in these nerves, and principally derived from the ganglia at their roots, are the most direct channels through which the processes of nutri- tion and secretion are influenced by mental emotions ; and that the office of the distinct visceral system, is rather to bring these functional changes into har- mony with each other than to supply any condition necessary for the separate operations themselves. 161. The Spinal Cord, with its encephalic continuation the Medulla Oblongata, may be regarded as constituting the essential part of the nervous system of Vertebrata. Although the Cerebral Hemispheres in Man bear so large a proportion to it in size, that the Spinal Cord seems but a mere append- age to them, the case is reversed when we look at the other extremity of the scale ; the Cerebral Hemispheres jn many Fishes being but ganglionic pro- tuberances from the Medulla Oblongata. Moreover, the fact that animals are capable of living without the brain, whilst they at once die if deprived of the spinal cord, sufficiently demonstrates this. The spinal cord, then, when viewed in relation to the nervous system of the Invertebrata, may be regarded as including their respiratory, stomato-gastric, and pedal ganglia. That these should be associated together, can scarcely be considered remarkable. It is obviously convenient that they should all be enclosed in the bony sheath provided for their protection; and their closer relation favours that sympathy of action, which is so important in animals of such complex structure and mutually dependent functions as the higher Vertebrata. An animal, either congenitally or experimentally deprived of its cerebral hemispheres, is very much in the condition of one of the Acephalous Mollusca. It can perform those respiratory movements on which depend the maintenance of its circula- tion, and consequently its whole organic life ; it can swallow food brought within its reach, and it can, in some degree, exert its locomotive powers to obtain it ; but it is unconscious of the direction in which these can be best employed, and is dependent upon the supplies of food that come within its grasp. The Acephalous Mollusca are so organized, that they find support from the particles brought in by their respiratory current ; but the more highly- organized Vertebrata are not capable of so existing, and they must have the*ir food provided for them by an exertion of the mental powers. So long as an encephalous Vertebrate animal is duly supplied with its requisite food, so long may it continue to exist ; and thus it is seen that the operations of the brain are rather connected with the intelligence than with the blind, undesigning instinct of the animal. 162. It is only in the Vertebrata, that the difference between the afferent and efferent fibres of the nerves has been satisfactorily determined. The merit of this, discovery is almost entirely due to Sir C. Bell. He was led to it by a chain of reasoning of a highly philosophical character; and though his first experiments on the spinal nerves were not satisfactory, he virtually determined the respective functions of their two roots, by experiments and pathological observations upon the cranial nerves, before any other physio- logist came into the field.* Subsequently his general views were confirmed by the very decided experiments of Mtiller; but, until very recently, some ob- * See British and Foreign Medical Review, vol. ix. p. 140, &c. 122 FUNCTIONS OF THE NERVOUS SYSTEM. scurity hung 1 over a portion of the phenomena. It was from the first maintained by Magendie, and has been subsequently asserted by other physiologists, that Jhe anterior and posterior roots of the nerves were both concerned in the "reception of sensations and in the production of motions; for that, when the anterior roots were touched, the animal gave signs of pain, at the same time that convulsive movements were performed; and that, on touching the poste- rior roots, not only the sensibility of the animal seemed to be affected, but mus- cular motions were excited. These physiologists were not willing, therefore, to admit more than that the anterior roots were especially motor, and the pos- terior especially sensory. But the recently attained knowledge of the reflex function of the spinal cord, enables the latter portion of these phenomena to be easily explained. The motions excited by irritating the posterior root are entirely dependent upon its connection with the spinal cord, arid upon the integrity of the anterior roots and of the trunks into which they enter ; whilst they are not checked by the separation of the posterior roots from the peri- pheral portion of the trunk. It is evident, therefore, that excitation of the posterior root does not act immediately upon the muscles through the trunk of the nerve, which they contribute to form, but that it excites a motor impulse in the Spinal Cord, which is propagated through the anterior roots to the peri- phery of the system. The converse phenomenon, the apparent sensibility of the anterior roots, has been still more recently explained by the experiments of Dr. Kronenberg ;* who has satisfactorily proved that it is dependent upon a branch of the posterior root passing into the anterior root at their point of inosculation, and then directing itself towards the cord ( 124). 163. On the other hand, the distinctness of the system of nerves concerned in the simply reflex actions, from those which minister to sensation and voli- tion by their connection with the brain, is by no means so obvious as in the Invertebrated classes. When first pointed out by Dr. Marshall Hall, who had grounded his opinion more upon physiological phenomena than upon anato- mical facts, the statement did not command general assent ; since, while the phenomena were admitted, the inferences which he drew from them by him were not regarded as necessary results. When, however, the anatomy of the nervous centres in Vertebrata was more closely inquired into (by Mr. Grain- ger, who had been partly anticipated by Bellingeri), it was found to present certain phenomena which might be regarded as supporting Dr. M. Hall's views ; and when the inquiry was extended to the Invertebrated classes, the confirmation was found to be still more decisive. In our previous sketch these doctrines have been treated as established ; since they have been found not only to correspond with the facts disclosed by anatomical research, but to be required by them. We shall now apply them to the nervous apparatus of the Vertebrata. 164. The Spinal Cord consists of two lateral halves; these are partially separated in the higher classes, by the superficial anterior and posterior fis- sures ; and in Fishes by an internal canal, which is continuous with the fourth ventricle.! This canal is evidently the indication of that complete separation of the two columns, which exists in the lower Articulata; and the fourth ven- tricle, which in many fishes remains unclosed (the cerebellum ngt being suffi- ciently developed to overlap it,) corresponds with the passage between the cords uniting the cephalic ganglia with the first subcesophageal, through which the oesophagus passed in all the Invertebrata. The two lateral halves have little connection with each other in Fishes, and the pyramidal bodies at their * Mailer's Archiv. 1839, Heft v.; and Brit, and For. Med. Rev., vol. ix. p. 547. 1 his canal may be traced in the Spinal Cord of Man and other Mammalia; but it is nearly obliterated. NERVOUS SYSTEM OF VERTEBRATA. 123 1 B apex scarcely decussate ; but, in ascending towards the higher classes, the communication between the two sides is more intimate, and a larger proportion of the pyramidal fibres crosses to the opposite sides. In all the Vertebrata, the true Spinal Cord contains gray substance, or something equivalent to it; thus possessing the character of a continuous ganglion. The proportion of the vertebral column which this ganglionic portion occupies, is, however, extremely variable ; depending principally on the position of the chief organs of locomotion. Thus, in the Eel, and other Vermiform Fishes, it is continued through the whole spinal canal; whilst in the Lophius and Tetraodon, whose body is less prolonged, and more dependent for its movements upon the anterior extre- mities, the true Spinal Cord scarcely passes out of the cranium. The quantity of gray matter is nearly uniform in every part of the cord, where there is no great diversity in the functions of the nerves which originate from each portion. In most Fishes, for example, the body is propelled through the water more by the lateral action of the flattened trunk (whose surface is extended by the dorsal and caudal fins erected upon prolongations of its vertebras), than by the movements of its extremities, which serve prin- cipally to guide it. Hence we usually find the amount of gray matter varying but little in different parts of the cord. But in the Flying-fish, and others whose pectoral fins are unusually powerful, a distinct ganglionic enlargement of the cord takes place where the nerves are given off. In Serpents, again, the spinal cord is nearly uniform throughout its entire length ; whilst in Amphibia it is so during the Tadpole condition, but presents enlargements corresponding to the anterior and poste- rior extremities, when these are developed ; at the same time becoming much shortened, as the tail is less important to locomotion, or is altogether atrophied. In Birds, the ganglionic en- Nervous Centres in Frog; A, olfactive ganglia; B, cere- bral hemispheres ; c, optic ganglia; D, cerebellum, so small as not to cover the 4th ventricle, or cavity left by the of the columns of largements are generally very perceptible; and bear a close relation in size, with the de- velopment of the locomotive organs with which they are connected. Thus, in Birds of active flight, and short power- less legs,, the anterior enlarge- ment is the principal ; but in those which are more adapted to run on land than to wing their way through the air, such as the whole tribe of Struthi- ous birds, the size of the pos- terior enlargement is very re- markable. Hence we have a right to infer, that the in- crease in the quantity of gray matter in the cord has some connection with the amount of Fig. 17. Transverse sections of human Spinal Cord at different po ; nts. showing proportional quantity and arrangement of gray and white mailer at each; after Solly: 1, opposite 11 ih dorsal vertebra; 2, opposite 10th dorsal; 3, opposite Sth dor- sal; 4, opposite 5th dorsal; 5, opposite 7th cervical; G, oppo- site 4th cervical ; 7. opposite 3d cervical ; 8, section of medulla pbloiigata through centre of corpus olivare. 124 FUNCTIONS OF THE NERVOUS SYSTEM. power to be supplied ; and this exactly corresponds with what has been observed in the Articulated classes, and especially in watching tlje metamor- phosis of Insects. In Birds and Mammalia, however, the whole amount of the gray matter in the spinal cord does not bear so large a proportion to the bulk of the nerves proceeding from it, as in the lower Vertebrata ; and the reason of this seems obvious. The actions of the locomotive organs are less and less of a reflex character, and are more directly excited by the will, and conse- quently by the brain, than in the inferior tribes; and just in proportion, there- fore, to the development of the Brain, will it become the centre of all the actions performed by the animal, and the Spinal Cord a mere appendage to it. Still, in all the Mammalia, even in Man, do we find these ganglionic enlargements of the spinal cord ; and in Man it is the posterior (or rather the inferior) one which contains the largest quantity of gray matter. In the cord of this class, too, the lateral halves are much more intimately united than in the classes below ; for not only is the central canal for the most part absent, but the two crescent-shaped plates of gray matter are united by a transverse lamella, which connects their centres like a commissure. 165. The Cord is traversed, not only by the anterior and posterior fissures, but by two furrows on each side, marking out three columns upon it. We have, therefore, on each half of the cord, an anterior, middle or lateral, and posterior column. The points of the crescentic lamellce of gray matter ap- proach these furrows pretty closely ; but elsewhere the gray matter is covered deeply by the fibrous columns. Each spinal nerve arises from two sets of roots. The anterior roots join the spinal Fig. IS. cord near the anterior furrow; and the posterior near the posterior furrow. Respecting their intimate connection with the principal divisions of the cord, a considerable diversity has existed among the statements of anatomists ; but it seems to be now satisfactorily ascertained, that, as in the Articulata, A transverse section of the Spinal Marrow.- a part Q f each rQot enters the mat . 1,1. the two halves of the spinal marrow; 2 the / r i anterior middle fissure; 3, the posterior middle I- ^.P 1 S^gllOHW portion of the COrd, sure; 4, the position of the cineritious matter to each whl / st a P^rt IS COlltmUOUS With its half of the spinal marrow; 5, the origin of orfe of Whit* OY ftbrOUS Columns. The COUrSC the anterior roots of a spinal nerve ; 6, the origin of of tile fibres which enter the gray one of the posterior roots. matter, has been lately displayed, in part, at least, by the researches of Dr. Stilling.* It appears that of the fibres of the posterior roots, some form loops in the gray matter, and become continuous with those of the anterior roots of the same side, as seen at A, fig. 19. Others cross the gray matter, and become continuous with those of the anterior roots of the opposite side, as seen at B. It can scarcely be doubted that these fibres, being unconnected with the brain, constitute the system to which reflex actions are due. Although Dr. Stillinp-'s inquiries have not proved the fact,t yet it may be inferred from physiological phenomena, as well as from the facts recently shown by Mr. Newport ( 143), that there are other fibres, which pass from the posterior roots into the anterior roots of other nerves above and below, both on the same and on the opposite >f the portions of the roots which are continuous with the -fibrous columns, it is stated by Sir C. Bell that the anterior fasciculi pass to the an- * Ueber die Textur und Function der Medulla Oblongata. - It may be thought that the mode of examination which he adopted, that of making very thin transverse sections of the Spinal Cord,-is not well fitted to display the con- nections of the roots with longitudinal fibres. NERVOUS SYSTEM OF VERTEBRATA. Structure of the Spinal Cord, ac- cordingto Stilling; A, posterior fibres continuous with the anterior of the same side, through the nucleus of the cord; B, posterior fibres continu- ous with the anterior of the opposite side. terior columns only, and that the posterior are Fig. 19. restricted to the lateral columns. On the other hand, Mr. Grainger and Mr. Swan maintain that both sets are connected with the lateral cclumns only ; the anterior and posterior lateral fissures definitely limiting the two roots. Perhaps both these statements are rather too exclusive. The anterior roots would seem to have a connection with both the anterior and lateral columns ; and the posterior cannot be said to be restricted to the lateral column, some of their fibres entering the posterior division of the cord. 166. As the white or fibrous portion of the Spinal Cord Jkcontinuous with the medullary matter of theJBln,the roots of the nerves which enter it are "flfcity thus brought into connec- tion with the Cerebral Hemispheres and Cere- bellum ; and the posterior division of these may, therefore, be regarded as conducting to the brain those impressions which there become sensa- tions ; whilst the anterior roots convey the motive influence, which has been propagated, by a volun- tary or emotional impulse, down the tract of the Spinal Cord with which they are continuous. On the other hand, the passage of one portion of each set of roots through the gray matter of the Cord, completes the nervous circle re- quired for the performance of reflex actions ; and by this they would seem to take place in Vertebrated animals, just as through the distinct system of excito-motor fibres in the Articulata ( 143). The fibres which pass continu- ously from the posterior to the anterior roots of the nerves on the same side, probably constitute the channel of those reflex actions, which can be excited in a part supplied by any compound nerve, by stimulating its afferent fibres, and thus causing a motor impulse to be transmitted from the Spinal Cord along its efferent portion. The fibres which cross to the opposite side, will produce similar movements in its corresponding parts. And the fibres, if such there be, that pass from the posterior (afferent) roots of each nerve, into the anterior (motor) roots of distant nerves, would convey to a great variety of muscles, the influence of a stimulus applied to a single afferent nerve. It follows, then, on this view of the character of the Spinal Cord, that the continuity of the fibrous tracts is all that is required to convey the influence of the brain to the parts below ; whilst the completeness of the nervous circle is all that is necessary for the performance of reflex actions excited through it. This is found to be strictly true ; the former having been observed in cases of disease, and the latter having been proved by experiment. As far as simple reflex actions are concerned, there is as much segmental independence in Vertebrata, as in the Articulata ; but these actions seldom have so completely the character of adaptation, and are of a more irregular and convulsive nature. Still, however, there is an essential correspondence between them ; and we may, therefore, regard the distinction between the reflex and voluntary movements as tte same in each group ; . the former predominating in Articulata, the latter in Verte- brata. On this view, then, each spinal nerve contains at least four sets of fibres. I. A sensory bundle passing upwards to the brain. 11* 126 FUNCTIONS OF THE NERVOUS SYSTEM. II. A motor set, conveying the influence of volition and emotion downwards from the brain. III. A set of excitor or centripetal fibres, terminating in the true spinal cord or ganglion, and conveying impressions to it. I V. A motor or centrifugal set, arising from the same ganglionic centre, and conveying the motor impulse reflected/rom it to the muscles. Of these, the first and third are united in the posterior or afferent roots ; the second and fourth in the anterior or efferent roots. 167. It is difficult to trace the course of the fibres within the Spinal Cord; and some recent experiments by Valentin, appear to prove, that Sir C. Bell was not altogether correct in his idea that the functions of the columns of the cord are respectively similar to those of the roots connected with them. Cases, indeed, are of no unfrequent occurrence, in which a portion of one of the columns has been almost entirely destroyed by injury ojjjlsease, without any corresponding loss of the function attributed to it. Such cases have kept alive, in the minds of many eminent practical men, a considerable distrust of the accuracy of Sir C. Bell's conclusions. We have seen that, in regard to the roots of the nerves, his first statements have been confirmed, and rendered more precise, by subsequent researches ; but it is not so in regard to the func- tions of the anterior and posterior divisions of the Spinal Cord. Bellingeri was led, by experiments on the spinal cord, to the conclusion, that the anterior roots of the nerves were for the flexion of the various articulations, and the posterior for their extension. He also was wrong, in extending an inference, founded on experiments on the Cord, to the roots of the nerves. The recent experiments of Valentin, whilst they fully confirm Sir C. Bell's determination of the functions of the roots of the nerves, coincide, to no small degree, w r ith Bellingeri's opinion in regard to the offices of the anterior and posterior divi- sions of the Cord. He obtained reason to believe that, in the Frog, neither the superior nor inferior strand of the cord (posterior and anterior columns in Man) solely possess motor functions ; but he found that, when the former were irritated, sensations predominated ; and when the latter, motions were chiefly excited. He further states that, if the superior strand (posterior column) be irritated at the point at which the nerves of either extremity are given off, that extremity is extended; and that if the inferior strand (anterior column) be irritated, the extremity is flexed. At their entrance into the spinal cord, there- fore, it would appear that the rrjotor fibres of the extensors pass towards the superior stratum (posterior column in Man), whilst those of the flexors are continuous with the inferior stratum (anterior column) ; their course being more altered, however, when they are examined far from the point of issue. This doctrine was confirmed by experiments on Mammalia ; and is borne out (according to Valentin) by pathological phenomena observed in Man. Accord- ing to this eminent physiologist, also, relaxation of the sphincters is analogous to the extended state of the extremities ; and he has noticed a manifest relaxa- tion of the sphincter ani in the frog, when the superior part of the spinal cord was irritated so as to produce extension of the limbs. These statements are entitled to considerable weight, on account of the quarter from which they come ; but they are not, perhaps, to be received altogether without hesitation, until confirmed by other physiologists, especially whilst the phenomena of reflex action are still so imperfectly known. For it is quite possible that, whilst stimulation of the anterior part of the cord may excite direct motions of flexion, in preference to those of extension, the movements of extension pro- duced by stimulating the posterior column may be of a reflex character. 168. There is no reason to believe, that the functions of the Spinal Cord are essentially different along its whole length. Everywhere it appears to consist of a ganglionic centre, supplying nerves to its particular segment ; and NERVOUS SYSTEM OF VERTEBRATA. 127 of con. ibres, by which the nerves proceeding from anyone division are brought into relation with distant portions of the organ, and with the large ganglionic masses at its anterior extremity. In this respect, then, it corre- sponds precisely with the doable nervous cord of the Articulata; the only prominent difference between the two being, that in the former the ganglionic matter is continuous from one extremity of the organ to the other, whilst in the latter it is interrupted at intervals ; and in the Mollusca, the centres are still further separated from each other. The connection of the Spinal Cord with the large ganglia contained within the cavity of the cranium, is effected by means of processes from its superior extremity, the arrangement of which is somewhat complex. This portion of the cord, which also lies within the cavity of the cranium, has been termed the Medulla Oblongaia. It has Fig. 20. Fig. 21. A posterior superior view of the Pons Varolii, the Cerebellum, and the Medulla Oblongata and Spinalis. 1, 1, the crura cerebri ; 2, the pens varolii or tuber-aii- nularis; 3, its middle fossa; 4, an oblique band of me- dullary matter seen passing from its side ; 5, the external surface of the cms cerebelli in its natural state ; G, the same portion deprived of outer layer; 7, the "nervous matter which united it to 4; 8, the trigeminus or fifth pair of nerves; 9, portion of the auditory nerve the white neurine is seen passing from the oblique band which comes from the corpus .restiforme to the trige- minus nerve in front, and the auditory nerve behind ; 10, 11, the superior portion of the hemispheres of the cerebellum; 12, lobulus amygdaloides ; 13, corpus oli- vare; 14. corpus pyramidale ; 15, medulla spinalis. A posterior view of the Medulla Ob- lonsrata, as split open vertically on the middle line At the bottom of the fis- sure is a succession of fasciculi which interlock and cross from right to left. The cerebellum has also been cut off from its crura with great care, so as to show plainly the three principal ele- ments in its composition. 1. the thalami nervi optici slightly separated; 2, the corpora geniculata; 3, the tubercula quadrigemina; 4, the pineal gland di- vided in the middle; 5, the aqueduct of Sylvius laid open; 6, the fasciculated portion of the crura cerebelli; 7, 8, 9, the internal, middle and external fasciculi of the crura cerebelli ; 10, rootoWne au- ditory nerve ; 11, corpus restiforme ; 12, posterior portion of the corpus pyra- midale ; 13, posterior middle fissure of the spinal marrow; 14, point of the cala- mus scriptorius; 15. 15, between th6e figures is seen the interlocking of the two halves of the me"dulla oblongata. 128 FUNCTIONS OF THE NERVOUS SYSTEM. Fig. 22. the anterior columns of the spinal marrow ; 7, the lateral columns. been supposed to be the peculiar seat of vitality ; but the only real foundation of this idea is, that it is the great centre of the Respiratory actions, on the con- tinuance of which all the other functions are depend- ent. The Brain may be removed from above, and nearly the whole Spinal Cord from below, without an immediate check being put upon all the phenomena of life. In this Medulla Oblongata,/owr different parts maybe distinguished on each side ; 1, The Anterior Pyramids, or Corpora Pyramidalia ; 2, The Olivary bodies, or Corpora Olivaria ; 3, The Restiform bodies, or Corpora Restiformia, otherwise called Processus a Cerebello ad Medullam Oblongatam; 4, The Pos- terior Pyramids, or Corpora Pyramidalia Posteriora. The connections of these with the Brain above, and with the Spinal Cord below, will be now traced.* An anterior view of the Me- 169. As our object, however, is rather Physiolo- duiia Obiongata and of the ter- -^ t j ian p ure }y Anatomical, we shall commence with a description of the motor and sensory tracts, which Mitischelh. 1. the pons va- r J . ' roiii; 2, the emminentia oiiva- may, according to Sir C. Bell,t be very distinctly sepa- ria; 3, the corpus pyramkiaie ; rated in the Pons Varolii. The Pons has been correctly 4, the corpus restiforme ; 5, the designated as the great Commissure of the Cerebel- decussation of Mitischeiii; 6, j um? enclosing the Crura Cerebri ; and its transverse fibres not only surround the longitudinal bands which connect the Cerebrum with the Spinal Cord, but pass through them, so as in some degree to isolate the two lateral halves from one another, and to form a complete septum between the anterior and posterior portions of each. The Motor tract is brought into view, by simply raising the superficial layer of the Pons, and tracing upwards and downwards the longitudinal fibres which then present themselves. It is then found, that these fibres may be traced upwards, chiefly into the Corpora Striata, whence they radiate to the Hemispheres ; and downwards, chiefly into the An- terior Pyramids. From this tract arise all the Motor nerves usually reckoned as Cerebral ; as will be seen in the accompanying Figure. The Sensory tract is displayed, by opening the Medulla Obiongata on its posterior aspect ; and then separating and turning aside the Restiform Columns, so as to bring into view the Posterior Pyramids which lie on the outside of the calamus scriptorius. On tracing their fibres upwards, it is found that they form a part of the posterior layer of the Crura Cerebri, ultimately passing on to the Thalami nervorum opticorum, whence they radiate to the Hemispheres. From this tract, no motor nerves arise ; but on tracing it downwards into the Spinal Cord, it is found that the sensory root of the fifth pair terminates in it, and that the posterior roots of the spinal nerves are evidently connected with its * Great diversities will b,e found in the accounts given of these connections by differ- ent authors ; some of which are attributable to a variation in the use of terms, which must not pass unnoticed. By the majority of Anatomists, the name of Corpora Reslifor- mia is given to the Cere.bellar Columns; and this designation, therefore, it seems advisa- ble to retain. Some, however, and amongst them Dr. J. Reid, in his late very excellent desertion of the Anatomy of the Medulla Obiongata, (Edinb. Med. & Surg. Journal, Jan. 1841,) give the name to the columns that pass up from the posterior division of the spinal cord into the crus cerebri, which are here called (after Sir C. Bell) the posterior pyramids; and apply the term Posterior Pyramids to the Cerebellar column. The truth is that, as Sir C. Bell has justly observed, all the tracts of fibrous matter connecting the Brain with the Spinal Cord have a somewhat pyramidal form; and it might be added that all have something of a restiform or cord-like aspect. j- Philosophical Transactions, 1835. NERVOUS SYSTEM OF VERTEBRATA. Fig. 23. 129 Course of the Motor tract after Sir C. Bell. A, A, fibres of the hemispheres, converging to form the ante- rior portion of the cms cerebri; B, the same tract where passing the eras cerebri; c, the right pyramidal body, a little above the point of decussation; D, the remaining part of thepons varolii. a portion having been dissected off to expose B. 1, olfactory nerve, in outline ; 2, union of optic nerves; 3, 3, motor oculi ; 4, 4, patheticus ; 5, 5, trigeminus ; 6, 6, its muscular division ; 7, 7, its sensory root ; 8, origin of sensory root from the posterior part of the medulla oblongata ; 9, abducens oculi; 10, auditory nerve; 11, facial nerve; 12, eighth pair; 13, hypoglossal ; 14, spinal nerves; 1.5, spinal accessory of right side, separated from par vagum and glosso-pharyngeal. continuation. Also forming part of the posterior division of the crus cerebri, and separated from the anterior by the transverse septum, is a layer of fibres which ascends from the Olivary bodies, some of which terminate in the Cor- pora Quadrigemina. 130 FUNCTIONS OF THE NERVOUS SYSTEM. Fig. 24. Course of the Sensory tract after Sir C. Bell. A, pons varolii ; B. B, sensory tract separated ; c, union and decussaiion of posterior columns ; D, D, posterior roots of spinal nerves ; E, sensory roots of fifth pair. 170. On tracing upwards the four divisions of the Medulla Oblongata, the following are found to be their chief connections with the brain. The fibres of the Anterior Pyramids for the most part enter the Crura Cerebri, passing through the Pons Varolii, and traversing, the Optic Thalami (which, it must be carefully borne in mind by the student, have no real connection with the Optic Nerves or with the sense of sight) ; after which they diverge and be- come intermingled with gray matter, thus forming the Corpora Striata, and finally radiate to the convolutions of the Cerebrum. The fibres of the Olivary body also pass into the Pons Varolii, and there divide into tw r o bands, of which one proceeds upwards and forwards to join the Crus Cerebri, whilst the other passes upwards and backwards into the Corpora Gluadrigemina. Of the true Restifdrm bodies, the fibres pass entirely into the Cerebellum. Finally, of the Posterior Pyramids, the fibres pass directly onwards through the Crura Cerebri into the Thalami, whence they radiate to the convolutions. 171. The downward course of these fibres into the Spinal Cord now re- mains to be traced; and their arrangement is by no means a simple one. The anterior Pyramids decussate, as is well known, at their lower extremity ; the principal part (but not the whole) of the fibres on each side, passing over to the other. The decussating fibres pass backwards as well as downwards, and enter, not the anterior column of the spinal cord (as commonly stated), but the middle column.* The smaller bundle of fibres, which do not decussate, passes downwards, along with those of the olivary bodies, to form the anterior column. The fibres descending from the Olivary bodies converge, as those of the pyramids pass backwards from between them, until they meet on the median line, forming the greater part of the anterior column. The fibres of * See Dr. J. Reid, in Edinb. Med. and Surg. Journal, Jan., 1841. NERVOUS SYSTEM OF VERTEBRATA. Fig. 25. Fig. 26. 131 A lateral view of the Spinal Marrow. &c.. of a new-born infant, to show the lateral fascicu- lus, which is then more apparent. 1, crura cerebri ; 2, 3, 4, one of the hemispheres of the cerebellum; 5, external fasciculus of the crus cerebelli; 6, lobulus amygdaloicles and nervi pneumogastrici : 7, point where the lateral column of the spinal marrow enters the cere- bellum; 8, pons varolii; 9, 10, 11, continuation of 7, or of the lateral fasciculus all the way down the spinal marrow in the new-born in- fant it is very nearly white, whilst the matter around is of a light gray ; 12. emminentia oli- varia; 13. corpora pyramidal: a; 14. corpus res- ti forme. A posterior view of the medulla spinalis. with tin- fasciculi of the corpora restiformia cut off from each side of the calamus scriptorius. From the top of this section as far as the lumbar portion of the medulla spin- alis these posterior fasciculi have been dissected out down to the axis of the medulla. 1, the pineal gland ; 2, the tubercula quadrigemina: 3, origin of the 4th pair of nerves ; 4, the valve of the vieussens turned up a little; 5. posterior portion of the crus cerebri ; 6. section of the crus cerebelli; 7, anterior portion of the crus cerebri; 8, section of the corpus restiforme on one side ; 9, the corpus restiforme untouched on the other side ; 10. a prominent lateral fasciculus on the floor of the calamus scriptorius; 11. point of the calamus from its point to the end of the medulla spinalis are seen the junctions of the fasciculi of each side, which make the axis of the medulla spinalis ; 12, the lateral fasciculus ; 13, the en- largement for the axillary nerves; 14, for the lumbar nerves. 132 FUNCTIONS OF THE NERVOUS SYSTEM. the Posterior Pyramids are stated by Sir C. Bell to decussate^ like those of the anterior ; they pass down chiefly into the posterior part of the middle column, forming part also of the posterior. The fibres of the Restiform, or Cerebellar columns, which, like those of the Olivary columns, do not decussate, mostly pass downwards into the posterior columns ; but a band, (which has been termed, from its curved aspect, the arciform layer) passes forwards into the anterior columns. 172. The following tabular view may assist, better than any delineations could do, in the comprehension of this very intricate piece of Anatomy ; the knowledge of which can be already applied to the explanation o/ many curious pathological phenomena, and cannot but assist in. the elucidation of others whose rationale is as yet obscure. SPIXAL CORD. MEDULLA OBLOXGATA. BIIAIX. I Arciform fibres of Cerebellar Columns ? Cerebellum Anterior Column < Olivary Columns - ... 5 Corpora Quadrigemina f Non-decussating portion of Ant. Pyramids 7 p, > Decussating portion of Ant. Pyramids J Cor P ora Stnata Middle Column Post. Pyramidal Columns (decussating) t 5-> S Portion of Post. Pyramids (non-decus- > Thalami Optici sating?) ) Restiform Columns Cerebellum VIII. Functions of the Spinal Cord. 173. The functions of the Nervous System in Vertebrated Animals are so complex in their nature, and our means of analyzing them are so imperfect, that the inquiry is confessedly one of the greatest difficulty, and needs all the light which can be thrown upon it from any source. The great accession to our knowledge of them, which has been made within the last few years, chiefly by the labours of Sir C. Bell and Dr. M. Hall, has so far changed the aspect of this department of Physiological Science, as to render it necessary for those who had previously studied it, to begin de novo. This is especially the case in regard to the actions dependent on the Spinal Cord ; which it seems desirable to consider in the first instance, in order that it may be clearly defined what the Brain does not do. By many, even in recent times, the Spinal Cord has been considered as a mere appendage to the Brain ; but the phenomena of its independent action render such an idea quite inadmissible. These phenomena have been especially pointed out by Dr. M. Hall; and it is mainly owing to his arguments, that Physiologists are now for the most part agreed in the general fact, that the Spinal Cord constitutes a distinct centre, or rather a collection of centres, of nervous influence, and that its ope- rations are carried on through the nervous trunks with which it is connected. It is further generally admitted that its functions are independent of the will ; and that they are in effect frequently opposed to those of the brain, which operates on the muscles either by a volitional or by an emotional impulse. And, lastly, its actions are-always (except when excited by a physical irrita- tion directly applied to itself) entirely of a reflex character: that is to say, the motor impulses which originate in it are not spontaneous, but result from the stimulus of impressions, conveyed to it by the afferent trunks, and ope- rating upon it, to use the expression of Prochaska, according to certain "pecu- liar laws written, as it were, by nature on its medullary pulp." It is not, however, universally admitted that these actions are independent of sensation; and some eminent physiologists, among whom may be named Dr. Alison, still hold that the intervention of sensation is necessary, in the case at least, of the ordinary associated movements, which have definite ends in view, and FUNCTIONS OF THE SPINAL CORD. 133 folloAv one another in regular succession, as those of Respiration, for an im- pression to give rise to that organic change in the Spinal Cord which shall terminate in a muscular motion.* It will be desirable, therefore, to consider the evidence upon which the statement rests, that reflex actions are independ- ent of sensation, though ordinarily accompanied by it. 174. In the first place, then, it has long been well known that, in the Hu- man Jbeing, the Spinal Cord does not by itself possess, in the remotest degree, the power of communicating sensory impressions to the mind ; since, when its lower portion has been severed from the brain by injury or disease, there is complete anaesthesia of all the parts of the body which derive their nerves exclusively from it. Hence^it might be inferred, that, throughout the Verte- brated classes, the spinal cord is equally destitute of sensibility; and that any movements produced by stimuli acting through it, are the results of a physical, and not of a sensorial change. This inference, however, has been disputed ; and, if unsupported by other evidence, it would not, perhaps, be entitled to rank as an ascertained truth. The very performance, by decapitated animals of inferior tribes, of actions which had not been witnessed in Man under similar circumstances, has been held to indicate, that the spinal cord in them has an endowment which his does not possess. The possibility of such an explana- tion however unconformable to that analogy throughout organized nature, which, the more it is studied, the more invariably is found to guide to truth could not be disproved. Whatever experiments on decapitated animals were appealed to, in support of the doctrine that the brain is the only seat of sensi- bility, could be met by a simple denial that the spinal cord is everywhere as destitute of that endowment as it appears to be in Man. The cases of pro- found sleep and apoplexy might be cited, as examples of reflex action without consciousness ; and these might be met by the assertion, that in such condi- tions sensations are /4he expiratory movement ; so that a sudden blast of air is forced up the air-passages, carrying before it any thing that may offer an obstruction. The difference between coughing and Sneezing consists in this, that in the latter, the communication between the larynx and the mouth is partly or entirely closed, by the drawing together of the sides of the velum palati over the back of the tongue ; so that the blast of air is directed, more or less com- pletely, through the nose, in such a way as to carry off any source of irrita- tion that may be present there. 190. The influence of the Spinal Cord, and of its system of nerves, on the movements of Respiration, affords an excellent example of the importance of this organ, as supplying the conditions immediately requisite for the mainte- nance of the organic functions. We have seen that, strictly speaking, the act of Respiration, as we commonly understand it, is not Respiration itself ; for this consists in the interchange of ingredients between the blood and the surrounding medium, which is effected in the air-cells of the lungs, and which takes place in the lower animals (as in plants) without any muscular effort. But, in proportion to the necessity for the energetic exercise of this function, do we find a special provision in the higher classes, for the constant renewal of that portion of the surrounding medium which is in contact with the aerat- ing surface; and this comes to be so necessary, that asphyxia might be pro- duced, without any interruption to the ingress of air through the trachea, by DEGLUTITION AND DEFECATION. 145 merely breaking the circle of nervous action, through which the movements of Respiration are effected. It is an interesting circumstance, however, which shows the provision made in the animal frame to meet its necessities, that a very small portion only of the nervous centres is involved in this action ; and that, even in the highest Animals, all the rest may be removed, or may be rendered functionally inactive, without checking it. This fact, which was ascertained by Legallois, harmonizes well with that which Comparative Anatomy has brought under our notice ; for it has been shown that, in the lowest groups of Mollusca, but a single ganglion exists ; and that this is almost exclusively concerned in regulating the entrance and egress of the currents of water, the most constant office of which is the aeration of the blood ( 133). X. Deglutition, Defecation, 8fc. 191. Another very important function of the Spinal Cord (and of the ganglia corresponding to it in the Invertebrata), is the control which it exer- cises over the entrance and termination of the Alimentary Canal ; and this reflex action might probably be traced in some animals, in which the neces- sity for that of Respiration does not exist. In all beings which are une- quivocally of an animal character, a stomach or digestive cavity exists ; and a means must be provided for the introduction of food into it. This is partly accomplished by the power with which its entrance is endowed, of contracting- upon, and of attempting to draw inwards, whatever comes in contact with it ; as we may readily observe in the Star-Fish, or Sea-Anemone, where the mouth is simply the aperture of the stomach. From the analogy of the higher animals, as well as from what has been observed in the lower, it seems pro- bable that this action is of a reflex character, depending upon an impression conveyed to the nervous centres, and reflected back to the muscular fibres. But we almost always find some more special apparatus than this for bring- ing food within reach of the orifice of the stomach. In the Sea-Anemone, the, Hydra, and other Polypes, for example, we find that aperture surrounded by tentacula ; which have an evident tendency to lay hold of any thing that touches them, so as to bring it, by their contraction, within reach of tjjeJfctrscles immediately surrounding the orifice. This is just the purpose of ^IsHfearyn- geal muscles of Man. The lower part of the oesophagus, near its te^ilhation in the stomach, has the same simple tendency to contraction from above down- wards (so as to convey into the stomach any thing which is brought within its reach) as have the muscles surrounding the mouth of the Polype ; but there is need of some more complex apparatus, for the purpose of laying hold of the food, and of conducting it into its grasp. This is provided for, in the higher animals, in the muscles of that funnel-like entrance to the oesophagus, which is called the Pharynx. The actions of these are most distinctly reflex ; and it is interesting to remark, that the movements can neither be caused nor con- trolled by the direct influence of the will. In the case of the movements of respiration, we found sufficient provision made for their constant maintenance ; and yet, for secondary purposes, they were placed in a considerable degree under the control of the brain. But here there are no secondary purposes to be answered ; the introduction into the stomach of food, brought by the will within reach of the pharyngeal muscles, is the only object contemplated by them ; and they are accordingly placed under the sole government of the Spinal Cord. No attempts, on our own part, will succeed in producing a really voluntary act of deglutition. In order to excite it, we must supply some stimulus to the fauces. A very small particle of sojid matter, or a little fluid, (saliva, for instance,) or the contact of the back of the tongue itself, will be 13 146 FUNCTIONS OF THE NERVOUS SYSTEM. sufficient ; but without either of these we, cannot swallow at will. Nor can we restrain the tendency, when it is thus excited by a stimulus ; every one knows how irresistible it' is, when the fauces are touched in any unusual man- ner ; and it is equally beyond the direct control of the will, in the ordinary process of eating, voluntary as we commonly regard this. The only mode in which the will can influence it, is by regulating the approach of the stimu- lus necessary to excite it ; thus, we voluntarily bring a morsel of food, or a little fluid, into contact with the surface of the fauces, and an act of deglutition is then involuntarily excited ; or we may voluntarily keep all stimulus at a distance, and no effort of the will can then induce the action. Moreover, this action is performed, like that of respiration, when the power of the will is sus- pended, as in profound sleep, or in apoplexy affecting only the brain ; and it does not seem to be at all affected by the entire removal of the brain, in an animal that can sustain the shock of the operation ; being readily excitable, on stimulating the fauces, so long as the nervous structure retains its functions. This has been experimentally proved by Dr. M. Hall; and it harmonizes with the natural experiment sometimes brought under our notice in the case of an anencephalous infant, in which the power of swallowing seems as vigorous as in the perfect one. But, if the nervous circle be destroyed, either by division of the trunks, or by injury of any kind to the portion of the nervous centres connected with them, the action can no longer be performed ; and thus we see that, when the effects of apoplexy are extending themselves from the brain to the spinal cord, whilst the respiration becomes stertorous, the power of Deglu- tition is lost, and then respiration also speedily ceases. 192. Our knowledge of the nerves specially concerned in this action is principally due to the very careful and well-conducted experiments of Dr. J. Reid.* The distribution of the Glosso-Pharyngeal evidently points it out as in some way connected with it; and Sir C. Bell, misled by imperfect know- ledge of its anatomy, pronounced it to be a muscular nerve, whose function was to excite the combined movements of the tongue and pharynx, which are required in deglutition, and also in some acts of respiration. He was not aware that such a combination of movements may be due as much to the excitor nerve, and its termination in the Spinal Cord, as to the motor, and its particular distribution to muscles. The function of the Glosso-Pharyngeal nerve hasf been for some time one of the qusestiones vexatx of physiology ; and the results obtained by different experimenters are so strangely at variance, as almost to lead to the belief that they have operated on different nerves. In this dilemma, we may advantageously have recourse to anatomical examina- tion of its distribution ; and this, when carefully conducted, discloses the im- portant fact, that the nerve scarcely sends any of its branches to the muscles which they enter ; but that these mostly pass through them, to be distributed to the superjacent mucous surface of the tongue and fauces. Further, when the trunk is separated from the nervous centres, irritation scarcely ever pro- duces muscular movements. Hence it is not in any great degree an efferent or motor nerve ; and its distribution would lead us to suppose its function to be, the conveyance of impressions from the surface of the fauces to the me- dulla oblonganta. This inference is fully confirmed by the fact, that, so long as its trunk is in connection with the Medulla Oblongata, and the other parts are uninjured, pinching, or other severe irritation of the Glosso-Pharyngeal will excite distinct acts of deglutition. Such irritation, however, may excite only convulsive twitches, instead of the regular movements of swallowing ; and it is evident that, here as elsewhere, the impressions made upon the extremities of the nerves are much more powerful exciters of reflex movement than those * Edinb. Med. and Surg. Journ., vol. xlix. DEGLUTITION AND DEFECATION. 147 made upon the trunk, though the latter are more productive of pain. It was further observed by Dr. Reid, that this effect was produced b*y pinching the pharyngeal branches only ; no irritation of the lingual division being effectual to the purpose. 193. If, then, the muscles of deglutition are not immediately stimulated to contraction by the Glosso-Pharyngeal nerve, it remains to be inquired, by what nerve the motor influence is conveyed to them from the Medulla Oblongata ; and Dr. Reid has been equally successful in .proving that this function is per- formed by the pharyngeal branches of the Par Vagum. Anatomical examina- tion of their distribution shows that they lose themselves in the musdes of the pharynx ; and whilst no decided indications of suffering can be produced by irritating them, evident contractions are occasioned when the trunk, separated from the brain, is pinched or otherwise stimulated. It appears, however, that neither is the Glosso-Pharyngeal the sole excitor nerve, nor are the pharyn- geal branches of the Par Vagum the sole motor nerves, concerned in deglu- tition ; for after the former has been perfectly divided on each side, the usual movements can still be excited, though with less energy ; and, after the latter have been cut, the animal retains the means of forcing small morsels through the pharynx, by the action of the muscles of the tongue and neck. From a careful examination of the actions of deglutition, and of the influence of various nerves upon them, Dr. Reid draws the following conclusions : The impres- sions are conveyed to the Medulla Oblongata chiefly through the Glosso-Pha- ryngeal, but also along the branches of the Fifth pair distributed upon the fauces, and probably along the branches of the Superior Laryngeal distributed upon the pharynx. The motor influence passes chiefly along the pharyngeal branches of the Vagus ; along the branches of the Hypoglossal, distributed to the muscles of the tongue, and to the sterno-hyoid, sterno-thyroid, and thyro- hyoid muscles ; along the motor filaments of the Recurrents, ramifying upon the larynx ; along some of the branches of the Fifth, supplying the elevator muscles of the lower jaw ; along the branches of the Portio Dura, ramifying upon the digastric and stylo-hyoid muscles, and upon the muscles of the lower part of the face ; and probably along some of the branches of the Cervical plexus which unite themselves to the descendens noni. 194. When the food has been propelled downwards by the pharyngeal muscles as far as their action extends, its further progress through the oeso- phagus is effected by the peristaltic movement of the muscular coat of the tube itself. This movement is not, however, due only to the direct stimulus of the muscular fibre by the pressur^ of the food, as it seems to be in the lower part of the alimentary canal ; for Dr. J. Reid has found, by repeater! experiment, that the continuity of the oesophageal branches of the Par Vagum with the Spinal Cord, is necessary for the rapid propulsion of the food ; so that it can scarcely be doubted, that an impression made upon the mucous surface of the oesophagus, conveyed by the afferent fibres of these nerves to the Medulla Oblongata, and reflected downwards along the motor fibres, is the real cause of the muscular contraction. If the Par Vagum be divided in the rabbit, on each side, above the oesophageal plexus, but below the pharyngeal branches, and the animal be then fed, it is found that the food is delayed in the oeso- phagus, which becomes greatly distended. Further, if the lower extremity of the par vagum be irritated, distinct contractions are seen in the oesophageal tube, proceeding from above downwards, and extending over the cardiac extremity of the stomach. We have here, then, a distinct case of reflex action without sensation, occurring as one of the regular associated movements in the natural condition of the animal body ; and it is very interesting to find this following upon a reflex action with sensation (that of the pharynx), and pre- ceding an action which is altogether unconnected w T ith the Spinal Cord, (that 148 FUNCTIONS OF THE NERVOUS SYSTEM. of the lower part of the alimentary canal.) The use of sensation in the former case will presently appear. The muscular fibres of the oesophagus are also excitable, though usually in a less degree, by direct stimulation ; for it appears that, in some animals (the Dog, for example), section of the pneumogastric does not produce that check to the propulsion of the food which it occasions in the Rabbit ; and even in the Rabbit, as Dr. M. Hall* has lately remarked, the simple contractility of the muscular fibre occasions a distinct peristaltic movement along the tube, after its nerves have been divided ; causing it to discharge its contents, when cut across. t 195. It will be desirable here to revert for a short time to the actions which, in the higher animals, precede those of Deglutition. There can be no doubt that, in the Human being, the motions adapted to the ingestion and mastica- tion of aliment originally result, in part at least, from distinct operations of the Will ; but it would appear almost equally certain, that, in time, they come to be of so habitual a character, that the will only exerts a general controlling influence over them, each individual act being excited through the shorter channel already alluded to ( 183). Every one is conscious that the act of mastication may be performed as well, when the mind is attentively dwelling on some other object, as when directed to it; but, in the former case, one is rather apt to go on chewing and re-chewing what is already fit to be swallowed, simply because the will does not exert itself to check the action, and to carry the food backwards within the reach of the muscles of deglutition, We now see why sensation should be associated with the latter process. The convey- ance of food backwards to the fauces is a distinctly voluntary act ; and it is necessary that it should be guided by the sensation there resulting from the contact which it induces. If the surface of the pharynx were as destitute of sensation as is the lower part of the oesophagus, we should not know when we had done what was necessary to excite its muscles to operation. The muscles concerned in the Mastication of food are nearly all supplied by the third branch of the Fifth pair, a large proportion of which is well known to have a motor character. Many of these muscles, especially those of the cheeks, are also supplied by the portio dura of the Seventh ; and yet, if the former be paralyzed, this cannot stimulate them to the necessary combined actions. Hence we see that the movements are of an associated character, their due performance being dependent on the part of the nervous centres from which the motor influence originates. If the Fifth pair, on the other hand, be uninjured, whilst the Seventh is paralyzed, the movements of Mastication are performed wUhout difficulty ; whilst those connected in any way with the Respiratory function, or with Expression, are paralyzed. 196. Comparative anatomy supplies us with the key to the explanation of these phenomena. It has been seen that, in the lower animals, the Respira- tory organs are completely unconnected with the mouth, and that a very * Third Memoir on the Nervous System, 201. f There are many cases in which this direct contractility does not manifest itself in the ordinary condition of the system, but in which it becomes evident when the muscu- lar structure has gained an increase of irritability by diseased action, as we frequently have to notice in the intestinal canal. For example ; in many cases of disease or injury of the Spinal Cord, the bladder ceases to expel its contents, through the interruption of the circle of reflex actions hereafter to be described; but, after a time, it ceases to become necessary to draw off the urine by the catheter; for the fluid is constantly expelled, as soon as it has accumulated in small quantities. In such cases, the mucous coat is found after death to be thickened and inflamed; and the muscular coat is greatly increased in strength, and contracted upon itself. Here, then, the muscularicoat, which is not excited to contraction as long as the mucous coat is in a healthy condition, acquires a degree of abnormal irritability which is sufficient to enable it of itself to expel the urine ; but this could not be the case, unless it had originally been possessed of independent contractility. DEGLUTITION AND DEFECATION. 149 distinct set of muscles is provided to keep them in action. These muscles have distinct ganglia as the centres of their operations ; and these ganglia are only connected indirectly with those of the sensori-volitional system. The same would appear to be the case in regard to the introduction of the food into the digestive apparatus. It has been shown that the muscles concerned in this operation have their own centres, the stomato-gastric and pharyngeal ganglia, which are not very closely connected with the cephalic, or with the respiratory, or with those of general locomotion. Now in the Vertebrata, the distinct organs have been so far blended together, that the same muscles serve the purposes of both ; but the different sets of movements of these muscles are excited by different nerves ; and the effect of division of either nerve is to throw the muscle out of connection with the function to which that nerve previously rendered it subservient, as much as if the muscle were separated from the nervous system altogether. There is an apparent exception to this view of the matter, in the case of the Portio Dura ; this being the source of those movements of the upper lip which, in many animals, are essential to the prehension of food. These movements, however, are dependent upon sensations conveyed through the Fifth pair,* being completely checked by division of its infra-orbital trunk ; and it can scarcely be doubted, from their general character, that they are of a strictly voluntary nature, and are not to be regarded as part of the reflex associated movements in which that nerve is concerned. 197. Now although, in the adult Human being, the movements required to convey the food to the pharynx are under the control of the Will, if not con- stantly dependent upon it, there is good reason to believe that this is not the case in regard to those remarkable associated movements which constitute the act of suction in the Infant. The experiments provided for us by nature, in the production of anencephalous monstrosities, fully prove that the nervous connection of the lips and respiratory organs with the Spinal Cord, is alone sufficient for its execution ; and Mr. Grainger has sufficiently established the same, by experiment upon puppies whose brain had been removed. He adds that, as one of the puppies lay on its side, sucking the finger which was pre- sented to it, it pushed out its feet in the same manner as young pigs exert theirs against the sow's dugs. On the whole, however, the act of suction belongs more to the Respiratory ganglion (so to speak) than to the Stomato-gastric system of nerves ; and hence we can understand why, even in the highest animals, it should be purely instinctive ; the movements of Respiration being so from the first, whilst those ordinarily concerned at a later period in the ingestion of the -food are more directed by the will. The actions of the mammary foetus of the kangaroo, described by Mr. Morgan, furnish a very interesting exemplification of the same function of the Spinal Cord ; this crea- ture, resembling an earth-worm in appearance, arid only about fourteen lines in length, with a brain corresponding in degree of development to that of a human fostus of the ninth week, executes regular, but slow, movements of respiration, adheres firmly to the point of the nipple, and moves its limbs when disturbed. The milk is forced into the oesophagus by a compressor muscle, with which the mamma of the parent is provided. "Can it be imagined," very justly asks Mr. Grainger, " that in this case there are sensation and volition, in what can be proved anatomically to be a foetus ?" 198. We now return to the question of the influence of the Spinal Cord * Hence originated one of Sir C. Bell's early errors. He found that an ass, in which the infra-orbital branch of the fifth was divided, would not pick up oats with its lip, although they were in contact with it; hence he concluded that its power of motion was destroyed, when it was in reality only the sensation necessary to excite the will to cause the motion that was deficient. 13* 150 FUNCTIONS OF THE NERVOUS SYSTEM. upon the lower part of the alimentary canal. It has been already stated, that the motor function of the Par Vagum appe'ars to terminate at different points in different animals ; and this may in part explain the great variety in the results obtained by different experimenters, in regard to the effect of section of the par vagum upon the function of digestion. Valentin agrees with Dr. Reid in stating, that distinct movements of the stomach may be excited in the rabbit by irritation of the par vagum ; and he adds, as a precaution, that the experiment should be performed very soon after death, as the irritability of the stomach is soon lost ; and that the stimulation of the nerve should not be per- formed too high up, but rather in the lower part of the neck, or in the thorax. Various experiments upon living animals have led to the belief, that the motions of the muscular parietes of the stomach, which perform a very important part in chymification, are due to the influence of this nerve ; food taken in shortly before or subsequently to its division, having been found to be only dissolved on the surface of the mass, where it was in contact with the mucous membrane. But these experiments have been made for the most part upon Herbivorous animals, such as horses, asses, and rabbits, whose food is bulky and difficult of solution, requiring to be constantly changed in its position, so that every part of it may be successively brought to the exterior. On the other hand, Dr. Reid found, in his experiments upon Dogs, that, after the first shock of the operation had gone off, solution of the food in the stomach, and absorption of chyle, might take place ; and hence it may be inferred, that no influence of this nerve upon the muscular parietes of the stomach is essential to digestion in that species. This conclusion harmonizes well, therefore, with the fact already stated respecting the absence of such influence in the lower part of its oesophagus ; and it may, perhaps, be explained by the consideration, that the natural food of the dog is much less bulky and more easy of solution than that of the animals already named ; so that there is not so much need of the peculiar movement which is in them so important an aid to the process of reduction. 199. In regard to the functions of the afferent portion of the gastric branches of the Par Vagum, there has also been considerable difference of opinion ; some physiologists maintaining that it is by impressions on them alone that the sense of Hunger or satiety is occasioned ; whilst others deny that it has any power of transmitting such impressions, and maintain that they do not originate in the stomach at all. Dr. Reid has arrived at the conclusion, from his numerous experiments, that the par vagum is the channel through which the mind becomes cognisant of the condition of the stomach ; but that it is not the sole excitor of the sense of hunger. Animals which have sustained section of the nerve on both sides will eagerly take food, if they have not received too great a shock from the operation ; but they seem to experience no feeling of satiety when the stomach is loaded. This inference is confirmed by Valentin, who mentions that puppies after the operation will take three times, and even more, the same quantity of milk as uninjured individuals of the same age ; so that the abdomen is greatly distended. The act of Vomiting has been now sufficiently shown to be excitable through the par vagum; an impression propagated through which to the Medulla Oblongata excites to contraction a considerable number of muscles. But, as in the case of hunger, although the sense of nausea and the tendency to vomit may be excited by various irritating causes operating through this nerve only, it may be produced also through other channels. Thus severe vomiting has been excited by the injection of a solution of tartar emetic or of emetin into the blood-vessels ; a fact of which it has been pro- posed to take advantage in extreme cases of narcotic poisoning, when the nervous system has become so torpid, that emetics administered in the ordinary manner are of no avail (See 300). 200. That the ordinary peristaltic movements of the intestinal canal, from DEGLUTITION AND DEFECATION. 151 the stomach to the rectum, may take place without any connection with the nervous system, being due to the direct stimulation of the contact of food, there is now ample evidence ; and though some may still be found who deny the Hallerian doctrine, that muscular fibre possesses in itself the property of con- tractility, so much additional evidence of its truth has been recently adduced, whilst the fact itself is so conformable to the analogy supplied by others, that it will be here unhesitatingly adopted (See Chapter V.). Mr. Grainger and some other physiologists have supposed that the peristaltic movements of the alimentary canal are due to a sort of reflex action, taking place through the ganglia of the Sympathetic system of nerves, especially, of course, the semiJunar. This supposition, however, has little or no evidence to support it; for it has been fully proved that the muscular contractions will continue long after the tube has been separated from its nervous connections through its whole extent ; and the only evidence in its favour is derived from the contractions which may sometimes be induced in parts of the tube which are at rest when the sympathetic nerves supplying them are irritated. Some very interesting experiments have been recently published by Valentin, by which the fact that such contractions may be induced (which has been denied by some) is clearly substantiated ; but it is also shown that the motor influence does not originate in the Sympathetic ganglia, but in the Spinal Cord. The following are the general results of upwards of three hundred experiments, so far as they apply to this subject. The pharynx may not only be excited to contraction by irri- tation of the pharyngeal branches of the Par Vagum, or of the roots of the Spinal Accessory, from which their motor power is derived (as will be hereafter explained), but also by stimulating the roots of the first two Cervical nerves; and the lower part of the cesophagus in the neck is made to contract peristalti- cally from above downwards, by irritation of the roots of the first three Cervical nerves, and of the cervical portion of the Sympathetic, through which last the former evidently operate. The thoracic portion of the oesophagus is made to contract, by irritation of the lowest Sympathetic ganglion of the neck, and of the higher thoracic ganglia, and also of the roots of the lower Cervical spinal nerves. Muscular contractions of the stomach are produced by irrita- tion of the roots of the 4th, 5th, 6th and 7th Cervical nerves, and of the first thoracic in the rabbit ; so that a distinct furrow is evident between the cardiac and pyloric portion of the viscus; and the lower the nerve irritated, the nearer the pylorus do the contractions extend. Irritation of the first thoracic ganglion of the Sympathetic produces the same effect. Contractions of the intestinal tube, varying in place according to the part of the Spinal Cord experimented on, may be excited by irritation of the roots of the dorsal, lumbar, and sacral nerves, and of the trigeminus ; and similar effects are produced by irritation of the lower part of the thoracic portion of the lumbar, and of the sacral portions of the Sympathetic, also of the splanchnic, and of the gastric plexus. 201. From these facts it is evident, that the movements of the Intestinal tube may be influenced by the Spinal Cord; and that what is commonly termed the Sympathetic nerve, is the channel of that influence, by the fibres which it derives from the Spinal system. But it by no means thence follows, that the ordinary peristaltic actions of the muscles in question are dependent on a stimulus reflected through the spinal cord, rather than on one directly applied to themselves. It is clear that, although these movements are of the first im- portance to the welfare of the system, such means of sustaining them are feeble, compared to those which we find provided for the maintenance of the distinctly reflex actions of deglutition, respiration, &c. The difficulty with which any evidence can be obtained of the connection, is a sufficient proof of this. On the other hand, we do know that these peristaltic movements are influenced 152 FUNCTIONS OF THE NERVOUS SYSTEM. by particular states of mind, or by conditions of the bodily system ; and the "Connection just traced satisfactorily accounts for this, and is itself sufficiently explained. The intestinal tube, then, from the stomach to the rectum, is not dependent upon the Spinal Cord for its contractility, but is enabled to propel its contents by its own inherent powers ; still we find that here, as in other instances, the nervous centres exert a general control over even the organic functions, doubtless for the purpose of harmonizing them with each other, and with the conditions of the organs of animal life. 202. On examining the outlets by which the excretions are voided, we find that they are placed, like the entrances, under the guardianship of the Spinal Cord; subject, however, to some control on the part of the Will. In the lowest animals, the act of discharging excrementitious matter is probably as involuntary as are the acts immediately concerned in the introduction of nutriment ; and is performed as often as there is any thing to be got rid of. In the higher classes, however, such discharges are much less frequent ; and reservoirs are provided yi which the excrementitious matter may accumulate in the intervals. The associated movements required to empty these, are completely involuntary in their character ; and are excited by the quantity, or stimulating quality, of the contents of the reservoir. But, had volition no control over them, great incon- veniences would ensue ; hence sensation is excited by the same stimulus which produces the movements ; in order that, by arousing the will, the otherwise involuntary motions maybe restrained and directed. There can be little doubt, from the experiments of Dr. M. Hall, as well as from other considerations, that the associated movements, by which the contents of the rectum and bladder are discharged, correspond much with those of Respiration, being in their own nature involuntary, but capable of a certain degree of voluntary restraint and assistance : whilst the discharge of the contents of the vesiculae seminales would seem to be completely automatic ; thus corresponding with the act of deglutition. On the other hand, the sphincters, which antagonize the expellent action, are usually maintained in a state of moderate contraction, so as to afford a constant check to the egress of the contents of the cavities; and this condition has been fully proved by Dr. M. Hall to result from their connection with the Spinal Cord, ceasing completely when this is interrupted. On the other hand, the sphincter is certainly in part controlled by the will, and is made to act in obedience to the warning given by sensation ; and this voluntary power is fre- quently destroyed by injuries of the Brain, whilst the Spinal Cord remains able to perform all its own functions, so that discharge of the urine and faeces occurs. In their moderate action, the expulsors and the sphincters may be regarded as balancing one another, so far as their reflex action is concerned, the latter having rather the predominance, so as to restrain the operation of the former. But when the quantity or quality of the contents of the cavity gives an excessive stimulus to the former, their action predominates, unless the will is put in force to strengthen the resistance of the sphincter ; this we are fre- quently experiencing, sometimes to our great discomfort. On the other hand, if the stimulus is deficient, the will must aid the expulsors, in order to overcome that resistance which is due to the reflex contraction of the sphincters ; of this also we may convince ourselves, when a sense of propriety, or a prospective regard to convenience, occasions us to evacuate the contents of the rectum or bladder without a natural call to do so. The muscular coat of the Bladder is commonly regarded as having, like that of the intestinal tube, no connection with the Spinal Cord ; but the experiments of Valentin have shown that a con- nection exists, as in the former case, through the sympathetic nerve, affecting not only the bladder but also the ureters. That physiologist states, that a very distinct and powerful peristaltic action of the ureter, proceeding from the kidneys to the bladder, may be produced, by irritating the abdominal ganglia PROTECTING AGENCY OF THE SPINAL CORD. 153 of the Sympathetic, or the roots of the superior abdominal Spinal nerves ; and that strong contractions of the bladder are excited, by irritation of the inferior portion of the abdominal Sympathetic, but especially of its sacral portion, and of the roots of the middle and inferior abdominal nerves of the Spine. In these, as in former cases, no effect is produced by irritation of the Spinal Nerves, unless the portion of the Sympathetic connected with the particular organ be entire. 203. The influence of the Spinal Cord on the Genital organs is of a simi- lar character. The muscular contractions involved in the Emissio Seminis are clearly of a reflex nature ; being independent of the will, and not capable of restraint by it, when once fully excited ; and being producible in no other way, than (like those concerned in Deglutition) by a particular local irritation. That this irritation need not amount to a sensation, is proved by the cases already referred to ( 182) ; and it has been also shown by experiment, that section of the Spinal Cord in the lumbar region does not prevent the act from being performed, the lower division only being concerned in the reflexion of the impression. It appears, also, from the experiments of Valentin, that the Spinal Cord may operate on the Genital organs through the Sympathetic sys- tem. Contractions were excited in the vas deferens and vesiculse seminales, especially of the Guinea Pig at the time of heat, by irritation of the inferior lumbar and highest sacral portions of the Sympathetic ; and the Fallopian tubes, as well as the Uterus itself, may be excited to contraction, by irritation of the same nerves as those which excite the rectum, namely, the lower lum- bar and first sacral nerves of the Spine. This last fact is important, in regard to the rationale of the operation of certain medicines, such as aloes, which are known to have an influence on both parts. In regard to the act of Parturition, there would seem reason to believe, from the evidence of cases of paraplegia, that, of the muscles whose operation is associated in it, the diaphragm, abdomi- nal muscles, &c., are called into action (as in Defecation) through the Spinal Cord ; but that the contractions of the Uterus itself are independent of all con- nection with the nervous centres. Of the reason why the muscles, which were up to that time inert, should then combine in this extraordinary manner, and with such remarkable energy, Physiology can afford no certain informa- tion. There can be little doubt, however, that the stimulus usually originates in the uterus, or in some of the neighbouring organs which are incommoded by the pressure ; but it may also result from some condition of the general system, in which the uterus itself is but little concerned. It is an interesting fact, which has been more than once observed, that the fetus may be expelled from the dying body of the mother, even after the respiratory movements have ceased. This would appear due to the contraction of the Uterine fibres alone, which, like those of the heart and alimentary canal, retain their irritability longer than those of the muscles supplied by the cerebro-spinal nerves ; and the power of these would be unopposed by the resistance which they ordina- rily have to encounter ; since the tone of all the muscles surrounding the outlet would be destroyed by the cessation of the activity of the Spinal system of nerves ( 207). *' XL Protecting Agency of the Spinal Cord. 204. From the foregoing details it appears, that one of the chief functions of the Spinal Cord is to control the orifices of the various open cavities of the body ; and this function evidently has safety, as well as convenience, in view. It has been manifestly designed by the All-wise Creator, .that the glottis should close against agents injurious to the organs within ; and that the effort to vomit should be excited, by the attempt to swallow substances so nauseous as to induce 154 FUNCTIONS OF THE NERVOUS SYSTEM. loathing. There is another protective influence exerted by it, of a still more remarkable nature. It has been ascertained by Dr. M. Hall that, if the func- tions of the Brain be suspended or destroyed, without injury to the Spinal system of nerves, the Orbicularis muscle will contract, so as to occasion the closure of the eyelids, upon the tarsal margin being touched with a feather. This fact is interesting in several points of view. In the first place, it is a characteristic example of pure reflex action, occurring under circumstances in which volition cannot be imagined to guide it, and in which there is no valid reason to believe that sensation directs it. Further, it explains the almost irre- sistible nature of the tendency to winking, which is performed at short inter- vals by the contraction of the Orbicularis muscle ; this is evidently a Spinal action, capable of being in some degree restrained (like that of respiration) by the will, but only until such time as the stimulus (resulting, perhaps, from the collection of minute particles of dust upon the eyes, or from the dryness of its surface in consequence of evaporation), becomes too strong to be any longer resisted. Again, we have in sleep or in apoplexy an example of this purely spinal action, unbalanced by the influence of the will, which, in the waking state, antagonizes it by calling the levator palpebrae into action. As soon as the will ceases to act, the lids droop, and close over the eye in order to protect it ; and if those of a sleeping person be separated by the hand, they will be found presently to return. Here, as in studying the respiratory and other movements, we are led to perceive, that it is the Brain alone which is torpid during sleep, and whose functions are affected by this torpidity. As Dr. M. Hall very justly remarks, the Spinal system never sleeps ; it is constantly in activity ; and it is thus that, in all periods and phases of life, the movements which are essential to its continued maintenance are kept up without sensible effort. 205. The closure of the Pupil against a strong light is another movement of the same protective tendency. The channel through which that just named is performed, is completed by the first branch of the Fifth and the Portio Dura of the seventh. The contraction of the pupil is immediately caused by the Third pair, or Motor Oculi, as is easily shown by irritating the trunk of that. nerve and observing the result. But it is not easy to speak with certainty as to the afferent nerve, by which the motor influence is excited. Although the contraction of the pupil is usually in close accordance with the sensation occasioned by the impression of light upon the retina, yet there is no want of evidence to prove that the sensation of light is not always necessary ; for, even when the sight of both eyes has been entirely destroyed by amau- rosis, the regular actions have been witnessed in the pupil, in accordance with varying degrees of light impinging on the retina. This fact may be explained in two ways. It may either be imagined that the requisite stimu- lus is not that of light conveyed through the optic nerve, but that of heat conveyed through the ophthalmic branch of the Fifth pair. Or it may be still supposed, that the motion results from an impression upon the retina, which impression, being conducted to the Brain, ordinarily produces a sensa- tion ; whilst in these curious cases, no sensation is produced, on account of a disordered state of the part of the Brain, in which the Optic nerve terminates ; though some filaments of that nerve, being connected with the Spinal Cord, and not with the Brain, can produce a reflex action through the , Third pair, although no sensation accompany it. In either view, the rarity of the occur- rence is at once accounted for ; since in most cases of amaurosis, the disease lies in the trunk of the nerve, and thereby checks both its spinal and its cerebral actions. 206. The physiologist has not at present any knowledge of any similar protective movements, in the Human being, designed to keep the organ of OTHER FUNCTIONS OF THE SPINAL CORD. 155 Hearing from injury ; but there can be little doubt that those which we are constantly witnessing in other animals, possessing large external ears, are reflex actions excited by the irritation applied to them. In regard to the Nose, we find a remarkably complex action that of Sneezing adapted to drive off' any cause of irritation ( 189). It will Hereafter be shown that the stimulus is conveyed, in this case, not through the Olfactory nerve, but through the Fifth pair ; so that it is not dependent upon the excitement of the sensation of smell. The act of Coughing, also, may be regarded of a protective character, being destined to remove sources of irritation from the air-passages. XII. Other Functions of the Spinal Cord. 207. The influence of the Nervous Centres in maintaining what is com- monly designated as the tone of the muscular system, was first distinctly limited by Dr. M. Hall to the Spinal Cord, and the system of nerves connected with it. By the expression in question is meant, that state of moderate con- traction which causes all the muscles to present a certain degree of firmness, by their antagonism with each other, when none of them are particularly con- tracted or relaxed. The following experiments by Dr. M. Hall clearly prove the influence of the Spinal Cord on this functional condition : " Two Rabbits were taken ; from one the head was removed ; from the other also the head was removed, and the spinal marrow was cautiously destroyed with a sharp instrument : the limbs of the former retained a certain degree of firmness and elasticity ; those of the second were perfectly lax." " The limbs and tail of a decapitated Turtle possessed a certain degree of firmness or tone, recoiled on being drawn from their position, and moved with energy on the application of a stimulus. On withdrawing the spinal marrow gently out of its canal, all these phenomena ceased. The limbs were no longer obedient to stimuli, and became perfectly flaccid, having lost all their resilience. The sphincter lost its circular form and contracted state, becoming lax, flaccid, and shapeless. The tail was flaccid and unmoved on the application of stimuli." It is proba- ble that this tonic contraction is strictly a reflex action ; an impression of the condition of the muscle, corresponding with the " muscular sense" of Sir C. Bell, but not necessarily accompanied by sensation, being conveyed to the Spinal Cord, and producing the stimulus to contraction. The want of this tone is seen in the relaxation of the sphincters ; and also in the distortion of the face, produced by paralysis of the Portio Dura, and resulting from the tonic contraction of the muscles on one side of the face, unbalanced by that of the other side. Cases have occasionally presented themselves, in which the portio dura has been paralyzed to the influence of the Will (owing to disease affecting its Cerebral termination), whilst its Spinal connections have not been affected ; so that the tone of the muscles has been preserved, and no distortion of the face has 'manifested itself, until the muscles were stimulated by a voluntary impulse, to which those of one side only would respond. 208. Nearly allied to this function of the Spinal Cord, is that by which it is subservient to the maintenance of the contractility of muscles paralyzed to the influence of the will. It is well known that, in ordinary cases of paralysis, the muscles lose their irritability in the course of a few weeks, so that no stimulus excites them to contraction; and it is also \Vfell known that their characteristic structure is so greatly affected, that, in progress of time, no true muscular fibres can be detected in their place. Experiments on animals, in which portions have been removed from the nerves supplying the limbs, con- duct to exactly the same result as the experiments made for us by diseased conditions in Man. Now Dr. M. Hall has pointed out that, in cases where the muscles are paralyzed to the influence of the will, through disease of the 156 FUNCTIONS OF THE NERVOUS SYSTEM. Brain or of the upper part of the Spinal Cord, but retain their power of reflex action, (the nervous circle which operates through the Spinal Cord not being interrupted) the contractility of the muscles is not diminished, but appears to be sometimes even increased ; and he has suggested that this fact may be made available as a means of diagnosis in obscure cases of paralysis. Thus, in some cases of Paraplegia, the reflex actions may be excited ; in others they cannot be. In the former, the disease must be in the dorsal or cervical portion of the Spinal Cord, leaving its lumbar portion free to carry on the reflex actions, though its connection with the brain is interrupted. In the latter, the disease is probably within the lumbar vertebrae, involving that portion of the Spinal Cord through which the reflex actions of the lower extremities are produced. In like manner, in paralysis of a single arm or of one leg, if the reflex actions, and the contractility of the muscles on the application of a direct stimulus (such as galvanism), remain unimpaired, the cause is probably seated in. the Brain ; whilst, if the tone of the muscles be completely lost, and no contraction can be induced in them, the cause of the paralysis is probably somewhere in the neighbourhood of the roots of the nerves of the part affected. There is no good ground for believing, however, that the contractility of the muscles is directly dependent on their connection with the Spinal Cord, a doctrine which is inconsistent (as will be shown hereafter) with well-established facts. It is well known that muscular structure requires, for its perfect nutrition, to be kept in a state of functional activity. If the muscles of the leg, for example, be disused for a long time, their nutrition is greatly impaired, and their con- tractility is almost suspended ; even though they retain their connection with the nervous centres, and the latter be in their normal condition. It is to be expected, then, that, if a muscle be completely put out of the pale of nervous influence, its nutrition should be speedily impaired, and its contractility alto- gether lost ; but if the influence of the Brain only be withdrawn from it, and its connection with the Spinal Cord be uninterrupted, it will be in a state of continual action, by the operation of various reflected stimuli ; and this action will be sufficient to maintain its nutrition, and to -prevent the loss of its con- tractility (See Chap. V.). 209. The fact, that the action of the Heart is in some degree under the control of the Spinal Cord, has long been known. It is not a little curious that, although its movements will continue regularly, after complete section of all its nerves, any sudden and severe impression upon a large part of the Nervous Centres, such as crushing the Brain or Spinal Cord, will produce a great diminution in their frequency, or will even occasion their entire cessa- tion, if the nervous connection be entire. It will be hereafter shown, that the influence is partly communicated by the Par Vagum ; but it appears, from the experiments of Valentin, that the Sympathetic is in part, as in the case of the motions of the alimentary canal, the channel by which it is transmitted. He found that, when the heart had ceased to beat, its contractions might be re- newed by irritation of the roots of the Spinal Accessory nerve, and of the first four Cervical nerves, and also of the first cervical ganglion of the Sympathetic. He tjiinks that he has also witnessed distinct contractions of the thoracic aorta, of the inferior cavity, and of the thoracic duct, upon irritation of the neigh- bouring portion of the Sympathetic system, which evidently derives its whole motor power from the Spinal Cord. The ductus choledocus has also been seen by him to contract on irritation of the right splanchnic nerve. 210. Lastly, we have to inquire how far the Reflex action of the Spinal Cord is concerned in the locomotive actions of the lower extremities in Man. It will be remembered that, in the Dytiscus whose head had been removed, ( 146,) the stimulus of the contact of water immediately excited regular and continued locomotive actions, which lasted for some time. So in the cases OTHER FUNCTIONS OF THE SPINAL CORD. 157 already quoted, ( 177, 8,) when the control of the will over the lower extre- mities was lost, powerful muscular actions were excited in them through the Spinal Cord alone. In the healthy condition of the Human system, when the Will is controlling all the movements which are not immediately concerned in the maintenance and regulation of the organic functions, no such actions can be excited ; but, in proportion as its control is lost, does the independent power of the Spinal Cord manifest itself. The more such actions are of a simple rhythmical character, similar to those of Respiration, the more does it seem that they may with probability be referred to the Spinal system ; and if we attribute to this (as we can scarcely help doing) the rapid vibration of the wings of Insects, there seems no reason why we should not extend the same view to the wings of Birds. Such an explanation of their movements will account for their occasional continuance, without apparent fatigue, during a period through which no known voluntary effort can endure ; for it is one of the attributes of the Spinal system of nerves, well pointed out by Dr. M. Hall, that the exercise of the muscles excited by it does not occasion fatigue, the sense of which is Cerebral only. It would seem to the Author more probable, however, that those movements which guide the body, and which must them- selves be directed by Sensation, are to be referred to a class intermediate between the Voluntary and the Reflex, which may be properly termed Instinc- tive. Several actions in Man, which were at first Voluntary, appear at last to be performed as instinctively, or intuitively, as they are in the lower animals from the commencement of their existence. (See Section xvm. of the present Chapter.) 211. It would not be right to conclude this account of the principal functions .of the Spinal Cord, without adverting to some of the leading Pathological applications of the physiological doctrines, which have been developed in it. A large part of these were first pointed out by Dr. M. Hall ;* and they are receiving continual and important extensions from his own labours and those of other practical inquirers. It may be remarked, in the first place, that the power of the whole Spinal system is capable of being morbidly diminished or augmented. It may even be for a time almost completely suspended, as in Syncope ; which state maybe induced by sudden and violent impressions, either of a mental or physical nature, that operate upon the whole nervous system at once, commencing, however, in the Brain. It is to be remarked that, in recovering from these, it is the Spinal system of which the activity is first renewed, the respiratory movements recommencing, and the power of swal- lowing being restored, before any voluntary actions can be performed. A cor- responding state may be induced in particular portions of the system, by con- cussion ; as is seen in severe injuries of the Spinal Cord, which are almost invariably followed for a time by the suspension of its functions. Again, the power of the whole Spinal Cord may be diminished by various causes, such as enfeebled circulation, pressure, &c.; and then we have torpidity of the whole muscular system. If oppression exists in the brain, the functions of the Medulla . oblongata will be especially affected ; and if it be prolonged and sufficiently severe, Asphyxia will result from the interruption of the respiratory movements which it occasions. 212. On the other hand, the excitability of the whole Cord, or of particular parts of it, may be morbidly increased. This is especially seen in Tetanus, Hydrophobia, and the artificial tetanus induced by Strychnine ; so that the slightest external stimulus is sufficient to induce reflex actions in their most terrific forms. It is interesting to remark, that, in these formidable diseases, * See especially his Treatise on the Diseases and Derangements of the Nervous System. 14 158 FUNCTIONS OF THE NERVOUS SYSTEM. the functions of the muscles controlling the various orifices are those most affected ; and it is by the spasms affecting the organs of respiration or deglu- tition that life is commonly terminated. Various remedial agents will probably be found to operate, by occasioning increased excitability in some particular segments of the Cord ; so that the usual stimuli applied to the parts connected with these, will occasion increased muscular action. This seems to be the case, for example, in regard to the influence of aloes on the rectum and uterus, cantharides on the neck of the bladder and adjoining parts, and secale cornu- tum on -the uterus. The mode of influence of cantharides is illustrated by a curious case, related by Dr. M. Hall, of a young lady who lost the power of reten- tion of urine in consequence of a fatty tumour in the spinal canal, which gradually severed the Spinal Cord, and induced paraplegia. The power of retaining the urine was always restored for a time by a dose of tincture of cantharides, which augmented the excitability of the segment of the cord, with which the sphincter vesicae is connected. The researches of Valentin, when grafted (as it were) on the doctrines of Dr. M. Hall, afford the key to the explanation of the numberless sympathetic influences of the organs of nutrition, &c., upon one another ; by showing that they are all connected with the Spinal Cord ; and that the muscular structure, with which they are all provided, may be excited to contraction through it. And, lastly, t % he more recent observations of Dr. M. Hall, in regard to the peculiar excitor power that belongs to the nervous fibres distributed on various serous and fibrous membranes, will probably lead, when they have been fully carried out, to the explanation of the various convulsive actions that result from pressure or irritation affecting these parts. XIII. Comparative Anatomy of the Encephalon. 213. The assistance which the Physiologist has hitherto derived from the study of the Comparative Anatomy of the Encephalon in Vertebrata, is not so great as might have been expected ; there can be little doubt, however, that much is yet to be learned from it. Certain general inferences appear well established; and it is chiefly in questions of detail that difficulties still exist. The Encephalon may be described as consisting of the Cerebral Hemispheres, the Cerebellum, and the Medulla Oblongata with its chain of ganglia. The relative proportion of the two former to the latter is such in Man, that their character would not be readily understood by the inspection of his Brain alone ; and it is one of the most interesting results of the comparison of it with the brains of animals of the inferior tribes that the great change which we there find in the proportion of the parts, makes evident the importance of what would have been otherwise considered subordinate appendages. This is peculiarly the case in Fishes. There may be noticed in the Encephalon of that class four distinct ganglionic enlargements; of which the posterior is usually on the median line, whilst the others are in pairs. The posterior, from its position and connections, is evidently to be regarded in the light of a Cerebellum; and it bears a much larger proportion to the rest, in this class, than in any other. The pair in front of this are not the hemispheres of the Cerebrum, as their large size in some instances (the Cod for instance) might lead us to suppose ; but they are immediately connected with the Optic nerve, which, in fact, terminates in them, and are therefore to be considered (like the chief part of the cephalic masses of In vertebrated animals) as Optic Ganglia. In front of these are the Cerebral Hemispheres, which are small, generally destitute of convolutions, and possess no ventricle in their interior, except in the Sharks and Rays, in which they are much more highly developed than in the Osseous Fishes. Anterior to these is another pair of ganglionic Pike. COMPARATIVE ANATOMY OF THE ENCEPHALON. Fig. 27. Cod, 159 Brains of Fishes, after Leuret; A, olfactive lobes or ganglia; B, cerebral hemispheres; c, optic lobes; D, cerebellum; o I, olfactory nerve ; o p, optic nerve; p a, patheticus; m o, motor oculi; a b, abducens ; t r t, trifacial ; fa, facial ; a M, auditory ; v a g, vagus ; 1 1 : tubercles or ganglia of the trifacial ; t r, tubercles of the vagus. enlargements, from which the Olfactory nerves arise; and* these are, therefore, correctly designated as the Olfactive tubercles or ganglia. In some instances, these ganglia are not immediately seated upon the prolonged spinal cord, but are connected with it by long peduncles ; this is the case in the Sharks : and we are thus led to perceive the real nature of the portion of the trunk of the Olfactory nerve in Man, which lies within the cranium, and of its bulbous expansion on the Ethmoid bone. Besides these principal ganglionic enlarge- ments, there are often smaller ones, with which other nerves are connected. Thus in the Shark, we find a pair of tubercles of considerable size, at the origin of the Trifacial nerves; and another pair in most Fishes, at the roots of the Vagi. In some instances, too, distinct Auditory ganglia present themselves. 214. The Optic Lobes of Fishes have no analogy whatever with the Thai- ami optici of Mammalia; the connection of which, with the Optic nerves, is apparent only. They are rather to be compared with the Tubercula Quadri- gemina, which are the real ganglia of the Optic nerve. Their analogy is not so complete, however, to these bodies in the fully-formed Brain of Man, as it is to certain parts which occupy their place at an earlier period. The Third Ventricle^ which is quite distinct from the Corpora Q,uadrigemina, is hollowed out, as it we%, from the floor of the Optic Lobes of Fishes; and the Anterior Commissure bounds its front : hence these must be considered as analogous to the parts surrounding the Third Ventricle, as well as to the Corpora Cluadri- gemina. This is made evident by the fact, observed by Miiller, that, in the Lamprey, there is a distinct Lobe of the third ventricle, replacing the Optic Lobes of other fishes, and partly giving origin to the optic nerves ; and a sepa- rate vesicle, analogous to the Corpora Quadrigemina. With this condition, the early state of the* Brain in the embryo of the Bird and Mammiferous 160 FUNCTIONS OF THE NERVOUS SYSTEM. Fig. 28. Human embryo of sixth week, enlarged about thret times; a, vesicle of corpora quad- rigemina; b, vesicle of cerebral hemispheres; c. vesicle of thalami optici and third ventricle ; rf, vesicle for cerebellum and medulla oblon- gata; e, auditory vesicle; f, olfactory fossa; A, liver; **, caudal extremity. (After Wag- ner.) animal, and even in Man himself, bears a very close correspondence. The Erice- phalon consists, at this time, of a series of vesicles, arranged in a line with each other, of which those that represent the Cerebrum are the smallest, whilst that which repre- sents the Cerebellum is the largest. The latter, as in Fishes, is single, covering the fourth ventricle on the dorsal surface of the Medulla Oblongata. Anterior to this, is the single vesicle of the Corpora Gluadri- gemina, from which the Optic nerve chiefly arises : this has in its interior a cavity, the ventricle of Sylvius, which exists even in the adult Bird, where the Corpora duadri- gemina are pushed, as it were, from each other by the increased development of the Cerebral hemispheres. In front of this the vesicle of the Third Ventricle, is which contains also the Thalami: as de- velopment proceeds, this, like the preced- ing, is covered by the enlarged hemi- spheres; whilst its roof becomes cleft anteriorly on the median line, so as to form the anterior entrance to the cavity. Still more anteriorly is the double vesicle, which represents the hemispheres of the Cerebrum: this has a cavity on each side, the floor of which is formed by the corpora striata. The cavity of the cerebral vesicles has at first no opening, except into that of the third ventricle ; at a later period is formed that fissure on the inferior and posterior side, whick (under the name of the fissure of Sylvius) enables the membranes enveloping the brain to be reflected into the lateral ventricles. 215. Thus it will be seen that the real analogy between the brain of the Human foetus and that of the adult Fish, is not so close as, from the resem- blance in their external form, might have been supposed. In the small pro- portion which the Cerebral Hemispheres bear to the other parts, there is evidently a very close correspondence ; and this extends also to the general simplicity of their structure, the absence of convolutions, and the deficiency of commissures. But there is a much nearer analogy between the fatal brain of the Fish, and the fatal brain of the Mammal ; indeed, at the earliest period of their formation, they could not be distinguished ; during their advance to the permanent condition, however, each undergoes changes, which are so much more decided in the higher animals than in the lower, that in the latter there seems but little departure from the festal condition, whilst in the former the condition appears entirely changed. Hence it is not correct to assert, as is frequently done, that the Brain, or any other organ, in the higher animals, passes through a series of forms, which are parallel to the permanent forms of the same organ in different parts of the animal scale : since the Ja.ct is rather, that the more nearly all are traced back to their first origin, the closer will their conformity be found to be ; the subsequent development of each taking place not only in various degrees, but in different modes or directions ; so that the resemblances presented by the higher, at different epochs of their evolution, to the permanent conditions of the lower, are often far from being complete.* * For a fuller examination of this interesting question, see Gfeneral and Comparative Physiology, 244. ^ COMPARATIVE ANATOMY OF THE ENCEPHALON. 161 Fig. 29. This we have seen to be the case in the present instance ; the vesicle of the Corpora Q,uadrigemina, and that of the Third Ventricle, uniting to form the Optic Lobes of Fishes, whilst in the higher Vertebrata they remain distinct ; so that there is no single part with which the Optic Lobes can be properly compared, either in the fetal or perfect state of the Human Brain. 216. The Brain of Reptiles does not show any con- siderable advance in its general structure above that of Fishes ; but the Cerebral Hemispheres are usually much larger in proportion to the Optic lobes ; whilst the Cerebellum is smaller. The very low develop- ment of the Cerebellum is especially seen in the Frog (Fig. 16), in which it is so small as not even to cover- in the Fourth Ventricle ; but it is common to nearly the whole group. The deficiency in commissures still exists to a great extent. The Anterior Commissure in front of the third ventricle, is the only uniting band which can be distinctly traced in Fishes ; and Reptiles have, in addition to this, a layer of uniting fibres which may be compared to the Fornix ; but as yet there is no vestige of a true Corpus Callosum, or great transverse commissure of the hemispheres. The distinction between the tubercula quadrigemina, and the parts enclosing the third ventricle, is more obvious than in Fishes ; in fact the Optic ganglia of Reptiles correspond pretty closely with the Vesicle of the tubercula quadrigemina in the brain of the foetal Mammal. 217. This is still more evident in Birds, in whose Encephalon the Tubercula duadrigemina or Optic Ganglia, and the Thalami with their included ventri- cle, are obviously very distinct parts. The Cerebral Hemispheres attain a great increase of development, and arch backwards, so as partly to cover the Optic ganglia ; and these are separated from one another, and thrown to either side. The Cerebellum also is much increased in size, proportionably to the Medulla Oblongata and its Brain of Turtle ; A, olfactive ganglia; B, cerebral hemi- spheres; c, optic ganglia; D, cerebellum. Fig. 30. thai Brain of Buzzard ; the olfactive ganglia are concealed beneath B, the hemispheres ; c, op- tic ganglia; D, cerebellum; g, pineal gland. The hemispheres, B, drawn to either side to show the subjacent parts; c, the optic lobes; D, cerebellum; thai, thalamus optic us ; e s } corpus striattun. 162 FUNCTIONS OF THE NERVOUS SYSTEM. ganglia; and it is sometimes marked with transverse lines, which indicate the intermixture of gray and white matter in its substance ; there is as yet, however, no appearance of a division into hemispheres. On drawing apart the hemispheres of the Cerebrum, the Corpora Striata, Optic Thalami, and Tubercula duadrigemina or Optic Ganglia, are seen beneath them; the size of the last still bears a considerable proportion to that of the whole Encephalon. The Optic Ganglia are still hollow, as they are in the embryo condition of Man. Indeed the Brain of the Human foetus about the twelfth week will bear comparison, in many respects, with that of the Bird. The Cerebral hemispheres, much increased in size, and arching back over the Fig. 31. e..- Brain of human embryo at twelfth week; A, seen from behind; B, side view; c, sectional view a cor- pora quadrigemina; b b, hemispheres ; rf, cerebellum; e, medulla oblongata ;/, optic thalamus; g, floor of third ventricle ; i, olfactory nerve. (After Tiedemann.) Thalami and Optic ganglia, but destitute of convolutions and imperfectly con- nected by commissures, the large cavity still existing in the Optic ganglia, and freely communicating with the third ventricle, and the imperfect evolu- tion of the Cerebellum, make the correspondence in the general condition of the two very considerable. 218. The Brain of the lowest Mammalia presents but a slight advance upon that of Birds, in regard both to the relative proportions of its parts, and to their degree of development. Thus, in the Marsupialia, the Cerebral hemispheres exhibit no convolutions ; and the great transverse commissure, the Corpus Fig. 32. Agr Upper and under surface of Brain of Rabbit; A, B, , as before ; o I, olfactive lobes; o Coptic nerve- m o motor ocuh ; c m corpora mamillaria ; c c, crus cerebri ; ,, pons varolii ; p a, patheticus ; t r i, trifa' cial ; a b, abducens ; fa c, facial ; a u, auditory ; a g, vagus ; ,, spinal accessory ; h y p, hypoglossal. COMPARATIVE ANATOMY OF THE ENCEPHALON. 163 Callosum, is deficient. There is gradually to be noticed, however, in ascend- ing the scale, a backward prolongation of the Cerebral hemispheres ; so that first the Optic ganglia, and then the Cerebellum, are covered by them. The latter partly shows itself, however, in all but the Q,uadrumana, when we look at the brain from above downwards ; in the Rabbit, which is among the lowest in this respect of the true Viviparous Mammajia, nearly the whole of the Cere- bellum is uncovered. In proportion to the increase of the Cerebral hemi- spheres, there is a diminution in the size of the ganglia immediately connected with the organs of sense ; and this in comparison, not only with the rest of the Encephalon, but even with the Spinal Cord ; so that in Man the Tubercula Q,uadri gemma are absolutely smaller than they are in many animals of far inferior size. The internal structure of the hemispheres becomes more com- plex in the same proportion as their size and the depth of the convolutions increase ; and in Man all these conditions present themselves in a far higher degree than in any other animal. In fact it is only among the Ruminantia, [Fig. 33. A view of the base of the Cerebrum and Cerebellum, together with their nerves; 1, anterior extremity of the fissure of the hemispheres of the brain; 2, posterior extremity of the same fissure; 3. the anterior lobes of the cerebrum; 4, its middle lobe; 5, the fissure of Sylvius; 6, the posterior lobe of the cerebrum; 7, the point of the infundibulum ; 8, its body; 9, the corpora albicantia; 10, cineritious matter; 11, the crura cerebri; 12, the pons varolii; 13, the top of the medulla oblongata; 14, posterior prolongation of the pons varolii; 15, middle of the cerebellum; 16, anterior part of the cerebellum; 17, its posterior part and the fissure of its hemispheres; 18, superior part of the medulla spinalis; 19, middle fissure of the medulla oblongata; 20, the corpus pyramidale; 21, the corpus restiforme; 22, the corpus olivare; 23, the olfactory nerve; 24, its bulb; 25, its external root; 23, its middle root; 27, its internal root; 28, the optic nerve beyond the chiasm; 29, the optic nerve before the chiasm; 30, the motor oculi, or third pair of nerves ; 31, the fourth pair, or pathetic nerves; 32, the fifth pair, or trigemini nerves; 33, the sixth pair, or motor externus; 34, the facial nerve; 35, the auditory the two making the seventh pair; 36, 37, 38, the eighth pair of nerves. (The ninth pair is not here seen.) ] 164 FUNCTIONS OF THE NERVOUS SYSTEM. Pachydermata, Carnivora, and Quadrumana, that regular convolutions can be said to exist. The correspondence between the bulbous expansion of the Olfactive Nerves in Mammalia, and the Olfactive lobes of the lower Vertebrata, is made evident by the presence, in both instances, of a cavity which commu- nicates with the lateral ventricle on each side ; it is in Man only that this cavity is wanting. The external form of the Corpora Q,uadrigemina of Mammalia, differs from that of the Optic ganglia of Birds, owing to the division of the former into anterior and posterior eminences, (the nates and testes) ; and there is also an internal difference, occasioned by the contraction of the cavity or ventricle, which now only remains as the Aqueduct of Sylvius. The Cere- bellum is chiefly remarkable for the development of its lateral parts or hemi- spheres ; the central portion, sometimes*called the vermiform process, is rela- tively less developed than in the lower Vertebrata, in which it forms the whole of the organ. XIV. Functions of the Cephalic Nerves. 219. Before proceeding to inquire into the functions of the different parts of the Encephalon, it seems desirable to bring together what is known in regard to the functions of the Nerves specially connected with them ; so that, by tracing their connections, we may be able to obtain some light upon this very obscure, though most interesting and important subject. 220. That the First pair, or Olfactory nerves, minister to the sense of Smell, has long been known; yet it could not be . predicated without experimental inquiry, that it is not a conductor of the impressions which produce ordinary sensation; nor that it is destitute of all power of exciting muscular movement, either by direct or reflex action. Anatomical examination of the distribution of this nerve, proves that it is not one which directly conveys motor influence to any muscles ; since all its branches are distributed to the membrane lining the nasal cavity. Experimental inquiry leads to the same result ; for no irri- tation of the peduncles or branches excites any muscular movement. Further, no irritation of any part of this nerve excites reflex actions through other nerves. Again, it is not a nerve of common sensation; for animals exhibit no [Fig- 34. A view of the First pair, or Olfactory Nerves, with the Nasal Branches of the Fifth pair : 1, frontal sinus; 2, sphenoidal sinus; 3, hard palate ; 4, bulb of the olfactory nerve; 5, branches of the olfactory nerve on the superior and middle turbinated bones; G, spherio-palatine nerves from the second branch of the fifth pair; 7, internal nasal nerve from the first branch of the fifth: 8, branches of 7 to the Schneiderian mem- brane; 9. ganglion of Cloquet in the foramen incisivum; 10, anastomosis of the branches of the fi.th pair on the inferior lurbinated bone.] FUNCTIONS OF THE CEPHALIC NERVES. 1C5 sign of pain when it is subjected to any kind of irritation. Neither the divi- sion of the nerve, nor the destruction of the olfactive ganglia, seems to incon- venience them materially. They take their food, move with their accustomed agility, and exhibit the usual appetites of their kind. The common sensibility of the parts contained in the olfactive organ is in no degree impaired, as is shown by the effect of irritating vapours ; but the animals are destitute of the sense of smell, as is shown by the way in which these vapours affect them. At first they appear indifferent to their presence, and then suddenly and vehe- mently avoid them, as soon as the Schneiderian membrane becomes irritated. Moreover, if two dogs, with the eyes bandaged, one having the olfactory nerves and ganglia sound, and the other having had them destroyed, are brought into the neighbourhood of the dead body of an animal, the former will examine it by its smell ; whilst the latter, even if he touches it, pays no attention to it. This experiment Valentin states that he has repeated several times, and always with the same results. Further, common observation shows that sensibility to irritants, such as snuff, and acuteness of the power of smell, bear no constant proportion to one another; and there is ample pathological evidence that the want of this sense is connected with some morbid condition of the olfactory nerves or ganglia. It is well known that Magendie has main- tained that the Fifth pair in some way furnishes conditions requisite for the enjoyment of the sense of smell, asserting that, when it is cut, the animal is deprived of this. But his experiments were made with irritating vapours, which excite sternutation or other violent muscular actions, not through the olfactory nerve, but through the fifth pair: and the experiments of Valentin, just related, fully prove that the animals are not sensitive to odours, strictly so called, after the Olfactory has been divided. It is by no means improbable, however, that the acuteness of the true sense of smell may be diminished by section of the Fifth pair ; since the olfactory membrane is no longer duly moist- ened by its proper secretion ; and, when dry, it is not so susceptible of the impressions made by those minute particles of odoriferous substances to which the excitement of the sensation must be referred. 221. That the Second pair, or Optic nerves, have an analogous character, appears alike from anatomical and experimental evidence. No chemical or mechanical stimulus of the nerve produces direct muscular motion, nor does it give rise, as far as can be ascertained, to indications of pain; whence it may be concluded that this nerve is not one of common sensation. That the ordi- nary sensibility of the eyeball remains, when the functions of the Optic nerve are completely destroyed, is well known, as is also the fact that division of it puts an end to the power of vision. Valentin states that, although the Optic nerve may, like other nerves, be in appearance completely regenerated, he has never been able to obtain any evidence that the power of sight has been in the least degree recovered. He remarks that animals suddenly made blind exhibit great mental disturbance and perform many unaccustomed movements ; and that the complete absence of the power of vision is easily ascertained. Morbid changes are sometimes observed to take place in eyes whose Optic nerve has been divided; but these are by no means so constant or so extensive as when the Fifth pair is paralyzed; and they may not improbably be attri- buted to the injury, occasioned by the operation itself, to the parts within the orbit. It is well known that, when amaurosis is produced by a morbid condi- tion of the Optic nerve alone, the eye retains its usual appearance ; but, if the amaurosis be complete, the texture of the Retina undergoes a remarkable change, ceasing to exhibit that peculiar structure which normally characterizes it. Neither primitive nervous fibrils nor nucleated globules can be distin- guished in it; and the yellow spot of Soemmering becomes paler, and is at last undistinguishable. But, if a very slight degree of sensibility to light 166 FUNCTIONS OF THE NERVOUS SYSTEM. remain, these changes are much less decided. Further, it is well known that, when the sight is destroyed by a disease or injury, which prevents the passage of light through the pupil, the whole eye becomes more or less atrophied ; and the Retina and Optic nerve, although previously sound, are found after death (if the morbid condition have lasted sufficiently long) to have lost their charac- teristic structure. It seems evident, then, that the continuance of the functional operations of nerves, is a necessary condition of the maintenance of their nor- mal organization ; and we can very well understand that this should be the case, from the analogy of other parts of ( the system. 222. The Optic nerve, though analogous to the Olfactory in all the points hitherto mentioned, differs from it in one important respect; that it has the power of conveying impressions which shall excite reflex muscular motions. This is especially the case in regard to the Iris, the ordinary actions of which are regulated by the degree of light impinging on the retina. When the optic nerve is divided, a contraction of the pupil takes place ; but this does not occur if the connection of this nerve with the third pair, through the nervous cen- tres, be in any way interrupted. After such division (if complete), the state of the pupil is not affected by variations in the degree of light impinging on the retina, except in particular cases in which it is influenced through other chan- nels. Thus, in a patient suffering under amaurosis of one eye, the pupil of the affected eye is often found to vary in size, in accordance with that of the other eye; but this effect is produced by the action of light on the retina of the sound eye, which produces a motor change in the third pair on both sides. Further, as has been formerly stated ( 205), the impression only of light upon the retina may give rise to contraction of the pupil, by reflex action, when the optic nerve is itself sound; whilst no sensations are received through the eye, in consequence of disease in the sensorial portion of the nervous centres. Another cause has been pointed out by Valentin, for the influence of light in causing contraction of the pupil, and vice versa; that, if the rays impinge upon the iris, a reflex stimulation is produced through the fifth pair; and he remarks that the susceptibility of the iris to this kind of influence seems much increased after the optic nerve has been divided. Besides the contractions of the pupil, another action, which has been sometimes spoken of as reflex, is produced through the optic nerve the contraction of the orbicularis under the influence of strong light, or when a foreign body is suddenly brought near the eye. But this cannot be produced by any mechanical stimulation, and it evidently in- volves sensation; in fact, it is a movement of an emotional kind (Sect, xvui.), produced by the painful effect of light, which gives rise to the condition well characterized by the term photophobia. The involuntary character of it must be evident to every one who has been engaged in the treatment of diseases of the eyes; and the effect of it is aided by a similarly involuntary movement of the eyeball itself, which is rotated upwards and inwards to a greater extent than the Will appears able to effect. 223. It will be convenient next to advert to the Auditory nerve, or Portio Mpllis of the Seventh ; the functions of which are easily determined by anato- mical examination of its distribution, and by observation of pathological pheno- mena, to be analogous to triose of the two preceding. Atrophy or lesion of the trunk destroys the sense of Hearing ; whilst irritation of it produces audi- tory sensations, but does not occasion pain. From experiments made upon the nerve before it leaves the cranial cavity, it appears satisfactorily ascer- tained, that this nerve has no motor power either of a direct or reflex charac- ter, and that it is not endowed with common sensibility. It is interesting to remark, that microscopic examination of its structure clearly indicates its intermediate character, between the nerves of special sensation issuing from the anterior part of the cranium, namely, the Optic and Olfactory, -and those FUNCTIONS OF THE CEPHALIC NERVES. 167 A view of the origin and distribution of the Portio Mollis of the Seventh pair or Auditory Nerve; 1, the medulla oblongata ; 2, the pons varolii ; 3, 4, the crura cerebelli of the right side ; 5. the eighth pair of nerves ; 6, the ninth pair; 7, the auditory nerve distributed to the cochlea and labyrinth; 8, the sixth pair of nerves; 9, the portio dura of the seventh pair; 10, the fourth pair; 11, the fifth pair.] whose function is to minister, either [Fig. 35. to common sensation, or to that of taste which approaches nearly to it, namely, the Fifth pair and the Glosso-pharyngeal, which issue from the posterior part of the Ence- phalon, and are more nearly analo- gous to the Spinal nerves. The primitive fibres are not so soft as those of the Olfactive, nor so slender as those of the Optic ; and they are softer than those of the Glosso-pha- ryngeal. Moreover, the Auditory never forms a plexus with the Fa- cial, to which there is no analogy in the Optic and Olfactive nerves, but to which a similar one exists in the Glosso-pharyngeal. This inter- mediate structural character is inte- resting, when we compare it with the intermediate character of the function ; for the impressions made upon the sense of Hearing are pro- duced through vibrations of a mate- rial fluid, instead of being, as in the case of Sight, the result of changes so subtle as to be almost inscrutable to our means of research, or, as in the case of Taste and Touch, being produced by the direct contact of the substance which gives rise to the sensation. 224. Passing by for the present the Motor nerves of the Orbit, as consti- tuting a distinct subject for future inquiry, we may advantageously proceed with the other Sensory nerves connected with the Encephalon. It should be noticed, however, that the Third pair, or Motor Oculi, certainly possesses some degree of sensibility, as is evidenced by the signs of pain given by the animal when it is cut or compressed ; but this sensibility is not nearly so great as that of the Fifth pair ; and it may be doubted whether it is possessed by it, in virtue of its direct connection with the nervous centres, or whether it does not derive it by its anastomosis with the Fifth pair, some filaments of which may pass backwards as well as forwards, so as to confer sensibility on the Third pair, both before and after their junction with it. No sensory fibres can be proved to exist in the Fourth and Sixth nerves. 225. We next come to the Fifth pair, or Trifacial, the true nature of the functions of which was ascertained in part by Sir C. Bell; his views receiving modification, however, from the experimental researches of others. As for- merly stated, it possesses two distinct sets of roots, of which one is much larger than the other ; on the larger root, as on the posterior root of the spinal nerves, is a distinct ganglion ; and the fibres arising from the smaller root do not blend with the others, until after the latter have passed through this ganglion. The trunk of the nerve separates, as is well known, into three divisions, the Ophthalmic, the Superior Maxillary, and the Inferior Maxillary ; and it can easily be shown by careful dissection, that the fibres of the smaller root pass into the third of these divisions alone. When the distribution of this nerve is carefully examined, it is found that the first and second divisions of it pro- ceed almost entirely to the skin and mucous surfaces, a very small proportion only of their fibres being lost in the muscles ; but of the branches of the third division, a large part are distinctly muscular. Hence analogy, and the facts 168 FUNCTIONS OF THE NERVOUS SYSTEM. Fig. 36. A diagram showing the Fifth pair of Nerves with its Branches ; 1, the origin of the nerve by two roots; 2, the nerve escaping from the crus cerebelli ; 3, the Gasserian ganglion ; 4, its ophthalmic division ; 5, the frontal nerve, giving off the supra-trochlear branch, and escaping on the forehead through the supra-orbital foramen; 6, the lachrymal nerve ; 7, the nasal fierve, passing at 8 through the anterior ethmoidal foramen, and giving off the infra-trochlear branch ; 9, the communication of the nasal nerve with the ciliary ganglion ; 10, a small portion of the third nerve with which the ganglion is seen communicating; the ganglion gives off the ciliary branches from its anterior aspect; 11, the superior maxillary nerve ; 12, its orbital branch; 13, the two branches communicating with Meckel's ganglion ; the three branches given off from the lower part of the ganglion are the posterior palatine nerves; 14, 14, the superior dental nerves, posterior, middle and anterior; 15. the infra-orbital branches distributed upon the cheek; 16, the inferior maxillary nerve ; 17, its anterior or muscular trunk; 18, the posterior trunk; the two divisions are separated by an arrow; 19, the gustatory nerve ; 20, the chorda tympani joining it at an acute angle ; 21, the sub-maxillary ganglion ; 22, the inferior dental nerve; 23, its mylo-hyoidean branch; 24, the auricular nerve, dividing behind the articulation of the lower jaw, to reunite and form a single trunk ; 25, its branch of communication with the facial nerve ; 26, its temporal branch. [Fig. 37. A view of the distribution of the Trifacral or Fifth pair; 1, orbit ;-2, antrum highmorianum ; 3, tongue; 4, lower jaw-bone; 5, root of the fifth pair, forming the ganglion of Gasser; 6, first branch of the fifth pair, or ophthalmic; 7, second branch of the fifth pair, or superior maxillary; 8, third branch of the fifth pair, or inferior maxil- lary; 9, frontal branch, dividing into external and internal frontal nerves ; 10, lachrymal branch of the fifth pair; 11, nasal branch; just under the figure is the long root of the lenticular or ciliary ganglion and a few of the ciliary nerves ; 12, internal nasal nerve, disappearing through the anterior ethmoidal foramen; 13, external nasal nerve; 14, external and internal frontal nerve ; 15, infra-orbitary nerve ; 16, posterior dental branches; 17, middle dental branch; 18, anterior dental nerve; 19, terminating branches of the infra-orbital nerve, called the labial and palpe- bral nerves; 20, subcutaneous malce, or orbitar branch; 21, pterygoid, or recurrent nerve, from Meckel's ganglion; 22, five anterior branches of the third branch of the fifth pair; 23, lingual branch of the fifth, joined by the chorda tym- pani; 24, inferior dental nerve; 25, its mental branches ; 26, superficial temporal nerve ; 27, auricular branches ; 28, mylo-hyoid branch.] FUNCTIONS OF THE CEPHALIC NERVES. 169 supplied by anatomical research, would lead to the conclusion, that the two first divisions are nerves of sensation only, and that the third division combines sensory and motor endowments. Such an inference is fully borne out by experiment. When the whole trunk is divided within the cranium (which Magendie, by frequent practice, has been able to accomplish), evident signs of acute pain are given. After the incision has been made through the skin, the animal remains quiet until the nerve is touched ; and when it is pressed or divided, doleful cries are uttered, which continue for some time, showing the painful effect of the irritated state of the cut extremity. The common sensi- bility of all the parts supplied by this nerve is entirely destroyed on the affected side. The jaw does not hang loosely, because it is partly kept up by the mus- cles of the other side ; but it falls in a slight degree ; and its movements are seen, when carefully observed, to be somewhat oblique. If the trunk be divided on each side, the whole head is deprived of sensibility ; and the ani- mal carries it in a curious vacillating manner, as if it were a foreign body. 226. If the anterior or Ophthalmic branch only be divided, all the parts supplied by it are found to have lost their sensibility, but their motions are unimpaired ; and all experiments and pathological observations concur in attributing to it sensory endowments only. The only apparent exception is in the case of the Naso-Ciliary branch ; since there is good reason to believe that the long root of the ciliary ganglion, and the long ciliary nerves, possess motor powers ; but these appear to be derived from the Sympathetic nerve. When the whole nerve, or its anterior branch, is divided in the rabbit, the pupil is exceedingly contracted, and remains immovable ; but in dogs and pigeons it is dilated. The pupil of the other eye is scarcely affected ; or, if its dimen- sions be changed, it soon returns to its natural state. The eyeball speedily becomes inflamed, however; and the inflammation usually runs on to suppura- tion and complete disorganization. The commencement of these changes may be commonly noticed within twenty-four hours after the operation ; and they appear to be due to the want of the protective secretion, which (as will be explained when the direct influence of the nervous system upon the organic functions is considered) is necessary to keep the mucous surface of the eye in its healthy condition, and which is not formed when the sensibility of that surface is destroyed. The Superior Maxillary branch, considered in itself, is equally destitute of motor endowments with the ophthalmic ; but its con- nections with other nerves, through the spleno-palatine ganglion and its anasto- mosing twigs, may introduce a few motor fibres into it. The Inferior Maxillary branch is the only one which possesses motor as well as sensory endowments from its origin ; but its different subdivisions possess these endowments in varying proportions, some being almost exclusively motor, and others as com- pletely of a sensory character. The latter is probably the nature of the Lin- gual branch; and there seems good reason to believe, as will presently be shown, that this ministers not only to the tactile sensibility of the tongue, but to the sense of taste. The muscles put in action by this division of the Fifth pair, are solely those concerned in the masticatory movements. 227. The Portio Dura of the Seventh pair, or Facial nerve, has been sup- posed, since the first researches of Sir C. Bell, to be a nerve of motion only; but some recent physiologists have maintained that it both possesses sensory endowments and arises by a double root. The latter assertion is quite falla- cious ; and the most carefully conducted experiments do not bear out the for- mer. By exposing the roots of the Seventh pair within the cranium, Valentin ascertained that it possesses no sensory endowments at its origin ; since, when these were touched, the animals gave no signs of pain, though violent mus- cular movements were excited in the face. Subsequently to its first entrance into the canal by which it emerges, however, it anastomoses with other nerves ; 15 170 FUNCTIONS OF THE NERVOUS SYSTEM. Fig. 38. The distribution of the Facial Nerve, and the branches of the Cervical Plexus; 1, the facial nerve, escaping from the stylo-mastoid foramen, and crossing the ramus of the lower jaw; the parotid gland has been removed in order to see the nerve more distinctly ; 2, the posterior auricular branch; the digastric and stylo-mastoid filaments are seen near the origin of this branch ; 3, temporal branches, communicating with (4) the branches of the frontal nerve; 5, facial branches, communicating with (6) the infra-orbital nerve; 7, facial branches, communicating with (6) the mental nerve; 9, cervico-facial branches, commu- nicating with (10) the superficialis colli nerve, and forming a plexus (11) over the sub-maxillary gland ; the distribution of the branches of the facial in a radiated direction over the side of the face, constitutes the pes anserimus; 12, the auricularis magnus nerve, one of the ascending branches of the cervical plexus ; 13, the occipitalis minor, ascending along the posterior border of the sterno-mastoid muscle ; 14, the super- ficial and deep-descending branches of the cervical plexus ; 15, the spinal accessory nerve, giving off a branch to the external surface of the trapezius muscle : 10, the occipitalis major nerve, the posterior branch of the second cervical nerve. and thus sensory fibres are introduced into it from many different sources anteriorly, from the Fifth pair; and posteriorly, from the Cervical nerves which cause irritation of several of its branches to produce pain. The number and situation of the anastomoses vary much in different animals ; so that it is impossible to make any very comprehensive statement in regard to them. Experimental researches leave no doubt that the Portio Dura is the general motor nerve of the face, ministering to the influence of Volition and of Emo- tion, and also being the channel of the Reflex movements concerned in respi- ration and other associated movements of the muscles, but not being in the least concerned in the act of mastication.* 228. The functions of the Glosso-Pharyngeal nerve have been heretofore * The distinctness of the Spinal and Cerebral portions of this nerve is made evident by the not nnfrequent occurrence of paralysis in either of them, without the other being affected. Thus we may see the mouth 'drawn to one side (in consequence of the loss of tone which the muscles have experienced), and all the Reflex and Emotional actions of the face performed only on one side; and yet Voluntary power may remain unaffected. Thus, in ordinary winking, the lid of the affected side does not close, though the patient can shut the eye by an effort of the will. On the other hand, the tone of the muscles may remain unimpaired, and all their Reflex and Emotional actions may be performed as usual, and yet distortion may be at once apparent, when Voluntary actions are attempted, in consequence of paralysis of the Cerebral portion of the nerve on one side. FUNCTIONS OF THE CEPHALIC NERVES. 171 alluded to in part ; but there still remain several questions to be discussed in regard to them. Reasons have been given for the belief, that it is chiefly an afferent nerve, scarcely having any direct power of exciting muscular con- traction, but conveying impressions to the Medulla Oblongata, which produce reflex movements of the other nerves ( 192). Some experimenters assert, that they have succeeded in exciting direct muscular actions through its trunk. This is by no means impossible ; but if the truth of the statement be admitted, it does not invalidate the inferences regarding the general function of the nerve, deduced by Dr. Reid from minute anatomical investigation, and from a large number of experiments. Much controversy has taken place on the question, whether this nerve is to be regarded as ministering, partly or exclu- sively, to the sense of Taste; and many high authorities have ranged them- selves on each side. The question involves that of the function of the Lingual branch of the Fifth pair ; and it is partly to be decided by the anatomical rela- tions of the two nerves respectively. The glosso-pharyngeal is principally distributed on the mucous surface of the fauces, and on the back of the tongue. According to Valentin, it sends a branch forwards, on either side, somewhat beneath the lateral margin, which supplies the edges and inferior surface of the tip of the tongue, and inosculates with the Lingual branch of the Fifth pair. On the other hand, the upper surface of the front of the tongue is sup- plied by this lingual branch. The experiments of Dr. Alcock, whose conclu- sions are borne out by Dr. J. Reid, decidedly support the conclusion, that the gustative sensibility of this part of the tongue is due to the latter nerve, being evidently impaired by division of it. Moreover, cases are by no means rare, in which the gustative sensibility of the anterior part of the tongue has been destroyed, with its tactual sensibility ; when there was no reason to suppose that any other than the Fifth pair of nerves was involved.* On the other hand, it is equally certain, that the sense of taste is not destroyed by section of the Lingual nerve on each side ; and it seems also well ascertained, that it is impaired by section of the Glosso-pharyngeal nerve. Considering how nearly allied is the sense of taste to that of touch, and bearing in mind the respective distribution of these two nerves, it does not seem difficult to arrive at the con- clusion, that both nerves are concerned in this function ; but there seems good reason to believe the Glosso-pharyngeal to be exclusively that through which the impressions made by disagreeable substances taken into the mouth are propagated to the Medulla Oblongata, so as to produce nausea and to excite efforts to vomit. 229. The functions of the Par Vagum at its roots have lately been made the subject of particular examination by Valentin; and he has arrived at the very interesting result, that it there possesses no motor power, but is entirely a sensory or rather an afferent nerve. He states that, if the roots be carefully separated from those of the Glosso-Pharyngeal, and (which is a matter of some difficulty) from those of the Spinal Accessory nerve, and be then irritated, no movements of the organs supplied by it can be observed. On the other hand, if the roots be irritated whilst in connection with the nervous centres, muscular contractions, evidently of a reflex character, result from the irritation ; and strong evidences of their sensibility are also given. On the other hand, again, when the roots of the Spinal Accessory nerve are irritated, no indications of sensation 'are given ; but the muscular parts supplied by the Par Vagum, as well as by its own trunk, are made to contract, even when the roots are sepa- rated from the nervous centres ; so that these roots must be regarded as the channel of the motor influence, transmitted to them from the Medulla Oblon- gata. When the Par Vagum swells into the jugular ganglion, an interchange * Romberg, in Mailer's Archiv. 1838, Heft iii. 172 FUNCTIONS OF THE NERVOUS SYSTEM. Fig. 39. [Fig. 40. Origin and distribution of the Eighth pair of nerves ; 1, 3, 4, the medulla oblongaja; 1, the corpus pyra- midale of one side; 3, the corpus olivare ; 4, the cor- pus restiforme; 2, the pons varolii; 5, the facial nerve ; 6, the origin of the glosso-pharyngeal nerve ; 7, the ganglion of Andersch ; 8, the trunk of the nerve ; 9, the spinal accessory nerve ; 10, the ganglion of the pneumogastric nerve; 11, its plexiform ganglion; 12, its trunk ; 13, its pharyngeal branch forming the pha- ryngeal plexus (14) assisted by a branch from the glosso-pharyngeal (8) and one from the superior laryn- geal nerve (15) ; 16, cardiac branches ; 17, recurrent laryngeal branch; 18, anterior pulmonary branches; 19, posterior pulmonary branches; 20, cesophageal plexus; 21, gastric branches; 22, origin of the spinal accessory nerve ; 23, its branches distributed to the sterno-mastoid muscle ; 24, its branches to the trape- zius muscle. A, view of the distribution of the Glosso-Pha- ryngeal Pneumogastric and Spinal Accessory NeBres,orthe Eighth pair; l,the inferior maxillary nerve; 2, the gustatory nerve; 3, the chorda-tym- pani; 4, the auricular nerve; 5, its communication with the portio dura; 6. the facial nerve coming out of the stylo-mastoid foramen; 7, the glonso- pharyngeal nerve; 8, branches to the stylo-pha- ryngeus muscle; 9, the pharyngeal branch of the pneumogastr c nerve descending to form the pha- ryngeal plexus; 10, branches of the glosso-pha- ryngeal to the pharyngeal plexus ; 11, the pneumo- gastric nerve; 12, the pharyngeal plexus; 13, the superior laryngeal branch; 14, branches to the pharyngeal plexus ; 15, 15, communication of the superior and inferior laryngeal nerves; 16, car- diac branches ; 17, cardiac branches from the right pneumogastric nerve ; 18, the left cardiac gan- glion and plexus; 19, the recurrent or inferior laryngeal nerve ; 20, branches sent from^the curve of the recurrent nerve to the pulmonary plexus; 21, the anterior pulmonary plexus; 22,22, the osso- phagcal plexus.] FUNCTIONS OF THE CEPHALIC NERVES. 173 of fibres takes place between it and the Spinal Accessory ; but many more fibres can be traced from the latter into the former than from the former into the latter. Hence it results that, of the branches into which the Par Vagum subsequently divides, many enjoy a high degree of motor power; whilst those of the Spinal Accessory do not appear to possess any great share of sensibility. The pharyngeal branches are among the most decidedly motor of all those given off from the Pn.eumogastric ; and these may in great part be traced backwards into the Spinal Accessory. Hence the idea of Arnold and Scarpa, that the Par Vagum and Spinal Accessory are together analogous to a spinal nerve, the former answering to the posterior roots, and the latter to the anterior, appears sufficiently probable. In regard to its trunk, however, the Par Vagum must of course be considered as a nerve of double endowments. The chief function of its afferent portion is to convey to the Medulla Oblongata the impression produced by venous blood in the capillaries of the lungs, or of carbonic acid in the air-cells ; this impression may give rise, as we have seen, to the respiratory movements, without producing sensation ; but, if it be from any cause stronger than usual, the sense of uneasiness which it occasions is very distressing. This impression may be imitated by pressure on the nerve, which induces an immediate inspiratory movement. That the nerve is capa- ble of conveying those impressions, which become sensations when, communi- cated to the sensorium, is further evident from the fact that, when its trunk is pinched, the animal gives signs of acute pain. Besides the pulmonary impressions, this nerve also conveys to the Medulla Oblongata those which originate in the mucous surface of the larynx, trachea and bronchi, as well as on the lower part of the ossophagus and on the walls of the stomach. The purpose of these is to stimulate various movements, which are performed through the motor portion of the nerve ; this excites the actions of the muscles of the pharynx and larynx, of the oesophagus, and, in some degree, of the stomach and respiratory tubes. 230. The section of the Par Vagum produces, as would readily be expected, great disorder of the functions of Respiration and Digestion, to which it minis- ters. It is an operation which has been very frequently performed; and the statements of its results vary considerably amongst each other, being generally influenced, in some degree, by the preconceived views of the experimenter.* The section of the Par Vagum, when practised with the view of ascertaining the influence of the nerve upon the lungs and stomach, is usually made in the neck, between the origins of the superior and inferior (or recurrent) laryngeal branches. Hence the muscles of the larynx are paralyzed; 'and, if the animal should struggle violently, the ingress of air is likely to be ob- structed by the flapping down of the arytenoid cartilages, and by the closure of the glottis. This is especially the case in young animals, in which the larynx is small. But in those that are full grown, and ha've a large larynx, an adequate quantity of air may still find its way through the aperture, if the animal refrain from any violent effort. In a considerable number of Dr. Reid's experiments, therefore, he did not find it necessary to introduce the trachea- tube, which other experimenters have generally employed. An opening was made into the trachea, however, in those instances in which, from any cause, the entrance of air was obstructed. 231. The real character of the morbid changes in the lungs which are induced by cutting the Par Vagum, the order in which they arise, and the causes to which they are immediately due, constitute very interesting subjects of * The Author employs, as in his opinion the most worthy of confidence, the experi- ments of Dr-. J. Reid, (Edinb. Med. & Surg. Journ., Vols. xiix. and li.,) on whose accu- racy he has strong personal reasons for placing reliance; and whose anatomical and pathological attainments are such as to render him fully competent to the task. 15* 174 FUNCTIONS OF THE NERVOUS SYSTEM. investigation; and the knowledge of them will probably throw light upon many ill-understood morbid phenomena. In the first place, it has been fully established by Dr. Reid, that section of the Vagus on one side only does not necessarily, or even generally, induce disease of that lung; and hence the important inference may be drawn, that the nerve does not exercise any imme- diate influence on its functions. When both Vagi are divided, however, the animal rarely survives long ; but its death frequently results from the disorder of the digestive functions. Nevertheless, the power of digestion is sometimes restored sufficiently to re-invigorate the animals ; and their lives may then be prolonged for a considerable time. In fifteen out of seventeen animals experi- mented on by Dr. Reid, the lungs were found more or less unfit for the healthy performance of their functions. The most common morbid changes were a congested state of the blood-vessels, and an effusion of frothy serum into the air-cells and bronchial tubes. In eight out of the fifteen, these changes were strongly marked. In some portions of the lungs, the quantity of blood was so great as to render them dense. The degree of congestion varied in different parts of the same lung ; but it was generally greatest at the most depending portions. The condensation was generally greater than could be accounted for by the mere congestion of blood in the vessels ; and probably arose from the escape of the solid parts of the blood into the tissue of the lung. In some instances, the condensation was so great that considerable por- tions of the lung sank in water and did not crepitate ; but they did not present the granulated appearance of the second stage of ordinary pneumonia. In five cases, in which the animals had survived a considerable time, portions of the lungs exhibited the second, and even the third stages of pneumonia, with puri- form effusion into the small bronchial tubes ; and, in two, gangrene had super- vened. 232. One of the most important points to ascertain, in an investigation of this kind, is the first departure from a healthy state, to decide whether the effusion of frothy reddish serum, by interfering with the usual change in the lungs, causes the congested state of the pulmonary vessels and the laboured respiration ; or whether the effusion is the effect of a previously congested state of the blood-vessels. The former is the opinion of many physiologists, who have represented the effusion of serum as a process of morbid secretion, directly resulting from the disorder of that function, produced by the section of the nerve. The latter appears the unavoidable inference from the carefully noted results of Dr. Reid's experiments. In several of these, only a very small quantity of frothy serum was found in the air-tubes, even when the lungs were found loaded with blood, and when the respiration before death was very laboured. This naturally leads us to doubt whether the frothy serum is the cause of the laboured respiration, and of the congested state of the pulmonary vessels, in those cases where it is present ; though there can be no doubt that, when once it is effused, it must powerfully tend to increase the difficulty of respiration, and still further to impede the cir- culation through the lungs. Dr. R. has satisfied himself of an important point, which has been overlooked by others that this frothy fluid is not mucus, though occasionally mixed with it; but that it is the frothy serum so frequently found in cases where the circulation through the lungs has been impeded before death. From this and other facts, Dr. R. concludes "that the congestion of the blood-vessels is the first departure from the healthy state of the lung, and that the effusion of frothy serum is a subsequent effect." 233. The next point, therefore, to be inquired into, is the cause of this con- gestion; and this is most satisfactorily explained upon the general principles regulating the circulation of the blood, by remembering that section of the Par Vagum greatly diminishes the frequency of the respiratory movements, and FUNCTIONS OF THE CEPHALIC NERVES. 175 that the quantity of air introduced into the lungs is, therefore, very insufficient for the due aeration of the blood. We shall hereafter see reason to regard it as one of the best-established principles in Physiology, that the activity of the changes which the blood undergoes, in the capillary vessels, does, in some way or other, regulate its movement through them; that, when these changes are proceeding with activity, the capillary circulation is proportionably acce- lerated; and that, when they are abnormally low in degree, the movement of the blood in the capillaries is stagnated. There is now abundant evidence, in regard to the Pulmonary circulation in particular, that, to prevent the admis- sion of oxygen in the lungs, either by causing the animal to breathe pure nitrogen or hydrogen, or by occlusion of the air-passages, is to bring the circu- lation through their capillaries to a speedy check. Hence we should at once be led to infer, that diminution in the number of Respiratory movements would produce the same effect; and as little or no difference in their frequency is produced by section of one Vagus only, the usual absence of morbid changes in the lung supplied by it is fully accounted for. The congestion of the ves- sels, induced by insufficient aeration, satisfactorily accounts, not only for the effusion of serum, but also for the tendency to pass into the inflammatory con- dition, sometimes presented by the lungs, as by other organs similarly affected. Dr. Reid confirms this view, by the particulars of cases of disease in the human subject, in which the lungs presented, after death, a condition similar to that observed in the lower animals after section of the Vagi ; and, in these individuals, the respiratory movements had been much less frequent than natural during the latter part of life, owing to a torpid condition of the nervous centres. The opinion (held especially by Dr. Wilson Philip) that section of the par vagum produces the serous effusion, by its direct influence on the function of Secretion, is further invalidated by the fact stated by Dr. Reid, that he always found the bronchial membrane covered with its true mucus, except when inflammation was present. 234. " The experimental history of the Par Vagum," it is justly remarked by Dr. Reid, " furnishes an excellent illustration of the numerous difficulties with which the physiologist has to contend, from the impossibility of insu- lating any individual organ from its mutual actions and reactions, when he wishes to examine the order and dependence of its phenomena." In such investigations, no useful inference can be drawn from one or two experiments only ;'in order to avoid all sources of fallacy, a large number must be made ; the points in which all agree must be separated from others in which there is a variation of results ; and it must be then inquired, to what the latter is due. These observations apply equally to the other principal subject of inquiry in regard to the functions of the Par Vagum, its influence upon the process of Digestion. The results obtained by different experimenters have led to differ- ences of opinion as to its action no less remarkable than those on the question just discussed. Dr. Wilson Philip has long maintained that the par vagum controls the secretion of gastric juice, which he stated to cease when the nerve is divided ; and he further stated, that the influence of galvanism, propagated along the nerve, would re-establish the secretion. This statement has been quoted and re-quoted as an established physiological position ; and, when united with the well-known fact that galvanism would excite muscular con- traction, it has seemed to Dr. W. Philip and other physiologists sufficient to establish the important position, that galvanism and nervous influence are identical. The statement, however, has been disputed by many other experi- menters, who have satisfied themselves that the secretion of gastric juice con- tinued, and that the impairment of the digestive power, which is certainly a result (for a time at least) of the operation, may be attributed to paralysis of the muscular coat of the stomach. 176 FUNCTIONS OF THE NERVOUS SYSTEM. 235. The experiments of Dr. Reid do not furnish grounds for a positive determination of the functions of the gastrk portion of the Par Vagum ; but they furnish important correction of the results obtained by others. He has succeeded, as formerly stated ( 198), in producing movements of the stomach, by irritation of the Vagi ; but that these movements may be excited in other ways, is evident from the fact that, in several of his experiments, food was digested and propelled into the duodenum subsequently to the operation. The same fact, which he appears to have fully substantiated, is an incontrovertible proof, that the secretion of gastric juice is not dependent on nervous influence supplied by the Par Vagum, though doubtless in part regulated by it. The first effects of the operation, however, are almost invariably found to be vomit- ing (in those animals capable of it), loathing of food, and arrestment of the digestive process ; and it is not until after four or five days, that the power seems re-established. In the animals which died before that time, no indica- tion of it could be discovered by Dr. R.; in those which survived longer, great emaciation took place ; but when life was sufficiently prolonged, the power of assimilation seemed almost completely restored. This was the case in four out of the seventeen dogs experimented on ; and the evidence of this restora- tion consisted in the recovery of flesh and blood by the animals, the vomiting of half-digested food permanently reddening litmus paper, the disappearance of a considerable quantity of alimentary matter from the intestinal canal, and the existence of chyle in the lacteals. It may serve to account in some degree for the contrary results obtained by other experimenters, to state that seven out of Dr. R.'s seventeen experiments were performed before he obtained any evidence of digestion after the operation ; and that the four which furnished this followed one another almost in succession ; so that it is easy to understand why those, who were satisfied with a small number of experiments, should have been led to deny it altogether. [*] 236. Another series of experiments was performed by Dr. Reid, for the pur- pose of testing the validity of the results obtained by Sir B. Brodie, relative to the effects of section of the Par Vagum upon the secretions of the stomach, after the introduction of arsenious acid into the system. According to that eminent Surgeon and Physiologist, when the poison was introduced after the Par Vagum had been divided on each side, the quantity of the protective [* M.Bernard has instituted fresh experiments to determine this still-debated question, making use of the artificial fistulous openings into the stomach, invented by M. Blondlot. A dog's digestion had been thus watched for'eight days, and had always been well effected. On the ninth day, after a day's fast, M. Bernard sponged out the stomach, which contracted on the contact of the sponge, and at once secreted a large quantity of gastric fluid; he then divided the pneumogastric nerves in the middle of the neck, and immediately the mucous membrane, which had been turgid, became pale, as if exsan- guine, its movements ceased, the secretion of gastric fluid was instantaneously put a stop to, and a quantity of ropy neutral mucus was soon produced in its place. After this, no digestion was duly performed, and milk was no longer coagulated; raw meat remained unchanged, and the food (meat, milk, bread and sugar, which the dog had before tho- roughly digested) remained for a long time neutral, and at last acquired acidity only from its own transformation into lactic acid. In the stomachs of other dogs after the division of the nerves, he traced the transformation of cane-sugar into grape-sugar in three or four hours; and in ten or twelve hours the transformation into lactic acid was complete. In others, when the food was not capable of an acid transformation, it remained neutral to the last. In no case did any part of the food pass through the peculiar changes of chymification. In a last experiment, he gave to each of two dogs, in one of which he had cut the nerves, a dose of emulsine and half an hour after, a dose of amygdaline (substances which are innocent alone, but when mixed produce hydrocyanic acid). The dog, whose nerves were cut, died in a quarter of an hour, the substances being absorbed unaltered and mixing in the blood: in the other, the emulsine was changed by the action of the gastric fluid before the amygdaline was administered, and it survived. Gazette Mid., Juin 1, 1844, from the Report of the Acad. des ScL, seance du 27 Mai, 1844. M. 0.] FUNCTIONS OF THE CEPHALIC NERVES. 177 mucous and watery secretions was much less than usual, although obvious marks of inflammation were present. In order to avoid error as much as pos- sible, Dr. Reid made five sets of experiments, employing two dogs in each, as nearly as possible of equal size and strength, introducing the same quantity of the poison into the system of each in the same manner, 'but cutting the Vagi in one, and leaving them entire in the other. This comparative mode of experimenting is obviously the only one admissible in such an investigation. Its result was in every instance opposed to the statements of Sir B. Brodie ; the quantity of the mucous and watery secretions of the stomach being nearly the same in each individual of the respective pairs subjected to experiment; so that they can no longer be referred to the influence of the Eighth pair of nerves. Moreover, the appearances of inflammation were, in four out of the five cases, greatest in the animals whose Vagi were left entire ; and this seemed to be referable to the longer duration of their lives after the arsenic had been introduced. The results of Sir B. Brodie's experiments may perhaps be ex- plained, by the speedy occurrence of death in the subjects of them,consequent (it may JDC) upon the want of sufficiently free respiration, which was carefully guarded against by Dr. Reid. 2*37. So far as the results of Dr. Reid's experiments may be trusted to, therefore, (and the Author is himself disposed to rely on them almost impli- citly,) all me arguments which have been drawn, in favour of the doctrine that secretion depends upon nervous agency, from the effects of lesion of the Vagi upon the functions of the stomach, must be set aside. That this nerve has an important influence on the gastric secretion, is evident from the deficiency in its amount, soon after the operation, as well as from other facts. But this is a very different proposition from that just alluded to; and the difference has been very happily illustrated by Dr. R. " The movements of a horse," he observes, "are independent of the rider on his back; in other words, the rider does not furnish the conditions necessary for the movements of the horse ; but every one knows how much these movements may be influenced by the hand and heel of the rider." It may be hoped, then, that physiologists will cease to adduce the oft-cited experiments of Dr. Wilson Philip, in favour of the hypothesis (for such it must be termed) that secretion is dependent upon nervous influence, and that this is identical with galvanism. Additional evi- dence of their fallacy is derived from the fact mentioned by Dr. Reid, that the usual mucous secretions of the stomach were always found ; and they are fur- ther invalidated by the testimony of Miiller, who denies that galvanism has any influence in re-establishing the gastric secretion, when it has been checked by secretion of the nerves. 238. It only remains to notice the influence of section of the Vagi upon the actions of the Heart. It has been already stated that mechanical irritation of these nerves, especially at these roots, has a tendency to excite or accelerate the heart's action. It remains to inquire, if its movements are dependent upon their influence ; or if these nerves form the channel through which they are affected by emotions of the mind, or by conditions of the bodily system. In regard to the first point, no doubt can be entertained, since the regular move- ments of the heart are but little affected by section of the Vagi. With respect to the second, there is more difficulty, since the number of causes which may influence the rapidity and pulsations of the heart, is very considerable. For example, when the blood is forced on more rapidly towards the heart, as in exer- cise, struggling, &c., the stimulus to its contractions is more frequently renewed, and they become more frequent; and when the current moves on more slowly, as in a state of rest, their frequency becomes proportionabry diminished. If the contractions of the heart were not- dependent upon the blood, and their ttumber were not regulated by the quantity flowing into its cavities, very 178 FUNCTIONS OF THE NERVOUS SYSTEM. serious and inevitably fatal disturbances of the heart's action would soon result. That this adjustment takes place otherwise than through the medium of the nervous centres, ' is evident from the fact that, in a dog, in which the par vagum and sympathetic had been divided in the neck on each side, violent struggling, induced by alarm, raised the number of pulsations from 130 to 260 per minute. It is difficult to ascertain, by experiment upon the lower animals, whether simple emotion, unattended with struggling or other exertion, would affect the pulsation of the heart, after section of the Vagi ; but when the large proportion of the Sympathetic nerves proceeding to this, organ is considered, and when it is also remembered that irritation of the roots of the upper cervi- cal nerves stimulates the action of the heart through these, we can scarcely doubt that both may serve as the channels of this influence, especially in such animals as the dog, in which the two freely inosculate in the neck. 239. In regard to the functions of the Spinal Accessory nerve, also, there has been great difference of opinion; the peculiarity of its origin and course having led to the belief that some very especial purpose is answered by it. We shall first examine what evidence of its character may be obtained from its anatomy only. Its filaments come off from the middle column of the Spinal Cord, most frequently as low down as the origins of the sixth and seventh Cervical nerves. In its course upwards to the foramen magnum, it lies between the posterior roots of the spinal nerves and the ligamentum denticu- latum. It sometimes receives filaments from these roots, and is generally connected especially with the first cervical. According to Bellingeri, however, who has paid great attention to the subject, the filaments coming from the pos- terior roots do not form part of the trunk of the nerve, but leave it again to enter the posterior root of the first cervical. It may be doubted whether this is entirely true ; as some experiments appear to show that the Spinal Acces- sory is in some degree a sensory nerve, even at its roots. As the trunk passes through 'the foramen lacerum, it divides into two branches ; of which the internal, after giving off some filaments that assist in forming the pharyngeal branch of the Par Vagum, becomes incorporated with the trunk of that nerve, whilst the external proceeds outwards, and is finally distributed to the sterno- cleido-mastoideus and trapezius muscles, some of its filaments inosculating with those of the cervical plexus. It is interesting to remark, that the junction of the anterior branch with the Par Vagum, beyond the point at which the latter swells out into its superior ganglion, increases the analogy, which has been sustained upon other grounds, between the compound trunk thus formed and that of the spinal nerves ; the Par Vagum being regarded as the sensory root, and the Spinal Accessory as the motor. According to Valentin, however, there is not a mere passage of filaments from the Spinal Accessory to the Par Vagum, but an absolute interchange ; the trunk of the former containing some sensory fibres derived from the latter. When the roots of the Spinal Acces- sory are irritated, as appears from the experiments of Valentin, no decided indications of sensation can be obtained ; but all the motor actions of the Par Vagum manifest themselves. When the external branch is irritated, before it perforates the sterno-mastoid muscle, vigorous convulsive movements of that muscle, and of the trapezius, are produced; and the animal does not give any signs of pain, unless the nerve is firmly compressed between the forceps, or is included in a tight ligature. Hence it may be inferred that the functions of this nerve are chiefly motor, and that its sensory filaments are few in number. Further, when the nerve has been cut across, or firmly tied, irritation of the lower end is attended by the same convulsive movements of tlie muscles ; whilst irritation of the upper end, in connection with the spinal cord, is unat- tended with any muscular movement. Hence it is clear that the motions occasioned by irritating it are of a direct, not of a reflex character. The same 1 FUNCTIONS OF THE CEPHALIC NERVES. 179 muscular movements are observed on irritating the nerve in the recently killed animal, as during life. 240. According to SirC. Bell, the Spinal Accessory is a purely Respiratory nerve, whose office it is to excite the involuntary or automatic movements of the muscles it supplies, which share in the act of respiration ; and he states that the division of it paralyzes the muscles to which it is distributed, as muscles of respiration ; though they still perform the voluntary movements, through the medium of the spinal nerves. Both Valentin and Dr. Reid, how- ever, positively deny that this is the case. Dr. Reid's method of experiment- ing was well adapted to test the truth of the assertion. Considering that, in the ordinary condition of the animal, it might be difficult to distinguish the actions of particular muscles, beneath the skin, when those in the neighbour- hood were in operation ; and also that the usual automatic movements might be stimulated by voluntary action, when the breathing might be rendered difficult ; he adopted the following plan : A small dose of prussic acid was given to an animal, in which the Spinal Accessory had been previously divided on one side ; and after the convulsive movements produced by it had ceased, the animal was generally found in a state similar to that which we sometimes see in apoplexy, the action of the heart going on, the respirations being slow and heaving, and the sensorial functions appearing to be completely suspended. The Respiratory movements always ceased before the action of the heart ; but they continued, in several of the animals experimented on, sufficiently long to allow the muscles of the anterior part of the neck to be laid bare, so that accurate observations could be made upon their contractions. In the dog and cat, the sterno-mastoid does not appear to have much participation in the ordinary movements of respiration ; for in several instances it could not be seen to contract on either side, though the head was forcibly pulled towards the chest at each inspiratory movement, chiefly by the action of the sterno- hyoid and thyroid muscles. In two dogs and one cat, however, in which the head was fixed, and these respiratory movements were particularly vigorous, distinct contractions were seen in the exposed sterno-mastoid muscles, syn- chronous with the other movements of respiration: these were, perhaps, somewhat weaker on the side on which the nerve had been cut, but were still decidedly present. In one of these dogs, similar movements were ob- served in the trapezius, on the side on which the nerve had been divided. As the condition of the animal forbade the idea that volition could be the cause of these movements, it can scarcely be questioned that Sir C. Bell's statement was an erroneous one. As far, therefore, as these experiments afford any positive data, in regard to the functions of this nerve, it may be concluded that they are the same as those of the cervical plexus, with which it anasto- moses freely. "Future anatomical researches," as Dr. Reid justly remarks, " may perhaps explain to us how it follows this peculiar course, without obliging us to suppose that it has a reference to any special function in the adult of the human species." Thus, the study of the history of development has accounted satisfactorily for the peculiar course of the recurrent laryngeal, which may be traced passing directly from the par vagum to the larynx, at a time when the neck can scarcely be said to exist, and when that organ is buried in the thorax. As this rises in the neck, the nerve which at first came off below the great transverse blood-vessels, has both its origin and its termina- tion carried upwards ; whilst it is still tied down by these vessels in the middle of its course. 241. The Hypoglossal nerve, or Motor Liriguse, is the only one which, in the regular order, now remains to be considered. That the distribution of this nerve is restricted to the muscles of the tongue, is a point very easily established by anatomical research ; and accordingly we find that, long before 180 FUNCTIONS OF THE NERVOUS SYSTEM. the time of Sir C. Bell, Willis spoke of it as the nerve of the motions of articu- lation, whilst to the Lingual branch of the fifth pair he attributed the power of exercising the sense of taste ; and he distinctly stated, that the reason of this organ being supplied with two nerves is its double function. The inference that it is chiefly, if not entirely, a motor nerve, which has been founded on its anatomical distribution, is supported also by the nature of its origin, which is usually from a single root, corresponding to the anterior root of the Spinal nerves. Experiment shows that, when the trunk of the nerve is stretched, pinched, or galvanized, violent motions of the whole tongue, even to its tip, are occasioned ; and also, that similar movements take place after division of the nerve, when the cut end most distant from the brain is irritated. In regard to the degree in which this nerve possesses sensory properties, there is some difference of opinion amongst physiologists, founded, as it would seem, on a variation in this respect between different animals. Indications of pain are usually given, when the trunk is irritated after its exit from the cranium ; but these may proceed from its free anastomosis with the cervical nerves, which not improbably impart sensory fibres to it. But in some Mammalia, the hypo- glossal nerve has been found to possess a small posterior root with a ganglion : this is the case in the ox, and also in the rabbit ; and in the latter animal, Valentin states that the two trunks pass out from the cranium through separate orifices, and that, after their exit, one may be shown to be sensory, and the other to be motor. Hence this nerve, which is the lowest of those that ori- ginate in the cephalic prolongation of the spinal cord generally known as the medulla oblongata, approaches very closely in some animals to the regular type of the spinal nerves ; and though in Man it still manifests an irregularity, in having only a single root, yet this irregularity is often shared by the first cervical nerve, which also has sometimes an anterior root only. 342. The Hypoglossal nerve is distributed not merely to the tongue, but to the muscles of the neck which are concerned in the movements of the larynx ; and the purpose of this distribution is probably to associate them in those actions which are necessary for articulate speech. Though all the motions of the tongue are performed through the medium of this nerve, yet it would appear, from pathological phenomena, to have at least two distinct connections with the nervous centres ; for in many cases of paralysis, the masticatory movements of the tongue are but little affected, when the power of articula- tion is much injured or totally destroyed ; and the converse may be occa- sionally noticed. When this nerve is paralyzed on one side, in hemiplegia, it will be generally observed that the tongue, when the patient is directed to put it out, is projected towards the palsied side of the face : that is due to the want of action of the lingual muscles of that side, which do not aid in pushing forward the tip; the point is consequently directed only by the muscles of the other side, which will not act in a straight direction, when unantagonized by their fellows. It is a curious fact, however, that the hypoglossal nerve seems not to be always palsied on the same side with the facial, but sometimes on the other. This has been suggested to be due to the origination of the roots of this nerve from near the point at which the pyramids of the medulla oblongata decussate : so that some of its fibres come off, like those of the spinal nerves, without crossing ; whilst others are transmitted to the opposite side, like those of the higher cerebral nerves ; and the cause of paralysis may affect one or other of these sets of roots mortfparticularly. Whatever may be the validity of this explanation, the circumstance is an interesting one, and well worthy of attention.* * It may be questioned, however, whether the Hypoglossal is really paralyzed on the opposite side from the facial in such cases. An instance has been communicated to the FUNCTIONS OF THE CEPHALIC NERVES. 181 [Fig. 41. The course and distribution of the Hypo-Glossal or Ninth pair of nerves; the deep-seated nerves of the neck are also seen; 1, the hypo-glossal nerve ; 2, branches communicating with the gustatory nerve; 3, a branch to the origin of the hyoid muscles ; 4, the descendens noni nerve; 5, the loop formed with the branch from the cervical nerves; 6, muscular branches to the depressor muscles of the larynx; 7, a filament from the second cervical nerve, and 8, a filament from the third cervical, uniting to form the communicating branch with the loop from the descendens noni; 9, the auricular nerve; 10, the inferior dental nerve; 11, its mylo-hyoideau branch; 12, the gustatory nerve; 13, the chorda-tympani passing to the gustatory nerve ; 14, the chorda-tympani leaving the gustatory nerve to join the sub-maxillary ganglion ; 15, the sub-maxil- lary ganglion ; 16, filaments of communication with the lingual nerve; 17, the glosso-pharyngeal nerve; 18, the pneumogastric or par vagum nerve; 19, the three upper cervical nerves; 20, the four inferior cer- vical nerves ; 21, the first dorsal nerve ; 22, 23, the.brachial plexus ; 24 ; 25, the phrenic nerve ; 26, the carotid artery ; 27, the internal jugular vein.] 243. The character of the Cephalic nerves, as distinguished from the Spinal, is a point of much interest, when considered in relation to Compara- tive Anatomy, and to Embryology. It appears, from what has heen already stated, that the Par Vagum, Spinal Accessory, Glosso-pharyngeal, and Hypo- glossal nerves, may be considered nearly in the light of ordinary Spinal nerves. They all take their origin exclusively in the Medulla Oblongata ; and the want of correspondence in position, between their roots and those of the Spinal nerves, is readi]y accounted for,^by the alteration in the direction of the columns of the Spinal Cord, which, as long since pointed out by Rosenthal, and lately stated prominently by Dr. Reid, not only decussate laterally, but, as it were, from behind forwards ( 171). The Hypoglossal, as just stated, not unfrequently possesses a sensory in addition to its motor root. The Glosso-pharyngeal, which is principally an afferent nerve, is stated by Arnold and others to have a small motor root ; at any rate, the motor fibres which belong to it are to be found in the Par Vagum. That the Par Vagum and a portion of the Spinal Accessory together make up a spinal nerve, has been already stated as probable. Leaving these nerves out of the question, author by Dr. W. Budd, in which the hypoglossal nerve was completely divided on one side; and yet the tip of the tongue, when the patient was desired to put it out, was some- times directed from and sometimes towards the palsied side ; showing that the muscles of either half are sufficient to give any required direction to the whole. 16 182 FUNCTIONS OF THE NERVOUS SYSTEM. therefore, we proceed to the rest. Comparative anatomy, and the study of Embryonic development, alike show that the Spinal Cord and Medulla Ob- longata constitute the most essential part of the nervous system in Vertebrata ; and that the Cerebral Hemispheres are superadded, as it were, to this. At an early period of development, the Encephalon consists chiefly of three vesicles, which correspond with the gang-home enlargements of the nervous cord of the Articulata, and mark three divisions of the cerebro-spinal axis ; and, in accordance with this view, the Osteologist is able to trace, in the bones of the cranium, the same elements which would form three vertebras, in a much expanded and altered condition. However improbable such an idea might seem, when the cranium of the higher Vertebrata alone is examined, it at once reconciles itself to our reason, when we direct our attention to that of Reptiles and Fishes ; in which classes the size of the Cerebral or hemispheric ganglia is very small, in comparison with that of the Ganglia of special sensa- tion ; and in which the latter evidently form but a continuation of the Spinal Cord, modified in its function : so that, when we trace upwards the cavity of the spinal column into that of the cranium, we encounter no material change, either in its size or direction. The three pairs of nerves of special sensation make their way out through these three cranial vertebrae respectively. At a later period of development, other nerves are interposed between these ; which, being intervertebral, are evidently more analogous to the Spinal nerves, both in situation and function. A separation of the primitive fibres of these takes place, however, during the progress of development, so that their distribution appears irregular. Thus, the greater part of the sensory fibres are contained in the large division of the Trigeminus ; whilst, of the motor fibres, the ante- rior ones chiefly pass forwards as the Oculo-motor and Patheticus ; and of the posterior, some form the small division of the Trigeminus, and others unite with the first pair from the medulla oblongata, to form the Facial. This last fact explains the close union of this nerve with those proceeding more directly from the medulla oblongata, which we find in Fishes and in some Amphibia. According to Valentin, the Glosso-pharyngeal is the sensory portion of the first pair from the medulla oblongata, of which the motor part is chiefly com- prehended in the Facial nerve. It is very interesting to trace this gradual metamorphosis from the character of the Spinal nerves, which is exhibited in the Cephalic, when they are traced upwards from the Medulla Oblongata; and this is shown, as formerly pointed out ( 223), as much in the nerves of special sensation as in the rest. Although we are accustomed to consider the Fifth pair as par excellence the Spinal nerve of the head, the foregoing statements, founded upon the history of its development, show that the nerves of the Orbit really belong to its motor portion ; they may consequently be regarded as alto- gether forming the first of the intervertebral or Spinal nerves of the cranium. The Facial and Glosso-pharyngeal appear to constitute the second ; whilst the Par Vagum and Spinal Accessory intervene between this and the true Spinal, of which the Hypoglossal may be considered as the first. / XV. Motor Nerves of the Orbit. 244. We now return to consider the functions of the Third, Fourth and Sixth pairs of nerves, together constituting the entire channel of the movements of the eyeball. Their particular functions are but ill understood, and the movements which they govern offer so many peculiarities, that the inquiry becomes a very complex one. It is of peculiar interest, however, both on account of its general bearing on the Physiology of the Nervous System, and at the present time, more especially, in consequence of the assistance iwhich a correct Jmowledge of these functions may afford, in the treatment of Stra- MOTOR NERVES OF THE ORBIT. 183 [Fig. 42. bismus, by the operation which has now been so extensively and (when exe- cuted with care and judgment) so successfully performed. 245. It will be recollected that, in the Human Orbit, six muscles for the movements of the eyeball are found; the four recti, and the two oblique mus- cles. The precise actions of these are not easily established by experiment on the lower animals; for in all those which ordinarily maintain the horizontal position, there is an additional muscle, termed the retractor, which embraces the whole posterior portion of the globe, and passes backwards to be attached to the bottom of the orbit. This muscle is most developed in Ruminating animals, which, during their whole time of feeding, carry their heads in a dependent position. In most Carnivorous animals, instead of the complete hollow muscular cone, (the base enclosing the eyeball, whilst the apex sur- rounds the optic nerve,) which we find in the Ruminants, there are four dis- tinct strips, almost resembling a second set of recti muscles, but deep-seated, and inserted into the posterior instead of the anterior portion of the globe. It is obvious that the actions of these must greatly affect the results of any opera- tions which we may perform upon the other muscles of the Orbit ; and, as it is impossible to divide the former, without completely separating the eye from its attachments, we have no means of correcting such results but by reasoning alone. Experiments upon animals of the order of Q,uadru- mana, most nearly allied to Man, would be more satisfactory, as in them the retractor muscle is almost or entirely absent. If the origin and insertion of the four Recti mus- cles be examined, however, no doubt can remain that each of them, act- ing singly, is capable of causing the globe to revolve in its own direction the superior rectus causing the pupil to turn upwards the internal rectus causing it to roll towards the nose and so on. A very easy and direct application of the laws of me- chanics w r ill further make it evident to us, that the combined action of any two of the Recti muscles will cause the pupil to turn in a direc- tion intermediate between the lines of their single action, and that any intermediate position may thus be given to the eyeball by these mus- cles alone. This fact, which has not received the attention it de- serves, leads us to perceive that the Oblique muscles must have some supplementary function' It may be objected that this is a theoretical statement only, and that there may be some practical obstacle to the performance of diagonal movements by the Recti muscles, w T hich renders the assistance of the Obliques essential for this purpose. But to this it may be replied, that no single muscle can direct the ball either downwards and inwards, or upwards and outwards, and that, as we have good reason to believe these movements to A view of the Third, Fourth and Sixth pairs of Nerves; 1, ball of the eye and rectus externus muscle ; 2, the superior maxilla; 3, the third pair, or motores oculi, distributed to all the muscles of the eye except the su- perior oblique and external rectus; 4, the fourth pair, or pathetici, going to the superior oblique muscle; 5, one of the branches of the seventh pair; 6, the sixth pair, or motor externus, distributed to the external rectus muscle; 7, spheno-palatine ganglion and branches; S, ciliary nerves from the lenticular ganglion, the short root of which is seen to connect it with the third pair.] 184 FUNCTIONS OF THE NERVOUS SYSTEM. be affected by the combination of the Recti muscles, there is no reason why the other diagonal movements should not also be due to them. 246. The action of the Superior Oblique muscle has been a patter of dis- pute. Unlike the other muscles which arise from the back of the orbit, its tendon is not inserted into the front hemisphere of the eye, but into a point behind its vertical axis ; and we should, therefore, be led to suppose that its operation is to move the pupil in a direction contrary to that in which its ten- don is inserted ; that is to say, as its tendon passes, from its insertion towards the trochea, upwards and somewhat inwards, we should suppose that, in shortening, it would draw the back of the eyeball in that direction, and turn the pupil in the contrary one namely, downwards and a little outwards. This theory of its action is borne out by experiments, both upon the muscle and the nerve which supplies it ; for by laying bare the muscle, without dis- turbing the eyeball or the neighbouring parts, and then exercising gentle traction upon it, so as to draw the tendon in the same manner as ordinary contraction of the muscle would have done, the eyeball is turned downwards and somewhat outwards. The same effect is produced when the Fourth pair of nerves is irritated, either mechanically or by galvanism, after it has been separated from the brain. On the other hand, the Inferior Oblique mus- cle may be shown, by experiments upon itself, to roll the eye upwards and inwards ; the inward movement is much greater than the outward movement caused by the Superior Oblique ; so-that these two muscles are not exactly antagonists of each other. 247. The distribution of nerves to these muscles is very peculiar. The Superior Oblique has a nerve for itself alone, namely, the Fourth pair ; this was formerly called the Patheticus nerve, from its being supposed to govern that rotation of the ball upwards and inwards, which gives a pathetic expres- sion to the countenance ; but, as just shown, its real action is the reverse. By Sir C. Bell, this nerve was considered as belonging to his Respiratory system; ahd he endeavoured to show, that the sudden movement of the pupil upwards and imvards, which takes place in coughing and sneezing, and the fixation of the ball in a similar position during sleep, are due to its operation. The ascertained action of the muscle, however, constrained him to suppose, that the operation of the nerve was not to cause contraction, but relaxation, of this ; by which the antagonist muscles might be free to occasion the movement. This idea affords a remarkable exemplification of the degree in which theory may, in some minds, usurp the place of observation. There is, as we have formerly seen, no ground for the assumption of a system of Respiratory nerves distinct from those forming the general Excito-Motor system, from which a part of every motor trunk in the body is derived ; and the supposition that the action of a nerve is ever to cause relaxation in a muscle, is at variance with all sound physiological induction. In this particular instance, it is at once refuted, by such experiments on the trunk of the nerve as those just adverted to. It may further be added, in regard to this nerve, that there is no decided reason to. believe that it contains any sensory fibres. Its distribution is entirely restricted to the Superior Oblique muscle ; but since in this, as in other mus- cles of the. orbit, there is certainly a degree of sensibility, as is experienced by the fatigue to which the long fixation or violent straining of them gives rise, it may be questioned whether the Fourth pair of nerves is entirely motor. Its course within the cranium renders it very unlikely that this point can be satisfactorily determined by experiment. Muhler states that a connection exists between this nerve and the ophthalmic branch of the Fifth pair ; so that it is not improbable that, as in other instances, its sensory endowments are derived from this source. 248. Tine same may be said of the Sixth pair, which is termed the M- MOTOR NERVES OF THE ORBIT. 185 ducens nerve, from its being solely distributed to the Rectus externus muscle. There is no reason to believe that the actions of either of the two last-mentioned nerves are ever involuntary; on the contrary, there will appear reason to suppose that they are, with a branch of the third pair, the sources of the voluntary movements of the eyes. Cases occasionally present themselves in which this nerve alone is paralyzed; and the outward motion of the ball is then almost entirely lost. 249. The three other Recti muscles, together with the Levator Palpebra3, and Inferior Oblique, are supplied by the Third pair, commonly termed Oculo- motor. The general question, how far this nerve is to be regarded as exclu- sively motor, has been already considered ( 224); that it is chiefly so, there can be no doubt. But we have now to inquire, whether there is any ground for believing that different branches of the nerve are subservient to motions of a different character some, for example, being more connected with the Reflex function of the Spinal Cord; others with that instinctive tendency which causes opposite muscular actions to take place in the two orbits by one effort of the will; and others being immediately directed and controlled by the will. It will be remembered that this nerve subdivides into two principal branches ; of which one supplies the Levator Palpebra3 and Superior Rectus; whilst the other is distributed to the Internal and Inferior Recti, and to the Inferior Oblique. Now the action of the former appears to be of a purely voluntary character. We have no instance of the upper lid being elevated by any other than an effort of the will ; and, if this be suspended, the Orbicularis may be made to depress it, by the reflexion of a stimulus applied to the edge of the tarsi. Moreover, when a strong light causes the lids to contract invo- luntarily, we feel conscious that a voluntary effort is required to keep them apart. The same may be said of the directly upward movement of the eye- ball, w r hich is caused by the Superior Rectus alone: it is never any thing but a voluntary act; for the upward and inward movement adverted to by Sir C. Bell, is evidently occasioned by the inferior oblique acting alone. On the other hand, it is certain that some, at least, of the actions of the second branch are of a simply reflex nature ; and that others cannot be said to be voluntary, but are rather of an instinctive character. It is from this branch that the twigs proceed, which enter the ciliary ganglion, and which govern the move- ments of the pupil ; movements which have been already shown to be of a simply reflex nature. Some have attempted to show, that the actions of the iris are in a slight degree voluntary, because, by an effort of the will, they could occasion contraction of the pupil ; but this so-called voluntary contraction is always connected with a change in the place of the eyeball itself, occasioned by an action of some of its muscles. It is principally noticed under the two following conditions. 1. When an object is brought very near the eye, and we steadily fix our attention upon it, the axes of the two eyes are made to converge ; and if this convergence be carried to a considerable extent, so that the pupils of both eyes are sensibly directed towards the inner canthus, a contraction of the pupil takes place. The final cause or purpose of this con- traction is very evident. When an object is brought near the eye, the rays proceeding from it would enter the pupil (if it remained of its usual size) at an angle of divergence so much greater than that which would allow them to be properly refracted to a focus, that indistinct vision would necessarily result. By the contraction of the pupil, however, the extreme or most divergent rays are cut off, and the pencil is reduced within the proper angle. The principle is precisely the same as that on which the optician applies a stop behind his lenses, which reduces their aperture in proportion to the shortness of their focal distance. 2. Contraction of the pupil is also noticed, when the eyeball is performing that rotation upwards and inwards which has been already 16* 186 FUNCTIONS OF THE NERVOUS SYSTEM. spoken of as occasioned by the contraction of the Inferior Oblique muscle ; and which, when performed along with violent respiratory actions, or during sleep, must be regarded as involuntary. This rotation also takes place, to a slight degree, when the eyelid is depressed, as in ordinary winking; and it is obvious that, in this manner, the surface of the eye is more effectually swept free from impurities which m&y have gathered upon it, than it would be by the downward motion ofthe lid alone. But the pupil is not contracted when the eyeball is voluntarily rotated upwards and inwards, an action which \rnay be effected by the Superior Rectus, some fibres of which are sufficiently rar removed from the central axis of the globe to give it an internal direction. There is good reason to believe, therefore, that the actions of the inferior branch of the Third nerve are in great part automatic, whilst those of the superior branch are purely voluntary. Upon this reasoning, Valentin has founded a very ingenious theory of the consensual movements of the eyeball, which will now be explained ; and this w r ill be conveniently followed by an inquiry into the nature of this class of movements, as distinguished from the Reflex on the one hand, and the Volitional on the other. / XVI. Consensual Movements of the Eye. ^ * 250. From the foregoing observations it appears, that the Rectus Superior, Obliquus Superior, and Rectus Externus, which are supplied from the superior branch of the Third pair, and by the Fourth and Sixth pairs, are all to be regarded as purely voluntary muscles ; and Valentin considers them analogous to the Extensors of the limbs, spine, &c., which are for the most part distin- guished by the same character. By the actions of these three muscles, singly or combined, the eyeball may be moved in nearly all directions. On the other hand, the Inferior and Internal Recti, and the Inferior Oblique, supplied by the inferior branch of the Third pair, are more or less automatic in their action ; and these are compared, by Valentin, to the flexors. By the single or combined actions of these muscles also, the eyeball may be moved towards almost any point, except in an upward and outward direction ; and any one who tries the experiment will find that this is, of all the movements of the eye, the one that is attended with the most constrained action of the muscles. 251. On studying the conjoint movements of the eyeball, we are led to observe^ the very curious fact, that they are not so much symmetrical as harmonious ; that is to say, the corresponding muscles on the two sides are rarely in action at once ; whilst such a harmony or consent exists between the actions of the muscles of the two orbits, that they work to one common purpose, namely, the direction of both eyes toward the required object. In order to study them properly, it is necessary to reduce them to seme kind of classification. 1. If one eye be rotated imvards, and the other outwards, the Internal Rectus of one eye, and the External Rectus of the other, a^e evidently put in action together. This movement is harmonious or consensual, but not symmetrical. 2. Both eyeballs are elevated, by the contraction of the two Superior Recti. 3. Both eyeballs are depressed; this is effected by the con- joint action of the Inferior Recti muscles ; and the movement is, like the preced- ing, both harmonic and symmetrical. 4. Both are drawn directly inwards and downwards, as when $e look at an object placed on or near the nose ; this movement is symmetrical,* }>ut not harmonic ; and it is effected by the action of the Internal Rectus, joined either with the Inferior Rectus or the Superior Oblique. 5. When one eye is rolled upwards and inwards, and the other upwards and outwards, the Inferior Oblique is probably operating on one side, whilst the Superior Rectus unites its action with that of the External on the other. And, 6, when one eye is drawn downwards and inwards, and the CONSENSUAL MOVEMENTS OF THE EYE. 187 other downwards and outwards, the Inferior Rectus Is probably operating, along with the Internal Rectus, on one side, whilst the Superior Oblique is the chief cause of the latter movement. All these movements may be voluntarily performed by Man ; but it is not so clear that the muscles by which they are effected, are equally influenced by volition in each case ; and there are some curious diversities in our power of operating on different muscles, which throw some light on the matter. Of those which are entirely subjected to the will, we can only put that pair in action together which will operate without de- stroying the symmetrical position of the two eyes, namely, the Superior Recti. We cannot voluntarily abduct both eyes, nor roll them downwards and out- wards by the conjoint action of the two External Recti or Superior Obliques. Nor, again, can we bring any of these voluntary muscles the Superior Obliques and Superior Rectus, for example to act against each other in the two eyes, so as to destroy their symmetry. Thus, as remarked by Valentin, in almost every movement, in which the harmony of the two eyes is preserved, whilst the symmetry is destroyed (as in those of the 1st, 5th, and Oth of the foregoing classes), one or more muscles of voluntary motion are acting on one eye, and one, or more of the automatic grouj5 are chiefly concerned in produc- ing the rotation of the faker. This idea is an extremely ingenious one, and will be found to be supported by other facts. 252. But there are two kinds of movement of the Eyeballs which are not at all voluntary. In the first of these, both eyeballs are rotated upwards and imuards, by the action of both Inferior Obliques. In the other, both eyeballs are directed inwards, by the action of both Internal Recti. Now in both these cases, the harmony of the movements is destroyed : but it is by two similar muscles, both acting automatically, and subjected, therefore, to the same stimu- lus. In the first of these cases, the stimulus may originate in some part very distant from the eye itself, and may be of a purely reflex kind ; as when the eye is rotated under the lid, in the acts of .sneezing, coughing, winking, &c. The latter we shall find to be another result of the same cause as that which secures the usual harmonic movements of the eyeball ( 255). 253. It may be stated as a physiological fact, that single vision with two eyes is dependent upon the formation of the image upon parts of the two retinae which are accustomed thus to act with each other. In many physio- logical works, it is asserted, that single vision is the result of the impressions being made on corresponding parts of the two retina,- that is to say, on parts equally distant from the axis, on one side or the other: but this seems to be disproved by the fact, that patients who have been long affected with Con- vergent Strabismus, and who see equally well with both eyes (as many do), are not troubled with double vision. On the other hand, when a person whose eyes look straight before him, is the subject of a disorder which renders their motions in any degree irregular, he is at once affected with double vision ; and the same has been noticed to be a common immediate result of the successful operation for the cure of strabismus, where vision is good in both eyes. Al- though the images were previously formed on parts of the retinae which were very far from corresponding with each other, yet no sooner is the position of the eyes rectified (so that the relation between the situation of the images is the same as it would have been in a sound eye"), than the patient sees double. Now in these cases the difficulty very speedily diminishes, and the patient soon learns to see single. It can scarcely be imagined, then, that to any other cause than habit, is to be attributed the long-discussed phenomenon of single vision with two eyes. The mind receives the two images, frequently com- bining them together (as Mr. Wheatstone's ingenious experiments with the Stereoscope have most satisfactorily shown, 339) to produce a picture in relief; and so long as these are conveyed to it in the accustomed manner, it / 188 FUNCTIONS OF THE NERVOUS SYSTEM. reconciles them together, even if the parts of the retinae on which they are formed do not correspond ; but if any circumstance break this chain, and cause the imag'es to be transmitted to the sensorium through a new channel, the mind requires some little time to adapt itself to this impression, as it does by habit to almost every other.* * That there is a greater tendency to consent between the images, when they are formed upon corresponding parts of the retinae, the Author readily admits; and he thinks that this is a principle of some importance, in explaining the re-adjustment of the eyes, after the operation for Strabismus. Every one who has seen much of this operation is aware, that the re-adjustment of the eye is not always immediate, but that, after the muscle has been freely divided, the eye often remains somewhat inverted for a few days, gradually acquiring its straight position. The Author has known one case, in which," after such a degree of temporary inversion as seemed to render the success of the operation very doubtful, eversion actually took place for a short time to a considerable extent; after which the axes became parallel, and have remained so ever since. Another argument, derived from the results of this operation, in favour of the con- sensual movement being chiefly dependent upon the place of the impressions on the retina is, that it is much more successful in those cases in which the sight of the most displaced eye is good, than in those in which (as not unfrequently happens from long disuse) it is much impaired. In cases of the latter class, the cure is seldom complete. There is another curious fact, which may be adverted to in reference to this subject: Strabismus not unfrequently arises from the formation of an opaque spot on the centre of the cornea, which prevents the formation of any images on the retina, except by the oblique rays; and nature seems to endeavour (so to speak) to repair the mischief, by causing the eye to assume the position most favourable for the reception of these. To one more point only, connected with the subject of Strabismus, would the Author now allude. He is well convinced, from repeated observation, that those Surgeons are in the right, who have maintained, in a recent controversy, that, in a large proportion of cases, strabismus is caused by an affection of both sets of muscles or nerves, and not of one only; and that it then requires, for its perfect cure, the division of the corresponding muscle on both sides. Cases will be frequently met with, in which this is evident; the two eyes being employed to nearly the same extent, and the patient giving to both a slight inward direction, when desired to look straight forwards. In general? however, one eye usually looks straight forwards, whilst the other is greatly inverted; and the sight of the inverted eye is frequently affected to a considerable degree by disuse ; so that, when the patient voluntarily rotates it into its proper axis, his vision with it is far from being distinct. Some Surgeons have maintained, that the inverted eye is usually the only one in fault, and consider that the division of the tendon of its Internal Rectus is sufficient for the cure. They would even divide its other tendons, if the parallelism be not restored rather than touch the other eye. The Author is himself satisfied, how- ever, that the restriction of the abnormal state to a single eye, is the exception, and not the rule, in all but very slight cases of strabismus; and to this opinion he is led both by the consideration of the mode in which strabismus first takes place, and by the results of the operations which have come under his notice. If the eyes of an infant affected with cerebral disease be watched, there will frequently be observed in them very irregu- lar movements; the axes of the two being sometimes extremely convergent, and then very divergent. This irregularity is rarely or never seen to be confined to one eye. Now, in a large proportion of cases of Strabismus, the malady is a consequence of some cerebral affection during infancy or childhood, which we can scarcely suppose to have affected one eye only. Again, in other instances we find the Strabismus to have resulted from the constant direction of the eyes to very near objects, as in short-sighted persons; and here, too, the cause manifestly affects both. Now it is easy to understand, why one eye of the patient should appear to be in its natural position, whilst the other is greatly inverted. The cause of strabismus usually affects the two eyes somewhat unequally, so that one is much more inverted than the other. We will call the least inverted eve A, and the other B. In the ordinary acts of vision, the patient will make most use "of the least inverted eye, A, because he can most readily look straight forwards or outwards with it; but to bring it into the axis, or to rotate it outwards, necessitates a still more decided inversion of B. This remains the position of things, the patient usually look- ing straight forwards with A, which is the eye constantly employed for the purposes of vision, and frequently almost burying under the inner canthus the other eye, B, the vision ip which is of very little use to him. When, therefore, the tendon of the internal rectus of B is divided, the relative position of the two is not entirely rectified. Some- times it appears to be so for a time; but the strabismus then begins to return, and it can CONSENSUAL MOVEMENTS OF THE EYE. 189 254. If this be admitted, we gain an important step in the explanation of the Consensual movements of the eye. The object to be attained is evidently this that the usual axes of the eye should always be directed towards the object to be viewed; and this, as we have seen, involves the necessity (in a great majority of cases), of unsymmetrical movements being performed by the two eyeballs. Now it is fair to argue from the facts already stated (respecting the distribution of the Third pair, and the known functions of its inferior branch), that, in directing our eyes by a voluntary effort to any par- ticular object, the will acts chiefly upon one eye, and that the other follows its direction by an automatic movement. This automatic movement appears to be governed by the relative place of the images upon the retinae. It is well known that, in children born blind, the movements are not consensual ; they are frequently very far from being so, in cases of congenital cataract, where a considerable amount of light is evidently admitted, but where no distinct image can be formed ; and in such cases, the movements are most consensual where the object is bright or luminous, and a more vivid impression therefore made upon the retina. It is no objection to this theory to say, that persons who have become blind may still move their eyes in a consensual manner ; since the habit of the association of particular movements having been once acquired, the known laws of nervous action account for its continuance ; and, as a matter of fact, a want of consent may be often noticed where the blind- ness is total. The peculiar vacant appearance, which may be noticed in the countenance of persons completely deprived of sight by amaurotic or other affections which do*not alter the external aspect of the eyes, seems to result from this, that their axes are parallel, as if the individual were looking into distant space, instead of presenting that slight convergence, which must always exist between them when the eyes are fixed upon a definite object. This convergence, which is of course regulated by the Internal Recti, varies in degree according to the distance of the object ; and it is astonishing how minute an alteration in the axes of the eyes is perceptible to a person ob- serving them. For instance, A sees the eyes of B directed towards his face, but he perceives that B is not looking at him ; he knows this by a sort of intuitive interpretation of the fact, that his face is not the point of convergence of B's eyes. But if B, who might have been previously looking at something nearer or more remote than A's face, fix his gaze upon the latter, so that the degree of convergence of the axes is'altered, without the general direction of the eyes being in the least affected, the change is at once perceived by the person so regarded ; and the eyes of the two then meet. 255. The foregoing considerations may be summed up in this simple state- ment ; that, when the axis of one eye is voluntarily directed towards an only be checked by division of the tendon of the other eye, A; after which, the cure is generally complete and permanent. That it has not been so in many of the cases on which operations have been performed, the author attributes, without the slightest doubt in his own mind, to the neglect of the second operation. As just now stated, the sight of the most inverted eye is frequently very imperfect; indeed it is sometimes impaired to such an extent, that the patients speak of it as entirely useless. That this impair- ment results in part from disuse merely, seems very evident, from the great improve- ment which often succeeds the rectification of the axes. The Author cannot help thinking it probable, however, that the same cause which produced the distortion of the eye may, in some instances at least, have affected the Optic nerve, as well as the Motor nerves of the orbit; and this idea is borne out by the fact of the restoration of sight, in certain cases of Amaurosis, by division of one or more tendons, where no Strabismus previously existed (See Adams on Muscular Amaurosis). It is interesting to remark that, in these cases, Strabismus was usually the first effect of the operation; but that the eye generally recovered its ordinary position within a short time, especially when the sight was improving. 190 FUNCTIONS OF THE NERVOUS SYSTEM. object, there is an instinctive tendency, on the part of the Nervous System and the Muscles by which it acts, to effect a consensual movement of the other ; so that its accustomed axis also shall be directed towards the object. This principle fully accounts for the only non-consensual movement which can be performed in any way voluntarily, that of both eyes inwards, or downwards and inwards, which is effected by the conjoint action of the Internal Recti. Some persons possess the power of performing this to a much greater degree than others ; but in all instances, the eyes must be fixed on an object; and thus the movement is as referable as any other to this principle. It is, per- haps, desirable to qualify the classification of the nerves and muscles of the Orbit proposed by Valentin, by admitting that all have, in some degree, a voluntary, and in some degree an automatic action ; but that voluntary power predominates in regard to one group, whilst the other is more commonly acted on by an automatic impulse. It is clear that the will must have some power over the Inferior Rectus, for example ; since both eyes can be voluntarily directed downwards. But the power of the Will over this muscle is much less than it is over the Superior Rectus ; as is shown by the fact that, if we direct the eyes downwards, and then close the lids, no effort of volition can prevent the eye from being rolled upwards by the Inferior Oblique ; and that, whilst the lids remain closed, the pupils cannot be directed downwards in any considerable degree. It is evident, then, that the impression of an object upon the retina is almost as necessary to occasion the combined action of the Infe- rior as it is to produce that of the Internal Recti. The case is very different in regard to the Superior Recti, which can be made to act together, in any degree, without the necessity of a visual impression. 256. It has been pointed out by Mliller, as an obvious reason for the sepa- ration of the 6th from the 3d pair of nerves, that there is usually a great tendency to consentient action between the nerves of the two sides, which pass off from the same point of the cerebro-spinal axis as we see in the case of Reflex movements of both sides (such as that of the pupil), which are excited by a stimulus applied to one only ; and that this holds good also in those movements of the eyes which are effected by the third pair exclusively such as the elevation or depression of both pupils ; but in the horizontal movements of the eyeballs, two different actions are being performed on the two sides respectively; and it may be conceived that this may be more readily accomplished by two different nerves than by branches of the same. We may admit some truth in this idea, without attributing much weight to it. It has been already stated as a result of Embryological research, that all the Nerves of the Orbit do in reality form part of the Spinal nerve, to which the Fifth pair alone has been commonly regarded as equivalent ; and it is well known that we can perform many different actions on the two sides, through the medium of similar nerves at the same time. It is remarkable, however, that there are some dissimilar movements which it is impossible to execute with any degree of rapidity, except by long practice : thus, if we move the right hand as if winding on a reel, and afterwards make the left hand revolve in a con- trary direction, no difficulty is experienced; but if we attempt to move the two at the same time, in contrary directions, we shall find it almost impracticable. 257. There can be no doubt that, in these and many other voluntary move- ments, we are guided by the sensations communicated through the afferent nerves, which indicate to the mind the state of the muscle. Many interesting cases are on record, which show the necessity of this muscular sense, for determining voluntary contraction of the muscle. Thus Sir C. Bell (who pro- minently directed attention to this class of facts, under the designation of the Nervous Circle,) mentions an instance of a woman who was deprived of it in her arms, without losing the motor power, and who stated that she was GENERAL FUNCTIONS OF THE ENCEPHALON. 191 obliged to keep her eyes constantly fixed on any thing (even her child) which she held in her hands, as she could not continue the muscular effort when no longer informed, by one sense or the other, that it was necessary. Now, the only real difference between the case of the ordinary muscles and that of the muscles of the eyeball, is (as Dr. Alison* has justly remarked) that the guiding sensations are those received through the Retina in the latter case, whilst in the former they are those of the muscles themselves. It may be asked in what such consensual movements, as those of the Eye, differ from those of a reflex character? The answer is, simply, that the former cannot be effected without consciousness, and some mental condition supervening upon it; whilst, in the latter, sensation has been shown not to be a necessary link. The former may be as much involuntary as the latter, as is shown in the effects of tickling, which could not be manifested in an unconscious individual. Here a condition, very much resembling an emotion, is produced; and from this, as from other emotions, various combined movements may result, with which Volition has nothing to do. The same may probably be said of the Instinctive actions of animals, which, as will presently appear, are probably to be referred to the same category with the purely Emotional acts of Man : in both, Sensation, and that usually of a special kirfd, is a necessary link. Further it would appear that actions, which were originally of a completely voluntary character, may come by habit to be performed within the shorter channel: thus, a musician will play a difficult piece, whilst keeping up a conversation on an entirely different subject; and here the muscular movements are guided, not only by the sensations produced by their own contraction, but also by the anti- cipation of the auditory sensations which will result from their operation. The same may be said of the action of the muscles of Voice ( 412). ;'" XVII. General Functions of the Encephalon. 258. The portion of the Nervous Centres contained within the cranium, and commonly designated collectively as the- Encephalon, may be regarded as con- sisting of four principal divisions: 1, the Cerebral Hemispheres, which, in the Mammalia, and especially in Man, constitute by far the largest portion of the whole ; 2, the Cerebellum, the complete separation of which from the Cerebrum, and its distinct connections with the Medulla Oblongata, mark it out as an organ of peculiar character ; 3, the Tuber cula Quadrigemina and other Ganglionic masses at the^Mfe of the brain, connected with the nerves of special sensation, and analogous to the Olfactive, Optic and Auditory ganglia of the lower ani- mals ; and 4, the Medulla Oblongata, or cranial prolongation of the Spinal Cord, which is connected, at its upper end, with the Ganglia of specral sensa- tion, with the Thalami optici, which may probably be regarded asfne corre- sponding recipients of ordinary sensory impressions, and with the Corpa Striata, through which the motor impulses are transmitted to it from the Hemi- spheres. It has been already shown that this last organ is peculiarly connected with the functions of Respiration and Deglutition ; and we shall next inquire what special function can be attributed to the ganglionic enlargements at its upper end. XVIIfc Functions of the Tubercula Quadrigemina, fyc. Emotional and Instinctive Actions. 259. The degree in which animals high in the scale of organization can perform the functions of life, without any other centre of action than the Gang- * Anatomical and Physiological Inferences from the Study of the Nerves of the Orbit, in Trans, of Royal Society of Edin., vol. xv. 192 FUNCTIONS OF THE NERVOUS SYSTEM. lia of Special sense, the Medulla Oblongata, and the Cerebellum, appears extra- ordinary to those who are accustomed to regard the Cerebral Hemispheres as the centre of all energy. From the experiments of Flourens, Hertwig, Ma- gendie, and others, it appears that not only Reptiles, but Birds and Mammalia, may survive for many weeks or months (if their physical wants be duly sup- plied) after the removal of the whole Cerebrum. It is difficult to substantiate the existence in them of actual sensation ; but their movements appear to be of a higher kind than those resulting from mere Reflex action. One of the most remarkable phenomena in such a being, is the power of maintaining its equilibrium, which could scarcely exist without consciousness. If it be laid upon the back, it rises again ; if pushed, it walks. If a Bird thus mutilated be thrown into the air, it flies ; if a Frog be touched, it leaps. Such a being, when violently aroused, has all the manner of an animal waking -from sleep ; and it manifests just about the same degree of consciousness with a sleeping Man, whose torpor is not too profound to prevent his suffering from an uneasy position, and who moves himself to amend it. The negative results of experi- ments of this kind are much more satisfactory than the positive ; that is to say, if we are able to substantiate the performance of a particular function, after the removal of a certain organ, we may be sure that the function is not dependent on that organ. But the converse does not hold good ; for it fre- quently happens that, when such violent operations are practised on the ner- vous centres, they occasion an amount of general disturbance, which suspends or modifies functions that have no immediate connection with the organ in question ; so that we cannot safely attribute the alteration in them to the loss of it. For example, Hertwig found that, upon removing the upper part of the hemispheres in a pigeon, the powers of sight and hearing appeared to be destroyed, and the animal sat in one spot, as if asleep ; but, being fed during a fortnight, the sensibility returned, and the bird lived for three months. 260. Among the ganglia of special sensation, the functions of the Optic Lobes, or Corpora Quadrigemina, have been chiefly examined. The researches of Flourens and Hertwig have shown, that their connection with the visual function, which might be inferred from their anatomical relations, is substan- tiated by experiment. The partial loss of the ganglion on one side produces partial loss of power and temporary blindness on the opposite side of the body, without necessarily destroying the mobility of the pupil ; but the removal of a larger portion, or complete extirpation of it, occasions permanent blindness and immobility of the pupil, with temporary muscular weakness, on the opposite sides. This temporary disorder of the muscular system sometimes manifests itself in a tendency to move on the axis, as if the animal were giddy. No disturbance of consciousness appears to be produced ; and Hertwig states that he never witnessed the convulsions which Flourens mentions as a consequence of the operation, and which were probably occasioned by his incision having been carried too deeply. We shall now inquire what inferences may be drawn from comparative observations, in regard to the more general function of these organs. 261. That a large number of the actions of the lower animals are immedi- ately prompted by Sensations, without the intervention of reasoning processes, is universally admitted ; and' to these actions the term Instinctive is ordinarily given. They appear to result from the direct operation of a mental condition, analogous to that which exists in Man, when the emotions, passions, or pro- pensities are so strongly excited, as to act at once on the body without the intervention of the Will. In the purely reflex movements, it has been shown that sensation is not a necessary link. On the other hand, in Voluntary acts, neither sensations nor emotions directly affect the body, but only serve to stimu- late the reasoning j^rocesses, and to supply motives to the judgment ; and the ^ EMOTIONAL AND INSTINCTIVE ACTIONS. 193 operations of this terminate in the formation of a Volition, the commands of which are conveyed to the muscle, through a channel structurally distinct, as cases of paralysis fully prove, from that which is the medium of Emotional actions. It cannot be doubted by any person, who has attentively studied the characters of the lower animals, that many of them possess psychical endow- ments, corresponding with those which we term the intellectual powers and moral feelings in Man ; but in proportion as these are undeveloped, in that proportion is the animal under the dominion of those Instinctive impulses, which, so far as its own consciousness is concerned, may be designated as blind and aimless, but which are ordained by the Creator for its protection front danger, and for the supply of its natural wants. The same may be said of the Human infant, or of the Idiot, in whom the reasoning powers are unde- veloped. Instinctive actions may in general be distinguished from those which are the result of voluntary power guided by reason, chiefly by the two follow- ing characters : Although, in many cases, experience is required to give the Will command over the muscles concerned in its operations, no experience or education is required, in order that the different actions, which result from an Instinctive impulse, may follow one another with unerring precision. 2. These actions are always performed 4)y the same species of animal, nearly, if not exactly, in the same manner; presenting no such variation in the means adapted to the object in view, and admitting of no such improvement in the progress of life, or in the succession of ages, as we observe in the habits of individual men, or in the manners and customs of nations, that are adapted to the attainment of any particular ends, by those voluntary efforts which are guided by reason. The fact, too, that these instinctive actions are often seen to be performed under circumstances rendering them nugatory, as reason informs us, for the ends which they are to accomplish (as when the Flesh-fly deposits her egg on the Carrion-plant instead of a piece of meat, or when the Hen sits on a pebble instead of her egg), is an additional proof, that the Instinctive actions of animals are prompted, like the consensual movements we have been recently inquiring into, by an impulse which immediately results from a particular sensation being felt, and not by anticipation of the effect which the action will produce. 262. The correspondence between the purely Emotional actions in Man, and those actions in the lower animals to which we give the name of Instinct- ive, may be made evident by a very simple illustration. The Cuttlefish is well known to discharge its ink, when pursued, and to tinge fhe water around with a colour so deep, as to enable it to escape under the cloud thus formed. Now it is not to be supposed that the Cuttle-fish has any notion of the purpose which this act will serve ; since its constancy and uniformity, and the provi- sion for its performance immediately on the emersion of the young animal from the egg, forbid our regarding it as the result of any act of reasoning. Further, the ink is an excretion which corresponds to the urine (having been found to contain urea) ; and every one knows how strong an impulse to discharge this is frequently caused by mental emotion. The same may be said of the strongly odorous secretions possessed by many Mammalia, which are dis- charged under similar circumstances, and evidently with the same object; though of that object, the animal itself be not conscious. The emotion of fear involuntarily opens the sphincters, and causes the contraction of the receptacle, in one case as in the other: and the great difference between the condition of Man, and that of the lower animals, in this respect, is simply that, in the former, the purely Emotional or Instinctive actions are few in comparison with the whole, whilst in the latter they constitute by far the largest part ; and also that Man has much greater power of controlling these actions by an effort of his Will, than that which the lower animals possess. Every one knows, how- 17 * 194 FUNCTIONS OF THE NERVOUS SYSTEM. ever, that Man is not unfrequently compelled by the strength of his Emotions to act against his Will. Thus, we see or, hear something ludicrous, which involuntarily produces laughter, although we may have the strongest motives for desiring to restrain it. On the other hand, a loathsome object, which excites the feeling of disgust, produces nausea and even vomiting. Or, again, a violent fright not unfrequently occasions convulsive movements; and these may be brought on, in some excitable states of the nervous system, by Emotions of a less powerful kind ( 298, 299).* 283. Dr. M. Hall is of opinion that the Spinal system of nerves constitutes the channel of Emotional actions. There is no other evidence for this, how- ever, than the occasional manifestation, in cases of paralysis, of reflex and emotional actions, when voluntary control is lost. Such cases only prove, however, that emotional actions are not volitional ; they are far from proving them to be Spinal. If the essential correspondence between the purely Emo- tional acts of Man, and the Instinctive acts of the lower animals be admitted, we may reasonably localize their centre more satisfactorily, in that chain of ganglionic masses, which only occupies the centre of the base of the brain in Man, but which, in the lower Vertebrata, possesses an aggregate dimension far exceeding that of the Cerebral hemispheres-. We are led to such a locali- zation by a very simple and satisfactory train of reasoning. The actions in question are not simply reflex ; since sensation, and something of the nature of emotion, both involving consciousness, are elements in their performance ; and, moreover, these sensations are rather of the special than of the common character, involving, therefore, the olfactory, optic, and auditory ganglia. No intelligent person can doubt, that, as we descend the scale of being, instinct is gradually superseding reason ; and that in the lowest Vertebrata, the mani- festations of the latter are extremely feeble, nearly all the actions of life being guided by the former. Now on looking at the Encephalon, we perceive a difference in the relative proportions of its principal divisions, so closely cor- responding with these, that it is difficult to imagine them unconnected. In proportion as we descend the scale, we find the Cerebral Hemispheres dimin- ishing in relative size, whilst the Ganglia at the origins of the nerves of special sensation increase to a remarkable degree ; and we cannot, therefore, but con- * It is a very interesting question, how far actions at first performed voluntarily by Man, may by habit cease to require an effort of the Will ; being prompted, like the In- stinctive class of movements, by the direct impulse of sensations. Thus we all know that, in walking along an accustomed road, we frequently occupy our minds with some continuous train of thought, and yet our limbs continue to move under us with regular- ity, until we are surprised by finding ourselves at the place of our destination, or perhaps at some other which we had not intended to visit, but to which habit has conducted us. Or we may read aloud for a long time, without having in the least degree comprehended the meaning of the words we have uttered; our attention having been closely engaged by some engrossing thoughts or feelings within. Or a Musician may play a well-known piece of music, whilst carrying on an animated conversation ; the Author has known a skilful performer who could play at sight whilst thus occupied. Now in such a case it would be said by some Metaphysicians (acknowledging, as most do, that the mind can- not will two different things at the same time) that the Volition is in a sort of vibratory condition between the two sets of actions, now prompting one, and now the other. But it would seem much more conformable to the analogy afforded by other psychical phe- nomena, to refer the habitual series of actions to the same division of the Nervous System with the Instinctive; and perhaps the term Automatic may be fairly applied to the whole of this group. It is well known that in cases of severe injury of the brain, in which In- telligence and Will seem completely in abeyance, habitual actions may be often excited. Thus, Dr. Perceval, in his Essay on Habit, mentions the case of a*nuff-taking countess, in whom, when seized with apoplexy, irritation of the nose with a feather produced con- traction of the forefinger and thumb of the right hand ; and Mr. Travers has recorded a similar fact m the case of a boy, who, when apparently insensible from depressed frac- ture of the skull assisted in removing his clothes, preparatorily to the required operation. EMOTIONAL AND INSTINCTIVE ACTIONS. 195 sicfcr it probable that these ganglia and tracts of gray matter, whose size is in Man so trifling, in comparison to the bulk of his Cerebral Hemispheres, are subservient to those Instinctive actions which are prompted by sensations, but in which volition does not partake. 204. It may be said that, in attributing to this division of the nervous centres a function different from that of the Spinal Cord, on the one hand, and of the Brain on the other, we are unnecessarily multiplying the systems of nervous fibres which must then be supposed to exist in every trunk ; one, namely, for reflex actions, another for the instinctive and emotional, and a third for the volitional. But the tendency of Neurological research has certainly been to show, that different functions are performed by the same trunk, in virtu^ of its containing fibres, which are connected with different divisions of the ner- vous centres ; and knowing, as we do, that these three distinct sources of action have a real existence, it cannot be regarded as improbable, that their channels also should be separate ( 227 note). Moreover, it has been seen ( 172) that there is a distinct group of fibres in the Medulla Oblongata, which has its ganglionic centre in the Corpora Gluadrigemina, and cannot be traced into the Cerebral hemispheres ; it is reasonable, therefore, to suppose that it is functionally as well as structurally distinct ; and no function can be attri- buted to- them with such probability as that of producing those instinctive and emotional movements of the body which are excited and directed through the sense of Sight. On turning to the Invertebrata, we find important con- firmation of these views in the fact, that, in general, the principal ganglionic masses, occupying the place of the Brain of higher animals, are closely con- nected with the organs of special sensation situated in the head, and are therefore analogous to the Optic and other ganglia in Vertebrata ; whilst scarcely any traces can be found of superadded ganglionic bodies, at all resembling the Cerebral hemispheres. The almost exclusively Instinctive character of the actions of such animals harmonizes well with the opinion that these ganglia are the chief sources of them. 265. The Emotions are concerned in Man, however, in many actions which are in themselves strictly voluntary. Unless they be strongly excited, so as to get the better of the will, they do not operate directly through the nervous trunks, but are subservient to the intellectual operations, to which they supply materials or motives. Thus, of two individuals, with differently constituted minds, one shall judge of every thing through the medium of a gloomy morose temper, which, like a darkened glass, represents to his judgment the whole world in league to injure him ; and all his determinations, being based upon this erroneous view, exhibit the indications of it in his actions ; which are themselves, nevertheless, of an entirely voluntary character. On the other hand, a person of a cheerful, benevolent disposition, looks at the world around as through a Claude Lorraine glass, seeing every thing in its brightest and sunniest aspect ; and, with intellectual faculties precisely similar to those of the former individual, he will come to opposite conclusions ; because the materials which form the -basis of his judgment are submitted to it in a very different form. Various forms of Moral Insanity exhibit the same contrast in a yet more striking light. We not unfrequently meet with individuals, still holding their place in society, who are accustomed to act so much upon feeling, and to be so little guided by reason, as to be scarcely regarded as sane ; and a very little exaggeration of such a tendency causes the actions to be so injurious to the individual himself or to those around him, that restraint is required, although* th'e intellect is in no way disordered, nor are any of the feelings perverted. Not unfrequently we may observe similar inconsistencies resulting from the habitual indulgence of one particular feeling, or a morbid exaggeration of it. The mother who, through weakness of will, yields to her 196 FUNCTIONS OF THE NERVOUS SYSTEM. instinctive fondness for her offspring, in allowing it gratifications which she knows to be injurious to it, is placing herself below the level of many less gifted beings. The habit of yielding to a natural infirmity of temper often leads into paroxysms of ungovernable rage, which, in their turn, pass into a state of maniacal excitement. It is not unfrequently seen, that a delusion of the intellect (constituting what is commonly known as Monomania), has in reality resulted from a disordered state of the feelings, which have represented every occurrence in a wrong light to the mind of the individual. All such conditions are of extreme interest, when compared with those which are met with amongst idiots, and animals enjoying a much lower degree of intelligence ; for the result is much the same, in 'whatever way the balance between the feelings and the judgment (which is so beautifully adjusted in the well-ordered mind of Man), is disturbed, whether by a diminution of the intelligence, or by an exaltation of the feelings. These views will probably be found correct, whatever be the truth of the speculation with which they have been here connected, as to the part of the Nervous system concerned in the performance of the purely Emotional actions. That their channel is alike distinct, how- ever, from that of the voluntary movements, and from that of reflex operations, must be apparent to any one who fairly weighs the evidence. XIX. Functions of the Cerebellum. 286. In regard to the particular purposes which are served by the Cere- bellum, physiologists are still much in the dark ; although there are not wanting those who consider them well ascertained. That this organ has some special function, distinct from that of the Cerebral hemispheres, can scarcely be doubted ; since its peculiar structure and position, its independent connections with the Medulla Oblongata, and its extremely variable size rela- tively to the remainder of the Encephalon, point it out as an instrument adapted to some particular purpose. We shall inquire briefly into the nature of the evidence respecting its function, which is supplied to us by Comparative Anatomy, by Experiment, and by Pathological phenomena. A Cerebellum is found in all Vertebrated animals ; although it is in some extremely small, looking like a little prominence on the Medulla Oblongata. When this is the case, it is observed that the whole mass is not a miniature (so to speak) of the large Cerebellum of Man, but that the central portion (termed the vermiform process) is the part most developed ; the lobes not presenting themselves until the organ has acquired an increased dimension. The following table, con- structed from materials contained in M. Serres' most valuable Comparative Anatomy of the Brain, will afford some idea, of the materials for speculating on the nature of the function of the Cerebellum, which we obtain from this source. The first column gives the diameter of the Spinal Cord at the second cervical vertebra ; in the two succeeding columns are stated the transverse and the antero-posterior diameters of the Cerebellum ; these dimensions are stated in hundred-thousandths of a metre. The fourth column expresses, in round numbers, the proportion which the diameters of the Cerebellum bear to that of the Spinal Cord ; the latter being reckoned as 1. FUNCTIONS OF THE CEREBELLUM. 197 MAMMALIA. Diameter of Spinal Cord at 2d Cervical Vertebra. Transverse Diameter of Cerebellum. Antero-posterior Diameter of Cerebellum. Proportions. Man . 1100 12,000 6000 11 5i Simia Rubia 900 4500 2443 5 o| Bear . 1300 5900 3500 4^ 2^ Dog . 1100 4200 2525 3 2-1- Dromedary 1900 7100 4600 3| 2 Kangaroo . 1200 3800 2600 3 i 2* BIRDS. Falcon 400 1350 1100 3 2 Swallow . 3175 500 600 3 3J Turkey . 500 1350 1600 2| 2 Ostrich 700 1750 2500 21 33 REPTILES. Crocodile . 300 500 400 If 1| Frog 300 300 200 FISHES. Shark 700 1700 . 3100 2 :! 3 Cod . 575 1350 1700 2 3 7 Turbot 500 750 900 li If Lamprey . 275 225 100 *-l 267- This table affords us much scope for interesting speculation, and maybe applied to the correction of hypotheses erected upon other foundations. Before we proceed to these, however, a few general remarks may be made upon it. In the first place, the proportional development of the Cerebellum is seen to be smallest in the vermiform Fishes, which approach most nearly to the Inver- tebrata ; but it is much greater in the higher Fishes than it is in Reptiles. If we consider in what particular, that may be reasonably supposed to have a connection with this organ, the former surpass the latter, we should at once be struck with their superiority in activity and variety of movement. Passing on to Birds, we remark that the average dimensions of the Cerebellum greatly surpass those of the organ in Reptiles ; but that they do not exceed those occasionally met with in Fishes. The greatest size is not found in those species which approach most nearly to the Mammalia in general conformation, such as the Ostrich ; but in those of most active arid varied powers of flight. Lastly, on ascending the scale of Mammiferous animals, we cannot but be struck with the rapid advance in the proportional size of the Cerebellum, that we observe, as we rise from the lowest, which are surpassed in this respect by many Birds, towards Man, in whom it attains a development which appears enormous, even when contrasted with that of the duadrumana. 268. We have next to inquire what evidence can be drawn from Experi- mental investigations on the same subject ; and in reference to this it is desirable to remark, in the first place, that the experimental mode of inquiry is perhaps more applicable to this organ than to other parts of the Encephalon ; inasmuch as it can be altogether removed with little disturbance of the actions immediately essential to life ; and the animals soon recover from the shock of the operation, and seem but little affected, except in some easily recognized particulars. The principal experimenters upon this subject have been Rolan- 17* 198 FUNCTIONS OF THE NERVOUS SYSTEM. do, Flourens, Magendie, and Hertwig. It is not to be expected that there should be an exact conformity among the results obtained by all. Every one who has been engaged in physiological experiments, is aware of the amount of difference caused by very minute variations in their circumstances ; in no department of inquiry is this more the case than in regard to the Nervous System ; and such differences are yet more likely to occur in experiments made upon the Nervous Centres than in those which concern their trunks. The investigations of Flourens are the most clear and decisive in their results ; and of these we shall accordingly take a general survey. He found that, when the Cerebellum was mechanically injured, the animals gave no signs of sensi- bility, nor were they affected with convulsions. When the Cerebellum was being removed by successive slices, the animals became restless, and their movements were irregular ; and by the time that the last portion of the organ was cut away, the animals had entirely lost the powers of springing, flying, walking, standing, and preserving their equilibrium, in short, of performing any combined muscular movements which are not of a simply reflex character. When an animal in this state was laid upon the back, it could not recover its former posture ; but it fluttered its wings and did not lie in a state of stupor. When placed in the erect position, it staggered and fell like a drunken man, not, however, without making efforts to maintain its balance. When threatened with a blow, it evidently saw it, and endeavoured to avoid it. It did not seem that the animal had in any degree lost voluntary power over its several mus- cles ; nor did sensation appear to be impaired. The faculty of combining the actions of the muscles in groups, however, was completely destroyed ; except so far as those actions (as that of respiration) were dependent only upon the Reflex function of the Spinal Cord. The experiments afforded the same results when, made upon each class of Vertebrated animals; and they have since been repeated, with corresponding effects, by Bouillaud and Hert- wig. The latter agrees with Flourens, also, in stating that the removal of one side of the Cerebellum affects the movements of the opposite side of the body ; and he further mentions that, if the mutilation of the Cerebellum have been partial only, its function is in great degree restored. 269. All these results are objected to by those who assert that the Cere- bellum is the seat of the sexual instinct ; on the ground that the observed aberrations of the motor functions are sufficiently accounted for, by the gene- ral disturbance which an operation so severe must necessarily induce. The fallacy of this objection, however, is shown by the fact, that the much more severe operation of removing the Hemispheres does not occasion such an aberration ; the power of performing the associated movements, and of main- taining the equilibrium, being remarkably preserved after the loss of them. 270. Upon comparing these results with the preceding table, a remarkable correspondence will be observed between them. The classes which have the greatest variety of movements, and which require for them the most perfect combination of a large number of separate muscular actions, have, taken col- lectively, the largest Cerebellum. Of all classes of Vertebrata, Reptiles are the most inert ; and their motions require the least co-ordination. The active predaceous Fishes far surpass them in this respect; and may be compared with Birds, in the energy of their passage through the water, and in their facility of changing their direction during the most rapid progression. The Cerebellum, accordingly, bears to the Spinal Cord in them, very much the same 'proportion as it does in Birds. On the other hand, the Flat Fish, which lie near the bottom of the ocean, and which have a much less variety of movement, have a very much smaller cerebellum: and the Vermiform Fishes, which are almost or completely destitute of fins, and whose progression is accomplished by flexion of the body, have a Cerebellum so small as to be FUNCTIONS OF THE CEREBELLUM. 190 scarcely discoverable : their motion being, like that of the Articulata, almost entirely of a reflex character, each segment being influenced by its own ganglionic centre, and the Spinal Cord constituting by far the largest propor- tion of the nervous centres. On looking at the class of Birds, we observe that the active predaceous Falcons, and the swift-winged Swallows (the per- fect control possessed by which over their complicated movements must hav% been observed by every one), have a Cerebellum much larger in proportion, than that of the Gallinaceous birds, \vhose powers of flight are small, or than that of the Struthious tribe, in which they are altogether absent. Lastly, on comparing its proportional size in the different orders of Mammalia, with the number and variety of muscular actions requiring combined movements, of which they are respectively capable, we observe an even more remarkable correspondence. In the hoofed Quadrupeds, in which the muscular appara- tus of the extremities is reduced to its greatest simplicity, and in which the movements of progression are simple, the Cerebellum is relatively smaller than it is found to be in some Birds ; but in proportion as the extremities acquire the power of prehension, and together with this a power of applica- tion to a great variety of purposes, still more, in proportion as the animal becomes capable of maintaining the erect posture, in which a constant mus- cular exertion, consisting of a number of most elaborately combined parts, is required, do AVC find the size of the Cerebellum, and the complexity of its structure, undergoing a rapid increase. Thus, even between the Dog and the Bear there is a marked difference ; the latter being capable of remaining for some time in the erect posture, and often spontaneously assuming it ; whilst to the former it is any thing but natural. In the semi-erect Apes, again, there is a very great advance in the proportional size of the Cerebel- lum ; and those which most approach Man in the tendency to preserve habitually the erect posture, also come nearest to him in the dimensions of this organ. Now it is evident that Man, although far inferior to many of the lower animals, in the power of performing various particular kinds of move- ment, far surpasses them all in the number and variety of the combinations which he is capable of executing, and in the complexity of the combinations themselves. Thus, if we attentively consider the act of walking in man, we shall find that there is scarcely a muscle of the trunk or extremities which is not actually concerned in it, some being engaged in performing the neces- sary movements, and others in maintaining the equilibrium of the body, which is disturbed by them. On the other hand, in the Horse or Camel, the mus- cular movements are individually numerous, but they do not require nearly the same perfect co-ordination. And in the Bird, the number of muscles em- ployed in the movements of flight, and in directing the course of these, is really comparatively small ; as may at once be perceived, by comparing the rigidity of the skeleton of the trunk of the Bird w r ith that of Man, and by remembering the complete inactivity of the lower extremities during the active condition of the upper. In fact, the motions of the wings are so simple and regular, as to suggest the idea, that, as in Insects, their character is more reflex than directly voluntary : an idea which is supported by the length of time during which they can be kept up without apparent fatigue, and also by the important fact already mentioned, wfiich experimental research has dis- closed ( 284). It is seen, then, that Comparative Anatomy fully confirms the idea, which Experimental physiology suggests, respecting the chief func- tions of the Cerebellum. 271. Some of Magendie's experiments indicate a further connection of this organ with the motor function, the nature of which is still obscure. This phy- siologist asserts that, if a wound be inflicted on the Cerebellum, the animal seems compelled by an inward force to retrograde movement, although making 200 FUNCTIONS OF THE NERVOUS SYSTEM. an effort to advance ; and that, if the Crus Cerebelli on one side be injured, the animal is caused to roll over towards the same side. Sometimes (if Ma- gendie's statements can be relied on), the animals made sixty revolutions in a minute, and continued this movement for a week without cessation. Division of the second Crus Cerebelli restored the equilibrium. Hertwig observed the s*me phenomenon, when the Pons Varoiii (which is nothing more than the commissure of the Cerebellum, surrounding the Crura Cerebri) was injured on one side ; and he has also remarked, that the movements of the eyes were no longer consensual. 272. On turning to Pathology for evidence of the functions of the Cere- bellum, we meet with much that seems contradictory. It must be remembered that a sudden effusion of blood, even to a small extent, in any part of the Encephalon, is liable to produce the phenomena of apoplexy or paralysis ; and inferences founded upon the phenomena exhibited after sudden lesions of this description are, therefore, much less valid than those based on the results of more chronic affections. ' In regard to these last, however, it is to be observed, that we are not yet in a condition to be able to state with precision, what amount of morbid alteration in any part of the nervous centres is com- patible with but slightly disturbed performance of its function ; and that cases are every now and then occurring, which would upset all our previous notions, if we were not aware that the same difficulty presents itself even in regard to the best-established results in Neurology. It is also to be remembered, that the results of disease, occasioning pressure, will be peculiarly liable to affect the Medulla Oblongata, as well as the Cerebellum 1 , and will thus occasion a greater loss of motor power than would be occasioned by the mere suspension of the function of the latter. 273. Pathological phenomena, when examined with these reservations, appear to coincide with the results of experiment, in supporting the conclu- sion, that the Cerebellum is not in any way the instrument of psychical opera- tions. Inflammation of the membranes covering it, if confined to that part, does not produce delirium ; and its almost complete destruction by gradual softening, does not appear necessarily to involve loss of intellectual power. " But," remarks Andral, " whilst the changes of intelligence were variable, inconstant and of little importance, the lesions of motion, on the contrary, were observed in all the cases [of softening which had come under his notice] ex- cept one ; and in this it is not quite certain that motion was not interfered with." In general, apoplexy of the Cerebellum is accompanied by paralysis ; but this is by no means usual in cases of chronic disease, in which there is rather an irregularity of movement, with a degree of restlessness, resembling that described by Flourens as resulting from partial injury of this organ. In a few cases in which both lobes of the Cerebellum have been seriously affected, the tendency to retrograde movement has been observed ; and instances are also on record, of the occurrence of rotatory movement, which has been found to be connected with lesion of the Crus Cerebelli on the same side. So far as they can be relied on, therefore, the results of the three methods of investi- gation bear a very close correspondence ; and it can scarcely be doubted, that they afford us some approximation to truth. 274. We have now to examine^ however, another doctrine regarding the functions of -the Cerebellum, which was propounded by Gall, and which is supported by the Phrenological school of physiologists. This doctrine that the Cerebellum is the organ of the sexual instinct is by no means incom- patible with the other ; and by some it has been held in combination with it. The greater number of phrenologists, however, regard this instinct as the exclusive function of the Cerebellum ; and assert that they can judge of its intensity, by the degree of development of the organ. We shall now examine FUNCTIONS OF THE CEREBELLUM. 201 the evidence in support of this position, afforded by the three methods of inquiry which have been already indicated. The results of fair observation as to the comparative size of the Cerebellum in different animals, can scarcely be regarded as otherwise than very unfavourable to the doctrine in question. In the greatest number of Fishes, it is well known that no sexual congress takes place ; the seminal fluid being merely effused, like any other excretion, into the surrounding water ; and being thus brought into accidental contact with the ova, of which a large proportion are never fertilized. On the other hand, in many Reptiles, the sexual instinct appears extremely strong ; and this is especially the case in the Frog, the whole system of which is endowed, at the season of fertility, with an extraordinary degree of excitability, analo- gous to a morbid condition that sometimes presents itself in the Human being. It has been remarked that, if the head of a male Frog be cut off, during the congress (which lasts for some time), his embrace will not be relaxed, and will even continue until the body of tile female is becoming gangrenous from the pressure ; thus showing that the action is one of a purely reflex character. Now, on comparing the size of the Cerebellum of the Frog with that of the Cod, (we exclude the higher Cartilaginous fishes, in which the reproductive function has a more elevated type,) we find that it is not above one-half the proportional size. Moreover, not only is the size much inferior, but the struc- ture is much less complicated in the former than in the latter. Again, in comparing the Gallinaceous Birds, which are polygamous, with the Raptorial and Insessorial tribes, which live in pairs, we find that the former, instead of having a larger cerebellum, have one of inferior size. Further, on looking at the Mammalia, the same disproportion may be noticed. A friend who kept some Kangaroos in his garden, informed the author that they were the most salacious animals he ever saw ; yet their cerebellum is one of the smallest to be found in the class. Every one knows, again, the salacity of Monkeys ; there are many which are excited to violent demonstrations, by the sight even of a human female ; and there are few which do not practise masturbation, when kept in solitary confinement : yet in them the cerebellum is much smaller than in Man, in whom the sexual impulse is much less violent. It has been supposed that the large size of the organ in Man is connected with his constant possession of the appetite, which is only occasional in others ; but this does not hold good ; since among domestic animals, there are many which are ready to breed throughout the year, Cats and Rabbits for instance ; and in these we do not find any peculiar difference in the size of the Cere- bellum. It is asserted, however, that the results of observation in Man lead to a positive conclusion, that the size of the Cerebellum is a measure of the intensity of the sexual instinct in the individual. This assertion has been met by the counter-statement of others, that no such relation exists. It is unfortunate that here, as in many other instances, each party has registered the observations favourable to their own views rather than those of an oppo- site character ; so that, until some additional evidence of a less partial nature has been collected, we must consider the question as sub judice. The author is by no means disposed to deny that such a correspondence may exist ; but on contrasting the degree of support which this part of phrenology really derives from pathological evidence, with that which the upholders of this view represent it to receive, he cannot but look with much distrust at all their observations on this subject. 275. It is stated in Phrenological works, as an ordinary result of disease of the Cerebellum, that there is an affection of the genital organs, manifesting itself in priapism, turgescence of the testes, and sometimes in seminal emis- sions. Now it is quite true that, in cases of apoplexy, in which these symp- toms manifest themselves, there is very commonly found to be effusion upon 202 FUNCTIONS OF THE NERVOUS SYSTEM. the Cerebellum, or in its substance ; but it is to be remembered, that in all such lesions the Medulla Oblongata is involved, and these symptoms, equally with paralysis, may be due to affection of that organ.* Further, the converse does not by any means hold good ; for the proportion of cases of disease of the Cerebellum, in which there is any manifest affection of the sexual organs, is really very small, being, according to the calculations of Burdach, not above one in seventeen. The same physiologist states that such affections do pre- sent themselves, although very rarely, \vhen the Cerebrum is the seat of the lesion. A large number of facts adduced by Phrenologists in support of their views such as the erections and emissions which often take place during hanging may be explained as well, or even better, on the hypothesis that the Cerebro-spinal axis (that is, the Spinal cord with the Medulla Oblongata) is the seat of this instinct. And this hypothesis is much more conformable to the results of experiment and disease than that which locates it in the Cere- bellum. For it has been found that mechanical irritation of the Spinal Cord, and disease in its substance, much more frequently produce excitement of the genital organs, than do lesions of the Cerebellum. This view is entertained by Miiller, and by most physiologists who have taken a comprehensive and unbiased survey of the phenomena in question. 276. Among the arguments adduced by Gall and his followers in proof of the connection between the Cerebellum and the sexual instinct, is one which would deserve great attention, if the facts stated could be relied on. It has been asserted, over and over again, that the Cerebellum, in animals which have been castrated when young, is much smaller than in those which have retained their virility, being, in fact, atrophied from want of power to act. Now it is unfortunate, that vague assertion, founded on estimates formed by the eye from the cranium alone, is all on which this position rests ; and it will be presently shown, how very liable to error such an estimate must be. The following is the result of a series of observations on this subject, suggested by M. Leuret,t and carried into effect by M. Lassaigne : The weight of the Cerebellum, both absolutely, and as compared with that of the Cerebrum, -was adopted as the standard of comparison. This was ascertained in ten Stallions, of the ages of from nine to seventeen years ; in twelve Mares, aged from seven to sixteen years ; and in twenty-one Geldings, aged from seven to seven- teen years. The average weight of the Cerebrum in the Stallions was 433 grammes; the greatest being 485 gr., and the least (which was in a horse of ten years old) being 350. The average weight of the Cerebellum was 61 gr.; the greatest being 65 gr., and the least 56 gr. The average proportion borne by the weight of the Cerebellum to that of the Cerebrum, was, therefore, 1 to 7-07 ; the highest (resulting from a very small Cerebrum) being 1 to 6-25 ; and the lowest (resulting from an unusually large Cerebrum) being 1 to 7'46. Throughout it might be observed, that the variation in the size of the Cerebellum was much less than in that of the Cerebrum. In the twelve Mares, the average weight of the Cerebrum was 402 gr. ; the highest being 432 gr. and the lowest 336 gr. That of the Cerebellum was 61 gr. ; the highest being 66 gr. (which was in the individual with the smallest Cerebrum), and the lowest 58 gr. * A case has been recently communicated to the Author, in which the sexual desire, which had been always strong through life, but which had been controlled within the limits of decency, manifested itself, during: a period of some months preceding death, in a most extraordinary degree; on post mortem examination, a tumour was found on the Pons Varolii. This fact harmonizes with the view given in the text ( 278), that the sexual instinct, if connected with the cerebellum at all, has its seat in the central lobe; but it also corresponds equally well with the idea, that the Medulla Oblongata is its centre. f Anat. Comp. du Systdme Nerveux, torn. i. p. 427. FUNCTIONS OF THE CEREBELLUM. 203 The average proportion of the weight of the Cerebellum to that of the Cere- brum was 1 to 6-59 ; the highest being 1 to 5*09, and the lowest 1 to 7. The proportion was, therefore, considerably higher in the perfect female than in the perfect male. In the twenty-one Geldings, the average weight of the Cerebrum was 419 gr. ; the highest being 566 gr., and the lowest 346 gr. The average of the Cerebellum was 70 gr. ; the highest being 76 gr., and the lowest 64 gr. The average proportion was, therefore, 1 to 5-97 ; the highest being 1 to 5-16, and the lowest 1 to 7'44. It is curious that this last was in the individual which had the largest Cerebellum of the whole ; but the proportional weight of the Cerebrum was still greater. 277. Bringing together the results of these observations, they are found to be quite opposed to the statement of Gall. The weight of the Cerebrum, reckoning the Cerebellum as 1, is thus expressed in each of the foregoing descriptions of animals. Average. Highest. Lowest. Stallions . . . 7-07 7-46 6.25 Mares . . . 6-59 7-00 5-09 Geldings . . 5-97 7-44 5-16 The average proportional size of the Cerebellum in Geldings, therefore, is so far from being less than that which it bears in entire Horses and Mares, that it is positively greater ; and this depends not only on diminution in the rela- tive size of the Cerebrum, but on its own larger dimensions, as the following comparison of absolute weights will show : Average. Highest. Lowest. Stallions ... 61 65 56 Mares ... 61 66 58 Geldings ... 70 76 64 The difference is so remarkable, and appears, from examination of the indi- vidual results, to be so constant, that it cannot be attributed to any accidental circumstance, arising out of the small number of animals experimented on. The average weight of the Cerebellum in the ten Stallions and twelve Mares is seen to be the same ; and the extremes differ but little in the two ; whilst the average in the Gelding is more than one-seventh higher, and the lowest is considerably above the average of the precedingpvhile the highest far exceeds the highest amongst the entire Horses. It is curious that Gall would have been much nearer the truth, if he had said that the dimensions of the cere- brum are usually reduced by castration ; for it appears from the following table that this is really the case : Average. Greatest. Least. Stallions ... 433 485 350 Mares ... 402 432 336 Geldings . . .419 566 346 The weight of the largest Cerebrum of the Gelding is far above the highest of the Stallions ; but it seems to be an extraordinary case, as in no other was the weight above 490 gr. If this one be excluded, the average will be reduced still further, being then about 412 ; this may be seen by looking ovrr the whole table, to give a very fair idea of the'usual weight in these animals, which is therefore less, by about one-twentieth, than the average of the Stal- lions. The increased size of the Cerebellum in Geldings may perhaps be accounted for, by remembering that this last class of horses is solely employed for its muscular power, and that the constant exercise of the organ is not unlikely to develop its size ; whilst Stallions, being kept especially for the 201 FUNCTIONS OF THE NERVOUS SYSTEM. purpose of propagation, are much less applied to occupations, which call forth their motor faculties. 278. The Author is far from denying in toto that any peculiar connection exists between the Cerebellum and the Genital system ; but if the evidence at present adduced, in support of the Phrenological position, be held sufficient to establish it, in defiance of so many opposing considerations, we must bid adieu to all safe reasoning in Physiology. The weight of testimony appears to him to be quite decided, in regard to the connection of the Cerebellum with the regulation of the motor function. How far this invalidates the moderate phrenological view, which does not regard the function of the Cerebullum as exclusively devoted to the sexual instinct, is a question well deserving of attention. There is nothing opposed to such an idea, in the results of the experiments already adverted to ( 268) ; since there is no evidence that sex- ual instinct remained after the removal of the Cerebellum ; but, on the other hand, there is no proof that it was destroyed. A circumstance which has been several times mentioned to him, that great application to gymnastic exercises diminishes, for a time, the sexual vigour, and even totally suspends desire, seems worthy of consideration in reference to such a view. If the Cerebel- lum be really connected with both kinds of functions, it does not seem unrea- sonable that the excessive employment of it upon one should diminish its energy in regard to the other. Further, it would seem by no means impro- bable that the Lobes are specially connected with the regulation and co-ordi- nation of movements ; whilst the Vermiform processes, which are very large in many animals in which the former scarcely present themselves, are the parts connected with the sexual function. As an additional argument in favour of the former part of this view, it may be stated, that in Man the lobes bear a larger proportion to the Vermiform processes than in any other animal ; and that they undergo their most rapid development during the first few years of life, when a large number of complex voluntary movements are being learned by experience, and are being associated by means of the muscular sensations accompanying them ; whilst in those animals which have, imme- diately after birth, the power of regulating their voluntary movements for definite objects, with the greatest precision, the Cerebellum is more fully developed at the time of birth. In both instances it is well formed and in active operation (so far as can be judged of by the amount of circulation through it), long before the s&xual instinct manifests itself in any perceptible degree. XX. Functions of the Cerebrum. 279. In regard to certain general positions, there is little difference of opin- ion amongst Physiologists upon this much-controverted subject; and it will be desirable to inquire what may be considered as firmly established before we proceed with details of a more questionable nature. We shall, as before, apply to Comparative Anatomy, to Experiment and to Pathology, for our chief data. Any genera] inferences, founded only upon observation of the phenomena presented by Man, must be looked upon with suspicion ; since every advance in Comparative Physiology leads us to perceive how close is the functional relation between organs that are really of analogous nature in different classes of animals; and how necessary, therefore^ it is to examine ai^f contrast all the facts which we can attain in regard to them, in order to impart to our conclu- sions the utmost validity of which they are capable. Our first general propo- sition is, that the Cerebrum is the sole instrument of Intelligence; by which term is implied the Voluntary adaptation of means to ends, in a manner imply- ing a perception of the nature of both. The actions performed by the lower FUNCTIONS OF THE CEREBRUM. 205 animals are often such as to leave us in doubt whether they are the result of a mere instinctive impulse or of an intelligent adaptation of means to ends ; and we are guided in our determination chiefly by the uniformity of these actions in the several individuals of the same species. If we analyze any of our own instinctive actions, we shall perceive the same absence of design on our own parts as that which we attribute to the lower animals. No one would assert that the tendency to sexual intercourse is the result of a knowledge of its con- sequences, and of a voluntary adaptation of means to ends ; or that, if we can imagine a man newly coming into the world, in the full possession of all his powers, he would wait to eat, when hungry, until experience had taught him that the swallowing of food would relieve the uneasy feeling. It has been already shown that, in the infant, the act of sucking may be performed even without a Cerebrum ( 197) ; and for this, and other similar actions, therefore, it is doubtful whether consciousness is a requisite condition. Adult animals, whose Cerebral hemispheres have been removed, will eat food that is put into their mouths, although they will riot go to seek it; and this is the case with many Human idiots. When the functions of the Brain are disturbed, or in partial abeyance, as in fever, we often see a remarkable return to the instinctive propensities in regard to food ; and the Physician frequently derives import- ant guidance, as to the patient's diet and regimen, (particularly as to the administration of wine,) from the inclination or disinclination which he mani- fests. The Intelligence of an animal may be further estimated by its degree of educability that is, the facility with which its natural habits may be changed by the influence of man, and the complication of the mental pro- cesses which it appears to perform under its new circumstances. We all know that Insects the most active of all Inv^rtebrated animals are but little susceptible of such influence. It may be doubted whether there ever was a case in which an Insect of any kind could be taught to recognize any one who had been in the habit of feeding it, or to show any other unequivocal mark of intelligence. Bees and other Insects which display much art in the construction of their habitations, and which execute a variety of most curious contrivances, beautifully adapted to variations in their circumstances, appear to be entirely guided in their operations by instinct; since all Bees act alike, under the same circumstances. We do not find one community or individual clever, and another stupid ; and for a Bee to be destitute of its peculiar ten- dency to build at certain angles, would be as remarkable as a Human being without a tendency to eat.* In Insects, as already stated, we can discover little or nothing that is analogous to the Cerebrum of Vertebrata; and it is manifest that their cephalic ganglia correspond chiefly with the ganglionic enlargements at the upper end of the Medulla Oblongata, which are connected with the organs of special sensation, and which have been stated in the pre- * The only manifestation of educability which the Author has ever noticed, during a pretty long familiarity with the habits of Bees, is the acquirement of a power of distin- guishing the entrance of their hive from that of others around. When a swarm is first placed in a new box, and the Bees have gone forth in search of food, they often seem puzzled, on their return, as to which is their own habitation; more especially if there be several hives, with similar entrances, in one bee-house; and it has been proposed to paint these entrances of different colours, in order to enable the Bee to distinguish them more readily. In a short time, however, even without such aid, the Bees are seen to dart from a considerable height in the air, directly down to their proper entrances; showing that they have learned to distinguish these by a memorial power. This the Author has ob- served most remarkably, in a case in which a hive is placed in the drawing-room of a house, the entrance to it being beneath one of the windows; the adjoining houses have windows precisely similar, except in the absence of this small passage; and he has often noticed that, when a new stock has been placed in this hive, the Bees are some days in learning the exact position of their house, considerably annoying the neighbours by flying in at their windows. 18 206 FUNCTIONS OF THE NERVOUS SYSTEM. ceding section, to be not improbably the centres of the instincts and emotions of higher animals. 280. On comparing Birds with Insects, we at once see a very remarkable difference in the character of their actions. Their instinctive tendencies are of nearly the same kind; and the usual arts which they exhibit in the con- struction of their habitations, in procuring their food, and in escaping from danger, must be regarded as intuitive, on account of the uniformity with which they are practised by different individuals of the same species, and the perfection Vith which they are exercised on the very first occasion. But in the adaptation of their operations to peculiar circumstances, Birds display a variety and fertility of resource far surpassing that which is manifested by Insects ; and it is not doubted, by those who have attentively observed their habits, that in such adaptations they are often guided by real intelligence. This must be the case, for example, when they make trial of several means, and select that one which best answers the purpose ; or when they make an obvious improvement from year to year in the comforts of their dwelling ; or when they are influenced in the choice of a situation by peculiar circumstances, which, in a state of nature, can scarcely be supposed to affect them. The complete domesticability of many Birds is in itself a proof of their possessing a certain degree of intelligence ; but this alone does not indicate the possession of more than a very low amount of it ; since many of the most domesticable animals are of the humblest intellectual capacity, and seem to become attached to Ma*n, principally as the source on which they depend for the supply of their animal wants. This is the case with most Herbivorous quadrupeds, and with Rabbits, Guinea-pigs, &c., as well as with the Gallinaceous Birds. The attachment of the Dog or the Elephant is evidently of a much higher kind, and involves a much larger number of considerations ; and their actions are evidently the result, in many instances, of a complex train of reasoning, differ- ing in no essential respect from that which Man would perform in similar circumstances. The epithet, " half-reasoning," commonly applied to these animals, does not express the whole truth ; for their mental processes are of the same kind with those of Man, and differ more in the degree of control which the animal possesses over them, than they do in their own character. We have no evidence that any of the lower animals have a voluntary power of guiding, restraining, or accelerating their mental operations, at all similar to that which Man possesses ; these seem to be of very much the same character as those which we perform in our dreams, different trains of thought com- mencing as they are suggested, and proceeding according to the usual laws, until some other disturb them. Although it is customary to regard the Dog and the Elephant as the most intelligent among the lower animals, it is not certain that we do so with justice ; for it is very possible that we are misled by that peculiar attachment to Man which in them must be termed an instinct, and which enters as a motive into a large proportion of their actions ; and that, if we were more acquainted with the psychical characters of the higher Gluadrumana, we should find in them a greater degree of mental capability than we now attribute to them. One thing is certain, that, the higher the degree of intelligence which we find characteristic of a particular race, the greater is the degree of variation which we meet with in the characters of individuals ; thus every one knows that there are stupid Dogs and clever Dogs, ill-tempered Dogs and good-tempered Dogs, as there are stupid Men and clever Men, ill-tempered Men or good-tempered Men. But no one could distinguish between a stupid Bee and a clever Bee, or between a good-tem- pered Wasp and an ill-tempered Wasp, simply because all their actions are prompted by an unvarying instinct. 281. Before inquiring into the comparative size of the Cerebrum, in different FUNCTIONS OF THE CEREBRUM. 207 animals, it is desirable to obtain a general notion of its structure. Three principal sets of fibres may be distinguished in the white or medullary sub- stance, of which the great mass of it is composed. These are the Ascending fibres, which proceed from the sensory tract, and diverge from the Thalami optici to the periphery, the Descending fibres, which converge from the peri- phery towards the Corpora Striata, and then pass downwards into the motor tract ; and the Commissural fibres, which establish the connection between the various parts of the periphery and of the substance of the brain. It is on the very large proportion which the commissural fibres bear to the rest, that the bulk of the brain of Man and of the higher animals chiefly depends ; and it is easy to conceive, that this condition has an important rela- tion with mental operations, whatever be our view of the functions of dif-* ferent parts of the Brain. The different relative distribution of the gray and white matter in the Cerebrum, from that which is elsewhere presented to us in ganglionic masses, naturally suggests the inquiry, how far this corre- sponds with what has been stated of their probable functions. The amount of ordinary vascular action in the gray substance, as compared with that which takes place in the white, is an important circumstance in favour of the view, that it is the part in which all changes originate, and that the fibrous portion, like the trunk of the nerve, serves only to conduct or transmit the influence of those changes. This position derives additional support from the effects of disease. It has been frequently remarked that, if we compare those cases of cerebral disease in which there is delirium, with those in which it does not occur, we shall find that it is most common in cases in which there is an inflammatory affection of the surface, or of the membranes, extending from, them into it ; whilst in deep-seated inflammation, the most important symptoms are those which result from sympathetic affections of the muscular system. It has been even proposed to establish a diagnosis between inflammation of the membranes (especially of the arachnoid), and inflammation of the substance of the brain, upon this general fact ; but it is to be remembered that (to use the words of Lallemand) " it is impossible that the arachnoid should be inflamed, without the surface of the brain in contact with it being also affected ; but its tissue not being altered, there merely results from this vicinity exaltation in its functions." All the cases, therefore, which have been referred to in sup- port of this diagnosis, really tend to establish the proposition, that the superficial portion of the Cerebrum is the part really affected. It is absurd to suppose that inflammation of the membranes, without any abnormal condition of the Brain itself, can seriously affect the mental operations. It has been further remarked, that arachnitis of the convexity of the Brain is more frequently attended with delirium and other symptoms of excitement than similar in- flammation affecting the base, in which coma supervenes earlier, with little or no previous disturbance of the intellect ; this, too, corresponds with the doc- trine just referred to ; since the influence of any effusion aboirt the origins of the nerves and the Medulla Oblongata, is well known to be prejudicial to their functions as conductors, even entirely suspending them; whilst, from the infe- rior vascularity of these parts, they are not so liable to manifest symptoms of excitement, from the contiguity of an inflamed membrane. In fact, inflamma- tion of the white substance of the Brain is itself attended rather with a state of torpor, or of partial suspension of its usual operations, than with excitement ; irregular convulsive actions are not unfrequently seen as a result of it ; but these are often manifested, when the power of the will over the muscles, is destroyed. It may not be difficult to account for this difference of symptoms, by reflecting, that a large proportion of the medullary substance of the Brain consists of a sort of extremely delicate areolar tissue, by which the fibres are connected together, and through which the blood-vessels are distributed ; and it is probably in this that the principal changes take place, of which the early 308 FUNCTIONS OF THE NERVOUS SYSTEM. stages of the inflammatory process consist. These changes, being accompanied by turgescence of the vessels, and by effusion into the. tissue surrounding them, must occasion a degree of pressure on the enclosed fibres, which de- stroys their conducting power, and thus cuts off the body from connection with the centre of the intellectual operations ; whilst they may at the same time give rise to many irregular and involuntary movements of the muscles, to which the fibres thus affected are distributed. 282. This view is further supported by the researches of Foville, on the alte- rations of the Brain which are connected with insanity. His observations are deserving of great confidence, both for the sake of his own high character and attainments, and on account of the careful manner in which they were made. *To avoid trusting to his memory for comparison, Foville has been in the habit of examining the brain of a person who died without any disease in this organ, at the same time with that of one who died insane. In acute cases of Insanity, he frequently found the cineritious portion intensely red, but without adhesion to the membranes ; whilst in chronic cases, he found the cortical substance indurated and adherent to the membranes. In nearly all cases of Insanity accompanied with general Paralysis, he has found the white portion of the brain injected and indurated ; and he conceives that the fibres had become adherent to each other. It has been supposed by Calmeil, that the paralysis of the insane is connected with disease of the cineritious substance ; but Foville states that he and his colleagues have made many hundreds of observations on cases, in which there were well-marked alterations of the cortical substance of the brain, without any other manifestations during life than disorder of the intellect. This view is supported by Bouillaud, and by several other eminent pathologists ; as is also the other part of the proposition, that morbid alterations in the medullary portion are connected with dis- order in the transmission of motor impulses to the muscles. 283. It is important to bear in mind the view to which we are thus con- ducted, in regard to the relative offices of the gray and white matter, when forming our opinions upon the functions of the Cerebrum in general, or of its several parts, from the various data supplied to us by Comparative Anatomy, and by experimental and pathological inquiry. For in regard to the first of these sources it is to be remarked, that the size of the brain does not, con- sidered alone, afford a means of judgment as to its power. The quantity of gray matter on its surface should rather be our guide ; and this we may judge of, not only by the depth of the layer, but by the complexity of the convolu- tions by which the surface is extended. * In no class, save in Mammalia, do we find the surface marked with convolutions; and in general we do not meet with that fissure between the hemispheres which greatly increases the extent of surface. In forming comparisons as to the connection between the size of the Brain, and the intelligence, in different animals, we must not be at all guided by its simple proportional dimensions ; since it is very evident, that it is rather the proportion of the bulk of the brain to that of the whole body, upon which we should found our comparison. But even this is not alto- gether a safe guide; and many Physiologists have endeavoured to com- pare the size of the brain with the aggregate bulk of the nerves proceeding from it. This is a much fairer measure ; but it cannot be taken without great difficulty. For all practical purposes, the comparison of the bulk of the Brain with that of the Spinal Cord will probably answer very well. The following table, the materials of which are drawn from M. Serre's Comparative Anatomy of the Brain, exhibits the three diameters of the Brain of a number of different animals, and the diameter of the Spinal Cord at the second cervical vertebrae. The last three columns present in round numbers, the three diameters of the Brain, reckoning that of the Spinal Cord as 1, for the sake of easy comparison. FUNCTIONS OF THE CEREBRUM. 209 Diameter of Spinal Cord. DIMENSIONS OF CEREBRUM. Proportional Dimensions. AHt.-post. Transv. Vertical. Man . 1100 17000 7500 9000 li 5i 1 6f 184 Dolphin Mandril 1000 950 9500 8100 5850 3200 8200 4900 1-9* 1-4 1 3 ilSl 15 Tiger 1600 9400 4250 6400 1 52 1 2f 14 Dromedary 1900 10500 5050 5800 1 5* 1 2f 13 Kangaroo 1200 5300 2350 3800 l-4f 12 1-3* Vulture 800 3200 2200 1550 14 1 2| 12 Falcon 500 1900 1450 1200 1 3| 13 1 2f Swallow . 175 1000 600 550 1 5f 3* 1-31 Pie . 450 2000 1400 1200 1 4| 3 1 2| Turkey 500 1750 1250 1200 1 3| 2* Parroquet . 400 2900 1400 1700 1 7* 3* 14! Tortoise . 300 1600 500 1 5| 1* Crocodile . 300 800 500 12* 1* Viper 200 600 300 12 li Frog . 300 500 400 1-lf -i* Shark 710 2300 IJOO 1 3s l 4 Cod . 575 725 800 11$ i| Lamprey . 275 400 300 1 li H Angler 400 400 300 1 1 if 284. As might be expected, the Brain of Man bears by far the highest proportion ; but this proportion is not so large in the transverse and vertical diameters, as in the antero-posterior ; in fact, in the proportion of the vertical diameter, the brain of Man is equaled by that of the Dolphin, and nearly so in that of the transverse diameter. In the complexity of the convolutions, however, and in the thickness of the gray matter, the Cerebrum of Man far surpasses that of this Cetaceous animal. In these respects the higher Q,uad- rumana present the nearest approach to it ; but their brain is much inferior in size. In descending the scale of Mammalia, there may be observed a gradual simplification in the general structure of the Brain, depending upon a great diminution in the amount of commissural fibres ; until in the Marsu- pialia the Brain presents nearly the same condition which it offers in Birds ( 218). These animals manifest a much lower degree of intelligence than many Birds evidently possess ; and it is interesting to remark, that their cere- bral hemispheres are proportionably smaller than those which we find in many Birds : the diminution in their relative size not being counterbalanced (as it is in some other instances) by increased complexity of structure. In the class of Birds we observe that the Vulture and the Falcon, whose preda- ceous instincts give them a considerable amount of general energy, are much inferior in the size of their brains to the Insessorial Birds, which are more intelligent ; and that of all, there is none in which the brain is so proportion- ally large, as it is in the Parrot tribe, the educability of which is familiar to every one ; whilst the easily domesticable, but unintelligent Turkey, has a brain of scarcely half the proportional size. The' very small size of the Cere- brum in Reptiles and Fishes, completely harmonizes with the same view ; these animals presenting for the most part but feeble indications of intelli- gence. Among Reptiles, the Tortoise has a Cerebrum comparable in length 18* 210 FUNCTIONS OF THE NERVOUS SYSTEM. to that of Birds ; but its breadth and depth are far less. The largest Cerebra among Fishes are found in the Shark tribe; the superior intelligence of which is well known to those who have had the opportunity of observing their habits ; and it is interesting to remark, that their surface occasionally presents an appearance of rudimentary convolutions. 285. Comparative Anatomy, then, fully bears out the general doctrine, that the Cerebrum constitutes the organ of Intelligence, as distinguished from those mere Instincts by which many of the lower animals seem to be almost entirely guided. By Intelligence, we do not mean, however, the reasoning faculties only, but the combination of those powers which are of an educable character, and which become the springs of voluntary action in very different proportions in different animals of the same tribe ; as distinguished from those which have an immediate relation to the wants of the corporeal system, and which are automatic and invariable in the several individuals of the same species. Observation of the Human species exhibits the same distinction. When the Brain is fully developed, it offers innumerable diversities of form and size, among various individuals ; and there are as many diversities of cha- racter. It may be doubted if two individuals were ever exactly alike in this respect. That a Brain which is greatly under the average size, is incapable of performing its proper functions, and that the possessor of it must neces- sarily be more or less idiotic, there can be no reasonable doubt. On the other hand, that a large well-developed Brain is found to exist in persons, who have made themselves conspicuous in thfc world by their attainments or their achievements, may be stated as a proposition of equal generality. In these opposite cases, we witness most distinctly the antagonism between the in- stinctive and voluntary powers. Those unfortunate beings, in whom the Brain is but little developed, are guided almost solely by their instinctive tendencies, which frequently manifest themselves with a degree of strength that would not have been supposed to exist : and occasionally new instincts present themselves, of which the Human being is ordinarily regarded as destitute.* On the other hand, those who have obtained most influence over the under- standings of others, have always been themselves persons of strong volitional powers ; in whom the instinctive tendencies have been subordinate to the wih 1 , and who have given their whole energy to the particular object of their pur- suit. It is very different, however, with those who are actuated by what is ordinarily termed genius; and whose influence is rather upon the feelings than upon the understandings, of those around them. Such persons are often very deficient in the power of even comprehending the ordinary affairs of life ; and still more commonly, they show an extreme want of judgment in the management of them, being under the immediate influence of\their pas- sions and emotions, and not having brought these under the control of their intelligent will. The life of a genius, whether his bent be towards poetry, music, painting, or pursuits of a more material character, is seldom one which can be held up for imitation. In such persons, the general, power of the mind being low, the Brain is not usually found of any great size. The mere comparative size of the Brain, however, affords no accurate measure of the amount of mental power ; we not unfrequently meet with men possessing large and well-formed heads, whilst their capacity is not greater than that of others, the dimensions of whose crania have the same general proportion, but are of much less absolute size. Large brains, with deficient activity, are * A remarkable instance of this has been recently published. A perfectly idiotic girl, in Paris, having been seduced by some miscreant, was delivered of a child without assistance. It was found that she had gnawed the umbilical cord in two ; in the same manner as is practised by the lower animals. It is scarcely to be supposed that she had any idea of the object of this separation. FUNCTIONS OF THE CEREBRUM. 211 commonly found in persons of what has been termed the phlegmatic tempera- ment, in whom the general processes of life seem in a torpid and indolent state ; whilst small brains and great activity betoken what are known as the sanguine and nervous temperaments. These distinctions come to be very important, where we proceed further in our inquiries, and attempt to deter- mine the particular modes of development of the Brain, which coincide with certain manifestations of the mind. 286. Having now inquired into the evidence of the general functions of the Cerebrum, which may be derived from examination of its comparative deve- lopment, we proceed to our other sources of information Experiment and Pathological phenomena. The effects of the entire removal of the Hemi- spheres have been already described ( 264). In these and similar experi- ments, it has been constantly remarked that injuries of the Cerebral substance do not occasion signs of pain, and that they do not give rise to any convulsive movements. Even the Thalami and Corpora Striata may be wounded, with- out the excitement of convulsions ; but if the incisions involve the Tubercula Q,uadrigemina, or the Medulla Oblongata, convulsions uniformly occur. This result perfectly accords with what has been observed in Man ; for it has been frequently remarked, when it has been necessary to separate protruded por- tions of the Brain from the healthy part, that it has given rise to no sensation, even in cases in which the mind has been perfectly clear at the time. The effect of pressure upon the Brain is well known to be the suspension of all its operations : this has been substantiated by experiments upon animals, and also by similar experiments on persons who have had a portion of th *cranium removed, so as to expose the membranes of the Brain. The pressure of the finger upon the membranes occasions a state of immediate unconsciousness, resembling profound sleep, which ceases as soon as the pressure is withdrawn. Such pressure will, of course, affect the whole Encephalon, and not the Cere- brum alone. Experiment does not throw much light on the particular func- tions of the Corpus Callosum and other Commissures; since they can scarcely be divided without severe general injury. It would appear, however, that the partial or entire absence of these parts, reducing the Cerebrum to the level of that of the Marsupial quadruped, or of the Bird, is by no means an unfre- quent cause of idiocy. 287. The information afforded by Pathological phenomena is far from being definite. Many instances are on record, in which extensive disease has oc- curred in one Hemisphere, so as almost entirely to destroy it, without either any obvious injury to the mental powers, or any interruption of the influence of the mind upon the body. But there is no case on record, of severe lesion of both hemispheres, in which morbid phenomena were not evident during life. , It is true that, in Chronic Hydrocephalus, a very remarkable alteration in the condition of the Brain sometimes presents itself, which might a priori have been supposed destructive to its power of activity ; the ventricles being so enormously distended with fluid, that the cerebral matter has seemed like a thin lamina, spread over the interior of the enlarged cranium. But there is no proof that absolute destruction of any part was thus occasioned; and it would seem that the very gradual nature of the change gives to the structure time for accommodating itself to it. This, in fact, is to be noticed in all dis- eases of the Encephalon. A sudden lesion, so trifling as to escape observation, unless this be very carfully conducted, will occasion very severe symptoms ; whilst a chronic disease may gradually extend itself, without any external manifestation. It will usually be found that sudden paralysis, of which the seat is in the Brain, results from some slight effusion of blood in the substance or neighbourhood of the Corpora Striata; whilst, if it follow disorder of the Brain of long standing, a much greater amount of lesion will usually present 212 FUNCTIONS OF THE NERVOUS SYSTEM. itself. In either case, the paralysis occurs in the opposite side of the body, as we should expect in the decussation of the pyramids ; but it may occur either in the same or on the opposite side of the /ace the cause of which is not very apparent. If convulsions accompany the paralysis, we may infer that the Corpora Quadrigemina, or the parts below, are involved in the injury ; and, in this case, it is usually found that the convulsions are on the paralyzed side of the body the effect of the lesion, both of the Cerebrum and of the Corpora Quadrigemina, being propagated to the opposite side by the decussation of the pyramids. Where, as not unfrequently happens, there is paralysis of one side, accompanying convulsions on the other, it is commonly the result of a lesion affecting the base of the Brain and Medulla Oblongata, on the side on which the convulsions take place. Here the effect of the lesion has to cross from the Brain, whilst its influence on the Medulla Oblongata is shown on the same side. Many apparent anomalies present themselves, however, which are by no means easy of explanation in the present state of our knowledge. 288. The general result of such investigations is, that the Cerebrum is the organ through which all those impressions are received which give rise to Voluntary actions ; and that it affords the power of occasioning muscular contraction, in obedience to the influence of the will : but that the fibres com- posing its medullary portion are not susceptible of being thrown into action by mechanical irritation, in the same manner as are those of the Spinal Cord and Nerves, a peculiarity which may, perhaps, be connected with the differ- ence of their structure, formerly explained ( 110). There is no positive reason for the belief, that the Cerebrum is essential to the purely Emotional actions ; and analogy, as we have seen, applied to the explanation of patho- logical phenomena, would lead to the belief that their channel is different. It can scarcely be denied, however, that in the Cerebrum resides that power by which the attention of the mind is directed to any sensation; and by which, through the medium of a brief reasoning process, a notion is formed regarding its nature : this operation is altogether designated as perception, which term, however, is also applied to its result. Now it will be presently seen, that the formation of such elementary notions in us, is often a complex process, though a rapid one ; whilst, in many of the lower animals, it appears to be very much simpler, as to all those points, at least, which concern the instinctive actions necessary for their well-being. Such intuitive perceptions occasionally take place in ourselves ; but it will probably appear, from exami- nation of them, that they are connected either with the mere Instincts, or with the Emotions. 289. Some metaphysicians have confounded Perception with Sensation ; but the difference may be easily made evident. In order that a sensation should be produced, a conscious state of the mind is all that is required. Its whole attention may be directed towards some other object, and the sensation calls up no new ideas whatever ; yet it will produce some change in the sensorium, which causes it to be (as it were) registered there for a time, and which may become the object of subsequent attention ; so that, when the mind is directed towards it, that idea or notion of the cause of the sensation is formed which constitutes a perception. For example, a student, who is directing his thoughts to some object of earnest pursuit, does not receive any intimation of the passage of time from the striking of a clock in his room. The sensation must be produced, if there be no defect in his nervous system. ; but it is not attended to, because the mind is bent upon another object. It may produce so little impression on the mind, as not to recur spontaneously, when the train of thought which previously occupied the mind has been closed, leaving the attention ready to be directed to any other object ; or, the impression having been stronger, it may 'so recur, and at once excite an idea FUNCTIONS OF THE CEREBRUM. 213 in the mind. Again, the individual may then be able only to say, that ho heard the clock strike ; or he may be able to retrace the number of strokes. Now, in either case, a simple perception is formed, without his being aware that any mental operation has intervened. He would say that he remembers hearing the clock strike ; but this would not express the truth. That which he remembers is a certain series of sonorous impressions, which was commu- nicated to his mind ; and he recognizes them as the striking of a clock, by a process in which memory and judgment are combined, which process may further inform him, that the sounds proceeded from his own particular clock. If he had never heard a clock strike, and the sound produced by it had never been described to him, he would not have been able to form that notion of the object giving rise to the sensation, which, simple as it appears to be at the time, is the result of complex mental operations. But when these operations have been frequently performed, the perception or notion of the object be- comes inseparably connected with the sensation ; and thus it is excited by the latter, without any knowledge, on the part of the individual, that a mental operation has taken place. Such perceptions are termed acquired, in contra- distinction to the intuitive perceptions, of which the lower animals seem to possess a large number. The idea of the distance of an object, for example, is one derived in Man from many sources, and is the result of a long experi- ence ; the infant, or the adult seeing for the first time, has to bring the senses of sight and of touch to bear upon one another, in order to obtain it ; but, when once the power of determining it is acquired, the steps of the process are lost 'sight of. In the lower tribes of animals, however, in which the young receive no assistance from their parents, there is an evident necessity for some immediate power of forming this determination ; since they would not be able to obtain their food without it. Accordingly they manifest in their actions a perception of governing idea of distances, which can only be gained by Man after long experience. A Fly-catcher, for example, just come out of its shell, has been seen to peck at an insect, with an aim as perfect, as if it had been all its life engaged in learning the art ( 341). 290. In some instances, animals learn that, by intuitive perception, at which Man could only arrive by the most refined processes of reasoning, or b the careful application of the most varied experience. Thus, a little fish, named the ChsRtodon rostratus, is in the habit of ejecting from its prolonged snout, drops of fluid, which strike insects that happen to be near the surface of the water, and cause them to fall into it, so as to come within its own reach. Now, by the laws of refraction of light, the place of the Insect in the air will not really be that, at which it appears to the Fish in the water ; but it will be a little below its apparent place, and to this point the aim must be directed. But the difference between the real and the apparent place will not be con- stant ; for the more perpendicularly the rays enter the water, the less will be the variation ; and, on the other hand, the more oblique the direction, the greater will be the difference. Now it is impossible to imagine but that, by an intuitive perception, the real plac^of the Insect is known to the Fish in every instance, as perfectly as it could be to the most sagacious Human mathematician, or to a clever marksman, who had learned the requisite allow- ance in each case by a long experience. In Man, the acquirement of per- ceptions is clearly a cerebral operation ; but their intuitional formation in the lower animals is probably to be regarded as one of those processes, to which the ganglia connected with the organs of special sense, that are in them of so great a proportional size, are subservient. The same may be said of many of the intuitive perceptions in Man, which, if analyzed, are found to be con- nected rather with the instinctive and emotional tendencies than with the intellectual powers ; the perceptions which minister to the exercise of these 214 FUNCTIONS OF THE NERVOUS SYSTEM. last, being the result of experience. Thus, it has been well remarked by Dr. Alison, that the changes which Emotions occasion in the countenance, ges- tures, &c., of one individual, are instinctively interpreted by others ; for these signs of mental affection are very early understood by young children, sooner than any associations can be supposed to have been formed, by experience, of their connection with particular modes of conduct ; and they affect us more quickly and strongly, and with nicer varieties of feeling, than when it is attempted to convey the same feelings in words, which are signs addressed to the intellect. 291. Many Physiologists and Metaphysicians are of opinion, that every sensation actually experienced may become the subject of a perception at any future time, though beyond the voluntary power of the memory to retrace ; and the phenomena of dreams and delirium, in which these sensations often recur with extraordinary vividness, afford much support to this doctrine. Some of the instances upon record are remarkable, as proving that the sensa- tions may be thus remembered, without any perceptions being attached to them ; these sensations having been of such a nature, as not to excite any notion or idea in the mind of the individual. A very extraordinary case of this kind has been recorded, in which a woman, during the delirium of fever, continually repeated sentences in a language unknown to those around her, which proved to be Hebrew and Chaldaic ; of these she stated herself, on her recovery, to be perfectly ignorant ; but on tracing her former history, it was found that, in early life, she had lived as servant with a clergyman, who had been accustomed to walk up and down the passage, repeating or reading aloud sentences in these languages, which she must have retained in her memory unconsciously to herself. Of the nature of the change, by which sensations are thus registered, it is in vain to speculate ; and it does not seem likely that we shall ever become acquainted with it. This is certain, how- ever, that disease or injury of the brain will destroy this power, or will affect it in various remarkable modes. We not unfrequently meet with cases, in which the brain has been weakened by attacks of epilepsy or apoplexy, in such a manner as to prevent the reception of any new impressions ; so that the patient does not remember any thing that passes from day to day ; whilst the impressions of events, which happened long before the commencement of his malady, recur with greater vividness than ever. On the other hand, the memory of the long-since-past is sometimes entirely destroyed ; whilst that of events which have happened subsequently to the malady is but little weakened. The memory of particular classes of ideas is frequently destroyed ; that of a certain language, or some branch of science, for example. The loss of the memory of words is another very curious form of this disorder, which is not unfrequently to be met with : the patient understands perfectly well what is said, but is not able to reply in any other terms than yes or no, not from any paralysis of the muscles of articulation, but from the incapability of expressing the ideas in language. Sometimes the memory of a particular class of words only, such as nouns or verbs, is destroyed ; or it may be impaired merely, fco that the patient mistakes the proper terms, and speaks a most curious jargon. These cases have a peculiar interest, in reference to the final subject of our inquiry. 292. That the different portions of the Cerebrum have different functions in the complex operations of thought, must be admitted to be by no means an improbable speculation ; and it is well known that, under the name of Phre- nology, or the Science of Mind, a systematic allocation has been made, of what have been regarded as the several fundamental powers and faculties of the mind, to certain parts of the Cerebral hemispheres. This was first attempted by Gall, who stated that he was guided in his determinations, by FUNCTIONS OF THE CEREBRUM. 215 observing on the heads of those who manifested any remarkable faculty or tendency, a corresponding prominence ; and to have found confirmation of his inferences, by comparing in like manner the skulls of the lower animals with their peculiar powers and dispositions. Both these branches of inquiry have been taken up by numerous observers ; and a large amount of evidence has been adduced by them in support of Gall's views, which appears in itself plausible, and which is regarded by many physiologists of much intelligence as quite decisive. Nevertheless, it does not appear that the doctrine is widely received amongst those whose peculiar attention to the Physiology and Patho- logy of the Nervous System gives them the highest authority on the subject ; and much additional proof would seem to be requisite, before it can take rank as substantially true. It may be freely admitted that Mankind is in the habit of forming an impression of an individual's intellectual capacity, by the height and expansion of his forehead ; and that a low forehead and crown, with great development of the occipital portion of the brain, generally accompany a cha- racter in which the influence of the animal passions is predominant ; and correspondences even more detailed may be admitted, without the inference being then conclusive, that these several parts are the distinct organs of the several faculties, or that the size of the organ is a measure of its functional power. It may be thought to be, in regard to the form of the head, very much as in respect to the character of the face, that we may draw from it a general idea as to the character of the mind, and may not unfrequently be able to predicate correctly some minute details ; and yet that an attempt to localize the organs more minutely, may be as destitute of truth as were the details of the system of Lavater. Moreover, a fundamental doubt hangs over every determination of function, which results from a comparison of the size of the supposed organ or region in different cases. If it be true that the gray matter only is the source of power, and that the white is merely a conductor, we have no right to assume that the total size of the organ affords a measure of its power, until it has been shown that the thickness of the cortical substance can be judged of by the size of the Brain, or of any part of it. Certainly there is a considerable variation in this respect among different individuals ; and it is yet to be proved that the relation is constant in different parts of the same individual Brain. Until this is substantiated, all inferences drawn from correspondence between the prominence of a certain part of the brain, and the intensity of a particular function, are invalid ; that is, if the general doctrine of the relative functions of the gray and white matter be true. Further, there is unfortunately a considerable uncertainty attending all Phrenological obser- vations, which are made upon the cranium, rather than upon the brain ; this we have seen from the discrepancy between the statements of Gall, and the facts ascertained regarding the comparative weight of the Cerebellum in castrated and entire horses. It appears to the Author, too, that Comparative anatomy and psychology are very far from supporting the system, when their evidence is fairly weighed.* It is a very curious circumstance, that the difference in the antero-posterior diameter, between the brain of Man and that - of the lower Mammalia, principally arises from the shortness of the posterior lobes in the latter, these being seldom long enough to cover the Cerebellum ; yet it is in these posterior lobes that the animal propensities are regarded by * Much is said by Phrenologists respecting M. Vimont's examination of this question, and of the affirmative decision to which he has come; but they are not so ready to men- tion, that M. Leuret, from at least equally extensive observations, has arrived at an oppo- site conclusion. Of these two, if authority is to decide the matter, the Author would certainly give the preference to M. Leuret, as a man of general eminence, and one who had a reputation to lose; whilst M. Vimont was previously unknown, and had only brought himself into notoriety by his advocacy of Phrenology. 216 FUNCTIONS OF THE NERVOUS SYSTEM. phrenologists as having their seat. On the other hand, the anterior lobes, in which the intellectual faculties are considered as residing, bear, in many ani- mals,' a much larger proportion to the whole bulk of the brain than they do in Man. Again, Comparative Anatomy and Experiment alike sanction the conclusion, that the purely Instinctive propensities have not their seat in the Cerebrum. These examples, and many similar ones that might easily be added, collectively show the uncertainty, to say the least, of the inferences which are by many regarded as firmly established. 293. The evidence of Pathology, again, tends to show that particular dis- orders of function may result from lesions of any part of the Cerebral hemi- spheres. This has been especially noticed, for example, in regard to the loss of the Memory of Words, which Phrenologists locate in the organ of Lan- guage ; there, of course, the lesion might be expected, on their system, to present itself; but this is by no means constantly, or even generally, the case. Phrenologists lay great stress on the effects of local injury, in causing loss of memory of a particular subject; but this principle, if carried to its full extent, would require us to regard each organ as split up into a large number of subdivisions the organ of language, for example, having one store-house for Latin, another for Greek, &c. ; either of which may be destroyed, without the other being affected. A very important source of evidence is that afforded by the correspondence between the several kinds of Monomania, and the forms of the brains of the persons exhibiting them; and the number of those who, having studied this question, have given in their adhesion to the phrenological system, is one of the most weighty evidences of its containing much truth. The doubts which have been expressed on the subject would have much less weight, if the coincidence of Phrenological determinations of character with truth, were more constant. The fairest tests of these are to be found, as Dr. Holland has justly remarked, " not in vague and ill-defined moral propensities, but in a few simple and well-marked faculties, such as those of numerical calculation, language or music, which have no others in actual opposition to them, and the degree of perfection in which can be clearly defined." We hear much from Phrenologists, as to their successful application of these tests ; but we do not hear of the instances of failure. The Author's own experience of their determinations, however, has certainly led him to the belief that failure is nearly as frequent as success. Without wishing to set himself up as an opponent to Phrenology, he perfectly agrees with Dr. Holland, in thinking that an impartial view of it requires, " not that the doctrine should be put aside altogether, but that great abatement should be made of its pretensions as a system." In particular, he thinks that those who pursue it are bound to make themselves first acquainted with what dan be established as the general functions of the Brain, before descending to particulars. XXI. General Recapitulation and Pathological Applications. 294. A general summary of the views here propounded, in regard to the Functions of the'Cerebro-Spinal division of the Nervous System, may proba- bly be useful in assisting the Student to gain clear ideas regarding them. The fibres of the Nervous trunks may be divided, according to the direction o their influence, into two classes the afferent or centripetal and the effe- rint or centrifugal. The afferent may be said td commence at the periphery, Specially on the skin, mucous surfaces, &c., and to terminate in the gray matter of the nervous centres ; whilst the efferent originate in that gray matter, and terminate in the muscles.* Every fibre runs a distinct course from its * The terms originate and terminate cannot be used with strict correctness ; since, as RECAPITULATION AND PATHOLOGICAL APPLICATIONS. 217 origin to its termination ; and it is not improbable that there are several distinct endowment! in the different fibres composing each trunk. There is no evi- dence that the fibrous structure serves any different purpose than that of a mere conductor; and there seems good reason to believe that all the active operations, of which the nervous system is the instrument, originate in the gray matter. A mass of gray matter, connected with nervous trunks, forms a ganglion. In the Invertebrata, the ganglia are frequently numerous, and are scattered through the system, without much connection with each other- each having a distinct function. In Vertebrated animals, on the other hand, they are united into one mass partly, it would seem, for the sake of the protection afforded them by the bony skeleton and partly in order that more complete consentaneousness of action may be attained. Still, several distinct divisions may be traced in the centres of the Cerebro-Spinal System partly by the determination of their respective functions, as indicated by observation and experiment and partly by the study of the distribution of the nerves proceeding from them. In this manner we arrive at the knowledge of several distinct ganglionic centres, of which the following may be considered as a general account. I. The true Spinal Cord, consisting of a nucleus of gray matter, receiving afferent fibres, and giving origin to efferent; by these it is connected with all parts of the body, but especially with the surface and muscles of the extremi- ties. The actions of this centre may be performed without consciousness on the part of the individual; and they consist in the reflexion of a motor im- pulse along. an efferent nerve, on the reception of a stimulus conveyed by an afferent or excitor nerve. These reflex movements can be best excited when the muscles are removed from the control of the Will, which otherwise gene- rally antagonizes them. Some of them are connected with the maintenance of the organic functions ; and others with the protection or withdrawal of the body from injury. Muscular movements may also be excited, by a stimulus directly applied to the Spinal Cord itself ( 157 212). II. The Medulla Oblongata, or cranial prolongation of the Spinal Cord. The actions of this do not essentially differ from those of the true Spinal Cord; but they are connected with different organs. This part consists chiefly of the centres of the nerves of Respiration and Deglutition two functions, of which the continual maintenance is essential to the life of the being; and it would seem as if these were placed within the cranium, to be more secured from accidental injury. The movements concerned in Respiration and Deglutition are, like those excited through the true Spinal Cord, of a strictly reflex cha- racter, oeing, in all instances, due to an impression, or stimulus, originating in the periphery of the system, which, being conveyed to the centre, excites there a motor impulse ; and they, also, are independent of Sensation ( 184 194). III. The ganglia of the nerves of Special Sensation, which form, as it were^ the continuation of the Medulla Oblongata. These, also, appear to minister to actions, which do not differ widely from the Reflex in character being almost necessarily excited by certain stimuli, and being only in a degree controllable by the will. But their actions differ in this, that they are attended with consciousness, and also, it would appear, with certain peculiar feelings.' Reasons have been given for the belief that these ganglia are the centres of those actions which are commonly termed instinctive in the lower animals, and consensual and emotional in ourselves. These all correspond in being formerly explained ( 112), there is probably never an actual termination of nervous fibres, either in the muscles or in gray matter; but they cease to run in their previous direction, after forming their terminal loops; and their course, as afferent or efferen, fibres, may consequently be said to begin or to end at these points, 19 218 FUNCTIONS OF THE NERVOUS SYSTEM. performed without any idea of a purpose, and without any direction of the will, being frequently in opposition to it (258 265). IV. The Cerebral Hemispheres, or Ganglia, which are evidently the instru- ments or organs of the intellectual faculties. These are connected by fibres of communication with almost all parts of the body ; and from their propor- tional size in Man, it seems probable that many of the nervous trunks are principally composed of such fibres. It is probably by them alone that ideas or notions of surrounding objects are acquired, and that these ideas are made the groundwork of mental operations. They would seem, also, to be the exclusive seat of Memory. The results of these operations are manifested on the bodily frame, through the Will, which is capable of acting, in greater or less degree, on all the muscles forming part of the system of Animal life (279292). V. The Cerebellum, which appears to be concerned in the regulation and harmonization of Muscular movement, whether instinctive or voluntary ( 266 278). Tabular View of the Nervous Centres. Cerebral Ganglia, the centres of the operations of Intelligence and Will. Nerves of Special sensa-^i Ganglia of Snerial SPHSP fNerves of Special sensa- tion.-Motor fibres min- ( \**> I tion.-Motor fibres min- . ^- the centres of Consensual, -^ , , ., " 5tinctive,and Emotional actions. ! f t gj em Cerebellic Ganglia, for harmonization of general muscular actions. Afferent and Motor Nerves J Respiratory I Afferent and Motor Nerves of Respiration, Deglutition, > and stomato-gastric 2 of Respiration, Deglutition, &c. > Ganglia. f &c. Trunks of Spinal nerves, composed""! o g ^ f Trunks of Spinal nerves, composed of fibres from true Spinal Cord, and | c o ^ g g | of fibres from true Spinal Cord, and from Cerebrum, Cerebellum, and I T?;= '& *s \ from Cerebrum, Cerebellum, and Medulla Oblongata; each group f c c g ] Medulla Oblongata; each group containing afferent and efferent $^ >~ T containing afferent and efferent fibres. J 5 S'oJ = U 295. The distinctness of the operations of these several centres is shown in various ways : but especially by conditions of the bodily system, in which one or more of them is in a state of inaction, whether temporary or perma- nent ; or is prevented, by the interruption of the usual channel of communi- cation, from operating on particular parts. Thus, in ordinary profound Sleep, which is a state of complete unconsciousness, it is evident that the Cerebral Hemispheres, and the Ganglia of Special Sense, are at rest ; as the Cerebel- RECAPITULATION AND PATHOLOGICAL APPLICATIONS. 219 Jum, also, may be considered to be : but the Medulla Oblongata and Spinal Cord must be in complete functional activity. The same is the case in pro- found Coma, resulting from effusion of blood, or from narcotic poisons, but not affecting the j)ower of breathing or swallowing. It may be frequently ob- served, that the sleep is not so profound as entirely to suspend the conscious- ness of the individual ; and that various movements of an adaptive character are performed, tending to relieve uneasiness resulting from various causes. In this condition it seems not improbable, that the sensory ganglia are in some degree awake, and that the movements are of an instinctive nature ; the mind of the individual not being sufficiently active to discern the cause of the uneasiness, or to employ his intelligence in the removal of it. Whenever Dreaming takes place, it is evident that the Cerebrum is in a state of partial activity. The state of Dreaming, and many forms of Insanity, have consider- able analogy with each other ; especially in the absence of the power, which is so characteristic of the well-regulated mind of Man, of controlling and regu- lating the current of thought. One idea calls up another, according to their previous associations ; and the most incongruous combinations are frequently the result ; but it will generally, if not always, be found, that the ideas them- selves have been previously in the mind, and that no entirely new train of thought is started. Of the degree in which, when the mind is thus closed to the external world, the hidden stores of Memory are opened to its search, many very curious instances are recorded. 296. The state of Somnambulism appears to be nearer to that of wakeful activity of the whole mind, than is that of Dreaming. In the latter condition, the individual is unconscious of external objects ; for, if they produce an effect upon him, it is in modifying the current of ideas, frequently in some very ludicrous manner : and he does not form any true perception or idea of their nature. But in Somnambulism, his senses are partly awake, so that impres- sions made upon them may be properly represented to the mind, and excite there the ideas with which they are connected ; moreover, the Cerebellum is also awake, so that the movements which the individual performs are per- fectly adapted to their object ; indeed it has frequently occurred, that the power of balancing the body has been so remarkably exercised in this condi- tion, that sleep-walkers have traversed narrow and difficult paths, over which they could not have passed in open day, when conscious of their danger. In ^Somnambulism, as in Dreaming, there is an evident want of voluntary control over the thoughts ; their succession is more influenced, however, by impres- sions received from without, than it is in dreaming ; and hence the mind may sometimes be easily guided into a particular train, by properly directing the impressions made upon the sensory organs. It may be remarked, however, that impressions which do not in some degree harmonize with the train of ideas, are not received by the mind ; or, at any rate, they are not applied to the correction of the erroneous notions which possess it. But there are many different shades in the condition of the mind, between Dreaming and Som- nambulism, the individual being, in some cases, much less conscious of ex- ternal objects than he is in others. In some instances it appears as if the mind was so wholly engrossed in a particular train of thought, that it could not be affected by any new sensations, so that there is even an unconscious- ness of those which produce pain ; this has its parallel in the waking state. A very remarkable characteristic of the state of Somnambulism, is the com- plete isolation which commonly exists between the trains of thought which then occupy the mind and its operations during the waking hours ; so that in neither state is there a remembrance of what passes in the other. There is usually this difference, however ; that the mental operations which take place in Somnambulism are, like those of dreaming, frequently suggested by 220 FUNCTIONS OF THE NERVOUS SYSTEM. what has previously been occupying the mind ; whilst these seem to leave no impression to be retraced in the waking state, though all that passes in one fit of Somnambulism may be recollected in the next. This has been most remarkably observed in the phenomena of that curious state ^jjjiich is known under the name of Double Consciousness ;* in this, the form of Somnambulism in which there is a consciousness of external impressions, seems to alternate with the condition of ordinary mental activity, and the individual leads (as it were) two distinct lives, recollecting in each condition what happened in previous states of the same character, but knowing nothing of the occurrences of the other.t 297. We have thus witnessed several varieties in the condition of the bodily system, depending upon partial or complete suspension of the functional activity of the Cerebrum, Cerebellum, and Sensory ganglia. There is no normal con- dition of the Spinal system, which at all corresponds with these ; since its operations are so closely connected with the maintenance of the organic functions, that the suspension of them necessarily induces the cessation of the latter. This is especially the case, however, in regard to the Respiratory ganglion ; for the whole remainder of the Spinal Cord maybe removed, with- out the interruption of the movements which are dependent on that segment of it. Cases have occurred, however, in which the natural performance even of these has been partially or entirely suspended ; and in which the main- tenance of life has for a time been effected, by a voluntary exertion of the muscles of Respiration. The influence of the Will upon the general motor apparatus of Man, seems to predominate so greatly over the Reflex action of the Spinal Cord, that few phenomena which are attributable to the latter ordinarily present themselves ; these are manifested, however, when the in- fluence of the Brain over any part is cut ofl^ as is seen in certain cases of paralysis. These morbid conditions present us, also, with illustrations of other effects of the interruption of the communication between the nervous centres and particular sets of muscles. Thus, the influence of the Will may be cut off, although that of the Instincts, Emotions, and Reflex Function may remain ; or the respondence of the muscles to Emotion may be prevented, whilst they are still capable of Voluntary control, or of Reflex action. Such cases seem to point very clearly to three distinct primary centres of nervous agency ; and to these, the Cerebrum, Sensory Ganglia, and Spinal Cord (including the Medulla Oblongata) have been here assigned as the instruments. We shall next inquire into some other morbid conditions of the system, which seem due to the irregular action of these ; and in this we shall be chiefly guided by the researches of Dr. M. Hall, which have been already slightly glanced at ('211, 298. Of the Convulsive diseases, it appears that the greater part, if not the whole, may be attributed to a morbid state of the Spinal System of nerves. So completely does the power of producing convulsive movements appear limited to that and to the Emotional system, (no mechanical irritation of the Cerebral substance being effectual in exciting such movements, 286) that, * Much interesting information on this and other subjects alluded to in this section may be found in Dr. Abercrombie's Treatise on the Intellectual Functions. f In regard to the curious phenomena which are produced by the so-called Mesmeric influence, the, author prefers not offering, for the present, any decided opinion." He has seen enough to satisfy him that they are deserving of investigation ; and to the inquiry into their true character and causes, he purposes to devote himself as opportunity may serve. But there are so many sources of fallacy in the mode in which such inquiries have been usually conducted, that he cannot satisfy himself by relying on the testimony of others, however elevated their characters may be above the mere suspicion of deception, and however firmly he might resf upon their evidence, in regard to subjects of a less complex nature. RECAPITULATION AND PATHOLOGICAL APPLICATIONS. 221 where convulsions present themselves during diseases which appear limited to the Brain, we may infer that one of these systems is involved. Dr. M. Hall has recently pointed out, that this complication is due to the impressions made upon the fibres of the Spinal nerves distributed upon the Dura Mater, and other serious and fibrous membranes ; for convulsive actions may be in- duced by pinching these membranes, or otherwise irritating them. Of the distinct forms or combinations, of which the class of convulsive disorders is composed, Tetanus is one of the most interesting and instructive. This disease is evidently dependent upon a state of undue excitability of the whole Spinal System ; and this may be produced by different causes. That which is termed the idiopathic form of the disease has its origin in the centres ; it may result in Man from the operation of various predisposing and exciting causes ; and may be artificially induced in Animals by the administration of Strychnia. In the traumatic form of the disease, the morbid state has its origin in a local injury; and the irritation propagated from this, and operating through the Spinal Cord, may be itself a cause of many of the convulsive movements. But, when the irritable state is once established in the nervous centres, con- vulsive action of the muscles may be excited by any stimuli, and even almost entirely without external causes. Hence it is that, whilst the amputation of the injured part is not unfrequently the means of saving the patient, if per- formed sufficiently early, it is attended with no benefit if delayed. The Cerebral apparatus is entirely unaffected in this disorder ; but the nerves of deglutition are usually those first influenced by it ; those of respiration, how- ever, being soon affected, as also those of the trunk in general. The condition termed Hydrophobia is nearly allied to that of traumatic Tetanus, differing chiefly in the mode in which the cerebro-spinal axis is affected. The irritable state of the nervous centres results from a local injury of a peculiar kind; and here, too, the early removal of the part is very desirable as a means of pre- vention ; although, when the malady has once reached the centres, it is of no use. The muscles of respiration and deglutition are, as in Tetanus, those spasmodically affected in the first instance ; but there is this curious difference- in the mode in which they are excited to action, that, whilst in Tetanus the stimulus operates through the true Spinal Cord (either centrally, or by being conveyed from the periphery), in Hydrophobia it is often conducted from the ganglia of special sense, or even from the brain ; so that the sight or sound of fluids, or even the idea of them, occasions equally with their contact, or with that of a current of air the most distressing convulsions. It would seem therefore, as if the Emotional system of nerves was involved in it. 299. Epilepsy is another convulsive disease, principally involving the Spinal Cord, but partly affecting the Brain. The predisposition to convulsive move- ments may depend upon many causes ; but the movements themselves are in general immediately excited by some local irritation, as by the presence of undi- gested matter in the stomach, of worms in the intestines, &c., although fre- quently also from causes purely mental. The convulsive movements usually affect the muscular system very extensively ; acting especially upon the mus- cles of ingestion and egestion. The Brain is evidently much concerned in the disease, however; as is evident from the numerous instances in which it has been clearly traced to some local affection of that organ, as well as from the loss of consciousness which accompanies the convulsion. Many forms of that protean- malady, Hysteria, are attended with a similar irritability of the Ner- vous Centres ; but there is this remarkable difference in the two cases, that the morbid phenomena of Hysteria, whilst they often simulate those of Teta- nus, Hydrophobia, Epilepsy, &c., are evidently dependent upon a state of the system of -a much less abnormal character, being relieved by very mild reme- dies, and being often capable of prevention by a strong effort of the will. Dr. 19* 222 FUNCTIONS OF THE NERVOUS SYSTEM. Hall has pointed out an important distinction between Epilepsy and Hysteria, which materially influences the proximate danger of the paroxysm of each respectively; in the former, the larynx is convulsively closed and partial asphyxia is the necessary result, if the access of air be too long prevented, so that venous congestion ensues, increasing the disorder of the nervous centres even to a fatal degree ; in Hysteria, on the contrary, much as the larynx is affected, it is not usually closed. Cases sometimes present themselves, how- ever, in which the Hysteric paroxysm assumes the Epileptic character, the larynx being closed during expiration, so as to produce alarming results. 300. The foregoing are the chief general spasmodic diseases in which the Spinal system of nerves is evidently involved ;* but there are many others of a more local character. Such are the various forms of Spasmodic Asthma, the attacks of which generally result from some internal irritation, either in the lungs themselves or in the digestive system, producing a reflex action upon the muscular fibres of the bronchial tubes. The Croup-like Convulsion, or Crow- ing Inspiration of Infants, again, is an obstruction to the passage of the air through the glottis, by a spasmodic contraction of the constrictors of the larynx. This spasmodic action may be induced by various kinds of irritation ; such as that occasioned by teething, by the presence of undigested food, or by intestinal disorder. In the crowing inspiration, the larynx is partially closed ; when the spasm is severe, however, there is complete occlusion of the passage ; and forcible efforts at expiration are made, which induce, as in epilepsy, a severe degree of venous congestion, and this reacts upon the nervous centres, aggra- vating the previous disorder of their condition. The present increased know- ledge of the functions of the laryngeal nerves, and of the symptoms of this disease, appears to render inadmissible the explanation of it given not long since by Dr. H. Ley, who attributed it to paralysis of the pneumogastric nerves occasioned by pressure. Spasmodic closure of the larynx may occur from other causes. When the rima-glottidis is narrowed, by effusion of fluid into the substance of its walls, it is very liable to be completely closed by spas- modic action, to which the unduly irritable condition of the mucous membrane will furnish many sources of excitement. Choking, again, does not result so much from the pressure of the Food on the air-passages themselves, as from the spasmodic action of the larynx, excited by this ; and the dislodgment of the morsel by an act of vomiting is the most effectual means of obtaining relief. Tenesmus and Strangury are well-known forms of spasmodic muscular con- traction, excited by local irritation acting through the Spinal system. The abnormal action which leads to Abortion is frequently excited in the same manner ; how far the uterus itself is called into contraction by the ordinary spinal nerves, is a question as yet undecided ; but the facts already stated leave no doubt, that stimuli operating on these may act upon it through the Sympa- thetic, into which their fibres pass ( 203). It will be borne in mind, how- ever, that, in abortion, as in ordinary parturition, many muscles are called in, * Chorea is ranked by Dr. M. Hall as a disease of the Spinal System of nerves; but this can scarcely be regarded as a correct determination. It is true that there is consi- derable irregularity in the ordinary Reflex actions; but the irregularity is still greater in those to which Volition or Emotion is the stimulus. Moreover, the body is at rest during sleep; and the "Spinal system never sleeps." The frequent origin "of the disease in causes which have excited strong mental emotions, and the effect of even moderate excite- ment of the feelings in greatly aggravating ihe movements of the body, seem to indicate the connection of this disease with the Emotional system of nerves. Stammering may be regarded as a sort of Chorea affecting the muscles of voice: of this, more hereafter (CHAP. vr.). In Paralysis Agitans, it may be usually observed that the voluntary actions are much more affected than the reflex; the latter, indeed, not in general manifesting any disturbance. An interesting and well-marked case of this disease has been mentioned to the author by Dr. W. Budd, in which softening was found in the Crura Cerebri. RECAPITULATION AND PATHOLOGICAL APPLICATIONS. 223 to aid the contractions of the uterus, which are strictly under the dominion of the Spinal system. There is a form of Incontinence of urine, which is very analogous to the morbid action just described ; the sphincter has its due power; but the stimulus to the evacuation of the bladder is excessive in strength and degree, owing to the acridity of the urine or other causes. The part of the bladder upon which this appears chiefly to act, is the trigonum (which is well known to be more sensitive to the irritation of calculi than the rest of the internal surface) ; and Sir C. Bell advises young persons who suf- fer during the night from this very disagreeable complaint, to lie upon the belly instead of the back, so that the contact of the urine with the trigonum may be delayed as long as possible. 301. One of the most familiar examples of the pathological excitement of the true Spinal system, is the act of Vomiting ; and, as Dr. M. Hall justly remarks, the special function of this system nowhere receives better illustra- tion. The act may be excited in various ways. Thus, it results from the tickling of the fauces with a feather or with the finger ; but if the feather be carried too far down, an act of deglutition is induced, instead 5f vomiting.* In this instance the glosso-pharyngeal, and perhaps also the fifth pair, are the nerves by which the stimulus is conveyed to the Medulla Oblongata. Vomit- ing, again, may be induced by substances introduced into the stomach ; and here the pneumogastric is evidently the exciter. When it takes place as a result of pregnancy, or of some intestinal irritation, the stimulus must be con- veyed either through one of the ordinary Spinal nerves or through the Sym- pathetic. But it may also be occasioned by the sight, smell, or taste of any disagreeable object, or by the mere conception of it, or by mental emotion simply. In this case, the stimulus appears to be received by the ganglia of special sense, and to be transmitted by them to the muscles concerned, as by the Spinal Cord or Medulla Oblongata in the former case. When Vomiting is excited by the introduction of emetic substances into the blodti ( 199), it is probable that their stimulation chiefly operates through the extended plexus of nerves, spread out by the Sympathetic upon the walls of the blood-vessels ; but the irritant action of the substance upon the nervous centres may be also concerned. In regard to the mechanism by which the act of Vomiting is produced, considerable difference of opinion has existed. The old doctrine was, that it was occasioned by the simple contraction of the stomach itself; but Magendie proved that this could not be the case, by substituting a bladder for the stomach of an animal, and then injecting a solution of tartarized anti- mony into its blood, which immediately caused the emptying of the bladder, by the pressure of the surrounding muscles ; these muscles he considered to be the diaphragm and abdominal muscles, the conjoint actions of which would be a peculiarity observed in no other instance. By Dr. M. Hall, on the other hand, it is maintained that the act of vomiting is, like the expulsion of the foetus, urine, fa3Ces, &c., an expiratory effort, modified in its effects by the peculiar condition of the sphincters. It bears, indeed, great resemblance to the act of coughing ; differing chiefly in this, that in Vomiting the larynx is closed during the whole operation, whilst it is only closed momentarily in coughing ; and also that in coughing the cardiac 'orifice of the stomach is closed, whilst in vomiting it is opened. In this view, the accuracy of which * This has been the cause of many accidents. Patients have tickled the fauces with a feather, in order to excite vomiting; and, having introduced it too far into the pha- rynx, it has been drawn out of their fingers, by the muscles of deglutition, and carried into the oasophagus. Similar accidents have occurred with the rectum-bougie, and female catheter, as well as with probes, &c., introduced into the male urethra; all the orifices being furnished with a kind of ingestive power, which is clearly the result of Reflex action. 224 OF SENSATION, AND THE ORGANS OF THE SENSES. has been proved by experiment, the diaphragm is quite inert. A curious case has been recorded by Drs. Graves and Stokes,* in which vomiting took place from the stomach of a man, who was found after death to be the subject of a very remarkable change in the relative position of the viscera, the stomach lying in the thorax, which cavity communicated with the abdomen by an opening in the diaphragm, giving passage to the oesophagus and duodenum. This case was regarded by its reporters as proving that vomiting might take place by the action of the stomach alone ; but it can scarcely be held to justify this conclusion ; since, the diaphragm being entirely passive, the abdominal muscles would have the same power of emptying the stomach as they would possess over the lungs. The conformity of the act of vomiting with that of expiration, is further shown by the ejection of the contents of the oesophagus, which will take place, when it is distended by the deglutition of food that can- not pass into the stomach, on account of an obstruction at the cardia. CHAPTER IV. ON SENSATION, AND THE ORGANS OF THE SENSES. I. Of Sensation in general. 302. BY the term Sensation is rightly understood that change in the con- dition of the mind, by which we become aware of an impression made upon some part of the body ; or, in a briefer form of expression, it may be denned to be the consciousness of an impression. Some physiologists have, it is true, spoken of a sensation without consciousness ; but it seems very desirable thus to limit the term ; since the word impression may be very well applied to designate the change produced in the afferent nerves by an external cause up to the point at which the mind becomes conscious of it. We have seen reason to believe that the impressions communicated to the Spinal Cord may there excite motor actions, without occasioning true Sensation ; and it would seem to be with the Brain only, that the Mind possesses the relation neces- sary for the production of such a 'change in it. Hence the Brain is spoken of as the Sensorium. For the reasons already given ( 261), it seems probable that the ganglia of Special Sensation share in this function with the Cerebral Hemispheres. The afferent nervous fibres, which connect the various parts of the body with the Brain, are termed sensory. This term has also been applied to those which terminate in the Spinal Cord ; but as the impressions which these convey do not produce sensations, it seems desirable to avoid thus designating them ; and the term excitor, proposed by Dr. M. Hall, is much preferable. Every afferent spinal nerve, therefore, is made up of sen- sory and of excitor fibres ; and these may be distributed in very different pro- portions to different parts. Of the excitor fibres, enough has been already said. Those parts of the body which are endowed with sensory fibres, and impressions on which, therefore, give rise to sensation, are ordinarily spoken of as sensible ; and different parts are spoken of as sensible in different de- * Dublin Hospital Reports, vol. v. OF SENSATION IN GENERAL. 225 grees, according to the strength of the sensation which is produced by a cor- responding impression on each. 303. In accordance with what was formerly stated ( 118) of the depend- ence of all nervous action on the continuance of the capillary circulation, especially at the extremities of the fibres, it is found that the sensory nerves are distributed pretty much in the same proportion as the blood-vessels ; that is to say, in the non-vascular tissues, such as the epidermis, hair, nails, car- tilage, and bony substance of the teeth, no nerves exist, and there is an entire absence of sensibility ; and in those whose vascularity is trifling, the sensibility is dull, as is the case with bones, tendons, ligaments, fibrous mem- branes, and other parts whose functions are simply mechanical, and even with serous and areolar membranes. Many of these textures are acutely sensible, however, under certain circumstances ; thus, although tendons and ligaments may be wounded, burned, &c., with little or no consciousness of the injury, they cannot be stretched without considerable pain ; and the fibrous, serous, and areolar tissues, when their vascularity is increased by inflammation, also become extremely susceptible of painful impressions. All very vascular parts, however, do not possess acute sensibility : the muscles, for instance, are fur- nished with a large supply of blood, to enable them to perform their peculiar function ; but they are not sensible in by any means the same proportion. Even the substance of the brain, and of the nerves of special sensation, ap- pears to be destitute of this property ; and the same may be said of the mu- cous membranes lining the interior of the several viscera, which, in the ordi- nary condition, are much less sensible than the membranes which cover those viscera, although so plentifully supplied with blood for their especial purposes. The most sensible of all parts of the body is the Skin, in which the sensory nerves spread themselves out into a minute network ; and even of this tissue, the sensibility differs greatly in different parts. The organs of special sensa- tion are, by the peculiar character of the nerves with which they are supplied, rendered sensible to impressions of a particular kind : thus, the eye is sensible to light, the ear to sound, &c. ; and whatever amount of ordinary sensibility they possess, is dependent upon other sensory nerves. The eye, for example, contrary to the usual notions, is a very insensible part of the body, unless affected with inflammation ; for though the mucous membrane which covers its surface, and which is prolonged from the skin, is acutely sensible to some kinds of impressions, the interior is by no means so, as is well known to those who have operated much on the eye. And there are many parts of the body, that are supplied with the common sensory nerves, which convey to the mind impressions of particular kinds with much greater readiness than they com- municate those of a different description. 304. It appears, then that the vascularity of a part is an essential condition of its sensibility ; but it does not follow that a tissue should be peculiarly sen- sible, because it is highly vascular ; since its large supply of blood may be required for other purposes. It is not simple vascularity, however, which is necessary, but rather an active capillary circulation ; any cause which retards this, deadens the sensibility, as is well seen in regard to cold ; and, on the other hand, an increase in its energy produces a corresponding increase in the sensibility, as is peculiarly evident in the active congestion which usually precedes inflammation. Acute sensibility to external impressions may arise, however, not only from abnormal activity of the circulation in the organ or part itself, but from the same condition affecting that part of the sensorium in which the impressions are received. Thus, in active congestion and inflam- mation of the brain, the most ordinary external impressions produce sensations of an unbearable violence ; and there are some peculiar conditions of the nervous system, known under the name of hysterical, in which the patients 226 OF SENSATION, AND THE ORGANS OF THE SENSES. manifest the same discomfort, even when the circulation is in a feeble, rather than an excited state. It is remarkable that the sensibility of the mucous membranes lining the internal organs, is less exalted by the state of inflamma- tion, than is that of most other parts ; and in this arrangement we may trace a wise and beneficent provision ; since, were it otherwise, the functions neces- sary to life could not be performed without extreme distress, with a very mo- derate amount of disorder in the viscera. If a joint is inflamed, we can give it rest ; but to the actions of the alimentary canal we can give little voluntary respite. 305. The feelings of Pain or Pleasure, which are connected with particular sensations, cannot, (for the most part at least) be explained upon any other principle than that of the necessary association of these feelings, by an original law of our nature, with the sensations in question. As a general rule it may be stated, that the violent excitement of any sensation is disagreeable, even when the same sensation in a moderate degree may be a source of extreme pleasure. This is the case alike with those impressions which are communi- cated through the organs of sight, hearing, smell, and taste, as with those that are received through the nerves of common sensation ; and there can be no doubt that the final cause of the association of painful feelings with such violent excitement, is to stimulate the individual to remove himself from what would be injurious in its effects upon the system. Thus, the pain resulting from violent pressure on the cutaneous surface, or from the proximity of a heated body, gives warning of the danger of injury, and excites mental operations destined to remove the part from the influence of the injurious cause ; and this is shown by the fact, that loss of sensibility is frequently the indirect occa- sion of severe lesions, the individual not receiving the customary intimation that an injurious process is taking place. Instances have occurred, in which severe inflammation of the membrane lining the air-passages has resulted from the effects of ammoniacal vapours, introduced into them during a state of syncope, the patient not receiving that notice of the irritation which would, in an active condition of his nervous system, have prevented him from inhaling the noxious agent. It is a general rule, with regard to all sensations, however, that their intensity is much affected by habit; being greatly diminished by frequent and continual repetition. This is partly due to the different degree of attention which the sensations excite in the mind ; but there are many facts which lead to the conclusion, that it is chiefly to be attributed to a change in the degree in which, after frequent repetition, they impress the consciousness itself. Thus, most persons are readily awoke from a sound sleep by a trifling noise, if the sound be of a kind which they are unaccustomed to hear ; but after a few repetitions, the sound loses its effect, unless its intensity be increased. Of this, every one has had experience, who has occasionally made use of an alarum to arouse him for a few mornings in succession. It is curious also, that the feelings of pain or pleasure, which unaccustomed sensations excite, are often exchanged for each other, when the system is habituated to them ; this is especially the case in regard to impressions communicated through the organs of smell and taste. There are many articles in common use among mankind, such as Tobacco, Fermented liquors, &c., the use of which cannot be said to produce a natural enjoyment, since it is at first unpleasant to most persons ; and yet it first becomes tolerable, then agreeable ; and at last the want of them is felt as a painful privation, and the stimulus must be applied in an increasing degree, in order to produce the usual effect. 306. The general law, that sensations are blunted by frequent repetition, may perhaps be connected with certain other general facts, which lie under the observation of every one. It is well known that the vividness of sensa- tions depends rather on the degree of change which they produce in the OF SENSATION IN GENERAL. 227 system than on the absolute amount of the impressing cause ; and this is alike the case with regard to the special and the ordinary sensations. Thus, our sensations of heat and cold are entirely governed by the previous condition of the parts affected ; as is shown by the well-known experiment of putting one hand in hot water, the other in cold, and then transferring both to tepid water, which will seem cool to one hand, and warm to the other. Every one knows, too, how much more we are affected by a warm day at the commencement of summer, than by an equally hot day later in the season. The same is the case in regard to light and sound, smell and taste. A person going out of a totally dark room into one moderately bright, is for the time painfully impressed by the light, but soon becomes habituated to it ; whilst another, who enters it from a room brilliantly illuminated, will consider it dark and gloomy. Those who are constantly exposed to very loud noises, become almost unconscious of them, and are even undisturbed by them in illness ;* and the medical student well knows, that even the effluvia of the dissecting-room are not perceived, when the organ of smell is habituated to them ; although an intermission of sufficient length would, in either instance, occasion a renewal of the first un- pleasant feelings, when the individual is again subjected to the impression. 807. Again, it is a well-known fact, that impressions made upon the organs of sense continue for a time, after the cause of the impression has ceased. It is in this manner that a musical tone, which seems perfectly continuous, results from a series of consecutive vibrations, following each other with a certain rapidity : and that a line or circle of light is produced by a luminous body moving with a certain velocity. Now there is reason to believe that changes, of which the effects thus transiently remain upon the nerves of sense, are more permanently impressed upon the sensorium; since, as formerly shown ( 291), we can only in this manner account for the phenomena of Memory, and for the effects produced upon this power, by material changes in the brain. Hence the diminution in the force of sensations, which is the consequence of their habitual recurrence, may be considered as resulting from these two general facts, the persistence of the impression made by them upon the sensorium, and the consequent absence of a change in its state, when a sensory impression is brought to it, which is of the same nature with one already registered there: the degree in which the consciousness is excited, being dependent, as just stated, not upon the absolute degree of the impressing cause, but upon the amount of change which it produces in the sensorial apparatus. In this respect there is a perfect conformity between the law of sensation, and that of muscular contraction ; for stimuli which excite the latter, usually lose their force in proportion to the frequency of their repetition. Indeed, both may be considered as results of the more general laws of vitality; for the actions of other tissues follow the same rule, as is shown by the tolerance that may be gradually established in the system of medicinal agents, poisons, &c., which would have at first produced the most violent effects, when given in the same amount. 308. It is through the medium of Sensation that we acquire a knowledge of the material world around us ; and that its changes excite mental operations in ourselves. The various kinds or modes of Sensation excite in us various ideas regarding the properties of matter ; and these properties are known to us only through the changes which they produce in the several organs. Thus a man totally blind from birth can form no idea of the nature of light or colours ; nor could one completely deaf have any just conception of musical * This fact is very well known in the manufacturing districts; where it is not at all uncommon for a family to live in the immediate vicinity of a forge-hammer; and those who are accustomed to the noise are unable to sleep anywhere else. 228 OF SENSATION, AND THE ORGANS OF THE SENSES. tones. It is well known that instances exist in which, from some imperfec- tion in organization, there is an incapacity for distinguishing colours or musical tones, whilst there is no want of sensibility to light or sound ; and that some persons are naturally endowed with a much greater range of the sensory faculties than others possess. Hence it does not seem at all improbable, that there are properties of matter of which none of our senses can take immediate cognizance ; and which other beings might be formed to perceive, in the same manner as we are sensible to light, sound, &c. Thus, it is well known that many animals are affected by atmospheric changes, in such a manner, that their actions are regarded by Man as indications of the probable state of the weather ; and the same is the case in a less degree with some of our own species, who are peculiarly susceptible of the same influences. Now the most universal of all the qualities or properties of matter, that, in fact, on which our notion of it is founded, is resistance; and it is this quality, of which the knowledge seems most universally diffused throughout the Animal kingdom. In the lowest tribes, we find that contact between their surface and some material body is required to produce sensation ; and beings which cannot be made conscious, in this manner, of the existence of something ex- ternal to themselves, do not deserve to be ranked in the Animal kingdom. Our difficulty lies (as heretofore remarked, 113), in ascertaining what are to be regarded, in such beings, as unequivocal indications of consciousness. Those animals which are fixed to one spot, can have few other ideas of matter than this most general one ; but in those which have the power of locomotion, the general sensibility of the surface doubtless communicates to them some notion of the character of the body over which they move, in the same manner as we learn it by passing the hand over its exterior. We shall presently see, however, that the idea of the shape of a body which we form from the touch, results from a very complex process ; which animals of the lowest grade can scarcely be supposed to exercise. There can be no doubt that, next to the mere sense of resistance, sensibility to temperature is the most universally diffused through the Animal kingdom; and probably the consciousness of luminosity is the next in the extent of its diffusion. There is good reason to believe, from observation of their habits, that many animals are susceptible of the influence, and are directed by the guidance of light ; whilst their organs are not adapted to receive true visual impressions, or to form optical images ; and such would seem to be the function of the red spots, frequently seen on prominent parts of Animalcules, the lower Articulata and Mollusca, and even of some Radiata. Wherever these are of sufficient size to allow their struc- ture to be examined, they are found to be largely supplied with nerves, but to be destitute of the peculiar organization which alone constitutes a true eye. The sense of Taste may be considered as a refined modification of that of touch ; and it is probable that this exists very low down in the animal scale, being obviously of great importance in the selection of food ; but the Anatomist has no means of ascertaining where this refinement exists, and where it does not ; since the organs of taste and touch are so similar. The sense of Hearing does not seem to be distinctly present among the Invertebrate animals, except in such as approach most nearly to the Vertebrata; it is not improbable, how- ever, that sonorous vibrations may produce an effect upon the system of those animals which do not receive them as sound ; and this would appear, from a fact subsequently to be mentioned ( 320), to be not improbably the case with regard especially to aquatic animals. The sense of Smell, which is concerned with one of the least general properties of matter, appears to be the least widely diffused among the whole ; being only possessed in any high degree by Vertebrated animals, and being but feebly present in a large proportion of these. OF SENSATION IN GENERAL. 229 309. Besides the various kinds of sensibility which have been just enume- rated, there are others which are ordinarily associated together along with the sense of material resistance (and its several modifications), and the sense of temperature, under the head of Common Sensation ; but several of them, especially those which originate in the body itself, can scarcely be regarded in this light. Such are the feelings of Hunger^and Thirst; that of Nausea; that of distress resulting from suspended aeration of the blood; that of "sink- ing at the stomach," as it is vulgarly but expressively described, which results from strong mental emotion; that of the venereal excitement, and perhaps some others. Now in regard to all these, it is impossible in the present state of our knowledge to say, whether their peculiarity results from the particular constitution of the nerves that receive and convey them, or only from a modifi- cation in the impressing causes, and in the mode in which they operate. Thus we have no evidence that the nervous fibrils, which convey from the lungs the sense of distress resulting from deficient aeration, may not be of a different character from those which convey from the surface of the air-pas- sages the sense of the contact of a foreign body. But as we know that all the trunks, along which these peculiar impressions travel, do minister to ordinary sensation, whilst the nerves of truly special sensation are not sensible to com- mon impressions, it is evident that the probability is in favour of the identity of the fibres which minister to these sensations with those of the usual sensory character. For the sense of temperature, however, it is not by any means certain that a special set of fibres does not exist ; for many cases are on record, in which it has been lost, whilst the ordinary sense of tact remained ; and it is sometimes preserved, when the anaesthesia is in other respects complete. 310. With regard to all kinds of sensation it is to be remembered, that the change of which the mind is informed, is not the change at the peripheral extremities of the nerves, but the change communicated to the sensorium ; hence it results, that external agencies can give rise to no kind of sensation, which cannot also be produced by internal causes, exciting changes in the condition of the nerves in their course. This very frequently happens in regard to the senses of sight and hearing; flashes of light being seen, and ringing sounds in the ears being heard, when no external stimulus has pro- duced such impressions. The production of odorous and gustative sensations from internal causes, is perhaps less common ; but the sense of nausea is more frequently excited in this manner than by the direct contact of the nauseating substance with the tongue or fauces. The various phases of common sen- sibility often originate thus : and it is an additional evidence in favour of the distinctness of the fibres which convey the impressions of temperature, that these are frequently affected, a person being sensible of heat or of chilli- ness in some part of his body, without any real alteration of its temperature, whilst there is no corresponding affection of the tactual sensations. The most common of the internal causes of these subjective sensations (as they have been termed, in contradistinction to the objective which result from a real material object), is congestion or inflammation ; and it is interesting to remark that this cause, operating through each nerve, produces in the sensorium the changes to which that nerve is usually subservient. Thus, congestion in the nerves of common sensation gives rise to feelings of pain or uneasiness ; but when occurring in the retina and optic nerve it produces flashes of light ; and in the auditory nerve it occasions a "noise in the ears." It maybe observed, also, of some external causes, that they may excite changes in the sensorium through several different channels ; and that in each case the sensation is characteristic of the particular nerve on which the impression is made. Thus pressure, which produces through the nerves of common sensation the feeling 20 230 OF SENSATION, AND THE ORGANS OF THE SENSES. of resistance, is well known to occasion, when exerted on the eye, the sensa- tion of light and colours; and, when made 4 with some violence on the ear, to produce tinnitus aurium. It is not so easy to excite sensations of taste and smell by mechanical irritation; and yet, as Dr. Baly* has shown, it may readily be accomplished in regard to the former. The sense of nausea may be easily produced, as is familiarly known, by mechanical irritation of the fauces. The stimulus of electricity still more completely possesses the power of affecting all the sensory nerves, with the changes which are peculiar to them ; for, by proper management, an individual may be made conscious at the same time of flashes of light, of distinct sounds, of a phosphoric odour, of a peculiar taste, and of pricking sensations, all excited by the same cause, the effects of which are modified, according to the respective peculiarities of the instruments through which it operates. But although there are some stimuli which can produce sensory impressions on all the nerves of sensation, it will be found that those, to which any one organ is peculiarly fitted to respond, produce little or no effect upon the rest. Thus the ear cannot distinguish the slightest difference between a luminous and a dark object. A tuning-fork, which when laid upon the ear whilst vibrating, produces a distinct musical tone, excites no other sensation when placed upon the eye than a slight jarring feeling. The most delicate touch cannot distinguish a substance which is sweet to the taste, from one which is bitter; nor can the taste (if the commu- nication between the mouth and the nose be cut off) perceive any thing peculiar in the most strongly-odoriferous bodies. 311. It may hence be inferred that no nerve of special sensation can, by any possibility, take on the function of another. How far the nerves of common sensation can, under any circumstances, perform the offices usually delegated to those of special sense, we are not yet in a condition to determine. Com- parative Anatomy seems to show that, in the lowest animals in which the rudiments of eyes can be detected, there is no distinction between the nerves proceeding to these organs, and the rest ; and there would appear some ground for the belief that, as in other cases, the special organs of sensibility are gradually elaborated, in ascending the Animal scale, from the more general apparatus, and are not merely superadded to it. Hence we may conceive the possibility (though there is no proof of the fact) that states of the system might occur, in which a change in the common sensory nerves might produce the sensation of light, sound, &c. But it is quite impossible (so far at least as our present knowledge of physical phenomena permits us to decide upon the im- possibility of any thing) that distinct visual impressions should be communi- cated to a nerve, except through the mediation of such an optical instrument as the eye ; or distinct sonorous impressions, except through such an acoustic instrument as the ear. Hence we must receive with the greatest caution the wonderful accounts of transference of sensation, many of which have undoubtedly been the offspring of deception. Still it may be objected that, as we are so totally destitute of real knowledge as to the mode in which vision is ordinarily produced by inverted images upon the retina, we have no right to assert that it may not take place in some other way, and perhaps this objec- tion should lead us to consider the phenomenon rather as extremely improbable, than as impossible. But the improbability maybe compared to that of a stone ascending like a balloon, or a piece of lead floating on the water ; for we have no more knowledge of the ultimate cause of that which \ve term the force of Gravitation, than we have of the nature of Sensation. 312. The peculiar aptitudes of the different Sensory nerves, to receive and convey impressions of various kinds, must be regarded as the result of proper- * Translation of Mailer's Physiology, p. 1062, note. OF SENSATION IN GENERAL. 231 ties inherent in themselves ; just as we consider the difference between the afferent nerves in general, and the motor nerves, to be one belonging to their own constitution. But it is probable that there are also different localities in the Sensorium, in which the changes to which they give rise are performed. This may be judged of from the fact, that the phenomena of subjective sensa- tion frequently originate in peculiar conditions of the encephalon itself, and not in the nervous trunks or organs of sense ; thus, in dreaming, we have frequently very vivid pictures of external objects presented to our minds ; and we sometimes distinctly hear voices and musical tones, or have perceptions (though this is less common) of tastes and odours. The phenomena of spec- tral illusions are very nearly connected with those of dreaming ; both may be in some degree influenced by external causes, acting upon the organs of sen- sation, which are misinterpreted (as it were*) by the mind, owing to its state of imperfect operation ; but both also may entirely originate in the central organs. There seems to be no difference, in the feelings of the individual, between the sensations thus originating, and those which are produced in the usual manner; for we find that, unless otherwise convinced by their own reason, persons who witness spectral illusions believe as firmly in the reality of the objects that come before their minds, as if the images of those objects were actually formed on their retina. This is another proof, if any were wanting, that the organ of sense, and the nerve belonging to it, are but the instruments by which certain changes are produced in the sensorium; of which changes, and not of the immediate impression of the object, the sensa- tion really consists. It seems to be by an innate law of our constitution, that these subjective sensations, whether originating in the central organs, or in the course of the nervous trunks, should be referred by the mind to the ordi- nary situations of the peripheral terminations of those nerves ; even though these should not exist, or should be destitute of the power of receiving impres- sions. Thus, after amputations, the patients are for some time affected with sensations (originating probably in the cut extremities of the nerves), which they refer to the removed extremities ; the same has been noticed in regard to the eye, as well when it has been completely extirpated, as when its powers have been destroyed by disease. The effects of the Taliacotian operation also exhibit the operation of this law in a curious manner ; for until the flap of skin, from which the new nose is formed, obtains vascular and nervous con- nections in its new situation, the sensation produced by touching it is referred to the forehead. Another interesting illustration of it may be obtained by the following very simple experiment : if the middle finger of either hand be crossed behind the fore-finger, so that its extremity is on the radial side of the latter, and the ends of the two fingers thus disposed be rolled over a marble, pea, or other round body, a sensation will be produced, which, if unconnected by reason, would cause the mind to believe in the existence of two distinct bodies ; this is due to the impression being made at the same time upon the radial side of the fore-finger, and the ulnar side of the middle finger, two joints which, in the natural position, are at a considerable distance. 313. The acuteness of particular sensations is influenced in a remarkable degree by the attention they receive from the mind. If the mind be entirely inactive, as in profound sleep, no sensation whatever is produced by ordinary impressions ; on the other hand, when the mind is from any cause strongly directed upon them, impressions very feeble in themselves produce sensations of even painful acuteness. Every one knows how much a slight itching of some part of the surface may be magnified, by the direction of the thoughts to it ; whilst as soon as they are forced by some stronger impression into an- other channel, the irritation is no longer felt. To the traveler in warm coun- tries, the shrill but feeble buzz of a single mosquito, accidentally enclosed 232 OF SENSATION, AND THE ORGANS OF THE SENSES. within the netting that surrounds his bed, becomes a source of almost inexpres- sible annoyance, when he is composing himself to sleep : and every one is aware how vividly other sounds are perceived, when they break in upon the stillness of the night, being increased in strength, not only by the contrast, but by absorbing the whole attention. An interesting experiment is mentioned by Miiller, which shows how completely the mind may be unconscious of im- pressions communicated to it by one organ of sense, when occupied, even without a distinct effort of the will, by those received through another. If we look at a sheet of white paper through two differently-coloured glasses at the same time, one being placed before each eye, the resulting sensation is sel- dom that of a mixture of the colours ; if the experiment be tried with blue and yellow glasses, for example, we do not see the paper of an uniform green, but the blue is predominant at one moment, and the yellow at another, or blue nebulous spots may present themselves on a yellow field, or yellow spots on a blue field. We perceive, from this experiment, that the attention may not only be directed to the impressions made on either retina, to the complete exclusion of those of the other, but it may be directed to those made on particular spots of either. This may be noticed, again, in the process by which we make ourselves acquainted with a landscape or a picture ; if our attention be directed to the whole field of vision at once, we see nothing distinctly ; and it is only by abstracting ourselves from the contemplation of the greater" part of it, and by directing our attention to smaller portions in succession, that we can obtain a definite conception of the details. The same is the case in regard to audi- tory impressions ; and here the power of attention in causing one sensation or series of sensations to predominate over others, which are really more in- tense, is often most remarkably manifested. When we are listening to a piece of music played by a large orchestra, for example, we may either attend to the combined effect of all the instruments, or we may single out any one part in the harmony, and follow this through all its mazes ; and a person with a practised ear (as it is commonly but erroneously termed, it being not the ear but the mind that is practised), can even distinguish the sound of the weakest instrument in the whole band, and can follow its strain through the whole performance. This attention to a single element can only be given, however, by withdrawing the mind from the perception of the rest ; and a musician who thus listens, will have very little idea of the rest of the harmonic parts, or of the general effect. In fact, when the mind is thus directed, by a strong effort of the will, into a particular channel, it may be almost considered as un- conscious quoad any other impressions. 314. The effects of this principle are manifested in regard to the sensations which originate within the system ; as well as in respect to those which are excited by external impressions. Every one is aware how difficult it is to keep the body perfectly quiescent,* especially when there is a particular motive for doing so, and when the attention is strongly directed to the object. This is experienced even whilst a Photogenic likeness is being taken, when the position is chosen by the individual, and a support is adapted to assist him in retaining it ; and it is still more strongly felt by the performers in the Tableaux Vivans, who cannot keep up the effort for more than three or four minutes. Now it is well known that, when the attention is strongly directed to an entirely different object, (when we are listening, for example, to an elo- quent sermon, or an interesting lecture,) the body may remain perfectly motionless for a much longer period; the uneasy sensations which would otherwise have occasioned the individual to change his position, not being felt : but no sooner is the discourse ended than a simultaneous movement of the * Of course the movements of respiration and winking are left out of the question. OF SENSATION IN GENERAL. 233 whole audience takes place, every one then becoming conscious of some discom- fort which he seeks to relieve. This is the case also in regard to the respi- ratory sensation ; in general it may be observed, that the usual reflex move- ments are not enough for the perfect aeration of the blood, and that a more prolonged inspiration, prompted by an uneasy feeling, takes place at intervals ; but under such circumstances as those just alluded to, this feeling is not expe- rienced until the attention ceases to be engaged by a more powerful stimulus, and then it manifests itself by the deep inspirations which accompany, in almost every individual, the general movement of the body. 315. It is curious that the constant direction of the attention to internal sensa- tions of a subjective kind, should sometimes occasion actual disorder of the parts to which these sensations are referred ; and yet this seems the only way of accounting for some of the phenomena of disease. Sometimes the cause of the sensation may exist in the trunk of the nerve, in some part of its course ; whilst in other instances, it may be confined to the sensorium. Pain of the testicle, for example, may be occasioned by irritation having its seat in the lower part of the spine, the organ itself being perfectly sound ; yet if that pain continue, it may become diseased. The following are some very interesting remarks on this subject, from the able pen of Dr. Holland.* " There is cause to believe the action of the heart to be quickened or otherwise disturbed, by the mere centering of consciousness upon it, without any emotion or anxiety." This is* specially the case where its impulses are irregular, or are so loud as to be audible. " The same may be said of the parts concerned in respiration. If this act be expressly made the subject of consciousness, it will be felt to undergo some change ; generally to be retarded at first, and afterwards quick- ened." " The act of swallowing is manifestly rendered more difficult, by the attention being fixed upon it ; and the same cause will often be found to render articulation less distinct, especially when there exists already some impediment to the function. A similar direction of consciousness to the region of the stomach, creates in this part a sense of weight, oppression, or other less defi- nite uneasiness ; and, when the stomach is full, appears greatly to disturb the due digestion of the food. The state and action of the bowels are much influ- enced by the same cause." A peculiar sense of weight and restlessness, approaching to cramp, is felt in a limb, to which the attention is particularly directed. " The attention concentrated, for so by an effort of will it may be, on the head or sensorium, gives certain feelings of tension and uneasiness, caused possibly by some change in the circulation of the part ; though it may be an effect, however difficult to be conceived, on the nervous system itself. Persistence in this effort, which is seldom, indeed, possible beyond a short time without confusion, produces results of much more complex nature, and scarcely to be defined by any common terms of language." These phenomena have an evident affinity with those of several morbid conditions. Thus the hypo- chondriac patient " in fixing his consciousness with morbid intentness on cer- tain organs, creates not merely disordered sensations, but often also disordered actions in them. There may be palpitation of the heart, hurried or choked respiration, flatulence and other distress of stomach, irritation of the bladder; all arising from this morbid direction of attention to the organs in question." In hysteria, again, " the instances are frequent, of attacks brought on by the mere expectation of them; or by irritation; or occasionally even a sort of morbid solicitation of the organs to these singular actions." These facts go a long way to explain the phenomena of Animal Magnetism, many of which are obviously to be referred to the exaggerated operation of the same principle. * Medical Notes and Reflection?, chap. v. 20* 234 OF SENSATION, AND THE ORGANS OF THE SENSES. We now proceed to consider in more detail the functions of the several Organs of the Senses, and shall commence with that of the most general character. II. Sense of Touch. 316. By the sense of Touch, as commonly understood, is meant that modifi- cation of the common sensibility of the body, of which the cutaneous surface is the especial seat. It derives its peculiar powers simply from the large amount of sensory nervous fibres, which are distributed in its substance ; and especially through the terminations (or rather the origins) of these in the papilla, which are little elevations of the surface of the cutis, easily percepti- ble by the aid of a lens, and each chiefly composed of a vascular plexus sur- rounding the extremity of the nervous fibril. The number of these papillae within any given area, pretty closely corresponds with the degree of sensibility of that part of the surface ; thus we find them most abundant on the hands, especially towards the points of the fingers, and on the lips and tongue. In some animals, especially those of the Feline tribe, the long vibrisse (commonly termed whiskers) evidently minister to sensation ; and it has been demon- strated that their pulps are largely supplied with nerves from the fifth pair. Some interesting observations have been made by Prof. Weber, on the sensi- bility of different parts of the skin. His mode of ascertaining this, was to touch the surface with the legs of a pair of compasses, the points of which were guarded with pieces of cork ; the eyes being closed at the time, the legs were approximated to each other, until they were brought within the smallest distance, at which they could be felt to be distinct from one another. The following are some of the results of the experiments. With the extremities of the fingers and the point of the tongue, the distance could be distinguished most easily in the longitudinal direction ; on the dorsum of the tongue, the face, neck, and extremities, the distance could be recognized best when the arms were placed tranversely. Point of middle finger - of aline Point of tongue - - ^ of a line Palmar surface of third finger 1 line Red surface of lips - 2 lines Palmar surface of middle finger 2 Dorsal surface of third finger 3 Tip of the nose - -3 Dorsum and edge of tongue 4 Part of the lips covered by skin 4 Palm of hand - - 5 Skin of cheek - - 5 Extremity of great toe - 5 Hard palate - - 6 Dorsal surface of forefinger 7 Dorsum of hand - - 8 Mucous membrane of gun's Lower part of forehead Lower part of occiput Back of hand Neck, under lower jaw Vertex Skin over Patella Sacrum acromion Dorsum of foot Skin over sternum Skin beneath occiput Skin over spine, in back Middle of the arm thigh 9 lines 10 12 14 15 15 16 1 18 18 20 24 30 30 30 It is curious that the distance between the legs of the compasses seemed to be greater (although really so much less), when it was felt by the more sensitive parts, than when it was estimated by parts of less distinct sensibility. As a general fact, it seems that the sensibility of the trunk is greater on the median line, both before and behind, and less at the sides. Differences of tempera- ture, and the weight of bodies, were, according to Prof. Weber's observations, most accurately recognized at the parts which were determined to be most sensible by the foregoing method of inquiry. 317. As already stated ( 308), the only idea communicated to our minds by the sense of Touch, when exercised in its simplest form, is that of Resistance ; but when the sensory surface and the substance touched are made to change SENSE OF TOUCH. 235 their place in regard to each other, we obtain the additional notion of Exten- sion or Space. By the various degrees of resistance which the sensory surface encounters, we estimate the hardness or softness of the body ; but in this we are assisted by the muscular sense ( 257), which makes us conscious of the degree of pressure we are employing. By the impressions made upon the' papillae, during the movement of the tactile surface over that which is being examined, the roughness, smoothness, or other peculiar characters of the latter are estimated. Our knowledge of form, however, is a very complex process, requiring not merely the exercise of the sense of touch, but also great atten- tion to the muscular sensations. It is chiefly, as formerly remarked, in the variety of movements of which the hand of Man is capable, that it is superior to that of any other animal ; and it cannot be doubted that this affords a very important means of acquiring information in regard to the external world, and especially of correcting many vague and fallacious notions, which we should derive from the sense of Sight, if used alone. On the other hand, it must be confessed, that our knowledge would have a very limited range, if this sense were the only medium through which we could acquire ideas. It is probably on the sensations communicated through the touch, that the idea of the mate- rial world, as something external to ourselves, chiefly rests ; but this idea is by no means a direct result of these sensations, being rather an instinctive or intui- tive perception excited by them. Every person who directs the least attention to the subject must perceive, how completely different are those notions of the primary or elementary properties of matter, which we base upon the informa- tion thus communicated to us from the sensations themselves ; and, as Dr. Alison has justly remarked, "a decisive proof of this being the true repre- sentation of this part of our mental constitution, is obtained by attending to the idea of extension or space ; which is undoubtedly formed during the exercise of the sense of touch ; and is no sooner formed, than it ' swells in the human mind to Infinity,' to which certainly no human sensation can bear any resem- blance." 318. That the conditions under which certain of the modifications of com- mon sensation operate, are in some respects different from those of ordinary Touch, is very easily shown. Thus, the feeling of tickling is excited most readily in parts which have the least tactual sensibility, the armpits, flanks, and soles of the feet ; whilst in the points of the fingers it cannot be excited. Moreover, the nipple is very moderately endowed with ordinary sensibility; yet by a particular kind of irritation, a very strong feeling may be excited through it. Again, in regard to temperature, it is remarked by Weber that the left hand is more sensitive than the right; although the sense of touch is undoubtedly the most acute in the latter. He states that, if the two hands, previously of the same temperature, be plunged into separate basins of warm water, that in which the left hand is immersed will be felt as the warmest, even though its temperature is somewhat lower than that of the other. In regard to the sensations of heat and cold, he points out another curious fact, that a weaker impression made on a large surface, seems more powerful than a stronger impression made on a small surface ; thus, if the forefinger of one hand be immersed in water at 104, and the whole of the other hand be plunged in water at 102, the cooler water will be thought the warmer; whence the well-known fact, that water in which a finger can be held, will scald the whole hand. Hence it also follows, that minute differences in temperature, which are imperceptible to a single finger, are appreciated by plunging the whole hand into the water ; in this manner, a difference of one-third of a degree may readily be detected, when the same hand is placed successively in two vessels. The judgment is more accurate, when the temperature is not much 236 OF SENSATION, AND THE ORGANS OF THE SENSES. above or below the usual heat of the body; just as sounds are best discrimi- nated, when neither very acute nor very grave. 319. The improvement in the sense of Touch, in those persons whose dependence upon it is increased by the loss of other senses, is well known ; this is doubtless to be in part attributed (as already remarked) to the increased attention which is given to the sensations, and in part to the increased develop- ment of the organ itself, resulting from the frequent use of it. The case of Saunderson, who, although he lost his sight at two years old, became Professor of Mathematics at Cambridge, is well known ; amongst his most remarkable faculties, was that of distinguishing genuine medals from imitations, which he could do more accurately than many connoisseurs in full possession of their senses. The process of the acquirement of the power of recognizing ele- vated characters by the touch, is a remarkable example of this improbability. When a blind person first commences learning to read in this manner, it is necessary to use a large type ; and every individual letter must be felt for some time before a distinct idea of its form is acquired. After a short period of diligent application, the individual becomes able to recognize the combina- tions of letters in words, without forming a separate idea of each letter ; and can read line after line, by passing the finger over each, with considerable rapidity. Now when this power is once thoroughly acquired, it is found that the size of the type may be gradually diminished ; and this seems to indicate, that the sensations themselves are rendered more acute, by the frequent appli- cation of them in this direction. As an instance of the correct notions which may be conveyed to the mind of the forms and surfaces of a great variety of objects, and of the sufficiency of these notions for accurate comparison, the Author may mention the case of a blind friend of his own, who has acquired a very complete knowledge of Conchology, both recent and fossil; and who is not only able to recognize every one of the numerous specimens in his own Cabinet, but to mention the nearest alliances of a Shell previously unknown to him, when he has thoroughly examined it by his touch. Many instances are on record, of the acquirement, by the blind, of the power of distinguishing the colours of surfaces, which were similar in other respects ; and, however won- derful this may seem, it is by no means incredible. For it is to be remem- bered that the difference of colour depends upon the position and arrangement of the particles composing the surface, which render it capable of reflecting one ray whilst it absorbs all the rest; and it is quite consistent with what we know from other sources, to believe that the sense of touch may become so refined as to communicate a perception of such differences. 320. The examples of peculiar acuteness of this sense, which we occasion- ally meet with among the lower animals, are very interesting, when viewed in connection with its improvability in Man. It was found by Spallanzani, that Bats, when deprived of sight, and (as far as possible) of hearing and smelling also, still flew about with equal certainty and safety, avoiding every obstacle, 'passing through passages only just large enough to admit them, and flying about places previously unknown, with the most unerring accuracy, and without coming into collision with the objects near which they passed. He also stretched threads in various directions across the apartment, with the same result. So astonished was he at these curious facts, that he was led to attribute the phenomenon to the possession of a sixth sense, unknown to Man. Cuvier was the first to appreciate the real value of these experiments, as affording a proof of the existence of the most exquisite tactile sensibility, over the whole surface of the flying membrane ; the naked surface and delicate structure of which appear well adapted to constitute the seat of so important a function. From this view, therefore, it would appear that it is by means of the pulsation of the wings on the air, that the propinquity of solid bodies is perceived, SENSE OF TASTE. 237 through the manner in which the air reacts on their surface. It is curious that the instance which (so far as we at present know) is most analogous to this, should be met with among the inhabitants of the deep. It is a fact well known to Whale-fishers, especially to those who pursue the Spermaceti Whale, that these animals have the power of communicating with each other at great distances. It has often been observed, for example, that, when a straggler is attacked, at the distance of several miles from a shoal, a number of its fellows bear down to its assistance, in an almost incredible short space of time. It can scarcely be doubted, then, that the communication must be made through the medium of the vibrations of the water, excited by the struggles of the animal, or perhaps by some peculiar movements especially designed for this purpose, and propagated through the fluid to the large cutaneous surface of the distant Whales; and this idea is fully confirmed by the fact, that the nerves which proceed to the skin, pass through the inner layers of blubber with scarcely any subdivision, but spread out into a network of extreme minuteness, as soon as they arrive at the surface. III. Sense of Taste. 3*21. That this sense may be really considered as a peculiar modification of that of Touch, appears from several considerations. In the first place, the actual contact of the object of sense, with the organ through which the impres- sion is received, is here necessary ; and this is the case in regard to no other sense. Moreover the intimate structure of the organ is nearly the same in both instances. Again, it appears from the considerations formerly alluded to ( 228), that there is no special nerve of taste; the gustative impressions nuulc upon the front of the tongue being conveyed by the lingual branch of the fifth pair; whilst those made upon the back of the organ are conveyed by the glosso-pharyngeal. The first of these nerves also ministers to ordinary tactile sensibility; the second appears to convey the impressions which produce nausea.* The papillae of the tongue are essentially the same in structure with those of the skin ; and although Anatomists have classified them, accord- ing to their differences of form and situation, there is no definite physiological evidence that they possess corresponding varieties of endowment, although this is quite possible. As a general rule, it is a necessary condition of the sense of Taste, that the object should either be in a state of solution, or should be soluble in the moisture covering the tongue ; if this be not the case, or if the tongue be dry, a simple feeling of contact is all that is produced. As in the case of touch, the idea of the character of the sapid body is very imperfect, unless it is made to move over the gustative surface ; and thus the taste is very much heightened by the compression and friction of the substance between the tongue and the palate. From all these circumstances it appears indis- putable, that a very strong analogy exists between Taste and Touch ; indeed it may be questioned whether they are not in reality more closely allied than is the sense of Temperature with that of Resistance. 322. Although the tongue seems to be the chief seat of Gustative sensibility, yet this is also possessed, though in a less degree, by the palate. But it is to be remarked that the sensations produced by most sapid substances are of a complex kind ; and are in great part due to the organ of Smell. Of this any one may convince himself, by closing the nostrils, and inspiring and expiring through the mouth only, when holding in the mouth, or even rubbing between * Indeed it may be questioned whether the glosso-pharyngeal is really a nerve of taste at all; since the experiments which would indicate that it is so, may be explained upon the supposition that nausea, rather than real gustative sensibility, was induced by the substances applied to the tongue after division of the lingual branch of the fifth pair. 238 OF SENSATION, AND THE ORGANS OF THE SENSES. the tongue and the palate, some sapid substance ; of which the taste is then scarcely recognized, although it is immediately perceived, when its effluvia are drawn into the nose. It is well known, too, that, when the sensibility of the Schneiderian membrane is blunted by inflammation, (as in an ordinary cold in the head,) the power of distinguishing flavours is very much diminished. In fact some physiologists are of opinion that all our knowledge of the flavour of sapid substances is received through the Smell ; and this is not improbably true : but it is to be remembered, that, besides flavour, a sapid body may excite various other sensations, as those of irritation and pungency ; and of these, it seems to be the true function of the sensory surface of the mouth to take cog- nizance. Such sensations are evidently not far removed from those of ordi- nary touch ; and correspond with those which may be excited in the nostrils through the medium of the Fifth pair. Taken in its ordinary compound acceptation, the sense of Taste has for its object to direct us in the choice of food, and to excite the flow of the mucus and saliva, which are destined to aid in the preparation of the food for Digestion. Among the lower Animals, the instinctive perceptions connected with this sense are much more remarkable than our own; thus an omnivorous Monkey will seldom touch fruits of a poi- sonous character, although their taste may be agreeable ; and animals, whose diet is restricted to some one kind of food, will decidedly reject all others. As a general rule it may be stated, that substances of which the taste is agreeable to us, are useful in our nutrition; and vice versa: but there are many signal exceptions to this, 323. Like other senses, that of Taste is capable of being rendered more acute; ^education ; and this on the principles already laid down with regard to touch.* The experienced wine-taster can distinguish differences in age, purity, place of growth, &c., between liquors that to ordinary judgments are alike ; and the epicure can give an exact determination of the spices that are combined in a particular sauce, or of the manner in which the animal, on whose flesh he is feeding, was killed. As in the case of other senses, more- over, impressions made upon the sensory surface remain there for a certain period ; and this period is for the most part longer than that which is required for the departure of the impressions made upon the eye, the ear, or the organ of smell. Every one knows how long the taste of some powerful substances remains in the mouth ; and even of those which make less decided impres- sions, the sensation remains to such a degree, that it is difficult to compare them at short intervals. Hence if a person be blindfolded, and be made to taste substances of distinct, but not widely-different flavours (such as various kinds of wine or of spirituous liquors), one after another in rapid succession, he soon loses the power of discriminating between them. In the same man- ner, the difficulty of administering very disagreeable medicines may be some- times got over, by either previously giving a powerful aromatic, or combining the aromatic with the medicine ; its strong impression in both cases preventing the unpleasant taste from exciting nausea. IV. Sense of Smell. 324. Of the nature of CEdorous emanations, the Natural Philosopher is so completely ignorant, that the Physiologist cannot be expected to give a definite account of the mode in which they produce sensory impressions. Although it may be surmised that they consist of particles of extreme minuteness, dis- solved as it were in the air, and although this idea seems to derive confirma- tion from the fact that most odorous substances are volatile, and vice versa, yet the most delicate experiments have failed to discover any diminution in weight, in substances that have been impregnating with their effluvia a large SENSE OF SMELL. 239 quantity oT air for several years ( 104, note) ; and there are some volatile fluids, such as water, which are entirely inodorous. The Schneiderian or Pituitary membrane is the seat of the sense of smell ; but it is probable that every part of it is not equally endowed with the faculty of distinguishing odours, which is a very different power from that of becoming sensible of irri- tation from them. The Olfactory nerves cannot be traced to the membrane covering the middle and inferior spongy bones, or to that which lines the dif- ferent sinuses, these parts of the surface being supplied by the fifth pair only ; and it is a matter of common experience, that we cannot distinguish faint odours, unless, by a peculiar inspiratory effort, we draw the air changed with them to the upper part of the nose. In animals living in the air, it is a neces- sary condition of the exercise of the sense of Smell, that the odorous matter should be transmitted by a respiratory current through the nostrils ; and that the membrane lining these should be in a moist state. Hence, by breathing through the mouth, we may avoid being affected by odours, even of the strongest and most disagreeable kind ; and in the first stage of a catarrh, when the ordinary mucous secretion is suspended, the sense of smell is blunted from this cause, as it afterwards is from the excess in the quantity of the fluid, which prevents the odoriferous effluvia from coming into immediate relation with the sensory extremities of the nerves. Hence we may easily compre- hend, that section of the fifth pair, which exercises a considerable control over the secretions, will greatly diminish the acuteness of the smell ; and it will have the further effect of preventing the reception of any impressions of irri- tation from acrid vapours, which are entirely different in their character from true odorous impressions, and which are not transmitted through the olfactory nerve ( 220). The nasal' passages may indeed be considered as nerving, in the air-breathing Vertebrata, two distinct offices ; jhey constitute the organ of smell, through the distribution of the olfactory nerve upon a part of their sur- face ; but they also constitute the portals of the respiratory organs, having for their office to take cognizance of the aeriform matter which enters them, and to give warning of that which would be injurious; this latter function is per- formed by the Fifth pair, as by the Par Vagum in the glottis. It is through this nerve, that the act of sneezing is excitable : the evident purpose of which is the ejection of a strong blast of air through the nasal passages, in such a manner as to drive out any offending matter they may contain. 325. The importance of the sense of Smell among many of the lower ani- mals, in guiding them to their food, or in giving them warning of danger, and also in exciting the sexual feelings is well known. To Man its utility is very subordinate under ordinary circumstances ; but it may be greatly increased when other senses are deficient. Thus, in the well-known case of James Mitchell, who was deaf, blind and: dumb, from his birth, it was the principal means of distinguishing persons, and enabled him at once to perceive the en- trance of a stranger. It is recorded that a blind gentleman, who had an anti- pathy to cats, was possessed of a sensibility so acute in this respect, that he perceived the proximity of one that had been accidentally shut up in a closet ' adjoining his room. Among Savage tribes, whose senses are more cultivated than those of civilized nations, more direct use being made of the powers of observation, the scent is almost as acute as in the lower Mammalia; it is as- serted by Humboldt, that the Peruvian Indians in the middle of the night can thus distinguish the different races, whether European, American Indian, or Negro.* The agreeable or disagreeable character assigned to particular * The author has been assured by a competent witness, that a lad in a state of som- nambulism had his sense of Smell so remarkably heightened, as to be able to assign (without the least hesitation) a glove placed in his hand~to its right owner, in the midst of about thirty persons, the boy himself being blindfolded. 240 X OF SENSATION, AND THE ORGANS OF THE SENSES. odours, is by no means constant amongst different individuals. Many of the lower animals pass their whole lives in the midst of odours, which are to Man (in his civilized condition at least) in the highest degree revolting ; and will even refuse to touch food, until it is far advanced in putridity. It more fre- quently happens in regard to odours and savours, than with respect to other sensory impressions, that habit makes that agreeable, and even strongly re- lished, which was at first avoided ; the taste of the epicure for game that has acquired thefumet, for olives, for assafcetida, &c., are instances of this. As to the length of time, during which impressions made upon the organ of smell remain u^>on it, no certain knowledge can be obtained. It is difficult to say that the effluvia have been completely removed from the nasal passages ; since it is not improbable that the odorous particles (supposing such to exist) are absorbed or dissolved by the mucous secretion ; it is probably in this manner that we may account for the fact, well known to every medical man, that the cadaverous odour is frequently experienced for days after a post-mortem ex- amination.* V. Sense of Vision. 326. The objects of this sense are bodies, which are either in themselves luminous, or which become so by reflecting the light that proceeds from others. Whether their light is transmitted by the actual emission of rays, or by the propagation of undulations analogous to those of sound, is a question at present keenly debated amongst Natural Philosophers ; but it is of little consequence to the Physiologist, which is the true solution ; since it is only with the laws which actually regulate the transmission of light, that he is concerned. These laws it may be desirable here briefly to recapitulate. 327. Every point of a luminous body sends off a number of rays, which diverge in every directidn, so as to form a cone, of which the luminous point is the apex. So long as these rays pass through a medium of the same density, they proceed in straight lines ; but, if they enter a medium of different density, they are refracted or bent, towards the perpendicular to the surface at the point at which they enter, if they pass from a rarer into a denser medium, and from the perpendicular, when they pass from a denser medium into a rarer. It is easily shown to be a result of this law, that, when parallel rays passing through air fall upon a convex surface of glass, they will be made to converge ; so as to meet at the opposite extremity of the diameter of the circle, of which the curve forms part. If, instead of continuing in the glass, they pass out again, through a second convex surface, of which the direction is the reverse of the first, they will be made to converge still more, so as to meet in the centre of curvature. Rays which are not parallel, but which are diverging from a focus, are likewise made to converge to a point or focus ; but this point will be more distant from the lens, in proportion as the object is nearer to it, and the angle of divergence consequently greater. The rays diverging from every point of a luminous object are thus brought to a corre- sponding focus ; and the places of all these foci hold exactly the same relation to each other, with that of the points from which the rays diverged ; so that a perfect image of the object is formed upon a screen held in the focus to the lens. This image, however, will be inverted ; and its size, in proportion to that of the object, will depend upon their respective distances from the lens. If their distances be the same, their size will also be the same ; if the object be distant, and the image near, the latter will be much the smaller ; and" vice versa. * This may partly be attributed also to the effluvia adhering to the dress. It has been remarked that dark cloths retain these more strongly than light. SENSE OF VISION. 241 328. There are two circumstances, however, which interfere with the perfec- tion of an image thus formed by a convex lens. The one is, that, if the lens constitute a large part of the sphere from which it is taken, the rays which fall near its margin are not brought to a focus at the same point with those which pass through its centre, but at a point nearer the lens. This difference, which must obviously interfere greatly with the distinctness of the image, is termed spherical aberration ; it may be corrected by the combination of two or more lenses, of which the curvatures are calculated to balance one another in such a manner that all the rays shall be brought to the same focus ; or by diminishing the aperture of the lens by means of a stop or diaphragm, in such a manner that only the central part of it shall be used. The latter of these methods is the one employed, where the diminution in the amount of light transmitted is not attended with inconvenience! The nearer the object is to the lens (and the greater, therefore, the angle of divergence of its rays), the greater will be the spherical aberration, and the more must the aperture of the diaphragm be contracted in order to counteract it. The other circumstance that interferes with the distinctness of the image, is the unequal refrangibility of the diffefently-coloured rays, which together make up white or colourless light ; the violet being more bent from their course than the blue, the blue more than the yellow, and the yellow more than the red ; the consequence of which will be, that the violet rays are brought to a focus much nearer to the lens than the blue, and the blue nearer than the red. If a screen be held to receive the image, in the focus of any of the rays, the others will make them- selves apparent as fringes round its margin. This difference is termed Chromatic Aberration. It is corrected in practice, by combining together lenses of different substances, of which the dispersive power (that is, the power of separating the coloured rays) differs considerably. This is the case with flint and crown glass, for instance, the dispersive power of the former being much greater than that of the latter, whilst its refractive power is nearly the same : so that, if a convex lens of crown glass be united with a concave of flint whose curvature is much less, the dispersion of the rays effected by the former will be counteracted by the latter, which diminishes in part only its refractive power. 329. The Eye may be regarded as an optical instrument of great perfec- tion, adapted to produce, on the expanded surface of the Optic nerve, a com- plete image or picture of luminous objects brought before it ; in which the forms, colours, lights and shades, &c., of the object are all accurately repre- sented. By the different refractive powers of the transparent media, through which the rays of light pass, and by the curvatures given to their respective surfaces, both the Spherical and Chromatic aberrations are corrected in a degree sufficient for all practical purposes ; so that, in a well-formed eye, the picture is quite free from haziness, and from false colours. The power by which it adapts itself to variations in the distance of the object, so as to form a distinct image of it, whether it be six inches, six yards, or six miles off, is extremely remarkable, and cannot be regarded as hitherto completely explained. It is obvious that, if we fix upon any distance as that for which the eye is naturally adjusted (say 12 or 14 inches, the distance at which we ordinarily read), the f rays proceeding from an object, placed nearer to the eye than this, would not be brought to a focus upon the retina, but Avould converge towards a point behind it; whilst, on the contrary, the rays from an object at a greater distance would meet before they reached the retina, and would have again diverged from each other when they impinge upon it; so that, in either case, vision would be indis- tinct. Now two methods of adaptation suggest themselves to the Optician. Either he may vary the distance between the refracting surface and the screen on which the image is formed, in such a manner, that the latter shall always 21 OF SENSATION, AND THE ORGANS OF THE SENSES. Fig. 43. A longitudinal section of the globe of the Eye; 1, the sclerotic, thicker behind than in front; 2, the cornea, received within the anterior margin of the sclerotic, and connected with it by means of ajbeveled edge; 3, the choroid, connected anteriorly with (4) the ciliary ligament, and (5) the ciliary processes; 6, the iris; 7, the pupil ; 8, the third layer of the eye, the retina, terminating anteriorly by an abrupt border at the com- mencement of the ciliary processes; 9, the canal of Petit, which encircles the lens (12); the thin layer in front of this canal is the zonula ciliaris, a prolongation of the vascular layer of the retina to the lens' 10, the anterior chamber of the eye, containing the aqueous humour; the lining membrane by which the humour is secreted is represented in the diagram; 11, the posterior chamber; 12, the lens more convex behind than before, and enclosed in its proper capsule; 13, the vitreous humour enclosed in the hyaloid membrane, and in cells formed in its interior by that membrane ; 14, a tubular sheath of the hyaloid mem- brane, which serves for the passage of the artery of the capsule of the lens ; 15, the neurilema of the optic nerve ; 16, the arteria centralis retinae, imbedded in its centre. be in the focus of the converging rays ; or, the distance of the screen remain- ing the same, he may vary the convexity of his lens, in such a manner as to adapt it to the distance of the object. It is not improbable, that both of these methods are employed in the eye, though no distinct evidence has been obtained of the operation of either. Several hypotheses have been proposed, to account for the phenomenon ; it is easily proved tjiat no one of them can alone be true ; but it cannot be readily shown that any of them is entirely false : and it would not seem unlikely, therefore, that all may participate, in various degrees, in the effect. The following are the principal of these. 1. An alteration in the form of the globe of the eye by the action of the muscle^, so that its antero- posterior diameter may be increased or diminished.* 2. A change in the con- vexity of the cornea. This might be very well connected with the last ; since, if the globe were converted into a spheroid, of which the antero-posterior diameter would be the longest, the curvature of the cornea would be increased ; whilst, if the antero-posterior diameter were shortened, the curvature would be diminished. 3. Change of position of the crystalline lens, by means of the ciliary processes. 4. Change of figure of the lens itself. That one or both of the last two are concerned in the effect, would appear from the fact, well known to every Oculist, that, after the removal of a cataract, the power of adapting the eye to distances is greatly diminished. 5. Change in the aper- ture of the pupil; the mode in which this could assist in accommodating the eye to variations of distance, is not very obvious. 330. Some curious circumstances relative to the connection between the optical adaptation of the eye to distances, and the changes in the direction of the axes of the two eyes, have been pointed out by Miiller. When both eyes * The influence of the muscles in altering the form of the globe may be better com- prehended, now that we know the mode in which this is kept in its place in the front of the orbit, by a fascia passing behind it, and attached anteriorly to the lids. SENSE OF VISION. [Fig. 44. 243 "'- -tf A Horizontal Section of the Eye-Ball; 1, sclerotic coat; 2, sheath of the optic nerve, or canal of Fon- tana; 3, circular venous sinns of the iris; 4, proper substance of the cornea; 5, arachnoidea oculi; 6, membrane of the anterior chamber of the aqueous humour; of the two dotted lines one points to the sup- 244 OF SENSATION, AND THE ORGANS OF THE SENSES. posed membrane of Descemet, the other to the supposed continuation of that membrane over the anterior surface of the iris; 7, choroid coat; 8, annulus albidus; 9, ciliary ligament; 10,10', ciliary body, consisting of (10') a pars non-fimbriata, and (10) a pars fimbriata formed by the ciliary process; 11, ora serrata of the ciliary body ; 12, iris; 13, pupil; 14, membrane of the pigment; 15, delicate membrane lining the posterior chamber of the aqueous humour; 16, membrane of Jacob; 17, the optic nerve surrounded by its neurilema; 17', the fibres of the optic nerve consisting of fasciculi of primitive tubules ; 18, central artery of the retina; 19, papilla cornica of the optic nerve ; 20, retina; the situation of its vascular layer is indicated by a dotted line ; 21, central transparent point of the retina ; 22, vitreous humour; 23, the hyaloid membrane ; 24, canalis hyaloideus ; 25, zonula ciliaris; in the plate, none of its fimbriated part is seen, being concealed by the ciliary processes; 26, canal of Petit; 27, crystalline lens; 28, anterior wall of the capsule of the lens; 29, posterior wall of the capsule of the lens; 30, posterior chamber of the aqueous humour; 31, ante- rior chamber of the aqueous humour.] are fixed upon an object, their axis must converge (as formerly explained, 254) so as to meet in it. The nearer the object, the greater must be the degree of convergence ; and when the object is brought within the ordinary distance of distinct vision, the convergence must very rapidly increase. Now this is precisely what takes place, in regard to alterations in the focus of the eye; for little change is required, when the object is made to approach from a considerable distance to a moderate distance ; but, when it is brought near the eye, the focus must be considerably lengthened, or the convexity of the eye increased, to cause the rays to meet on the retina : and hence it may be surmised, that the same cause is acting to produce both changes. But that the convergence of the axis is not itself in any way the occasion of the altera- tion of the focus of the eye, is shown by the fact, that the adaptation is as perfect in a person who only possesses or uses one eye, as it is when both are employed; and also by the power which is possessed by some persons of altering the focus of the eye by an effort of the will, whilst the convergence remains the same. In regard to the adaptation of the eyes to varying distances, it is further to be remarked, that, when an object is being viewed as near to the eye as it can be distinctly seen, the pupil contracts in a considerable degree. The final cause of this change is evidently to exclude the outer rays of the cone or pencil, which, from the large angle of their divergence, would fall so obliquely on the convex surface of the eye, as to be much affected by the spherical aberration; and to allow the central rays only to enter the eye, so as to preserve the clearness of the image. The channel through which it is effected is evidently the same as that by which the convergence of the eyes is produced, namely, the inferior branch of the third pair of nerves ; to the action of which, the sensations upon the retina form the stimulus, in the Same manner as they do to the ordinary variation in the diameter of the pupil under the influence of light. 331. The ordinary forms of defective vision, which are known under the names^of myopia and presbyopia, or short-sightedness and long-sightedness, are entirely attributable to defects in the optical adaptation of the eye. In the former, its refractive power is too great; the rays from objects at the usual distance are consequently brought too soon to a focus, so as to cross one another and diverge before they fall upon the retina; whilst the eye is adapted to bring to their proper focus on the retina only those rays which were previously diverging at a large angle, from an object in its near proxi- mity. Hence a short-sighted person, whose shortest limit of distinct vision is not above half that of a person of ordinary sight, can see minute objects more clearly; his eyes having, in fact, the same magnifying power which those of the other would possess, if aided by a convex-glass that would enable him to see the object distinctly at the shorter distance. But as the myopic structure of the eye incapacitates its possessor from seeing objects clearly, at even a moderate distance, it is desirable, to apply a correction ; and this is done by simply interposing a concave lens, of which the curvature is properly SENSE OF VISION. 245 Fig. 45. adapted to compensate for the excess of that of the organ itself, between the object and the eye. On the other hand, in the presbyopic eye, the curvature and refractive power are not sufficient to bring to a focus on the retina, rays which were previously divergent in a considerable or even in a moderate degree; and indistinct vision in regard to all near objects is, therefore, a necessary consequence, whilst distant objects are well seen. This defect is remedied by the use of convex lenses, which make up for the deficiency of the curvature. We commonly meet with myopia in young persons, and with presbyopia in old ; but this is by no means the invariable rule ; for even aged persons are sometimes short-sighted; and long-sightedness is occasionally met with amongst the young. In choosing spectacles, for the purpose of correcting the errors of the eye, it is of great consequence not to make an over-compen- sation ; for this has a tendency to increase the defect, besides occasioning great fatigue in the employment of the sight. It may be easily found, when a glass of the right power has been selected, by inquiring of the individual whether it alters the apparent size of the objects, or only renders them distinct. If it alter the size (increasing it if it be a convex lens, and diminishing it if it be a concave), its curvature is too great; whilst if it do not disperse the haze, it is not sufficiently powerful. In general it is better to employ a glass which somewhat under-compensates the eye, than one which is of a curvature at all too high ; since, with the advance of years in elderly persons, a progressive increase in power is required ; and, as young persons grow up to adult age, they should endeavour to dispense with the aid of spectacles. 332. Many other interesting inquiries, re- specting the action of the eye as an optical instrument, suggest themselves to the physical philosopher ; but the foregoing are the chief in which the physiologist is concerned , and we shall now proceed, therefore, to consider the share, which the retina and optic nerve per- form in the phenomena of vision. The Optic Nerve, at its entrance into the eye, divides itself into numerous small fasciculi of ultimate fibrils ; and these spread themselves out, and inosculate with each other by an exchange of fibrils, so as to form a net-like plexus, which is the outer layer of the true retina. From this plexus, in which the fibres are lying in the a plane of the surface of the vitreous humor, very large number of fibrils arise, in a direction perpendicular to that surface, so as all to be directed towards the centre of the eye. These pass through a delicate layer of cellular tissue, containing a minute plexus of blood-vessels ; and from this every fibril receives a sheath, which envelops its extremity, thus forming a minute papilla. The surface of the retina nearest the vitreous humor, is entirely com posed of these papillae, which are closely set together ; a layer of cells is interposed between them, however ; and these cells are regarded Papilla? of the retina of the Frog, seen by Valentin as analogous to those of ganglionic from the side ^ ur " ed t( > w * rds the vitre ' J , , ?. , j r f ous humor; the four hipher rows are matter; but other Microscopists dissent from geen sidew ' ayg Magni ^ ed 300 times this opinion, considering them as belonging {After T reviranus.) rather to the vitreous humor. In the retina of 21* Part of the retina of a Frog, seen from the outer surface. Magnified 300 times. (After Treviranus.) Fig. 46. 246 OF SENSATION, AND THE ORGANS OF THE SENSES. [Fig. 47. the Frog, the diameter of the ultimate nervous fibres is stated by Treviranus at about y^Q-th of an inch ; whilst that of the papillae is about i th of an inch. In Birds and Mammalia, A portion of the Retina of an Infant, with its vessels injected and magnified 25 diameters. An outline of the natural size of this piece is seen just below the main cut.] however, the papillas, as well as the nervous fibrils, are much smaller; in the former the diameter of the papillae is stated at from about __^.__th to fi - 3 J 5o tn f an inch; in the Rabbit at yYQ-oth of an inch ; and in Man at from the ^^--th to grV^h f an inch.* An attempt has been made to show, that the size of the papillae determines that of the smallest object, which can be seen by the unaided eye ; and it is a curious fact, that the calculation long ago made by Smith, in regard to the size of the most minute sensitive point upon the retina, founded upon the dimensions which the image of the minutest visible object will possess, co- incides exactly with the measurement of Weber. There is no doubt, however, that, under favour- able circumstances, the eye will take cognizance of objects much smaller than those on which Smith's calculation was founded. The follow- ing statements on this interesting subject com- prehend the result of numerous inquiries re- cently made by Ehrenberg, with the view of establishing the limits of Human Vision, as a datum from which to calculate the ultimate power of the Microscope.! 333. In opposition to the generally-received opinion, Ehrenberg arrived at the conclusion that, in regard to the extreme limits of vision, there is little difference amongst persons of ordinarily good sight, whatever may be the focal distance of their eyes. The smallest square magnitude usually visible to -the naked eye, either of white particles on a black ground, or of black upon a white or light-coloured ground, is about the 4 ^th of an inch. It is possible, by the greatest condensation of light, and excitement of the attention, to recognize magnitudes between the j^-jth and j^th of an inch; but without sharpness or certainty. Bodies which are smaller than these cannot be discerned with the naked eye when single ; but may be seen when placed in a row. Parti- cles which powerfully reflect light, however, may be distinctly seen, when not half the size of the least of the foregoing ; thus, gold dustf of the fineness of of an inch, may be discerned with the naked eye in common daylight, e delicacy of vision is far greater for lines than for single articles ; opaque threads of T -oVo- tn f an inch in diameter may be discerned with the naked eye, when held towards the light. Such threads are about half the diameter of the Silkworm's fibre. It is evident, from these facts, that the images of such particles formed upon the retina, must be considerably smaller than the diameter of the papillae. Still it is by no means improbable that, when we are looking at a continuous surface, the diameter of the papillae will regulate our * This is the diameter assigned by Weber to what he terms the globules of the Retina; there can be little doubt, however, that these are identical with the papillas, since the latter are very apt to separate, in eyes which are examined even a short time after death, from the fibres beneath. j- Taylor's Scientific Memoirs, vol. i. p. 576. t Ehrenberg mentions that he obtained the finest particles of gold, by scraping gilt brass ; by filing pure gold he always obtained much coarser particles. SENSE OF VISION. 247 power of distinguishing minute parts of that surface ; since, as Weber justly remarks, two impressions falling upon one of these points, can scarcely affect the sensorium otherwise than with one sensation. The degree in which the attention is directed to them, has a great influence on the readiness with which very minute objects can be perceived ; and Ehrenberg remarks that there is a much greater difference amongst individuals in this respect than there is in regard to the absolute limits of vision. Many persons can distinctly see such objects, when their situation is exactly pointed out to them, who cannot other- wise distinguish them ; and the same is the case with persons of acuter per- ception, with respect to objects at distances greater than those at which they can see most clearly. " 1 myself," says Ehrenberg, " cannot see ^yL-^th of an inch, black on white, at twelve inches distance ; but having found it at from four to five inches distance, I can remove it to twelve inches, and still see the object plainly." Similar phenomena are well known in regard to a bal- loon, or a faint star, in a clear sky ; or a ship in the horizon : we easily see them after they have been pointed out to us ; but the faculty of rapidly descrying depends on the habit of using the eyes in search of such objects ( 313). 334. The sense of Vision depends, in the first place, on the transference to our minds of the picture which is formed upon the retina ; this picture puts us in possession of the outlines, lights and shades, colours and relative posi- tions of the objects before us ; and all the ideas respecting the real forms, dis- tances, &c., of bodies, which we found upon these data, must be considered in the light of perceptions either instinctive or acquired. Many of these are derived through the combination, in our minds, of the visual sensations, with those derived from the sense of touch. Thus, to take a most simple illustra- tion, the idea of smoothness is one essentially tactile ; and yet it constantly occurs to us on looking at a surface which reflects light in a particular man- ner. But if it were not for the association, which experience leads us to form, of the connection between polish as seen by the eye, and smoothness as felt by the touch, we should not be able to determine, as we now can do, the existence of both these qualities, from an impression communicated to us through either sense singly. The general fact that, in Man, the greater part of those notions of the external w r orld, by which his actions in the adult state are guided, are acquired by the gradual association of the sensations commu- nicated by the sight and by touch, is substantiated by amply sufficient evidence. This evidence is chiefly derived -from observations made upon persons born blind, to whom sight has been communicated by an operation at a period of life which enabled them to give an accurate description of their sensations. The case recorded by Cheselden is one of the most interesting of these. The youth (about 12 years of age) for some time after tolerably distinct vision had been obtained, saw every thing^fotf, as in a picture ; simply receiving the con- sciousness of the impressions made upon his retina : and it was some time before he acquired the power of judging, by his sight, of the real forms and distances of the objects around him. An amusing anecdote recorded of him, shows the complete want of natural or intuitive connection which there is in Man, between the ideas formed through visual and through tactile sensations. He was well acquainted with a Dog and a Cat by feeling; but could not remember their respective characters when he saw them. One day, when thus puzzled, he took up the Cat in his arms, and felt her attentively, so as to associate the two sets of ideas ; and then, setting her down, said, " So, puss, I shall know you another time." A similar instance has come under the Author's own knowledge ; but the subject of it was scarcely old enough to present phenomena so striking. One curious circumstance was remarked of him, which fully confirms (if confirmation were wanting) the view here given. 248 OF SENSATION, AND THE ORGANS OF THE SENSES. For some time after the sight was tolerably clear, the lad preferred finding his way through his father's house, to which he, had been quite accustomed when blind, by touch rather than by sight, the use of the latter sense appearing to perplex rather than to assist him ; but, when learning a new locality, he employed his sight, and evidently perceived the increase of facility which he derived from it. 335. The question has been proposed, whether a person born blind, who was able by the sense of touch to distinguish a cube from a sphere, would, on suddenly obtaining his sight, be able to distinguish them by the latter sense. This question was answered by Locke in the negative ; and probably with justice. It is no real objection to such a reply, that a netv-born animal seeks the nipple of its mother, when informed of its proximity by sight ; for all that is indicated by this fact is, that the sensation excites an intuitive feeling of desire, which gives rise to movements adapted to gratify it. Such instinctive actions, founded upon intuitive perceptions, are, as already pointed out, much more numerous in the lower animals than in the higher, and in the young of the Human species than in the adult ( 259) ; and they do not afford any proof that definite notions, such as we acquire, of the forms and properties of exter- nal objects, are possessed by the animals which exhibit them. We shall now examine, a little more in detail, into the means by which we gain such notions, and the data on which they are founded. 336. The first point to be determined, is one which has been a fruitful source of discussion, the cause of erect vision, the picture upon the retina being inverted. Many solutions of it have been attempted ; but they are for the most part rather specious than really satisfactory. That which has been of late years the most in vogue, is founded upon what was styled the Law of Visible Direction, which has been supported by Sir D. Brewster and other eminent Philosophers. This law affirms, that every object is seen in the direction of the perpendicular to that point of the retina, on which its image is formed ; or, in other words, that, as all the perpendiculars to the several points of the inner surface of a sphere meet in the centre, the line of direction of any object is identical with the prolonged radius of the sphere, drawn from the point at which its image is made upon the retina. Upon close examina- tion, however, it is found that this law cannot be optically correct ; since the lines of direction cross each other at a point much anterior to the centre of the globe; as may be determined by drawing a diagram upon a large scale, and laying down the course of the rays received by the eye, according to the cur- vatures and refractive powers of its different parts/ In this manner it has been determined by Volkmann, that the lines of direction cross each other in a point a little behind the crystalline lens; and that they will thus fall at such different angles on different points of the retina, that no general law can be laid down respecting them. It may be questioned, moreover, whether any such law would afford any assistance in explaining the phenomenon; since, after all, it is requisite to assume an intuitive applicatjon of it, in supposing the mind to derive its ideas of the relative situations of objects, from the imagined line of direction -A much simpler and more, direct explanation may be given. We must remember that which we have had occasion to notice in regard to all the other senses, the broad line of distinction between the sensation and the perception or elementary notion; and this is still more clearly shown by the complete absence of any relation, but such as experience ' develops, between the perceptions derived through the sight and those acquired from the touch. Hence there is no more difficulty in understanding that an inverted picture upon the retina should convey to us a notion of the external world, which harmonizes with that acquired through the sense of touch, than there is in comprehending the formation of any of those intuitive SENSE OF VISION. 249 perceptions of animals which are so much more removed from the teachings of our own experience ( 290). 1 It is justly remarked by Miiller that, " if we do see objects inverted [or rather, if the picture on the retina is inverted] the only proof we can possibly have of it, is that afforded by the study of the laws of Optics ; and, if every thing is seen reversed, the relative position of the objects remains unchanged. Hence it is, also, that no discordance arises between the sensations of inverted vision and those of touch, which perceives every thing in its erect position ; for the images of all objects, even of our own limbs, on the retina, are equally inverted, and therefore maintain the same relative position. Even the image of our hand, when used in touch, is inverted." From what has been stated, it would appear quite conceivable, that a person just endowed with sight, should not at first know by his visual powers, whether a pyramid placed before his eyes is the same body, and in the same position, as one with which he has become acquainted by the touch ; and, if this be admitted, the inference necessarily follows, that the notion of erectness, which we form by the combined use of our eyes and our hands, is really the product of experience in ourselves, whilst it is probably innate or intuitional in the lower animals. 337. The cause of single vision with the two eyes has, in like manner, been the subject of much discussion; since the mode in which we are affected! by the two simultaneous impressions, is quite different from that in which we derive our knowledge of external things through the other senses. Some have even asserted, that we do not really employ both eyes simultaneously, but that the mind is affected by the image communicated by one only ; and this idea might seem to be confirmed by the fact heretofore mentioned ( 313) respecting the alternate use of the two eyes, when they are looking through two differently-coloured media. But it is easily disproved in other ways. It will presently be shown, that all our estimates of the forms of bodies depend on the combination by the mind of the images simultaneously transmitted by the two eyes ; and our knowledge of distances is in great part obtained in like manner. / The condition of Single Vision has been already stated ( 253) to be probably this, that the two images of the object should be formed on parts of the two retina? which are accustomed to act in concert ; and reasons were given for the belief, that habit is the chief means by which this conformity is produced. ; There can be no doubt, however, that double images are continu- ally being conveyed to our minds ;' but that, from their want of force and dis- tinctness, and from the attention being fixed on something else, we do not take cognizance of them. This may be sho\vn by a very simple experiment. If two fingers be held up before the eyes, one in front of the other, and vision be directed to the more distant, so that it is seen singly, the nearer will appear double ; while, if the nearer one be regarded more particularly, so as to appear single, the more distant will be seen double. A little consideration will show, therefore, that our minds must be continually affected with sensations which cannot be united into the idea of a single image ; since, whenever we direct the axes of our eyes towards any object, every thing else will be represented to us as double ; but we do not ordinarily perceive this, from our minds being fixed upon a clear and distinct image, and disregarding, therefore, the vague undefined images formed by objects at a different focus. Of this it is very easy to convince one's self. It is, moreover, evident from this experiment, that double vision cannot result from want of symmetry in the position of the images upon the retina, to which some have attributed it; for it answers equally well, if the line of the two fingers be precisely in front of the nose, so that the inclination of both eyes towards either object is equal; the position of the images of the second object must then be at the same distance on each side from the central line of the retina, and yet they are represented to the 250 OF SENSATION, AND THE ORGANS OF THE SENSES. mind as double. It is, moreover, easily shown that, in the lower animals whose orbits are not directed forwards as in us, but sideways in a greater or less degree, whenever an object is so situated as to be seen by both eyes, the points of the two retinae on which its images are formed, must be very far from possess- ing this symmetry. 338. Many attempts have been made to explain the phenomena of single vision, by the peculiar decussation of the optic nerves ; and an interesting correspondence between the varieties in the degree of decussation, and the position of the eyes, in several animals, has been pointed out by Mr. Solly and Mr. Mayo. It is stated by Mr. Mayo that the Optic nerve in Man con- sists of three tracts; of which the internal one is strictly commissural, con- necting together the two retinae anteriorly, and the two optic ganglia posteri- orly; the middle tract decussates, and is believed by Mr. M. to supply that part of the retina which lies on the inner side of each ball, between its ante- rior border and the insertion of the optic nerve ; whilst the external tract does not decussate, but passes on to supply the exterior portion of the retina on the same side. Thus the right optic nerve supplies the right side of each ball ; whilst the left supplies the left side. On the other hand, in most of the Osse- ous Fishes, the decussation is complete ; each nerve passing entirely to the eye of the opposite side. From these and other data, it has been concluded, that each nerve is used in looking towards the opposite side. This is evidently true of the Osseous Fishes, whose two eyes, being directed sideways, have two entirely different spheres of vision. And it is -also true of Man, if Mr. M.'s account of the distribution of the nerve be correct ; since, when we look at an object held directly in front of the face, at the level of the eyes, and at the nearest point for distinct vision, almost the whole of that portion of the right retina, which lies to the outside of the entrance of the optic nerve, is directed to the left ; and the exactly different, complementary, or inner portion of the left retina, which is supplied by the same nerve, is likewise directed to the left. On this supposition, all the rays entering the two eyes from any one point, will be brought to a focus on fibrils belonging to the same nerve ; though these are in Man, as in other animals whose spheres of vision are nearly or partly coincident, distributed to distinct visual organs.* It is obvious, how- ever, that this or any similar explanation must be insufficient to explain the phenomenon of single vision ; since the images formed upon the two retinae are necessarily different, and must be combined or harmonized by an act of the mind, as will be shown in the succeeding paragraphs. 339. We shall next consider the mode in which our notion of the solid forms and relative projection of objects is acquired ; on which great light has recently been thrown by the interesting experiments of Mr. Wheatstone.t It is perfectly evident, both from reason and experience, that the flat picture upon the retina, which is the only object of our sensation, could not itself convey to our minds any notion but that of a corresponding plane surface. In fact, any notion of solidity which might be formed by a person who had never had the use of more than one eye, would entirely depend upon the combination of his visual and tactile sensations. This idea is fully confirmed by the case already referred to, as recorded by Cheselden. The first visual * The late Dr. Wollaston was subject to a curious affection of vision, which consisted m his not being able to see more than half of an object, the loss being sometimes on one side, and sometimes on the other. The Author has met with several cases of this disorder, which has been termed hemiopia. Dr. W. thought that they might be explained by the decussation of the optic nerve; but Mr. Mayo states that he has known instances of a parallel affection, involving alternately the centre and the circumference of the retina, and therefore not attributable to any such structural arrangement. f Philosophical Transactions, 1838. SENSE OF VISION. 251 idea formed by the youth was, that the objects around him formed a flat sur- face, which touched his eyes, as they had previously been in contact with his hands ; and after this notion had been corrected, through the education of his sight by his touch, he fell into the converse error of supposing that a picture, which was shown to him, was the object itself represented in relief on a small scale. But where both eyes are employed, it has been ascertained by Mr. Wheatstone that they concur in exciting the perception of solidity or pro- jection, which arises from the combination of two different images in the mind. It is easily shown that any near object is seen in two different modes by the two eyes. Thus, let the reader hold up a thin book, in such a manner that its back shall be exactly in front of his nose, and at a moderate distance from it ; he will observe, by closing first one eye and then the other, that his perspective view of it (or the manner in which he would represent it on a plane surfabe) is very different, according to the eye with which he sees it. With the right eye he will see its right side, very much foreshortened ; with the left, he will gain a corresponding view of the left side ; and the apparent angles, and the lengths of the different lines will be found to be very different in the two views. On looking at either of these views singly, no other notion of solidity can be acquired from it than that to which the mind is conducted by the association of such a view with the touch of the object it represents. But it is capable of proof, that the mental association of the two different pictures upon the retinae does of itself give rise to the idea of solidity. This proof is afforded by Mr. Wheatstone's ingenious instrument, the Stereoscope. 340. The Stereoscope essentially consists of two plane mirrors, inclined with their backs to one another at an angle of 90. If two perspective draw- ings of any solid object, as seen at a given distance with the two eyes respect- ively, be placed before these mirrors, in such a manner that their images shall be made to fall upon the corresponding parts of the two retinae, in the same manner as the two images formed by the solid object itself would have done, the mind will perceive, not a single representation of the object, nor a confused union of the two, but a body projecting in relief, the exact counterpart of that from which the drawings were made. Mr. Wheatstone further shows, by means of the Stereoscope, that similar images, differing to a certain extent in magnitude, when presented to the corresponding parts of the two retinae, give rise to the perception of a single object, intermediate in size between the two monocular pictures. Were it not for this, objects would appear single only when at an equal distance from both eyes, so that their pictures upon the retina are of the same size ; which will only happen when they are directly in front of the median line of the face. Again, if pictures of dissimilar objects be simultaneously presented to the two eyes, the conse- quence will be similar to that which is experienced when the rays come to the eye through two differently-coloured media; the two images do not coalesce, nor do they appear permanently superposed upon one another ; but at one time one image predominates to the exclusion of the other, and then the other is seen alone ; and it is only at the moment of change that the two seem to be intermingled. It does not appear to be in the power of the will, Mr. Wheatstone remarks, to determine the appearance of either ; but, if one picture be more illuminated than the other, it will be seen during a larger proportion of the time. :Many other curious experiments with this simple instrument are related by Mr. Wheatstone ; and they all go to confirm the general conclusion, that the combination of the images furnished by the two eyes is a mental act, resulting from an inherent law of our psychical constitu- tion ; and that our perceptions of the solidity and projection of objects, near enough to be seen in different views with the two eyes, result from this cause. In regard to distant objects, however, the difference in the images formed by 252 OF SENSATION, AND THE ORGANS OF THE SENSES. the two eyes is so slight, that it cannot aid in the determination ; and hence it is, that,\vhilst we have no difficulty in distinguishing a picture, however well painted, from a solid object, when placed near our eyes, (since the idea which might be suggested by the image formed on one eye, will then be corrected by the other,) we are very liable to be misled by a delineation, in which the perspective, light and shade, &c., are faithfully depicted, if we are placed at a distance from it, and are prevented from perceiving that it is but a picture. In this case, however, a slight movement of the head is sufficient to undeceive us ; since by this movement a great change would be occasioned in the perspective view of the object, supposing it to possess an uneven sur- face ; whilst it scarcely affects the image formed by a picture. In the same manner, a person who only possesses one eye, obtains, by a slight motion of his head, the same idea of the form of a body which another would acquire by the simultaneous use of his two eyes. 341. The appreciation of the distance of objects may be easily shown to be principally derived from the association, in the mind, of visual and tactual sensations assisted, in regard to near objects, by the muscular sensations derived from the convergence of the eyes. Thus, an infant, or a person who has but recently acquired sight, evidently forms very imperfect ideas regard- ing the distance of objects ; and it is only after long experience that a correct notion is formed. The assistance which is given by the joint use of both eyes, is evident from the fact that, if we close one eye, we are unable to exe- cute with certainty many actions which require a precise appreciation of the distance of near objects, such as threading a needle, or snuffing a candle. In regard to distant objects, our judgment is chiefly founded upon their appa- rent size, if their actual size be known to us ; but if this is not the case, and if we are so situated that we cannot judge of the intervening space, we prin- cipally form our estimate from the greater or less distinctness of their colour and outline. Hence this estimate is liable to be greatly affected by varying states of the atmosphere ; as is well known to every one who has visited warmer latitudes. The extreme clearness of the air sometimes brings into an apparently near proximity a hill that rises beyond some neighbouring ridge (the intervening space being hidden, so as not to afford any datum for the esti- mate of the distance of the remote hill) ; and which, by a slight haziness, is carried to three or four times the degree of apparent remoteness. It is pro- bable that, in the lower animals, the perception of distance is much more intuitive than it is in ourselves. 342. Our estimate of the real size of an object is manifestly connected Avith that of its distance. The apparent size is dependent upon the angle at which its rays diverge, to impinge upon the cornea ; this angle increases with the proximity, and diminishes with the remoteness of the object. Our estimate of the comparative size of near objects, of whose distances we can become aware by the inclination of the optic axes, is much more correct than that which we form when one or both are far removed ; since, when we are uncer- tain as to its distance, we cannot form a judgment of the real size of a body from the angle at which its rays diverge. Hence our estimate of the size of objects, even moderately distant, is much influenced by states of the atmo- sphere. Thus, if we walk across a common in a fog, a child approaching us appears to have the size of a man, and a man seems like a giant ; since the indistinctness of the outline excites in the mind the idea of distance ; and an object seen under a given visual angle at a distance, must of necessity be much larger than one of which the apparent size is the same, but which is much nearer. The want of innate power in Man to form a true conception of either size or distance, is well shown by the effect produced on the mind unpre- pared for such delusions, by a skilfully-painted picture ; the view of which is SENSE OF VISION. 253 / so contrived, that its distance from the eye cannot be estimated in the ordinary manner; the objects it represents are invested by the mind with their real sizes and respective distances, as if their real image was formed upon the retina.* 343. From all these considerations, we are led to perceive the truth of the quaint observation made by Dr. Brown, that " vision is, in fact, the art of seeing things which are invisible ;" that is, of acquiring information by means of the eye, which is neither contained in the sensations of sight themselves nor logically deducible from the intimations which those sensations really con- vey. We cannot too constantly bear in mind, in treating of this subject, that we do not take cognizance by our optic nerves, as we do by the nerves of touch, of material bodies themselves, but of the pictures or images formed by those objects ; and whatever be the notions suggested by the picture, that can never be transformed into any thing else. These notions appear to be, in the lower animals, entirely of an intuitional or instinctive character ; in Man they are so in a much less degree ; and although it is impossible to come to a pre- cise conclusion on the subject, from the want of sufficient data, it is indubita- ble that a large part of the knowledge of the external world, which he derives in the adult condition from the use of his eyes alone, is really dependent upon the early education of his perceptive powers, in which process the sensations conveyed by different organs are brought to bear upon one another. 344. The persistence, during a certain interval, of impressions made upon the retina, gives rise to a number of curious visual phenomena. The pro- longation of the impression will be governed, in part, by its previous duration. Thus, when we rapidly move an ignited point through a circle, the impression itself is momentary, and remains but for a short time ; whilst, if we have been for some time looking at a window, and then close our eyes, the impression of the dark bars traversing the illuminated space is preserved for several seconds. Such phenomena can here only be briefly adverted to. One of these is the combination, into one image, of two or more objects presented to the eye in successive movements : but these must be of a kind which can be united ; otherwise a confused picture is produced. Thus in a little toy, called the Thaumatrope, which was introduced some years ago, the two objects were painted on the opposite sides of a card, a bird, for instance, on one, and a cage in the other ; and, when the card was made (by twisting a pair of strings) to revolve about one of its diameters, in such a manner as to be alternately presenting the two sides to the eye at minute intervals, the two pictures were blended, the bird being seen in the cage. A far more curious illusion, how- ever, was that first brought into notice by Mr. Faraday ; who showed that, if two toothed wheels, placed one behind the other, be made to revolve with equal velocity, a stationary spectrum will be seen ; whilst if one be made to revolve more rapidly than the other, or the number of teeth be different, the spectrum also will revolve. The same takes place when a single wheel is made to revolve before a mirror; the wheel and its image answering the pur- pose of the two wheels in the former case. On this principle a number of very ingenious toys have been constructed ; in some of these, the same figure or object is seen in a variety of positions ; and the impressions of these, pass- ing rapidly before the eye, give rise by their combinations to the idea that the object is itself moving through these positions. Similar illusions may be produced in regard to colour. 345. When the Retina has been exposed for some time to a strong impres- * This delusion has been extremely complete, in some of those who have seen the panoramic view of London in the Coliseum. A lively and interesting account of it is given in the Journal of the Parsee Shipbuilders, who recently visited England. 22 254 OF SENSATION, AND THE ORGANS OF THE SENSES. sion of some particular kind, it seems less susceptible of feebler impressions of the same kind. Thus, if we look at any Brightly luminous object, and then turn our eyes on a sheet of white paper, we shall perceive a dark spot upon it ; the portion of the retina which had been affected by the bright image not being able to receive an impression from the fainter rays reflected by the paper. The dark spectrum does not at once disappear, but assumes different colours in succession, these being expressions of the states through which the retina passes, in its transition to the natural condition. If the eye has received a strong impression from a coloured object, the spectrum exhibits the complementary colour ;* thus, if the eye be fixed for any length of time upon a bright red spot on a white ground, and be then suddenly turned so as to rest upon the white surface, we see a spectrum of a green colour. The same expla- nation applies to the curious phenomenon of coloured shadows. It may not un- frequently be observed at sunset, that when the light of the sun acquires a bright orange colour from the clouds through which it passes, the shadows cast by it have a blue tint. Again, in a room with red curtains, the light which passes through these produces' green shadows. In both instances, a strong impres- sion of one colour is made on the general surface of the retina ; and at any particular spots, therefore, at which the light is colourless but very faint, that colour is not perceived, its complement only being visible. The correctness of this explanation is proved by the fact that, if the shadow be viewed through a tube, in such a manner that the coloured ground is excluded, it seems like an ordinary shadow. It is not unlikely that, as Miiller suggests, the pre- dominant action of one colour on the retina disturbs (as it were) the equili- brium of its condition, and excites in it a tendency to the development of a state corresponding to that which is produced by the impression of the com- plementary colour ; for the latter is, according to him, perceived even where it does not exist ; as when the eye, after receiving a strong impression from a coloured spot, and directed upon a completely dark surface or into a dark cavity, still perceives the spectrum. Upon these properties of the eye are founded the laws of harmonious colouring, which have an obvious analogy with those of musical harmony. All complementary colours have an agree- able effect when judiciously disposed in combination ; and all bright colours which are not complementary have a disagreeable effect if they are predomi- nant : this is especially the case in regard to the simple colours, strong com- binations of any two of which, without any colour that is complimentary to either of them, are extremely offensive. Painters, who are ignorant of these laws, introduce a large quantity of dull gray into their pictures, in order to diminish the glaring effects which they would otherwise produce ; but this benefit is obtained by a sacrifice of the vividness and force, which may be secured in combination with the richest harmony, by a proper attention to physiological principles. 346. Some persons, who can perfectly distinguish forms, are deficient, through some original peculiarity in the constitution of the retina, in the power of discriminating colours. This is most commonly seen in regard to the complementary colours, especially red and green ; such persons not being able to perceive cherries amidst the leaves on a tree, except by the difference oi their form. Several distinct varieties of this affection may be distinguished, however ; and these have been classified by Leebeck.t * By the complementary colour is meant that which would be required to make white or colour ess light, when mixed with the original. As red, blue and yellow are the pri- mary or elementary colours, red is the complement of green (which is composed of yel- low and blue); blue is the complement of orange (red and yellow); and yellow of purple (red and blue) ; and vice versa in all instances f Mailer's Physiology, p. 1213. SENSE OF HEARING. 255 347. Amongst other curious phenomena of Vision, is the vanishing of images which fall at the entrance of the optic nerve ; as is shown in the fol- lowing experiment. Let two black spots be made upon a piece of paper, about four or five inches apart ; then let the left eye be closed, and the right eye be strongly fixed upon the left-hand spot. If the paper be then moved backwards and forwards, so as to change its distance from the eye, a point will be found at which the right-hand spot is no longer visible ; though it is clearly seen when the paper is brought nearer or removed further. In this position of the eye and object, the rays from the right-hand spot cross to the nasal side of the globe, and fall upon the point of the retina, which has just been mentioned. The phenomenon is not confined to that spot, however; nor is it correct to say, as is sometimes done, that the retina is not sensible to light at that point ; since, if such were the case, we should see a dark spot in our field of view, whenever we use only one eye. The fact is, that a similar phe- nomenon may occur under somewhat different conditions, in any division of the retina, especially in its lateral parts. Thus, if we fix the eye for some time, until it is fatigued, upon a strip of coloured paper lying upon a white surface, the image of the coloured object will in a short time disappear, and the white surface will be seen in its place ; the disappearance of the image, however, is only of a few seconds' duration. The truth seems to be, that there is a tendency in the retina, to the propagation, over neighbouring parts, of impressions which occupy a large proportion of its surface ; and that this tendency is the strongest around the point at which the optic nerve enters, so that the state of this part will generally become similar to that of the surround- ing portion of the retina. Hence, when we are using one eye only, we do not perceive any dark spot in the field, but only a certain degree of indistinct- ness in a portion of the image. 348. Under particular circumstances, we may receive a visual representa- tion of the retina itself; as is shown by the experiment of Purkinje. " If, in a room otherwise dark, a lighted candle be moved to and fro, or in a circle, at a distance of six inches before the eyes, we perceive, after a short time, a dark arborescent figure ramifying over the whole field of vision ; this appearance is produced by the vasa centralia distributed over the retina, or by the parts of the retina covered by those vessels. There are, properly speaking, two arborescent figures, the trunks of which are not coincident, but on the con- trary arise in the right and left divisions of the field, and immediately take opposite directions. One trunk belongs to each eye, but their branches inter- sect each other in the common field of vision. The explanation of this phe- nomenon is as follows : By the movement of the candle to and fro, the light is made to act on the whole extent of the retina, and all the parts of the mem- brane which are not immediately covered by the vasa centralia are feebly illuminated ; those parts, on the contrary, which are covered with those ves- sels, cannot be acted on by the light, and are perceived, therefore, as dark arborescent figures. These figures appear to lie before the eye, and to be suspended in the field of vision."* We have thus another demonstration of the fact that, in ordinary vision, the immediate object of our sensation is a certain condition of the retina, which is excited by the formation of a luminous image. VI. Sense of Hearing. 349. In the Ear, as in the Eye, the impressions made upon the sensory nerve are not at once made by the body which originates the sensation; but they are propagated to it, through a medium capable of transmitting them. * Mailer's Physiology, p. 1163. 256 OF SENSATION, AND THE ORGANS OF THE SENSES. Here too, therefore, we take cognizance by the mind, not of the sonorous object, but of the condition of the auditory nerve ; and all the ideas we form of sounds, as to their nature, intensity, direction, &c., must be based upon the changes which they produce in it. The complex contrivances which we meet with in the organ of hearing among higher animals, are evidently intended to give them greater power of discriminating sounds than is pos- sessed by the lower tribes ; in which last it is reduced to a form so simple, that it may be questioned whether they can be said to possess an organ of hearing, if by this term we imply any thing more than the mere consciousness of sonorous vibrations. There is a considerable difference, however, between the Eye and the Ear, in regard to the special purposes for which they are respectively adapted. In the former we have seen that the whole object of the instrument was to direct the rays of light received by it, in such a manner as to occasion them to fall upon the expansion of the optic nerve in a similar relative position, and with corresponding proportional intensity to that which they possessed when issuing from the object. We have no reason to believe any thing of this kind to be the purpose of the Ear; indeed it would be incon- sistent with the laws of the propagation of sound. Sonorous vibrations having the most various directions, and the most equal rate of succession, are trans- mitted by all media without modification, however numerous their lines of intersection; and wherever these undulations fall upon the auditory nerve, they must cause the sensation of corresponding sounds. Still it is probable that some portions of the complex organ of hearing, in Man and in the higher animals, are more adapted than others to receive impressions of a particular character; and that thus we may be especially informed of the direction of a sound by one part of the organ, of its musical tone by another, and of some other of its qualities by a third. In our inquiries into this ill-understood sub- ject, we shall commence with a brief survey of the comparative structure of the organ. 350. The essential part of an Organ of Hearing being obviously a nerve, endowed with the peculiar property of receiving and transmitting sonorous undulations, it is by no means indispensable that a special provision should be made for this purpose ; since the Auditory nerve, if merely in contact with the solid parts of the head, will be affected by the vibrations, in which it is continually participating. Hence we must not imagine the sense to be absent, wherever we cannot discover a special organ. It is among the highest only of the Invertebrate animals, that any such special organ presents itself; and then only in a very simple form. Thus in the Crustacea and Cephalopoda, the ear consists of a small cavity excavated in the solid frame-work of the head ; this cavity is lined with a membrane, on which the nerve is distributed ; and it is filled with a watery fluid. In some instances, the cavity is com- pletely shut in by its solid walls ; and the sonorous vibrations can then only be communicated through these : but in the higher forms of this apparatus, there is a small aperture covered with a membrane, upon which the external me- dium can at once act. In tracing this most simple into the more complex forms, it is at once seen, that the cavity corresponds with the vestibule of the ear of higher animals, and its opening with thefenestra ovalis. In the lowest Cyclostome Fishes, the organ is but little more complicated ; from the vestibule proceeds a single annular passage, which may be considered as a semicircular canal ; and the auditory nerve is distributed minutely upon its lining mem- brane, as upon that of the vestibule itself. In species a little higher in the scale, two such canals exist; these are present in the Lamprey. And in all the rest of the class, three semicircular canals are found, holding the same direc- tion in regard to each other as they do in Man. Within the vestibular sac of Fishes are found calcareous concretions, which are pulverulent in the Carti- SENSE OF HEARING. 257 laginous, but hard and stony in the Osseous tribes; to these the name of Oto- lithes has been given. Some rudiments of a tympanic cavity may be found in Fishes; but there is no vestige of a cochlea; in several tribes, the organ of hearing possesses a peculiar connection with the air-bladder, which appears to be a foreshadowing of the Eustachian tube of higher classes. 351. In the true Reptiles, a considerable advance is constantly to be found in the character of the Ear; a tympanic cavity being added, with a drum and a chain of bones ; and a rudiment of the cochlea being generally discoverable. Among the Amphibia, however, which are in so many respects intermediate [Fig. 48. An imaginary figure or plan of the Cochlea; this figure is designed to show how the twoscalaeof the coch- lea communicate in its summit ; the parietes of the scala vestibuli are supposed to be removed ; 1. 1, the osseous portion of the lamina spiralis; its small end is the hamulus cochleae; 2, 2, the dark ground here represents the membranous portion of the cochlea or the zona membranacea; 3, the commencement of the scala tympani ; 4, its external edge; 5, its internal edge ; 6 corresponds to the modiolus around which the lamina spiralis is wound ; 7, its summit; 8, the point of communication 9f the two scalae.] [Fig. 49. A view of the axis of the Cochlea and the Lamina Spiralis, showing the arrangement of the three Zones the osseous zone and the membrane of the vestibule have been removed; 1, the natural size of the pans; the other figure is greatly magnified; 2, trunk of the auditory nerve ; 3, the distribution of its filaments in the zona ossca; 4, the nervous anastomosis of the zona vesicularis; 5, the zona membranacea ; 6, the osseous tissue of the modiolus ; 7, the opening between the two scalce.l 22* 258 OF SENSATION, AND THE ORGANS OF THE SENSES. between the true Reptiles and Fishes, there is a remarkable variation in this respect some having a tympanum, and some being completely destitute of it. Wherever a tympanic cavity distinctly exists, there is an Eustachian tube connecting it with the fauces. This cavity, in the true Reptiles, not only possesses the fenestra ovalis (or opening into the vestibule) but the fenestra rotunda (or opening into the cochlea). The membrana tympani is usually visible externally ; but it is sometimes covered by the skin. In Birds, the structure of the ear is essentially the same, as in the higher Reptiles. A dis- tinct cochlea exists, though its form is not spiral but nearly straight : of its character, however, there can be no doubt ; a division into two passages, by a membranous partition on which the nerve is spread out, being evident. More- over, the tympanum communicates with cavities in the cranial bones, which are thus filled with air; and these, by increasing the extent of surface, pro- duce a more powerful resonance. There is no external ear, except in a few species of nocturnal Birds. In Mammalia, the organ of hearing is usually formed upon the same plan as it presents in Man ; in the Monotremata, how- ever, it more approaches that of Birds. The cochlea of the Mammalia in general is a spiral, forming about two turns and a half; the partition which divides its canal is partly osseous, partly membranous ; and its two passages communicate with the tympanic cavity and the vestibule respectively. The cavity of the tympanum is very large in some species, extending even into the contiguous bones. All the Mammalia, except the aquatic tribes, have an external ear ; and this is sometimes of an enormous size in proportion to the dimensions of the body, as it is in the Bats. The labyrinth of the higher Vertebrata contains no otolithes. The Cochlea divided parallel with its axis, through the centre of the Modiolus; after Breschet; 1, the modiolus; 2, the infundibulum in which the modiolus terminates; 3, 3, the cochlear nerve, sending its fila- ments through the centre of the modiolus ; 4, 4, the scala tympani of the first turn of the cochlea; 5, 5, the scala vestibula of the first turn; 6, section of the lamina spiralis, its zonula ossea; one of the filaments of the cochlear nerve is seen passing between the two layers of the lamina spiralis to be distributed upon the membrane which invests the lamina; 7, the membranous portion of the lamina spiralis ; 8, loops formed by the filaments of the cochlear nerve ; 9, 9, scala tympani of the second turn of the cochlea; 10, 10, scala vesubula of the second turn; the septum between the two is the lamina spiralis; 11, the scala tympani of he rema.nmg half turn; 12, the remaining half turn of the scala vestibula; the dome placed over this half turn is the cupola: 13, the lamina of bone which forms the floor of the scala vestibula curving spirally round to constitute the infundibulum (2) ; 14, the helicotrema through which a bristle is passed; its lower extremity issues from the scala tympani of the middle turn of the cochlea. 352. The ultimate terminations of the fibres of the auditory nerve in minute papillae are best seen in the lamina spiralis of the cochlea, and its membranous prolongation. Much diversity exists, however, as to the interpretation of the appearances there seen; some observers affirming that there are no free or papillary terminations, and that the nervous fibres all return by loops; whilst SENSE OF HEARING. 259 Fig. 51, [Fig. 52. Papillre of the Auditory Nerve, on a Segment of the Spiral Lamina of the Cochlea of a young Mouse ; the lower portion is the osseous, and the higher the membranous part of the lamina. Mag- nified 300 times. After Treviranus. The Auditory Nerve taken out of the Cochlea; 1, 1, 1, the trunk of the nerve ; 2, 2, its filaments in the zona ossea of the lamina spiralis; 3, 3, its anastomoses in the zona vesicularis.] [Fig. 53. A highly magnified view of a small piece of the Lamina Spiralis, showing the globular structure of the Nerves, and the manner in which they leave their Neurilema as they anastomose; the natural size of the piece is seen on the side of the figure; 1, portion of the auditory nerve; 2, 2, osseous canals in the zona ossea of the lamina spiralis; 3, 3, anastomoses in the zona mollis ; 4, 4, the neurilema leaving the nervous loops and interlocking to form the layer of the zona membranacea.] others state that the papillae are clearly to be distinguished. The fact appears to be that, as in the retina, the fibres do form a minute plexus ; but that fibres are connected with this, which end, or rather commence, in papillae. The auditory nerve is also very minutely distributed on the membrane lining the vestibule and semicircular canals ; and in the ampullae or dilated extremities of the latter, there are little projections of this membrane internally, which are largely supplied with nerves. 353. In order to gain any definite idea of the uses of different parts of the Ear, it is necessary to bear in mind, that sounds may be propagated amongst solid or fluid bodies in 'three ways, by reciprocation, by resonance, and by conduction. 1. Vibrations of reciprocation are excited in a sounding body, when it is capable of yielding a musical tone of definite pitch, and 200 OF SENSATION, AND THE ORGANS OF THE SENSES. [Fig. 54. The soft parts of the Vestibule taken out of their bony case, so as to show the distribution of the Nerves in the Ampullae; 1, the superior semicircular membranous canal or tube; 2, the external semicircular tube; 3, the inferior semicircular tube; 4, the tube of union of the superior and inferior canals; 5, the sacculus ellipticus; 6, the sacculus sphericus; 7, the portio dura nerve; 8, the anterior fasciculus of the auditory nerve; 9, the nerve to the sacculus sphericus; 10, 10, the nervous fasciculi to the superior and external ampullne; 11, the nerve to the sacculus elliplicus; 12, the posterior fasciculus of the auditory nerve, furnishing 13, the filaments of the sacculus sphericus, and 14, the filaments of the cochlea, cut off.] [Fig. 55. The Ampulla of the External Semicircular Membranous Canal, showjng the mode of termination of its Nerve.] another body of the same pitch is made to sound near it. Thus if two strings of the same length and tension be placed alongside of each other, and one of them be sounded with a violin-bow, the other will be thrown into reciprocal vibration ; or if the same tone be produced near the string in any other man- ner, as by a flute, or a tuning-fork, the same effect will result. 2. Vibrations of resonance are of somewhat the same character ; but they occur when a sounding body is placed in connection with any other, of whi^n'one or more SENSE OF HEARING. 261 Fig. 56. The labyrinth of the Left Ear, laid open in order to show its cavities and the Membranous Labyrinth ; after Breschet; 1, the cavity of the vestibule, opened from its anterior aspect in order to show the three-cor- nered form of its interior, and the membranous labyrinth which it contains; the figure rests upon the com- mon saccule of the membranous labyrinth the sacculus communis ; 2, the ampulla of the superior or per- pendicular semicircular canal, receiving a nervous fasciculus from the superior bronch of the vestibular nerve; 3, 4, the superior or perpendicular canal with its contained membranous canal; 5, the ampulla of the inferior or horizontal semicircular canal, receiving a nervous fasciculus from the superior branch of the vestibular nerve; 6, the termination of the membranous canal of the horizontal semicircular canal in the sacculus communis; 7. the ampulla of the middle or oblique semicircular canal, receiving a nervous fasciculus from the inferior branch of the vestibular nerve; 8, the oblique semicircular canal with its membranous canal; 9, the common canal, resulting from the union of the perpendicular with the oblique semicircular canal; 10, the membranous common canal terminating in the sacculus communis; 11, the otoconite of the sacculus communis seen through the membranous parietes of that sac ; a nervous fasci- culus from the inferior branch of the vestibular nerve is seen to be distributed to the sacculus communis near to the otoconite; the extremity of the sacculus above the otoconite is lodged in the superior ventricle of the vestibule, and that below it in the inferior ventricle ; 12, the sacculus proprius situated in the ante- rior ventricle; its otoconite is seen through its membranous parietes, and a nervous fasciculus derived from the middle branch of the vestibular nerve, is distributed to it; the spaces around the membranous labyrinth are occupied by the aqua labyrinth!; 13, the first turn of the cochlea; the figure is situated in the scala tympani; 14, the extremity of the scala tympani corresponding with the fenestra rotunda; 15, the lamina spiralis; the figure is situated in the scala vestibuli ; 16, the opening of the scala vestibuli into the vestibule; 17, the second turn of the cochlea; the figure is placed upon the lamina spiralis, and, therefore, in the scala vestibuli, the scala tympani being beneath the lamina; 18, the remaining half turn of the cochlea; the figure is placed in the scala tympaui; 19, the lamina spiralis terminating in a falciform ex- tremity ; the dark space included within the falciform curve of the extremity of the lamina spiralis is the helicotrema ; 20, the infundibulum. parts may be thrown into reciprocal vibration, even though the tone of the whole be different, or it be not capable of producing a definite tone at all. This is the case, for example, when a tuning-fork in vibration is placed upon a sound-board ; for even though the whole board have no definite fundamental note,* it^tt divide itself into a number of parts, which will reciprocate the * The fundamental note of & body is the lowest tone which it will yield, when the whole of it is in vibraftj^ together. By dividing the body into two or more distinct parts, it may be made to^pre a great variety of sounds. Thus, if a stretched string be divided by a bridge into two equal parts, each will sound the octave of the fundamental note, or the 8th note above it. If it be divided into three parts, each will give the 12th above the fun- damental note ; if into four, the 15th or double octave will be heard ; if into five, the 17th ; if into six, the 19th; if into seven, the 20 (flat seventh above the second octave); if into eight, the 22dor triple octave. A string forcibly set in vibration has a tendency to sound these harmonics with the fundamental note, by spontaneous division into several distinct segments of vibration ; as may be easily made evident, by striking one of the lower keys of the piano, and listening to the sounds heard whilst the fundamental note is dying away. 262 OF SENSATION, AND THE ORGANS OF THE SENSES. original sound, so as greatly to increase its intensity ; and the same sound- board will act equally well for tuning-forl^s of several different degrees of pitch. When a smaller body is used for resonance, however, it is essential that there should be a relation between its fundamental note and that of the sonorous body; otherwise no distinct resonance is produced. Thus, if a tunino-fork in vibration be held over a column of air in a tube, of such a length that the same note would be given by its vibration, its sound will be reciprocated. And if it be held over a pipe, the column of air in which is a multiple of this, the column will divide itself into that number of shorter parts each of which will reciprocate the original sound, and the total action will be one of resonance. But if the length of the pipe bear no such correspondence with the note sounded by the tuning-fork, no resonance is given by the column of air it contains. 3. Vibrations of conduction are the only ones by which sounds can strictly be said to be propagated. These are distinguishable into various kinds, into which it is not requisite here to inquire. It should be remarked, however, that all media, fluid, liquid, or solid, are capable of trans- mitting sound in this manner, a vacuum being the only space through which it cannot pass. The transmission is usually much more rapid through solid bodies than through liquid ; and through liquid than through gaseous. The greatest diminution in the intensity of sound is usually perceived, when a change takes place in the medium through which it is propagated, especially from the aeriform to the liquid. 354. The detailed application of these principles has been most elaborately worked out by Miiller; and the following statement of what may be regarded as the present condition of our knowledge of the subject, is little more than an abstract o his results. Considering it desirable, in the first place, to establish the conditions under which those animals hear, that are constantly immersed in water, he made a series of experiments, from which he draws the following conclusions : i. Sonorous vibrations, excited in water, are imparted with con- siderable intensity to solid bodies. n. Sonorous vibrations of solid bodies are communicated with greater intensity to other solid bodies brought in contact with them than to water ; but with much greater intensity to water than to atmospheric air. in. Sonorous vibrations are communicated from air to water with great difficulty, with very much greater difficulty than they are pro- pagated from one part of the air to another ; but their transition from air to water is much facilitated by the intervention of a membrane extended between them. iv. Sonorous vibrations are not only imparted from water to solid bodies with definite surfaces, which are in contact with the water, but are also returned with increased intensity by these bodies to the water ; so that the sound is heard loudly in the vicinity of those bodies, in situations where, if it had its origin in the conducting power of the water alone, it would be faint. v. Sonorous undulations, propagated through water, are partially reflected by the surfaces of solid bodies. vi. Thin membranes conduct sound in water without any loss of its intensity, whether they be tense or lax. From m., iv., and vi., we learn the mode in which the sound is conducted to the ear, in aquatic animals not breathing atmospheric air. The labyrinth of such is either entirely enclosed within the bones of the head, as in the Cephalopoda, and in the Cyclostome and Osseous Fishes ; or, its cavity being prolonged to the surface of the body, it is there brought into communication with the con- ducting medium, by means of a membrane, besides receiving the vibrations through the medium of the solids of the body, as is the case in Cartilaginous Fishes and Crustacea. It would seem as if, in the Osseous Fishes, the reso- nance of the cranial bones, in which the labyrinth is imkrdded, were sufficient to give the requisite increase of intensity to the sound ; whilst in the Cartila- ginous orders, the softness of these bones renders some other means necessary. SENSE OF HEARING. 263 In addition to this, we find in many Fishes a communication with the air- bladder ; which, indeed, seems to have in these but little other use. The mode in which this increases by resonance the intensity of the sounds, will appear from the following experimental conclusions. vn. When sonorous vibrations are communicated from water to air enclosed in membranes or solid bodies, a considerable increase in the intensity of the sound is produced, by the resonance of the air thus circumscribed. vm. A body of air enclosed in a membrane, and surrounded by water, also increases the intensity of the sound by resonance, when the sonorous undulations are communicated to it by a solid body. From these observations it .may be concluded, that the air- bladder of Fishes, in addition to other uses, serves the purpose of increasing by resonance the intensity of the sonorous undulations, communicated from the water to the body of the Fish. Moreover, as the conducting and resonant power of the air in the air-bladder is greater in proportion to its density, the influence of this organ on the perception of sounds will, of course, be greater in deep waters, where the pressure upon it is considerably increased. 355. Most animals living in air, are provided with an opening into the ves- tibule, covered by a thin membrane ; and, in the majority of cases, with the tympanic apparatus also. The following experimental results bear upon the manner in which the Ear of such animals is affected by sound. ix. Sonorous undulations, in passing from air directly into water, suffer a considerable dimi- nution in their strength; while, on the contrary, if a tense membrane exists between the air and the water, the sonorous undulations are communicated from the former to the latter medium with great intensity. x. The sonorous vibrations are also communicated, without any perceptible loss of intensity, from the air to the water; when, to the membrane forming the medium of communication, there is attached a short solid body, which occupies the greater part of its surface, and is alone in contact with the water. xi. A small solid body, fixed in an opening by means of a border of membrane, so as to be movable, communicates sonorous vibrations from air on one side to water or the fluid of the labyrinth on the other, much better than solid media not so constructed. But the propagation of sound to the fluid is rendered much more perfect, if the solid conductor, thus occupying the opening, is by its other end fixed to the middle of the tense membrane, which has atmospheric air on both sides. The fact stated in ix. is evidently one of great importance in the physiology of hearing ; . and fully explains the nature of the process in those animals which receive the sonorous vibrations through air, but which have no tympanic apparatus. In x. we have the elucidation of the action of the fenestra ovalis, and of the movable plate of the stapes which occupies it, in animals living in air but destitute of tympanic apparatus ; this is naturally the case in many Amphibia ; and it may happen as the result of disease in the human subject. In xi. we have a very interesting demonstration of the pur- pose and action of the tympanum, in the more perfect forms of the auditory apparatus. We are now prepared to inquire, in somewhat more of detail, into the action of the different parts of this apparatus; and it will be better to com- mence with that of the Internal Ear, the accessory organs being afterwards considered. 356. The object of the Membrana Tympani is evidently to receive the sonorous undulations from the air, in such a manner as to be thrown by them into a reciprocal vibration, which is to be communicated to the chain of bones. This membrane is, in its usual state, rather lax than tense ; and this laxity is found by experiment to be, for a small membrane, the best condition for the propagation of ordinary sounds. This is easily rendered sensible in one's own person ; for an increased tension may be given to the membrana tym- pani, either by holding the breath and forcing air into the Eustachian tube, 264 OF SENSATION, AND THE ORGANS OF THE SENSES. so as to distend it from within, or Ipy exhausting the cavity so as to cause the external air to make increased pressure upon it. In either case the hearing is found immediately to become indistinct. It is observed, however, that grave and acute sounds are not equally affected by this action ; for the experimenter renders himself deaf to grave sounds, whilst acute sounds are heard even more distinctly than before. This fact is easily understood, by referring to the laws of Acoustics already mentioned. The greater the tension to which the membrana tympani is subjected, the more acute will be its fun- damental tone ; and as no proper reciprocation can take place in it, to any sound lower than its fundamental tone, its power of repeating perfectly the vibrations proper to the deeper notes will diminish. The nearer a sound approaches to the fundamental note proper to the tense membrane, the more distinctly will it be heard. On the other hand, when the membrane is in its natural lax condition, its fundamental note is very low, and it is capable of repeating a much greater variety of sounds ; for, when it receives undulations of a higher tone than those to which the whole membrane would reciprocate, it divides itself into distinct segments of vibration, which are separated by lines of rest ; and every one of these reciprocates the sound,* at the same time rendering it more intense by multiplication. These facts enable us to under- stand the influence of the tensor tympani muscle, in modifying the tension of the membrane, and thus causing it to vibrate in reciprocation to sounds having a great variety of fundamental notes. Moreover, the fact that some persons are deaf to grave sounds, whilst they readily hear the more acute, is thus accounted for. The tensor tympani, like the iris, is probably excited to opera- tion by a reflex action ; and it is by no means improbable that one of its func- tions may be to prevent the internal ear from being too violently affected by loud sounds, by putting the membrana tympani into such a state of tension as not readily to reciprocate them. 357. The uses of the Tympanic cavity are very obvious. One of its pur- poses is to render the vibrations of the membrane quite free ; and the other, to isolate the chain of bones in such a manner as to prevent their vibrations from being weakened by diffusion through the surrounding solid parts. As to the objects of the Eustachian tube, however, opinions have been much divided. From the experiments of Miiller, it appears that it does not increase the intensity of sound, but that it prevents a certain degree of dulness which would attend it if the cavity of the tympanum were completely closed ; of this dulness we are conscious, when any tumefaction of the fauces causes an occlusion of the extremity of the tube. It has been supposed that, among other uses, this canal serves for the conduction of the speaker's voice to his ears ; but this is certainly not the case in any considerable degree ; for, when the Eustachian tubes are obstructed by disease, the patient hears his own voice well, though other sounds are indistinct ; and it is easily shown, that its transmission is chiefly accomplished in other ways. The common idea is, that it serves the same purpose with the hole in an ordinary drum ; the effect of which js generally supposed to be, the removal of the impediment to the vibrations* of the membrane, that would be offered by the complete enclosure of the air within. It does not appear, however, that any such impediment is really offered ; and the effect of the hole in the drum seems rather to be the * This is very easily proved by experiments on a membrane stretched over a resonant cavity; if light sand be strewed upon it, and a strong musical tone be produced in its vicinity, the membrane will immediately be set in vibration, not as a whole (unless its fundamental note be in unison with that sounded), but in distinct segments, of which every one reciprocates the sound; from the vibrating parts, the sand will be violently thrown off; but it will settle on the intermediate lines of rest, forming a variety of curious figures, which are known as the nodal lines. SENSE OF HEARING. 265 communication, to the ear of the auditor, of the sonorous vibrations of the contained air ; which are thus transmitted directly through the atmosphere, instead of being weakened by transmission through the walls of the instru- ment. Hence there is no real analogy in the two cases. The principal object of the Eustachian tube (which is always found where there is a tympanic cavity) seems to be the maintenance of the equilibrium between the air within the tympanum and the external air ; so as to prevent inordinate tension of the membrana tympani, which would be produced by too great or too little pres- sure on either side, and the effect of which would be imperfection of hearing. It also has the office of conveying away mucus secreted in the cavity of the tympanum, by means of cilia vibrating on its lining membrane ; and the deaf- ness, consequent on occlusion of this tube, is in part explicable by the accu- mulation which will then take place in the tympanum. 858. From what has been stated, it is evident that sonorous undulations taking place in the air, will be propagated to the fluid contained in the laby- rinth, through the tympanum, the chain of bones, and' the membrane of the fenestra ovalis to which the stapes is attached, without any loss, but rather an increase of intensity. Why water should be chosen as the medium through which the impression is to be made upon the nerve, it is impossible for us to say with any thing like certainty, in our present state of ignorance as to the physical character of that impression. But, the problem being to communi- cate to water the sonorous undulations of air, the experimental results already detailed satisfactorily prove, that, whilst this may be accomplished, in a degree sufficient for the wants of the inferior animals, by the simple interposi- tion of a tense membrane between the air and the fluid, the tympanic appa- [Fig. 57. A view of the labyrinth of the Left Side, laid open in its whole extent so as to show its Structure; these figures are all magnified ; 1, the thickness of the outer covering of the cochlea; 2, 2, the scala vesti- buli or upper layer of the lamina spiralis; 3, 3, the scala tympani or lower layer of the lamina spiralis; 4, the hamulus cochlese ; 5, centre of the infundibulum ; 6, the foramen rotundum communicating with the tympanum; 7, the thickness of the outer layer of the vestibule; 8, the foramen rotundum; 9, the fenestra ovalis; 10, the orifice of the aqueduct of the vestibule ; 11, the inferior semicircular canal; 12, the superior semicircular canal; 13, the external semicircular canal; 14, the ampulla of the inferior canal; 15, the am- pulla of the superior canal ; 16, the common orifice of the superior and inferior canals ; 17, the ampulla of the external canal.] 23 266 OF SENSATION, AND THE ORGANS OF THE SENSES. ratus of the higher classes is most admirably adapted for this purpose. The fenestra ovalis is not, however, the only cjiannel of communication between the tympanum and the labyrinth; for there is, in most animals, a second aper- ture, the fenestra rotunda, leading into the cochlea, and simply covered with a membrane. It is generally supposed that, the labyrinth being filled with a nearly incompressible fluid, this second aperture is necessary to allow of the free vibration of that fluid, the membrane of the fenestra rotunda being made to bulge out, as that of the fenestra ovalis is pushed in. It may, however, be easily shown by experiment, as well as by reference to comparative anatomy, that no such contrivance is necessary ; for sonorous undulations may be excited in a non-elastic fluid, completely enclosed within solid walls at every part, except where these are replaced by the membrane through which the vibra- tions are propagated; and this is precisely the condition, not only of the Invertebrated animals, but even of Frogs ; in which last a tympanic apparatus exists, without a second orifice into the labyrinth. Moreover it is certain, that the vibrations of the air in the cavity of the tympanum, must of themselves act upon the membrane of the fenestra rotunda; and this is perhaps the most direct manner in which the fluid in the cochlea will be affected ; although it will ultimately be thrown into much more powerful action, by the transmission of vibrations from the vestibule. For it has been satisfactorily determined by experiment (xn.), that vibrations are transmitted with very much greater intensity to water, when a tense membrane, and a chain of insulated solid bodies, capable of free movement, are successively the conducting media, than when the media of communication between the vibrating air and the water are the same tense membrane, air, and a second membrane : or, to apply this fact to the organ of hearing, the same vibrations of the air act upon the fluid of the labyrinth with much greater intensity, through the medium of the chain of auditory bones and the fenestra ovalis, than through the medium of the air of the tympanum and the membrane closing the fenestra rotunda. The fenestra rotunda is not to be considered as having any peculiar relation with the cochlea; since, in the Turtle tribe, the former exists without the latter. 359. In regard to the functions of particular parts of the labyrinth, no cer- tainty can be said to exist. From the experimental results already stated, it appears likely that, the greater the extension of the cavity into the dense sub- stance of the bone, the greater will be the resonance communicated to the fluid, and thence transmitted to the nerves exposed to its influence. It is commonly supposed that the Semicircular Canals have for their peculiar function, the reception of the impressions by which we distinguish the direc- tion of sounds ; and it is certainly a powerful argument in support of this view, that, in almost every instance in which these parts exist at all, they hold the same relative position to each other as in Man, their three planes being nearly at right angles to one another. The idea, however, must be regarded as a mere speculation, the value of which cannot be decided without an increased knowledge of the laws, according to which sonorous vibrations are transmitted. Regarding the special function of the Cochlea, there is precisely the same uncertainty. This part of the organ is peculiar in one respect, that the expansion of the auditory nerve is here spread out (upon the lamina spiralis) in closer proximity with the bone itself, than it is in any other part of the labyrinth ; so that the vibrations of the bone will be more directly communi- cated to the nerve. It is not easy to see, however, what can be the peculiar object of this disposition, in regard to the function of hearing. By M. Duges it is surmised, that by the cochlea we are especially enabled to estimate the pitch of sounds, particularly of the voice ; and he adduces, in support of this idea, the fact, that the development of the cochlea follows a very similar proportion with the compass of the voice. This is much the greatest in the SENSE OF HEARING. 267 Mammalia ; less in Birds ; and in Reptiles, which have little true vocal power, the cochlea is reduced to its lowest form, disappearing entirely in the Amphibia. That there should be an acoustic relation between the voice and ear of each species of animals, cannot be regarded as improbable ; but the speculation of M. Duges can at present only be received as a stimulus to further inquiry. 360. We have now to consider the functions of the accessory parts, the External Ear, and the Meatus. The Cartilage of the external ear may propa- [Fig. 58. [Fig. 5U. A view of the Left Ear in its natural state ; 1, 2, An anterior view of the External Ear, as well the origin and termination of the helix; 3, the as of the Meatus Auditorius, Labyrinth, &c.; l,the anti-helix; 4, the anti-tragus; 5, the tragus; 6, the opening into the ear at the bottom of the concha,; lobus of the external ear; 7, points to the%capha 2, the meatus auditorius externus or cartilaginous and is on the front and top of the pinna; 8, the canal; 3, the membrana tympani stretching upon concha; 9, the meatus auditorius externus.] its ring; 4, the malleus; 5, the stapes; 6, the laby- rinth.] gate sonorous vibrations in two ways, by reflection and by conduction. In reflection, the concha is the most important part, since it directs the reflected undulations towards the tragus, whence they are thrown into the auditory passage. The other inequalities of the external ear cannot promote hearing by reflection ; and the purpose of the extension of its cartilage is evidently to receive the sonorous vibrations from the air, and to conduct them to its point of attachment. In this point of view, the inequalities become of importance ; for those elevations and depressions upon which the undulations fall perpen- dicularly, will be affected by them in the most intense degree ; and in conse- quence of the varied form and position of these inequalities, sonorous undula- tions, in whatever direction they may come, must fall advantageously upon some of them. The functions of the Meatus appear to be threefold. The sonorous undulations entering from the atmosphere are propagated directly, without dispersion, to the membrana tympani : the sonorous undulations received on the external ear, are conveyed along the walls of the meatus to the membrana tympani : the air which it contains, like all insulated masses of air, increases the intensity of sounds by resonance. That, in ordinary hearing, the direct transmission of atmospheric vibrations to the membrana tympani, is the principal means of exciting the reciprocal vibrations of the latter, is suffi- ciently evident ; the undulations which directly enter the passage, will pass straight on to the membrane ; whilst those that enter obliquely will be reflected 268 OF SENSATION, AND THE ORGANS OF THE. SENSES. from side to side, and at last will fall obliquely on the membrane, thus perhaps contributing to the notion of direction. The power of the lining of the meatus to conduct sound from the external ear, is made evident by the fact, that, when both ears are closely stopped, the sound of a pipe having its lower extremity covered by a membrane, is heard more distinctly, when it is applied to the cartilage of the external ear itself, than when it is placed in contact with the surface of the head. The resonant action of the air in the tube is easily demonstrated, by lengthening the passage by the introduction of another tube ; the intensity of external sounds, and also that of the individual's own voice, as heard by himself, are then much increased. 361. 'Many facts prove, however, that the fluid of the labyrinth may be thrown into vibration in other ways than by the tympanic apparatus. Thus in Osseous Fishes, it is only by the vibrations transmitted through the bones of the head that hearing can take place. There are many persons, again, who can distinctly hear sounds which are thus transmitted to them ; although, through some imperfection of the tympanic apparatus, they are almost insensible to those which they receive in the ordinary way. It is evident, where this is the case, that the nerve must be in a state fully capable of functional activity ; and, on the other hand, where sounds cannot thus be perceived, there will be good reason to believe that the nerve is diseased. 362. A single impulse communicated to the Auditory nerve, in any of the foregoing modes, seems to be sufficient to excite the momentary sensation of sound; but most frequently a series of such impulses is concerned, there being but few sounds which do not partake, in a greater or less degree, of the character of a tone. Any continuous sound or tone is dependent upon a suc- cession of such impulses ; and its acuteness or depth is governed by the rapidity with which they succeed one another. It is not difficult to ascertain by experiment, what number of such impulses or undulations are required to give every tone which the ear can appreciate. Thus, if a circular plate, with a number of apertures at regular intervals, be made to revolve over the top of a pipe through which air is propelled, a succession of short puffs will be allowed to issue from this ; and, if the revolution is sufficiently rapid, these impulses will unite into a definite tone. In the same manner, if a spring be fixed near the edge of a revolving toothed wheel, in such a manner as to be caught by every tooth as it passes, a succession of clicks will be heard ; and these too, if the revolution of the wheel be sufficiently rapid, will produce a tone. The number of apertures in the plate which pass the orifice of the pipe in a given time, or the number of teeth which pass the spring being known, it is easy to see that this must be the number of impulses required to produce the given tone. Each impulse produces a double vibration, forwards and backwards ;* hence the number of impulses is always half that of the single vibrations. The maximum and minimum of the intervals of successive pulses, still appreciable by the ear as determinate sounds, have also been determined by M. Savart more satisfactorily and more accurately than had previously been done. If their intensity is great, sounds are still audible which result from the succession of 24,000 impulses in a second ; and this, probably, is not the extreme limit in acuteness of sounds perceptible by the ear. From some observations of Dr. Wollaston's, it seems probable that the ears 'of different individuals are differently constituted in this respect, some not being able to hear very acute tones produced by Insects, or. even Birds, which are distinctly audible to others. Again, the sound resulting frn 16 impulses per. second, is not, as has been usually supposed, the lowest appreciable note ; on the con- trary, M. Savart has succeeded in rendering tones distinguishable, which were *This is seen when a string is put in vibration, by pulling it out of the straight line. SENSE OF HEARING. 269 produced by only 7 or 8 impulses in a second ; and continuous sounds of a still deeper tone could be heard, if the individual pulses were sufficiently prolonged. In regard, however, to the precise time during which a sonorous impression remains upon the ear, it is difficult to procure exact information, since it departs more gradually than do visual impressions from the eye. This is certain, however, that it is much longer than the interval between the successive pulses in the production of tones ; since it was found by M. Savart, that one or even several teeth might be removed from the toothed wheel, without a perceptible break in its sound, showing that, when the tone was once established, the impression of it remained during an intermission of some length. 363. The Ear may, like the Eye, vary considerably, as regards general acuteness, amongst different individuals ; and its power may be much increased by practice. A part of this increase depends, however, as in other instances, upon the greater attention which its fainter indications receive ; but a part, also, upon an increased use of the organ. The power of hearing very faint sounds is as different from the power of distinguishing musical tones, as the power of discerning very minute objects, or of seeing with very faint degrees of light, is from that of distinguishing colours. Many persons are altogether destitute of what is termed a musical ear ; whilst others are endowed with it in a degree, which is a source of great discomfort to them, since every dis- cordant sound is a positive torment. The power of distinguishing the direc- tion of sounds appears to be, in Man at least, for the most part acquired by habit. It is some time before the infant seems to know any thing of the direc- tion of noises which attract his attention. Now although there can be no question that this perception is acquired by attention to certain variations in the impression made upon the nerve, through the medium either of the tym- panic apparatus or of the bones of the head, yet it is equally evident that there can be nothing in these variations themselves adequate to excite the idea, and that it must therefore be either intuitive or acquired by habit. This is a consideration of some importance, in regard to the similar question as to the sense of Visual direction. In some cases we are probably assisted by the relative intensity of the sensations, communicated by the two ears respectively. The idea of the distance of the sonorous body is another acquired perception, depending principally upon the loudness or faintness of the sound, when we have no other indications to guide us. In this respect there is a great simi- larity between the perception of distance of an object, through the Eye, by its size, and through ^he Ear by the intensity of its sound. When we know the size of the object, or are acquainted with the usual intensity of its sound, we can judge of its distance ; and vice versa, when we know its distance, we can at once form an idea of its real from its apparent size, and of its real Strength of tone from that which affects our ears. In this manner, the mind may be affected with corresponding deceptions through both senses ; thus, in the Phantasmagoria, the figure is gradually diminished while its distance re- mains the same, and it appears to the spectators to recede, the illusion being more complete, if its brightness be at the same time diminished ; and the effect of a distant full military band gradually approaching, may be alike given by a corresponding crescendo of concealed instruments. It is upon the com- plete imitation of the conditions, which govern our ideas of the intensity and direction, as well as of the character, of sounds, that the deceptions of the Ventriloquist are founded. 364. Some facts of much interest have lately been ascertained, in regard to an occasional variation in the rapidity of the perception of sensory impres- sions, received through the Eye and through the Ear. These facts are the result of comparisons made amongst different astronomical observers, who may 23* 270 OF MUSCULAR CONTRACTILITY. be watching the same visual phenomena, and timing their observations by the same clock ; for it has been remarked, that some persons see the same phenomenon, a third or even half of a second earlier than others. There is no reason to suppose from this, however, that there is any difference in the rate of transmission of the sensory impressions in the two nerves. The fact seems rather to be, that the sensorium does not readily perceive two different impres- sions with equal distinctness ; and that, when several impressions are made on the nerves at the same time, the mind takes cognizance of one only, or perceives them in succession. When, therefore, both sight and hearing are directed simultaneously to one object, the communication of the impression through one sense will necessarily precede that made by the other. The interval between the' two sensations is greater in some persons than in others ; for some can receive and be conscious of many impressions, seemingly at the same moment ; whilst in others a perceptible space must elapse. 365. Amongst other important offices of the power of Hearing, is that of supplying the sensations by which the Voice is regulated. It is well known that those who are born entirely deaf, are also dumb, that is, destitute of the power of forming articulate sounds ; even though not the least defect exist in their organs of voice. Hence it appears that the vocal muscles can only be guided in their actijn by the sensations received through the Ears, in the same manner as other muscles are guided by the sensations received through themselves ( 399). On this point, more will be said hereafter ( 413). CHAPTER V. OF MUSCULAR CONTRACTILITY. I. Of Contractility in general. 366. THE Nervous System has no power of occasioning movement in any part of the body, save by exciting to contraction certain structures, to which the term Muscular is given. That one tissue should possess within itself the property of contractility on the application of a stimulus, is no more wonderful than that another should be capable of conveying sensory or motor influences, or another of separating a peculiar secretion from the blood. Such contrac- tile tissues are found in Vegetables as well as in Animals ; and instances of their operation have been already referred to ( 13). The only essential difference between the contractility* of Muscular Fibre, and that of the cells of the Sensitive Plant, is that the former can be excited by the stimulus of innervation, as well as by those of a physical or chemical nature, which will act upon the latter. Muscular structure, as heretofore remarked, is employed in * The peculiar operation of this property in Muscular Fibre, the ordinary contraction of which alternates with relaxation, has occasioned the distinctive term Irritability to be applied to it. This term has been employed, however, in so many different 'senses (being, by some Physiologists, used almost synonymously with the more general one of Vitality), that it seems desirable to avoid adopting it for such a purpose. MUSCLES OF ANIMAL LIFE. 271 the Animal body, not only as the instrument of the operation of the Nervous System upon the external world, in which respect alone its action can be said to form part of the Functions of Animal Life, but also to execute many of those interior movements which the peculiar conditions of Animal existence require for its own maintenance, such as the propulsion of the food along the alimentary canal, and that of the blood through the vascular system. The muscles concerned, however, in these operations, which are so immediately connected with the maintenance of the Organic functions, differ essentially from those strictly forming part of the apparatus of Animal life, both in their own structure, and in the manner in which their contractility is called into operation. The former are (like the contractile tissues of Plants) much more susceptible than the latter, of being excited to action by a stimulus immediately applied to themselves, and are with difficulty shown to be in any degree under the influence of nerves ( 201) ; whilst the latter are readily thrown into violent contraction, by a stimulus conveyed to them through the nervous system. Hence a physiological distinction may be made, between these two groups of muscles ; which is fully borne out by differences in the structure and arrangement of their component parts. By some, the two classes have been spoken of as those of Involuntary and Voluntary Muscles ; but this distinction is not correct ; since every muscle ordinarily termed voluntary, is susceptible of being called into action involuntarily. It is better to found the distinction upon their nearer or more remote concern- in the functions of Or- ganic Life : those which are immediately involved in their maintenance, and over which the will can never exert any influence, the Heart, and Muscular coat of the Intestinal Canal, for instance, being designated as the Muscular System of Organic Life ; and those which can be employed by the Nervous System to execute the commands of the Will, being included in the Muscular System of Animal life. The structure peculiar to the latter will first be de- scribed ; as it is evidently that which is most characteristic of Muscle. II. Muscles of Animal Life. 367. When we examine an ordinary Muscle (from one of the extremities, for example), with the naked eye, we observe that it presents a fibrous appearance ; and that the fibres are arranged with great regularity in the direction in which the muscle is to act. Upon further examination it is found, that these fibres are united together in fasciculi or bundles of larger or smaller size, by means of areolar tissue ; ana when the Microscope is applied to the smallest fibre which can be seen with the naked eye, it is seen itself to consist of a fascicu- lus, composed of a number of cylindrical fibres lying in a parallel direction, and closely bound together. These pri- mitive fibres present two sets of markings Fig. 60. or striae ; one set longitudinal, the other transverse or annular. By more closely examining these fibres, when separated from each other, it is frequently seen that each may be resolved into fibrillg, by the splitting of its contents in a longi- tudinal direction, as shown in Fig. 60. These fibrillae have a peculiar beaded ap- pearance, which will be presently noticed more particularly. It not unfrequentlv happens, however, that when a fibre is , the fibres se drawn apart, its contents separate in the bundles of fibriiUe. direction of the transverse striae; forming a 272 OF MUSCULAR CONTRACTILITY, Fig. 61. Portion of Human Muscular Fibre, separating into disks, by cleavage in direction of transverse Striae. After Bowman. series of discs, as shown in Fig. 61 [and Fig. 62]. This cleavage is just as natural as the former, though less frequent ; and it leads us to a view of the composition of Muscular Fibre, somewhat different from the one commonly adopted. To use the words of Mr. Bowman,* it would be as proper to say, "that the fibre is a pile of discs, as that it is. a bundle of fibrillx; but in fact it is neither the one nor the other, but a mass in whose structure there is an intimation of the existence of both, and a tendency to cleave in the two direc- tions. If there were a general disintegration along all the lines in both direc- tions, there would result a series of particles, which may be termed primitive particles or sarcous elements, the union of which constitutes the mass of the [Fig. 62. 7 8 Fragments of Striped Elementary Fibres, showing a cleavage in opposite directions ; magnified 300 dia- meters; 1, longitudinal cleavage ; the longitudinal and transverse lines are both seen; some longitudinal lines are darker and wider than the rest, and are not continuous from end to end ; this results from par- tial separation of the fibrillae; 6, fibrillee. separated from one another by violence at the broken end of the fibre, and marked by transverse lines equal in width to those on the fibre; 7, & represent two appear- ances commonly presented by the separated single fibrillae, (more highly magnified;) at 7 the borders and transverse lines are all perfectly rectilinear, and the included spaces perfectly rectangular; at 8 the borders are scalloped, the spaces bead-like; when most distinct and definite, the fibrilla presents the former of these appearances: 2, transverse cleavage; the longitudinal lines are scarcely visible ; 3, incomplete fracture following the opposite surfaces of a disc, which stretches across the interval and retains the two fragments in connection ; the edge and surface of this disc are seen to be minutely granular, the granules corresponding in size to the thickness of the disc, and to the distance between the faint longitudinal lines; 4, another disc nearly detached; 5, detached disc more highly magnified, showing the sarcous elements.] * See Bowman on the Minute Structure and Movements of Voluntary Muscle, in Phil. Trans. 1840. His description is here followed by the Author, as that most gene- rallyreceived amongst Physiologists. An entirely different account, however, has been given by Dr. Barry (Phil. Trans. 1842). The Author cannot satisfy himself, that either of these explains all the appearances which are presented by this interesting object. MUSCLES OF ANIMAL LIFE. 273 fibre. These elementary particles are arranged and united together in the two directions. All the resulting discs as well as fibrillse are equal to one another in size, and contain an equal number of particles. The same particles compose* both. To detach an entire fibrilla, is to abstract a particle of every disc; and vice versa. 1 '' 368. The elements of Muscular Fibre are bound together, in the perfect condition of the fibre, by a very delicate tubular sheath, which seems to answer to the tube of nervous fibre. This cannot always be readily brought into view ; but it is occasionally seen with great distinctness : thus, when the two ends of a fibre are drawn apart, its contents will sometimes separate with- Fig. 63. out the rupture of the sheath, which then becomes evident; and this, during the act of contraction, may sometimes be observed to rise up in wrinkles upon the surface of the fibre, as spen in Fig. 66. This sheath is quite distinct from the areolar tissue, which binds the ,. .1? i i Fibre of Human Muscle broken across; the frag- fibres mtO fasciculi; and it has been menta connected by the uiitorn sarcolemma. After termed, for the sake of distinction, the Bowman. Sarcolemma. Its existence may be de- monstrated in any muscular fibre, by subjecting it to the action of fluids, which occasion a swelling of its contents ; this is especially the effect of acids and alkalies, and may be well produced by the citric and tartaric acids, and by potash. For a time, the Sarcolemma yields to the distension which takes place from within ; but at last it bursts at particular points, and a sort of hernia of its contents takes place, making the existence of a perfect envelop in all other parts quite evident. This membrane is itself perfectly transparent, and has nothing to do with the production of either the longitudinal or the trans- verse stria3. There is no reason to believe that it is perforated either by nerves or by capillary vessels; in fact it seems to be an effectual barrier between the real elements of Muscular structure, and the surrounding parts. That it has no share in the contraction of the fibre, is evident from the fact just mentioned, respecting the condition which it occasionally presents when the fibre is much shortened. 369. Muscular Fibres are commonly described as cylindrical ; but there is reason to believe that they are rather of a polygonal form, than sides being flattened against those of adjoining fibres. In some instances the angles are sharp and decided; in others they are rounded off, so as to leave spaces between the contiguous fibres for the passage of vessels. In Insects, the fibres often present the form of flattened bands. Their size varies considerably in different classes of animals ; and even in the same animal, and the same mus- cle. The following table gives illustrations of these varieties; the extremes are those met with by Mr. Bowman himself; but other observers speak of dimensions more widely separated. Fractions of an inch. BIRDS Mole Mouse Owl - Chaffinch Heron 274 OF MUSCULAR CONTRACTILITY. REPTILES FISH INSECTS Frog Lizard Boa - Skate Cod - Sprat Staghorn Beetle Blue-bottle Fly to ik cle and the same fasciculus, but even in the same ' bowing St?S 7 ~, . ,.~, r -I i mi n tlie aggregation or bead-like n- fibre m different parts of its length. The figures brilte . After Bowman, indicate the number of striae in TTJ ! jr3 . of an inch. Maximum. Minimum. Mean. Human 15-0 6-0 9-4 Other Mammalia 15-0 6-7 10-9 Birds 14-0 7-0 10-4 Reptiles 20-0 6-7 11-5 Fish 18-0 7-5 11-1 Insects 16-0 4-5 9-5 The extremes in the same specimen, however, were in no instance so widely apart as the table indicates for the Class ; the greatest proportion between the maximum and minimum being, except in Insects, as 2 to 1. 371; The general opinion as to the disposition of the fibres during the con- 276 OF MUSCULAR CONTRACTILITY. traction of muscle, has been that of Prevost and Dumas, who stated that they are thrown into a sinuous or zigzag flexure. Recent observations, however, have fully demonstrated the incorrectness of this view ; the improbability of which might have been suspected from the consideration that fibres in this state of flexure could not be imagined to be exerting any force of traction.* Prof. Owen has noticed that,- in the contracted state of the very transparent muscles of some Entozoa, each separate fibre, which may be seen with great distinctness, presents a knot or swelling in the middle, besides being generally thickened ; but that it is simply shortened without falling out of the straight line. Dr. A. Thomson remarked the same thing in the Frog; single fibres, whilst continuing in contraction, being simply shortened, without falling into zigzag line ; and he was led to suspect, from this and other circumstances, that the zigzag arrangement was not produced until the act of contraction had ceased. The recent inquiries of Mr. Bowman have proved most satisfactorily, that, in a state of contraction, there is an approximation of the transverse strise, and a general shortening of the fibre ; and that its diameter is at the same time increased ; but that it is never thrown out of the straight line, except when it has ceased to contract, and its two extremities are still held in proxi- mity by the contraction of other fibres. The whole process may be distinctly seen under the Microscope in a single fibre isolated from the rest ; it is, of course, desirable to select the specimen from those animals in which the con- tractility of the Muscle is retained for the longest period after death, which is particularly the case in Reptiles among Vertebrata, and in most Invertebrata (Mr. Bowman particularly recommends' the Crab and Lobster) ; but the change has been fully proved to differ in no essential degree, in the warm-blooded Vertebrata. The contraction usually commences at the extremities of the fibre ; but it frequently occurs also at one or more intermediate points. The first appearance is a spot more opaque than the rest, caused by the approxi- mation of a few of the segments of some of the fibrillse: this spot usually extends in a short time through the whole diameter of the fibre, and the shading, caused by the approximation of the transverse striaB, increases in intensity. The strise are found to be two, three, or even four times as nume- rous in the contracted as in the uncontracted part, and are also proportionally narrower and more delicate. The line of demarkation between the contracted and uncontracted portions is well defined ; but, as the process goes on; fresh striae are absorbed (aS-it were) from the latter into the former. The contracted parts augment in thickness; but not in a degree commensurate with its diminished length ; so that its solid parts lie in smaller compass than before, the fluid which previously intervened between them, being -pressed out in bullse under the sarcolemma (Fig. 66). The force with which the elements Fig. 66. Muscular Fibre of Dytiscus, contracted in the centre ; the stria; approximated ; the breadth of the fibre increased ; and the sarcolemma raised in bullee on its surface. After Bowman. * By Prevost and Dumas it was imagined that the muscular fibres themselves were passive agents in contraction; and that the real power was given by an attractive force, analogous to or identical with that of electricity, existing between the nervous fibres, which were stated by them to be disposed in parallel rows, transversely to the direction of the muscle. Other Physiologists, however, have shown that this was a hasty assump- tion ; and it is completely disproved by the numerous facts which prove that muscular contractility does not depend on nervous agency ( 380385). MUSCLES OF ANIMAL LIFE. 277 Muscular fibre of Skate, in a state of rest (1), and in three different stages of contraction (2, 3, 4). After Bowman. of the fibre thus tend to approximate -is Fig. 67. evidently considerable ; for if the two ex- tremities be held apart, the fibre is not unfrequently ruptured. This corresponds with the appearances found in the muscles of persons who have died from tetanus ; for in the ruptured fibres of those muscles, which had been the subjects of the spas- modic action, the striae have been observed to approximate so closely as to be scarcely distinguishable. When the contraction is not very decided, the dark and elevated spot appears to play like a wave along the fibre, before it involves the whole diameter in any part ; and even when considerable traction is being exercised, there is con- tinual interchange in the elements by which it is effected, the disks at one end of the contracted part receding from each other, whilst at the other end new disks are being received into it. 372. The foregoing description is chiefly derived from the appearances- presented by muscular fibre, when spontaneously pass- ing into that state of contraction which is termed the rigor mortis ; and it has not been fully demonstrated that the pheno- mena of contraction, excited by the agency of the nerves, are precisely similar. Mr. Bowman has remarked, however, that stimuli of various kinds, directly applied to them, produce corresponding effects, although, in the case of galvanism, the change is too rapid for its steps to be followed ; and that, from the appearances presented by muscles which have been affected with tetanic spasms, the contraction produced by nervous agency may be inferred to correspond in character. It now remains, therefore, to inquire what is the cause of the zigzag arrangement which is often seen in the fibres. This may be easily produced by approximating the ends of a fasciculus, after the irritability of its fibres has ceased ; and it would not seem unlikely that the passage of vessels or nerves should determine the points; at which the flexures take place. Hence it appears that the sinuous or zigzag arrangement is that into which fibres are naturally thrown, if, on elongation following contraction, they are not at once stretched by antagonist muscles.- Many facts support the opinion, which has long been held by several physi- ologists, that, when an entire muscle is contracting, all its fasciculi are not in contraction at once ; but that there is a continual interchange in the parts by which the tension is effected ; some relaxing, whilst others are shortening. When the ear is applied to a muscle in vigorous action, an exceedingly rapid, faint, silvery vibration is heard ; which seems to be attributable to this constant movement in its substance. Now, on examining a muscle, of which some fasciculi present the zigzag arrangement, others will be seen (if the two ex- tremities have not been purposely approximated) to be quite straight, and in a state of contraction ; and it thence appears, that the former appearance is presented by bundles of fibres, which have either not yet entered into con- traction, or which have relaxed after undergoing it, but of which the extremi- ties are still approximated by the agency of other contracting fibres. From the fact that a single muscular fibre, isolated from all other tissues, can pass 24 278 OF MUSCULAR CONTRACTILITY. into a state of complete contraction, when subjected to excitement of some kind, the very important inference may be drawn, that the property of contractility is inherent in the tissue itself, and is not dependent (as some physiologists have supposed) upon nervous agency, though usually called into action by it in the living body. This inference will be shown to be fully borne out by physiological facts. The result of various experiments made for the purpose, leads to the conclusion, that the total bulk of a muscle in con- traction is not less than when it is in a relaxed state ; or that the difference, if any exist, is extremely trifling. 373. All muscular fibres are attached at their extremities to tissues of the ordinary fibrous character ; and most commonly to that which is known as tendinous structure. The component fibres of this are arranged, with great regularity, parallel to each other ; and they are attached to the end of the sarcolemma, which terminates abruptly ; so that the muscular fibre does not taper to a point, as some have supposed. The line of demarkation between the muscular and tendinous structure, is always very distinct (Fig. 68). Fig. 68. Attachment of Tendon to Muscular Fibre, in Skate. After Bowman. 374. The Sarcolemma of the Muscle, like the tube of the Nerve ( 110), appears to be the part first formed ; being distinctly visible long before any [Fig. 69. Stages of the development of striped Muscular Fibre ; 1, arrangement of the primitive cells in a linear series, after Schwann; 2, the cells united; the nuclei separated, and some broken up; longitudinal lines becoming apparent. from a foetal calf three inches long; 3, 4, transverse stripes apparent; in 3, the nuclei are internal, and bulge the fibre ; in 4, they are prominent on the surface, from a fcetal calf of two months old; 5, transverse stripes, fully formed and dark; nuclei disappearing from view, from the human infant at birth; 6, elementary fibre from the adult, treated with acid, showing the nuclei, magnified about 300 diameters, after Bowman.] MUSCLES OF ORGANIC LIFE. 279 traces of fibrillse can be observed in it. This tube takes its origin, like the ducts of Plants, in cells laid end to end, the cavities of which coalesce, by the disappearance of the partitions, at a subsequent period ; and the nuclei of these original cells may be distinctly seen, for some time after the appearance of the striae, which indicate the formation of the fibrillae in their interior. In an early stage of the development of the fibres, indeed, these bodies project considerably from their sides : in this respect as well as in others, there is a close correspondence between the temporary character of the Muscular fibre of Animal life, and the permanent condition of that of Organic life. In the fully-formed muscle of Animal life, they are not perceptible, except when a peculiar method has been adopted for bringing them into view. This method consists in treating the fibre with weak acids, which render the nuclei more opaque, whilst the surrounding structure becomes more transparent. They are usually numerous in proportion to the size of the fibre ; when the fibre is small, as in Birds or Mammalia, they lie at or near its surface ; but in those of greater bulk, as in Fishes and Reptiles, they are intermingled with the fibrillae through the whole thickness of the fibre, and are brought into view by a transverse section. It would seem probable that these bodies are continually exercising their functions, that of giving origin to new cells, and thence to new muscular tissue ; since their amount is far greater in the adult than in the fetus, their number relatively to the bulk of the fasciculi (which is in the fetus about one-third of that of the adult) being nearly the same at the two periods. In the Larvae of several Insects, perfect and imperfect fibres may often be found lying side by side. III. Muscles of Organic Life. 375. The Muscular fibre of Organic life is very different from that which has been thus fully described. It consists of a series of tubes which do not present transverse striae, and in which the longitudinal striae are very faint ; these tubes are usually much flattened, and cannot be shown to contain dis- tinct fibrillae. Their size is usually muchness than that of the fibres of Animal life ; but, owing to the extreme variation in the flattening which they undergo, it is difficult to make a precise estimate of their dimensions. Those of the alimentary canal of Man are stated by Dr. Baly to measure from about ^ iVo to ??T j-o part of an inch. They sometimes present markings, which indicate [Fig. 70. Another view of the stages of development of Muscular Fibre; 1, a muscular fibre of animal life enclosed in its sheath or myolemma; 2, an ultimate fibril of the same; 3, a more highly magnified view of 1, showing the true nature of the longitudinal striae, as well as the mode of formation of the transverse striae ; the myolemma is here so thin as to permit the ultimate fibrils to be seen through it; 4, a muscular fibre of Organic Life with two of its nuclei; taken from the urinary bladder, and magnified 600 diameters; 5, muscular fibre of organic life from the stomach, magnified the same.] 280 OF MUSCULAR CONTRACTILITY. a granular arrangement in their interior ; and these markings have occasion- ally a degree of regularity which approaches that of the stria? on the Muscular Fibre of Animal life. In most instances, the nuclei of the cells in which they originate, are very perceptible ; and from their similarity to the imperfectly formed fibres of Animal life, it would seem that they are rather to be com- pared with these than with their fibrillaz, to which some have considered them analogous. These fibres are, like those of the other muscles, arranged in a parallel manner into bands or fasciculi ; but these fasciculi are generally inter- woven into a network, not having any fixed points of attachment, but contract- ing against each other. This kind of structure is that which forms the muscular coat of the oesophagus, stomach, [Fig. 70, (4 and 5,)] intestinal tube, bladder, and pregnant uterus ; it is found, also, in no inconsiderable amount, in the trachea and bronchial tubes. The pharyngeal muscles, however, belong to the former system.* The fibres of the Uterus somewhat differ in aspect from those of other parts ; being much broader at their centre, and tapering off at their extremities into what appear to be cylindrical parts. In the Heart, both the striated and non-striated muscular fibres are found ; and this accords with the structure of the organ, which affords some fixed points, whilst much of its action resembles (except in its greater degree of vigour) that of the muscular coat of the intestines. It is a curious and interesting fact, that in Articulata, whose animal life is so predominant ( 22), the Animal muscular fibre is formed as perfectly as in the highest Vertebrata ; whilst in Mollusca, whose character is exactly the reverse ( 28), scarcely any striated fibres can be detected. It seems probable that the contractility possessed by the skin (which gives rise to the state termed culis anserina, under the influence of cold, or of depressing mental emotions), and that which is peculiar to the Dartos (by which the scrotum is thrown into wrinkles), are alike due to the action of fibres of this description, intermingled with the other fibrous tissues of which these parts are chiefly composed. The middle coat of the Arteries will be hereafter shown to have a strong analogy to this form of muscular tissue, both in structure and properties (Chap. ix). IV. Properties of Muscular Fibre. 376. Muscles may be thrown into contraction, so long as they preserve their vitality, by stimuli applied to themselves. Mechanical and chemical influences, cold, heat, and electricity, produce this effect. They do not lose their vitality immediately on the general death of the system, which must be considered as taking place when the circulation ceases without a power of renewal ; in cold-blooded animals it is retained much longer after this period than in the higher Vertebrata, in some of which it disappears within an hour. From experiments on the bodies of executed criminals, who were previously in good health, Nysten ascertained that in the Human subject, the contractility of the several muscular structures departs in the following time and order. The left ventricle of the heart first ; the intestinal canal at the end of 45 or 55 minutes ; the urinary bladder nearly at the same time ; the right ventricle after the lapse of an hour ; the oesophagus at the expiration of an hour and a half; the iris a quarter of an hour later ; the muscles of Animal life somewhat later ; and lastly, the auricles of the heart, especially the right, which in one instance contracted under the influence of galvanism 16| hours after death. The muscles of young animals generally retain their contractility for a longer * The distinctness between the two is remarkably shown in bodies infested with the Trichina spiralis, which, whilst it profusely infests the foraier, is seldom or never found in the latter; so that there is a definite line of demarkation, even in closely contiguous parts, such as at the lower edge of the inferior constrictor of the pharynx. PROPERTIES OF MUSCULAR FIBRE. 281 time than those of adults ; on the other hand, those of Birds lose their con- tractility sooner than those of Mammalia. Hence, as a general rule, the dura- tion of the contractility is inversely to the amount of respiration. Muscular contractility is deadened by many substances, especially by those which have a narcotic or sedative action on the nervous s^tem. In carbonic acid gas, hydrogen, carbonic oxide, or sulphurous acid gas, muscles contract very feebly, or not at all, when stimulated ; whilst in oxygen they retain their contractility longer than usual. Narcotic substances, such as a watery solution of opium, when applied directly to the muscles, have an immediate and powerful effect in diminishing or even destroying their contractility ; this effect is also pro- duced, though in a less powerful degree, by injecting these substances into the blood. In the same manner, venous blood, charged with carbonic acid, and deficient in oxygen, has the effect of a poison upon muscles ; diminishing their contractility, when it continues to circulate through them, to such a degree, that they sometimes lose it almost as soon as the circulation ceases, as is seen in those who have died from gradual and therefore prolonged Asphyxia. The unfavourable influence of venous blood is also shown in the Morbus Cceruleus ; patients affected with which are incapable of any considerable muscular exertion. Most of the stimuli which occasion muscular contraction, when directly applied to their fibres, operate also when applied to their motor nerves ; but the same does not hold good in regard to those agents which diminish contractility. It is a fact of some importance, in relation to the dis- puted question of the connection of muscular contractility with the nervous system, that when, by the application of narcotic substances to the nerves, their vital properties are destroyed, the contractility of the muscle may remain for some time longer ; and the latter must, therefore, be independent of the former. Hence we should conclude, that contractility must be a property really inherent in Muscular tissue, which may be called into action by various stimuli, and which may be weakened by various depressing agents, applied to itself ; and that the nerves have the power of conveying the stimuli, which call the property into action, but have little or no other influence on it. 377. It seems to be a general law of Muscular Contraction, that it shall alter- nate with relaxation at no long intervals. This is most evident in the action of the Heart, and in the peristaltic movements of the Alimentary canal. In these parts, the whole or a large part of the fibres seem to contract together, and then shortly to relax ; but this is probably no less true, as has been just shown, of the individual fibres of those muscles which are kept in a state of contraction by an effort of the will ; since none of them appear to remain in a contracted state for any length of time. The peculiar contractility of Muscular tissue, like the vital properties of other parts, is diminished by want of action ; and in this, as in other cases, it is quite clear that the cause of its loss is to be found in the alteration of the nutritive processes, which results from the ces- sation of the usual operations of the part ( 221). In persons whose lower extremities have, from any cause, been long disused, not only does the bulk of the muscles much diminish, but their characteristic structure in great part disappears, degenerating into fat mixed with ordinary fibrous tissue. On the other hand, a frequently-renewed exercise of muscular contractility increases the power, by stimulating the increased nutrition of the muscles, which be- come more developed, and consequently more powerful; this is welt seen in the arms of the Smith or Waterman, and in the legs of the Opera-dancer. But the exercise must not be too constant ; for it appears to be during the intervals of rest, that the increased nutrition chiefly takes place ; and if the action have been of a violent character, the contractility of the muscle is for a time exhausted, and can only be restored by an interval of inactivity. A great variety of evidence has been, for some time, conducting physiologists to the 24* 282 OF MUSCULAR CONTRACTILITY. opinion, that every act of Muscular contraction necessarily involves the death and disintegration of a certain amount of Muscular Tissue ( 77) ; and it has been recently argued by Liebig, that this disintegration, resulting from the action of Oxygen upon the elements of which the tissue is composed, is the real source of the mechanical power ; by setting at liberty (so to speak) the Vital Force, which was previously employed, in a latent manner, in holding together the components of the structure. Certain it is, that the amount of Muscular power exercised by an animal, bears a very close correspondence (other things being equal), on the one hand to the measure of oxygen intro- duced into the system by the lungs, and on the other to the amount of those excretions, which seem especially produced by this metamorphosis : and this is true, as a general fact, whether we compare together different animals, or different states of the same animal (See Chap. vm. Sect. i). 378. The effects of stimuli locally applied to portions of the Muscles of Animal Life, are very different from those which result from their application to the muscles of Organic life. If, for example, we irritate mechanically a portion of the Biceps, the fasiculus of fibres which is touched will contract, but the surrounding parts will be unaffected, and the contracted fasciculus will soon relax ; in fact, the only way to call the whole muscle into contrac- tion at once, is to stimulate it through its nerves. On the other hand, if we apply a similar irritation to the intestinal canal, when in a state of equal con- tractility, the fasciculus which is stimulated shortens in a much greater degree ; and propagates its action in a wave-like manner to other bundles of fibres; so that successive contractions and relaxations may be produced, through a considerable part of the canal, by a single prick with the point of a scalpel ; but the contractions into which these same fibres are thrown, by irri- tating their nerves, are for the most part feeble and undecided ( 200). It is, indeed, a curious fact, corroborative of what has been just said of the influence of narcotics, that the ganglionic nerves lose their power of exciting these muscles to contraction, when themselves irritated, much sooner than the muscles lose their power of contraction, when directly stimulated. 379. There can be no doubt that it is through the motor or efferent nerves, that contraction is ordinarily excited in the muscles of the first class, in the living body ; and these nerves may, as formerly shown, convey the influence of volition, of emotional or instinctive operations of mind, or of the reflex action of the Spinal Cord. As the effect produced upon the muscle is in all instances similar, there can be little doubt that the stimulus actually commu- Fig. 71. Form of the terminating loops of the Nerves in the Muscles. After Burdach. PROPERTIES OF MUSCULAR FIBRE. 283 nicated by the nerve is of the same character, whatever may have been its source. The motor nerves cannot be properly said to terminate in the muscles ; for the trunks form a kind of network in their substance, the fibres which they send off returning again to themselves, by loops, or to other trunks. In what manner the stimulus is conveyed and communicated, can only be at present a matter of speculation. That the influence is of an electrical kind, has been supposed by some ; principally on account of the similarity between the muscular contractions, excited by galvanism transmitted through the nerves, and those ordinarily produced by voluntary direction. But it is to be remembered that other agents, both physical and chemical, may produce the same effect ; and there are objections, which at present appear insuperable, to the belief that nervous influence and electricity are identical, whatever may be the analogy in their mode of operation. The muscles of the second class appear to be, in the living body, much seldomer called into contraction through their nerves, than they are by stimuli applied directly to themselves. The will has no power over them ; and they would seem to be rather affected by those emotional conditions of mind which volition cannot imitate. This influence is continually experienced in the action of the heart, and probably also affects the movements of the intestinal tube. 380. The continual and evident influence of the Nervous System upon Muscular Contractility, has led many physiologists to the belief, that the latter is dependent upon the agency of the former. Two views upon this question have been commonly taught, to both of which it seems necessary to devote a brief consideration. The first of these is, that Muscular contractility is de- rived from some influence or energy communicated from the Brain or Spinal Cord. This opinion is evidently analogous to that which attributes the vital properties of other parts to the Nervous System alone ; and it is open to the same objection, in limine, which has been applied to the latter, the improba- bility that any one of the solid textures of the living body should have for its office to give to any other the pow-er of performing any vital action. More- over, it is inconsistent with the fact that, in Vegetables, tissues endowed with a high degree of contractility exist, and manifest their property when a stimulus is directly applied to themselves ; which, nevertheless, can have no depend- ence whatever upon a nervous system. In the lower classes of Animals, too, there is good reason to believe that the property is much more universally diffused through their tissues than nervous agency can be. Again, the action of the heart may be kept up, in the highest animals, by taking care that the current of the circulation be not interrupted, for a long time after the removal of the brain and spinal cord ; it may even continue when com- pletely separated from the body, which shows that the ganglionic system cannot supply any influence necessary to it ; and there are many instances in which the human foetus has come to its full size, so that its heart must have regularly acted, without the existence of a brain or spinal cord. Further, the irritability of muscles of the first class continues for a long time after their nerves are divided, and may be called into action by stimuli directly applied to the parts themselves, or to their nerves below the section, so long as their nutrition is unimpaired. 381. The loss of the irritability of Muscles, within a few weeks after the section of their nerves, on which great stress has been laid by Miiller in support of a modified form of the above doctrine, (it being maintained by this distinguished physiologist, that, if muscular irritability is not dependent on the Brain and Spinal Cord, they supply some influence essential to its exercise,) is clearly due to the alteration in their nutrition, consequent upon their disuse. This has been recently proved to demonstration, by the very inge- 284 OF MUSCULAR CONTRACTILITY. nious experiments of Dr. J. Reid.* " The spinal nerves were cut across, as *they lie in the lower part of the spinal canal, in four frogs ; and both posterior extremities were thus insulated from their nervous connections with the spinal cord. The muscles of one of the paralyzed limbs were daily exercised by a weak galvanic battery ; while those of the other limb were allowed to remain quiescent. This was continued for two months ; and at the end of that time, the muscles of the exercised limb retained their original size and firmness and contracted vigorously, while those of the quiescent limb had shrunk to at least one-half of their former bulk, and presented a marked contrast with those of the exercised limb. The muscles of the quiescent limb still retained their contractility, even at the end of two months : but there can be little doubt that, from their imperfect nutrition, and the progressing changes in their physical structure, this would in no long time have disappeared, had circumstances permitted the prolongation of the experiment. "t This experiment satisfactorily explains the fact observed by Dr. M.'Hall, and heretofore adverted to, ( 177, 208,) that in cases in which the cause of the paralysis is situated in the Brain, and in which the Spinal Cord and its nerves are unaffected, the irritability of the muscles of the paralyzed part is not destroyed even after a considerable lapse of time. For, if the capability of performing reflex actions still exist, on the part of the nervous system, it is manifest that the muscles will be con- tinually excited to action through this channel ; and that their nutrition and vital properties will thereby be preserved, as they were in Dr. Reid's experi- ments by the artificial excitement of galvanism. Hence Dr. M. Hall's opinion, that the property of Muscular contractility is derived from the Spinal Cord is no more tenable than that which locates it in the Brain. 382. The loss of contractility from section of the nerves, takes place more speedily in warm-blooded Vertebrata, all whose vital operations are performed with a much greater activity than in Reptiles and other cold-blooded animals. Dr. Reid found that, in a Rabbit, a portion of whose sciatic nerve had been removed on one side, the muscles of that leg were but very feebly excited to contraction by Galvanism, after the lapse of seven weeks. The change in their nutrition was evident to the eye, and was made equally apparent by the balance. The muscles of the paralyzed limb were much smaller, paler and softer than the corresponding muscles of the opposite leg ; and they scarcely weighed more than half, being only 170 grains, whilst the others were 327 grains. It was found, also, that a perceptible difference existed in the size of the bones of the leg, even after so short an interval had elapsed ; the tibia and fibula of the paralyzed limb weighing only 81 grains, whilst those of the sound limb weighed 89 grains. On examining the muscular fibres with the micro- scope, it was found that those of the paralyzed leg were considerably smaller than those of the sound limb, and presented a somewhat shriveled appearance ; and that the longitudinal and transverse striae were much less distinct. 383. Another equally satisfactory proof, that the loss of Contractility, which follows the severance of the connection between the Nervous centres and the Muscle, is not immediately due to the interruption of any influence communi- cated by the former, has been given by the experiments of Dr. J. Reid. It * Edinburgh Monthly Journal of Medical Science, May, 1841. f A fact of an exactly parallel character has fallen under the Author's observation, in a case ol Hysteric Paraplegia, in which one leg was occasionally affected with severe cramps. The muscles of this leg suffered much less diminution of size and firmness than those of the other; so that there was a difference of more than an inch in the cir- cumference of the limbs. But since the paraplegia has been partially recovered from, some degree of voluntary power having been established in both limbs, and the muscles of both having been exercised in the same degree, they have greatly improved in' size and firmness, and there is now little or no perceptible difference between them. PROPERTIES OF MUSCULAR FIBRE. 285 was asserted by Mr. Earle (and the statement has been repeated by Mil Her) that, if the irritability of a muscle, whose nerves have been divided, be exhausted by repeated stimulation, it cannot be recovered. Dr. J. Reid has shown, however, that the means employed by Mr. E. to exhaust the irrita- bility were such as would probably induce an inflammatory condition of the muscles, and would thereby interfere with the nutritive processes, which would be necessary to re-establish the irritability during the state of subse- quent quiescence. And he has further proved, that if the contractility be exhausted by means which have no such unfavourable tendency, and the other conditions favour the normal performance of the nutrient processes, the irritability is restored, and remains for some time. His first experiments were on cold-blooded animals, and they would in themselves be sufficiently satisfac- tory ; but he has since repeated them in the Rabbit, and established the fact beyond all doubt.* " The sciatic nerve was divided in the Rabbit, and a por- tion of it removed. One wire from two galvanic batteries, consisting of thirty pairs of plates, was applied over the course of the nerve ; and the other wire was applied over the foot, which was kept moist, until the muscles had ceased to contract. Three days after this, a weaker battery was used, and the mus- cles of the limb had recovered their contractility, and contracted powerfully. The more powerful battery was used as before, until the muscles had ceased to respond to the excitement ; and three days after this, they had again recov- ered their contractility." It seems scarcely possible to draw any other infer- ence from these experiments than that Contractility is a property inherent in Muscular tissue, and that the agency of the Nervous system upon it is merely to call it into active operation. 384. The second doctrine above referred to ( 380), as having been taught by some physiologists, is that Muscles, though not dependent on nerves for vital power, are yet dependent upon them for the exercise of that power, all stimuli, which excite muscles to contraction, operating first on the nervous filaments which enter muscles, and through them on the muscular fibres. The facts which have been already stated, in regard to the ordinary action of the Muscles of Organic life, furnish a sufficient answer to this hypothesis. It is with great difficulty that these can be made to display their irritability, by any stimuli applied to their nerves ; whilst they manifest it strongly, when the stimulus is directly applied to themselves. Even in the Muscles of Animal life, individual fasciculi may be thrown into action in the same manner; although the entire mass cannot be put into combined operation, except by a stimulus simultaneously communicated to the whole, which the nerve affords the readiest means of effecting. Perhaps the most satisfactory disproof of it, however, is to be found in the observation of Mr. Bowman, already cited ( 371), that a single fibre, completely isolated from all its connections, may be seen with the microscope to pass into a state of contraction, under the influ- ence of direct irritation. Further, it has been experimentally ascertained, that there are some chemical stimuli, which will produce the contraction of muscles when directly applied to them, but of which the influence cannot be transmitted through the nerves ; this is especially the case with regard to acids. 385. When all these considerations are allowed their due weight, we can scarcely do otherwise than acquiesce fully in the doctrine of Haller, which involves no hypothesis, and which is perfectly conformable to the analogy of other departments of Physiology. He regarded every part of the body which is endowed with Irritability, as possessing that property in and by itself; but considered that the property is subjected to excitement and control from the * Loc. cit. 286 OF MUSCULAR CONTRACTILITY. Nervous System, the agency of which is one of the stimuli that can call it into operation. It may be desirable briefly to recapitulate the facts by which this doqtrine is supported. 1 . The existence in Vegetables of irritable tissues, which are excited to contraction by stimuli directly applied to themselves, and can be in no way dependent upon, or influenced by, a Nervous system. 2. The existence in Animals of a form of Muscular tissue, which is especially con- nected with the maintenance of the Organic functions, and which is much more readily excited to action by direct stimulation than it is by nervous agency. 3. The fact that, by the agency of these, the organic functions may go on (as long as their other requisite conditions are supplied) after the removal of the nervous centres, and when none were ever present ; rendering it next to certain, that their ordinary operations are not dependent upon any stimuli received through the nerves, but upon those directly applied to themselves. 4. The persistence of irritability in muscles, for some time after the nerves have ceased to be able to convey to them the effects of stimuli ; this is con- stantly seen in regard to the Sympathetic system of nerves, and the muscles of Organic life upon which they operate; and it may also be shown to occur with respect to the Cerebro-Spinal system, and the muscles of Animal life, by the agency of narcotics. 5. The persistence of irritability in the muscles, after their complete isolation from the nervous centres, so long as their nutri- tion is unimpaired; and the effects of frequent exercise, in preventing the impairment of the nutrition and the loss of irritability. 6. The recovery of the irritability of muscles, when isolated from the nervous centres, after it has been exhausted by repeated stimulation ; this also depends upon the healthy performance of the nutritive actions. 7. The contraction of muscular fibre under the microscope, when completely isolated from all other tissues. In the words of Dr. Alison, then, " the only ascertained final cause of all endow- ments bestowed on Nerves in relation to Muscles, in the living body, appears to b.e, not to make Muscles irritable, but to subject their irritability, in different ways, to the dominion of the acts and feelings of the Mind," to its volitions, emotions, and instinctive determinations. 386. There can be no doubt, however, that the Nervous System is capable of exerting an influence upon the property itself ; for we find that sudden and severe injuries of the Nervous Centres have power to impair, directly and instantaneously, or even to destroy the Contractility of the whole Muscular System ; so that death immediately results, and no irritability subsequently remains. It is in this manner, that the sudden destruction of the Brain and Spinal Cord, especially of the latter, occasions the immediate cessation of the heart's action ; though they may be gradually removed, without any consider- able effect upon it. Severe concussion has the same effect ; hence the Syn- cope which immediately displays itself. It is sometimes an important question in Forensic Medicine, whether an individual, who has died from the effects of a blow upon the head, could have moved from the place where the blow was inflicted. If there be found, as is frequently the case, no sensible disorgan- ization of the Brain, the death must be attributed to the concussion, and must have been in that case immediate. If, on the other hand, effusion of blood has taken place within the cranium to any considerable extent, it is probable that the first effects of the blow were in some degree recovered from, and that the circulation was re-established. It is not essential, however, that the im- pression should be primarily made upon the Cerebro-Spinal system. The well-known fact of sudden death not unfrequently resulting from a blow on the stomach, especially after a full meal, without any perceptible lesion of the viscera, clearly indicates that an impression upon the widely-spread coeliac plexus of Sympathetic nerves (which will be much more extensively commu- nicated to them, when the stomach is full, than when it is empty), may cause PROPERTIES OF MUSCULAR FIBRE. 287 the immediate cessation of the heart's action, in the same manner as a violent injury of the Brain or Spinal Cord. Now it is interesting to remark that, in all these cases, the whole vitality of the system appears to be destroyed at once ; for the processes which would otherwise succeed the injury, and which, after other kinds of death less sudden in their character, produce evident changes in the part of the surface that has immediately received it, are here entirely prevented. An instance is on record, in which a criminal under sentence of death determined to anticipate the law by self-destruction. Having no other means of accomplishing his purpose, he stooped his head and ran violently against the wall of his cell ; he immediately fell dead ; and no mark of contusion showed itself on his forehead. The same absence of the usual results is to be noticed, in the case of blows on the stomach. Yet it is well known, that many of the ordinary vital processes will take place in the injured parts, after death of a more lingering nature ; the vitality of the individual organs not being destroyed, immediately on the severance of the chain which binds together the different functions. 387. The influence of severe impressions on the Nervous System in dimi- nishing, where it does not altogether destroy, Muscular Contractility, is well seen in the effect of severe injuries affecting vital organs, or extending over a large part of the surface, in depressing the heart's action. This is a well- known result of severe burns, especially in children, whose nervous system is more susceptible of such impressions than that of the adult ; also of the rupture of the alimentary canal, of the bladder or uterus ; and of the shattering of one of the extremities, by violence affecting a large part of their substance. In all these cases, the sufferer is in the same condition with one who has received a severe blow on the head, that does not quite stun him; the shock mime* diately diminishes the muscular contractility of the whole system ; and its influence on the heart, which of course manifests itself most conspicuously, produces a degree of depression, which is frequently never recovered from, and which at any rate renders necessary the employment of stimulants, for the purpose of counteracting this very dangerous effect.* Excessive mental emotion, of a kind not in itself depressing, may occasion the sudden cessation of the heart's action, and a general loss of muscular contractility ; and it is well known that muscular power is greatly diminished by emotions which produce no other direct action. 388. There is no evidence that Muscular Irritability can be increased by any cause operating through the nervous system. It is quite true that, under the stimulus of alcohol, nitrous oxide, &c., or of some purely mental excite- ment, individuals can perform actions requiring a degree of strength, which they cannot exert under any other circumstances. But it does not hence follow, that the irritability is increased ; since the energy of the action may be due solely to the power of the stimulus, by which it is excited. It is well known that stimulating agents, which thus temporarily increase Muscular power, primarily excite the nervous system ; as is shown by the increased mental activity, which results from the moderate use of alcohol, nitrous oxide, opium, &c. ; and it does not seem necessary, therefore, to go further in search * The large quantity of stimulus which can be borne even by children, suffering under severe burns, is very extraordinary. There can be no doubt that many lives have been saved by the judicious administration of them, to an amount which would, a priori, have been judged in itself fatal ; but that. many more have been sacrificed to neglect, even on the part of those whose duty it is to watch the indications with the closest attention. The Author's, observation leads him to believe, that Hospital-Nurses very commonly make up their minds that children, who have met with severe burns, must die ; and that, unless closely watched, they neglect the means of which Science and Experience alike dictate the free employment. 288 OF MUSCULAR CONTRACTILITY. of an explanation of their effect on muscular action. It is worthy of remark, that whilst the influence of general depressing causes acting through the Nervous System, is primarily manifested on the muscles of Organic life, that of stimulants chiefly shows itself in the muscles subjected to the will. 389. The last general question now to be considered regarding Muscular Contractility, is that which relates to the Rigor Mortis. The temporary stif- fening of the body after death, from a general contraction of its muscles, is a phenomenon which is rarely absent ; though it may be so slight, and may last for so short a time, as to escape observation. The period which elapses before its commencement, is as variable as its duration ; and both appear to be in some degree dependent upon the vital condition of the body at the time of death. When the fatal termination has supervened on slow and wasting disease, occa- sioning great general depression of the vital powers, the rigidity usually develops itself very early, and lasts for a short time. In diseases which pow- erfully affect the nervous energy, such as Typhus, this is the case ; even though they have not been of long duration. Thus, after death from Typhus, the limbs have been sometimes known to stiffen within 15 or 20 minutest The same is observed in infants and in old people. On the other hand, where the general energy has been retained up to a short period before death, the rigidity is much later in coming on, and lasts longer ; this happens, for example, in many cases of Asphyxia and Poisoning, in which it has been said not to occur at all. The commencement of the rigidity, however, is not usually prolonged much beyond seven hours ; but twenty or even thirty hours may elapse before it shows itself. Its general duration is from twenty-four to thirty-six hours ; but it may pass off much more rapidly, or it may be prolonged through seve- ral days. An attempt has been made to connect it with the lowering of the temperature of the dead body ; but with this it does not seem to have any rela- tion. It occurs in cold-blooded Vertebrata, and even in Invertebrata, as well as in warm-blooded animals; and it has frequently been noticed to commence in the latter Jong before the heat has entirely departed from the body. Moreover, it appears first upon the trunk, which is the region last deserted by the caloric. It first affects the neck and lower jaw, and seems gradually to travel down- wards; but, according to some observers, the lower extremities are stiffened before the upper. In its departure, which is immediately followed by decom- position, the same order is observed. It affects all the muscles nearly alike ; but the flexors are usually more contracted than the extensors, so that the fingers are somewhat flexed on the palm, and the forearm on the arm; and the lower jaw, if previously drooping, is commonly drawn firmly against the upper. It is remarkable that it is equally intense in muscles which have been para- lyzed by Hemiplegia, provided that no considerable change has taken place in their nutrition. 390. The Muscular contraction which gives rise to the Rigor Mortis, appears to be of the same kind with that which takes place under the influence of nervous agency, though differing as to its conditions. When very strong, it renders the muscles prominent, as in voluntary contraction; and the compara- tive observations of Mr. Bowman, upon the state of muscular fibre passing into this condition, and upon that which presented various degrees of contrac- tion from ordinary causes, leave no doubt as to their correspondence. It is to be remarked, however, that the tendency of the muscle to contract upon the ordinary stimuli, appears to be almost invariably lost, or greatly diminished, before the rigor mortis commences. This statement holds good in regard to animals of different classes, as well as with respect to Man under various con- Ihus, in Birds whose muscles most speedily lose their contractility, the cadaveric rigidity is most quickly exhibited; whilst in Reptiles it is much longer m commencing, the irritability of the muscles being more persistent. PROPERTIES OF MUSCULAR FIBRE. 289 An attempt has been made to show a correspondence^ between the rigor mortis and the coagulation of the blood in the vessels ; and there is certainly evidence enough to make it appear that some analogy exists between these two actions, though they are far from being identical. After those forms of death, in which the blood does not coagulate, or coagulates feebly, the rigidity commonly manifests itself least ; but this is by no means an invariable rule. It seems probable that, as the coagulation of the blood will be shown to be the last act of its vitality, so the stiffening of the muscles is the expiring effort of theirs. The property to which it is due, however, would appear to be of a different character from ordinary irritability. This may be inferred from the fact, that the rigidity does not ordinarily come on until after the contractility has departed sometimes for a considerable period ; and also from the circum- stance, observable in most cases of Asphyxia, that the rigidity is very decided and prolonged, whilst the contractility is speedily lost. This property, to which the name of Tonicity has been given, is probably the same as that which partly occasions the retraction of the muscle when divided during life ; the degree of this retraction being much greater than that seen in a muscle which has been for some time dead. Moreover, this kind of tonic contraction is more directly excited by heat than that which results from ordinary con- tractility ; and it is not excited by galvanism or mechanical irritation, which so powerfully act on the latter. It is interesting, moreover, to remark, that the state of habitual preponderance during sleep, of the flexor over the extensor muscles (which last are the stronger), is explicable by attributing it to the same property ; the manifestations of which thus correspond, whether the con- tractility of the muscles be in a dormant or unexcited state, or whether it have altogether departed from the tissue. 391. It is necessary to bear in mind, when the phenomena of cadaveric rigidity are brought into question in juridical investigations, that a state at first sight corresponding to it may supervene immediately upon death, from some peculiar condition of the nervous and muscular systems at the moment. This has been observed in some cases of Asphyxia ; but chiefly when death has resulted from apoplexy following chronic ramollissement of the brain or spinal cord. This contraction, which is obviously of a tetanic character, ceases after a few hours, and is then succeeded by a state of flexibility, after which the ordinary rigidity supervenes. The following case illustrates the nature of the inquiries, to which this condition may give rise.* The body of a man was found 1 in a ditch, with the trunk and limbs in such a relative posi- tion, as could only be maintained by the stiffness of the articulations. This stiffness must have come on at the very moment when the body took that posi- tion ; unless it could be imagined that the body had been supported by the alleged murderers, until the joints were locked by cadaveric stiffness. A post-mortem explanation showed, that there was no necessity for this supposi- tion, obviously a very improbable one in itself; by affording sufficient evi- dence that apoplexy, resulting from chronic disease, was the cause of death. A case occurred a few years since in Scotland, in which the same plea was raised. The body was found in a position in which it could have only been retained by rigidity of the joints ; and it was pleaded, on the part of the pri- soner, that death had been natural, and had resulted from fracture of the pro- cessus dentatus, causing sudden pressure upon the spinal cord, whence the spasmodic rigidity would naturally result. Proof was deficient, however, as to the existence of this lesion before death ; and the position of the body rather resembled that into which it might have been forced during the rigidity, than that in which it would probably have been at the moment of death. There * Annales d'Hygiene, torn. vii. 25 290 OF MUSCULAR CONTRACTILITY. were also marks of violence, and many other suspicious circumstances ; but the prisoner Avas acquitted, chiefly from want of evidence against him. What seemed to indicate that the rigidity was of the ordinary cadaveric nature, was, that there was no evidence of the body having become flexible and again stiffened ; as it would probably have done, had the rigidity been of the spas- modic character. V. Energy and Rapidity of Muscular Contraction. 392. There can be no question that, in the living body, the energy of Mus- cular contraction is determined (other things being equal), by the supply of arterial blood, which the muscle receives. It is well known that, when a ligature is applied to a large arterial trunk in the Human subject, there is not only a deficiency of sensibility in the surface, but also a partial or complete suspension of muscular power, until the collateral circulation is established. It is evident, however, that a portion of this effect is to be ascribed to the interruption in the functions of the nervous trunks, which is due to the same cause ; since muscles, taken from the human body, after the circulation has entirely ceased, retain their contractility for some time. The influence of this supply of arterial blood is twofold ; it supplies the materials for the nutrition of the tissue ; and it furnishes (what is perhaps more immediately necessary) the supply of oxygen required for that metamorphosis of the tissue which is probably an essential condition of the generation of its contractile force ( 377). As this oxygen is taken in through the lungs, and as the greater part of it is thrown off when united with carbon into carbonic acid, by the same channel, we should expect to find a very close correspondence between the amount of muscular power developed in an animal, and the quantity of oxygen consumed in its Respiration : and this is in reality the case. We find, for example, that in Birds and Insects, whose respiration is the highest, the muscular power is greater in proportion to their size than in any other animals. In the Mam- malia, and certain Fishes that might be almost called warm-blooded, it is only in a degree inferior. But in the cold-blooded reptiles, Fishes and Mollusca, the muscular power is comparatively feeble ; though even here we trace gra- dations, which accord well with the relative quantities of oxygen consumed. But in proportion to the feebleness of the power, do we usually find its dura- tion greater ( 376) ; so that it is not so immediately dependent upon the supply of oxygen, in cold-blooded, as in warm-blooded animals. Thus, it is found that Frogs are still capable of voluntary movement, after the heart has been cut out ; they can move limbs which are connected with the trunk by . the nerves alone : and that this power is not altogether due to the blood which may remain in the capillary vessels, is shown by the experiment of Miiller, who found the muscles still contractile, after he had expelled all the blood, by forcing a current of water into an artery, until it escaped from the divided veins. It seems probable that the Muscles of Organic life are less dependent upon a supply of arterialized blood, than are those of Animal life ; for the Heart will continue to contract, when the blood in its vessels is entirely venous, and when the circulation in it has come to a stand. Still the dependence of its action upon a constant supply of arterial blood, is very close ; and in all animals, however different the plans of their circulation, we find a provision for this supply, by a special arrangement of the coronary arteries.* That the heart's action comes to an end much sooner, after the destruction of animal life by pithing, when the coronary arteries have been tied, than when they are left untouched, has been proved by the experiments of Mr. Erichsen.t * Dr. M. Hall's Gulstonian Lectures, pp. 23, 24. f Medical Gazette, July 8, 1842. ENERGY AND RAPIDITY OF MUSCULAR CONTRACTION. 291 In an animal that has been pithed, but whose heart has been left intact, arti- ficial respiration will easily keep up its action for an hour, or an hour and a half. But when the coronary arteries were tied, a mean of six experiments give a duration, for the ventricular action, of only 23 minutes after the liga- tures were applied, and 32 after the pithing; and in no instance was it prolonged more than 31 minutes after the application of the ligature, or 37 minutes after the pithing. On the other hand, when the aorta was tied, so that the coronary arteries were distended with blood, the circulation being carried on through them alone, the right ventricle continued to act up to the 82d minute. 393. There is a remarkable difference in the degree of irritability in the two sides of the heart, to which Dr. M. Hall has directed attention. In the warm-blooded Vertebrata, the right side of the heart will act on the stimulus of venous blood ; whilst the left side requires the stimulus of arterial. In Fishes, on the other hand, whose heart corresponds to the right side only of that of Man, the whole is put in action by venous blood. In Reptiles, one auricle is sufficiently stimulated by venous blood, whilst the other requires arterial ; and the ventricle is excited to action by a mixed fluid. In all these cases, there must be a marked difference in the properties of the several parts ; some being sufficiently affected by a stimulus, which is totally inoperative on others. This is still more remarkably exemplified by the fact, that the mus- cular fibre of Frogs would be thrown into a state of permanent and rigid contraction (through the powerful operation of its property of Tonicity), by the stimulus of a fluid no hotter than the blood which ordinarily bathes the muscles of Birds. Now in those warm-blooded animals which pass the winter in a state of torpidity, the respiration is very slow and imperfect, and the blood is very imperfectly arterialized. There must, therefore, be a change in the properties of the left ventricle, by which it becomes capable of action on a more feeble stimulus, thus resembling the ventricle of Reptiles. This change Dr. M. Hall designates as an increase of Irritability ; considering that, if mus- cular action be excited by a more feeble stimulus, the property to which that action is due, must be itself more exalted. Physiologists have been so long accustomed, however, to consider the irritability of the muscles in warm-blooded animals as greater than that of cold-blooded, on account of the greater energy and rapidity of their contractions when excited, that it seems undesirable to modify the term in the manner proposed by Dr. Hall. No one will assert that the vitality of the Muscle is exalted, when it is reduced to the condition of that of the Reptile ; and, as Irritability is strictly a vital property, it cannot be correctly spoken of in that manner. The general principle, however, laid down by Dr. M. Hall, that the facility with whic.h the muscular system may be excited to contraction, or, in other words, the feebleness of the stimulus required for the purpose, is inversely as the respiration of the animal, is, no doubt, generally correct. 394. A curious question has been lately raised, the decision on which is of some importance in our determination of the nature of the force by which the contraction of muscles is occasioned. This isy whether the power of a muscle is greater or less at different degrees of contraction, the same stimulus being applied. This seems to have been determined by the ingeniously-devised experiments of Schwann.* He contrived an apparatus, which should accu- rately measure the length of the muscle, and at the same time, the weight which it would balance by its contraction. Having caused the muscle of a frog to shorten to its extreme point, by the stimulus of galvanism applied to the nerve, so that no further stimulation could lift a weight placed in the * MQller's Physiology, p. 903. 292 OF MUSCULAR CONTRACTILITY. opposite scale, he allowed the muscle to relax until it was extended to a certain point, and then ascertained the weight which would balance its power. The same was several times repeated, as in the following manner. The length of the muscle in its extreme state of contraction, at which no additional force could be exerted by it, being represented by 14, it was found that, when it had been extended to 17, it would balance a weight of 60 ; when its length increased to 19-6, it would balance a weight of 120 ; and at 22-5, it would balance 180. In another experiment, the muscle at 13-5, balanced 0; at 18-8, it balanced 100; and at 23-4, it balanced 200. Hence it appears that an uniform increase of force corresponds with a nearly uniform increase in the length of the muscle ; or, in other words, that when the muscle is nearly at its full length, its contractile power is the greatest. In later experiments upon the same muscle, this uniform ratio seemed to be departed from ; but by com- paring the results in a considerable number of instances, it was constantly found that in those experiments which were performed the soonest after the preparation of the frog, and in which, therefore, the normal conditions of the system were the least disturbed, the ratio was very closely maintained. It has been hence inferred by Miiller, that the power which causes the contrac- tion of a Muscle must be very different in its character, from any of the forces of attraction known to us ; since these all increase in energy as the attracted parts approach each other, in the inverse ratio of the square of the distance ; so that the power of a Muscle, if operated on by any of these, ought to increase, instead of regularly diminishing, with its degree of contraction. But it is to be remembered that, as the observations of Mr. Bowman have clearly shown, there must be a considerable displacement of the constituents of every fibre during contraction (371) ; so that it is easy to understand that the greater the contraction, the more difficult must any further contraction become. If, be- tween a magnet and a piece of iron attracted by it, there were interposed a spongy elastic tissue, the iron would cease to approach the magnet at a point, at which the attraction of the magnet would be balanced by the force needed to compress still further the intermediate substance. VI. Applications of Muscular Power. 395. The energy of Muscular contraction is of course to be most remark- ably observed in those instances in which the continual exercise of particular parts has occasioned an increased determination of blood towards them, and in consequence a permanent augmentation in their bulk. This has been the case, for example, with persons who have gained their livelihood by exhibit- ing feats of strength. Much will, of course, depend on the mechanically advantageous application of muscular power ; and in this manner effects may be produced, even by persons of ordinary strength, which would not have been thought credible. In lifting a heavy weight in each hand, for example, a person who keeps his back perfectly rigid, so as to throw the pressure verti- cally upon the pelvis, and only uses the powerful extensors of the thigh and calf, by straightening the knees (previously somewhat flexed), and bringing the leg to a right angle with the foot, will have a great advantage over one who uses his lumbar muscles for the purpose. A still greater advantage will be gained, by throwing the weight more directly upon the loins, by means of a sort of girdle, shaped so as to rest upon the top of the sacrum and the ridges of the ilia ; and by pressing with the hands upon a frame, so arranged as to- bring the muscles of the arms to the assistance of those of the legs ; in this manner, a single man of ordinary strength may raise a weight of 2000 Ibs. ; whilst few who are unaccustomed to such exertions, can lift more than 300 Ibs. in the ordinary mode. A man of great natural strength, however, has been known to lift 800 Ibs. with his hands : and the same individual APPLICATIONS OF MUSCULAR POWER. 293 performed several other curious feats of strength, which seem deserving of being here noticed. "1. By the strength of his fingers, he rolled up a very large and strong pewter dish. 2. He hroke several short and strong pieces of tobacco-pipe, with the force of his middle finger, having laid them on the first and third finger. 3. Having thrust in under his garter the bowl of a strong tobacco-pipe, his legs being bent, he broke it to pieces by the tendons of his hams, without altering the bending of the knee. 4. He broke such another bowl between his fist and second fingers, by pressing them together sideways. 5. He lifted a table six feet long, which had half a hundred-weight hanging at the end of it, with his teeth, and held it in that position for a considerable time. It is true, the feet of the table rested against his knees ; but, as the length of the table was much greater than its height, that performance required a great strength to be exerted by the muscles of his loins, neck and jaws. 6. He took an iron kitchen poker, about a yard long, and three inches in circumference, and, holding it in his right hand, he struck it on his bare left arm between the elbow and the wrist, till he bent the poker nearly to a right angle. 7. He took such another poker, and, holding the ends of it in his hands, and the middle of it against the back of his neck, he brought both ends of it together before him ; and, what was yet more difficult, he pulled it straight again."* Haller mentions an instance of a man, who could raise a weight of 300 Ibs., by the action of the elevator muscles of his jaw ; and that of a slender girl, affected with tetanic spasm, in whom the extensor muscles of the back, in the state of tonic contraction or opisthotonos, resisted a weight of 800 Ibs., laid on the abdomen with the absurd intention of straightening the body. It is to be recollected, that the mechani- cal application of the power developed by muscular contraction, to the move- ment of the body, is very commonly disadvantageous as regards force; being designed to cause the part moved to pass over a much greater space than that through which the muscle contracts. Thus the temporal muscle is attached to the lower jaw at about one-third of the distance between the condyle and the incisors ; so that a shortening of the muscle to the amount of half an inch, will draw up the front of the jaw through an inch and a half; but a power of 900 Ibs. applied by the muscle, would be required to raise 300 Ibs. bearing on the incisors. In the case of the forearm and leg, the disproportion is much greater ; the points of attachment of the muscles, by which the knee and elbow-joints are flexed and extended, being much closer to the fulcrum, in comparison with the distance of the points on which the resistance bears. 396. The energy of muscular contraction appears to be greater in Insects, in proportion to their size, than it is in any other animals. Thus a Flea has been known to leap 60 times its own length, and to move as many times its own weight. The short-limbed Beetles, however, which inhabit the ground, manifest the greatest degree of muscular power. The Lucanus cervus (Stag Beetle) has been known to gnaw a hole of an inch diameter, in the side of an iron canister in which it had been confined. The Geotrupes stercorarius (Dung or shard-borne Beetle) can support uninjured, and even elevate, a weight equal to at least 500 times that of its body. And a small Carabus has been seen to draw a weight of 85 grains (about 24 times that of its body) up a plane of 25 ; and a weight of 125 grains (36 times that of its body) up a plane of 5 ; and in both these instances the friction was considerable, the weights being simply laid upon a piece of paper, to which the insect was attached by a string. 397. The rapidity of the changes of position of the component particles of muscular fibres, may, as Dr. Alison justly remarks,t be estimated, though * Desaguliers' Philosophy, vol. ii. f Cyclopaedia of Anatomy and Physiology, Art. Contractility. 25* 294 OF MUSCULAR CONTRACTILITY. it can hardly be conceived, from various well-known facts. The pulsations of the heart can sometimes be distinctly numbered in children, at more than 200 in the minute ; and as each contraction of the ventricles occupies only one-third of the time of the -whole pulsation, it must be accomplished in ff | th of a minute, or T Vth of a second. Again, it is certain that, by the movements of the tongue and other organs of speech, 1500 letters can be distinctly pro- nounced by some persons in a minute : each of these must require a sepa- rate contraction of muscular fibres ; and the production and cessation of each of the sounds, implies that each separate contraction must be followed by a relaxation of equal length ; each contraction, therefore, must have been effected in Vo tn P art of a minute, or in the T \>th of a second. Haller calculated that, in the limbs of a dog at full speed, muscular contractions must take place in less than the o^th of a second, for many minutes at least in succession. All these instances, however, are thrown into the shade by those which may be drawn from the class of Insects. The rapidity of the vibrations of the wings may be estimated from the musical tone which they produce ; it being easily ascertained, by experiments, what number of vibrations are required to produce any note in the scale. From these data, it appears to be the neces- sary result that the wings of many Insects strike the air many hundred, or even many thousand times in every second. The minute precision with which the degree of muscular contraction can be adapted to the designed effect, is in no instance more remarkable than in the Glottis. The musical pitch of the tones produced by it, is regulated by the degree of tension of the chordae vocales, which are possessed of a very considerable degree of elasti- city ( 402). According to the observations of Miiller,*the average length of these, in the male, in a state of repose, is about /^hs f an i nc ^ 5 whilst, in the state of greatest tension it is about T 9 ^o tns 5 tne difference being therefore or one-fifth of an inch : in the female glottis, the average dimensions are about yVV tns ' anc ^ T 6 o 3 o tns respectively ; the difference being thus about one-eighth of an inch. Now the natural compass of the voice, in most per- sons who have cultivated the vocal organ, may be stated at about two octaves, or 24 semitones. Within each semitone, a singer of ordinary capability could produce at least ten distinct intervals ; so that of the total number, 240 is a very moderate estimate. There must, therefore, be at least 240 different states of tension of the vocal cords, every one of which is producible by the will, without any previous trial ; and the whole variation in the length of the cords being not more than one-fifth of an inch, even in man, the variation required to pass from one interval to another, will not be more than one-twelve hundredth of an inch. And yet this estimate is much below that which might be truly made from the performances of a practised vocalist.! 398. Of the different associations of muscular actions which are employed for various purposes in the living body, it would be out of place here to speak ; since these associations depend upon the nervous rather than upon the mus- cular system ; and the most important of them have already been considered in detail. It may be mentioned, however, that the aptitude which is acquired by practice, for the performance of particular actions, that were at first ac- complished with difficulty, seems to result as much from a change, which the continual repetition of them occasions in the muscle, as in the habit which the nervous system acquires, of exciting their performance. Thus almost every * Physiology, p. 1018. f It is said that the celebrated Mad. Mara was able to sound 100 different intervals be- tween each tone. The compass of her voice was at least three octaves, or 22 tones ; so that ihe total number of intervals was 2200, all comprised within an extreme variation of one- eighth of an inch ; so that it might be said that she was able to determine the contractions of her vocal muscles to the seventeen-thousandth of an inch. SENSIBILITY OF MUSCLES. 295 person learning to play on a musical instrument, finds a difficulty in causing the two shorter fingers to move independently of each other and of the rest ; this is particularly the case in regard to the ring-finger. Any one may satisfy himself of the difficulty, by laying the palm of the hand fiat on a table, and raising one finger after the other, when it will be found, that the ring-finger cannot be lifted without disturbing the rest evidently from the difficulty of detaching the action of that portion of the extensor communis digitorum, by which the movement is produced, from that of the remainder of the muscle. Yet to the practised musician, the command of the will over all the fingers becomes nearly alike ; and it can scarcely be doubted, that some change takes place in the structure of the muscle, which favours the isolated operation of its several divisions. f VII. Sensibility of Muscles. 399. Muscles are much less sensible to external impressions than many other parts of the body ; this is seen in amputations, in which the severest pain caused by the section is that of the incision through the skin. It is well known that a needle passed through the skin may be carried into the sub- stance of a muscle with scarcely any further pain ; and the heart laid bare has been observed to possess but a very slight degree of sensibility. This is easily accounted for by our knowledge of the distribution of the nerves ; for although every principal trunk may contain motor and sensory nerves in equal proportion, we know that in its various subdivisions these may be very unequally distributed. Thus, in the third division of the Fifth pair, it has been ascertained that the fibres from the motor root chiefly pass into the mus- cular branches, whilst those of the sensory root predominate in those supplying the surface ; and in the Par Vagum, a difference in the endowments of the several branches has been equally substantiated. Again, in the Orbit we find some muscles supplied by nerves which are exclusively motor, and scarcely receiving any sensory branches from the Fifth pair. Knowing as we do, that the general surface of the body would not derive any advantage from receiving the motor division of the Spinal nerves, and must, on the contrary, be largely supplied from the sensory, it cannot be doubted that, with regard to the subjacent muscles, the case is entirely the reverse. 400. Muscles are endowed, however, with the power of originating sensa- tions indicative of their ow^n condition; and these sensations differ so far from those conveyed by the usual sensory organs, that it has been proposed to designate the channel through which they are received by a peculiar name, the Muscular Sense. It may be questioned, however, whether this is de- sirable. The property by which we estimate the force with which muscles are contracting, which enables us to compare different degrees of weight and resistance, and to acquire a knowledge of the distances and relative positions of bodies brought in contact with the surface, appears to be the same with that by which we become conscious of fatigue from continued muscular action, and experience pain \vhen the muscles are spasmodically contracted, as in ordinary cramp. Of the importance of this sense in guiding and regulating muscular contraction, instances have already been given ( 257). It is, in addition, one of the principal means by which we acquire our notions of the external world. All our ideas offeree and of resistance are derived from it. When we put our muscles in action to raise a weight, or to push from us an obstacle, we should have no idea without this sense of the effort required; and it is obvious on a little consideration that no accounts of any natural forces (such as that of Gravitation, Magnetic Attraction, &c.) could convey a distinct idea to our minds, were it not for our means of estimating them by the same 296 OF THE VOICE AND SPEECH. method. It is by the sensibility of the muscles that we become conscious of the existence and direction of motion, to which the whole body is being pas- sively subjected. When sitting upright in a carriage, which is suddenly drawn forwards, we are thrown in the contrary direction ; and it is only by a certain muscular effort that we can regain our position. After some little time, however, we become so habituated to the sensation which this occasions, that we are unconscious of it, except by a certain degree of fatigue which results if it be greatly prolonged. But, when the motion suddenly ceases, we are thrown forwards, showing that the effort itself- had continued ; and not unfrequently the feeling motion is then experienced for a few seconds, as if the sensation remained, and were more perceptible when the cause of it had ceased. CHAPTER VI. OF THE VOICE AND SPEECH. I. The Larynx, and its Actions. 401. The sounds produced by the organ of Voice constitute the most important means of communication 'between Man and his fellows ; and the power of speech has, therefore, a primary influence, as well on his physical condition, as on the development of his mental faculties. Hence, although it only depends on one particular application of muscular force, comparable to that by which other volitional or emotional movements are effected, it seems right in treating of the Physiology of Man, to make it an object of special consideration. In order to understand the nature of the Organ of Voice as a generator of Sound, it is requisite to inquire, in the first instance, into the sources from which sounds at all corresponding to the human voice are else- where obtained. It is necessary to bear in mind that Vocal Sounds and Speech, or Articulate Language, are two things entirely different ; and that the former may be produced in great" perfection where there is no capability for the latter. Hence we should at once infer that the instrument for the pro- duction of Vocal Sounds was distinct from that by which these sounds are modified into articulat espeech ; and this we easily discover to be the case, the Voice being unquestionably produced in the Larynx whilst the modifica- tions of it, by which language is formed, are effected for the most part in the Oral cavity. The structure and functions of the former, then, first claim our attention. 402. It will be remembered that the Windpipe is surmounted by a stout bony annulus, termed the Cricoid cartilage, which serves as a foundation for the superjacent mechanism. This is embraced (as it were) by the Thyroid, which is articulated to its sides by its lower horns, round the extremities of which it may be regarded as turning, as on a pivot. In this manner the lower front border of the thyroid cartilage, which is ordinarily separated by small intervals from the upper margin of the thyroid, may be made to approach it or recede from it ; as any one may easily ascertain, by placing his finger against the little depression which may be readily felt externally, and observing its THE LARYNX, AND ITS ACTIONS. [Fig, 72. 297 A front view of the Thyroid Cartilage; 1, left half of the cartilage ; 2, anterior projecting angle ; 3, superior margin ; 4, its notch , 5, inferior mar- gin; 6, 6, cornu majus of each "half; 7, 7, cornu minus of each half. A front view of the Cricoid Cartilage ; 1, its internal face ; 2, the cavity of the larynx as formed by this cartilage ; 3, its inferior surface ; 4, the little head or convexity for articulating with the arytenoids; 5, ihe surface of the supe- rior edge for the attachment of the lateral crico- arytenoid muscles.] Fig. 73. External and sectional views of the Larynx, after Willis ; A n B, the cricoid cartilage ; E c G, the thyroid cartilage; G, its upper horn ; c, its lower horn, where it is articulated with the cricoid; F, the arytenoid cartilage; E F, the vocal ligament; A K, crico-thyroideus muscle; F e m, thyro-arytenoideus muscle; x. e, crico-arytenoideus lateralis ; s, transverse section of arytenoideus transversus ; m n, space between thyroid and crycoid; B L, projection of axis of articulation of arytenoid with thyroid. changes of size, whilst a range of different tones is sounded: it will then be observed that, the higher the note, the more the two cartilages are made to approximate, whilst they separate in proportion to the depth of the tones.* * In making; this observation, it is necessary to put out of view the general movement of the larynx itself, which the finger must be made to follow up and down. 298 OF THE VOICE AND SPEECH. Upon the upper surface of the back of the cricoid, are seated the two small Arytenoid cartilages ; these are fixed in one direction by a bundle of strong [Fig. 74. A posterior view of the left Arytenoid Carti- lages ; 1, its posterior face ; 2, the summit ; 3, the base and cavity for articulating with the cricoid cartilage ; 4, its external angle ; 5, its internal angle. An anterior view of the left Arytenoid Carti- lages ; 1, its anterior face. The other references as in the accompanying figure.] ligaments, which tie them to the back of the cricoid ; but they have some power of moving in other directions upon a kind of articulating surface. The direction of the surface, and the mode in which these cartilages are otherwise attached, cause their movement to be a sort of rotation in a plane, which is Fig. 75. Bird's-eye view of Larynx from above, after Willis; G E H, the thyroid cartilage, embracing the ring of the cricoid r u x to, and turning upon the axis x z, which passes through the lower horns, c, Fig. 73; N F, N F, the aryterioid carti- lages, connected by the arytenoideus transversus; T v, T v, the vocal ligaments ; NX, the right crico- arytenoideus lateralis (the left being removed) ; v kf, the left thyro-arytenoideus, (the right being removed) ; N /, N I, the crico-arytenoidei postici ; B B, the crico-arytenoid ligaments. [Fig. 76. A vertical section of the Larynx to show its internal surface ; 1, section of the root of the tongue; 2, os hyoides; 3-, the muciparous gland of the epiglottis ; 4, top of the epiglottis cartilage ; 5, a section of its anterior face ; 6, a fold of mu- cous membrane from the arytenoids to the epi- glottis; 7, superior vocal ligament; 8, section of thyroid cartilage ; 9, ventricle of Galen or Mor- gagni; 10, lower vocal ligament; 11, arytenoid cartilages; 12, inside of the cricoid cartilage ; 13, its posterior portion ; 14, lining membranes of the trachea; 15, end of the cornu majus of the os hyoides ; 16, cornu majus of the thyroid cartilage ; 17, mucous membrane of the pharynx ; IS, oeso- phagus ; 19, thyroid gland.] THE LARYNX, AND ITS ACTIONS. 299 nearly horizontal, but partly downwards ; so that their vertical planes may be made to separate from each other, and at the same time to assume a slanting position. This change of place will be better understood, when the action of the muscles is described. To the summit of the arytenoid cartilages are attached the chordae vocctles, or Vocal Ligaments, which stretch across to the front of the thyroid cartilage ; and it is upon the condition and relative situation of these ligaments that their action depends. It is evident that they may be ren- dered more or less tense by the movement of the thyroid cartilage just described, being tightened by the depression of its front upon the cricoid cartilage, and slackened by its elevation. On the other hand, they may be brought into more or less close apposition, by the movement of the arytenoid cartilages ; being made to approximate closely, or to recede in such a manner as to cause the rima glottidis to assume the form of a narrow V, by the revolution of these cartilages. We shall now inquire into the actions of the muscles upon the several parts of this apparatus; and first into those of the larynx alone. 403. The depression of the front of the Thyroid cartilage, and the conse- quent tension of the vocal ligaments, are occasioned by the conjoint action of the Crico-thyroidei on both sides ; and the chief antagonists to these are the Thyro-ctrytenoidei, which draw the front of the Thyroid back towards the Arytenoid cartilages, and thus relax the vocal ligaments. These two pairs of muscles may be regarded as the principal governors of the pitch of the notes, which, as we shall hereafter see, is almost entirely regulated by the tension of the ligaments ; their action is assisted, however, by that of other muscles presently to be mentioned. The Arytenoid cartilages are made to diverge from each other, by means of the Crico-arytenoideus posticus of each side, which proceeds from their outer corner, and turns somewhat around the edge of the Cricoid, to be attached to the lower part of its back ; its action is to draw the outer corner backwards and downwards, so that the points to which the vocal ligaments are attached, are separated from one another, and the rima glottidis is thrown open. This will be at once seen from the subjoined dia- gram, in which the direction of traction of the several muscles is laid down. The action of this muscle is partly "antagonized by that of the Crico-arytenoi- Fig. 77. Part of Fig. 75 enlarged, to show the direction of the muscular forces, which act on the Arytenoid Car- tilage ; Q N v s, the right arytenoid cartilage; T v, its vocal ligament; B R s, bundle of ligaments uniting it to cricoid ; c p, projection of its axis of articulation; h g, direction of the action of the thyro-arytenoideus ; N x, direction of crico-arytenoideus lateralis; N w, direction of crico-arytenoideus posticus; N T, direction of arytenoideus transversus, After Willis. 300 OF THE VOICE AND SPEECH. deus lateralis, which runs forwards and downwards from the outer corner of the Arytenoid cartilage ; and its action, with that of its fellow, will be to bring the anterior points of the Arytenoid cartilages into the same straight line, at the same time depressing them, and thus to close the Glottis. This muscle is assisted by the Arytenoideus transversus, which connects the posterior faces of the Arytenoid cartilages, and which, by its contraction, will draw them to- gether. By the conjoint action, therefore, of the Crico-arytenoideus lateralis, and of the Arytenoideus transversus, the whole of the adjacent faces of the Arytenoid cartilages will be pressed together; and the points to which the vocal ligaments are attached will be depressed. But if the Arytenoideus be put in action in conjunction with the Crico-arytenoidei postici, the tendency of the latter to separate the Arytenoid cartilages being antagonized by the former, its backward action only will be exerted ; and thus it may be caused to aid the Crico-thyroideus in rendering tense the vocal ligaments. This action will be further assisted by the Sterno-thyroideus, which tends to depress the Thyroid cartilage, by pulling from a fixed point below ;* and the Thyro-hyoi- deus will be the antagonist of this, when it acts from a fixed point above, the Os Hyoides being secured by the opposing contraction of several other mus- cles. The respective actions of these muscles will be best comprehended by the following table. Govern the Pitch of the Notes. ^ CDenress the front of the Thyroid cartilage on the > C CRICO-THYROIDEI | .... 3 Cricoid, and stretch the vocal ligaments; assisted S^STERXO-IHYROIDEI 3 by the Arytenoideus and Crico-arytenoidei postici. ("Elevate the front of the Thyroid cartilage, and draw I ) THTRO HYi OII)EI }'") il towards lhe Arytenoids, relaxing the vocal liga- Govern the Aperture of the Glottis. l> 5. CHICO-ARYTENOIDEI POSTICI '. . . . . Open the Glottis. C CRICO-ABYTENOIDEI LATEBALES? . . C Press together the inner edges of the Ary- 5T ABYTENOIDECS J " " C tenoid cartilages, and close the Glottis. F 404. The muscles which stretch or relax the Vocal ligaments, are entirely concerned in the production of Voice ; those which govern the aperture of the Glottis have important functions in connection with the Respiratory actions in general, and stand as guards (so to speak) at the entrance to the lungs. Their separate actions are easily made evident. We can close the aperture of the Glottis, by an exertion of the will, either during inspiration or expiration ; and it is a kind of spasmodic movement of this sort, which is concerned in the acts of Coughing and Sneezing ( 189), as well as in the more prolonged impedi- ments to the ingress and egress of air, which have been already noticed as resulting from disordered states of the Nervous system ( 300). A slight examination of the recent Larynx is sufficient to make it evident that, when once the borders of the Rima Glottidis are brought together by muscular action, the effect of strong aerial pressure on either side, whether produced by an expulsory blast from below, or by a strong inspiratory effort, occasioning a partial vacuum below, and consequently an increased pressure above, will * This is not usually reckoned as one of the principal muscles concerned in regulating the voice ; but that it is so, any one may convince himself by placing his finger just above the sternum, whilst he is sounding high notes; a strong feeling of muscular tension is then at once perceived. THE LARYNX, AND ITS ACTIONS. 301 P be to force them into closer apposition. With this action, th^u, the muscles which regulate the tension of the vocal ligaments have nothing to do. In the ordinary condition of rest, it seems probable that the Ary enoid cartilages are considerably separated from each other ; so as to cause a wide opening to intervene between their inner faces, and between the vocal ligaments, through which the air freely passes ; and the vocal ligaments are at the same time in a state of complete relaxation. In order to produce a vocal sound, it is not sufficient to put the ligaments into a state of tension ; they must also be brought nearer to each other. That the aperture of the Glottis is greatly narrowed during the production of sounds, is easily made evident to one's self, by com- paring the time occupied by an ordinary expiration with that required for the passage of the same quantity of air during the sustenance of a vocal tone. Further, the size of the aperture is made to vary in accordance with the note which is being produced ; of this, too, any one may convince himself, by noting the time during which he can hold out a low and a high note ; from which it will appear, that the aperture of the Glottis is so much narrowed in producing a high note, as to permit a much less rapid passage of air than is allowed when a low one is sounded. This adjustment of the aperture to the tension of the Vocal Ligaments, is a necessary condition for the production of a clear and definite tone. It further appears that, in the narrowing of the Glottis which is requisite to bring the vocal ligaments into the necessary approxima- tion, the upper points of the Arytenoid cartilages are caused to approximate, not only by being made to rotate horizontally towards each other, but also by a degree of elevation ; so that the inner faces of the Vocal Ligaments are brought into parallelism with each other, a condition which may be ex- perimentally shown to be necessary for their being thrown into sonorous vibration. 405. We have now to inquire what is the operation of the Vocal Ligaments in the production of sounds; and in order to comprehend this, it is necessary to advert to the conditions under which tones are produced, by instruments of various descriptions, having some analogy with the Larynx. These are chiefly of three kinds, strings, flute-pipes, and reeds or tongues. The Vocal Liga- ments were long ago compared by Ferrein to vibrating Strings ; and at first sight there might seem a considerable analogy, the sounds produced by both being elevated by increased tension. This resemblance disappears, however, on more accurate comparison ; for it may be easily ascertained by experiment, that no string so short as the vocal ligaments could give a clear tone at all to be compared in depth with that of the lowest notes of the human voice ; and also, that the scale of changes produced by increased tension is fundamentally different. When strings of the same length, but of different tension, are made the subject of comparison, it is found that the number of vibrations is in pro- portion to the square roots of the extending forces. Thus, if a string extended by a given weight produce a certain note, a string extended by four times that weight will give a note, in which the vibrations are twice as rapid, and this will be the octave of the other. If nine times the original weight be employed, the vibrations will be three times as rapid as those of the fundamental note, producing the twelfth above it. Now by fixing the larynx in such a manner that the vocal ligaments can be extended by a known weight, Miiller has ascertained that the sounds produced by a variation of the extending force will not follow the same ratio ; and therefore the condition of these ligaments cannot be simply that of vibrating cords. Further, a cord of certain length, which is adapted to give out a clear and distinct note, equal in depth to tfce lowest of the human voice, may be made by increased tension to produce all the superior notes, which, in stringed instruments, are ordinarily obtained by 26 302 OF THE VOICE AND SPEECH. t shortening the strings.* But it does not follow that a short string, which, with moderate tension, naturally produces a high note, should be able, by a diminution of the tension, to give out a deep one ; for, although this might be theoretically possible, yet it cannot be accomplished in practice ; since the vibrations become irregular on account of the diminished elasticity .t These considerations are in themselves sufficient to destroy the supposed analogy; and to prove that the Chordse Vocales cannot be reduced to the same category with vibrating strings. 406. The next kind of instrument, with which some analogy might be sus- pected, is the Flute-pipe, in which the sound is produced by the vibration of an elastic column of air contained in the tube ; and the pitch of the note is determined almost entirely by the length of the column, although slightly modified by its diameter, and by the nature of the embouchure or mouth from which it issues. This is exemplified in the German Flute, and in the English Flute or Flageolet ; in both of which instruments, the acting length of the pipe is determined, by the interval between the embouchure and the nearest of the side apertures ; by opening or closing which, therefore, a modification of the tone is produced. In the Organ, of which the greater number of pipes are constructed upon this plan, there is a distinct pipe for every note ; and their length increases in a regular scale. It is, in fact, with flute-pipes as with strings, that a diminution in length causes an increase in the number of vibrations, in an inverse proportion ; so that of two pipes, one half being the length of the other, the shorter will give a tone which is the octave above the other, the vibrations of its column of- air being twice as rapid. Now there is nothing in the form or dimensions of the column of air between the larynx and the mouth, which can be conceived to render it at all capable of such vibrations as are required to produce the tones of the Human voice ; though there is some doubt whether it is not the agent in the musical tones of some Birds. The length of an open pipe necessary to give the lowest G of the ordinary bass voice, is nearly six feet; and the conditions necessary to produce the higher notes from it, are by no means those which we find to exist in the process of modulating the human voice. 407. We now come to the third class of instruments, in which sound is pro- duced by the vibration of Reeds or Tongues ; these may either possess elasticity in themselves, or be made by elastic tension. The reeds of the Mouth-^Eoli- na, Accordion, Seraphine, &c., are examples of instruments of this character, in which the lamina vibrates freely in a sort of frame that allows the air to pass out on all sides of it through a narrow channel, thus increasing the strength of the blast ; whilst in the Hautboy, Bassoon, &c., and in Organ-pipes of similar construction, the reed is attached to one end of a pipe. In the former kind, the sound is produced by the vibration of the tongue alone, and is regulated entirely by its length and elasticity; whilst in the latter, its pitch is dependent upon this conjointly with the length of the tube, the column of air contained in which is thrown into simultaneous vibration. Some interesting researches on the effect produced on the pitch of a sound given by a reed, through the union of it with a tube, have been made by M. W. Weber; and, as they are important in furnishing data, by which the real nature of the vocal organ * Thus in the Piano-forte, where there are strings for each note, a gradual shortening is seen from the lowest to the highest; and in the Violin the change of tone is produced by stopping the strings with the finger, so as to diminish their acting length. f Thus it would be impossible to produce good Bass notes on the strings of a Violin, by diminishing their tension ; the length afforded by the Violoncello or Double Bass is requisite. The striking difference between the tone of the Bass strings in the Grand Piano-forte and the small upright Piccolo, is another exemplification of the same princi- ple ; being chiefly due to the length and tension of the former, as contrasted with the shortness and slackness of the latter. THE LARYNX, AND ITS ACTIONS. 303 may be determined, their chief results will be here given. I. The pitch of a reed may be lowered, but cannot be raised, by joining it to a tube. II. The sinking of the pitch of the reed thus produced, is at the utmost not more than an octave. III. The fundamental note of the reed thus lowered, may be raised again to its original pitch, by a further lengthening of the tube ; and by a further increase, is again lowered. IV. The length of tube necessary to lower the pitch of the instrument to any given point, depends on the relation which exists between the frequency of the vibra- tions of the tongue of the reed, and those of the column of air in the tube, each taken separately. From these data, and from those of the preceding paragraph, it follows that, if a wind-instrument can, by the prolongation of its tube, be made to yield tones of any depth in proportion to the length of the tube, it must be regarded as a flute-pipe ; whilst, if its pitch can only be lowered an octave or less (the embouchure remaining the same) by lengthening the tube, we may be certain that it is a reed instrument. The latter proves to be the case in regard to the Larynx. 408. Between the action of the Chordae Vocales, however, and that of an ordinary Reed, there appears to be a marked difference ; but this difference is really by no means considerable. In a reed, elasticity is a property of the tongue itself, when fixed at one end, the other vibrating freely ; but by a mem- branous lima, fixed in the same manner, no tone would be produced. If such a lamina, however, be made elastic by a moderate degree of tension, and be fixed in such a manner as to be advantageously acted on by a current of air, it will give a distinct tone. It is observed by Miiller, that membranous tongues made elastic by tension, may have either of three different forms. I. That of a band extended by a cord, and included between two firm plates, so that there is a cleft for the passage of air on each side of the tongue. II. The elastic membrane may be stretched over the half or any portion of the end of a short tube, the other part being occupied by a solid plate, between which and the elastic membrane a narrow fissure is left. III. Two elastic membranes may be extended across the mouth of a short tube, each covering a portion of the opening, and having a chink left open between them. This last is evidently the form most allied to the Human Glottis; but it may be made to approximate still more closely, by prolonging the membranes in a direction parallel to that of the current of air, so that not merely their edges, but their whole planes shall be thrown into vibration. Upon this principle, a kind of arti- ficial Glottis has been constructed by Mr. Willis ; the conditions of action and the effects of which are so nearly allied to that of the real instrument, that the similar character of the two can scarcely be doubted. The fol- lowing is his description of it. " Let a wooden pipe be prepared of the form of Fig. 78, having a foot C like that of an organ-pipe, and an upper opening, long and narrow, as at B, with a point A rising at one end of it. If a piece of leather, or still better, of sheet India Rubber, be doubled round this point, and se- cured by being bound round the Fig. 78. Fig. 79. Artificial Larynx. After Willis. 304 OF THE VOICE AND SPEECH. pipe at D with strong thread, as in Fig. 79, it will give us an artificial glottis with its upper edges G H, which may be made to vibrate or not, at pleasure, by inclining the planes of the edges. A couple of pieces of cork may be glued to the corners, to make them more manageable. From this machine, various notes may be obtained by stretching the edges in the direction of their length G H ; the notes rising in pitch with the increased tension, although the length of the vibrating edge is increased. It is true that a scale of notes equal in extent to that of the human voice cannot be obtained from edges of leather ; but this scale is much greater in India Rubber than in leather ; and the elas- ticity of them both is so much inferior to that of the vocal ligaments, that we may readily infer that the great scale of the latter is due to its greater elastic powers." By other experimenters, the tissue forming the middle coat of the arteries has been used for this purpose, in the moist state, with great success ; with this, the tissue of the vocal ligaments is nearly identical. It is worthy of remark, that in all such experiments, it is found that the two membranes may be thrown into vibration, when inclined towards each other in various degrees, or even when they are in the same plane, and their edges only approximate ; but that the least inclination from each other (which is the position the vocal ligaments have during the ordinary state of the glottis, 404), completely pre- vents any sonorous vibrations from being produced. 409. The pitch of the note produced by membranous tongues, may be affected in several ways. Thus, an increase in the strength of the blast, which has little influence on metallic reeds, raises their pitch very considerably ; and in this manner the note of a membranous reed may be raised by semitones to as much as a fifth above the fundamental. The addition of a pipe has nearly the same effect on their pitch as on that of metallic reeds ; but it cannot easily be determined with the same precision. The effect of the junction of a pipe with a double membranous tongue, is well shown in the Trumpet, Horn, and other instruments, which require the vibration of the lips, as w^ell as a blast of air, for the production of their sound, having no reed of their own. By some, these instruments have been classed with Flute-pipes ; but the condi- tions of their action are entirely different. The mouth-piece of the horn or trumpet is incapable of yielding any tone, when a current of air is merely blown through it ; and the lips are necessary to convert it into a musical reed, being rendered tense by the contraction of their sphincter, partly antagonized by the slightly dilating action of other muscles. The variation of the tension of the lips is effected by muscular effort ; and several different notes may be produced with a pipe of the same length ; but there is a certain length of the column of air, which is the one best adapted for each tone ; and different in- struments possess various contrivances for changing this. It has been recently ascertained, that the length of the pipe prefixed to the reed has also a consider- able influence on its tone, rendering it deeper in proportion as it is prolonged, down to nearly the octave of the fundamental note ; but the pitch then sud- denly rises again, as in the case of the tube placed beyond the reed. The researches of Miiller, however, have not succeeded in establishing any very definite relation between the length of the two tubes, in regard to their influence on the pitch of the reed placed between them. 410. From the foregoing statements it appears, that the true theory of the Voice may now be considered as well established, in regard to this essential particular, that the sound is the result of the vibrations of the vocal ligaments, which take place according to the same laws with those of metallic or other elastic tongues ; and that the pitch of the notes is chiefly governed by the ten- sion of these laminae. With respect, however, to the modifications of these tones, induced by the shape of the air-passages, both above and below the larynx, by the force of the blast, and by other concurrent circumstances, little THE LARYNX, AND ITS ACTIONS. 305 is certainly known. Hence it is, that on the theory of the production of what are called falsetto notes, there is much difference of opinion amongst Physio- logists. Some have contended, that these tones are produced by the vibration of the vocal ligaments along only a part of their length ; but this is certainly untrue. By Muller it is believed, that in the falsetto notes merely the thin border of the glottis vibrates, so that the fissure remains distinctly visible; whilst in the production of the ordinary vocal tones, the whole breadth of the vocal ligaments is thrown into strong vibrations, which traverse a wider sphere, so that a confused motion is seen in the lips of the glottis, rendering its fissure obscure. That the tension of the vocal cords is not diminished (as it ought to be if only a part of their length were being used), but is progressively in- creased, as we pass from the ordinary to the falsetto scale, any one may convince himself, by placing his finger on the interval between the thyroid and cricoid cartilages, as formerly described ( 402).* A very important adjunct to the production of the higher notes, has been pointed out by Muller, as being afforded by the modification in the space included between the two sides of the thyroid cartilage, which is effected by the thyro-arytenoidei. He had experimentally ascertained, that the introduction of a hollow plug into the upper end of the pipe beneath his artificial larynx (and therefore just below the reed), by dimin- ishing its aperture, produced a considerable elevation of the tone. The action may be imitated in the human larynx, when made the subject of experiment, by compressing the thyroid cartilage laterally ; and in this manner, the natural voice could be made to extend through a range that could otherwise be only reached by a falsetto. 411. The strength of the tone produced in the larynx is much increased by the resonance of the elastic tissue, which it contains in various other parts; but still more, perhaps, by that produced by the air in the trachea, bronchi, and pulmonary cells. This comes to be of great importance in the phenomena of auscultation. The aerial resonance is loudest, where any large body of air is collected together, as in the trachea, the larger bronchi, an emphysematous dilatation, or a cavity resulting from tubercular softening. On the other hand, solidification of the pulmonary tissue will produce a resonance of a somewhat different kind. The influence of the prefixed and superadded tubes in modifying the tones produced by the Human larynx, has been found by Prof. Muller not to be at all comparable to that which they exercised over the artificial larynx; the reason of which difference does not seem very apparent. It appears, however, that there is a certain length of the prefixed tube, as there is a certain distance of the vibrating laminae, and a certain length or form of the tube above, which is most favourable to the production of each note ; and the downward movement of the whole vocal organ, which takes place when we are sounding deep notes, and its rise during the eleva- tion of the tones, have been supposed to have the purpose of making this adjustment in the length of the trachea; but this requires the supposition that the real length of the trachea is shortened whilst it appears extended, for which there seems no foundation. It is considered by Mr. Wheatstone, that the column of air in the trachea may divide itself into harmonic lengths, and may produce a reciprocation of the tone given by the vocal ligaments ( 353) ; and in this manner he considers that the falsetto notes are to be explained. It * That the falsetto voice differs in some essential particular from the natural, is evident from this, that many persons who possess a considerable range of both, are yet unable to unite them, so as to sing through the whole scale without a marked interruption. Thus a gentleman of the Author's acquaintance has a bass voice, ranging from the lowest E oi the Square Piano to the second D above; and a falsetto ranging from the A below this to the E of the octave above, so as to give a compass of three octaves on the whole ; yet the two registers cannot be smoothly blended. 26* 306 OF THE VOICE AND SPEECH. may be added, that the partial closing of the epiglottis seems to assist in the production of deep notes, just as the partial covering of the top of a short pipe fixed to a reed will lower its tone ; and that something of this kind takes place during natural vocalization, would appear, from the retraction and depression of the tongue which accompany the lowering of the front of the head, when the very lowest notes are being sounded. The arches of the palate and uvula become contracted during the formation of the higher tones; but no difference can be perceived in their state, whether these tones be falsetto or not ; hence it would appear, that they have no concern in this peculiarity ; and the purpose of their increased tension is probably to maintain their power of resonance. The experiments of Savart have shown that a cavity which only responds to a shrill note when its walls are firm and dry, may be made to afford a great variety of lower tones, when its walls are moistened and relaxed in various degrees. This observation may probably be applied also to the trachea. 412. These and numerous other muscular actions which are employed in the production and regulation of the voice, are effected by an impulse which can scarcely be termed Voluntary, and the nature of which is a curious sub- ject for inquiry. It may be safely affirmed that the production of sounds is in itself an Instinctive action ; although the combination of these, whether into music or articulate language, is a matter of acquirement. Now it might be supposed that the Will has sufficient power over the vocal muscles, to put them into any state requisite for its purposes, without any further condition ; but a little self-experiment will prove that this is not the case. No definite tone can be produced by a Voluntary effort, unless that tone be present to the mind, during however momentary an interval, either as immediately conveyed to it by an act of Sensation, recalled by an act of Conception, or anticipated by an effort of the Imagination. When thus present, the Will can enable the muscles to assume the condition requisite to produce it ; but under no other circumstances does this happen, except by a particular mode of discipline presently to be adverted to. The action itself, therefore, must be reduced to the class of consensual movements ; and we must suppose that the Will is exercised in preparing the conditions requisite for it, rather than in directly exciting it. That those who are unfortunately labouring under congenital deafness, are thence debarred from learning the use of Voice in the ordinary manner, is well known ; the consensual action cannot be excited, either through sensations of the present, or conceptions of the past ; and the imagination is entirely destitute of power to suggest that which has been in no shape expe- rienced. But such persons may be taught to speak in an imperfect manner, by causing them to imitate particular muscular movements, which they may be made to see ; and it is evident, that they must be guided in the imitation and ordinary performance of those movements, by the common muscular sen- sations which accompany them, and not by the sensations conveyed through the Auditory nerve, which are ordinarily by far the most precise guides. Many instances, indeed, are on record, in which persons entirely deaf were enabled to carry on a conversation in the regular way ; judging of what was said, by the movements of the lips and tongue, which they had learned to connect with particular syllables ; and regulating their own voices in reply, by their voluntary power, guided by muscular sensation.* [In the foregoing account of the Physiology of Voice, the Author has been chiefly guided by the excellent paper by Mr. Willis in "the transactions of the Cambridge Philo- sophical Society, vol. iv., and by the elaborate investigations of Mailer and his coadju- tors, as detailed in the Fourth Book of his Physiology.] * See Johnstone on Sensation, p. 128. OF ARTICULATE SOUNDS. 307 II. Of Articulate Sounds. 413. The larynx, as now described, is capable of producing those tones of which Voice fundamentally consists, and the sequence of which becomes Music : but Speech consists in the modification of the laryngeal tones, by other organs intervening between the Glottis and the Os Externum ; so as to produce those articulate sounds of which Language is formed. It cannot be questioned that Music has its language ; and that it is susceptible of expressing the emotional states of the mind, among those at least who have been accustomed to associate these with its varied modes, to even a higher degree than articu- late speech. But it is incapable of addressing the intellect, by conveying definite ideas of objects, properties, actions, &c., in any other way than by a kind of imitation, which may be compared to the signs used in hieroglyphic writing. These ideas it is the peculiar province of articulate language to convey ; and we find that the vocal organ is adapted to form a large number of simple sounds, which may be readily combined into groups, forming words. The number of combinations which can be thus produced is so inexhaustible, that every language has its own peculiar series ; no difficulty being found in forming new ones to express new ideas. There is considerable diversity in differ- ent languages, even with regard to the use of the simplest of these combinations ; some of them are more easy of formation than others, and these accordingly enter into the composition of all languages ; whilst of the more difficult ones, some are employed in one language, some in another, no one language pos- sessing them all. Without entering into any detailed account of the mechanism required to produce each of these simple sounds, a few general considerations will be offered in 'regard to the classification of them ; and the peculiar defect of Articulation, termed Stammering, will be briefly treated of. 414. Vocal sounds are divided into Vowels and Consonants ; and the dis- tinctive characters of these are usually considered to be, that the Vowels are produced by the Voice alone, whilst the sound of the Consonants is formed by some kind of interruption to the voice, so that they cannot be properly ex- pressed, unless conjoined with a vowel. The distinction may be more cor- rectly laid down, however, in this manner ; the Vowel sounds are continuous tones, modified by the form of the aperture through which they pass out ; whilst in sounding Consonants, the breath suffers a more or less complete in- terruption in its passage through parts anterior to the larynx. Hence the really simple Vowel sounds are capable of prolongation during any time that the breath can sustain them ; this is not the case, however, with the real Diphthongal sounds (of which it will presently appear that the English i is one) ; whilst it is true of some Consonants. It seems to have been forgotten by many of those who have written upon this subject, that the laryngeal voice is not essential to the formation of either vowels or consonants ; for all may be sounded in a whisper. It is very evident, therefore, that the larynx is not primarily concerned in their production ; and this has been fully established by the following experiment. A flexible tube- was introduced by M. Deleau through his nostril into the pharynx, and air was impelled by it into the fauces ; then, closing the larynx, he threw the fauces into the different posi- tions requisite for producing articulate sounds, when the air impelled through the tube became an audible whisper. The experiment was repeated, with this variation, -that laryngeal sound^ were allowed to pass into the fauces ; and each articulated letter was then heard double, in a proper voice and in a whisper. 415. That the Vowels are produced by simple modifications in the form of the external passages, is easily proved, both by observation and by imitative 308 OF THE VOICE AND SPEECH. experiment. When the mouth is opened wide, the tongue depressed, and the velum palati elevated, so as to give the freest possible exit to the voice, the vowel a in its broadest form (as in ab) is sounded.* On the other hand, if the oral aperture be contracted, the tongue being still depressed, the sound oo (the continental u) is produced. If attention be paid to the state of the buccal cavity, during the pronunciation of the different vowel sounds, it will be found to undergo a great variety of modifications, arising from varieties of position of the tono-ue, the cheeks, the lips, and velum palati. The position of the tongue is, indeed, one of the primary conditions of the variation of the sound ; for it may be easily ascertained that, by peculiar inflexions of this organ, a great diversity of vowel sounds may be produced, the other parts remaining the same. Still there is a certain position of all the parts, which is most favourable to the formation of each of these sounds; but this could not be expressed without a lengthened description. The following table, slightly altered from that of Kempelen, expresses the relative dimensions of the buccal cavity and of the oral orifice, for some of the principal of these ; the number 5 expressing the largest size, and the others in like proportion : Vowel. Sound. Size of oral opening. Size of buccal cavity. a as in ah 5 5 a as in name 4 2 e as in theme '3 1 o as in cold 2 4 oo as in cool 1 5 These are the sounds of the five vowels, a, e, i, o, u, in most Continental lan- guages ; and it cannot but be admitted that the arrangement is a much more natural one than that of our own vowel series. The English a has three distinct sounds capable of prolongation ;t the true broad a of ah, slightly modified in/r ; the a of fate, corresponding to the e of French ; and the a of fall, which should be really represented by au. This last is a simple sound though commonly reckoned as a diphthong. In Kempelen's scale, the oral orifice required to produce it would be about 3, and the size of the buccal cavity 4.J On the other hand, the sound of the English i cannot, like that of a true vowel, be prolonged ad libitum; it is in fact a sort of Diphthong, result- ing from the transition from a peculiar indefinite murmur to the sound of e, which takes its place when we attempt to continue it. The sound oy or oi, as in oil, is a good example of the true diphthong ; being produced by the transition from au to e. In the same manner, the diphthong ou, which is the same with ow in owl, is produced in the rapid transition from the broad a of ah, to the oo of cool. Much discussion has taken place as to the true charac- ter of y, when it commences a word, as in yet, yawl, &c. ; some having main- tained that it is a consonant, (for the very unsatisfactory reason, that we are in the habit of employing a rather than an, when we desire to prefix the indefi- * This sound of the vowel a is scarcely used in our language, though very common in most of the continental tongues; the nearest approach to it in the English is the a in far : but this is a very perceptible modification, tending towards au. j- The short vowel sounds, as a in fat, e in met, o in pot, &c., are not capable of pro- longation. + The mode of making a determination of this kind may here be given, for the sake of example. If the broad a be sounded, the mou.th and fauces being opened wide, and we contract the oral orifice by degrees, at the same time slightly elevating the point of the tongue, we gradually come to the sound of u; by still further contracting the orifice, and again depressing the tongue, \ve form oo. On the other hand, in sounding e, the tongue is raised nearly to the roof of the mouth; if it be depressed, without the position of the lips being altered, au is given. OF ARTICULATE SOUNDS. 309 nite article to such words,) whilst others regard it as a peculiar vowel. A slight attention to the position of the vocal organs during its pronunciation, makes it very clear, that its sound in such words really corresponds with that of the long (English) e ; the pronunciation of the word yawl being the same as that of eaul, when the first sound is not prolonged, but rapidly transformed into the second. The sound of the letter w, moreover, is really of the vowel character, being formed in the rapid transition from oo to the succeeding vowel; thus wall might be spelt ooall. Many similar difficulties might be removed, and the conformity between spoken and written language might be greatly increased (so as to render far more easy the acquirement of the former from the latter), by due attention to the state of the vocal organs in the pro- duction of the simple sounds. 416. It is not very difficult to produce a tolerably good artificial imitation of the Vowel sounds. This was accomplished by Kempelen, by means of an India-rubber ball, with an orifice at each end, of which the lower one was attached to a reed; by modifying the form of the ball, the different vowels could be sounded during the action of the reed. He also employed a short funnel-like tube, and obtained the different sounds by covering its wide open- ing to a greater or less extent. This last experiment has been repeated by Mr. Willis ; who has also found that the vowel sounds might be imitated, by drawing out a long straight tube from the reed. In this experiment he arrived at a curious result: with a tube of a certain length, the series of vowels, i, e, a, 0, u, was obtained, by gradually drawing it out; but, if the length was increased to a certain point, a further gradual increase would produce the same sequence in an inverted order, u, o, , e, i; a still further increase would produce a return to the first scale, and so on. When the pitch of the reed was high, and the pipe short, it was found that the vowels o and u could not be distinctly formed, the proper tone being injured by the elongation of the pipe necessary to produce them ; and this, Mr. Willis remarks, is exactly the case with the Human voice, most singers being unable to pronounce u and o upon their highest notes. 417. The most natural primary division of the Consonants is into those which require a total stoppage of the breath at the moment previous to their being pronounced, and which, therefore, cannot be prolonged ; and those in pronouncing which the interruption is partial, and which can, like the vowel sounds, be prolonged ad libitum. The former have received the designation of explosive; and the latter of continuous. In pronouncing the explosive con- sonants, the posterior nares are completely closed, so that the exit of air through the nose is altogether prevented ; and the current may be checked in the mouth in three ways, by the approximation of the lips, by the approxima- tion of the point of the tongue to the front of the palate, and by the approxi- mation of the middle of the tongue to the arch of the palate. In the first of these modes, we pronounce the letters b, and p; in the second, &c " pounds J ^thrown off by other excretions. But in regard to the Herbivorous animals, the case is different. They per- spire much more abundantly, and their temperature is thus continually kept down. They consequently require ' a more active combustion, to develop sufficient bodily heat ; and the materials for this are supplied, as we have seen, by the non-azotized portions of their food, rather than by the metamor- phosis of their own tissues, which takes place with much less rapidity than in the Carnivorous tribes. Hence we may thus express the destination of this part of their food ; that of the azotized matter, here much smaller in amount, will be the same as in the preceding case. Starch, oil, and J partly I Fatty and"^ but chiefly CCarbonic acid and Water, dis- other non-azotized > converted < other an i- > th ro wn off< engaged by the respiratory compounds ) into f mal tissues^ directly as (^process. The proportion of v the food deposited as fat, will depend in part upon the surplus which remains, after the necessary supply of materials has been afforded to the respiratory process. Hence, the same quantity of food being taken, the quantity of fat will be increased by causes that check the perspira- tion, and otherwise prevent the temperature of the body from being lowered, so that there is need of less combustion within the body to keep up its heat. 324 OF DIGESTION AND NUTRITIVE ABSORPTION. This is consistent with the teachings of experience respecting the fattening of cattle ; for it is well known that this may bfe accomplished much sooner, if the animals are shut up in a warm dwelling and covered with cloths, than if they are freely exposed in the open air. Now the condition of Man may be regarded as intermediate between these two extremes. The construction of his digestive apparatus, as well as his own instinctive propensities, point to a mixed diet as that which is best suited to his wants. It does not appear that a diet composed of ordinary vegetables only, is favourable to the full development of either his bodily or mental powers ; but this cannot be said in regard to a diet of which bread is the chief ingredient, since the gluten it contains appears to be as well adapted for the nutrition of the animal tissues, as does the flesh of animals. On the other hand, a diet composed of animal flesh alone is the least economical that can be conceived ; for, since the greatest demand for food is created in him (taking a man of average habits in regard to activity and the climate he inhabits), by the necessity for a supply of carbon and hydrogen to support his respiration, this want may be most advantageously fulfilled by the employment of a certain quantity of non-azotized food, in which these ingre- dients predominate. Thus it has been calculated, that some fifteen pounds of flesh contain no more carbon than four pounds of starch. A savage with one animal and an equal weight of starch, could support life for the same length of time, during which another restricted to animal food, would require five such animals, in order to procure the carbon necessary for respiration. Hence we see the immense advantage as to economy of food, which a fixed agricul- tural population possesses over the wandering tribes of hunters, which still people a large part both of the old and new continents. The mixture of the azotized and non-azotized compounds (gluten and starch), that exists in wheat flour, seems to be just that which is most useful to Man ; and hence we see the explanation of the fact, that, from very early ages bread has been regarded as the "staff of life." In regard to the nutritious properties of different arti- cles of vegetable food, these may be generally estimated by the proportion of azote they contain ; which is in almost every instance less than that existing in good wheat flour. 434. Besides these substances, there are certain Mineral ingredients, which may be said to constitute part of the food of Animals; being necessary to'their support, in the same manner as other mineral substances are necessary to the support of Plants. Of this kind are common salt, and also phosphorus, sulphur and lime, either in combination or separate. The uses of Salt are very numerous and important. It consists of two substances of opposite qualities, muriatic acid and soda; and the former is the essential ingredient in the gastric juice ; whilst the latter performs a very important part in the pro- duction of bile. Phosphorus is chiefly required to be united with fatty matter, to serve as the material of the nervous tissue ; and to be combined with oxygen and lime, to form the bone-earth, by which the bone is consolidated. Sulphur exists in small quantities in several animal tissues ; but its part is by no means so important as that performed by phosphorus. Lime is required for the con- solidation of the bones, and for the production of the shells and other hard parts that form the skeletons of the Invertebrata. To these ingredients we may also add Iron, which is a very important element in the red blood of Ver- tebrated animals. These substances are contained, more or less abundantly, in most articles generally used as food; and where they are deficient, the ani- mal suffers in consequence, if they are not supplied in any other way. Thus common Salt exists, in no inconsiderable quantity, in the flesh and fluids of animals, in milk, and in the egg : it is not so abundant, however, in plants ; and the deficiency is usually supplied to herbivorous animals by some other means. Thus salt is purposely mingled with the food of domesticated animals ; NATURE OF THE DIGESTIVE APPARATUS. 325 and in most parts of the world inhabited by wild cattle, there are spots where it exists in the soil, and to which they resort to obtain it. Such are the " buffalo licks" of North America. Phosphorus exists also in the yolk and white of the Egg, and in Milk, the substances on which the young animal subsists during the period of its most rapid growth ; and it abounds, not only in many animal substances used as food, but also (in the state of phosphate of lime or bone-earth) in the seeds of many plants, especially the grasses. In smaller quantities it is found in the ashes of almost every plant. When flesh, bread, fruit, and husks of grain, are used as the chief articles of food, more phosphorus is taken into the body than it requires ; and the excess has to be carried out in the excretions. Sulphur is derived alike from vegetable and animal substances. It exists in flesh, eggs, and milk ; also in the azotized com- pounds of plants ; and (in the form of sulphate of lime) in most of the river and spring-water that we drink. Iron is found in the yolk of egg, and in milk, as well as in animal flesh ; it also exists in small quantities in most vege- table substances used as food by Man, such as potatoes, cabbage, peas, cucum- bers, mustard, &c.; and probably in most articles from which other animals derive their support. Lime is one of the most universally diffused of all mineral bodies ; for there are very few animal or vegetable substances, in which it does not exist. It is most commonly taken in among the higher animals, combined with Phosphoric acid, so as to form bone earth ; and in this state it exists largely in the seeds of most grasses, especially in wheat-flour. If it were not for their deficiency in Phosphate of Lime, beans and peas would be more nutritious than wheaten-flour, the proportion of azotized matter they contain being much larger. A considerable Quantity of lime exists, in the state of carbonate and sulphate, in all hard water. 435. The introduction of alimentary matter into the system, is accomplished in Animals by the reception of food into an internal cavity, where it is sub- jected to a preparatory process, to which nothing analogous exists in Plants, and which is termed Digestion. This process may be said to have" three different purposes in view ; the reduction of the alimentary matter to a fluid form, so that it may become capable of absorption ; the separation of that portion of it which is fit to be assimilated or converted into organized texture, from that which cannot serve this purpose, and which is at once rejected ; and the alteration (when required) of the chemical constitution of the former, which prepares it for the important changes it is subsequently to undergo. The simplest conditions requisite for the accomplishment of these purposes are the following : a fluid capable of performing the solution and of effecting the required chemical changes ; a fluid capable of separating the unorgan- izable matter, by a process analogous to chemical precipitation ; and a cavity or sac, in which these operations may be performed. In the lowest Animals, we find this cavity formed on a very simple plan ; being evidently nothing else than an inversion of the external integument, communicating with the exterior by one orifice only, through which the food is drawn in, and the excrementitious matter rejected. The fluid necessary to dissolve the food, which is known by the name of gastric fluid or juice, and that required to separate the portion which is to be thrown off, which is known as the bile, are secreted in the walls of the stomach. In the Sea-Anemone, which affords a very characteristic example of this type of structure, it cannot be ascertained that the very rapid solution of food, which takes place in the digestive cavity, is assisted by any movement of its walls. In Polypes of a higher con- formation, however, the digestive cavity is provided with a second orifice; the stomach opens into an intestinal tube, through which the excrement is rejected in little pellets; and the food, before entering the true digestive cavity, is submitted to a powerful gizzard or triturating apparatus. Still the 28 326 OF DIGESTION AND NUTRITIVE ABSORPTION. [Fig. 80. A view of the Organs of Digestion, opened in nearly their wholg length ; a portion of the O3sophagus has been removed on account of want of space in the figure; the arrows indicate the course of substances along the canal ; 1, the upper lip, turned off the mouth ; 2, its frsenum; 3, the lower lip, turned down ; 4, its frsenum ; 5, 5, inside of the cheeks, covered by the lining membrane of the mouth; 6, points to the operting of the duct of Steno; 7, roof of the mouth; 8, lateral half arches; 9, points to the tonsils; 10, velum pendulum palati; 11, surface of the tongue; 12, papillae near its point; 13, a portion of the trachea; 14, the oesophagus; 15, its internal surface; 16, inside of the stomach; 17, its greater extremity or great cul-de-sac; 18, its lesser extremity or smaller cul-de-sac ; 19, its lesser curvature ; 20, its greater curvature ; 21, the cardiac orifice ; 22, the pyloric orifice; 23, upper portion "of duodenum; 24, 25, the remainder of the duodenum; 26, its valvulse conniventus; 27, the gall bladder; 28, the cystic duct; 29, division of hepatic ducts in the liver; 30, hepatic duct; 31, ductus communis choledochus; 32, its opening into the duodenum; 33, ductus Wir- sungii, or pancreatic duct; 34, its opening into the duodenum; 35, upper part of jejunum; 36, the ileum; 37, some of the valvute conniventus; 38, lowej extremity of the ileum ; 39, ileo-colic valve ; 40, 41, caecum, or caput coli ; 42, appendicula vermilbrmis ; 43, 44, ascending colon ; 45, transverse colon ; 46, 47, descend- ing, colon; 48, sigmoid flexure of the colon ; 49, upper portion of the rectum; 50, its lower extremity; 51, portion of the levator-ani muscle ; 52, the anus.] bile, like the gastric juice, is secreted in the walls of the stomach ; as may be distinctly perceived in many of these animals, on account of their transparency, and the bright yellow colour of the fluid. As we ascend the animal series, MASTICATION AND DEGLUTITION. 327 we find no essential change in the character of the digestive apparatus. The biliary follicles are gradually collected into a glandular mass, which is altoge- ther removed from the walls of the stomach, and which pours its secretion into the intestinal tube, at a short distance from its commencement ; the gastric juice, however, is still secreted in minute sacs imbedded in the substance of the membrane ; the form and arrangement of these will be hereafter described (CHAP. xii.). Several accessory glands are added, the uses of which are not accurately known ; and particular modifications of the apparatus are adapted to peculiarities in the nature of the food, or in the mode of its ingestion. As a general rule it may be stated, that the digestive apparatus is most simple in Carnivorous animals, in which it has to effect little change upon the aliment, except solution, in order to bring it to the state fit for absorption ; whilst it is most complex in those that feed upon Vegetable matter, which needs to un- dergo a greater change, both in its chemical composition and in the mechanical arrangement of its components, before it can be rendered subservient to animal nutrition. II. Mastication and Deglutition. 436. The first step in the process of reduction is the Mastication of the food, and the impregnation of its comminuted particles with the salivary secre- tion. Mastication is evidently of great importance, in preparing the sub- stances to be afterwards operated on, for the action of their solvent ; and it exactly corresponds with the trituration, to which the Chemist would submit any solid matter, that he might present it in the most advantageous form to a digestive menstruum. The complete disintegration of the alimentary matter, therefore, is of great consequence ; and, if imperfectly effected, the subsequent processes are liable to derangement. This derangement we continually meet with : for there is not, perhaps, a more frequent source of Dyspepsia (difficult digestion), than imperfect mastication, whether resulting from the haste with which the food is swallowed, or from the want of the proper instruments. The disintegration of the food by mechanical reduction is manifestly aided by Insalivation : it is doubtful, however, to what degree the saliva has any chemical effect upon it. It has been ascertained that this fluid has the power of converting starch into sugar, a conversion which does take place in the stomach ; but from the experiments of Berzelius and Miiller, it is doubtful whether the solution of other alimentary substances is more facilitated by the impregnation of them with saliva, than if pure water only had been employed.* The chemical nature of the Salivary secretion will be described at the same time with the structure of the gland itself (CHAP. xn.). 437. When the reduction of the food in the mouth has been sufficiently accomplished, it is carried into the oesophagus by the action of Deglutition. The share which the nervous system has in this action has been already stated ( 191) ; and it here only remains to define more precisely the different move- ments which are concerned in it. These were first described in detail by Ma- gendie ; but his account requires some modification', through the more recent observations of Dzondi.t The first stage in the process is the carrying back of the food until it has passed the anterior palatine arch ; this, which is effected by the approximation of the tongue and palate, is a purely voluntary move- * A different result has been given, however, by the recent experiments of Dr. Wright ; and perhaps it would be correct .to say, that the peculiar animal matter of the Saliva, being itself in a state of change, is capable of acting on substances with which it is brought in contact somewhat in the manner of a ferment: thus commencing the pro- cess which is to be carried on in the stomach ( 456). f Miiller's Physiology, p. 501. 338 OF DIGESTION AND NUTRITIVE ABSORPTION. [Fig. 81. A view of the Muscles of the Tongue, Palate. Larynx and Pharynx as well as the position of the upper portion of the (Esophagus, as shown by a vertical section of the head; 1, 1, the vertical section of the head; 2, points to the spinal canal ; 3, section of the hard palate ; 4, inferior spongy bone ; 5. middle spongy bone ; 6, orifice of the right nostril ; 7, section of the inferior maxilla ; 8, section of the os hyoides ; 9, section of the epiglottis; 10, section of the ericoid cartilage; 11, the trachea, covered by its lining membrane; 12, section of sternum; 13, inside of the upper portion of the thorax; 14, genio-hyo-glossus muscle; 15, its origin; 16, 17, the fan-like expansion of the fibres of this muscle; 18, superficialis linguae muscle; 19, ver- ticales linguse muscle; 20, genio-hyoideus muscle; 21, mylo-hyoideus muscle; 22, anterior belly of digas- tricus ; 23, section of platysma myodes ; 24, levator menti ; 25. orbicularis oris ; 26, orifice of Eustachian tube; 27, levator palati; 28, internal pterygoid; 29, section of velum pendulum palati, and azygos uvulse muscle; 30, stylo-pharyngeus; 31, constrictor pharyngis superior; 32, constrictor pharyngis medius; 33, insertion stylo-pharyngeus; 34, constrictor pharyngis inferior; 35, 36, 37, muscular coat of oesophagus; 38> thyreo-arytenoid muscle and ligaments, and above is the ventricle of Galen; 39, section of arytenoid carti- lage ; 40, border of sterno-hyoideus.] ment. In the second stage, the tongue is carried still further backwards, and the larynx is drawn forwards under its root, so that the epiglottis is pressed down over the rima glottidis. The muscles of the anterior palatine arch con- tract after the morsel has passed it, and assist its passage backwards ; these, with the tongue, cut off completely the communication between the fauces and the mouth. At the same time, the muscles of the posterior palatine arch con- tract in such a manner as to cause the sides of the arch to approach each other like a pair of curtains ; so that the passage from the fauces into the posterior nares is nearly closed by them ; to the cleft between the approximated sides, the uvula is applied like a valve. A sort of inclined plane, directed obliquely downwards and backwards, is thus formed; and the morsel slides along it into the pharynx, which is brought up to receive it. Some of these acts may be performed voluntarily ; but the combination of the whole is instinctive. The third stage of the process, the propulsion of the food down the oesophagus, then commences. This is accomplished in the upper part by means of the constrictors of the pharynx ; and in the lower by the muscular coat of the oeso- phagus itself. When the morsels are small, and are mixed with much fluid, the undulating movements from above downwards succeed each other very ACTION OF THE STOMACH. 329 rapidly ; this may be well observed in Horses whilst drinking ; large morsels, however, are frequently some time in making their way down. Each portion of food and drink is included in the contractile walls, which are closely applied to it during the whole of its transit. The gurgling sound which is observed when drink is poured down the throat of a person in articulo mortis, is due to the want of this contraction. The whole of the third stage is completely in- voluntary. The usual peristaltic movements of the oesophagus are reversed in Vomiting ; and this reversion has been observed, even after the separation of the stomach from the oesophagus, as a consequence of the injection of tartar- ized antimony into the veins. At the point where the oesophagus enters the stomach, the cardiac orifice of the latter, there is a sort of sphincter, which is usually closed. This opens when there is a sufficient pressure on it, made by accumulated food ; and afterwards closes, so as to retain the food in the stomach. The opening of the cardia is one of the first acts which take place in vomiting. When the sphincter is paralyzed by division of the pneumo- gastric nerve, the food regurgitates into the oesophagus. III. Action of the Stomach. 438. A remarkable opportunity of ascertaining the condition of the Stomach during Digestion, has lately presented itself, in a case in which a large fistu- lous aperture remained after a wound that laid open the cavity, but in which the general health has been completely recovered, so that the process may be considered as normally performed.* " The inner coat of the stomach, in its natural and healthy state, is of a light or pale pink colour, varying in its hues, according to its full or empty state. It of a soft or velvet-like appearance, and is constantly covered with a very thin, transparent, viscid mucus, lining the whole interior of the organ. By applying aliment or other irritants, to the in- ternal coat of the stomach, and observing the effect through a magnifying glass, innumerable lucid points, and very fine nervous or vascular papillae, can be seen arising from the villous membrane, and protruding through the mucous coat, from which distils a pure, limpid, colourless, slightly viscid fluid. The fluid thus excited is invariably distinctly a,cid. The mucus of the stomach is less fluid, more viscid or albuminous, semi-opaque, sometimes a little saltish, and does not possess the slightest character of acidity. The gastric fluid never appears to be accumulated in the cavity of the stomach while fasting ; and is seldom, if ever, discharged from its proper secerning vessels, except when excited by the natural stimulus of aliment, mechanical irritation of tubes, or other excitants. When aliment is received, the juice is given out in exact pro- portion to its requirements for solution, except when more food has been taken than is necessary for the wants of the system." That the quantity of the Gastric Juice secreted from the walls of the stomach depends rather upon the general requirements of the system, than upon the quantity of food introduced into the digestive cavity, is a principle of the highest practical importance, and cannot be too steadily kept in view in Dietetics. A definite proportion only of aliment can be perfectly digested in a given quantity of the fluid ; the action of which, like that of other chemical operations, ceases after having been exer- cised on a fixed and definite amount of matter. " When the juice has become * See the case of Alexis St. Martin, with the observations and experiments of Dr. Beau- mont, republished in this country by Dr. A. Combe. [A very extended examination of the phenomena of gastric digestion has been made by M.Blondlot. The chief subject of experiment was a dog. in which he maintained, without affecting the health, a fismlous opening into the stomach for more than two years. His examinations have furnished many new and important facts, and have confirmed those of Dr. Beaumont made on Alexis St. Martin in nearly every point.] 28* 330 OF DIGESTION AND NUTRITIVE ABSORPTION. saturated, it refuses to dissolve more ; and, if an excess of food has been taken, the residue remains in the stomach, or passes into the bowels in a crude state, and becomes a source of nervous irritation, pain and disease, for a long time." The unfavourable effect of an undue burthen of food upon the stomach itself interferes with its healthy action ; and thus the quantity really appropriate is not dissolved. The febrile disturbance is thus increased ; and the mucous membrane of the stomach exhibits evident indications of its morbid condition. The description of these indications, given by Dr. Beaumont, is peculiarly graphic, as well as Hygienically important. 439. "In disease or partial derangement of the healthy function, the mucous membrane presents various and essentially different appearances. In febrile conditions of the system, occasioned by whatever cause, obstructed perspira- tion, undue excitement by stimulating liquors, overloading the stomach with food, fear, anger, or whatever depresses or disturbs the nervous system, the villous coat becomes sometimes red and dry, at other times pale and moist, and loses its smooth and healthy appearance ; the secretions become vitiated, greatly diminished, or even suppressed ; the coat of mucus scarcely perceptible, the follicles flat and flaccid, with secretions insufficient to prevent the papillae from irritation. There are sometimes found on the internal coat of the stomach eruptions of deep-red pimples, not numerous, but distributed here and there upon the villous membrane, rising above the surface of the mucous coat. These are at first sharp-pointed, and red, but frequently become filled with white purulent matter. At other times, irregular, circumscribed red patches, varying in size and extent from half an inch to an inch and a half in circum- ference, are found on the internal coat. These appear to be the effects of congestion in the minute blood-vessels of the stomach. There are also seen at times small aphthous crusts, in connection with these red patches. Abra- sion of the lining membrane, like the rolling up of the mucous coat into small shreds or strings, leaving the papillae bare for an indefinite space, is not an uncommon appearance. These diseased appearances, when very slight, do not always affect essentially the gastric apparatus. When considerable, and particularly when there are corresponding symptoms of disease, as dryness of the mouth, thirst, accelerated pulse, &c. no gastric juice can be extracted by the alimentary stimulus. Drinks are immediately absorbed or otherwise disposed of; but food taken in this condition of the stomach remains undigested for twenty-four or forty-eight hours, or more, increasing the derangement of the alimentary canal, and aggravating the general symptoms of disease. After excessive eating or drinking, chymification is retarded ; and, though the appe- tite be not always impaired at first, the fluids become acrid and sharp, excori- ating the edges of the aperture, and almost invariably producing aphthous patches and the other indications of a diseased state of the internal membrane. Vitiated bile is also found in the stomach under these circumstances, and floc- culi of mucus are more abundant than in health. Whenever this morbid condition of the stomach occurs, with the usual accompanying symptoms of disease, there is generally a corresponding appearance of the tongue. When a healthy state of the stomach is restored, the tongue invariably becomes clean."* * Dr. A. Combe's commentary on the above passage is too apposite to be omitted. "Many persons who obviously live too freely, protest against the fact, because they feel no immediate inconvenience, either from the quantity of food, or the stimulants in which they habitually indulge; or, in other words, because they experience no pain, sickness, or headache, nothing, perhaps, except slight fulness and oppression, which soon go off. Observation extended over a sufficient length of time, however, shows that the conclu- sion drawn is entirely fallacious, and that the real amount of injury is not felt at the moment, merely because, for a wise purpose, nature has deprived us of any conscious- ACTION OF THE STOMACH. 331 440. In regard to the cause of the sense of Hunger, many different theories have been propounded. The following positions may be considered as well ascertained. The sense of Hunger, although referred to the stomach, is governed by the condition of the system at large ; being increased, when the demand for Nutrition is greater than that which the Blood can supply ; and being diminished, when such an addition is made to the nutritive ingredients contained in the latter, as renders it adequate for this purpose, even though this addition be not made through the introduction of food in the usual man- ner. It is, however, immediately dependent on some condition of the stomach itself; for it is abated, if not arrested, by section of the eighth pair of nerves ( 199) ; and it may be temporarily alleviated by introducing into its cavity matter which is not alimentary, but which causes pressure on its walls, and probably a flow of gastric juice. It may subside instantaneously under the influence of mental emotion, or of other strong impressions on the nervous system. It is easy to prove that many of the causes which have been assigned for it, are but little, or not at all, concerned in the production of the sensation. Thus, mere emptiness of the stomach does not produce it ; since, if the pre- vious meal have been sufficient, the food passes from its cavity some time before a renewal of hunger is felt. It cannot be due to the action of the gastric fluid upon the coats of the stomach themselves ; since this fluid is not poured into the stomach, except when the production of it is stimulated by the irrita- tion of its secreting follicles. By Dr. Beaumont it is thought, that the distension of these follicles by the secreted fluid is the proximate cause of hunger ; but there is no more reason to believe that the secretion of Gastric fluid is accumu- lating during the intervals when it is not required, than there is in regard to Saliva, the Lachrymal fluid, or any other secretions which are occasionally poured out in large quantities under the influence of a particular stimulus; and, moreover, it is difficult to imagine how mental emotion, or any impression on the nervous system alone (which is able, as is well known, to dissipate the keenest appetite in a moment), can relieve such distension. 441. It may, perhaps, be a more probable supposition, that there is a cer- tain condition of the Capillary circulation in the Stomach, which is prepara- tory to the secretion, and which is excited by the influence of the Sympathetic nerves, that communicate (as it were) the wants of the general system. This condition may be easily imagined to be the proximate cause of the sensation of hunger, by acting on the Par Vagum. When food is introduced into the stomach, the act of secretion is directly excited; the capillary vessels are gradually unloaded; and the immediate cause of the impression on the par vagum is withdrawn. By the conversion of the alimentary matter into mate- rials fit for the nutrition of the system, the remote demand also is satisfied ; and thus it is, that the condition of the stomach just referred to, is permanently relieved by the ingestion of substances that can serve as food. But if the ingested matter be not of a kind capable of solution and assimilation, the feel- ing of hunger is only temporarily relieved, and soon returns in greater force than before. The theory here given seems reconcilable with all that has been said of the conditions of the sense of hunger ; and particularly with what is ness of either the existence or the state of the stomach during health. In accordance with this, Dr. Beaumont's experiments prove, that extensive erythematic inflammation of the mucous coat of the stomach was of frequent occurrence in St. Martin, after excesses in eating, and especially in drinking, even when no marked general symptom was present to indicate its existence. Occasionally, febrile heat, nausea, headache, and thirst were complained ot* but not always. Had St. Martin's stomach, and its inflamed patches, not been visible to the eye, he too might have been pleased that his temporary excesses did him no harm; but, when they presented themselves in such legible cha- racters, that Dr. Beaumont could not miss seeing them, argument and supposition were at an end, and the broad fact could not be denied." 332 OF DIGESTION AND NUTRITIVE ABSORPTION. known of the effect produced upon it by nervous impressions, which have a peculiar influence upon the capillary circulation. It also corresponds exactly with what we know of the influence of the nervous system, and of mental impressions, upon other secretions ($ 426). 442. The sense of Hunger, like other sensations, may not be taken cogni- zance of by the mind, if its attention be strongly directed towards other objects ; of this fact, almost every one engaged in active occupations, whether mental or bodily, is occasionally conscious. The nocturnal student, who takes a light and early evening meal, and, after devoting himself to his pursuits for several hours uninterruptedly, retires to rest with a wearied head and an empty stomach, but without the least sensation of hunger, is frequently prevented from sleeping by an indescribable feeling of restlessness and deficiency; and the introduction of a small quantity of food into the stomach will almost instan- taneously allay this, and procure comfortable rest. Many persons, again, who desire to take active exercise before breakfast, are prevented from doing so by the lassitude and even faintness which it induces, the bodily exercise increasing the demand for food, whilst it draws off the attention from the sen- sation of hunger. The Author may be excused for mentioning the following circumstance, which some years ago occurred to himself; and which seems to him a good illustration of the principle, that the sense of hunger originates in the condition of the general system, and that its manifestation through a peculiar action in the stomach, is to be regarded as a secondary phenomenon, . adapted, under ordinary circumstances, to arouse the mind to the actions necessary for the supply of the physical wants, but capable of being over- looked, if the attention of the mind be otherwise directed. He was walking alone through a beautiful country, and with much to occupy his mind; and, having expected to meet with some opportunity of obtaining refreshment on his road, he had taken no food since his breakfast. This expectation, how- ever, was not fulfilled ; but, as he felt no hunger, he thought little of the dis- appointment. It was evening before he approached the place of his destina- tion, after having walked about twenty miles, resting frequently by the way ; and he then began to feel a peculiar lassitude, different from ordinary fatigue, which rapidly increased, so that during the last mile he could scarcely support himself. The " stimulus of necessity," however, kept him up ; but on arriving at his temporary home, he immediately fainted. It is obvious that, in this case, the occupation of the mind on the objects around, and on its own thoughts, had prevented the usual warning of hunger from being perceived; and the effect which succeeded was exactly what was to be anticipated, from the exhaustion of the supply of food occasioned by the active and prolonged exer- tion. 443. The conditions of the sense of Thirst appear to be very analogous to those of hunger. This sense is not referred, however, to the stomach, but to the fauces. It is generally considered that it immediately results from an impression on the nerves of the stomach ; since, if liquids are introduced into the stomach through an oesophagus tube, they are just as effectual in allaying thirst, as if they are swallowed in the ordinary manner. It may, however, be doubted whether the sense of thirst is not even more immediately connected with the state of the general system than that of hunger; for the immediate relief afforded by the introduction of liquid into the stomach is fully accounted for, by the instantaneous absorption of the fluid into the veins, which is known to take place, when there is a demand for it, not only from Dr. Beaumont's observations, but from many experiments made with reference to this particu- lar question. This demand is increased with almost equal rapidity, by any excess in the amount of the fluid excretions ; and it may be satisfied without ACTION OF THE STOMACH. 333 the introduction of water into the stomach* ( 464). Thirst may also be pro- duced, however, by the impression made by peculiar kinds of food or drink upon the walls of the alimentary canal; thus salted or highly-spiced meat, fermented liquors when too little diluted, and other similarly irritating agents, excite thirst ; the purpose of which is obviously to cause ingestion of fluid, by which they may be diluted. 444. The food which is propelled along the 03sophagus, enters the Stomach through its cardiac orifice, in successive waves ; and it is immediately sub- jected to a peculiar peristaltic movement, which has for its object to produce the thorough intermixture of the gastric fluid with the alimentary mass, and also to aid the solution of the latter by the gentle trituration to which it is thus [Fig. 82. A front view of the Stomach, distended by flatus, with the Peritoneal Coat turned off; 1, anterior face of the oesophagus ; 2, the cul-de-sac, or greater extremity ; 3, the lesser or pyloric extremity; 4, the duodenum ; 5, 5, a portion of the peritoneal coat turned back ; 6, a portion of the longitudinal fibres of the muscular coat; 7, the circular fibres of the muscular coat; 8, the oblique muscular fibres, or muscle of Gavard; 9, a portion of the muscular coat of the duodenum, where its peritoneal coat has been removed.] A view of the interior of the Stomach, as given by the removal of its anterior parietes ; 1, oesophagus ; 2, cardiac orifice of the stomach ; 3, its greater extremity, or cul-de-sac; 4, the greater curvature ; 5, line of the attachment of the omentum majus; 6, the muscular coat; 7, the anterior cut edge of the mucous coat; 8, the rugae of the mucous coat; 9. the lesser curvature; 10, the beginning of the duodenum; 11, pyloric orifice, or valve ; 12, the first turn of the duodenum downwards.] * This was among the remarkable results of the injection of fluid into the veins, in the Asiatic Cholera. 334 OF DIGESTION AND NUTRITIVE ABSORPTION, [Fig. 84. A view of -the interior of the Stomach and Duodenum in situ, the inferior portion of each having been removed; 1, 1, the under side of the liver; 2, the gall bladder; 3, 3, the lesser curvature and anterior faces, as seen from below ; 4, the rugae, about the cardiac orifice ; 5, the pyloric orifice ; 6, the rugse, and thickness of this orifice ; 7, 7, the duodenum ; 8, lower end of the right kidney.] subjected. The muscular fasciculi composing the human stomach, are so disposed as to shorten its diameter in every direction ; and by the alternate contraction and relaxation of these bands, a great variety of motion is induced in this organ, sometimes transversely, and at other ' times longitudinally. ** These motions," Dr. Beaumont remarks, " not only produce a constant dis- turbance or churning of the contents of the stomach, but they compel them, at the same time, to revolve about the interior from point to point, and from one extremity to the other." In addition to these movements, there is a constant agitation of the stomach, produced by the respiratory muscles. The motions of the stomach itself are not performed on any very exact plan, and are much influenced by the character of the ingesta, the state of the general system, and by other circumstances. The following is the ordinary course, however, of the revolutions of the food. " After passing the oesophageal ring, it moves from right to left, along the small arch ; thence, through the large curvature, from left to right. The bolus, as it enters the cardia, turns to the left, passes the aperture,* descends into the splenic extremity, and follows the great curvature towards the pyloric end. It then returns, in the course of the smaller curvature, makes its appearance again at the aperture in its descent into the great curvature, to perform similar revolutions. These revolutions are completed in from one to three minutes. They are probably induced in a great measure, by the circular or transverse muscles of the stomach. They are slower at first, than after chymification has considerably advanced ;" at which time also there is an increased impulse towards the pylorus. It is probable that, from the very commencement of chymification, until the organ becomes empty, portions of chyme are continually passing into the duodenum ; for the bulk of the alimentary mass progressively diminishes, and this the more rapidly as the process is nearer its completion. 445. The accelerated expulsion appears to be effected by a peculiar action of the transverse muscles ; and especially of that portion o them, which sur- rounds the stomach at about four inches from its pyloric extremity. This band is so forcibly contracted in the latter part of the digestive process, that it almost separates the two portions of the stomach into a sort of hour-glass form ; and Dr. B. states that when he attempted to introduce a long thermometer tube into the pyloric portion of the stomach, the bulb was at first gently * The fistulous orifice in St. Martin's stomach, through which these observations were made. ACTION OF THE INTESTINAL TUBE. 335 resisted, then allowed to pass, and then grasped by the muscular parietes beyond, so as to be drawn in : whence it is evident that the contraction has for its object, to resist the passage of solid bodies into the pyloric extremity of the stomach, at this stage of digestion, whilst the matter which has been reduced to the fluid form is pumped away (as it were) by the action of that portion of the viscus. These peculiar motions continue, until the stomach is perfectly empty, and not a particle of food or chyme remains. Of the degree in which they are dependent upon the influence of the Nervous System, some idea has been already given ( 235) ; there is yet much to be learned, how- ever, especially in regard to the degree in which the movements may be checked or altered, by impressions transmitted through the nervous system. It is stated by Brachet that, in some of his experiments upon the Par Vagum, some hours after the section of the nerve on both sides, the surface only of the alimentary mass was found to have undergone solution, the remainder of the mass remaining in the condition in which it was first ingested ; and if this statement can be relied on, it would appear that the movements of the stomach, like those of the heart, can be readily affected by a strong nervous impression. It may be partly in this manner, therefore, and not by acting upon the secre- tions alone, that strong Emotions influence the process, as they are well known to do. On the other hand, the moderate excitement of pleasurable emotions may be favourable to the operation, not only by giving firmness and regularity to the action of the heart, and thence promoting the circulation of the blood, and the increase of the gastric secretion, but also in imparting firm- ness and regularity to the muscular contractions of the stomach. IV. Action of the Intestinal Tube. 446. The pulpy substance to which the aliment is reduced, by the me- chanical reduction and chemical solution it has undergone in the mouth and stomach, is termed chyme. The consistency of this will of course vary in some degree with the quantity of fluid ingested; in general it is grayish, semifluid, and homogeneous ; and possesses a slightly acid taste, but is other- wise insipid. Dr. Beaumont describes it as varying in its aspect, from that of cream, which it presents when the food has been of a rich character, to that of gruel, which it possesses when the diet has been farinaceous. The passage of the chyme through the pyloric orifice is at first slow ; but when the digestive process is nearly completed, it is transmitted in much larger quantities. From the time that the ingested matter enters the intestinal canal, it is propelled by the simple peristaltic action of its muscular coat, which is directly excited by the contact either of this matter, or of the secretions which are mingled with it ;* and all that is not absorbed is thus conducted to the rectum, its expulsion from which is due to an action of a distinctly reflex kind, excited through the nervous centres ( 202). During its progress through the intestinal tube, the product of the gastric operation undergoes very important changes. The chyme is mingled in the duodenum with the biliary and pancreatic secretions, which effect an immediate alteration both in its sensible and chemical properties. The nature of this alteration can be best estimated, by mingling bile with chyme removed from the body. This has been done by several experimenters on the lower animals ; and by Dr. Beau- mont in the case already referred to, which afforded him the means of obtaining not only chyme, but bile and pancreatic fluid. The effect of this admixture * The bile seems to have an important share in producing this effect; since, when the ductus choledochus is tied, constipation always occurs. The action of mercury as a purgative appears to take place through the increase of the hepatic and other secretions which it induces. 336 OF DIGESTION AND NUTRITIVE ABSORPTION. was to separate the chyme into three distinct parts, a reddish-brown sedi- ment at the bottom, a whey-coloured fluid in the centre, and a creamy pellicle at the top. The central portion is probably that which is absorbed as chyle ; the sediment, partly consisting of the insoluble portion of the food, and partly of tne biliary matter itself, is evidently excrementitious ; the creamy or oily portion is probably taken up by the lacteals, and appears as fatty matter in the fluid drawn from them. It is not until the food has passed the orifice of the Ductus Choledochus, that the absorption of chyle begins, the lacteals not being distributed upon the Stomach, or the higher part of the Duodenum. 447. By the gradual withdrawal of their fluid portion, the contents of the alimentary canal are converted into a mass of greater consistence ; and this, as it advances through the small intestines, assumes more and more of a fsecal character. A part of the faeces, however, may be derived from the secretions of the enteritic mucous membrane, and of its glandulse ; the surface of the former with its simple follicles, probably secretes nothing else than mucus ; but the glandulae, with which it is so thickly studded, appear to serve as the channel for the elimination of putrescent matter from the blood. There can be no doubt, that a large quantity of fluid is poured out by these glandulse, when they are in a state of irritation from disease, or from the stimulus of a purgative medicine ; since the amount of water discharged from the bowels is often much greater than that which has been ingested, and must be derived from the blood. The secretion of the caecum has bejsn ascertained to be in herbivorous animals, distinctly acid during digestion ;" and there is reason to believe that the food there undergoes a second process analogous to that to which it has been submitted in the stomach, and fitted to extract from it what- ever undissolved alimentary matter it may still contain. There is no evidence, however, that this is the case in Man, whose ccecum (commonly termed the appendix cosci vermiformis) is very small, compared to that of most herbivorous animals. V. Nature of Chy unification. 448. The causes of the reduction of the food in the Stomach have long been a fruitful source of discussion amongst physiologists ; and various hypotheses have been devised to account for it. Some have compared the Stomach of Man to the Gizzard of a fowl, and have supposed that the trituration of the food between its walls was the essential element in the process ; but this doctrine is completely incompatible with the fact, that digestible substances, inclosed in metallic balls with perforations in their sides, are still dissolved by the power of the gastric fluid, though the walls of the stomach do not come in contact with them. Others, again, have imagined that the process of digestion is one of putrefaction ; but this idea, putting aside its inherent absurdity, is proved to be incorrect by the fact, that the gastric juice has a decidedly anti- septic quality. Others, in despair of obtaining any other solution, have attri- buted the operation to the direct agency of the vital principle ; forgetting that, as long as the aliment remains within the stomach and intestinal canal, it can no more be the subject of any peculiarly vital process, than if it were in contact with the skin, of which the mucous membrane is out an internal reflexion. The theory of chemical solution, which was at first regarded by many as quite untenable, has been of late years so much strengthened by new facts and arguments, that there now appears no valid reason for withholding our assent from it ; even though it cannot yet give a complete explanation of the complex phenomena in question. The chief opposition to this theory has arisen from the difficulty of imagining that any simply chemical solvent should have the power of acting on so great a variety of substances, and of reducing them to NATURE OF CHYMIFICATION. 337 a state so homogeneous. This difficulty, however, seems now in a great degree removed, by the discovery of the close Chemical relation that subsists between the various substances of each of the groups already enumerated ( 430) ; which renders it easy to conceive that the changes involved in their reduction may be of a very simple character. 449. The first series of facts which will be here adduced, as throwing light on the process of chymification, is that which has been obtained by the expe- riments of Dr. Beaumont upon the individual already alluded to (434). By introducing a tube of India-rubber into the empty stomach, he was able to obtain a supply of Gastric Juice whenever he desired it ; for the tube served the purpose of stimulating the follicles to pour forth their secretion, and at the same time conveyed it away. This fluid, of which the existence has been denied by some physiologists, is not very unlike saliva in its appearance ; it is, however, distinctly acid to the taste ; and chemical analysis shows that it contains a considerable proportion of free muriatic acid, and also some acetic acid. The former must evidently be derived from the decomposition of the muriate of soda contained in the blood, the remote source of which is the salt ingested with the food. The latter is an organic compound, probably formed at the expense of some of the saccharine matter of the previous aliment. Of equal importance with the free acids, is an animal matter, soluble in cx>ld water, but insoluble in hot, bearing considerable resemblance to albumen. Of this more will be said hereafter. Besides these principal ingredients, the gastric fluid contains 'muriates and phosphates of potass, soda, magnesia, and lime. It possesses the power of coagulating albumen in an eminent degree ; it is powerfully antiseptic, checking the putrefaction of meat ; and it is effec- tually restorative of healthy action, when applied to old foetid sores and foul ulcerating surfaces. It may be kept for many months, if excluded from the air, without becoming foetid. [The most remarkable results of M. Blondlot's investigations* relate to the compo- sition of the gastric fluid, and different as his conclusions may be from those usually received, yet the large quantity of fluid he was enabled to collect in a purer state than any one hitherto has collected it, entitles his account to every consideration. He very carefully distilled on a sand-bath 3875 grains of pure gastric fluid obtained after feeding his dog with raw meat; he repeated the distillation, and repeated the whole experiment, several times, with the gastric fluid of other animals as well as of the same dog, and the constant result was, that the product of the distillation did not once exhibit the slightest acid reaction ; but the residue in the retort was always strongly acid. It was thus proved that the acid of the gastric fluid cannot be either the hydrochloric or the acetic, for both these are volatile at the boiling point of water, and would have distilled over. A further proof that it is neither of these nor lactic acid, was furnished by the fact that no effervescence is produced when chalk, marble, or any other carbonate of lime is added to the gastric fluid: and it was this fact which chiefly led M. Blondlot to his con- clusion, that the true and almost only source of the acidity of healthy gastric fluid is the presence of superphosphate and biphosphate of lime. The evidence which he gives in addition to the above is: 1st. There is no acid salt, except this superphosphate of lime which could retain its acidity and remain in contact with carbonate of lime without exciting decomposition; 2d. Sulphuric acid, added to gastric fluid, produces an abundant precipitate of sulphate of lime, and oxalic acid a similar one of oxalate of lime. 3d. Potash, soda, ammonia, and lime-water, produce abundant precipitates of neutral phos- phate of lime. 4th. The calcined ash of gastric fluid was not deliquescent, was dissolved without effervescence by a few drops of hydrochloric acid, with which it formed chloride of calcium; it had, therefore, contained neutral phosphate of lime, the excess of the acid having been decomposed in the calcination. The general conclusion of his analysis is, that the gastric fluid is composed of ninety- nine parts of water, with one part of superphosphate of lime, superphosphate of am- monia, chloride of sodium, mucus, an aromatic, and a peculiar principle. Similar results were obtained from the analysis of the gastric fluid of several animals. M. C.] 450. The Gastric Juice obtained from the stomach, was found by Dr. Beau- mont to possess the power of dissolving various kinds of alimentary substances, * Traite Analytique de la Digestion, Paris, 1844. 29 338 OF DIGESTION AND NUTRITIVE ABSORPTION. when these were submitted to its action at a constant temperature of 100 (which is about that of the stomach), and were frequently agitated. The solution appeared to be in all respects as perfect as that which naturally takes place in the stomach ; but required a longer time. This is readily accounted for, when we remember that no ordinary agitation can produce the same effect with the curious movements of the stomach ; and that the continual removal from its cavity of the matter which has been already dissolved, must aid the operation of the solvent on the remainder. The following is one out of many experiments detailed by Dr. Beaumont. "At llj o'clock, A, M., after having kept the lad fasting for 17 hours, I introduced a gum-elastic tube, and drew off one ounce of pure gastric liquor, unmixed with any other matter, except a small proportion of mucus, into a three-ounce vial. I then took a solid piece of boiled recently-salted beef, weighing three drachms, and put it into the liquor in the vial ; corked the vial tight, and placed it in a saucepan filled with water, raised to the temperature of 100, and kept at that point on a nicely-regulated sand-bath. In forty minutes, digestion had distinctly com- menced over the surface of the meat. In fifty minutes, the fluid had become quite opaque and cloudy ; the external texture began to separate and become loose. In sixty minutes, chyme began to form. At 1 o'clock, p. M., (digestion having progressed with the same regularity as in the last half-hour), the cel- lular texture seemed to be entirely destroyed, leaving the muscular fibres loose and unconnected, floating about in fine small shreds, very tender and soft. At 3 o'clock, the muscular fibres had diminished one-half since the last exami- nation. At five o'clock they were nearly all digested ; a few fibres only remaining. At 7 o'clock the muscular texture was completely broken down, and only a few of the small fibres could be seen floating in the fluid. At 9 o'clock every part of the meat was completely digested. The gastric juice, when taken from the stomach, was as clear and transparent as water. The mixture in the vial was now about the colour of whey. After standing at rest a few minutes, a fine sediment of the colour of the meat subsided to the bottom of the vial. A piece of beef, exactly similar to that placed in the vial, was introduced into the stomach, through the aperture, at the same time. At 12 o'clock it was withdrawn, and found to be as little affected by digestion as that in the vial ; there was little or no difference in their appearance. It was returned to the stomach ; and, on the string being drawn out at 1 o'clock, p. M., the meat was found to be all completely digested and gone. The effect of the gastric juice on the piece of meat suspended in the stomach was exactly similar to that in the vial, only more rapid after the first half-hour, and sooner completed. Digestion commenced on, and was confined to, the surface entirely in both situations. Agitation accelerated the solution in the vial, by removing the coat that was digested on the surface, enveloping the remainder of the meat in the gastric fluid, and giving this fluid access to the undigested portions."* 451. Many variations were made in other experiments, some of which strikingly displayed the effects of thorough mastication in aiding both natural and artificial digestion. The following table exhibits some of the most inte- resting results of these experiments. It may also be regarded as affording some approximation to the relative solubility of different kinds of aliment ; but a more accurate series of experiments, conducted with an express view to the determination of the quantity of albumen formed in each case, is still required for this purpose. The proportion of gastric juice to aliment, in artificial diges- tion, was generally calculated at one ounce of the former to one drachm of the latter. In several of the experiments, the limited extent of the powers of the solvent was very evident ; its character thus corresponding exactly with that of ordinary chemical agents. * Experiments 2 and 3, of First Series. 4 NATURE OF CHYMIFICATIONT. Mean Time of Chymification. 339 ARTICLES OF DIET. In Stomach. In Vials. Preparation. H. M. Preparation. H. M. Beef, with salt only Boiled 2-45 Boiled 9- 30 Beef-steak Broiled 3- Masticated 8- 15 Beef, fresh, lean, dry Beef, old, hard, salted Roasted Boiled 3-30 4- 15 Roasted 7-45 Mutton, fresh Broiled 3- Masticated 6-45 Do. do. Boiled 3- Do. do. Roasted 3- 15 Lamb, fresh Broiled 2-30 Venison steak Broiled 1-35 Pork, recently salted Raw 3- Raw 8-30 Do/ do. Fried 4- 15 Do. do. Boiled 4- 30 Masticated 6-30 Pork, fat and lean Roasted 5- 15 Pig, sucking Roasted 2- 30 Veal ; fresh Broiled 4- Do. do. Fried 4- 30 % Liver, beef's, fresh Broiled 2- Cut fine 6- 30 Heart, animal Fried 4- Entire piece 13-30 Brains, animal Boiled 1-45 Boiled 4- 30 Pig's feet, soused Boiled 1- Tripe, soused Boiled 1- Eggs, whipped Raw 1-30 Whipped 4- Do. fresh Raw 2- Raw 4- 15 Do. do. Soft boiled 3- Soft boiled 6- 13 Do. do. Hard boiled 3-30 Hard boiled 8- Do. do. Roasted 2- 15 Do. do. Fried 3-30 Turkey, wild Roasted 2- 18 Do. domestic Roasted 2-30 Goose, wild Roasted 2-30 Ducks, domestic Roasted 4- Fowls, do. Boiled 4- Do. do. Roasted 4- Ducks, wild Roasted 4-30 4 Trout, salmon, fresh Boiled 1-30 Boiled 3-30 Cod-fish, cured, dry Boiled 2- Boiled 5- Salmon, salted Boiled 4- Boiled 7-45 Oysters, fresh Do. do. Raw Stewed 2-55 3-30 Raw, entire Stewed 7-30 8-25 Sago Boiled 1-45 Boiled 3- 15 Tapioca Boiled 2- Boiled 3-20 Cabbage, with vinegar Raw 2- Shaved 10- 15 Do. " do. Boiled 4-30 Boiled 20- Beans Boiled 2-30 Parsnips Boiled 2-30 Mashed 6-45 Potatoes Roasted 2-30 Bread, wheat, fresh Baked 3- 30 Masticated 4- 30 Potatoes Boiled 3- 30 Mashed 8-30 Chicken-soup Boiled 3- Soup, beef, vegetables and bread Boiled 4- Gelatin Boiled 2-30 Boiled 4- 45 Milk Boiled 2- Boiled 4- 15 Cheese, old, strong Raw 3-30 Masticated 7- 15 Suet, mutton, boiled Boiled 4- 30 Divided 10- 340 OF DIGESTION AND NUTRITIVE ABSORPTION. 452. That the foregoing table can only be regarded as approximative, is shown by the fact substantiated by Dr. Beaumont, that the rapidity of diges- tion varies greatly according to the quantity eaten, the nature and amount of the previous exercise, the interval since the preceding meal, the state of health and of the weather, and the condition of the mind. In scarcely any of the experiments have these circumstances been carefully noted ; and, as Dr. B. himself remarks, "the only way of insuring 4 minuteness and accuracy, as to the relative digestibility of "different kinds of diet, would be to try the effect of the gastric juice, in a series of experiments, first on one article of diet, and then on another, repeating and adapting them to meet all the various condi- tions of the stomach, and the vicissitudes and irregularities of the system, until the whole range should be completed, a Herculean task, which it would take years to accomplish." Some important inferences, however, may be drawn from the foregoing results. It seems to be a general rule, that the flesh of wild animals is more easy of digestion than that of the domesticated races which approach them most nearly. This may, perhaps, be partly attributed to the small quantity of fatty matter that is mixed up with the flesh of the former, whilst that of the latter is largely pervaded by it. For it appears from Dr. B.'s experiments, that the presence in the stomach of any substance which is difijcult of digestion, interferes with the solution of food that would otherwise be soon reduced. It seems that, on the whole, beef is more speedily reduced than mutton, and mutton sooner than either veal or pork. Fowls are far from possessing the digestibility that is ordinarily imputed to them; but turkey is, of all kinds of flesh except venison, the most soluble. Dr. B. has also ascertained that moderate exercise facilitates digestion, though severe and fatiguing exercise retards it. If even moderate exercise be taken immediately after a full meal, however, it is probably rather injurious than beneficial ; but if an hour be per- mitted to elapse, or if the quantity of food taken has been small, it is of decided benefit. 453. The presence of Bile in the stomach has been regarded by some phy- siologists as an ordinary occurrence during digestion ; but according to Dr. B. this is not the case, except in morbid conditions of the organ, or after a long perseverance in the use of fat or oily food. It is not impossible that the con- version of such food may be aided by the bile, the free alkali of which will have a chemical operation upon it. Dr. A. Combe suggests whether the circumstance of the peculiar digestibility of a piece of fat bacon in certain forms of dyspepsia, may not be accounted for by the presence of bile in the stomach in this condition. Dr. B.'s experiments further show that bulk is as necessary for healthy digestion, as the presence of the nutrient principle itself. This fact has been long known by experience to uncivilized nations. The Kamschatdales, for example, are in the habit of mixing earth or saw-dust with the train-oil, on which alone they are frequently reduced to live. The Ved- dahs, or wild hunters of Ceylon, on the same principle, mingle the pounded fibres of soft and decayed wood with the honey, on which they feed when meat is not to be had ; and on one of them being asked the reason of the prac- tice, he replied, "I cannot tell you, but I know that the belly must be filled." It is further shown by Dr. B., that soups and fluid diet are not more readily chymified than solid aliment, and are not alone fit for the support of the system ; and this, also, is conformable to the well-known results of experience ; for a dyspeptic patient will frequently reject chicken-broth, when he can retain solid food or a richer soup. Perhaps, as Dr. A. Combe remarks, the little support gained from fluid diet, is due to the rapid absorption of the watery part of it ; so that the really nutritious portion is left in too soft and concentrated a state to excite the healthy action of the stomach. 454. From the foregoing statements we may conclude, that the process by NATURE OF CHYMIFICATION. 341 which the food is dissolved in the Gastric fluid is of a purely Chemical nature, since it takes place out of the living body as well as in it, allowance being made for the difference in its physical condition. That the natural process of digestion is imitated, when the food is submitted to the action of the gastric juice in a vial, not only in regard to the disintegration of its particles, but as to the change of character which they are made to undergo, is proved by the fact, that the artificial chyme thus formed exhibits the same changes as the real chyme, when submitted to the action of the bile ( 446). The process of digestion, however, may be freely conceded to be vital, in so far as it is dependent upon the agency of a secreted product, which vitally alone (so far at least as we at present know) can elaborate ; and all for which it is here contended is, that, when this product is once formed, it has an agency upon the alimentary matter, which, though not yet fully understood, is conformable, in all that is known of its operation, to the ordinary laws of chemistry. Thus, Digestion is conformable to Chemical solution, -first, in the assistance which both derive from the minute division of the solids submitted to it ; secondly, in the assistance which both derive from the successive addition of small por- tions of the comminuted solid to the solvent fluid, and from the thorough inter- mixture of the two by continual agitation; thirdly, in the limitation of the quantity of food on which a given amount of gastric juice can operate, which is precisely the case with chemical solvents; fourthly, in the assistance which both derive from an elevation of temperature,* the beneficial influence of heat being only limited, in the case of digestion, by its tendency to produce decomposition of the gastric fluid ; -fifthly, in the different action of the same solvent upon the various solids submitted to it. 455. We have now to inquire what information has been obtained, with regard to the chemical nature of the organic principle, which performs so important a part in the digestive process. It may be considered a well-esta- blished fact, that diluted acids alone have no power of chymifying alimentary substances, although capable of partially dissolving some of them ; but that their presence in the gastric fluid is essential to its effectual action. Thus Muller states that, when small pieces of meat, or small cubes of coagulated white of egg, have been macerated for some time in equal quantities of much- diluted muriatic, acetic, tartaric, and oxalic acids, a precipitate or turbidity may be produced by the ordinary re-agents ; but that the masses are not per- ceptibly changed, the cubes of coagulated white of egg preserving their angles and edges for weeks. Small pieces of meat were also placed in a solution of common salt, and submitted to the action of a powerful galvanic battery, which would set free muriatic acid ; without the change being perceptibly accele- rated. From the subsequent experiments of Eberle and Schwann, however, it appears that, although acids alone have so little power of digesting food, they act energetically when combined with mucus of the stomach.t The following is an outline of these experiments. The mucous membrane of the fourth stomach of the calf, being dissected from the other coats, and washed with cold water until it no longer gives evidence of containing a free acid, is macerated in water acidulated with muriatic acid; and after some time, the * The influence of temperature is remarkably shown in some of Dr. B.'s experiments. He found that the gastric juice had scarcely any influence on the food submitted to it, when the bottle was exposed to the cold air, instead of being kept at a temperature of 100. He observed on one occasion, that the injection of a single gill of water at 50 into the stomach, sufficed to lower its temperature upwards of 30; and that its natural heat was not restored for more than half an hour. Hence the practice of eating ice after dinner, or even of drinking largely of cold fluids, is very prejudicial to digestion. f By Eberle it was stated that the acidulated mucus of any membrane is an efficient solvent; but this has been found by Muller and Schwann to be an error, only the mucus of the stomach possessing this property. 29* 342 OF DIGESTION AND NUTRITIVE ABSORPTION. liquor being filtered is found to have the property of reducing most alimentary substances, submitted to its action at a slightly elevated temperature ; even though the membrane have been previously dried. Pieces of meat and of hard-boiled egg are softened within twelve hours; and they disappear almost entirely after twelve hours more : the fluid acquires a peculiar sourish odour, which is not, however, of a putrescent character. When the fluid is not acidulated, it is found to have no more solvent power than simple water would have. It was stated by Schwann, as the result of his experiments, that the quantity of free acid remains the same at the end of the digestive process as it was at its commencement ; whence he inferred, that the acid does not enter into combination with the substances dissolved: but this was probably an error, resulting from the weakness of the organic base with which the acid combines. 456. The active agent in the process appears to be an organic compound, to which the name of pepsin has been given. The properties of this have been investigated by Wasmann, who first succeeded in obtaining it in an isolated state ;* his observations were made upon the mucous membrane of the stomach of the Pig, which greatly resembles that of Man. When this mem- brane is digested in a large quantity of water at from 85 to 95, many other matters are removed from it besides pepsin ; but if this water be removed, and the digestion be continued with fresh water in the cold, very little but pepsin is then taken up. Pepsin appears to be but sparingly soluble in w r ater; when its solution is evaporated to dryness, there remains a brown, grayish, viscid mass, with the odour of glue, and having the appearance of an extract. The solution of this in water is turbid, and still possesses a portion of the charac- teristic power of pepsin, but greatly reduced. When strong alcohol is added to a fresh solution of pepsin, the latter is precipitated in white flocks, \vhich may be collected on a filter, and produce a gray compact mass when dried. Pepsin enters into chemical combination with many acids, forming compounds which still redden litmus paper; and it is when thus united with acetic and muriatic acids, that its solvent powers are the greatest. " In regard to the solvent power of pepsin for coagulated albumen, it was observed by M. Was- mann that a liquid which contains 17-10,000ths of acetate of pepsin, and 6 drops of hydrochloric acid per ounce, possesses a very sensible solvent power, so that it will dissolve a thin slice of coagulated albumen in the course of 6 or 8 hours' digestion. With 12 drops of hydrochloric acid per ounce, the white of egg is dissolved in 2 hours. A liquid which contains 3 gr. of acetate of pepsin, and to which hydrochloric acid and white of egg are alternately added, so long as the latter dissolves, is capable of dissolving 210 grains of coagulated white of egg at a temperature between 95 and 104. It would appear, from such experiments, that the hydrochloric acid is the true solvent, and that the action of the pepsin is limited to that of disposing the white of egg to dissolve in hydrochloric acid. The acid when alone dissolves white of egg by ebulli- tion, just as it does under the influence of pepsin; from which it follows that pepsin replaces the effect of a high temperature, which is not possible in the stomach. The same acid with pepsin dissolved blood, fibrin, meat, and cheese; while the isolated acid dissolved only an insignificant quantity at the same temperature ; but when raised to the boiling point, it dissolved nearly as much, and the part dissolved appeared to be of the same nature. The epider- mis, horn, the elastic tissue (such as the fibrous membrane of arteries) do not * Graham's Elements of Chemistry, [Am. Ed. p. 695.] It is considered by Liebig, how- ever, that Pepsin has no proper existence as such; and that the dissolving power of the animal membrane is due to the state of decomposition, which has been induced in it by exposure to air. It does not appear, however, that any other membrane than that of the Stomach can undergo this change. NATURE OF CHYMIFICATION. 343 ) dissolve in a dilute acid containing pepsin. M. Wasmann has remarked that the pepsin of the stomach of the pig is entirely destitute of the power to coagu- late milk, although the pepsin of the stomach of the calf possesses it in a very high degree ; from which he is led to suppose, that the power of the latter depends upon a particular modification of pepsin, or perhaps upon another substance accompanying it, which ceases to be formed when the young animal is no longer nourished by the milk of its mother."* It appears from the recent inquiries of Liebig and others, that the solvent principle is a protein-compound in a state of change; and that, like ferments in general, it possesses the property of exciting change in other substances, with which it is brought in contact. Hence we should regard the Digestive process, as result- ing from the combined actions of Fermentation and Chemical solution; the alimentary substances being first made to undergo an incipient fermentation, by the agency of the pepsin, which so alters their condition, as to dispose them to solution in hydrochloric and acetic acids, with which they form definite chemical compounds. This view harmonizes completely with the fact just stated, that a small quantity of pepsin will perform its part in the digestion of an unlimited quantity of aliment, which is analogous to what we know of the action of true ferments ; whilst only a definite quantity can be dissolved in a limited amount of acid, which is the case with all acts of proper chemical solution. 457. Our knowledge of the nature of the Digestive process has lately- received another important addition from the discovery, that the chief proxi- mate principles of the animal tissues, and those which have been regarded as most nutritious among vegetables, have almost identically the same chemical composition. This conformity will appear from the following comparative analysis, lately executed in the laboratory of Liebig. ALBUMEN. FIBRIN. { A ^ CASEIN. Of E<:s. Of Serum. Carbon 54-56 54-48 54-84 54-96 Nitrogen 15-72 15-70 15-83 15-80 Hydrogen 6-90 7-01 7-09 7-15 Oxygen ""] Phosphorus j> 22-82 22-81 22-24 22-09 Sulphur J 100-00 100-00 100-00 100-00 The proportion of carbon to nitrogen in all these substances, is that of 8 equi- valents of the latter to one of the former. They differ slightly in the quantity of phosphorus and sulphur which they contain ; but agree in many other im- portant chemical properties. Thus, they all dissolve, with the aid of heat, in concentrated muriatic acid ; and the solutions, kept for a time at a pretty high temperature, first assume a beautiful lilac, and then a rich violet-blue colour. At this stage of the decomposition, each of the three substances reacts in the same way with carbonate of ammonia and other reagents. The parallel vege- table principles are vegetable fibrin (a constituent of gluten first properly dis- tinguished by Liebig), gluten itself, vegetable albumen, and legumin ; this last is termed by Liebig vegetable casein, from its holding the same relation to vegetable albumen that animal casein does to animal albumen. The following is the elementary composition of these substances : * Graham, op. cit. [Am. Ed. p. 696 ] 344 OF DIGESTION AND NUTRITIVE ABSORPTION. VEGETABLE FIBRIN. GLUTEN. VEG. ALBUMEN. LEGUMEN. Carbon . . . . . 54-60 55-22 55-01 54-14 Nitrogen . . . 15-81 15-98 15-92 15-67 Hydrogen . . . . 7.30 7-42 7-23 7.15 Oxygen . . Phosphorus !'.!}> 22-28 21-38 21-84 23-03 Sulphur J Thus it appears that the azotized vegetable principles may be converted into those organic compounds which have been ordinarily considered as peculiar to animals, without any essential change in their chemical composition. 458. When Albumen or Fibrin is dissolved in a moderately strong solution of caustic potass, and is heated to about 120, the small portions of phosphorus and sulphur it contains are separated in the form of phosphate of potass and sulphuret of potassium ; and when this solution is saturated with acetic acid, a gelatinous substance is precipitated, which is the same in aspect and consti- tution, whether obtained from fibrin or albumen. To this the term Protein has been given. After being washed, it is still gelatinous, of a grayish colour, and semi-transparent ; when dried, it is yellowish, hard, easily pulverized, tasteless, insoluble in water and alcohol ; and, like fibrin and albumen, it is not fusible by heat without decomposition. The formula for Protein, accord- ing to Mulder, is, 40 Carbon, 31 Hydrogen, 5 Nitrogen, 12 Oxygen.* It may be obtained equally well from the globulin of blood, from the casein of milk, and from vegetable albumen ; whence it is evident that these substances are all to be regarded as modifications of a common principle. The nature of these modifications may be partly understood from the fact that Protein unites, according to strictly chemical principles, with many inorganic substances; forming new compound acids when combining with acids ; and acting in some degree as an acid, when brought into relation with bases, such as the oxides of lead and silver, of which one atom combines with 10 of protein. Viewed as chemical compounds, fibrin and albumen may be regarded as the products of the union of protein with definite proportions of sulphur and phosphorus. The following are their formulae, according to Mulder. Fibrin, and the albumen of eggs . 10 Pro. + 1 Sulph. -f | Phos. Albumen of Serum ..... 10 Pro. + 2 Sulph. + Phos. 459. From these facts, taken in combination with those already mentioned, it seems scarcely possible to resist the conclusion that the process of Digestion (strictly so called) is one of a purely Chemical nature. The conversion of the azotized animal and vegetable substances into Albumen, can scarcely be viewed in any other light ; for the change of form and of external characters is in no instance so great as that which starch and gum undergo during their con- version into sugar, which is well known to be of a strictly chemical nature. The albumen thus formed is dissolved in the water that has been ingested, and in the gastric secretion; and becomes one of the most important and characteristic ingredients of chyle. According to Dr. Prout, the albumen which is first formed in the stomach, differs from the principle elsewhere known under that name, in its imperfect coagulation, when acted on either by acids or heat. This is noticed, even when pure albumen has been introduced into the stomach ; for it is first coagulated, and then dissolved, so as to present the same characters with the albumen formed from other substances. In this process it appears to enter into chemical combination with a large quantity of * Liebig takes rather a. different view of its composition, which is, however, equally conformable with analytic results. His formula is, 48 Carbon, 36 Hydrogen, 6 Nitro- gen, 14 Oxygen. NATURE OF CHYMIFICATION. 345 water. "The solid and tenacious albumen is thus reduced to the weakest possible state, to the state as it were of infancy ; in short, to a state precisely analogous to that of the weak sugars and other organic compounds, as compared with the strong and perfect varieties of the same substances." * 460. In regard to the operation of the Digestive process upon the non-azotized vegetable matters, which belong to the class of saccharine compounds, less cer- tain information has been obtained. As already stated ( 430) it seems to be the prevalent belief of Chemists of the present day, that they are incapable of undergoing conversion into protein-compounds. By Dr. Prout, however, it is distinctly asserted that, where no azotized matter* existed in the food, (the ani- mal being fed, for example, upon pure starch,) and where no albumen was found in the stomach, that principle was distinctly traceable in the contents of the duodenum, after the admixture of the biliary and pancreatic fluids. Still there is not sufficient evidence that such a conversion takes place; for the albumen might have been derived from the blood circulating in the walls of the cavity, the fluid portion of which would naturally undergo an interchange with the thick gummy solution in the intestine, by a process of Endosmose. By Dr. Proutt it is supposed that, "under ordinary circumstances, the azote is principally furnished by a highly azotized substance (organized urea?) secreted from the blood, either into the stomach or duodenum,;]: or into both these localities ; and that the portion of the blood thus deprived of its azote is sepa- rated from the general mass of blood by the liver, as one of the constituents of the bile ; which secretion, as a whole, is remarkably deficient in azote." As the Saccharine principles cannot be distinctly recognized either in the blood or chyle (in a state of health), it is doubtful whether they can be absorbed without undergoing conversion. Their simplest transformation is into lactic acid; which may probably be absorbed directly from the stomach, by the blood-vessels of its villi. But there is evidence that they may be also con- verted into oleaginous compounds; and they probably form a considerable part of the fatty matter contained in the chyle, the quantity of which seems often greater than that pre-existing in the food. The possibility of such a conversion (which has been denied by some eminent chemists), has recently been demonstrated by the careful repetition of the old experiment of Huber ; who showed that Bees, when fed upon honey alone, have the power of form- ing wax to an amount much greater than that which the honey contained. The oleaginous compounds forming part of the food are probably absorbed as such ; and, in common with those produced by the transformation just de- scribed, are either used for the maintenance of the respiratory process, or are deposited as fat. The question whether they can ever, by any addition of highly-azotized matter, be converted into protein-compounds, and thus be applied to the nutrition of the azotized tissues, still, in the Author's opinion, remains undecided; although there are not wanting those who speak quite decidedly upon the impossibility of such transformation. VI. Lacteal and Lymphatic Absorption. 461. Although there can be no doubt that the Mucous membrane is capable of absorbing by its whole surface, it can scarcely be questioned that this func- tion is most energetically performed by the villi which cover it. These are short processes, from a quarter of a line to a line and a half in length ; giving to the membrane, where they are most numerous, a fleecy appearance. In * Bridgewaler Treatise, p. 503* f On Stomach and Urinary Diseases, [Am. ed., p. 370, note.] * May this be the function of the glands of Brunner, which are situated in the duode- dum and commencement of the jejunum only? w. u. c. 346 OF DIGESTION ANI> NUTRITIVE ABSORPTION. Fig. 85. Fig. 86. Vessels of an Intestinal Villus of a Hare, from a dry preparation by Dollinger; 1, 1, veins filled with white injection; 2, 2, arteries injected red. Magnified about 45 diameters. A. apex of intestinal villus from the duodenum of human female ; B, a mesh of the vascular net- work, 1. 1. filled up with delicate cellular tissue. 2. Magnified about 45 diameters. (After Wagner.) Fig. 87. Man thoy are commonly cylindrical or nearly so ; but in many of the lower animals they are spread out into broader laminae at their base, and are con- nected together so as to form ridges or folds. It was formerly believed that the villi were not supplied with blood-vessels. So far is this from being the case, however, that in each villus there is a minute plexus of blood-vessels, of which the larger branches may even be seen with the naked eye, when they are distended with blood. It can scarcely be doubted that through these capillaries takes place the absorption of fluid from the digestive cavity, which will be immediately stated to convey a portion of their contents directly into the veins ( 463). The Lacteal vessels, which are distributed upon the walls of the intestine, but not upon those of the stomach, occupy the interior of the villi of the former ; each lacteal tube commencing, as it were, in the midst of the tissue of a villus. The accompanying figure represents the appearance offered by the incipient lacteals in the villi of the jejunum of a young man, who had been hung soon after taking a full meal of farinaceous food. The lacteal that issued from each villosity arose by several smaller branches, in some of which free extremities could be traced, whilst others anastomozed with each other. It is certain that the lacteals never open by free orifices on the surface of the intestine, as was formerly imagined; and the same is true of the lymphatics, which originate in the sub- stance of the various tissues. From the recent ob- servations of Mr. Goodsir,* it appears that the villi are One of the intestinal villi, with the commencement of a lacteal. After Krause. * Edinb. New Phil. Journal, July, 1842. LACTEAL AND LYMPHATIC ABSORPTION. 347 enclosed in a very delicate membrane (analogous to that which lies under the epidermis and epithelium in the skin and mucous membrane, ( 640) ; and that when digestion is not going on, they are covered by an epithelium. The space between the reticulations of the blood-vessels and lymphatics, towards the extremity of each villus, is occupied, whilst the absorption of chyle is taking place, by a number of spherical vesicles or cells, varying in diameter from the l-1000th to 1 -2000th of an inch, and containing an opalescent fluid. At the part where the vesicles approach the granular texture of the substance of the villus, minute granular or oily particles are seen. When the intestine contains no more chyme, the vesicles disappear almost entirely, the lacteals empty themselves, and the villi become flaccid ; the epithelium, which had fallen off' during the process of Absorption, is then renewed. The vesicles at the extremities of the villi can scarcely be regarded in any other light than as cells, whose lives have but a very brief duration, selecting from the materials in contact with the surface of the villi, and appropriating these to their own growth, then liberating them, by solution or disruption of the cell-wall, in a situation where they can be absorbed by the lacteals. 462. In regard to the degree in which the function of Nutritive Absorption is performed by the Lacteals and by the Venous System respectively, con- siderable difference of opinion has prevailed. When the Absorbent vessels were first discovered, and their functional importance perceived, it was ima- gined that the introduction of alimentary fluid into the vascular system took place by them alone. A slight knowledge of Comparative Anatomy, however, might have sufficed to correct this error ; since no lacteals exist in the Inver- tebrated animals, the function of Absorption being performed by the Mesenteric veins alone ; from which it is evident that the veins do possess the power of absorption : and it is scarcely to be supposed that they should not exercise this power in Vertebrated animals also, since their disposition on the walls of the intestinal cavity is evidently favourable to it. On the other hand, the introduction of a new and distinct system of vessels would seem to indicate that they must have some special purpose ; and there can be no doubt that the absorption of nutritive matter is that for which they are peculiarly designed. The fluid found in the lacteals is almost invariably the same ; being that to which the name chyle has been applied, and which may be regarded as im- perfectly elaborated blood. It appears from the uniformity of its composition, which forms a striking contrast with the diversity of the food from which it is obtained, that the lacteals have in some degree the power of selecting the par- ticles of which it is composed ; and that, whilst they take up such a proportion of each class of alimentary materials, as will rightly blend with the rest in the nutritious fluid, they reject not only the remainder, but also (for the most part at least) any other ingredients which may be contained in "the fluid of the intestines. Such may be stated as the general result of the experiments that have been made to determine their function ; though it is unquestionable that extraneous substances, especially of a saline nature, occasionally find their way into this system of vessels. This may not improbably be due to a cor- respondence in the size and form of the ultimate particles of such substances, with those of the materials normally absorbed by the lacteals.* From Mr. Goodsir's observations, it would appear that the power of selection is a pecu- liar vital endowment of the cells at the extremities of the villi, rather than of * Experiments upon the function of Absorption in Plants, whose radical vessels have a corresponding power of selection, appear likely to assist in elucidating this interesting subject. By the experiments of Dr. Daubeny it has been ascertained, that if a plant absorb any particular saline compound, it can also be made to absorb those which are isomorphous with it, though it will reject most others. See Princ. of Gen. and Comp. Phys. 294. 348 OF DIGESTION AND NUTRITIVE ABSORPTION. the lacteals ; and it appears strictly analogous to the property which is pos- sessed by the different cells of Plants, of selecting, from the pabulum common to all of them, the materials requisite for the elaboration of their own peculiar products, such as colouring matter, starch, oil, &c. 463. On the other hand, the Veins seem to be less concerned in nutritive absorption, but take up from the alimentary canal a portion of almost any fluid matters which it may contain. This seems to have been established by the carefully conducted experiments of MM. Tiedemann and Gmelin, who mingled with the food of animals various substances, which, by their colour, odour, or chemical properties, might be easily detected in the fluids of the body. After some time the animal was examined ; and the result was, that unequivocal traces of the substances were not unfrequently detected in the venous blood and in the urine ; whilst it was only in a very few instances, that any indi- cation of them could be discovered in the chyle. The colouring matters em- ployed were various vegetable substances ; such as gamboge, madder, and rhubarb : the odorous substances were camphor, musk, assafoetida, &c. ; while, in other cases, various saline bodies, such as muriate of barytes, acetate of lead and of mercury, and some of the prussiates, which might easily be detected by chemical tests, were mixed with the food. The colouring matters, for the most part, were carried out of the system, without being received either into the veins or lacteals ; the odorous substances were generally detected in the venous blood and in the uterine, but not in the chyle ; whilst of the saline substances many were found in the blood and in the urine, and a very few only in the chyle. A similar conclusion might be drawn from the numerous instances in which various substances introduced into the intestines have been detected in the blood, although the thoracic duct had been tied ; but these results are less satisfactory, because even if there is no direct communication (as maintained by many) between the lacteals and the veins in the mesenteric glands, the partitions which separate their respective contents are evidently so thin, that transudation may readily take place through them. It would seem probable, that substances perfectly dissolved in the fluids of the stomach, are taken into the blood-vessels so copiously distributed on its walls, by the simple and necessary process of Endosmose ; in this manner we may account for the fact, that saline substances are for the most part readily absorbed into the blood ; and there seems reason to believe that the albuminous portion of the chyme, together with the saccharine principles or the products of their trans- formation, may thus be introduced directly into the circulating current, without passing through the lacteals. On this subject there is much need of further information. VII. Absorption by the General Surface. 464. The Mucous Membrane of the alimentary canal is by no means the only channel through which nutritive or other substances may be introduced into the system. In the lowest tribes of animals, and in the earliest condition of the higher, it would seem as if Absorption by the external surface is almost equally important to the maintenance of life, with that which takes place through the internal reflexion of it forming the walls of the digestive cavity. In the adult condition of the higher animals, however, the special function of the latter is so much exalted, that it usually supersedes the necessity of any other supply; and the function of the cutaneous and pulmonary surfaces may be considered as rather that of exhalation than of absorption. But there are peculiar conditions of the system, in which the imbibition of fluid through these surfaces is performed with great activity, supplying what would otherwise be a most important deficiency. It may take place either through the direct ABSORPTION BY THE GENERAL SURFACE. 349 application of fluid to the surface, or even through the medium of the atmo- sphere, in which a greater or less proportion of watery vapour is usually dis- solved. This absorption occurs most vigorously when the system has been drained of its fluid, either by an excess of the excretions, or by a diminution of the regular supply. It may be desirable to adduce some individual cases, which will set this function in a striking point of view. It is well known that shipwrecked sailors, and others, who are suffering from thirst, owing to the want of fresh water, find it greatly alleviated or altogether relieved, by dipping their clothes into the sea, and putting them on whilst still wet, or by frequently immersing their own bodies. Dr. Currie relates the case of a patient labour- ing under dysphagia in its most advanced stage ; the introduction of any nutriment, whether solid or fluid, into the stomacn, having become perfectly impracticable. Under these melancholy circumstances, an attempt was made to prolong his existence by the exhibition of nutritive enemata, and by im- mersion of the body, night and morning, in a oath of milk and water. During the continuance of this plan, his weight, which had previously been rapidly diminishing, remained stationary, although the quantity of the excretions was increased. How much of the absorption which must have taken place, to replace the amount of excreted fluid, is to be attributed to the baths, and how much to the enemata, it is not easy to say ; but it is important to remark that " the thirst, which was troublesome during the first days of the patient's ab- stinence, was abated, and, as he declared, removed by the tepid bath, in which he had the most grateful sensations." " It cannot be doubted," Dr. Currie observes, " that the discharge by stool and perspiration exceeded the weight of the clysters ;" and the loss by the urinary excretion, which increased from 24oz. to 36oz. under this system, is only to be accounted for by the cutaneous absorption. Dr. S. Smith mentions that a man who had lost nearly 31bs. by perspiration, during an hour and a quarter's labour in a very hot atmosphere, regained 8oz. by immersion in a warm bath at 95, for half an hour. The experiments of Dr. Madden* show that a positive increase usually takes place in the weight of the body during immersion in the warm bath, even though there is at the same time a continual loss of weight by pulmonary exhalation, and by transudationt from the skin. This increase was, in some instances, as much as 5 drachms in half an hour ; whilst the loss of weight during the previous half hour had been 6 drachms : so that if the same rate of loss were continued in the bath, the real gain by absorption must have been nearly an ounce and a half. Why this gain was much less than in the cases just alluded to, is at once accounted for by the fact, that there was no deficiency in the latter case in the fluids naturally present in the body. 465. The quantity of water which may be imbibed from the vapour of the atmosphere, would exceed belief, were not the facts on which the assertion rests, beyond all question. Dr. Dill relates the case of a diabetic patient, who for five weeks passed 24 Ibs. of urine every twenty-four hours ; his ingesta during the same period amounted to 22 Ibs. At the commencement of the disease he weighed 145 Ibs. ; and when he died, 27 Ibs. of loss had been sus- tained. The daily excess of the excretions over the ingesta could not have been less than 4 Ibs. ; making 140 Ibs. for the thirty-five days during which the complaint lasted. If from this we deduct the amount of diminution which the weight of the body sustained during the time, we shall still have 113 Ibs. to be accounted for, which can only have entered the body from the atmo- * Prize Essay on Cutaneous Absorption, pp. 5963. f That part of the function of cutaneous transpiration, which consists in simple exha- lation, is of course completely checked by such immersion; but that which is the result of an actual secreting process in the cutaneous glands (CHAP, xn.) is increased by heat, even though this be accompanied with moisture. 30 350 ON DIGESTION AND NUTRITIVE ABSORPTION. sphere. A case of ovarian dropsy has been recorded, in which it was observed that the patient, during eighteen days, drank 692 oz. or 43 pints of fluid, and that she discharged by urine and by paracentesis 1298 oz. or 91 pints, which leaves a balance of 606 oz. or 38 pints to be similarly accounted for.* The following remarkable fact is mentioned by Dr. Watson in his Chemical Essays. " A lad at Newmarket having been almost starved, in order that he might be reduced to a proper weight "for riding a match, was weighed at 9 A. M., and again at 10 A. M.; and he was found to have gained nearly 30 oz. in weight in the course of this hour, though he had only drunk half a glass of wine in the interim. A parallel instance was related to the Author by the late Sir G. Hill, then Governor of St. Vincent. A jockey had been for some time in training for a race in which that gentleman was much interested, and had been reduced to the proper weight. On the morning of the trial, being much oppressed with thirst, he took one cup of tea ; and shortly afterwards his weight was found to have increased ft Ibs.; so that he was incapacitated for riding. Nearly the whole of the increase in the former case, and at least three-fourths of it in the latter, must be attributed to cutaneous absorption ; which function was probably stimulated by the wine that was taken in the one case, and by the tea in the other. 466. Not only water, but substances dissolved in it, may be thus introduced. It has been found that after bathing in infusions of madder, rhubarb, and tur- meric, the urine was tinged with these substances ; and that a garlic plaster affected the breath, when every care was taken, by breathing through a tube connected with the exterior of the apartment, that the odour should not be received into the lungs.t Gallic acid has been found in the urine, after the external application of a decoction of a bark containing it ; and the soothing influence, in cases of neuralgic pain, of the external application of cherry- laurel water, is well known. Many saline substances are absorbed by the skin, when applied to it in solution ; and it is interesting to remark that, con- trary to what happens in regard to the absorption of these from the alimentary canal, they are for the most part more readily discoverable in the absorbents than in the veins. This is probably due to the fact, that the imbibition of them is governed entirely by physical laws ; in obedience to which, they pass most readily into the vessels which present the thinnest walls and the largest surface. In the intestines, the vascular plexus on each villus is far more extensive than the ramifying lacteal which originates in it ; and as the walls of the veins are thin, there is considerable facility for the entrance of saline . and other substances into the general current of the circulation ; but in the skin, the lymphatics are distributed much more minutely and extensively than the veins; and soluble matters, therefore, enter them in preference to the veins. The absorbent power of the Lymphatics of the Skin is well shown by the following experiment. A bandage having been tied by Schreger round the hind-leg of a Puppy, the limb was kept for twenty-four hours in tepid milk ; at the expiration of this period, the lymphatics were found full of milk, whilst the veins contained none. In repeating this experiment upon a young man, no milk could be detected in blood drawn from a vein. It has been shown by Miiller that when the posterior extremities of a Frog were kept for two hours in a solution of prussiate of potass, the salt had freely penetrated the lymphatics, but had not entered the veins. It does not follow, however, from these and similar experiments, that in all tissues the lymphatics absorb more readily than the veins; for as the capillary blood-vessels in the lungs are much more freely exposed to the surface of the air-cells than are the lymphatics, we should, on the principles just now stated, expect the former to * Madden, loc. cit. f Dunglison's Physiology, vol. L p. 644. ABSORPTION BY THE GENERAL SURFACE. 351 absorb more readily. This appears from experiment to be the fact ; for, when a solution of prussiate of potass was injected by Mayer into the lungs, the salt could be detected in the serum of the blood much sooner than in the lymph, and in the blood of the left cavities of the heart, before it had reached that of the right. 467. It is not, however, from the external world alone, that Animals derive the materials of their Nutrition. It has been stated ( 84) that the necessity for a constant supply of food, arises from the continual decomposition which is taking place within the living body ; and it will be hereafter shown that this decomposition is connected with the death of the cells, of which the several parts are constructed, these having an independent life of their own, and con- sequently a limited duration, which has no immediate connection with that of the organism at large ( 646). In every portion of the body, therefore, mate- rials for nutrition are continually being set free ; and we find a peculiar pro- vision for the re-introduction of these into the circulating fluid. All animals which have a lacteal system have also a lymphatic system, closely correspond- ing to it in aspect, but consisting of vessels that are distributed through the whole body, instead of on the intestinal surface only, permeating almost every tissue, and in many forming a most minute plexus. These vessels pass, like the lacteals, through conglobate glands, in which they are brought into inti- mate relation with blood-vessels ; and they empty their contents into the same receptacle, so as to pour them into the blood in precisely the same manner. The evident conformity in the nature of the fluid which these two sets trans- mit, joined to the fact of the fluid Lymph, like the Chyle, being conveyed into the general current of the circulation just before the blood is again trans- mitted to the system at large, almost inevitably leads to the inference, that the lymph is, like the chyle, a nutritious fluid, and is not of an excrementitious character, as formerly supposed. The following is the most recent compara- tive analysis of the two, as performed by Dr. G. O. Rees ; the fluids were procured from the lacteal and lymphatic vessels of a donkey, previously to their entrance into the thoracic duct ; the animal was killed seven hours after a full meal. Chyle. Lymph. Water 90-237 98-536 Albuminous matter - 3-516 1-200 Fibrinous matter 0-370 0-120 Animal extractive matter, soluble in water and alcohol 0-332 0-240 Animal extractive matter, soluble in water only 1-233 1-319 Fatty matter - 3-601 a trace Salts ; Alkaline chloride, sulphate and carbonate, with > n-711 0-585 traces of alkaline phosphate, oxide of iron 5 100-00 100-00 From this analysis it appears that the chief chemical difference between the chyle and lymph consists in a much larger proportion of assimilable sub- stances, albumen, fibrin, and fatty matter, contained in the former: the nature and amount of the less clearly-defined animal principles, and of the saline ingredients, appear to be nearly identical in both. 468. Hence it can scarcely be doubted that, to use Dr. Prout's expression, " a sort of digestion is carried on in all parts of the body ;" and that such of the products of that digestion as are fit to be again converted into organized tiss.ue, are re-introduced into the current of the circulation by the Lymphatics ; whilst those which have undergone too great a degree of decomposition, are carried off by the Excreting processes. In this way, Animals, when deprived 352 ON DIGESTION AND NUTRITIVE ABSORPTION. of a sufficient supply of food, may live upon their own flesh ; and there seems no reason why it should not be as capable of re-conversion into living tissues, as is that of the animals on which they feed. A very similar phenomenon is observed in Vegetables.* It may be stated, then, as a general fact, that the function of the Absorbent System is to take up, and to convey into the Circu- lating apparatus, such substances as are capable of appropriation to the nutri- tive process ; whether these substances be directly furnished by the external world, or be derived from the disintegration of the organism itself. We have seen that, in the Lacteals, the selecting power is such, that these vessels are not disposed to convey into the system any substances but such as are de- stined for this purpose ; and that extraneous matters are absorbed in preference by the Mesenteric Veins. The case is different, however, with regard to the Lymphatics ; for there is reason to believe that they are more disposed than the veins to the absorption of other soluble matters ; especially when these are brought into relation with the skin, through which the lymphatic vessels are very profusely distributed. 469. Since the time of Hunter, who first brought prominently forwards the doctrine alluded to, it has been commonly supposed that the function of the Lymphatics is to remove, by interstitial absorption, the effete matter, which is destined to be carried out of the system; and any undue activity in this pro- cess (such as exists in ulceration), or any deficiency in its energy (such as gives rise to dropsical effusions, and other collections of the same kind), have been attributed to excess or diminution in the normal operation of the Absorb- ent System. From what has been stated, however, it appears that the special function of the Lymphatics, like that of the Lacteals, is nutritive absorption ;t * See Principles of General and Comparative Physiology, 403. f The Author, at the time of the publication of the First Edition of this work, believed this view to be altogether novel; he has since learned, however, that a similar doctrine had been put forward by Dr. Moultrie, of South Carolina, in the American Journal of the Medical Sciences, for the year 1827. [In the American Journal of the Medical Sciences, for 1827, Dr. James Moultrie pub- lished an essay on the " Uses of the Lymph," in which, amongst other things, attempted to be sustained, will be found the following views. 1. The lacteals and lymphatics do not constitute, as they are supposed to do, the absorbent system of the animal economy ; they do not, as the absorbent theory supposes, remove from the organs the " cast off molecules" of which they are composed, or carry out of the body the "effete" particles disintegrated by the act of the assimilative function. The one is engaged in the preparation and introduction of chyle, and chyle only, into the blood; the other in elaborating an organizable product a recrementitious secretion destined to unite with it for objects of a common and nutritious nature. 2. The primary object of the lymph, and that for which it is made to commingle with the chyle in the thoracic duct, is the vitalization of the latter fluid. 3. The truly "effete" matter of the body is the carbonaceous element of the venous blood, to which may be added the urea or azotic element of the urine. Than these, we know of nothing to which that term can be applied. 4. The venous and not the lacteal or lymphatic system, therefore, is the " absorbent system," in any disintegratory or effete sense of the phrase. 5. Nature, in effecting the elimination of excrementitious matter from the constituency of the solid or fluid parts, appears to aim at restoring to the physical universe, the matter temporarily borrowed for subsistence, in a state of elementary simplicity, or an approximation there- to ; that is, the carbon as carbon, the azote as azote, and hydrogen and oxygen as hydro- gen and oxygen. The lungs she uses as one medium of escape ; the kidneys as a second ; and the skin as a third, &c. Hence, the carbonic acid gas of respiration; the urea of the kidneys, and the aqueous exhalations of the skin, pulmonary transpiration and urine. These doctrines have been regularly taught by Dr. M., in his course of lectures on physiology, delivered in the Medical College of the State of South Carolina, since the establishment of the College in 1833. They have also been recently enforced in a bro- chure published by Dr. M., in which he asserts and vindicates his claim to their paternity. On the Organic Functions of Animals. By JAMES MOULTBIE, M. D., etc., Charleston, S. C. 1844. M. C.] SUPPLY OF FOOD REQUIRED BY MAN. 353 and that the reception of any other substances into their interior, must he looked upon as resulting simply from the permeability of their walls. This statement applies to the not unfrequent occurrence of the absorption of bile and other fluids, from the walls of the cavities in which they were collected ; with regard to the absorption of pus, however, which has been occasionally noticed to take place, both from internal collections, and from open ulcers, it may be remarked, that the lymphatic vessels were not improbably laid open by ulceration ; since in no other way can be understood the entrance of glo- bules so large as those of pus into their interior. If this view of the function of the Lymphatics be correct, it follows that we must attribute to the Blood- vessels the absorption of the truly effete particles ; and in this there would seem no improbability. We know that venous blood contains the elements of two important excretions, that of the lungs and that of the bile, in a far higher amount than does arterial blood ; and we shall hereafter see, that there is a certain portion of the fluid which consists of "ill-defined animal principles" that seem ready to be thus thrown off. Moreover, the materials of a large part of the excretory products are probably derived from the processes of assimilation themselves ; each tissue appropriating' the principles which it needs, and leaving the remainder in the fluid as superfluous matter. It may be further remarked, that the reciprocal part which Hunter imagined the Arteries and Lymphatics to perform in the function of Nutrition, is quite inconsistent with what is now known of the nature of that process ; for, as will subsequently appear, it entirely consists in a reaction between the tissues and the nutritious fluid, in which the vessels have no share save as the channels of supply. When these channels are obstructed, or the supply of new matter is cut off in any other way, the removal of the old by interstitial absorption becomes evident ; and that this is accomplished at least as much by the veins as by the lymphatics, appears from the fact, that in some tissues, in which it may take place with rapidity, lymphatics do not exist. VIII. Supply of Food required by Man. 470. The quantity of food required for the maintenance of the Human body in health, varies so much with the age, sex and constitution of the indi- vidual, and with the circumstances in which he may be placed, that it would be absurd to attempt to fix any standard, which should apply to every particu- lar case. The appetite is the only sure guide for the supply of the wants of each ; but its indications must not be misinterpreted. To eat when we are hungry, is an evidently natural disposition ; but to eat as long as we are hungry may not always be prudent. Since the feeling of hunger does not depend so much upon the state of fulness or emptiness of the stomach, as upon the condition of the general system, it appears evident that the ingestion of food cannot at once produce the effect of dissipating it, though it will do so after a short time ; so that, if we eat with undue rapidity, we may continue swallowing food long after we have taken as much as will really be required for the wants of the system ; and every superfluous particle is not merely use- less but injurious. Hence, besides its other important ends, the process of thorough mastication is important, as prolonging the meal, and giving time to the system to become acquainted (as it were) that the supply of its wants is in progress ; so that its demand may be abated in due time to prevent the inges- tion of more than is required. It is very justly remarked by Dr. Beaumont, that the cessation of this demand, rather than the positive sense of satiety, is the proper guide. " There appears to be a sense of perfect intelligence con- veyed to the encephalic centre, which, in health, invariably dictates what quantity of aliment (responding to the sense of hunger and its due satisfaction) 30* 354 ON DIGESTION AND NUTRITIVE ABSORPTION. is naturally required for the purposes of life ; and which, if noticed and pro- perly attended to, would prove the most salutary monitor of health, and effectual preventive of disease. It is not the sense of satiety, for this is be- yond the point of healthful indulgence, and is Nature's earliest indication of an abuse and overburden of her powers to replenish the system. It occurs immediately previous to this ; and may be known by the pleasurable sensa- tions of perfect satisfaction, ease and quiescence of body and mind. It is when the stomach says, enough ; and it is distinguished from satiety by the differ- ence of sensations, the latter saying too much." Every medical man is well aware how generally this rule is transgressed ; some persons making a regular practice of eating to repletion ; and others paying far too little atten- tion to the preliminary operations, and thus ingesting more than is good for them, even though they may actually leave off with an appetite. 471. Although no universal law can be laid down for individuals, however, it is a matter of much practical importance to be able to form a correct average estimate ; and there is no more difficulty in accomplishing this than there is in forming tables of mortality. It is by estimating the average duration of life that Insurance Companies make their calculations of the probable duration of any one and though the actual result may, in every individual case, be dif- ferent from that estimate, yet it holds good perfectly well for a large number. It is from the experience afforded by the usual consumption of food by large bodies of men, therefore, that our data are obtained ; and these data are suf- ficient to enable us to predict with tolerable accuracy what will be required by similar aggregations, though they can afford no guide to the consumption of individuals. We shall first consider the quantity sufficient for men in regular active exercise ; and then inquire how far that may be safely reduced for those who lead a more sedentary life. The Diet-scale of the British Navy may be advantageously taken as a specimen of what is required for the first class. It is well known that an extraordinary improvement has taken place in the health of seamen during the last 80 years ; so that three ships can now be kept afloat with only the same number of men, which were formerly required for two. This is due to the improvement in the quality of the food, in combi- nation with other prophylactic means. At present it may safely be affirmed, that it would not be easy to conceive of any diet-scale more adapted to answer the required purpose. The health of crews that have been long afloat, and have been exposed to every variety of external conditions, appears to be pre- served (at least when they are under the direction of judicious officers), to the full as well as that of persons subject to similar vicissitudes on shore ; and there can be no complaint of insufficiency of food, although the allowance cannot be regarded as superfluous. It consists of from 31 to 35^ ounces of dry nutritious matter daily ; of this 26 oz. are vegetable ; and the rest animal ; 9 oz. of salt meat, or 4 oz. fresh, being the allowance of the latter. This is found to be amply sufficient for the support of strength ; and considerable variety is produced by exchanging various parts of the diet for other articles. This, however, is sometimes done erroneously ; thus 8 oz. of fresh vegetables, which contain only 1 oz. of solid nutriment, are exchanged for 12 oz. of flour, which is almost all nutritious. Sugar and Cocoa are also allowed; partly in exchange for a portion of the Spirits formerly served out, the dimi- nution of which, especially in the case of boys, has been attended with great benefit. 472. A considerable reduction in this amount is of course admissible, where little bodily exertion is required, and where there is less exposure to low tem- peratures. In the case of Prisoners, the diet should of course be as spare as possible, consistently with health ; but it should be carefully modified, in indi- vidual cases, according to several collateral circumstances, such as depression SUPPLY OF FOOD REQUIRED BY MAN. 355 of mind, compulsory labour, previous intemperate habits, and especially the length of confinement. It has been supposed by some, that prisoners require a fuller diet than persons at large ; this is probably erroneous : but more variety is certainly desirable, to counteract, as far as possible, the depressing influence of their condition upon the digestive powers. The circumstances which oc- curred at the Mil Ibank Penitentiary in 1823, form a lamentable warning against the reduction of the diet-scale to an insufficient amount. The allowance to the prisoners had formerly been from 31 to 33 oz. of dry nutriment daily, and the prison was considered healthy ; but in 1822, it was reduced to 21 oz. " The health of the prisoners continued unbroken for nearly six months ; but scurvy then showed itself unequivocally ; and out of 860 prisoners, 437, or 52 -per cent, were affected with it. The effect of previous confinement here became remarkable ; for those were chiefly attacked who had been in the prison for two years, a year, or six months. Again, the prisoners employed in the kitchen, who had 8 oz. of bread additional per day, were not attacked, except three who had only been there a few days. After the epidemic had spread to a great extent, it was found that the addition of 8 oz. to the daily allowance of vegetable food, and oz. to the animal, facilitated the operation of the reme- dies which were used for the restoration of the health of the prisoners. The effects of confinement have been further shown in the experience of the Edin- burgh House of Refuge, which was first established in 1832, for the reception of beggars during the cholera, and which has been continued to the present time. The diet was at first a quart of oatmeal porridge for each person, morning and evening; and at dinner 1 oz. of meat, in broth, with 7 oz. of bread ; making altogether about 23 oz. of solid food a day. During some months, this diet seemed to answer very well ; the people went out fatter than they came in, owing to the diet being better than that to which they had been accustomed ; but afterwards, a proneness to disease manifested itself in those who had been residents there for a considerable time, and the diet was, there- fore, somewhat increased, with good effect. The quantity of animal food was probably here too small ; and the total weight might still have been sufficient, if it had been differently apportioned. In a Convict-ship, which took out 433 prisoners to New Holland in 1802, the mortality was very trifling, and the general health good ; although these prisoners were supported on 16 oz. of vegetable food, and 7 5 oz. of animal food per day, a quantity which was found to be perfectly sufficient for them. The aged inmates of work-houses, especially those who have been accustomed to poor food during their whole lives, require much less than this ; their vital functions being comparatively inactive, and their amount of labour or exercise small. In the Edinburgh work-house, of which the inmates have usually good health, they are fed upon oatmeal-porridge morning and evening, with barley-broth at dinner; the total allowance of dry nutriment is about 17 oz., namely, 13 oz. vegetable, and 4 oz. animal. It is a curious effect of insufficient nutriment, as shown by the recent inquiries of Chossat,* that it produces an incapability of digesting even the limited amount supplied. He found that, when turtle-doves were supplied' with limited quantities of corn, but with water at discretion, the whole amount of food taken was scarcely ever actually digested ; a part of it being rejected by vomiting, or passing off by diarrhoea, or accumulating in the crops. It seems as if the vital powers were not sufficient to furnish the requisite supply of gastric fluid, when the body began to be enfeebled by insufficient nutrition ; or perhaps we might well say, the materials of the gastric fluid were wanting. Hence the loathing of food, often manifested by those who have been subjected to the influence of an insufficient diet-scale in our prisons and poor-houses, * Recherches Experimentales sur 1'Inanition, 1843. 356 ON DIGESTION AND NUTRITIVE ABSORPTION. which has been set down to caprice or obstinacy, and punished accordingly, may be actually a proof of the deficiency of the supply, which we might expect to have been voraciously devoured, if really less than the wants of the system require. 473. The smallest quantity of food upon which life is known to have been supported with vigour, during a prolonged period, is that on which Cornaro states himself to have subsisted. This was no more than 12 oz. a day, chiefly of vegetable matter, for a period of 58 years. There is only one instance on record in which his plan was followed ; and there are probably few who could long persevere in it, at least among those whose avocations require much mental or bodily exertion. It is certain, however, that life, with a moderate amount of vigour, may be preserved for some time with a very limited amount of food ; this appears from the records of shipwreck and similar disasters. In regard, however, to those who have been stated to fast for a period of months or even years, taking no nutriment, but maintaining an active condition, it may be safely asserted that they were impostors, probably possessing unusual powers of abstinence, which they took care to magnify. The instances in which the life of man, or of other mammalia, has been prolonged to the greatest extent without water, are those in which, from the peculiarity of the circum- stances, the cutaneous exhalation must have been reduced to a very small amount, or in which there may have been an actual absorption of water by the skin and lungs. Thus, Fodere mentions that some workmen were extricated alive, after fourteen days' confinement in a cold damp cavern, in which they had been buried under a ruin. And there is a well-known case of a hog, which was buried in its stye for 160 days, under thirty feet of the chalk of Dover clifT, and was dug out alive at the end of that time, reduced in weight from 160 Ibs. to 40 Ibs.: here the temperature would be kept up by the non- conducting power of the chalk around ; and the air surrounding the animal would soon become sufficiently charged with fluid to resist further evaporation. The time during which life can be supported under total abstinence, is usually stated to vary from 8 to 10 days ; the period may be greatly prolonged, how- ever, by the occasional use of water, and still more by a very small supply of food. In a case recorded by Dr. Willan, of a young gentleman who starved himself under the influence of religious delusion, life was prolonged for 60 days ; during the whole of which time nothing else was taken than a little orange juice. In a somewhat similar case which occurred under the Author's notice, in the person of a young French lady, more than 15 days elapsed between the time that she ceased to eat regularly and the time of her being compelled to take nourishment ; during this period she took a good deal of exercise, and her strength seemed to suffer but little, although she swallowed solid food only once, and then in small quantity. If the cessation of muscular exertion be complete, it seems that life is usually more prolonged than where exercise of any kind is performed ; and this is what might naturally be expected. In certain states of the system, commonly known as Hysterical, there is frequently a very remarkable disposition for abstinence, and power of sustaining it. In a case of this kind, which occurred under the Author's own notice, a young lady who had suffered severely from the tetanic form of hysteria was unable to take food for three weeks. The slightest attempt to introduce a morsel of solid matter into the stomach, occasioned very severe vomiting and retching ; and the only nourishment taken during the period mentioned, was a cup of tea once or twice a day, on many days not even this being swallowed. Yet the strength of the patient rather increased than diminished during this period ; her muscles became firmer and her voice more powerful. It may be well to remark that, under such circumstances, the continual persuasions of anxious SUPPLY OF FOOD REQUIRED BY MAN. 357 friends are very injurious to the patient ; who is much more likely to come round, if left entirely to herself. 474. Of the quantity which can be devoured at a time, it is scarcely the place to speak ; since such feats of gluttony only demonstrate thdtextraordinary capacity which the stomach may be made to attain by continual practice. Many amusing instances are related by Captain Parry in his Arctic Voyages ; in one case, a young Esquimaux, to whom he had given (for the sake of curiosity) his full tether, devoured in four-and-twenty hours, no less than 35 Ibs. of various kinds of aliment, including tallow candles. A case has recently been published of a Hindoo, who can eat a whole sheep at a time ; this pro- bably surpasses any other instance on record. The half-breed voyageurs of . Canada, according to Capt. Franklin, and the wandering Cossacks of Siberia, as testified by Capt. Cochrane, habitually devour a quantity of animal food which would be soon fatal to any one unused to it. The former are spoken of as very discontented when put on a short allowance of 8 Ibs. of meat a day ; their usual consumption being from 12 to 20 Ibs. That a much larger quantity of food than that already specified, may be taken with perfect freedom from injurious consequences, under a particular system of exercise, &c., appears from the experience of those who are trained for feats of strength, pugilistic encounters, &c. The ordinary belief, that the athletic constitution cannot be long maintained, appears to have no real foundation ; nor does it appear that any ultimate injury results from the system being persevered in for some time. That trained men often fall into bad health on the cessation of the plan, is probably owing in part to the intemperance and other bad habits of persons of the class usually subjected to this discipline. The effects of trainers' regi- men are, hardness and firmness of the muscles, clearness of the skin, capability of bearing continued severe exercise, and a feeling of freedom and lightness (or 'corkiness') in the limbs. During the continuance of the system, it is found that the body recovers with wonderful facility from the effects of injuries ; wounds heal very rapidly ; and cutaneous eruptions usually disappear. Clear- ness and vigour of mind, also, are stated to be results of this plan ; and it is probable that, where persevering attention and intense application are neces- sary, a modification of this system, in which due allowance should be made for the diminished quantity of exercise, would be found advantageous.* * The method of training employed by Jackson (a celebrated trainer of, prize-fighters in modern times), as deduced from his answers to questions put to him by John Bell, was to begin on a clear foundation, by an emetic and two or three purges. Beef and mutton, the lean of fat meat being preferred, constituted. the principal food; veal, lamb and pork, were said to be less digestible ("the last purges some men"). Fish was said to be " a watery kind of diet ;" and is employed by jockeys who wish to reduce weight \ty sweating. Stale bread was the only vegetable food allowed. The quantity of fluid permitted was 3 pints per diem ,- but fermented liquors were strictly forbidden. Two full meals, with a light supper, were usually taken. The quantity of exercise employed was very consider- able, and such as few men of ordinary strength could endure. This account corresponds very much with that which Hunter gave of the North American Indians, when about to set out on a long march. 358 OF THE CIRCULATION OF BLOOD. CHAPTER IX. OF THE CIRCULATION OF BLOOD. I. Of the Circulation in General. 475. THE Circulation of nutritive fluid through the body has for its object, to convey to every part of the organism the materials for its growth and reno- vation, as well as the supply of oxygen which is required for the performance of its vital actions : and also to carry off the particles which are set free by the disintegration or waste of the system, and which are to be removed from it by excreting processes. Of these processes, the one most constantly in operation as well as most necessary for the maintenance of the purity of the blood, is the extrication of carbonic acid from the respiratory organs ; and this is made sub- servient to the introduction of oxygen into the system. The extent, therefore, to which a circulating apparatus is developed in the animal kingdom, is partly dependent upon the degree in which the function of nutritive absorption is limited to one part of the body, and partly upon the arrangement of the excret- ing surfaces, and especially of the respiratory apparatus. Where the digestive cavity itself extends through the whole system, so that every part can absorb at once from its parietes, and where the whole external surface is adapted, by its softness and permeability, to expose the fluids of the body to the aerating medium around, there is no necessity for any transmission of fluid from one part to another ; and accordingly, in the lowest animals, which are thus formed, no true circulation exists. Again, in the insect tribes, in whose bodies the absorption of fluid can only take place at fixed points, there is a circulation, for the purpose of transmitting the absorbed matter to the remote parts of the body ; but, as every part of the interior is permeated by air, the second of the above- named purpose is already answered ; and the circuit of the blood through the vessels, therefore, is not accomplished with the energy and activity which, from the vigorous movements performed by these little beings, might have been supposed necessary. On the other hand, in the Mollusca, the absorption of fluid is liraited, and the respiratory action equally so ; and among these we find the circulation performed with nearly as much vigour as it is in the Vertebrata. * 476. In Man, as in other Vertebrated animals, there is a regular and con- tinuous movement of the nutritive fluid through the vascular system; and upon the maintenance of this, the activity of all parts of the organism is de- pendent. The course of the Blood may be likened to the figure 8 ; for there are two distinct circles of vessels through which it is transmitted; and the Heart is placed at the junction of these. The Systemic and Pulmonary circu- lations are entirely separate, and might be said to have distinct hearts ; for the left and right sides of the heart, which are respectively appropriated to these, have no direct communication with each other, (in the perfect adult condition, at least,) and are merely brought together for economy of material. At an early period of fetal life, as in the permanent state of the Dugong, the heart is so deeply cleft, from the apex towards the base, as almost to give the idea of two separate organs. Each system has its own set.of arteries, or efferent vessels, and veins or afferent trunks; thse communicate at their central OF THE CIRCULATION IN GENERAL. 359 extremity by the Heart ; and at their peripheral extremity by the Capillary vessels, which are nothing else than the minutest ramifications of the two sys- tems, inosculating into a plexus. The systemic arteries all proceed from one trunk, the Aorta; which first ascends, and gives off -branches to the head and superior extremities ; then descends through the thorax and abdomen, giving off branches to the parts near which it passes ; and terminates in the two large trunks that proceed to the inferior extremities. Although the diameters of the branches, at each subdivision, together exceed that of the trunk, yet there is but little real difference in their size. For, according to a well-known geo- metrical law, the areas of circles are as the squares of their diameters; and, as the calibre of a tube is estimated by its area, not by its diameter, it follows that, in comparing the size of a trunk with that of its branches, we are to square the diameter of the former, and compare the result with the sum of the squares of the diameters of the branches. When this is done, there is found to be a very close correspondence. The following table gives the result of eight measurements, taken with a view to determine the question. The first three were taken from the mesenteric artery of a Sheep ; the next three from the aorta and iliac arteries ; the last two from the Horse.* TRUNK. Diameter. I. 9 II. 7.2 III. IV. V. VI. VII. VIII. 3.5 7.0 17 10 4.5 8 Square. 81 51.64 12.25 49 289 100 20.25 64 BRANCHES. Diameters. 7.5+5 6+4 3+2 5 + 5 10+10+9.5 7+7+2 3.5 + 3 4+7 Sum of Squares. 81.25 52 13 50 290.25 102 21.25 65 The discrepancy between the two results must be considered extremely small, Fig. 88. Web of Frog's foot, stretching between two toes, magnified 3 diameters showing the blood-vessels and their anastomoses; 1, 1, veins ; 2, 2, 2, arteries. (After Wagner.) * Ferneley, in Medical Gazette, Dec. 7, 1839. 360 OF THE CIRCULATION OF BLOOD. when it is stated that the unit, in the above measurements, is no more than one-fortieth of an inch; and when it is remembered that any error in the measurement is greatly increased in the calculation.* 477. The ultimate ramifications of the Arteries usually pass so insensibly into those of the Veins, that the line of demarkation between them cannot be distinctly drawn. Hence there is no ground for the assumption that the Capillaries form a distinct system of vessels, in which the Arteries terminate and the Veins arise. They are in no respect different, except in size, from the other vessels. Their anastomosis is very frequent, so that a minute net- work is formed by them ; but this is also seen in the distribution of the larger vessels. It has been maintained by some, that they are mere passages, chan- neled out of the tissues through which they convey the blood ; but this, again, is incorrect, for recent microscopical observations have shown, that they have Fig. 89. a A Capillary circulation in a portion of the web of a .Frog's foot, magnified 110 diameters; 1, trunk of vein ; 2, 2, 2, its branches ; 3, 3, pigment-cells. (After Wagner.) * From Mr. Paget's observations, it appears that there is seldom an exact equality be- tween the area of the trunk and that of its branches; but the area sometimes increases, and sometimes diminishes ; the former being the general rule for the subdivision of the aorta and its principal branches in the upper extremities ; the latter in the lower. The following Table shows the relative areas of several arterial trunks, and of the branches proceeding from them. Trunk. Arch of Aorta Innominata - ... Common carotid ... External carotid Subclavian - Abdominal Aorta, to last lumbar art. , just before dividing Common Iliac External Iliac Branches. 1-055 1-147 1-013 1-19 1-055 1-183 893 982 1-15 OF THE CIRCULATION IN GENERAL. 301 distinct parietes, and that these are composed of a fibrous structure analogous to the muscular. Their mode of origin, again, refutes such a supposition; for there can be no doubt that they are produced, in any newly forming tissue, not by the retirement of its cells, one from the other, so as to leave passages between them, but by the formation of communications among cer- tain cells, whose cavities become connected with each other, so as to constitute a plexus of tubes, of which Fig. 90. the original cell-walls become the parietes. The pro- cess may be most clearly traced in Plants, in which (among the Phanerogarnia at least) a distinct system of capillary vessels exists ; but it may also be seen in the germinal membrane of the Ovum, in which the capillaries distinctly originate before the trunks ; and this view of it is confirmed by the fact, that the nuclei or cytoblasts of the original cells, may often be seen imbedded in the walls of the fully-formed capillary vessels. In regard to the size of the capillary vessels, there is considerable variation ; some being so small, as only to admit globules of blood in single file ; whilst First appearance of blood- others, passing directly between arteries and veins, vessels in the vascular layer admit several rows at once. From the measurements of ' he 7 gem ? na ' '' bnn ? ( e TTT i it/f n 11- i a Fowl at the 36th hour of m- of Weber, Muller, and others, it appears that the cuba uon. (After Wagner.) capillaries in Man vary from about ^Tn^ to T3Vo tn of an inch ; whilst the blood-corpuscles vary from about T ^j- 7 th to o-j V^ tn f an inch in diameter. As the capillaries cannot be examined in the human body until after death, and then only by means of forcible injections, these measurements may be somewhat inaccurate. To the larger tubes, (which may perhaps be more numerous in cold-blooded than in warm-blooded Verte- brata), some would deny the term capillary; but in the sense in which that word is here employed, it is strictly applicable to all those minute vessels which connect the arterial and venous systems. 478. The Size of the Capillary vessels in any part is continually under- going variation ; sometimes all of them enlarging or contracting simultaneously ; and one sometimes contracting, whilst others enlarge. In regard to the first of these phenomena, more will be said hereafter ; the second is here noticed, because it explains an occasional appearance, on which some have founded their belief in the non-existence of distinct parietes to these vessels. In watching the capillary circulation in any transparent part, we not unfrequently see the globules of blood running into passages of the tissue, which we did not perceive before ; but on a more careful examination, the observer may satisfy himself that these passages existed previously, and that the fluid part of the blood was transmitted through them ; the stoppage of the red particles being in a great measure dependent on some partial or local impediments. The compression of one of the small arteries, for instance, will generally occasion an oscillation of the globules of blood in the smallest capillaries, which will be followed by the disappearance of some of them ; but when the obstruction is removed, the blood soon regains its former velocity and force, and flows exactly into the same passages as before.* It may also be frequently observed, that the rate of motion is very different in the different parts of the network ; and that an entire stagnation of the current sometimes takes place in some particular tube, the motion of the globules recommencing, but in an opposite direction. Irregularities of this kind, however, are more frequent when the Heart's action is partially interrupted ; as it usually is by the pressure to which the * Dr. Allen Thomson, in Cyclop, of Anat. and Phys., Art. Circulation. 31 362 OF THE CIRCULATION OF BLOOD. Tadpole or other animal must be subjected, in order to allow microscopic observations to be made upon its circulation. Under such circumstances, the varieties in the capillary circulation, induced by causes purely local, become very conspicuous ; for when the whole current has nearly stagnated, and a fresh impulse from the heart renews it, the movement is not by any means uniform (as it might have been expected to be) through the whole plexus supplied by one arterial trunk, but is much greater in some of the tubes than it is in others ; the variation being in no degree connected with their size, and being very different at short intervals. 479. The opinion was long entertained, that there are vessels adapted to the supply of the white or colourless tissues ; which carry from the arteries the liquor sanguinis, or fluid portion of the blood, leaving the globules behind. Many objections might be raised against such a supposition ; one of the most obvious of which is the mechanical obstacle that would be created at the entrances to such a system of tubes, by the retention of the globules in the larger vessels from which they diverged. No such vessels have ever been observed ; and it may be safely affirmed, that the supposition of their existence is not required. For any one who observes the smaller capillary vessels may perceive, that the current of blood which passes through them is entirely free from colour, as the corpuscles themselves appear to be when spread out in a single layer. Tissues which are rather scantily permeated by such vessels, therefore, may still be white ; and it is only where the network is very close, and the quantity of blood which passes through it is consequently great, that a perceptible colour will be communicated by the red corpuscles. On the other hand, the supposition that Nutrition can only be carried on by means of Capillary vessels, is entirely gratuitous, as will be hereafter shown (CHAP. xi.) ; and it would appear from the late researches of Mr. Toynbee,* that car- tilages in general, the true cornea, crystalline lens, and vitreous humour, together with the epidermic appendages, are entirely destitute of them. He has demonstrated, by means of injections, that the arteries, which previous anatomists had supposed to penetrate into their substance, either as serous vessels, or as red vessels too minute for injection, actually terminate in veins before reaching them ; he also shows that, around these non-vascular tissues, there are numerous vascular convolutions, large dilatations, and intricate plexuses of blood-vessels (Fig. 112) : the object of which he believes to be to arrest the progress of the blood, so that its nutrient portion may penetrate into and be diffused through them. There is, as will hereafter appear, no essential difference between the nutrition of the non-vascular tissues, and that of the islets in the midst of the network of capillary vessels which traverses the most vascular (Fig. 89). In both cases the nutrient materials conveyed by the blood are absorbed by the cells of the tissue immediately adjoining 'the vessels, and are imparted by them to others which are further removed ; and the only variation that exists, is in the amount of the portion of tissue,' which has to be thus traversed. There is great variety in this respect, among the tissues that are traversed by vessels ; and we are only required to extend our ideas from the largest of the islets which we find in these, to the still more isolated struc- tures, of which the non-vascular tissues are composed. In the Vegetable Kingdom, as among the lowest Animals, there are entire organisms of con- siderable size, throughout which nutriment is conveyed by mere transudation from cell to cell ; and this seems to be the case, in "those parts of the highest Animals in which the vital changes are least active. * Philosophical Transactions, 1841. * ACTION OF THE HEART. 363 II. Action of the Heart. 480. The Heart is endowed in an eminent degree with the property of irritability, by which is meant the capability of being easily excited to move- ments of contraction alternating with relaxation ( 366). Thus, after the Heart has been removed from the body, and has ceased to contract, a slight irritation will cause it to execute, not one movement only, but a series of alternate contractions and dilatations, gradually diminishing in vigour until they cease. To this property, the contact of blood with the membrane lining its cavity, appears to be the usual stimulus ; and, when this is withdrawn, its action will cease after a certain time ; whilst its movements may be prolonged, by means of artificial respiration (which assists in maintaining the circulation through the lungs), for a much greater duration, even after the Brain and Spinal Cord have been removed, and when animal life is, therefore, completely extinct. Hence we see that the Irritability of this organ must be an endow- ment properly belonging to it,' and not derived from the Nervous System. Like the contractility of other muscles, it can only be sustained for any great length of time by a supply of Arterial blood to its own tissue ( 392). It is much less speedily lost in cold-blooded animals, however, than in warm- blooded ; the heart of the Frog, for example, will go on pulsating for many hours after its removal from the body ; and it is stated by Dr. Mitchell* that the heart of a Sturgeon, which he had inflated with air, continued to beat until the auricle had absolutely become so dry as to rustle during its movements. It is commonly supposed that when it is empty of blood, the contact of air with its internal cavities is the stimulus by which the irritability is excited ; but Dr. J. Reid has proved that this is not a sufficient explanation, by placing under an air-pump a Frog's heart in a state of activity, which still continued after the receiver had been exhausted.! It is thought by Dr. Alison, that the succession of movements may be in some degree accounted for, by the pecu- liar arrangement of the fibres of the heart, which may cause one set, in con- tracting, to press on and irritate another ; and this idea may be considered as by no means unworthy of adoption, although it can scarcely account for the whole of the phenomena. In all experiments made upon the cause of the Heart's contraction, "it must be carefully borne in mind, that the slightest dis- turbance of the organ will frequently renew its motions, after they have ceased for some time ; the neglect of which fact has led to several erroneous conclu- sions. It has been thought by some that the contraction of the ventricle is the necessary sequence of the contraction of the auricle, a doctrine which might seem to follow inevitably from the circumstance (ascertained by Dr. Knox) that when the irritability is nearly exhausted, contractions excited in the auricle are sometimes followed by contractions of the ventricle, when irritation of the outer surface of the ventricle itself produced no effect. But it is to be remembered that the irritability of the internal surface is much greater than that of the external ; and that the movement of the auricle will excite that of the ventricle, by forcing blood into its cavity, and thus renewing the usual stimulus. That this is the true explanation, is shown by the facts observed by Dr. Reid and others, that the usual relation between the movements of the auricles and ventricles is often so much disturbed, when the irritability is becoming exhausted, that these do not regularly alternate with each other, that the- contraction of the auricle frequently ceases before that of the ven- tricle, on the left side particularly, and that both sets of movements will con- tinue, when the auricle and ventricle have been separated from each other. * American Journal of the Medical Sciences, vol. vii. p. 58. f Cyclopaedia of Anatomy and Physiology, vol. ii. p. 611. 364 OF THE CIRCULATION OF BLOOD. 481. It was formerly supposed, that the movements of the Heart were de- pendent upon its connection with the centres of the Cerebro-Spinal nervous system ; and the experiments of Legallois and others, who found that they were arrested by crushing, or otherwise suddenly destroying, large portions of these centres, appeared to favour the supposition. But it has been shown by Dr. Wilson Philip and his successors in the same inquiry, that the whole Cerebro-Spinal axis might be gradually removed, without any such conse- quence ; which fact harmonizes perfectly with the " experiments prepared for us by Nature," in the production of monsters destitute of these centres, which nevertheless possessed a regularly pulsating heart. It has latterly been the fashion with many, however, to attribute the action of the Heart to the Gan- glionic system ; but of this there is no sufficient evidence. As has already been generally remarked on this subject, the possibility of exciting the action of the heart through the Sympathetic nerve, shows that this may have an influence on its movements ; whilst the great difficulty with which any evi- dence to this effect can be procured, is in itself a sufficient proof, that this influence cannot be nearly adequate to the constant maintenance of this ener- getic function ( 209). It would appear, however, that changes in the Gan- glionic nerves, like strong impressions upon the cerebro-spinal system, may have the effect of impeding or even checking the Heart's action ; for a case has lately been recorded, in which the movements were occasionally checked for an interval of from 4 to 6 beats, its cessation of action giving rise to the most fearful sensations of anxiety, and to acute pain passing up to the head from both sides of the chest, these symptoms being connected, as it proved on a post-mortem examination, with the pressure of an enlarged bronchial gland upon the great cardiac nerve.* It may be surmised, that in many cases of angina pectoris, in which no lesion sufficient to account for death could be discovered, some affection of the cardiac plexus might have been traced on a more careful examination. Brachet has asserted that, by section of the cardiac ganglion, he has caused the movements of the heart to be suddenly arrested ; but this result has not been confirmed by other experimenters. 482. When the Heart is exposed in a living animal, and its movements are attentively watched, they are seen to be of a peculiarly rhythmical character ; one series following another with great regularity. In an active and vigorous state of the circulation, however, they are so linked together, that it is not easy to distinguish them into periods. A case has recently fallen under the notice of Prof. Cruveilhier, in which the heart was exterior to the chest, having escaped from it by a perforation in the superior part of the sternum ; and his observations upon it may be perhaps regarded as more satisfactory than such as are made after the very severe operation required for the artificial exposure of the organ ; although they are liable to some exception, from the very early age of the subject of them, which had only been born nine hours. His con- clusions will be here adopted, with such additional remarks as are suggested by the experimental researches of others, who have made this question a subject of special attention.! It is universally admitted, that both Auricles contract, and also dilate simultaneously; and that both Ventricles do the same : also that the systole or contraction of the ventricles corresponds with the projection of blood into the arteries, causing the pulse ; whilst the diastole or dilatation of the ventricles coincides with the collapse of the arteries. It is further admitted, that the contraction of the Ventricles, and that of the Auri- cles, alternate with one another ; each taking place (for the most part, at least), during the dilatation of the other. But it is a question, whether there is any * Mailer's Archiv. 1841, heft iii.; and Brit, and For. Med. Rev., Oct. 1841. t sp e also another case, recently observed by M. Monod, in Bullet, de 1'Acad. de Med. Fevr., 1843; and Edinb. Med. and Surg. Journ., July 1843. ACTION OF THE HEART. 365 interval between them. In the case just alluded to, the contraction of the Ventricles is stated to have been precisely synchronous with the dilatation of the Auricles ; and the dilatation of the Ventricles to have been performed at the same time with the contraction of the Auricles, no period of repose inter- vening between the two sets of actions. It appears, however, from the con- current testimony of numerous experimenters, that whilst the contraction of the Ventricle immediately succeeds that of the Auricle, an interval, which is usually, however, extremely brief, may elapse between the partial dilatation of the Ventricles and the succeeding- systole of the Auricles. The Ventricular diastole max be distinguished into two stages, of which the first immediately succeeds its systole, and manifests itself in the recession of the Heart's apex from the front of the chest ; whilst the second is attended with an enlargement of the heart in all its dimensions, and is synchronous with the Auricular con- traction. It is between these two that the interval of *repose occurs where it can be observed. The following tabular view will, perhaps, make this account more intelligible ; it is framed in such a manner as to commence with the Auricular contraction ; but when considering the Sounds of the heart, it will be necessary to commence with the Ventricular systole. Auricles. Ventricles. Contraction. 2d stage of dilatation. Dilatation. Contraction. Pulse. 1st stage of dilatation. Brief interval of Repose. Contraction. 2d stage of dilatation. Dilatation. Contraction. Pulse. 483. The duration of the contraction of the Ventricles is, according to Cruveilhier, double that of their Dilatation, and the same holds good of the Auricles. In the Systole of the Ventricles, their surface becomes rugous; the superficial veins swell ; the carnese columnae of the left ventricle are deline- ated ; and the curved fibres of the conical termination of the left ventricle, which alone constitutes the apex of the heart, become more manifest. During their contraction, every diameter of the Ventricles is lessened ; their shorten- ing is the most sensible change ; but this is owing to the vertical diameter being the greatest. The lower extremity of the left ventricle, or, in other words, the apex of the heart, describes a spiral movement from right to left, and from behind forwards. It is to this slow, gradual, and as it were succes- sive spiral contraction, that the forward movement of the apex of the heart is owing, and its consequent percussion against the thoracic parietes. The ventricular systole is not accompanied by a projection of the entire heart for- wards (as some have maintained) ; for it is exclusively the spiral contraction, which determines the approach of the apex of the heart and the thoracic parietes. The Diastole of the heart, according to Cruveilhier, has the rapidity and energy of an active movement, triumphing over pressure exercised upon the organ, so that the hand closed upon it is opened with violence. This is an observation of great importance ; but of the cause to which this active dila- tation is due, no definite account can be given. It may partly be explained, perhaps, by the elasticity of the tissue, interwoven with muscular fibre in the substance of the heart; and this may be the cause of the first Ventricular dila- tation, the second being produced by the ingress of blood occasioned by the auricular systole. But the dilatation of the Auricles appears to be much greater than can be accounted for by any vis a tergo (which, as will hereafter appear, is extremely small in the venous system), or by the elasticity of it! substance ; for it was observed in this case to be so great, that the right auricle 31* 366 * OF THE CIRCULATION OF BLOOD. seemed ready to burst, so great was its distension, and so thin were its walls. Moreover, the large Veins near the heart contract simultaneously with the auricular Systole, and not with its Diastole ; so that they can have no influence in causing its dilatation. The Ventricular diastole is accompanied with a projection of the heart downwards ; this motion was at its maximum when the child was placed vertically, and was very strongly marked. 484. When the ear is applied over the cardiac region, during the natural movements of the Heart, two successive sounds are heard ; each pair of which corresponds with one pulsation. The whole interval between one beat of the Heart, and the next, may be divided into four parts ; of which ^ie two first are occupied by what is commonly known as the first sound ; the third, by the second sound ; whilst the fourth is a period of repose. The first sound is dull and prolonged ; it is evidently synchronous with the impulse of the Heart against the parietes of the chest, and also with the pulse, as felt rtfear the heart ; it must, therefore, be produced during the Ventricular Systole. The second sound follows so immediately upon the conclusion of the first, that it can scarcely be imagined to take place during the auricular systole, as some have supposed, but must be assigned to the period of the first stage of the Ventricular Diastole. This, indeed, may now be regarded as clearly established ; for it has been fully demonstrated, that the second sound is due to the sudden filling-out of the Semilunar valves of the aorta and pulmonary artery, with blood ; when the outward current through them has ceased, and the incipient dilatation of the ventricles occasions a vacuum behind them. If one of these valves be hooked back by a curved needle against the side ofthe artery, so that a reflux of blood is permitted, the sound is entirely suppressed. 485. The first sound cannot be so readily or satisfactorily accounted for. That it is partly due to the Impulse of the apex of the Heart, seems proved by the fact that, when this impulse, is prevented, the sound is much diminished in intensity ; and also by the circumstance, that, when the Ventricles contract with vigour, the greatest intensity of the sound is over the point of percussion. But that it is not entirely due to this cause is also evident, from the fact, that a sound may still be heard, when the Heart is contracting out of the body : as in the case observed by Prof. Cruveilhier. This sound has been attributed by some experimenters to the flapping-back of the auriculo-ventricular valves ; by others, to the muscular contraction of the walls of the ventricles; by others again to the rush of blood along the irregular walls of the ventricles, and through the comparatively narrow orifices of the aorta and pulmonary artery. This last is probably the most consistent with truth ; as would appear from the following interesting observations made by Cruveilhier. By apply- ing the finger to the origin of the pulmonary artery (which is situated in front ofthe aorta, and completely conceals it), a perfectly distinct vibratory fremisse- ?nent, corresponding with the ventricular diastole, was perceived : but no such vibratory thrill could be felt by the finger, when applied to any part of the base of the ventricles : whence it was evident, that no action takes place in the mitral and tricuspid valves, which can give rise to the same palpable effects as those produced by the semilunar valves. The same was ascertained regard- ing the valvular sound, which could be distinctly heard by laying the finger against the origin of the pulmonary artery, and applying the ear to it as to a stethoscope : whilst nothing of the kind could be perceived in the region of the auriculo-ventricular valves. Hence it seems quite certain, that the natural first sound cannot be dependent in any way upon the action of the mitral and tricuspid valves. It appeared, on the contrary, that the maximum intensity of the first sound was in precisely the same situation as the maximum intensity |f the secondnamely, at the origin of the large arteries; and that it diminished, as the ear was carried from the base towards the apex of the ACTION OF THE HEART. 367 heart. Moreover, the first sound was observed to be of exactly the same character with the second (the complicating effect of the impulse being here withdrawn), except as to its intensity, which was less, and its duration, which was greater. 486. Hence, although these observations do not entitle us to dehy the par- ticipation of the muscular contraction, and of the movement of the blood over the ventricular walls, in the production of the first sound, they establish (if correct), that the principal cause of it exists at the entrances to the arterial trunks ; and it does not seem that any other reason can be assigned for it than the prolon^d rush of blood through their orifices, and the throwing back of the SemiluMir valves ; which, in suddenly flapping down again, produce the second sound. That an exaggeration of the first sound, not essentially differ- ing from it in character, is often produced by disease of the sigmoid valves, wMch causes an obstruction of their orifice, has long been known ; and in such cases, the character of the second sound is also changed. Indeed, M. Cruveil- hier states it as, in his opinion, an uniform occurrence, that disease of the Semilunar valves alters both sounds. When this disease is such as to prevent the valves from effectually closing, a reflux of blood takes place into the ven- tricle at the time of its diastole ; causing a rushing sound, more or less pro- longed, to be heard in the intervals of the pulse, instead of with it. These considerations appear to prove almost incontestably, that the cause of the first sound, and Hiat of the second, are very closely allied ; and this view, which if correct, is of great importance in the explanation of numerous morbid phe- nomena, harmonizes well with the known effect of a slight obstruction in a tube, through which fluid is being rapidly forced, in producing a prolonged sound, very analogous to the first sound of the heart. The following table may assist the student in connecting the sounds of the Heart with its movements. FIRST SOUND. Ventricular Systole, and Auricular Diastole. Impulse of apex against parietes of chest. Pulsation in arteries. SECOND SOUND. First stage of Ventricular Diastole. INTERVAL. Short repose ; then Auricular Systole, and second stage of Ventricular Diastole. 487. The course of the circulating fluid through the Heart, and the action of^ts different valves, will now be briefly described. The Venous blood, which is returned by the ascending and descending Vena Cava, enters the right Auricle during its diastole ; and, when it contracts, is forced between the Tricuspid valves, into the Ventricle. The reflux of blood into the veins, during the auricular systole, is prevented by the valves with which they are furnished ; but these valves are so formed, as not to close accurately, especially when the tubes are distended ; so that a small amount of reflux usually takes place, and this is much increased when there is any obstruction to the pulmonary circu- lation. Whilst the right Ventricle is contracting upon the blood that has entered it, the carnese columns, which contract simultaneously with its proper walls, put the chordae, tendinese upon the stretch ; and these draw the flaps of the Tricuspid valve into the auriculo-ventricular axis. The blood then getting behind them, and being compressed by the contraction of the ventricle, forces the flaps together in such a manner as to close the orifice ; but they do not fall suddenly against each other, as is the case with the semilunar valves, since they are restrained by the chordse tendinese ; whence it is that no sound is produced by their closure. The blood is expelled by the ventricular systole into the Pulmonary Artery, which it distends, passing freely through the Semi- lunar valves ; but as soon as the vis a tergo ceases, and reflux might take place by the contraction of the arterial walls, the valves are filled out h| the back- ward tendency of the blood, and completely check the return ofjlny portion 368 OF THE CIRCULATION OF BLOOD. Fig. 91. The Anatomy of the Heart; 1, the right auricle; 2, the entrance of the superior vena cava; 3, the entrance of the inferior cava; 4, the opening of the coronary vein, half closed by the coronary valve; 5, the Eustachian valve; 6, the fossa ovalis, surrounded by the annulis ovalis; 7, the tuberculum Loweri; 8, the musculi pectinati in the appendix auriculae; 9, the auriculo-venlricular opening; 10,. the cavity of the right ventricle; 11, the tricuspid valve, attached by the chorda? tendineEe to the carneae columnae (12); 13, the pulmonary artery, guarded at its commencement by three semilunar valves; 14. the right pulmonary artery, passing beneath the arch and behind the ascending aorta; 15, the left pulmonary artery, crossing in front of the descending aorta; *, the remains of the ductus arteriosus, acting as a ligament between the pulmonary artery and arch of the aorta; the arrows mark the course of the venous blood through the right side of the heart; entering the auricle by the superior and inferior cava, it passes through the auriculo- ventricular opening into the ventricle, and thence through the pulmonary artery to the lungs ; 16, the left auricle; 17, the openings of the four pulmonary veins; 18, the auriculo- ventricular opening; 19, the left ventricle; 20, the mitral valve, attached by its chordae tendineae to two large columnar carnerc. which project from the walls of the ventricle; 21, the commencement and course of the ascending aorta behind the pulmonary artery, marked by an arrow ; the entrance of the vessel is guarded by three semilunar valves; 22, the arch of the aorta. The comparative thickness of the two ventricles is shown in the dia- gram. The course of the arterial blood through the left side of the heart is marked by arrows. The blood is brought from the lungs by the four pulmonary veins into the left auricle, and passes through the auriculo- ventricular opening into the left ventricle, whence it is conveyed by the aorta to every part of the body. of it into the ventricle. The blood, after having circulated through the lungs, returns as Arterial blood, by the Pulmonary Veins, to the left Auricle ; whence it passes through the mitral valves into the left Ventricle, and thence into the Aorta, in the same manner with that on the other side, as just described. 488. There are, however, some important differences in the structure and functional actions of the two divisions of the Heart, which should be here adverted to. The walls of the left Ventricle are considerably thicker than those of the right; and its force of contraction is much greater. The follow- ing are the comparative results of M. Bizot's recent measurements, taking the average of males from 16 to 89 years. Base. Middle. Apex. Left Ventricle 4 lines 5*- lines 3| lines Right Ventricle 1 La lines If lines 1^- lines In the female, the average thickness is somewhat less. It will be seen that the point of greatest thickness in the left Ventricle is near its middle ; while in the right, it is nearer the base. The thickness of the former goes on increasing during all periods of life, from youth to advanced age ; whilst that of the righ| is nearly stationary. The left Auricle is somewhat thicker than the right; the average thickness of the former being, according to Bouillaud, ACTION OF THE HEART. 369 a line and a half; whilst that of the latter is only a line. In regard to the relative capacities of the right and left cavities, much difference of opinion has prevailed. The right Auricle is generally allowed to be more capacious than the left; and the same is commonly taught of the right Ventricle. So much fallacy may arise, however, from the peculiar condition of the animal at the moment of death, that this is not easily proved, and is indeed by no means certain. Many eminent Anatomists maintain, that the two cavities are equal. The capacity of each of the cavities may be estimated, in the full-sized Heart, at about two ounces ; that of the Auricles being probably a little less ; and that of the Ventricles a little greater. That the Ventricles receive more blood from the Auricles than the latter could transmit to them by simply emptying themselves once, seems therefore probable ;. and may be accounted for by the fact already stated, regarding the slight intermission in the Ventricular Diastole, during which more blood may enter the Auricle from the veins. 489. There is a well-known anatomical difference between the Auriculo- Ventricular valves on the two sides, which has given rise to the diversity of name. This seems, from the researches of Mr. King,* to be connected with an important functional difference. The Mitral valve closes much more per- fectly than the Tricuspid ; and the latter is so constructed", as to allow of con- siderable reflux, when the cavities are greatly distended. Many occasional causes tend to produce an accumulation of blood in the venous system, and in the right side of the Heart : thus, any obstruction to the pulmonary circulation, cold, compression of the venous system by muscular action, &c., are known to favour such a condition. This is a state of peculiar danger, from the lia- bility which over-distension of the Ventricular cavity has to produce a state of muscular paralysis ; and in the structure of the Heart itself, there seems to be a provision against it. For, when the Ventricle is thus distended, the Tricuspid valves do not close properly ; and a reflux of blood is permitted, not only into the Auricle, but also (through the imperfect closure of their valves under the same circumstances) into the large veins. This is proved by the fact, several times observed by Dr. J. Reid in his experiments upon Asphyxia, &c., that, when the action of the Right Ventricle had ceased from over-disten- sion, he could frequently re-excite it, not merely by puncturing its walls, but by making an opening in the Jugular vein. This fact evidently affords an indication of great importance in the treatment of Asphyxia ; and it explains the reflux of blood, or venous pulse, which is frequently observed in cases of pulmonary disease, and which, according to Mr. King, always exists, though in a less striking degree. 490. It is not quite certain whether the Ventricles empty themselves com- pletely at each contraction ; but it seems probable that the blood which they contain is not entirely forced into the arteries. The quantity which is pro- pelled by each Ventricle, at every stroke, may be estimated, therefore, at from 1 oz. to 2 oz. If we adopt the lower of these numbers, we shall find that, reckoning 75 pulsations of the Heart to a minute, 112 oz., or. 7 Ibs., of blood pass through each Ventricle in that time ; and, on the higher estimate, 150 oz., or 9 Ibs. 6 oz., would pass through in the same period. Now the whole quantity of blood contained in the human body, according to the estimate of Haller (which is considered by Dr. Allen Thomson to be near the truth), is about one-fifth of the weight of the body or 28 pounds in a person weighing 140 Ibs.t This quantity would pass through the Heart, therefore, in four minutes, on the lower of the two preceding estimates, or in three minutes on the higher ; and would circulate afresh, fifteen or twenty times in an hour. * Guy's Hospital Reports, vol. ii. j- Cyclopaedia of Anatomy, Art. Circulation. See also 581. - 370 OF THE CIRCULATION OF BLOOD. It would appear, however, that this estimate of the rapidity of the circulation is very far from the truth ; for recent experiments have shown that substances introduced into the Venous circulation, may be detected in the remotest parts of the Arterial circulation, even in animals larger than Man, in less than half a minute. The earliest of such experiments were those of Hering,* who endeavoured to ascertain the rapidity of the circulation, by introducing Prus- siate of Potash into one part of the system, and drawing blood from another. He states that he detected this salt in blood drawn from one of the Jugular veins of the Horse, within 20 or 30 seconds after it had been introduced into the other ; in which brief space the blood must have been received by the Heart, must have been transmitted through the Lungs, have returned to the Heart again, have been sent through the Carotid artery, and have traversed its capillaries. From experiments of a similar nature upon other veins, he states that the salt passed from the Jugular vein into the Saphena in 20 se- conds; into the Masseteric artery in from 15 to 20 seconds ; into the External Maxillary artery in from 10 to 25 seconds : and into the Metatarsal artery in from 20 to 40 seconds. An attempt has been made to invalidate the inference which seems inevitably to flow from these experiments, in regard to the rate of the circulation, by attributing the transmission of the salt to the permea- bility of the animal tissues ;t but it has never been shown that even Prussiate of Potash (which is probably more transmissible through this channel than any other salt) can be carried from one part to another with a rapidity at all proportional to this. The only mode in which this property can be conceived materially to facilitate the transmission of the salt through the vascular system, would be by allowing it to pass through the septum of the auricles, and thus to make its way from the right to the left side of the heart, without passing through the pulmonary circulation ; and this it could scarcely do, to the large amount which is evidently transmitted, in so short a time. 491. The experiments of Hering have been recently fully confirmed by those of Mr. Blake,J who varied them by employing different substances, and took other precautions against sources of fallacy. Ten seconds after having injected a solution of Nitrate of Baryta into the Jugular vein of a horse, he drew blood from the Carotid artery of the opposite side ; after allowing this to flow for five seconds, he substituted another vessel, which received the blood that flowed during the five ensuing seconds ; and the blood that flowed after the twentieth second, by which time the action of the Heart had stopped, was received into a third vessel. These different specimens were carefully ana- lyzed. No trace of Baryta could be detected in the blood which had escaped from the artery between the tenth and fifteenth second after the injection of the poison; but in that which was drawn between the fifteenth and the twentieth second, the salt was found to be present and in greater abundance than in the blood which had subsequently flowed. Moreover, the coincidence between the cessation of the Heart's action, and the diffusion of the salt through the arterial blood, bears a striking correspondence ; and it may be hence in- ferred that the arrestment of its muscular movement is due to the effect of this agent upon its tissue, when immediately operating upon it through the capil- laries of the coronary artery. This conclusion is borne out by a variety of other experiments ; which show that the time of the agency of other poisons, that suddenly check the Heart's action (which is the especial property of mineral poisons), nearly coincides, in different animals, with that which is required to convey them into the Arterial capillaries. And it seems to derive full confirmation from the fact that poisons which act locally on other parts, * Tiedemann's Zeitschrift, vol. iii. p. 85. f See Dr. Allen Thomson, Joe. tit. * Edinb. Med. and Surg. Journal, Oct., 1841. ACTION OF THE HEART. 371 [Fig. 92. give the first indications of their operation in the same period, after they have been introduced into the Venous circulation. Thus, in the Horse, the time that is required for the blood to pass from the Jugular vein into the capillary terminations of the Coronary arteries, is 16 seconds ; as is shown by the power of Nitrate of Potass to arrest the Heart's action within that time ; and Nitrate of Strychnia, injected into a vein, gave the first manifestation of its action on the Spinal Cord in precisely the same number of seconds. In the Dog, the Heart's action was arrested by the Nitrate of Potass in 11 or 12 seconds ; and the tetanic convulsions occasioned by Strychnia also commenced in 12 seconds. In the Fowl, the former period was 6 seconds, and the latter 6 ; in the Rabbit, the first was 4, and the other 4| seconds. From these experiments, it seems difficult to resist the conclusion, that the rapidity of the Circulation is very much underrated in any estimate that we found upon the capacity of the Heart, and its number of pulsations in a given time ; and that some other force than its contractions, must have a share in producing the movement of the blood through the vessels. 492. The force with which the heart propels the blood, may be estimated in two ways ; either by ascertaining the height of the column of that fluid, which its contractile action will support ; or by causing the blood to act upon a shorter column of mercury. The former me- thod was the one adopted by Hales, who intro- duced a long pipe into the carotid artery of a horse, and found that the blood would some- times rise in it to the height of ten feet. From parallel experiments upon sheep, oxen, dogs, and other animals, and by comparing the cali- bre of their respective vessels with that of the human aorta, Hales concluded that the usual force of the heart in man would sustain a column of blood 7| feet high, the weight of which would be about 4 Ibs. 6 oz. The second me- thod is that more recently adopted by Pois- seuille ; and the instrument which he contrived for carrying it into practice (termed by him the Hsemadynamometer) has been the means of aiding many valuable inquiries on the physio- logy of the circulation. The result of his ex- periments is Very nearly the Same as that Of bent glass tube, filled with mercury in Hales; his estimate of the force With which the lower part, a d e. The horizontal part the blood is propelled into the aorta being 4 Ibs. 3 oz. The backward pressure upon the walls of the heart, or in other words the force which they have to overcome in propelling the blood to prevent its coagulation, when blood, is properly estimated by multiplying the the blood presses on the fluid in the pressure of blood in the aorta into the surface > tal limb ' the rise of the mercury of a plane passing through the base and apex of the left ventricle ; by which calculation it is found to be about 13 Ibs.* The pressure ap- * The extreme latitude of the estimates which have been made of this force, has been a subject of not undeserved ridicule. Borelli imagined it to be 180,000 Ibs.; whilst by Keill it was supposed to be no more than from 5 to 8 ounces. HoemadynamometerofPoisseuille. A &, is provided with a brass head, which fits into the artery. A small quantity of a solution of the carbonate of soda is in- terposed between the mercury and the towards e. measured from the level to which h has fa]|en towards fl> gives the pressure under which the blood moves.] 372 OF THE CIRCULATION OF BLOOD. pears, from the experiments of Poisseuille, to be very nearly equal for equal surfaces, throughout the larger arterial branches, since it diminishes regularly in proportion to their calibre ; in the radial artery at the wrist, it was estimated by him at 4 drachms. 493. The number of contractions of the heart in a given time, is liable to great variation, within the limits of ordinary health, from several causes ; the chief of these are, diversities of age, of sex, of muscular exertion, of the con- dition of the mind, of the state of the digestive system, and of the period of the day. Putting aside the other causes of uncertainty, the following table may be regarded as an approximation to the average frequency of the pulse, at the several ages specified in it. Beats per minute. In the foetus in utero . . 140 150 Newly-born infant . . 130 140 During the first year . . 115 130 During the second year . . 100 115 During the third year . . 90 100 About the seventh year . . 85 90 . Age of puberty . . 8085 Manhood . . 70 80 Old age . . 5065 The difference caused by sex is very considerable, especially in adult age ; it appears from the inquiries of Dr. Guy,* that the pulse of the adult female ex- ceeds in frequency the pulse of the adult male, at the same mean age, by from 10 to 14 beats in a minute. The effect of muscular exertion in raising the pulse is well known; as is also the fact, which is one exemplification of it, that the pulse varies considerably with the posture of the body. The amount of this variation has been made the subject of extensive inquiry by Dr. Guy, and the following are his results. In 100 healthy males, of the mean age of 27 years, in a state of rest, the average frequency of the pulse was, when stand- ing, 79 ; when sitting, 70 ; and when lying, 07 per minute. Several excep- tions occurred, however, to the general law ; and when these were excluded, the average numbers were, standing, 81 ; sitting, 71 ; and lying, 66 ; so that the difference between standing and sitting was 10 beats, or jth of the whole ; the difference between sitting and lying was 5 beats, or T ^th of the whole ; and the difference between standing and lying was 15 beats, or -}th of the whole. In 50 healthy females, of the same mean age, the average pulse when standing was 89 ; when sitting, 81 ; and when lying, 80. When the exceptions (which were more numerous in proportion than in males) were excluded, the averages were, standing, 91 ; sitting, 84 ; lying, 79 ; the difference between standing and sitting was thus 7 beats, or T L th of the whole ; that between sitting and lying was 4, or ^ r st f tne whole ; and that between standing and lying was 11, or |th of the whole. In both sexes, the effect produced by change of posture in- creases with the usual frequency of the pulse ; whilst the exceptions to the general rule are more numerous as the pulse is less frequent. The variation is temporarily increased by the muscular effort involved in the absolute change of the posture ; and it is only by the use of a revolving board, by which the position of the body can be altered without any exertion on the part of the subject of the observation, that correct results can be obtained. That the differ- ence between standing and sitting should be greater than that between sitting and lying, is just what we should expect, when we compare the amount of muscular effort required in the maintenance of the two former positions respectively. * Guy's Hospital Reports, vol. iii., p. 312. MOTION OF THE BLOOD IN THE VESSELS. 373 494. The Pulse is well known to be much accelerated by Mental excite- ment, especially by that of the Emotions ; it is also quicker during Digestion ; but on neither of these points can any exact numerical statement*be given. The diurnal variation of the pulse, however, has been made the subject of observation by Dr. Guy ;* and, as the result of his inquiries have much in- terest, although (from having been made only on his own person) they may ultimately require some modification, they will be here stated. " 1. The pulse of a healthy male in a state of rest, unexcited either by food or exercise, is most frequent in the morning, and gradually diminishes as the day advances. 2. The pulse diminishes in frequency more rapidly in the evening than in the morning. 3. The diminution in the frequency of the pulse (after excite- ment) is more regular and progressive in the evening than in the morning. 4. The effect of food is greater and more lasting in the morning than in the evening ; and in some instances, the same food, which in the morning pro- duces an effect considerable both in amount and duration, has no effect what- ever in the evening." It may be hoped that, ere long, this interesting and important subject will receive further elucidation. [Dr. Valleix has recently publishedf a series of interesting observations on the fre- quency of the pulse in newly-born infants, and in children aged from seven months to six years. He obtained the following results: 1. At birth the pulse is less frequent than at six months; the number of beats in a minute may be stated with considerable exact- ness to be between 90 and 100. 2. Increase of temperature, even in the slightest degree, invariably produces a notable acceleration of the pulse. The exact ratio between the degree of elevation of temperature and the increase in the frequency of the pulse, is not yet accurately ascertained. 3. Although the observations of Dr. Valleix show a progres- sive diurnal diminution in the frequency of the pulse, still, he thinks, that it would be premature to conclude that these facts support those of Dr. Guy. Dr. Valleix examined his patients in the morning after they had been eating, and to this fact, he thinks, should be attributed the acceleration of the pulse in the early part of the day, and its subsequent diminution, towards evening. 4. The slightest muscular effort in children is sufficient to augment considerably the number of pulsations. The same is true of any moral emo- tion. 5. The influence of sex on the pulse is very marked in young children. The pulse is much more frequent in young girls than in boys of the same age. 6. During sleep there is a decided diminution in the number of beats. 7. Between 7 and 27 months there is no sensible change in the frequency of the pulse. Its mean may be stated at 126 beats in the minute, without distinction of sex. If sex be considered, it would be 121 for males and 128 for females. These numbers express the frequency of the pulse at this age under ordinary circumstances, but if a state of perfect calm is supposed, the numbers would be 119 for the males, and 124 for females. 8. After some observations, not very numerous, however, the pulse would appear to range a little above 100 till six years of age. 9. The mean number of inspirations in a minute in children aged from 7 months to two years and a half, is between 30 and 32, and is to number of pulsations : : 1 : 4. M. C.] III. Causes influencing the Circulation in the Arteries and Capillaries. 495. That the movement of the Blood through the Arterial trunks and the Capillary tubes is, in Man, and in other warm-blooded animals, chiefly de- pendent upon the action of the Heart, there can be no doubt whatever. It can be easily shown by experiment, that, if the Arterial current be checked, the Capillaries will immediately cease almost entirely to deliver the blood into the veins, and the Venous circulation will be instantaneously arrested. And it has also been proved, that the usual force of the Heart is sufficient to propel the blood, not only through the Arterial tubes, but through the Capillaries, into the Veins ; since even a less force wityserve to propel warm water through * Op. cit., vol. iv. p. 69. f [Memoires de la Societe Medicale d'Observation de Paris. Tome Deuxieme, IS p. 300, et seq.] 32 374 OF THE CIRCULATION OF BLOOD. the vessels of an animal recently dead.* But there are certain " residual phenomena" even in Man, which clearly indicate that this is not the whole truth ; and that forces existing in the Blood-vessels have a considerable influ- ence in producing both local and general modifications of the effects of the Heart's action. There are also indications of the nature of an influence, in which the blood-vessels do not partake, arising from those changes occurring between the Blood and the Tissues, that constitute the processes of Nutrition, Secretion, &c. Such, for instance, would appear to be the interpretation of the fact, that whilst any variations in the action of the Heart affect the whole system alike, there are many variations in the Circulation, which, being very limited in their extent, cannot be attributed to such central disturbances, and must therefore be dependent on causes purely local. Of the nature of these influences, and of the mode of their operation, we shall probably arrive at a more correct knowledge, if we examine the phenomena of the Circulation in those beings in which the moving power is less concentrated than it is in the higher Animals ; for just as we find in the latter, that the development of special absorbent vessels does not exclude the function of absorption from being still performed by the general vascular system ( 463), so may we here be led to perceive that there is a generally diffused force, to which alone the Circulation of the nutritious fluid in the lowest organisms is due, and w r hich is not alto- gether replaced by the special organ of impulsion, that is developed in the centre of the system in the higher. 496. The ascent of the sap in Vegetables is probably to be regarded as due, in part, to the vis a tergo occasioned by the action of Endosmose at the roots ; and in part, to the demand for fluid, occasioned by the vital processes taking place in the leaves. For if the stem of a Vine, in which the sap is rising, be cut across, the sap will continue to flow for some time from the top of the lower portion ; and its force of ascent may be shown to be very considerable, by tying over the cut surface a piece of 'bladder, which will be speedily burst, or by affixing to it a bent tube, containing a column of mercury, which will be raised to the height of forty inches or more. On the other hand, the attrac- tive force of the leaves is shown by the fact, that if the lower end of the upper division be put into water, it will continue to absorb, as long as the vital actions of the leaves are being performed with vigour ; but, if the branch be carried into a dark room, the exhalation from the leaves is immediately checked, and absorption is checked also. The influence of the actions at the periphery of the circulating system in maintaining the flow of fluid towards the part, is further shown by the fact, that if a shoot of an evergreen species be grafted on a stock of one with deciduous leaves, a continual and unwonted ascent of sap will be kept up in the latter through the winter ; this being evidently due to the demand occasioned at its summit. Again, the recommencement of the annual flow of sap in an ordinary tree has been found to take place, in the first instance, not at its roots, but in the neighbourhood of the buds ; for their expansion, under the influence of the returning warmth, exhausts the fluid from the vessels of their neighbourhood ; this, again, occasions a demand from below; and thus the motion is gradually propagated to the roots. Now it has been experimentally ascertained, that, if a branch of a vine growing in the open air be trained into a hot-house, it may be made to vegetate during the winter, and to draw up fluid through the stems and roots, whose condition has not been changed. It is evident, then, that in Plants, the demand for fluid in the organs to which it is distributed by the vascular system, is one of the chief forces by which the supply is obtained. * See Dr. Williams's Principles of Medicine, [Am. ed., by Dr. Clymer, p. 155, note.] MOTION OF THE BLOOD IN THE VESSELS. 375 497. This is still more evidently the case in regard to the Circulation of nutritious or elaborated sap, which takes place in the under surface of the leaves and in the bark. The object of this movement is not to convey the fluid in a direct line from one point to another (as in the case with the ascend- ing current), but to supply every part with materials for its growth, or for the production of its peculiar secretions. Hence the vessels in which it takes place, form a minutely anastomozing network, instead of consisting of a system of straight and distinct tubes. Through this network, the latex or elaborated sap is seen to move, exactly as does the blood through the capillary vessels of animals. The movement takes place, under favourable circumstances, with considerable rapidity ; it is accelerated by heat, and retarded by cold ; and it is subject to all those minor irregularities (such as the cessation of movement, or change in the direction of the current, in a particular channel), which are so constantly to be noticed by any one who attentively watches the capillary circulation of Animals, and which clearly prove the operation of some causes independent of the heart's action ( 478). The general direction of the ela- borated sap, through this capillary system, is downwards; but that the force of gravity cannot have much to do with the movement, is shown by the fact that, in a dependent branch, it has to ascend towards the stem, which it will do without interruption from this cause. 498. In the lowest Animals, the movement of the circulating fluid seems as independent of any central organ of impulsion as it has been shown to be in Plants. Thus, in the living Sponge, a current of water is continually flowing through the tubes and channels, by which its substance is traversed, the fluid being taken in by the small orifices, and ejected in powerful streams from the large ones ; and yet the most attentive examination has not revealed any mechanical cause for this movement. In some of the compound Polypifera, a similar current may be seen ; and it is curious that, in many species, its direc- tion undergoes a periodical change ; being reversed at intervals of a certain number of seconds. In the Star-fish and Sea-Urchin tribe, a complex circula- tion of blood takes place, through regular vessels ; and here we find some indication of a contractile cavity, by the power of which it may be, in some degree, kept up ; but its feeble pulsations can scarcely be regarded as having any great share in the movement of the fluid which passes through it. In the Articulated* series, there is, with a few exceptions, an absence of any cen- tral organ of impulsion, possessed of power sufficient to carry the blood through the vascular system, by its contractions alone. In many of the aquatic worms and larvae, the movement of the blood, and the pulsations of the dorsal vessel, may be distinctly seen : and the thinness of the walls of the latter, and the character of its movements seem clearly to show, that these can scarcely be regarded as propulsive, but that they merely result from the variations in the current which passes through it, the sides flapping together when there is an outward flow, and bulging out when there is an influx. It is in these Articulata, in which there is a provision for respiration throughout the whole structure, as is especially the case in Insects, that the absence of any central impulsive power is most remarkable. In the Crustacea, and in the Mollusca in general, the respiration is aquatic, and is restricted to a particular organ ; and in these, the heart is found to be more muscular, and the circulation to be more under its control. It is curious to remark, however, that, in some of the lower Mollusca, which exhibit a tendency to aggregation into compound struc- tures, like those of the Polypifera, there is the same want of definiteness in the course of the circulation, as has been just stated to exist in the latter group ; the flow of blood through their complex apparatus of nutritive organs, being arrested at regular intervals, and then recommencing in the reverse direction. 376 OF THE CIRCULATION OF BLOOD. 499. Even in Vertebrated animals, we find indications of the same defi- ciency of central power, over the peripheral circulation. When we look at the simple, thin-walled heart of Fishes, for example, it seems impossible that it should have rnuch power over the current of blood flowing back to it by the veins ; for of this blood, a considerable portion has to pass through three sets of capillaries, between its ejection from the heart and its return to it. It is first transmitted through the respiratory capillaries, for the purpose of aera- tion ; the confluent vessels, which collect the arterial blood from these, termi- nate in the general systemic trunk or Aorta, in which, as in the veins of Man, there is an absence of pulsation, and by these it is distributed to the systemic capillaries ; and the blood which, after passing through these, returns from the posterior part of the body, and from the viscera, passes through ariother set of capillaries, those of the liver and kidneys, before it returns to the heart. Even in the warm-blooded Vertebrata, in which the respiratory circulation is separately performed, the blood, which is returned from the intestines, passes into a trunk, the Vena Portse, which again subdivides into capillary ramifi- cations, being transmitted over the plexus of biliary ducts, of which the liver is chiefly composed ; and thus the Vena Portse, as Hunter justly observed, should be considered rather in the light of an artery,* resembling as it does the aorta of Fishes. Considering the small amount of pressure which is exerted by the blood, upon the sides of the vessels that are formed by the reunion of capillaries, it seems impossible to imagine that the vis a tergo, derived from the impulsive action of the Heart, can be alone sufficient to maintain the portal circulation. 500. We have next to consider the influence of the Arterial tubes, on the flow of blood through them. This influence is exerted by the middle or fibrous coat, which alone is possessed of contractile properties. We find in this coat, a layer of annular fibres, possessing no small resemblance to that of which the muscular coat of the alimentary canal is composed. On the outside of this, is a layer of yellow elastic tissue, which is much thicker in the larger arteries, in proportion to their size, than in the smaller. To this last tissue is due the simple elasticity of the arterial walls, which is a physical property that per- sists after death, until a serious change takes place in their composition : whilst to the one first mentioned, we are to attribute the property which they unquestionably possess in common with proper muscular tissue, of con- tracting on the application of a stimulus, so long as their vitality remains. These two endowments exist, in various proportional degrees, in the different parts of the Arterial system. Thus it was justly remarked by Hunter, that elasticity, being the property by which the interrupted force of the Heart is made equable and continuous, is most seen in the large vessels more imme- diately connected with that organ. On the other hand, the contractility is most observable in the smaller vessels, where it is more required for regulating the flow of blood towards particular organs. 501. It is easily shown that the action of the Elasticity of the Arterial tubes is one of a purely physical character ; and that its purpose is to convert the intermitting impulses, which the fluid receives from the heart, into a continu- ous current. The former are very evident in the larger trunks; but they diminish with the subdivision of these, until they entirely disappear in the capillaries, in which the stream is usually equable or nearly so. We may imagine a powerful forcing-pump injecting water, by successive strokes, into a system of tubes with unyielding walls ; the flow of fluid at the farther * That it conveys venous blood, is no reason to the contrary; since this is the case with the pulmonary artery. The character of an artery is derived from the division of its current into several diverging streams. MOTION OF THE BLOOD IN THE ARTERIES. 377 extremities of these tubes, would be as much interrupted as its entrance into them. But if an air-vessel (like that of a fire-engine) were placed at their commencement, the flow would be in great degree equalized ; since a part of the force of each stroke would be spent upon the compression of the air included in it ; and this force would be restored by the elasticity of the air during the interval, which would propel the stream, until directly renewed by the next impulse. A much closer imitation of the natural apparatus would be afforded, by a pipe which had elastic walls of its own ; if water were forced by a syringe into a long tube of caoutchouc, for example, the stream would be equalized before it had proceeded far. This effect is found to be accomplished, at any point of the Arterial circulation, in a degree proportionate to its distance from the Heart ; and it is another effect of the same cause, that the pressure of the blood upon the walls of the arteries (as shown by the experiments of Poisseuille) is nearly the same ail over the system. It is to the distension of the arterial tubes, both in their length and calibre, that their pulsation is due. Their elongation is the more considerable of the two effects ; and it causes the artery to be lifted from its seat and to become curved. The transverse dilatation has been denied by some physiologists ; but it has been recently proved to take place, by an ingenious experiment of Poisseuille's. The increase of capacity, however, is not more than one-tenth; so that the increase of diameter will not be so much as one-twentieth, a quantity scarcely perceptible to ordinary measurement. The transmission of the pulse-wave through the whole system is not instantaneous, but takes place in an appre- ciable time. The pulsation of the large arteries near the Heart is synchronous with the Ventricular systole ; but that of other arteries is somewhat later, the difference varying with their distance, and amounting in some instances to between one-sixth and one-seventh of a second. 502. It has been denied by many Physiologists, that the middle coat of the arteries possesses any property, which can be likened to Muscular Contrac- tility; and it will therefore be desirable to enter somewhat in detail into the question. That it cannot be readily stimulated to contraction, through the medium of its nerves, is universally admitted; but the same is the case in regard to the Muscular coat of the alimentary canal, which contracts most vigorously on the direct application of stimuli to itself; and Valentin and others have recently succeeded in producing evident contractions in both, by irritation of the Sympathetic nerve, and of certain roots of the Spinal nerves ( 209). Further, although many experimenters have failed in producing contractions of this tissue, by stimuli directly applied to itself, yet others have distinctly witnessed them ; and, in any question of this kind, the positive evi- dence must be held to outweigh the negative. Thus Verschuir states, that he has seen arteries contract, when stimulated by the mineral acids, by electricity, and by the application of the point of a scalpel. Dr. Thomson also saw them contract, on the application of ammonia, and when punctured with the point of a fine needle, in the living body. It has been ascertained by the direct and careful experiments of Poisseuille, that, when the artery is dilated by the blood injected into it from the heart, it reacts with a force superior to the impressing impulse ; and he has also* shown that, if a portion of an artery from an animal recently dead (in which the vital contractility seems to be pre- served), and one from an animal that has been dead some days (in which nothing but the elasticity remains), be distended with an equal force, the former becomes much more contracted than the latter, after the distending force is removed. 503. Several experiments also indicate the existence of a power of slow contraction in the arteries, which has been distinguished by the appellation 32* 378 OF THE CIRCULATION OF BLOOD. Tonicity; but which does not seem any thing else than a particular mani- festation of the general property of vital contractility, and is certainly of a nature quite distinct from ordinary elasticity. Thus, when a ligature is placed upon an artery in a living animal, the part of the artery beyond the ligature becomes gradually smaller, and is emptied to a certain degree, if not completely, of the blood it contained. Again, when part of an artery in a living animal is isolated by means of two ligatures, and is punctured, the blood issues from the orifice, and the enclosed portion of the artery is almost completely emptied of its contents. The exposure of arteries to the air was found by Hunter to occa- sion their contraction to such an extent, that obliteration of their tube was the result; and this statement has been subsequently confirmed. Further, every surgeon knows, that the contraction of divided arteries is an efficient means of the arrest of hemorrhage from them, especially when they are of small calibre ; so that, in the case of the temporal artery, for example, the complete division of the tube is often the readiest means of checking the flow of blood from it, when it has been once wounded. This contraction is much greater than could be accounted for by the simple elasticity of the tissue ; anti is more decided in small than in large vessels. The empty condition of the arteries, generally found within a short time after death, seems to be in part due to the same cause ; since their calibre is usually much diminished and is sometimes completely obliterated. A remarkable example of the same slow contraction, is that which takes place in the end of the upper portion of an arterial trunk, when the passage of blood through it is interrupted by a ligature ; for the cur- rent of blood then passes off by the nearest large lateral branch ; and the tube of the artery shrivels, and soon becomes impervious, from the point at which the ligature is applied, back to the origin of that branch. This last fact is important, as proving how little influence the vis a tergo possesses over the calibre of arterial tubes ; since, without any interruption to the pressure of blood occasioned by it, the tube becomes impervious. It is to the moderate action of the Tonicity of arteries, that their contraction upon the stream of blood passing through them (which serves to keep the tubes always full) is due. If the tonicity be excessive, the pulse is hard and wiry ; but if it be deficient, the pulse is very compressible, though bounding, and the flow of blood through the arteries is retarded. Dr. Williams has performed some ingenious experiments, which prove that the force required to propel fluid through a tube, whose sides are yielding, is very much greater than that which will carry it through an even smaller tube, with rigid parietes ; conse- quently, a loss of tonicity in the blood-vessels retards the flow of blood through them; whilst an increase hastens it. The Tonicity of the arteries differs from their ordinary Contractility, in being augmented by cold, and diminished by warmth. Hence cold and heat are two most valuable remedial agents, when this property is deficient or in excess. 504. It is still to be inquired, in what manner the Contractility of the Arte- ries is to be regarded as influencing the flow of Blood through them. It is at once evident, that any general contraction of the arterial tubes would have rather the effect of opposing than of assisting the flow ; but if the fibrous coat of the Arteries is in some degree disposed to the alternate contraction and relaxation, which are so remarkable in the Heart, they may exert a force which shall be supplementary to that of the Heart's impulse, relaxing to receive the blood from it, and contracting upon their contents, with a power superior to that by which they were distended. It is difficult to say whether or not this be the case ; though there would certainly appear some evidence in favour of the supposition. The loss of the Heart's power over the currents of blood, in proportion to their degree of subdivision, occasioned by the increased fric- MOTION OF THE BLOOD IN THE CAPILLARIES. 379 tion to which they will be subjected, would seem to require some compensat- ing power, in order that the perfect equality of pressure may be obtained, which has been spoken of as existing in all parts of the arterial system. In no other way than this, can the fibrous coat of the Arteries be regarded as having any propulsive power over their contents ; except by a peristaltic or vermicular movement, resembling that which takes place in the alimentary canal ; and of such there is no evidence whatever. A very important use may be assigned to this muscular coat, which has been generally overlooked by physiologists, that of regulating the diameter of the tubes, in accordance with the quantity of blood to be conducted through them to any part, which will depend upon its peculiar circumstances at the time. Such local changes are continually to be observed, in the various phases of normal life, as well as in diseased states ; and they will be found to be constantly in harmony with the particular condition of the processes of Nutrition, Secretion, &c., to which the Capillary circulation ministers. Of this kind are the enlargement of the trunks of the Uterine and Mammary arteries, at the epochs of pregnancy and lactation ; the enlargement and strongly increased pulsation of the Radial artery, when there is any active inflammation in the thumb ; the enormous diameter which the Spermatic artery will attain, when the testicle is greatly increased in size by diseased action ; and many other similar phenomena. In such cases, it cannot be the action of the Heart that increases the calibre of the vessels; since this is commonly unaltered, and is itself unable, as we have just seen, even to maintain their permeability. It must, therefore, be by a power inherent in themselves, that their dilatation is effected. The minute dis- tribution of the Sympathetic nerve upon the walls of the arteries, -the known power which this has of producing contractions, alike in their fibrous coat, and in the muscular tunic of the intestinal canal, and various phenomena, which indicate the power of certain states of mind over the dimensions of the arte- ries, in particular parts of the body at least, render it highly probable, that the calibre of the arteries is regulated in no inconsiderable degree through its intervention.* The permanent dilatation, however, which is seen in the arte- ries supplying parts that are undergoing enlargement, must be due, not to simple dilatation merely, but to increased nutrition ; since we find that their walls are thickened as well as extended. And, on the other side, when slow contraction occurs in these tubes, as a consequence of disease, it must be in part occasioned by atrophy ; since their nutrition is so much diminished, that in time they almost entirely disappear, a portion of a large artery occasionally shriveling into a ligamentous band. 505. We now come to the last head of the inquiry into the powers which convey the blood through the Capillary system ; that, namely, which con- cerns the agencies existing in the capillaries themselves. Many discussions on this subject may be found in physiological writings; and it has so imme- diate a bearing on one of the most important questions in Pathology, the nature of inflammation, that it deserves the fullest attention. The chief question in debate, is the degree in which the Capillary circulation is influ- enced by any other agency than the contractile power of the Heart and Arte- rial system ; some physiologists maintaining, that this alone is sufficient to account for all the phenomena of the Capillary circulation ; and others assert- ing, that it is necessary to admit some supplementary force, which may be exerted either to assist, retard, or regulate the flow of blood from the Arteries into the Veins. We shall first inquire what evidence there is of the existence * For Anatomical evidence to this effect, see Henle on the Contractility of the Blood- vessels, in Casper's Wochenschrift, May, 1840, and Brit, and For. Med. Rev., vol.x. p. 551. 380 OF THE CIRCULATION OF BLOOD. of any such force ; and, when led to an affirmative conclusion, we shall examine into its nature. No physiological fact is more clearly proved than the existence, in the lower classes of Animals, as well as in Plants, of some power independent of a vis a tergo, by which the circulating fluid is caused to move through their vessels ( 496 498). This power seems to originate in them- selves, and to be closely connected with the state of the Nutritive and Secreting processes : since any thing which stimulates these to increased energy, accele- rates the circulation ; whilst any check to them, occasions a corresponding stagnation. It may be convenient to designate this motor force, by the name of capillary power ; it being clearly understood, however, that no mechanical propulsion is thence implied. On ascending the Animal scale, we find the power which, in the lower organisms, is diffused through the whole system, gradually concentrated in a single part ; a new force, that of the Heart, being brought into operation, and the Circulation placed, in a greater or less degree, under its control. Still there is evidence, that the movement of blood through the capillaries is not entirely due to this ; since it may continue after the ces- sation of the Heart's action, may itself cease in particular organs when the Heart is still acting vigorously, and is constantly being affected in amount and rapidity, by causes originating in the part itself, and in no way affecting the Heart. The chief proofs of these statements will now be adverted to. 506. The movement of the blood in the Capillaries of cold-blooded animals, after complete excision of the Heart, has been repeatedly witnessed. In warm- blooded animals, this cannot be satisfactorily established by experiment, since the shock occasioned by so severe an operation much sooner destroys the gene- ral vitality of the system ; but it may be proved in other ways to take place. After most kinds of natural death, the arterial system is found, subsequently to the lapse of a few hours, almost or completely emptied of blood ; this is partly, no doubt, the effect of the tonic contraction of the tubes themselves ; but the emptying is commonly more complete than could be thus accounted for, and must, therefore, be due to the continuance of the capillary circulation. Moreover, when death has taken place suddenly from some cause, (as, for instance, a sudden electric shock,) that destroys the vitality of the whole system at once, the arterial tubes are found to contain their due proportion of blood. Further, it has been well ascertained, that a real process of secretion not unfre- quently continues, after general or somatic death; urine has been poured out by the ureters, sweat exuded from the skin, and other peculiar secretions formed by their glands ; and these changes could not have taken place, unless the capillary circulation were still continuing. In the early embryonic condi- tion of the highest animals, the movement of blood seems to be unquestionably due to some diffused power, independent of any central impulsion ; for it may be seen to commence in the Vascular Area, before the development of the Heart. The first movement is towards, instead of from the centre ; and even for some time after the circulation is fairly established, the walls of the Heart consist merely of vesicles loosely attached together, and can hardly be sup- posed to have any great contractile power. 507. The last of these facts may be said not to have any direct bearing on the question, whether the Capillary power has any existence in the adult condi- tion ; but the phenomena occasionally presented by the Foetus, at a later stage, appear decisive. Cases are of no very unfrequent occurrence, in which the heart is absent during the whole of embryonic life, and yet the greater part of the organs are well developed. In most or all of these cases, however, a perfect twin fcstus exists ; of which the placenta is in some degree united with that of the imperfect one ; and it has been customary to attribute the circulation in the latter to the influence of the heart of the former, propagated through the pla- MOTION OF THE BLOOD IN THE CAPILLARIES. 381 cental vessels. The supposition has not been disproved (however improbable it might seem) until recently ; when a case of this kind occurred, which was submitted to the most careful examination by an accomplished anatomist ;* and this decisive result was obtained, that it seemed impossible for the heart of the twin foetus to have occasioned the movement of blood in the imperfect one ; and that -some cause, present in the latter, must have been sufficient for the propulsion of blood through its vessels. It was a very curious anomaly in this case, that the usual functions of the Arteries and Vpins must have been reversed ; for the Vena Cava, receiving its blood from the Umbilical Vein nearly as usual, had no communication with the Arterial system (the Heart being absent), except through the Systemic Capillaries ; to which, therefore, the blood must have next proceeded, returning to the placenta by the Umbilical Artery. This view of the course of the blood was confirmed by the fact, that the veins were everywhere destitute of valves. It is evident that a single case of this kind, if unequivocally demonstrated, furnished all the proof that can be needed of the existence, even in the highest animals, of a capillary power, which, though usually subordinate to the Heart's action, is sufficiently strong to main- tain the circulation by itself, when the power of the central organ is diminished. In this, as in many other cases, we may observe a remarkable power in the living system, to adapt itself to exigences. In the acardiac Foetus, the capil- lary power supplies the place of the Heart, up to the period of birth ; after which, of course, the circulation ceases, for want of due aeration of the blood. It has occasionally been noticed, that a gradual degeneration in the structure of the Heart has taken place during life, to such an extent that scarcely any mus- cular tissue could at last be detected in it, without any such interruption to the circulation, as must have been anticipated, if it were the sole impelling force. 508. It is equally capable of proof, on the other hand, that the Capillaries may, by an influence peculiar to them, afford a complete check to the circu- lation in the part ; even when the Heart's action is unimpaired, and no mechanical impediment exists to the transmission of blood. Thus, cases of spontaneous gangrene of the lower extremities are of no unfrequent occurrence, in which the death of the solid tissues is clearly connected with a local decline of the circulation ; and in which it has been shown by examination of the limb after its removal, that both the larger tubes and the capillaries were completely pervious ; so that the cessation of the flow of blood could not be attributed to any impediment, except that arising from the cessation of some power which exists in the capillaries, and which is necessary for the main- tenance of the current through them. The most remarkable evidence on this point, however, is derived from the phenomena of Asphyxia, which will be more fully explained in the succeeding Chapter. At present it may be stated as a fact, which has now been very satisfactorily ascertained, that if admission of air into the lungs be prevented, the circulation through them will be brought to a stand, as soon as the air which they contain has been to a great degree deprived of its oxygen, or rather has become loaded with carbonic acid ; and this stagnation .will, of course, be communicated to all the rest of the system. Yet, if it have not continued sufficiently long to cause the loss of vitality in the nervous centres, the movement may be renewed by the admission of air into the lungs. Now, although it has been asserted, that the stagnation is due * See Dr. Houston in the Dublin Medical Journal, 1837. An attempt has been recently made by Dr. M. Hall (Edinb. Monthly Journal, 1843) to disprove Dr. Houston's infer- ences; but a most satisfactory reply has been made by Dr. Houston, at the Meeting of (he British Association, August, 1843, and published in the Dublin Journal, Jan., 1844. See also Edinb. Med. and Surg. Journ., July, 1844. 382 OF THE CIRCULATION OF THE BLOOD. to a mechanical impediment, resulting from the contracted state of the lungs in such cases, this has heen clearly proved not to be the fact, by causing animals to breathe a gas destitute of oxygen, so as to cause Asphyxia in a different manner ; the same stagnation results as in the other case. The influence of the prolonged application of cold to a part, may be quoted in support of the same general proposition; for, although the calibre of the vessels may be diminished by this agent, yet their contraction is not sufficient to account for that complete cessation of the flow of blood through them which is well known to occur, and to terminate in the loss of their vitality. 509. Many of the facts which indicate the influence of the Capillaries on the amount and rapidity of the circulation through them, have been already adverted to. It is a general principle, unquestioned by any physiologists, and embodied in the ancient aphorism Ubi stimulus ibi fluxus, that when there is any local excitement to the processes of Nutrition, Secretion, &c., a determination of blood towards the part speedily takes place, and the motion of blood through it is increased in rapidity ; and although it might be urged that this increased determination may not be the effect, but the cause, of the increased local action, such an opinion could not be sustained without many inconsistencies with known facts. For it is known that such local determina- tions may take place, not only as a part of the regular phenomena of growth and development (as in the case of the entire genital system at the time of puberty and of periodical heat, the uterus after conception, and the mammae after parturition), but also as a consequence of a strictly local cause. Thus, the student is well aware that after several hours' close application, there is commonly an increased determination of blood to the brain, causing a sense of oppression, a feeling of heat, and frequently a diminished action in other parts ; and, again, when the capillary circulation is being examined under the micro- scope, it is seen to be quickened by moderate stimuli, and equally retarded by depressing agents. All these facts harmonize completely with the phenomena, which are yet more striking in the lower classes of organized beings, and which are evidently the results of the same laws. 510. If the phenomena which have been here brought together be con- sidered as establishing the existence, in all classes of beings possessing a circulating apparatus, of a Capillary power, which affords a necessary con- v dition for the movement of the nutritious fluid through those parts in which it ^ comes into more immediate relation with the solids, the question still remains ' j open, as to its nature. That the Capillaries possess a contractile power, far - higher in degree than that of the large Arteries, and more easily excited than * that of the smaller, appears scarcely to admit of doubt ; though to what it is " due may be reasonably questioned. It has been recently asserted by Schwann, that they possess the same kind of fibrous tissue in their walls as do the large vessels : and this cannot be regarded as improbable. It is not possible, how- ever, that their contractility could have any influence in aiding the continuous motion of blood through them ; unless it were exercised in a very different manner from that of which observation affords us evidence. For, when we are microscopically examining the Capillary circulation of any part, it is at once seen that the vessels present no obvious movement ; and that the stream, now rendered continuous by the elasticity of the arteries, passes through them as through unelastic tubes. The only method in which the contractility of the Capillaries could produce a regular influence on the current of blood, would be an alternate contraction and dilatation, or a peristaltic movement ; and of neither of these can the least traces be discerned. Hence we should altogether dismiss from our minds the idea of any mechanical assistance, afforded by the action of the Capillaries, to the movement of the blood. That the contractile coat of the Capillaries has for its office, to regulate the MOTION OF THE BLOOD IN THE CAPILLARIES. 383 calibre of the vessels, can scarcely be doubted ; but any general permanent contraction would only occasion an obstacle to the circulation, as is shown by the effects of stimulating injections, which, if thrown into the vessels before their vitality has been lost, will not pass through the capillaries. It would appear, therefore, to be through their action on this coat that local stimuli occasion a contraction of the capillaries ; their effect, however, is different from what might have been anticipated ; for, instead of the capillary circulation being retarded, it is accelerated, at least until an abnormal condition results from their continued operation. Here, again, is another evidence, that some- thing different from mechanical power must be the agent that operates in all the foregoing cases. 511. The nature of this agent is at present very obscure ; and it may- not be in our power for some time to unveil it. The conditions of its action, how- ever, lie open for investigation ; and it appears from the foregoing facts that a very simple and constant expression of these may be given. Whilst the injection of blood into the capillary vessels of every part of the system is due to the action of the heart, its, rate of passage through those vessels is greatly modified by the degree of activity in the processes, to which it should normally be subservient in them ; the current being rendered more rapid by an increase in their activity, and being stagnated by their depression or total cessation. Thus it seems that "the capillaries possess a distributive power over the blood, regulating the local circulation independently of the central organ, in obedience to the necessities of each part."* If this be true, it is evident that the dilata- tion or contraction of the capillaries will only have a secondary influence on the movement of the blood through them. The former condition is usually an indication of diminished vital energy ; and when it is observed, it is almost invariably accompanied by a retardation or partial stagnation of the current ; on the other hand, the application of a moderate stimulus, which excites the contractility, accelerates for a time the motion of the blood, by rendering more energetic that reaction between the fluids and the surrounding tissues, which ^)is the condition that really has the most influence over the current. It is not ^enough to object to such a doctrine, that we know nothing of the mode in which i this reaction affects the movement of the blood ; since we are equally ignorant f\)f the modus operandi of many other causes whose real existence is fully Acknowledged, as, for instance, the effect of a stimulus applied to a motor j*Yierve, in causing contraction of the muscle supplied by it. 512. An attempt has been made by Dr. Alison, to give more precision to foregoing statement, by attributing the effect to a series of " vital attrac- s and repulsions," created by the operations to which the blood in the pillaries is subservient. He considers that the particles of blood are drawn irds the solids surrounding the capillaries, so long as they have not come close relation with them ; but that, after accomplishing the purposes of circulation, they are again repelled by the same property. It is very possible that these attractions and repulsions may have a real existence, and may be the operative causes in producing the phenomena in question, without being essentially different in character from those which are witnessed in physics and chemistry : it seems desirable, therefore, not to apply to them the term vital, which denotes, if it mean any thing, that they are to be referred to a set of laws entirely distinct. That alterations in the chemical state of the blood (involving, of course, important changes in its vital properties), are capable of exercising a most important effect on the capillary circulation, is shown not merely by the phenomena of Asphyxia, already referred to, but by the curious fact recently ascertained by Dr. J. Reid, that the blood, when imperfectly arterialized, is * See Palmer's edition of Hunter, vol. iii., p. 232. Note by Mr. P. 384 OF THE CIRCULATION OF BLOOD. retarded in the systemic capillaries, causing an increased pressure on the walls of the arteries. He found that, when the Ingress of air through the trachea of a dog was prevented, and the Asphyxia was proceeding to the stage of in- sensibility, the attempts at inspiration being few and laboured, and the blood in an exposed artery being quite venous in its character, the pressure upon the arterial walls, as indicated by the haemadynamometer applied to the femoral artery, was much greater than usual. Upon applying a similar test to a vein, however, it was found that the pressure was proportionably diminished ; whence it became apparent, that there was an unusual obstruction to the passage of venous blood through the systemic capillaries. After this period, however, the mercury in the hsemadynamometer applied to the artery began to fall stea- dily, and at last rapidly, in consequence of the diminished force of the heart, and the retardation of the blood in the pulmonic capillaries ; but, if atmo- spheric air was admitted, the mercury rose very speedily, showing that the renewal of the proper chemical state of the blood restored the condition neces- sary for its circulation through the capillaries. 513. It can be scarcely doubted, that it is by some influence exercised over, the molecular actions, to which the Blood is subject in the Capillaries, that the Nervous system can operate on the functions of Nutrition, Secretion, &c., in the manner already alluded to (Chap, vn.) ; and this influence may be not improperly termed vital, if by so designating it we merely imply that its nature and mode of operation are unknown, but that it is closely connected with those actions which are altogether peculiar to living beings. The fol- lowing experiment, made by Dr. Wilson Philip, exhibits, in a convincing manner, the possibility of such an influence. " The web of one of the hind legs of a frog was brought before the microscope ; and while Dr. Hastings observed the circulation, which was vigorous, the brain was crushed by the blow of a hammer. The vessels of the web instantly lost their power, the circulation ceasing; an effect which cannot arise, as we have seen, from the ceasing of the action of the heart. [Dr. P. here refers to experiments, by which it was ascertained that the circulation in the capillary vessels of the frog will continue for several minutes after the interruption of the heart's action.] In a short time the blood again began to move, but with less force. This experiment was repeated with the same result. If the brain is not com- pletely crushed, although the animal is killed, the blow, instead of destroying the circulation, increases its rapidity."* We are not hence to conclude, how- ever, that the Nervous system supplies any influence which is essential to the continuance of the Circulation ; since it is only by such sudden and severe injuries to the nervous centres, as instantaneously destroy the vitality of the whole system ( 386), that the movement of the blood is arrested. The experi- ments of Miiller and others satisfactorily prove, that mere action of the Nerves does not produce any direct effect upon the Capillary circulation ; and this corresponds with the well-known fact, that the Nutritive processes may con- tinue as usual, after this action has been suspended. All the facts which bear upon the question of the connection between nervous agency and the Capil- lary Circulation, have an equal relation to the functions of Nutrition and Secretion in general. IV. Of the Venous Circulation. 514. The Venous system takes its origin in the small trunks that are formed by the re-union of the Capillaries ; and it returns the blood from these to the Heart. The 'structure of the Veins is essentially the same with that of ^Experimental Inquiry into the Laws of the Vital Functions, 4th edition, p. 52. VENOUS CIRCULATION. 385 the Arteries ; but the fibrous tissue, of which their middle coat is /made up, bears more resemblance to the areolar tissue of the skin than it does either to muscular fibre, or to ,the true elastic tissue. The Elasticity of the Veins, however, is shown by the jet of blood which at first spouts out in ordinary venesection; when, by means of the ligature, a distension has been occasioned in the tubes below it. A slight Contractility on the application of stimuli has been observed; but this is not so decided as in the Arteries. The whole capacity of the Venous system is considerably greater than that of the Arte- rial ; the former is usually estimated to contain from 2 to 3 times as much blood as the latter, in the ordinary condition of the circulation ; and when we consider the great proportion which the Veins, in almost every part of the body, bear to the arteries, we shall scarcely regard even the larger of these ratios as exaggerated. Of course the rapidity of the movement of the blood in the two systems, will bear an inverse ratio to their respective capacities ; thus if, in a given length ; the Veins contain three times as much blood as the Arteries, the fluid will move with only one-third of the velocity. Even at their origins in the Capillary plexus, the Veins are larger than are the Arteries which termi- nate in the same plexus ; so that, wherever the arterial and venous networks form distinct strata, they are readily distinguished from each other. The Veins are remarkable for the number of valves which they contain, formed of duplicatures or loose folds of the internal tunic, between the component laminae of which contractile fibres are interposed ; arid for the dilatations behind these, which, when distended, give them a varicose appearance. The valves are single in the small veins, the free edge of the flap closing against the opposite wall of the vein ; in the larger trunks they are double ; and in a few instances they are composed of three flaps. The object of these valves is evi- dently to prevent the reflux of blood ; and we shall presently see that they are of important use in assisting in the maintenance of the venous circulation. They are most numerous in those Veins which run among parts affected by muscular movement ; and they are not found in the veins of the lungs, of the abdominal viscera, or of the brain. 515. The movement of the blood through the Veins is, without doubt, chiefly effected by the vis a tergo or propulsive force ; which results from the action of the Heart and Arteries, and from the additional power generated in the Capillary vessels. This is shown by the immediate arrestment of it, which takes place when these forces are suspended. There are some concurrent causes, however, which are supposed by some to have much influence upon it, and of which the consideration must not be neglected. One of these, is the suction power attributed to the Heart ; acting as a vis a fronte, in draw- ing the blood towards it. It is very doubtful how far the Auricles have such a power of active dilatation as that which would be required for this purpose ; and no sufficient evidence has been given that the current of blood at any dis- tance from the Heart is affected by it. Indeed, for a reason to be presently given, this may be regarded as impossible. Another important agency has been found by some physiologists, in the Inspiratory movement ; this is sup- posed to draw the blood of the Veins into the chest, in order to supply the vacuum which is created there, at the moment of the descent of the Dia- phragm. That the movement in question has some influence on the flow of Venous blood into the chest, is evident from the occurrence of the respiratory pulse, long ago described by Haller ; which may be seen in the veins of the neck and shoulder in thin persons, and in those especially who are suffering from pulmonary diseases. During Inspiration, the Veins are seen to be partially emptied : whilst during Expiration they become turgid, partly i consequence of the accumulation from behind, and of the check in front ; and partly (it may be) in some cases, through an absolute reflux from the veins 33 386 OF THE CIRCULATION OF BLOOD. within the chest ( 489). It was maintained by Sir D. Barry, that, the suction of the blood towards the chest in Inspiration, is one of the most important causes of the maintenance of the Venous circulation ; but several considera- tions agree in pointing to the conclusion, that no great influence can be rightly attributed to it. The Pulmonary circulation, being entirely within the chest, cannot be affected by variations in atmospheric pressure ; and it may be further remarked, that the whole mechanism of respiration is so different in Birds from that which exists in Mammalia, that no vacuum can ever be said to exist in their chests, although the venous circulation is performed as actively as usual. The Venous circulation of the fetus, also, is independent of any such agency. Again, it has been shown experimentally by Dr. Arnott and others, that no suction-power exerted at the farther end of a long tube, whose w r alls are so deficient in firmness as are those of the Veins, can occasion any accele- ration in a current of fluid transmitted through it ; for the effect of the suction is destroyed, at no great distance from the point at which it is applied, by the flapping together of the sides of the vessel. There can be no question about the fact, however, that in the immediate neighbourhood of the chest, the flow of blood towards the heart is aided by Inspiration and impeded by Expiration ; for Sir D. Barry's experiment, which consisted in introducing one extremity of a tube into the Jugular vein of a Horse, and the other into water, which exhibited an alternate elevation and depression with inspiration and expira- tion, has been repeated and confirmed by several physiologists. It is evident, that the suction of blood into the chest will aid the flow through the Veins, by removing the obstacle to it in front ; although it does not exercise any more direct influence over the current at a distance. On the other hand, the expiratory movement, while it directly causes accumulation in the Veins, will assist the Heart in propelling the blood into the Arteries ; and by the combined action of these two causes is produced, among other effects, the rising and sinking of the Brain, synchronously with expiration and inspiration, which are observed when a portion of the cranium is removed. 516. One of the most powerful of the general causes which influence the Venous circulation, is doubtless the frequently recurring action of the Muscles upon their trunks. In every instance that Muscular movement takes place, a portion of the Veins of the part will undergo compression ; and as the blood is prevented by the valves in the veins from being driven back into the small vessels, it is necessarily forced on towards the Heart. As each set of muscles is relaxed, the Veins compressed by it fill out again, to be again compressed by the renewal of the force. That the general Muscular movement is an important agent in maintaining the Circulation at a point above that at which it would be kept by the action of the Heart and Capillaries alone, appears from several considerations. The pulsations are diminished in frequency by rest, accelerated by exertion, and very much quickened by violent effort. In all kinds of exercise, and in almost every sort of effort, there is that alternate contraction and relaxation of particular groups of Muscles, which has been just mentioned, as affecting the flow of blood through the Veins ; and there can be little doubt, that the increased rapidity of the return of blood through them, is of itself a sufficient cause for the accelerated movements of the Heart. When a large number of Muscles are put in action after repose, as is the case when we rise up from a recumbent or a sitting posture, the blood is driven to the Heart with a very strong impetus ; and if that organ should be diseased, it may arrive there in a quantity larger than can be disposed of; so that sudden death may be the result. Hence the necessity for the avoidance of all sudden and violent movements on the part of those who labour under either a func- tional or structural disease of the centre of the circulation. That Gravity has an influence in modifying the Circulation in particular parts, is a fact well PECULIARITIES OF CIRCULATION. 387 known, especially in certain diseased states ; and the keeping an inflamed part at as high a level as possible, is often one of the most important therapeutic means that can be adopted. V. Peculiarities of the Circulation in different parts. 517. In several portions of the Human body, there are certain varieties in the distribution, and in the functional actions of the Blood-vessels, which should not be omitted in a general account of the Circulation. Of these, we have in the first place to notice the apparatus for the Pulmonary circulation ; the chief peculiarity of which is, that venous blood is sent from the heart, through a tube which is Arterial in its structure, whilst arterial blood is returned to the heart, through a vessel whose entire character is that of a Vein. The move- ment of the blood through these is considerably affected by the physical state of the Lungs themselves ; being retarded by any causes which can occasion pressure on the vessels (such as over-distension of the cells with air, ob- struction of their cavity by solid or fluid depositions, or by foreign substances injected into them, &c.) ; and proceeding with the greatest energy and regu- larity, when the respiratory movements are freely performed. The Portal circulation, again, is peculiar, in being a kind of offset from the general or systemic circulation ; and also in being destitute of valves; and it may be sur- mised with much probability, that the purpose of their absence is, to allow of an unusually free passage of blood from one part of that system to another, during the very varying conditions to which it is subjected ( 709). 518. Another very important modification of the circulating system, is that which presents itself within the Cranium. From the circumstance of the cranium being a closed cavity, which must be always filled with the same total amount of contents, the flow of blood through its vessels is attended with some peculiarities. The pressure of the atmosphere is here exerted, rather to keep the blood in the head than to force it out ; and it might accordingly be inferred that, whilst the quantity of cerebral matter remains the same, the amount of blood in the cranial vessels must also be invariable. This inference appears to derive support from the experiments of Dr. Kellie.* On bleeding animals to death, he found that, whilst the remainder of the body was completely ex- sanguine, the usual quantity of blood remained in the arteries and veins of the cranium ; but that, if an opening was made in the skull, these vessels were then as completely emptied as the rest. It is not to be hence inferred, how- ever, that the absolute quantity of blood within the cranium is not subject to variation ; and that in the states of inflammation, congestion, or other morbid affections, there is only a disturbance of the usual balance of the arterial and venous circulation. The fact in all probability is rather that the softness of the cerebral tissue, and its varying functional activity, render it peculiarly liable to undergo alterations in bulk ; and that the amount of the cerebro-spinal fluid varies considerably at different times ; so that the quantity of blood may thus, even in a healtfiy condition, be continually changing. Moreover, in disordered states of the circulation, the quantity of blood in the vessels of the cranium may be for a time diminished by sudden extravasation, either of blood or serum, into the cerebral substance ; and the amount of interior pressure upon the walls of the vesssels may also be considerably altered, even when there is no differ- ence in the quantity of fluid contained in them.t 519. The Erectile tissues constitute another curious modification of ordinary vascular apparatus. The chief of these are the Corpora Cavernoss * Edinburgh Medico-Chirurgical Transactions, vol. i. + The results of the more recent experiments of Dr. G. Burrows (Med. Gaz., April May, 1843) fully confirm the views stated above. 388 ON RESPIRATION. in the penis of the male, and in the clitoris of the female ; the collection of similar tissues round the vagina, and in the nymphae of the female ; and the nipple in both sexes. In all these situations, erection may be produced by local irritation ; or it may take place as a result of certain emotional conditions of the mind; the influence of which is probably transmitted through the sympa- thetic nerve, as it may be experienced even in cases of paraplegia. The erectile tissue appears essentially to consist of a plexus of varicose veins, enclosed in a fibrous envelop. According to Gerber,* this plexus is traversed by numerous contractile fibres, which are analogous to those that form the dartos ; and to the contraction of these is probably to be attributed that obstruction to the return of blood by the veins, which is the occasion of the turgescence. The proxi- mate cause of the erection of the penis, has been stated by some to be, the action of the ischio-cavernosi muscles ; and by others it has been attributed to the compression of the vena dorsalis penis against the symphysis pubis. But it is obvious that nothing analogous to this can apply to the other erectile organs, especially to the nipple. In the penis, according to Miiller, there are two sets of arteries ; of which one, destined for the nutrition of the tissues, communi- cates with the veins in the usual way, through a capillary network ; whilst the others pass off as large branches and penetrate the cavernous substance in a helicine manner, communicating abruptly with the venous cells. It would seem not improbable, that these last are not ordinarily pervious to blood ; but that the same change in the contractile fibres which impedes the return of the blood by the veins, may also permit it to enter more freely from the helicine arteries. This double communication, however, is denied by Valentin, who gives a different explanation of the appearances described by Miiller. The arteries are protected in such a manner, that, even when the veins are most compressed and the erection most complete, they are still quite pervious. CHAPTER X. ON RESPIRATION. I. Nature of the Function; and Provisions for its Performance. 520. IT is obvious that the Nutritive fluid, in its circulation through the capillaries of the system, must undergo great alterations, both in its physical constitution, and in its vital properties. It gives up to the tissues with which it is brought into contact, some of its most important elements ; and, at the same time, it is made the vehicle of the removal, from these tissues, of ingre- dients which are no longer in the state of combination, that fits them for their offices in the Animal Economy. To separate these ingredients from the general current of the circulation, and to carry them out of the system, is the great object of the Excretory organs; and it is very evident that the importance of the respective functions of these will vary with the amount of the ingredient which they have to separate, and with the deleterious influence which its * Op. cit., p. 298. ON RESPIRATION IN GENERAL. 389 retention would exert on the welfare of the system at large. Of all these injurious ingredients, Carbonic Acid is without doubt the most abundantly introduced into the nutritive fluid ; and it is also most deleterious in its etiects on the system, if allowed to accumulate. One of the most important changes which result from its retention, is the stagnation of the blood, both in the sys- temic and pulmonary capillaries ; for there is evidence that, if the process of aeration, by which the venous blood brought to the lungs is converted into arterial, be in any way checked, the flow of blood through the pulmonary capillaries speedily ceases ; and that if venous blood be propelled through the system, in place of arterial, it is transmitted with difficulty through the sys- temic vessels. The cause is the same in both instances; the normal changes, which the blood ought to undergo in these vessels, are prevented ; and there is consequently a cessation of that capillary power which has been shown to be one of the most important of the forces by which the blood is kept in motion ( 511). 521. We find, accordingly, that the provision for the removal of carbon from the blood, surpasses in extent that which is made for any other excretion. The two largest glands in the body the Liver and the Lungs are designed for this purpose ; but their operation is made subservient, in each case, to other objects. By the Liver, the carbon is excreted, with other elements, in the form of a fluid, which has important uses in the digestive function ; whilst by the Lungs (which will be presently seen to have in all essential points a glan- dular structure) it is thrown off in a gaseous form, and thus is made subser- vient, according to the laws of the mutual diffusion of gases, to the introduction of oxygen into the system, and consequently to the maintenance of the animal temperature, as well as of the stimulating properties of the blood. It is evi- dent, then, that any circumstances which check the excretion of carbonic acid by the lungs, will have an immediately injurious effect upon the system at large; by causing the accumulation, in the fluid upon which it is. dependent for the performance of its vital actions, of an agent that so seriously injures its vivifying properties. But this is not the only mode in which the cessation of this function becomes injurious. The exclusion of a constant supply of oxygen from the blood, even though the removal of the carbonic acid were provided for by other means, deprives it of its due power of nourishing and exciting to action the tissues and organs to which it is afterwards distributed ; for it would appear that this element is, throughout animated nature, a stimu- lant as essential to the energy of its operations, as caloric is to all, and light to many of these. Further, in those animals in which (as in Man) the whole current of blood passes through the Respiratory apparatus, any stagnation in its capillaries must derange, and soon check, the systemic circulation. There are some animals, however, (such as Reptiles,) in which only a portion of the blood that has returned from the system is transmitted to the lungs by each impulse of the heart; so that their pulmonary circulation is in some respects upon the footing of the portal circulation in other animals: in such, therefore, the interruption of the pulmonary circulation will not immediately suspend the movement of the blood through the systemic vessels; and in the Batrachia, whose soft moist skin allows the air to act with tolerable freedom upon the blood contained in its vessels, life may be prolonged for a considerable time, even after the complete removal of the lungs, provided the temperature be low.* But if, under these circumstances, the skin be covered with any unc- tuous substance, preventing the transmission of air, death speedily ensues. 522. The necessity for the Aeration of the Circulating fluid, is most remark- * See Edwards on the Influence of Physical Agents on Life (Translation by Hudgkin), p. 32. 33* 390 ON RESPIRATION. ably exemplified, in the provision which is made for it in every living being; such provision being more universally found than that for any other function, except for the ingestion of aliment, and for the perpetuation of the race. Even in Plants, a true Respiration is continually going on, although its effects are sometimes obscured by those of a converse change, which is subservient to a different purpose. It has been ascertained, that the absorption of oxygen, and the extrication of carbonic acid never cease during the life of the plant, taking place under all conditions, by day and by night, in sunshine and in shade. This is their true Respiration. But Plants obtain from the atmo- sphere a large proportion of the carbon which they require as food ; and this they procure, by decomposing the carbonic acid of the air, absorbing or fixing its carbon, and setting free its oxygen. Now to this process, which is only performed by the green parts of plants, and under the influence of light, the name Digestion has not improperly been given. A healthy Plant will fix in this manner much more carbon than it sets free by Respiration ; so that its effect upon the atmosphere is, on the whole, to aid in purifying it from the deleterious ingredient so largely imparted to it by Animal Respiration, Com- bustion, &c. The Fungi, however, derive their support, like Animals, only from matter which has been previously organized: and their respiration is uncompensated by the fixation of carbon from the atmosphere. The same is the case, during the processes of flowering and germination in the higher Plants ; for certain chemical conversions are then taking place, which involve the liberation of a large amount of carbonic acid, and a corresponding absorp- tion of oxygen, without any counterbalancing change.* In no Plants is there any distinct respiratory circulation ; since the nutritious fluid can be brought into close relation with the air, in almost every part of its course. There is, however, a rudiment of an internal respiratory apparatus, in a system of air- vessels, or tracheae, composed of membranous tubes kept pervious by an elastic spiral fibre which winds within them, and closely resembling the air- tubes of Insects. 523. In the Animal kingdom we almost universally find distinct organs for the aeration of the blood ; these are always formed upon the same general plan, being essentially composed of a membranous prolongation of the external surface, adapted by its vascularity and permeability, to bring the blood into close relation with the surrounding medium. But as this medium may be either air or water, we find two principal forms of the apparatus ; one of them adapted for each kind of respiration. In aquatic animals, the membrane is usually prolonged externally into tufts or fringes, which are so arranged as to expose the greatest amount of surface to the water ; each filament of which these are composed includes an afferent and efferent capillary vessel ; and it is whilst the fluid is passing through them, that its aeration is accomplished. The collection of tufts or fringes constitutes what are known as gills; and though their arrangement varies considerably, their essential character is but little different throughout the classes of animals that possess them. On the other hand, in air-breathing Animals, the aerating surface is reflected in- wardly, forming passages or chambers, into which the air is received, and on the walls of which the blood is distributed in a minute capillary network. Such a conformation is found even among some of the lower Articulata, which have a series of air-sacs disposed along each side of the body, one for every segment. In Insects we find, instead of these sacs, a system of prolonged tubes, ramifying through the body, and carrying air into its minutest portions. Even in some Mollusca, such as the Snail and other terrestrial Gasteropods, we find a provision for aerial respiration ; a large cavity being formed in the * See Principles of General and Comparative Physiology, 440 etseq. NATURE OF THE FUNCTION. 391 back, communicating with the air, and having a beautifully-reticulated plexus of blood-vessels on its walls. In none of the Invertebrata, however, does the respiratory apparatus communicate with the mouth ; which is an organ solely appropriated, in them, to the ingestion of food. In the Mollusca, indeed, the channel through which the water, that has passed over the aerating surface, leaves the chamber (formed by a fold of the mantle or general envelop) that contains the gills, is the same as that through which the excrementitious matter is discharged from the intestine ; and the gills themselves are very commonly situated in the neighbourhood of the anal orifice. This fact is inte- resting, in regard to the character of the temporary respiratory apparatus of the Human embryo. In Fishes and the larvae of Batrachia, which are the highest animals that breathe by gills, these organs are so disposed in connection with the cavity of the mouth, that fresh currents of water are continually being forced over them by its muscles ; and thus the energy of their action is greatly increased. Moreover, the whole blood which is propelled from the heart, proceeds first to the respiratory organs, instead of passing through them on its return from the systemic circulation, as in most of the aquatic Invertebrata. Still, as the quantity of oxygen which the blood can obtain in this manner is very small, being limited to that contained in the atmospheric air dissolved in the water, the amount of aeration must be considered as low. 524. In the lowest Vertebrata that possess any thing like a pulmonary cavity, this has a structure as simple as that of the air-sac of the Snail. This is the case in many Fishes, where it is known as the air-bladder ; it is fre- quently single in this class, and communicates with the intestinal canal near the stomach, or is altogether destitute of outlet. In others, however, it is double, and its duct opens into the oesophagus near the mouth ; so that its analogy to the lungs of higher animals is very evident. The Batrachia begin life as fishes, breathing by gills during their tadpole state ; but at the time that the legs are developed and the tail has decreased, the pulmonary organs also are evolved, and the course of the blood is altered, so that it is no longer transmitted through the gills, which speedily shrivel and disappear ( 42). There are some species, however, whose metamorphosis is checked, so that in their permanent condition both lungs and gills are present ; but the former are then present in a very rudimentary form, not being more developed than the air-sacs of many Fishes. The lungs of Reptiles are, almost universally, simple sacs with little subdivision into cells. Where such subdivision exists, it is usually at the upper part of each lung, the rest being one undivided bag, on the walls of which the pulmonary vessels are distributed. They afford us, therefore, a good opportunity of studying the distribution of these vessels ; and the accompanying figures represent the course of the circulation as observed in them. It will be seen that the trunk of the pulmonary artery runs along one side of the sac, and that of the pulmonary vein along the other (Fig. 93) ; and that numerous branches arise from the former, which subdivide into capillaries that ramify over the whole surface, and then reunite into small veins which terminate in the latter. The islets of parenchyma left between the capillary vessels, are seen to be much smaller than those which are usually to be observed in the systemic circulation (Figs. 94, 95) ; so that the mem- brane is more copiously traversed by vessels than any other that is known. The walls of the capillaries, moreover, are. much less distinct than those of the systemic circulation. These two conditions are obviously favourable to the exposure of the largest possible quantity of blood to the influence of the air ; but as the surface is not an extensive one, the amount which can be thus exposed at any one time is very limited ; and the pulmonary artery is in fact one of the smaller branches of the aorta, which conveys a mixed fluid to the system at large. 392 ON RESPIRATION, Fig. 93. Fig. 04. Lung of Triton cri&tatus, magnified about 3 dia- meters ; a, pulmonary artery ; 6, pulmonary vein. Portion of the lung of the same animal, more highly magnified ; the vessels, finely injected with size and vermilion, form a network so minute that the parenchyma is only seen in small islets in its interstices. (After Wagner.) Fis. 95. Portion of the lung of a living Triton, as seen under the microscope wilh a power of 150 diameters: a, pulmonary vein, receiving blood from the large trunk c, and a smaller vessel d. (After Wagner.) PROVISIONS FOR THE FUNCTION. 393 525. In the warm-blooded Vertebrata, which Fig. 96. have a complete double circulation, namely, birds and mammalia, a much larger extent of surface is provided for the aeration of the blood; the whole current of which is transmitted to the lungs, before circulating again through the sys- tem. This increase is provided in birds, partly by the more minute subdivision of the lungs into cells, and partly by the addition of a number of large air-sacs, which are disposed in various parts of the body, and even in the interior of the long bones. Hence it happens, that the amount of Potion of the Lung of a p/^, the ... I'll- terminal vessels being filled with respiration is greater m this class than in any mercury . A , natural 8ize . B) mode . other, although the form of the apparatus is not ra tei y magnified. (After Wagner.) nearly so concentrated as in the mammalia ; nor is the mechanism of the chest so well adapted to a constant exchange of the air contained in its cavities ( 48). In mammalia the lungs are proportionally smaller, and the whole respiratory apparatus is restricted to the thorax ; but the minute subdivision of their cavity, and the mechanism by which a continual interchange of air is provided for, render them very efficient for their designed purpose. In regard to the intimate structure of the lungs of man and of the mammalia, it is difficult to speak with confidence. It was maintained by Reissessen, and has been repeated by other anatomists, that the air-cells of the lungs are in reality the globular dilatations of the extremities of the ultimate ramifications of the bronchial tubes, analogous to the milk-cells of the mammary gland (Fig. 161) ; but it has been objected, that they are much more numerous than these ramifications can be supposed to be ; and there seems much reason to believe that every tube leads to a clus- ter of cells, communicating with each other. The recent inquiries of Mr. Addison* have shown that Reissessen's account is true of the foetal lung ; in which the ultimate subdivisions of the bronchial tubes terminate without anas- tomosis in closed extremities. But when an animal has respired, these termi- nations undergo a great change ; for the membrane composing each of them offers but a feeble resistance to the pressure of the air, and is pushed forwards and distended laterally into rounded inflations, forming a series of cells, which are moulded by mutual pressure into various angular forms, and which com- municate freely with one another by large oval apertures. The passages thus formed do not communicate with each other, otherwise than by their connec- tion with the same bronchial tube ; and the blood-vessels lie between the con- tiguous walls of each two of them, so that the capillary stream is exposed to air on either side. It appears from the researches of M. Bourgery,t that the development of the air-cells continues in the human subject up to the age of thirty, at which time the capacity for respiration is the greatest ; it subsequently decreases, especially in persons who suffer from cough, the violence of which expiratory effort frequently causes rupture of the air-cells, and thus gradually produces that emphysematous state of the lungs which is so common in elderly persons. The power of increasing the volume of air taken in, by a forced inspiration, is much less in the old person than in the child, though the average amount of air inspired may be the same ; hence the young person possesses a greater capacity of respiration, as it were, in reserve ; whilst the old man has little, and is therefore unfit for great exertion. 526. The lungs are developed, in the first instance, as diverticula from the * Philosophical Transactions, 1842. j- Archives Generates de Medecine, Mars, 1843. 394 ON RESPIRATION. Fig. 97. cesophageal tube. In the chick, about the fourth day, a little sacculus is de- scribed as shooting forth at its posterior and inferior part ; and this soon sub- divides at its lower part into two, at the same time becoming more separated from the tube by a constriction around the neck, which soon elongates so as to form the trachea. On the fifth or sixth day, the lung of one side is completely distinct from that of the other, and each is attached to the common pedicle by a peculiar branch, the future bronchus. The upper portion has much thicker walls than the lower, and these appear to contain a large quantity of vesicular parenchyma, in which the ramifications of the bronchial tubes subsequently extend themselves. About the tenth or eleventh day of incubation, these rami- fications possess nearly their permanent character and situation. The first trace of the glottis appears about the fifth day ; it is then a mere slit in the walls of the ossophagus, resembling that by which the ductus pneumaticus of some fishes opens into the alimentary canal. The formation of the cartilaginous [Fig. 98. First appearance of the Lungs ; a, in a Fowl at four days ; b, in a Fowl at six days; c. termination of bronchus in very young Pig. (After Rathke.) The Larynx, Trachea and Bronchiae, deprived of their fibrous covering, and with the outline of the Lungs; 1, 1, outline of the upper lobes of the lungs; 2, outline of the middle lobe of the right lung; 3, 3, outline of the inferior lobes of both lungs; 4, outline of the ninth dorsal vertebra, showing its relation to the lungs and the vertebral column; 5, thyroid cartilage; 6, cricoid cartilage; 7, trachea; 8, right bronchus; 9, left bronchus; 10, crico-thyroid ligament; 11, 12, rings of the trachea; 13, first ring of the trachea; 14, 1 ast ring of the trachea, which is corset-shaped; 15, 16, a complete bronchial cartilaginous ring; 17, one which is bifurcated; 18, double bifurcated bronchial rings; 19, 19, smaller bronchial rings; 20, depressions for the course of the large blood-vessels.] NATURE OF THE FUNCTION. [Fig. 99. 395 A view of the Bronchise and Blood- Vessels of the Lungs as shown by dissection, as well as the relative position of the Lungs to the Heart; 1, end of the left auricle of the heart; 2, the right auricle ; 3, the left ventricle with its vessels; 4, the right ventricle with its vessels; 5, the pulmonary artery; 6, arch of the aorta; 7, superior vena cava; 8, arteria innominata; 9, left primitive carotid artery; 10, left sub-clavian artery; 11, the trachea; 12, the larynx; 13, upper lobe of the right lung; 14, upper lobe of the left lung; 15, trunk of the right pulmonary artery ; 16, lower lobes of the lungs. The distribution of the bronchia and of the arteries and veins, as well as some of the air-cells of the lungs, are also shown in this dissection.] rings of the trachea does not commence until after the twelfth day, when they first appear as transverse striae on the median line of the front only ; they gradually become solid, and extend themselves on either side, until they nearly meet at last on the median line on the back or vertebral side of the tube. The history of the process in the human embryo, appears to be very nearly the same. The first appearance of the lungs takes place about the sixth week, at which time they are simple vesicular prolongations of the oesophageal mem- brane. Their surface, however, soon becomes studded with numerous little wart-like projections, and these are caused by the formation of corresponding enlargements of their cavity. These enlargements soon become prolonged, and. develop corresponding bud-like enlargements from their sides ; and in this manner the form of the organ is gradually changed, a progressive increase in their bulk taking place from above downwards, in consequence of the exten- sion of the bronchial ramifications from the single tube at the apex. At the same time, however, a corresponding increase in the amount of the parenchy- matous tissue of the lung is taking place ; for this is deposited in all the inter- stices between the bronchial ramifications, and might be compared with the soil filling up the spaces amongst the roots of a tree. It is in this parenchyma that the pulmonary vessels are distributed, and the portion of it which extends beyond the terminations of the bronchial tubes, seems to act as the nidus for their further extension. It can be easily shown that, up to a late period of the 396 ON RESPIRATION. development of the lungs, the dilated terminations of the bronchi constitute the only air-cells (Fig. 97, c) : but, as already mentioned, the parenchyma subse- quently has additional cavities formed within it. It is a fact of some interest, as an example of the tendency of certain diseased conditions to produce a return to forms which are natural to the foetal organism, or which present themselves in other animals, that up to a late period in the development of the human em- bryo, the lungs do not nearly fill the cavity of the chest, and the pleura of each side contains a good deal of serous fluid. 527. The network of vessels on the walls of the air-cells is described by Reissessen as so minute, that the diameter of the meshes is scarcely so great as that of the capillary vessels which form it. According to Mr. Addison, the capillaries in the lung of a Toad admit, in their natural state, no more than one, or at most two rows of blood-corpuscles ; and the islets of tissue between them are comparatively large : whilst, if the lung be congested or inflamed, five or six rows of corpuscles are seen in the vessels ; and the islets of tissue are almost entirely obliterated. The diameter of the pulmonary vesicles is about twenty times greater than that of the capillaries which are distributed upon their parietes ; varying (according to the measurement of Weber) from the -nf th to the g\th of an inch. There is no evidence that the alteration in the size of the air-cells, which takes place during the respiratory process, is due to any other cause than the simple elasticity of their walls ; but the bron- chial tubes certainly possess a considerable amount of contractility, which can scarcely be regarded as otherwise than muscular. From the experiments of Dr. C. B. Williams,* it appears that all the air-tubes are endowed with a con- siderable amount of irritability, which may be excited by electrical, chemical, or mechanical stimuli, applied to themselves, but not readily (if at all) excitable through their nerves. This contractility resembles that of the intestines or arteries more than that of the voluntary muscles or heart ; the contraction and relaxation being more gradual than that of the latter, though less tardy than that of the former. It is chiefly manifested in the smaller bronchial tubes ; since, in the trachea and the larger bronchi, the cartilaginous rings prevent any decided diminution in the calibre of the tube. Wedemeyer did not succeed in producing any distinct contraction of the fibres of the trachea and larger bronchi ; but he states that tubes of less than a line in diameter could be per- ceived to contract gradually under the stimulus of galvanism, until their cavity was nearly obliterated. It is remarked by Dr. Williams, that the irritability of the bronchial muscles is soon exhausted by the action of a stimulus ; but that it may in some degree be restored by rest, even when the lung is removed from the body. When the stimulation is long continued, however, as by intense irritation of the mucous membrane during life, the contractile tissue passes into a state which resembles that of the tonic contraction of muscular fibre ( 390). The contractility is greatly affected by the mode of death, and is remarkably diminished by the action of vegetable narcotics, particularly stramonium and belladonna ; whilst it seems to be scarcely at all affected by hydrocyanic acid. These facts are very important, as throwing light upon cer- tain diseased conditions. It has long been suspected, that the dyspnoea of Spas- modic Asthma depends upon a constricted state of the smaller bronchial tubes, excited through the nervous system, frequently by a stimulating cause at some distance ; and there can now be little doubt that this is the case. That they should not be readily excited to contraction by a galvanic stimulus applied to their nerves, is no valid argument against this view ; as it was long held that the muscular coat of the alimentary canal also was completely removed from nervous influence, which is now well known to be not by any means the case. * Athenaeum Report of the Meeting of the British Association, 1840, p. 802. NATURE OF THE FUNCTION. 397 The peculiar influence of stramonium and belladonna, in diminishing the contractility of these fibres, harmonizes remarkably with the well-known fact of the relief frequently afforded by them in this distressing malady. 528. Notwithstanding the high degree of contractility which the bronchial tubes have been shown to possess, there is no valid reason for the belief, that they contribute by any rhythmical movement to the exchange of the contained air, which, in the healthy state, is continually taking place. For it can be scarcely imagined that they should, by any power of their own, contract and dilate uniformly with the contraction and expansion of the chest, unless their muscles were equally subject with those of the thorax, to the influence of the nervous system ; which all experiments concur in showing not to be the case. The lungs themselves, then, are to be regarded as quite passive in the move- ments of respiration ; the renewal of their contained air being accomplished by the action of the muscles external to the thorax, or partly forming its pari- etes. The lung completely fills the cavity of the pleura, in the healthy state at least ; so that, when this is enlarged, a vacuum is produced, which can only be filled by a corresponding enlargement of the lung ; and to produce this, the air rushes down the trachea, and passes to the remotest air-cells. The dis-' tension of the whole tissue of the lung, which is effected in this manner, is much more complete than that which could be occasioned by simple insuffla- tion from the trachea ; a fact of which it has been proposed to take advantage in juridical inquiries in regard to suspected cases of Infanticide, where the lungs are found to float, and the defence is set up that the child was still-born, and that air was blown into the chest for the purpose of resuscitating it. It has been ascertained by the experiments of Mr. Jennings,* that if a piece of lung, which has been filled with air by insufflation, be exposed to great pres- sure, the air may be expelled from it sufficiently to cause it to sink in water ; but that no pressure can produce the same effect upon that which has been filled by a natural inspiratory effort. It is a serious objection to the use of this test in juridical investigations, however, that the early inspiratory efforts of the infant are often so feeble as to produce but a very imperfect dilatation of the air-cells ; so that the lung of an infant which has naturally inspired cannot, by such means, be distinguished from one that has been artificially inflated. The fact ascertained by Mr. J., however, is one of much physiological interest. Owing to the freedom with which the air enters the lungs, when there is no abnormal obstruction, the external surface is always in contact with the walls of the chest, so that the pulmonary and costal pleura glide over one another with every inspiration and expiration. The smooth and moistened character of their surface prevents the movement from producing any sound ; but it becomes evident when the friction is increased, either by the dryness that is commonly one of the early changes produced by inflammation, or by the rough deposit that subsequently appears. 529. The complete dependence of the expansion of the Lungs upon the production of a vacuum in the chest, is well shown by the effect of admission of air into the pleural cavity. When an aperture is made on either side, so that the air rushes in at each inspiratory movement, the expansion of the lung on that side is diminished, or entirely prevented, in proportion to the size of the aperture. If air can enter through it more readily than through the trachea, an entire collapse of the lung takes place ; and by making such an aperture on each side, complete asphyxia is produced. But if it be too small to admit the very ready passage of air, the vacuum produced by the inspira- tory movement is more easily filled by the distension of the lungs than by the rush of air into the pleural cavity ; so that a sufficient amount of change takes * Transactions of the Provincial, Medical and Surgical Association, vol. n. 34 ON RESPIRATION. place for the maintenance of life. This is frequently observed in the case of penetrating wounds of the thorax, in the surgical treatment of which it is of great importance to close the aperture as completely as possible ; when this has been accomplished, the air that had found its way into the cavity is soon absorbed, and the lung resumes its full play. Where one lung is obstructed by tubercular deposit, or is prevented in any other way from rightly dis- charging its function, an opening that freely admits air into the pleural cavity of the other side, is necessarily attended with an immediately fatal result ; an'd in this manner it not unfrequently happens, that chronic pulmonary diseases suddenly terminate in Asphyxia, a communication being opened by ulcera- tion, between a bronchial tube and the cavity of the thorax. 530. The dilatation of the -chest during Inspiration, is chiefly accomplished by the contraction of the Diaphragm, which, from the high arch that it pre- viously formed, becomes nearly plane ; in this change of figure, it presses on the abdominal viscera, so as to cause them to protrude, which they are enabled to do by the relaxation of the abdominal muscles. In ordinary tranquil breathing, the action of the diaphragm is alone nearly sufficient to produce the necessary exchange of air ; but, when a full inspiration is required, the cavity of the chest is dilated laterally, as well as inferiorly. This is accom- plished by the Intercostal muscles, the Scaleni, Serrati, and others ; which, by elevating the ribs, bring them and their cartilages more nearly into the same direction, and thus separate them more widely from the median line. Expi- ration is chiefly effected by the contraction of the abdominal muscles, which at the same time force up the diaphragm by their pressure on the viscera, and depress the ribs ; in the latter movement they are aided by the Longissimus Dorsi, Sacrolumbalis, &c., and also by the elasticity of the cartilages of the ribs, with that of the air-cells and air-tubes themselves. It is difficult to form an estimate, by observations on one's self, of the usual number and degree of the respiratory movements; since the direction of the attention to them is certain to increase their frequency and amount. In general it may be stated that from 14 to 18 alternations usually occur in a minute ; of these, the ordi- nary inspirations involve but little movement of the thorax; but a greater exertion is made at about every fifth recurrence. The average numerical proportion of the respiratory movements, to the pulsations of the heart, is about 1 to 5 or 4 ; and when this proportion is widely departed from, there is reason to suspect some obstruction to the aeration of the blood, or some disorder of the nervous system. Thus in Pneumonia, in which a greater or less amount of the lung is unfit for its office, the number of respirations in- creases in a more rapid proportion than the acceleration of the pulse ; so that the ratio becomes as 1 to 3, or even 1 to 2, in accordance with the degree of engorgement.* In Hysterical patients, however, a similar increase, or even a greater one, may take place without any serious cause ; thus Dr. Elliotsont mentions a case, in which the respiratory movements of a young female, through nervous affection, were 98 or even 106, whilst the pulse was 104. On the other hand, the respirations in certain typhoid conditions and in nar- cotic poisoning become abnormally slow, owing to the torpid condition of the nervous centres, the proportion being 1 to 6, or even 1 to 8; and in such cases, the lungs not unfrequently become oedematous, from the cause formerly mentioned (231 and 232). 531. The amount, also, of the Respiratory Movements is affected by various morbid conditions ; thus when dislocation of the spine takes place above the * See a paper by Dr. Hooker, on the Relation between the Respiratory and Circulating Functions, in the Boston (N. E.) Medical and SurgicalJournal; an abstract of which will be found in the British and Foreign Medical Review, vol. vi. p. 263. f Physiology, p. 215, note. NATURE OF THE FUNCTION. 399 origin of the intercostal nerves, but below that of the phrenic, so that the former are paralyzed, the respiratory movement is confined to the diaphragm ; and as this is insufficient, serum is effused into the lungs, and a slow Asphyxia supervenes, which usually proves fatal in from three to seven days. Even where the muscles and nerves are all capable of action, the full performance of the inspiratory movements is prevented by the solidification or engorgement of any part of the lung, which interferes with its free distension ; or by adhe- sions between the pleural surfaces, which offer a still more direct impediment. When these adhesions are of long standing, they are commonly stretched into bands, by the continual tension to which they are subjected. If the impeding cause affect both sides, the movements of both will be alike interfered with; but if one side only is affected, its movements will be diminished, whilst those of the other remain natural ; and the physician hence frequently derives an indi- cation of great value, in regard to the degree in which the lung is incapable of performing its functions. It is to be remembered, however, that the action both of the diaphragm and of the elevators of the ribs may be prevented, by pain either in the muscles themselves or in the parts which they move ; thus the descent of the diaphragm is checked by inflammation of the abdominal viscera or of the peritoneum; and that of the intercostals by rheumatism, pleuritis, pericarditis, or other painful disorders of the parts forming the parietes of the thorax. 532. In regard to the capacity of the Lungs, the quantity of air introduced and expelled at each ordinary respiratory movement, and the amount that remains after expiration, great discrepancy exists in the statements of the vari- ous experimenters who have endeavoured to ascertain them. This discrepancy has doubtless arisen in part from the circumstance already mentioned, that attention to the respiratory movements will render them fuller and more fre- quent : and in part, also, from the degree of effort that is required, to draw air from any kind of apparatus adapted to afford a measurement of the quantity inhaled; which effort will of itself cause the distension of the chest to be much greater than natural. The experiments of Messrs. Allen and Pepys seemed to give 16 cubic inches as the average quantity taken in at each inspiration : whilst those of Menzies (who is followed by Dr. Bostock) caused him to rate it at 40 cubic inches. The most recent experiments on the subject are those of Mr. Coathupe,* in which the Author has much reason to feel con- fidence. According to his estimate, about 286 cubic feet, or 460,224 cubic inches of air, pass through the lungs in 24 hours ; reckoning the average number of inspirations at 16 per minute, this would give 20 cubic inches as the amount inhaled at each. According to the experiments of Allen and Pepys, the quantity of air remaining in the lungs of a stout full-grown man after death, is about 100 cubic inches ; this is probably less than the amount that remains after ordinary expiration.! * Athenaeum Report of Meeting of the British Association, 1839, p. 702. j- [ Many very interesting and practically important results have been obtained by Mr. Hutchinsori,t with his spiromefer, an instrument by which the capacity of respiration is measured by the quantity of air expired in a full and forcible expiration. Among these the chief is the fact of the existence of an intimate relation between this capacity and the height of the individual examined. In 1088 healthy men from five to more than six feet in height, he found the capacities of respiration as follows: in men of 5 feet, 135 cubic inches; of 5 feet 1 in., 177 c. i.; of 5ft. 2 in., 173 c. in.; of 5 ft. 3 in., 184 c. i.; of 5 ft. 4 in., 193 c. in.; of 5 ft. 5 in., 208 c. in.; of 5 ft. 6 in., 204 c. in.; of 5 ft. 7 in., 224 c. i.; of 5 ft. 8 in., 220 c. i.; of 5 ft. 9 in., 229 c. i.; of 5 ft. 10 in., 246 c. i.; of 5 ft. 1 1 in., 254 c. i.; of 6 ft., 255 c. i.; of upwards of 6 ft., 260 c. i. These numbers are such that it may be generally stated that for every additional inch of height from 5 to 6 feet, eight additional cubic inches of air, at 60, are given out by a forced expiration. And the [* Lancet, July 27 and Aug. 3, 1844.] 400 ON RESPIRATION. II. Chemical Phenomena of Respiration. 533. We naturally pass from the foregoing inquiries to those that relate to the alterations in the air, which are effected by Respiration. It was formerly supposed that the blood arrived at the lungs charged with Carbon, that this carbon was united in their cells to the Oxygen of the atmosphere, and that in this manner a certain amount of the Oxygen of the inspired air was being continually converted into Carbonic acid, which thus replaced it in the expired air. Subsequent researches, however, appear to have satisfactorily proved, that this is not a true account of the changes which take place in the Lungs ; and that it would be more correct to say, that the blood comes to the lungs charged with Carbonic acid, formed by the union of carbon and oxygen in the systemic capillaries ; this it imparts to the inspired air, at the same time abstracting from it a volume of Oxygen which is always as large, and usually greater. Hence it is not correct to speak of a certain quantity of the inspired oxygen as being converted into carbonic acid in the lungs ; but it should rather be said, that a certain quantity of oxygen is absorbed, and a certain quantity (generally less than the equivalent bulk) of carbonic acid exhaled. The pro- portion of these quantities is by no means constant ; varying with different species, and with the same animal at different ages and at different periods of the year. According to Dr. Edwards, the quantity of oxygen which entirely disappears from the air is sometimes as much as one-third of the whole ; it is greatest in the young animal, and is sometimes almost imperceptible in the adult. It appears probable that a part of this Oxygen is made to combine with Hydrogen set free in the systemic capillaries ; and that the water thus gene- rated forms part of that exhaled from the lungs. A sort of combustion of Hydro-Carbon thus appears to be continually going on in the body at large, the products of which are got rid of by the lungs; and this process is mainly, if not solely, instrumental in the maintenance of Animal Heat. (See Chap, xm.) 534. The quantity of Carbonic Acid excreted by the lungs has been esti- mated by some experimenters at as much as 39,600 cubic inches in twenty- four hours; this amount of gas would contain 5,148 grains, or 11 ounces (Troy) of solid carbon. This estimate was formed upon the results of an expe- riment continued during a short time ; in which, from the nature of the appa- ratus employed, the respirations were to a certain degree laborious, and the quantity of air renewed at each movement was therefore greater ; and there are several reasons for regarding it as much too high. In Mr. Coathupe's experiments, great care was taken to render the inspiration as free as possible from effort ; and the measuring process was continued for a much longer time. According to his statement, the. quantity of Carbonic acid generated in twenty- four hours is about 17,856 cubic inches ; this will contain 2,616 grains, or 5| ounces of solid carbon, a quantity, which we may very well imagine to be thus excreted, and which corresponds very closely with the results obtained by MM. Prevost and Dumas.* The proportion of carbonic acid contained in results of the examinations are so nearly uniform that disease may be suspected in any man who cannot blow out nearly so many cubic inches as the average of those of ihe same height, even when, by external measurement, his chest appears to be of full size. Indeed, in general, the size of the chest affords no good indication of the capacity of expi- ration. The only exceptions among healthy men" to the general rule of the direct pro- portion between the height of the body and the capacity, are in the cases of fat men whose capacity is always low. M. C.] ' * M M. Andral and Gavarret state the following as the results of experiments made on seventy-two persons (thirty-six males and thirty-six females), to determine the quantity of carbonic acid exhaled in breathing. These experiments were made as nearly as pos- sible under the same circumstances, as regards health, time of the day, amount of exer- CHEMICAL PHENOMENA OF RESPIRATION. 401 the expired air appears from these estimates to be about 4 per cent, on the whole ; but single experiments give a much higher estimate. Thus, in one of those made by Allen and Pepys, in which fresh air was taken in at every inspiration, the proportion was 8 parts in every 100. They found, however, that if the air be already charged in some degree with carbonic acid, the quan- tity excreted is much less ; for when 300 cubic inches were respired for three minutes, only 28 cubic inches of carbonic acid were found in it, although the rate of its production in a parallel experiment was 32 cubic inches in a minute. Knowing, then, the necessity of a free excretion of carbonic acid, we are led by this fact to perceive the high importance of ventilation ; for it is not suf- ficient for health, that a room should contain the quantity of air requisite for the support of its inhabitants during a given time ; since after they have remained in it but a part of that time, the quantity of carbonic acid which its atmosphere will contain, will be large enough to interfere greatly with the due aeration of their blood, and thus to cause oppression of the brain and the other morbid affections that result from the accumulation of carbonic acid in the cir- culating fluid. On the other hand, it has been ascertained by the recent expe- riments of Dr. Boswell Reid that, if the carbonic acid be removed as fast as it is formed, an animal may remain in a limited quantity of air, without much inconvenience, until nearly the whole of its oxygen is exhausted ; thus showing that the respirability of air does not depend so much upon the pro- portion of oxygen it contains, as upon its freedom from contamination with carbonic acid or other poisonous gases. 535. Although the statements just given may be regarded as representing the average amount of carbonic acid evolved during the twenty-four hours, tion, &c. ; and were repeated several times on each individual. 1. At all ages beyond eight years, the exhalation is greater in males than in females. 2. In mules, it regularly increases in quantity from eight to thirty years of age ; from thirty to forty it is stationary, or diminishes a little; from forty to fifty the diminution is greater; and from fifty to extreme age, it goes on diminishing, till it scarcely exceeds the quantity at ten years. 3. The quantity of carbonic acid exhaled in one hour by males of different ages, is as follows; at eight years, 77-5 grains; at fifteen, 135 grains; at twenty, 176-7 grains; between thirty and forty, 189 grains; between forty and sixty, 156 grains; between sixty and eighty, 142-5 grains; and in a man of a hundred and two, it was only 91 5 grains. 4. In females, nearly the same proportionate increase goes on to the time of puberty ; when the quantity abruptly ceases to increase, and remains stationary so long as they continue to menstruate. When, however, menstruation has ceased, the exhalation of car- bonic acid begins again to augment, and then, again, in advancing years, decreases as it does in men. Thus before puberty the quantity of carbon exhaled by girls in an hour is 99 grains; and so it remains during the continuance of menstruation; afterwards, from thirty-eight to forty-nine years of age, it increases to 130 grains: from fifty to sixty it again falls to 1 13 grains ; from sixty to eighty it is reduced to 105 grains ; and in a woman of eighty-two, it was only 93 grains. 5. Should menstruation temporarily cease at any time, the exhalation of carbonic acid immediately undergoes an increase, precisely as at the final cessation of the function. 6. In pregnancy, the exhalation is equal to that which is natural soon after cessation of the menstruation. 7. The more robust the indi- vidual, cseteris paribus, the more carbonic acid is exhaled; but the differences are not con- stant. 8. The influences of the weights of individuals, of the capacities of their chests, and of the extent of the respiratory movements, are not great. 9. The maximum observed was in a strong man of twenty-six, who exhaled at the rate of 21ci-5 grains of carbon per hour. Another robust man of sixty years of age exhaled at the rate of 209 grains per hour; another, of similar constitution, and sixty-three years of age, exhaled at the rate of 190 grains per hour; and an old man of ninety-two, who preserved an uncommon degree of energy, and who, in his younger days, had boasted of uncommon muscular powers, still consumed at the rate of 151 grains per hour. On the other hand, a slender man of forty-five, in the enjoyment of good health, only consumed 139-5 grains per hour. These facts demonstrate the influence of individual constitution on the amount of carbonic acid generated. Interesting as these results undoubtedly are, they require to be confirmed by a more extensive series of experiments, before they can be received as physiological truths. 24* 402 ON RESPIRATION. the amount is subject to great variation under particular circumstances. Thus, during a state of muscular activity it is greatly increased. Mr. Newport has noticed that in Insects the difference is enormous, their respiration being as feeble as that of cold-blooded animals when they are at rest, and more energetic (the quantity of oxygen consumed in proportion to their size being greater) than that of any other animals, when they are in active movement. In Man the difference is not so great, and its exact amount cannot be readily estimated ; but it is unquestionable that an increase does take place. It has been ascer- tained by Dr. Prout, however, that, if the exercise be prolonged so as to occa- sion fatigue, a diminished consumption of oxygen takes place ; he also states that the exhilarating passions increase, whilst the depressing passions (as also the use of alcohol and tea) diminish, the quantity of carbonic acid exhaled. There is little doubt that there is a great diminution, also, during sleep ; this may be partly due to the total cessation of muscular exertion, and partly to the greater retention of the heat which is the consequence of it. For it appears that the amount of carbonic acid produced is greatly influenced by the tem- perature ; in the Guinea-pig, according to Crawford, the quantity exhaled at 104 is only half that which is generated at 55.* The final cause or purpose of this connection will be evident, when we consider the subject of Animal Heat. 536. It has been supposed, until recently, that the azote of the air undergoes no change through Respiration ; but the experiments of Dr. Edwards have shown that, although its quantity may remain nearly the same, there is a con- tinual absorption and a continual exhalation of the gas. If the absorption be the more active, there will be a disappearance of azote from the air ; if exha- lation predominate, the proportion of this gas will be increased. Even in the same animal, there may be a variation in this respect at different periods of the year, and even at different parts of the day. Thus in nearly all the lower animals on which he experimented, there was an augmentation in the quantity of azote during the summer, sometimes equaling-, in the course of the day, the whole bulk of the animal. On the other hand, towards the end of October, he found that a diminution of the nitrogen began to be apparent ; and this, continued until the following spring. 537. The reaction which takes place between the air and the blood, is easily explained upon physical principles. If the Blood come to the Lungs charged with Carbonic acid, and is exposed in their cells to the influence of atmo- spheric air, which is a mixture of Oxygen and Nitrogen, an endosmose and exosmose of gases will take place, according to certain fixed laws.t The Car- bonic acid of the blood will pass out, to be replaced by Oxygen and Nitrogen ; and the quantity of the former which enters will be much greater than that of the latter, on account of the superior facility with which oxygen passes through porous membranes. If the venous blood also contain Nitrogen as well as car- bonic acid, this also will pass out, to be replaced by the Oxygen of the air. Thus, there will be a continual Exosmose of Carbonic acid and Nitrogen, and a continual Endosmose of Oxygen and Nitrogen ; and the relative quantities of these gases exhaled and absorbed will be subject to continual variation from * The experiments of Dr. Malcolm (Edinb. Monthly Journal, Jan., 1843), appear to show that the proportion of carbonic acid exhaled is greatly diminished in typhus fever. According to Dr. Prom's experiments, the average proportion generated in healthy respi- ration, between 11 A. M. and 1 P. M., is about 3-96 per cent, of the whole inspired air. But in some severe cases of fever, the proportion was as little as 1-18 per cent.; but in general it was about 2-50. The proportion did not seem to be much influenced by the number of respirations; being about the same when they were only 20 as when 48 per minute. f See Principles of General and Comparative Physiology, 4379. EFFECTS OF RESPIRATION ON THE BLOOD. 403 secondary causes. It appears that, in general, even in herbivorous animals, the Exhalation of Nitrogen predominates over the absorption ; but nrrni inquiries have shown, that their food ordinarily contains a supply of azotized matter amply sufficient for their wants. It is yet a matter of doubt, however, whether the Absorption would not predominate, when there is a deficiency of azotized matter in the aliment. Such we may imagine to be the case in Insects which feed upon the saccharine juices of plants ; the waste of their muscular tissue being, from the activity of their movements, excessively rapid. III. Effects of Respiration on the Blood. 538. That an important change is effected in the character of the Blood, by exposure to atmospheric air in the lungs, has been known, from the time when it was first ascertained that it is regularly transmitted to those organs. The most obvious part of this change is the alteration in its colour, from the dark purple of the venous fluid, to the rich crimson of the arterial. But this alteration is only the index of changes far more important which occur in its chemical constitution. Respecting the nature of these changes, there has been, as formerly stated, much difference of opinion ; some maintaining that the carbonic acid exhaled is formed in the lungs ; and others, that it is con- tained in the venous blood, and is truly excreted from it. The latter opinion, which was long since brought forwards by La Grange and Hassenfratz, has recently obtained such full confirmation from the experiments of Spallanzani, Edwards, Miiller, Bischoff, Magnus, and others, as to have a full claim for adoption as a physiological truth. These experiments are of two kinds ; first, those which show that an exhalation of carbonic acid may continue for a long time, when the animal is breathing an atmosphere in which no oxygen exists ; and, secondly, those which prove that much more carbonic acid exists in an uncombined state in venous blood than in arterial, whilst more oxygen exists in a similar condition in arterial blood than in venous. The results of these will now be briefly stated. 539. It was stated, Jong since, by Spallanzani, that Snails might be kept for a long period in Hydrogen, without apparent injury to them ; and that during this period they disengaged a considerable amount of Carbonic acid. Dr. Edwards subsequently ascertained that, when frogs were kept in hydrogen for several hours, the quantity of carbonic acid exhaled was fully as great as it would have been in atmospheric air, or even greater ; this latter fact, if correct, may be accounted for, by the superior displacing power, which (on the laws of the diffusion of gases), hydrogen possesses for carbonic acid. Col- lard de Martigny repeated this experiment in nitrogen, with the same results. In both sets of experiments, the precaution was used of compressing the flanks of the animal, previously to immersing it in the gas, so as to expel from the lungs whatever mixture of oxygen they might contain. These experiments have been since repeated by Miiller and Bergemann, who took the additional precaution of removing, by means of the air-pump, all the atmospheric air that the lungs of the frog might previously contain, together with the carbonic acid that might exist in the alimentary canal. They found in one of their experi- ments, that the quantity of carbonic acid exhaled in hydrogen was nearly a cubic inch in 6 hours ; and in another, that nearly the same amount was given off in nitrogen, but this required rather a longer period. It appears from the table of their results,* that the amount was not ordinarily greater in the experiments which were prolonged for twelve or fourteen hours than in those which were terminated in half the time ; hence it may be inferred, that the * Miiller's Physiology, p. 338. 404 ON RESPIRATION. quantity which the blood is itself capable of disengaging is limited, and that the absorption of oxygen is necessary to enable carbon to be set free from the tissues. An exception may be taken to all these experiments, on the ground that they were made upon cold-blooded animals ; and that in the warm-blooded tribes the character of the change may be different. It is scarcely probable, however, that the uniformity of Nature should be thus violated. There is no difference in kind between the alterations effected in the air by the respiration of warm-blooded, and by that of cold-blooded animals ; the only variation is in degree. Nor is there any appreciable difference in the character of the changes effected upon their venous blood, by the action of oxygen or of other gases. It is impossible, however, for an adult Bird or Mammal to sustain life for any considerable time, in an atmosphere deprived of oxygen ; since the greatly increased rapidity and energy of all their vital operations, necessitates a much more constant supply of this vivifying agent than is needed by the inferior tribes ; and, as we shall presently see, the capillary action necessary for the passage of the blood through the lungs will not take place without it. But Dr. Edwards has shown, that young Mammalia can sustain life in an atmosphere of hydrogen or nitrogen, for a sufficient length of time to exhale a sensible amount of carbonic acid ; so that the character of the process is clearly proved to be the same in them as in Reptiles and Invertebrata. 540. That the change which Venous Blood undergoes in the lungs is to be explained upon principles of a purely chemical and physical nature, is evi- dent from the fact, that the same changes will take place when it is exposed to the air out of the body, even through the medium of a thick membrane, such as a bladder. Such changes, however, only affect the surface of the fluid ; but this is exactly what we should expect, since the air has no access to the part beneath. The Blood, whilst circulating through the capillaries of the Lungs, is divided into an innumerable multitude of minute streamlets, each so small as to admit but a single layer of its corpuscles ; and in these, therefore, the surface which is placed in contact with the air is so enormously extended, as to be almost beyond calculation. Hence, then, we can at once understand how a change may be instantaneously effected in it ; which would occupy several hours, when the blood is less advantageously exposed to the influence of oxygen. The ultimate comparative analysis of Venous and Arte- rial blood indicates the predominance of Carbon in the former, and of Oxygen in the latter ; and it would appear, from the experiments of Michaelis, that it is in the composition of the Red particles, that the principal difference exists.* Venous Blood. Carbon. Nitrogen. Albumen 52-650 15-505 Cruor 53-231 17-392 Fibrin 50-440 17-267 Hydrogen. 7-359 7-711 8-228 Oxygen. 24-486 21-666 24-065 Total in 300 parts . .156-321 50-164 23-298 70-217 Jlrterial Blood. Carbon. Nitrogen. Hydrogen. Albumen 53-009 15-562 6-993 Cruor 51-382 17-253 Fibrin Total in 300 parts 51-374 155-765 8-354 7-254 22-601 Oxygen. 24-436 23-011 23-785 71-232 Mailer's Physiology, p. 323. EFFECTS OF RESPIRATION ON THE BLOOD. 405 The analysis of Marcet gives a more decided predominance of Carbon in Venous blood and of Oxygen in Arterial; according to him, venous blood contains 557 per cent, of carbon, and only 21-7 per cent, of oxygen; whilst arterial blood contains only 50-2 per cent, of carbon, but as much as 26-3 per cent, of oxygen. The discrepancy between these results is probably to be accounted for by the fact to be presently noticed, regarding the facility with which important changes are effected in the gaseous contents of the blood, by a short exposure of it to the atmosphere. The analysis of Dr. Marcet proba- bly over-states the difference between arterial and venous blood, as that of Michaelis underrates it ; but from these and other data, the general fact of the predominance of oxygen in the former, and of carbon in the latter, may be confidently stated. Here, then, we have an important confirmation of the doctrine, that there is an absolute removal of oxygen from the air, during the process of respiration ; and not a mere conversion of this gas into carbonic acid. 541. In what precise form the variable amount of these bodies is contained in the Blood, has not yet been clearly shown. That they must be partly com- bined with its other ingredients, and not merely dissolved in the fluid, is clear, from the changes which they produce in its aspect and properties; these changes are the most evident in the red corpuscles ; but they are also con- siderable in the fibrinous portion of the blood. Indeed, as the Invertebrata do not possess red corpuscles, it can only be upon the Liquor Sanguinis that their respiration operates. There seems good reason to believe, that the red Cor- puscles are the chief carriers of oxygen from the lungs to the tissues, and of carbonic acid from the tissues to the lungs; whether or not we hold with Liebig, that they possess this power in virtue of the iron which enters into their composition. The numerous experiments of Scudamore, Clanny,Bischoff and others, have shown that a small quantity of these gases may be removed from fresh-drawn blood by exposing it to a vacuum. But the amount thus obtained is small in proportion to that which may be procured by treating it with hydrogen or nitrogen : for these gases possess, according to the laws of mutual diffusion already referred to, a much greater power of displacing the carbonic acid and oxygen diffused through the blood, than is exerted by a vacuum. Carbonic acid, however, may be obtained from venous blood in con- siderable amount, by agitating it with atmospheric air, the oxygen and nitrogen of which have a powerful displacing influence upon it; and it is probable that a large quantity is thus removed, during the flow of blood from the vein in ordinary bleeding, especially when the fluid does not spout forth in a full stream, but trickles down the arm in a shallow current. Hence, in all experi- ments upon the gaseous contents of the blood, it is essential that it should flow direct from the orifice into the gas which is to operate upon it ;* and to the neglect of this precaution may be traced much of the discrepancy that has prevailed among the several results which have been made public. The quantity of carbonic acid that may be obtained from venous blood by continued agitation of it with atmospheric air, is stated by Miiller at half a cubic inch from seven cubic inches of the fluid; but when it is agitated with hydrogen, the quantity of carbonic acid obtained is sometimes as much as one-sixth of the volume of the blood. Hence we understand the mode in which the respira- tion of hydrogen is a powerful cause of the extrication of carbonic acid from the lungs of those animals which can support life for some time without oxy- gen. The most important and satisfactory experiments that have been hith- erto made upon the gases of the blood, are those of Magnus. He has shown that carbonic acid, oxygen, and nitrogen, may be extracted both from arterial * An apparatus contrived for this purpose by Dr. Stevens, is described by him m the Phil. Trans., 1834. 406 ON RESPIRATION. and venous blood, but in varying proportion. The amount of oxygen in arte- rial blood equals at least one-third, and frequently almost one-half that of the carbonic acid ; whilst in venous blood the oxygen is scarcely ever more than one-fourth, and often less than one-fifth, of the carbonic acid. The proportion of nitrogen seems to be continually varying, without any fixed law ; it is some- times as little as one-twelfth of the whole quantity of gas extracted from the blood; and sometimes nearly a fourth. 542. That the change of the colour of Venous blood to that of Arterial, is principally due to the replacement of its carbonic acid by oxygen, is very easily shown. The simple removal of the carbonic acid by hydrogen will not produce the alteration ; it has been observed by Magnus, however, that a slight change of colour takes place in blood under the vacuum of an air-pump, although it does not nearly acquire the arterial tint. This falls in with what is known of the influence of carbonic acid on the blood : in common with other acids it has a blackening effect upon it, so that arterial blood when ex- posed to it becomes venous, and venous blood is rendered still darker ; but the simple removal of it is not sufficient to restore the original hue. This restora- tion may be effected in two ways, -either by the addition of saline matter to the blood, or by exposing the fluid to oxygen. The presence of a certain amount of saline matter appears, from the experiments of Dr. Stevens, to be a condition necessary for the due influence of oxygen upon the colouring matter of the blood ; since, if it be deficient, the contact of oxygen will not produce its usual effect. On the other hand, the addition of saline matter (especially nitre) will occasion a decided change of hue, without any extrication of car- bonic acid or absorption of oxygen. Hence it appears that the presence of saline matter in the blood is an essential condition for the due effect of the process of oxygenation ; and that the change of colour may be regarded as resulting from the conjoint operation of the removal of carbonic acid, and the absorption of oxygen.* * [Some experiments by Schererf both confirm the opinion of Nasse, that the change from the arterial to the venous colour of the blood depends in great measure on the form, of the blood-corpuscles, and explain most of the observations of Dr. Sievens on the effects of distilled water and salts upon the blood. Their general conclusions are: 1. That when fresh-stirred and bright-red ox-blood is mixed with distilled water, it acquires a dark-red colour, and its corpuscles, by imbibing water, become spherical, and at last vanish. But, 2. That if, after the change has begun, and not gone far, a concentrated solution of a neutral salt be added, the blood-corpuscles again acquire their natural form, and the bright-red colour is restored. 3. That when oxygen is passed through blood darkened by the addition of distilled water, it is not changed in colour, and the blood- corpuscles do not reappear; but that the same kind of blood, mixed with a small quan- tity of milk, or oil, or finely-powdered chalk, or gypsum, soon regains its bright red colour. 4. Again, by the long-continued contact of concentrated saline solutions with the blood-corpuscles, they become jagged and decomposed, and the blood becomes black; and those which have been reddened by the action of salts, become black again on being expanded by the imbibition of water. 5. By adding carbonic acid to bright-red blood, its corpuscles change their biconcave for a biconvex form, and at the same time its colour changes from red to black. So that there are always changes in the shape of the blood- corpuscles, coincident with the changes in the colour of the mass of blood ; whenever they are dilated, as by distilled water or carbonic acid, the dark colour is produced; whenever they are contracted into the biconcave form, the bright-red colour is restored. Mulder, also,t espouses the opinion of the changes of colour in the blood being im- mediately due to physical rather than to chemical changes of the corpuscles, and has added many facis to those just quoted in disproof of the opinion of Liebig.that the changes are due to the alternate production of the carbonate of the protoxyde, and of the peroxyde, f [ Henle and Pfeufler, Zeitschrift, &c., and Oesterr. Medic. Wochenschrift, Nov. 4, 1843. B. & F. Med. Rev., vol. xix, p. 253.] * [ Verslag van de Vertiende Vergadering van het Nederlandsche Instituut in ' Het In- stituut,' 1844, No. iv, and Physiologische Scheikunde, pp. 361-77. Lond. Med. Gaz., No. 13, Dec. 1844.] EFFECTS OF RESPIRATION ON THE BLOOD. 407 543. The aeration of the blood may take place, not only by means of the lungs, but also through the medium of the cutaneous surface. In some of the lower tribes of animals, indeed, this is a very important part of their respira- tory process ; and even in some Vertebrata, the cutaneous respiration is capa- ble of supporting life for a considerable time. This is especially the case in the Batrachia, whose skin is soft, thin, and moist ; and the effect is here the greater, since the blood which circulates through the system is, from the small proportion of it that has passed through the lungs, very imperfectly arterial- ized. By the experiments of Bischoff it was ascertained that, even after the lungs of a Frog had been removed, a quarter of a cubic inch of carbonic acid was exhaled from the skin, during eight hours. Experiments which have been made on the Human subject leave no room for doubt, that a similar pro- cess is effected through the medium of his general surface ; for, when a limb has been enclosed for some hours in an air-tight vessel containing atmospheric air freed from carbonic acid, a sensible amount of this gas has been found to of iron in the blond-corpuscles, as they pass alternately through the systemic and the pulmonary capillaries. His chief facts' are 1. That the elementary composition of the colouring matter is the same, whether obtained from arterial or from venous blood, viz., C. 44, H. 44, N. 6, O. 6, Fe. 2. That the change from dark to bright blood is effected as completely by the agency of a neutral salt as by oxygen. 3. That if the iron were present in the blood as an oxyde (and especially as a peroxyde), it should be easily extracted by weak acids; but he has found that well-prepared hsamatine may be digested in diluted hydrochloric or sulphuric acid for several days without the iron in it being in the least diminished. After being so treated, he has obtained, after incineration, the regular pro- portion of 9-49 per cent, of oxyde.* If strong sulphuric acid be poured on dried blood, or dried pure haematine,and kept on it for some days, and then water be added, hydrogen is evolved, and sulphate of peroxyde of iron is found in the solution, which could not happen if the iron had been at first in the form of peroxyde.j- 5. The iron may thus be all extracted from the blood, or from haematine, (though not, as some say, without affecting the colour,) and the other constituents may be obtained in a separate form. Numerous analyses of this constituent, by Van Goudoever, regularly yielded the same equivalents of the elements, viz., C. 44, H. 44, N. 6, O. 6; but if the iron had been united with this in the form of Fe. 2, O. 3, and in the proportion of one equivalent to two, there should have remained only four and a half equivalents of oxygen. Mulder concludes, therefore, that iron is present in hsematine, as iodine is in sponge, or sulphur in cystine, or arsenic in cacodyl. His notion of the mode in which the changes of colour are effected is, that when the corpuscles of the venous blood are exposed in the lungs, oxy-proteine is formed by the oxydation of the fibrin proteine of the liquor san- guinis, or perhaps, by the oxydation of the outer layer of the cell membrane of the corpuscles. If formed in the liquor sanguinis, its peculiar plasticity would lead to its being deposited in a thin layer on the corpuscles. In either such case, the dark corpus- cles would, after respiration, be invested by a thin layer of white and imperfectly trans- parent oxy-proteine, or buffy coat, through which they would look bright-red, as dark blood does when contained in a vessel of milk-white glass. But, in the systemic capil- laries, the oxy-proteine may be consumed in nutrition, and the darkness of the corpuscles will then again appear unveiled. Moreover, since it appears that, in the biconcave form, the corpuscles, by reflecting more light, are always bright, and in the biconvex form always dark, it may be that in the arterial blood they are not only buffed, but also cupped, by the oxy-proteine, by the plastic properties of which, moreover, it is easy, on this pretty theory, to explain the ready adhesion of the corpuscles in inflammatory blood. Diluted acids, which make bright blood dark, may do so by making the outer layer of the corpuscles transparent, as they do fibrin before dissolving it; and concentrated solutions of neutral salts may make it bright by making the same layer contract. M. C.] * [Liebig adduces the possibility of extracting iron from dried blood as one of the proofs of its being in an oxydized state; but Mulder says this iron must have been extracted from some other constituent of the blood; for others, besides the globules, even pure serum, contain iron.] f [When the blood or its colouring matter has been exposed to the air prepared in it, the iron must always, according to Liebig's view, be in the state in which he supposed it to be in arterial blood.] 408 ON RESPIRATION. be generated. Moreover, it has been observed not unfrequently, that the livid tint of the skin which supervenes in Asphyxia, owing to the non-arterialization of the blood in the lungs, has given place after death to the fresh hue of health, owing to the reddening of blood in the cutaneous capillaries by the action of the atmosphere upon them. (See also 726.) 544. We have no means of ascertaining the usual amount of carbonic acid excreted through the skin, except by determining the whole quantity disen- gaged from the body, and subtracting the portion exhaled from the lungs. This determination has been attempted in various ways. By Liebig the total quantity of carbon in the food consumed by a certain number of soldiers, was compared with that excreted in the faeces and urine ; and an excess of 13-9 oz. daily for each man was found in the former ; which excess is regarded by him as the amount disengaged in the form of carbonic acid, by the lungs and skin. The experiment, however, was far from being exactly conducted ; as many items among the ingesta are set down by guess merely ; and no exact estimate was made of the quantity of carbon in the urine. The amount con- tained in the solid matter excreted from the skin, too, was altogether neglected. The estimate is in all probability much too high. Another mode of deter- mining the total amount of carbon thrown off in the form of carbonic acid, in the twenty-four hours, has been recently tried by Professor Scharling. He constructed an air-tight chamber, of dimensions sufficient to allow an indi- vidual to remain in it for some time without inconvenience ; and so arranged that he could eat and drink, read, or sleep, within it. This was connected with an apparatus, by which the air was continually renewed ; and the air drawn off was carefully analyzed, In order to determine the quantity of car- bonic acid contained in it. The following are the principal results deduced from his experiments. The amount given off during sleep and when fasting, was the least ; and after a meal it was always the highest. (It may be re- marked, however, that the construction of the apparatus did not admit of active exercise, which would probably increase the quantity of carbonic acid gene- rated, to a still higher proportion.) Children exhaled more carbonic acid, in proportion to their weight, than adults. The total quantity of carbon thus extricated in the twenty-four hours, allowing seven hours for sleep in the adult, and nine hours for the child, appears to be as follows : In the adult male, from 7 oz. to 7 ; in the adult female, about 5d oz. ; and in the child of ten years old, from 4 oz. to 4 oz.* These comparative results accord well with those of Dr. Prout, and of MM. Andral and Gavarret, upon the exhalation of carbonic acid by the Lungs alone ( 534, note, 535) : and also with that already given of the actual quantity of carbon thrown off by them ; for if this be estimated at 5<| oz., the remaining 2 oz. may very fairly be supposed to be exhaled from the Skin in a similar form. 545. From the facts which have been stated, and from many others of the same kind, the conclusion seems indisputable, that the changes produced by Respiration are of the following nature. The Arterial blood propelled from the heart to the System contains a large proportion of oxygen, either free or in loose combination with it ; and also a certain amount of carbonic acid. During its passage through the systemic capillaries, it loses a part of its oxygen, and acquires a great increase in its amount of carbonic acid, together with some addition to its water; and it returns to the heart in the state of Venous blood, its colour having been darkened by the loss of its oxygen, and by the influence of the acid. In the Lungs, to which it is then transmitted, it undergoes, by exposure to the atmosphere, the converse change to that which took place in the systemic capillaries ; a large part of its carbonic acid and * Annalen der Cheinie und Pharmacie, xlv. p. 214. EFFECTS OF RESPIRATION ON THE BLOOD. 409 water being removed, and a- considerable addition being made to the amount of oxygen which it contains : its arterial hue and character are thus restored. It may be observed, then, that the blood, by its alternate passage through the systemic and pulmonary capillaries, serves to bring the two into close rela- tion ; and that in this manner, the oxygen of the air is enabled to act upon the minutest portions of those tissues of the body that are most distant from the lungs, as completely as it can do by being directly introduced into their sub- stance, as it is in Insects. It is interesting to remark that these are the only Invertebrata in which there is an active respiration ; and it would seem as if the universal permeation of their tissues by tracheae is a compensating struc- ture, making up for that deficiency in the carrying power of the blood, which may be attributed to the absence of red corpuscles ( 576). 546. We have now to consider the results of the cessation of the Respiratory function, and the consequent retention of carbonic acid in the blood. If this be sufficiently prolonged, a condition ensues to which the name of Asphyxia has been given ; the essential character of which is the cessation of muscular movement, and shortly afterwards of the circulation ; with an accumulation of blood in the venous system. The time which is necessary for life to be de- stroyed by asphyxia varies much, not only in different animals, but in different states of the same. Thus, Warm-blooded animals are much sooner asphyxiated than Reptiles or Invertebrata; on the other hand, a hybernating Mammal supports life for many months, with a respiration sufficiently low to produce speedy asphyxia if it were in a state of activity. And among Mammalia and Birds, there are many species which are adapted, by peculiarities of conforma- tion, to sustain a deprivation of air for much more than the average period.* Excluding these, it may be stated as a general fact, that, if a warm-blooded animal, in a state of activity, be deprived of respiratory power, its muscular movements (with the exception of the contraction of the heart) will cease within five minutes, often within three ; and that the circulation generally fails within ten minutes. Many persons, however, are capable of sustaining a deprivation of air for three, four, or even five minutes, without insensibility or any other injury ; but this power, which seems possessed to the greatest degree by the divers of Ceylon, can only be acquired by habit. The period during which remedial means may be successful in restoring the activity of the vital and animal functions, is not, however, restricted to this. Cases are not unfrequent, of the revival of drowned persons after a submersion of half an hour; and more than one has been credibly recorded, in which above three-quarters of an hour had elapsed. It is not improbable, however, that in some of these cases a state of Syncope had come on at the moment of immer- sion, through the influence of fear or other mental emotion, concussion of the brain, &c, ; so that, when the circulation was thus enfeebled, the deprivation of air would not have the same injurious effect as when this function was in full activity. The case would then closely resemble that of a hybernating animal ; for in both instances the being might be said to live very slowly, and would therefore not require the usual amount of vital stimuli. The condition of the still-born infant is in some respects the same ; and re-animation has been successfully attempted, when nearly half an hour had intervened between * Thus, the Cetacea contain far more blood in their vessels than do any other Mamma- lia; and these vessels are so arranged, that both arteries and veins are in connection with large reservoirs or diverticula. The reservoirs belonging to the former are usually full; but when the Whale remains long under water, the blood which they contain is gradually introdiBed into the circulation, and, after becoming venous, accumulates i the reservoirs connected with the venous system. By means of this provision, the Whale can remain under water for more than an hour. 35 410 ON RESPIRATION. birth and the employment of resuscitating means, and when probably a much longer time had elapsed from the period of the suspension of the circulation. 547. It has now been sufficiently proved, both by experiment and by patho- logical observation, that the first effect of the non-arterialization of the blood in the lungs, is the retardation of the fluid in their capillaries ; of which the accumulation in the venous system, and the deficient supply to the arterial, are the necessary consequences. It is some time, however, before a complete stagnation takes place from this cause : since, as long as the proportion of oxygen which remains in the air in the lungs is considerable, and that of the carbonic acid is small, so long will some imperfectly arterialized blood find its way back to the heart, and be transmitted to the system. This blood will have a depressing influence upon the functions of the brain and of the mus- cular system ; which influence is aided by the diminution that gradually takes place in the quantity of blood propelled through the aorta; and the actions of the respiratory muscles and of the heart will therefore soon be enfeebled. . The cessation of the heart's contraction^ due to two distinct causes, acting on the two sides ; for on the right side it is the result of the over-distension of the walls of the ventricle, owing to the accumulation of venous blood; and on the left to deficiency of the stimulus necessary to excite the movement. The property, of contractility is not finally lost nearly as soon as the movements cease; for: the action of the right ventricle may be renewed, for some time after it has ceased, by withdrawing a portion of its contents, either through the pulmo- nary artery, their natural channel, or, more directly, by an opening made in , its own parietes, in the auricle, or in the jugular vein ( 489). On the other hand, the left ventricle may be again set in action, by renewing its appropriate stimulus of arterial blood. Hence, if the stoppage of the circulation have not been of too long continuance, it may be renewed by artificial respiration ; for the replacement by oxygen of the carbonic acid in the air-cells of the lungs, restores the circulation through the pulmonary capillaries ; and thus at the same time relieves the distension of the right ventricle, and conveys to the left the due stimulus to its actions. 548. Of the mode in which the pulmonary circulation is stagnated by the want of oxygen, and renewed by its ingress into the lungs, no other explana- tion can be given, than that which has been heretofore offered of the capillary circulation in general; namely, that the performance of the normal reaction between the blood and the surrounding medium (whether this be air, water, or solid organized tissue), is a condition necessary to the regular movement of the blood through the extreme vessels.* This view has recently obtained additional support from the experiments of Dr. J. Reid on the Respiration of Azote.t He found that, when the ordinary respiration of an animal is inter- rupted, and the Asphyxia is proceeding to the stage of insensibility, the first effect upon the arterial system is an increased distension (as indicated by the hsemadynamometer), even although the blood is at that time nearly venous in its character ; this indicates that the fluid, now so perverted, is unable to pass with facility through the systemic capillaries, in consequence of its not being in a state fit for the performance of its normal actions. As the stagna- tion in the pulmonary capillaries becomes more complete, however, less and less blood is returned from the lungs to the heart ; and, the systemic arteries being gradually unloaded without being refilled, the pressure of the blood upon their walls diminishes, and at last is no longer experienced. Its dimi- nution is not arrested by causing the animal to breathe nitrogen, although the respiratory movements are renewed, thus proving that the stagnation of the * For a fuller discussion of the Pathology of Asphyxia, see the Author's essay on the subject in the Library of Practical Medicine, vol. iii. f Edinb. Med. and Surg. Journal, April, 1841. EXHALATION AND ABSORPTION BY THE LUNGS. 411 blood in the capillaries of the lungs is not due (as some have supposed) to a mechanical impediment: but the pressure is immediately increased by the admission of atmospheric air, which occasions the renewal of the pulmonary circulation, and the consequent increase in the supply of aerated blood to the systemic arteries. It has been recently shown by Mr. Wharton Jones,* that the capillary circulation in a frog's foot is retarded or even checked by the direction of a stream of carbonic acid gas against the membrane ; and he attri- butes this stagnation to the disposition thus produced in the red corpuscles, to aggregate together and to adhere to the walls of the vessel so as to choke up its calibre. IV. Exhalation and Absorption by the Lungs. 549. The alteration in the proportions of its usual gaseous ingredients, is by no means the only change which the blood undergoes in the Lungs. It parts, also, with a considerable amount of water, in the form of vapour; this usually contains a certain proportion of animal matter; and it is sometimes charged with volatile substances, which have been elsewhere introduced into the blood, or which have been formed during its assimilation. It may also absorb from the atmosphere volatile matter diffused through it. Both these changes are probably to be explained upon simple physical principles ; bein^ dependent on the exposure of the blood to the atmosphere, with a very exten- sive surface, and through a membrane of great permeability. Of the fluid ordinarily exhaled with the breath, a part doubtless proceeds from the moist 'lining of the nostrils, fauces, &c. ; but it is indisputable that the greater pro- portion of it comes from the lungs ; since, when the respiration is entirely performed through a canula introduced into the trachea, the amount of watery vapour which the breath contains is still very considerable. The quantity which thus passes off is by no means trifling ; probably between 16 and 20 ounces in the twenty-four hours. It is not so liable to variation under the influence of temperature, the movement of the surrounding air, and other similar causes, as is the cutaneous transpiration ; for air, which has found its way into the air-cells of the lungs, will, under almost all circumstances, be nearly the same in regard to such conditions, and will, therefore, dissolve an equal amount of watery vapour. It is considered by Dr. Prout, that the prin- cipal source of this vapour is not the blood properly so called, but the chyle and lymph which have just been introduced into it from the thoracic duct ; a loss of a portion of their fluid being required to give them sufficient concentra- tion. A process very analogous takes place in Plants ; for a very large pro- portion of the water taken up in the crude sap, is parted with in the leaves. But it is probable that a part, at least, of the water thrown off by the lungs is generated by the union of Oxygen and Hydrogen, during the course of the Circulation ( 533). The fluid thrown off from the Lungs is not pure water. It holds in solution, as might have been expected, a considerable amount of car- bonic acid, and also some animal matter; the exact nature of the latter, which, according to Collard de Martigny, constitutes about 3 parts in 1000, has not been ascertained. If the fluid be kept in a closed. vessel, and be exposed to an elevated temperature, a very evident putrid odour is exhaled by it. Every one knows that the breath itself has, occasionally in some persons, and con- stantly in others, a fetid taint ; when this does not proceed from carious teeth, ulcerations in the air-passages, disease in the lungs, or other similar causes, it must result jfzm the excretion of the odorous matter, in combination with watery vapour, from the pulmonary surface. That this is the true account of * Brit, and For. Med. Rev., vol. xiv. p. 600. 412 ON RESPIRATION. it seems evident, from the analogous phenomenon of the excretion of turpen- tine, camphor, alcohol, and other odorous substances, which have been intro- duced into the venous system, either by natural absorption, or by direct injection ; and also from the suddenness with which it manifests itself, when the diges- tive apparatus is slightly disordered. 550. The Lungs are capable, under peculiar circumstances, of absorbing fluid from the atmosphere." Thus Dr. Madden* has shown that, if the vapour of hot water be inhaled for some time together, the loss by exhalation is found to be so much less than usual as to indicate that the cutaneous transpiration is partly counterbalanced by pulmonary absorption ; the pulmonary exhalation being at the same time entirely checked. It it probable that, if the quantity of fluid in the blood had been previously diminished by excessive sweating, or by other copious fluid secretions, the pulmonary absorption would have been much greater. Still, in the cases formerly mentioned (465), in which a large in- crease in weight could only be accounted for on the supposition of absorption of water from the atmosphere, it seems probable that the cutaneous surface was chiefly concerned ; for it can only be when the air introduced into the lungs is saturated with watery vapour that the usual exhalation will be checked, or that any absorption can take place. That absorption of volatile matter dif- fused through the air is, however, continually taking place by the lungs, is easily demonstrated. A familiar example is the effect of the inhalation of the vapour of turpentine upon the urinary excretion. It can only be in this manner that those gases act upon the system which have a noxious or poison- ous effect, \vhen mingled in small quantities in the atmosphere. Of these, sulphuretted hydrogen is one of the most powerful in its action ; for it has been found that air impregnated with TI L-Qth part of it will kill a bird in a very short time, and that a quantity but little more than double, namely ^th part, will soon kill a dog. This gas is exhaled in large quantities from many forms of decomposing animal and vegetable matter, and it has recently been shown (by Professor Daniell) to be absorbed by the water of the estuaries of those African rivers, whose mouths are regarded as among the most pestilential spots upon the surface of the globe. Carburetted hydrogen is another gas whose effects are similar, but a larger proportion is required to destroy life. 551. Carbonic acid gas also appears to be absorbed by the lungs, when a large proportion of it is contained in the atmosphere. The accumulation of this gas in the blood, when the respired air is charged with it even to a mode- rate amount, might be attributed to the impediments thus offered to its ordinary exhalation ; but the following experiment appears to prove, that it may be actually absorbed into the blood, and that it will thus exert a real poisonous influence, and not merely produce an asphyxiating effect. It was found by Rolando, that the air-tube of one lung of the land tortoise may be tied without apparently doing any material injury to the animal, as the respiration performed by the other is sufficient to maintain life for some time ; but, having contrived to make a tortoise inhale carbonic acid by one lung whilst it breathed air by the other, he found that the animal died in a few hours .t Cyanogen is another gas which has an actively poisonous influence upon animals when absorbed into the lungs ; its agency, also, is of a narcotic character. It is singular that the effects of the respiration of pure oxygen should not be dissimilar. At * Prize Essay on Cutaneous Absorption, p. 55. f The fatal result of breathing the fumes of charcoal is, therefore, not simple asphyxia, such as would result from breathing hydrogen or nitrogen. Other volatile products are set free in the combustion of charcoal, besides carbonic acid. Mr. Cd|thupe (loc. cit.) states these to be Carbonate, Muriate and Sulphate of Ammonia, Carbonic Oxide, Oxygen, Nitrogen, Watery vapour, and Empyreumatic Oil: to these Sulphurous Acid may appear to be properly added. ORGANIZABLE PRINCIPLES. 413 first, the rapidity of the pulse and the number of the respirations are increased, and the animal appears to suffer little or no inconvenience for an hour ; but symptoms of coma then gradually develop themselves, and death ensues in six, ten, or twelve hours. If the animals are removed into the air before the insensibility is considerable, they then quickly recover. When the body is examined, the heart is seen beating strongly, while the diaphragm is motion- less ; the whole blood in the veins, as well as in the arteries, is of a bright scarlet colour ; and several of the membranous surfaces have the same tint. The blood is observed to coagulate with remarkable rapidity ; and it is to the alteration in its properties, occasioned by the hyper-arterialization, and indicated by this condition, that we are probably to attribute the fatal result. There can be no doubt that, in this instance, an undue amount of oxygen is absorbed. Death is also caused by the inhalation of several gases of an irritant character, such as sulphurous, nitrous and muriatic acids ; but it is doubtful how far they are absorbed, or how far their injurious effects are due to the abnormal action which they excite in the lining membrane of the air-cells and tubes. It cannot be doubted, that miasmata and other morbific agents diffused through the at- mosphere, are more readily introduced into the system through the pulmonary surface than by any other ; and our aim should therefore be directed to the discovery of some counteracting agents, which can be introduced in the same manner. The pulmonary surface affords a channel for the introduction of cer- tain medicines that can be raised in vapour when it is desired to affect the system with them speedily and powerfully ; such are iodine, mercury, tobacco, stramonium, &c. CHAPTER XI OF NUTRITION. 551. THE Function of Nutrition essentially consists of the conversion of the fluid alimentary materials, prepared by the digestive process, and introduced into the system by absorption, into organized tissue, possessed of certain pro- perties which inorganic matter never exhibits, and which, being neither phy- sical nor chemical, are termed Vital. We shall hereafter see reason to believe that the manifestation of these vital properties, which gives rise to the various phenomena of life, is to be considered as the result of the process of organiza- tion, to which matter is subjected in the living body ( 560). I. Organizable Principles. 552. It has been shown ( 467) that the Chyle taken up by the lacteals is composed of water holding albumen and saline matter in solution, and having oily particles suspended in it. Albumen may be regarded as the proximate element, at the expense of which, in conjunction with fatty matter (which never itself undergoes organization as such, 16), all the tissues of the animal body are ultimately formed. In this assumption we seem justified by two very obvious considerations. First, in the egg of a bird, (or any other oviparous 35 414 OF NUTRITION. animal,) we find that, putting aside the fatty matter of the yolk, albumen is the sole organic compound, at the expense of which all its tissues are to be formed ; so that, by the wonderful processes of chemical and vital transformation, which take place during the period of incubation, the albumen which it contained at first is metamorphosed into bone, cartilage, nerve, muscle, tendon, ligament, membrane, areolar tissue, gelatinous matter, horny substance, feathers, &c., &c. Secondly, a similar metamorphosis appears to be continually taking place in the body of the adult animal; for every protein compound employed as food appears to be reduced to the form of albumen in the digestive process ; so that this becomes the essential constituent of whatever fluid is absorbed for the nutrition of the tissues. It ,is true that gelatin taken in as food may be absorbed and carried into the current of the circulation ; but there is no doubt that it is incapable of being applied to the re-construction of any but the gelatinous tissues ; and it seems questionable whether, even in these, it exists in a con- dition that can rightly be termed organized. Moreover, as it is clear that the gelatinous tissues may be formed at the expense of albumen, we are justified in regarding this substance as the common pabulum for all. Hence albumen seems to hold very much the same position in the animal economy, with gum in the vegetable. As long, however, as albumen remains in the state regarded by chemists as characteristic of it, it exhibits no tendency to become organized ; and it is only when it has been subjected to certain peculiar vital influences, and perhaps undergone a change in its chemical constitution, or, in other words, has become converted into fibrin, that any such tendency manifests itself. The properties of albumen may be studied in the white of egg, or in the serum of blood, from both of which situations it may be obtained in a pure state by very simple means. In the animal fluids it exists in a soluble state ; and even when it has been dried (at a temperature of 126), it is readily dis- solved again in water, forming a glairy, colourless, and nearly tasteless fluid. In this condition it is always combined with a small quantity of free soda ; to the separation of which (whether by the agency of heat or acids), its coagula- tion is thought by many chemists to be due. On this view, pure albumen is not soluble in water ; its solution being only accomplished by union with an alkali. When dissolved in water it coagulates at 158 ; a very dilute solution, however, does not become turbid until it is boiled. When the coagulation of albumen takes place rapidly, a coherent mass is formed which shows no trace whatever of organization ; but, when the process is more gradual, minute granules present themselves, which do not, however, exhibit any tendency towards a higher form of structure. It is thrown down from its solution, in a coagulated state, by alcohol, creosote, and by most acids (particularly nitric) with the exception of the acetic. These precipitates are definite compounds of the acids with the albumen, which here acts the part of a base. On the other hand, coagulated albumen dissolves in caustic alkalies, and neutralizes them ; so that it must here act as an acid. A solution of albumen in water is precipi- tated by acetate of lead, and by many other metallic solutions : and insoluble compounds are formed, of which one the albuminate of the chloride of mer- cury is of much interest, as being that which is produced by the mixture of a solution of albumen with one of corrosive sublimate. Albumen, both in its soluble and insoluble state, always contains a certain amount of sulphur ( 458), which blackens metallic silver. Soluble albumen dissolves phosphate of lime ; and about two per cent, of this salt may be separated from it in its coagulated state. 553. Subsequently to its introduction into the living system, Albumen under- goes a very peculiar modification, by which it is converted into Fibrin. As already mentioned ( 457) it appears from the analyses of Mulder and Scherer, that the ultimate composition of these two substances is the same ; but their ORGANIZABLE PRINCIPLES. 415 properties are widely different: according to Dumas, however, there is a marked difference in composition between Fibrin and the various forms of Albumen, the former having less Carbon, and more Nitrogen, than the latter. The following are the results of his analyses. From serum. From egs:s. Carbon 58-32 53-37 53-50 52-78 Hydrogen 7-29 7-10 7-05 Nitrogen 15-70 15-77 15-77 10-78 Oxygen ~) Sulphur I 23-69 23-76 23-68 21-48 Phosphorus . . . .J 100-00 100-00 100-00 100-00 It is not, perhaps, of any great moment whether this difference has a real existence or not ; for the conversion of Albumen into Fibrin is unquestionably a process much more of vital than of chemical transformation. We shall presently see, that Fibrin may be regarded as Albumen in which the process of Organization has begun ; its molecules being ready to assume the peculiar arrangement that is so designated : this arrangement takes place most com- pletely, when the fibrinous mass is in contact with a living tissue, and is, therefore, to a certain degree, under its influence. Fibrin, like albumen, may exist in a soluble or in a coagulated state ; its soluble form only occurs, however, in the living Animal Fluids, the chyle, lymph, and blood ; and it seems to be the intermediate condition between the soluble albumen, and the solid organized substances which are formed from it. When withdrawn from the blood-vessels, the Blood soon coagulates (as do also the Chyle and Lymph, when they contain sufficient fibrin, 564 and 565) ; and this coagulation is entirely due to a change in the condition of the Fibrin, the particles of which have a tendency to aggregation. The Fibrin may be obtained in a separate form, by stirring fresh-drawn blood with a stick, to which it adheres in threads ; these contain some fatty matter which is to be washed out with alcohol. In this condition it possesses the softness and elasticity which characterize the flesh of animals, and contains about three-fourths of its weight of water. It may be deprived of this water in dry air, and then becomes a hard and brittle substance ; but, like flesh, it imbibes water again when moistened, and recovers its original softness and elasticity. When burned, it always leaves, like albu- men, a portion of phosphate of lime. Fibrin is insoluble in alcohol and ether, and also, under ordinary circumstances, in water; but when long boiled in water, especially under pressure, its nature is altered, and it becomes soluble. This is also the case with coagulated albumen. Fibrin, like albumen, unites with acids as a base, forming definite compounds ; and with bases as an acid. Its correspondence with albumen has been recently proved by the fact (first stated by M. Denis), that it may be entirely dissolved in a solution of nitrate of potash ; and that this solution is coagulated by heat, and greatly resembles a solution of albumen. This is only true, however, of the ordinary Fibrin of Venous blood ; for that which is obtained from arterial blood or from the buffy coat, or which has been exposed for some time to the air, is not thus soluble. This is an important and interesting circumstance. The difference appears to depend upon the larger quantity of oxygen contained in the latter ; for a solution of Venous Fibrin in nitre, contained in a deep cylindrical jar, allows a precipitate in fine flocks to fall gradually, provided the air have access to the surface, but not if it be prevented from coming in contact with the fluid ; 416 OF NUTRITION. this precipitate is insoluble in the solution of nitre, and possesses the properties of arterial fibrin.* Hence it may be inferred, that the Fibrin of Venous blood most nearly resembles albumen ; whilst that of Arterial blood, and of the Buffy coat, contains more oxygen, and is more highly animalized. It is evident, from this circumstance, that the matter of the Red Corpuscles is by no means the only constituent of the Blood, which undergoes a change in the respiratory process ( 540) .t When decomposition commences in a coagulum of Fibrin withdrawn from the body (and even in the greatly debilitated living body, in which the fibrin appears to be imperfectly formed), a granular mode of Aggre- gation is evident in the particles of the mass, thus showing its affinity to Albumen, when its peculiar vital characters have departed, or are possessed by it in an inferior degree. 554. It appears to be in the Fibrin of the Blood that all the organized constituents of the body have their immediate origin ; and it may hence be designated as the plastic or organizable constituent of the nutritious fluid. To use a rather homely illustration, Albumen, Fibrin, and Organized tissue, stand in much the same relation to each other, with raw cotton, spun yarn, and the woven fabric. That the particles of perfectly elaborated Fibrin are capable, in solidifying, of spontaneously assuming a definite arrangement, cannot now be questioned. In the ordinary Crassamentum of healthy Blood ( 582), this arrangement can be seen, by examining thin slices under the microscope ; especially after the clot has been hardened by boiling. A num- ber of fibres, more or less distinct, may be seen to cross one another ; forming, by their interlacement, a tolerably regular network, in the meshes of which the red corpuscles are entangled-. This fact was known to Haller ; but it has been generally overlooked by subsequent physiologists, until attention was drawn to it by the inquiries of Messrs. Addison, Gulliver, and others. It is * Scherer, Chemisch-physiologische Untersuchungen ; Annalen der Chemie, Oct., 1841; quoted in Graham's Chemistry, p. 1025. t It appears, from the recent inquiries of Mulder, that the Protein-base of Fibrin may exist in different states of definite combination with Oxygen. When Fibrin is long boiled, so that part of it is dissolved, the soluble portion is found to consist of Triioxide of Protein,- whilst the insoluble residue is in the state of Deutoxf.de. When Albumen is boiled for a sufficient length of time, a Tritoxide of Protein is formed and dissolved in like manner; but the residue is simple Albumen. Tritoxide of Protein may be further produced, by exposing Fibrin to Oxygen gas. It is found in small quantity in healthy blood; its amount being greater in arterial than in venous: but it forms a much larger proportion of the Buffy coat, of which 14 per cent, may be dissolved by a quarter of an hour's boiling. This soluble matter has been mistaken for Gelatin ; but its composition is altogether different; and it is slated by Mulder to agree exactly with that which Dumas has analyzed as Fibrin. These Oxides of Protein are regarded by Mulder as formed at the expense of the Fibrin during its passage through the Lungs; and as being the real materials of the nutritive process ; and he denies that any other constituent of the blood undergoes oxidation in the pulmonary capillaries. In this view he is probably as much in error, as are those who deny that any element but the Red Corpuscles is concerned in the respiratory process. How far he is correct in asserting, that the oxides of Protein alone are the materials of the nutritive processes, further researches alone can deter- mine. It appears to the Author, however, that the doctrine is inconsistent with known facts in regard to the organizability of true Fibrin. And it may be remarked, that the differences of opinion which prevail amongst the most eminent Chemists,on topics which lie at the foundation of Physiological Chemistry, should lead us to hesitate before ad- mitting the novelties continually put forth by one or other of them as the basis of our deductions. To him it appears, that the direct observation of Vital phenomena, about which there can be little or no difference of opinion, is a much surer basis for theorizing than those Analyses of the Chemist in which the utmost delicacy of manipulation is required to insure even tolerably accurate results, and as to the details of which, a new set of statements is put forth from some of the great Continental Laboratories, almost every month. For the detailed account of Mulder's researches, see the Annalen der Chemie und Pharmacie, Band xlvii. ORGANIZABLE PRINCIPLES. 417 in the Buffy Coat ( 588), however, that the fibrous arrangement is best seen; on account, as it would appear, of the stronger attraction which the particles of fibrin have for one another, when its vitality has been raised by the increased elaboration to which it has been subjected. That there are varieties of plastic! I \/ in the substance, which, on account of its power of spontaneously coagulating, we must still calljibrin, appears from this fact among others, that, in tuber- culous subjects, the quantity of fibrin in the blood is higher than usual (Andral and Gavarret), although its plasticity is certainly below par. It is easy to understand, that its plasticity may be increased as that it may be diminished ; and this either in the general mass of the blood, or in a local deposit. In fact, the adhesions which are formed by the consolidation of coagulable lyrnph, or in other words, of liquor sanguinis, whose plasticity has been heightened by the vital actions in the capillaries of the part on which it has been effused, often acquire very considerable firmness, before any vessels have penetrated them ; and this firmness must depend upon that mutual attraction of the par- ticles for one another, which, in aplastic deposits, is altogether wanting, and which, in cacoplastic deposits, is deficient. A very interesting example of a structure entirely composed of matted fibres, and evidently originating in the simple consolidation of Fibrin, has lately been discovered by the writer. This is found in the membrane adherent to the interior of the Egg-shell (Membrana putaminis) ; and also in that which forms the basis of the Egg-shell itself. Between the two, there is no essential difference ; as may be seen by examin- ing " an egg without shell," as it is commonly termed, (or rather one in which the shell-membrane has been unconsolidated by the deposition of calcareous matter) ; or by treating the egg-shell with dilute acid, so as to remove the particles of carbonate of lime, which are deposited in the interstices of jhe network. The place of the shell is then found to be occupied by a membrane of considerable firmness, closely resembling that which surrounds the albumen of the egg, but thicker and more spongy. After maceration for a few days, either of these membranes may be separated into a number of laminae ; each of which (if sufficiently thin) will show a beautiful arrangement of reticulated fibres. It is impossible to refuse to such a structure the designation of an organized tissue, although it contains no vessels, and must be formed by the simple consolidation of Fibrin, poured out from the lining membrane of the oviduct of the bird. It is probably in the same manner that the Chorion of the Mammiferous animal originates; since this is a new envelop, formed around the ovum, during its passage along the Fallopian tube. In the latter, for an ulterior purpose, vessels are afterwards developed, by extension from the contained ovum ; and by the nutrition they supply, its size is increased, and changes take place in its texture. But in the Egg-membrane of the Bird there is no need of vessels ; because no subsequent change in its texture is required, and its duration is sufficient for the purpose it has to answer. 555. The completeness of the transformation of Fibrin into simple Fibrous Tissue, appears to depend upon two circumstances in particular ; the perfect elaboration of the Fibrin itself, and the vitality of the surface upon which the concretion takes place. When the Fibrin is highly elaborated, it will coagu- late in the form of a definite network of minute fibrillaB, even upon a dead surface, as a slip of glass; this is the case, for instance, with the Fibrin of the buffy coat of the Blood, or with that of the Liquor Sanguinis (coagulable lymph), poured out for the reparation of an injured part. But in the ordinary Fibrin of the blood, the fibrillation is less distinct when the concretion takes place upon a dead surface. When it occurs in contact with a living surface, however, the coagulation takes place more gradually ; and it seems as if the particles, having more time to arrange themselves, become aggregated into more definite forms, so that a more regular tissue is produced just as crystals 418 OF NUTRITION. are most perfectly formed when the crystalline action takes place slowly. It was formerly imagined, that the Muscular tissue is the only one produced at the expense of the Fibrin of the blood ; the other tissues being formed from its Albumen. This, however, is unquestionably erroneous/ There is no proof whatever that Albumen, as long as it remains in that condition, ever becomes organized ; whilst, on the other hand, there is abundant evidence that the plasticity of any fluid deposit that is, its capability of being meta- morphosed into organized tissue is in direct relation with the quantity 'of Fibrin which it contains. Thus the Liquor Sanguinis or Coagulable Lymph, thrown out for the reparation of injuries, contains a large amount of Fibrin; and this substance is converted, not at first into muscular fibre, but (whatever may be the tissue to be ultimately produced in its place), into a fibrous net- work, which fills up the breach, and holds together the surrounding structure. This may be regarded as a simple form of areolar tissue ; which gradually becomes more perfectly organized, by the extension of vessels and nerves into its substance; and in \vhich other forms of tissue may subsequently make their appearance. This process will be more particularly described here- after; it is at present noticed here as an illustration of the general fact, that fibrin is to be regarded as the plastic element of the nutritive fluids. The change from Albumen to Fibrin is, therefore, the first important step in the process of Assimilation. It commences in the Absorbent system, ( 564), and it continues in the Blood ; for the quantity of Fibrin it contains is always kept up, in health, to a certain standard, although there must be a continual withdrawal of it for the nutritive processes, without a correspondingly regular supply from the chyle ; and we find it, moreover, undergoing a sudden and remarkable increase, under the influence of local agencies. The mode in which this conversion is effected, will be better discussed hereafter ; when the chief circumstances under which it occurs, have been inquired into ( 579). II. Formation of Cells. 556. A very large proportion of the Vegetable Organism (in the simplest Plants, the entire structure) is made up of cells or vesicles; wnich are minute closed sacs, whose walls are composed in the first instance of a delicate mem- brane, frequently strengthened, at a period long subsequent to their first formation, by some internal deposit. The form of these cells is extremely variable, and depends chiefly upon the degree and direction of the pressure, to which they have been subjected at the period of their origin, and subse- quently to it. Sometimes they are spheroidal ; sometimes cubical or prismatic ; sometimes cylindrical ; and sometimes very much prolonged. These cells may undergo various transformations. One of the most common, is the conversion of several into a continuous tube or Duct. This is principally seen in the vessels through which the sap ascends the stem ; these appear to have been formed by the breaking down of the transverse partitions, between a regular series of cylindrical cells laid end to end ; and the remains of such partitions may frequently be seen in them. The ducts which convey the ascending sap do not inosculate with each other, their purpose being merely to carry it direct to the leaves ; but the vessels through which the descending or elabo- rated sap flows, are of very different character; for their purpose is to distribute the nutritious fluid through the tissues ( 497) ; and they anastomose very freely, just as do the capillaries of Animals. The network which they form, however, can be as clearly traced to an origin in cells whose cavities were originally distinct, as can the bundles of straight non-communicating ducts. Another important transformation of the original cells, is that by which the FORMATION OF CELLS. 419 Woody Fibres, which compose nearly all the fibrous textures of Vegetables, are produced. These fibres are still cells, but their form is very much elon- gated ; they have a fusiform or spindle shape, being tubes drawn to a point at each end; at first they are quite pervious, like ordinary cells; but in the older wood, their cavity is filled up by interior deposit. There seerns reason to believe, that fasciculi of these fibre-cells originate within certain of the ordi- nary cells of the primary cellular tissue ; for in the young Plant, the latter alone can be detected ; and it is not until the operation of the leaves has fairly commenced, that any true woody structure is formed. Thus, cells or vesicles may be regarded as the primordia of all the Vegetable tissues. The next question is, how are Cells formed ? 657. Cells appear to originate in two modes ; either in the midst of an organizable fluid, under the influence of a living solid tissue with which it is in contact, or in the interior of previously-formed cells. Both these modes may be observed in the Animal as well as in the Vegetable organism ; and the right comprehension of them is of the utmost importance. It has been already remarked, that Gum holds the same rank in the economy of the Plant as Albumen does in that of the Animal ; and the glutinous compound which exists in the elaborated sap, and which is especially abundant in parts where organ- ization is taking place with rapidity, may be compared with fibrin. This glu- tinous sap undergoes a sort of coagulation when withdrawn from the vessels, and it may be sometimes perceived to resolve itself into distinct organic forms. The process of organization may be observed with the greatest facility in the embryonal sac, previously to fecundation. This contains, when first developed, a consistent gummy fluid, slightly wanting in transparency, but not exhibiting any distinguishable granules ; the addition of tincture of iodine produces a sort of granular coagulum, of a pale yellow. The first perceptible stage of organ- ization is the appearance in this fluid of a number of extremely minute granules, which render it opalescent and almost opaque. The fluid then takes from iodine a somewhat darker tinge, and the granules, when, their small size per- mits their colour to be distinguished, seem to become of a dark brownish-yellow. Single, larger, and more sharply-defined granules are next evident in the mass, and these soon present a regular form, and increase in size 1 apparently from the coagulation of the minuter granules around the larger ones. These bodies usually assume a flattened disc-like form, with a circular or oval outline ; and as they speedily become subservient to the formation of cells, they have been termed cytoblasts or cell-germs. From the surface of each of these, a delicate transparent membrane is seen to project, as a watch-glass does from the dial ; and this is the commencement of the cell. The membrane gradually projects more and more, and extends beyond the cytoblast, which is at last seen as a mere spot upon its walls. It is some time in acquiring consistence ; for even after the cell has arrived at nearly its full size, it may be made to dissolve by agitation in the surrounding fluid. In fact, a disappearance not unfrequently takes place, as a part of the natural course of vital phenomena ; the cell-walls melting away before they have acquired consistence enough to be permanent. When the cell is complete, the granular cytoblast commonly disappears ; some- times, however, it remains in the wall of the cell, where (in the orders Orchideas and Cactese) it was long since described by Dr. R. Brown, under the name of the nucleus. By Schleiden, the original observer of these phenomena,* it is considered that the function of the cytoblast is complete with the formation of the primordial cell ; but there is strong reason to believe, that the granules of which it is composed are the germs of new cells afterwards to be developed * See Muller's Archiv., 1808, p. 137, Taylor's Scientific Memoirs, vol. ii., and the Brit. and For. Med. Rev., vol. ix. p. 499. 420 OF NUTRITION'. within the parent vesicle ; and that, even when it disappears, it is by a resolu- tion into its component granules, which may act as well separately as in appo- sition. It is not to be inferred from the preceding account, that cells are ever formed by the mere apposition of particles, at all in the manner of the aggre- gation of molecules in a crystal ; since there appears sufficient reason to believe that in this instance, as in all others, germs are present in the fluid, which were prepared by previously existing cells, and which grow and develop themselves in virtue of their peculiar inherent powers. 558. We are thus brought to the second mode in which Cells may be de- veloped, which is within the parent vesicles or primordial cells ; the granules contained in these being apparently the germs from which they originate. This is probably the mode which is always followed when a tissue previously existing has to be extended or partially renewed ; the former one being adopted where a structure entirely new has to be evolved, which is normally the case in certain phases of Vegetable life, but is less common in Animals. The secondary cells developed within a parent vesicle, originating in the granular germs which it includes, at first grow at the expense of the fluid it contains, and afterwards by absorbing nutrient materials through its walls. When they have undergone great increase in size, they distend the original vesicle in such a manner that its limits are no longer apparent. The pressure to which they are subjected during their development, determines their form, as in the pre- vious case. If the original cell be spherical, and the pressure be equal on all sides, they also will be spherical until their sides are flattened against each other, when they will become rhomboidal dodecahedrons. If, on the other hand, the pressure be predominant in one direction, or there be any traction in another, the newly-forming cells will be elongated in the direction of least re- sistance ; and this elongation may be carried to such an extent as to impart to them the fibrous character. The development of cells within cells is most distinctly seen in the case of the spore or pollen-grain ; the granules contained in which are clearly the germs of the cells that compose the tissue of the em- bryonic structure. These cells, when fully evolved, in their turn produce others in their interiors ; and in this manner a complex and extensive organism may be developed from a single cell-germ. This, in fact, is what takes place in the lowest plants, in which the cell-germs or reproductive granules are set free from the parent vesicle, before they are themselves developed into cells ; and each one of them, imbibing nutriment from the air and moisture around, may ultimately evolve itself into a complete individual. In the higher Crypto- gamic Plants, on the contrary, the parent vesicle or spore does not rupture, but the new cells of the embryo are developed within it, at last distending its walls so much that they can be no longer traced ; and it would seem as if it served to elaborate for them, from the surrounding elements, the nutriment they re- quire. In the flowering plants, a further supply of this nutriment is provided in the ovule, where materials previously elaborated are stored up, to be absorbed through the wall of the parent cell, and to be subservient to the development of its contained germs. This process, although forming a part of the function of reproduction, is in reality essentially the same with the ordinary nutritive operations ; for in these the circulating fluid supplies the pabulum or organiza- ble matter ; whilst the cells already formed contain the germs, which, with the assistance of this, evolve themselves into new cells, and thus become the means of the extension of the original structure. 559. The Animal body exhibits phenomena of a character essentially the same. Even in the fully-formed organism, many parts may be found, which are composed, more or less evidently, of isolated cells or vesicles, analogous to those of Plants ; and it has been clearly proved that, in its early condition, the whole fabric has this character. In fact, it has been shown by the re- FORMATION OF CELLS. 421 searches of Barry, Schwann, and Valentin, that the whole structure originates in a single cell ; that this cell gives birth to others analogous to itself, and these again to many future generations ; and that all the varied tissues of the Animal body are developed from these, although no difference can be, in the first instance, observed among them. The multiplication of cells appears to take place upon the plan just stated, two or more being produced within the parent vesicle ; and this alike in the earliest condition of the embryo, and in the more advanced stages of the formation of its tissues. (See Plate I., Figs. 9 12, and Explanation.) The organizable fluid, then, prepared by the digestive process, is converted into organized tissue, by supplying the materials for this continual reproduction; the formation itself is dependent upon the powers of the solid texture. 560. This is not, however, the only mode in which new cells are produced in the Animal body ; for they may originate in Fibrin, from nuclei or cyto- blasts, which are formed by the aggregation of minute granules ; just as do those of the Plant in the organizable gummy fluid of the ovule. Both require for their perfect performance, that the fluid should be in contact with a living tissue ; and, when this condition is supplied, there seems to be no necessity for any further assistance ; but traces of cellular organization may often be discovered in Fibrin, even after it has been drawn from the vessels of the living body, though more frequently, perhaps, in that which has coagulated within the vessels after death.* The cells, where they present themselves, possess nuclei, and have all the characters of being in progress of development ; and frequently the nuclei can be distinguished when no cells yet appear. It is, however, when Fibrin is effused, in the form of Coagulable Lymph, on a cut surface, or on an inflamed membrane, that this process of organization most unequivocally displays itself. Soon after the coagulation, a number of granular bodies may be seen in the mass ; and these soon present appearances which indicate that they serve as nuclei for the formation of cells. In this condition they are known as Exudation-Corpuscles. The layers of these soon acquire such a consistence, that they may be peeled off in cohering shreds from the membranes to which they are attached ; and the cells when first formed present a ruddy yellow colour, which corresponds with that of the Chyle-globules. The Exudation-Cells are laid flat over one another, forming many super- imposed layers, which unite into membranous expansions, bearing a strong resemblance to the layers of flat cells of which Epithelium is composed ; their margins are at first rounded, and they are united by a connecting medium, which gradually disappears, leaving the sides coherent to each other, so that the figure of the disk is changed into a polygon. These cells afterwards give place to the various forms of tissue that are to present themselves in the new fabric, by a series of changes which will be hereafter more fully described. If the general principle be correct, that no Cell can be produced, save from a germ prepared by a pre-existing cell, it is obvious that such germs must be contained in the Liquor Sanguinis, and must escape from the blood-vessels which pour it forth. When their extreme minuteness is considered, this may not be deemed improbable. The formation of similar cells in the Clot of Blood drawn directly from the vessels, and even in that which has coagulated within them, seems to add weight to this idea. We have reason to believe that such granules are being continually set free by the rupture of the White Corpus- cles of the Blood ( 577) ; and reasons will hereafter be given for the belief, that they may be the germs of the Epithelial Cells, to which the layers o Exudation Cells bear so strong a resemblance. It is well known, that character of organisms of a low grade is very much influenced by the circum- * See Mr. Gulliver's Appendix to Gerber's General Anatomy, p. 31. 30 422 OF NUTRITION. stances under which they are developed ; hence there is no a priori objection to the belief (which other circumstances seem to favour), that Pus Corpuscles, and Tubercular matter, are abnormal forms of the same elements, as those which would otherwise produce a well-formed layer of Exudation Cells ( 609, 610). 561. From what has been stated, it appears evident that the process of Nutrition mainly consists in the growth of the individual cells composing the fabric ; and that these derive their support from the organic compounds with which they are supplied by the blood, just as the cells composing the simplest Plants derive theirs from the inorganic elements which surround them : and as different species of the latter select and combine these, in such modes and proportions, as to give rise to organisms of very diversified forms and proper- ties, so it is easily intelligible that the different parts of the fabric of the highest Animals should exercise a similar selective power, in regard to the materials with which the blood supplies them. The structure composing every separate portion of the body has (what may be termed) a special affinity for some par- ticular constituents of the blood ; causing it to abstract from that fluid, and to convert into its own substance certain of its elements. The conversion is termed Assimilation. The property by which the cells of the Animal or Vegetable structure are enabled to perform it, is one of which we are not likely ever to know more. It will probably long remain an ultimate fact in Physio- logy, that cells have the power of growing from germs, of undergoing certain transformations, and of producing germs that will develop other cells similar to themselves ; just as it is an ultimate fact in Physics, that masses of matter attract each other ; or in Chemistry, that the molecules of different substances have a tendency to unite so as to form a compound different from either of the elements. It is of such ultimate facts as these that the science of Vitality essentially consists ; since the Physical and Chemical phenomena which occur in living bodies are not strictly removable from the laws of Inorganic Nature. The conditions under which this Assimilating power operates, however, are freely open to our investigation ; and it is a great step in the progress of the inquiry, to become aware that these are so closely conformable throughout the organized world, as they have been shown to be. It may be stated as a general fact, that in assimilating, or converting into its own substance, matter which was previously unable to exhibit any of the manifestations of life, every cell thereby participates in the process of organization and vitalization ; for, by the new circumstances in which the matter is placed, its properties undergo a change, or, to speak more correctly, properties which were previously dor- mant are caused to manifest themselves. No matter that is not in a state of Organization can exhibit those properties which, from their being peculiar to living bodies, and altogether different from Physical and Chemical, are termed Vital; and it may also be asserted that no matter which exhibits perfect organization, is destitute of the peculiar vital properties belonging to its kind of structure.* As a corollary to this general fact, it may be stated that no organism can be produced by any fortuitous combination of inorganic matter ; since, even for the generation of the simplest cell, there is required a cell previously existing, to furnish the germ. 562. But this view also leads us to admit greater probability to the idea that beings of the highest degree of organization may, by a perversion of their assimilating processes, give origin to structures of a lower grade ; and it is difficult to say that this is not the case in certain diseases with which the * For a fuller consideration of this question, and the grounds upon which this view is supported, the reader is referred to the Article Life in the Cyclopaedia of Anatomy and Physiology ; and to the Chapter on the Nature and Causes of Vital Actions," in his Principles of General and Comparative Physiology. ELABORATION OF CHYLE AND LYMPH. 423 medical man is familiar. Thus, in the various forms of Cancer, it has been shown by Miiller and others, that the new growth consists of a mass of cells ; which, like the Vegetable Fungi, develop themselves with great rapidity ; and which destroy the surrounding tissues by their pressure, as well as by abstracting from the blood the nourishment which was destined for them. These parasitic masses have a completely independent power of growth and reproduction ; and it seems difficult to refuse them the title of distinct exist- ences. They can be propagated by inoculation, which conveys into the tissues of the animal operated on the germs of the peculiar cells that constitute this morbid growth ; and these soon develop themselves into a new mass. It seems to be by the diffusion of the germs produced in one part through the whole fabric, by the circulating current, that the tendency to re-appearance (which is one great feature in the malignant character of these diseases) is occasioned. Yet there is no evidence that the first production of a Cancerous growth is due to germs introduced from without ; in fact, as it appears to the Author, the history of its origin, as well as the analogy of similar cases, makes it far more probable that the Cancer-cell is but a degenerated form of the ordi- nary tissue of the body, being, in fact, a cell which possesses, to an unusual degree, the power of reproduction instead of undergoing those transformations by which it would be converted into other kinds of tissue ( 645). Several instances have been recently published of the occurrence of Vegetable organ- isms as parasites upon the Animal body ; that in some of these a true Plant, possessing a regular apparatus of nutrition and reproduction, has arisen from a germ introduced from without, there can be little question ; but in other instances (as in the case of the crusts of Porrigo favosa] it has been assumed that the organization is Vegetable, because it consists of a mass of cells capable of extending themselves by the ordinary process of multiplication. But it must be remembered that the cellular organization is common to Animals as well as to Plants ; being the only form that manifests itself at an early period of development in either kingdom ; and remaining throughout life in those parts, which have .not undergone a metamorphosis for special purposes. , Hence to speak of Porrigo favosa, or any similar disease, as produced by the growth of a Vegetable within the Animal body, appears to the Author a very arbitrary assumption ; the simple fact being, in regard to this and many other structures of a low type, that they present the simplest or most general kind of organization.* Their nature must generally be decided by their Chemical constitution ( 16) ; and this, in the case of the Porrigo favosa, appears to be unquestionably Animal. III. Elaboration of Chyle and Lymph. 563. The Chyle, as first absorbed in the Lacteals, is very different, both in its physical and chemical characters, from that which may be obtained from the larger absorbent trunks, and from the Thoracic Duct ; for during its pas- sage through these vessels, and their ganglia or glands, it undergoes important alterations, which gradually assimilate it to Blood. The chyle drawn from the lacteals that traverse the intestinal walls, contains Albumen in a state of complete solution ( 467) ; and it is entirely destitute of the power of coagula- tion, no fibrin being present in it. The Salts, also, are completely dissolved ; but the Oily matter presents itself in the form of globules of variable size.t It * The Author is strongly inclined to bel'ieve that the propagation of many diseases by inoculation, essentially consists in the implanting of cell-germs from one animal in the body of another. The structure of the Vaccine Vesicle appears to him to point clearly to such a view. f These oily globules are more abundant in the Chyle of Man and of the Carnivora, 424 OF NUTRITION. is generally supposed that the milky colour of the chyle is owing to these ; but Mr. Gulliver has recently pointed out* that it is really due to an immense mul- titude of far more minute particles, which he describes as forming the mole- cular base of the chyle. These molecules are most abundant in rich, milky, opaque chyle ; and in poorer chyle, which is semi-transparent or opaline, the particles float thinly or separately in the transparent fluid, and often exhibit the vivid motions common to the most minute molecules of various substances. Such is their minuteness, that, even with the best instruments, it is impossible to form an exact appreciation either of their form or their dimensions. They seem, however, to be generally spherical ; and their diameter may be estimated at between 1 -36,000th and 1 -24,000th of an inch. Their chemical nature is as yet uncertain : they are remarkable for their unchangeableness, when sub- jected to the action of numerous other re-agents, which quickly affect the proper Chyle-corpuscles ; and they are readily soluble in Ether, the addition of which causes the whole molecular base instantly to disappear, not a particle of it remaining ; whence it may be inferred that they consist of oily or fatty matter. The milky colour, which the serum of blood sometimes exhibits, is due to an admixture of this molecular base ; it is most common in young ani- mals that are suckling; but it is not uncommon in adults, and is not to be attributed to an absorption of milk into the chyle, as the physical properties of the two are quite different.. 564. During the passage of the Chyle through the absorbents on the intes- tinal edge of the Mesentery, towards the Mesenteric Glands, its character changes in several important particulars. The presence of Fibrin begins to manifest itself, by the slight coagulability of the fluid when withdrawn from the vessels ; and while this ingredient increases, the Albumen and the Oil- globules gradually diminish in amount. The Chyle drawn from the neigh- bourhood of the mesenteric glands exhibits the Corpuscles regarded as charac- teristic of that fluid ; these are peculiarly abundant in the fluid drawn from the glands themselves ; and they are constantly found in it, through its whole subsequent course. The Chyle-corpuscles are much larger than the molecules just described, and an examination of their characters presents no difficulty. The diameter varies from 1-71 10th to l-2600th of an inch; the average being about l-4600th. They are usually minutely granulated on ths surface, seldom exhibiting any nuclei, even when treated with acetic acid; but sometimes three or four central particles may be distinguished within them. During the passage of the Chyle through the mesenteric glands, a further increase in the proportion of Fibrin takes place ; and the resemblance of the fluid to blood becomes more apparent. The Chyle drawn from the vessels intermediate between these and the central duct, possesses a pale reddish-yellow colour ; and, when allowed to stand for a time, undergoes a regular coagulation, sepa- rating into dot and serum. The former is a consistent gelatinous mass, which, when examined with the microscope, is found to include the Chyle-corpuscles, each of them being surrounded by a delicate film of oil: the Fibrin of which it is principally composed, differs remarkably from that of the blood, in its infe- rior tendency to putrefaction ; whence it may be inferred that it has not yet undergone its complete vitalization. The serum contains the Albumen and Salts in solution, and a proportion of the Chyle-corpuscles suspended in it. It is curious, however, that considerable differences in the perfection of the coagu- lation, and in its duration, should present themselves in different experiments. Sometimes the chyle sets into a jelly-like mass, which, without any separation into coagulum and serum, liquefies again at the end of half an hour, and than in that of the Herbivora; their diameter has been observed to vary from l-25,000th to l-2000th of an inch. * Dublin Medical Press, Jan. 1, 1840, and Gerber's General Anatomy, Appendix, p. 88. ELABORATION OF CHYLE AND LYMPH. 425 remains in this state. This change takes place in the true coagulum also, if it be kept moist for a sufficient length of time. The Chyle from the Recepta- culum and Thoracic Duct coagulates quickly, often almost instantaneously ; and few or none of the corpuscles remain in the serum. 565. It is to be remembered that the Lacteals are the Lymphatics of the intestinal walls and mesentery ; performing that function of Interstitial Absorp- tion, which is elsewhere accomplished by vessels that are not concerned in the introduction of alimentary substances from without. During the intervals of digestion, they contain a fluid which is in all respects conformable to the Lymph of the Lymphatic trunks. The aspect of the Lymph greatly differs from that of the Chyle, the former being nearly transparent, whilst the latter is opaque or opalescent ; and this difference is readily accounted for, when the assistance of the microscope is sought, by the entire absence from the Lymph of that molecular base which is so abundant in the Chyle, A considerable number of corpuscles are generally present in it ; and these seem to corre- spond in all respects with the white or colourless corpuscles of the Blood ( 577). Their amount, however, is extremely variable ; as is also that of the oil-glob- ules, which sometimes occur, whilst in other instances none can be discovered. Lymph coagulates like chyle ; a colourless 61ot being formed, which encloses the greater part of the corpuscles. 560. The nature and source of the peculiar globules of the Chyle, are as yet matters of doubt; some light, however, has been thrown on their history, by recent investigations ; and much may be said of them, which, if not absolutely proved, can scarcely be regarded as improbable. The process of their forma- tion bears a striking analogy to that of the cytoblasts of Plants, as observed by Schleiden ( 557). They appear in the midst of a fluid crowded with minute granules, and appear to be themselves at first composed of an aggregation of smaller particles. Various stages of development present themselves, how- ever, in these bodies ; and the larger ones, which are chiefly to be met with in the Thoracic Duct, are evidently cells, bearing a strong resemblance to the Lymph-corpuscles, and to the Colourless corpuscles of the blood. Like the latter, they contain three or four large central particles, which are distinctly brought into view, when they are treated with acetic acid. Their diameter, too, is about the same ; being usually between the 1 -2600th and the 1 -2900th of an inch, or rather larger than the Red corpuscles of Human blood. It will be hereafter pointed out, that there is strong reason to believe their functions to be the same ( 578). 567. The changes which the Chyle is observed to undergo, in its passage from the Intestinal villi to the Thoracic Duct, suggest some inferences in regard to the possible transformation of Fatty matter into a Protein-compound. That the Chyle-corpuscles are not identical in chemical composition with the Mole- cular base is quite certain, from the completely different effects of re-agents upon the two respectively; but it maybe surmised that, as they appear to consist of an altered form of Albumen, the soluble Albumen and the Fatty matter are both concerned in their production. It has been stated that, whilst the Fibrin increases, the Oil-globules undergo an evident diminution ( 564), and that the quantity of Albumen lessens. It is not conceivable that the Fibrin should be at once formed at the expense of the Oil-globules ; since Albumen, which is a mere chemical compound, ready to undergo organization and vitalization, is always the preceding grade. The Fibrin must, therefore, be produced at the expense of the Albumen ; whilst new Albumen is elabo- rated from the Oily matter. Of the process by which the latter important change is accomplished, we are yet entirely ignorant ; but the evident altera- tion which takes place in the proportion of azotized ingredients would seem to show, that Nitrogen is in some manner communicated to the Chyle during its 36* 426 OF NUTRITION. progress along the Lacteals. No source for this nitrogen can be suggested except the Blood: and the influence of the blood upon the contents of the absorbent vessels, must be in part communicated through the vasa vasorum distributed upon their walls, (since in the cold-blooded Vertebrata there are no lymphatic glands,) but chiefly in the Lymphatic Glands, where the blood-ves- sels and absorbents come into extremely close relation. The idea that the Blood is deprived, in the Mesenteric vessels, of fcome of its azote, seems to derive important confirmation from the fact, that the secretion of the Liver, which is chiefly formed from blood that has returned from these vessels, con- sists almost entirely of unazotized ingredients (Chap. xn.). It may be con- ceived, then, that whilst the Albuminous matter originally present in the Chyle is being converted into Fibrin, new Albumen is being formed at* the expense of the Fatty matter. The same account is applicable to the Lymphatics, a part of whose functions it is, to bring the oily matter stored up in the Adipose tissue within the sphere of the nutritive operations ( 468) ; and the variation in the circumstances which may render this necessary, fully accounts for the varia- tion in the amount of Oily and Albuminous matter presenting itself in this fluid. It must be acknowledged, however, that the views here offered are in great part hypothetical. They derive some confirmation, however, from the circumstance recently pointed out by Mr. G. Ross,* -that the constituents of fatty matter, added to those of uric acid, would very nearly give the atomic constituents of albumen; whence it might be surmised that, when there is a demand for Protein-compounds in the system, azotized matter, which would otherwise be excreted, may be united with non-azotized compounds taken in as food, in order to supply its wants. The fact, which constitutes an important feature in the Physiology of Secretion ( 648), that a separation of Protein- compounds into two such classes of bodies is continually taking place in the living economy, would seem to render the possibility of their union greater. An important source of fallacy, however, attends all deductions founded upon the differences observed in the Chyle in the several parts of its course through the Lacteals; viz., that we cannot be at all sure how far this is dependent upon an actual interchange of ingredients with the Blood, taking place by Imbibition or Endosmose through the very thin parietes of the contiguous vessels. The whole question offers a very wide scope for further inquiry. 568. The fluid drawn from the Thoracic Duct, and from the Absorbent ves- sels which empty their contents into it, is frequently observed to present a decided red tinge, which increases on exposure to the air. This tinge is due to the presence of true Blood-corpuscles ; but these are somewhat modified in form and size, being a little smaller than the ordinary Blood-discs, and fre- quently angular, granulated, or indented at the edges. By Mr. Lanet it is stated that this intermixture is accidental ; and that it results from the absorp- tion of Blood-particles into the Lymphatics, at the points where the latter are divided, in making the sections necessary to expose the centres of the Absorbent system : and he mentions a striking fact in illustration of his view. He con- siders that the alteration in the character of the corpuscles is due to the action of the Chyle on the Blood, since many other fluids will produce analogous effects ; and he states that shortly after a flow of chyle into the blood, a large number of such altered discs may be seen in the circulating fluid. On the other hand, Mr. Gulliver and several eminent observers, regard these blood- discs as true constituents of the fluid of the absorbents ; and suppose that they are in process of formation. Reasons will hereafter be given, however, for the belief that the red Blood-discs are not formed from the Chyle-corpuscles ; * Lancet, 1842-3, vol. i. f Cyclopaedia of Anatomy and Physiology, vol. iii. p. 220. PROPERTIES OF THE BLOOD. 427 so that Mr. Lane's view is probably the correct one. Even if the Blood-discs are not introduced into the Lymphatics during the operation of exposing tha Thoracic Duct, it may not be considered as improbable that, in those animals in which the Lymphatics have several communications with the Veins, they should naturally obtain an entrance in various parts of the system. Such communications, according to Gerber, decidedly exist in the Horse ; and it is in the Chyle of that animal, that the rosy tint, and the Blood-corpuscles which occasion it, have been chiefly observed. 509. The following table, slightly modified from that of Gerber, presents in a concise form, a view of the relative proportions of the three chief ingre- dients in the Chyle, in different parts of the absorbent system, and thus gives an idea of its advance in the process of assimilation. In the afferent or periphe- ("Fat, in maximum quantity (numerous fat or oil ral Lacteals (from the In-J g lobules > x i v T./T < Albumen m minimum quantity. SET) Few or no ch y le - COT P scks - In the efferent or central LacteaJs (from the Me- senteric glands to the Thoracic Duct). In the Thoracic Duct. JFibrin, almost entirely wanting. "Fat, in medium quantity (fewer oil-globules). Albumen, in maximum quantity. Chyle-corpuscles very numerous, but imperfectly developed. _Fibrin, in medium quantity. "Fat, in minimum quantity (fewer or no oil- globules). Albumen, in medium quantity. Chyle-corpuscles numerous, and more distinctly cellular. _Fibrin, in maximum quantity. IV. Physical and Vital Properties of the Blood. 570. The Blood, whilst circulating in its vessels, is composed of a fluid, in which a large number of corpuscles or particles of a red colour are suspended. The fluid portion, which is known under the name of Liquor Sanguinis, essentially consists of Fibrin and Albumen, with Saline matter, dissolved in water ; and this, when effused without an intermixture of corpuscles, is known under the name of Coagulable Lymph. The red Blood-particles (commonly, but erroneously, termed globules] are flattened discs, which, in Man and most of the Mammalia, have a distinctly circular outline. In the discs of Human blood, when examined in its natural condition, the sides are somewhat con- cave ; and there is a bright spot in the centre, which has been regarded by many as indicating the existence of a nucleus, although it is in reality due simply to the greater thinness of the disc at that part. The form of the disc is very much altered by various reagents ; for the membrane which composes its exterior is readily permeable by fluids ; so that, if the discs be put into water, a powerful endosmose takes place towards the interior, causing the particles to assume a globular form ; whilst, if they be treated with syrup, or with a thick solution of albumen, they will be more or less completely emptied, so as to present a shrunken appearance. Hence, in examining the Blood, it is necessary to dilute it with a fluid as nearly as possible of the same character with ordinary serum.* In regard to the existence of a nucleus in the corpus- * By Wagner, the filtered serum of frog's blood is recommended for this purpose. Weak solutions of salt or sugar, and urine, answer tolerably well ; but Mr. Gulliver re- marks that all addition must be avoided when it is intended to measure the corpuscles, 428 OF NUTRITION. Fig. 100. Corpuscles of Human Blood, magnified about 500 diameters; A, single red particles; 1, 1, their flattened face; 2, a particle seen edgeways, three-quarter view; B, aggregation of particles in a columnar form; c, co- lourless corpuscles. (After Wagner.) Fig. 101. cles of Mammalia, there has been considerable difference of opinion amongst microscopists ; some main- taining that it can be brought into view by treating them with acetic acid, whilst others deny that any de- finite appearance is thus produced. The researches of Mr. Gulliver and Mr. Wharton, however, appear to the Author quite conclusive as to the non-existence, in the red parti- cles of Mammalia (even in the oval discs of the Camelidas), of any thing at all analogous to the nucleus, which is to be seen in the blood- discs of Oviparous Vertebrata. The corpuscles, when emptied of their coloured contents by the action o'f water, exhibit no trace of it in Mammalia ; though the same process brings them clearly into view in the oval blood-discs of all other Vertebrata. The central matter in the Mammiferous blood-disc, which undergoes coagulation under the influence of acetic acid, is therefore not to be regarded as a true cell-nucleus, which is evidently the character of the central spot on other blood-discs ; but is rather a collection of albuminous particles, which are coagulated by the action of the acid upon them, just as they would be in the liquor sanguinis or in the serum of the blood. 571 . In all Oviparous Vertebrata, without any known exception, the red corpuscles are oval, the propor- tion between their long and short diameters, however, being much subject to variation ; and their nu- clei may always be brought into view by treatment with acetic acid, when not at first visible. In the red particles of the Frog, which are far larger than those of Man, a nu- cleus can be observed to project somewhat from the central portion of the oval, even during their circu- lation, and it is rendered extremely distinct by the action of acetic acid ; this dissolves away the remainder of the particle and gives an increased opacity to the nucleus, which is then seen to consist of a granular substance. In the still larger blood-disc of the Proteus and Siren, this appearance is yet more distinct ; the structure of the nucleus being so evident, without the addi- tion of acetic acid, that its granules can be counted.* or to ascertain their true forms; as the serum of one Mammal reacts injuriously on the blood of another. See Philos. Magaz., Jan. and Feb., 1840. * As Professor Owen's interesting account of the blood-discs of the Siren may not be generally accessible (Penny Cyclopedia, Art. Siren'), the leading facts in it will be here stated. This animal agrees with the Proteus and other species in being perenni branchi- ate (43); and, as in all its congeners yet examined, the blood-discs are of very large dimensions. They are usually of an oval form, the long diameter being nearly twice the short; and the nucleus projects slightly from each of the flattened surfaces. Considera- ble variety in the form of the disc presented itself, some of the corpuscles being much less oval than others; but the nucleus did not partake of these variations in nearly the Particles of Frogs Blood; 1. 1. their flattened face; 2, particle turned nearly edgeways ; 3, lymph-globule ; 4, blood-corpuscles, altered by dilute acetic acid. Mag- nified 500 diameter. (After Wagner.) PROPERTIES OF THE BLOOD. 429 572. The form of the Red Corpuscles is not unfrequently seen to change during their circulation ; but this is generally in consequence of pressure, from the effects of which, however, they quickly recover themselves. In the narrow capillary vessels, they sometimes become suddenly elongated, twisted, or bent, through a narrowing of the channel ; and this may take place to such a degree as to enable the disc to pass through an aperture which appears very minute in proportion to its diameter.* When undergoing spontaneous decomposition, the blood-discs become granulated, and sometimes (as long ago noticed by Hewson),even mulberry-shaped ; and particles in which these changes appear to be commencing may be found in the blood at all times. It has been ascer- tained that bile and urea exert a peculiar solvent power on the blood-corpuscles ; and hence we can understand one of the modes in which a retention of these substances in the circulating fluid ( 649) proves so injurious. The size of the blood-discs is liable to considerable variation, even in the same individual ; and this is at once understood when they are considered as cells in different stages of growth. There are, however, limits to this variation for each species ; and the blood-discs of one tribe of Mammalia can rarely be confounded with those of another. The diameter of the corpuscles bears no constant relation to the size of the animal, even within the limits of the same class ; thus, although those of the elephant are the largest among Mammalia (as far as is hitherto known), those of the Mouse tribe are far from being the smallest, being in fact more than three times the diameter of those of the Musk Deer, there is, however, a more uniform relation between the size of the animal and that of its blood-discs, when the comparison is made within the limits of the same order. In man, the diameter varies from about l-4000th to l-2800th of an inch ; the average diameter is probably about 1 -3400th. a. The following measurements of the blood-discs of various animals, are chiefly given on the authority of Mr. Gulliver. The diameter of the corpuscles in the Quad- rumana is generally about the same with that of the Human blood-discs; there is, how- ever, a slight diminution among the Lemurs, and there is more variation among them, than among the Monkeys. Among the Cheiroptera, the diameter of the corpuscles is somewhat less than in the preceding order, the average being about l-4300th of an inch. The blood-discs of the Mole are still smaller, averaging only the l-4750th of an inch; those of the Hedge-hog, however, are larger, being about l-4100th. Among the Planti- grade Carnivora, the average is about l-3800th, and from this none depart very widely: but among the Digitigrade species there is a considerable range; in the Weasel tribe, the average is about l-4800th ; in the Feline, it is about l-4400th ; in the Dog tribe,f there is a range of averages from l-3400th to l-4100th ; and in the Seal, the average is about l-3300th. Observations on the blood-discs of the Cetacea are much required. Among the Pachyderrnata, the average, excluding the Elephant (the diameter of whose blood- same degree. The nucleus is clearly seen to consist of a number of moderately-bright spherical granules, of which from 20 10 30 could be seen in one plane or focus, the total number being of course much greater. When removed from the capsule, the nuclei are colourless, and the component granules have a high refracting power. Viewed in situ, they present a tinge of colour lighter than that of the surrounding fluid, and dependent upon the thin layer of that fluid interposed between the nucleus and the capsule. As the fluid contents of the blood-disc in part evaporate during the process of desiccation, the capsule falls into folds in the interspace between the nucleus and the outer margin; these folds generally take the direction of straight lines, three to seven in number, radiating from the nucleus. * " Blood-corpuscles are repeatedly found, quite unaltered in appearance, on the mucous surfaces, when no solution of continuity whatever can be detected in the vessels." Gul- liver, in Gerb. Gen. Anat., p. 78. f Two facts of much interest in Zoology have been brought to light by Mr. Gulliver's examination of the diameter of the blood-corpuscles of this tribe. The difference between those of the Dog and Wolf is not greater than that which exists among the varieties of the Dog, whilst the discs of the Fox are much smaller. The discs of the Hyaena are far more approximate to those of the Canidae than they are to those of the Felidae. 430 OF NUTRITION. discs is about l-2745th of an inch), and the Rhinoceros (in which they are about l-3765th), may be stated at about l-4200th; and there is les* variation than might have been ex- pected, from the different size and conformation of the several species examined. Among the Ruminantia, the corpuscles are for the most part smaller than in other orders; and there is more relation between their diameter and the size of the animal, than is else- where observable. Excluding the Camelidae (which are zoologically intermediate between the Ruminantia and Pachydermata), we find a range of sizes extending from the l-3777th to the l-12325th of an inch; the former is the diameter in one of the larger Deer: the latter in the Musk Deer, which is the smallest of the whole order. In the Camel tribe, the average of the long diameter of the oval corpuscles is l-3400th of an inch: whilst that of the short diameter is l-6300th; and this is no where widely departed from: the length of the discs is, therefore, not quite twice their breadth. Among the Rodentia, the discs are rather large, especially considering the small size of most of the species. In the Capybara, which is the largest animal of the order, they average l-3216ih ; and in the Mouse family (the smallest of Mammalia) they are as much as l-3800th. In the Squirrels, the diameter is rather less; but in scarcely any of the whole order is it under l-4000th. Among the Edentata, the only species yet examined, is one of the Arma- dillos, in which the diameter of the corpuscles is about the same as in the Quadrumana. In the Marsupialia the range is nearly the same as among the Rodentia. b. In BIRDS, according to the observations of Mr. Gulliver, the long and short diameters of the corpuscles usually bear to each other the proportion of 1 or 2, to 1 ; and this is the general relation among Oviparous Vertebrata, with the exception of some of the Crocodile tribe, in which the length is sometimes three times the breadth. The size of the corpuscles of Birds has generally more relation to that of the species, than in Mam- malia. No instance has yet been detected, of the occurrence of comparatively small corpuscles in the larger species, and of large corpuscles among smaller animals, which has been seen to be common among the former class; the blood of the Humming-birds, however, has not yet been examined. The largest discs are found among the Cursores ; those of the Ostrich have an average long diameter of l-1649th of an inch, and a short diameter of l-3000th ; and among the larger Raptures, Grallatores, and Natatores, the dimensions are but little inferior. The least dimensions hitherto observed are among the small Passerine birds; in which the corpuscles have a long diameter of about l-2400th of an inch, and a transverse diameter of from l-3800th to l-4800th. Circular discs may be occasionally observed in some species, agreeing with the others in every particular but their form; and every gradation may be noticed between these and the regular oval corpuscles. c. The large size of the blood-discs in REPTILES, especially in Batrachia, and above all, in the Perennibranchiate species of the latter, has been of great service to the Phy- siologist; by enabling him to ascertain many particulars regarding their structure, which could not have been otherwise determined with certainty. Among other facilities which this occasions, is that of procuring their separation from the other constituents of the blood ; for they are too large to pass through the pores of ordinary filtering-paper, and are therefore retained upon it, after the liquor sanguinis has flowed through. The blood-discs of the warm-blooded Vertebrata cannot be thus separated. The oval cor- puscles of the Frog have a long diameter of about l-1000th, and a transverse diameter of about l-lSOOth^of an inch; those of the Salamander or Water-newt, are rather smaller. The long diameter of the corpuscles of the Proteus is staled by Wagner at l-337th of an inch ; that of the Siren is about l-435th, the short diameter being about 1-SOOth of an inch; the extremes of variation, however, are very wide. The long diameter of the nuclei is about l-1000th or l-l 100th, and the short diameter about l-2000th; hence it is about three times as long, and nearly twice as broad, as the entire Human blood-disc, thus having six times its superficies; its thickness is about l-3800th of an inch. 573. In regard to the Chemical constitution of red Blood-corpuscles, it is difficult to speak with defmiteness ; since there are three parts in each disc which are essentially different in character, and which may have a very dif- ferent composition. These parts, the capsule, the nucleus, and the con- tained matter, cannot be separated without the use of chemical reagents, which must alter their respective properties. Two proximate principles have been obtained from the blood-discs : these are designated as hssmatosine and globu- line. To the h&matosin the red colour of the blood is due, although it con- stitutes not more than a 20th or 25th part of the whole mass of dried globules. When separated from the globulin, it is of a dark-brown hue, and is tasteless and insoluble in water, alcohol, and ether ; but is readily soluble in water or PROPERTIES OF THE BLOOD. 431 alcohol that contain alkalies or acids, whence it may be supposed to unite with these, like albumen, as an acid or a base. In composition, however, it differs considerably from both protein and albumen ; its formula being 44 c, 22 H, :J N, 6 o, with a single proportional of iron. When burned, it yields a notable quantity of peroxide of iron ; and one atom of this is considered to be present in combination with one of the animal compound, which is analogous to protein. The red colour is not due, however, as formerly supposed, to the presence of this peroxide ; for M. Scherer has recently found, that the metal may be entirely dissolved away by the agency of acids, and that the animal matter, afterwards boiled in alcohol, colours the spirit intensely red. The globulin, which is the principal constituent of the corpuscles, has not yet been isolated ; but from its properties in combination, it is inferred to differ but little from protein. It may perhaps be doubted, whether these two principles have a separate existence ; or whether they are not rather results of the chemical processes employed to obtain them. When the blood-discs are separated from the other constituents of the fluid, and are immersed in water, they soon absorb so much as to become globular ; and the continuance of the endosmose occa- sions the diffusion of their contents (by the rupture of the capsule) through the water, in which the greater part dissolves. This solution exhibits the same changes of colour under the influence of oxygen, acids, saline matter, &c., as the blood undergoes in similar circumstances ( 542). When it is heated, the matter of the globules is coagulated and forms an insoluble pre- cipitate ; both in its soluble and coagulated states, it exhibits similar effects with reagents, as does albumen in the same conditions. 574. The question of the origin of the red Blood-corpuscles is a very in- teresting one, and cannot yet be regarded as completely determined. That they are to be regarded as nucleated cells, conformable in general character with the isolated cells, which constitute the whole of the simplest Plants ( 555), and having each an independent life of its own, there can now be no reasonable doubt. From this we should infer that they have the power of reproducing themselves ; and the recent observations of Dr. Barry and other Microscopists have confirmed the statement long ago made to that effect by Leeuwenhoek. The first change which takes place is the appearance of delicate radiating lines between the nucleus and the periphery ; dividing the disc into several segments, usually six in number (Fig. 22, Plate I.). The margin is soon observed to become crenated, by indentations at corresponding points ; and these indentations become deeper, until a complete separation takes place, forming six young cells or discs (, b, c, d, e). It is next to cer- tain that these are developed within the parent cell or disc, from some of the granules on the margin of its nucleus ; just in the same manner as rings of cells will be hereafter described ( 745) as arising from the Germinal Spot. From this fact, connected with what has been already stated of the continual decomposition of the blood-discs ( 572), we may infer that each cell has a determinate period of existence ; and that whilst some are decaying from age, their place is being supplied by young ones in process of growth. Between the small newly-generated disc, and the full-sized corpuscle, we should expect to find every intermediate size ; and this is exactly what presents itself. That the corpuscles may be generated with great rapidity under peculiar circum- stances, will hereafter appear ( 594) ; and their amount may undergo a rapid diminution also, without any evident abstraction of them from the circulating fluid. Appearances have been seen by Wagner, Gulliver and others, in the blood of Batrachia, which seem to indicate that the Colourless corpuscles ( 577) serve as the nuclei of cells, which, when fully developed, may become Red blood-discs ; but in the Mammalia it is scarcely possible to imagine that this can occur ; since the diameter of the colourless corpuscles is very con- 432 OF NUTRITION. slant ; whilst that of the blood-discs is so variable, that the former, though sometimes the smaller, are in other instances far larger than the latter. If it be admitted that the red corpuscles have the power of reproduction, like other isolated cells, it does not seem necessary to seek elsewhere for the source of their constant renewal ; and various facts, hereafter to be stated, appear to the Author strongly indicative of the entire functional as well as structural differ- ence, between the red and the colourless corpuscles of the blood of Vertebrata. 575. That the red Blood-discs, when first formed in the embryo, have an origin common to that of all other tissues, cannot be doubted. They are pro- duced, in the embryo of the Bird, in the portion of the germinal membrane which afterwards becomes the area vasculosa ; this consists of delicate cells very uniformly disposed ; and whilst capillary vessels are being formed by the union of the cavities of these, blood-discs seem to be developed from the granules or cell-germs they contain. These changes take place about the second or third day of incubation ; but it is not until some days afterwards that the discs assume their characteristic form.* As at this period no special organs exist in the embryonic structure, it is evident that the blood must be formed by the cells of the germinal membrane, at the expense of the albumi- nous alimentary materials which they absorb from the yolk ; hence we may Fig. 102. & Production of Blood-Corpuscles in Chick, on the fourth day of incubation ; a, particles fully formed : b. particles in progress of formation ; c, similar particles, altered by dilute acetic acid so as to display their nuclei. (After Wagner.) infer that no special organ can be needed for this purpose in the adult, and that the assignment of the manufacture of blood-corpuscles by some physiolo- gists to the Spleen, by others to the Thymus, must be incorrect. The cor- * Mr. Macleod gives the following history of the development of the blood-corpuscles in the Chick. In blood withdrawn from the heart, on the third day, and diluted with serum, or from the germinal membrane or allantois, and diluted with fluid albumen, *' a number of small granules are seen floating about the field : these enlarge and become clearer in the centre ; this enlargement goes on very rapidly, and when they have gained to about twice their original size, the central clear part becomes dull. This dullness slightly increases, and in a short time it is seen to be distinctly granular: whilst the borders are observed to be well-defined, smooth, and clearer than the central part. The enlargement of these bodies, with the granular appearance of their centre, seems not to depend on the aggregation of granules round a central one, but on a property which they have in themselves of enlarging and presenting that figure. During all this time they are quite spherical and of good consistence, as they do not lose their form by con- siderable pressure. In the second stage, the central portion gradually becomes less opaque, and ceases to appear granular, the external portion at the same separating in some degree from the central part. The blood-corpuscle, in this stage of development, has the appearance of a slightly flattened round cell, formed of a somewhat delicate but elastic membrane, with a nucleus in the centre. At this time a number of these bodies, being close together in the field, present a yellowish colour. The cell is disc-like, rather concave, but the nuclus convex. In the third stage, one side of the corpuscle gradually elongates, giving it a pear-shaped appearance; the opposite side then elongates itself in a similar manner, and to the same degree. The concavity between the nucleus and border disappears, and the whole becomes slightly convex. The hue at the same time gradually becomes redder." (London and Edinburgh Monthly Journal, September, 1842.) PROPERTIES OF THE BLOOD. 433 puscles are generally larger in the embryo than in the adult, especially soon after the period of their first formation ; it was remarked by M. Prevost that in the foetal goat they were at first twice the size of those of the mother. Mr. Gulliver has observed however, that at a later period of utero-gestation they are sometimes smaller than the average dimension of the adult : but perhaps all such observations are to be received with hesitation, owing to the fact mentioned by him, that the variety in the magnitude of the foetal corpuscles is much greater than in the full-grown animal. 576. In regard to the uses of the red Blood-corpuscles in the animal econ- omy, it appears to the Author that a definite conclusion may be now arrived at. Their existence in the circulating fluid is confined to the Vertebrated classes ; the corpuscles which are seen in the blood of the Invertebrata being analogous rather to the colourless corpuscles, presently to be described as present in the blood of the higher animals. Hence the inference appears irresistible, that they are not essentially necessary to the production of the organizable elements of the blood, or of the organized tissues. The red cor- puscles are most abundant in those classes among Vertebrata, which maintain the highest temperature ; thus, they are somewhat more numerous, in propor- tion to the whole bulk of the blood, in Birds than in Mammalia ; and far more in the latter than in Reptiles and Fishes. As it is evident that they undergo very important changes in the pulmonary and systemic capillaries, their colour being changed from purple to red in the former, and from red to purple in the latter; it seems highly probable that they have for their principal office the introduction of oxygen into the blood that circulates through the systemic capillaries, and the removal of the carbonic acid set free there ; to serve as the medium, in fact, for bringing the tissues into relation with the air, the influence of which is necessary for the maintenance of their vital activity. In the Invertebrata generally, whose respiration is very feeble, this end will be sufficiently answered by the fluid plasma of the blood ; the altera- tions in which, under the influence of the air, have been already noticed ( 540 and 553). And in Insects, the only class whose respiration is at all active, we find the air directly conveyed into the tissues; the circulating fluid not being employed as its carrier. By Liebig it is supposed, that the iron in the red corpuscles is the real agent in the respiratory process ; for if its original state be the protoxide, it may become the peroxide by uniting with an additional atom of oxygen, or the protocarbonate by the addition of an atom of carbonic acid. The former change is supposed by him to take place in the lungs, to which the blood comes charged with carbonic acid ; the carbonic acid is given up by the iron, and replaced by an equivalent of oxygen taken in from the air; whilst in the systemic capillaries, the converse change takes place, the oxygen being imparted to the tissues, and being replaced by carbonic acid which is given up by them to be conveyed out of the system. It is stated by Liebig that there is far more than sufficient iron in the whole mass of the blood, to convey in this manner all the oxygen and carbonic acid, which are interchanged between the pulmonary and systemic capillaries. The speculation is certainly an ingenious one ; but it can scarcely be yet received as a physiological fact. In addition to their uses in the respiratory process, it would not seem unlikely that the red corpuscles may be of impor- tant assistance in promoting the movement of the fluid in which they are suspended ; for, if it be true that this partly depends upon the chemical con- dition of the blood in respect to the tissues which it supplies, any attractions or repulsions arising out of this may be more powerfully exercised upon a solid corpuscle than upon the constantly-shifting particles of the fluid. Hence, perhaps, the local congestions of anaemic patients. 577. Besides the red particles of the Blood, there are others which possess 37 434 OF NUTRITION. no colour, and which seem to have a function altogether different ; these are known as the white or colourless corpuscles. Their existence has long been recognized in the blood of the lower Vertebrata, where, from being much smaller than the red corpuscles, they could be readily distinguished. But it is only of late, chiefly through the researches of Gulliver, Addison,* and others, that they have been recognized in the blood of Man and other Mam- malia; their size being nearly the same with that of the red corpuscles; and the general appearance of the two (owing to the circular form of the latter, and the absence of a proper nucleus,) being less distinct. It is remarkable that, notwithstanding the great variations in the size of the red corpuscles in the different classes of Vertebrata, the dimensions of the colourless corpuscles are extremely constant throughout; their diameter seldom being much greater or less than l-3000th of an inch. This has been observed even in those ani- mals, the Musk-Deer, and the Proteus, which present the widest departure from the general standard in the size of their red corpuscles ; so that the colourless corpuscle is as much as four times the diameter of the red in one instance; whilst it is not one-eighth of the long diameter of the red in the other. Hence it would seem very improbable that the red can ever be con- verted into the white, or the white into the red. The aspect of the two, under the Microscope, is very different. Instead of presenting a distinct central nucleus, like the red corpuscles of the Oviparous Vertebrata, or being en- tirely destitute of granular contents, as are those of Mammalia when unaffected by reagents, the colourless corpuscles are studded with minute granules, which may be occasionally seen in active motion within them, and which are discharged when the corpuscles are .treated with liquor potassse. They possess, moreover, a higher refracting power than the red corpuscles; and are further distinguished from them by their greater firmness, and by the absence of any dis- position to adhere to each other ; so that, when a drop of recent blood is placed between two strips of glass, and these are gently moved over one anoher, the white corpuscles may be at once recog- nized by their solitariness, in the midst of rows and irregular masses formed by the aggregation of the red. These white corpuscles of the blood correspond so closely, in all their characters, with the Lymph-corpuscles, that it is difficult to regard them as otherwise than identical ; and, as already pointed out ( 566), the corpuscles of the Chyle appear to be of "the same character, though less perfectly formed. The colourless corpuscles may be readily distinguished in the circulat- ing Blood, in the capillaries of the Frog's foot ; and it is then observable, that they occupy the exterior of the current, where the motion of the fluid is slow, whilst the red corpuscles move rapidly through the centre of the tube. The colourless cor- puscles, indeed, often show a disposition to adhere to the walls of the vessels ; which is manifestly increased on the A small venous trunk a, from the web of the Frog's loot, magnified 350 diameters; b. b, cells of pavement-epithelium, containing nuclei. In the space between the current of oval blood-corpus- cles and the walls of the vessels, the round, transparent, white corpuscles are seen. (After Wagner.) * Transactions of the Provincial Medical Association, 1842 and 1843. PROPERTIES OF THE BLOOD. 435 application of an irritant. Hence the idea naturally arises, that (to use the words of Mr. Wharton Jones) "there is some reciprocal relation between the colourless corpuscles, and the parts outside the vessels, in the process of nutrition." What that relation is, we shall now proceed to inquire. 578. In regard to the purpose of the colourless corpuscles in the animal economy, a view has been brought forward by the Author,* which increased consideration has only served to strengthen ; and which he advances here with some degree of confidence that it will be found, on attentive examination, war- ranted by a large number of physiological analogies, though not capable of being directly proved. That it may be rightly understood, a general sketch of certain known operations of cells in Plants and Animals will be first given. It is not difficult, on taking a comprehensive survey of the assimilating pro- cesses, to find a number of examples in which cells are developed in a tempo- rary manner ; growing, arriving at maturity, and then disappearing, apparently without having performed any particular function. In the albumen of the seed, for instance, this often takes place to a remarkable extent. In the Yolk of the Egg there is a similar transitory development of cells, of which several generations succeed each other, without any permanent structure being the result. In the Germinal Vesicle, again (according to Dr. Barryt), several annuli of cells are seen to occupy its cavity, when it is prepared for fecundation; and the oldest and largest of these contain another generation : yet all these dis- appear by liquefaction, as soon as the two permanent cells begin to be developed in the centre ( 746). Further, in the subsequent development of all the cells which are descended from these, and form the " mulberry mass," the same process is repeated ; a great number of temporary cells being produced, only to liquefy again as soon as the two permanent central cells make their appear- ance. It can scarcely be imagined by the well-judging physiologist, that all this cell-life comes into existence without some decided purpose ; and if we can assign to it an object, the fulfilment of which is consistent with the facts supplied by analogy elsewhere, this may be reasonably considered as having a fair claim to be received as a physiological induction. In all these instances, and in many more which might be quoted, the crude alimentary materials are being prepared to undergo conversion into permanent and regularly-organized structures. The very first union of the inorganic elements into the simplest proximate principles, is effected by the cell-life of plants. The change of these principles into the peculiar compounds which form the characteristic secretions of plants, is another result of their cell-life. And there seems equal ground for the belief, that the change of these proximate principles into the peculiar glutinous sap, which is found wherever a formation of new tissue is taking place, is equally dependent upon the agency of cells. Thus, the starchy fluid which is contained in the ovule, previously to its fecundation, is probably not in the state in which it can be immediately rendered subservient to the nutrition of the embryo ; and the development of successive generations of cells, which exert upon it their vitalizing influence, may be reasonably regarded as the means by which the requisite change is effected. Exactly the same may be said of the albuminous matter contained in the yolk of the egg, which is certainly not in a condition in which it can be immediately applied to the purposes of nutrition ; and its conversion may be regarded as commencing with the development of transitory cells within its own substance, and as being completed by means of the cells forming the inner layer of the germinal mem- brane, by which it is subsequently taken up and introduced into the current of * Report on Cells, in British and Foreign Medical Review, Jan., 1843. f Euibryological Researches. Third Series. 436 OF NUTRITION. blood flowing through the vascular area ( 761). A similar purpose is proba- bly answered by the transitory cells developed within the germinal vesicle, and by those which appear at a similar period in the evolution of the descendants of the "twin cells" produced in it. Many similar examples have been else- where adduced.* We have thus a class of facts, which indicates that the con- version of the Chemical compound into the Organizable principle the aplastic into the plastic material is effected in the particular situations where it is most wanted, by the vital agency of transitory cell-life ; that is, by the production of cells which are not themselves destined to form an integral part of any, perma- nent structure, but which, after attaining a certain maturity, reproduce them- selves and disappear ; successive generations thus following one another until the object is accomplished, after which they altogether vanish. We shall now consider another class of facts, which seems to indicate that a change of this kind is being continually effected in the nutritious fluids of Animals, during their circulation through the body : by Cells, which are either carried about with them, or which are developed for the purpose in particular situations, as in plants. The former is the more common occurrence ; since the conditions of animal life, usually involving a general movement of the body, require also a general reparation of its parts, and an adaptation of the circulating fluid there- fore to the wants of the whole fabric. 579. It has been already shown, that Cells, which seem identical with the white corpuscles of the Blood, are to be met with in the Chyle and Lymph, fluids in which the elaboration of plastic fibrin is going on (564) ; and that such an elaboration must be continually taking place in the blood itself, to supply the plastic material which is being as continually drawn off by the nutritive processes. Hence there would seem reason for attributing this im- portant function to these floating cells ; the number of which present in the fluids seems to bear a very close relation with the energy of this elaborating process. It is a fact of great physiological interest and importance, that, whilst the colourless corpuscles are to be met with in the nutritious fluids of all ani- mals which possess a distinct -circulation, the red corpuscles are restricted to the blood of Vertebrata. This observation, which was first put forth by Wag- ner,t has been confirmed by the Author, who had been previously struck with the very close analogy between the floating cells carried along in the current of the circulation in some of the very transparent aquatic larvae (especially those of the Culicidae), and the lymph-corpuscles of the frog. Now it is evident from this fact, that, as the blood of Vertebrata is distinguished from their chyle chiefly by the presence of red corpuscles in the former and by their absence in the latter, the nutritious fluid of invertebrated animals is rather analogous (as Wagner has remarked) to the Chyle and Lymph, than to the blood of Vertebrata. Or, to put the same idea in another form, the presence of the colourless corpuscles in the nutritious fluid appears to be the most general fact in regard to its character * There are probably cases, however, in which cells are very rapidly called into exist- ence, without that preparatory elaboration of their nutrient materials, which we regard as due to the vital operations of a preceding generation. Thus the Bovista giganteum, a large fungus of the puff-ball tribe, has been known to increase, in a single fright, from a mere point to the size of a huge gourd, estimated to contain 47,000,000,000 cellules. In such a case it is difficult to suppose than any but the most rapid mode of generating cells can have been in operation; and the idea that these could not have been developed by any such elaborate process as that just alluded to, is borne out by the fact of their ex- tremely transitory character, the decay of such a structure being almost as rapid as its production. The same may be remarked of those fungous growths in the animal body, which sprout forth most rapidly. Hence the apparent exception assists in proving the rule. | Physiology, by Willis, Part ii. PROPERTIES OF THE BLOOD. 437 throughout the whole animal scale ; whilst the presence of red corpuscles in that fluid is limited to the vertebrated classes. Hence it would not be wrong to infer, that the function of the colourless corpuscles must be of a general character, and intimately connected with the nutritious properties of the circu- lating- fluid ; whilst the function of the red corpuscles must be of a limited character, being only required in one division of the animal kingdom. Fur- ther, it has been noticed by Mr. Gulliver, that in the very young embryo of the Mammalia, the white globules are nearly as numerous as the red particles : this, Mr. Gulliver has frequently noticed in foetal deer of about 1 inches long. In a still smaller foetus, the blood was pale from the preponderance of the white corpuscles. It is, therefore, a fact of much interest that, even in the Mam- miferous embryo, at the period when growth is most rapid, the circulating fluid has a strong analogy to that of the Invertebrata. It then, too, bears in other respects the most striking analogy to chyle ; since it consists of the fluid elaborated from the organizable matter supplied by the parent, and directly introduced into the current of the circulation. The function of the placental vessels may be regarded as double : for they are at the same time the channel through which the alimentary materials supplied by the parent are introduced into the circulating system of the foetus, and the medium of aerating the fluid which has traversed the foetal system. Hence the placenta may be regarded as at once the digestive and the respiratory apparatus of the foetus ; and the fluid circulating through the cord, as at once chyle and blood. It is not until the pulmonary and lacteal vessels of the embryo have commenced their inde- pendent operation, that the distinction between the blood and the chyle of the foetus becomes evident ; and we should expect, therefore, to find that the circu- lating fluid, up to the time of birth, contains a large proportion of white cor- puscles, which is actually the case. There is a gradual decrease, however, in their proportional number, from the earlier to the later stages of embryonic life, in accordance with the diminishing energy of the formative processes. It has been also observed by Wagner,* that the number of colourless corpuscles is always remarkably great in the blood of well-fed frogs just caught in the summer season ; and that it is very small in those that had been kept long without food, and in those examined during the winter. The most remarka- ble evidence, however, of the connection between the generation of white cor- puscles in the blood and the production of fibrin, is derived from the pheno- mena of Inflammation. A decided increase in the normal proportion of fibrin in the blood (from 2 to 3 parts in 1000), may probably be looked upon as the essential indication of the existence of the inflammatory condition ( 591 a}. That this production of fibrin is due to a local change can scarcely be doubted, since it is frequently observed to commence before any constitutional symp- toms manifest themselves ; and it may be regarded, in fact, as one cause of these symptoms. Now the recent microscopic observations of Mr. Addisont and Dr. Williams,;}; which were made independently of each other, have established the important fact, that a great accumulation of white corpuscles takes place in the vessels of an inflamed part: this seems to be caused at first by a determination of those already existing in the circulating fluid towards the affected spot ; but partly by an actual increase or generation of these bodies, which appear to have the power of very rapidly multiplying themselves. The accumulation of white corpuscles may be easily seen, by applying irri- tants to the web of a frog's foot. Mr. Addison has noticed it, in the human * Op. cit. p. 245. f Medical Gazette, Dec., 1840; Jan. and March, 1841. t Medical Gazette, July, 1841; and Principles of Medicine, [Amer. ed. by Dr. Clymer, pp. 214, 215.] 438 OF NUTRITION. subject, in blood drawn by the prick of a needle from an inflamed pimple, the base of a boil, the skin in scarlatina, &c. And the Author, without any know- ledge of these observations, had remarked a very obvious difference between the proportions of white corpuscles, in blood drawn from a wound in the skin of a frog immediately upon the incision being made, and in that drawn a few minutes after ; and had been led, like the observers just quoted, to refer this difference to a determination of white corpuscles to a part irritated. The ab- solute increase, sometimes to a very considerable amount, in the quantity of white corpuscles in the blood of an inflamed subject, has been verified by Mr. Gulliver and several other observers. These facts, therefore, afford strong ground for the belief, that the production of fibrin in the blood is closely con- nected with the development of the white corpuscles ; and when we consider them in connection with the facts previously urged, there scarcely appears to be a reasonable doubt, that the elaboration of fibrin is a consequence of this form of cell-life, and is, in fact, its express object. This view derives further confirmation from the following recent experiment of Mr. Addison's.* "Pro- vide six or eight slips of glass, such as are usually employed for mounting microscopical objects : and as many smaller pieces. Having drawn blood from a person with rheumatic fever, or any other inflammatory disease, place a drop of the colourless liquor sanguinis, before it fibrillates, on each of the large slips of glass ; cover one immediately with one of the smaller slips, and the others one after another at intervals of thirty or forty seconds : then, on examining them by the microscope, the first will exhibit colourless blood corpuscles in various conditions, and numerous white molecules distributed through a more or less copious fibrous net-work ; and the last will be a tough, coherent, and very elastic membrane, which cannot be broken to pieces nor resolved into smaller fragments, however roughly or strongly the two pieces of glass be made to rub against each other. This is a * glaring instance' of a compact, tough, elastic, colourless, and fibrous tissue, forming from the colourless elements of the blood; and the several stages of its formation may be actually seen and determined. Numerous corpuscles may be observed, in all these preparations, to have resolved themselves, or to have fallen down into a number of minute molecules, which are spread out over a somewhat larger area than that occu- pied by the entire corpuscles ; and although still retaining a more or less per- fectly circular outline, yet refracting the light at their edges, in a manner very different from that in which the corpuscles themselves are seen to do. It is from these and various other larger and more irregular masses of molecules or * Transactions of the Provincial Medical Association, 1843. j- A different view of the cause of the production of fibrin, however, has been enter- tained by some eminent physiologists; and it does not seem right to allow the opinions of Wagner, Henle, and Wharton Jones, to pass without notice, even though they appear to the Author to be easily set aside. By these observers, the elaboration of fibrin has been attributed to the red corpuscles, and' has been regarded as one, at least, of their special functions. Nearly all the arguments, however, which have led us to assign this duty to the white corpuscles, tell equally against the doctrine now under consideration. The pre- sence of fibrin in the circulating fluid may be regarded as a universal fact; but the red corpuscles are restricted to vertebrated animals: how, then, is the plastic element elabo- rated in the invertebrata! The number of the red corpuscles in the blood of different classes bears an obvious relation to their amount of respiratory power, and to the func- tional activity of the several organs, which is closely connected with the amount of oxygen introduced into the system; but it does not bear the same relation with the activity of the formative processes, which may be taking place energetically, (as in the development of the embryo, or in the reparation of parts in the adult), in a state of functional quiescence. The pathological evidence that the red corpuscles are not the elaborators of the fibrin, appears to the Author to be quite conclusive. Whilst the quantity of fibrin is so remark- ably increased in inflammation, the number of red corpuscles' undergoes no decided PROPERTIES OF THE BLOOD. 439 disintegrated corpuscles, that the fibrinous filaments shoot out on all sides, as from so many centres ; or frequently the filaments are more copious in two opposite directions."! 580. Besides the red corpuscles and the colourless globules, it is stated by Mr. Gulliver,* that the blood obtained from Mammals after death not unfre- quently contains an abundance of white matter, generally presenting the form of spherules, having a diameter of from l-4000th to 1-1 750th of an inch. They frequently seem to have a semi-fluid consistency, especially in the blood of the rnesenteric veins, in which the white matter is found most abundantly. The appearance of these globules (which appear to be the same with the fibrinous globules of Mandl) is probably to be attributed to Pathological changes, and especially to the existence of Tubercular disease ; a large proportion of the granular matter of caseous tubercles consists of similar corpuscles. The milky serum, which sometimes occurs when patients are bled not long after a meal, and which is seen not unfrequently in the blood drawn from young ani- mals, is usually found, by the highest powers of the microscope, to present the granular base, which has been already described in the account of chyle ( 563]. The appearance seems due, however, in some instances, to the diffu- sion 01 oily matter, in a less finely divided state than usual, through the fluid ; in some of these cases, a very large amount of fat has been shown, by chemical analysis, to exist in it.t The only visible constituents of ordinary blood which remain to be noticed, are the corpuscles of the Spleen ( 708) and of the Supra- Renal capsules ( 711) which may be frequently observed in the Splenic and Supra Renal veins respectively. According to Schultz, the red corpuscles of the Portal vein differ from those of the rest of the vascular system, in their tendency to decay; their colouring matter is soluble in the liquor sanguinis, which is not the case elsewhere ; and they seem to have lost a portion of their contractility. 581. Having now separately considered the chief Organic Elements which enter into the composition of the Blood, we are prepared to inquire into the characters of this fluid as a whole. The analysis of M. LecanuJ is usually regarded as the most complete and satisfactory. The following is his account of the composition of the fluid, obtained from two stout and healthy Men. Water 780-145 785-590 Fibrin 2-100 3-565 Albumen 65-090 69-415 Colouring matter (globules) .... 133-000 119-626 Fatty crystallizable matter .... 2-430 4-300 change. Again, the augmentation of the fibrin is not incompatible with a chlorotic state of the blood; the peculiar characteristic of which is a great diminution in the proportion of red corpuscles. By such alterations, the normal proportions between the fibrin and the red corpuscles, which may be stated as A: B, may be so much altered, as to become, in inflammation, 3 A : B, in chlorisis, A : B. Again, in fever, the characteristic alteration in the condition of the blood appears to be an increase in the amount of red corpuscles, with a diminution in the quantity of fibrin ; yet if a local inflammation should establish itself during the course of the fever, the proportion of fibrin will rise; and this without any change in the amount of corpuscles. Lastly, the effect of loss of blood has been shown by Andral's investigations to be a marked diminution in the number of red cor- puscles, with no decided reduction in the quantity of fibrin, even when this is much above its normal standard; and in this condition of the blood, it has been observed by Remak that the colourless corpuscles are very numerous. * Gerber's Anatomy, Appendix, p. 21. f For some very interesting observations lately made on this subject, see 711, note. i Journal de Pharmacie, Nos. ix. and x., 1831. 440 OF NUTRITION. Oily matter ...... Extractive matter soluble in water and aleohol Albumen combined with soda Chloride of sodium potassium Carbonates Phosphates I. of potash and soda Sulphates J Carbonates of lime and magnesia Phosphates of lime, magnesia, and Peroxide of iron Loss ..... iron L 1-310 1-790 1-265 8-370 2-100 2-400 2-270 1-920 2-010 7-304 1-414 2-586 100-000 100-000 Hence it is seen that the proportion of the elements of Blood is subject to con- siderable variation within the limits of ordinary Health ; we shall hereafter find that, in Disease, these variations are far more decided, and that they have a constant and evident connection with the morbid condition of the system. The amount of solid matter in the Blood appears to be in general greater in the Male than in the Female. The following table exhibits the general results of the inquiries of Denis on this head: it represents the Maximum and Mini- mum of each of the chief constituents of the Blood ; and it will be seen that, whilst the Water predominates in the Female, the other elements (with a slight exception in favour of the albumen) are in largest amount in the Male. Water Albumen Globules . Fibrin . MALE. FEMALE. Maximum. Minimum. Maximum. Minimum. 805 63 186 4 732 48-5 110-5 2 848 68 167 3-1 753 50 71-4 2 The Specific Gravity of course varies with the amount of solid matter contained in it ; in the Human subject, the average is probably about 1050. The esti- mate formerly given ( 490) of the proportional amount of Blood in the body, has recently been confirmed by a very ingenious experiment of Valentin's, in which the calculation was based on Specific Gravity. A certain quantity of blood was taken away from the jugular vein of an animal of known weight ; and a known measure of blood-warm distilled water was slowly injected, by the orifice of the vein, towards the heart. Some minutes afterwards, another portion of the blood was withdrawn, and carefully weighed. The two quan- tities were then evaporated in dry air, until the residue no longer lost weight ; and the whole amount of the previously-contained mass of blood could then be calculated, by a simple mathematical formula, from the degree of attenuation in the fluid, produced by the injected water. This was found to be between one-third and one-fourth, or about two-sevenths, of the whole weight of the body. 582. When the Blood is drawn from the body, and left to itself, its organic elements speedily undergo a new arrangement. The Fibrin coagulates, and PROPERTIES OF THE BLOOD. 441 separates itself from the fluid in which it was previously dissolved ; and during its coagulation it attracts the red particles ; these are included in areoke or meshes of the Clot, the substance of which has a tendency to assume a fibrous arrangement ( 554) ; and they usually group themselves together in columnar masses, resembling piles of money (Fig. 100, B). The Coagulum or clot be- comes dense in proportion to the amount of Fibrin it contains ; and the Albu- minous and Saline matter still dissolved in the water are separated from it, constituting what is called the Serum. This separation will not occur, how- ever, if the coagulation takes place in a shallow vessel ; nor if the amount of Fibrin should be small, or its vitality low. A homogeneous mass, deficient in firmness, presents itself under such circumstances ; though the solid part of this may pass into a state of more complete condensation, after the lapse of a certain time. That the coagulation is due to the Fibrin, and that the Red particles are merely passive in the process, appears from several considera- tions. A microscopical examination of the Clot shows that it has the same texture with Fibrin when coagulating by itself; the Corpuscles clustering together in the interspaces of the network, and not being uniformly diffused through the whole mass. Their Specific Gravity being greater than that of the Fibrin, they are usually most abundant at the lower part of the clot ; and the upper surface is sometimes nearly colourless, especially when the coagu- lation has taken place slowly ; yet this upper part is much firmer than the under, showing that the Fibrin alone is the consolidating agent. This has been proved to demonstration by an experiment of. Muller's. He placed the blood of a Frog, diluted with water (or still better with a very thin syrup) on a paper filter, of sufficiently fine texture to keep back the Corpuscles ; and the Liquor Sanguinis, having passed through the filter completely unmixed with them, presented a distinct coagulum, although from the diluted state of the fluid, this did not possess much consistency. Owing to the more minute size of the Blood-discs of warm-blooded animals, this experiment cannot be performed with their blood ; but there is no reason to believe its constitution to be different. In fact, the sole agency of the Fibrin in coagulation is very easily proved in another way. If fresh-drawn blood be continually stirred with a stick, the Fibrin will adhere to it in strings during its coagulation ; and the red particles will be left suspended in the serum, without the slightest tendency to coagulate. Moreover, if a solution of any salt, that has the pro- perty of retarding the coagulation (such as carbonate of potash or sulphate of soda), be added to the blood, the Corpuscles will have time to sink to the lower stratum of the fluid, before the clot is formed ; the greater part of the Coagulum is then entirely colourless, and is found by the microscope to con- tain few or no red particles.* 583. That the coagulation of the Fibrin is not, as some have supposed, a proof of the death of the blood, but is rather an act of vitality, appears evident from what has been already stated ( 554) of the incipient organization which may be detected even in an ordinary clot ; and still more from the fact that, if the effusion of Fibrin takes place upon a living surface, its coagulation is the *[M. Figuierj- has suggested an easy method for the rough analysis of the blood. By adding to one volume of defibrinated blood, two volumes of a solution of sulphate of soda, of sp. gr. making 16 to 18 in Baume's areometer, the corpuscles will separate (as Berzelius showed), and may, with hardly an exception, be all collected on a filter. Thus their quantity may be estimated, as that of the fibrine may very roughly by what is obtained by whipping. The quantity of albumen may be estimated by boiling the serum ; and the water, by evaporating a separate portion of blood. M. C.] f Report of the Academie des Sciences du 8, Juillet 1844; and, in full, in the Ann. de Chimie et de Physique, Aout, 1844. 442 OF NUTRITION. first act of its conversion into solid tissues possessing a high degree of vitality. It is absurd to suppose that the blood dies, in order to assume a higher form. When withdrawn from the body, however, the Coagulation of the Blood is the last act of its life ; for, if not within the influence of a living surface, it soon passes into decomposition. Instances occasionally present themselves in which the Blood does not coagulate after death ; and in most of these there has been some sudden and violent shock to the Nervous system, which has destroyed the vitality of solids and fluids alike. This is generally the case in men and animals killed by lightning, or by strong electric shocks ; and in those poisoned by prussic acid, or whose life has been destroyed by a blow on the epigastrium. It has also been observed in some instances of rupture of the heart, or of a large aneurism near it ; and a very interesting phenomenon then not unfre- quently presents itself, the coagulation of the Blood which has been effused into the pericardium (the effusion having taken place during the last moments of life), whilst that in the vessels has remained fluid. In several of the in- stances in which the blood has been found uncoagulated in the vessels, many hours after death, a portion withdrawn from the body has clotted ; this would seem to indicate that there is some absolutely depressing influence exercised by the surrounding tissues under such circumstances, an influence of which manifest evidence is afforded, by the sudden destruction of the muscular con- tractility, the arrestment of the capillary circulation, and other phenomena of like nature. It appears, however, that simple arrestment of Nervous influence favours the coagulation of the blood in the vessels ; clots being found in their trunks within a few minutes after the Brain and Spinal marrow have been broken down. 584. The length of time which elapses before Coagulation, and the degree in which the Clot solidifies, vary considerably ; in general, they are in the inverse proportion to each other. Thus, if a large quantity of blood be with- drawn from the vessels of an animal at the same time, or within short inter- vals, the portions that last flow coagulate much more rapidly, but much less firmly, than those first obtained. In Blood drawn during Inflammatory states, again, the coagulation is usually slow, but the clot is preternaturally firm; especially at its upper part, where the Buffy coat ( 588) or colourless stratum of Fibrin, gradually contracts, and produces the cup, which is usually regarded as indicative of a high degree of Inflammation. Except, under the peculiar circumstances just stated, the Blood withdrawn from the body always coag- ulates;* whether it be kept at rest or in motion; whether its temperature be high or low ; and whether it be excluded from the air, or be admitted to free contact with the atmosphere.! The Coagulation may be accelerated or retarded, however, by variation in these conditions. Thus, if the Blood be continually * Some diseases may, perhaps, be an exception; non-coagulation of the Blood is said to be characteristic of the Scurvy, but this is erroneous. In very severe forms of Typhus, the same has been stated to occur. f f[An inexplicable case, in which the complete coagulation of the blood did not take place till fifteen days after its abstraction, has been published by Dr. Pollit, who adduces this case in proof that there is no blood which becomes putrid before it has coagulated. The patient was a man 37 years old, with acute pneumonia. The blood of a first bleeding was drawn into a block-tin vessel, and set in a temperature between 46 and 52. It remained liquid for eight days, the blood-corpuscles having settled to the bottom, with the liquor san- guinis floating above them, exactly like serum pressed from a clot. On the ninth day, a membranous pellicle began to form on the surface of the fluid, and this becoming thicker, and increasing in tenacity and consistence, acquired at last- all the characters of the most genuine bufify coat. The serum began to ooze from the clot on the fifteenth day : and on Gazzetia Medica di Milano,Gennaio 20, 1844. PROPERTIES OF THE BLOOD. 443 gitated in a bottle, its coagulation is delayed, though it will at last take place in shreds or insulated portions ; but that rest is not the cause of its coagula- tion (as some have supposed), is proved by the fact that, if a portion of blood be included between two ligatures in a living vessel, it will remain fluid for a long time. Again, the Coagulation is accelerated by moderate heat, and jetarded by cold ; but it is not prevented by even extreme cold ; for, if blood be frozen immediately that it is drawn, it will coagulate on being thawed. Moreover it is accelerated by exposure to air, but it is not prevented by com- plete exclusion from it, as is proved by its taking place in a vacuum, or in a shut sac within the dead body: complete exclusion from the air, however, retards the change ; as has been shown by causing Blood to flow into a vessel containing oil, which will form an impervious coating on its surface, and will occasion the coagulation to take place so slowly, that the Red particles have time to subside, and the upper stratum of the clot is colourless.* An extrica- tion of Carbonic acid usually takes place to a slight degree during coagula- tion ; but this is not a constant occurrence ; and the process is not prevented, even by agitating Carbonic acid with the Blood. 585. The proportions of Serum and Clot which present themselves after coagulation are liable to great variation, independently of the amount of the several ingredients characteristic of each ; for the Coagulum may include not only the Fibrin and Red particles, but also a large proportion of the Serum, entangled as it were in its substance. This is particularly the case when the coagulation is rapid ; and the clot then expels little or none of it by subsequent contraction. On the other hand, if the coagulation be slow, the particles of Fibrin seem to become more completely aggregated, the coagulum is denser at first, and its density is greatly increased by subsequent contraction. When a firm fresh clot is removed from the fluid in which it is immersed, its concre- tion is found to continue fqr 24 or even 48 hours, serum being squeezed out in drops upon its surface ; and in order, therefore, to form a proper estimate of the relative proportions of Crassamentum and Serum, the former should be cut into slices, and laid upon bibulous paper, that the latter may be pressed from it as completely as possible. According to the experiments of Mr. Thackrah, Coagulation takes place sooner in metallic vessels than in those of glass or earthenware, and the quantity of Serum separated is much less ; in one instance the proportion of Serum to Clot was as 10 to 24, when the blood coagulated in a glass vessel; whilst a portion of the same Blood, coagulating in a pewter vessel, gave only 10 of Serum to 175 of Clot. The Specific Gravity of Blood is no measure of its coagulating power; for a high specific gravity may be due to an excess in the amount of globules, which form the heaviest part of the blood; and may be accompanied by a diminution in the quantity of fibrin, which is the coagulating element. 586. The Serum, when completely separated from the Crassamentum, may be said to contain all the Albumen and Saline matter of the Blood ; together with a portion of the Fatty matter (of which some adheres to the Fibrin), and those " ill defined animal principles" which are included under the designa- the clot being now taken out of the vessel, it was found that the upper four-fifths of it consisted of buffy coat, and only the lower fifth clot coloured with corpuscles. The serum which continued to be expressed was perfectly transparent, and the blood did not show signs of putrefaction till a month after it had been drawn from the body. A small quantity drawn from another pneumonic patient, and placed under the same circum- stances, was completely coagulated in two hours, and was quite putrid in fifteen days. In fifteen bleedings of the same patient in the following eight days, the blood drawn gradually lost its indisposition to coagulate, the whole process being completed each time in twelve hours or less. The patient recovered. M. C.] * Babington in Medico-Chirurgical Transactions, vol. xvi. 444 OF NUTRITION. tion " extractive matter." Its Specific Gravity may be stated at about 1030 in health; and it contains about 9^ per cent, of solid matter. When it is heated to 160, its Albumen coagulates and the remaining fluid may be sepa- rated by pressure. This fluid still contains some Albumen, which is held in solution by free alkali ; for if the latter be neutralized by Acetic acid, a further precipitation takes place on the application of heat. The fatty matter, which may be separated by ether, seems to be nearly allied to the several fatty sub- stances formed in the body ; for some chemists have determined it to consist of Oleine, Margarine, and Stearine, the constituents of ordinary fat ; whilst others regard it as analogous to Cerebrine, the fatty matter of the brain ; and others, again, consider it to bear a closer resemblance to Cholesterine, the fatty matter of the bile. The Extractive, matter partly consists of Lactic acid, partly of a substance called Osmazome ; it is believed by Berzelius that this portion of the blood contains the resultants of the acts of decomposition continually taking place in the body ; and that it is chiefly, therefore, from this that the excre- tions are formed. 587. It cannot be doubted that, upon the due admixture in the Blood of all these elements, the regular performance of its actions is dependent. In regard to its physical properties merely, it is easily shown that a slight alteration may produce the most injurious consequences ; for a certain degree of viscidity has been found (by the experiments of Poisseuille) to favour the passage of fluid through capillary tubes; and thus, if the viscidity of the blood be diminished by a loss of part of its Fibrin, stagnation of the current, and extravasation of a portion of the contents of the vessels, will be the result. This has been fully proved by the numerous experiments of Magendie ; and the fact is one of very important Pathological applications ( 592 6). In regard to the effect of alterations in the amount of the Red Particles, our information is less satis- factory, since it is almost impossible to deprive Blood of these, without at the same time defibrinizing it. It appears, however, from the experiments of Dieffenbach on transfusion, that they are more effectual as stimuli to the Heart's action, than is any other constituent of the blood ; and, if the hypo- thetical account of their use already offered ( 576) have any correctness, they must be important agents in the maintenance of the Capillary Circulation also. The rapidity with which they may be decomposed and reconstituted, is made remarkably evident by the experiments of Magendie, who found that, when the Blood of one animal was injected into the veins of another having discs of very different size and form (care being taken to prevent the coagulation of the Fibrin during the operation), the original Red particles soon disappeared, and were replaced by those characteristic of the species, in whose veins the fluid was circulating. The Albumen of the Blood seems to be chiefly of im- portance as the material from which Fibrin is elaborated. The continual drain upon the Fibrin, which is taking place in the capillaries of the system, is made evident by the remarkable fact, that Arterial blood contains a much larger proportion of it than Venous, the excess being usually about one-fifth. 588. The Crassamentum not unfrequently exhibits, in certain disordered conditions of the Blood, a layer of Fibrin nearly free from colour; and this is known as the Buffy coat. The presence of this has been frequently regarded as a sign of the existence of Inflammation, occasioning an undue predominance of Fibrin ; but this idea is far from being correct, since, as will presently appear, (589), it may result from a very opposite condition of the Blood. A similar colourless layer of Fibrin is always observable, when the Coagulation of the blood is retarded by the addition of agents that have the power of delay- ing it ( 582) ; and since, in Inflammatory states of the system, the blood is generally long in coagulating, it has been supposed that the separation of the PROPERTIES OF THE BLOOD. 445 red particles is due to this cause alone. Dr. Alison,* however, maintains that there must be an absolute tendency to separation between the two compo- nents of the clot, in order to account for the phenomena sometimes presented by it; and he adduces the two following reasons in support of this view. " 1, The formation of the Buffy coat, though no doubt favoured or rendered more complete by slow coagulation, is often observed in cases where the coagulation is more rapid than usual; and the colouring matter is usually observed to retire from the surface of the fluid in such cases, before any coagu- lation has commenced. 2. The separation of the Fibrin from the colouring matter in such cases takes place in films of blood so thin as not to admit of a stratum of the one being laid above the other ; they separate from each other laterally, and the films acquire a speckled or mottled appearance, equally characteristic of the state of the blood with the buffy coat itself." It appears from the observations of Mr. Wharton Jones, that the red corpuscles of Inflam- matory Blood have an unusual attraction for each other, which occasions their coalescence in piles and masses ; so that by this character, the state of the Blood may be detected, from the examination of no more than a single drop of the fluid. Now if we consider, in connection with this increase in the mutual attraction of the Blood discs, the increase in the mutual attraction of the par- ticles of Fibrin (which causes the coagulation of Inflammatory blood to be so much firmer and more decidedly fibrous than that of the healthy fluid), we have a cause sufficient to explain the phenomena noticed by Dr. Alison ; without the necessity of resorting to the idea of an absolute repulsion being present between the two constituents. It is in the Buffy Coat of Inflammatory Blood, that we see the clearest indications of organization ever presented by the circulating fluid. The fibrous network is frequently extremely distinct; and it commonly includes a large number of white corpuscles in its meshes. 589. When the Buff arises from other causes, however, its appearance is less characteristic. It appears from the researches of Andral, that the essen- tial condition of its production is an increase in the quantity of Fibrin in proportion to the Red Corpuscles; and not a simple increase of Fibrin. When the Blood contains an excessive quantity of Fibrin, it coagulates slowly; thus the blood of a patient labouring under Rheumatism coagulates more slowly than that of one affected with Typhoid fever. The increase may occur in two ways ; either by an absolute increase in the Fibrin, the amount of the corpuscles remaining unchanged, or not being augmented in the same proportion; or \>y a diminution of the Corpuscles, the quantity of Fibrin remaining the same, or not diminishing in the same proportion. Hence in severe Chlorosis, in which the latter condition is strongly developed, the buffy coat may be as well marked as in the severest Inflammation. Unless the composition of the blood be altered in one of these two ways, it is stated by Andral that the buffy coat is never formed ; the influence of circumstances which favour it not being sufficient to produce it when acting alone. The absence of these circumstances may prevent it, however, when it would otherwise have been formed ; thus, when the Blood flows slowly, the buff is not properly produced ; because the slow discharge gives one portion time to coagulate before another ; and only the blood last drawn furnishes the Fibrin at the upper part of the vessel. Again, in a deep narrow vessel, the buff will form much more decidedly than in a broad shallow one ; because the thickness of the Fibrinous crust will be greater. If the blood be agitated during its co- agulation, the Corpuscles are mixed up with the Fibrin, and the crust is imperfect and soft. The process of the formation of the Buffy coat may be best studied by treating ordinary Blood with some of those agents which re- * Outlines of Physiology, 3d edition, p. 89. 38 446 OF NUTRITION. tard its coagulation. Of these, the Sulphate of Soda is stated by Andral to be the best, producing no alteration in the character of the elements, but simply delaying their change of state ; and the following is his account of the appear- ances observed. After a few minutes the blood separates into two parts ; the lower one contains the globules collected together into a soft mass ; the upper one is at first transparent and resembles serum, but soon becomes opaque. At this period a number of globular white corpuscles may be seen in it with the microscope; and these form the first degree of solidification of the fibrin. After 48 hours, the fluid contains numerous flocci like spiders'-webs ; which flocci are chiefly composed of the Fibrin set free from the white corpuscles : this is the second stage of solidification. After 96 hours, the fluid recovers its transparency and contains no trace of separate corpuscles ; but the flocci are more numerous, more firm, and constitute an irregular web composed of fibres arranged in various directions. Where the web is thickest, several reticulated layers may be seen, the one placed over the other ; and in the midst of the distinct fibres, several strings of corpuscles, still retaining their globular character, are perceived. This state of organization is intermediate between that, in which the Corpuscles are altogether separate, and that in which a firm coagulation of the Fibrin takes place ; and it may be seen to be that, through which the buffy coat passes in the progress of its formation V. Pathological Changes in the Blood. 590. From the part which the Blood performs in the ordinary processes of Nutrition, it cannot be doubted that it undergoes important alterations, when these processes take place in an abnormal manner. These alterations must be sometimes the causes, and sometimes the effects, of the morbid phenomena, which constitute what we term the Disease. Thus, when some local cause, affecting the solid tissues of a certain part of the body, produces Inflammation in them, their normal relation to the blood is altered ; the consequence is, that the blood, in passing through them, undergoes a different set of changes from those, for which it is originally adapted ; and thus its own character undergoes a change, which soon becomes evident throughout the whole mass of the cir- culating fluid, and is, in its turn, the cause of morbid phenomena in remote parts of the system. On the other hand, the strong analogy between many Constitutional diseases, and the effects of poisonous agents introduced into the blood, appears clearly to point to the inference, that these diseases are due to the action of some morbific matter, which has been directly introduced into the current of the circulating fluid, and which has affected both its physical and its vital properties.* Here, then, is a wide field for investigation, of which the surface can scarcely be said to be yet broken up, and which must yield an abundant harvest to those who shall cultivate it with intelligence and zeal. The only connected researches which have been yet published, on the changes which the blood undergoes in disease, are those of MM. Andral and Gavarret ;t and these are confined to the alterations which take place in the * This doctrine has been recently brought prominently for ward / in a Paper on Sym- metrical Diseases, read by Dr. William Budd before the Medico-Chirurgical Society, Dec. 16, 1841. The Author ingeniously proves, that the symmetry of many diseases (such as certain forms of cutaneous eruptions,' rheumatism, &c.) which do riot imme- diately depend upon external causes, necessarily involves the idea of the conveyance of the morbific agent in the circulating fluid; the palsy produced by lead is a very interest- ing example, in which the agent is known to be mingled with the blood, and to be depo- sited in the parts affected, which are generally, if not always, symmetrical. f An account of these inquiries will be found in the Provincial Medical and Surgical Journal for May, June, and July, 1841; in the Annales des Sciences Naturelles, Dec. 1840, and March 1841 ; and in the Ann. de Chimie, Tom. Ixxv. They have since been published in a separate form, under the title of "Essai d'Hematologie Pathologique." PATHOLOGICAL CHANGES IN THE BLOOD. 447 proportions of the Organic elements of the fluid. It is of course necessary to determine, in the first instance, what are the usual or normal proportions ; and the following is estimated by them, from numerous analyses, as the ordi- nary quantity* of each element in 1000 parts of healthy blood. Fibrin 3 Globules 127 Solid matter of serum 80 Water 790 The proportion of Fibrin may vary, within the limits of health, from 2 to 3, or even from 2 to 4: but these last extremes are rarely met with in perfect health ; and must be considered, when they occur, as exceptions dependent on idiosyncrasy. The extremes in the proportion of Globules, in the physio- logical condition of the system, are fixed at 110 and 140; the latter converging towards the condition of the blood characteristic of Plethora. 591. The inquiries which have been made by Andral, in regard to the alterations which these proportions undergo in various states of Disease, have already led to results of great interest and value ; and there can be no doubt that these will be greatly extended, when this simple analytical process shall have been more generally employed. As an instance of the erroneous conclu- sions into which we may be led, by merely attending to the size of the crassa- rnentum, it may be remarked that the existence of a large Clot does not by any means necessarily imply the presence of an increased amount of Fibrin ; since it may depend upon the retention within it of a large proportion of Serum, consequent upon the deficient contractile power of the clot, which results from a diminution in the proportion of Fibrin. When the clot is dense and contains but little Serum, it may be judged to contain a full proportion of Fibrin, even though it may itself be small. Before entering upon the con- sideration of the alterations in the Blood which are affected by particular mor- bid states, Andral notices two extraneous causes, usually operating in disease, which may affect the result. These are, abstinence from food, and loss of blood by venesection. It has been commonly stated, that they have a tendency to diminish the proportion of all the solid elements of the blood; but this is not the case ; for they seem especially to act upon the Globules, the quantity of Fibrin remaining nearly the same, unless the hemorrhage have been very * The analysis of the Blood with reference to the quantity of the chief proximate ele- ments which it may contain, is very easily 'accomplished; and as the determination of this is the point of most practical importance, the method adopted in the inquiries of MM. Andral and Gavarret will be here detailed. The blood is caused to flow into two different vessels; into one vessel, the first and last quarters of the blood are received; and into the other, the second and third quarters: in this manner, the similarity of the two quantities is secured as far as possible. The blood in one vessel is allowed to coag- ulate spontaneously; that in the other is beaten with a small rod, in order to separate the fibrin. When the first portion is completely coagulated, the serum is carefully separated from the crassamentum; and there are then dried and weighed, 1. The Fibrin obtained by the rod; 2. The whole Crassamentum; 3. The Serum. The weight of the sepa- rated fibrin gives us the quantity of it contained in the clot. The weight of the serum after complete desiccation, gives us the proportional quantity of solid matter contained in its water. The quantity of water driven off" from the clot in. drying, gives us the amount of serum it contained; from which may be estimated the proportion of the solid matter of the serum that the crassamentum included. Hence, by deducting from the weight of the whole dried clot, first, that of the fibrin separated by agitation, and then that of the solid elements of the serum, ascertained by calculation, we obtain the weight of the globules. In order to ascertain the whole amount of solid matter in the serum, that which was ascertained by calculation to exist in the fibrin, is added to that which was obtained from the separate serum. The proportion of organic and of inorganic matter in, this solid residuum, is ascertained by incinerating it in a crucible; by which the whole of the former will be driven off, and the latter will be left. 448 OF NUTRITION. severe, in which case the Fibrin and the solid matter of the Serum are also reduced in amount. The extreme variations of each ingredient noticed by Andral were, Fibrin, from 0-9 to 10-0 in every 1000 parts of Blood; the Globules, from 21 to 185; the solid parts of the Serum, from 57 to 104: and the Water from 725 to 915. The smallest proportion of Globules, and the largest amount of Water, were presented in a case of severe uterine hemor- rhage. a The most important fact substantiated by Andral is one that had been previously suspected, the invariable increase in the quantity of Fibrin during acute Inflammatory affections; the increase being strictly proportional to the intensity of the Inflammation, and to the degree of symptomatic Fever accompanying it. " The augmentation of the quantity of Fibrin is so certain a sign of Inflammation, that, if we find more than 5 parts of fibrin in 1000, in the course of any disease, we may positively affirm that some local inflammation exists." Several cases are mentioned, in which an increase to 7 or 7 parts took place, without any apparent cause; but in which it afterwards proved that severe local inflammation was present: and thus we are furnished with a pathognomonic sign of great importance. The average augmentation of Fibrin in inflammation may be estimated at 7; the minimum at 5; the maximum at 10. It does not appear that in. robust athletic persons, the proportion of Fibrin is greater than in those of feeble consti- tution ; in the latter it is the Globules that are deficient ; and it is rather from this dispro- portion, than from, an absolute excess of Fibrin, that their greater liability to Inflamma- tory affections arises. Diseases which commence at the same time as the Inf 1 ammation, or co-exist with it, do not prevent the characteristic increase of the Fibrin ; thus in Chlorotic females, the proportion rises to 6 or 7, under this influence. The augmenta- tion is observed at the very outset of the affection ; the quantity increases with its progress; and a decrease shows itself when the disease begins to abate.* When the disease presents alternations of increase and decline, these are marked by precisely cor- responding changes in the quantity of Fibrin. It appears that the rise of the quantity of Fibrin above the normal standard is not immediately checked by venesection ; this does not prove, however, that bleeding is useless ; but only that it cannot arrest instanter the tendency to the production of an increased quantity of Fibrin. It is a curious fact, that an augmentation is commonly observable during the advanced stage of Phthisis, in spite of the deterioration which the blood must then have undergone; this is probably depend- ent upon the development of local inflammation around the tubercular deposits. Some experiments performed by M. Andral on the blood of pregnant women seem to lead to the conclusion that, during the first six months, the Fibrin is below the normal standard ; and that it subsequently varies, usually undergoing an augmentation between the sixth and seventh, and the eighth and ninth months. There is also a diminution in the Globules ; and these circumstances combined favour the production of the buffy coat ( 589). b. It appears obvious, from what has been just stated, that the increase in the quantity of Fibrin is not dependent upon the febrile condition, which is secondary to the local inflammation, but upon the Inflammation itself. This conclusion is confirmed by the interesting fact that, in idiopathic Fever, the proportion of Fibrin is diminished, instead of undergoing an increase. This diminution was constantly observed by Andral in the premonitory stage of Continued Fever; in some instances the amount was no more than 1-6 parts in 1000. The proportion of Globules was found to have usually, but not con- stantly, undergone an increase; as had also thatttf the solid parts of the Serum. In ordinary Continued Fever, in which there was no evident complication from local disease, the quantity of Fibrin varied from 4-2 to 2-2; that of the Globules from 185-1 to 103 6 (excluding a case in which their amount was only 82-5, which was that of a Chlorotic female); that of the solid matter of the Serum, from 98-7 to 90-9; and that of the Water from 725-6 to 851-9. Hence the quantity of solid matter appears to be usually increased ; but the peculiar condition of the disease may probably be stated to be, an increase in the * By experiments on animals, M. Andral has ascertained that no circumstance of pre- vious debility or privation prevent this characteristic change. Having ascertained the amount of fibrin in the blood of three dogs to be 2-3, 2-2, and 1-6 (the natural range for these animals), he deprived them, completely or partially of food. On the fourteenth day, the proportion of fibrin had risen, in the first, to 4-5: and in the second, to 4: these animals had no food. In the third dog, which was supplied with a very small quantity of food daily, the same condition developed itself at a later period; the blood on the four- teenth day exhibiting only 1-8 parts of fibrin; but on the twenty-second day presenting 3-3 parts. In all these instances, the elevation in the proportion of fibrin was coincident with Inflammatory changes in the stomach. PATHOLOGICAL CHANGES IN THE BLOOD. 449 proportion of the Globules to the Fibrin. When, however, a local Inflammatory affection develops itself during the course of the Fever, the amount of Fibrin increases; but its augmentation seems to be kept down by the febrile condition. In Typhoid Fever,* the decrease in the proportion of Fibrin is much more decidedly marked; this dues not depend upon abstinence; for it ceases as soon as a favourable change occurs in the disease, long before the effect of food could show itself. In the various cases examined by Andral, the blood furnished a maximum of 37 of Fibrin, and a minimum of 0-9; in this last case, the Typhoid condition existed in extreme intensity, yet the patient recovered. The proportion of Globules varies considerably; in an early stage of the disease it is usually found to be absolutely high; and it always remains high relatively to the amount of Fibrin. In Typhoid Fever, then, the abnormal condition of the Blood, in regard to the disproportion between the Globules and the Fibrin, is more strongly marked than in ordinary Continued Fever: yet the usual augmentation of Fibrin will take place, if a local inflammation develops itself. In the Eruptive Fevers, it does not appear that the proportion between the Fibrin and the Globules undergoes so striking a change, as in Ordinary continued Fever; but the number of cases examined was too small to admit of decided conclusions. It was evident, however, that the specific Inflammations proper to, and characteristic of, these Fevers, have not the same effect in occasioning an increase of the Fibrin, as an intercurrent Inflammation of an extraneous character. By the expe- riments of Magendie it has been ascertained, that one of the effects of a diminution in the proportion of Fibrin is a tendency to the occurrence of Hemorrhage or of Conges- tion, either in the parenchymatous tissue, or on the surface of membranes: these con- ditions are well known to be of frequent occurrence, as complications of febrile disorders. A marked diminution of Fibrin was noticed also in many cases of the disorder termed Cerebral Congestion, which commences with headache, vertigo, and tendency to epis- taxis, and not unfrequently passes into coma and apoplexy. In Apoplexy, the diminution of Fibrin was still more striking; and in general, there was found to be an increase of the Globules. In one instance, the quantity of Fibrin on the second day of the attack was found to have fallen to 1-9, whilst that of the Globules had risen to 175-5; but on the third day, when the patient's consciousness began to return, the quantity of Fibrin was 3-5, whilst that of the Globules had fallen to 137-7. It would seem, from the great change in the character of the Blood, which was noticed in this and in other instances, that the want of due proportion between the Fibrin and the Globules was the cause, rather than the effect, of the Apoplectic attack. c. The amount of Globules seems to be subject to greater variation within the limits of ordinary health, than is that of Fibrin. In the condition which is ordinarily termed a highly sanguineous temperament, or Plethora, it is chiefly the former that undergoes an increase. Plethoric persons are not more liable to Inflammation, than are those of weaker constitution; but, from the quantity of Fibrin in their blood being small rela- tively to that of the Globules, they are liable to Congestion, especially of the brain, and to apoplexy or other Hemorrhage. The effect of Bleeding in diminishing this tendency is now intelligible; since we know that loss of blood reduces the number of Globules. On the other hand, in that temperament,f which, when exaggerated, becomes Anaemia, there is a marked diminution of the globules; this temperament may lead to two dif- ferent conditions of the system. In Chlorosis, the Globules are diminished, whilst the Fibrin remains the same; so that the clot, though small, is firm, and not unfrequently exhibits the buffy coat; in some extreme cases of this disease, the Globules have been found as low as 27. The influence of the remedial administration of Iron, in increasing the quantity of Globules, was rendered extremely perceptible by Andral's analyses; in one instance, after iron had been taken for a short time, the proportion of Globules was found to have risen from 49-7 to 64-3; whilst in another, in which it had been longer con- tinued, it had risen from 46-6 to 95-7. On the other hand, Bleeding reduced still lower the proportion of Globules; thus, in one instance, their amount was found, on a second bleeding, to have sunk from 62-8 to 49. The full proportion of Fibrin in the blood of Chlorotic patients accounts for the infrequency of Hemorrhage in them; whilst it also leads us to perceive that they may be, equally with others, the subjects of acute Inflam- mation, which we know to be the fact. A diminution of Globules may also coexist with a diminution in the amount, or in the degree of elaboration, of the Fibrin ; and this degree seems to be characteristic of Scrofula. Andral has noticed a diminution in the proportion of Globules in other Cachectic states, resulting from the influence of various depressing causes on the nutritive powers ; as in a case of Diabetes Mellitus, in which * M. Andral confines this term to the species characterized by ulceration of the mucous follicles of the intestinal canal. f The term lymphatic has been applied to this temperament; by which term was meant a predominance of lymph in the absorbent vessels. 38* 450 OF NUTRITION. the patient was much exhausted; a case of Aneurismal dilatation of the Heart inducing Dropsy; and in several cases of Cachexia Saturnina. d. The chief class of cases, in which any marked change has been observed in the amount of solid matter in the Serum, is that of Albuminuria, or Bright's disease of the Kidney. The diminished Specific Gravity of the Serum was long ago pointed out by Dr. Christison; but Andral remarks that this is not an accurate criterion. since, if there be a diminished amount of Globules (as is not unfrequenlly the case in this disease), the pro- portion of water in the whole will be increased, and the specific gravity of the serum thus lowered, without any alteration in its proper quantity of solid matter. According to Andral, the diminution in the amount of Albumen in the Serum is exactly proportional to the quantity contained in the urine. A case is related by him, under this head, which affords an interesting exemplification of the general facts, that have been already attained by his investigations. A woman who had been suffering from Erysipelas of the face, and who had lost blood both by venesection and by leeches, became the subject of Albu- minuria. The blood drawn at this time exhibited a considerable diminution in the pro- portion of Globules, as well as of Albumen, a fact which the previous loss of blood fully accounted for. After a short period, during which she had been allowed a fuller diet, another experimental bleeding exhibited an increase in the proportion of Globules. Some time afterwards, when the Albumen had disappeared from the Urine, some more blood was drawn ; and it was then observed that the Albumen of the Serum had returned to its due proportion, but that the Globules had again diminished, whilst there was a marked increase in the quantity of Fibrin. This alteration was fully accounted for by the fact, that, in the interval, several Lymphatic ganglia in the neck had been inflamed and had suppurated; and that the patent had been again placed on very low diet. "Thus," observes Andral, "we were enabled to give a complete explanation of the remarkable oscillations which were presented, in the proportion of the different elements of the blood drawn at three different times from the same individual; and thus it is that, the more extended are our inquiries, the more easy does it become to refer to general principles the causes of all those changes in the composition of the blood, which, from the frequency and rapidity with which they occur, seem at first sight to baffle all rules, and to take place, as it were, at random. In the midst of this apparent disorder, there is but the fulfilment of laws,- and in order to obtain these, it is only necessary to strip the phenomena of their complications, and reduce them to their simplest form." 592. That the Blood is subject to a great variety of other morbid alterations, which are sometimes the causes, and sometimes the results, of Disease, cannot be for a moment doubted. But our knowledge of the nature of these changes is as yet very insufficient. The great amount of attention which is being directed by Chemical Pathologists to the subject, however, will doubtless ere long produce some important results. Among the most frequent causes of depravation in the character of this fluid, we must undoubtedly rank the re- tention, in the Circulating current of matters which ought to be removed by the Excreting processes. We have already seen, that a total interruption to the excretion of Carbonic Acid by the lungs, will occasion death in the course of a very few minutes ( 546) ; and even when only a slight impediment is offered it, so that the quantity of Carbonic Acid always contained in arterial blood is augmented to but a small degree, a feeling of discomfort and oppres- sion, increasing with the duration of the interruption, is speedily produced. The results of the retention of the materials of the Biliary and Urinary excre- tions will be hereafter considered ( 661 and 670) ; and at present it will be only remarked that such retention is a most fertile source of slight disorders of the system, that it is largely concerned in producing many severe diseases, and that if complete^ it will most certainly and rapidly produce a fatal result. The most remarkable cases of depravation of the Blood, by the introduction of matters from without, are those in which these substances act as ferments, exciting such Chemical changes in the constitution of the fluid, that its whole character is speedily changed, and its vital properties are altogether destroyed. Of such an occurrence, we have characteristic examples in the severe forms of Typhoid fever, commonly termed malignant; in Plague, Glanders, Pustule Maligne, and several other diseases ; in some of which we can trace the direct introduction of the poison into the blood, whilst in others we must infer (from ORIGIN OF THE SOLID TISSUES. 451 the similarity of result) that it has been introduced through some obscure channel, probably. the lungs. The final symptoms which are common to all these diseases have been well described by Dr. Williams,* under the title of Necrsemia, or death by 'depravation of the blood. " Almost simultaneously, the heart loses its power, the pulse becomes very weak, frequent, and un- steady : the vessels lose their tone, especially the capillaries of the most vas- cular organs, and congestions occur to a great amount; the brain becoming inactive, and stupor ensues ; the medulla is torpid, and the powers of respira- tion and excretion are imperfect : voluntary motion is almost suspended ; se- cretions fail; molecular nutrition ceases; and at a rate much more early than in other modes of death, molecular death follows close on somatic death, that is, structures die and begin to run into decomposition as soon as the pulse and breath have ceased ; nay, a partial change of this kind may even precede the death of the whole body ; and parts running into gangrene, as in the carbuncle of plague, the sphacelous throat of malignant scarlatina, and the sloughy sores of the worst forms of typhus, or the putrid odour exhaled even before death by the bodies of those who are the victims of similar pestilential disease, are so many proofs of the early triumph of dead over vital chemistry." " The appearance of petechiae and vibices on the external surface, the occurrence of more extensive hemorrhage in internal parts, the general fluidity of the blood, and frequently its unusually dark or otherwise altered aspect, its poisonous properties as exhibited in its deleterious operation on other animals, and its proneness to pass into decomposition, point out the Blood as the first seat of disorder ; and by the failure of its natural properties and offices as the vivifier of all structure and function, it is plainly the medium by which death begins in the body." VI. Origin of the Solid Tissues Separative Processes. 593. It has been shown that the Blood contains a substance (Fibrin), which is prepared to become organized, or to take upon itself that peculiar kind of molecular arrangement, anatomically characterized as structure ; and which possesses what the physiologist terms vital properties ; but it has also been shown, that the conversion of this Fibrin into any higher form of organized structure than simple fibrous tissue, requires the influence of a previously-ex- isting organism. In the development of the Embryo, the germ of the first cell appears to be supplied by the male parent ; whilst the nutriment at the expense of which it is evolved is supplied by the female. In the subsequent growth of the organism, the materials are derived from the food ingested ; and the conversion of these into organized tissue depends upon the properties of the structure already formed, which, whilst itself decaying, liberates the germs of new cells, and thus makes preparation for its renewal. The pro- cesses by which these cells are converted into the several kinds' of organic structure, that compose the fabric of the higher animals, are in many instances very complex ; and can only be traced by an attentive examination of their several stages. Whether they are observed, however, in the first Develop- ment of the Embryo, or in the Reproduction of lost parts, they seem to be essentially the same. In fact, among the lowest tribes of Animals, we find these two conditions blended, as it were, together ; for the process of repara- tion may be carried in them to such an extent as to regenerate the whole organism from a very small portion of it. In the Hydra, or Fresh-water Polype, there would seem to be scarcely any limit to this power ; for, if the body of the animal be minced into the smallest possible fragments, every * Principles of Medicine, [Am. Ed. by Dr. Clymer, p. 373.] 452 OF NUTRITION. one of these can produce a new and perfect being. In this manner no less than forty have been artificially generated from a single individual. In ascending the Animal scale, we find this reparative power less conspicuous, because exercised with regard to smaller parts only of the body; but the greater complexity of the changes involved in the process, renders it in reality not less considerable than in the lower classes. Thus, the restoration of a bone destroyed by Necrosis is a much more extraordinary operation, than the growth of an entire Polype from a single fragment ; since it involves a far greater amount and variety of actions. Numerous and well-authenticated instances are on record of the reunion of parts that had been entirely separated from the body, and of the restoration of all their vital properties ; and this could only take place through the perfect reproduction of a large number of very different structures. The reappearance of Fungous growths, whose organization is of a low character, is a fact with which every surgeon is fami- liar; and cases occasionally, though rarely, present themselves, in which re- production of a whole member takes' place even in the Human subject.* 594. Before proceeding to describe in detail the mode in which the primor- dial cells are converted into the several varieties of tissue, it may be desirable to take a general survey of the conditions under which the reparative processes are carried on, a question of great practical importance, on which very mis- taken notions are prevalent. It is a general opinion among British surgeons, (founded upon what they believe, but erroneously, to have been the doctrine of Hunter), that inflammation is essential to the process of reparation. There is no doubt that, as generally conducted, the healing of wounds is attended by a greater or less degree of inflammation ; but it does not thence follow that this morbid condition is essential to the renewal of the healthy state ; and in fact it can be shown that, in the majority of cases, the inflammation is injurious rather than beneficial. The following important conclusions are drawn by Dr. Ma- cartneyt from a very philosophical comparative survey of the operations of reparation and inflammation, as performed in the different classes of animals : "That the powers of reparation and reproduction are in proportion to the indisposition or incapacity for inflammation ; that inflammation is so far from being necessary to the reparation of parts, that, in proportion as it exists, the latter is impeded, retarded, or prevented ; that, when inflammation does not exist, the reparative power is equal to the original tendency to produce and maintain organic form and structure ; and that it then becomes a natural func- tion, like the growth of the individual, or the reproduction of the species." 595. Guided chiefly by Dr. Macartney's views, which have derived impor- tant confirmation from recent observations, we shall treat of the reparative pro- cesses under three distinct heads : First, the adhesion of the sides of a wound by a medium of coagulable lymph, or of a clot of blood. Second, reparation without any medium of lymph or granulations, the cavity of the wound being filled by a natural process o/ growth from its walls. Third, reparation by means of a new, vascular, and organized substance, termed granulations. The first of these modes of reparation, is that which is ordinarily termed union by the first intention; of this kind of adhesion, the 'healing of the incision made in venesection, which usually takes place almost without consciousness on the part of the patient, and with scarcely any inflammation, is a characteristic ex- ample : the white line of cicatrix which is left, marks the formation of new substance, and is the result of the want of that perfect approximation of the lips of the wound, which may frequently be obtained in parts where pressure * See, on the whole of the subject of the comparative powers of Reparation in the Animal series, the Author's Principles of Gen. and Com. Physiol. 586, 587. j- Treatise on Inflammation, p. 7. ORIGIN OF THE SOLID TISSUES. 453 can be more firmly applied, and where the space to be filled up is proportion- ably thinner. This mode of union is ordinarily considered by British surgeons to be the result of an adhesive inflammation. In so regarding it, they con- ceive that they are following out the views of Hunter ; but he expressly states that wounds may heal without any pain or constitutional disturbance, the re- union proceeding "as if nothing had happened ;" so that he in effect admits that reparation of this kind may take place without inflammation. It is well known that if a slight wound which is thus healing be provoked to an increased degree of inflammation, its progress is interrupted ; and all the means which the surgeon employs to promote union, are such as tend to prevent the acces- sion of this state. The doctrine that the effusion of lymph for the reparation of the tissues is not to be regarded as necessarily a result of the inflammatory process, is not so novel as its opponents have regarded it, since it has been maintained by. many eminent observers, even from the earliest times. The only case in which the occurrence of inflammation can be regarded as salutary, is that in which there is a deficiency of fibrin in the blood, causing a deficient organizability of the lymph. It has been seen that the amount of fibrin is rapidly increased by inflammation ; and the surgeon well knows that a wound with pale flabby edges, in a depressed state of the system, will not heal until some degree of inflammation has commenced. 596. When the Liquor Sanguinis of the Blood, known as Coagulable Lymph, is effused between the two edges of a wound, or upon the surface of a membrane lining a closed sac, the following appears to be the history of its organization. The new matter, which is poured out in a fluid state, and which seems to have been subjected to the peculiar influence of the white corpuscles that rapidly collect in large numbers at the injured spot, undergoes a Coagulation resem- bling that of Blood ; the Serum, being set free by the concretion of the Fibrin, is absorbed ; and the fibrino'us coagulum speedily obtains an almost membra- nous density. If examined with a microscope at the commencement of the process of organization, it is seen to contain a large number of the exudation- corpuscles already mentioned ( 660) ; these originating probably in the granules set free by the rupture of white corpuscles. In"a short time, these corpuscles present the appearance of regular cells, disposed in layers, and adhering together by an intermediate unorganized substance ; bearing, in fact, a strong resemblance to the cells of tesselated epithelium. Some hours later, the mass exhibits an evidently fibrous character ; which is probably due to the further elaboration of the plastic material, by the cells just mentioned. Between the fibres, a considerable amount of unorganized substance yet remains ; and they may be readily separated, or torn in any direction. A vascular rete next makes its appearance, in connection with the vessels of the subjacent surface ; the first appearance of this network is in the form, of transparent arborescent streaks, which push out extensions on all sides ; these encounter one another, and form a complete series of capillary reticulatipns, the distribution of which very nearly resembles that which has been seen in the villi of the intestines (Fig. 60). From the observations of Mr. Travers* it appears, that isolated globules enter these capillary tubes, and perform an oscillatory motion in them for some hours, before any series of them passes into it ; so that we cannot regard the new channel as burrowed out by a string or file of red corpuscles, pushed out from the nearest capillary by vis a tergo, as some have maintained. And he has further established two important facts, in the history of the Repara- tion of Tissues, which correspond with the observations just cited: 1. That the Liquor Sanguinis first effused is not sufficiently organizable to become an entirely new and permanent tissue ; although adequate both to afford nutrition * Physiology of Inflammation and the Healing Process. 454 OF NUTRITION. to the old, and to form a new tissue of a temporary character : and, 2. That the generation of the new tissues is preceded by the collection of a large num- ber of white corpuscles, in a nearly stationary condition, in the blood-vessels immediately subjacent ; and by the appearance of a large number of similar cells in the newly-forming tissue ; the two together constituting what Mr. T. has aptly called " the new lymph-bed of organization." The views formerly advanced (579) respecting the function of the Colourless Corpuscles, are thus strikingly confirmed. This process of Reparation appears to be conformable, in all essential particulars, with that which has been observed in the first Developement of new parts, such as the toes of the larva of the Water-Newt. 597. Although many have doubted whether effusions of Blood could thus become organized, there seems no valid reason to think that its Fibrin would comport itself in any other way, when Red particles are included in its coagu- lum, than when they are absent. That large masses of extravasated Blood should exhibit little or no tendency to organization, will not be considered sur- prising ; when it is remembered that only their surface can be in that relation with a living membrane, which has been stated to be essential to the further vitalization of the effused Fibrin ( 555). It has been proved in many instances, however, that Coagula of Blood completely enclosed within the body possess an incipient vascularity, being capable of injection from the surface beneath ;* and there is no valid reason to deny that the thin layer of Blood which remains between the lips of an incised wound, when these are closely brought together, is the medium of their reunion. It is unquestionable, however, that the Fibrin of an ordinary Blood-clot is less highly-elaborated, and consequently less sus- ceptible of organization than that of the Liquor Sanguinis, which is poured forth after an injury, and which has been subjected to the local action that is its immediate result. 598. To the second mode of Reparation, attention has recently been strongly directed by Dr. Macartney ; and as this, too, is a strictly Physiological action, and is one which the surgeon should aim at producing, it will be here dis- cussed somewhat in detail. The Surgeon has, until recently, regarded the processes of Granulation and Suppuration, which are attended with much local inflammation, and with a considerable amount of Constitutional disturb- ance when the surface is large, as the only means by which an open wound can be filled up. Occasional instances, however, have not been wanting, in which large open wounds have closed up under the dry clot of blood, by which they were at first covered over, without any suppuration, or other symptom of inflammation ; and in these it has been found that the new surface much more nearly resembles the ordinary one, than does the Cicatrix which follows granulation. To Dr. Macartney, however, is due the merit of explaining the rationale of this action ; which is precisely analogous to that which is concerned in the ordinary processes of growth, and to that reproduction of \vhole parts which takes place in the lower animals without inflammation. It is termed by him the modelling process ; and he remarks as characteristic of it that, when it goes on perfectly, and without Inflammation, the patients are so completely free from uneasy sensations, as only to be aware of the extent of the injury by their own examination. In this process, the surfaces of the wound do not unite by vascular connection, even when they lie in contact ; nor is the space between them filled up with coagulable lymph ; but they are smooth and red, moistened with a fluid, and presenting the appearance of one of the natural mucous membranes. " It might be anticipated that, as this mode of reparation bears so strong a resemblance to the natural formation and * For well-established cases of this sort see communications, by Mr. Dalrymple in the Medico-Chirurgical Transactions, Vol. xxiii; and in Lancet, March 23, 1844. ORIGIN OF THE SOLID TISSUES. , 455 development of parts, it is the slowest mode ; but this is of little account, when compared with its great advantages in being unattended with pain, inflamma- tion, and constitutional sympathy, and leaving behind it the best description of cicatrix." In the case of large burns on the trunk in children, the differ- ence between the two modes of Reparation will frequently be that of life and death ; for it often happens that the patient sinks under the great constitu- tional disturbance occasioned by a large Suppurating surface, although he has survived the immediate shock of the injury. 599. The most effectual means of promoting this kind of Reparative process, and of preventing the interference of Inflammation, vary according to the na- ture of the injury. The exclusion of air from the surface, and the regula- tion of the temperature appear the two points of chief importance. By Dr. Macartney, the constant application of moisture is also insisted on.* He states that the immediate effects of injuries, especially of such as act severely upon the sentient extremities of the nerves, are best abated by the action of " steam at a high but comfortable temperature, the influence of which is gently stimu- lant, and at the same time extremely soothing. After the pain and sense of injury have passed away, the steam, at a lower temperature, may be con- tinued ;" and, according to Dr. M., no local application can compete with this, when the Inflammation is of an active character. For subsequently restraining this, however, so as to promote the simple Reparative process, Water-dressing will, he considers, answer sufficiently well; its principal object being the constant production of a moderate degree of Cold, which diminishes, whilst it does not extinguish, sensibility and vascular action, and allows the Reparative process to be carried on as in the inferior tribes of animals. The reduction of the heat in an extreme degree, as by the application of ice or iced water, is not here called for, and would be positively injurious ; since it not only renders the existence of Inflammation in the part impossible, but, being a direct seda- tive to all vital actions, suspends also the process of restoration. The efficacy of Water-dressing in injuries of the severest character, and in those which are most likely to be attended with violent Inflammation (especially wounds of the large joints) has now been established beyond all question ; and its employ- ment is continually becoming more general. Other plans have been proposed, however, which seem in particular cases to be equally effectual. To Dr. Green- how, of Newcastle, for instance, it was accidentally suggested, a few years since,tto cover the surface of recent burns with a liquefied resinous ointment; and he states that in this manner Suppuration may be prevented, even where large sloughs are formed ; the hollow being gradually filled up by new tissue, which is so like that which has been destroyed that no change in the surface manifests itself, and none of that contraction which ordinarily occurs even under the best management, subsequently takes place. A plan has, moreover, been proposed for preventing suppuration, and promoting reparation by the modeling process, which consists in the application of warm dry air to the wounded surface. The experiments made on this have not been entirely satisfactory, but they seem to show that though the process of healing is much slower under treatment of this kind, it is attended with less constitutional dis- turbance than is unavoidable in the ordinary method ; and it may, therefore, be advantageously put in practice in those cases in which the condition of the patient requires every precaution against such an additional burthen, as after amputation in a strumous subject. But of the superiority of this treat- ment to the water-dressing, no evidence has yet been adduced. 600. The third method of Reparation that by Granulation appears to be * Treatise on Inflammation, p. 178. f Medical Gazette, Oct. 13, 1638. 456 OF NUTRITION. a means employed by Nature for the purpose, under the unfavourable circum- stances of Irritation or a continuance of Inflammation ; proving that parts, previously in a healthy state, are disposed to heal in despite of many impedi- ments thrown in their way. The Granulation-structure is a special one, formed for a temporary purpose. It is endowed with higher vascularity and a more rapid power of growth, than is possessed by any modification of ordi- nary tissue ; but it is very easily destroyed by injury, or by a higher degree of inflammation. The existence of Granulations has been supposed to be necessary to fill up deficiencies ; this, however, is not altogether true ; as we occasionally find very considerable vacancies filled with lymph, which gra- dually becomes organized, without being converted into granulations ; and the void may be also supplied by the process of natural growth just described. Moreover, it is only in the beginning that granulations take the place of the natural structure ; for the approximation of the edges of a wound filled with them, requires that they should be removed by interstitial absorption ; so that wounds healed by this process do not exhibit any remains of the new medium. This approximation somewhat resembles that which occurs in open wounds that have never inflamed, being the result of the natural processes of growth, and it does not take place until the Inflammation has in great degree subsided ; but it differs from the modeling processes in this, that, as the wound is oc- cupied by granulations, its closure takes place prematurely, as it were ; so that, when the granulations are subsequently absorbed altogether, a contracted cicatrix is the result. It will be presently seen, that the formation of the Granulation-structure is intimately connected with the elaboration of Pus ; and' this process, accompanied as it is with such great constitutional disturb- ance, and involving such a loss of nutritious material, cannot but be regarded as an action to be altogether avoided, if possible. 601. We shall now consider, more in detail, the nature of the process of Granulation, and of the Suppuration which usually accompanies it. Its com- mencement is exactly conformable to the first stage of ordinary reunion by the first intention ; for liquor sanguinis is thrown out, in \vhich exudation-corpus- cles present themselves in large numbers. According to Gerber, the trans- formation of these into a sort of imperfect Epithelium may be seen to take place within half an hour. New layers are in the mean time developed, and the most superficial of the exudation-corpuscles, which are exposed to the contact of air, change their character fin the mode to be presently described ( 609), and become Pus-Globules ; whilst those in close contact with the subja- cent surface take a share in the process of reparation. A new layer of exuda- tion-corpuscles is next deposited over this, of which the outer portion degene- rates as before into pus-globules, whilst the inner part gives origin to a kind of areolar tissue, forming Granulations. These Granulations are themselves ex- tremely vascular ; and, as recently shown by Mr. Listen,* the vessels of the subjacent tissue are much enlarged, and assume a varicose character. The bright red colour of the Granulations, however, does not depend on their vas- cularity alone ; for the cells themselves, especially those most recently evolved, are of nearly as deep a colour as the blood-globules : and the superficial bleed- ing which follows even the slightest touch of the granulating surface, does not proceed from blood shed from the new-formed vessels only ; for the red fluid shed in this manner contains, besides blood-discs, newly-developed red cells, ruddy cytoblasts, pale granules and reddish serum. It is a common property of animal cytoblasts, that they present a red colour on their first formation, when in contact with oxygen ; but this hue they lose again, whether they ad- vance to perfect development and become integral parts of a living tissue, or * Medico-Chirurgical Transactions, vol. xxiii. VARYING ACTIVITY OF THE NUTRITIVE PROCESSES. 457 die and degenerate. The process of Granulation and Suppuration appears to differ from that of simple Reparation (the modeling process of Dr. Macartney) in this, that a large part of the exudation-corpuscles deposited on the wounded surface degenerate into pus in the former case, whilst none are thus wasted in the latter ; hut that the existence of inflammation occasions a more copious supply of fibrin in the former case, and increases its tendency to become or- ganized ; the filling-up of a wound with granulations being thus a much more rapid process than that renewal of the completely-formed Tissues which may take place in the absence of Inflammation. The imperfect character of the granulation-structure is shown, by the almost complete disappearance of it, after the wound has closed over. The portion of it in immediate contact with the subjacent tissue, however, appears to undergo a higher organization ; for it becomes the medium by which the Cicatrix is made to adhere to the bottom of the wound. It is very liable to undergo changes which end in its disinte- gration ; as is evident from the known tendency to re-opening, in wounds that have been closed in this manner. VII. Varying Activity of the Nutritive Processes. 602. Without any change in the character of the nutritive processes which we have been describing, there may be considerable variations in their degree of activity; and this, either as regards the entire organism, or individual parts, though most commonly the latter. These variations may be so considerable as to constitute Disease ; though there are some which take place as part of the regular series of physiological phenomena. Thus, the Nutritive processes should have a degree of activity more than sufficient to supply the waste of the body, during the whole period of infancy, childhood and adolescence, until, in fact, its full dimensions are attained ; whilst, on the other hand, they are usually less rapid than the disintegrating processes in old age, so that the bulk of the body diminishes. Now as the waste of the body, so far from being more rapid in old age than in childhood, is much less so, it follows that the difference in the activity of the nutritive processes in these two states must be very considerable ; and this is manifested, not only in the greater demand for food which exists in the child (relatively to the bulk of its body), but also in the greater quickness and facility with which injuries are repaired. Local variations may also occur, as part of the regular train of vital actions in the adult ; thus we perceive an enormous increase in the amount of tissue con- tained in the Uterus and Mammary glands during pregnancy, and a decrease in the bulk of the Thymus gland after the first year of infancy. Now in these cases we see, that increased nutrition is invariably connected wit'h increased Functional activity, and diminished nutrition with diminished functional acti- vity ; and this we shall find to be the constant rule, in regard also to those vari- ations which must be considered as abnormal. 603. Increased Nutrition, or Hypertrophy, is never known to affect the whole body, to a degree sufficient to constitute disease. It cannot be produced as a consequence of the ingestion of an undue supply of food ; for this does not increase the formative activity of the tissues, but merely renders the blood richer in nutritive materials ; a part of which the excreting organs are called on to be continually removing, without its being rendered subservient to the wants of the body ( 679) ; whilst another part may be employed in the nutri- tion of one particular tissue, the Adipose, which has a tendency to increase with the superfluity of non-azotized food, provided that the requisite amount of cellular tissue be generated to hold the fatty matter ( 433). But examples of Hypertrophy of particular tissues or organs are very common. Thus any 39 458 OF NUTRITION. particular set of Muscles, which is subjected to frequent and energetic use, acquires a great increase in bulk ; as we s6e in the arms of a Blacksmith or Waterman, the legs of an Opera-dancer, &c. The hypertrophy of these muscles is a consequence of their increased functional activity ; which, being produced by an exertion of the will, and unaccompanied with any injurious effects on the system, can scarcely be regarded as morbid. But there are many instances in which the involuntary muscles acquire a greatly increased strength, in consequence of an obstruction to their action, which results from disease. Thus we see the right ventricle of the Heart become hypertrophied (and dilated at the same time), where chronic pulmonary disease produces a difficulty in the propulsion of the blood through the vessels of the lungs ; the muscular fibres of the Bladder become enormously hypertrophied, when stric- ture, diseased prostate, or other causes produce a demand for increased expul- sive force on the part of that organ ; and those of the Stomach also become so, in cases of stricture of the pylorus. As an instance of hypertrophy of a Se- creting organ in consequence of an undue excitement of its function, we may notice the enlargement which usually takes place in the Kidney, when its fellow is incapacitated by disease. And the Nervous system presents us with a very remarkable case of hypertrophy of a part, resulting from over-excite- ment of its function ; for if young persons, who naturally show precocity of intellect, are encouraged rather than checked in the use of their brain, the increased nutrition of the organ (which grows faster than its bony case) occa- sions pressure upon its vessels, it becomes indurated and inactive, and fatuity and coma are the result. Local hypertrophy may be induced also by local congestions ; but in such cases it will usually be found that the form of tissue produced is of the lowest kind, unless the functional activity of the part be increased by the congestion. Thus, when disease of the Heart produces long-continued congestion of the Lungs, Liver, Spleen, &c., the bulk of these organs increases ; but chiefly by the production of an additional amount of interstitial Areolar tissue, which may result (as we .have seen) from the simple consolidation of Fibrin ; and partly also (in the case of the spleen espe- pecially) by the gorging of their distensible veins with blood. One of the least explicable cases of Hypertrophy, is that which takes place in the Thyroid gland, causing Bronchocele. So little is known of the normal office of this organ, that it cannot be determined whether its increased size be due to an increased activity of its functional operations, or to an unusual formative acti- vity in its tissue, depending on some hidden cause. The connection of this disorder with causes which affect the whole constitution rather than individual parts, would seem to indicate the former. 604. When the Waste of the Tissues is more rapid than their replacement by Nutrition, Atrophy is said to take place ; and this may affect either the whole body, or individual parts. General Atrophy, Marasmus, or emaciation, may result from an insufficient supply of plastic matter, from want of forma- tive power in the tissues themselves, or from their too rapid disintegration. The insufficiency of the supply of nutritive matter may depend either on de- ficiency in the azotized substances ingested as food, or on imperfect perform- ance of those processes by which they are converted into the plastic element, Fibrin. Hence, even when there is an ample supply of food, atrophy may take place to a very severe extent, in consequence of disordered digestion, or of want of vital power in the fibrin-elaborating cells. Again, we have reason to believe that the formative power in the tissues themselves may be diminished, so as to check the process of Nutrition, even when the plastic material is sup- plied ; thus there seems to be a complete stoppage of this action in Fever, and a diminution of it in that irritable state of the system which results from ex- VARYING ACTIVITY OF THE NUTRITIVE PROCESSES. 459 cessive and prolonged bodily exertion or anxiety of mind, especially when accompanied by want of sleep. It is difficult to separate this cause, however, from mal-assimilation on the one hand, or from too rapid decay of the tissues on the other : for we know that, in such states, there is a tendency to imper- fect elaboration of the Fibrinous element, and at the same time an unusually rapid disintegration as manifested by the increased amount of Urea in the urine. The influence of excessive waste in causing Atrophy of the body, is well shown in the cases of Diabetes mellitus and colliquative Diarrhoea ; in both these, the increase and depravation of the secretions are undoubtedly to be regarded as the effects, and not the causes, of the textural changes with which they are associated. Colliquative Diarrhoea is a constant occurrence on the last day or two of life, in animals reduced by Starvation ; and is accom- panied by that foetid odour of the body which indicates that decomposition is already going on throughout the system. The same thing occurs as the ordinary termination to many diseases of exhaustion ; in which Inanition is unquestionably the immediate cause of death. 605. Partial Atrophy may occur in consequence of disuse of the organ affected, occasioning inactivity in its formative processes; or as a result of a deficiency of nutriment, occasioned by an obstruction to the circulation. Of the operation of the former cause, we have many examples in the ordinary processes of the economy. Thus the Uterus is atrophied, relatively to its previous condition, as soon as parturition has taken place ; and the Mammary glands, when lactation has been discontinued. It is probably in part to this cause, and in part to the diversion of the blood into other channels, that we are to attribute the atrophy of many parts, as the development of the system ad- vances, which at an earlier period were of large comparative size, such as the Corpora Wolffiana, the Suprarenal capsules, and the Thymus gland. Many instances might be adverted to, of the influence of suspension of func- tional activity, as a result of disease or injury, in producing local atrophy. One of the most common cases, is the atrophy of Muscles which is consequent upon their disuse. This disuse will produce the same effect, whether it be occasioned by paralysis, which prevents the nervous centres from exciting the muscles to contraction ; or by anchylosis, which interposes a mechanical im- pediment to their use ; or by fractures or other accidents, the reparation of which requires the limb to be kept at rest. Or even if, without having suffered from any injury, a limb be fixed during some time in one posture, its muscles will become atrophied, as is seen in the case of the Indian Fakirs. (See 382). Similar facts may be adduced, in regard to Atrophy of Nerves, from interrup- tion of their normal function. Thus when the Cornea has been rendered so opaque by accident or disease, that no light can penetrate to the interior of the eye, the Retina and the Optic nerve lose, after a time, their characteristic structure ; so that scarcely a trace of the peculiar globules of the former, or of the nerve-tubes of the latter, can be found in them. These and similar facts are readily understood, when connected by the general principle formerly laid down, that every proper vital operation involves an act of nutrition ; in such a manner that, whilst the vital properties of any part are dependent upon its due nutrition, the amount of its nutrition will in return depend upon the degree in which these properties are exercised. Partial Atrophy may depend, however, upon causes of a purely mechanical nature ; such, for example, as produce an interruption of the current of Blood through the part. This may result from changes in the Arteries supplying it ; such as ossification, or other forms of obstruction. Or it may be consequent upon disease in the part itself; as when the deposits produced by Inflammation tend to contract, and thus to press upon the vascular structure, which frequently happens in the lungs, liver, and kidneys ; or when the inflammation occurs in the vessels themselves, 460 OF NUTRITION. causing adhesion of their walls, and obliteration of their tubes ; or when a new growth absorbs into itself all the nutritive materials which the Blood supplies.* VIII. Abnormal Forms of the Nutritive Process. 606. Under the preceding head, we have considered the chief variations in the degree of activity, that are witnessed in the ordinary or normal conditions of the Nutritive process, that is, those conditions in which the products are adapted, by their similarity of character, to replace those which have been removed by disintegration. But we have now to consider those forms of this process, in which the products are abnormal, being different from the tissues they ought to replace. We shall confine ourselves to a brief examination of the two most important of these states : that which is termed Inflammation ; and that which gives rise to Tubercular deposit. The former results from an excess of the plastic element in the blood ; the latter from a depraved condition of it, whereby its plasticity is impaired or destroyed. Notwithstanding all the attention which has been given to the state of the vessels in Inflammation, a careful consideration of its phenomena, with the light which recent investiga- tions have thrown upon these, leads us to attach comparatively little importance to this, and to seek for the essential character of the process elsewhere. The researches of Addison, Williams, Barry, Gulliver, Andral and others, all seem to point to the following conclusions. 1. That there is a peculiar afflux or determination of the White Corpuscles of the Blood towards the inflamed part. 2. That the total amount of these Corpuscles in the circulating blood undergoes a great increase. 3. That the quantity of Fibrin in the Blood aug- ments, in proportion to the extent and intensity of the Inflammation ; and this, even when it was previously, from the influence of some other morbid condi- tion, below the usual standard. With its quantity, its plasticity, or tendency to organization, also increases in a healthy subject. Now when these facts are compared together, and are connected with those formerly adduced, in regard to the probable function of the White Corpuscles of the blood, they lead almost irresistibly to the conclusion, that the process of Inflammation essentially con- sists in an undue stagnation of these Corpuscles in the vessels of the part, an excessive multiplication of them by the ordinary process of generation, and a consequent over-production of Fibrin. By these changes, and by the results which follow them, Inflammation may be distinguished from the various forms of Hyperaemia and Congestion. To the results, then, we shall next direct our attention. 607. It may be inferred from various phenomena, that whilst the formative power of the Blood is increased in Inflammation, that of the Tissues is dimin- ished. Certainly this is the case in regard to the system at large, when febrile irritation has been established ; for, notwithstanding the increased Plasticity of the Blood, we see the body wasting, instead of increasing in vigour. And it may be inferred, also, in regard to the tissues of the part affected, from the tendency to Atrophy and Disintegration which they exhibit ; and which is greater (leading even to the death of whole parts) in proportion as the inflam- mation is more intense, and as the tendency to the deposit of new products is the more decided. That a Stagnation of Blood takes place in the vessels of the inflamed part, is another general fact, which throws some light upon the nature of the process ; for this stagnation is obviously favourable to the trans- udation of the fluid Plasma of the blood, through the walls of the vessels, into * See on this subject Dr. Williams's Principles of Medicine, Chap, iv.: to which the Author is partly indebted lor the preceding paragraphs. ABNORMAL FORMS OF THE NUTRITIVE PROCESS. 461 the surrounding tissue, or upon a neighbouring surface. This deposition of the Fibrinous element, possessing a high degree of plasticity, and capable of spontaneously passing into simple forms of tissue (which may be gradually replaced by higher forms, when penetrated by vessels from the surrounding parts), may be regarded as the first characteristic result of Inflammation, ft is by the deposition, and subsequent organization, of plastic matter in the sub- stance of organs, that their tissues become consolidated ; and by its deposition and subsequent organization upon their free surfaces, that false membranes and adhesions are formed. It appears probable, from the recent inquiries of Mr. Robinson,* that this deposition may be attributed to physical causes. It is well known, that simple Congestion will occasion transudation of the serous portion of the Blood ; and if the return of the Blood by the veins of a part be com- pletely prevented, a greater or less proportion of fibrin also may be poured forth. Now when the quantity of Fibrin in the blood is greatly augmented, and the firmness of the walls of the vessels in the inflamed part is diminished by the alterations taking place in their tissue, it is easy to understand that the dispo- sition to the effusion of Fibrin will be much increased. Sometimes the Fibrin is diluted with a large quantity of Serum ; and is poured into a cavity (as that of a serous sac) in the form of a liquid, which afterwards separates into clot and serum. 608. Should the Inflammation increase in intensity, a complete stagnation of blood in the tissue most affected, or even in an entire organ, will be the result ; and this will occasion its death. If a large part be thus entirely de- stroyed at once, the process is termed Gangrene; and it separates from the living part at a line where the Inflammation is less intense, and where there is a deposit of Fibrin, which serves the important purpose of closing the mouths of the blood-vessels that are laid open by the process. If the destruction of tissue, however, be interstitial, the dead parts are not thus thrown off, but are taken up by the absorbent process ; and thus the cavity of an Jlbscess, or of an Ulcer is formed. This cavity is usually bounded by tissue that has been consolidated by the effusion of Fibrin ; a fact readily accounted for on the principles just stated. For the death and removal of tissue take place where the Inflammation has been most intense, and the stagnation most complete ; which is in the centre of the inflamed spot ; and the fibrinous effusion, the result of moderate inflammation, is poured into the surrounding tissue. The elements of Liquor Sanguinis are poured into the central, as well as the peri- pheral, portion of the inflamed tissue ; but they assume a different form that of Pus. It would appear as if the influence of the surrounding death and decay produces a degradation of their character ; so that they become entirely aplastic or unorganizable, although immediately derived from Blood highly charged with Fibrin. 609. Between Coagulable Lymph and Purulent effusions, there are many degrees of transition ; the very same deposit being frequently organizable at one part, presenting the character of a tough fibrous membrane, interspersed with corpuscles, whilst it is friable in another, from want of complete fibril- lation in the fluid portion of the effusion, and is entirely destitute of tenacity in a third portion, especially the superficial part, or free surface, of the deposit. When examined by the Microscope, Pus is found to be characterized by the presence of a number of cells of a peculiar aspect, having a very tuberculated or mulberry surface ; these are seen floating in a fluid, termed liquor puris, which is of an albuminous or low fibrinous character, being entirely destitute of organizability. Now the production of Pus in an inflamed part, or in other words, the act of Suppuration, may be due to one of three causes, viz., the * Medico-Chirurgical Transactions, vol. xxvi., p. 51. 39* 462 OF NUTRITION. intensity of the inflammation ; the presence of air, which becomes a source of irritation ; and a previously vitiated state of the blood. Various attempts have been made to show, that the Pus-globule is a degenerated red or white corpus- cle of the Blood ; it seems more probable, however, that it does not escape from the vessels as a complete cell, but as a cell-germ, which may have had its origin in a white corpuscle of the blood ; and which, under favourable cir- cumstances, might have produced an Exudation-corpuscle ( 560). At any rate, it must be regarded as a degenerated form of cell ; and the liquor puris must be considered as analogous to the plasma of the Blood in a degenerated state. In what manner the Inflammatory process determines the formation of the Pus-cell, and the consequent degradation of the product, we are at present unable to state ; but that the degree of irritation in the part has an influence upon it, is evident from the effects of the contact of air upon inflamed surfaces, causing those elements to take the form of Pus, which would otherwise have been thrown out as a plastic deposit. This circumstance would seem to indi- cate, beyond all doubt, that the Exudation and Pus-corpuscles, the plastic Lymph and the aplastic Liquor puris have the same origin; but that their character is determined by local circumstances. There is great reason to believe, that when Pus is introduced into the Blood, it may induce such a change in the character of the fluid as speedily to impair its vital properties ; so that the Pus-corpuscles will rapidly propagate themselves in the Blood, and the plas- ticity of the Liquor Sanguinis will be diminished. In this manner the whole system will be seriously affected, and there will be a tendency to deposits of Pus in various organs especially in those which, like the Lungs and Liver, serve as emunctories to the system without any previous inflammatory changes in these parts. It has been ascertained by Mr. Addison, that if a drop of Pus be treated with Liquor Potassae, it entirely loses its opaque character, and be- comes clear and transparent, like Mucus, with whose tenacity and elasticity also, it becomes endowed. If it be then treated with acetic acid, it recovers somewhat of its former opacity; and, when pressed into a thin film, exhibits a distinct fibrillation. 610. In persons of that peculiar constitution which is termed Scrofulous or Strumous, we find an imperfectly organizable or Caco-plastic deposit, or even an altogether aplastic product, known by the designation of Tubercular .* matter, frequently taking the place of the normal elements of Tissue ; both in the ordinary process of Nutrition, and still more when Inflammation is set up. From an examination of the Blood of Tuberculous subjects it appears, that the Fibrinous element is not deficient in amount, but that it is not duly elaborated ; so that the coagulum is loose, and the red corpuscles are found to bear an abnormally low proportion to it. We can understand, therefore, that such a constant deficiency in the Plasticity must affect the ordinary nutritive process; and there will be a liability to the deposit of caco-plastic products, without Inflammation, instead of the normal elements of tissue. Such appears to be the history of the formation of Tubercles in the lungs and other organs, when it occurs as a kind of metamorphosis of the ordinary Nutritive process ; and in this manner it may proceed insidiously for a long period, so that a large part of the tissue of the lungs shall be replaced by an amorphous deposit, without any other ostensible sign than an increasing difficulty of respiration. It is in the different forms of Tubercular deposit, that w'e see the gradation most strikingly displayed between the plastic and the aplastic formations. In the semi-transparent, milliary, gray, and tough yellow forms of Tubercle, we find traces of organization in the form of cells and fibres, more or less obvious ; these being sometimes almost as perfectly formed as those of Plastic Lymph, at least on the superficial part of the deposit, which is in immediate relation with the living structures around ; and sometimes so degenerated as scarcely FORMATION OF THE TISSUES. 463 to be distinguishable. In no instances do such deposits ever undergo further organization; and, therefore, they must be regarded as caco-plaslic. But in the opaque, crude, or yellow Tubercle, we do not find even these traces of definite structure ; for the matter of which it consists is altogether granular, more resembling that which we find in an albuminous coagulum. The larger the proportion of this kind of matter in a tubercular deposit, the more is it prone to soften, whilst the semi-organized tubercle has more tendency to con- traction. This is entirely aplastic. Now although Tubercular matter may be slowly and insidiously deposited, by a kind of degradation of the* ordinary Nutritive process, yet it cannot be doubted that Inflammation has a great ten- dency to favour it ; so that a larger quantity may be produced in the lungs, after a Pneumonia has existed for a day or two, than it would have required years to generate in the previous mode. But the character of the deposit still remains the same ; and its relation to the plastic element of the blood is shown by the interesting fact, of no unfrequent occurrence, that, in a Pneumonia affecting a Tuberculous subject, Plastic Lymph is thrown out in one part, whilst Tubercular matter is deposited in another. Now Inflammation, pro- ducing a rapid deposition of Tubercular matter, is peculiarly liable to arise in organs, which have been previously affected with chronic Tubercular deposits, by an impairment of the process of textural Nutrition ; for these deposits, acting like foreign bodies, may of themselves become sources of irritation ; and the perversion of the structure and functions of the part renders it pecu- liarly susceptible of the influence of external morbific causes. These views, at which several recent Physiologists and Pathologists have arrived on inde- pendent grounds, seem to reconcile or supersede all the discordant opinions which have been upheld at different times regarding the nature of Tubercle ; and lead to the soundest views with respect to the treatment of the Diathesis. IX. Formation of the Tissues. 611. From the primordial cells, of which the whole fabric of the embryo, or the tissue of a newly-formed part, is composed, all the Animal tissues, various as they are in structure and in properties, are gradually elaborated. Their variety is much greater than that which exists in Plants ; and this is exactly what we should expect, when we take into account the much greater number of entirely different functions to be performed. When we contrast the fabric of an Animal with that of a Plant, we are struck with this important dif- ference in their conformation, that whilst the latter is made up solely of ele- ments which are to perform their several parts in the performance of the Nutritive and Reproductive operations (the only exception being in the case of those more solid portions of the fabric, which are destined to give mechanical support to the remainder), the former is composed of a much greater variety of parts, which are adapted to move upon each other. Now this purpose requires, not only the addition of certain new tissues, to which nothing ana- logous is to be found in Plants, for creating and exercising the motor power, but also an adaptation of the whole structure to this new condition. The tissues of Plants entirely consist of cells, or simple modifications of them. Some of these cells being strengthened by internal deposits, form the solid woody frame- work of the stem and branches ; which gives support to their wide-sprsading foliage and numberless blossoms. Others coalesce, by the disappearance of their intervening partitions into tubes ; which serve for the conveyance of fluid between the most distant parts. But the great bulk of the fabric still consists of cells, closely adherent to each other, and actively participating in the various operations of organic life. In like manner, in the Animal body, a certain part of the cells have contributed to form the solid Osseous and Cartilaginous frame- work, which not only gives support and protection to the body, but contributes 464 OF NUTRITION. to its power of movement, by affording fixed points for the attachment of its muscles. Others again have coalesced into Vessels, as in plants, for the rapid conveyance of fluids. Others, too, after a similar coalescence, have developed new and remarkable products in the interior of the tubes thus formed ; and become transformed into those Nervous and Muscular tissues, to which nothing analogous is found in Plants, and which are the peculiar instruments of Animal life. Yet still there remains a large number of unchanged Cells scattered through the body ; which perform, as in Plants, the essential part in the func- tions of Nutrition, Reproduction, &c. These, however, could not be held together in their constantly varying relative positions, without some inter- vening substance altogether different from true cellular tissue. It must be capable of resisting tension with considerable firmness and elasticity ; it must admit free movement of the several parts upon one another ; and it must still hold them sufficiently close together to resist any injurious strain upon the delicate vessels, nerves, &c., which pass from one to another, as well as to prevent any permanent displacement. Now all these offices are performed in a remarkably complete degree, by the Jireolar Tissue ( 637); the reason of whose restriction to the Animal kingdom, notwithstanding the purely physical nature of its functions, is thus evident. And as necessity arises, in certain parts, for tissues which shall exercise a still greater power of resistance to tension, and which shall thus communicate motion (as in the case of Tendons), or shall bind together organs that require to be united (as in the case of Liga- ments and Fibrous Membranes), so do we find peculiar tissues developed, that shall serve these purposes in the most effectual manner. Hence these tissues also, although not endowed with any properties that are peculiarly animal, are nevertheless restricted to the Animal Kingdom, as completely as are the Mus- cular and Nervous Tissues, which make up the essential parts of the appara- tus of Animal Life. 612. That all the Animal tissues are in the first instance developed from Cells, was the doctrine put forth by Schwann, who first attempted to generalize on this subject. By subsequent research, however, it has been shown that this statement was too hasty; and that, although many tissues retain their original cellular type, through the whole of life, and many more are evidently generated from Cells and are subsequently metamorphosed, there are some in which no other cell-agency can be traced than that which was concerned in the preparation of the plastic material. This would appear to be certainly the case, in regard to the very delicate structureless lamella of membrane, now known under the name of Basement or Primary Membrane, which is found (beneath the Epidermis or Epithelium), on all the free surfaces of the body. No vestige of cell-structure can be seen in this membrane ; and it would rather appear to resemble that, of which the walls of the cells are themselves con- stituted.* In some instances it presents a somewhat granular appearance; and is then supposed by Henle to consist of the coalesced nuclei of cells, whose development has been arrested. This, however, is quite hypothetical; and all we can say is, that the Basement membrane is probably formed by the con- solidation of a layer of the plastic element, which may, in certain cases, include a large number of granules that may serve for the development of new cells. Possibly it is in these granular germs, sometimes scattered through the mem- brane, and in other instances collected into certain spots,! that the cells of the superjacent Epithelium or Epidermis take their origin; and if this be the * See a Paper by the Author, on the Microscopic Structure of Shells, &c., in the Annals of Natural History, Dec., 1843. The inner layer of the Shells of Mollusca, after treatment with a dilute acid, yields specimens of Basement Membrane, in a form well adapted for examination. j- See Goodsir, in Trans, of Roy. Soc. of Edinb., 1842. FORMATION OF THE TISSUES. 465 case, we must regard the Basement membrane as a transitional rather than as a permanent structure, continually disintegrating, and yielding up its con- tained cell-germs on its free surface, and as constantly being renewed from the blood beneath. For the Epidermic structures appear to constitute an exception to the general rule, that the Tissues reproduce themselves ; since they are cast off, without leaving their germs behind them; and the cells which replace them must be derived from new germs more directly supplied from the blood than is elsewhere the case. In the case of the other tissues, whose disintegration takes place inter stitially (so to speak), it would seem probable that in the very act of the dissolution of the parent-structure, the germs of the new structures destined to replace it are set free ; as happens in the reproduction of the simple Cellular Plants. 013. It would seem doubtful, also, in regard to the simple Fibrous tissues, whether they are generated by a metamorphosis of Cells, in the same manner as the Osseous, Muscular and Nervous ; or whether they are not produced, like the Basement Membrane, by the consolidation of a plastic fluid which has been elaborated by cells. The latter view is the one which the Author has been led to regard as most probable, from the results of his own observa- tions, coupled with those of Messrs. Addison and Gulliver previously adverted to. The Membrane of the Egg-shell, whose structure has been already described ( 554), appears to him to have essentially the same constitution with the simple Fibrous tissues, which it resembles also in its tenacity ; whilst its origin can scarcely be supposed to be different from that of the fibrous net- work in the buffy coat of the blood, or in the bands formed by the coagulation of Lymph upon an inflamed membrane. The appearances which the Fibrous tissues display, and which have been quoted in proof of their Cellular origin, are not inconsistent with this view. For in the reticulated structures just adverted to, certain bodies are seen which appear to be nuclei or imperfectly formed cells, originating probably in germs set free by the rupture of the white corpuscles of the blood, and which closely correspond with the nuclear corpuscles, which may be brought into view in the Fibrous tissues ( 637). Mr. Addison's observation, too, that the fibres formed in the Liquor Sanguinis, during its coagulation, often seem to radiate from the remains of the white cor- puscles that have ruptured, or from the little aggregations of granules they contained, gives the explanation of several of the appearances which have led to the belief in the production of Areolar tissue by Cell-transformation. As an additional argument in support of this view, the appearances presented by the semi-fibrous Cartilages may be adduced. In the cartilages of the ribs, for instance, a more or less distinct fibrous appearance may be frequently seen in the intercellular substance ; this is sometimes so faint, that it might be con- sidered as an illusion occasioned by the manipulation to which the section has been subjected; but it is often so well-defined, as almost to present the appear- ance of the true fibrous structure. No indication of the direct operation of cells in the development of these fibres has ever been witnessed ; and we can scarcely do otherwise than regard them as produced by the regular arrange- ment and consolidation of the particles of the blastema or plastic element, in virtue of its own inherent powers. In many instances, Fibres, like mem- branes, appear to originate in the nuclei of cells, whose development has been checked; the fibres which occasionally encircle the fasciculi of Areolar tissue ( 637) appear to have this origin. 614. The transformation of the elements of Blood into Organized Tissues is not confined to the form and structure which these present, but extends also to their Chemical constitution. In the greater number of them, the Protein composition prevails ; and there is reason to believe that this is the case with the organized portion of all those which are formed by the transformation of 466 OF NUTRITION. Cells. Very frequently, however, a deposit is formed within these cells, through a secreting process effected by themselves ( 651) ; which may have an entirely different character. Thus the cells of Adipose tissue elaborate Fatty matter, the cells of Epidermis appear to draw off Horny matter, and the cells of the Epithelium fill themselves with products of various kinds, which were either pre-existing in the Blood, or are generated by a simple transforma- tion of its elements ( 649). These last might be regarded, equally with the contents of the more permanent Tissue-cells, as products of Nutritive action ; but it will be more convenient to consider them with a view to their destination, which is altogether different. 615. The composition of the greater part of the Fibrous tissues, however, is very different ; for they all yield to boiling water the substance called Gela- tin, which does not seem capable of the same degree of organization with the Protein compounds. This may be obtained by boiling portions of Skin, Areo- lar tissue, Serous membrane, Tendon, Bone, &c., in water, for some time ; after which the decoction is allowed to cool, when it solidifies into a jelly of greater or less thickness. Some tissues dissolve readily in this manner, and little resi- dual (or fibrinous) substance is left ; this is especially the case with areolar tissue, serous membranes, and (in a less degree) with skin. Others require a long boiling for the extraction of any Gelatin, and even then it is obtained in but small quantity ; of this kind are the elastic fibrous tissue and some forms of cartilage. A peculiar modification of this principle exists in most of the permanent cartilages ; and has received the name of Chondrin. Gelatin is not found in the blood, nor in any of the healthy fluids ; and most Chemists are of opinion that it is rather a product of the operation practised to separate it, than a real constituent of the living solids. This idea seems inconsistent, however, with the fact that the gelatinous tissues will exhibit, without any preparation, the best marked of the chemical properties which are regarded as characteristic of Gelatin, that, namely, of forming a peculiar insoluble compound with Tannin ; and the Tanno-Gelatin which may be obtained by precipitating Gelatin from a solution, and that which results from the action of Tannin on Animal membrane, appear to be precisely analogous in every respect, save the presence of structure in the latter, and its absence in the former. Gelatin is very sparingly soluble in cold water ; by contact with which, however, it is caused to swell up and soften. It is readily dissolved by hot water ; and forms so strong a jelly on cooling, that 1 part in 100 of water becomes a consistent solid. Its reaction with Tannic acid is so distinct that 1 part in 5000 of water is at once detected by infusion of Galls. The fol- lowing are the results of four analyses of Gelatin, by Scherer and Mulder. X SCHERER, MULDER. Carbon . . . 50-557 50-774 50-048 50-048 Hydrogen . . 6-903 7-152 6.477 6-643 Nitrogen . . 18-790 18-320 18-350 18-388 Oxygen . . . 23-750 23-754 25-125 24-921 The formula adopted by the former is 48 C, 41 H, 7k N, 18 O ; that of the latter is 54 C, 42 H, 9 N, 20 O. Neither of these can be yet regarded as satisfactorily determined ; and it is therefore useless to speculate upon the mode in which Gelatin is produced by a metamorphosis of Protein-compounds. That it cannot be converted, in the living body, into Albumen or Fibrin, would appear from the considerations already stated (554). A kind of sugar, termed Glycicoll, may be obtained from Gelatin, by the action of Sulphuric acid, or by boiling it in caustic Alkali : this substance crystallizes in large prisms, which are colourless, taste sweet, and feel gritty' between the teeth ; FORMATION OF THE TISSUES. PIGMENT-CELLS. 467 it is soluble in 4 parts of water, and is taken up in small quantity by alcohol. This fact is one of much interest, in regard to certain Pathological relations of Gelatin. [The following table presents, perhaps, the best general view of the various tissues. No satisfactory arrangement can be constructed, based on any one principle of classifi- cation. TABULAR VIEW OF THE TISSUES OF THE HUMAN BODY. 1. Simple membrane, homogeneous, or nearly so, Examples. Posterior layer of the employed alone, or in the formation of com- cornea. Capsule of the lens. pound membranes. Sarcolemma of muscle, &c. 2. Filamentous tissues, the elements of which are White and yellow fibrous tissues. real or apparent filaments. Areolar tissue. 3. Compound membranes, composed of simple Mucous membrane. Skin. True or membrane, and a layer of cells, of various secreting glands. Serous and sy- forms (epithelium or epidermis), or of areo- novial membranes, lar tissue and epithelium. 4. Tissues which retain the primitive cellular Adipose tissue. Cartilage. Gray structure as their permanent character. nervous matter. 5. Sclerous or hard tissue. Bone. Teeth. 6. Compound tissues. a. Composed of tubes of homogeneous mem- Muscle. Nerve, brane, containing a peculiar substance. b. Composed of white fibrous tissues and Fibro-cartilage. cartilage. M. C.j 616. There are several instances in which Nucleated Cells, resembling those of the primordial fabric, are seen even in the adult body. The most striking examples of this are to be found among the inferior members of the class of Fishes. Thus, in the Myxinoid family, there is no true Vertebral column, but its place is occupied by a gelatinous tube, termed the chorda dorsalis ; which consists entirely of nucleated cellular tissue, and which is precisely analogous to the structure occupying the same situation in the early Embryo of higher animals ( 760). In the Short Sun-fish, a corresponding form of tissue forms a thick covering to the body, replacing the true skin. And in the Lancelot (a little fish which is deficient in so many of the characters of the Vertebrated division, that many naturalists have doubted its right to a place in the class), a considerable portion of the fabric is made up of a similar paren- chyma. We shall find, however, that even in Man a considerable part of the fabric is made up of Cells ; and that these perform some of the most important offices in the economy. 617. The Pigment-cells, which give colour to the Skin, and of which the Pigmentum Nigrum of the eye is entirely composed, usually exhibit the ori- ginal form of the cell with little alteration. On the choroid coat of the eye they are seen as a kind of pavement, having somewhat of a polyhedral shape, and lying in a very regular manner with some intercellular substance interposed between them. In the Skin of Man, they are scattered through the ordinary epidermic cells ; and its colour is determined by that of their contents. There is no distinct coloured layer, as was formerly supposed ; but the cells are more closely aggregated in some parts than in others. This is as much the case in the European, however, as in the Negro ; in the former, they are concerned in producing the spots termed freckles, and others of a similar kind. In some animals, the Pigment cells of the skin frequently undergo a change of form ; being elongated in many directions into hollow fibres, which, meeting other formations of the same kind, produce a more or less perfect network of star- shaped cells. This change is best seen in the skin of the Batrachia, where the cells are frequently isolated : a good example of it is shown in Fig. 89 (p. 360.) The black colour is given by an accumulation within the cell, of a 468 OF NUTRITION. number of rounded granules, which, when separately viewed, are found to be transparent, not black and opaque ; these granules are flat oval corpuscles, measuring about l-20,000th of an inch in diameter, and about a quarter as much in thickness ; they exhibit a very active molecular movement when set free by the bursting of the cell, and this has even been noticed while they are enclosed. The chemical nature of the black pigment has not yet been made evident ; it has been shown, however, to have a close relation to that of the Cuttle-fish ink (which derives its colour from pigment-cells lining the ink-bag) ; and to contain a larger proportion of Carbon than most other organic sub- stances, every 100 parts containing 58<| of this element. The nucleus of the pigment cells may generally be traced as a clear spot. 618. The Fat-cells, of which Adipose tissue is composed, also permanently exhibit the original type of structure in its simplest form. This tissue is usually diffused over the whole body, filling up interstices, and forming a kind of pad or cushion for the support of movable parts. Even in cases of great ema- ciation, some Fat is always left ; especially at the base of the heart, around the origin of the large vessels ; in the orbit of the eye ; in the neighbourhood of the kidney; in the interior of the bones ; and within the spinal canal, between the periosteum and the dura mater. The Fat Cells are usually spherical or spheroidal ; sometimes, however, when closely pressed together without the intervention of any intercellular substance, they become polyhedral. The nucleus is not always to be distinguished ; perhaps in consequence of its having passed to the interior of the cell. [The Fat-cell is composed of the adipose tissue, a closed vesicle formed by a membrane of extreme tenuity, and the material which it contains, ihefat. The membrane is per- fectly homogeneous and transparent, about the 2