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 THE LIBRARY 
 
 OF 
 
 THE UNIVERSITY 
 OF CALIFORNIA 
 
 PRESENTED BY 
 
 PROF. CHARLES A. KOFOID AND 
 MRS. PRUDENCE W. KOFOID 
 
 > > 
 
ANIMAL PHYSIOLOGY 
 
 BY 
 
 WILLIAM B. CARPENTER, M.D, 
 
 F.R.S. F.G.S. F.L.S. 
 
 REGISTRAR OF THE UNIVERSITY OF LONDON. 
 
 THOROUGHLY REVISED, AND PARTLY RE-WRITTEN. 
 
 LONDON : 
 H. G. BOHN, YORK STREET, COVENT GARDEN. 
 
 1859. 
 
 
LONDON: 
 
 R. CLAY, PRINTER, BREAD STREET HILL. 
 
TO 
 
 SIR JAMES CLARK, BART. M.D. F.R.S. 
 
 PHYSICIAN IN OKDINABY TO THE QUEEN AND TO PRINCE ALBERT, 
 ETC. ETC. 
 
 MY DEAR SIR JAMES, 
 
 I cannot more appropriately inscribe this Treatise, 
 having for its object the general diffusion of sound Physio- 
 logical knowledge, than to one whose Professional eminence 
 is founded on his enlightened application of it to the pre- 
 vention and cure of Disease, and who has ever been the 
 consistent advocate of Liberal Education. 
 
 The grateful sense I entertain of many acts of personal 
 kindness, makes me feel additional pleasure in paying this 
 humble tribute. 
 
 Believe me to remain, 
 
 MY DEAR SIR JAMES, 
 Your obliged Friend and Servant, 
 
 WILLIAM B. CAKPENTEK. 
 
 UNIVERSITY HALL, LONDON, 
 January } 1859. 
 
 
PREFACE. 
 
 THE issue of the present Volume may be considered as an 
 attempt to supply what the Author has long considered to be 
 a deficiency in the literature of this country, that, namely, 
 of an Educational Treatise on Animal Physiology, which 
 should at the same time communicate to its readers the facts 
 of greatest importance as regards their practical bearing, 
 and present these in such a form as to place the learner 
 in possession of the essential principles of Physiological 
 Science. 
 
 The Author has followed the general plan of the Treatise on 
 Animal Physiology contributed by Professor Milne-Edwards, 
 one of the most eminent Naturalists in France (in which 
 country it is not thought beneath the dignity of men of the 
 highest scientific reputation to write elementary books for 
 the instruction of the beginner), to the " Cours Elementaire 
 d'Histoire Naturelle" adopted by the French Government as 
 the text-book of instruction in the Colleges connected with 
 the University of Paris, which requires from every Candidate 
 for its Degree of " Bachelor of Sciences " a competent know- 
 ledge both of Animal and of Vegetable Physiology. He has 
 also had at his disposal the admirable series of Illustrations 
 prepared for that work, which, as a whole, are unsurpassed 
 either in beauty or in exactness. 
 
 In carrying-out this plan, however, the Author has entirely 
 followed his own judgment ; and has made so much more use 
 
 b 
 
PREFACE. 
 
 THE issue of the present Volume may be considered as an 
 attempt to supply what the Author has long considered to be 
 a deficiency in the literature of this country, that, namely, 
 of an Educational Treatise on Animal Physiology, which 
 should at the same time communicate to its readers the facts 
 of greatest importance as regards their practical bearing, 
 and present these in such a form as to place the learner 
 in possession of the essential principles of Physiological 
 Science. 
 
 The Author has followed the general plan of the Treatise on 
 Animal Physiology contributed by Professor Milne-Edwards, 
 one of the most eminent Naturalists in France (in which 
 country it is not thought beneath the dignity of men of the 
 highest scientific reputation to write elementary books for 
 the instruction of the beginner), to the " Cours Elementaire 
 d'Histoire Naturelle " adopted by the French Government as 
 the text-book of instruction in the Colleges connected with 
 the University of Paris, which requires from every Candidate 
 for its Degree of " Bachelor of Sciences " a competent know- 
 ledge both of Animal and of Vegetable Physiology. He has 
 also had at his disposal the admirable series of Illustrations 
 prepared for that work, which, as a whole, are unsurpassed 
 either in beauty or in exactness. 
 
 In carrying-out this plan, however, the Author has entirely 
 followed his own judgment ; and has made so much more use 
 
 6 
 
PREFACE. 
 
 of his own materials than of those supplied by the treatise of 
 Professor Milne-Edwards, that the work may be regarded 
 as almost entirely original. The present Edition, too, has 
 undergone very considerable modifications ; the first chapter, 
 which now contains a complete outline of the Elementary 
 Tissues of the Animal Body, and the last, in which a com- 
 prehensive sketch is given of the principal phenomena 
 of Eeproduction and Development throughout the Animal 
 Kingdom, having been entirely re- written and illustrated with 
 numerous additional figures. In order to make room for the 
 large amount of new matter now introduced (not less than 
 one-fifth of the entire volume), the second chapter, contain- 
 ing a General View of the Animal Kingdom, has been much 
 abridged ; a change the Author has the less regretted being 
 obliged to make, since there are now before the public several 
 excellent Elementary Treatises on Zoology, which had no 
 existence at the time this volume originally appeared. 
 
 Everyone who desires to see the study of Physiology duly 
 appreciated as a branch of General Education, must feel 
 gratified at the progress which has been made of late years in 
 the public recognition of its value. The University of London 
 led the way, by the introduction of Animal Physiology into 
 the programme of study to which all Candidates for its Degree 
 of Bachelor of Arts are required to conform. The Universi- 
 ties of Oxford and Cambridge have since admitted it as one 
 of the subjects which Candidates may select for their Bachelor 
 of Arts Examination, and in which they may obtain Honours. 
 And in many of the large Public Educational Institutions 
 with which this country is now so abundantly furnished, it 
 forms a part of the regular course of instruction. 
 
 It has been the Author's steady aim, not merely to adapt 
 his treatise to the wants of those who wish to acquire a 
 general knowledge of the principal facts and doctrines of 
 Physiological Science, but also to render it suitable to that 
 
PREFACE. Vli 
 
 which he considers a far more important purpose of the study, 
 namely, the culture and discipline of the Mind itself. Having 
 been satisfied, by no inconsiderable experience of different 
 modes of Education, that Natural Science, if judiciously 
 taught, is second in value to no other subject as an educational 
 means, and that it may be made to call forth a more varied 
 and wholesome exercise of the mental powers than almost any 
 other taken singly, he has kept this purpose constantly in 
 view; and he trusts that the experience of intelligent In- 
 structors will be found so far to concur with his own, that the 
 study of Physiology may be still more generally introduced 
 into Popular Education. It can only be by the general diffu- 
 sion of sound information on this subject, that the Public 
 Mind can be led to understand the difference between 
 Rational Medicine, and that Empiricism which now presents 
 itself under so many different forms ; that it can appreciate 
 the true value of measures of Sanitary Reform, the efficiency 
 of which must depend upon the amount of support they 
 receive from an intelligent public opinion ; and that it can 
 be preserved from those Epidemic Delusions, whose preva- 
 lence, from time to time, is not less injurious to the minds 
 of which they lay hold, than is that of Epidemic Diseases to 
 the bodies of those who suffer from them. 
 
 He has only further to add that, whilst keeping in view 
 the most important practical applications of the Science of 
 Physiology, he has not thought it desirable to pursue these 
 too far; since they constitute the details of the Art of pre- 
 serving Health, which is founded upon it, and which may be 
 much better studied in a distinct form, when this outline of 
 the Science has been mastered. And, for the same reason, 
 he has adverted but slightly to those inferences respecting the 
 Infinite Power, Wisdom, and Goodness, of the Great First 
 Cause, which are more obvious, although, perhaps, not really 
 more clear and valid, in this Science, than in any other. 
 
Vlll PREFACE. 
 
 Believing, as he does, that such inferences are more satisfac- 
 torily based upon the general manifestations of Law and 
 Order, than upon individual instances of Design, he has 
 thought it the legitimate object of this treatise to lay the 
 foundation for them, by developing, so far as might be, the 
 Principles of Physiology, leaving it to special treatises on 
 Natural Theology, to build-up the applications. 
 
 UNIVERSITY HALL, LONDON, 
 Jan. 1859. 
 
CONTENTS. 
 
 INTRODUCTION 1 
 
 CHAPTER I. 
 
 ON THE VITAL OPERATIONS or ANIMALS, AND THE INSTRUMENTS 
 
 BY WHICH THEY ARE PERFORMED 17 
 
 CHEMICAL CONSTITUTION OF THE ANIMAL BODY ... 31 
 
 STRUCTURE OF THE PRIMARY TISSUES . 36 
 
 CHAPTER II. 
 
 GENERAL VIEW OF THE ANIMAL KINGDOM 84 
 
 VERTEBRATA . 84 
 
 MAMMALS '. 89 
 
 BIRDS 92 
 
 REPTILES 93 
 
 BATRACHIA 97 
 
 100 
 
 ARTICULATA 102 
 
 INSECTS 104 
 
 ARACHNIDA 105 
 
 CRUSTACEA 106 
 
 CIRRHIPEDA 109 
 
 MYRIAPODA 110 
 
 ANNELIDA ib. 
 
 ENTOZOA Ill 
 
 MOLLUSCA 112 
 
 CEPHALOPODA 116 
 
 PTEROPODA 117 
 
 GASTEROPODA 118 
 
 CONCHIFERA ib. 
 
 TUNICATA ]21 
 
 POLYZOA . 122 
 
CONTENTS. 
 
 CHAPTER II. Continued. 
 
 PAGE 
 
 RADIATA 123 
 
 ECHINODERMATA 125 
 
 ACALEPHJS . 128 
 
 POLYPIFERA 129 
 
 PROTOZOA 135 
 
 RHIZOPODA 136 
 
 INFUSORIA 139 
 
 PORIFERA 140 
 
 CHAPTER III. 
 
 NATURE AND SOURCES OF ANIMAL FOOD 142 
 
 CHAPTER IV. 
 
 DIGESTION AND ABSORPTION 162 
 
 PREHENSION OF FOOD 163 
 
 MASTICATION 166 
 
 INSALIVATION 176 
 
 DEGLUTITION 178 
 
 DIGESTIVE APPARATUS 181 
 
 GASTRIC DIGESTION : CHYMIFICATION 188 
 
 INTESTINAL DIGESTION : CHYLIFICATION 193 
 
 DEFECATION 195 
 
 ABSORPTION OF NUTRITIVE MATERIAL 196 
 
 SANGUIFICATION 199 
 
 CHAPTER V. 
 
 OF THE BLOOD, AND ITS CIRCULATION 202 
 
 PROPERTIES OF THE BLOOD 203 
 
 CIRCULATION OF THE BLOOD 216 
 
 CIRCULATING APPARATUS OF THE HIGHER AKIMALS . . 222 
 
 FORCES THAT MOVE THE BLOOD 232 
 
 COURSE OF THE BLOOD IN THE DIFFERENT CLASSES OF 
 
 ANIMALS . . . , , 240 
 
CONTENTS. XI 
 
 CHAPTER VI 
 
 PAGE 
 
 OF RESPIRATION 258 
 
 NATURE OP THE CHANGES ESSENTIALLY CONSTITUTING 
 
 RESPIRATION 259 
 
 STRUCTURE AND ACTIONS OF THE RESPIRATORY APPARATUS 265 
 
 CHAPTER VII. 
 
 OF EXCRETION AND SECRETION 292 
 
 GENERAL PURPOSES OF THE EXCRETING PROCESSES . . ib. 
 NATURE OF THE SECRETING PROCESS. STRUCTURE OF THE 
 
 SECRETING ORGANS 298 
 
 CHARACTERS OF PARTICULAR SECRETIONS 304 
 
 CHAPTER VIII. 
 GENERAL REVIEW OF THE NUTRITIVE OPERATIONS. FORMATION 
 
 OF THE TISSUES 316 
 
 GENERAL REVIEW OF THE NUTRITIVE OPERATIONS . . ib. 
 
 FORMATION OF THE TISSUES 317 
 
 REPAIR OF INJURIES 323 
 
 CHAPTER IX. 
 
 ON THE EVOLUTION OF LIGHT, HEAT, AND ELECTRICITY BY 
 
 ANIMALS 327 
 
 ANIMAL LUMINOUSNESS ib. 
 
 ANIMAL HEAT 332 
 
 ANIMAL ELECTRICITY 340 
 
 CHAPTER X. 
 
 FUNCTIONS OF THE NERVOUS SYSTEM 345 
 
 STRUCTURE AND ACTIONS OF THE NERVOUS SYSTEM IN THE 
 
 PRINCIPAL CLASSES OF ANIMALS 350 
 
 FUNCTIONS OP THE SPINAL CORD. REFLEX ACTION . . 374 
 FUNCTIONS OF THE GANGLIA OF SPECIAL SENSE. CON- 
 SENSUAL ACTIONS 380 
 
 FUNCTION OF THE CEREBELLUM. COMBINATION OF MUS- 
 CULAR ACTIONS 384 
 
 FUNCTION OF THE CEREBRUM.- INTELLIGENCE AND WILL . 385 
 
Xii CONTENTS. 
 
 CHAPTER XI. 
 
 PAGE 
 
 ON SENSATION, AND THE ORGANS OF THE SENSES 387 
 
 SENSE OF TOUCH 390 
 
 SENSE OF TASTE 395 
 
 SENSE OF SMELL 398 
 
 SENSE OF HEARING 401 
 
 SENSE OF SIGHT 413 
 
 CHAPTER XII. 
 
 OF ANIMAL MOTION, AND ITS INSTRUMENTS 443 
 
 CONTRACTILE TISSUES. MUSCULAR CONTRACTILITY . . 444 
 
 APPLICATIONS OF MUSCtfLAR POWER. BONES AND JOINTS . 453 
 
 MOTOR APPARATUS OF MAN. SKELETON AND MUSCLES . 464 
 OF THE ATTITUDES OF THE BODY, AND THE VARIOUS KINDS 
 
 OF LOCOMOTION 489 
 
 CHAPTER XIII. 
 
 OF THE PRODUCTION OF SOUNDS : VOICE AND SPEECH .... 513 
 
 CHAPTER XIV. 
 
 OF INSTINCT AND INTELLIGENCE 525 
 
 MANIFESTATIONS OF INTELLIGENCE 546 
 
 CHAPTER XV. 
 
 OF REPRODUCTION 552 
 
 GEMMIPAROUS OR NON-SEXUAL REPRODUCTION .... 553 
 
 SEXUAL REPRODUCTION, OR GENERATION 557 
 
ANIMAL PHYSIOLOGY. 
 
 ENTBODUCTIOK 
 
 THE importance of the study of Animal Physiology, as a 
 branch of General Education, can scarcely be over-estimated ; 
 and it is remarkable that it is not more generally appreciated. 
 It might have been supposed that curiosity alone would have 
 led the mind of Man to the eager study of those wonderful 
 actions by which his body is constructed and maintained ; 
 and that a knowledge of those laws, the observance of which 
 is necessary for the due performance of these actions, in other 
 words, for the maintenance of his health, would have been 
 an object of universal pursuit. That it has Dot hitherto been 
 so, may be attributed to several causes. The very familiarity 
 of the occurrences is one of these. We are much more apt 
 to seek for explanations of phenomena that rarely present 
 themselves, than of those which we daily witness. The Comet 
 excites the world's curiosity, whilst the movements of the 
 sun, moon, and planets are regarded as things of course. We 
 almost daily see vast numbers of animals of different tribes, 
 in active hfe around us ; their origin, growth, movements, 
 decline, death, and reproduction, are continually taking place 
 under our eyes ; and there seems to common apprehension 
 nothing to explain, where everything is so apparent. And of 
 Man too, the ordinary vital actions are so familiar, that the 
 study of their conditions appears superfluous. To be born, 
 to grow, to be subject to occasional disease, to decline, to die, 
 is his lot in common with other animals ; and what know- 
 ledge can avail (it may be asked) to avert the doom imposed 
 on him by his Creator 1 
 
2 INTRODUCTION. 
 
 In reply to this it is sufficient to state, that millions annually 
 perish from a neglect of the conditions which Divine wisdom 
 has appointed as requisite for the preservation of the body from 
 fatal disease ; and that millions more are constantly suffering 
 various degrees of pain and weakness, that might have been 
 prevented by a simple attention to those principles which it is 
 the province of Physiology to unfold. From the moment of 
 his birth, the infant is so completely subjected to the in- 
 fluence of the circumstances in which he is placed, that the 
 future development of his frame may be said to be governed 
 by them ; and thus it depends, in great part, upon the care 
 with which he is tended, and the knowledge by which that 
 care is guided, whether he shall grow up in health and vigour 
 of body and mind ; or shall become weakly, fretful, and self- 
 willed, a source of constant discomfort to himself and to 
 others ; or shall form one of that vast proportion, whose lot 
 it is to be removed from this world before infancy has ex- 
 panded into childhood. The due supply of warmth, food, and 
 air are the principal points then to be attended to ; and on 
 every one of these the greatest errors of management prevail. 
 Thousands and tens of thousands of infants annually perish 
 during the few first days of infancy, from exposure to cold, 
 which their feeble frames are not yet able to resist ; and at 
 a later period, when the infant has greater power of sustain- 
 ing its own temperature, and is consequently not so liable to 
 suffer from this cause, the seeds of future disease are sown, 
 by inattention to the simple physiological principles, which 
 should regulate its clothing in accordance with the cold or 
 lieat of the atmosphere around. Nor is less injury done by 
 inattention to the due regulation of the diet, as to the quan- 
 tity and quality of the food, and the times at which it should 
 be given ; the rules for which, simple and easy as they are, 
 are continually transgressed through ignorance or carelessness. 
 And, lastly, one of the most fertile sources of infantile dis- 
 ease, is the want of a due supply of pure and wholesome air ; 
 the effects of which are sure to manifest themselves in some 
 way or other, though often obscurely and at a remote period. 
 It is physiologically impossible for human beings to grow up 
 in a sound and healthy state of body and mind, in the midst 
 of a close, ill-ventilated atmosphere. Those that are least 
 able to resist its baneful influence, are carried off by the dis- 
 
INTRODUCTION. 3 
 
 eases of infancy and childhood ; and those whose native 
 vigour of constitution enables them to struggle through these, 
 become the victims, in later years, of diseases which cut short 
 their term of life, or deprive them of a large part of that 
 enjoyment which health alone can bring. 
 
 !Nor is the effect of these injurious causes confined to 
 infancy, though most strikingly manifested at that period. 
 " The child is father to the man," in body as well as in mind ; 
 but the vigorous health of the adult is too often wasted and 
 destroyed by excesses, whether in sensual indulgence, in 
 bodily labour, or in mental exertion, to which the very feeling 
 of buoyancy and energy often acts as the incentive ; and the 
 strength which, carefully husbanded and sustained, might 
 have kept the body and mind in activity and enjoyment to 
 the full amount of its allotted period of "threescore years 
 and ten," is too frequently dissipated in early manhood. Or, 
 again, the want of the necessary conditions for the support of 
 life, the warmth, food, and air, on which the body depends 
 for its continued sustenance, no less than for its early deve- 
 lopment, may cause its early dissolution, even where the 
 individual is guiltless of having impaired its vigour by his 
 own transgressions. 
 
 These statements are not theoretical merely : they are based 
 upon facts drawn from observations carried on upon the most 
 extensive scale. Wherever we find those conditions, which the 
 Physiologist asserts to be most favourable to the preservation 
 of the health of the body, most completely fulfilled, there do 
 sickness and mortality least prevail A few facts will place 
 this subject in a striking light. " The average mortality of 
 infants among rich and poor in this country (and with little 
 variation throughout Europe) is about one in every four and 
 a-half before the end of the first year of existence. So directly, 
 however, is infant life influenced by good or bad management, 
 that, about a century ago, the workhouses of London presented 
 the astounding result of twenty-three deaths in every twenty- 
 four infants under the age of one year. For a long time this 
 frightful devastation was allowed to go on, as beyond the reach 
 of human remedy. But when at last an improved system of 
 management was adopted in consequence of a parliamentary 
 inquiry having taken place, the proportion of deaths was 
 speedily reduced from 2,600 to 450 in a year. Here, then, 
 
 B 2 
 
4 INTRODUCTION. 
 
 was a total of 2,150 instances of loss of life, occurring yearly 
 in a single institution, chargeable, not against any unalterable 
 decrees of Providence, as some are disposed to contend as an 
 excuse for their own negligence ; but against the ignorance, 
 indifference, or cruelty of man. And what a lesson of vigi- 
 lance and inquiry ought not such occurrences to convey, 
 when, even now, with all our boasted improvements, every 
 tenth infant still perishes within a month of its birth ! " l 
 
 The effect of attention to cleanliness and ventilation in the 
 reduction of an excessive infantile mortality, has been equally 
 shown in the experience of the Dublin Lying-in Hospital. 
 At the conclusion of 1782, it was found that out of 17,650 
 infants born alive, no fewer than 2,944, or one in every six, 
 had died within the first fortnight. By the more efficient 
 ventilation of the wards, the proportion of deaths during the 
 first fortnight was at once reduced to 419 out of 8,033, or 
 but little more than one in twenty; and it has subsequently 
 been still further diminished. 
 
 In the island of St. Kilda, the most northern of the Heb- 
 rides, according to the statement of a gentleman who visited 
 it in 1838, as many as eight out of every ten children die 
 between the eighth and twelfth day of their existence ; in 
 consequence of which terrible mortality, the population of the 
 island is diminishing rather than increasing. This is due, 
 not to anything injurious in the position or atmosphere of the 
 island ; for its " air is good, and the water excellent : " but 
 to the " filth in which the inhabitants live, and the noxious 
 effluvia which pervade their houses." The huts are small, low- 
 roofed, and without windows ; and are used during the winter 
 as stores for the collection of manure, which is carefully laid 
 out upon the floor, and trodden under foot, till it accumulates 
 to the depth of several feet. The clergyman, who lives 
 exactly as those around him do, in every respect, except as 
 regards the condition of his house, has reared a family of four 
 children, all of whom are well and healthy ; whereas, accord- 
 ing to the average mortality around him, at least three out of 
 the four would have been dead within the first fortnight. 
 
 It is not a little remarkable that a recent sanitary inquiry 
 carried out by order of the Danish government, into the con- 
 
 1 Dr. A. Combe on the Physiological and Moral Management of 
 Infancy. 
 
INTRODUCTION. 5 
 
 dition of the Icelandic population, should have disclosed the 
 existence of almost precisely similar habits of life among 
 them, with almost precisely the same results. The dwellings 
 of the great bulk of the peasantry seem as if constructed for 
 the express purpose of poisoning the air which they contain. 
 They are small and low, without any direct provision for 
 ventilation, the door serving alike as window and chimney ; 
 the walls and roof let in the rain, which the floor, chiefly 
 composed of hardened sheep's-dung, sucks up ; the same 
 room generally serves for all the uses of the whole family, 
 and not only for the human part of it, but frequently also for 
 the sheep, which are thus housed during the severest part of 
 the winter. The fuel employed in this country chiefly con- 
 sists of cow-dung and sheep's-dung, caked and dried ; and 
 near the sea-coast, of the bones and refuse of fish and sea- 
 fowl ; producing a stench, which to those unaccustomed to it 
 is completely insupportable. In addition to this, the people 
 are noted for their extreme want of personal cleanliness ; the 
 same garments (chiefly of black flannel) being worn for 
 months without having even been taken off at night. Although 
 the Icelanders enjoy an almost complete exemption from 
 many diseases (such as consumption) which are very fatal 
 elsewhere, and the number of births is fully equal to the 
 usual average, the population of the island does not increase, 
 and in some parts actually diminishes. This result is in great 
 measure due, as at St. Kilda, to the very high rate of infantile 
 mortality; a large proportion of, all the 'infants born being 
 carried off before they are a fortnight old. It is in the little 
 island of Westmannoe, and the opposite parts of the coast of 
 Iceland, where the bird-fuel is used all the year round, instead 
 of (as elsewhere) during a few months only, that the rate is 
 the highest; the average mortality for many years having 
 been sixty-four out of every hundred, or nearly two out of 
 three, of all the infants born in these localities. 
 
 But it is yet more remarkable that the immediate cause of 
 the high rate of infantile mortality should have been pre- 
 cisely the same in the Workhouses of London, the Lying-in 
 Hospital of Dublin, and the close filthy huts of -the peasantry 
 of Iceland and St. Kilda ; for it was almost entirely referrible 
 to one single disease, " Trismus nascentium," or, " Lock-jaw 
 of the New-born ; " and this disease has diminished in exact 
 
6 INTRODUCTION. 
 
 proportion to the improvement of the places it previously 
 infested, in respect to ventilation and cleanliness. Thus, it is 
 BO rare for a case of it now to occur in London, that many 
 practitioners of large experience have never seen the disease. 
 In the Dublin Lying-in Hospital, the number of deaths from 
 it has been reduced to three or four yearly. And there can- 
 not be a reasonable doubt, that, by due attention to the same 
 conditions, it might be exterminated from Iceland and from 
 St. Kilda. There is scarcely, in fact, a disease incident to 
 humanity, which is more completely preventable than this ; 
 and yet the annual sacrifice of life which it formerly caused 
 in our own country alone, might have been reckoned by tens 
 of thousands. 
 
 Although the peculiar susceptibiltty of the constitution of 
 children, gives to foul air and other causes of disease a much 
 more destructive influence over them, than the like causes 
 have over persons more advanced in life, yet it is now well 
 ascertained that the rate of mortality among different classes 
 of the community varies in a degree which bears a very close 
 relation to the nature of the conditions under which they live. 
 Thus, whilst the annual average number of deaths in the whole 
 of England and Wales is about 22 out of every thousand 
 persons living, there are localities in which the annual 
 average exceeds 50 in a thousand, and others in which it falls 
 as low as 1 1 in a thousand. And it is not a little remarkable, 
 that the difference is almost entirely referrible to the mortality 
 produced by Fevers and allied diseases, which, as experience 
 has now fully demonstrated, are absolutely preventible by due 
 attention to the ordinary conditions of health. 
 
 As the population of England and Wales may at present be 
 estimated at about twenty millions, and its actual mortality at 
 about 440,000, what maybe termed its inevitable mortality 
 arising from diseases that would not be directly affected by 
 sanitary improvements would be only one half, or 220,000 ; 
 so that the same number of lives may be considered to be 
 annually sacrificed by the public neglect of the means of pre- 
 serving them, the deaths from typhus alone being no fewer 
 than 50,000. But as it is scarcely to be supposed that every 
 part of our population could be placed in conditions as favour- 
 able as those which prevail where the rate of mortality is 
 the lowest, we may take 13 per thousand as the average to 
 
INTRODUCTION. 7 
 
 which, it may be safely affirmed, on the basis of actual expe- 
 rience, that the annual mortality may be reduced, by such 
 efficient sanitary measures as render the dwellings of the mass 
 of the population fit for human -habitation ; this would give 
 an annual mortality for England and Wales of 260,000, 
 showing a saving of 180,000 lives annually in that one por- 
 tion of the British empire. And it must be remembered 
 that this amount of mortality represents a vastly greater 
 amount of sickness, since, for every death, there are numerous 
 cases of severe illness ; so that it would be scarcely too much 
 to affirm that at least a million out of the whole number of 
 such cases annually occurring, are preventible, like the 
 180,000 deaths, by adequate provisions for the supply of pure, 
 air and water, and by efficient sewerage for the removal of 
 decomposing matters. It cannot be doubted that, even in 
 a mere pecuniary point of view, the expense of such arrange- 
 ments would be amply compensated by the prevention of a 
 vast amount of that loss of productive labour of various 
 kinds, which is at present due to disease ; and, considered on 
 the large scale, as a question of social economy, the import- 
 ance of sanitary legislation can scarcely be over-rated. But 
 much cannot be expected to be done in this direction, until 
 such an intelligent public opinion shall have been created, by 
 the general diffusion of sound physiological information, as 
 shall be sufficiently forcible to bear down the self-interested 
 opposition of those, who do not see that the value of their 
 property will be permanently increased at least in proportion 
 to the amount of money judiciously expended upon it. 
 
 A more remarkable illustration of what is to be effected by 
 sanitary improvements can scarcely be adduced, than that 
 which is presented by the comparison between the locality 
 termed "the Potteries," in the immediate vicinity of Ken- 
 sington, and the " Model Lodging-houses," which have been 
 erected in various parts of the Metropolis. The site of the 
 group of dwellings constituting the former is far from being 
 insalubrious in itself, and rows of handsome houses are rising 
 up in its immediate neighbourhood; but the condition of 
 these dwellings is most filthy. A few years ago, as many as 
 3,000 pigs were kept in this locality (the number has since 
 been somewhat diminished) ; and the boiling of fat and other 
 offal, which is carried on by some of the pig-feeders, some- 
 
8 INTRODUCTION. 
 
 times taints tlie air for a mile round. Very few of the tene- 
 ments have any water-supply ; the wells are useless, or worse 
 than useless, through the contamination of their water with 
 putrescent liquid which filters down into them ; and the 
 drainage of the dwellings both for men and pigs is almost 
 entirely superficial, being chiefly discharged into a stagnant 
 piece of water called the "Ocean," which is covered with 
 a filthy slime and bubbles with poisonous gases, and very 
 commonly has dead dogs or cats floating on its surface. It is 
 difficult to conceive anything more horribly offensive than 
 the rears of some of the houses, whose yards are filled with 
 ordure and other filth collected for manure, which is here 
 .stored for weeks, or even months, until an opportunity occurs 
 for selling it. And even the public ways are generally 
 covered with black putrescent mire. Now, during ten 
 months of the year 1852, when no epidemic prevailed, as 
 many as forty deaths occurred in the Potteries, out of a 
 population of about one thousand, the mortality being 
 thus at the rate of 48 per thousand annually ; and no 
 fewer than four-fifths of these deaths occurred at, or beneath, 
 jive years of age. In the first ten months of 1849, when 
 cholera was prevalent, the number of deaths was fifty, or 
 about one in twenty of the whole population, twenty-one of 
 these being due to cholera and diarrhoea, and twenty-nine to 
 typhus and other diseases. On the other hand, in the whole 
 population of the " Model Lodging-houses," amounting to 
 1,343, only seven deaths took place in the whole twelve 
 months of 1852, or at the rate of scarcely more than 5 per 
 thousand; and although they contain a large proportion of 
 children, yet only half the number of deaths occurred below 
 ten years old. During the prevalence of the cholera-epidemic, 
 no cases of that disease occurred among them, although it was 
 raging in their various neighbourhoods ; and from the time 
 that their drainage has been rendered thoroughly efficient, no 
 case of fever has presented itself among their inmates. 
 
 The experience of Cholera-epidemics is peculiarly valuable, 
 on account of the marked tendency of this disease to search 
 out and expose defects, which have continued to produce 
 other diseases year after year, without having been suspected 
 as the causes of them. The greatest severity in each visita- 
 tion has shown itself in identical localities, provided those 
 
INTRODUCTION. 9 
 
 remained in the same foul state as at first ; whilst new loca- 
 lities have been affected, just in proportion to the degree in 
 which they have participated in the same conditions ; and those 
 originally attacked have escaped, wherever they had adopted 
 the requisite means of purification. Thus, at Newcastle- on- 
 Tyne and Gateshead, the first outbreak occurred in the very 
 same streets, and even in the same houses, in the three visi- 
 tations of 1831, 1848, and 1853. An outbreak which 
 occurred in 1853, at Luton, in Bedfordshire, vrhere, out of 
 a population of 126 persons, inhabiting twenty-five houses, 
 no fewer than fifty-four attacks of choleraic disease, fifteen of 
 them fatal, took place within three weeks, was most dis- 
 tinctly traceable to defect of sewerage, which had been pre- 
 viously manifesting its malign influence on the general health 
 of the town. And the fearful pestilence which devastated 
 the neighbourhood of Golden Square (London) in the autumn 
 of 1854, was no less distinctly traceable to the contamination 
 of the pump-water by the bursting of a sewer into the well. 
 On the other hand, Exeter and Nottingham, which suffered 
 severely in the first epidemic, escaped comparatively un- 
 harmed in the subsequent visitations ; and this result is 
 plainly due to the sanitary improvements which had been 
 made in the interval. In 1832 there perished of the epide- 
 mic in Exeter, as many as 402, out of a population of 
 28,000, or no fewer than one in seventy ; and a vast amount of 
 suffering, with a heavy expense, was entailed upon the town. 
 In 1848-9, on the other hand, out of a population of about 
 32,600, there were but 44 deaths, or less than one in seven 
 hundred; and upwards of one-half of these occurred in 
 a single parish, that lies very low, and in the midst of putrid 
 exhalations from the city drains. In Nottingham, with a 
 population of 50,000, there were 296 fatal cases of cholera in 
 1832, nearly all of these being in the lower part of the town, 
 which was ill-drained, extremely filthy, and densely popu- 
 lated ; but in 1848-9, though the population had increased to 
 58,000, the number of deaths from cholera was no more than 
 18, all of these occurring in localities, which, in spite 
 had been done, retained much of their previous filth. 
 
 The foregoing are only samples of a vast number of 
 which might be adduced, in proof of the absolute preventi- 
 bility of Cholera, and of other diseases of the same class. It 
 
10 INTRODUCTION. 
 
 may be well to subjoin a few additional facts, derived from 
 the cholera-experience of 1848-9, which, from its general 
 diffusion, tested, in a very remarkable degree, the relative 
 healthfulness of different provincial towns, and of different 
 metropolitan districts. Thus, among the whole population of 
 the ten towns of Exeter, Derby, Cheltenham, Leicester, 
 Nottingham, Eochdale, Norwich, Preston, Halifax, and Bir- 
 mingham, amounting to 657,000, there were no more than 
 238 deaths from cholera ; whilst, in an equal population 
 inhabiting the towns of Newcastle-under-Lyne, Plymouth, 
 Brighton, Merthyr Tydvil, Portsea, Tynemouth, Wigan, Hull, 
 Wolverhampton, and Leeds, the number of deaths was no 
 fewer than 10,415, or forty-three times as great. So again, in 
 twenty-five Metropolitan districts, chiefly on the north side of 
 the Thames, having a total population of about 310,000, the 
 number of deaths from cholera was only 389 ; whilst in 
 twenty-two districts, almost entirely on the south side of the 
 river, the number of deaths, out of a population of almost 
 exactly the same amount, was 5,932, or more than twelve 
 times as great. In no instance is there the least difficulty in 
 accounting for these contrasts. They all point to the same 
 general conclusion; that, namely, of the immense influence 
 which is exercised over human health by the purity of the air 
 that is breathed, and of the water that is drunk ; and it is 
 because these two conditions are in a great degree capable of 
 public regulation, that legislative interference has so much in 
 its power, and is so imperatively called for by the interests of 
 humanity, which speak solemnly and distinctly to all who 
 claim the rights of property in the foul " plague-spots " which 
 deface our country, of their bounden duty to render them not 
 unfit for human occupation. 
 
 But although the magnitude of the evils resulting from the 
 neglect of the conditions of Public Health, gives to this sub- 
 ject the first claim on our consideration, yet it is not the less 
 important that every individual should acquire as much 
 knowledge of the constitution of his body, and of the right 
 means of keeping it in working order, as will save him from 
 seriously damaging either himself or other people by his 
 ignorance of such matters. It is less than ten years since a 
 fearful sacrifice of life occurred among the deck-passengers on 
 board the Irish steamer " Londonderry," who were ordered 
 
INTRODUCTION. 1 1 
 
 below "by the Captain on account of the stormy character of 
 the weather, and on whom the hatches were closed down, 
 although the cabin which was crowded by them had scarcely 
 any other means of ventilation. Out of 150 of these unfor- 
 tunates, no fewer than 70 died of suffocation before the 
 morning, a catastrophe only second to that which occurred 
 in the "Black Hole of Calcutta," in which 123 out of 146 
 died during one night's confinement in a room eighteen feet 
 square, provided with only two small windows. Yet the 
 Captain of the " Londonderry " was acquitted of all blame ; 
 since he had done what seemed to him best for the welfare of 
 his passengers, the result being due simply to his astound- 
 ing ignorance of the fact that men cannot live without having 
 air to breathe. Not a year passes without the occurrence of 
 numerous deaths from the like cause ; and yet these are 
 really insignificant, when compared with the vast amount of 
 disease which is constantly attributable to inattention, on the 
 part of individuals, to those simple means of securing an 
 adequate supply of air which are within the reach of every 
 one. And when we bear in mind that the respiratory func- 
 tion is only one of the processes whose due performance has 
 to be provided for, and that the regulation of the food and 
 drink, of the excretions, of clothing and temperature, of 
 exercise (bodily and mental) and repose, and of the repro- 
 ductive functions, all fall within rules which it is the pro- 
 vince of Physiology to prescribe, we see how vain it is to 
 expect that the body can be maintained in health, without 
 some acquaintance with that science, or at least with the 
 rules which it lays down. For, although it is quite true that 
 man has within himself certain instincts which afford him a 
 considerable measure of guidance in all these particulars, 
 hunger and thirst, for example, leading him to take the 
 sustenance which his body requires, weariness tempting him to 
 needed repose, and so on, yet it is no less certain that in a 
 state of artificial civilisation these instincts are so often over- 
 borne by acquired tastes, or by the pressure of other circum- 
 stances, that they cannot alone be safely relied on. Hence it 
 is all the more important that the rules for preserving health 
 should be based on an intelligent knowledge of Physiological 
 principles ; otherwise, }ike the natural instincts, they are likely 
 to be put aside as occasion prompts ; whereas, in proportion as 
 
12 INTRODUCTION. 
 
 the individual is possessed of their rationale, will he be likely 
 to shape his conduct in accordance with them. 
 
 The general principles of Physiological science, again, will 
 be likely to be thoroughly apprehended, in proportion as they 
 are based on an extended recognition of the phenomena 
 which they comprehend. Every physiologist is now satisfied 
 that the life or vital actions of no one species of animal can 
 be correctly understood, unless compared with those of other 
 tribes of different conformation. Hence, for the student of 
 physiology to confine himself to the observation of what 
 takes place in Man alone, would be as absurd as for the astro- 
 nomer to restrict himself to the observation of a single planet, 
 or for the chemist to endeavour to determine the properties of 
 a metal by the study of those of that one only. There is not 
 a single species of animal, that does not present us with a set 
 of facts which we should never learn but by observing it ; 
 and many of the facts ascertained by the observation of the 
 simplest and most common animals, throw great light upon 
 the great object of all our inquiries, the Physiology of Man. 
 For though in him are combined, in a most wonderful and 
 unequalled manner, the various faculties which separately 
 exhibit themselves in various other animals, he is not the 
 most favourable subject for observing their action ; for the 
 obvious reason that his machinery (so to speak) is rendered 
 too complex, on account of the multitude of operations it has 
 to perform : so that we often have to look to the lowest and 
 simplest animals for the explanation of what is obscure in 
 man, their actions being less numerous, and the conditions 
 which they require being more easily ascertained. 
 
 The diffusion of Animal life is only one degree less exten- 
 sive than that of vegetable existence. As animals cannot, 
 like plants, obtain their support directly from the elements 
 around, they cannot maintain life, where life of some kind 
 has not preceded them. But vegetation of the humblest 
 character is often sufficient to maintain animals of the highest 
 class. Thus the lichen that grows beneath the snows of 
 Lapland, is, for many months in the year, the only food of 
 the rein-deer ; and thus contributes to the support of human 
 races, which depend almost solely upon this useful animal for 
 their existence. JSTo extremes of temperature in our atmo- 
 sphere seem inconsistent with animal life. In the little pools 
 
INTRODUCTION. 13 
 
 formed by the temporary influence of the sun upon the sur- 
 face of the arctic snows, animalcules have been found in 
 a state of activity ; and the ocean of those inhospitable regions 
 is tenanted, not only by the whales and other monsters which 
 we think of as their chief inhabitants, whose massive forms 
 are only to be encountered " few and far between," but by 
 the shoals of smaller fishes and inferior animals of various 
 kinds upon which they feed, and through vast fleets of which 
 the mariner sails for many miles together. 
 
 On the other hand, even the hottest and most arid portions 
 of the sandy deserts of Africa and Asia are inhabited by 
 animals of various kinds, provided that vegetables can find 
 sustenance there. The humble and toilsome ants make these 
 their food, and become in turn the prey of the cunning ant- 
 lion and of the agile lizard j and these tyrants are in their 
 turn kept under by the voracity of the birds which are 
 adapted to prey upon them. The waters of the tropical ocean 
 never acquire any high temperature, owing to the constant 
 interchange which is taking place between them and those of 
 colder regions ; but in the hot springs of various parts of the 
 world, we have examples of the compatibility of even the 
 heat of almost boiling water with the preservation of animal 
 life. Thus in a hot spring at Manilla which raises the ther- 
 mometer to 187, and in another in Barbary whose usual tem- 
 perature is 172, fishes have been seen to flourish. Fishes 
 have been thrown up in very hot water from the crater of a 
 volcano, which, from their lively condition, was apparently 
 their natural residence. Small caterpillars have been found 
 in hot springs of the temperature of 205; and small black 
 beetles, which died when placed in cold water, in the hot 
 sulphur baths of Albano. Intestinal worms within the body 
 of a carp have been seen alive after the boiling of the fish for 
 eating ; and the inhabitants of some little snail- shells, which 
 seemed to have been dried up within them, have been caused 
 to revive by placing the shells in hot water for the purpose of 
 cleaning them. 
 
 The lofty heights of the atmosphere, and the dark and 
 rayless depths of the ocean, are tenanted by animals of 
 beautiful organisation and wonderful powers. Vast flights of 
 butterflies, the emblems of summer and sunshine, may some- 
 times be seen above the highest peaks of the Alps, almost 
 
14 IXTKODTJCTIOX. 
 
 touching with their fragile wings the hard surface of the 
 never-melting snow. The gigantic condor or vulture of the 
 Andes has been seen to soar on its widely-expanded wings 
 far above the highest peak of Chimborazo, where the baro- 
 meter would have sunk below ten inches. The existence of 
 marine fishes has been ascertained at a depth of from 500 to 
 600 fathoms ; and in the deep recesses of those caverns in 
 Styria and Carniola, which are inhabited by the curious 
 Proteus (ZooL. 532), numerous species of insects are found, 
 all of which, however, like the Proteus, are blind. 
 
 Having thus glanced at some of those facts which demon- 
 strate the practical importance of the study of Physiology, 
 and having indicated the mode in which that study should 
 be pursued, it remains to offer a few observations upon its 
 value with reference to the culture and discipline of the 
 mind itself. One of its great advantages is, that it not 
 only calls forth, in a degree second to no other, both the 
 observing and the reasoning powers ; but that it offers so 
 much that is attractive by its novelty to those who enter 
 upon it seriously, and make it an object of regular pursuit. 
 For it affords abundant opportunities, even to the beginner, 
 of adding to the common stock of information respecting the 
 structure and habits of the vast number of living beings that 
 people our globe. The immense variety of the objects which 
 come under the investigation of the physiologist, so far from 
 discouraging the learner, should have the effect of stimulating 
 his exertions, by. opening to him new fields for productive 
 cultivation. Of by far the larger part of the organised crea- 
 tion, little is certainly known. Of no single species, of 
 none of our commonest native animals, not even of Man 
 himself, can our knowledge be regarded as anything but im- 
 perfect. Of the meanest and simplest forms of animal life, we 
 know perhaps even less than we do of the more elevated and 
 complex ; and it cannot be doubted that phenomena of the 
 most surprising nature yet remain to be discovered by patient 
 observation of their actions. It was not until very recently, 
 that the existence of a most extraordinary series of metamor- 
 phoses, more wonderful than those of the insect, has been 
 discovered in the jelly-fish of our seas, in the barnacles that 
 
INTRODUCTION. 15 
 
 attach, themselves to floating pieces of timber, and in the crabs, 
 lobsters, and shrimps of our shores. The very best accounts 
 we have, of the structure, habits, and economy of the lower 
 tribes of animals, have been furnished to us by individuals 
 who did not think it beneath them to devote many years to 
 the study of a single species ; and as there are very few 
 which have been thus fully investigated, there is ample 
 opportunity for every one to suit his own taste in the choice 
 of an object. 
 
 And none but those who have tried the experiment, can 
 form an estimate of the pleasure which the study of Nature 
 is capable of affording to its votaries. There is a simple 
 pleasure in the acquisition of knowledge, worth to many far 
 more than the acquisition of wealth. There is a pleasure in 
 looking in upon its growing stores, and watching the expan- 
 sion of the mind which embraces it, far above that which the 
 miser feels in the grovelling contemplation of his hard-sought 
 pelf. There is a pleasure in making it useful to others, com- 
 parable at least to that which the man of generous benevo- 
 lence feels in ministering to their relief with his purse or his 
 sympathy. There is a pleasure in the contemplation of beauty 
 and harmony, wherever presented to us. And are not all 
 these pleasures increased, when we are made aware, as in the 
 study of Nature we soon become, that the sources of them 
 are never-ending, and that our enjoyment of them becomes 
 more intense in proportion to the comprehensiveness of our 
 knowledge ? And does not the feeling that we are not look- 
 ing upon the inventions or contrivances of a skilful human 
 artificer, but studying the wonders of a Creative Design 
 infinitely more skilful, immeasurably heighten all these 
 sources of gratification? If it is not every one who can 
 feel all these motives, cannot every one feel the force of 
 some ? 
 
 There is certainly no science which more constantly and 
 forcibly brings before the mind the power, the wisdom, and 
 the goodness of the Creator. For whilst the Astronomer has 
 to seek for the proofs of these attributes in the motions and 
 adjustments of a universe, whose nearest member is at a 
 distance which imagination can scarcely realize, the Physio- 
 logist finds them in the meanest worm that we tread beneath 
 our feet, or in the humblest zoophyte dashed by the waves 
 
16 INTRODUCTION. 
 
 upon our shores, no less than in the gigantic whale, or massive 
 elephant. And the wonderful diversity which exists amongst 
 the several tribes of animals, presents us with a continual 
 variety in the mode in which these adjustments are made, 
 that prevents us from ever growing weary in the search. 
 
 But it is not only in affording us such interesting objects 
 of regular study, that the bounty of Nature is exhibited. 
 Perhaps it is even more keenly felt by the mind which, 
 harassed by the cares of the world, or vexed by its disap- 
 pointments, or fatigued by severer studies, seeks refuge in 
 her calm retirement, and allows her sober gladness to exert 
 its cheering and tranquillizing influence on the spirit. 
 
 " With tender ministrations, thou, Nature, 
 Healest thy wandering and distracted child ; 
 Thou pourest on him thy soft influences, 
 Thy sunny hues, fair forms, and breathing sweets, 
 The melody of woods, and winds, and waters. 
 Till he relent, and can no more endure 
 To be a jarring and dissonant thing 
 Amidst the general voice and minstrelsy, 
 But bursting into tears wins back his way, 
 His angry spirit healed and harmonized 
 By the benignant touch of love and mercy." 
 
 COLERIDGE. 
 
DISTINCTIVE CHARACTERS OF ORGANIZED BODIES. 17 
 
 CHAPTER I. 
 
 OF THE VITAL OPERATIONS OF ANIMALS, AND THE INSTRUMENTS BY 
 WHICH THEY ARE PERFORMED. 
 
 1. LIVING beings, whether belonging to the Animal or to 
 the Vegetable kingdom, are distinguished from the masses of 
 inert matter of which the Mineral kingdom is made up, by 
 peculiarities of form and size, of structure, of elementary 
 composition, and of actions. Wherever a definite form is 
 exhibited by Mineral substances, it is bounded by plane 
 surfaces, straight lines, and angles, and is the effect of the 
 process of crystallization, in which particles of like nature 
 arrange themselves on a determinate plan, so as to produce a 
 regular aggregation; and there is, probably, no Inorganic 
 element or combination which is not capable of assuming such 
 a form, if placed in circumstances adapted to the manifestation 
 of its tendency to do so. The number of different crystalline 
 forms is by no means large ;' and as many substances crystal- 
 lize in several dissimilar forms, whilst crystals resembling one 
 another in form often have a great diversity of composition, 
 there is no constant correspondence between the crystalline 
 forms and the. essential nature of the greater number of 
 mineral substances. If that peculiar arrangement of the 
 molecules which constitutes crystallization should be wanting, 
 so that simple cohesive attraction is exercised in bringing 
 them together, without any general control over their direc- 
 tion, an indefinite or shapeless figure is the result. "With 
 this indefiniteness of form, there is an absence of any limit 
 whatever in regard to size : a crystal may go on increasing 
 continuously, so long as there is new material supplied ; but 
 this new material is deposited upon its surface merely, and 
 its addition involves no interstitial change ; the older particles, 
 which were first deposited, and which continue to form the 
 nucleus of the crystal, remaining just as they were. In Or- 
 ganized bodies, on the other hand, we meet with convex 
 surfaces and rounded outlines, and with a general absence of 
 angularity ; and the simplest grades, both of Animal and of 
 
18 DISTINCTIVE CHARACTERS OP ORGANIZED BODIES. 
 
 Vegetable life, present themselves tinder a shape which ap- 
 proaches more or less closely to the globular. From the 
 highest to the lowest, each species has a certain characteristic 
 form, by which it is distinguished ; this form, however, often 
 presents marked diversities at different periods of life, and 
 it is also liable to vary within certain limits among the 
 individuals of which the species is composed. The size of 
 Organized structures, like their form, is restrained within 
 tolerably definite limits, which may nevertheless vary to a 
 certain extent among the individuals of the same species. 
 These limits are most obvious in the higher animals, whilst 
 they seem almost to disappear among certain members 
 both of the Animal and the Vegetable kingdoms, which tend 
 to increase themselves almost indefinitely by a process of 
 gemmation or budding, so as to produce aggregations of 
 enormous size. Such aggregations, however, being formed 
 by the repetition of similar parts, which can maintain their 
 existence when detached from one another, may, in some 
 sense, be regarded as clusters of distinct organisms, rather 
 than as single individuals. Such is the case, for example, 
 with the wide-spreading forest-tree, and with those enormous 
 masses of coral of which reefs and islands are composed in 
 the Polynesian Archipelago. For every separate leaf-bud of 
 the tree, like every single polype of the coral, if detached 
 from its stock, can, under favourable circumstances, perform 
 all the functions of life, and can develop itself into a new 
 fabric resembling that from which it was separated. 
 
 2. The differences between Organized and Inorganic bodies, 
 in regard to their structure, are much more important than 
 those which relate to their external configuration. . Every 
 particle of a mineral substance, in which there has not been 
 a mere mixture of components, exhibits the same properties 
 as those possessed by the whole ; the minutest atom of car- 
 bonate of lime, for instance, has all the properties of a crystal 
 of calc-spar, were it as large as a mountain. Hence it is the 
 essential nature of an Inorganic body that each of its particles 
 possesses a separate individuality, and has no relation but 
 that of juxtaposition to the other particles associated with 
 itself in one mass. The Organized structure, on the other 
 hand, receives^ its designation from being made up of a greater 
 or less number of dissimilar parts or organs ; each of these 
 
DISTINCTIVE CHARACTEES OF ORGANIZED BODIES. 19 
 
 being the instrument of some special action or function, which. 
 it performs under certain conditions ; and the concurrence of 
 all these actions being necessary to the maintenance of the 
 structure in its normal or regular state. Hence there , is a 
 relation of mutual dependence among the parts of an Organized 
 fabric, which is quite distinct from that of mere proximity ; 
 and this relation is most intimate, not in the case of those 
 beings which have the greatest multiplication of parts, but 
 among those in which there is the greatest dissimilarity 
 among the actions of the several organs. Thus it has been 
 just shown that among Plants and Zoophytes, a small fraction 
 of an organism may live independently of the rest; the 
 necessary condition being that it shall either itself contain 
 all the organs essential to life, or shall be capable of pro- 
 ducing them, as when the leaf-bud develops rootlets for 
 its nutrition. This "vegetative repetition," and consequent 
 capacity of sustaining the loss of large portions of the fabric, 
 still shows itself in animals much higher in the scale than 
 Zoophytes ; thus it is not uncommon to meet with Star-fish 
 in which not only one or two, out of the five similar arms, 
 but even three or four, have been lost, without the destruction 
 of the animal's life ; and this is the more remarkable, as these 
 arms are not simply members for locomotion or prehension, 
 but are really divisions of the body, containing prolongations 
 of the stomach. In like manner, many of the Worm tribes, 
 whose bodies show a longitudinal repetition of similar parts, 
 can lose a large number of their joints without sustaining any 
 considerable damage. In the bodies of the higher animals, 
 however, where there are few or no such repetitions (save 
 in the two lateral halves of the body), and where there is, 
 consequently, a greater diversity in character and function 
 between the different organs, the mutual dependence of their 
 actions upon one another is much more intimate, and the loss 
 of a single part is much more likely to endanger the existence 
 of the whole. Such structures are said to be more highly 
 organized than those of the lower classes ; the principle of 
 " division of labour " being carried much further in them, 
 a much greater variety of objects being attained, and a much 
 higher perfection in the accomplishment of them being thus 
 provided for. Thus the individuality of a plant or a zoo- 
 phyte may be said to reside in each of its multiplied parts ; 
 
 c2 
 
20 DISTINCTIVE CHARACTERS OF ORGANIZED BODIES. 
 
 whilst that of one of the higher animals resides in the sum 
 of all its organs. 
 
 3. The very simplest Organized fabric is further dis- 
 tinguished from Inorganic bodies by marked differences in 
 regard to intimate structure and consistence. Inorganic sub- 
 stances can scarcely be regarded as possessing a structure, 
 since their perfection consists in their homogeneousness and 
 their solidity. It is the essential character of Organized 
 fabrics, on the other hand, that they are formed by a com- 
 bination of solid and liquid components, so intimately 
 combined and arranged as to impart a heterogeneous cha- 
 racter to almost every portion of their substance ; and in all 
 the parts which are most actively concerned in the vital 
 operations, softness of texture seems an essential condition, 
 those parts only being so consolidated as to acquire anything 
 comparable to the density of mineral bodies, which are 
 destined to possess the simply physical property of resistance, 
 so as to be subservient either to support, to protection, or 
 to mechanical movement. A comparison between the pulpy 
 portion of the leaves of Plants and the heartwood of the stem, 
 between the membranous tissues of the Coral-polypes and the 
 stony masses which they form, between the firm shell of the 
 Crab or the Oyster and the substance of the included body, 
 or between the solid bones of Man and the flesh which clothes 
 them, will serve to* illustrate this principle. It is in such 
 solidified portions of the Organized fabric, that the greatest 
 resemblance exists to Inorganic bodies; but even these 
 portions all pass through the condition of soft tissue, the 
 consolidation of which is effected by the deposit of some 
 hardening material (generally carbonate or phosphate of lime), 
 in its interstices. It is by the reaction which is continually 
 taking place' between the solid and the liquid parts of 
 Organized structures, that their integrity is maintained. For 
 we shall find it to be a result of their peculiar composition, 
 that they are prone to continual decay ; and this decay would 
 speedily destroy them altogether, if it were not compensated 
 by new formation. The materials for their reproduction 
 must always be presented to the tissues in a liquid state, and 
 all the dead and decomposing matter must be reduced to the 
 same form, in order that it may be carried off; so that the 
 intermingling or mutual penetration of solids and liquids, in 
 
DISTINCTIVE CHARACTERS OF ORGANIZED BODIES. 21 
 
 the minutest parts of Organized bodies, is a necessary condition 
 of their existence. 
 
 4. Organized structures are further distinguished from In- 
 organic masses by the peculiarity of their chemical constitution. 
 This peculiarity does not consist, however, in the presence of 
 any elementary substances which are not found elsewhere ; 
 for all the elements of which Organized bodies are composed, 
 exist abundantly in the world around. This, indeed, is a 
 necessary consequence of the mode in which they are built 
 up ; for that which the parent communicates in giving origin 
 to a new being, is not the structure itself, but the capacity to 
 form that structure from materials supplied to it ; and it is 
 by progressively converting these materials to its own use, 
 that the germ develops itself into the complete fabric. JS~ow 
 out of about seventy simple or elementary substances which 
 are known to occur in the Mineral world, not above twenty 
 present themselves as constituents of Vegetable and Animal 
 fabrics ; and many of these occur there in extremely minute 
 proportion. Some of them, indeed, appear to be introduced 
 merely to answer certain chemical or mechanical purposes ; 
 and the composition of the parts which possess the highest 
 vital endowments is extremely uniform. They are nearly all 
 formed at the expense of certain " organic compounds," which 
 are made up of the four elementary substances, oxygen, hy- 
 drogen, carbon, and nitrogen ; and these elements appear to 
 be united, not as in the case of inorganic compounds, two 
 by two, or after the binary method, but all four together, 
 so as to form a compound atom of great complexity. Thus 
 common nitre is regarded as a binary compound of nitric acid 
 and potass, since it can be decomposed into those two con- 
 stituents and can be re-formed by their union; and in the 
 same manner, its nitric acid is a binary compound of nitrogen 
 and oxygen, whilst its potass is a binary compound of potassium 
 and oxygen. But neither albumen nor gelatine, which are 
 the principal materials of the animal tissues, can be resolved 
 into any two other substances, by the union of which it can 
 be re-formed ; and when once it has been decomposed by che- 
 mical agencies, no means known to the chemist can reproduce 
 it. Albumen can, in fact, be generated only by the living 
 Plant, at the expense of the carbon, hydrogen, oxygen, and 
 nitrogen, which it draws from the elements around; and 
 
22 DISTINCTIVE CHARACTERS OP LIVING ORGANISMS. 
 
 gelatine can only be formed in the animal body by a meta- 
 morphosis of the albumen which it derives from the Plant. 
 The peculiar mode in which the elements of these substances 
 are held together, renders them very prone to decomposition ; 
 so that Organized bodies, when no longer alive, rapidly pass 
 into decay, unless they are secluded from the contact of 
 oxygen, or are kept at a very low temperature. Such decay, 
 however, is continually taking place during life, and would 
 make itself obvious if its products were not carried out of 
 the system as fast as they are generated within it. It 
 essentially consists in the resolution of the four principal 
 components of organic compounds carbon, hydrogen, oxy- 
 gen, and nitrogen, in combination with oxygen drawn from 
 the atmosphere into the three binary compounds, water, 
 carbonic acid, and ammonia, which thus restore to the In- 
 organic world the original materials of Organized fabrics, in 
 the very forms from which those materials were first derived 
 by the agency of the growing Plant. (See VEGET. PHYSIOL.) 
 5. It is, however, by their peculiar actions, that living 
 Organisms are most completely differentiated from the inert 
 bodies of which the Mineral kingdom is composed. There 
 can be no doubt that of many of the changes which take 
 place during the life of an Organized being, a large proportion 
 (especially in the Animal kingdom) are effected by the direct 
 agency of physical and chemical forces ; and there is no 
 reason to believe that these forces have any other. operation 
 in the living body, than they would have out of it under 
 similar circumstances. Thus the propulsion of the blood by 
 the heart, through the large vessels, is a purely mechanical 
 phenomenon ; as is also the movement of the limbs by the 
 lever-action of the forces brought to bear on their bones. 
 So, again, the digestive operations which take place in the 
 stomach are of a purely chemical nature ; and the interchange 
 of gases between the air and the blood, which takes place in 
 the act of respiration, must be regarded in the same light. 
 But after every possible allowance has been made for the 
 operation of physical and chemical forces in the living or- 
 ganism, there still remain a large number of phenomena 
 which cannot be in the least explained by them, and which 
 must be regarded as the result of an agency that differs from 
 these as they differ from each other ; and this agency, which 
 
DISTINCTIVE CHARACTERS OF LIVING ORGANISMS. 23 
 
 is recognised by the effects it produces in the same manner 
 as we recognise heat or electricity by their effects may be 
 conveniently designated vital force. 1 Thus, to revert to our 
 previous illustrations, the mechanical power employed in the 
 propulsion of the blood, or in the movements of the limbs, is 
 evolved by muscular contraction, a phenomenon altogether 
 peculiar to the living muscle ; and the muscle derives its pro- 
 perty of contractility from the previous development of its 
 peculiar tissue in the act of nutrition. So the solvent 
 fluids by which the digestion of food is accomplished, are 
 separated from the blood by an act of secretion, which can 
 only be performed by a glandular apparatus in the living 
 walls of the alimentary canal. And the materials for the 
 nutrition of the muscular tissue, and for the secretion of the 
 digestive solvent, as of all the other acts of nutrition and 
 secretion which are continually going on in the living body, are 
 derived from the blood, a liquid which possesses properties 
 very different (as we shall hereafter see) from any mere 
 mixture of chemical compounds, and which is prepared by 
 actions totally beyond the power of the chemist to imitate, 
 the laboratory of the living organism being requisite for their 
 performance. 
 
 6. The whole assemblage of vital actions whiqh is per- 
 formed by the living Animal, may be arranged under two 
 principal groups ; one of them consisting of those which are 
 directly concerned in the development and maintenance of 
 its Organized fabric ; the other including all those by which 
 it is brought into conscious relation with the world around. 
 The former group includes the acts of digestion, absorption, 
 and assimilation, by which the nutritive materials are pre- 
 pared for becoming part of the living fabric ; the circulation 
 of the assimilated materials through the body ; their conver- 
 sion, by the act of nutrition, into the solid textures ; the 
 formation of various secretions, having various purposes to 
 serve in the economy ; the removal, by the acts of respiration 
 
 1 The Author has elsewhere given his reasons for the belief, that 
 Vital force bears the same " correlation " to the Physical and Chemical 
 forces, as the latter bear to each other ; but the discussion of this sub- 
 ject is not suited to an elementary treatise ; and the essential peculiarity 
 of the manifestations of vital force in the phenomena of life, requires 
 that it should be treated as belonging to a distinct category. 
 
24 DISTINCTIVE CHAEACTERS OF ANIMALS. 
 
 and excretion, of the effete matters with, which the blood be- 
 comes charged by the decomposition continually going on in 
 the body ; the maintenance of animal-heat by the same process ; 
 and the act of reproduction, whereby the race is perpetuated, 
 in spite of the limited duration of the individual. The fore- 
 going, which are for the most part common to the Animal and 
 the Plant, are termed Organic Functions, or Functions of 
 Vegetative Life. But, in addition to these, it is the character- 
 istic of Animals generally, that they are sensible to impressions 
 made by surrounding objects, so that they possess some con- 
 sciousness of what is going on about them ; and that they also 
 possess the power of re-acting on those objects by movements 
 of their own, so as to change either their own places, or 
 the places of surrounding objects in relation to them- 
 selves. These two functions, sensibility and the power of 
 spontaneous motion, being peculiar to animals, are distin- 
 guished as Animal Functions, or Functions of Animal Life. In 
 the higher, animals, they are the most important and charac- 
 teristic phenomena of their existence ; so that it would seem 
 as if the whole assemblage of organic functions had no other 
 destination in them, than to build up and keep in order the 
 apparatus by which the functions of animal life are performed. 
 But this state of things is entirely reversed among those 
 lower tribes of animals which border most closely on the 
 Vegetable kingdom ; for we find that among such, the mani- 
 festations of sensibility and power of spontaneous movement 
 are so feeble, that it may be doubted whether these attributes 
 are really present in them ; and even in higher orders, there 
 are many in which the proper animal powers are in such a 
 low grade of development, that they appear as if they were 
 destined merely to minister to the organic functions. 
 
 7. Thus, although the characteristic difference between the 
 Animal and the Vegetable kingdom, taking each as a whole, 
 may be truly said to consist in the possession by the former 
 of endowments which do not exist in the latter, this does not 
 express the essential difference between Animals and Plants ; 
 since, while there are many tribes among the former in which 
 the proper animal powers are reduced to so low a degree as 
 to prevent it from being certainly affirmed that they are 
 present at all, there are many tribes among the lower plants 
 which exhibit a power of spontaneous movement fully as 
 
DISTINCTIVE CHAEACTERS OF ANIMALS. 25 
 
 great as that which, exists among the lowest animals ; so that 
 no positive line can be drawn between the two kingdoms on 
 the basis of this distinction alone. There is another very 
 important physiological difference, however, between the two 
 kingdoms, which seems to afford an adequate means of 
 settling the true place of those tribes whose position would 
 otherwise be doubtful. This lies in the nature of their food, 
 and the source from which it is obtained. If or although it .is 
 now known that the primary tissues of plants are originally 
 formed of the same albuminous material as are those of 
 animals (the cellulose layers which constitute the great 
 bulk of the vegetable fabric being a subsequent deposit), yet 
 this material is generated in the Plant by the combination of 
 the elements which it obtains from the carbonic acid, water, 
 and ammonia of the soil or of the atmosphere ; whilst the 
 Animal is destitute of all power of thus forming it for itself, 
 and is hence entirely dependent upon the plant for its sup- 
 plies of nutriment. Thus, whilst the very humblest forms of 
 Vegetation, in common with the highest, are found to have 
 the power of decomposing carbonic acid under the influence 
 of sunlight, setting free its oxygen and retaining its car- 
 bon, the humblest forms of Animal life, in common with 
 the highest, derive their nutriment either directly from 
 plants, or from the bodies of other animals which have sub- 
 sisted on vegetable food, whilst they produce a converse 
 change in the atmosphere by their respiration, absorbing 
 from it oxygen, and giving forth to it carbonic acid. This 
 criterion will serve, it is believed, to distinguish the very 
 lowest forms of Animal life from those humble forms of Vege- 
 tation which they most closely resemble in the simplicity of 
 their organization ( 128); and its application will generally 
 be found to be very easy. There is now no longer any doubt 
 that a large proportion of the beings formerly ranked as 
 Animalcules, are really to be regarded as Plants, notwithstand- 
 ing that they possess a power of active and apparently 
 spontaneous movement, far greater than that of many unques- 
 tionable animals. And generally it may be said that the 
 presence of a bright-green or bright-red colour in any of these 
 simple organisms, where it is not derived from coloured sub- 
 stances taken- in as food, affords a strong probability of their 
 vegetable character; these colours being produced in the 
 
26 DISTINCTIVE CHARACTERS OP ANIMALS. 
 
 course of that series of chemical changes, by which, under the 
 influence of light, the living plant can unite, inorganic elements 
 into organic compounds. 
 
 8. Not only do Animals differ from Plants in the nature 
 and sources of their aliment, but also in the mode in which, 
 it is taken into their bodies ; and this difference is related 
 alike to the character of the food of animals, and to the general 
 conditions of animal existence. For the Plant extends its 
 roots through the soil in search of liquid, and spreads out its 
 leaves to the air for the purpose of imbibing some of its 
 gaseous ingredients. But the Animal could not so exist, and 
 be at the same time endowed with the power of moving from 
 place to place ; nor could it appropriate solid nutriment, if it 
 were not provided with some peculiar means of receiving and 
 preparing this. For these purposes, animals (with few 
 exceptions) are provided with an internal cavity or stomach 
 into which the food is received from time to time, in which 
 it can be carried about in the general movements of the body, 
 and within which it can be prepared for being received by 
 absorption into the current of nutrient liquid which circu- 
 lates through the body. This stomach is nothing else than a 
 bag formed by the prolongation of the external covering of 
 the body into its interior ( 36); its cavity receives the food 
 introduced into it by the mouth ; its walls pour out or secrete 
 a fluid which acts upon the food in such a manner as to dis- 
 solve it ; and through its walls are absorbed those portions 
 of the food which are fit to be employed as nutriment, while 
 the remainder is cast forth from the cavity, either by the 
 aperture which first admitted it, or by a distinct orifice. The 
 exceptional cases, in which no stomach exists, chiefly occur 
 in one particular tribe of animals, the Entozoa ( 105), which 
 live either in the intestinal canal or in the substance of the 
 tissues of other animals, and which are supported by the 
 nutrient juices of these ; such an organ obviously not being 
 required by creatures which have no power of locomotion, 
 and which can imbibe liquids already prepared for their use, 
 through the whole of the soft surface of their bodies. But 
 there is a large tribe of very simple animals, the Rhizopoda 
 ( 129), in which, notwithstanding the absence of any regular 
 stomach, the food is.. received into the very substance of the 
 jelly-like particle of which the body consists ; a mouth and 
 
DISTINCTIVE CHARACTERS OP ANIMALS. 27 
 
 stomach, being extemporized, as it were, on each occasion that 
 aliment is ingested ; and an anal orifice being extemporized 
 in like manner, when the indigestible residue has to be cast 
 forth. All true Animalcules ( 133) have a proper mouth, 
 into which food is drawn by the current created by the cilia 
 ( 45) wherewith it is fringed ; and this mouth leads to the 
 general cavity of the body, within which the food is subjected 
 to the digestive process. In Zoophytes ( 121) which possess 
 a proper stomach, this organ forms so large a part of the 
 animal, that its entire body may be almost said to consist of 
 the stomach and of the prehensile appendages by which it 
 draws in its food. But in all the higher tribes, the stomach, 
 with the alimentary canal proceeding from it, are suspended 
 freely within the general cavity of the body ; and we shall 
 find that the space that surrounds these viscera is extremely 
 important in the economy of all but vertebrated animals, as 
 being a sort of reservoir into which the nutrient materials 
 prepared by the digestive process first transude, and from 
 which it is carried into the remoter parts of the system. In 
 vertebrated animals, this cavity called in them the peritoneal 
 cavity, from its being lined with a serous membrane ( 28), 
 termed the peritoneum *is not subservient to the same pur- 
 poses ; the nutrient materials being taken up from the walls 
 of the digestive cavity, both by. the blood-vessels and by 
 special absorbents, and being by them carried into the current 
 of the circulation. It is obvious that until they have found 
 their way, through one or other of these channels, into the 
 general system, the nutrient materials introduced as food into 
 the stomach of an animal are not within its body, properly so 
 called, any more than a fluid is within a plant when it bathes 
 the exterior of its roots, or within an entozoon when in con- 
 tact with the soft surface of its integument. In each case, 
 the absorption of the fluid is first requisite ; and it is with 
 this that its application to the requirements of the living 
 body really commences. 
 
 9. But further, when we compare together, not the lowest, 
 but the highest members of the Vegetable and Animal king- 
 doms respectively those in which their respective attributes 
 are most characteristically displayed, we find that they 
 present such differences as to render it quite impossible to 
 confound the one with the other. Although it is easy even 
 
28 DISTINCTIVE CHARACTERS OF ANIMALS. 
 
 for the scientific naturalist to mistake a Protophyte (or one 
 of the simplest forms of vegetation) for an Animalcule, and 
 although Zoophytes are continually ranked in the popular mind 
 with the Plants they so much resemble in form, no one is in 
 any danger of confounding the Oak and the Elephant, the Palm 
 and the Whale. For among the higher Animals, not only the 
 principal organs, but the greater part of their elementary 
 parts or tissues, are formed upon a plan entirely different 
 from that which prevails in Plants. All the arrangements 
 of their organism or corporeal edifice are made for the pur- 
 pose of enabling them to perform, in the most advantageous 
 manner possible, those peculiar functions with which they have 
 been endowed, to receive sensations, to feel, think, and 
 will, and to move in accordance with the directions of the 
 instinct or the judgment. For these purposes we find a 
 peculiar apparatus, termed the Nervous system, adapted. This 
 apparatus consists of a vast number of fibres, spread out over 
 the surface of the body, and especially collected in certain 
 parts, called Organs of Sense (such as the eye, nose, ear, 
 tongue, lips, and points of the fingers). These have the 
 peculiar property of receiving impressions which are made 
 upon their extremities, and of conveying them to the central 
 masses of nervous matter (known in the higher animals as 
 the Brain and Spinal Cord*), by the instrumentality of which 
 they are communicated to the mind. 
 
 10. From the Nervous centres, other cords proceed to the 
 various Muscles, by which the body is moved. These muscles, 
 commonly known as " fiesh," are composed of a tissue which 
 has the power of contracting suddenly and forcibly, when 
 peculiar stimuli are applied to it. In this respect, it bears a 
 resemblance to the contractile tissues by which the move- 
 ments of plants are produced (YEGET. PHYS. 390) ; but it 
 differs from them in being thrown into action, not only by 
 stimuli that are applied directly to itself, but by an influence 
 conveyed through the nervous system. Thus, in an animal 
 recently dead, we may excite any muscles to contraction, by 
 sending a current of electricity into the nerves supplying 
 them ; and in a living animal we may do the same by simply 
 touching those nerves. But the stimulus which these nerves 
 ordinarily convey, originates in an act of the mind, which is 
 connected in some mysterious and inscrutable manner with 
 
DISTINCTIVE CHAKACTEES OP ANIMALS. 29 
 
 the central masses of the nervous system. Thus, we desire 
 to perform a certain movement or set of movements; this 
 desire leads to an act of volition or will; and the will causes 
 a certain force or motor impulse to issue from the brain and 
 travel along the nerves, so as to produce the desired motion, 
 by exciting contractions in the muscles that perform it. Or, 
 again, a certain sensation calls forth an emotion, which 
 prompts a certain muscular movement, and may even cause it 
 to take place against the will, as when a strong sense of the 
 ludicrous produces laughter, in spite of our desire (owing 
 to the unfitness of the time and place) to restrain it; for 
 the emotion, like the act of volition, produces a change in 
 the nervous centres, which causes a motor impulse to travel 
 along the nerves, and thus calls the muscles into contraction. 
 And it seems to be in the same manner that those instinctive 
 actions are produced, which, although few in adult Man when 
 compared with those resulting from his will, predominate in 
 his infant state, and through the whole life of the lower 
 animals (Chap. xiv.). "We shall also find that the nervous and 
 muscular systems of animals are concerned in a class of actions 
 with which the mind has no necessary connexion; these autom- 
 atic actions, such as those of swallowing (195) and breathing 
 ( 340), having for their object to assist in the performance of 
 the organic functions, and to protect the body from danger. 
 
 11. In the higher Animals, then, the presence of this 
 Nervo-Muscular apparatus is an essential and obvious dis- 
 tinction between their structure and that of Plants ; and we 
 find that it constitutes a large part of the bulk of the body. 
 Thus the whole interior of the skull of Man is occupied by his 
 brain ; his limbs are composed of the muscles, and of the bones 
 which support them and which are put in motion by them ; 
 and it is only in the interior of his trunk, that we find organs 
 corresponding with those which form the entire fabric of the 
 Plant. These organs of Nutrition have for their main pur- 
 pose, to supply the wants of the organs of animal life ; every 
 exercise of which is accompanied by a certain decay or wear 
 of their structure, and which consequently require to be con- 
 tinually nourished and repaired, by the materials provided 
 by what may be termed the vegetative organs. But in 
 the lower of tribes of Animals, we do not find the animal 
 functions to possess this predominance. In fact, among the 
 
30 DISTINCTIVE CHARACTERS OF ANIMALS. 
 
 many which, are fixed to one spot during nearly their whole 
 lives, and which grow and extend themselves like plants, the 
 movements of the body are but few in number, and trifling 
 as to their variety ; these movements are only destined to 
 assist in the performance of the organic functions, as by 
 bringing food to the mouth, and water to the respiratory 
 organs ; and the nervo-muscular apparatus by which they 
 are effected, bears so small a proportion to the organs of 
 nutrition, as to seem like a mere appendage to them, and is 
 sometimes altogether undiscoverable. This is the case, for 
 example, in the lowest kinds of shell-fish, such as the Oyster, 
 and in the Coral-polypes. 
 
 12. Hence we perceive, as we descend the Animal scale, a 
 nearer and nearer approach to the character of Plants ; and 
 this we shall find to be the case, not only in the general 
 arrangement of the organs, but also in the nature of the 
 elementary tissues of which these are composed. For in the 
 higher animals, the whole organism is constructed in such a 
 manner as to admit a free motion in its individual parts. 
 The different portions of the skeleton or hard framework are 
 connected with each other by flexible ligaments, which are 
 adapted to resist a very powerful strain; the muscles are 
 attached to these by fibrous cords or tendons, which, also, 
 can support a vast weight ; and the several muscles and 
 other parts, which need to be mutually connected, but also 
 require a certain power of moving independently of one 
 another, are bound together by a very elastic loosely-arranged 
 tissue, consisting of fibres crossing and interlacing in every 
 direction, the interstices between which are filled with fluid. 
 Now to these fibrous tissues, there is nothing analogous in 
 plants, because no freedom of motion is required, or even 
 permitted, among their parts ; and we find them bearing a 
 less and less proportion to the whole, as we descend the 
 animal scale. On the other hand, we find the various forms 
 of true cellular tissue, such as predominate in plants (VEGET. 
 PHYS. Chap, in.), becoming mere and more abundant, as we 
 pass from the highest to the lowest animals, and having 
 more and more important duties to fulfil. But even in the 
 highest Animals, as will hereafter appear, they are the im- 
 mediate instruments of the most important among the organic 
 functions, just as they are in Plants. 
 
CHEMICAL CONSTITUENTS : ALBUMEN. 31 
 
 Chemical Constitution of the Animal Body. 
 
 13. By far the larger proportion of the Animal fabric is 
 formed at the expense of the substance termed Albumen; the 
 composition and properties of which, therefore, claim our 
 first attention. The fundamental importance of albumen in 
 the animal economy, is shown by the fact that it constitutes, 
 with fat, and a small proportion of certain mineral ingredients, 
 the whole of that mass of nutrient material stored up in the eggs 
 of oviparous animals, which, being appropriated by the germ 
 to the building up of its fabric, is converted by it into the 
 bones, muscles, nerves, tendons, ligaments, glands, mem- 
 branes, &c. of the embryo. "We find it also constituting a 
 large proportion of the solid matter of the blood and other 
 nutrient fluids of the adult animal ; and it is the fundamental 
 form to which the various azotized substances employed as 
 food ( 153) such as animal flesh, or the gluten of bread 
 are first reduced by the act of digestion. It is composed 
 of 49 carbon, 36 hydrogen, 14 oxygen, 6 nitrogen, with a 
 minute proportion of sulphur ; it is generally blended, also, 
 with more or less of fatty matter, and with saline and earthy 
 substances. 
 
 14. Albumen may exist in two states, the soluble and 
 insoluble. In the animal fluids it exists in its soluble 
 form ; and is united (as an acid to its base) with about 1 
 per cent, of soda, forming an albuminate of soda. It is not 
 altered by being dried at a low temperature, but still retains 
 its power of being completely dissolved in water. "When a 
 considerable quantity of it exists in a fluid (as in the white of 
 the egg), it gives to it a glairy tenacious character ; but it is 
 nearly tasteless. "When such a fluid is exposed to a tempe- 
 rature of about 150, a coagulation or 'setting' takes place, as 
 in the familiar process of boiling an egg. But if the albumen 
 be present in smaller quantity, the fluid does not form a 
 consistent mass, but only becomes turbid ; and this only after 
 being boiled. Albumen which has been dried at a low 
 temperature, however, may be heated to the boiling point of 
 water, without passing into the insoluble condition ; a fact 
 which is of peculiar interest in relation to the power which 
 the Tardigrada (ZooL. 841) possess, of sustaining a very 
 high temperature without the loss of their vitality, when 
 
32 CHEMICAL CONSTITUENTS : ALBUMEN, CASEIN. 
 
 their bodies have been completely dried up in the first 
 instance. No trace of organization can be detected in 
 coagulated albumen, which seems to be composed only of a 
 mass of granules ; and in this respect it differs in an im- 
 portant degree from fibrin as we shall presently see. 
 Albumen may also be made to coagulate readily by the action 
 of acids, especially the nitric (aqua-fortis) ; so that a very 
 small quantity of it may be detected in water, by the tur- 
 bidity produced by adding to it a drop or two of nitric acid, 
 and then heating it. Now, when thus coagulated, albumen 
 cannot be dissolved again by any ordinary process ; but its 
 solution may be accomplished by rubbing it in a mortar with 
 a caustic alkali, potass or soda. From this solution it may be 
 precipitated again on the addition of an acid in sufficient 
 quantity to neutralise the alkali. Albumen is distinguished, 
 then, by its peculiar property of coagulating on the applica- 
 tion of heat, or on being treated with certain acids. 
 
 15. Nearly allied to albumen is the substance termed 
 Casein, which replaces it in milk ; and this is specially 
 worthy of notice here, because it is the sole form in which 
 the young Mammal receives albuminous nourishment during 
 the period of suckling, in which it draws its sustenance from 
 its parent. Like albumen, this substance may exist in two 
 forms, the soluble, and the insoluble or coagulated ; and the 
 presence of a small quantity of free alkali seems essential to 
 its continuance in the soluble form. Casein differs from 
 albumen, however, in this, that it does not coagulate by 
 heat, and that it is precipitated from its solution by organic 
 acids, such as the acetic and lactic, which have no coagulating 
 action on albumen. It is further remarkable for the facility 
 with which its coagulation is effected by the contact of 
 certain animal membranes ; as we see when a small piece of 
 rennet (which is the dried stomach of the calf) is put into a 
 large pan of milk in the process of cheese-making, the ' curd* 
 which then separates being composed of casein entangling the 
 oily particles of the milk. In the coagulated state, casein 
 differs but very little from albumen, and is readily converted 
 into it by the gastric fluid. It is remarkable for its power of 
 dissolving the earthy phosphates, as much as 6 per cent, of 
 phosphate of lime being usually obtainable from it ; and it is 
 in this combination, that the large quantity of bone-earth 
 
CHEMICAL CONSTITUENTS I CASEIN, STNTONIN, FIBRIN. 33 
 
 required for the consolidation of the skeleton of the young 
 animal, is introduced into its system. A substance resembling 
 casein is obtainable from the serum of the blood, especially in 
 pregnant females ; and also from the serous fluid which 
 occupies the interstices of the tissues. It is found, also, 
 mingled with albumen, in the yolk of the egg, forming a 
 compound which (before its true character was known) has 
 been distinguished as vitellin. Now as all the liquids con- 
 taining casein have it for their special function to supply 
 formative materials to rapidly-growing tissues, we may with 
 much probability regard it as still more closely related to 
 them than is albumen itself. It differs from albumen but little, 
 if at all, in the ultimate proportions of its elements ( 13). 
 
 16. The substance of which muscles are composed, has 
 been commonly considered to be Fibrin ( 17) ; but it differs 
 essentially from fibrin in its properties, and is now dis- 
 tinguished as Syntonin. Its chief peculiarity is its solubility 
 in very dilute muriatic acid (1 part to 100 of water), and its 
 precipitation in the form of a jelly when the acid is neutra- 
 lised ; this jelly treated with dilute alkalies forms a solution 
 which coagulates by heat ; and thus it seems to be reduced 
 nearly to the condition of albumen. This is, in fact, very 
 much what takes place in the act of digestion of flesh-meat ; 
 the muscle-substance being first dissolved by the muriatic or 
 other acid of the gastric fluid, and the solution being then 
 rendered alkaline by the mixture of bile and other secretions 
 in the small intestine. 
 
 17. In the blood and other nutrient fluids of the animal 
 body, there is found a substance which is so closely related to 
 albumen in its ultimate chemical composition, as not to be dis- 
 tinguishable from it with any certainty ; but which, though 
 fluid whilst circulating in the living vessels, coagulates spon- 
 taneously after having been for a short time withdrawn from 
 them, the coagulum or clot being distinguished from that of 
 albumen or fibrin by the fibrillar arrangement of its particles, 
 which indicates an incipient organization. This substance, 
 termed Fibrin, may be obtained in a separate form, by 
 stirring fresh-drawn blood with a stick, to which it adheres 
 in threads. In this condition it possesses the softness and 
 elasticity which characterise the flesh of animals, and con- 
 tains about three-fourths of its weight of water. It may be 
 
 D 
 
34: CHEMICAL CONSTITUENTS : FIBRIN. 
 
 deprived of this water by drying, and then becomes a hard 
 and brittle substance ; but, like dried flesh, it imbibes water 
 again when moistened, and recovers its original softness and 
 elasticity. From the recent experiments of Dr. Richardson, 
 it appears that the coagulation of blood-fibrin depends upon 
 the escape of ammonia, being accelerated by such conditions 
 as favour the liberation of this gas, and retarded or prevented 
 by such as cause its retention in the liquid; whilst, even 
 after the clot has been formed, it may be dissolved by 
 ammonia, forming again when that gas is set free. Fibrin 
 differs from syntonin or muscle-substance in not being dis- 
 solved by very dilute muriatic acid, but being merely caused 
 to swell up into a gelatinous mass, which contracts again 
 when more acid is added. It combines with the earthy 
 phosphates, of which as much as 2^ per cent, is sometimes 
 found in the ash left by its combustion. 
 
 18. There can be no doubt that fibrin is formed in the 
 blood and in the other fluids in which it presents itself, at 
 the expense of albumen. What is its precise destination, 
 cannot as yet be clearly specified ; but there are several 
 circumstances which point to the conclusion that it is to be 
 regarded as a transitional stage in the metamorphosis of 
 albumen into the simple fibrous tissues ( 23.) Thus, when 
 the ordinary clot of blood is examined microscopically, it is 
 found to consist, not, like an albuminous coagulum, of a 
 homogeneous mass of granules, but of a network of im- 
 perfectly-formed fibres, enclosing the red corpuscles in its 
 interstices. A much more distinct network of the same kind 
 may be seen in the colourless coagulum formed by the liquid 
 which may be skimmed off the surface of the blood drawn 
 from persons suffering under any severe inflammation*; such 
 blood coagulates slowly, and its red corpuscles and the fluid 
 in which they float have an unusual tendency to separate 
 from each other ; and the fibrin previously dissolved in the 
 latter sets into definite fibres, which continue for some days 
 to increase in firmness. It is a liquid of the same kind, 
 charged with fibrin in a peculiarly " plastic " condition, that is 
 poured forth for the formation of new tissue when the repa- 
 rative processes are at work for the healing of a wound or the 
 reunion of divided parts ; and it is by a plug of coagulated 
 fibrin, which gradually comes to present a more and jnore 
 
CHEMICAL CONSTITUENTS : FIBRIN, GELATIN. 35 
 
 distinctly fibrous structure, that the mouths of divided blood- 
 vessels are closed up, when the flow of blood from them 
 spontaneously stops. In all such cases, the fibrous network, 
 if formed out of connexion with a living body, passes after a 
 time into decay j but if it be formed in apposition with living 
 parts, blood-vessels gradually extend into it from these, its 
 nutrition is maintained and improved, and it progressively 
 comes to present the ordinary characters of the simple fibrous 
 tissues ( 22). 
 
 19. Although the tissues most actively concerned in 
 carrying on the vital operations, retain for the most part the 
 composition 'of albumen, yet that very large proportion of the 
 fabric of the higher animals whose offices are essentially 
 mechanical, has a very different chemical constitution. If we 
 boil down either their bones, their skin, or their internal 
 membranes, we shall get a considerable quantity of the sub- 
 stance scientifically termed Gelatin, familiarly glue. Though 
 consisting of the same elements as albumen, its composition is 
 simpler, because these elements are united in smaller propor- 
 tions ; the atom or combining equivalent of gelatin being 
 made up of 13 Carbon, 10 Hydrogen, 5 Oxygen, 2 Nitrogen. 
 The distinctive character of gelatin consists in its solubility 
 in warm water, its coagulation on cooling into a uniform jelly 
 which can be liquefied again by warmth, and its formation of 
 a peculiar insoluble compound with tannin. Gelatin is very 
 sparingly soluble in cold water, though made to swell up and 
 soften by prolonged contact with it. A solution of only one 
 part of gelatin in 100 of hot water is sufficiently strong for 
 the whole to form a consistent jelly on cooling. The re- 
 action of gelatin with tannin is so decided, that the presence 
 of only one part in 5000 of water is at once detected by 
 infusion of galls ; and it is in this action that the process of 
 tanning consists, the gelatinous fibre of the skin, which 
 would speedily pass into decay, being converted into a com- 
 paratively unchangeable substance. The different tissues 
 which have gelatin for their base, yield it to boiling water 
 with different degrees of facility ; this diversity apparently 
 depending in some degree upon the definiteness of their 
 organization. Thus the " sound " or air-bladder of the cod, 
 sturgeon, and other fish, which, when dried and cut into 
 strips, is known as isinglass, is very readily acted on ; the 
 
 D2 
 
36 CHEMICAL CONSTITUENTS : GELATIN, CHONDRIN. 
 
 same is the case with the animal substance of bones from 
 which the earthy matter has been removed ; and in each case 
 the fibrous texture of the living tissue is but very imperfectly 
 developed. For the extraction of gelatin from the skin, the 
 ligaments, the tendons, and various internal membranes, 
 whose fibrous texture is more pronounced ( 29), a much 
 longer action of boiling water is required. 
 
 20. A peculiar modification of gelatin, which presents 
 itself in Cartilage (or gristle), is distinguished as Chondrin. 
 This requires longer boiling than gelatin for its solution in 
 water; as is seen when a knuckle of veal or of mutton 
 is cooked, the tendons and ligaments about the joint 
 being almost reduced to pulp, whilst the cartilages are scarcely 
 at all softened. The essential properties of chondrin are 
 nearly the same as those of gelatin, and its composition 
 seems nearly identical; but it is thrown down from its 
 solution by muriatic and acetic acids and some other reagents, 
 which do not disturb a solution of gelatin. 
 
 21. It is not yet fully known how the material of the 
 gelatinous tissues is produced in the animal body. There 
 can be no doubt of its being producible from albumen ; since 
 we find it in large proportion in the tissues of animals that 
 have never received gelatin into their bodies in any shape. 
 And although carnivorous animals will receive it as part of 
 their aliment, yet there is strong reason to believe that the 
 gelatin which is thus supplied to them does not really serve 
 to nourish their bodies, but that it is speedily decomposed 
 and got rid of ( 159). It maybe considered as quite certain 
 that the albuminous tissues cannot be formed by the meta- 
 morphosis of gelatin ; whilst conversely, looking to the fact 
 that in the egg and in milk no gelatin is provided for the 
 young animal, although the gelatinous tissues form a yet 
 larger proportion of its body than they do in the adult, we 
 seem entitled to question whether it is possible that these 
 tissues can be formed in any other way than at the expense 
 of the albuminous constituents of the blood. 
 
 Structure of the Primary Tissues. 
 
 22. In considering the structure of the " primary tissues," 
 of which the various organs of animals are composed, it will 
 be convenient first to treat of those which are subservient 
 
PRIMARY TISSUES : SIMPLE FIBROUS TISSUES. 
 
 37 
 
 merely to the physical actions of the framework ; as, for ex- 
 ample, by holding its parts together, by communicating motion, 
 or by giving them mechanical support and protection. The 
 several parts of the body, even to the very minute divisions 
 of its organs, are held together by what may be termed, in 
 contradistinction to Muscular and Nervous fibre, the simple 
 fibrous tissues ; and these are merely endowed, like ordinary 
 cords, with the power of resisting tension or strain, either 
 without themselves yielding to it at all, or with a certain 
 amount of elasticity, which enables them first to yield to 
 a certain degree, and then to recover their previous state. 
 These two qualities are characteristic of two distinct forms of 
 simple fibrous tissue, the white and the yellow. 
 
 23. The White fibrous tissue presents itself under various 
 forms, being sometimes composed of fibres so minute as to be 
 scarcely distinguishable, but 
 more commonly presenting 
 itself under the aspect of 
 flattened bands, which are 
 but imperfectly divided into 
 fibres, and have more or less 
 of a wavy aspect (fig. 1). This 
 tissue is resolved, by long 
 boiling, into gelatine ; and 
 when treated with acetic acid, 
 it swells up and becomes 
 transparent, by which peculiarity it can be readily dis- 
 tinguished from the other kind, to be next described. The 
 Yellow fibrous tissue presents 
 itself in the form of long, 
 separate, clearly defined fibres, 
 which sometimes branch, and 
 which break short off when 
 overstrained, their extremi- 
 ties being disposed to curl up 
 (fig. 2). They are, for the most 
 part, between 1-5, 000th and 
 1-1 0,000th of an inch in 
 diameter ; but they are often Fig - 2 -- Y * MO * FIBROUS TISSUE. 
 met with both larger and smaller. This kind of tissue un- 
 dergoes but very little change from long boiling, and it is 
 
 Fig. 1. WHITE FIBROUS TISSUE. 
 
38 PKIMARY TISSUES : AEEOLAR TISSUE. 
 
 not acted on by acetic acid. It is but little prone to decom- 
 position, and will exhibit its peculiar elasticity long after it 
 has been separated from the body, provided it be kept moist. 
 These two forms of tissue exist separately in certain parts 
 of the fabric, but they are much more frequently combined ; 
 and the proportion of the yellow elastic tissue which exists in 
 any such combination, may be readily determined under the 
 microscope by the use of acetic acid, which renders all the 
 white fibrous structure so transparent, that the yellow fibres 
 are seen completely isolated in the midst of it. 
 
 24. One of the tissues which is composed of such an 
 admixture of white and yellow (or non-elastic and elastic) 
 fibres, is the one which was formerly called "cellular," but 
 which is now more correctly designated as Areolar. 1 This 
 is composed of a mesh-work of fibres, and of bands of fibrous 
 membrane, which are interwoven in such a manner as to leave 
 very numerous interstices and cavities amongst them, having 
 a tolerably free communication with each other (fig. 3). These 
 
 Fig. 3. PORTION OP AREOLAR TISSUE. 
 
 cavities are filled during life with a serous fluid ; 2 and it is a 
 necessary result of the communication between them, that if 
 an accumulation of this fluid takes place to an undue extent, 
 
 1 From the Latin areola, a small open space. 
 
 2 A fluid resembling the serum of the blood, diluted with water 
 ( 236). 
 
PRIMARY TISSUES : AREOLAR TISSUE. 39 
 
 as in dropsy, it descends by gravity to the lowest situation. 
 Hence, the legs swell more frequently than any other parts. 
 In its natural state, this tissue possesses considerable elas- 
 ticity ; hence, when we press upon any soft part, and force out 
 the fluid beneath into the tissue around, the original state 
 returns as soon as the pressure is removed. But in dropsy, 
 it appears as if the elasticity of the fibres were impaired or 
 destroyed by their being over-stretched ; for when we press 
 with the finger upon a dropsical part, a pit remains for some 
 time after the finger has been removed. 
 
 25. This Areolar tissue is diffused through almost the 
 whole fabric of the adult animal, and enters into the compo- 
 sition of almost every organ. It binds together the minute 
 parts of which the muscles are composed ; it lies amongst the 
 muscles themselves, connecting them together, but yet per- 
 mitting them sufficient freedom of motion ; it exists in large 
 amount between the muscles and the skin j it forms sheaths 
 to the blood-vessels and nerves, and so connects them with 
 the muscles that they shall not be strained or suddenly bent 
 by the movements of the latter ; and it enters into the struc- 
 ture of almost every one of the organs which are contained 
 in the cavity of the trunk, uniting its parts to each other, and 
 keeping the whole in its place. But it is a great mistake to 
 assert, as it was formerly common to do, that it penetrates the 
 harder organs, such as bone, teeth, and cartilage. Its purpose 
 obviously is to allow a certain amount of motion among the 
 parts it unites ; and we find that the more free this motion is 
 required to be, the larger is the proportion borne by the 
 yellow or elastic fibres, to the white or non-elastic. 
 
 26. Although the Areolar tissue contains a very large 
 number of blood-vessels and nerves, yet it does so merely 
 because it furnishes the bed or channel in which they are 
 conducted to the parts where they are really wanted. Its 
 own vitality is low, and its sensibility very slight. It is 
 quickly reproduced after injury ; and it is by its means that 
 losses of substance are repaired in tissues of a more elaborate 
 kind, which are not so easily regenerated. 
 
 27. The continuity or connectedness of this tissue over the 
 whole surface of the body, admits air to pass readily from 
 one part to another ; and the inflation or blowing-up of its 
 cavities with air, which has sometimes happened accidentally, 
 
40 PRIMARY TISSUES I SEROUS MEMBRANES. 
 
 and has sometimes been purposely effected, does not produce 
 any disorder in the general functions of the body. In blow- 
 ing the nose violently, some part of the membrane lining its 
 cavity has occasionally given way, so as to allow air to pass 
 into the areolar tissue of the face, and especially into that 
 contained in the eyelids, which is particularly loose ; an enor- 
 mous swelling of these parts then takes place, presenting a 
 very frightful appearance, but not attended with the least 
 danger, and subsiding of itself in a few days. This swelling 
 presents a character to the touch quite different from that 
 which would be occasioned by a similar distension with liquid; 
 for it gives somewhat of the crackling feel that is occasioned 
 by pressing on a blown bladder. A similar inflation of the 
 areolar tissue of the body has sometimes occurred from the 
 formation of an aperture, by disease or injury, in the walls of 
 the lungs or air-passages, and the consequent escape of air 
 during the act of breathing : in one remarkable case of this 
 kind, the skin of the whole body was so tightly distended 
 with air as to resemble a drum. It is intentionally practised 
 by butchers, who " blow up " the areolar tissue of their veal, 
 in order to increase its plumpness of aspect; and the in- 
 flation of the areolar tissue of the head, in the living state, 
 has been sometimes practised by impostors, in order to excite 
 commiseration. 
 
 28. Fibres and shreds of nbro-membrane, resembling those 
 of which areolar tissue is composed, may be so interwoven as 
 to form a continuous sheet of membrane, having a smooth 
 and glistening surface j and in this manner are produced the 
 Serous Membranes that line the different cavities in which the 
 viscera (or organs contained within the skull, the chest, and 
 the abdomen) are lodged. The peculiar manner in which 
 these membranes are arranged, will be explained hereafter 
 ( 43). One of their surfaces is always free or unattached, 
 whilst the other is in contact with the outer wall of the 
 cavity ; and from the free surface, which is covered with a 
 layer of flattened epithelium-cells (fig. 10), a serous fluid is 
 exhaled, which adds to its smoothness. It is by an accumula- 
 tion of this fluid, that dropsies of the cavities are produced, 
 such as water on the brain, or in the chest. 
 
 29. By the union of fibres of a stronger kind, those firmer 
 tissues are produced, which are employed wherever a greater 
 
FIBROUS MEMBRANES AND LIGAMENTS. 41 
 
 strain has to be borne. This is the case with the Ligaments, 
 which bind together the bones at the joints, the Tendons, by 
 which the muscles are usually attached to the bones, and the 
 tough Fibrous Membranes that envelope and protect many 
 of the most important viscera. In these any considerable 
 amount of elasticity would be misplaced ; and we conse- 
 quently find that they are chiefly or entirely composed of the 
 white fibrous tissue. Whenever an elastic ligament is re- 
 quired, however, we find the white replaced by yellow. One 
 of the best examples of this is seen in the ligament of the 
 neck of many quadrupeds, commonly known as the paxy- 
 waxy ; which is given to the large herbivorous quadrupecls, 
 such as the ox, to assist them in supporting their heavy 
 heads with as little exertion as possible ; whilst carnivorous 
 quadrupeds, such as the lion and tiger, are endowed with it 
 to give them additional power of carrying away heavy bur- 
 dens in their mouths. In Man we scarcely find a trace of 
 it. This yellow fibrous tissue is found, moreover, in the walls 
 of the arteries ( 248), to which it gives their peculiar elas- 
 ticity; and it also forms the vocal cords of the larynx ( 681). 
 It is by the same kind of elastic ligament that the claws of 
 the Feline tribe are drawn back into their sheaths when not 
 in use, being projected (when required) 'by muscular action ; 
 and that the two pieces of the shell of Bivalve Mollusks are 
 united at the hinge, and are at the same time kept apart for 
 the admission of water between them, except when the 
 animal forcibly draws them together by its adductor muscle 
 ( 113). 
 
 30. All these fibrous tissues, then, are concerned in actions 
 purely mechanical; and there is nothing in their properties 
 which is so distinct from those of inorganic substances, as to 
 require to be considered as vital. We may consider them, 
 therefore, as among the lowest forms of animal tissue ; and 
 accordingly we find that, when the higher forms degenerate 
 or waste away, these appear in their place. Such a degene- 
 ration may take place simply from want of use. Thus if, 
 from palsy or want of power of the nerves, the muscles of 
 the legs are disused for several years, they will lose their 
 peculiar property of contractility ( 5) ; and it will be found 
 that scarcely any true muscular structure remains, but that it 
 is replaced by some form of fibrous tissue. Or again, if the 
 
42 BASEMENT MEMBRANE I CELLS. 
 
 front of the eye be so injured by accident or disease, that light 
 cannot pass through it to make its impression on the nerve, 
 that nerve, being thrown into disuse, will gradually degenerate 
 into fibrous tissue. Moreover, this change may take place as 
 a part of the regular actions of life ; for there are certain 
 organs in the young animal previous to birth, which are not 
 required afterwards ; and these degenerate in like manner, 
 gradually wasting away, and leaving only traces behind them, 
 tubes shrivelling into fibrous ligaments, and glandular 
 structures remaining only as areolar tissue. 
 
 31. Along every free surface of the body, both external 
 and internal, is spread out a delicate structureless layer, which 
 is termed the Basement or Primary Membrane. This forms 
 the outer layer of the True Skin, lying between it and the 
 Epidermis or scarf-skin ( 37) ; in the same manner it 
 underlies the Epithelial layer of the Mucous membranes 
 which line the open cavities of the body ( 39), and of the 
 Serous membranes which line its closed cavities ( 43) ; 
 and it occupies the same position in the walls of the blood- 
 vessels, gland-ducts, and other tubes. It is difficult to sepa- 
 rate it, in any of these parts, from the tissues with which it 
 is in contact j and its characters may be well studied by dis- 
 solving the calcareous part of an oyster or mussel-shell in 
 dilute acid, when it will be found that layers of a thin trans- 
 parent membrane are left, which have been thrown off at 
 each act of shell-formation, from the surface of the mantle. 
 This elementary membrane, like that which forms the walls 
 of cells ( 32), is remarkable for the readiness with which 
 
 it is permeated by fluid, al- 
 though no visible pores can 
 be seen in it. 
 
 32. A considerable part of 
 the fabric of even the highest 
 Animal is formed, like the 
 entire organism of the Plant, 
 of Cells, either unchanged or 
 in some way metamorphosed. 
 A cell is a minute bag or 
 
 Fig. 4._N UCLE A TED CELLS; a a, nuclei. ^.^ formed Q a stmcture . 
 
 less membrane, and having its cavity filled with fluid of some 
 kind. In some part of its interior, most commonly adhering 
 
CELLS; THEIR MODE OF MULTIPLICATION. 43 
 
 to its wall, there is usually to be observed a solid collection 
 of granular matter, which is termed the nucleus (fig. 4, a a). 
 The typical form of the cell is globular or oval (fig. 5) ; but 
 when a number of cells are in contact with each other, and 
 are pressed together, their sides become . flattened ; so that 
 when they are cut across no intervals are seen between them, 
 but their walls are everywhere in contact (fig. 6), just as in 
 
 Fig. 5. Fig. 6. 
 
 ROUNDED CELLS IN CARTILAGE POLYGONAL CELLS FROM CAR- 
 
 OF BAT'S EAR. TILAGE IN MOUSE'S EAR. 
 
 the section of a vegetable pith. The chemical composition of 
 the nucleus differs from that of the cell-wall ; for whilst the 
 latter is dissolved by acetic acid, the former (like the yellow 
 elastic tissue, with which its substance appears to have some 
 relationship) is unchanged by it. "When the formation of a 
 cell is complete, and it is not destined to reproduce its kind, 
 the nucleus frequently disappears ; this is the case, for 
 example, with the red corpuscles of the blood of Mammalia 
 ( 229), and also with Fat-cells ( 46). 
 
 33. New cells may originate in one of two very distinct 
 modes ; either from a pre-existing cell, or by an entirely new 
 production in the midst of an organizable fluid or blastema. 
 The most remarkable example of the first process is presented 
 in the early development of the germ, which entirely consists 
 of an aggregation of cells, every one of which undergoes 
 successive subdivisions into two, so that the total number 
 in the germ-mass is repeatedly doubled (Chap. xv.). The 
 same method of multiplication by binary subdivision may be 
 seen to continue throughout life in Cartilage-cells ( 47), the 
 growth of which almost exactly repeats the history of the 
 growth of the lowest forms of Sea-weeds. The process of sub- 
 division seems to commence in the nucleus, which begins to 
 separate itself into two equal parts, and each of these draws 
 
44 MULTIPLICATION AND NEW PRODUCTION OF CELLS. 
 
 around it a portion of the contents of the cell ; so that the 
 cell-wall, which is at first merely doubled inwards by a sort 
 of hour-glass contraction, at last forms a complete partition 
 between the two halves of the original cavity. The process 
 may be repeated either in the same or in a transverse direc- 
 tion, so as to produce four cells, which may be either arranged 
 in a single line o o o o or may form a cluster gg ; and 
 another subdivision of each cell will, of course, again double 
 the entire number. In other cases, however, the nucleus 
 appears to break up at once into several fragments, each of 
 which may draw around it a portion of the contents of the 
 parent- cell, which becomes invested by a cell-wall of its own ; 
 and thus the cavity of the parent-cell may at once become 
 filled with a whole brood of young cells, without any successive 
 subdivision. Generally speaking, the former method seems 
 to prevail in structures which, like Cartilage, have a com- 
 paratively permanent destination ; whilst the latter is followed 
 in cases in which the cells thus formed are destined only 
 for a transitory existence. This is the case especially in Can- 
 cerous structures, which are particularly distinguished by 
 their proneness to the rapid production of cells within cells. 
 
 34. The production of new cells in the midst of an or- 
 ganizable blastema or formative fluid, such as is poured out 
 from the blood for the reparation of an injury, is a very 
 different process. This blastema, when first effused, is an 
 apparently homogeneous semi-fluid substance ; as it solidifies, 
 however, it becomes dimly shaded by minute dots, and as it 
 is acquiring further consistence, some of these dots seem to 
 aggregate, so as to form little round or oval clusters, bearing 
 a strong resemblance to cell-nuclei. These bodies appear to 
 be the centres of the further changes which take place in the 
 blastema ; for if it be about to undergo development into a 
 fibrous tissue ( 18), they seem to be the centres from which 
 the fibrillation spreads ; whilst, if a cellular structure is to be 
 generated, it is from them that the cells take their origin. 
 The first stage of the latter process appears to consist in the 
 accumulation of the substance which the cell is to include, 
 about each nucleus, and around this the cell-membrane is 
 subsequently developed. It is in this mode that the de- 
 velopment of new structures, for the filling up of losses of 
 substance, is provided for; and it appears, from recent 
 
ISOLATED CELLS OF ANIMAL FLUIDS. 45 
 
 inquiries, that the blastema will resolve itself into fibres or 
 into cells, according as the wound is completely secluded from 
 the air, or is exposed to it. It is under the former condition 
 that losses of substance are most rapidly and most completely 
 repaired ; whilst it is under the latter that inflammation is 
 most likely to arise, in consequence of the bad effect pro- 
 duced by the contact of air with the raw surface; the 
 process of healing, when thus interfered with, going on less 
 iavourably as well as more slowly. 
 
 35. The very simplest and most independent condition of 
 the animal Cell, is probably to be found in the nutritive fluids 
 of the body ; in which we meet with floating cells that are 
 completely isolated from each other, and which are conse- 
 quently just as self-sustaining as are the separate vesicles of 
 the Yeast-plant, of the Eed Snow, or of other simple cellular 
 Plants. These cells are of two classes. In the blood of 
 animals generally, and in the chyle and lymph of Yertebrata, 
 we find a larger or smaller proportion of colourless corpuscles, 
 which are usually nearly spherical in form, and which exhibit 
 various stages of development into cells, being sometimes 
 little else than collections of granules, without any distinct 
 enveloping membrane, whilst, in other instances, there is a 
 distinct cell-wall, cell-cavity, and nucleus. These bodies, if 
 watched under a sufficiently powerful microscope, may often 
 be seen to undergo very curious changes of form, resembling 
 those of the Amoeba ( 129). Besides the foregoing, however, 
 the blood of Vertebrated animals contains a far larger pro- 
 portion of red corpuscles, which are flattened disks, sometimes 
 circular but more commonly oval, having pellucid and colour- 
 less walls, but having their cavities filled with a peculiar 
 coloured fluid. As these will be more fully described here- 
 after ( 229), it is not requisite to do more than notice them 
 here as constituting a most important part of the animal 
 organism, probably not less than a twelfth part of the entire 
 weight of Man and the higher animals, being thus composed 
 of nothing else than these isolated cells. 1 
 
 36. Next in independence to the cells or corpuscles float- 
 ing in the animal fluids, are those which cover the free 
 
 1 The entire weight of the blood of Man seems to be about one-sixth 
 part of that of the body ; and the moist corpuscles constitute about half 
 the entire weight of the blood. 
 
46 SKIN AND MUCOUS MEMBRANES. 
 
 membranous surfaces of the body, and which, form the 
 Epidermis, or superficial layer of the skin, and the Epithelium 
 of the internal membranes. And it will be convenient here 
 to consider the entire structure of the Skin, the Mucous 
 Membranes, and the Serous Membranes, which are complex 
 fabrics, chiefly made up of the elementary tissues already 
 described. These membranes may each be considered as 
 composed of three principal parts, namely, the superficial 
 layer or layers of cells, the basement-membrane whereon the 
 cells lie, and the subjacent texture covered by this, which 
 consists of fibrous tissue compactly interwoven and traversed 
 by blood-vessels, nerves, absorbents, and also containing 
 glands of various kinds. The Skin and Mucous Membrane 
 may, in fact, be regarded as belonging to one and the same 
 type ; for they are continuous with each other wherever one 
 of the open cavities of the body communicates with the 
 surface, as at the mouth, nostrils, and anus; and in the 
 Hydra ( 121) it has been experimentally found that the 
 membranous layer covering the body may be made to change 
 places with that which lines the stomach, without any sensible 
 disturbance in the functions of either. The difference between 
 the two essentially consists in this ; that the Skin, being 
 destined especially for the reception of sensations, and for the 
 protection of the soft parts beneath, is more copiously furnished 
 with nerves than with blood-vessels, and has its surface 
 covered by a firm, dry cuticle ; whilst the Mucous Membrane, 
 ministering especially to the organic functions, is comparatively 
 little supplied with nerves, but is abundantly furnished with 
 blood-vessels, and in certain parts with absorbents, whilst its 
 cellular layer is soft and easily permeable by liquids. Both 
 in the skin and in mucous membrane we find a multitude of 
 minute glands, for the separation of particular fluids from the 
 blood ; the nature of these differs with the locality. 
 
 37. The fibrous mesh-work of the Cutis or True- Skin is con- 
 tinuous with that of the Areolar tissue which lies immediately 
 beneath it; so that the two textures are not separated one 
 from the other by any definite boundary (as the examination 
 of a vertical section (fig. 7) clearly proves), but are dis- 
 tinguishable only by the compactness of the one, as contrasted 
 with the looseness of the other. The outer surface of the 
 Cutis usually presents numerous minute elevations or papillce 
 
STRUCTURE OF THE SKIN. 
 
 47 
 
 (fig. 7, i i), which are commonly arranged in rows ; of these, 
 some are organs of touch, being furnished with sensory nerves 
 that end upon a peculiar cushion-like organ in their interior 
 ( 490); but into others no nerves can be traced, so that, 
 as these are copiously supplied with blood-vessels, it is pro- 
 bable that they minister to the .nutrition of the epidermis. 
 
 Fig. 7. VERTICAL SECTION OP THE SKIN, 
 
 Showing the different structures which it contains. A, Epidermis ; a a, its outer 
 surface ; a b, its horny layer ; b c its inner soft layer, dipping down into the 
 hollow between the papillae ; B, Cutis ; d, arterial twig supplying its vascular 
 papilla ; e e, perspiratory glandulae ; /, cluster of fat-cells ; g g, perspiratory duct, 
 traversing the true skin ; h, its continuation through the epidermis ; * , tactile 
 papillae, with their nerves. 
 
 This is the more probable from the fact that we find these 
 vascular papilla very large and full of blood-vessels in the 
 interior of corns, warts, and other such productions, formed 
 by a "hypertrophy" or over-nutrition of the epidermis in 
 particular spots ; and also in situations in which the ordinary 
 epidermis is very thick, as it is on the black pads of the foot 
 of the dog or cat. And a highly vascular structure of the 
 
48 STRUCTURE OF THE SKIN. 
 
 same kind is found in the matrix or receptacle of the growing 
 roots of nails, hoofs, horns, &c. which are only modified forms 
 of epidermis. Imbedded in the substance of the cutis wo 
 find, in most situations, the perspiratory glands (fig. 7, e e.), by 
 which the watery fluid that is continually being exhaled from 
 the skin, is separated from the blood ( 371) ; these send forth 
 their secretion by canals (g 7i), which traverse the epidermis 
 in a corkscrew-like manner, and then open upon its surface 
 by oblique valvular orifices. In the Cutis, also, are lodged 
 the hair-follicles ( 38), which are really pits or depressions 
 of its surface, with a vascular papilla at the bottom of each, 
 supplying nutriment for the abundant development of the 
 cells in which the hair originates, as will be presently 
 described. Wherever the hair-fol- 
 licles occur, there do we also find 
 sebaceous follicles (fig. 8, a a) ; these 
 are peculiar glandule, secreting fatty 
 matter, which is poured into the hair- 
 canal, so as to come through it to the 
 surface of the epidermis ; and the use 
 of this secretion, which is particularly 
 abundant in the dark skins of the 
 natives of warm climates, is to pre- 
 vent the cuticle and the hair from 
 being too much dried up by exposure 
 to air. The surface of the Cutis is 
 covered by a layer of basement-mem- 
 brane ( 31), which is not traversed 
 either by blood-vessels, nerves, or 
 absorbents ; so that none of these 
 pass into the epidermis which lies on 
 its outer side. 
 
 38. The Epidermis, otherwise 
 termed the cuticle, or " scarf-skin," is 
 OF THE HTTMAN composed of numerous layers of nu- 
 
 SCALP; a a, sebaceous glands; cleated Cells : of which WC find those 
 6, a hair, with its follicle c. ^ immediate contact with the bage , 
 
 ment-membrane to be nearly spherical ; those a little removed 
 from it to be rendered polygonal by the mutual pressure of 
 their sides ; those nearer the outer surface to be flattened, 
 and this in an increased degree, as we pass from within 
 
STRUCTURE OP THE SKIN I EPIDERMIS. 49 
 
 outwards, until we arrive at layers composed entirely of dry 
 flat scales, which show but little indication of ever having 
 been cells. There is no doubt, however, that all these forms 
 are but different stages of the existence of one and the 
 same set of epidermic cells ; these taking their origin in the 
 formative fluid exuded on the surface of the basement-mem- 
 brane, and being progressively carried towards the surface by 
 the . successive development of new layers beneath them, 
 whilst the layers above them are thrown off, or are worn 
 away ; and at the same time undergoing a change of form, in 
 the first instance from mutual pressure, and afterwards from 
 the loss of their contained fluid. At the same time they are 
 rendered more firm in texture, by the formation of a horny 
 secretion in their interior ; so that the outer layers of epi- 
 dermis form a consistent membrane, which is raised from the 
 surface of the Cutis when fluid infiltrates between them (as 
 when the hand has been long soaked in water), or is poured 
 out by the vessels of the latter (as when a blister is applied) ; 
 whilst the soft internal layers remain in contact with the 
 basement-membrane. The number of layers varies greatly in 
 different parts, being usually found to be greatest where 
 there is most pressure or friction, as if the irritation deter- 
 mined an increased supply of blood to the spot, and thus 
 favoured an augmented development of epidermic cells. 
 Thus, on the soles of the feet, particularly at the heel and 
 the ball of the great toe, the Epidermis is extremely thick ; 
 and the palms of the hands of the labouring man are 
 distinguished by the horny hardness of their thick cuticle. 
 It was formerly supposed that a special layer of a soft 
 spongy tissue, termed the rete mucosum, intervenes between 
 the Cutis and the Epidermis ; and that this was the special 
 seat of the colour of the skin in the dark races. It is 
 now well ascertained, however, that this supposed rete con- 
 sists of nothing else than the newly-forming soft layers 
 of the true epidermis; and that the colouring matter is 
 diffused through the epidermic cells, so as to tinge the 
 entire thickness of the cuticle, although its presence is 
 particularly obvious in the deeper layers. The Nails may 
 be considered as nothing more than an altered form of 
 Epidermis ; when examined near their origin, they are found 
 to consist of cells which gradually dry into scales that remain 
 
 E 
 
50 EPIDERMIC APPENDAGES : NAILS, HAIR, &C. 
 
 coherent; and when thin sections are treated by a dilute 
 solution of soda, these scales swell out again (as do also those 
 of the cuticle) into globular cells. A new production is 
 continually taking place in the groove of the skin in which 
 the root of the nail is imbedded, and also from the whole of 
 the surface beneath it ; the former adds to the length of the 
 nail ; the latter to its thickness. The structure of Hairs is 
 essentially the same. The base of each is formed of a " bulb," 
 which consists of a mass of epidermic cells developed from 
 the vascular papilla at the bottom of the hair follicle (fig. 
 8, c) ; and as this narrows into the " shaft " of the hair, a 
 difference shows itself between the cortical or outer layer, and 
 the medullary or pith-like substance of the interior. The 
 former, which is continuous with the outer layers of the epi- 
 dermis, is composed of flattened scales, arranged in an imbri- 
 cated (tile-like) manner, so that the surface of the hair is 
 usually marked by transverse jagged lines ; the latter consists 
 of cells which frequently retain their spheroidal form, like the 
 inner layers of the epidermis j but in the human hair these 
 cells are elongated into fibres. It is very seldom that there is 
 any canal in the interior of the Hair, although irregular spaces 
 are not unfrequently left by the drying-up of the fluid con- 
 tents of the cells. The structure of Quills is essentially the 
 same as that of hairs on a large scale ; and we there see the 
 difference very distinctly marked between the cortical portion 
 which forms the "barrel" of the quill, and the medullary 
 portion which forms the white pith-like substance of the 
 stem of the feather. The Scales, where, they are really epi- 
 dermic appendages, as is the case in serpents and lizards, are 
 formed upon the same pattern ; and we have a good example 
 of the detachment of the entire epidermis at once (reminding 
 us of the casting of the shell of the crab and lobster) in the 
 " sloughing " of the snake. 
 
 39. The Mucous Membranes form a sort of internal skin, 
 lining those cavities of the body which open on its surface ; 
 and the elements of which they are composed are essentially 
 the same, though combined and arranged in a different 
 manner, in accordance with their difference of function. The 
 principal part of the thickness of every ordinary mucous 
 membrane is made up, as in the skin, by the consolidation of 
 areolar tissue, the fibres of which are continuous with those 
 
MUCOUS MEMBRANES : EPITHELIUM. 61 
 
 of the ordinary areolar tissue on which the membrane rests ; 
 this layer is copiously furnished with blood-vessels, but it is 
 seldom supplied with many nerves. Thus the mucous mem- 
 brane lining the stomach possesses in health so little sensi- 
 bility, that we are not aware of the contact of the substances 
 taken in as food, unless they are of an acrid character, or of 
 a temperature very diiferent from that of the body; and 
 though the mucous membrane lining the air-passages is very 
 susceptible of certain kinds of irritation, yet it has but little 
 ordinary sensibility in the state of health, except near the 
 entrance to the windpipe. The large supply of blood which 
 these membranes receive, has reference to their active partici- 
 pation in the functions of secretion and absorption. One 
 secretion is common to all, that of the mucus by which they 
 are covered ; this serves to protect them from the irritation 
 that would otherwise be produced by the contact of solid or 
 liquid substances, or even of air, with their free surfaces; 
 and we see the results of its deficiency, in the inflammation 
 which attacks the membrane, sometimes proceeding to its 
 entire destruction, when from any cause the secretion is 
 checked, as it sometimes is by injuries of the nerves sup- 
 plying the part. 
 
 40. In every mucous membrane, as in the skin, the fibrous 
 texture is bounded on the free surface by basement-mem- 
 brane, beyond which no blood-vessels pass. And the surface 
 of the basement-membrane is covered by cells, arranged either 
 in a single layer or in multiple layers, constituting the 
 Epithelium. This, although answering to the Epidermis in 
 structure and position, has a very different character ; for its 
 cells neither dry up nor become horny ; nor do they adhere 
 in such a manner as to form a continuous membrane, except 
 in the interior of the mouth and oesophagus (gullet), where 
 the epithelium is endowed with somewhat of the firmness 
 of cuticle, in order to resist the abrading contact of hard 
 substances. The epithelium cells of mucous membranes are 
 commonly somewhat flattened ; but in some situations, as on 
 the villi of the intestinal canal (fig. 9, d), they have more of 
 a cylindrical, or rather conical shape, their smaller extremities 
 being in contact with the basement-membrane. The epi- 
 thelial cells are frequently cast off, like the epidermic, espe- 
 cially from the parts that are most concerned in secretion ; 
 
 E2 
 
52 MUCOUS MEMBRANES : EPITHELIUM. 
 
 and they are as continually replaced by newly-formed cells, 
 which are produced on the surface of the basement-mem- 
 brane, at the expense of the fluid that transudes through 
 it from the blood-vessels copiously distributed to its under 
 surface. 
 
 41. Mucous membrane may either exist in the condition of 
 a simple expanded surface, or may have a much more complex 
 arrangement, by which its surface is greatly increased. The 
 simple mucous membrane, such as that which lines the nose 
 and air-passages, is found, for the most part, where no ab- 
 sorption has to be performed, and where only a moderate 
 amount of secretion is necessary. But where it is to absorb 
 as well as to secrete, it is usually involuted or folded upon 
 itself, in such a manner as to form a series of little projec- 
 tions, and also a number of minute pits (fig. 9). These pro- 
 
 Fig. 9. DIAGRAM KEPRESENTING THE Mucous MEMBRANE OP THE 
 INTESTINAL CANAL. 
 
 a a, absorbent vessels; ft 6, basement membrane; c c, epithelium-cells of level 
 surface of membrane ; d d, cylindrical epithelium-cells of villus ; e e, secreting 
 cells of follicle. 
 
 jections sometimes have the form of long folds ; in other 
 instances they are narrow filaments, crowded together so as 
 almost to resemble the pile of velvet. In either case, the 
 absorbent surface is vastly increased ; but chiefly so by these 
 filaments, which are termed mlli, and act as so many little 
 rootlets. On the other hand, it is in the pits or follicles, that 
 the production of the fluid which is to be separated or 
 secreted from the blood, chiefly takes place. Not only are 
 the flat expanded surfaces of the mucous membrane covered 
 with epithelium cells, but the villi also are sheathed by 
 them; and the secreting follicles are lined by the same. 
 
STRUCTURE OF GLANDS. SEROUS MEMBRANES. 53 
 
 The cells covering the villi (fig. 9, d) perform the important 
 function of selecting and absorbing certain nutritious ele- 
 ments of the food, which they communicate to the absorbent 
 vessels in the interior of the villi. On the other hand, the 
 epithelium-cells of the follicles (e) seem to be the real agents 
 in the secreting process ; drawing from the blood, as materials 
 for their own growth, certain elements contained in it ; and 
 falling off, when mature, so as to discharge these substances 
 as the product of secretion, giving place to a fresh crop or 
 generation of cells, which go through a series of changes 
 precisely similar to the preceding. 
 
 42. Now these follicles are the simplest types or examples 
 of all the Glandular structures, by which certain products aro 
 separated from the blood, some to be cast forth from the body 
 as unfit to be retained in it, and some to answer particular 
 purposes in the system. In all of them the structure ulti- 
 mately consists of such follicles, sometimes swollen into 
 rounded vesicles, and sometimes extended into long and 
 narrow tubes. Each follicle, vesicle, or tube, is composed of 
 a layer of basement-membrane, lined with epithelium-cells, 
 and surrounded on the outside with minutely distributed 
 blood-vessels ; and it seems to be by the peculiar powers of 
 these cells, that the products of the secreting action, whether 
 bile, saliva, fatty matter, or gastric fluid, are formed (see 
 Chap. VIL). Hence we see that the act of Secretion is, in 
 animals as in plants, really performed by cells. It is neces- 
 sary to bear in mind, however, that a simple transudation of 
 the watery parts of the blood may take place without any 
 proper secreting action, in the dead as in the living body ; it 
 is in this manner that the serous fluid of areolar tissue and 
 serous membrane is poured out, and that the watery portion 
 of the urine is separated. 
 
 43. The Serous Membranes which line the closed cavities of 
 the body, though composed of the same elements as the skin 
 and mucous membranes, have- a much simpler structure, and 
 can scarcely be said to minister directly to any important 
 vital function. The tissue of which Serous membrane is 
 principally composed, scarcely diifers, except in its greater 
 density, from the laxer areolar tissue whereby the membrane 
 is attached to the walls which it covers like plaster ; it is but 
 sparingly supplied either with blood-vessels or absorbents ; and 
 
Fig. 10. 
 
 PAVEMENT EPITHE 
 
 54 ARRANGEMENT OF SEROUS MEMBRANES. 
 
 it contains very few nerves. The smooth surface of the mem- 
 brane forms one unbroken plane, being neither raised into 
 villi, nor depressed into follicles ; and its basement-membrane 
 is covered with a single layer of flat epithelium-cells, which 
 are closely applied to it and to each other, 
 like the pieces of a pavement (fig. 10). It 
 is with such a membrane that every one of 
 those great cavities is lined, which contains 
 important viscera ; and it is also continued 
 on to the outer surface of these viscera, 
 so as to afford them an external coating 
 over every part save that by which they 
 are attached. Thus the heart is suspended 
 freely, by the large vessels proceeding from 
 CELLS OF^SEROUS its summit, within a bag or sac of fibrous 
 MEMBRANE. membrane peculiar to itself, which is termed 
 
 the pericardium. The cavity of this bag is completely lined 
 by the serous membrane (fig. 11, p'\ which closely embraces 
 
 the vessels, and which then 
 bends down over the surface 
 of the heart, so as to enclose 
 it in the envelope p. Hence 
 it will be seen that this 
 membrane, whilst including 
 the heart, and allowing it to 
 communicate with its vessels, 
 forms a completely shut sac; 
 and it may be likened to a 
 common double cotton or 
 woollen night-cap, which has 
 a similar cavity between its 
 two layers, the head being 
 
 j-'Ag. ** J-'i A. VJJVAUO. vx i nr* JL X.AJ.VAAI.'IIJ J -- _ ... ^^ 
 
 a a, auricles ; v v, ventricles ; b, pulmonary really On the GUtSlue 01 
 artery ; c, aorta; pp>, pericardium. ^ whi]jgt geeming to be 
 
 within the envelope. The two layers of the pericardium, 
 though separated in the diagram for the sake of distinctness, 
 are really in mutual contact, save when separated by the in- 
 terposition of fluid poured out in disease. Each of the lungs, 
 in like manner, is suspended in a closed sac of its own, termed 
 the pleura; and the surface of the lung is covered by a serous 
 membrane, which is reflected over the wall of the pleural cavity. 
 
 Fig. 11. DIAGRAM OF THE PERICARDIUM. 
 
SEROUS AND SYNOVIAL MEMBRANES. 55 
 
 A similar arrangement exists in the great cavity of the ab- 
 domen ; but the number and the complex relations of the 
 viscera which this contains, give to the disposition of ita 
 serous membrane, termed the peritoneum, a peculiar complica- 
 tion. The cavity of the skull also is lined by a serous mem- 
 brane, termed the arachnoid, and this is prolonged over the 
 surface of the brain, and enters its lateral ventricles ( 458). 
 The chief purpose of these membranes appears to be to faci- 
 litate the movements of the included organs, by forming 
 smooth surfaces which shall freely glide over each other ; this 
 is evidently of great importance, where such constantly- 
 moving organs as the heart and lungs are concerned. Their 
 surfaces are kept constantly moist with a serous fluid which 
 exudes from the blood ; but in the state of health this fluid 
 does not accumulate in their cavities, being absorbed as fast 
 as it is poured out. Various forms of dropsy, however, 
 such as "water on the brain," "water on the chest," and 
 " ascites," or dropsy of the abdomen are the result of the 
 increased outpouring of fluid into the serous cavities of the 
 arachnoid, the pericardium, the pleura, and 
 the peritoneum respectively. 
 
 44. Nearly allied to the Serous mem- T / 
 branes are the Synovial, which form closed 
 sacs in the interior of joints, covering the 
 ends of the cartilages, and then lining the 
 fibrous capsule which passes from one bone 
 to the other. The mode of their arrange- 
 ment will be understood from the accom- 
 panying diagram ; in which a a represent 
 the extremities of the two bones which 
 are jointed together, b b the layers of car- Fi s- 12 
 
 tilage with which they are severally covered, DIAGRAM OF THE STRUC- 
 
 1,, , , . , ,. * ,, '. , ' TURE OF A JOINT. 
 
 and the dotted line c c the synovial mem- 
 brane, which is seen to form the sac or 
 bag cf cf, whilst at the points cccc it is 
 reflected upon the cartilages of the joints. 
 In point of fact, however, the Synovial 
 membrane is not ordinarily traceable as a 
 distinct layer over the surface of these 
 cartilages, but seems to have become incorporated with them ; 
 for though in the embryo its presence may be distinctly proved 
 
 cartilage ; b b, layer of 
 cartilage closely co- 
 vered with synovial 
 membrane ; c c 1 c, re- 
 flected layer of syno- 
 vial membrane form- 
 ing synovial capsule. 
 
56 SYNOVIAl MEMBRANES. CILIATED EPITHELIUM: 
 
 by the continuity of its blood-vessels over the entire car- 
 tilage, yet these are found to retreat gradually as the joint 
 is brought into use, until at last they only form a circle round 
 the border of the cartilage. Some of the Synovial mem- 
 branes, as that of the knee-joint, are furnished with little 
 fringe -like projections, somewhat resembling the mlli of 
 mucous membranes ( 41) ; these are extremely vascular, 
 and are furnished with an epithelium, which very readily 
 falls off; and there is a strong probability that they are 
 concerned in the secretion of the synovial fluid, which is 
 much denser than the ordinary serous transudation, having 
 from 6 to 8 per cent, of additional albumen, and presenting a 
 glairy appearance like that of white of egg. It is interesting 
 to see that the same purpose may thus be served by the 
 extension of the membrane in either direction, either out- 
 ivards into a villous filament, or inwards into a follicle ; the 
 function being determined in each case rather by the 
 attributes of the cells, and by the 
 supply of blood, than by the form 
 which the secreting surface may 
 happen to present. 
 
 45. The cells of Epithelium, 
 whether flattened or cylindrical, 
 are observed to be furnished in 
 particular situations with a fringe 
 of delicate filaments, which are 
 termed cilia. These, although of 
 extreme minuteness, are organs of 
 great importance in the animal 
 economy, on account of the extra- 
 ordinary motor powers with which 
 they are endowed. The form of 
 the cilia is usually a little flattened, 
 and tapering gradually from the 
 base to the point. Their size is 
 extremely variable ; the largest that 
 
 their cina are seen at o, tneir nave been observed being about 
 nuclei at c -, at a is shown'or.e of l-500th of an inch in length, and 
 
 these cells unusually elongated. ^ gmaUest M^OOOth. When in 
 
 motion, each filament appears to bend from its root to its 
 point, returning again to its original state, like the stalks of 
 
 B 
 
 Fig. 13. CILIATED EPITHELIUM 
 
 CELLS; as seen sideways at A, 
 
CILIARY MOVEMENT. 57 
 
 corn when depressed by the wind; and if a number be 
 affected in succession with this motion, the appearance of 
 progressive waves following one another is produced, as when 
 a corn-field is agitated by repeated gusts. When the ciliary 
 motion is taking place in full activity, however, nothing can 
 be distinguished save the whirl of particles in the surround- 
 ing liquid; and it is only when the rate of movement 
 slackens, that the shape and size of the individual filaments, 
 and the manner in which their stroke is made, can be made 
 out. The motion of the cilia is not only quite independent 
 (in all the higher animals at least) of the will of the animal, 
 but is also independent even of the life of the rest of the 
 body ; being seen to continue after the death of the animal, 
 and even going on with perfect regularity in parts separated 
 from the body. Thus, isolated epithelium-cells have been 
 seen to swim about actively in water, by the agency of their 
 cilia, for some hours after their detachment from the mucous 
 membrane of the nose; and the regular movement of cilia 
 has been noticed fifteen days after death, in the body of a 
 tortoise in which putrefaction was already far advanced. In 
 the gills of the Eiver Mussel, which are amongst the best 
 objects for the study of this most curious phenomenon, the 
 movement endures with similar pertinacity. The purpose of 
 this remarkable agency is obviously to propel fluids over the 
 surfaces which are furnished with cilia. We find it taking 
 the most important share in the functions of life among the 
 lowest classes of animals. Thus, in Animalcules of various 
 kinds, the cilia are the sole instruments, not merely for the 
 production of those currents in the water which may bring 
 them the requisite supplies of air and food, but also for pro- 
 pelling their own bodies through the liquid. In most 
 Zoophytes, and in the inferior Mollusks, which pass their 
 lives with little or no change from one spot to another, the 
 motion of the cilia lining the alimentary canal and clothing 
 the gills (where such have a special existence), draws into the 
 mouth the minute currents which serve as food, and also 
 renews the layer of water in contact with the respiratory 
 surface. The gills of Fishes are not furnished with cilia, 
 another provision being .made by muscular action for conti- 
 nually driving fresh streams of water over them ; but the 
 motion may be very well seen upon the gills of the young 
 
58 
 
 CILIA. FAT CELLS. 
 
 Tadpole or larva of the "Water Newt, which hang down as fringes 
 on either side of the neck. In the higher air-breathing 
 animals, the function of the cilia is much more limited. They 
 clothe the mucous membrane which lines the air-passages ; 
 and their function appears to be, in that and other cases, to 
 prevent the accumulation of the secretion with which the 
 membrane is kept moist, by keeping up a continual onward 
 movement of it towards the outlet of the passage. In some 
 other cases, however, we find the ducts of secreting organs 
 furnished with cilia, whose action is obviously to assist in 
 carrying the products of secretion towards their outlet. 
 
 46. Passing on, now, to those tissues of animals of which 
 cells constitute the permanent components, instead of being 
 successively thrown off and replaced as they are in the 
 Epidermis and Epithelium, we may first notice the Adipose 
 tissue, or Fat, in which the oily and fatty matters of the body 
 are for the most part contained. This tissue is composed of 
 minute cells or vesicles (fig. 14), having no communication 
 with each other, but lying side by side in the meshes of the 
 
 areolar tissue, which serves 
 to hold them together, and 
 through which also the blood- 
 vessels find their way to 
 them. From the fluid in these 
 vessels, the fatty matter is 
 separated in the first place by 
 the secreting action of the 
 cells ; and it is prevented 
 from making its way through 
 the very thin walls of the 
 cells, by the simple expedient of keeping these constantly 
 moist with a watery fluid, the blood. 1 The blood-vessels 
 have also the power of taking back the fatty matter again 
 into the circulation, when it is wanted for other purposes in 
 the economy. These deposits of fatty matter answer several 
 important objects. They often assist the action of moving 
 parts, by giving them support without interfering with their 
 free motions ; thus the eye rests on a sort of cushion of fat, 
 on which it can freely turn, and through which the muscles 
 
 1 Thus oil will nob pass into blotting-paper, if this have been 
 previously moistened with water. 
 
 Fig. 14. FAT CELLS, HIGHLY 
 
 MAGNIFIED. 
 
FAT. CARTILAGE. 59 
 
 pass that keep it in play. It also affords, by its power of re- 
 sisting the passage of neat, a warm covering to animals that 
 are destined to live in cold climates ; and it is in these that 
 we find it accumulated to the largest amount. Further, 
 being deposited when nourishment is abundant, it serves as a 
 store of combustive material, which may be taken back into 
 the system, and made use of in time of need. The causes 
 which peculiarly contribute to the production of fat, will be 
 considered hereafter ( 162). 
 
 47. Another tissue of which cells form the principal part, 
 is that termed Cartilage or gristle. Its simplest state is that 
 of a mass of firm substance, composed of 
 chondrin (20), through which are scat- 
 tered a number of cells, at a greater or less 
 distance from one another. In the simple 
 cellular cartilages, such as those which 
 cover the ends of the bones where they 
 glide over one another so as to form 
 moveable joints, no trace of structure can 
 be seen in the intervening substance. Fig. is. SECTIOK OF 
 But in cartilages which have to resist not CARTILAGE, 
 
 , . . Showing its cells imbed- 
 
 only pressure but also extension or strain, ded in intercellular sub- 
 we find the space between the cells partly stance - 
 occupied by fibres, which resemble those of ligaments ; and 
 such are termed fibro-cartilages. They are found in Man be- 
 tween the vertebrae of which the spinal column is made up 
 ( 71); and also uniting the bones of the pelvis ( 645). 
 Sometimes, where elasticity is required, the fibres are those 
 of the yellow fibrous tissue ( 23) ; this is the case with the 
 cartilage which forms the external ear. Cartilage is not 
 penetrated by blood-vessels, at least in its natural state. The 
 blood is brought to its surface by a set of vessels which bulge 
 out into dilatations or swellings upon it, so that a large quan- 
 tity of fluid comes into the immediate neighbourhood of the 
 cartilage, being only separated from it by the thin walls of 
 the vessels ; and it appears that this fluid, or so much of 
 it as is required, is absorbed by the nearest cells, and trans- 
 mitted by them to the cells in the interior, so that the whole 
 substance is nourished. This is precisely the mode in which 
 the interior of the large sea-weeds (whose tissue consists of 
 cells imbedded in a gelatinous substance, and therefore bears 
 
60 CAETILAGE. BONE. 
 
 a close resemblance to animal cartilage) obtains its nourish- 
 ment from the surrounding fluid. 
 
 48. The permanent Cartilages seem to undergo very little 
 change from time to time. Their wear is slow ; and, being 
 purely mechanical, it is confined to the surface. It is replaced 
 by the materials absorbed from the blood, which are employed 
 in the development of new cells, sometimes within the old 
 ones, sometimes in the space between them. When a portion 
 of cartilage has been destroyed, however, by disease or injury, 
 it is not renewed by true cartilaginous structure, but by what 
 seems a condensed areolar tissue. Although cartilage does 
 not usually contain vessels, yet these may be rapidly deve- 
 loped in its substance, by a process which will be described 
 hereafter ( 393), when it becomes inflamed. This may be 
 often seen to take place. The front of the eye is formed by 
 a transparent lamina of a substance somewhat resembling 
 cartilage, which bulges like a watch-glass : this, which is 
 termed the cornea ( 533), is properly nourished only by 
 vessels that bring blood to its edge, where it is connected 
 with the tough membrane that forms the white of the eye. 
 But when the cornea becomes inflamed, minute vessels may 
 be seen to spread over it, proceeding from its circular edge 
 towards its centre ; and at last some of these often become of 
 considerable size. Under proper treatment, however, these 
 vessels gradually shrink and disappear ; and the cornea 
 becomes nearly as transparent as before. 
 
 49. Many parts exist in the state of Cartilage in the young 
 animal, which are afterwards to become Bone; and it has 
 been commonly believed that all bone has its origin in a 
 cartilaginous structure. This, however, is not the fact, as 
 will be presently shown. Before attempting to explain the 
 formation of Bone, it will be desirable to describe its 
 structure. When we cut through a fully formed bone, such 
 as that of the thigh, we find that the shaft or elongated 
 portion is a hollow cylinder ; of which the walls are formed 
 by what appears to be solid bone ; whilst the interior is filled, 
 in the living state,* by an oily substance laid up in cells, and 
 termed marrow. Towards the extremities, however, the struc- 
 ture of bone is very different. The outside wall becomes 
 thinner ; and the interior, instead of forming one large cavity, 
 is divided into a vast number of small chambers, like .those 
 
STRUCTURE OF BONE. 
 
 61 
 
 of areolar tissue, by thin bony partitions, which cross each 
 other in every direction, forming what is called the " cancel- 
 lated" structure. These chambers or cancelli are filled with 
 marrow, like the central cavity, with which they communi- 
 cate. In the flat bones, moreover, such as those of the 
 head we find that the two surfaces are composed of dense 
 plates of bone, like that which forms the shaft of the long 
 bones ; but that between them there is a layer of cancellated 
 structure, filled in like manner with marrow. But when we 
 examine with the microscope a thin section of even the 
 densest bony matter, we find it traversed by a network of 
 minute canals, continuous with the central cavity. These 
 canals usually run, in the shafts 
 of long bones, in the direction 
 of their length; and are con- 
 nected, every here and there, by 
 cross branches (fig. 16). They 
 are termed the Haversian canals, 
 after the name of their disco- 
 verer, Havers. The lining mem- 
 brane of the large central cavity 
 is copiously supplied with blood- 
 vessels; and this sends off pro- 
 longations into the cancelli at 
 
 the extremities of the bone, and Fif? - ".-DIAGRAM REPRESENTING 
 
 ' THE STRUCTURE OP A PORTION OS 
 
 into the Haversian Canals. Thus THE SHAFT OF A LONG BONE. 
 
 hlonrl is rrmvpvprl into fhp in a b d> the surface as seen in 
 Lb COnvtyeu IE in- transverse section; b e f c, surface 
 
 terior of the bone ; but no vessels 
 
 can be traced absolutely into its 
 
 texture, so that all the spaces 
 
 which lie between the Haversian 
 
 canals are as destitute of vessels as 
 
 is healthy cartilage. These spaces are provided with nutriment 
 
 by the following very remarkable arrangement. 
 
 50. When we cut across the shaft of a long bone, and 
 examine a thin section with a microscope, we of course see 
 the open extremities of the Haversian canals (fig. 17, a) ; 
 just as we see the cut ends of the ducts and vessels of 
 wood, when we make a transverse section of a stem. 
 Around each of these apertures, the bony matter is arranged 
 in concentric rings, which are marked out and divided 
 
 seen in longitudinal section ; i, Ha- 
 versian systems cut across, each 
 having an Haversian canal in its 
 centre ; g v, Haversian systems cut 
 longitudinally { /, lamellae near the 
 surface of bone, destitute of Haver- 
 sian systems. 
 
62 
 
 STRUCTUEE OP BONE. 
 
 by circles of little dark spots; and when these spots are 
 examined with a higher magnifying power, it is seen that 
 they are small flattened cavities, from which proceed a number 
 of extremely minute tubules (A). These tubules pass out 
 
 a 
 
 Fig. 17. TRANSVERSE SECTION op BONE. 
 
 Showing the concentric rings round a a, the Haversian canals. At A are seen 
 some of the cavities with their radiating tubes, more highly magnified. 
 
 from the two flat sides of each cavity; one set passes inwards, 
 towards the centre of the ring, and the other outwards, to- 
 wards the ring that next surrounds them. These minute 
 tubuli, which are far smaller than the smallest blood-vessels, 
 may thus be traced into every part of the substance of the 
 bone ; and those proceeding from different rings are so con- 
 nected with each other, that a communication is established 
 between the innermost and the outermost circles. The tubuli 
 which open upon the sides of the Haversian canals, are thus 
 enabled to take up the nourishment with which they are 
 
COMPOSITION OF BONE. 63 
 
 supplied by the blood-vessels, and to transmit it to the 
 outer circles, or those furthest removed from those vessels ; 
 and in this manner, a much more active nutrition takes place 
 in bone than that which is performed in cartilage. It has 
 been proved by various experiments, that the substance of 
 bone is undergoing continual change ; and it is owing to the 
 comparative activity of its nutritive processes, that bone is so 
 readily and perfectly repaired, when it has been broken by 
 violence or has been injured by disease. 
 
 51. But the peculiarity of Bone consists, not so much in 
 this remarkable arrangement of its organic structure, as in its 
 solidity and firmness. This is given to it by the union of 
 a large quantity of mineral matter with the organic substance 
 of its tissue. The mineral matter of bones consists almost 
 entirely of two compounds of Lime; the carbonate, with 
 which we are familiar in the form of limestone and chalk ; 
 and the phosphate, which is seldom found as an ingredient of 
 rocks or soils, except where it has been derived from animal 
 remains. The latter greatly predominates, at least in the 
 bones of the higher animals. We. may easily separate the 
 animal and the mineral portions of the bony tissue. If we 
 soak a small bone for some time in muriatic acid much 
 diluted with water, the compounds of lime are entirely 
 removed from it, and -the organic substance remains; 
 the latter is now quite flexible, and almost transparent, so 
 that the distribution of its vessels (if they have been pre- 
 viously injected with colouring matter) may be distinctly 
 seen. On the other hand, if we subject a bone to strong 
 heat, the animal portion will be burnt out, and the earthy 
 matter will remain. The form of the bone will be still 
 retained ; but the cohesion between the earthy particles is so 
 slight, that the least touch will break them asunder. Thus 
 we see that the hardness of bone, or power of* resisting pres- 
 sure, is given by the earthy matter; whilst its tenacity, or 
 power of holding together, depends upon the animal portion. 
 Although the animal substance which remains after the solu- 
 tion of the mineral matter, has been commonly described as 
 Cartilage, yet it is not so in reality ; for it consists not of 
 chondrin, but of gelatin ; and instead of being made up of an 
 aggregation of cells united by an intervening substance, it may 
 be torn into layers of an indistinctly-fibrous matting. In fact, 
 
64 COMPOSITION AND DEVELOPMENT OF BONE. 
 
 it corresponds closely with the "white fibrous tissue ( 23), both 
 in structure and composition ; and so far from this view of its 
 nature being inconsistent with the history of the formation of 
 bone, it will be found to be in entire harmony with it. The 
 proportion which the mineral bears to the animal substance of 
 bone is very constant, when the proper "osseous tissue alone is 
 taken into account ; being almost exactly two of the former to 
 one of the latter, or 66f per cent, to 33|- per cent. But when 
 the composition of entire bones, including the contents of the 
 Haversian canals and cancelli, is compared, the proportion of 
 mineral to animal matter is found to vary greatly in different 
 classes of animals, in the same animal at different ages, and 
 even in different bones of the same individual ; the mineral 
 matter predominating in bones of a compact texture, and the 
 animal in those whose substance is more spongy. 
 
 52. In the first development of the embryo, a sort of mould 
 of cartilage is laid down for the greater part of the bones; 
 though, in the case of the fiat bones, this mould is generally 
 limited to the central portion, the place of their marginal part 
 being occupied by a fibrous membrane only. The process of 
 ossification, or bone-formation, commences with the deposit 
 of calcareous matter in the intercellular substance of the 
 cartilage, so as to form a sort of network, in the interspaces 
 of which are seen the remains of the cartilage-cells. The 
 tissue thus formed can scarcely be considered as true bone, 
 for it contains neither lacunce nor canaliculi. Before 
 long, however, it undergoes very important changes; for 
 many of the partitions are removed, so that the minute 
 chambers which they separated coalesce into larger ones ; and 
 thus are formed the cancelli of the spongy substance, and the 
 Haversian canals of the more compact. These are at first 
 much larger than they are subsequently to become ; for they 
 are gradually narrowed by deposits of true bony tissue, 
 which successively take place upon their interior walls, at the 
 expense of the materials supplied by the blood brought 
 thither by their contained vessels ; and it is by this forma- 
 tion of concentric layers around the cavities of the Haversian 
 canals, that the appearance of concentric rings is produced, 
 which we have just seen to be presented by transverse sec- 
 tions of long bones. In old bones the Haversian canals are 
 so nearly filled by these deposits, that there is barely room 
 
DEVELOPMENT OF BONE : OSSIFICATION. 65 
 
 for the blood-vessels to pass along them. And it is through 
 their complete blocking up, by a continuance of the same 
 growth, that the supply of blood is cut off from the interior 
 of the bone which forms the antlers of the deer, so that they 
 die and fall off ; their shedding and renewal being an annual 
 process. 1 Whilst the formation of the Haversian canals and 
 cancelli is being effected by the partial removal of the first 
 formed partitions, a complete cavity is formed in the centre of 
 the shaft of every long bone (at least in Mammals and Birds), 
 by the entire removal of the solid tissue. This cavity is at 
 first not much larger than one of the Haversian canals ; but 
 as the bone grows in diameter by additions to the exterior of 
 its shaft, so is the cavity in its interior augmented by the 
 removal (by absorption) of the first-formed bone ; and this 
 double process continues until the bone has attained its full 
 diameter. The formation of new bone on the exterior of 
 the shaft seems to be the result of the consolidation- of the 
 fibrous tissue of the periosteum (or membrane covering the 
 bone) by calcareous deposit ; the lacunae being probably the 
 cavities of cells which were entangled in the fibres, and the 
 canaliculi being outgrowths from these ; and new fibrous 
 tissue being formed on the outside of the periosteum, to replace 
 that which has been taken into the bone. Thus it comes to 
 pass, that after a time none of the bone first formed in its 
 cartilaginous mould any longer remains, the whole of it 
 having been removed by absorption ; since the central cavity 
 of the perfect bone is much larger than the entire carti- 
 laginous shaft in which it originated. And thus it also 
 comes to pass, that (as gelatin is the basis of fibrous tissue) 
 bones yield gelatin, not chondrin, upon being long boiled. 
 The increase of the shaft in length, however, is the result of 
 a different process. In all bones of any considerable dimen- 
 sions, the process of ossification commences in more than one 
 point at a time. In the long bohes, there are usually three 
 such points; one for the shaft, and the others for the two 
 
 1 It is commonly stated that the death of the anclers is due to the 
 formation of a bony ring at their base, which cuts off the supply of 
 blood from the " velvet" which covers them ; but though this may con- 
 tribute to produce the effect, it is by no means the sole cause, as the 
 interior of the antlers is supplied with blood from the vessels of the 
 bone from which they sprout, and not from those of the " velvet" 
 only. 
 
 F 
 
66 DEVELOPMENT OF BONE : OSSIFICATION. 
 
 extremities. Long after the ossification of the shaft and of 
 the extremities has been completed, these parts remain sepa- 
 rated from each other by the interposition of a thin layer of 
 unconsolidated cartilage ; so that, although the bone appears 
 firm and complete, its three portions fall apart, if it be 
 macerated sufficiently long in water for the cartilage to 
 decay. Now it is by the progressive consolidation of the 
 cartilage at these two junctions, and by the continual forma- 
 tion of new cartilage as the old is taken into the bone, that 
 the length of the shaft continues to increase up to adult 
 age ; and then, its full size having been attained, the whole 
 thickness of the intervening layer of cartilage is replaced by 
 bone, so that the shaft and extremities become firmly con- 
 solidated. The general history of the formation of the fiat 
 bones is nearly the same." In these, when they are large, or 
 have projecting out-growths, there are several centres of ossi- 
 fication ; and although the first ossification takes place in the 
 substance of cartilage, yet the subsequent growth seems to 
 be effected mainly by the consolidation of fibrous mem- 
 brane. 
 
 53. The foregoing description applies chiefly to those 
 higher and more complete forms of Bone, which are found in 
 Birds and Mammals. In Reptiles and Fishes, the process of 
 ossification is stopped short, as it were, at an early period ; 
 and thus the texture of their bones resembles that which we 
 find the skeleton to present in the earlier life of the higher 
 animals. The long bones of Eeptiles (with one remarkable 
 exception in the Pterodactylus, 669, which is adapted to 
 the life of a Bird) have no one central cavity, but are pene- 
 trated by numerous large Haversian canals, like those of very 
 young bone ; and various pieces remain separate in them 
 throughout life, which, originating in distinct centres of ossi- 
 fication, subsequently coalesce in Birds and Mammals. This 
 permanent separation is still more remarkable in the bones 
 of Fishes ; and it is consequently in them that we can best 
 study the real composition of the skeleton, every piece 
 which originates in a distinct centre of ossification, being, 
 in the eye of the philosophical anatomist, a separate bone. 
 Further, there is a large group of Fishes in which the 
 skeleton retains the cartilaginous character through life; a 
 certain quantity of mineral matter being deposited in the 
 
BONES OF FISHES : TEETH. 67 
 
 cartilage, but its conversion into true bony structure never 
 taking place. In a few, not even a firm cartilage is produced; 
 and all the trace of a skeleton is a cylinder formed of -hex- 
 agonal cells, resembling those of the pith of plants, which 
 takes the place that is generally occupied by the " bodies " of 
 the vertebrae ( 71). Such a cylinder, which is termed the 
 chorda dorsalis, precedes the formation of the vertebral 
 column in other vertebrated animals ( 757). In the curious 
 Amphioxus (ZooL. 642), even this is wanting; and the 
 only rudiment of the bony skeleton is to be found in the 
 fibrous sheath that surrounds the nervous centres, and sends 
 off prolongations between the successive transverse bands of 
 muscles, which are attached to these, as they are in other fishes 
 to the ribs and the spines of the vertebrae. 
 
 54-. In connexion with the structure of Bone, it will be 
 convenient to describe that of Teeth, although the general 
 description of the form and development of these organs will 
 be more appropriately given in connexion with the account 
 of their instrumental uses ( 181 183). The principal part 
 of the substance of all teeth is made up of a solid tissue, 
 which has been appropriately called Dentine. Of this sub- 
 stance, one variety, which is peculiarly close in texture, and 
 susceptible of a high polish, is familiarly known as "ivory* 
 The more perfect forms of dentine, such as present them- 
 selves in Man and the Mammalia generally, consist of a. hard 
 transparent substance formed by the union of animal matter 
 and calcareous salts (chiefly phos- _ T ^_ mr __ TTTTT ^_^ n _ T ^. r ^ rimr _ TTr . i 
 phate of lime), in the proportion 
 of about 28 of the former to 72 of 
 the latter; the mineral matter thus 
 bearing a somewhat larger ratio 
 to the organic, than it'' does an 
 bone. This dentinal substance is 
 traversed by minute tubuli of 
 about l-10,000th of an inch in 
 diameter, which appear as dark 
 lines, generally very close to- Fig. is. 
 
 gether ; these pass in a radiating PORTION OF DENTINE (highly magni- 
 
 manner from the central cavity fied), showing Us tubular structure. 
 of the tooth, diverging from each other as they approach 
 its exterior; but when seen in only a small part of their 
 
 F 2 
 
68 STRUCTURE OF TEETH. 
 
 course, they appear to be nearly parallel (fig. 18), though 
 usually more or less wavy. They occasionally divide into 
 two branches, which continue to run, at a little distance from 
 one another, in the same parallel direction ; and they also 
 frequently give off small lateral branches, which again send 
 off smaller ones. In some animals the tubuli may be traced 
 at their extremities into minute cavities analogous to the 
 lacunse of bone ; and the lateral branchlets also occasionally 
 terminate in similar cavities. Thus the whole tooth may be 
 likened, in some degree, to a single Haversian system in 
 bone ; the central cavity, which is lined by a vascular mem- 
 brane, representing the Haversian canal, while the radiating 
 tubuli of the former correspond with the radiating canaliculi 
 of the latter ; the chief difference lying in the absence of 
 lacunae along the course of the radiating tubes. In a large 
 proportion of Fishes, however, there is no single central cavity, 
 but the whole tooth is traversed by a system of medullary 
 canals, not only resembling the Haversian, but actually con- 
 tinuous with those of the bone on which the tooth is im- 
 planted; and as each of these is the centre of a distinct 
 system of radiating tubuli, the resemblance of their dentine 
 to bone , is very close. A somewhat similar condition of the 
 dentine (obviously a lower or less specialized form of this 
 substance) presents itself in certain Reptiles and Mammals. 
 In the Teeth of Man and most other Mammals, and in those 
 of many Reptiles and some Fishes, we find two other sub- 
 stances, one of them harder and 
 the other softer than dentine. 
 The former, which is called 
 Enamel, consists of long pris- 
 matic cells, which pass from one 
 surface to the other of the thin 
 layer formed by this substance 
 over the crown, or sometimes in 
 the interior of the tooth ( 182). 
 These prisms are usually hex- 
 Fig. 19. agonal in form, as is seen in 
 
 PORTION OF ENAMEL (highly magni- transverse section (fig. 19): and 
 fied), showing its component prisms. 
 
 their course is usually more or 
 
 less wavy. In teeth which have to sustain an extraordinary 
 amount of compression (as is especially the case with those of 
 
TEETH. MUSCLE AND NERVE. 69 
 
 the Eodentia), the enamel-prisms cross and interlace with one 
 another, iii such a manner as to prevent that separation 
 which would readily occur if the direction of all of them 
 were the same. Of all the tissues of the animal body, the 
 Enamel is the most remarkable for the predominance of 
 mineral ingredients ; these amount to no fewer than 98 
 parts in 100, leaving when removed only 2 per cent, of 
 organic matter. The softer component of Teeth, known as 
 the Cementum, or Crusta petrosa, possesses the essential 
 characters of true bone ; but when only a thin layer of it is 
 present, we do not find it traversed by medullary canals, its 
 system of lacunae and canaliculi being then in relation to the 
 nearest vascular surface, as is the case also with very thin 
 laminae of ordinary bone, such as we find in the scapula 
 (blade-bone) of a Mouse. 
 
 55. We come, lastly, to the two tissues which are of the 
 highest importance in the Animal fabric, and to which all the 
 rest are merely subsidiary ; namely, the Muscular and the 
 Nervous. It is through the instrumentality of these, that 
 all the actions are performed which essentially constitute 
 Animal life ; for the nervous apparatus is the medium by 
 which the consciousness of the individual is affected by what 
 takes place around him, or within his own body, and by 
 which, in his turn, he originates movements in his body, 
 and through it in things external to it ; whilst the muscles 
 are, so to speak, the servants of the nerves, doing, with a 
 force of their own, the work which the nerves direct. The 
 relation between the two may be likened to that of the rider 
 and his horse, or of the engine-driver and his locomotive ; for 
 the nerves can put forth no motor power by themselves; 
 whilst, on the other hand, the muscles (with certain excep- 
 tions) remain inert except when stimulated to contract by the 
 agency of the nerves. The muscles use the tendons and the 
 framework of bones, joints, &c., for the mechanical appli- 
 cation of their power, as will be shown hereafter (Chap, xn.); 
 but these parts of the fabric have not the slightest power of 
 originating motion by themselves. Hence, all Animal Force 
 takes its rise in one or other of these two tissues ; and we 
 shall find that the special purpose of the whole apparatus of 
 Organic life, is, by providing materials for their nutrition and 
 renovation, to build them up in the first instance, and then 
 
70 STRUCTURE OF MUSCLE. 
 
 to keep them in working order. For every development of 
 animal force involves a change of state of the Nervo-mus- 
 cular substance : a certain amount of it ceasing to exist as 
 living tissue, and passing into the condition of dead matter ; 
 and its elements resolving themselves, under the influence of 
 the free oxygen brought to them by the blood, into new combi- 
 nations, which are carried forth from the body as quickly as 
 possible. Consequently, if the ISTervo-muscular tissues be not 
 renewed as rapidly as they are used up, their powers must 
 speedily fail from the progressive loss of their substance. In 
 this particular they are on a different footing from the other 
 elementary parts of the organism ; for although each of these- 
 seems to have a certain term of life, the length of which is 
 in some degree related inversely to its functional activity, 
 those which live the fastest having the. shortest individual 
 duration, and vice versd, there are none which are called 
 upon to give forth their whole vital energy in one effort, and 
 which may thus have their existence as parts of the living 
 organism terminated at any moment by a demand for their 
 peculiar power. 
 
 56. Muscular Fibre presents itself under two forms, which 
 are ordinarily very distinct from each other ; although it is 
 probable that they may ultimately prove to be but modifi- 
 cations of one and the same. The first, which is known as 
 the striated fibre, is that of which all those muscles are com- 
 posed, which constitute what is commonly designated as 
 "flesh" or the "lean" of meat. If any "joint" of meat 
 be even cursorily examined, it will be seen that its whole 
 substance is made up of distinct masses,' held loosely together 
 by areolar tissue ; and these masses, which are known as 
 " muscles," are easily isolated from each other by dissection. 
 Every such Muscle is formed by the union of a number of 
 bundles, having a generally parallel arrangement, which are 
 closely bound together by areolar tissue, and are themselves 
 composed of bundles still more minute, united in a similar 
 manner. These, again, may be separated in the same way ; 
 and at last we come to the primitive fibres of which this 
 tissue is composed. Each of these primitive fibres termi- 
 nates at either extremity in tendinous fibre, which unites 
 with other fibres to form the tendinous cords or bands, that 
 are attached to the points of the skeleton which the muscle 
 
STHIATED MUSCULAR FIBRE. 71 
 
 has to bring together. The muscular fibre itself consists of 
 a delicate membranous tube, enclosing a great number of 
 fibrillce, or extremely minute fibrils, which are not capable 
 of further division (fig. 20). The peculiar transverse marking 
 
 Fig. 20. STRIATED MUSCULAR FIBRE SEPARATING INTO FIBRILLJE. 
 
 or striation by which this form of muscular fibre is characterised, 
 is found, when the fibre is separated into its fibrillee, to be due 
 to the peculiar markings which every fibril presents. These 
 markings, consisting of alternate light and dark spaces, give 
 to the fibril a beaded appearance ; but this is only an optical 
 deception, since its form is in reality cylindrical, or nearly 
 so. It is easy to see how the correspondence of the light 
 and dark spaces respectively, throughout the whole bundle of 
 the fibril, will give rise to the banded appearance which the 
 entire fibre presents. The form and diameter of the fibres 
 vary considerably, both in different tribes, and in different 
 parts of the same animal. In the higher classes, their form 
 usually approaches a cylinder; but the parts which press 
 against one another are somewhat flattened, so that it is more 
 or less prismatic. In Insects, on the other hand, the fibrillse 
 are arranged in flat bands, so that the fibre often consists of 
 but a single layer of them. The diameter of the fibres in Man 
 averages about 1 -400th of an inch, and does not differ very 
 widely in either direction ; in the cold-blooded Vertebrata, 
 however, the average size is greater, and the extremes are 
 also wider ; the diameter of the fibres varying in the Frog 
 from l-100th to l-1000th of -an inch, and in the Skate from 
 l-65th to l-300th of an inch. The diameter of the fibrils is 
 nearly the same in all classes, seldom departing much from 
 1-1 0,000th of an inch ; and the average olistance of the dark 
 striee from each other is nearly the same. 
 
72 NON-STRIATED MUSCULAR FIBRE. 
 
 57. The other form of Muscular Fibre, which, from the 
 absence of transverse striation, is distinguished as smooth or 
 non-striated, is found not in large masses, but in thin layers, 
 forming part of the wall of various hollow organs, such as the 
 stomach and intestinal canal, the bladder, the principal gland- 
 ducts, and the larger blood-vessels. In all these situations it 
 is so exclusively concerned in the performance of the vege- 
 tative or nutritive functions, and it is so entirely withdrawn 
 from the influence of the will, that it has been frequently 
 designated as " the muscular fibre of organic life ;" the striated 
 fibre, of which the voluntary muscles are composed, being 
 distinguished as the "muscular fibre of animal life." But 
 these designations are not by any means consistent with the 
 facts of the case ; for in a large proportion of the Molluscous 
 classes, the muscles of animal life are composed of non- 
 striated fibre, whilst the heart of Man and of other Verte- 
 brata, though a muscle of organic life, is made up of striated 
 fibre. In fact, the employment of the one or of the other 
 kind of fibre would seem to be chiefly determined by the 
 kind of contraction which is required from it ( 59). The 
 non-striated fibres are arranged, like those of the other 
 muscles, in a parallel manner into bands or bundles; but 
 c these bundles, instead of being them- 
 selves grouped into larger ones having 
 a like parallel arrangement, are gene- 
 rally interwoven into a kind of network, 
 having no fixed points of attachment. 
 The form of the individual fibres is 
 much more variable than that of the 
 striated kind, being often very much 
 flattened out ; and hence their general 
 dimensions cannot well be estimated. 
 By macerating a portion of this kind 
 of tissue in dilute nitric acid, each fibre 
 fibre, showing, a a, the ma y ^Q re solve<! into bundles of long 
 
 spindle-shaped cells, and, . ,, -, -, , -,. n i . - 
 
 b 6, the elongated nuclei ; spindle-shaped bodies, which, contain- 
 So?et!Mv Ce ^'iS? n a S elongated staff-shaped nuclei 
 similar ceil treated with (fig. 21), may be regarded as cells, al- 
 though it is difficult to distinguish their 
 
 walls from their contents. This form of muscular tissue is 
 commonly mingled with a large quantity of the ordinary fibrous 
 
MUSCULAR CONTRACTION. 73 
 
 structure ; and we find it dispersed in small quantity through 
 the latter in the skin, to which (especially in particular 
 regions) it gives a contractility that is manifested under the 
 influence of cold or of mental emotions, and thus produces 
 that general roughness and rigidity of the surface which is 
 known as cutis anserina, or " goose's skin." 
 
 58. Under the influence of certain exciting causes, or 
 stimuli (Chap, xu.), striated muscular fibres suddenly and for- 
 cibly contract. Their two ends approach one another, and their 
 striae become closer ; but they bulge out in the middle to a 
 corresponding degree. This causes a like change in the bundles 
 which are made up of these fibres ; and thus the whole muscle, 
 when shortened by the drawing together of its two ends, is 
 greatly enlarged in diameter, especially towards its middle. 
 Of this any one may convince himself, by bending his fore- 
 arm upon the arm (as when the hand is brought to the 
 mouth), and feeling the fleshy mass upon the front of the 
 latter. The muscle, in fact, does not in the least degree 
 change its own bulk in the act of contraction ; for its enlarge- 
 ment in diameter is exactly equivalent to the shortening of 
 the distance between its extremities. The contraction of a 
 muscular fibre is ordinarily followed, after a short interval, 
 by its relaxation ; of this we have a remarkable illustration in 
 the contractions excited by the electric stimulus. But relax- 
 ation of individual fibres is not incompatible with the con- 
 tinuance of the state of contraction of the muscle as a whole. 
 For it appears that when an ordinary muscle is thrown into 
 contraction, all its fibres do not usually contract together, but 
 only a small part of them ; and that, as long as its contraction 
 is maintained so as to exert a constant force, a continual in- 
 terchange is taking place in the action of the fibres by which 
 this is kept up those which have been shortened becoming 
 slack, and being replaced (as it were) by others, which pass 
 into the contracted state for a time, and then relax again, 
 being succeeded by another set. Now as the ends of those 
 fibres which are actually in a relaxed condition, are brought 
 near together by the contraction of the rest, the fibre is 
 thrown out of the straight line, and assumes a wavy or zigzag 
 form, which was formerly supposed to be the state of con- 
 traction, but is now known to be otherwise. This peculiar 
 arrangement gives place to the straight form, either when the 
 
74 MUSCULAR CONTRACTION. NERVOUS TISSUE. 
 
 fibre passes into the state of contraction, or when, by the 
 relaxation of the whole muscle, its ends are separated again 
 to their full extent. 
 
 59. Now the alternate contraction and relaxation, which 
 is thus made to produce a continued contraction in ordinary 
 muscles, elsewhere occasions a different effect. Thus in the 
 heart, all the fibres of the ventricles seem to contract to- 
 gether and all to relax together, those of the auricles contract- 
 ing whilst the others are relaxing, and vice versd; and in this 
 way the alternate contractions and dilatations of that most 
 important organ are continually kept up. Again, in the muscular 
 coat of the intestinal canal, we observe the contraction of each 
 part to be almost immediately followed by its relaxation ; but 
 the peculiarity of its movement is, that the contraction is pro- 
 pagated on (as it were) to the succeeding part, which in its turn 
 contracts and then relaxes, producing the same action in the 
 part that follows it, and so on along the whole canal. This 
 peristaltic motion ( 215), as it is called, is obviously adapted 
 to propel the contents of the intestinal tube from one ex- 
 tremity of it to the other ; just as the peculiar action of the 
 heart is adapted to receive and propel the blood alternately, 
 or as the mode of contraction of the ordinary muscles enables 
 them to keep up a continued strain for a great length of time. 
 It is much less rapid and energetic than the action of the 
 heart ; for it is the characteristic of the non-striated fibre, that 
 its contraction follows much less closely on the application of 
 the stimulus, and is much less rapidly succeeded by relaxa- 
 tion, than that of the striated fibre. 
 
 60. The Nervous tissue consists of two distinct structures, 
 of one of which the trunks of the nerves are entirely made 
 up, whilst the other enters largely into the composition of 
 the ganglia or centres of action ( 61). The former, termed 
 the white or fibrous tissue, consists of straight fibres, lying 
 side by side, and bound together by areolar tissue into 
 bundles (fig. 22); these, again, are united with others into a 
 larger group ; and by the union of a considerable number of 
 such groups, the nervous trunks are formed, which are dis- 
 tributed through the body, especially to the skin and muscles. 
 Nervous Fibre, like muscular, presents itself in the higher 
 animals under two forms, of which one may be considered as 
 more completely developed than the other ; these are known 
 
STRUCTURE OF TUBULAR NERVE-FIBRES. 75 
 
 as the tubular and the gelatinous. The " tubular " fibres are so 
 named because each poss.' ;*es a distinct tubular sheath of a 
 delicate structureless membrane (fig. 22, A), which encloses the 
 proper nerve-substance, and isolates it completely from the 
 
 Fig. 22. STRUCTURE OF NERVE-TUBES. 
 Tubular Nerve-fibres ; A, from a nerve-trunk; B, from the substance of the brain. 
 
 blood-vessels and other surrounding structures ; this tube 
 does not either branch or unite with others, and there is 
 reason to believe it to be continuous from the origin to the 
 termination of the nerve-trunk. Within the tube is a hollow 
 cylinder of a material known (after its discoverer) as the 
 " white substance of Schwann ;" and this encloses a sort of 
 central pith, which is transparent and semi-fluid in the living 
 state, but undergoes a kind of coagulation into a granular sub- 
 stance after death, and under the influence of chemical 
 re-agents. There is reason to believe that this central pith or 
 " axis-cylinder " is the essential component of the nervous 
 fibre, and that the hollow cylinder which surrounds it serves 
 only to isolate it more completely; for we not unfrequently 
 see the former to be alone continued, both the tubular sheath 
 and the white substance stopping short ; and this at either 
 extremity of the fibre, where it separates itself from those 
 with which it is bound up in the nerve-trunk. The proper 
 form of the fibre seems always to be truly cylindrical ; though 
 
76 TUBULAR AND GELATINOUS NERVE-FIBRES. 
 
 it is very liable to be altered by manipulation, a small excess 
 of pressure in one part forcing the contents of the tube 
 towards some other where they are more free to distend it, 
 and thus producing a swelling. The greater delicacy of 
 the tubular sheath in the fibrous substance of the brain 
 and spinal cord, renders its fibres peculiarly susceptible of 
 this kind of alteration, so that they often present under the 
 microscope a somewhat beaded appearance (fig. 22, B) ; when 
 carefully examined, however, without any previous disturb- 
 ance, these fibres are found to be as cylindrical as those of 
 the nerve-trunks. The diameter of the nerve-tubules is 
 usually between l-2000th and l-4000th of an inch ; but it 
 may be somewhat greater or considerably less than this 
 average. They are larger in the. nerve- trunks than they are 
 near their central termination in the brain ; and it is a remark- 
 able circumstance that the fibres of the nerves of " special 
 sense" are considerably smaller than the average in every 
 part of their course. The " gelatinous" fibres cannot be shown 
 to consist of the same variety of parts as the preceding ; for 
 neither the tubular sheath nor the white substance of 
 Schwann can be distinguished in them. They are flattened, 
 soft, and apparently homogeneous, sometimes showing a dis- 
 position to split into very delicate fibrillse ; being of a 
 yellowish-grey colour, they are sometimes designated the 
 grey fibres. Their diameter averages between the 1 -4000th 
 and l-6000th of an inch. As these "gelatinous" fibres 
 form a considerable proportion of the trunks of the Sympa- 
 thetic system of nerves ( 461), they have been supposed to 
 belong properly to it, and to minister exclusively to the 
 organic functions, like the non-striated muscular fibre ( 57); 
 but^there is no doubt that this is an incorrect notion, and 
 that even the fibres of the ordinary nerve-trunks may present 
 the " gelatinous " aspect, probably from incompleteness of 
 development. 
 
 61. In the central organs of the Nervous system namely, 
 the brain and spinal cord of the Vertebrata, the ganglia or 
 knot-like swellings on the nervous cords which take their 
 place in the lower animals, and similar ganglia belonging to 
 the Sympathetic system we find a form of nervous tissue 
 altogether distinct from the preceding ; which, from its con- 
 sisting of large cells or vesicles, is generally known as the 
 
VESICULAR OR GANGLIONIC NERVE-SUBSTANCE. 77 
 
 vesicular. These nerve-vesicles, sometimes known as gan- 
 glion-globules, may be regarded as originally spherical, or 
 nearly so, in form (fig. 23, a); but they often present one or 
 more prolonged extensions ; and as these when single re- 
 semble tails, and when multiple are like the rays proceeding 
 from a star, the cells are said in the first case to be "caudate," 
 
 Fig. 23. VESICULAR NERVE-SUBSTANCE. 
 
 A, combination of Ganglion-cells (of which one is shown separately at a, more highly 
 magnified), and Nerve-fibres in the grey substance of the brain, which is also 
 traversed by a capillary vessel, b; B B, Ganglionic cells with caudate pro- 
 longations. 
 
 and in the second to be stellate (B). These prolongations 
 have been traced into continuity, in some instances, with the 
 axis-cylinders of nerve-tubes, whilst in other cases they seem 
 to unite with those proceeding from other vesicles. It is not by 
 any means certain, however, that the nerve- tubes thus connect 
 themselves with the nerve-vesicles in all instances ; since it 
 frequently appears as if the former passed in among the 
 latter, without coming into direct continuity with them. 
 Sometimes a ganglion-cell seems to lie in the course of a 
 tubular fibre, which enlarges to envelope it, and then con- 
 tracts again to its former dimensions. There can be no 
 reasonable doubt, however, that in some way or other the 
 nerve-fibres and the nerve-vesicles come into some kind of 
 communication in the ganglionic centres. The vesicles are 
 
78 STRUCTURE OF GANGLIA. NERVOUS ACTION. 
 
 filled with a finely-granular substance, which extends into 
 their prolongations ; and in the warm-blooded Vertebrata they 
 contain pigment-granules, which give them 
 a reddish or yellowish-brown colour; so 
 that the aggregations of vesicular substance 
 which we find in the larger nervous centres, 
 are distinguishable by their greyish hue. 
 This "grey matter,"as it is frequently called, 
 is disposed on the surface of the brain; 
 but it occupies the interior of the spinal 
 cord, and holds the same position in the 
 smaller ganglionic centres (fig. 24). It 
 is not only, however, in the central organs 
 that nerve- vesicles are found; for they 
 present themselves also in certain situa- 
 Fig. 24. THIN SLICE OF tions at the other extremities of the nerve- 
 
 E THE M,^HE"IC fibreS ' ThuS We filld a }S B P r POrtion 
 
 SYSTEM, showing the of the retina ( 535), which is commonlv 
 flK* e aoiVt ne described as a mere expansion of the optic 
 giionic ceils. nerve, to be composed of nerve-vesicles 
 
 that are scarcely distinguishable from those of the brain; 
 and it is probable that the ultimate branches of other sensory 
 nerves have some such termination. Wherever we meet with 
 vesicular substance, we find it imbedded in a minute net- 
 work of blood-vessels; and a copious supply of oxygenated 
 blood is requisite to the due performance of its actions. 
 
 62. There can be no doubt that the special office of the 
 'Neive-Jibres is to convey the influence of the changes which 
 are effected in one part of the system, to other and remote 
 parts ; just as the wires of a galvanic battery conduct the 
 electric influence from the instrument which excites it, to 
 some distant point where it is to be applied to some use. 
 The effects of such changes in the state of the Nervous 
 System are propagated in two opposite directions ; the im- 
 pressions made upon the skin and other parts possessed of 
 sensibility, being conveyed towards a portion of the nervous 
 centres called the sensorium, and there giving rise to sensa- 
 tions; and the influence of the emotions or volitions to 
 which these sensations give rise ( 7), being propagated from 
 the central organs to the muscles, which they excite to con- 
 traction. And by the discoveries of Sir C. Bell, hereafter to- 
 
ACTIONS OF NERVOUS SYSTEM. 79 
 
 be described it has been fully proved that these opposite 
 changes are conducted by two different sets of fibres ; one 
 conveying to the central organs those which originate in the 
 circumference; and the other conveying to the circum- 
 ference those which originate in the centre ( 451). The 
 transmission of these changes is completely interrupted by 
 division of the nervous trunk, or by pressure upon it ; and it 
 sometimes happens that one set of conducting fibres is thus 
 affected, whilst the functions of the other are not impaired ; 
 so that a limb may retain its sensibility and yet be totally 
 destitute of the power of motion, or may be completely 
 obedient to the will though totally destitute of sensibility. In 
 Yertebrated animals, we find some nerves in which there is 
 only one set of fibres, so that the trunk is only sensory or only 
 motor ( 459); but in general, the two sets are bound up 
 together in the same sheath. 
 
 63. The motor fibres may be considered as originating in 
 the vesicular substance of the central organs, and as termi- 
 nating in the muscles- ; the power which is generated in the 
 former being conveyed by their means to the apparatus through 
 which it operates to produce mechanical motion. When the 
 nerve-trunks reach the muscles, they divide into branches 
 which penetrate their substance, and these again subdivide 
 and ramify minutely, so that at last the fibres may often be 
 observed running singly, passing amongst the muscular fibres, 
 but not appearing to penetrate their tubular sheaths. These 
 terminal fibres seem often to double back upon themselves, so 
 as to form loops, either re-entering the branch from which 
 they issued, or connecting themselves with some neighbour- 
 ing branch ; so that the ultimate distribution of the motor 
 nerves in the muscular substance, is a sort of plexu,s or net- 
 work. The sensory fibres, on the other hand, may be con- 
 sidered as originating in the sensory surfaces, such as the 
 skin, the interior of the nose, the lining membrane of the 
 cavities of the internal ear, the retina of the eye, &c. ; and 
 as passing towards the central organs, conveying to these the 
 impressions they have received, which impressions may either 
 affect the consciousness, or may excite respondent move- 
 ments, or may act in both modes, through the instrumentality 
 of the vesicular substance to which they are transmitted. 
 The immediate dependence of the functional activity of this 
 
80 SIMPLIFICATION OF STRUCTUBE IN LOWEST ANIMALS. 
 
 substance upon .the supply of blood which it receives, is 
 shown by the fact, that if this supply be temporarily cut off, 
 either by failure of the heart's action (as in fainting), or by 
 pressure on the blood-vessels which convey it, immediate 
 insensibility, with loss of all power of motion, is the result. 
 And the same is the case with regard to the organs of sense ; 
 for if the circulation through them be interrupted, no sensory 
 impression can be made upon the nerve-fibres which originate 
 in them, as we see when the movement of blood in a limb is 
 suspended by pressure upon its artery. 
 
 64. The foregoing constitute the principal tissues among 
 the higher animals, in which the principle of division of labour 
 is most fully carried out, every component part having its 
 own peculiar structure and its own special action. As we de- 
 scend in the scale, we find these distinctions less and less 
 obvious, so that when we come down to Zoophytes ( 121), we 
 meet with but little differentiation either in the textures or in 
 the actions of the several parts of the body ; the whole sub- 
 stance of these animals being composed of a tissue, which 
 very closely resembles that which is first formed in higher 
 animals for the reparation of wounds, having the appearance 
 of a solidified blastema ( 34), with nuclear particles, in 
 various phases of development into cells and fibres, more 
 or less thickly scattered through it ; and this substance 
 being everywhere contractile, and everywhere (at least in 
 many instances) equally capable of participating in the func- 
 tions of nutrition and reproduction. And when we pass still 
 lower, to that simplest type of animal life, which is pre- 
 sented to us in the Rhizopods ( 129), we do not meet with 
 even this amount of definite structure, but find the entire sub- 
 stance of their bodies composed of an apparently homogeneous 
 jelly, which, like the more organized tissue of the Zoophytes, 
 is everywhere contractile, and which has also the power of 
 performing every operation required for its growth and main- 
 tenance as a living being. In such creatures there is not the 
 slightest vestige of a Nervous system ; and it remains a question 
 whether, in consequence of this deficiency, they are altogether 
 destitute of consciousness, or whether this endowment is dif- 
 fused, as it were, through the whole substance of their bodies. 
 
 65. Every component part of the fabric must be regarded 
 
INDEPENDENT VITALITY OF PARTS OF ORGANISM. 81 
 
 as having a life of its own, which it maintains by drawing to 
 itself the nutrient material supplied by the circulating cur- 
 rent ; but as the continuance of its vital activity is dependent 
 upon the continuance of its nutrition, the life of no tissue 
 can be prolonged for any considerable period after the circu- 
 lation has ceased. But after the movement of 'the blood has 
 come to an end, though the body as a whole is dead, its part& 
 may remain alive for a certain time, and may perform their 
 functions, so long as they are supplied with the necessary 
 materials. Thus, various secretions, the growth of hair, and 
 muscular movements, have been observed to take place in 
 dead bodies. But they cannot continue, because the neces- 
 sary Conditions are withheld by the stoppage of the circu- 
 lation, a function which thus binds, as it were, into one 
 whole the scattered elements, and causes the different opera- 
 tions to minister one to another. As every component part 
 has an independent life, so has it a limited duration, quite 
 irrespective of that of the organism as a whole. Thus the 
 cells which float separately in the blood, seem to be con- 
 tinually undergoing change, dying, and giving place to new 
 ones. We have seen that the cells of the epidermis and of 
 some parts of the epithelium are being constantly thrown off 
 and renewed. The duration of the cells of fat and cartilage- 
 appears to be much greater; in fact, we have no precise- 
 knowledge of their term of life. That of the bony tissue is 
 probably greater still ; yet there is adequate evidence that 
 it is by no means indeterminate. But that of the muscular 
 and nervous tissues seems to depend almost entirely on the- 
 use that is made of them. Thus we may justly say, how- 
 ever startling the assertion may seem, that death and decay 
 are continually going on in every living animal body, and are 
 essential to the activity of its functions. 
 
 66. Many animals are reduced to a state of apparent death 
 by dryness, by cold, or by exclusion of "the air. A curious 
 example of the first kind is furnished by the Tardigrada 
 (ZOOLOGY, 841) ; some species of which may not only be 
 completely dried up, but may even be exposed in that state 
 to a temperature much exceeding that of boiling water, 
 without losing the power of recovery when moistened. A 
 similar power of revival after being dried up is possessed by 
 the common Wheel Animalcule, and probably also by the 
 
 G 
 
82 SUSPENDED ANIMATION. 
 
 eggs of many minute Entomostracous Crustacea (ZOOLOGY, 
 883, 931). It is unquestionable that many Fishes, especially 
 those of fresh- water lakes, will revive on being thawed after 
 having been completely frozen ; and the same has been ascer- 
 tained of certain Caterpillars. The Snail, when retiring for 
 the winter, seals the orifice of its shell with an impervious 
 lid ; and in this cavity it may remain shut up for years, until 
 re-excited to activity by warmth and moisture. Animals in 
 such states of torpidity strongly resemble seeds that are pre- 
 vented from germinating, apparently for unlimited periods, 
 by being kept at a moderate temperature, and excluded 
 from the influence of air and moisture, which, with adequate 
 warmth, would call them into active growth, but which, at 
 a lower temperature, would occasion their decomposition. 
 There are no positive facts which enable us to say how long 
 Animals may remain in a parallel condition ; but there seems 
 no reason why it might not be indefinitely prolonged. 
 
 67. The death of the body, then, does not consist in the 
 mere suspension of its vital activity; for so long as that 
 activity may be renewed when the requisite conditions are 
 supplied, so long must the organism be considered as alive, 
 however death-like its condition may seem. Among warm- 
 blooded animals, such a suspension, if complete, cannot be 
 endured for more than a very brief period, without the 
 extinction of life ; for the substance of_ their tissues is so 
 prone to decomposition, that it speedily passes into decay 
 unless prevented from doing so either by a reduction of tem- 
 perature, or by complete drying -up, or by entire seclusion 
 from air; and although each of these methods, practised 
 upon animal substances already dead, may prevent the occur- 
 rence of decomposition for almost unlimited periods, yet 
 neither can be applied to the living tissues of any of the 
 higher animals, without occasioning the entire loss of their 
 vitality, as we see (in regard to cold) in the loss of members by 
 " frost-bite." Such parts die, because not only is their vital 
 activity suspended, but their vital properties are annihilated. 
 Their death, however, does not necessarily involve that of 
 the organism as a whole ; since the stoppage of their function 
 may not disarrange the general train of vital operations, or 
 their duty can be discharged by other organs. And among 
 many of the lower animals, we find that there is a provision 
 
DECAY CONSTANT DURING LIFE. 83 
 
 for their replacement by ordinary acts of growth ; and that 
 even when the body has been so severely injured that the 
 organic functions are seriously disturbed for a time (as 
 when a Hydra is divided into two or more pieces, 122), 
 the vitality of the individual parts is sufficiently enduring, 
 and their reparative powers sufficiently energetic, to enable 
 them to reproduce all that is wanting for the completion of 
 the organism, and for the renewal of its ordinary actions. 
 Among the higher animals, the death of the organism at 
 large may be said to take place when the circulation finally 
 ceases ; since, as we have just seen, every individual part 
 must ere long lose its peculiar functional activity, and the 
 entire body be subject to decay. 
 
 68. Prom what has been stated, it will be seen that Life 
 cannot be regarded as a condition in which decay is resisted ; 
 for an incessant decay is taking place in every living organism 
 as a necessary condition of its vital activity, being only 
 checked when that activity is itself suspended. But it is a 
 condition in which, by the wonderful harmony and mutual 
 adaptation of the operations of the different parts, the repa- 
 rative action of the Organic Functions is made to countervail 
 the destructive action involved in the exercise of the Animal 
 Faculties ; whilst the latter, in their turn, serve to furnish 
 the conditions requisite for the maintenance of the former. 
 So long as all these actions go on with regularity and com- 
 pleteness, so long the whole body lives ; but if any one of the 
 more important among them be interrupted, the stoppage of the 
 whole is the result. This relation of mutual dependence is most 
 intimate in the higher animals ; in which, by the differentia- 
 tion of the several tissues and organs, and the specialization 
 of their functions, the division of labour is carried to its 
 greatest extent, so that no part can entirely fulfil the duty of 
 any other. On the other hand, it is among those lowest 
 forms of animal life, in which there is the greatest multipli- 
 cation of similar parts, and the greatest diffusion of the same 
 endowments amongst them all, that we find the dependence 
 of the several parts of the organism upon each other to be 
 the slightest, and severe injuries to be tolerated with the 
 least general disturbance. 
 
84 PRINCIPAL TYPES OP ANIMAL STRUCTURE. 
 
 CHAPTEE II. 
 
 GENERAL VIEW OF THE ANIMAL KINGDOM. 
 
 69. WHEN we examine the Animal Kingdom as a whole, it . 
 is easy to distinguish in it four general plans or types of struc- 
 ture, by which, with almost infinite variations in detail, the 
 formation of the several beings that compose it has been 
 guided. As specimens of these four plans or types, we may 
 name four animals which are familiar to almost every one, 
 the Dog, the Lobster, the Snail, and the Star-fish. The dif- 
 ferences by which these types are distinguished, are mani- 
 fested in the arrangement of the different organs of the body ; 
 and particularly in the form of the .nervous system and its 
 instruments. It has been already stated ( 4 ) that the power 
 of feeling, and of spontaneous motion, is that which peculiarly 
 distinguishes the Animal from the Plant; and as these powers 
 are possessed in very different degrees, and exercised in very 
 different modes, by the various tribes of animals, whilst the 
 operations' of -nutrition are performed, as in plants, in a much 
 more uniform manner, they afford us a satisfactory means of 
 separating these tribes from one another. For the nervous 
 system is the organ to which these powers are due ; and we 
 find it presenting forms so different in the four great divisions 
 already alluded to, that we can at once distinguish them by 
 this alone, even where (as sometimes happens) there may be 
 such a blending, in a particular animal, of the general characters 
 of two of them, as to lead us to hesitate in assigning its precise 
 place in the animal kingdom. 
 
 70. The highest of these four divisions is that denominated 
 VERTEBRATA, or Vertebrated Animals; it receives its name 
 from the structure characteristic of it, the possession of a 
 jointed back-bone or vertebral column, which will be pre- 
 sently described. This is the group to which Man belongs ; 
 and all the animals it contains bear a greater or less resem 
 blance to him in structure. We notice in regard to their 
 external form, that they are alike on the two sides of their 
 body; every part having its fellow on the other side. This 
 
VERTEBRATED TYPE OF STRUCTURE. 85 
 
 " bi-lateral symmetry " extends to the arrangement of those 
 internal parts which are connected with the functions of 
 animal life ; namely, the nervous system, the organs of sense, 
 and the muscular apparatus. But it does not always extend 
 to the organs of nutrition, which are unequally disposed on 
 the two sides : thus, in Man, the heart and stomach are on the 
 left side, and the liver on the right, while the lungs are much 
 larger on the right side than on the left. But in many of the 
 lower Vertebrata, there is an almost perfect symmetry in the 
 disposition of these organs, as there is also in the early embryo 
 of those in which this symmetry is subsequently departed 
 from ; so that it may be truly said that this symmetry is cha- 
 racteristic of the Vertebrate type, although for special purposes 
 it is frequently superseded. 
 
 Fig. 25. SKELETON op THE OSTRICH. 
 
 71. In all Vertebrated animals, the skeleton is chiefly 
 internal (fig. 25); and consists of bones, which are capable of 
 
86 VERTEBRAL COLUMN. 
 
 growing, and of being reproduced after injury, like any other 
 part of the living tissue ; being copiously supplied with blood- 
 vessels, which penetrate into their interior. These bones give 
 support, and afford points of attachment, to the soft parts, in the 
 limbs (where they exist) as well as in the trunk ; but the former 
 are not unfrequently wanting, as in Serpents : and we must look 
 in the trunk, therefore, for that peculiar arrange- 
 ment which is characteristic of this division of 
 the Animal Kingdom. The back-bone, as it is 
 commonly termed, is found in all Vertebrated 
 animals ; though in a few among them (the 
 lowest Fishes) it is very imperfect ( 53). It 
 consists of several pieces jointed together, so as 
 to possess great flexibility; whilst they are so 
 firmly connected by ligaments, that they cannot 
 easily be torn asunder or displaced. The number 
 of these pieces varies considerably ; in Man there 
 are only 33 ; in some long- tailed Mammals there 
 are more than 70 ; but in many Serpents there 
 are several hundred. Each of them is termed a 
 vertebra; and the whole structure, composed of the 
 ^JRAif coIuM TE ~ lin ^' e( ^ Ver tehra3, i s termed the vertebral column 
 (fig. 26). The ordinary character of the vertebras 
 is, that each is perforated by an aperture, which, united to the 
 corresponding apertures of those above and below it, forms 
 a continuous canal ; and in this canal, one of the most im- 
 portant parts of the nervous system, the spinal 
 cord (commonly but erroneously termed the spinal 
 marrow), is contained. The solid portion of the 
 vertebra (fig. 27, a) is termed its body; and the 
 projections, b and c, are termed its processes, the 
 former spinous, the latter transverse. The row 
 of spinous processes forms the ridge which we 
 P ass i n g down the back; it is seen on the 
 right-hand side of fig. 26. To the. transverse 
 processes the ribs are attached. The vertebral column is ex- 
 panded (as it were) at its upper extremity, to form the skull ; 
 in the large cavity which it contains, the brain is lodged ; and 
 its bones are so arranged as to give protection to the organs of 
 sense also. At the opposite extremity we see it contracted 
 into the tail; which is composed of a series of vertebrae 
 
NERVOUS SYSTEM OF VEBTEBRATA. 
 
 87 
 
 resembling those of the back, but simpler in their form, and 
 not possessing a cavity for the spinal cord. We commonly 
 find that in those animals in which the skull is very large, 
 the tail is short; and that where the tail is very long or 
 powerful, the head is small. Thus in man and in the apes, 
 the head is large, and there is no 
 external appearance of a tail ; but there 
 are some very imperfect vertebrae at the 
 lower end of the spinal column, which 
 constitute the rudiment of it. In the 
 long-tailed monkeys and in the kan- 
 garoo (whose tail is like a third hind- 
 leg), the head is comparatively small. 
 But this rule does not hold good uni- 
 versally. 
 
 72. The Nervous system of Verte- 
 brated animals consists of a Brain and 
 Spinal Cord (fig. 28), which are lodged 
 within the skull and vertebral column ; 
 and of nervous trunks proceeding from 
 these, which are distributed to all parts 
 of the body. The Brain is not (as 
 commonly reputed) a single organ, but 
 is composed of a number of ganglionic 
 masses, differing considerably in their 
 functions. Thus each of the nerves 
 of special sense (smell, sight, hearing, 
 and taste) has its own proper centre ; 
 and there is another of considerable 
 size, which seems to perform the same 
 office in regard to common sensation. 
 These are found in Vertebrata generally ; 
 and their proportionate size corresponds 
 with the relative development and ac- 
 tivity of the several organs of sense 
 with which they are connected. The 
 bulk of the brain of Man, however, is * 
 made up by two large masses of nervous 
 matter, which are known as the Cerebral Hemispheres; these, 
 as will be shown hereafter (chap, x.), are so small in the brains 
 of Fishes as to be scarcely distinguishable ; and their relative size 
 
88 NERVOUS SYSTEM OF VERTEBRATA. 
 
 and complexity of structure increase as we ascend the scale, 
 in pretty close accordance with the increase of the intelligence 
 or reasoning faculty. There is also another large nervous 
 mass, called the Cerebellum ; the function of which seems to 
 consist in the regulation of the more complex movements. 
 The Spinal Cord is made up of a longitudinal succession of 
 independent centres, of which one corresponds with each of 
 the vertebral segments of the body. 
 
 73. The distinguishing feature of the Nervous system in 
 Vertebrata is, that its several centres are thus united into one 
 large mass, instead of forming a number of separate small 
 masses or ganglia, as we shall find that they do in the lower 
 classes of animals : and that it is inclosed in the bony casing 
 which has been described as peculiarly destined for its pro- 
 tection, instead of being enveloped with all the other organs 
 in a hard covering, as in the Lobster, or of being entirely 
 destitute of protection, as in the Slug. That it should receive 
 this peculiar protection is quite necessary, in consequence of 
 the much higher development which it attains, and the much 
 greater importance which it possesses, in this division of the 
 animal kingdom, than in any other. In all but the very 
 lowest Vertebrata, all five kinds of sensation exist ; namely, 
 sight, hearing, smell, taste, and touch. We find in this group 
 more intelligence than in any other ; that is to say, the animals 
 composing it act more with a designed adaptation of means to 
 ends ; instead of being impelled by a blind instinct to perform 
 actions of whose objects they are not aware. And we find, by 
 observing and comparing the structure and actions of the dif- 
 ferent groups, that the intelligence gains upon the instinct, as 
 we ascend from the lowest Fishes towards Man, in whom the 
 intelligence is at its highest ; whilst we observe a similar 
 increase in the proportion which the brain bears to the rest 
 of the nervous system. Hence we conclude, that the brain is 
 the organ of intelligence, or of the reasoning faculties. 
 
 74. The general arrangement of the other organs in Verte- 
 brated animals, is shown in fig. 29. At m is seen the mouth, 
 forming the entrance to the digestive cavity, of which the 
 termination is at the opposite extremity of the body ; i, i, is 
 the intestinal canal, and I, the liver : these organs occupy the 
 part of the body which is called the abdomen or belly. The 
 mouth also opens, however, into the windpipe, or trachea, t, 
 
GENERAL STRUCTURE OP VERTEBRATA. 89 
 
 which conducts air into the lungs, p ; these organs, with the 
 heart, h, are contained in the portioy. of the trunk called the 
 
 s I 
 
 Fig. 29. DIAGRAM, SHOWING THK POSITION op THE PRINCIPAL ORGANS IN 
 VERTEBRATA. 
 
 thorax, or chest. At 6 is seen the position of the brain ; and 
 at s that of the spinal cord. 
 
 75. The foregoing characters apply, with greater or less 
 modification as to details, to the classes of Mammals (com- 
 monly termed Quadrupeds), Birds, Reptiles, and Fishes; and 
 these further agree in the following points, all of which, 
 therefore, enter into our idea of a Vertebrated animal. The 
 number of limbs or members never exceeds four ; and of 
 these, two, or even all four, may be absent. In all the 
 classes just named, four is the general number; and the 
 absence of two or more is the exception. Thus in Mammals, 
 we find all four present in every tribe save that of Whales, 
 which want the hinder pair ; though the upper or anterior 
 pair may take the form of arms, wings, legs, or fins, accord- 
 ing to the element which the animal is formed to inhabit. 
 In Birds we find the posterior pair invariably present in the 
 form of legs ; whilst the anterior pair, though almost always 
 developed into wings, is absent in a few instances. In Eeptiles 
 we find considerable variety ; all four members are present in 
 the Turtle tribe, and in most Lizards, as well as in the Frog 
 tribe ; but they are entirely absent in the whole tribe of Ser- 
 pents ; and there are Lizards which have only one pair. And 
 in Fishes, we usually find two pairs, constituting the pectoral 
 and ventral fins ; but one or both pairs are sometimes absent, 
 as in the Eel, Lamprey, &c. We have further to remark, in 
 regard to the general characters of Vertebrated animals, that, 
 
90 CLASSES OF VERTEBEATA : MAMMALS. 
 
 with one exception, they have all red blood ( 226) ; and 
 that they possess a complex apparatus for circulating this 
 through the body. 
 
 76. The four principal modifications under which the Ver- 
 tebrated type presents itself, constituting the classes of MAM- 
 MALS, BIRDS, REPTILES, and FISHES, are respectively character- 
 ised by the mode in which the principal functions of life are 
 performed in each.* Thus there are some Vertebrated animals 
 which produce their young alive, and which nourish them 
 afterwards by suckling; while the greater part rear them 
 from eggs which contain a store of nutritive matter, and 
 do not afford them any further nourishment from their own 
 bodies. Again, some breathe air; whilst others live con- 
 stantly in water, and have no direct communication with the 
 atmosphere. Some, moreover, have the power of keeping 
 up a high temperature, so that their bodies always feel warm 
 to the touch ; whilst the temperature of others varies with 
 that of the atmosphere, so that their bodies give a feeling of 
 coldness : the former are termed warm-blooded the latter 
 cold-blooded. There is a like difference in their mode of life ; 
 some of them being destined to live on the surface of the 
 earth, whilst others are chiefly inhabitants of the air, and 
 others again are the tenants of the ocean. 
 
 77. MAMMALS are distinguished from all other Vertebrata 
 by the first of the characters just adverted to ; being the only 
 animals that produce their young alive, and nourish them 
 afterwards by suckling. Like Birds and Reptiles, they 
 breathe air by means of lungs ; and, in common with Birds, 
 they are warm-blooded and have a complete double circula- 
 tion of their blood, carried on by a heart with four cavities. 
 They are for the most part quadruped (that is, four-footed), and 
 are destined to live upon the surface of the earth ; but Man, 
 and the Apes that approach nearest to him, are biped, having 
 the power of walking on two limbs, and of using the others 
 for different purposes ; whilst the Bat tribe have the two 
 arms converted into wings, which enable them to fly through 
 the air like birds (for which the older naturalists mistook 
 
 * Many Zoologists range the Frogs and their allies in a separate class, 
 under the name of AMPHIBIA; but when looked at from a physiological 
 point of view, the author does not see that they require to be separated 
 from the true Reptiles. 
 
GENERAL STRUCTURE OF MAMMALS. 91 
 
 them); and the Whale tribe are adapted in their general 
 form to lead the life of fishes (among which they are still 
 commonly ranked by persons ignorant of natural history). 
 Notwithstanding these marked differences in external form, 
 there is a great correspondence as to internal structure ; for 
 bats and whales, as well as ordinary quadrupeds, produce 
 their young alive, and suckle them afterwards ; they are also 
 warm-blooded, breathing air, and having an active circulation. 
 The bodies of Mammals are, for the most part, more or less 
 completely covered with hair, which serves to keep in their 
 warmth ; and this is seldom absent, except in such as inhabit 
 warm climates and do not require this provision. In the 
 Whales, the same end is answered by the thick layer of oil in 
 the substance of the skin, constituting the blubber ; and Man 
 is left to form a protective covering for his body by the exer- 
 cise of his own ingenuity. The general arrangement of the 
 
 Sub-maxillary Gland Parotid Gland 
 
 Windpipe .. ~"*v.. 
 
 ' 
 
 Pharynx 
 
 -Oesophagus 
 
 Gall-Bladder 
 
 Colon .^, _ 
 Caecum.... 
 Small Intestines 
 
 Fig. 30. INTERIOR OF A MONKEY. 
 
 internal organs of Mammals will be seen from the accom- 
 panying figure of the body of a Monkey, laid open in such 
 
92 GENERAL STRUCTURE OF BIRDS. 
 
 a manner as to exhibit the chief of them. The cavity of 
 the trunk is completely divided, by the muscular partition 
 termed the diaphragm, into two portions the thorax, con- 
 taining the heart and lungs ; and the abdomen, containing 
 the digestive apparatus. It is chiefly by the alternate con- 
 traction and relaxation of this muscle, that the act of 
 breathing is performed in Mammals, as will be explained 
 hereafter ( 331). 
 
 78. In BIRDS there is a much closer conformity to one 
 general plan than we find among Mammals. The covering of 
 feathers, by which we ordinarily distinguish the members of 
 this class, prevails universally ; and there is no wide depar- 
 ture from the typical form. This class belongs to the 
 oviparous division of the Vertebrata ; the young being reared 
 from eggs. But it is distinguished from Reptiles, which are 
 also oviparous and air-breathing, by being warm-blooded; 
 and by having a very energetic instead of a very slow circu- 
 lation. The warmth of the maternal body, moreover, is im- 
 parted to the egg in the act of incubation ; and without the 
 heat thus communicated (unless it be supplied from some 
 other source) the embryo cannot be developed. The covering 
 of feathers is given, not only to keep in the heat of the body, 
 which is even greater than that of Mammals, but also 
 to afford the required surface for the wings, on which the 
 Bird is supported and propelled through the air. The 
 feathered portion of the wings is stretched out upon the 
 bones which answer to those of our arm, and is moved by its 
 muscles. The wings are very small, or are entirely absent, in 
 the Ostrich and a few other birds, which present the nearest 
 approach to the Mammalia in their internal structure ; and 
 these cannot rise from the ground, but run swiftly along it, 
 by means of their powerful legs. In the Penguin, also, the 
 wings are small ; and they are used as fins, by the assistance 
 of which this bird, which can neither walk nor fly with 
 rapidity, can swim very quickly through the water. 
 
 79. Generally speaking, Birds are characterized by the 
 extraordinary power of motion which they possess, and by 
 the great acuteness of the sense of sight, by which their 
 movements are chiefly directed. They are also remarkable for 
 their instinctive actions, which are chiefly related to their care 
 of their young, for whom they usually construct a protective 
 
GENERAL STRUCTURE OF BIRDS. 
 
 93 
 
 nest. The nutritive functions are performed with extra- 
 ordinary activity in Birds, that the means may be supplied 
 for the maintenance of their locomotive activity. Their blood 
 is particularly rich in red particles, and its heat is usually 
 considerably above that of Mammals. Its circulation is very 
 energetically carried on ; and although the lungs themselves 
 are constructed upon a type inferior to that of Mammals, 
 and the mechanism of respiration is less complete, yet, by an 
 extension of the respiratory organs through the whole fabric, 
 the aeration of the blood is carried on with unequalled 
 energy ( 326). 
 
 80. The arrangement of the organs contained in the cavity 
 of the trunk of Birds differs from that which has beeD 
 described in Mammals, chiefly 
 in this, that there is usually 
 no diaphragm to separate the 
 chest from the abdomen, and 
 that although the lungs them- 
 selves are confined to the upper 
 part of this cavity, they are con- 
 nected with a series of air-sacs 
 which are distributed through 
 the whole of it. In the accom- 
 panying figure, which repre- 
 sents the internal organs of the 
 Ostrich, the heart is seen at a, 
 the stomach at 6, and the in- 
 testinal tube at c. The windpipe, 
 d, opens into the lungs, e, which 
 are themselves small, and are 
 attached to the ribs, instead of 
 lying freely in the cavity of the 
 chest : but the space they would 
 
 Fig. 31. LUNGS OF THE OSTRICH. 
 
 , 
 
 ; ///, air-cells,in which are also 
 seen tne tubes b ^ which these air ' 
 
 cellg communicate W uh the lu-ngs. 
 
 . the lieart ; b > the stomach ; c c, the 
 
 .. . , j fm j intestines; d, the trachea; e, the 
 
 Otherwise have OCCUpied IS filled lungs; 
 n-n Vnr fhp IflTfTP flir rpll<3 f f 
 
 up by tne large air- eiis, /,/, 
 
 which communicate freely with 
 
 the lungs and with each other, and which even occupy a large 
 
 part of the cavity of the abdomen, as seen in the figure. 
 
 81. In the class of EEPTILES we find a variety of form so 
 remarkable, that, if we were influenced by this alone, we 
 should scarcely regard the animals it contains as belonging to 
 
94 GENERAL STRUCTURE OP REPTILES. 
 
 the same group j yet the structure of the internal organs, on 
 which classification is founded, is essentially alike in all; 
 and their physiological condition presents no important dis- 
 similarity. Four obviously different tribes, Turtles, Lizards, 
 Serpents, and Frogs, are brought together by the following 
 characters. They are all oviparous, in this respect agreeing 
 with Birds and Fishes ; but they are cold-blooded, and have 
 not a complete apparatus for the double circulation of the 
 blood, in which respect they differ from Birds; and they 
 breathe air by means of lungs, instead of breathing water by 
 gills, in which respect they differ from Fishes. But by the 
 lowest group, that of Frogs and their allies, this class is 
 united to that of Fishes in a most remarkable manner ; for 
 these animals in their young state breathe by gills, and 
 lead the life of a fish ; and some of them retain their gills 
 during the whole of life, even after the lungs are developed 
 ( 87). The first three of the tribes just mentioned un- 
 dergo no such change : and they further agree in this, that 
 they breathe air during the whole of their lives, coming forth 
 from the egg in the same condition as that in which they are 
 subsequently to live, and also in having their bodies covered 
 with horny scales or plates, whilst the skin of the Frog tribe 
 is soft and unprotected. 
 
 82. The class of Eeptiles presents a marked contrast to 
 that of Birds, in the comparative slowness and feebleness of 
 its movements, the dulness of its sensibility, and the in- 
 activity of its organic functions. As there is no fixed tempe- 
 rature to be maintained, one important source of demand for 
 food is withdrawn; and when not excited to activity by 
 external warmth, these animals may pass long periods without 
 fresh supplies of food. Their blood is very poor in red 
 corpuscles, and its circulation is comparatively languid. A 
 reduction of the temperature of their bodies to within a few 
 degrees of freezing point, induces complete torpidity, which 
 continues until they are roused by a renewal of warmth. 
 
 83. The Turtle tribe is peculiarly distinguished by the 
 inclosure of the body in a bony covering ; of which the 
 upper arched portion (termed the carapace] is formed by 
 the coalescence of the ribs with a set of bony plates deve- 
 loped in the substance of the skin; whilst the lower fiat 
 plate (termed the plastron), which is often incomplete, is 
 
STRUCTURE OP TURTLES AND LIZARDS. 95 
 
 formed by an expansion of the sternum or breast-bone, which 
 is spread out sideways, instead of being raised into a project- 
 ing keel as in Birds. The carapace and 
 plastron are covered with large horny 
 plates, variously arranged in the dif- 
 ferent species, and constituting what is 
 commonly called tortoise-shell. These 
 plates are often very beautifully disposed, 
 forming a kind of tesselated pavement ; 
 as in the common Tortoise (fig. 32), 
 which is often preserved alive in our 
 gardens. 
 
 84. In the tribe of Lizards, the body 
 has no such covering ; but these animals, 
 having more activity than the tortoises 
 
 / !_ i. i n i \ -ui j Fig. 32. TORTOISE. 
 
 (which are proverbially slow), are enabled 
 to make their escape from danger, whilst the latter are obliged 
 to trust to their bony casing for protection from it. In their 
 general form, Lizards approach Mammals, being four-footed, 
 and living for the most part on land ; but they differ from 
 them not only in their essential reptilian characters, but also 
 in several others of less consequence. Their bodies are 
 usually covered with scales, which lap over one another like 
 the tiles of a roof ; but in the Crocodile tribe, many parts of 
 
 Fig. 33. CROCODILE. 
 
 the surface are covered with large knotted horny plates, that 
 meet at their edges like the scales of tortoise-shell, and afford 
 an almost impenetrable covering. Although some of the 
 Lizard tribe spend a large part of their time in water, they 
 all breathe air ; but, as their respiration is very inactive, they 
 can remain for long periods beneath the surface, without 
 being obliged to come up to breathe. 
 
 85. The tribe of Serpents may be regarded as lizards with- 
 out feet ; their spinal column is immensely prolonged ; and 
 
96 
 
 STRUCTURE OF SERPENTS. 
 
 their ribs are also very numerous ; and they are able to crawl 
 upon the points of these, using them almost as Centipedes do 
 their legs (fig. 42). But in general the movement of their 
 
 Fig. 34. ANATOMY OF A COLUBER 
 
 bodies is accomplished by their being drawn-up into folds, 
 and then straightened so as to project the head. The pro- 
 longed form of the body in Serpents occasions a curious 
 variation in the arrangement of the principal organs, which 
 is shown in the accompanying figure. The oesophagus or 
 
STRUCTURE OF SERPENTS AND BATRACHIA. 97 
 
 gullet, oe, which leads from the mouth to the stomach, is a 
 long and very wide canal, being even larger than the stomach 
 at its commencement ; a portion of it is removed at oe', in. 
 order to show the heart, &c., which would otherwise be con- 
 cealed by it. The stomach, i, is long and narrow ; and the 
 intestinal tube, i', after making a few turns or convolutions, 
 passes backwards in a straight line, to terminate in the cloaca, 
 cl, which opens externally by the orifice, an. The liver, /, is 
 also much lengthened. From the mouth also proceeds the 
 long windpipe, t t, which conveys air to the lungs, or rather 
 to the single lung ; for the lung on the left side, p' } is scarcely 
 at all developed, whilst that on the right, p, extends along a 
 great part of the body. At o is seen the ovarium, in which 
 the eggs, o' o', are produced ; and this also is very much 
 lengthened, extending from the cloaca a good way up the 
 body, so as nearly to meet the lung. The other references 
 are to the parts of the heart, and the principal vessels ; the 
 structure and arrangement of which will be explained here- 
 after ( 284). 
 
 86. The Batrachia, or animals of the Frog tribe, are 
 readily distinguished from all the preceding, by their soft 
 naked skins ; even when the form of the body, as in the com- 
 mon Salamander or Water-Newt, resembles that of the lizards. 
 They are also remarkable for the metamorphosis which they 
 undergo in the early part of their lives ; for they come forth 
 from the egg in a condition which is, in all essential particu- 
 lars, that of a fish, and undergo a gradual series of changes, 
 by which their form and structure become assimilated to those 
 of the true reptiles. This change is most complete in the 
 Frogs and Toads ; the early form of which is known as the 
 tadpole. The principal stages of this change are represented 
 in figs. 35 to 39 ; in which, however, the relative sizes are 
 not preserved, the tadpoles being much larger in proportion 
 (for the sake of displaying their form and the gradual 
 development of their legs) than the complete frog. Soon 
 after the young tadpole has come forth from the egg, it pre- 
 sents the form which is shown in fig. 35 ; its head and 
 trunk are large, and the latter is prolonged into a flattened 
 tail, by which the little animal swims freely through the 
 water. There is not the least appearance of limbs or mem- 
 bers. It breathes by gills, which are long fringes, hanging 
 
 H 
 
98 
 
 METAMORPHOSIS OF BATRACHIA. 
 
 loosely in the water on either side of the head. At a later 
 period, however, these gills, which are merely temporary, 
 disappear ; and the breathing is carried on by another set, 
 which are situated behind the head, and are covered in by a 
 fold of skin; the water gains access to these by passing 
 through the mouth, exactly as in Fishes. The form is then 
 that which is represented in fig. 36. In a short time after- 
 wards, the animal still breathing by its gills, the hind-legs 
 begin to sprout forth, as it were, at the base of the tail ; this 
 
 Fig. 36. 
 
 Fig. 37. 
 
 Fig. 35. 
 
 Fig. 39. 
 
 stage is shown in fig. 37. At a still later period, the fore- 
 legs begin to be developed, as seen in fig. 38 ; and from that 
 time they are nourished at the expense of the tail, which 
 gradually disappears, as seen in fig. 39, a, b. During ^this 
 period, other important changes are taking place in the inte- 
 rior of the body ; the chief of which are the development of 
 the lungs and the gradual disuse of the gills, so that the 
 animal becomes fitted to live on land and breathe air, and is 
 no longer capable of remaining long under water without 
 coming to the surface to respire. 
 
 87. The metamorphosis in other members of the group is 
 
PERENNIBRANCHIATE BATRACHIA. 
 
 99 
 
 less complete than in the Frog, being checked at a less 
 advanced stage. Thus in the common Water-Newt, the tail 
 is retained during the whole of life, and the animal continues 
 to be an inhabitant of the water, though breathing air alone. 
 There are some very curious animals, however, in which the 
 change is stopped, as it were, at a much earlier period, so that 
 the gills also are retained ; and in these, the lungs are suffi- 
 ciently developed to enable the animals to breathe air, so that 
 they can live either on land or in water. Such Batrachia are 
 scientifically known as perennibranchiate, this term express- 
 ing the persistency of their gills. In fig. 40 is represented 
 
 Fig. 40. AXOLOTL. 
 
 an animal of this kind, the Axolotl, which inhabits some of the 
 lakes of Mexico. And in fig. 41 is shown the form of a still 
 more remarkable animal, the Lepidosiren, or mud-fish, recently 
 
 
 Fig. 41. LEPIDOSIREN. 
 
 brought from th.3 rivers of Africa, the metamorphosis of 
 which appears to be checked at a still earlier period, so that 
 it is very difficult to decide whether it should be regarded as 
 
 H 2 
 
100 STBUCTUEE OP FISHES. 
 
 a Fish or as a Reptile, so complete is the mixture of charac- 
 ters which it presents. 
 
 88. The class of PISHES is distinguished from all other 
 Vertebrata, by the adaptation of the animals composing it to 
 breathe by means of water in their adult state, so as to be 
 capable of living in that element only. Like Reptiles, they 
 are oviparous and cold-blooded ; and in these characters they 
 differ completely from the "Whales and other Mammals, 
 which are, like them, inhabitants of the great deep, but which 
 are warm-blooded, viviparous, and air-breathing animals. 
 There is a simple external character, by which the members 
 of the two classes may be at once distinguished. The animals 
 of the "Whale tribe are, like fishes, chiefly propelled through 
 the water by means of a flattened tail ; but in the former the 
 tail is flattened horizontally, so that its downward stroke may 
 serve to bring the animal to the surface to breathe ; whilst in 
 Fishes it is flattened vertically, that its strokes from side to 
 side may simply propel the fish through the water. A 
 flattening or compression of the body is seen more or less 
 in almost all fishes, and is intimately connected with the 
 nature of their motion through the element they inhabit ; as 
 it serves the double purpose of diminishing the resistance 
 which is offered to their progress, and of increasing the extent 
 of the oar-like surface, by the lateral stroke of which the 
 body is propelled forwards (Chap. xn.). This stroke is given 
 by- a series of muscles of great power, which pass from the 
 prolonged extensions of one vertebra to those of another, and 
 altogether make up the principal part of the bulk of the 
 animal. The fins which represent the limbs are not so much 
 used in propelling the Fish, as in changing its direction 
 either laterally or vertically. Thus in the lowest group of 
 the Vertebrated series, the act of motion is chiefly performed 
 by the vertebral column itself, instead of being committed to 
 the limbs, as in Mammals, Birds, and most Reptiles. The 
 larger number of Fishes swim with great activity ; and their 
 lives may be said to be passed in seeking their subsistence 
 and in flying from their enemies. 
 
 89. Fishes are for the most part very voracious, and their 
 food consists in great part of the members of their own class. 
 In seeking it, they appear to be chiefly guided by the sight ; 
 for their eyes are usually large and highly developed, while 
 
STBUCTUEE OF FISHES. 101 
 
 the other organs of sense are formed upon a very inferior 
 type. They swallow it without much division in the mouth ; 
 but it seems to undergo rapid digestion. The blood of some 
 Fish, whose muscular activity is peculiarly great, is rich in 
 red corpuscles, and of a temperature not much lower than 
 that of Mammals ; but, generally speaking, it contains much 
 less solid matter than that of the warm-blooded Vertebrata, 
 and its temperature follows that of the surrounding medium. 
 
 90. Although Fishes breathe by gills instead of by lungs, 
 these gills are connected with the mouth, so that the water 
 which passes over them is received into it, in the same man- 
 ner as the air is in the higher Vertebrata. This is a character 
 which distinguishes the position of the gills of fishes from 
 that of the corresponding organs of any of the inferior tribes. 
 They are lodged in a cavity on each side of the throat ; and this 
 cavity opens outwardly, either by one large valve-like aperture 
 on either side, or by several; through these apertures the 
 streams of water which have been taken in by the mouth, 
 and forced over the gills by the action of its muscles, make 
 their exit. 
 
 9 1 . All Fishes are oviparous ; and the number of eggs which 
 they produce is generally prodigious. It is very seldom that 
 after the eggs have been deposited and fertilized, the parents 
 take any further concern in regard to them ; though there 
 are a few instances in which a kind of nest is made, and 
 others in which the egg is retained and hatched within the 
 body, so that the young comes forth alive. This last is the 
 case with the Sharks and Eays, which, notwithstanding that 
 their skeleton is cartilaginous, are higher than Fishes generally 
 in several other parts of their organization. 
 
 92. All the animals which are destitute of a vertebral 
 column are called Invertebrata ; and this division into the 
 Vertebrated and Invertebrated groups was formerly regarded 
 as the first step in the classification of the animal kingdom. 
 But it was pointed out by Cuvier ? that in the Invertebrated 
 division are comprehended three groups, of which the mem- 
 bers differ as much from one another as they do from Verte- 
 brated animals ; and that each of these ought, therefore, to 
 rank with the first, as a primary division. This is evident 
 to those who are but slightly acquainted with the structure 
 of the animals already named ( 69) as characteristic speci- 
 
102 
 
 GENERAL STRUCTURE OF ARTICULATA. 
 
 mens of these divisions ; and it will become more apparent 
 as we proceed. 
 
 93. In the second division, that of ARTICULATA, or Articu- 
 lated (jointed) animals, we find a conformation very different 
 
 from that which has been just described. The 
 exterior of the body is still perfectly symme- 
 trical, as in the Vertebrata ; and the interior is 
 even more symmetrical ; for the organs that 
 represent the heart and lungs are equally dis- 
 posed on the two sides of the central line of the 
 body. But the skeleton, instead of being internal, 
 is external; and is composed of a series of pieces 
 jointed together, which form a casing that in- 
 cludes the whole body. In general, these pieces 
 are very similar to each other ; so that the whole 
 body appears like the repetition of a number of 
 similar parts, as we see in the Centipede (fig. 42). 
 The limbs are usually very numerous, where 
 they exist at all ; and they have a jointed cover- 
 ing, like that of the body. But in the lower 
 tribes of this group, such as Leeches and Worms, 
 the limbs or members are but slightly developed, 
 or are altogether absent; and in the highest, 
 which approach most nearly to the Yertebrata 
 in their general organization, the number of 
 members is much reduced, although it is never 
 less than six. The hard matter of which the 
 external skeleton is composed, undergoes little 
 or no change when it is once fully formed ; and, 
 in order to accommodate it to the increasing size of the 
 animal, this covering is thrown off and renewed at intervals 
 during the period of growth. 
 
 94. The nervous system consists of a series of separate 
 ganglia, which are arranged in a cord or chain along the 
 central line of the body. There is usually a pair of large 
 ganglia in the head, bearing a resemblance (in their peculiar 
 connexion with the eyes) to the ganglionic centres of the 
 optic nerves in Vertebrata ; and there is commonly one for 
 each segment or division of the body, from which the nerves 
 pass to supply its muscles, as they do from the spinal cord of 
 Yertebrata. The cord which connects these ganglia is double, 
 
GENERAL STRUCTURE OF ARTICULATA. 103 
 
 and the ganglia themselves are composed of two halves, 
 which have little connexion with each other. The chain 
 thus formed (fig. 43) passes along the 
 under-side of the trunk of the animal 
 (as seen at g, fig. 44), not on what 
 seems its back ; and by the presence 
 of this double chain of ganglia an 
 Articulated animal may be distin- 
 guished, even when, in its general 
 structure, it should seem to belong to 
 the group of Mollusca ( 102). 
 
 95. The general arrangement of the 
 organs in the Articulata is shown in 
 the accompanying figure of a Cray- 
 fish. The mouth, situated on a pro- 
 jecting head, opens into s, the stomach, 
 from which passes backwards the in- 
 
 , . , , , r . . , , . ., Fig. 43. NERVOUS SYSTEM OF 
 
 testmal tube, i, ?., to terminate at the AN INSECT. 
 
 opposite extremity of the body. The 
 
 upper part of the tube is surrounded by the liver, /, which is 
 here very large. In the head are seen the ganglia, c; and 
 along the under- side of the body is seen the chain of ganglia, 
 
 Fig. 44. DIAGRAM SHOWING THE POSITION OF THE PRINCIPAL ORGANS IN 
 THE ARTICULATA. 
 
 g. The blood is nearly colourless, and is usually impelled 
 through the body not by a single organ or heart, but by a 
 succession of contractile cavities, one for each segment, which 
 open into one other longitudinally, forming what is known as 
 the dorsal vessel; in the Cray-fish and its allies, however, one 
 part of this, k, is specially enlarged, so as in great degree 
 to serve as a heart for the system generally. The respiratory 
 organs are not connected with the mouth ; and are not usually 
 
104 STRUCTURE OF INSECTS. 
 
 restricted to one part of the body, but are diffused either on 
 its outside or through its substance. 
 
 96. The organs of sense, in this group, are less numerous 
 than in Yertebrata, and are inferior in perfection ; those of 
 sight are the most developed, and are formed upon a very 
 peculiar plan ( 573); but all organs of special sense appear 
 wanting in the lowest tribes. Yet we find that the muscular 
 power is very great; for the animals of this group, taken as a 
 whole, can move faster in proportion to their size, and possess 
 greater strength, than those of any other. We observe, too, 
 that with little or no intelligence, they are prompted to the 
 most remarkable actions by instinct alone. They seem to act 
 like machines, doing as they are prompted, without choice, or 
 knowledge of the end to be gained ; and consequently the dif- 
 ferent individuals of the same species have not that difference 
 of capacity and of disposition, which we see in animals whose 
 endowments are higher. 
 
 97. In the highest division of the Articulated series, we 
 easily recognise, as forms quite distinct from each other, the 
 Insects, the Spiders, the Crustaceans animals (crabs, lobsters, 
 &c.), and the Centipedes. The class of INSECTS is distinguished, 
 for the most part, by the presence of wings ; but to this there 
 are exceptions. It includes those of the higher Articulata, 
 which breathe air by means of air-tubes distributed through 
 the body ( 320), which have no more than six legs, and 
 whose body, in its perfect form at least, manifests a division 
 into three distinct parts the head, thorax, and abdomen 
 (fig. 45). To the thorax alone are attached the six legs, as 
 well as the wings ; and its cavity is principally occupied by 
 the muscles that move them : the abdomen contains the 
 organs of digestion and reproduction, as in vertebrated animals. 
 In the greater part of this class, the young animal comes forth 
 from the egg in a condition very different from that which it 
 is ultimately to possess; and it undergoes a complete meta- 
 morphosis, the larva which the egg produces bearing a close 
 resemblance in form to the lower Articulata, and only attain- 
 ing the condition of the imago or perfect insect by passing 
 again into a state of inactivity, during which the store of 
 nutriment which it has acquired is applied to the development 
 of new organs. This pupa or chrysalis condition may be 
 considered as a sort of postponed completion of the embryonic 
 
STRUCTURE OF INSECTS AND ARACHNIDA. 
 
 105 
 
 life, which was interrupted at a very early period. In some 
 tribes, however, the general form is the same from the first, 
 and the wings are the only parts deficient ; these gradually 
 
 Antennae _ 
 
 Eyes 
 
 Head 
 
 1st pair of Legs 
 
 1st pair of Wings - 
 2nd pair of Legs - 
 
 2nd pair of Wings " 
 3rd pair of Legs 
 
 Tibia 
 
 Tarsus 
 
 Abdomen 
 
 Fig. 45. SKELETON OF AN INSECT. 
 
 make their appearance, and the insect is then complete. Such 
 is the case with the Grasshopper and Cricket; and a change 
 of this kind is termed an incomplete metamorphosis. 
 
 98. The animals of the class ARACHNIDA, which includes 
 the spiders, scorpions, and mites, are, like Insects, articulated, 
 breathing air, and possessing legs, but the number of these 
 legs is never less than eight; there is an entire absence of 
 wings, and the head is united with the thorax, so that the 
 body seems to be formed of two principal divisions, the 
 cephalo-thorax (as it is termed), and the abdomen. In fig. 46 
 we have a representation of the arrangement of the parts con- 
 
106 
 
 STRUCTURE OF ARACHNIDA AND CRUSTACEA. 
 
 tained in these cavities. At c t is seen the cephalo-thorax 
 opened from below, and giving attachment to the legs ; at m 
 is shown the place of the mandibles or jaws ; at p is seen one 
 
 pa ab pa s 
 
 p pa t a I s ma o 
 
 Fig. 46. ANATOMY OF SPIDER. 
 
 of the palpi, which are appendages to the mouth ; pa is the 
 foremost leg ; t, the large nervous mass, from which the legs 
 are supplied; a, the collection of ganglia supplying the 
 abdomen ; a 6, the abdomen ; p a, the respiratory chambers ; 
 s s, the stigmata or openings into these ; Z, the leaf-like folds 
 within them ( 323) ; m a, the muscles of the abdomen ; a n, 
 the termination of the intestine ; /, the spinnerets ; o, the 
 ovaries ; and o r, the opening of the oviduct. 
 
 99. The class of CRUSTACEA, of which the Crab, Lobster, 
 and Cray-fifth are the best-known forms, differs from both 
 the preceding, in being adapted to breathe by means of gills, 
 and thus to reside in or near water, instead of inhabiting 
 the air. Moreover, the body is inclosed in a hard covering, 
 which generally contains a good deal of carbonate of lime, and 
 which is thrown off at regular intervals. This covering also 
 incloses the members, which are never less than ten in 
 number, and are frequently more numerous. There is great 
 variety of form among the animals of this group, which is 
 altogether one of great interest. In the Crab tribe, the head, 
 thorax, and abdomen are all drawn together, as it were, into 
 one mass and the general arrangement of the organs it con- 
 tains is exhibited in the succeeding figure, which shows them 
 nearly as they are found to lie, when the upper part of the 
 shell, or carapace, is removed. At t there is left a portion of 
 
STRUCTURE OF CRUSTACEA. 
 
 107 
 
 the membrane which, lines the carapace and covers in the 
 viscera. On the central line, at c, is seen the heart, which in 
 the Crustacea is large and powerful in its action ; from it 
 there passes forwards the artery a o, which supplies the eyes 
 
 Fig. 47. ANATOMY OF A CRAB. 
 
 and the front of the body ; whilst the artery a a passes to the 
 lower and hinder parts ; at b are seen the gills of the left side 
 in their natural position ; whilst at b' are seen those of the 
 right side, turned back to show their under-surface, and to 
 disclose the lower portion of the shell, fl. At e is seen the 
 stomach, situated close behind the mouth; and at TTL are 
 pointed out its powerful muscles, by the action of which the 
 food is ground down. The bulky ovary is seen on either side 
 of the stomach ; and the space between this and the edge of 
 the shell is occupied by the very large liver, / o. 
 
 100. In most of the Crustacea, however, the body is more 
 prolonged. In some, as the Lobster, there is an indication of 
 a division of the body into three parts, representing the head, 
 thorax, and abdomen of insects ; whilst in others, as the Sand- 
 
108 STRUCTURE AND METAMORPHOSIS OF CRUSTACEA. 
 
 hopper, the rings or segments are almost as similar to each 
 other as they are in the centipede tribe. There is no class in 
 which we find the same parts exhibiting so great a variety of 
 forms, and rendered subservient to so many uses. Thus in 
 the Crab and Lobster the members of the first pair are not 
 used for walking, but form the claws or arms by which the 
 food is seized ; in the Cray-fish, these members may be used 
 either as legs or claws ; whilst in the Sand-hopper, they 
 closely resemble the other legs. And the jaws of the higher 
 Crustacea, of which there are several pairs, are really meta- 
 morphosed legs ; as may be seen by comparing them with the 
 corresponding appendages of the Limulus or king-crab, the 
 first joints of which act as jaws, whilst the remaining portions 
 of these members serve either as legs for locomotion, or as 
 claws for prehension. 
 
 101. Most of the Crustacea, like insects, come forth from 
 
 the egg in a state very different from 
 their adult form ; and afterwards undergo 
 a series of changes, which are in some 
 instances so remarkable as to approach 
 the complete metamorphosis of insects, 
 and which end in the production of the 
 complete form. An early form of the 
 common crab, at a time when it is of 
 the minute size indicated on the scroll, 
 is shown in fig. 48. The immature 
 Crustacea of different tribes bear much 
 more resemblance to each other, than do 
 the forms into which they are ulti- 
 mately to be developed ; and the dif- 
 ferences they afterwards present are 
 chiefly due to a variety in the amount 
 of growth which the different parts 
 undergo. 
 
 102. It is one of the most remarkable results of modern 
 zoological research, that in immediate connexion with the 
 class of Crustacea, if not as actual members of it, we have 
 to place a group of animals which were for some time asso- 
 ciated with the Mollusca; their bodies being inclosed in 
 shells, which do not fit closely around them, nor give more 
 than a general protection to their members. This group is 
 
STKUCTUKE OP CIRBHIPEDA. 109 
 
 the Barnacle tribe, forming the class CIBRHIPEDA, or tendril- 
 footed animals. They agree with the lower Mollusca, in 
 being fixed to one spot during all but the earliest period of 
 their lives; the shell being sometimes attached by a long 
 membranous or leathery tube, as that of the Barnacle 
 (fig. 49) ; and sometimes being itself fixed on the surface of 
 
 Fig. 49. SHELL or Fig. 50. -BODY OP 
 
 THE BARNACLE. THE BARNACLE. 
 
 a rock, or on another shell, as is that of the Balanus or 
 acorn-shell. In both cases, the form and structure of the 
 animal are essentially the same. When taken from the shell 
 (in which it lies doubled up, as it were) and spread out, its 
 articulated nature is evidenced by its division into segments, 
 and by the regularity of the arrangement of their tendril-like 
 appendages. These are not formed like legs, since they could 
 be made no use of, the animal being incapable of moving 
 from place to place ; but they serve to produce currents in 
 the surrounding water, by which food is brought to the 
 mouth, and the blood is submitted to the influence of a 
 fresh supply of air. The nervous system of this group 
 is formed precisely upon the plan of that of the Articulata 
 generally ( 94) : and if any doubt could have remained as to 
 its true place in the series, it is removed by the knowledge 
 of the fact, that the animals composing it bear a strong 
 resemblance in their early condition to some of the lower 
 Crustacea, possessing eyes and legs, and swimming freely 
 about; and that they attain their adult form by passing 
 
110 STRUCTURE OF MYRIAPODA AND ANNELIDA. 
 
 through a series of metamorphoses, in which they lose their 
 eyes and legs, and become fixed for the remainder of their 
 lives. 
 
 103. We now pass back to another class of the higher 
 group of Articulata, adapted to breathe air and to inhabit 
 the land, the MYRIAPODA or Centipede tribe (fig. 42). Both 
 these names are derived from the great number of legs 
 possessed by these animals, which often amount to 60 pairs 
 or even more. In. this class we see a more perfect equality 
 of the segments or divisions of the body than in any others 
 among the higher Articulata; and the similarity is scarcely 
 less complete in the internal arrangement, than it is in the 
 external form. In its lower tribes (fig. 51), the legs are so 
 
 Fig. 51. IULUS. 
 
 weak as scarcely to be able to sustain the body, which moves, 
 therefore, partly in the manner of that of a worm. The 
 animals of this class undergo 110 proper metamorphosis ; but 
 there is a considerable adoption to the number of their seg- 
 ments and legs after they have come forth from the egg. 
 
 104. We now pass to the lower division of Articulata, in 
 which the body possesses no jointed members ; and the animals 
 belonging to this group are for the most part included in the 
 class of ANNELIDA, the Leech and Worm tribe. We here find 
 the body enveloped, not in a hard casing, formed of distinct 
 pieces united by a flexible membrane, but in a skin which 
 is altogether flexible, and which gives little indication of a 
 division into segments. This class includes several distinct 
 tribes, which all agree, however, in the long worm-like form 
 of the body, and in the similarity of the different ganglia 01 
 their nervous system. The Earth-worm and its allies are 
 adapted to live on land and to breathe air ; but the greater 
 number of Annelids are purely, aquatic ; and these breathe 
 by gills, which form tufts that are disposed on various parts 
 of the body. In the Nereis, or Sea-centipede (fig. 52), these 
 
STRUCTURE OF ANNELIDA AND ENTOZOA. Ill 
 
 tufts axe arranged regularly on the several segments, and the 
 animal can swim by the motion that it gives them ; besides 
 these, it has a kind of bristle-shaped appendage, that seems 
 
 Fig. 52. NEREIS. 
 
 like a rudimentary leg, which assists it in crawling. Eut 
 there are others of these marine- worms, that form a tubular 
 shell, in which they reside during the greatest part of their 
 lives; and in these the gills, if disposed along the body, 
 would have been removed from the access of water ; they are 
 therefore arranged round the head, often forming (as in the 
 fSerpulce, fig. 145) tufts of great brilliancy and elegance. 
 
 105. Eelow the Annelida are other worm-like tribes of yet 
 greater simplicity of conformation, but still presenting the 
 same general plan of structure. Of one of these the common 
 Leech may be taken as an example; of another, the Tape- 
 
 Fig. 53. TAPE-WORM. 
 
 worm (fig. 53). This last belongs to a group termed ENTOZOA, 
 from the circumstance that they inhabit the bodies of other 
 animals. They are remarkable for the very low development 
 of their digestive apparatus, their nourishment being appa- 
 
112 GENERAL STRUCTURE OF MOLLUSCA. 
 
 rently imbibed through, the whole surface of their bodies 
 from the juices in the midst of which they live ; whilst, on 
 the other hand, their reproductive apparatus is enormously 
 developed, the multiplied segments of the Tape-worm (for 
 example) containing this alone, and the head (as it is com- 
 monly termed, though really the body) being able to repro- 
 duce these to an indefinite extent after they have been 
 thrown off. The group of ROTIFERA, or Wheel-Animalcules, 
 which is one of great interest to the Microscopist, also belongs 
 to this lower section of the Articulated sub-kingdom. 
 
 106. The general character of the animals composing the 
 group or division MOLLUSCA, is, in many respects, the very 
 opposite of that which prevails in the Articulated animals. 
 The body is soft (whence the name of the group is derived), 
 neither possessing an internal skeleton, nor any proper ex- 
 ternal skeleton. In some of the most characteristic specimens 
 of the group, such as the Slug, there is no hard frame- work 
 or skeleton whatever, the body being alike destitute of 
 support and protection. In most Mollusks, however, the 
 body has the power of forming a shelly covering, which serves 
 for its protection ; but this does not give any assistance in its 
 movements by affording fixed points for the attachment of the 
 muscles ; in fact, when the animal puts itself in motion, it is 
 obliged to make its locomotive organs project beyond the 
 shell. We must not regard the shell as an essential part of 
 the Molluscous animal ; because there are many tribes entirely 
 destitute of it ; and also because some of the Articulata have the 
 power of forming a shell ( 102), which bears a close resem- 
 blance to that produced by the animals of this group. Not un- 
 
 Fig. 54. TESTACEILA. 
 
 frequently we see that, of two animals whose general structure 
 is almost exactly the same, as that of the Snail and Slug, 
 
STRUCTURE OF MOLLUSCA. 113 
 
 one possesses a shell into which it can withdraw its whole 
 body for the sake of protection, whilst the other has none; 
 and several intermediate forms exist; in which the shell 
 bears a larger or smaller proportion to the body, sometimes 
 being able to contain nearly the whole of it, and sometimes 
 being a mere rudiment, as in the Testacella (fig. 54). 
 
 107. The external form of the body of Mollusks is subject 
 to great variation ; and generally has a good deal to do with 
 the degree in which the organs of sense and the instruments 
 of motion are developed in the particular animal. For these 
 are almost always symmetrical, being arranged with equality 
 on the two sides of a middle line ; whilst the rest of the 
 body, containing the organs of nutrition, is often unequal on 
 the two sides. But in the lower Mollusca, which have little 
 or no power of moving from place to place, even this degree 
 of symmetry is altogether lost. Few of the Mollusca have 
 any powers of active movement ; in fact, the term sluggish- 
 ness, derived from a characteristic member of the group, very 
 well expresses their general habit. The Gasteropods, which 
 may be regarded as the types of the whole series, crawl upon 
 a fleshy disk, by the successive contractions and relaxations 
 of which they advance slowly along the surface over which 
 they move ; this kind of action is easily studied, by causing 
 a Snail or Slug to crawl upon a piece of glass, and by looking 
 through this at the under side of its foot. Hence, there is a 
 great contrast between the inertness of the Mollusca, and the 
 high activity of the Articulata. This contrast shows itself in 
 the structure of their bodies ; for whilst the chief part of the 
 interior of an Insect is made up of the muscles which move 
 its legs and wings, the apparatus of nutrition being small, 
 the chief part of the bulk of a Slug or Snail is given by its 
 very complex apparatus for nutrition there being no other 
 muscles (except some small ones connected with the mouth 
 and head) than the fleshy disk already mentioned. The blood 
 of the Mollusca is nearly colourless, as it is in the Articulata ; 
 but the organ by which it is circulated through the body is 
 much more powerful and complete, bearing more resemblance 
 to the heart of Vertebrated animals. The skin is usually 
 thick and spongy in its texture ; having muscular fibres inter- 
 woven in its substance, so that it can contract or extend itself 
 in any part ; and having the power of exuding shelly matter 
 
114 STEUCTURE OF MOLLUSCA. 
 
 from its surface, in those species which form such a pro- 
 tection. This envelope, which is called the mantle, is very 
 loosely applied round the parts which it contains; and it 
 frequently extends itself into folds or duplicatures, which 
 wrap round the gills, and sometimes meet and adhere so as to 
 inclose them, within a cavity of their own. In the Cuttle- 
 fish, the water within this cavity is renewed from time to time 
 by the muscular movements of its walls ; but usually a 
 current of fluid is kept up over the surface of the gills, by the 
 action of the cilia ( 45) with which they are covered. 
 
 vb ab b ov 
 
 Fig. 55. ANATOMY OF TURBO PICA. 
 
 108. The accompanying figure of the interior of a Turbo 
 show the very large size of the digestive apparatus, and 
 of the other organs of nutrition. The muscular disk or foot 
 is seen at/>; and this carries the operculum o, which serves 
 to cloge the mouth of the shell when the body of the animal 
 is drawn within it. At t is shown the proboscis, on either 
 side of which are the tentacula or feelers, ta, bearing the eyes 
 at y. Just behind the tentacula is seen the large cephalic 
 ganglion, sending nerves to the eyes ; and behind this again 
 are the salivary glands. The mantle, m, is opened and folded 
 back to show the respiratory cavity, in which lie the gills 
 
STEUCTURE OP MOLLUSCA. 
 
 115 
 
 b b; to this cavity, water has access by means of a wide slit, 
 of which the edge, /, of the mantle forms one part of the 
 border, whilst at d is seen a fringed membrane that forms 
 another part. At c is seen the heart, which receives the 
 blood from the gills by v b, the branchial vein, and then 
 transmits it to the body generally ; at e, far up in the spire, 
 are the stomach and liver ; at cc, the anal orifice of the intes- 
 tine within the branchial cavity, and at ov the oviduct, which 
 opens in the same situation. 
 
 109. Thus it is seen that, whilst the body of an Articu- 
 lated animal may be compared to that of a man in whom the 
 apparatus of nutrition (contained in the chest and abdomen) 
 is of the smallest possible size, but whose limbs are strong, 
 and his movements agile, the body of a Mollusk resembles 
 that of a man " whose god is his belly," his digestive appa- 
 ratus becoming enormously developed, whilst his limbs are 
 feeble, and his movements heavy. Such varieties, in a greater 
 or less degree, are continually presenting themselves to our 
 notice. 
 
 110. The nervous system of the Mollusea generally consists 
 of a single ganglion or pair of ganglia, which are placed in the 
 head, or (when that is deficient) in the neighbourhood of the 
 mouth ; and of two or more separate ganglia, which are found 
 in different parts of the body, and are connected with the 
 preceding by nervous cords. The 
 
 former correspond to those con- 
 tained in the head of Insects ; but 
 of the latter, one only is connected 
 with the foot or organ of motion, 
 the remainder having for their 
 function to regulate the action of 
 the gills, and to perform other 
 movements connected with the 
 operations of nutrition. In fig. 56 
 is represented one of the simpler 
 forms of this nervous system, 
 that of the Pecten or Scallop-shell; 
 A A are the ganglia near the mouth, 
 from which the organs of sense 
 are supplied; B is the ganglion connected with the gills; 
 and c is that from which power is given to the foot. The 
 
 i2 
 
 Fig. 56. NERVOUS SYSTEM OF 
 PECTEN. 
 
116 GENERAL STRUCTURE OF MOLLUSCA. CEPHALOPODS. 
 
 two first lie wide apart, but are connected by an arched 
 band that passes over the gullet, e. The organs of sense in 
 the higher forms of Mollusca are more developed than those 
 of motion. They serve to direct the animal to its food, and 
 to warn it of danger ; but there seems an absence, in all save 
 the highest species, of that ready and acute sensibility which 
 is so remarkable in the preceding groups ; and the variety of 
 impressions which they can receive appears to be but small. In 
 no instance has a special organ of smell been certainly dis- 
 covered the organ of hearing is always imperfect, and fre- 
 quently absent altogether ; and the eyes are very often wanting. 
 In the lower Mollusca there are no certain indications of the 
 existence of any organs of special sense ; and there is probably 
 but a limited amount of general sensibility. 
 
 111. As the Articulata are divided into two subordinate 
 groups, according to the presence or absence of articulated 
 limbs or members, so may we arrange the Mollusca in two 
 subdivisions, according to the presence or absence of a dis- 
 tinct head, that is, a projecting part of the body, containing 
 the mouth or entrance to the digestive cavity, and also bearing 
 the organs of sense which guide the animal in the discovery 
 and selection of its food. In the higher Mollusca, there is a 
 distinct head, furnished with eyes, and sometimes with im- 
 perfect ears ; but in the lower, the entrance to the digestive 
 cavity or stomach is buried deep among other parts, and is 
 guarded by no other organs of sense than the tentacula or 
 sensitive lips. These are termed acephalous, or headless 
 Mollusca : and among the lowest of them ( 114), we meet 
 with composite fabrics, formed by the process of multiplica- 
 tion by budding, which was formerly regarded as peculiar to 
 Zoophytes. The highest group of Mollusca, in regard to the 
 approach of several parts of its structure to that of Verte- 
 brated animals, is the class of CEPHALOPODA, or Cuttle-fish 
 1f ibe : which receives its name from the peculiar arrangement 
 of the arms or feet around the mouth, which is the cha- 
 racteristic of its members (fig. 57). The common Cuttle-fish 
 and its allies are destitute of any external protection; but 
 they usually have a flat shell, commonly known as the cuttle- 
 fish bone, inclosed in a fold of the mantle, and lying along 
 the back. In the Calamary, this is horny in its texture, and 
 is sufficiently flexible to offer no resistance to the action of 
 
STRUCTURE OF CEPHALOPODS AND PTEROPODS. 117 
 
 the fin-like tail, by which the animal is propelled through the 
 water very much in the manner of a fish. The Pearly 
 Nautilus is the only type now existing of an inferior order of 
 
 Fig. 57. CALAMARY. 
 
 Cephalopods, which approaches the Gasteropods in many parts 
 of its organization. The body is inclosed in the last chamber 
 of a shell (usually spiral in form), 
 the cavity of which is divided by 
 numerous transverse partitions ; and 
 such shells, the fossilized remains of 
 very numerous forms of this group 
 that existed in the ancient seas, con- 
 stitute the nautilites, ammonites, 
 belemnites, &c., which abound in 
 many rocks (fig. 58). The Cuttle- 
 fish are animals of considerable 
 activity; their mouth is furnished Fig SS 
 with a horny beak, strongly resem- 
 bling that of the parrot ; and their arms are provided with 
 a series of very curiously constructed suckers, by the action 
 of which they can take a very firm 
 hold of anything which they desire 
 to grasp. 
 
 112. The class of PTEROPODA, or 
 wing-footed Mollusks, consists of 
 but few species, and the animals 
 which it contains are all of them of 
 small size ; but the individuals are 
 often very numerous, whole fleets 
 of them being sometimes seen 
 covering the ocean, especially in 
 the Arctic and Antarctic regions, 
 where they constitute one of the Fig> 59 -- HTAI -* A - 
 principal articles of food to the Whale. The general form of the 
 body usually differs but little from that represented in fig. 59. 
 
118 STRUCTURE OP GASTEROPODS AND BIVALVES. 
 
 On either side, a little behind the head, the mantle is extended 
 into a fin-like expansion, by the aid of which the animal can 
 swim through the water. The hinder part of the body is 
 usually inclosed, more or less completely, in a shell, which 
 is commonly of extreme thinness and delicacy. The head is 
 not furnished with long arms, to grasp the food ; but it has 
 a number of minute sucking disks, by which it can lay firm 
 hold of whatever it attacks : whilst its powerful rasp-like 
 tongue is set to work upon it. The class GASTEROPODA con- 
 tains those animals which, like the Snail and Slug, crawl 
 upon a fleshy disk on the under side of their bodies ; and 
 the number of distinct forms which it includes is very large. 
 The greater part of them are inhabitants of the sea-shore, 
 rivers, lakes, &c. ; some have the power of swimming freely 
 through the open sea; and the proportion of those that 
 breathe air and live on land, is comparatively small. The 
 general structure of the animals of this group has been 
 already described ( 108). Some of them form shells, whilst 
 others are destitute of them. The shells are composed of a 
 single piece, or are univalve, except in one tribe ; 
 and they have usually more or less of a spiral 
 formation (fig. 60). The animals of this class all 
 possess a distinct head ; and this is generally 
 furnished with eyes, as well as with tentacula. 
 They have often a powerful masticating ap- 
 
 paratus, and are voracious in their habits ; 
 
 Fig. GO. SHELL some of them feed upon vegetable matter, others 
 
 OF PALUDINA. ^^ animalg> 
 
 113. The Acephalous Mollusca are divided into two groups, 
 those which form shells, and those which do not. The 
 former are termed CONCHIFERA, or shell-bearing animals ; and 
 this class includes all the Mollusca that form a shell composed 
 of two parts or valves fitted together (which shell is termed 
 bivalve), as well as some others whose general structure is the 
 same, but whose shell is formed in several pieces, or multivalve. 
 The two valves of a bivalve shell (fig. 61) are connected by a 
 hinge, where they are united by a ligament, which, by its 
 elasticity, keeps them apart while it holds them together. This 
 is their usual condition when the animal is alive ; and in this 
 manner the water which is required for their respiration, and 
 also to convey their supply of food, has free access to the internal 
 
STRUCTURE OF CONftHIFERA, OR BIVALVES. 119 
 
 parts. But when any alarm or irritation causes the animal to 
 close its shell, it does so by means of a muscle (sometimes 
 
 Fig. 61. SHELL op TRIDACNE. 
 
 single, sometimes double), which stretches across from one 
 valve to the other, and which, by contracting, draws them 
 together. Each valve is lined by an extended fold or lobe 
 of the mantle. In the higher tribes of the class, these lobes 
 are united along their edges, leaving apertures for the ingress 
 and egress of water (which are sometimes prolonged into 
 tubes, fig. 150), and another for the foot. But in the Oyster 
 and its allies, which have no foot, or a very small one, the 
 mantle-lobes are quite disunited. The accompanying diagram 
 (fig. 62) gives a general idea of the arrangement of the 
 organs in one of the higher acephalous Mollusca, the Mactra, 
 which is among those having two muscles for the drawing 
 together of the valves. The upper end, as represented in 
 this figure, is that which is considered as the anterior end or 
 front of the animal, being that nearest which the mouth lies ; 
 and the posterior extremity (the lowest in the figure) is that 
 at which the intestinal canal terminates, and at which the 
 respiratory tubes are formed. JSear the anterior muscle, we 
 find the mouth, or entrance to the stomach ; it is furnished 
 with four riband-shaped tentacula, of which one is seen in 
 the figure ; and these seem to possess peculiar sensitiveness. 
 Near the mouth lie the anterior ganglia of the nervous 
 system, which represent the brain of higher animals ; and 
 these are connected by long cords with the posterior ganglion, 
 which lies near the posterior muscle. The stomach, intes- 
 tines, and liver occupy the central portion of the cavity of 
 the shell ; and the intestinal tube is seen to pass backwards, 
 
120 
 
 STRUCTURE OP CONCHIFERA OR BIVALVES. 
 
 terminating near one of the canals or siphons, which also 
 carries out the water that has been taken in through the other 
 for the purposes of respiration. The figure also shows the 
 large fleshy foot, by which this animal can move itself along 
 
 Foot - 
 
 Intestine T. 
 
 Stomach - 
 
 Gills. 
 
 Mantle 
 
 Anas 
 
 Respiratory Tubes. 
 Fig. 62. ANATOMY OF MACTRA. 
 
 the ground, or bore into sand or mud. The heart and circu- 
 lating system are less complete than in the Gasteropoda ; but 
 are far higher in character (as are most of the other parts 
 
STRUCTURE OP TUNICATA. 121 
 
 of the nutritive apparatus) than the corresponding parts in 
 Articulated animals, in which the apparatus for locomotion so 
 much predominates. 
 
 114. The group" of Acephalous Mollusks which are desti- 
 tute of the power*of forming a shell, includes two classes, of 
 Avhich one does not depart widely from the general Molluscan 
 type, whilst the other presents 'so strong a general resem- 
 blance to Zoophytes, that until recently it has been universally 
 ranked with it. The first of these classes receives its name 
 TUNICATA from the circumstance that the mantle, instead of 
 secreting a shell, is very commonly condensed into a tough 
 leathery or cartilaginous tunic. Many of these animals live 
 separately, and have the power of freely moving through the 
 water. Others are associated in compound masses, of which, 
 however, the individuals are not connected by any internal 
 union. But others form really composite structures, like 
 those of Zoophytes ( 124) ; each individual being able to 
 live by itself alone, but being connected by a stem and vessels 
 with the rest. The general structure of the individuals is 
 the same, however, in the single and in the composite 
 animals of this class, and may be understood from the accom- 
 
 t 
 
 Fig. 63.--SOCIAL ASCIDIANS. 
 
 panying figure (fig. 63). The cavity of the mantle possesses, 
 as in the former instance, two orifices ; by one of which, 5, a 
 current of water is continually entering, whilst by the other, 
 a, it is as continually flowing out. These orifices lead into a 
 large chamber, the lining of which, folded in various ways, 
 constitutes the gills ; and at the bottom of this chamber lie 
 the stomach, e, and the intestinal canal, i, which terminates 
 near the aperture for the exit of the water. All these parts 
 
122 STRUCTURE OF TUNICATA AND POLYZOA. 
 
 are covered with cilia, by the action of which a continual 
 stream is made to flow over the gills and to enter the stomach ; 
 and the minute particles which the water brings with it, and 
 which are adapted to serve as food, are retained and digested 
 in the stomach. Even these animals, fixed to one spot during 
 all but the early part of their lives, and presenting but very 
 slight indications of sensibility, possess a regular heart and 
 system of vessels ; and these vessels form part of the stem, t, 
 by which the compound species are connected. A single 
 nervous ganglion is found between the two orifices ; this 
 seems to receive sensory fibres from tentacula situated around 
 the oral orifice, and to transmit motor filaments to the mus- 
 cular coat which underlies the outer tunic, so that any irrita- 
 tion applied to the former occasions a contraction of the 
 latter, which tends to expel the offending particle. This 
 class is one of particular interest to the naturalist, since we 
 see in it the tendency to the formation of compound struc- 
 tures, by a process resembling that of the budding of plants, 
 which is essentially characteristic of Zoophytes; this ten- 
 dency, however, is more fully manifested in the succeeding 
 class. 
 
 115. The animals forming the class POLYZOA (more com- 
 monly known as Bryozoa) are seldom or never found solitary ; 
 since, in consequence of their universal tendency to multiply 
 by gemmation, they form clusters or colonies of various kinds. 
 The body of each individual is inclosed in a sheath or " cell," 
 which is sometimes horny, sometimes calcareous ; and the 
 composite skeleton formed by the aggregation of these, which 
 has sometimes a branching or leaf-like form, but sometimes 
 possess the compactness of a stony coral, is known as the 
 " polyzoary." In their general structure the animals of this 
 class possess considerable analogy to the Tunicata ; but the 
 Molluscan type presents itself under a more degraded aspect, 
 no vestige of a heart or of blood-vessels being here dis- 
 cernible, and the general structure being so simplified as to 
 manifest no great degree of elevation above that of Polypes. 
 The typical structure of these animals may be understood 
 from that of the Eowerbankia (fig. 64), which is one of those 
 whose cells are not in contact with each other, but grow forth 
 at intervals from a creeping stem. The mouth, a, is situated 
 in the midst of a circle of arms fringed with cilia; these 
 
STRUCTURE OF POLYZOA AND RADIATA. 
 
 123 
 
 arms do not serve, however, like those of polypes, to grasp 
 the food ; but the vibration of their cilia produces a powerful 
 current which brings both food and oxygen. 
 The mouth leads by a large funnel-shaped 
 oesophagus or gullet, to a gizzard, b ; in which 
 the particles of food that enter it are ground 
 down, by the action of its muscular walls and 
 of the tooth-like processes that line it. Below 
 this gizzard is the true digestive stomach, c, 
 around which the rudiment of a liver may 
 be traced ; and from this stomach there passes 
 upwards an intestinal tube, which terminates 
 by a distinct orifice at d, on the outside of the 
 circle of arms. The digestive apparatus is evi- 
 dently formed, therefore, upon a much higher 
 plan in these animals than it is in the true 
 polypes, which have no true anal orifice. The 
 Molluscan character of these animals is further 
 shown by the presence of a single nervous 
 ganglion, situated between the two orifices, 
 as in the Tunicata ; this acts upon a complex 
 a, oesophagus ; b, giz- apparatus of muscles, by which the animal 
 rorifice s ofnfe h s i can be either drawn into its cell or projected 
 tiue. forth from it, with great rapidity. 
 
 116. The fourth subdivision, that of EADIATA, includes 
 those animals which have the parts of the body arranged in 
 a circular manner around a common centre, so as to present a 
 radiated or rayed aspect. This arrangement is well seen in 
 the common Star-fisti (fig. 65), which has five such rays, all 
 having a precisely similar structure, and thus repeating each 
 other in every respect. The mouth of this animal is in the 
 centre ; and it opens into a stomach, which occupies the cen- 
 tral disk, and sends prolongations into the rays. The nervous 
 system is, in like manner, composed of a repetition of similar 
 parts. A plan of it is seen in fig. 66 ; where a shows the 
 position of the mouth, which is surrounded by a ring or 
 nervous cord, having five ganglia, corresponding to the five 
 arms. From each of these ganglia proceeds a branch along 
 its arm, terminating in a little organ at its extremity, which 
 is believed to be an imperfectly-developed eye. No other 
 organs of special sense can be detected in any of these ani- 
 
 Fig. 64. 
 
 BOWERBANKIA. 
 
124 
 
 STRUCTURE OF RADIATA. 
 
 mals ; and it is onjy in a few that even these imperfect eyes 
 can be discovered. In the inferior Eadiata, not the slightest 
 
 Fig. 65. SHELL OF STAR FISH. 
 
 traces of a nervous system have yet been discovered; and 
 it is very doubtful whether any such structure exists in 
 them. It is only among the higher 
 Radiata that any locomotive power 
 exists ; and this is usually so feeble 
 that the animals remain in the same 
 locality during the greater portion 
 of their lives. Generally speaking, 
 there is a period in the history of 
 each species, in which there is a 
 more active movement, that serves 
 to prevent the accumulation of indi- 
 viduals in one spot ; but this move- 
 ment is of a purely automatic 
 character, rather resembling that of the " zoospores " of plants, 
 than the intentional change of place of the higher animals. 
 
 Fig. 66. NERVOUS SYSTEM 
 OF STAR FISH. 
 
STRUCTURE OF RADIATA. ECHINODERMATA. 125 
 
 117. The circular arrangement of the organs of Eadiated 
 animals is a striking point of resemblance to the Vegetable 
 kingdom ; and it has frequently caused mistakes to be made 
 in regard to the Sea- Anemones and other large polypes, 
 which, when their mouths are open and their arms spread 
 out, look so much like the blossoms of some of the Com- 
 posite tribe of plants, as to have received the name of animal 
 flowers. But there is yet a stronger analogy between the 
 lower members of the Eadiated group and the Vegetable 
 kingdom; for among the former, as in the latter, we find 
 a union of many individuals, which are capable of existing 
 separately, into one compound structure, having a plant-like 
 form. This is the nature of the stem of Coral (fig. 76); 
 which is, in fact, the skeleton of one of these compound 
 systems, consisting of a number of polypes united by a jelly- 
 like flesh ; just as the woody stem of a tree is the skeleton 
 that supports a vast number of buds, each of which is capable 
 of living by itself. This aggregation results from the in- 
 definite multiplication of parts by the process of gemmation 
 or budding, and from the persistence of the connexion 
 between these parts, notwithstanding that, if separated, 
 they can maintain an independent existence. To the tree- 
 like fabrics thus produced, the name Zoophytes (animal 
 plants) is commonly given; and ordinary observers often 
 find it difficult to get rid of the idea of their vegetable 
 origin. The animals that formed them are, of course, fixed 
 to one spot during all but the earliest periods of life ; and 
 the amount of movement which they perform, for the pur- 
 pose of obtaining and securing their food, is very little 
 greater than that which is witnessed in the Sensitive plant 
 and Venus's fly-trap. 
 
 118. The class of ECHINODERMATA receives its name from 
 the prickly character of its covering, which is evident enough 
 in the Echinus or Sea- Urchin, and in the Star-fish; but there 
 are other animals, sufficiently resembling these in general 
 structure to be united in the same class, which have a body 
 entirely soft, namely, the Holothurice (fig. 67), commonly 
 termed Sea-Cucumbers. This class ranks as the highest 
 among the Eadiata, in regard to general complexity of struc- 
 ture. The skeleton of the Sea-Urchin, Star-fish, and other 
 animals resembling them, is a box-like shell or " test," formed 
 
126 
 
 STRUCTUKE OF ECHINODERMATA. 
 
 of a great number of pieces, very regularly arranged and 
 united together (fig. 69, e ) ; but these pieces are for the most 
 
 Fig. 67. HOLOTHURIA. 
 
 part only repetitions of one another ; and the different 
 portions have not that variety of uses which we see in higher 
 animals. With the exception of 
 the tribe of Encrinites or lily-like 
 animals (fig. 68), of which there are 
 very few now existing, but which 
 were very abundant in former ages, 
 all the animals belonging to this 
 class are unattached, and are capa- 
 ble of moving freely from place to 
 place. Their motions are very 
 sluggish, however, and are princi- 
 pally effected by means of an im- 
 mense number of minute tubular feet 
 (fig. 68, c), furnished with suckers 
 at their extremities, which can be 
 projected from almost any part of 
 the body. These are seen in rows 
 on the under side of each arm of 
 the Star-fish; they are put forth 
 through rows of very minute aper- 
 tures in the shell of the Sea- 
 Urchin (commonly termed the Sea- 
 
 Fig. 68. ENCRINITE. 
 
STRUCTURE OF ECHINODERMATA. 
 
 127 
 
 Egg) ; and they are also arranged in rows on the surface of the 
 body of the Holothuria, as seen in fig. 67. 
 
 119. The radiated arrangement is very evident in the 
 whole bodies of the Star-Fish (fig. 65), and Echinus or Sea- 
 Urchin (fig. 69); but in the Holothuria (fig. 67) it is nearly 
 confined to the parts about the mouth; which, however, 
 exhibit it so completely, that such an animal cannot be mis- 
 taken for one of the Articulated series, even though, as some- 
 times happens, the body is prolonged into a worm-like 
 form. The digestive apparatus in this class has usually a 
 high degree of complexity, as will be seen by the accompany- 
 
 . INTERIOR, OP ECHINUS. 
 
 ing figure (fig. 69), which shows the interior of an Echinus, 
 whose globular shell has been sawn across its equator, so as 
 
128 STRUCTURE OP ECHINODERMATA AND ACALEPH^. 
 
 to allow of the separation of its two halves. The mouth, &, 
 situated at one of the poles of the shell, is surrounded by a 
 very curious apparatus of jaws and teeth (fig. 69), which 
 forms what is termed the "lantern;" from the mouth com- 
 mences the long narrow oesophagus, m, that leads to the 
 stomach, n, which is merely a dilated portion of the fl.1imp.n- 
 tary tube ; the continuation of this, o } <?, r, forms the intestinal 
 canal, which winds once round the shell, and then doubles 
 back and winds in the opposite direction, terminating at the 
 anal orifice, s, which is situated at the opposite pole. The 
 intestine is held in its place by a double fold of "the mem- 
 brane lining the shell, resembling the mesentery of higher 
 animals ; the blood is distributed over this membrane, to be 
 exposed to the aerating influence of the water admitted into 
 the cavity of the shell ; and the water is kept in movement 
 by the cilia with which the membrane is clothed. Round the 
 anus, s, are seen the five branching ovaries, each of which 
 discharges its contents by a distinct orifice. The circulating 
 apparatus is imperfect, the blood not being impelled by a 
 distinct heart ; still, however, it moves in great part of its 
 course through proper vessels, and not through mere chan- 
 nels in the tissue. In the Star-fish, however, the body is 
 very much flattened ; and the stomach, instead of having a 
 separate intestinal tube with a distinct orifice, is a mere bag 
 with a single aperture, which serves both to take in food and 
 to cast forth the indigestible remains. This character will be 
 found to prevail among all the inferior Radiata. 
 
 120. The radiated structure is also well seen in the greater 
 number of animals forming the group of ACALEPH.E, or Sec,* 
 Nettles. Their bodies are entirely soft and jelly-like ; contain- 
 ing so small a quantity of solid matter, that, when upon being 
 taken out of the water their fluid drains away, there is 
 scarcely anything left; hence they are commonly termed 
 Jelly-Fish. They derive their other name of Sea-Nettles from 
 the stinging power which most of them possess. They are 
 formed to float freely in the water; but they do not in 
 general possess any means of actively propelling themselves 
 through it. The radiated arrangement is very regularly pre 
 served in some of this group, whilst it is less evident in 
 others. The accompanying figure (fig. 70) represents one of 
 the Medusa tribe, as seen floating in water. The umbrella- 
 
STRUCTURE OP ACALEPH^. 129 
 
 shaped disc above contains the stomach, which is placed in 
 the centre, and which opens by a single orifice or mouth, 
 directed downwards. Around the stomach are four chambers, 
 in which the eggs are prepared. The mouth is surrounded by 
 four large tentacula, which bring to it the necessary supply of 
 
 Fig. 70. PELAGIA. 
 
 food; and other tentacula are seen, in this species, to be 
 hanging from the edge of the disc. In the edge of this disc, 
 the nutritious fluid, which flows in channels prolonged from 
 the stomach and excavated out of the soft tissues, seems to be 
 exposed to the influence of the surrounding water ; but 
 nothing like a heart or a regular circulation exists. Eecent 
 discoveries in regard to the developmental history of the 
 Medusae and their allies, have rendered it very doubtful 
 whether the Acalephce should continue to take rank as a dis- 
 tinct class ; since many of them constitute only a particular 
 phase in the life of the Hydroid Zoophytes ( 124). 
 
 121. The class of POLYPIFERA, or coral-forming animals, 
 commonly known as Zoophytes, includes two principal tribes, 
 which differ from one another in structure to such a degree as to 
 
130 
 
 STRUCTURE OF HYDRA. 
 
 Fig. 71. HYDRA, OR FRESH-WATER 
 POLYPE. 
 
 require separate notice. The group of Hydrozoa, or Hydroid 
 Zoophytes, so named from the little Hydra, or fresh-water 
 
 polype, which may be regarded 
 as its type, will be first described 
 on account of its near con- 
 nexion with the preceding. The 
 Hydra (fig. 71) is a solitary 
 polype, not at all uncommon 
 in ponds or other collections of 
 fresh water, where it is found 
 attached to aquatic plants, or 
 to floating sticks, straws, &c., 
 by means of a kind of sucker 
 at its lower extremity, stretching 
 out its tentacles in search of its 
 food, which consists of minute 
 aquatic worms and insects. These 
 are securely laid hold of by one 
 or more of the tentacles, and are 
 drawn into the mouth, a, which 
 leads to the stomach or general 
 cavity of the body, in which they are digested, and from 
 the walls of which the nutritious portions are absorbed, the 
 portions of the food which are not capable of being digested 
 being cast out through the mouth. 
 
 122. The Hydra multiplies in two ways ; namely, by gem- 
 mation or budding, and by a proper generative process. Little 
 bud-like processes are developed from various parts of the 
 walls of the stomach, which gradually assume the form of the 
 parent, possessing a mouth surrounded by tentacles, and a 
 digestive cavity which is at first in connexion with that 
 of the parent ; the communication is gradually cut off, how- 
 ever, by the closure of the canal of the footstalk of the young 
 polype; and ere long the footstalk itself separates, and the 
 young polype henceforth leads an entirely independent life. 
 Not unfrequently, however, the young polype itself puts forth 
 buds before its separation ; and as many as nineteen young 
 Hydrse, in different stages of development, have been seen to 
 be thus connected with one and the same stock. Another 
 very curious endowment of the Hydra depends upon the same 
 facility of developing the whole structure from any part of it ; 
 
HYDRA, AND HYDROID ZOOPHYTES. 131 
 
 for into whatever number of parts its body may be cut up, 
 each, under favourable circumstances, can give origin to 
 a new and entire polype, so that thirty or forty individuals 
 may thus be produced by the division of one. 
 
 123. The proper generative process, here reduced to its 
 utmost simplicity, consists in the development of a germ-cell 
 and of sperm-cells in the substance of the wall of the stomach, 
 the former being produced near the footstalk, the latter just 
 beneath the arms. The egg which is evolved from the former, 
 being fertilized by the products set free from the latter, gives 
 origin to a young Hydra, which resembles its parent. The 
 two reproductive processes, however, are performed under 
 very different conditions; 
 
 for whilst multiplication by 
 gemmation is favoured by 
 warmth and a copious sup- 
 ply of food, the true gene- 
 rative process seems to be 
 brought about by a lower- 
 ing of the temperature, and 
 to have for its object the 
 perpetuation of the race 
 through the winter, the egg 
 being capable of enduring a 
 degree of cold which would 
 be fatal to the polype itself. 
 
 124. The group of Hy- 
 drozoa is for the most part 
 made up of composite fabrics 
 more or less resembling the 
 Campanularia (fig. 72), 
 which may be likened to 
 a Hydra whose buds do not 
 detach themselves, but re- 
 main in connexion with the 
 stock that produced them ; 
 
 the whole plant-like StniC- Fig. 72. CAMPANULARIA. 
 
 ture, moreover, being strengthened by the consolidation of its 
 external layer into a horny sheath, which retains its form 
 after the destruction of the soft parts. Thus each comes to 
 consist of a stem and branches, on the sides or ends of which 
 
 K2 
 
132 REPRODUCTION OP HYDROID ZOOPHYTES. 
 
 are a number of little cells or bell-shaped chambers, with 
 their mouths upwards, every one of them containing a polype 
 that bears a strong resemblance to the Hydra. Each of these 
 polypes is capable of living independently of the rest, obtains 
 its nourishment by means of its own arms, and digests it in 
 its own stomach ; but all are connected by a canal that passes 
 along the stem and branches, in which a kind of circulation 
 takes place, that strongly reminds us of that of the compound 
 Tunicata ( 114). This plant-like structure extends itself by 
 budding; new branches are formed from those previously 
 existing ; and these are enlarged at a certain point into cells, 
 in which after a time polypes make their appearance. 
 
 125. Besides the cells containing the polypes, however, we 
 find capsules in which are evolved buds of a different nature, 
 that form within themselves the generative products. These 
 buds in some instances assume the form of Medusce, and, 
 becoming detached from the stalk that put them forth, swim 
 about freely, living upon food obtained by themselves, and 
 setting free either sperm-cells or germ-cells, by the concur- 
 rence of whose contents eggs are formed, from which new 
 polype-growths arise. In other instances the Medusoid bodies 
 give forth their generative products, without ever leaving the 
 capsules in which they were themselves developed. And in 
 other cases, again, it does not seem that any Medusoid form 
 intervenes at all, the germ-cells and sperm-cells being evolved 
 from the Zoophytic structure itself. But since it is also 
 known that even the most characteristic Medusan forms are 
 evolved as buds from a Zoophytic stock (Chap, xv.), and since 
 those composite forms of Acalephse whose structure has until 
 lately been most obscure, turn out to be, as regards their 
 essential characters, Hydrozoa organized for floating, there 
 seems to be no longer any sufficient ground for ranking the 
 Acalephse as a separate class. 
 
 126. It is not, however, by animals of this very simple 
 structure, that the massive stony fabrics are built up, which 
 constitute the coral islands of the Pacific Ocean, and of which 
 a large portion of our limestone rocks seems to be composed. 
 These are constructed by animals belonging to the group of 
 Anthozoa, and formed upon the same general plan with the 
 Sea-Anemone, a plan which is higher than that of the Hydra, 
 inasmuch as we find the interior of the body containing other 
 
STRUCTURE OF ANTHOZOA : - SEA- ANEMONE. 
 
 133 
 
 cavities around the stomach, which are destined to pre- 
 pare the generative products. In fig. 73, we have a repre- 
 sentation of the Sea- Anemone, as seen from above ; showing 
 its mouth in the centre, surrounded by its numerous radi- 
 ating tentacula ; these are often brightly coloured, and give to 
 the animal the appearance of a beautiful flower. In fig. 74, a 
 similar animal is represented, cut open to show its interior. 
 
 Fig. 73. SEA-ANEMONE, seen Fig. 74. SECTION OF SEA-ANEMONE. 
 
 from above. a, cavity of stomach; b, surrounding 
 
 chambers. 
 
 The mouth is seen to open into a rounded stomach, a, which 
 has no other orifice outwards ; and round this stomach there is 
 a series of radiating membranous partitions, which divide the 
 space intervening between it and the outer covering of the 
 body into numerous chambers, b. Within these chambers, and 
 attached to their partition- walls, are found the bodies which 
 are commonly designated ovaries, but which contain sperm- 
 cells or germ-cells according to the sex. It is doubtful 
 whether these two products are ever formed by the same 
 individual, as they are in the Hydra. The Sea- Anemone does 
 not usually multiply itself by budding, though some species- 
 do so ; but large numbers of young are produced from the 
 eggs, which are fertilized and partly developed whilst still 
 within the ovarian chambers, and these make their way into- 
 the stomach through an aperture at its deepest point, and 
 finally escape by the mouth. 
 
 127. The Sea- Anemone itself, like the Hydra, is a solitary 
 animal, capable of shifting its place at will; and it forms no 
 stony skeleton or support. But there are other animals of the 
 same general structure, which have the power of depositing 
 stony matter in the membrane of their base or foot, and in 
 the membranous partitions between the chambers ; and this 
 stony deposit forms a Coral or Madrepore, such as is shown 
 
134 
 
 ANTHOZOA I STONY CORALS. 
 
 in the accompanying figure (fig. 75). The particular arrange- 
 ment of the radiating plates of the Madrepore (shown at the 
 top of each stem) is the result of the 
 form of the soft structures by which 
 it was deposited; and wherever we 
 see a structure of this kind in coral, 
 whether upon a large or a small 
 scale, we may infer that it was formed 
 by an animal nearly allied in structure 
 to the Sea- Anemone. Of the stone 
 depositing coral -animals, a large 
 number are often* associated in a com- 
 pound structure, as in fig. 76 ; this 
 consists of a stony tree-like stem and 
 branches ; but instead of the soft ani- 
 ^ matter k e i ng contained in its 
 interior, as in the Hydrozoa, it usually forms a kind of flesh 
 
 Fig. 76. STEM OF CORAL. 
 
 that clothes the surface, and connects together the different 
 
STRUCTURE OF PROTOZOA. 
 
 135 
 
 polypes ; and new branches, are formed either by the sub- 
 division of the polypes, or by gemmation from the connecting 
 substance. 
 
 128. When we pass from Zoophytes to animals of still 
 simpler organization, we lose all trace of definite symmetry, and 
 find ourselves amid forms which cannot be referred to any 
 particular plan of growth. These, moreover, are for the most 
 part distinguished by an extreme simplicity of structure ; no 
 such differentiation of parts exhibiting itself among them, as 
 is shown in the " organs " of even the simplest Zoophyte or 
 Worm. Hence they are appropriately designated PROTOZOA. 
 They may, in fact, be considered as essentially consisting of 
 homogeneous particles of a jelly-like substance, to which the 
 name of Sarcode has been given ; and the chief modification 
 this undergoes, consists in the consolidation of certain parts 
 of it by the deposit of horny, calcareous, or siliceous matter, 
 so as to form a skeleton. This may take place on the outer 
 surface only, so as to form shells very like those of Mollusks 
 in miniature, as we see among Foraminifera (fig. 78); or it 
 may occur in the midst of the fleshy substance, so as to 
 form an internal network, such as presents itself in the 
 Sponge. The endowments of the " sarcode " are very extra- 
 ordinary ; and will be best understood by observation of the 
 life-history of one of those simplest Protozoa, in which the 
 whole body consists of but a minute particle of it. 
 
 >. 77. RHIZOPODA : A, Amoeba; B, Actinophrys. 
 
 129. Such an example is afforded by the Amoeba (fig. 77 A), 
 a creature frequently to be met with in great abundance 
 in fresh and stagnant waters, vegetable infusions, &c. Its 
 
136 RHIZOPODA : AIKEBA; ACTINOPHRTS. 
 
 organization is so low, that there is not even that distinct 
 differentiation into containing and contained parts which is 
 necessary to constitute a cell ( 32) ; for although the super- 
 ficial layer of the sarcode possesses more consistence than the 
 interior, it is nevertheless obvious that it has not the tenacity 
 of a membrane, since (as will be presently seen) it does not 
 oppose the passage of solid particles into the interior. How- 
 ever inert this creature may seem when first glanced at, its 
 possession of vital activity is soon made apparent by the 
 movements which it executes and the changes of form it 
 undergoes; these being, in fact, parts of one and the same 
 set of actions. For the shapeless mass puts forth one or 
 more finger-like prolongations, which are simply extensions 
 of its gelatinous substance in those particular directions; 
 and a continuation of the same action, first distending the 
 prolongation, and then, as it were, carrying the whole body 
 into it, causes the entire mass to change its place. After 
 a short time another prolongation is put forth, either in the 
 same or in some different direction ; and the body is again 
 absorbed into it, so as to shift its place still more. It is by 
 means of this movement that the creature obtains its supplies 
 of food ; for when, in the course of its progress, it meets with 
 a particle appropriate for its nutriment, its gelatinous body 
 spreads itself over this, so as to envelope it completely ; and 
 the substance (sometimes animal, sometimes vegetable), thus 
 taken into this extemporized stomach, undergoes a sort 01 
 digestion therein, the nutrient material passing into the sub- 
 stance of the sarcode, and any indigestible portion making its 
 way to the surface, from some part of which it is (as it were) 
 finally squeezed out. 
 
 130. Many other forms of this group, which has received 
 the designation of Rhizopoda, have less power of moving from 
 place to place, but obtain their food by a modification of the 
 same arrangement : of this we have an example in ActinopJirys 
 (fig. 77 B). The body being stationary, its gelatinous substance 
 extends itself into long filaments, termed pseudopodia : these 
 often divide themselves again like the roots of a tree (whence 
 the designation of the group), so as to form threads of ex- 
 treme tenuity; and sometimes these threads meet again and 
 coalesce, so as to form a sort of irregular network. When any 
 minute animal or vegetable organism happens to come in contact 
 
RHIZOPODA I FORAMINIFERA. 137 
 
 with one of these threads, it is usually held by adhesion to 
 it, and the filament forthwith begins to retract itself; as it 
 shortens, the surrounding filaments also apply themselves to 
 the captive particle, bending their points together, so as gra- 
 dually to inclose it, and themselves retracting until the prey 
 is brought to the surface of the body ; and the substance of 
 the threads being itself drawn into that of the body, the 
 entrapped particle is embedded along with this, and under- 
 goes digestion in the surrounding sarcode, any indigestible 
 particle being subsequently extruded from the surface of the 
 body, just as 4 in the Amoeba. The reproduction of these 
 creatures, so far as is yet known, is effected by sell-division, 
 like .that of the Infusoria ( 135); but there is reason to 
 believe that a " conjugation," or reunion of two individuals, 
 sometimes occurs, and that this is to be looked on as repre- 
 senting the sexual propagation of higher animals. 
 
 Fig. 78. FORAMINIPERA. 
 
 A, Oolina ; B, C, Nodosaria ; D, Cristellaria ; E, Polystomella ; F, Dendritina , 
 G, Glolngerina ; H, Textularia; I, Quinqueloculina. 
 
 131. This Bhizopod type of animal life is manifested in 
 two groups of great interest, which are characterised by 
 the possession of hard shells, formed by the consolidation 
 of the external layer of sarcode. The Foraminifera have 
 calcareous shells, which often bear a strong resemblance to 
 those of Nautili, &c. in miniature (fig. 78), but which really 
 have an entirely different relation to the animals that form 
 them. For whilst the Nautilus occupies only the last or 
 outer chamber of its shell, the chambers previously formed 
 
138 
 
 FORAMINIFERA AND POLYCYSTINA. 
 
 being empty and deserted, each chamber of the Rotalia, or 
 any other Foraminiferous shell, is occupied by a segment of 
 sarcode, which is to a great degree independent of the rest, 
 and is only connected with those on either side of it by 
 delicate threads of the same substance ; and the extension of 
 the shell is due to the formation of an additional segment of 
 sarcode on the outside of the last-formed chamber. Each 
 segment has usually the power of putting forth its own 
 " pseudopodia " through minute apertures in the shell, and 
 thus of drawing in its own nourishment through these ; but 
 even when (as sometimes happens) these * food-collecting 
 threads are put forth from the last chamber alone, the nutri- 
 ment there obtained is transmitted to the segments within by 
 percolation through the substance of the sarcode, and not 
 through any tubular canal. The accumulation of the shells 
 of Foraniinifera in some parts of the existing sea-bottom is 
 very remarkable; and similar accumulations in past ages 
 have formed no unimportant part of the crust of the earth 
 a large part of the Chalk-formation having had its origin in 
 them, as well as nearly the whole of the Nummulitic limestone 
 by which it was succeeded. 
 
 132. But animals whose essen- 
 tial structure seems to be nearly 
 the same, may form siliceous in- 
 stead of calcareous shells; and 
 thus are produced those beautiful 
 organisms, known tinder the 
 name of Polycystina (fig. 79), 
 which are occasionally found in 
 the existing seas, but whose re- 
 mains are met with under a far 
 greater variety of forms in certain 
 of the newer marine deposits. 
 There is not in these the same 
 
 t -i i J* '. Fig. 79. POIYCYSTIKA. 
 
 tendency to lorrn composite A _, 
 
 i ji TI- v ,. A, Podocyrtis ; B, Rhopalocanium. 
 
 structures by the multiplication 
 
 of segments, as in the Foraminifera ; but the complication of 
 the individual form is often much greater. Yet, however 
 complex the form, the essential composition of these crea- 
 tures seems to retain the same attribute of simplicity, which 
 cannot be conceived capable of further reduction. 
 
INFUSORY ANIMALCULES. 139 
 
 133. The Animalcules to which the name of INFUSORIA 
 may be properly restricted (the jRotifera, or Wheel- Animal- 
 cules, 105, whose organization is much higher, together with 
 many organisms whose true nature is vegetable, being ex- 
 cluded), present an advance upon the simplicity of the Khizo- 
 poda in this, that whilst their bodies consist for the most 
 part of sarcode, and present scarcely anything that can be 
 termed a distinction of organs, their external surface is con- 
 densed into a membrane too firm to admit either of indefinite 
 extension into pseudopodia, or of the passage of alimentary 
 particles through it ; and consequently the form of the body, 
 although not insusceptible of being temporarily changed by 
 pressure, possesses a considerable degree of constancy for each 
 species (fig. 80). A mouth, or definite aperture for the in- 
 gestion of food, is provided; with an additional orifice in 
 some instances, through which indigestible or effete matters 
 may be discharged from the interior. Into this mouth, ali- 
 
 Fig. 80. INFUSORY ANIMALCULES. 
 
 i. Monads ; n. Trachelis anas ; in. Enchelis, discharging faecal matter; iv. Para- 
 nuEcium; v. Kolpoda; vi. Trachelis fasciolaris. 
 
 mentary particles are drawn by the agency of the cilia with 
 which some part of the surface of the body is provided; 
 these cilia being always so disposed as to serve at the same 
 time for the general locomotion of the animalcule, and for the 
 production of currents that shall bring food to its interior. 
 
 134. Although most Infusoria move freely through the 
 water in which they live, yet certain kinds of them attach 
 themselves by footstalks to marine plants or other floating 
 bodies, during at least a part of their lives ; and in this con- 
 dition bear no slight resemblance to Zoophytes, though of far 
 simpler organization. It is in these sessile forms that the 
 agency of the cilia in creating currents which bring food to 
 
140 INFUSORIA. PORIFERA OR SPONGES. 
 
 the mouth, becomes most conspicuous. The alimentary par- 
 ticles introduced into the mouth commonly have to pass 
 down a short canal before they enter the general cavity of 
 the body ; and within this cavity a number of minute par- 
 ticles are commonly aggregated into a sort of little pellet, as 
 may be well seen when Infusoria are fed with carmine or 
 indigo. One after another of these pellets being thus intro- 
 duced into the interior, which is occupied by a soft sarcode, 
 each seems to push onwards its predecessors ; and a kind of 
 circulation is thus occasioned in the contents of the cavity. 
 The pellets that first entered make their way out after a time 
 (their nutritive materials having been yielded up), generally 
 by a distinct anal orifice, sometimes, however, by any part of 
 the surface indifferently, and sometimes by the mouth. 
 
 1 35. The multiplication of Infusoria ordinarily takes place 
 by spontaneous fission, precisely after the manner of the 
 multiplication of ordinary cells ( 33). This process, under 
 favourable circumstances, may be performed with such 
 rapidity, that, according to the computation of Ehrenberg, no 
 fewer than 268 millions might be produced in a month by 
 the repeated subdivision of a single Paramecium. Sometimes, 
 instead of undergoing subdivision into two equal parts, the 
 Animalcule puts forth a bud, which gradually increases, and 
 then detaches itself from the parent stock. Whether any- 
 thing equivalent to the sexual generation of higher animals 
 occurs among Infusoria, is yet uncertain ; but recent re- 
 searches afford a probability in the affirmative. 
 
 136. In the tribe of PORIFERA, or Sponges, we seem to 
 have the connecting link between Protozoa and Zoophytes. 
 For their animality does not lie so much in the individual 
 particles, as in those aggregations whioh begin to shadow 
 forth that distinction into organs which is carried out more 
 completely among Zoophytes : and there is a large section of 
 the last-named group, in which the polypes are connected 
 together, not by a regular stony or horny stem, but by a 
 sponge-like mass; while the extension of the fabric is provided 
 for by the budding out of this spongy portion of it, the 
 orifices of whose canals after a time become furnished with 
 polype-mouths. The true Sponge (fig. 81) consists of a fleshy 
 substance, composed of an aggregation of particles of sarcode, 
 supported upon a skeleton which usually consists of a net- 
 
PORIFEBA OR SPONGES. 141 
 
 work of horny fibres, strengthened by spicules of mineral 
 matter, sometimes calcareous, but more commonly siliceous. 
 The entire mass is traversed by a great number of canals, 
 which may be said to commence in the small pores upon its 
 surface, and which discharge themselves into the wide canals 
 that terminate in the large orifices, or vents, that usually pro- 
 ject more or less from the surface 
 of the Sponge. Through this sys- 
 tem of canals, there is continually 
 taking place, during the living state 
 of the animal, a circulation of water, 
 which is drawn in from without 
 through the minute pores, then 
 passes into the large canals, and is pig gl _ SPONQE 
 
 ejected in a constant stream from 
 
 the vents. The immediate cause of this movement seems 
 to lie in the vibration of cilia so extremely minute that their 
 existence can only be detected by the most careful micro- 
 scopic examination. Its purpose is evidently to convey to 
 the animal the nutriment which it requires, and to carry oif 
 the matter which it has to reject. No distinct indications of 
 sensation, or of power of locomotion, have been seen in the 
 Sponge : but changes in the form of its projecting vents 
 may be seen to take place from time to time, if it be watched 
 sufficiently long. 
 
 1 37. The reproduction of the Sponge is commonly effected 
 by the budding forth of little particles of sarcode, from the 
 layer which lines the larger canals ; these become furnished 
 with cilia, and, when detached and carried out by the current 
 that issues from the vents, swim freely about for some time ; 
 so as, before fixing themselves and beginning to develope 
 into Sponges, to spread the race through distant localities. 
 But it appears that Sponges are also reproduced by a true 
 sexual process ; " sperm-cells " and " germ-cells " being pro- 
 duced (as in the Hydra, 123) in different parts of the 
 organism, and a true embryo taking its origin in the action 
 of the contents of the former upon those of the latter. 
 
142 NATURE AND SOURCES OP ANIMAL FOOD. 
 
 138. We thus conclude our general survey of the Animal 
 Kingdom ; which, it is hoped, will be found to answer the 
 purpose for which it was designed, that of giving such an 
 amount of preparatory knowledge respecting the principal 
 types of animal structure, as may enable even the beginner to 
 comprehend what will hereafter be stated of their physiological 
 actions. It has not been attempted to observe any proportion 
 in the notice of these several types ; the higher forms having 
 been slightly passed over, because the details of their vital 
 phenomena will constitute the principal subject of the follow- 
 ing pages ; whilst some among the lower have been more 
 fully treated, because the ordinary reader cannot be expected 
 to have even that outline-acquaintance with their nature and 
 actions, which he can scarcely help possessing in the case of 
 animals that are familiar to him. 
 
 CHAPTEE III. 
 
 NATURE AND SOURCES OF ANIMAL FOOD. 
 
 139. BEFORE we examine the nature of the process by 
 which the food of animals is prepared for absorption into 
 their bodies, it will be desirable to consider the characters of 
 the aliment itself, and the purposes to which it is to be appro- 
 priated. The term food or aliment may be applied to all 
 those substances which, when introduced into the living 
 body, serve as materials for its growth, or for the repair of 
 the losses which it is continually sustaining ( 55). When 
 animals are deprived of these materials, we see their bodies 
 progressively diminishing in bulk, their strength decreases, 
 and death at last takes place, after sufferings more or less 
 prolonged. In warm-blooded animals, however, a yet more 
 urgent demand for food is created by the requirements of the 
 heat-producing process ; and many substances are fitted to 
 supply this, which cannot serve for the nourishment of the 
 tissues. 
 
 140. The demand of the body for food is made known by 
 a peculiar sensation, which has its seat in the stomach, namely, 
 hunger. It is increased by mental and bodily exercise, and 
 
NATURE AND SOURCES OF ANIMAL FOOD. 143 
 
 by everything which augments the general energy of the 
 system ; whilst, on the contrary, everything which tends to 
 retard the operations of life, such as bodily and mental inac- 
 tivity, sleep, or depression of spirits, tends also to render the 
 demand for food less imperious. Thus, cold-blooded animals, 
 particularly Reptiles, can sustain a very prolonged abstinence, 
 when the general activity of their functions is kept down by 
 a low temperature; and hybernating Mammals, which pass 
 the winter in a state of torpidity, require no food during the 
 continuance of their lethargy. But with this exception, 
 warm-blooded animals require a constant supply of nutriment, 
 not merely for the maintenance of their proper heat, but also 
 for the repair of the waste resulting from that continuous 
 activity which the uniform temperature of their own bodies 
 enables them to keep up. This is the case with Man and 
 the Mammalia generally, and still more with Birds, whose 
 temperature is higher, and whose movements are more active 
 and energetic. It is also more the case with young animals 
 than with adults; since in the former the changes in the 
 tissues, in consequence of the increase they are undergoing, 
 take place with much more rapidity than in the latter, the 
 bulk of whose bodies remains stationary. Hence, if children, 
 young persons, and adults be shut up together, and deprived 
 of food, the younger will usually perish first, and the adults 
 will survive the longest. The Italian poet Dante has given 
 a terrible picture of such an occurrence, in his history of the 
 imprisonment of Count Ugolino and his children. 
 
 141. The difference in the demand for food between the 
 young growing animal and that which has arrived at maturity, 
 is very remarkable in the case of Insects. There are no 
 animals more voracious than the larva or caterpillar; and 
 there are none that can sustain abstinence, with little dimi- 
 nution of their activity, better than the imago or perfect 
 insect. The larvaa of the Flesh-fly, produced from the eggs 
 laid in carrion, are said to increase in weight 200 times in 
 the course of 24 hours ; and their voracity is so great as to 
 have caused Linnaeus to assert, that three individuals and 
 their immediate progeny (each female giving birth to at least 
 20,000 young, and a few days sufficing for the production of 
 a third generation) would devour the carcase of a horse with 
 greater celerity than a Hon. The larva of the Silk-worm 
 
144 NATURE AND SOURCES OF ANIMAL FOOD. 
 
 weighs, when hatched, about 1-1 00th of a grain ; previously 
 to its first metamorphosis it increases to 95 grains, or 9,500 
 times its original weight. The comparative weight of the 
 full-grown caterpillar of the Goat-moth to that of the young 
 one just crept out of the egg, is said to be as 72,000 to 1. 
 For this enormous increase a very constant supply of material 
 is necessary, and many larvae perish if left unsupplied with 
 food for a single day. On the other hand, a black beetle 
 (Melasoma) has been known to live seven months, pinned 
 down to a board ; and another beetle (Scarabseus) has been 
 kept three years without food, and this without manifesting 
 any inconvenience or loss of activity. There are many perfect 
 insects which never eat after their last change, but die as soon 
 as they have performed their part in the propagation of the race. 
 
 142. The nature of the food of animals is as various as the 
 conformation of their different tribes. It always consists, 
 however, of substances that have previously undergone organ- 
 ization. There are some apparent exceptions to this, in the 
 case of animals which seem to derive their support, in part at 
 least, from mineral matter. Thus, the Spatangus (an animal 
 allied to the Echinus, 119) fills its stomach with sand; but 
 it really derives its nourishment from the minute animals 
 which this contains. The Earthworm and some kinds of 
 Beetles are known to swallow earth ; but only to obtain from 
 it the remains of vegetable matter that are mixed with it. 
 By some races of Man, too, what seems to be mineral matter 
 is mixed with other articles of food, and is said to be nutri- 
 tious ; this may be beneficial, in part, by giving bulk to the 
 aliment, and thus exciting the action of the stomach ( 205); 
 but it has been found, in one case at least, that the supposed 
 earth consists of the remains of animalcules, and contains no 
 inconsiderable portion of organic matter. 
 
 143. There are many instances in which, no obvious sup- 
 plies of food being afforded, the mode of sustenance is obscure ; 
 and it has been frequently supposed that, in such cases, the 
 animals are sustained by air and water alone. But it will 
 always be found that, where food is taken in no other way, 
 a supply of the microscopic forms of animal or vegetable life 
 is introduced by ciliary action ( 45); and it is on these, 
 indeed, that a large proportion of the lower forms of aquatic 
 animals depend entirely for their support. 
 
NATURE OF THE FOOD OF ANIMALS. 145 
 
 144. The first division of aliments is naturally into those 
 which are derived from the Animal and Vegetable kingdoms 
 respectively. Wherever plants exist, we find animals adapted 
 to make use of the nutritious products they furnish, and to 
 restrain their luxuriance within due limits. Thus among 
 Mammals, the Dugong (an animal having the general form 
 and structure of the whale, but adapted to a vegetable diet) 
 browses upon the sea-weeds that grow beneath the surface 
 of the tropical ocean ; the Hippopotamus roots up with his 
 tusks the plants growing in the beds of the African rivers, 
 and fills his huge paunch, not only with these, but with the 
 decaying vegetable matter which he finds in the same situa- 
 tion ; the Antelopes, Deer, Oxen, and other Ruminants, crop 
 the herbage of the plains and meadows ; the Giraffe is enabled 
 by his enormous height to feed upon the tender shoots which 
 are above the reach of ordinary quadrupeds ; the Sloths, living 
 entirely in trees, and hanging from their branches, strip them 
 completely of their leaves ; the Squirrels extract the kernels 
 of the hard nuts and seeds ; the Monkeys devour the soft 
 pulpy fruits ; the Boar grubs up the roots and seeds buried 
 under the soil ; the Reindeer subsists during a large part of 
 the year upon a lichen that grows beneath the snow; and 
 the Chamois finds a sufficient supply in the scanty vegetation 
 of Alpine heights. Not less is this the case among 'Birds ; 
 but in the classes of Reptiles and Fishes, the number of 
 vegetable-feeders, and consequently the variety of their food, 
 is much less. 
 
 145. Among Insects, a very large proportion derive their 
 food entirely from Plants, and many from particular tribes of 
 plants only; so that, if from any cause these should fail, the 
 race may for a time disappear. There is probably not a 
 species of plant which does not furnish nutriment for one or 
 more tribes of insects, either in their larva state or their per- 
 fect condition ; and in this manner it is prevented from mul- 
 tiplying to the exclusion of others. Thus, on the Oak no less 
 than two hundred kinds of caterpillars have been estimated 
 to feed ; and the Nettle, which scarcely any beast will touch, 
 supports fifty different species of insects, but for which 
 check it would speedily annihilate all the plants in its neigh- 
 bourhood. The habits and economy of the different races 
 existing on the same plant, are as various as their structure. 
 
 L 
 
146 VORACITY OF INSECTS. 
 
 Some feed only upon the outside of the leaves ; sortie upon 
 the internal tissue ; others upon the flowers or on the fruit ; 
 a few will eat nothing but the bark ; while many derive their 
 nourishment only from the woody substance of the trunk. 
 
 146. The excessive multiplication of certain tribes of 
 Insects has sometimes had the effect of devastating an entire 
 country. Thus the " plague of locusts " is not unfrequently 
 repeated in tropical countries, and is dreaded by the inhabi- 
 tants even more than an earthquake. These insects are of 
 such extreme voracity that no green thing escapes them; 
 and when their numbers are so increased that they fly in 
 masses which look like dark clouds, and cover the ground 
 where they alight for miles together, it may be easily con- 
 ceived that the devastation they create must produce incal- 
 culable injury. The north of Africa and the west of Asia are 
 the countries most infested by these pests. It is related by 
 Augustin, that a plague, induced partly by the famine they 
 had created, and partly by the stench occasioned by their 
 dead bodies, carried off 800,000 inhabitants from the kingdom 
 of Numidia and the adjacent parts. They occasionally attack 
 the south of Europe. It is recorded that Italy was devastated 
 by them in the year 591 ; and that a prodigious number both 
 of men and beasts perished from similar causes, no less 
 than 30,000 persons in the kingdom of Venice alone. These 
 tremendous swarms usually advance towards the sea ; and 
 being there checked, and having completely exhausted the 
 country behind them, they themselves die of famine, or are 
 blown into the sea by a gale. In 1784 and 1797, they de- 
 vastated Southern Africa ; and it is stated by Mr. Barrow 
 (in his Travels in that country) that they covered a surface 
 of 2,000 square miles; that, when cast into the sea by a 
 strong wind from the north-east, and washed upon the beach, 
 they formed a line fifty miles long, and produced a barrier 
 along the coast three or four feet high ; and that, when the 
 wind again changed, the stench created by the putrefaction 
 of their bodies was perceived at a distance of 150 miles 
 inland. A similar event occurred in the Earbary States in 
 1799, and was followed, as in the other cases, by a plague. 
 
 147. We have occasionally an example of similar devasta- 
 tion in our own country, though on a smaller scale. Thus, 
 a few years ago, the turnip-crops of some parts of England 
 
VORACITY OF INSECTS. 147 
 
 were almost entirely destroyed by the larvse of an insect 
 called the " turnip-fly." The parent insects were seen buzzing 
 over the fields, and depositing their eggs in the plants, \vhicli 
 they do not themselves employ as food ; and in a few days all 
 the soft portions of the leaves were destroyed, and nothing 
 but the skeletons and stalks were left. Some kinds of timber 
 occasionally suffer to no less an extent from the devastations 
 of insects, whose operations are confined to the wood, and do 
 not manifest themselves externally, until the tree is seen to 
 languish and at last to die. The pine-forests of the Hartz 
 mountains in Germany have been several times almost de- 
 stroyed by the ravages of a single species of beetle, less than a 
 quarter of an inch in length. The eggs are deposited beneath 
 the bark; and the larvae, when hatched, devour the sap- 
 wood and inner bark (the parts most concerned in the func- 
 tions of vegetation) in their neighbourhood. It was estimated 
 that, in the year 1783, a million and a half of pines were 
 destroyed by this insect in the Hartz alone ; and other forests 
 in Germany were suffering at the same time. The wonder is 
 increased, when it is stated that as many as 80,000 larvae are 
 sometimes found on a single tree. 
 
 148. But every class in the Animal Kingdom has its car- 
 nivorous tribes, which are adapted to restrain the too rapid 
 increase of the vegetable-feeders (by which a scarcity of their 
 food would soon be created), or to remove from the earth the 
 decomposing bodies that might otherwise be a source of dis- 
 ease or annoyance. The herbivorous races, being for the most 
 part very prolific, would very rapidly increase to such an 
 extent as to produce an absolute famine, if not kept in check 
 by the races appointed to limit their multiplication. Thus, 
 the myriads of Insects which find their subsistence in our 
 forest-trees, if allowed to increase without restraint, would 
 soon destroy the life that supports them, and must then all 
 perish together ; but another tribe (that of the insectivorous 
 Birds, as the woodpecker) is adapted to derive its subsistence 
 from them, and thus to keep their numbers within salutary 
 bounds. Their occasional multiplication to the enormous 
 extent mentioned in the preceding paragraphs, is probably 
 due in general to the absence of the races that should keep 
 them in check. This may occur from accidental causes, or 
 may be produced by the interference of Man. Thus, a set of 
 
 L2 
 
148 BALANCE AMONG DIFFERENT RACES. 
 
 ignorant farmers have imagined that a neighbouring rookery 
 was injurious to them, because they saw the rooks hovering 
 over the newly-sown corn-fields, and seeming to pick the 
 grains out of the ground ; and having extirpated the rookery, 
 they have found in the course of a year or two that they 
 have done themselves an immense injury, the roots of their 
 corn and grasses being devoured by the grubs of cockchafers 
 and other insects, the multiplication of which was before 
 prevented by the rooks, whose natural food they are. 
 
 149. On the other hand, by an intelligent application of 
 this principle, the excessive multiplication of insects has been 
 prevented where it had already commenced. Thus, no means 
 of extirpating the larvae of the turnip-fly was found so suc- 
 cessful, as turning into the fields a number of ducks, which 
 quickly removed them from the plants. And in the island of 
 Mauritius, the increase of locusts, which had been accidentally 
 introduced there, and which were becoming quite a pest, was 
 checked by the introduction from India of a species of bird, 
 the grakle, which feeds upon them. 
 
 150. Of the carnivorous tribes themselves, however, the 
 increase might be so great as to destroy all the sources of their 
 food, were it not that they are kept in check by others, larger 
 and more powerful than themselves, which, not being prolific,, 
 are not likely ever to gain too great a power. Thus, among 
 birds, the eagles, falcons, and hawks rear only two or three 
 young every year, whilst many of the smaller birds produce 
 and bring up four or five times that number. The following 
 is a curious instance of the system of checks and counter- 
 checks, by which the "balance of power" is maintained 
 amongst the different races. A particular species of moth 
 having the fir-cone assigned to it for the deposition of its eggs r 
 the young caterpillars, coming out of the shell, consume the 
 cone and superfluous seed ; but, lest the destruction should 
 be too great, another insect of the ichneumon kind lays its eggs 
 in the caterpillar, inserting its long tail in the openings of 
 the cone until it touches the included insect, its own body 
 being too large to enter. Thus it fixes upon the caterpillar 
 its minute egg, which, when hatched, destroys it. 
 
 151. The peculiarity of the agency of Insects, in the 
 economy of nature, has been justly remarked to consist in their 
 power of very rapid multiplication, in order to accomplish a 
 
VARIATIONS IN POWER OP ABSTINENCE. 149 
 
 certain object, and then in their as rapidly dying off. In this re- 
 spect they resemble the Fungi among plants. (BOTANY, 789.) 
 
 152. There are great variations in the degree of power 
 possessed by animals of different species to sustain abstinence 
 from food, which appear to be related to their respective 
 habits of life ; such as most easily obtain a constant supply 
 of food being immediately dependent upon it, and vice versd. 
 Thus, among the Iarva3 of Insects, those that feed upon vege- 
 tables or dead animal matter (in the neighbourhood of which 
 their eggs are usually deposited by the parent) speedily die if 
 placed out of reach of their aliment ; whilst those that lie in 
 wait for living prey, the supply of which is uncertain, are able 
 to endure a protracted abstinence, even to the extent of ten 
 weeks, without injury. Again, carnivorous Birds and Mam- 
 mals are generally able to exist for some time without food ; 
 their natural habits leading them to glut themselves upon the 
 carcase of the animal they have destroyed, in such a manner 
 as to prevent them from requiring any new supply for some 
 time : thus the wild cat has been kept twenty days without 
 food, the dog has lived for thirty-six days in the same circum- 
 stances, and the eagle for a similar period. But some herbi- 
 vorous animals, such as the camel and the antelope, whose 
 habits are such as to keep them out of the reach of food for 
 several days together, are able to endure a similar abstinence ; 
 whilst among the insectivorous Mammals, which naturally 
 take food often, and but little at a time, the power of absti- 
 nence is much less, the mole, for instance, perishing in 
 confinement, if not fed once a day, or even more frequently. 
 
 153. We have next to consider the different substances 
 used as food, in regard to their chemical composition ; and to 
 inquire for what purposes in the nutrition of the body they are 
 respectively destined. The Vegetable tissues are chiefly made 
 up of the three components, oxygen, hydrogen, and carbon ; 
 the oxygen and hydrogen having the same proportions as in 
 water. Their composition being thus nearly the same as that 
 of starch, gum, and sugar (into which, indeed, they may for 
 the most part be converted by a simple chemical process), 
 alimentary substances of this kind form a natural group to 
 which we may give the name of Saccharine (sugary). But in 
 many vegetable substances used as food, there is a considerable 
 quantity of oily matter, stored up in cells ; and the same kind 
 
150 ORGANIC CONSTITUENTS OF ANIMAL FOOD. 
 
 of matter constitutes the principal part of the fat of animals. 
 Of these oily and fatty matters, also, the chemical elements, 
 oxygen, hydrogen, and carbon, are the only ingredients ; but 
 they are combined in proportions different from the last, the 
 two latter predominating considerably. Hence they consti- 
 tute another group of alimentary materials, to which the 
 term Oleaginous may be given. Lastly, most Vegetables con- 
 tain, in greater or less amount, certain compounds which 
 consist of the four elements, oxygen, hydrogen, carbon, and 
 nitrogen, of which the animal tissues are composed. These 
 compounds exist most largely in the corn-grains, and also in 
 the seeds of the pea and bean tribe ; but there are few vege- 
 table substances used as food by animals, that do not contain 
 them in some small amount. The gluten of wheat, the legu- 
 min of peas, and other vegetable substances of this kind, 
 together with the flesh of animals, the composition of which 
 ( 13) is identical with theirs, are united into a third group, 
 to which the name Albuminous is given. We cannot pro- 
 perly include in this group, however, the gelatinous portions 
 of the animal tissues, which exist largely in gristle, bone, the 
 skin, and other parts ; because gelatin (the substance that 
 forms glue), though it agrees with albumen in being made up 
 of the/<mr ingredients just named, differs from it extremely 
 in the proportions of those elements ( 19) ; so that, although 
 there is good reason to believe that gelatin may be formed out 
 of albumen, it does not seem that any albuminous compound 
 can be formed out of gelatin. Hence we must consider the 
 gelatinous compounds separately. 
 
 1 54. Of these four groups, the last two are distinguished as 
 azotized compounds, or substances that contain azote or nitro- 
 gen ; whilst the first two are spoken of as non-azotized, being 
 destitute of this element. The distinction is a very important 
 one j and must be kept steadily in view in considering the ulti- 
 mate destination of each kind of food. It is obvious from what 
 has been already stated as to the composition of the animal tis- 
 sues ( 1 3 21), that azotized compounds must supply the chief 
 materials for their nutrition and re-formation. The non-azotized 
 substances must be for the most part destined, unless converted 
 into azotized compounds within the living body, either to be 
 simply deposited in its interstices, or to be thrown off from it 
 again without ever actually forming part of its organised 
 structure. 
 
DESTINATION OP NON-AZOTIZED ALIMENTS. 151 
 
 155. Now, in regard to the non-azotized, or the saccharine 
 and oleaginous groups of alimentary substances, it appears to 
 be an established fact, that none of the higher animals can be 
 permanently supported upon them alone. Thus, dogs that 
 have been fed on sugar and starch only, do not survive long ; 
 and it is evident, before their death, that their tissues are 
 gradually undergoing decay. It has been thought that such 
 results might be partly explained upon the fact, that animals 
 fed upon one simple substance soon become disgusted with it, 
 and will even refuse it altogether ; but the experiments have 
 been repeated with a combination of various non-azotized sub- 
 stances, and the same result has occurred. Still it is too much 
 to affirm, as some have done, that these substances do not con- 
 tribute in any degree to the nutrition of the animal tissues; 
 since there is ample evidence that the presence of fatty matter in 
 the blood is a condition essential to the production of newly 
 forming tissue ; and we find that either oleaginous substances, 
 or substances belonging to the saccharine group which can be 
 readily converted into fat within the body, constitute an im- 
 portant part of the food of Man, and of animals generally. 1 
 
 156. That such a conversion can take place, has been de- 
 monstrated by experiments carefully conducted upon bees, 
 which have been found to generate wax when fed upon sugar 
 only and also upon cows, which give off in their milk so 
 much larger a quantity of butter than can be produced at the 
 expense of the fat contained in their food, that there is no 
 other mode of accounting for its presence, than by regarding 
 it as generated from the starchy portion of their diet. And 
 the fattening power of starchy and saccharine articles of diet 
 is well known to breeders of cattle ; though the articles which 
 contain oily matter in addition seem to possess a higher value 
 in this respect. 
 
 157. But if these non-azotized compounds, which exist so 
 largely in the food of herbivorous animals, are not destined 
 to form any other permanent part of the animal organism 
 than the oleaginous contents of the fat-cells ( 46), the ques- 
 tion again arises, what becomes of them ? It is not enough 
 
 1 The value of cod-liver oil, which is now so extensively used in the 
 treatment of diseases of imperfect nutrition, seems to depend upon the 
 readiness with which it can be digested and assimilated, so as to furnish 
 the supply of fat required by the formative processes. 
 
152 DESTINATION OF NON-AZOTIZED ALIMENTS. 
 
 to say that they are deposited as fat ; since it is only when 
 a large quantity of them is taken in, that there is any in- 
 crease in the quantity of fat already in the body. "We shall 
 hereafter see that they are used up in the process of respira- 
 tion, one great object of which is, to produce a certain amount 
 of heat, sufficient to keep up the temperature of the body, in 
 warm-blooded animals, to a high standard. "We might almost 
 say with truth, that a great part of the oleaginous and sac- 
 charine principles is burned within the body, for this pur- 
 pose. The process will be hereafter considered more in 
 detail ( 412, 413) ; and at present we need only stop to 
 remark upon the adaptation between the food provided for 
 animals in different climates, and the amount of heat which 
 it is necessary for them to produce. Thus the bears, and 
 seals, and whales, from which the Esquimaux and the Green- 
 lander derive their support, have an enormous quantity of 
 fat in their massive bodies : this fat is as much esteemed as 
 an article of food amongst these people, as it would be thought 
 repulsive by the inhabitants of southern climates ; and by the 
 large quantity of it they consume, they are able to support 
 the bitterness of an Arctic winter, without appearing to suffer 
 more from the extreme cold than do the residents in more 
 temperate climes during their winter. On the other hand, 
 the antelopes, deer, and wild cattle, which form a large pro- 
 portion of the animal food of savage or half-cultivated nations 
 inhabiting tropical regions, possess very little fat; and the 
 comparatively small supply of carbon and hydrogen, of which 
 the combustion is required to keep up the bodily temperature 
 of the inhabitants of those regions, is derived from the flesh of 
 these animals, in the manner that will be presently 'explained. 
 158. The application of the substances forming the albu- 
 minous group, to the support of the animal body, by affording 
 the materials for the nutrition and re-formation of its tissues, 
 needs little explanation. The proportions of the four ingre- 
 dients of which they are al 1 composed, are so nearly the same, 
 that no essential difference appears to exist among them ; and 
 it is a matter of little consequence, except as far as the gra- 
 tification of the palate is concerned, whether we feed upon 
 the flesh of animals (syntonin, 16), upon the white of egg 
 (albumen, 13), the curd of milk (casein, 15), the grain of 
 wheat (gluten), or the seed of the pea (legumin). All these 
 
DESTINATION OF NOX-AZOTIZED ALIMENTS. 153 
 
 substances are reduced in the stomach to the form of albumen ; 
 which is the raw material out of which the various fabrics of 
 the body are constructed. But the rule holds good with re- 
 gard to these also, that by being made to feed constantly on 
 the same substance, boiled white of egg for instance, or meat 
 deprived of the principle that gives it flavour, an animal may 
 be effectually starved ; its disgust at the food being such, that 
 even if it be swallowed it is not digested. It is very interest- 
 ing to remark that, in the only instance in which Nature has 
 provided a single article of food for the support of the animal 
 body, she has mingled articles from all the three preceding 
 groups. This is the case in Milk ; which contains a consider- 
 able quantity of the albuminous substance, casein, that forms 
 its curd ; a good deal of oily matter, the butter ; and no in- 
 considerable amount of sugar, which is dissolved in the whey. 
 The proportions of these vary in different Mammalia, being 
 related as it would seem to the habits of the young animal 
 thus sustained, while they depend in part upon the nature of 
 the food supplied to the animal that forms the milk ; but the 
 three substances are thus combined in every instance. 
 
 159. But although the greater part of the organised tis- 
 sues of animals have a composition nearly allied to that of 
 albumen, many of them also contain a large quantity of 
 gelatin ( 19). It seems certain that this gelatin may be pro- 
 duced out of albuminous substances ; since in animals chat 
 are supported on these alone, the nutrition of the gelatinous 
 tissues does not seem to be impaired. But it appears equally 
 certain, that gelatin cannot be applied to the nutrition of the 
 albuminous tissues. Many series of experiments have been 
 made on this subject, with a view of determining how far 
 gelatin-soup made from crushed bones (such as that which 
 long constituted a principal article of diet in the hospitals of 
 Paris) is adequate for the support of the body in health. 
 The result of these has been uniformly the same, namely, 
 that although gelatin may be advantageously mixed with 
 albumen, fibrin, gluten, &c., and those other ingredients which 
 exist in meat-soup and bread, yet that, when taken alone, it 
 has little (if any) more power of sustaining life, than sugar 
 or starch possesses. Although it might have been thought 
 likely that gelatin employed as food might be applied within 
 the body to the nutrition of its gelatinous tissues, yet there 
 
154 SOURCES OF DEMAND FOR ALIMENT. 
 
 is strong reason to believe that these, like the albuminous, 
 are formed at the expense of the albuminous matter of the 
 blood, and that gelatin thus introduced undergoes a rapid 
 decomposition, yielding up a considerable part of its carbon 
 and hydrogen to the combustive process, which is the only 
 function to which it affords any substantial pabulum. Con- 
 sequently the current idea regarding the nutritive value of 
 jellies of various kinds, has little or no real foundation. 
 
 160. It has been already stated ( 68) that all the living 
 tissues of the body are continually undergoing a sort of death 
 and decay ; and that they do this the more rapidly, in pro- 
 portion as they are called upon for the discharge of their 
 functions. The need of material capable of replacing that 
 which has been lost, is consequently the chief source of the 
 constant demand for aliment. Even in young, actively 
 growing animals, the quantity required for the increase of 
 their bodies constitutes but a very small proportion of that 
 which is taken in ; of the remainder, a part is at once re- 
 jected as indigestible ; and the rest is appropriated to the 
 repair of the waste which is continually going on. This waste 
 is much greater in young animals than in adults ; for all their 
 vital processes are more actively and energetically performed : 
 their movements are quicker in proportion to their size ; and 
 injuries are more speedily repaired. To remove the products 
 of this decomposition is the special object of the various pro- 
 cesses of excretion ; and among these, the respiration, by 
 which a large quantity of carbon and hydrogen is carried 
 off in the form of carbonic acid and water, is of the most 
 constant importance, on account of the heat which it thus 
 enables the animal body to maintain. This temperature, in 
 Carnivorous animals, appears to be sufficiently kept up by 
 the combustion of the carbon and hydrogen set free by the 
 decay (or metamorphosis, as it may be termed) of their tis- 
 sues ; but this combustion goes on with much more rapidity, 
 in consequence of their almost unceasing activity, than it does 
 in the Herbivorous animals, which lead comparatively inac- 
 tive lives. Every one who has visited a menagerie must have 
 noticed the continual restlessness of the Tigers, Leopards, 
 Hyenas, &c., which keep pacing from one end of their narrow 
 cages to tjie other ; and it would seem as if this restlessness 
 were a natural instinct, impelling them to use muscular exer- 
 
NUTRITION OF CARNIVOROUS ANIMALS. 155 
 
 tion sufficient for the metamorphosis of an adequate amount 
 of tissue, that enough carbon and hydrogen may be set free 
 for the support of the respiratory process. And we see a cor- 
 responding activity in the Human hunters of the swift-footed 
 antelope and agile deer, which answers a similar purpose ; and 
 which is remarkably contrasted with the stupid inertness of 
 the inhabitants of the frigid zone, that is only occasionally 
 interrupted by the necessity of securing the supplies of food 
 afforded by the massive tenants of their seas. 
 
 161. The nutrition of the Carnivorous races may, then, be 
 thus described. The bodies of the animals upon which they 
 feed, contain flesh, fat, &c., in nearly the same proportion as 
 their own ; and all, or nearly all, the aliment they consume, 
 goes to supply the waste in the fabric of their own bodies, 
 being converted into its various forms of tissue. After having 
 remained in this condition for a certain time, varying ac- 
 cording to the use that is made of them, these tissues un- 
 dergo another metamorphosis, which ends in restoring them 
 to the condition of inorganic matter ; and thus give back to 
 the mineral world the materials which were drawn from it by 
 plants. Of these materials, part are burned off, as it were, 
 within the body, by union with the oxygen of the air taken 
 in through the lungs, from which organs they are discharged 
 in the form of carbonic acid and water : the remainder are 
 carried off in the liquid form by other channels. Hence 
 we may briefly express the destination of their food in the 
 following manner : 
 
 ( Carbonic acid and 
 
 \ water thrown Off 
 
 
 
 other excretions. 
 
 162. But in regard to the Herbivorous animals, the case 
 is different. They perspire much more abundantly, and their 
 temperature is thus continually kept down ( 372). They 
 consequently require a more active combustion, to de- 
 velop sufficient bodily heat ; and the materials for this are 
 supplied, as we have seen, by the non-azotized constituents of 
 their food, rather than by the metamorphosis of their own 
 tissues, which takes place with much less rapidity than in 
 the carnivorous tribes. Hence we may thus express the 
 
156 NUTRITION OF HERBIVORA AND OF MAN. 
 
 destination of this part of their food ; that of the albuminous 
 matters, here much smaller in amount, being the same as in 
 the preceding case : 
 
 Starch, oil, and 1 } partly ( Fatty and \ but chiefly f Carbonic acid and water, 
 other non-azo- > converted < other animal [thrown off< disengaged hy the respi- 
 tized compounds J into ( tissues, ) directly as (. ratory process. 
 
 The proportion of the food deposited as fat, will depend in 
 part upon the surplus which remains, after the necessary sup- 
 ply 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 perspiration, 
 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. This is consistent with the teach- 
 ings of experience respecting the fattening of cattle ; for it is 
 well known that this may be accomplished much sooner, if 
 the animals are shut up in a warm dwelling and are covered 
 with cloths, than if they are freely exposed in the open air. 
 
 163. Now the condition of Man may be regarded as inter- 
 mediate between these two extremes. The construction of 
 his digestive apparatus, as well as his own instinctive pro- 
 pensities, 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 develop- 
 ment of either his bodily or his mental powers ; but this 
 cannot be said in regard to a diet of which the corn-grains 
 furnish the chief ingredient, since the gluten they contain 
 appears to be as well adapted for the nutrition of the animal 
 tissues, as is 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 to the climate under which he lives) by the ne- 
 cessity for a supply of carbon and hydrogea 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 ingredients predominate. Thus it has been 
 calculated that, since 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 
 
COMPOSITION OP ARTICLES OF HUMAN FOOD. 
 
 157 
 
 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 agricultural 
 population possess over the wandering tribes of hunters which 
 still people a large part both of the Old and New Continents. 
 164. The following Table exhibits the proportions of albu- 
 minous, starchy or saccharine, fatty, and saline substances, 
 contained in various articles ordinarily used as food by Man ; 
 together with the proportion which water bears in each case 
 to the solid constituents of the food, which becomes a most 
 important element of consideration when the nutritive value 
 of different kinds of food is compared : 
 
 Substances, 100 parts. 
 
 jj 
 
 "5 
 
 F 
 
 Albumi- 
 nous sub- 
 stances. 
 
 & 
 
 S Id 
 
 o><a 
 
 i 
 
 K 
 
 
 
 1 
 
 . J. * 
 J % 
 u 
 
 A 
 
 o 
 
 S 
 
 
 
 'c' 
 
 3 S 
 H e S 
 
 Human Milk 
 
 89 
 
 3 5 
 
 4 2 
 
 3 
 
 2 
 
 11 4 
 
 3 5 
 
 14 9 
 
 Cow's Milk 
 
 8fi 
 
 4.5 
 
 5 
 
 4.1 
 
 0.7 
 
 14.8 
 
 4 5 
 
 19 3 
 
 Skimmed Milk 
 Butter Milk 
 
 87 
 
 87 
 
 4.5 
 4.5 
 
 5.0 
 5.0 
 
 2.7 
 0.5 
 
 0.7 
 0.7 
 
 11.5 
 6.0 
 
 4.5 
 4 5 
 
 16.0 
 10 5 
 
 Beef and Mutton 
 
 7? 
 
 19.0 
 
 
 5 
 
 2.0 
 
 12 
 
 19 
 
 31 
 
 Veal 
 
 77 
 
 19 
 
 
 1 
 
 0.6 
 
 2 4 
 
 19 
 
 21 4 
 
 Poultry 
 
 74 
 
 21.0 
 
 
 3.0 
 
 1.2 
 
 7 2 
 
 21.0 
 
 28 2 
 
 
 20 
 
 0.8 
 
 
 70.0 
 
 1.3 
 
 168 
 
 8 
 
 168 8 
 
 Cheese (Cheddar) 
 (Skimmed) 
 Butter 
 
 .36 
 44 
 IS 
 
 29.0 
 45.0 
 
 .'..' 
 
 30.0 
 6.0 
 830 
 
 4.5 
 5.0 
 2.0 
 
 72.0 
 14.4 
 99 
 
 29.0 
 45.0 
 
 101.0 
 59.4 
 199 
 
 Eggs 
 White of Egg 
 
 74 
 
 78 
 
 14.0 
 20.0 
 
 ... 
 
 10.5 
 
 .5 
 
 .6 
 
 25.0 
 
 14.0 
 20.0 
 
 39.0 
 gO o 
 
 Yolk of Egg 
 White Fish 
 
 52 
 
 79 
 
 lti.0 
 19 
 
 ... 
 
 30.0 
 1 
 
 .3 
 2 
 
 72.0 
 2 4 
 
 16.0 
 19 
 
 88.0 
 21 4 
 
 
 78 
 
 17.0 
 
 
 4.0 
 
 .4 
 
 9 6 
 
 17 
 
 26 6 
 
 Eel 
 
 80 
 
 10 
 
 
 8.0 
 
 3 
 
 19 2 
 
 10 
 
 29 2 
 
 Wheat Flour 
 
 IS 
 
 11.0 
 
 70.0 
 
 2.0 
 
 .7 
 
 74.8 
 
 11.0 
 
 85 8 
 
 
 15 
 
 10.0 
 
 70.0 
 
 2.4 
 
 2.0 
 
 75 8 
 
 10 
 
 85 8 
 
 Oat-meal 
 
 15 
 
 12 
 
 62 
 
 6 
 
 3 
 
 76 4 
 
 12 
 
 88 4 
 
 Rye-meal 
 
 15 
 
 9.0 
 
 66.0 
 
 2.0 
 
 1.8 
 
 70 8 
 
 9 
 
 79 8 
 
 
 14 
 
 9 
 
 65 
 
 80 
 
 1 7 
 
 84 2 
 
 9 
 
 93 2 
 
 Rice 
 
 14 
 
 7 
 
 76 
 
 3 
 
 3 
 
 76 7 
 
 7 
 
 83 7 
 
 Haricots 
 
 10 
 
 23.0 
 
 45 
 
 3.0 
 
 3.6 
 
 52 2 
 
 23 
 
 75 2 
 
 Peas 
 
 13 
 
 22 
 
 58 
 
 2.0 
 
 3 
 
 62 8 
 
 22 
 
 84 8 
 
 Beans 
 
 14 
 
 24 
 
 44 
 
 1 4 
 
 3 6 
 
 47 4 
 
 24 
 
 71 4 
 
 Lentils 
 
 14 
 
 29.0 
 
 44 
 
 1.5 
 
 2 3 
 
 47.6 
 
 29 
 
 76 6 
 
 Wheat-bread 
 
 44 
 48 
 
 9.0 
 5 3 
 
 49.0 
 
 4/i 
 
 1.0 
 1 
 
 2.3 
 1 4 
 
 51.4 
 48 4 
 
 9.0 
 5 3 
 
 60.4 
 53 7 
 
 Potatoes 
 
 74 
 
 20 
 
 23.0 
 
 0.2 
 
 0.7 
 
 23.5 
 
 2 
 
 25 5 
 
 Green Vegetables 
 
 86 
 18 
 
 2.0 
 
 4.0 
 82 
 
 0.5 
 
 0.7 
 
 5.0 
 82 
 
 2.0 
 
 7.0 
 82 
 
 
 
 
 
 
 
 
 
 
 * The value of the Fat is stated in this column according to its 
 Jieating equivalent of starch, which is larger in the ratio of 24 to 1. 
 Hence, in the last column, the proportion of nutriment in aliments 
 containing fat, comes to be greater than the weight of their solids 
 would indicate. 
 
158 ECONOMY OF HUMAN DIET. 
 
 Those articles of food in which the nitrogenous compounds 
 predominate, are especially fitted for the maintenance of the 
 solid fabric of the body ; whilst those in which the carbon- 
 aceous compounds are in largest excess, are those which are 
 most effective as supplying materials for the combustive pro- 
 cess. Conspicuous among the former are the various kinds 
 of animal flesh, as also the white of eggs ; whilst among the 
 latter the most noticeable are bacon and butter, rice and 
 potatoes, the former consisting almost wholly of fat, the latter 
 being chiefly composed of starch. Of all single articles of 
 food, good wheaten bread, in which the proportion of nitro- 
 genous to carbonaceous components is about as 5.7 to 1, 
 seems to be the one best suited to the ordinary wants of 
 Man ; but this acquires much additional value from the con- 
 current use of a moderate amount of fatty matter in the form 
 of butter. 
 
 165. If the more highly azotized forms of food be em- 
 ployed exclusively, a great excess of them must be consumed 
 to supply the carbon needed for respiration ; whilst if the 
 more carbonaceous kinds of food be used as the sole susten- 
 ance, unless the quantity ingested be large enough to afford 
 the requisite supply of azotized material for the maintenance 
 of the tissues, their nutrition must be imperfectly effected, 
 and the strength must fail Not only in the instance just 
 cited, but in a variety of others, the instincts of mankind 
 have led to such a combination of different articles of diet, 
 as includes in their appropriate proportions the albuminous, 
 the saccharine, and the oleaginous principles. Thus with 
 meat we eat potatoes ; and with the white meats which are 
 deficient in fat, we eat bacon. We use melted butter with 
 most kinds of fish, or fry them in oil ; whilst the herring, the 
 salmon, and the eel, are usually fat enough in themselves, and 
 are dressed and eaten alone. A similar adjustment is made 
 when we mix eggs and butter with sago, tapioca, and rice ; 
 when we add oil and the yolk of an egg to salad ; when we 
 boil rice with milk, and combine cheese with maccaroni. 
 Bacon and greens, and pork and pease-pudding, again, are 
 combinations founded in taste, which approve themselves to 
 the judgment; as is also the Irish dish termed kolcannon, con- 
 sisting of potatoes and cabbage, with a little bacon or fat pork. 
 So are the mixture so common in Ireland and Alsace, of butter- 
 
ECONOMY OF HUMAN DIET. 159 
 
 or curdled-milk with potatoes ; and the combination of 
 rice and fat, which is the staple of the diet of many Eastern 
 nations. Even the morsel of butter or the bit of cheese 
 which the English labourer eats with his hard-earned bread, 
 are not matters of luxury, but have a positive importance ; 
 and the existence of these tastes and habits shows how by 
 long experience Man has at last learned to adjust the com- 
 position of his food, so as best to maintain the health and 
 vigour of his body. With a difference of requirement comes 
 a difference of tastes. Thus men who are going through a 
 very laborious course of exertion, prefer meat to bread or 
 vegetables, feeling it to be more sustaining to their strength. 
 On the other hand, those who are continuously exposed to 
 the severity of an Arctic winter, eat with relish large masses 
 of fat, on which they would look with disgust under other 
 circumstances. The quantity of work which a man can do, 
 and his power of sustaining extreme cold, both depend in 
 great part, as has now been abundantly proved, upon the 
 adequacy of the sustenance he takes : the demand, in the first 
 case, being for albuminous material to supply the waste of his 
 tissues ; whilst in the second it is for combustive material 
 suitable to generate heat in large measure, a purpose which 
 is far more efficiently answered by oleaginous substances, than 
 by those of a starchy or saccharine nature. Experience fur- 
 ther shows that the healthy condition of the blood of Man 
 can only be maintained by the use of fresh vegetables as part 
 of his ordinary diet. "When these are withdrawn for any 
 length of time, the disease known as Scurvy is certain to 
 appear, unless lemon -juice or some other efficacious anti- 
 scorbutic be employed as a substitute. This is a fact of the 
 utmost importance in provisioning ships for long voyages ; 
 the tendency to scurvy being increased by confinement 
 and insufficient ventilation, and by the exclusive use of salt 
 provisions. 
 
 166. Besides these organic substances, there are certain 
 Mineral ingredients, which may be said to constitute a 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, lime, and iron, either in combina- 
 tion or separate. The uses of Salt are very numerous and 
 
160 MINERAL INGREDIENTS OF FOOD. 
 
 important. It exists largely in the blood, and in the various 
 animal fluids which are secreted from it ; and it is also an 
 essential ingredient of most of the solid tissues. Its presence 
 obviously tends to prevent that spontaneous decomposition 
 lo which organic substances are liable. 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 bones 
 are consolidated. Sulphur exists in small quantities in several 
 animal tissues ; but its part seems by no means so important 
 as that performed by phosphorus. Lime is required for the 
 consolidation of the bones, and for the production of the 
 shells and other hard parts that form the skeletons of the 
 Invertebrata. Of the limestone rocks of which a great part 
 of the crust of our globe is composed, a very large proportion 
 is made up of the remains of animals that formerly existed in 
 the ocean. Thus some almost entirely consist of masses of 
 Coral, others of beds of Shells, and others of the coverings 
 of minute Foraminifera (131). To these mineral ingredients 
 we may also add Iron, which is a very important element in 
 the red blood of Vertebrated animals. 
 
 167. These substances are contained, more or less abun- 
 dantly, in most articles generally used as food ; and where 
 they are deficient, the animal suffers in consequence, if they 
 be not supplied in any other way. 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 herbi- 
 vorous animals by some other means. Thus salt is purposely 
 mingled with the food of domesticated animals ; 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. Phos- 
 phorus 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 ; 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 ; and 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 
 
MINERAL INGREDIENTS OF ANIMAL FOOD. 161 
 
 of food, more phosphorus is taken into the body than it 
 requires ; and the excess has to be carried out in the excre- 
 tions. Sulphur is derived alike from vegetable and animal 
 substances. It exists in flesh, eggs, and milk ; also in the 
 azotized compounds of plants ; and (in the form of sulphate 
 of limp.) 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 
 vegetable substances used as food by man, such as potatoes, 
 cabbage, peas, cucumbers, mustard, &c. ; and probably in 
 most articles from which other animals derive their support. 
 
 168. Lime is one of the most universally diffused of all 
 mineral bodies ; there being very few animal or vegetable 
 substances in which it does not exist. It is most commonly 
 taken in, among the higher animals, combined with phos- 
 phoric acid, so as to form bone-earth, in which state it exists 
 largely in the seeds of most grasses. A considerable quantity 
 of lime exists, moreover, in the state of carbonate and sul- 
 phate, in all hard water. 
 
 169. When an unusual demand exists for lime, however, 
 for a particular purpose, an increased supply must be afforded. 
 Thus a hen preparing to lay, is impelled by her instinct to 
 eat chalk, mortar, or some other substance containing the car- 
 bonate of lime which is required for the consolidation of the 
 shell ; and if this be withheld, the egg is soft, its covering 
 being composed of animal matter alone, not consolidated by 
 the deposit of earthy particles. The thickness of the shells 
 of aquatic Mollusks depends greatly upon the quantity of 
 lime in the surrounding water. Those which inhabit the sea, 
 find in its waters as much as they require ; but those that 
 dwell in fresh- water lakes, which contain but a small quan- 
 tity of lime, form very thin shells ; whilst, on the other hand, 
 those that inhabit lakes in which, from peculiar local causes, 
 the water is loaded with calcareous matter, form shells of 
 remarkable thickness. 
 
 170. The mode in which the Crustacea, whose calcareous 
 shell is periodically thrown off ( 99), are able to renew it 
 with rapidity, is very curious. There is laid up in the walls 
 of their stomachs a considerable supply of calcareous matter, 
 in little concretions, which are commonly known as " crabs' 
 eyes." When the shell is cast, this matter is taken up by 
 
162 DIGESTION AND ABSORPTION. 
 
 the blood, and is thrown out from the surface, mingled with 
 animal matter. This hardens in a day or two, and the new 
 covering is complete. The concretions in the stomach are 
 then found to have disappeared ; but they are gradually 
 replaced, before the supply of lime they contain is again, 
 required. 
 
 CHAPTER IV 
 
 DIGESTION AND ABSORPTION. 
 
 171. HAVING now considered the nature of the food of 
 Animals, and the sources from which it is obtained, we have 
 next to consider the process by which the aliment is received 
 into their bodies, and prepared to form a part of their own 
 fabric. This process, termed Digestion, is naturally divided, 
 among the higher animals at least, into various stages. In 
 the first place, there is the prehension or laying hold of the 
 aliment, and its introduction into the mouth or entrance to 
 the digestive cavity. In the mouth it usually undergoes a 
 preparation ; which consists partly in its being cut, ground, 
 or crushed, by mechanical action, into minute pieces ; and 
 partly in the working-up of these pieces with a fluid that is 
 poured into the mouth, the saliva. These two processes are 
 termed mastication and insalivation ; similar processes are 
 performed, in some animals, in a part of the digestive tube 
 intermediate between the mouth and the stomach, and even 
 in the latter itself. The stomach is usually situated at some 
 distance from the mouth, and is connected with a tube called 
 the oesophagus or gullet ; and the passage of the food into 
 this, constituting the act of swallowing, is termed deglutition. 
 The food, having arrived in the stomach, is acted-upon by a 
 peculiar fluid which it contains, and much of its alimentary 
 portion is dissolved, so that a pulpy mass is formed which is 
 termed chyme ; hence this process, which is the first stage of 
 digestion properly so called, is termed chymijlcation or the 
 manufacture of chyme. The chyme, which passes into the 
 intestines, is further acted-on by secretions that are poured 
 into them ; and a certain nutritive combination of albuminous 
 
PEEHENSION OF FOOD. 163 
 
 and fatty ^ matters, termed chyle, is separated from the matters 
 that are to be thrown off : this process, which is the second 
 stage of true digestion, is termed chylification. The rejected 
 portions of the food, with secretions poured into the alimen- 
 tary canal, find their way out through the intestinal tube ; 
 and are voided at its terminal orifice by the act of defecation. 
 And lastly, the nutritive materials are taken up by absorption 
 into vessels that are distributed upon the walls of the diges- 
 tive cavity, and undergo a gradual change, by which they are 
 converted into blood. These two processes are called absorp- 
 tion and sanguification (or manufacture of blood). Each of 
 the foregoing stages will now be separately considered. 
 
 Prehension of Food. 
 
 172. The introduction of aliment within the entrance to the 
 digestive cavity is accomplished in various methods in dif- 
 ferent animals. In the Mammalia in general, the aperture of 
 whose mouth is guarded by fleshy lips, these, with the jaws 
 and teeth, are the chief instruments of this operation. But 
 in Man and the Monkey tribe the division of labour is 
 carried further ; the food being laid hold of by the anterior 
 members, or hands, and by them carried to the mouth. 
 "Where the hand has the power of grasping, and especially 
 where the thumb can be opposed to the fingers, the action of a 
 single member is sufficient j but there are several animals 
 which, like the Squirrel, use both limbs conjointly to hold 
 their food, the extremity not having itself the power of grasp- 
 ing. The Ant-eaters, Woodpeckers, Chameleons, and other 
 insect-eating animals, obtain their food 
 by means of a long extensible tongue ; 
 this either serving to transfix the insect, 
 or being covered with a viscid saliva 
 which glues it to the surface. The Giraffe 
 uses its long tongue to lay hold of the 
 young shoots on which it browses ; and 
 the Elephant employs its trunk, which is 
 nothing else than a prolonged nose, for 
 every kind of prehension (fig. 82). Many 
 of the Invertebrata are furnished with 
 
 T, ji T -, , -, . ,1 -i HEAD OF ELEPHANT. 
 
 little appendages round their mouths by 
 
 which the food is conveyed into them ; such are the palpi 
 
 M2 
 
164 
 
 .RECEPTION OF SOLID AND LIQUID ALIMENT. 
 
 of Insects, of which a pair is attached to each jaw (fig. 84); 
 the tentacula of Mollusks, which are sometimes extremely 
 prolonged, as in the Cuttle-fish tribe (fig. 85); and the 
 similar organs of the polypes (fig. 71). 
 
 Fig. 84. JAWS OF THE 
 SAME INSECT. 
 
 Fig. 85. LOLIGOPSIS. 
 
 173. The reception of liquids is accomplished in two ways. 
 Sometimes the liquid is made to fall into the mouth, simply 
 by its own weight (fig. 86) ; in other instances it is drawn or 
 pumped up into this cavity, either by the expansion of the 
 chest, which causes a rush of air towards the lungs, or by 
 the movement of the tongue, which, being drawn back like a 
 piston, produces the action of sucking. Some of the lower 
 animals are destined to be entirely supported by liquids which 
 they find in plants, or which they draw from the bodies of 
 other animals whereon they live as parasites. This is the 
 case with many Insects ; and their mouth, instead of present- 
 ing the ordinary structure, is formed into a sort of tube or 
 trunk, very much extended, through which the juices are 
 drawn up according to the wants of the animal. Such a 
 conformation exists in the butterfly and moth tribe, whose 
 
SUCTION OP LIQUIDS. 
 
 165 
 
 trunk, when not in use, is coiled up in a spiral beneath the 
 head ; as is shown in fig. 87, representing the head of a 
 
 Fig. 87. TRUNK OF A 
 BUTTERFLY. 
 
 Fig. 86. CHIMPANZEE DRINKING. 
 
 Butterfly, a, of which the eye is seen at c, the base of the 
 antennae at b, the palpi at <?, and the trunk at d. In some of 
 the Fly tribe, the trunk attains a length several times greater 
 than that of the body, as shown in fig. 88, representing a 
 
 Fig. 88. NEMESTRINA LONGIROSTRIS. 
 
 dipterous (two-winged) insect from the Cape of Good Hope, 
 which sucks the juices of a single kind of flower, the length 
 of whose tube just equals that of its long proboscis. 
 
166 
 
 DEVELOPMENT OF TEETH. 
 
 Mastication. 
 
 174. The act of Mastication, or the mechanical division of 
 the alimentary matter, is effected in most of the higher animals, 
 by the Teeth ; which are implanted in the jaws, and are so fixed 
 as to act against one another, with a cutting, crushing, or 
 a grinding power, according 
 
 to the nature of the food on 
 which they have to operate. 
 The manner in which they 
 are formed is worthy of 
 note. In Man, who may be 
 taken as a fair example, 
 c each tooth is developed in 
 
 6 _. the interior of a little mem- 
 
 Fig. 89. DEVELOPMENT OF TEETH. , , . , . , n 
 
 a, the gum ; b, the lower jaw ; c, angle of the DrailOUS SRC, which IS lodged 
 jaw ; d, dental capsules. in the tl^knegg O f the j aw . 
 
 bone ; as seen in the accompanying figure, which represents 
 half the lower jaw of a very young infant, from which the 
 outside has been removed. This sac, which is named the 
 dental capsule (a, fig. 90), is composed of two membranes, 
 abundantly furnished with blood-vessels ; and it encloses in 
 its interior a little bud-like protuberance, b, in which ramify 
 a great number of nervous filaments and minute vessels, c. 
 The matter composing this little body, which is termed the 
 pulp, is gradually converted into the dentine ( 54) of the tooth, 
 d a which in Man constitutes nearly its whole 
 
 structure ; this conversion takes place first 
 at its highest points, d, d. The crown or 
 upper portion of the tooth receives a 
 covering of enamel ( 54). Gradually the 
 process of conversion extends more and 
 more to the interior of the pulp ; and at 
 last the whole is changed into dentine, 
 with the exception of a small portion 
 that still remains, occupying what is termed the cavity of 
 the tooth, which is frequently laid open by decay of its 
 external wall. The fang of the tooth, which is the part 
 last formed, receives an envelope of cementum ( 54), which 
 invests it up to the part at which the enamel begins. As the 
 
 Fig. 90. DENTAL 
 CAPSULE. 
 
DEVELOPMENT OF TEETH. 167 
 
 root of the tooth is developed, the crown is gradually pushed 
 upwards, so as to press against the upper portion of the 
 capsule and the gum by which this is covered. These 
 parts yield slowly to the pressure ; and the tooth makes its 
 way to the surface ; or, in common language, is cut. 
 
 175. The process of "cutting teeth" is usually not a severe 
 one in the healthy and well-managed infant ; but it occasions 
 the death of vast numbers of children who are injudiciously 
 treated ; and it is especially fatal to those who have a ten- 
 dency to disease of the nervous system. The irritation 
 caused by the pressure of the tooth against the gum, is 
 liable to excite, in such cases, convulsive actions of various 
 kinds, on the principles hereafter to be explained ( 473) ; 
 and, as the removal of the source of irritation is of the 
 most urgent importance, the lancing of the gums, doing 
 that in an instant which the pressure of the tooth might not 
 accomplish for days, is a measure of most obvious utility ; 
 however unnecessary it may seem, in ordinary cases, to in- 
 terfere with the course of nature. But it is of the utmost 
 importance at the same time to bring the nervous system into 
 a less excitable condition ; and no measure is commonly more 
 efficacious in this respect, than removal into a fresh and pure 
 atmosphere. 
 
 176. At the same time that the development of the tooth 
 is thus taking place, the bone of the jaw is becoming hardened, 
 and closes round its root, forming a complete socket. This 
 partly interrupts the passage of vessels and nerves to the 
 tooth, which, when once fully formed, seems to acquire no 
 further growth, and to possess but little power of repairing 
 injuries occasioned by disease or accident. Hence a tooth 
 which is broken or decayed, is not restored as a bone would 
 be. Still, however, its root or fang is penetrated by a small 
 nerve and artery, which are distributed to the membrane 
 that lines the cavity ; and it is to the action of air upon the 
 former, when the cavity is laid open by decay, that the pain 
 of tooth-ache is chiefly due. The remedies which are most 
 effectual in removing this pain, such as kreosote, nitric acid, 
 or a heated wire, are those which destroy the vital power of 
 the nerve. 
 
 177. But there are teeth, in many animals, which never 
 cease to grow, and in which the central cavity is always filled 
 
168 TEETH OF RODENTS. MOTION OF JAWS. 
 
 with pulp. Such have no proper root ; for additional matter 
 is being continually formed at their base, and thus the whole 
 tooth is pushed upwards. This is the case with the Elephant's 
 tusks ; and also with the large teeth that occupy the front of 
 the jaw in Rabbits, Squirrels, Rats, and other gnawing ani- 
 mals (fig. 91). The 
 upper edges of these 
 teeth are being con- 
 stantly worn away by 
 use : and they are 
 kept up to their 
 proper level by the 
 growth of the tooth 
 
 Fig. 91. JAW AND TEETH OF RABBIT. from below. But it 
 
 sometimes happens that one of these teeth is broken off ; and 
 the one opposite to it in the other jaw is then thrown into dis- 
 use. It continues, however, to grow up from below; but, 
 not being worn down at the top, its length increases greatly, 
 so that it may become a source of great inconvenience to the 
 animal 
 
 178. The teeth are but passive instruments in the act of 
 mastication. They are put in movement by the jaws in which 
 they are fixed ; and these are made to act against each other 
 by various muscles. The upper jaw is usually fixed to the 
 head; and has not, therefore, any power of moving inde- 
 pendently of it. But the lower jaw is connected with the 
 skull by a regular joint on either side ; and is so moved by 
 the muscles attached to it, as to cut, crush, or grind the food, 
 according to the nature of the teeth. 
 
 179. There is considerable variety, in different animals, as 
 to the extent of motion which the lower jaw possesses. In 
 the purely Carnivorous quadrupeds, it has merely a hinge-like 
 action, that of opening and shutting ; and by the sharpness of 
 the edges of the molar teeth, it is thus rendered a powerful 
 cutting instrument. But in the Herbivorous animals, which 
 have to grind or triturate their food 1 ., 'tween the roughened 
 surfaces of their molars, such a limited motion would be of 
 no avail ; and we accordingly notice, if we watch an ox or a 
 horse whilst masticating its food, that the lower jaw has con- 
 siderable power of motion from side to side. On the other 
 hand, in the Rodents, or gnawing animals furnished with 
 
MOVEMENTS OF LOWER JAW. 
 
 169 
 
 two large front teeth, the lower jaw has no power of moving 
 from side to side, but is rapidly drawn backwards and for- 
 wards ; and, as the ridges of the molar teeth are arranged in 
 the opposite direction, they become very powerful filing in- 
 struments, by which the toughest vegetable substances are 
 quickly reduced. 
 
 180. In the Human jaw, there is a moderate power of 
 motion in all these different directions ; and it is furnished 
 with all the muscles by which they are effected in the 
 different animals that perform them ; but these are not so 
 large or strong. The most powerful of the muscles of the 
 lower jaw, in all animals, is that by which it is drawn up 
 against the upper, so as to close the mouth. This arises from 
 the side of the skull in the region of the temple, and is hence 
 called the temporal muscle. It covers at its origin a large 
 surface of bone ; but its fibres approach one another as they 
 descend, and pass under a bony arch (which may be felt 
 between the cheek and the ear), to attach themselves to 
 a process or projection of the 
 
 lower jaw (a, fig. 92), about 
 an inch in front of the joint. 
 As the distance from the ful- 
 crum of the point a, at which the 
 power is applied, is thus much 
 less than that of the front oi 
 the jaw b, where chiefly the 
 resistance is encountered, the 
 power of the muscle is applied 
 at a mechanical disadvantage ; 
 and, to overcome a given resist- 
 ance, the muscle must itself be 
 several times more powerful. 
 Thus the Tiger and Lion, which 
 can lift and carry away the bodies of animals weighing several 
 hundred pounds, must possess temporal muscles that shall 
 contract with a force of two thousand, or even more. 
 
 181. In Man, as in most of the other Mammalia, there are 
 three kinds of teeth, adapted for different purposes. The 
 first terminate in a thin cutting edge, and are intended simply 
 to divide the food introduced into the mouth ; these are termed 
 incisor teeth (fig. 93). Others have more of a conical form,. 
 
 Fig. 92.- HUMAN SKULL. 
 
170 
 
 DIFFERENT KINDS OF TEETH. 
 
 and in many animals (especially those of carnivorous habits) 
 project far beyond the former ; they are adapted not to cut 
 the food, but, by being deeply fixed in it, to enable the 
 animal to tear it asunder : these are termed canine teeth. 
 The teeth of the third kind have large irregular flattened 
 surfaces, and are adapted to bruise and grind the food ; these 
 are called molar (or mill-like) teeth. The manner in which 
 these different teeth are implanted in the jaw, varies with the 
 form of their crowns, and is in accordance with their several 
 uses. The incisors, whose action tends as much to bury 
 them in their sockets as to draw them forth, have but a single 
 root or fang of no great length. The canine teeth, on which 
 there is often considerable strain, penetrate the jaw more 
 deeply than the incisors ; especially when they are large and 
 
 Molars. Bicuspid. Canine. 
 
 Fig. 93. HUMAN TEETH. 
 
 long, as in the Cat tribe (fig. 94). And the molars, whose 
 action requires great firmness, have two, three, or even four 
 roots or fangs, which spread out from each other ; and these 
 at the same time increase the solidity of their attachment to 
 the jaw, and prevent the teeth from being forced into their 
 sockets by any amount of pressure. 
 
 182. The arrangement of the dental apparatus varies, in 
 different Mammalia, according to the nature of the aliment 
 on which they are destined to feed ; and this correspondence 
 is so exact, that the anatomist can generally determine by the 
 simple inspection of the teeth of an animal, not only the 
 nature of its food, but the general structure of the body, and 
 even its ordinary habits. Thus, in those that feed exclusively 
 on animal flesh, the molar teeth are so compressed as to form 
 
DIFFERENT KINDS OF TEETH. 
 
 171 
 
 cutting edges, which work against each other like the blades 
 of a pair of scissors (fig. 94) ; whilst in animals that live on 
 insects, these teeth are raised into conical points, which lock 
 
 Fig. 05. 
 Fig. 94. TEETH OF CARNIVOROUS ANIMAL TEETH OF INSECTIVOROUS ANIMAL. 
 
 into corresponding depressions in the teeth of the opposite 
 jaw (fig. 95). When the nourishment of the animal con- 
 sists principally of soft fruits, these teeth are simply raised 
 into rounded elevations (figs. 97, 98) and when they are 
 
 Fig. 96. 
 TEETH OF HERBIVOROUS AMMAL. Fig. 97. TEETH OF FRUGIVOROUS ANIMAL. 
 
 destined to grind harder vegetable substances, they are termi- 
 nated by a large flat and roughened surface (figs. 96, 99). 
 The roughness of this surface is maintained by the peculiar 
 arrangement of the three substances of which the tooth is 
 composed. The enamel, instead of covering its crown, is 
 arranged in tipright plates, which are dispersed through the 
 tooth ; and the space between them is filled up by plates of 
 ivory and of cementum ( 54). These last, being softer than 
 the enamel, are worn down the soonest ; and thus the plates 
 of enamel are left constantly projecting, so as to form a rough 
 surface admirably adapted to the grinding action which the 
 tooth is destined to perform. The mode in which these 
 plates are disposed, affords a most characteristic distinction 
 between the two species of Elephant at present existing, 
 
172 
 
 DIFFERENT KINDS OF TEETH. 
 
 namely, the African and the Indian ; as also between each 
 of these and the great extinct species known as the Mam- 
 moth (fig, 99). In the great gnawing teeth of the Kabbit, 
 
 Fig. 98. MOLAR TOOTH OF MASTODON. 
 
 1. 2. 3. 
 
 Fig. 99. MOLAR TEETH OF ELEPHANTS. 
 
 1, African Elephant; 2, Indian Elephant ; 
 
 3, Mammoth. 
 
 &c., the front surface only is covered with enamel ; and as 
 this is worn away more slowly than the ivory, it stands up as 
 a sharp edge (fig. 91), which is always retained, however 
 much the tooth may be worn away. 
 
 183. Of all the teeth, the molars may be regarded as the 
 most useful. They are seldom absent in the Mammalia ; and 
 their office is usually essential to the proper digestion of the 
 food. Animal flesh (the most easily digested of all substances) 
 needs but to be cut in small pieces ; but the hard envelopes 
 
 of beetles and other insects 
 must be broken up ; and the 
 tough woody structure of the 
 grasses, and the dense coverings 
 of the seeds and fruits on which 
 the herbivorous animals are 
 supported, must be ground 
 down. The incisors and canines 
 Fig. IOO.-SKULL OF BOAR. are ^fly employed among 
 
 Carnivorous animals for the purpose of seizing their living 
 prey, and are never deficient in them ; but they are less re- 
 quired in Herbivorous animals ; and either or both kinds are 
 not unfrequently deficient. Sometimes, however, they are not 
 
SUCCESSION OP TEETH. WHALEBONE. 173 
 
 only present in the latter, but are largely developed, serving as 
 weapons of attack and defence ; as in the Boar (rig. 100). 
 
 184. In the Mammalia in general, as in Man, the teeth are 
 not much developed at the time of birth, that they may not 
 interfere with the act of sucking; and they do not make 
 their appearance above the gum, until the time approaches 
 when the young animal has to prepare its own food, instead 
 of simply receiving that which has been prepared by its 
 parent. The teeth which are first formed are destined to be 
 shed after a certain period, and to be replaced by others. 
 They are called milk-teeth ; and in Man they are twenty in 
 number, namely, four incisors in the front of each jaw, and 
 two canines and four molars on each side. These begin to fall 
 out at about the age of seven years ; previously to which, 
 however, the first of the permanent molars appears above the 
 gum, behind those of the first set. The incisors and canines 
 of the first set are replaced by incisors and canines respec- 
 tively ; but the molars of the first set are replaced by teeth 
 like small molars, having only two fangs ; these are called 
 false molars, or, more properly, bicuspid teeth (fig. 93). The 
 second of the true molars does not make its appearance until 
 all the milk-teeth have been shed ; since it is only then that 
 the jaw becomes long enough to hold any additional teeth. 
 The third does not usually come up until the growth of the 
 jaw is completed ; and as this time corresponds with that at 
 which the mind as well as the body is matured, they are 
 commonly known as wise or wisdom teeth. There are then 
 thirty-two teeth in all, or sixteen in each jaw ; namely, four 
 incisors, two canines, four bicuspid, and six true molars. In 
 extreme old age, these teeth fall out like those of the first 
 set ; but they are not replaced by others, and their sockets 
 are gradually obliterated. 
 
 185. There are a few Mammalia which do not possess teeth. 
 This is the case with the common Whale, in which they are 
 replaced by an entirely different structure. From the upper 
 jaw (fig. 102) there hang down into the mouth a number of 
 plates of a fibrous substance (fig. 101), to which we give the 
 name of whalebone, though it is really analogous to the gum 
 of other animals. The fibres of these plates are separate at 
 their free extremities, and are matted (as it were) together, so 
 as to form a kind of sieve. Through this sieve the Whale 
 
174 ABSENCE OF TEETH IN WHALE, ANT-EATEB, ETC. 
 
 draws water in enormous quantities, whenever it is in want 
 of food ; and in this manner it strains out, as it were, the 
 minute gelatinous animals upon which it lives, from the water 
 of the seas it inhabits. The water thus taken in is expelled 
 from the nostrils or blow-holes, which are situated at the top 
 
 Fig. 102. SKULL OF WHALE. 
 
 Fig. 101. WHALEBONE. 
 
 of the head. Most of the Whale tribe have short fringes of 
 this kind in the roof of the mouth ; but in none, except the 
 Balcena, or Greenland Whale, is it long enough to make it 
 worth separating ; all the other species having teeth, either in 
 one or both jaws. It is a curious fact, that the rudiments of 
 teeth may be discovered in both jaws of the young Greenland 
 whale, although they are never to be developed. And the 
 rudiments of incisor teeth in the upper jaw, and of canine 
 teeth in both jaws, may also be discovered in the young of the 
 Ruminant quadrupeds (oxen, sheep, &c.), though they never 
 show themselves above the gum. 
 
 186. The Ant-eaters, also, are destitute of teeth, and usually 
 obtain their food by means of their long extensible tongues, 
 
 which are covered with a viscid 
 saliva; this being pushed into 
 the midst of an ant-hill, and 
 then drawn into the mouth, 
 brings into it a large number 
 
 Fig. 103. SKULL OF THE ANT-EATER. 5? . , , . , 
 
 of these insects, which are 
 
 sufficiently bruised between the toothless jaws (fig. 103). 
 Lastly, may be mentioned as a curious exception to the general 
 rules respecting the teeth of Mammalia, the remarkable Orni- 
 thorhyncus of New Holland (ZOOLOGY, 317), which feeds, 
 
TEETH OF KEPTILES AND FISHES. 
 
 175 
 
 like the duck, upon the water-insects, shell-fish, and aquatic 
 plants, that it obtains from the mud, into which it is continu- 
 ally plunging its singular bill; and its jaws, entirely destitute 
 of teeth, are famished with horny ridges, by which it can in 
 some degree masticate its food. 
 
 187. Among Birds, there is an entire absence of teeth; 
 and the mechanical division and the reduction of food is per- 
 formed in the stomach, in the manner hereafter to be men- 
 tioned ( 200). The mouths of almost all Reptiles, excepting 
 the Turtle tribe, are furnished with numerous teeth (fig. 
 104) ; but these are not 
 adapted for much variety of 
 purposes, being principally 
 destined to prevent the 
 escape of the prey which 
 the animals have secured ; 
 and their shape is conse- 
 quently nearly uniform, being for the most part simply 
 conical. There are some Lizards, however, which are herbivo- 
 rous ; and these have large rough teeth, somewhat resembling 
 the molars of Mammalia. The Iguanodon, an animal of this 
 tribe, attained a gigantic size in past ages of the world. 
 
 188. In Fishes, the teeth are commonly very numerous (fig. 
 105), but they have for their object only to separate and retain 
 
 Fig. 104. HEAD OF GAVIAL. (Crocodile 
 of the Ganges.) 
 
 Fig. 105. HEAD OF SHARK. 
 
 their food ; and there is little variety in their form. Fre- 
 quently they have no bony attachment, being only held by 
 the gum, as in the Shark ; and they are consequently often 
 torn away, but they are as readily replaced. Sometimes, bow- 
 
176 MASTICATING INSTRUMENTS OF INVERTEBRATA. 
 
 ever, the tooth seems like a continuation of the bone of the 
 jaw, not being in any way separated from it, and the tubular 
 structure of the latter being continued into it without any 
 interruption. The teeth of fishes are often set, not only upon 
 the proper jaw-bones, but upon the surface of the palate, and 
 even in the pharynx or swallow. 
 
 189. In the Invertebrata there are generally no proper 
 teeth ; in the Articulated and sometimes in the Molluscous 
 series, however, we meet with firm horny jaws, which are 
 often furnished with projections that answer the same pur- 
 pose ; and in most Gasteropods we find a very curious organ, 
 commonly designated as the tongue, more correctly the 
 palate, the surface of which is beset with innumerable tooth- 
 like points (fig. 106), by whose rasping action the food is 
 reduced. These teeth present great varieties of form and 
 
 arrangement in the different 
 genera and species of this group ; 
 and these varieties appear to 
 bear some relation to the nature 
 of the food on which the animals 
 respectively live. It is remark- 
 able that in an animal so low 
 in the scale as the Echinus or 
 Sea- Urchin ( 1 19), a very com- 
 plex dental apparatus should 
 exist. This consists of five long 
 hard teeth, which surround the 
 mouth ; and these are fixed in 
 a framework which is worked 
 
 - 
 
 and thus serve effectually to grind down the food. 
 
 Insalivation. 
 
 190. The act of mastication is connected with another; 
 which is also of great importance in preparing for the sub- 
 sequent process of digestion. This is the blending of the 
 saliva with the food, during its reduction between the teeth, 
 an act which is termed insalivation. The saliva is separated 
 from the blood, by glands which are situated in the neigh- 
 
SECRETION OF SALIVA, AND ITS USES. 177 
 
 bourhood of the mouth ; of these there are three pair in Man, 
 two beneath the tongue (fig. 107), and one in the cheek, each 
 pouring-in its secretion by a separate canal. The salivary fluid 
 is principally composed of water, in which a small quantity 
 of animal matter and some saline substances (chiefly common 
 salt) are dissolved ; the whole amount of these, however, is 
 not more than 1 part in 100. The secretion of saliva is not 
 constantly going on ; but the fluid is formed as it is wanted. 
 The stimulus by which the gland is set in action may be simply 
 the motion of the jaws ; thus, on first waking in the morning, 
 the mouth is usually dry, but it is soon rendered moist by the 
 movements which take place in speaking. The contact of 
 solid substances with the membrane lining the mouth appears- 
 also to excite the flow; hence dryness of the mouth may 
 often be remedied for a time, when no water is at hand, 
 by taking a pebble into its interior, and moving this from 
 side to side. There are certain substances, however, whose- 
 presence in the mouth has a special influence in provoking 
 an increased secretion of saliva ; and every one knows, too, 
 that the simple idea of savoury food will excite an increased 
 flow, making the " mouth water " as it is popularly termed. 
 These are instances of the power of the nervous system, 
 through which such impressions are conveyed, over the act of 
 secretion. 
 
 191. In the case of farinaceous or starchy food, the admix- 
 ture of saliva occasions the commencement of that chemical 
 change in which its digestion consists, namely, its conversion 
 into sugar ; but in general, the benefit derived from this pro- 
 cess of insalivation is just that which is obtained by the 
 chemist, when he bruises in a mortar, with a small quantity 
 of fluid, the substances he is about to dissolve in a larger 
 amount of the same. If the preliminary operations of masti- 
 cation and insalivation be neglected, the stomach has to do the 
 whole of the work of preparation, as well as to accomplish 
 the digestion ; thus more is thrown upon it than it is adapted 
 to bear ; it becomes over- worked, and manifests its fatigue by 
 not being able to discharge even its own proper duty. Thus 
 the digestive function is seriously impaired, and the general 
 health becomes deranged in consequence. A malady of this 
 kind is very prevalent in the United States ; and is almost 
 universally attributed by medical men, in part at least, to the 
 
 N 
 
178 
 
 DEGLUTITION OR SWALLOWING. 
 
 general habit of very rapidly eating or rather " bolting " the 
 meals. There is another evil attendant on this practice, that 
 much more food is swallowed than is necessary to supply the 
 wants of the system ; for the sense of hunger is not so readily 
 abated by food which has not been prepared for digestion ; 
 and thus the feeling of satiety is not produced, until the 
 stomach has already received a larger supply than it is well 
 able to dispose of. Imperfect mastication of the food is very 
 apt to occur, in persons who are losing their teeth by old age 
 or decay; and where these are not replaced by artificial means, 
 the next best remedy is to cut the food into very small por- 
 tions, before it is taken into the mouth, and to masticate it 
 there as thoroughly as possible. 
 
 Deglutition. 
 
 192. In the Mammalia, the cavity of the mouth is guarded 
 behind by a sort of moveable curtain, which is known as the 
 veil of the palate (fig. 107); and this hangs down during 
 
 Veil of the palate 
 
 Nose 
 
 Pharynx 
 
 Salivary glands 
 Os hyoides 
 
 Larynx 
 Thyroid gland 
 
 (Esophagus 
 
 mm ma 
 
 Trachea 
 Fig. 107. PERPENDICULAR SECTION OF THE MOUTH AXD THROAT. 
 
 mastication, in such a manner as to prevent any of the food 
 from passing backwards. This partition, which does not exist 
 
DEGLUTITION OB SWALLOWING. 179 
 
 in Birds and other animals that do not masticate their food, 
 hangs from the arch and sides of the palate, so as to touch 
 the tongue by its lower border ; but it can be lifted in such a 
 manner as to give the food free passage beneath it, into the 
 top of the gullet. When mastication is completed, the food 
 is collected on the back of the tongue into a kind of ball ; 
 and this, being carried backwards by the action of its muscles, 
 presses against the partition just mentioned, and causes it to 
 open. The food thus passes into a sort of funnel, formed by 
 the expansion of the top of the oesophagus or gullet; this 
 cavity, termed the pharynx, communicates above with the 
 nostrils, and in front with the larynx, which is at the top of 
 the trachea or windpipe. The oesophagus is a long and narrow 
 tube, which descends from the pharynx to the stomach, lying 
 just in front of the vertebral column, and behind the heart 
 and lungs. It is surrounded by muscular fibres, disposed in 
 various ways ; by the action of which the food that has once 
 passed into the pharynx is propelled downwards to the 
 stomach. 
 
 193. But in order to reach this tube, the alimentary ball 
 must pass over the glottis or aperture of the larynx. With 
 a view to prevent its falling-in, the larynx is drawn, in the 
 very act of swallowing, beneath the base of the tongue ; and 
 this action presses down a little valve-like flap, the epiglottis, 
 upon the aperture, so as in general effectually to prevent any 
 solid or fluid particles from entering it. But it sometimes 
 happens that, if the breath be drawn-in at the moment 01 
 swallowing, a small particle of the food, or a drop of fluid, is 
 drawn into the glottis ; and this action (commonly termed 
 "passing the wrong way,") excites a violent coughing, the 
 object of which is to drive up the particle, and to prevent it 
 from finding its way into the lower part of the windpipe. It 
 may also happen that a larger substance may slip backwards, 
 by its own weight, into the glottis, when there was no 
 intention of swallowing, and when the larynx was conse- 
 quently not drawn forwards beneath the tongue. The presence 
 of such a substance in the windpipe excites a violent and fre- 
 quently almost suffocating cough ( 342) ; the effect of which 
 is sometimes to drive it up through the glottis, and thus to 
 get rid of the source of irritation. 
 
 194. The act of swallowing is itself involuntary, and may 
 
 N2 
 
180 MOVEMENTS OF DEGLUTITION. 
 
 be even made to take place against the will. This may seem 
 contrary to every one's daily experience ; but it is nevertheless 
 true. The movement by which the food is carried back, 
 beneath the arch of the palate, into the pharynx, is effected by 
 the will ; but when the food has arrived there, it is laid hold of, 
 as it were, by the muscles of the pharynx, and is then carried 
 down involuntarily. It has several times happened, that a 
 feather, with which the back of the mouth was being tickled 
 to excite vomiting, having been introduced rather too far, has 
 been thus grasped by the pharynx, and has been swallowed. 
 Moreover, we cannot perform the act of swallowing, without 
 carrying something backwards upon the tongue ; and it is the 
 contact of this something, even if it be only a little saliva, 
 with the membrane lining the pharynx, that produces the 
 muscular movement in question. 
 
 195. This action is one of the kind now denominated reflex 
 ( 430). It is produced through the nervous system ; for if 
 the nerves supplying the part be divided, it will not take 
 place. But it does not depend upon the Brain ; for it may 
 be performed after the brain has been removed, or when its 
 power has been destroyed by a blow. It is caused by the 
 conveyance to the top of the Spinal Cord, of the impression 
 made on the lining of the pharynx ; this impression, brought 
 thither through one set of nerves, excites in the spinal cord a 
 motor impulse; which, being transmitted thence through 
 another set of nerves, calls the muscles into action. 
 
 196. This action is, therefore, necessarily connected with 
 the impression, so long as this portion of the spinal cord, and 
 the nerves proceeding from it, are capable of performing their 
 functions : and it is one of those to which we may give the 
 name of instinctive, to distinguish it from those which are 
 effected by an effort of the Will, intentionally directed to 
 accomplish a certain purpose. It may even take place without 
 the animal being aware of the contact of any substance to be 
 swallowed with the lining of the pharynx ; for there is good 
 reason to believe that when the brain has been destroyed, or 
 paralyzed by a blow, all sensibility is destroyed ; and we have 
 also sufficient reason to consider it as suspended in profound 
 sleep or apoplexy, in which states swallowing is still per- 
 formed. In the severest cases of apoplexy, however, the 
 power of swallowing is lost ; and this is a symptom of great 
 
DEGLUTITION DIGESTIVE APPARATUS. 
 
 181 
 
 danger, since it shows that not the brain alone, but the upper 
 part of the spinal cord, is suffering from the pressure ; and 
 that the movements of respiration, which depend upon a 
 similar action of the nervous system ( 340), will probably 
 soon cease, so that death must ensue. 
 
 Digestive Apparatus. 
 
 197. The food, thus propelled downwards by the action of 
 the muscles of the pharynx and of the oesophagus (gullet), 
 
 Large Intestine 
 
 Spleen 
 
 .----Colon 
 
 - Small Intestine 
 - - Colon 
 
 Small Intestine Rectum 
 
 Fig. 108. DIGESTIVE APPARATUS op MAH. 
 
 arrives, in Man and the Mammalia, at the stomach ; which is 
 a large membranous bag, placed across the upper part of the 
 
182 FORM OF THE STOMACH. 
 
 abdomen (fig. 108). The form of this stomach varies much, 
 according to the nature of the aliment to be digested. Where 
 the food is animal flesh, which is easily dissolved, the stomach 
 is small, and appears like a mere enlargement of the alimentary 
 tube this is the case in the Cat tribe, for example. In Her- 
 bivorous animals, on the contrary, the stomach is very large, 
 the food being delayed there a long time on account of the 
 difficulty with which it is digested ; and the principal part of 
 its cavity is not a simple enlargement of the alimentary tube, 
 but a bag or sac that bulges out, as it were, on the left side of 
 that canal. By the degree of this bulging, we can judge of 
 the nature of the food on which the animal is destined to 
 live. Thus in Man (fig. 108), the large end of the stomach, 
 situated on the left side (the right side of the figure as we 
 look at it), is moderately developed; showing, as we might 
 expect from the form of his teeth, as well as from his natural 
 tastes, that he is adapted for a diet in which animal and 
 vegetable food are mixed. In the purely carnivorous tribes, 
 this large end of the stomach is almost deficient ; whilst in 
 the herbivorous races, it is enormously developed, and some- 
 times forms a distinct pouch. 
 
 (Esophagus 
 
 Intestine ^,^_^^^^mi 
 
 Pylorus 4thStom. 2d Stom. 1st Stom. 
 Fig. 109. STOMACHS OF THE SHEEP. 
 
 198. The most complex form of the stomach among Mam- 
 mals, is that which we find in the animals that ruminate or 
 chew the cud. It possesses, in fact, no less than four distinct 
 cavities, through all of which the food has to pass during the 
 
STOMACH OF RUMINANTS. 183 
 
 process of digestion. The external appearance of the stomach 
 of the Sheep is seen in fig. 109 ; and its interior is displayed 
 in fig. 110. The food of the Kuminant animals is not 
 chewed by them before it is first swallowed. In their wild 
 state, they are peculiarly exposed to the attacks of their car- 
 nivorous enemies, when they come down from their rocky 
 heights to browse upon the rich pastures of the valleys. If 
 they were then obliged to masticate every mouthful, they 
 would be subjected to long-continued danger at every meal ; 
 but, by the curious construction of the digestive apparatus, 
 this is spared to them ; for they are enabled to swallow their 
 food as fast as they can crop it, and afterwards to return it to 
 their mouths, so as to masticate it at their leisure, when they 
 have retreated to a place of safety. The crude unmasticated 
 food, which is brought-down by the oesophagus, first enters the 
 large cavity on the left side, which is commonly termed the 
 paunch. It is there soaked, as it were, in the fluid secreted 
 
 Heed 
 
 Intestine Honeycomb Paunch 
 
 Fig. 110. SECTION OF THE STOMACHS OF THE SHEEP. 
 
 by its walls ; and is then transmitted to the second cavity, 
 which, from the sort of network produced by the irregular 
 folding of its lining membrane, is called the reticulum or 
 honey-comb stomach. This stomach also has a direct commu- 
 nication with the oesophagus, and appears destined especially 
 to receive the fluid that is swallowed; for this passes im- 
 mediately into it, without going into the first stomach at all. 
 The folds of its lining membrane present a large surface, 
 through which fluid may be absorbed into the system. It is 
 
184 ACT OF RUMINATION. 
 
 here that we find the curious arrangement of water-cells in the 
 stomach of the Camel, by which that animal is enabled to 
 retain a supply of water for several days. These cells corre- 
 spond with the little pits which are seen in the honey-comb 
 stomach of the Sheep, but are much deeper, and their orifices 
 may be closed by the action of a set of muscular fibres which 
 pass in every direction round each, so as to form a net-work 
 including these orifices in its meshes. 
 
 199. After the food has been macerated in the fluids of the 
 first and second stomachs, it is returned to the mouth by 
 a reversed peristaltic action of the oesophagus, which brings 
 it up as a succession of globular pellets, that are formed by 
 compression in a sort of mould at the lower end of the oeso- 
 phagus. These pellets are subjected within the mouth to 
 mastication and insalivation ; and the food is then ready for 
 the real process of digestion. It is this mastication which is 
 commonly known as the " chewing of the cud ; " and the 
 animal, whilst performing it, seems the very picture of placid 
 enjoyment. When again swallowed, the food is directed, by 
 a peculiar valvular groove at the bottom of the cesophagus, 
 into the third stomach, commonly termed the manyplies, 
 from the peculiar manner in which its lining membrane is 
 arranged. This presents a number of folds, lying nearly close 
 to one another, like the leaves of a book, but all directed, by 
 their free edges, towards the centre of the tube, a narrow 
 fold intervening between each pair of broad ones. The food 
 has, therefore, to pass over a large surface, before it can reach 
 the outlet of the cavity ; and this leads to the fourth stomach, 
 commonly termed the reed. This is the seat of the true digestive 
 process, the gastric juice ( 204) being formed here only ; 
 and it is from this that the rennet is taken, which is used 
 in making cheese to cause the milk to coagulate or curdle. 
 In the sucking animal, the milk passes directly into this 
 fourth stomach, without entering either the first or second 
 stomachs, and without being delayed in the third, the folds 
 of which adhere together so as to form a narrow undivided 
 tube. The paunch is at that time comparatively small, being 
 of less size than the reed; and its dimensions increase, as 
 soon as the young animal begins to distend it by swallowing 
 solid vegetable matter. 
 
 200 In the digestive apparatus of Birds, we find a con- 
 
DIGESTIVE APPARATUS OF BIRDS. 
 
 185 
 
 siderable modification of form, resulting from the fact that, as 
 these animals do not masticate their food, they require some 
 
 CEsophagus 
 
 Ventriculus\ 
 Succenturiatusj 
 
 Gizzard 
 
 Pancreas 
 
 Duodenum 
 
 Coeca 
 
 Large Intestine 
 
 Ureter 
 
 Oviduct 
 
 Cloaca 
 Anus 
 
 Fig. 111. DIGESTIVE APPARATUS OF FOWL. 
 
 other means of reducing it. This means is provided for them 
 in their stomach. In the tribes whose food is of such a 
 nature as to require being moistened before it is rubbed down, 
 and especially in those which feed upon grains, the oesopha- 
 gus has a pouch-like dilatation, termed the crop or craw 
 \&g. Ill); in this it is retained, and exposed to the action 
 
186 TRITURATING ACTION OF GIZZARD. 
 
 of fluid secreted by its walls, just as it is in the paunch of 
 ruminant quadrupeds. This crop is of enormous size in some 
 of the granivorous (grain-eating) birds, such as the Turkey. 
 The second stomach (or ventriculus succenturiatus) is the one 
 in which the gastric juice is secreted; but this is seldom 
 large enough to retain the food, which passes-on through it 
 to the gizzard, a hollow muscle, furnished with a hard tendi- 
 nous lining. In the granivorous birds this is extremely 
 strong and thick ; and pieces of gravel are swallowed by 
 them, which, being worked-up with the food by the action of 
 the gizzard, assist in its reduction. In the rapacious flesh- or 
 fish-eating birds, however, no such assistance is required, the 
 food being easy of solution ; the walls of their gizzard are 
 thin, possessing but few tendinous fibres ; and the three 
 cavities of the stomach are almost united into one. 
 
 201. Various experiments have been made to test the 
 mechanical powers of the gizzard of Birds. Balls of glass 
 which they were made to swallow with their food, were soon 
 ground to powder ; and the points of needles and of lancets, 
 fixed in a ball of lead, were blunted and broken-off by the 
 power of the gizzard, whilst its own internal coat did not 
 appear to be in the least injured. On the other hand it has 
 been ascertained, that grain enclosed in metal balls which 
 protected it from the mechanical action of the gizzard, but 
 which were perforated so as to afford the gastric fluid free 
 access to their contents, was not in the least digested ; so that 
 the utility, and even the necessity of this operation, become 
 evident. 
 
 202. As there are few animals, save the Mammalia, that 
 perform any proper masticaton in their mouths, the grinding 
 down of their food (where it is of such a nature as to require 
 it) must be performed in the stomach ; and accordingly we 
 find many tribes, belonging to different divisions of the animal 
 kingdom, in which a gizzard, or something analogous to it, 
 exists. It is possessed by almost all Cephalopods, and by 
 many of the Gasteropods. In the walls of the stomach of 
 some of these last, there is a considerable amount of mineral 
 matter deposited, intermixed with the hard tendinous fibres 
 of which they chiefly consist. A powerful gizzard is also 
 found in many Insects, but here it is placed above the diges- 
 tive stomach (fig. 112, c). The accompanying figure exhibits 
 
REDUCING APPARATUS OF INSECTS, ETC. 
 
 187 
 
 the alimentary canal of a Beetle, from its commencement to 
 its termination. At a is seen the head, bearing the jaws, &c. ; 
 from this the gullet passes straight backwards, and is dilated 
 
 into a crop at 6, below 
 which is the gizzard, c. This 
 opens at its lower end into 
 the tme digestive stomach, 
 d; which is surrounded by 
 an immense number of little 
 follicles or bags, by which 
 the secretion of the gastric 
 juice is effected ( 204). 
 Into the lower end of this, 
 the long vessels, e, open, 
 which constitute in Insects 
 ths only rudiment of a liver 
 ( 358). In many of the 
 Crustacea, the walls of the 
 stomach are beset with re- 
 gular rows of teeth, which 
 are moved by the action of 
 powerful muscles. These 
 teeth are cast or shed at the 
 same time with the shell. 
 In the W keel- Animalcules, 
 the place of the gizzard is 
 occupied by a curious pair of 
 jaws, armed with teeth by 
 the working of which, the 
 food is effectually crushed. 
 In the Bryozoa, a gizzard 
 exists between the oesopha- 
 gus and the true digestive 
 stomach ; and the stomach 
 itself is surrounded by the 
 little follicles which secrete 
 the bile, and pour it into 
 that cavity (115). 
 203. In animals which subsist exclusively on flesh, how- 
 ever, no such complicated apparatus exists. Thus in Serpents 
 (fig. 34), the stomach is but a slight dilatation of the alirnen- 
 
 Fig. 112. DIGESTIVE APPARATUS OF 
 BEETLE. 
 
188 
 
 DIGESTIVE APPAKATUS GASTRIC DIGESTION. 
 
 tary tube ; and it is not easy to say where it commences and 
 terminates. In Spiders and Scorpions, too, which live upon 
 the juices they suck from other animals, the alimentary tube 
 is very simple ; and it is scarcely dilated into a proper sto- 
 mach. And in most of the Eadiated classes, we find the 
 stomach to possess only one orifice, through which the undi- 
 gested residue of the food is cast out, as well as fresh sup- 
 plies taken in. But this stomach is not always a simple 
 bag ; thus in the Star-fish it sends prolongations into the 
 rays, the use of which is at present un- 
 determined. There are certain animals in 
 which no digestive cavity exists : their 
 sustenance being derived either from the 
 juices prepared by other animals, in 
 whose tissues or cavities they are im- 
 bedded, and being introduced by absorp- 
 tion through the whole surface, as is the 
 case in the lower Entozoa (fig. 53) ; or 
 from particles which are drawn into the 
 midst of the soft gelatinous substance of 
 their bodies, and undergo a sort of diges 
 tion there, as is the case with the Rhizo- 
 poda ( 129). 
 
 Gastric Digestion : Chymification. 
 
 204. The food which has been re- 
 duced in the mouth by the action of the 
 teeth, or in the stomach itself by the 
 movement of its own tendinous walls, is 
 prepared for the real process of digestion; 
 by which it is converted into a fluid, 
 and thus made fit to be truly received 
 into the system, by being absorbed into 
 its vessels. The chief agent in the 
 digestive process is a fluid termed the 
 gastric juice, which is secreted or sepa- 
 as seen in a vertical section rated from the blood by a vast number 
 * C dSe^ m aJTd ^ little bags or follicles (fig. 113), im- 
 twenty diameters at B. bedded in the walls of the stomach. 
 When the cavity is empty, this fluid is secreted in very small 
 quantities ; but, like the salivary secretion, it is poured out 
 
 Fig. 113. 
 GASTRIC FOLLICLES, 
 
SENSE OF HUNGER SECRETION OP GASTRIC JUICE. 189 
 
 in abundance when the lining membrane is stimulated by 
 the contact of food, especially solid food. Only a limited 
 quantity is secreted at any one time ; and this quantity is 
 just that which is sufficient to dissolve food enough for the 
 supply of the natural wants of the system. The contact of 
 any solid substances with the interior of the stomach, is suffi- 
 cient to produce a flow of this fluid into its cavity ; but the 
 secretion soon ceases if the substance be not of an alimentary 
 nature. 
 
 205. The sense of hunger appears due to the distension of 
 the blood-vessels of the stomach, which takes place in pre- 
 paration for the secretion of the gastric fluid. This deter- 
 mination of blood towards the stomach seems to occur when- 
 ever the body needs a fresh supply of nourishment ; and it 
 ceases as soon as a sufficient amount of gastric fluid has been 
 drawn off. Hence it is, that hunger is relieved by eating ; 
 and hence it is, also, that hunger is for a time relieved by 
 taking solid substances into the stomach, even though they 
 contain no nourishing matter. It is from having experienced 
 this, that savage nations are in the habit of mixing indiges- 
 tible solid matter with the fluids that sometimes constitute 
 their principal articles of food. Thus the Kamschatdales mix 
 earth or saw-dust with the train-oil on which alone they are 
 frequently reduced to live ; and the Veddahs, or wild hunters 
 of Ceylon, mix the pounded fibres of soft or decayed wood 
 with the honey on which they feed when meat is not to be 
 had. One of them being asked the reason of the practice, 
 replied, " I cannot tell you, but I know that the belly must 
 be filled." It has been found by experiment, that soups and 
 other forms of liquid aliment are not alone fit for the support 
 of the system, even though they may contain a large amount 
 of nutritious matter ; and the medical man well knows, that 
 many persons have stomachs too weak and irritable to retain 
 " slops" (as they are commonly termed), who can yet digest 
 solid food of a simple kind. All these instances show, that 
 the contact of a solid substance with the walls of the stomach, 
 is the proper stimulus or excitement to the secretion of the 
 gastric fluid. 
 
 206. This fluid, when poured upon the food, is thoroughly 
 mixed-up with it by a peculiar movement of the walls of the 
 stomach, which is continually bringing fresh portions of the 
 
190 PROPERTIES OF GASTRIC JUICE. 
 
 alimentary mass into contact with its sides, so that the whole 
 is after a time equally 'exposed to the influence of the gastric 
 secretion. If this movement were not to take place, only the 
 outside of the mass would be digested, and the central portion 
 would remain but little affected. 
 
 207. The nature of the gastric fluid, and the mode of its 
 operation upon the food, have been studied by withdrawing 
 a portion of it from the stomach, and by observing its pro- 
 perties and actions out of the body. A sufficient quantity 
 for this purpose cannot be easily procured. Spallanzani, an 
 Italian physiologist of the last century, contrived to obtain 
 it, by causing birds and other animals to swallow sponges to 
 which pieces of thread were attached ; these, when they had 
 remained long enough in the stomach to cause a secretion of 
 the gastric juice, were drawn up again ; and the fluid they 
 had absorbed was pressed out into vessels, in which its pro- 
 perties could be examined. More recently, however, an 
 advantageous opportunity has presented itself for obtaining 
 supplies of gastric fluid in a less objectionable manner. A 
 young man, named Alexis St. Martin, received a very severe 
 wound in his left side, by the bursting of a gun ; and al- 
 though this wound laid open the cavity of his stomach, he 
 recovered his health completely, and subsequently married 
 and had a family. There remained, however, an aperture in 
 his stomach, which would not close up ; and through this 
 orifice, which was usually covered by a bandage, the contents 
 of the stomach could be drawn out. The gastric juice was 
 obtained by introducing an India-rubber tube into the sto- 
 mach when it was empty, and by moving it about within the 
 cavity ; the contact of the tube then excited the follicles to 
 secretion (on the principle already mentioned, 204) ; and 
 the fluid thus poured into the stomach was drawn off through 
 the tube. 
 
 208. The Gastric Juice is very like saliva in its appearance, 
 but it is distinctly acid to the taste ; and it is found, by 
 chemical examination, to contain a considerable quantity of 
 muriatic acid * in an uncombined state. Besides this, it con- 
 tains a considerable quantity of a peculiar animal substance 
 which seems like altered albumen, and which has been desig- 
 nated pepsin; as well as other ingredients of less importance. 
 
 * Muriatic acid is commonly known as spirit of salt. 
 
ACTION OF GASTRIC JUICE. 191 
 
 This fluid possesses the power of dissolving albuminous sub- 
 stances of various kinds, when these are submitted to its 
 action at the constant temperature of 100 (which is about 
 that of the stomach), and are frequently shaken-up with it. 
 The solution appears to be in all respects as perfect as that 
 which naturally takes place in the stomach, but requires a 
 longer time. It does not seem, however, that the gastric juice 
 has a special solvent power for any other than albuminous 
 substances. Gelatinous and saccharine matters are taken-up 
 by it, as by other watery fluids ; but neither starchy nor 
 oleaginous substances undergo any other change by its action, 
 than consists in the separation of their particles by the solu- 
 tion of the membranes and fibres which held them together. 
 There is every reason to believe that what is true of artificial 
 is true of natural digestion ; and that so far from the whole 
 operation being performed in the stomach, as was formerly 
 supposed, gastric digestion is limited to the solution of the 
 albuminous, gelatinous, and saccharine constituents of the 
 food. 
 
 209. With regard to the precise mode in which the gastric 
 fluid acts in dissolving albuminous substances, there is yet 
 some uncertainty ; although there can be no longer any rea- 
 sonable doubt, that the operation is of a purely chemical 
 nature. An artificial gastric fluid, capable of effecting all 
 that can be done by that which is secreted in the living 
 stomach, may be made, by macerating (or soaking) a portion 
 of the membrane lining the stomach of a pig, or of the fourth 
 stomach of a calf (even after it has been washed and dried) 
 in water, which dissolves a portion of the pepsin ; and by 
 then acidulating this solution with muriatic or acetic acid. 
 It has been proved that both the acid and the pepsin are 
 essential to the process of solution ; for the acidulated fluid 
 without the animal matter acts extremely slowly upon pieces 
 of meat, hard-boiled egg, &c., submitted to it ; and water in 
 which the stomach has been macerated, but which contains 
 no acid, will not act at all. But the acidulated water alone 
 will readily dissolve the substances just mentioned, at a higher 
 temperature ; and thus it appears that the acid is the real sol- 
 vent ; and that the pepsin has for its office to produce some 
 change in the albuminous substances, by which they are more 
 readily dissolved. The recent inquiries of Liebig and other 
 
192 GASTRIC DIGESTION : CHYMIFICATION. 
 
 Chemists, render it probable that this change is of the nature 
 of fermentation. 
 
 210. It is a fact of great practical importance, that a cer- 
 tain quantity of the gastric fluid can act only upon a limited 
 amount of alimentary matter ; so that, if more food be taken 
 into the stomach than the gastric fluid can dissolve, it remains 
 there undigested. Now it has been already mentioned, that 
 the quantity of the gastric fluid secreted at any one time, is 
 proportional, not to the amount of food in the stomach, but 
 to the wants of the system ; so that, if more food be swal- 
 lowed than is required to repair the waste of the body, it 
 lies for some time unchanged in the stomach, and becomes a 
 source of irritation which prevents the due discharge of its 
 functions ; and the evil goes on increasing with every addi- 
 tion to the contents of the cavity. This may not be felt by 
 the individual at the time ; but it leaves permanent effects, 
 which manifest themselves sooner or later in derangement of 
 the general health. The, habit of taking more food than is 
 really necessary, and of irritating the stomach by stimulating 
 substances or fluids (such as pepper, mustard, spirits, &c.), is 
 a fertile source of disease. The injurious effects of these are 
 manifested by the thirst which is the consequence of their 
 use, and which is a call (as it were) on the part of the stomach, 
 to prevent their irritating action by diluting them with water. 
 
 211. By the solution of its albuminous portion, and the 
 separation of its other component particles, the food is re- 
 duced in the stomach to a kind of pulp, which is termed 
 chyme. The consistence of this will of course vary accord- 
 ing to the nature of the food, and the quantity of fluid in the 
 stomach ; but in general it is grayish, semi-fluid, and uniform 
 throughout. When the food has been of a rich character, the 
 aspect of the chyme resembles that of cream ; but when the 
 food has consisted of farinaceous substances (rice, potatoes, 
 &c.), the chyme is more like gruel. At the point where the 
 stomach opens into the intestinal canal, which is called the 
 pylorus, there is a kind of valve, which permits the chyme 
 to pass as fast as it is formed, but closes against the portions 
 of the food which are yet solid and undigested ; and thus the 
 chyme escapes from the stomach in successive waves, slowly 
 at first, but afterwards more rapidly, as the digestive process 
 approaches its completion. 
 
INTESTINAL DIGESTION. 193 
 
 Intestinal Digestion; Chylification. 
 
 212. The process of digestion is by no means completed in. 
 the stomach j for much of the matter which escapes from it 
 in the chyme, is destined to undergo a further change whilst 
 passing through the intestinal canal ; especially in the her- 
 bivorous tribes, whose food, being less digestible than that of 
 the carnivorous races, requires to be longer delayed in the 
 intestinal canal, in order that it may yield up its nutritious 
 portion. Hence we find this canal of enormous extent in 
 most animals whose food is vegetable, being in the Sheep 
 about twenty-eight times the length of the body ; in the 
 purely carnivorous animals, on the other hand, it is compara- 
 tively short, being in the Lion only about three times the 
 length of the body, while in the Serpent it runs almost 
 straight from one extremity to the other ; and in animals 
 which live on a mixed diet, it is of medium length, being 
 in Man about six times as long as his body. The intes- 
 tinal tube is usually distinguished into the small and the 
 large intestine ; of which the small is the first portion, and 
 the large the second. The former, as shown in fig. 108, is 
 disposed in a convoluted or twisted manner, so that a great 
 extent of it may be packed within a small compass ; it 
 usually forms about three-fourths of the whole length of the 
 canal It is held in its place by a serous membrane termed 
 the peritoneum, which forms an immense number of folds 
 that suspend it (as it were) from the vertebral column ; but 
 these still allow it a considerable power of movement. 
 
 213. Soon after passing from the stomach into .the intes- 
 tinal canal, the food is mingled with three secretions, which 
 have an important influence on the changes it is further to 
 undergo ; these are the Bile, the Pancreatic fluid, and the In- 
 testinal juice. The two former are prepared by two large glan- 
 dular masses, the Liver and the Pancreas (or sweetbread), 
 which, in all the higher animals, are completely detached 
 from the alimentary canal, and send their secretions into it 
 through special ducts ; the latter, like the gastric juice, is 
 formed in little follicles lodged in the wall of the canal itsel 
 The peculiar matter which forms the chief solid constituent of 
 bile, is essentially a soap formed by the union of two resinoid 
 acids, with soda as a base ( 364). The composition of ttie 
 
194 BILIARY, PANCREATIC, AND INTESTINAL SECRETIONS. 
 
 pancreatic fluid closely corresponds with that of saliva, which 
 it much resembles in appearance. The intestinal juice, like 
 the gastric, is a nearly colourless, somewhat viscid fluid, con- 
 taining an organic compound not far removed from albumen ; 
 but it differs from the gastric juice in being alkaline instead 
 of acid. The relative offices of these three fluids have not 
 yet been determined with certainty ; but there appears good 
 reason to believe : (1) that the bile, by its alkalinity, neutralizes 
 the acidity which the chyme derives from the gastric juice, 
 and that this neutralization favours the metamorphosis of 
 starch into sugar, which has been almost suspended in the 
 stomach ; (2) that the bile aids the pancreatic fluid in re- 
 ducing the oleaginous particles to the condition of an emul- 
 sion, that is, in bringing them into a state of very minute 
 division, in which they remain suspended in the albuminous 
 solution ; (3) that the pancreatic fluid aids the salivary mat- 
 ter which was swallowed with the food, in the transforma- 
 tion of starch into sugar ; (4) that the intestinal juice has a 
 solvent power for albuminous substances which is scarcely 
 inferior to that of the gastric juice, with a power of converting 
 starch into sugar which is scarcely inferior to that of saliva 
 or pancreatic fluid. The fluid of the Small Intestine, com- 
 pounded of the salivary, gastric, intestinal, biliary, and pan- 
 creatic secretions, appears to possess a far greater digestive 
 power than that of the stomach, being capable of dissolving, 
 or at any rate of reducing to an absorbable condition, nutri- 
 tious substances of every class. This process goes on during 
 the passage of the alimentary mass along the small intes- 
 tine ; and the nutritious materials are progressively with- 
 drawn by absorption, partly into the blood-vessels, which 
 appear to receive whatever are in a state of perfect solution 
 ( 218), and partly into the lacteal absorbents, which take up 
 nothing but that peculiar emulsion of albumen and fatty matter 
 which is termed chyle ( 222). 
 
 214. At the extremity of the Small Intestine, there is a 
 kind of pouch, called the coecum ; which in some animals 
 seems almost like a second stomach, and which is furnished 
 with one or more little appendages, termed coeca* This is very 
 small in Man, and does not seem to perform any important 
 
 * The word ccecum is used in Anatomy to denote a tube closed at one 
 extremity. 
 
PERISTALTIC MOVEMENT DEFECATION. 195 
 
 function ; but in most herbivorous animals it is larger (as 
 in the Monkey, fig. 30) ; and it is found to secrete an acid 
 fluid, which resembles the gastric juice, and which may have 
 for its office to perform a second digestion upon the sub- 
 stances which have escaped the first. These coeca are some- 
 times very large in the intestinal canal of Birds (fig. 111). 
 From the coecum, the Large Intestine ascends as high as the 
 liver, crosses the upper part of the abdomen, and then 
 descends again, as shown in fig. 108 ; this portion is termed 
 the colon ; and it terminates in the rectum, which forms the 
 extremity of the intestinal tube. 
 
 215. The alimentary mass is propelled along the first part 
 of the intestinal canal, and the residue left after the absorp- 
 tion of the nutritive materials is carried along the continua- 
 tion of it, by the contraction of its muscular coat, producing 
 what is termed the peristaltic motion of the bowels. The fibres 
 of this muscular coat are chiefly arranged in a ring-like 
 manner around the tube j so that, when they contract, they 
 narrow the diameter of the tube. They are stimulated to 
 contract by the contact of the solid or liquid matter passing 
 through it (Chap, xn.) j and their contraction forces this matter 
 
 .onwards, into the succeeding portion of the tube. This con- 
 tracts in its turn, so as to propel its contents further ; and thus 
 the mass is gradually driven from one extremity of the canal 
 to the other. The peristaltic movement does not seem to 
 depend (as do the contractions of the muscles concerned in 
 swallowing, 195) upon the nervous system; for it will take 
 place after the intestinal tube has been completely separated 
 from the principal nervous centres ; and also after the death 
 of the animal, if this have been produced by a sudden cause. 
 Thus, if a Rabbit be killed by a smart blow at the top of the 
 neck, and the abdomen be immediately opened, the peristaltic 
 movement will be seen in vigorous action, especially if the 
 animal have eaten a full meal an hour or two previously. 
 
 Defecation. 
 
 216. In passing through the large intestine, the undigested 
 residue is still more completely deprived of the nutritive 
 matter it may contain ; and its fluid portion is absorbed, so 
 that it becomes more solid. It is allowed to accumulate in the 
 rectum, until its bulk occasions inconvenient pressure upon 
 
 o2 
 
196 
 
 DEFECATION LACTEAL ABSORPTION. 
 
 the surrounding parts ; and it is kept-in by a circular muscle 
 or sphincter, which surrounds the outlet of the alimentary 
 canal. But when the accumulation has taken place beyond 
 this amount, it excites a reflex action ( 195) in the muscles 
 that surround the abdomen; and these make pressure suf- 
 ficient to overcome the resistance of the sphincter, and to 
 force out the contents of the rectum. 
 
 Absorption of Nutritive Material. 
 
 217. We have only now to inquire into the mode, by which 
 the nutritive matter extracted from the food is taken-up from 
 the alimentary canal and applied to the nutrition of the body. 
 In all Vertebrated animals, there exists a special set of vessels 
 termed Absorbents; of which those forming one division, 
 
 Thoracic Mesenteric 
 Aorta Duct Glands 
 
 Origins of 
 Lacteal Vessels 
 
 Intestine 
 
 i .' / 
 
 Lymphatic Mesentery 
 Vessels 
 
 Fig. 114. CHYLE-VESSELS. 
 
 known as Lacteals, from the milk-like character of their con^ 
 tents, originate in the numberless villi or minute projections 
 with which the mucous membrane that lines the small intes- 
 tine is covered ( 41). During the act of digestion, the 
 
ABSORPTION BY LACTEALS AND BLOOD-VESSELS. 197 
 
 epithelium-cells, which clothe the extremity of each villus 
 (fig. 9), become distended with an opalescent fluid, the chyle 
 ( 222), which they select from the contents of the small 
 intestine ; and this is subsequently given up by them to a 
 lacteal tube, which, without any open mouth, commences in 
 the midst of each villus. The vessels which thus originate, 
 unite into minute trunks, and these again into larger ones ; and 
 these pass between the two layers of the mesentery (or fold of 
 peritoneum by which the intestines are suspended, 212) 
 towards the lower part of the spinal column : where they 
 deliver their contents into a sort of reservoir, which thus 
 becomes the receptacle for all the chyle that has been collected 
 from the alimentary canal (fig. 114). In traversing the me- 
 sentery, the lacteals of the higher animals pass through little 
 knot-like bodies of a peculiar nature, which are called mesen- 
 teric glands. These appear to afford the means for the per- 
 formance, within a more concentrated space, of the assimi- 
 lating action which is carried on during the passage of the 
 chyle through the lacteal system ; for in Reptiles, in which 
 these glands do not exist, the absorbent vessels are much 
 more extended and spread out than they are in Birds and 
 Mammals. 
 
 218. Near the surface of each of these villi, moreover, lies 
 a minute network of Blood-vessels ; and there is now no 
 longer any doubt that these receive, by simple imbibition,* 
 any substances, whether alimentary or otherwise, which exist 
 in a state of perfect solution in the contents of the intestinal 
 canal. For a great variety of such substances have been 
 detected, by chemical analysis, in the blood which is returned 
 from the walls of the intestines by the mesenteric veins ; 
 whilst it is seldom that anything is found in the lacteals, 
 save the proper constituents of chyle. It is through this 
 channel that poisonous substances are taken into the circula- 
 tion; and these may be absorbed from the walls of the 
 stomach (on which there are no villi or lacteals), without ever 
 passing from it into the intestinal tube. Hence it is a great 
 
 * That tendency called Endosmose which thinner liquids have to 
 pass-towards and mix-with such as are more viscid, even through an 
 intervening membrane, seems to be the physical cause (as experi- 
 ment indicates) of this imbibition ; which is greatly promoted by the 
 movement of blood in the vessels. 
 
198 ABSORPTION BY LYMPHATICS. 
 
 mistake to characterise the lacteals (with the lymphatics) as 
 Absorbents in any exclusive sense ; the fact being that their 
 function is limited to a special selective absorption, whilst 
 the more general action is performed by the blood-vessels. 
 
 219. But the reservoir above-mentioned receives, not only 
 the lacteal vessels that bring nutritious matter from the intes- 
 tinal tube, but also lymphatics, which are absorbent vessels 
 of similar character, that originate in every part of the body. 
 These, also, pass through a set of (so-called) glands, in their 
 way towards this receptacle ; and the structure of these glands, 
 of which many are seated in the neck, some in the arm-pit, 
 others in the groin, &c., is exactly the same as that of the 
 mesenteric glands. The fluid they convey, which resembles 
 very dilute liquor sanguinis ( 229), seems evidently destined 
 to be again applied to the purposes of nutrition. There is 
 some obscurity as to its source ; but it seems probable that 
 it may partly consist of the residual fluid, which, having 
 escaped from the blood-vessels into the tissues, and having 
 furnished the latter with the materials of their nutrition, is 
 now to be returned to the former ; and partly of those par- 
 ticles of the body, which, though they have lost their vitality 
 in the course of the change it is continually undergoing, 
 have not undergone a degree of decay that unfits them for 
 serving, like the dead bodies of other animals, as a material 
 for reconstruction by the organizing process. The lymphatics, 
 being copiously distributed in the true Skin, absorb substances 
 which are introduced into its tissue ; and if these substances 
 be of an irritating nature, they may occasion an inflammatory 
 action in the absorbents and their glands. Thus when poisoned 
 wounds in the hand have been received, as in opening the 
 bodies of men or animals that have died of particular diseases, 
 the effect is usually manifested at first by heat and pain in the 
 arm, along which the inflamed absorbents can be traced as 
 hard cords ; and the glands in the arm-pit swell and become 
 tender. 
 
 220. The lymphatics do not appear destined, however, to 
 absorb from the surface of the skin ; this function being per- 
 formed by the blood-vessels which are distributed abundantly 
 in its substance. It is a fact now well established, that when 
 the quantity of fluid in the body has been greatly reduced, 
 absorption of water through the skin may take place to a 
 
ABSORPTION THROUGH SKIN THORACIC DUCT. 199 
 
 considerable amount. Thus there is a case recorded by Dr. 
 Currie, of a patient who suffered under obstruction of the 
 gullet, of such a kind that no nutriment, either solid or fluid, 
 could be received into the stomach ; and who was supported 
 for some weeks by immersion of his body in milk and water, 
 and by the introduction of nutritive liquids into the lower 
 end of the intestine. During this time, his weight did not 
 diminish ; and it was calculated by Dr. Currie, that from one 
 to two pints of fluid must have been daily absorbed through 
 the skin. The patient's thirst, which had been very trouble- 
 some previously to the adoption of this plan, was removed by 
 the bath, in which he experienced the most refreshing sensa- 
 tions. 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. 
 
 221. From the receptacle into which the chyle, and a con- 
 siderable proportion of the contents of the lymphatics, are 
 delivered, a tube passes upwards in front of the spine (fig. 
 114) ; and this tube, called the Thoracic Duct, conveys these 
 nutritious fluids to the point where they are to be delivered 
 into the current of blood. This delivery takes place at the 
 angle where two great veins unite, a point at which there is 
 less resistance than in any other part of their walls. These 
 veins are the Jugular, which brings the blood from the neck, 
 and the Subclavian, which conveys it from the arm, of the 
 right side (fig. 122) ; on the left side there is a smaller duct, 
 which receives some of the lymphatics of the left side, and 
 opens into the blood-vessels at a corresponding point between 
 the left jugular and subclavian veins. 
 
 Sanguification. 
 
 222. The Chyle of Vertebrated animals, as taken-up by the 
 lacteals, may be regarded as blood in an early stage of its 
 formation, with a large excess of fatty matter. It contains 
 about 90 parts of water in 100 ; about 3J parts of albumen, 
 and the same of fat ; and about 3 parts of other animal and 
 saline matter. Its appearance and characters differ, according 
 to the part of the lacteal system from which it is drawn. If 
 obtained near the surface of the intestines, before it has passed 
 
200 PROPERTIES OF CHYLE SANGUIFICATION. 
 
 through the glands, it is entirely destitute of that power of 
 spontaneously coagulating, or dotting, which is so remarkable 
 in blood : and when examined with a microscope, it is seen 
 to present a number of oily globules of various sizes ; together 
 with an immense number of very minute particles or mole- 
 cules, which also seem of a fatty nature ; and to these last, 
 whose diameter is between l-24,000th and l-36,000th of an 
 inch, the milky whiteness which characterises chyle appears 
 principally due. But the chyle drawn from the lacteals, after 
 they have passed through the mesenteric glands, possesses the 
 power of coagulating slightly ; hence it would seem that some 
 of its albumen has undergone a transformation into fibrin 
 ( 17). At the same time, a great increase is observed in 
 the number of certain floating corpuscles, which are occa- 
 sionally to be noticed in the first chyle, but which are very 
 abundant in the fluid drawn from the glands and from the 
 lacteals that have passed through them ; of these, which bear 
 a strong resemblance to the colourless corpuscles of the blood 
 ( 234), the average diameter is about 1-4, 600th of an inch. 
 By the time that the chyle reaches the central receptacle, its 
 power of coagulating has still further increased ; so that its 
 resemblance to blood, except in regard to colour, is much 
 stronger. The proportion of fibrin and albumen which it 
 contains, is much greater than that which existed in the first 
 chyle, whilst the amount of oily matter is less. 
 
 223. There can be little doubt that the change which the 
 chyle undergoes in its passage through the lacteals, is partly 
 due to the influence of the living walls of these vessels upon 
 the fluid in contact with them, and partly to that of the 
 colourless corpuscles which float in the fluid, and which form 
 the principal constituents of the absorbent glands. The whole 
 apparatus, indeed, may be looked upon as one great Assimi- 
 lating Gland, having for its function to make blood out of 
 crude nutriment ; provided-for in the higher Vertebrata by the 
 convolution of the lacteals in the mesenteric glands, and in the 
 lower, by the simple extension of the vessels themselves. It is 
 probable that, by being brought into very close neighbourhood 
 with the blood in these glands, the chyle may be made to 
 undergo some further change ; although, as each fluid is con- 
 tained in its own tubes, which do not communicate, there can 
 be no proper intermixture. 
 
ASSIMILATING GLANDS ABSORPTION IN INVERTEBRATA. 201 
 
 224. There are certain glandular bodies, disposed in various 
 parts of the system, which seem to discharge a similar office ; 
 withdrawing the raw material (so to speak) from the general 
 current of the circulation, and returning it again in a state of 
 higher elaboration. Such are the Spleen, the Thyroid and 
 Thymus glands, and the Supra-Eenal capsules. Besides these, 
 the Liver probably exerts an assimilating action upon the crude 
 materials which are made to pass through its substance, almost 
 immediately after having been received into the blood-current, 
 and before they are allowed to pass into the general circula- 
 tion ; the whole of the blood returned by the gastric and 
 mesenteric veins from the walls of the alimentary canal, being 
 conveyed through the liver by the portal system, in its way to 
 the heart ( 267). 
 
 225. In the Invertebrated animals, neither lacteals nor 
 lymphatics exist; and the blood-vessels, whose absorbent 
 powers are to a certain extent restricted in the higher animals, 
 have to perform the functions of these. There are animals, 
 however, which are destitute not only of lacteal and lymphatic 
 vessels, but even of blood-vessels ; and in these, as in the 
 Cellular Plants, there is but little transmission of fluid from one 
 part of the body to the other ; for every portion, both of the 
 internal surface (or lining of the stomach), and of the external 
 surface which is bathed in the surrounding fluid (for most of 
 these animals are aquatic), seems equally to possess the power 
 of absorption ; and the parts to whose nourishment the fluid 
 thus received into the body is to be appropriated, are in the 
 immediate neighbourhood of those which have absorbed it. 
 This is the case, for example, in the Hydra and Sea- Anemone, 
 and, more or less, in all the Polypes ; as well as in the lower 
 "Worms. Between these, therefore, and the Cellular Plants, a 
 remarkable analogy exists in regard to the mode in which the 
 nutriment is absorbed and applied ; the difference being, that 
 the Animal possesses a digestive cavity, lined by an inward 
 extension of the external surface, which does not exist in 
 Plants ( 8). And it is upon the walls of this cavity, that 
 the absorbent vessels of the higher Animals (whether lacteals 
 or blood-vessels) are distributed, collecting the nourishment 
 in contact with them ; just as the roots of a Plant, spread 
 through the soil, draw up that which it contains. But among 
 those lowest animals in which the digestive cavity altogether 
 
202 OF THE BLOOD, AND ITS CIRCULATION. 
 
 disappears ( 203), the function of absorption is not in any 
 way limited ; since every part seems to have the power of re- 
 ceiving from without, and of assimilating to its own substance, 
 the nutrient materials which it needs. 
 
 CHAPTER V. 
 
 OF THE BLOOD, AND ITS CIRCULATION. 
 
 226. The processes that have been already explained, have 
 for their object to prepare the nutritious fluid, which supplies 
 the materials for the growth of the several parts of the body, 
 and which is conveyed through them by the apparatus to be 
 presently described. In Man and the higher animals, this 
 fluid, which is known as the Blood, has a red colour, and con- 
 tains a large quantity of solid matter. The redness of the blood 
 has been mentioned as a distinctive character of the Yerte- 
 brated classes ( 75) ; it exists in Mammalia, Birds, Eeptiles, 
 and Fishes, and in these alone. In the Molluscous classes, as 
 also in most of the Articulated, the nutritious fluid is nearly 
 colourless ; and it will hereafter appear that this fluid bears, 
 in some respects, a stronger resemblance to the chyle and 
 lymph of the Yertebrata, than to their blood ( 234). There is 
 an apparent exception in the case of certain marine "Worms, 
 the fluid circulating in whose vessels has a reddish hue ; this 
 does not depend, however, upon the presence of any red par- 
 ticles, such as are characteristic of the blood of Yertebrata 
 ( 229), but upon a reddish tinge in the fluid itself, which 
 does not seem altogether to answer to the character of 
 blood ( 294). 
 
 227. The blood of all the higher animals exists in two 
 different states. When it is drawn from a slight scratch or 
 other wound of the skin, it is of a bright red hue ; whilst that 
 which is drawn in bleeding from the arm, is of a dark purple. 
 The former is termed arterial blood, because it is contained, for 
 the most part, in the tubes which are called Arteries, and 
 which are conveying it from the heart to the tissues it has to 
 nourish. The latter is called venous blood, because it is drawn 
 from the Yeins, by which it is returned from the tissues to 
 the heart, after having performed its part in them. Hence it 
 
VENOUS AND ARTERIAL BLOOD. 203 
 
 is evident that this change of character has been produced 
 during the passage of the blood through the tissues ; and so 
 important is the alteration, that the blood which has been 
 subjected to it is not fit to pass again into the arteries of the 
 body, until it has been renewed by exposure to air in the 
 Lungs. In their vessels, the contrary change of which the 
 nature will be presently explained ( 253) is effected, the 
 dark hue of venous blood giving place to the bright red of the 
 arterial fluid ; this is again changed during the passage of the 
 blood through the body, to be again restored in the lungs. 
 The same is the case in regard to Fishes, whose gills perform 
 the same function as the lungs of air-breathing Vertebrata. 
 And among the Invertebrated classes, although the deteriora- 
 tion of the blood in its passage through the body is not made 
 manifest by any change of colour, yet its renewal by exposure 
 to air in the respiratory organs is not less requisite. 
 
 228. Hence the continual movement of the blood is neces- 
 sary for two purposes in particular; -first, to convey the 
 nutritive materials from the place where they are received and 
 prepared, to that in which they are appropriated, and thus 
 to afford to every organ a constant supply of the materials 
 which it requires ; and, second, to carry this fluid, at regular 
 intervals, to certain organs by whose instrumentality it may 
 be exposed to the influence of the air, so as to regain the 
 qualities it has lost, and part with what it has taken-up to its 
 prejudice. But there are many other objects fulfilled by it, 
 which will unfold themselves as we proceed. 
 
 Properties of the Blood. 
 
 229. When the circulating blood of a red-blooded animal 
 is examined with a microscope, it is seen to consist of two 
 distinct parts ; a clear and nearly colourless fluid, to which 
 the name of liquor sanguinis (or liquor of the blood) is given ; 
 and of an immense number of rounded particles floating in 
 this fluid, which are often termed the globules of the blood. The 
 shape and size of these particles are, for the most part, very 
 uniform in animals of the same species ; but in no instance 
 are they globular ; and it is better, therefore, to term them 
 corpuscles. In Man and most other MAMMALS, they are 
 nearly flat discs, resembling pieces of money, but usually 
 exhibiting a slight depression towards the centre (fig. 115). 
 
204 BLOOD-DISCS OF MAN AND MAMMALS. 
 
 No nucleus can be distinguished in them, but they present a 
 dark central spot, which is an optical effect of their bi-concave 
 form ; and this spot may be made to disappear by the addition 
 
 T> 
 
 Fig. 115. RED CORPUSCLES OF HUMAN BLOOD. 
 
 Seen separately at A, a a showing the front view, b the profile or edge view, and * a 
 three-quarter view; at B united with each other so as to form columns likepilis 
 of money; at c in a state of alteration such as exposure to air will produca; 
 D shows a colourless corpuscle, or lymph-globule. 
 
 of water to the liquid in which they are suspended, the discs 
 first becoming flat, then bulging-out on either side, and at 
 last swelling so as to burst. The reason of this will be pre- 
 sently explained (231). In MAN and MAMMALS generally, 
 the diameter of these blood-discs varies from about l-2800th to 
 1 -4000th of an inch; but in the small Musk-deer, it is less 
 than 1-1 2,000th. In the Camel tribe, the discs are oval, as 
 in the lower Vertebrata. 
 
 230. In Birds, Reptiles, and Fishes, the blood-particles 
 present some curious differences from those of Mammalia. 
 In the first place, they are much larger ; their form, also, is 
 oval instead of being round ; and instead of being depressed 
 in the centre, they bulge-out on each side. This bulging is 
 
 Fig. 116. BLOOD CORPUSCLES OF PIGEON. 
 
 At A are seen the red corpuscles a, b, and the colourless, or lymph globules c, c; at 
 B, a red corpuscle treated with acetic acid ; and at c, the same treated with water, 
 so as to render the nucleus more distinct. 
 
 evidently occasioned by the presence of a nucleus which is 
 more solid than the rest ; the nucleus, however, is not so well 
 
BLOOD-DISCS OF BIRDS AND REPTILES. 
 
 205 
 
 seen in the corpuscles of circulating or of freshly-drawn blood, 
 as it is in that of blood which has been drawn for some little 
 time ; and it is best brought into view by treating the blood 
 either with water or with acetic acid. The long diameter of 
 the oval discs of BIRDS (fig. 116) varies from about 1-1 7 00th 
 to 1 -2400th of an inch ; and the short diameter from about 
 
 Fig. 117. BLOOD CORPUSCLES OF FROO. 
 
 At A are seen the red corpuscles a, b, and the colourless corpuscle c ; at B, a red 
 corpuscle treated with acetic acid. 
 
 l-300th to l-4800th. Thus the discs, though much longer than 
 those of Man, are not in general much broader. In REPTILES, 
 
 Fig. 118. BLOOD CORPUSCLES OF PROTEUS. 
 
 , b t red corpuscles ; a*, corpuscle showing the nucleus ; c, colourless corpuscle 
 d, red corpuscle treated with water. 
 
206 BLOOD-DISCS OF REPTILES AND FISHES. 
 
 there is considerable diversity as to the size of the discs ; but 
 the largest particles are found in the group of Amphibia, and 
 especially in those species which retain their gills through 
 life. The oval discs of Frogs (fig. 117) have a long diameter 
 of about 1-1 000th of an inch, and a transverse diameter of about 
 1-1 800th. Those of the perennibranchiate Amphibia ( 87) 
 may even be distinguished by the naked eye ; those of the 
 Siren having a long diameter of about 1 -435th of an inch, whilst 
 in the Proteus (fig. 118) the long diameter is stated occasionally 
 to reach 1 -337th of an inch. In FISHES, also, the size of the 
 blood-discs is variable ; they are 
 sometimes smaller (fig. 1 1 9), though 
 generally larger, than those of the 
 Frog ; but they never approach those 
 of the last-named remarkable ani- 
 mals. Hence the great size of the 
 Fig. ii9. BLOOD CORPUSCLES OP blood-discs of the curious Lcpido- 
 RoACH - siren (fig. 41) is strongly indicative 
 
 a, a, b, red corpuscles ; c, colour- n f xi^ PprvHlinTi flffim'tiPQ nf fhnt 
 less corpuscle ; d, red corpuscle Ol tjie -ttepttUan affinities 01 tnat 
 treated with water. Species. 
 
 231. It is by observing the large blood-discs of the Frog, 
 and still better those of the Proteus and Siren, that we can 
 obtain the best information as to their structure. They are 
 evidently fattened cells, having an envelope or cell-wall, which 
 consists of an extremely delicate membrane, and which con- 
 tains a fluid. The nucleus consists of an assemblage of minute 
 granules, which seem adherent to each other and to the wall 
 of the cell ; and it corresponds, in all essential particulars, to 
 the nuclei of the cells of other Animal tissues ( 32). The 
 fluid contained in the cells has a red colour ; and it is to this 
 that the peculiar hue of the blood of Vertebrata is owing. 
 When we are looking at a single layer of blood-discs, how- 
 ever, their red colour is not apparent, but they have rather 
 a yellowish tint; and it is only when we look through a 
 number at once, that the characteristic hue is seen. The 
 fluid is of about the same density as that in which the par- 
 ticles float; and thus neither will have a tendency to pass 
 towards the other. But, if we dilute the liquor sanguinis 
 with water, the fluid outside the cells will have a tendency 
 to pass towards their interior, according to the law of Endos- 
 mose. The cells will in consequence be first distended, and 
 
STRUCTURE AND COMPOSITION OF RED CORPUSCLES. 207 
 
 will then burst ; and their contents will be diffused through 
 the surrounding fluid, whilst their membranous walls will 
 subside to the bottom. On the other hand, if the liquor 
 sanguinis be rendered denser than the fluid in the blood- 
 discs, as by the admixture of gum or syrup, the latter will 
 pass towards it, and the cells will become still more flattened, 
 and more or less completely emptied. The flexibility and 
 elasticity of the blood-discs are well seen, in watching (with 
 a microscope) its flow through the minute vessels ; for if one 
 of them meets with an accidental obstruction to its progress, 
 its form becomes accommodated to that of the space left for 
 it to pass, and it makes its way through a very small aperture, 
 recovering its usual form immediately afterwards. 
 
 232. The Eed Corpuscles differ considerably in chemical 
 composition from the liquid in which they float. Of the 
 solid residue obtained by drying, about one-eighth is formed 
 by their cell-walls, the remainder being yielded by the cell- 
 contents. The latter portion seems to consist chiefly of a 
 mixture of two components, which have been named globulin 
 and hcematin. The former is a colourless substance, nearly 
 allied to albumen in composition, but differing from it in 
 some of its reactions ; its most characteristic peculiarity, how- 
 ever, being its power of crystallizing. Its crystals, the form 
 of which varies in different animals, are usually tinged deeply 
 with hsematin, from which they cannot easily be freed. The 
 composition of hsematin, to which alone the colour of the red 
 corpuscles (and consequently of the whole mass of the blood) 
 is due, is notably different from that of the albuminoid 
 compounds ; the proportion of carbon to the other components 
 being much greater, and a definite quantity of iron being an 
 essential part of it. This iron, in a certain state of oxidation, 
 has been supposed to be the source of the red colour; but 
 such is certainly not the case ; and this hue must be, like the 
 colours of Plants, a peculiar attribute of the organic compound 
 which presents it. Besides their globulin and haematin, the 
 red corpuscles contain a certain proportion of fatty and 
 mineral matters. The former, which are united with phos- 
 phorus, are of a kind which are scarcely traceable in the 
 liquor sanguinis ; and the latter are remarkable as having 
 potass for their principal base, whilst the base of the salts of 
 the liquor sanguinis is chiefly soda. Hence it appears that 
 
208 PROPORTION OP RED CORPUSCLES. 
 
 the Red Corpuscles draw into themselves nearly the whole 
 of the iron, phosphorus, and potass, which the chyle pours 
 into the circulating current ; and that they modify a large pro- 
 portion of the solid matter of the blood, that which they con- 
 tain being notably different in composition from that of the 
 liquor sanguinis, which does not differ, save in the proportion 
 of its components, from the liquid portion of Chyle or Lymph. 
 
 233. The proportion of Red Corpuscles to the whole mass 
 of the blood varies greatly in different animals, and even in 
 different states of the same animal. It is greatest in those 
 which have the highest muscular vigour and activity, and 
 which consume the largest quantity of oxygen by respiration ; 
 hence these particles are rather more numerous in the blood 
 of Birds than in that of Mammals, and far more abundant 
 in these last than in Reptiles or Fishes. Again, they are 
 more numerous in Men of ruddy complexion, strong pulse, 
 and active habits, than in those of pale skins, languid circu- 
 lation, and comparatively feeble powers. In a healthy Man 
 they seem to constitute ajbout half the mass of the circulating 
 blood ; but they contain as much as three-fourths of its solid 
 matter, the proportion of dry corpuscles being about 150 in 
 1000 parts of blood, whilst that of the other solid matters 
 is about 50. A very marked decrease occasionally presents 
 itself in disease ; the proportion of dry corpuscles being some- 
 times reduced as low as 27. When too abundant, they pro- 
 duce what is known as the plethoric condition of the body, 
 in which haemorrhage from the bursting of a blood-vessel is 
 liable to occur. Their number is effectually reduced by bleed- 
 ing ; and the aspect of those who have suffered from extreme 
 loss of blood, gives sufficient evidence that the deficiency is 
 not made-up for a long period. The most effectual means of 
 restoration, in cases where the proportion of blood-corpuscles 
 is too low, is a highly nutritious diet, with the administration 
 of iron as a medicine ; for this substance seems to have the 
 power of hastening the reproduction of the corpuscles, being 
 itself an essential ingredient in their contents ; and there 
 are facts which show its remarkable power of increasing their 
 amount in proportion to the mass of the blood. 
 
 234. It appears that the red corpuscles, like other cells, 
 have a certain allotted term of life ; and as they are con- 
 tinually dying, they must be as continually reproduced. The 
 
COLOURLESS CORPUSCLES USES OF RED CORPUSCLES. 209 
 
 mode in which this reproduction is effected has not yet been 
 clearly made out ; but there is strong reason to believe that the 
 red corpuscles are developed from the corpuscles of the chyle 
 and lymph ( 222) which are continually being poured into 
 the circulating current, and of which isolated examples, known 
 as the white or colourless corpuscles, are met with in every 
 drop of blood that is examined under the microscope. The 
 size of these is pretty much the same in all Vertebrata, their 
 diameter being usually about 1 -3000th of an inch. In the 
 blood of Man and the Mammalia in general (fig. 115, D) they 
 are not easily distinguished from the red particles ; their 
 diameter being nearly the same, while the colour of single 
 discs of the two kinds is not very dissimilar. But in the lower 
 Vertebrata, whose blood has large oval red particles, the differ- 
 ence between the two kinds is very obvious ; and the resem- 
 blance which the colourless globuler (c, figs. 116-119) bear to 
 those of the chyle and lymph, i? very striking. Similar colour- 
 less particles exist, to a variaole amount, in the nutritive fluid 
 of Invertebrated animals ; so that in this, as in some other 
 respects, that fluid bears a stronger resemblance to the chyle 
 and lymph of the Vertebrata, than it does to their blood, 
 which is characterised by the presence of the red particles. 
 
 235. Physiologists are now generally agreed, that one of 
 the functions of the Red Corpuscles is to convey oxygen from 
 the lungs to the tissues and organs through which the blood 
 circulates, and to bring back the carbonic acid which is set 
 free in these, so as to deliver it at the lungs. For although 
 it is certain that the liquor sanguinis can also convey these 
 gases, yet experiment shows that the red corpuscles can take 
 up, bulk for bulk, a much larger proportion of them ; and 
 that the blood which is richest in these particles is, therefore, 
 most fit to serve as the medium for the transmission between 
 the respiratory organs and the body at large. Now it is in 
 the nervo-muscular apparatus that there is the greatest demand 
 for oxygen; for this apparatus is not capable of vigorous 
 action, unless oxygen be freely supplied to it. The quantity 
 of this it requires, however, depends upon the exercise of its 
 powers ; for when at rest, it needs little or no more than is 
 made use of by the other tissues ; but whilst in activity, it 
 needs a greatly-increased supply. The quantity of oxygen 
 which the animal takes-in by its lungs, and the amount of 
 
210 USES OF EED CORPUSCLES LIQUOR SANGUINIS. 
 
 carbonic acid which, it gives-off by the same channel, vary, 
 therefore, with the muscular exertion it makes. This variation 
 is most easily observed and measured in Insects ; and it is found 
 in them to be enormous ( 308). As, however, the blood of 
 the Invertebrata does not contain these red particles, to which 
 so important a function has been assigned, it may be asked, 
 how the conveyance of oxygen to their tissues is provided 
 for. The reply is very simple. In Insects, and other ARTI- 
 CULATA which have active powers of motion, the air is con- 
 veyed to the tissues, not through the medium of the blood, 
 but directly through air-tubes which convey it to every part 
 of the body ( 321). And in the MOLLUSCOUS classes, as 
 among the Crustacea also, the nervo-muscular system forms 
 so subordinate a part of the general mass of the body, and its 
 movements are so sluggish, that the quantity of oxygen which 
 $he fluid part of the blood conveys to them, is sufficient for their 
 need. 
 
 236. Of the properties of the Liquor Sanguinis, whilst it 
 is circulating in the vessels, the microscope tells us nothing ; 
 since it constantly remains in the state of a transparent fluid. 
 But if the blood be withdrawn from the living body, it soon 
 undergoes a very curious and important change. A large 
 portion of it passes into the solid state, forming the crassa- 
 mentum or clot ; whilst there remains a transparent liquid of 
 a yellowish hue, which is termed the serum. It is evident 
 that the clot contains all the red particles ; but it is easily 
 proved that its coagulation is not due to them. For the blood 
 of a Frog, or of any other animal having blood-discs suffi- 
 ciently large, may be caused to pass through filtering-paper, 
 which will retain and collect its blood-discs, allowing the 
 liquor sanguinis to flow through it ; and this fluid will coagu- 
 late just as completely as if these particles were retained in 
 it. Again, in certain conditions of the blood (generally result- 
 ing from disease), even when the coagulation is allowed, to 
 take place in the ordinary manner, the fibrin and the red 
 particles separate from one another, the latter gradually 
 subsiding, whilst the former are left at the surface ; and the 
 upper part of the clot is then nearly colourless, exhibiting 
 what is commonly known as the huffy coat or crust ; whilst 
 the lower part of it includes the red particles, and has a very 
 deep colour. The buffy coat, being composed almost exclu- 
 
LIQUOR SANGUINIS COAGULATION. 211 
 
 sively of the fibrous network, is very firm in its texture, 
 being sometimes almost leathery in its character ; whilst the 
 lower part of the clot, which is chiefly composed of the red 
 particles, loosely bound together by scattered fibres, is very 
 soft, and easily broken asunder. This effect may be also 
 produced, by acting on healthy blood with certain substances 
 which retard its coagulation, such as a strong solution of 
 Glauber's salt ; for if sufficient time is allowed, the red par- 
 ticles will subside in consequence of their greater specific 
 gravity, leaving a colourless layer of fibrin above them. It 
 is of the liquor sanguinis, in a concentrated form, that those 
 exudations consist, which are poured out from the blood for 
 the repair of injuries, and which pass spontaneously into the 
 condition of a simple form of tissue ( 393). 
 
 237. When a very thin slice of the clot is examined with 
 a microscope, it is found to be made up of a net-work of an 
 imperfectly fibrous character, interlacing in every direction, 
 and including the blood-discs in its meshes. These fibres are 
 produced by the spontaneous change in the fibrin of the blood, 
 from the fluid to the solid form. So long as the blood is 
 circulating in the vessels of the living body, so long does its 
 fibrin remain dissolved in the watery part of it ; but so soon 
 as it is withdrawn from these, and is allowed to remain at 
 rest, it undergoes this remarkable change. If fresh-drawn 
 blood be continually stirred with a stick or beaten with twigs, 
 the fibrin coagulates in irregular strings, which adhere to the 
 stick or twigs ; and it does not then include the red particles, 
 which are left behind in the fluid. In this manner it may be 
 completely separated from the other elements of the blood, 
 which have not in themselves the least tendency to coagulate 
 spontaneously. Although forming a large proportion of the 
 substance of the clot, the fibrin, when dried, does not consti 
 tute more than from 2 to 3 parts by weight in 1000 of blood. 
 This proportion is augmented to 6, 8, or even 10 parts, in 
 severe inflammatory diseases. 
 
 238. "When the fibrin and the red particles have both been 
 separated from the blood, there remains a fluid, the serum. 
 in which a good deal of albumen is dissolved, together wii-n 
 fatty matter, and other organic substances ; with the addition 
 of saline matter, of which a considerable proportion is chloride 
 of sodium, or common salt. The proportion which the solid 
 
 p2 
 
212 SERUM USES OF BLOOD. 
 
 matter of the serum bears to the whole mass of blood, in 
 health, is about 53 parts in 1000 ; and of these about 40 
 parts are albumen, 8 parts saline matter, and 5 parts fat, with 
 certain ill-defined substances, of which some appear to be 
 organic compounds that are undergoing metamorphosis into 
 solid tissues, whilst others are the products of the decay of 
 the tissues, which are being progressively withdrawn and 
 eliminated by the excretory organs. 
 
 239. The influence of the Blood as a whole upon the 
 animal as well as on the nutritive functions, is easily proved. 
 When an animal is bled largely, it is gradually weakened as 
 the flow proceeds, and at last it seems to lose all consciousness 
 and power of movement. If allowed to remain in this con- 
 dition, it seldom or never recovers of itself. But if we inject 
 into its veins, by small quantities at a time, blood similar to 
 that which it has lost, the apparent corpse becomes as it were 
 reanimated, and all its functions are completely re-established. 
 The importance of the red particles is manifestly seen in the 
 effect of this remarkable operation, which is called the trans- 
 fusion of blood ; for if, instead of blood freshly obtained from 
 another living animal, we inject serum without these particles, 
 the effect is but little greater than if so much water were 
 introduced, and the animal dies of the haemorrhage. By this 
 operation, practised on the Human subject, many valuable 
 lives have been saved, that would otherwise have been de- 
 stroyed by loss of blood. Again, if, by mechanical means, as 
 by tying the principal blood-vessel going to any organ, we 
 cause a permanent diminution to any considerable extent, in 
 the quantity of blood with which it is supplied, a decrease in 
 its size is soon apparent, and it may even shrink almost to 
 nothing. On the other hand, we observe that, the more active 
 the function of a part, the larger is the quantity of blood with 
 which it is supplied. Thus, when the antlers of the Stag, 
 which fall off every year, are being renewed, the arteries that 
 supply the parts of the skull from which they spring, are 
 greatly increased in size ; but they shrink again, as soon as 
 the growth of the horns is completed for that year. A similar 
 increase takes place among animals that suckle their young, 
 in the size of the arteries that supply the mammary glands, 
 """by which the milk is formed ; and these also shrink, when 
 this liquid is no longer required. 
 
USES OP SEPARATE CONSTITUENTS OF BLOOD. 213 
 
 240. The following appear to be the chief uses of the 
 principal constituents of the Blood, considered separately, in 
 the general economy : The fibrin is the material which is 
 most assimilated to the condition of the solid tissues, having 
 the power of passing from the liquid state into a low and 
 simple form of organization. It was formerly supposed to be 
 the nutritive material at the expense of which the solid 
 tissues generally are immediately produced; the muscular 
 substance, in particular, being regarded as chemically identical 
 with it. But there is now good reason to think that the 
 greater part of the tissues form themselves at the expense of 
 the albumen of the serum and perhaps of the globulin of the 
 red corpuscles j and that the purpose of the fibrin is chiefly 
 to give origin to those simple forms of fibrous or connective 
 substance, the production of which is the first step in the 
 reparation of injuries. Were it not for its power of coagula- 
 tion, the slightest cut or scratch might become fatal, from the 
 gradual draining-away of the blood ; and such, in fact, has 
 actually happened, in cases of disease in which the fibrin is 
 deficient. The presence of fibrin also gives a degree of vis- 
 cidity to the blood, which, as experiment proves, favours 
 (instead of resisting, as might have been expected) its passage 
 through capillary tubes ; and thus, when there is a deficiency 
 in this ingredient, local stagnations and obstructions in the 
 circulation of the blood are very liable to occur. The albumen 
 of the blood may be considered, like that of the egg, as the 
 raw material, at the expense of which (in combination with 
 fat) every other organic compound in the body is generated. 
 It is, as we have seen, the substance to which all the tissue- 
 forming elements of the food are reduced in the process of 
 digestion ; and in this condition it seems to be continually 
 appropriated by the acts of self-formation that are taking 
 place, with varying rapidity, throughout the body, just as the 
 albumen of the egg is appropriated by the self-formative 
 operations of the embryo. There is strong reason to believe that 
 a large proportion of the solid tissues regenerate themselves 
 by the direct appropriation of this material ; and if (as has 
 been already stated to be probable) the simple fibrous tissues 
 find their material in the fibrin, and the muscular substance 
 in the globulin of the red corpuscles, it is from the albumen 
 that these substances are themselves elaborated, both of them 
 
214 USES OF SEPARATE CONSTITUENTS OF BLOOD. 
 
 being, as it were, in process of organization. The albumen 
 of the blood further serves to supply the albuminoid matters 
 which are required as constituents of various secretions, espe- 
 cially those which are concerned in the digestive process, as 
 the saliva, the gastric juice, and the pancreatic fluid. A 
 large amount is daily drawn-off for the production of the 
 peculiar ferments contained in these secretions, whose action 
 upon the food is necessary for its reduction to the form in 
 which alone it can be received into the circulating current. 
 Hence the making of new blood involves a considerable ex- 
 penditure of the old. 
 
 241. The liquid in which the fibrin and albumen are dis- 
 solved, has a considerable power of absorbing gases ; and this 
 is greatly increased by the presence of the saline matters 
 which it holds in solution. Hence the liquor sanguinis not 
 only sustains the nutrition of the body, but can also serve, to 
 a considerable extent, as a medium of communication between 
 the lungs and the tissues. In this kind of activity, however, 
 it is completely surpassed by the red corpuscles ( 235). 
 Independently of their use in ministering to the function of 
 Inspiration, there seems reason to believe that the red cor- 
 puscles are also subservient to that of Nutrition ; for a certain 
 conformity which exists between the organic and mineral sub- 
 stances they contain ( 232), and the composition of Muscle 
 and Nerve, taken in connexion with the manifest relation 
 between their number and the activity of the Nervo-muscular 
 apparatus, makes it probable that they have it for their especial 
 office to prepare the materials which are to be used in its pro- 
 duction and renewal of those tissues. The saline matter of the 
 blood has many important offices : thus it furnishes the mineral 
 ingredients which are requisite for the production of the tissues 
 and secretions ; it helps to preserve the organic substances from 
 decomposition ; and, in conjunction with the albumen, it keeps 
 up the density of the serum to the point at which it is equi- 
 valent to that of the contents of the red corpuscles, without 
 which balance the condition of the latter would be seriously 
 impaired ( 231). Finally, \hsfatty matters of the blood are 
 subservient to two very important functions the maintenance 
 of heat, and the formation of tissue. They maintain the 
 combustive process, whenever there is a deficiency of more 
 readily combustible material; and they also take part with 
 
ASSIMILATING AND SELF-PURIFYING POWER OF BLOOD. 215 
 
 albumen in the formation of all new tissue, its nuclear par- 
 ticles being always found to include fat-granules. 
 
 242. The presence of a due proportion of the foregoing 
 substances in th.e blood is an essential condition of health ; 
 and we find it provided-for in the marvellous power which 
 the blood, like any solid tissue, seems to have of making itself 
 from the materials supplied to it, and of getting rid of what 
 is superfluous or unsuitable. Thus an excess of albuminous 
 matter in the food does not seem to produce more than a very 
 limited increase in the quantity of albumen in the blood, the 
 surplus being made to undergo changes within the body, 
 which issue in its being removed by the excretory organs. 
 An excess of any of the saline compounds is very speedily 
 strained off (as it were) into the urine. And an excess of fatty 
 matters is drawn off either by the formation of fat as a tissue, 
 or by the augmented activity of the liver in producing bile. 
 This conservative power is still more remarkably shown in 
 the completeness with which the poisons that are generated 
 in the body by the decay of its tissues, and which are received 
 into the current of the circulation for the purpose of being 
 conveyed to the several excreting organs, are drawn off from 
 it, so as to leave the blood pure. Thus, carbonic acid is being 
 continually produced in such large quantities, that its accu- 
 mulation in the blood, even for five minutes, would be fatal ; 
 yet by the aerating process to which the lungs are subservient, 
 it is got rid of as fast as formed, so that the blood is restored 
 to its previous purity. In like manner, the urea, which is 
 one of the products of the wear and tear of the muscles 
 consequent upon their use, is so perfectly and constantly 
 eliminated by the kidneys, that its detection in the circulating 
 current is a matter of difficulty, although we know that it 
 must always be passing through this. 
 
 243. Thus the circulating current may be likened to a 
 tidal river running through the midst of a large town, and 
 supplying it with the water needed for the drink of its 
 human and other inhabitants, as well as with that which is 
 required for the various manufacturing and cleansing opera- 
 tions carried on within its precincts ; the same stream also 
 receives the drainage of the town, and consequently becomes 
 charged with the products of animal and vegetable decompo- 
 sition, and the foul refuse of manufactories ; and as the flow 
 
216 CIRCULATION OF THE BLOOD. 
 
 of the tide brings back a large proportion of what is carried 
 down at ebb, the waters speedily become so contaminated with 
 hurtful and offensive matters as to be unfit for use, unless 
 means be provided for getting rid of these as fast as they are 
 poured in. The perfection with which this requirement is 
 fulfilled in the Animal body, while it excites our admiration, 
 should also incite us to imitation, so far as the art of Man 
 can hope to imitate the works of the Divine Artificer. 
 
 Circulation of the Blood. 
 
 244. In some of the lower tribes of Animals, the blood 
 appears to circulate in channels which are merely excavated 
 in the substance of their tissues and organs. But among all 
 the Vertebrata, and even in most of the Invertebrated classes, 
 the movement of the blood takes place in a very complicated 
 apparatus, which is composed, 1st, of a system of tubes or 
 canals which serve to convey it through every part of the 
 structure, and 2d, of a special organ for the purpose of 
 giving motion to that liquid. These canals are known as the 
 blood-vessels ; and this special organ is the heart. 
 
 245. The Heart is the centre of the circulating apparatus. 
 It is a kind of % fleshy bag, communicating with the blood- 
 vessels : and it alternately dilates to receive the blood, which 
 is conveyed to it by one set of these ; and then contracts so as 
 to force it out into another set of tubes. In this manner a 
 continual current is kept up. All but the lowest animals 
 have a heart, or something which represents it. Such an 
 organ exists, not merely among all the Vertebrated classes, 
 but in all the Mollusca, and in the higher Articulata. But, 
 as will presently appear, there is a great diversity in its form, 
 and in the complexity of its construction ; for whilst, in its 
 simplest condition, it possesses but one cavity, communicating 
 with both sets of vessels, it contains, in its highest forms, four 
 different chambers, each of which has its own peculiar function. 
 
 246. The two sets of blood-vessels just adverted-to are, 1st, 
 the Arteries, which convey the blood from the heart into the 
 several parts and organs of the body ; and 2d, the Veins, 
 which collect the blood that has been distributed through 
 these, and return it to the heart. The Arterial system, as it 
 issues from the heart, consists of one or more large trunks, 
 which divide into branches, very much in the manner of the, 
 
DISTRIBUTION OF ARTERIES AND VEINS. 217 
 
 stem of a tree ; these branches again subdivide into others 
 more numerous but smaller, and these again into twigs still 
 more numerous and more minute ; until almost every portion 
 of the body is so penetrated with them, that not even a 
 trifling scratch, cut, or prick, can be made, without wounding 
 some one of these small divisions (fig. 120). The Venous 
 system presents a corresponding distribution, but it is destined 
 for an opposite purpose ; and we must regard it as commencing 
 in the tissues by the minuter canals, which run together like the 
 I 
 
 Fig. 120. DISTRIBUTION OF THE SMALLER BLOOD-VESSELS IN THE MEMBRANE 
 
 BETWEEN TWO OF THE TOES OF THE HIND FOOT OF THE COMMON FROG; a O, 
 
 veins ; b b, arteries. 
 
 little rivulets that form the origin of a mighty river, or like the 
 smallest fibres of which the roots of a tree are made up. The 
 larger canals thus formed gradually unite with each other as 
 they approach the heart, towards which they all tend, just as 
 the various tributary streams pour their contents into one prin- 
 cipal channel : and at last all the veins empty into the heart, 
 by one or two large trunks, the blood which they have conveyed 
 from the several parts of the body ; just as all the tributaries 
 which have arisen over a wide extent of country, pour into 
 the ocean the water they have collected, by one mouth which 
 is thus common to all of them. 
 
 247. Although the number of the Arterial branches increases 
 
218 AREAS OF ARTERIAL TRUNKS AND BRANCHES. 
 
 so vastly, as we proceed from their origin towards their termi- 
 nation, yet their capacity does not, at least in any considerable 
 degree ; that is, the first or main trunk will allow as much 
 fluid to pass through it in a certain time, as will the whole of 
 the first set of branches into which it divides, or the still more 
 numerous subordinate branches into which these diverge. Or, 
 to put this fact in another form, if we cut across the main 
 trunk, and compare the area, or space included within its 
 circular walls, with the sum of the areas of all the branches it 
 supplies at a certain distance say a foot from the heart, we 
 shall find them precisely equal ; and the same will hold good, 
 if the comparison be made with the sum of the areas of the 
 more numerous but smaller branches at a greater distance from 
 the main trunk. It is quite true that, when an artery divides 
 into branches, the combined size of these seems to be greater 
 than that of the trunk ; but this is only because the compa- 
 rison is made, not between the areas of their circles, but their 
 diameters. Thus, an artery of 1O1 lines in diameter, may 
 divide into three branches, two of them having a diameter of 
 7 lines, and the third a diameter of 2 lines ; and yet these 
 will convey no more blood than the single trunk. Fof, 
 according to a simple rule in geometry, the areas of circles are 
 to each other as the squares of their diameters. The area of 
 the trunk is expressed, therefore, by the square of 10-1, 
 which is almost exactly 102. The area of each of the two 
 large branches, in like manner, is expressed by the number 
 49, which is the square of 7 ; and that of the smaller one by 
 4, the square of 2 ; and the sum of these (49+49+4) is ex- 
 actly 102, making the combined areas of the branches the same 
 as that of the trunk. In like manner, one of the branches of 7 
 lines diameter might subdivide into two branches of a little 
 less than 5 lines each ; for, as the square of 5 is 25, and twice 
 that number is equal to 50, the combined areas of the two 
 branches of 5 lines each, exceed by very little the area of the 
 trunk of 7 lines. Hence it results, that the pressure of the 
 blood upon the walls of the arteries will be everywhere 
 almost exactly the same ; a conclusion which is confirmed 
 by experiment. 
 
 248. There are certain differences in the structure and dis- 
 tribution of the Arteries and Veins, which it is desirable to 
 mention. The Arteries receive the blood pressed out from 
 
STRUCTURE AND DISTRIBUTION OF ARTERIES AND VEINS. 219 
 
 the heart, and must be strong enough to resist the force of its 
 contraction ; otherwise, as there is a considerable impediment 
 to its onward flow, produced by the minuteness of the tubes 
 through which it has to pass, and the friction to which it is 
 subjected against their sides, their walls would give way, and 
 they would burst They have, accordingly, a tough elastic 
 fibrous coat, which contains also more or less of non-striated 
 muscular fibre. On the other hand, the Veins receive the 
 blood after the heart's power over it has been almost ex- 
 pended in forcing it through the capillary system, and when 
 it is consequently moving much more slowly. They are very 
 large in proportion to the arteries ; so that, if we were to cut 
 across a limb at any place, and to estimate the respective areas 
 of all the veins and arteries, we should find that of the veins 
 two or three times as great as that of the arteries. Hence the 
 pressure on their walls is much less ; and their strength does 
 not require to be so great. Accordingly we find their walls 
 much thinner, and the tough elastic fibrous coat almost entirely 
 wanting. 
 
 249. The difference in the force with which the blood 
 presses on the walls of the arteries and veins, is seen when 
 these vessels are wounded. If a small incision be made into 
 an artery, the blood spouts from it to a great distance ; but if 
 a similar incision be made in a vein, the blood merely flows 
 out, unless we stop its passage to the heart, by making pres- 
 sure on the vein above the orifice, as in ordinary blood-letting 
 ( 277). Hence much greater pressure is requisite to check 
 bleeding from an artery, than to stop bleeding from a vein ; 
 and it frequently happens that no amount of pressure can 
 prevent the continued drain of blood from the former, so that 
 it becomes necessary to stop the flow of blood through the 
 artery altogether, by tying a ligature tightly round it. 
 
 250. The Arteries are for the most part so distributed, that 
 their trunks lie at a considerable distance from the surface of 
 the body, so as to be secluded from injury ; and they are often 
 specially protected by particular arrangements of the bony 
 parts. Of the Veins, on the other hand, a large proportion lie 
 near the surface, and they are consequently more liable to be 
 injured ; but, for the reason just stated, wounds in them are 
 of comparatively little consequence. 
 
 251. The ultimate ramifications of the Arteries are conti- 
 
220 
 
 CAPILLARY BLOOD-VESSELS. 
 
 rnious with the commencing twigs of the Venous system. The 
 communication is established by means of a set of extremely 
 minute vessels, which are termed Capillaries.* These capil- 
 laries form a network, which is to be found in almost every 
 
 part of the body (fig. 121). 
 It is in them alone that the 
 blood ministers to the opera- 
 tions of nutrition and secre- 
 tion. Even the walls of the 
 larger blood-vessels are inca- 
 pable of directly imbibing 
 nourishment from the blood 
 which passes through them ; 
 but are supplied with minute 
 branches, which proceed from 
 neighbouring trunks, and form 
 a capillary network in their 
 substance. The diameter of 
 the capillaries must of course 
 bear a certain proportion to 
 
 J . A , . . *, ,f , . , 
 
 that 01 tllC DlOOd-dlSCS WIUCI! 
 
 ho vp f n rqqq tVirmio-Ti fhpTn 
 naVG t0 P aSS mrou g n 
 
 in Man they are Commonly 
 i_ j_ i O^AA.L'U t. 
 
 irom about 1 - 2oOOth to 
 
 1_1 600th of an inch in dia- 
 . 
 
 meter. In the true capilla- 
 ries, it would seem that only one row or file of these particles 
 can pass at a time ; but we frequently see vessels passing 
 across from the arteries to the veins, which will admit 
 several rows. There seems, however, to be a considerable 
 difference in the diameter of the same capillary at different 
 times ; a change sometimes taking place from causes which 
 are not yet understood, f The rate at which the blood moves 
 
 * From the Latin capilla, hair ; so named on account of their being, 
 like hairs, of very minute size. Their diameter is really, however, far 
 less than that of ordinary hairs. 
 
 f The circulation of 'the blood in the Frog's foot, the tail of the 
 Tadpole, the gills of the larva of the Water-Newt, the yolk-bag of 
 embryo Fish, and other appropriate subjects for the observation, is one 
 of the most beautiful and interesting spectacles that the Microscope can 
 open to us. Details of the various modes of exhibiting it will be found 
 in the Author's treatise on " The Microscope and its Revelations," 
 Chap, xviii. 
 
 Fig. 121. PORTION OF THK MEMBRANE 
 
 BETWEEN THE TOES Of THE HIND 
 
 FOOT OF THE FROG, more highly 
 magnified than in fig. 120, showing the 
 network of Capillaries that traverses it ; 
 a, small venous trunk; b b, branches 
 communicating with the capillaries ; 
 c intervening tissue covered with epi- 
 thehum cells. 
 
RESPIRATORY CIRCULATION. 221 
 
 through the capillaries of a warm-blooded animal, has been 
 determined by microscopic examination to be about 3-100ths 
 of an inch per second. From the comparison of this rate with 
 that of the flow of blood through the larger arteries, which has 
 been found by experiment to be nearly 12 inches per second, 
 it appears that the area of the capillary system must be 
 nearly four hundred times as great as that of the vessels 
 which supply it with blood. 
 
 252. Thus the Arterial and Venous systems communicate 
 with each other at their opposite extremities ; their large 
 trunks through the medium of the heart j and their ultimate 
 subdivisions through the capillaries. Hence we may consider 
 this double apparatus of vessels as forming a complete circle, 
 through which the blood flows in an uninterrupted stream, 
 returning continually to its point of departure ; and the term 
 circulation is therefore strictly applicable. 
 
 253. But the conveyance of the nutritive fluid to the several 
 organs of the body, for their support and maintenance, is not 
 the only object to which its circulation has to minister. It is 
 requisite that the blood should be continually exposed to the 
 influence of the air, by which it may get rid of the carbonic 
 acid with which it has become charged during its circulation 
 in the system, and may take-in a fresh supply of oxygen to 
 replace that which has been withdrawn from it. In order to 
 effect this exposure, the blood is conveyed to a particular organ, 
 in which it is made to pass through a special set of capillary 
 vessels, that bring it into almost immediate contact with 
 air. In the lower tribes, in which this aeration is (from 
 various causes hereafter to be explained) much less constantly 
 necessary than in the higher, we find the respiratory organ 
 supplied by a branch from the general circulation ; and the 
 blood which has passed through it, and which has been sub- 
 jected to the invigorating influence of the air, is mingled in 
 the heart with that which has been deteriorated by circulating 
 through the system, which is again supplied with this mixed, 
 half-aerated blood. But in the highest classes, there is a dis- 
 tinct circle of vessels subservient to the respiratory function : 
 namely, an arterial trunk issuing directly from the heart, and 
 subdividing into branches which terminate in the capillary 
 system of the respiratory organ ; a set of capillaries, in which 
 the aeration of the blood takes place ; and a system of veins 
 
OTHER USES OP THE CIRCULATION. 
 
 which collects the blood from, these, and returns it to the 
 heart. This circuit of the blood is sometimes called the lesser 
 circulation; to distinguish it from that which it makes 
 through the general system, which is called the greater 
 circulation. 
 
 254. Although carbonic acid is one of the chief impurities 
 with which the blood becomes charged during its circulation, 
 in consequence of the changes of composition which are con- 
 tinually taking place in the living body, it is by no means the 
 only one ; and other organs are provided, besides the lungs, 
 for removing the noxious matters from the current of the cir- 
 culation as fast as they are introduced into it. Thus, in the 
 course of its movement through the general system, the blood 
 is made to pass through the liver, the kidneys, and the skin, 
 each of which has its special purifying office ; these organs, 
 however, have no such special circulation of their own as the 
 respiratory apparatus of higher animals possesses, though the 
 liver, as we shall hereafter see ( 267), is peculiarly supplied 
 by a sort of offset from the general circulation, so that the 
 blood from which its secretion is formed is venous instead of 
 arterial, like that transmitted to the lungs. 
 
 255. The course which the blood takes, and the structure 
 of the apparatus which is subservient to its movement, differ 
 very greatly in the several classes of animals. The chief of 
 these differences will be pointed out hereafter ; and it will be 
 preferable to commence with the highest and most complex 
 form of the circulating system, such as we find in Man, that 
 it may serve as a standard of comparison with which the 
 rest may be contrasted. 
 
 Circulating Apparatus of the Higher Animals. 
 
 256. In Man, and those animals which approach him most 
 nearly in structure, the heart is situated between the lungs in 
 the cavity of the chest, which is termed by anatomists the 
 thorax. Its form is somewhat conical ; the lower extremity 
 tapering almost to a point, and the upper part being much 
 larger. The lower end is quite unattached, and points rather 
 forwards and to the left ; during the contraction of the heart, 
 it is tilted forwards, and strikes against the walls of the chest, 
 between (in Man) the fifth and sixth ribs. It is from the 
 large or upper extremity that the great vessels arise ; and 
 
CIRCULATING APPARATUS IN MAN. 
 
 223 
 
 these, being attached to the neighbouring parts, serve to 
 suspend the heart, as it were, in a cavity in which its 
 movements may take place freely. This cavity is lined by 
 a smooth serous membrane ( 43), which, near its top, is 
 
 ac vj 
 
 vl 
 
 Fig. 122. LUNGS, HEART, AND PRINCIPAL VESSELS op MAN. 
 
 a r, right auricle; IT, right ventricle; vl, left ventricle; a, aorta; vc, vena cava; 
 ac, carotid arteries ; vj, jugular veins ; as, subclavian artery ; vs, subclavian veins ; 
 t, trachea. 
 
 reflected downwards over the vessels, and covers the whole 
 outer surface of the heart. Hence as the surface of the heart, 
 and the lining of the cavity in which it works, are alike 
 smooth, and are kept moist (in health) with a fluid secreted 
 for the purpose, there is as little interruption as possible from 
 friction in the working of this important machine. 
 
 257. The heart may be described as a hollow muscle, 
 which, in Birds and Mammalia, as in Man, is divided into 
 four distinct chambers. This division is effected by a strong 
 vertical partition, that divides the entire heart into two halves, 
 which are almost exactly similar to each other, excepting in 
 the greater thickness of the walls on the left side ; and each 
 of these halves (which do not communicate with one another) 
 is again subdivided by a transverse partition, into two cavities, 
 
224 STRUCTUEE OP THE HEART. 
 
 of which the upper one is termed the auricle, and the lower 
 the ventricle. Thus we have the right and left auricles, and 
 the right and left ventricles. Each auricle communicates 
 with its corresponding ventricle, by an aperture in the 
 
 Superior Pulm. Pulmonary 
 
 vena cava art. Aorta artery 
 
 7 Pulmonary veins 
 Pulmonary veins 
 
 Right auricle (^^BT /HHW- Left auricle 
 
 Tricuspid valves - ~ SftlS ' Mitral valve 
 
 Inferior vena cava ; ,^^ 
 
 / x'HHffiSlw / "^*-.. 
 
 ^ ^^ Left ventricle 
 
 Right ventricle 
 
 Partition Aorta 
 Fig. 123. IDEAL SECTION op THE HUMAN HEART. 
 
 transverse partition, which is guarded by a valve. The walls 
 of the ventricles are much thicker than those of the auricles ; 
 and for this evident reason, that the ventricles have to 
 propel the blood, by their contraction, through a system of 
 remote vessels ; whilst the auricles have only to transmit the 
 fluid that has been poured into them by the veins, into the 
 ventricles, which dilate themselves to receive it. The difference 
 in the thickness of the walls of the left and the right ventricles 
 is explainable on the same principle ; for the left ventricle 
 has to send the blood, by its contractile power, through the 
 remotest parts of the body; whilst the right has only to 
 transmit it through the lungs, which, being much nearer, 
 require a far less amount of force for the circulation of the 
 blood through them. 
 
 258. The arterial system of the greater circulation entirely 
 springs from one large trunk, which is called the aorta (see 
 figs. 122-124); this originates in the left ventricle, and is 
 the only vessel which passes forth from that cavity. It first 
 ascends towards the bottom of the neck ; then forms what is 
 termed the arch, a sudden curve, which gives it a downward 
 
ARTERIAL SYSTEM OF MAX. 
 
 225 
 
 direction ; and then descends along the front of the spinal 
 column, behind the heart, as far as the lower part of the 
 
 Anterior 
 tibial artery 
 
 Art. of foot 
 
 Peroneal 
 
 artery 
 
 Fig. 124. ARTERIAL SYSTEM OF MAX 
 
 Q 
 
22 G AETERIAL SYSTEM OF MAN. 
 
 trunk, where it divides into two great branches, which 
 proceed to the lower extremities. From the arch of the 
 aorta are given off the arteries which supply the head and 
 upper extremities. These are, the two carotids, which ascend 
 on either side of the neck; and the two stibclavian, which 
 pass outwards beneath the clavicles, so as to arrive at the 
 arms, becoming successively in their course the axillary and 
 brachial arteries, as they pass through the axilla or arm-pit, 
 and along the arm. The subclavian and carotid arteries of 
 the right side arise together from the aorta, in Man, by a 
 common trunk ; but this arrangement varies much in different 
 Mammals. Thus in the Elephant, the two carotids arise by 
 a common trunk, the two subclavians separately. In some 
 of the Whale tribe, all four are separate. In the Bat, the 
 subclavian and carotid of the left side arise from a common 
 trunk, like those of the right. And in those Euminating 
 animals which possess a long neck, all four arteries come off 
 from the aorta together, by a large trunk, which first gives off 
 the subclavians on either side, and then divides into the 
 carotids. All these varieties occasionally present themselves in 
 Man; a fact of no small interest. 
 
 259. The descending aorta, in its progress along the trunk, 
 gives-off several important branches; as the cceliac, from 
 which the stomach, liver, and spleen are supplied ; the renal, 
 to the kidneys; and the mesenteric, to the intestines. It 
 divides at last into the two iliac arteries ; which, after giving 
 off branches for the supply of the lower bowels, pass into the 
 thighs, where they become the femoral arteries ; and these 
 again subdivide into branches for the supply of the leg. 
 
 260. For the sake of comparison, a figure of the arterial 
 system of a Bird is introduced ; from which it will be seen 
 that by far the larger proportion of its blood is distributed 
 to its upper extremities. In Man, the descending aorta is 
 evidently the continuation of the aortic arch ; and the parts 
 which it supplies receive far more blood than the head and 
 upper extremities, the locomotion of biped man being per- 
 formed almost entirely by his lower limbs. In Quadrupeds, 
 which require nearly as much strength in their fore feet as in 
 their hind, the subclavian arteries bear a larger proportion to 
 the iliac. But in Birds, the function of locomotion is almost 
 entirely performed by the wings ; and their powerful muscles, 
 
ARTERIAL SYSTEM OP BIRD. 
 
 227 
 
 which, constitute the mass of flesh lying on the breast, are 
 supplied with blood by the arteries of the upper extremities, 
 which here possess a manifest predominance. The aorta, soon 
 after its origin, subdivides into three large branches ; of which 
 the first two (one on either side giving-off the subclavian and 
 carotid arteries) convey the blood to the head, the wings, and 
 
 Lingual artery 
 
 Aorta 
 
 Sacral artery Cloaca 
 
 Fig. 125. AKTERIAI, SYSTEM OF BIRD. 
 
 the muscles lying on the thorax ; whilst the middle one curves 
 backwards and downwards, and becomes the descending aorta. 
 ISTow that which is here the continuation of the great side 
 
228 DISTRIBUTION OF AKTERIES. 
 
 branch, is neither the carotid nor the subclavian, both of 
 which are subordinate branches given-off from it; but it is 
 the trunk which distributes the blood to the muscles of the 
 breast, and which in Man is a subordinate branch of the sub- 
 clavian artery (the mammary). The descending aorta is seen 
 to lose itself almost entirely in supplying the viscera of the 
 trunk ; so that the branches into which it divides at last for 
 the supply of the legs, are very small. These limbs, in birds, 
 are usually required only for the support of the body at times 
 of rest, and are seldom much concerned in locomotion ; so that 
 they possess little muscular power, and require but a small 
 supply of blood. 
 
 261. It is very interesting to trace such differences in the 
 arrangement of the vascular system, corresponding with vari- 
 ations in the general plan of structure, yet not exhibiting 
 any actual departure from the general type. Thus, there is 
 probably not a single large artery in Man, to which a corre- 
 sponding branch might not be found in the Bird ; on the 
 other hand, there is perhaps not a single large artery in the 
 Bird, to which there is not an analogous branch in Man. 
 The chief difference consists in the relative sizes of the seve- 
 ral trunks ; and these correspond closely with the amount of 
 tissue they have respectively to supply. Here, then, we have 
 one example, out of many that might be adverted-to, of that 
 Unity of Design which we see everywhere prevalent through- 
 out nature ; manifesting itself in the close conformity of a 
 great number of apparently-different structures to one general 
 plan, whilst there is, at the same time, an almost infinite 
 variety in the details. 
 
 262. There is a very interesting peculiarity in the distribu- 
 tion of the arteries, by which the due circulation of blood in 
 their branches is provided for, even though there should be 
 an obstruction in the main trunk. The branches which are 
 given-off from it at different points, have frequent communica- 
 tions or anastomoses with each other ; so that blood may pass 
 from an upper part of a main artery into the lower, by means 
 of these lateral communications, even though its flow through 
 the trunk itself should be completely stopped. 
 
 263. These anastomoses are very numerous in the arteries 
 of the limbs, and particularly about the joints ; and it is well 
 that they are so ; for, by relying on the maintenance of the 
 
ANASTOMOSES OP ARTERIES ANEURISM. 229 
 
 circulation through them, the Surgeon is often able to save a 
 limb, or even a life, which would otherwise be sacrificed. 
 Arteries are liable to a peculiar disease, termed aneurism, 
 which consists in a thinning-away, or rupture, of the tough 
 fibrous coat, and a great dilatation of the other coats, so that 
 a pulsating tumour is formed. This change takes place most 
 frequently at the bend of the thigh, the ham, the shoulder, 
 and the elbow ; where the artery, in the working of these 
 joints, often has to undergo sudden twists. The result of the 
 disease Avould be generally fatal, in consequence of the gradual 
 thinning-away of the walls of the tumour, which at last 
 bursts, allowing the blood to escape from the arterial trunk 
 with such rapidity as, if unchecked, to cause almost instanta- 
 neous death. In order to prevent this, the surgeon ties the 
 artery at some little distance above the aneurism, that is, he 
 puts a thread round it, which is drawn so tight as to prevent 
 the passage of any blood to the aneurism. The circulation in 
 the lower part of the limb is at first retarded ; its temperature 
 falls ; and it becomes more or less insensible. But after the 
 lapse of a few hours, the circulation becomes quite vigorous, 
 the pulsations strong, the temperature rises, and the numb- 
 ness passes off; and as the main trunk still continues com- 
 pletely obstructed, this can only have been brought about by 
 the flow of blood through the anastomoses, which must in 
 that short period have undergone considerable enlargement. 
 Examination of the vessels after death shows that this has 
 been actually the case. Even the aorta has thus been tied in 
 dogs, without causing death ; the anastomoses of the branches 
 given-off from its upper part, with those proceeding from the 
 lower, being sufficient to maintain the circulation in the latter, 
 when the current through the main trunk is obstructed. 
 
 264. A very complex series of anastomoses, forming a com- 
 plete network of large tubes, is found in several situations, 
 where it seems desirable that the flow of blood to a particular 
 organ should be retarded, whilst a large amount is to be 
 allowed to pass through. Thus in animals which keep their 
 heads near the ground for some time together, as in grazing, 
 the arteries which supply the brain suddenly divide, on their 
 entrance within the skull, into a great number of branches, 
 by the anastomoses of which a complex network is formed ; 
 and from this network, by the reunion of its small vessels, 
 
230 PECULIARITIES OF DISTRIBUTION OF ARTERIES. 
 
 originate the trunks which supply the brain in the usual 
 manner. The object of this apparatus appears to be, to pre- 
 vent the influence of gravitation from causing a too great rush 
 of blood towards the brain, when the head is in a depending 
 position ; for the rapidity of its flow will be checked, as soon 
 as it enters the network, and is distributed through its 
 numerous canals. A similar conformation is found in the 
 blood-vessels of the limbs of the Sloth, and of some other 
 animals which resemble that animal in the sluggishness of 
 their movements ; and its object is probably to prevent the 
 muscles from receiving too rapid a supply of blood, which 
 would give them what (for these animals) would be an undue 
 energy of action ; whilst, by the very same delay, their power 
 of acting is greatly prolonged, as we find it to be in Eeptiles, 
 whose circulation is languid ( 284). 
 
 265. In the Whale tribe, and some other diving animals 
 that breathe air, we find a curious distribution of the blood- 
 vessels, which has reference to their peculiar habits. The 
 intercostal arteries (which are sent-off from the aorta to the 
 spaces between the ribs on each side) are enormously dilated, 
 and are twisted into thousands of convolutions, which are 
 bound together into a mass by elastic tissue. This mass, 
 which is of considerable bulk, lies at the back of the chest, 
 along both sides of the vertebral column ; and it serves as a 
 reservoir, in which a great quantity of arterial blood may be 
 retained. The veins also have very large dilatations, which 
 are capable of being distended, so as to hold a considerable 
 amount of venous blood ; and thus, while the animal is pre- 
 vented from breathing by its submersion in the water, the 
 circulation through the capillaries of the system is sustained, 
 by the passage of the blood stored up (as it were) in the 
 arterial system, into the venous reservoirs. If this provision 
 did not exist, the whole circulation would come to a stand, 
 in consequence of the obstruction it meets with in the lungs, 
 when the breathing is stopped. 
 
 266. With regard to the Venous system, there is little to be 
 added to what has been already stated ( 248-250) as to its 
 general character and distribution. The large proportion which 
 its capacity bears to that of the arterial system, is shown by 
 the fact, that every main artery is accompanied by a vein (fre- 
 quently by two) considerably larger than itself ; and that the 
 
DISTRIBUTION OF VEINS PORTAL SYSTEM. 231 
 
 superficial veins, which lie just beneath, the skin, are capable 
 of conveying at least as much more. The veins of the body 
 in general unite in two large trunks, the superior and inferior 
 vena cava ; which meet as they enter the right auricle of the 
 heart (fig. 123). The superior vena cava is formed by the 
 union of the veins which return the blood from the neck (the 
 jugulars) with those which convey it from the arms (the 
 subclavians), as shown in fig. 122; and the inferior cava 
 (v c, fig. 122) receives the blood from the trunk, the organs 
 contained in the abdomen, and the lower extremities. 
 
 267. There is, however, an important peculiarity in the 
 distribution of the veins of the Intestines, which should not 
 pass unnoticed. Instead of delivering their blood at once into 
 the inferior vena cava, these veins unite into a trunk, called 
 the Vena Portce (fig. 134), which enters the liver and subdivides 
 into branches, whence a capillary network proceeds that per- 
 meates the whole of its mass. It is from the venous blood, 
 as it traverses this network, that the secretion of bile is 
 formed ; and the blood which is brought by the hepatic artery 
 serves chiefly to nourish the liver, no bile being formed from 
 it, until it has become venous. The blood is carried-off from 
 this double set of capillaries by the hepatic vein, which conveys 
 it into the inferior vena cava. In Fishes, not only the blood 
 of the intestines, but that of the tail and posterior part of the 
 body, enters this "portal" system, which is distributed to 
 their kidneys as well as to their liver. Thus all the blood 
 which flows through the portal system, has to go through two 
 sets of capillaries, between each period of its leaving the heart 
 by the aorta, and its return to it by the vena cava. 
 
 268. We have yet to notice the lesser circulation, which 
 is confined to the Lungs only. The venous blood which is 
 returned to the heart by the vense cavae, enters the right 
 auricle, and thence passes into the right ventricle. By the 
 contraction of this last cavity, it is expelled through the. pul- 
 monary artery (fig. 123), which soon divides into two main 
 trunks that proceed to the right and left lungs respectively. 
 The right trunk again subdivides into three principal branches, 
 which are distributed to the three lobes or divisions of the 
 right lung ; whilst the left divides into two branches, which 
 are in like manner distributed to the two lobes of the left 
 lung. The capillaries, into which these branches ultimately 
 
232 LESSER CIRCULATION FORCES THAT MOVE THE BLOOD. 
 
 subdivide, are distributed upon the walls of the air-cells (fig. 
 162), and the character of the blood is in them converted, by 
 exposure to the air, from the dark venous to the bright arte- 
 rial. From this capillary network the pulmonary veins arise ; 
 and the branches of these unite into trunks, of which two 
 proceed from each lung, to empty themselves into the left 
 auricle (fig. 123). This auricle delivers the blood, now arte- 
 rialized or aerated ( 253), into the left ventricle, whence the 
 aorta arises ; and by the contraction of this cavity, it is 
 delivered through that vessel to the system at large. It 
 will be observed that the vessel which proceeds from the 
 heart to the lungs is called the pulmonary artery, although it 
 carries dark or venous blood. This is because it conveys the 
 blood from the heart towards the capillaries. And, for a 
 similar reason, the vessels which return the blood from the 
 capillaries to the heart are termed pulmonary veins, although 
 they carry red or arterial blood. 
 
 Forces that move the Blood. 
 
 269. The mechanical action, by which the blood is caused 
 to circulate in the vessels, is easily comprehended. The cavi- 
 ties of the heart, as already explained ( 245), contract and 
 dilate alternately, by the alternate shortening and relaxation 
 of the muscular fibres that form their walls (Chap, xn.) ; and 
 the force of their contraction is sufficient to propel the blood 
 through the vessels which proceed from them. The two 
 ventricles contract at the same moment ; the auricles contract 
 during the relaxation of the ventricles, and relax whilst the 
 ventricles are contracting. The series of movements is there- 
 fore as follows : The auricles being full of the blood which 
 they have received from the venae cavse and pulmonary veins, 
 discharge it by their contraction into the ventricles, which 
 have just before emptied themselves into the aorta and pul- 
 monary artery, and which now dilate to receive it. When 
 filled by the contraction of the auricles, the ventricles contract 
 in their turn, so as to propel their blood into the great vessels 
 proceeding from them ; and whilst they are doing this, the 
 auricles again dilate to receive the blood from the venous 
 system, after which the whole process goes-on as before. It 
 is when the ventricles contract, that we feel the beat of the 
 heart, which is caused by the striking of its lower extremity 
 
MECHANISM OF THE HEART. 
 
 233 
 
 Fig. 126. 
 
 against the walls of the chest ; and it is by the same action 
 that the pulse in the arteries is produced ( 276V 
 
 270. The combined actions 
 of each auricle and its ventri- 
 cle, may be illustrated by an 
 apparatus like that repre- 
 sented in fig. 126. It con- 
 sists of two pumps, a and 
 b, of which the pistons move 
 np and down alternately ; 
 and these are connected with 
 a pipe c /, in which there are 
 two valves d and e, opening 
 in the direction of the arrow. 
 The portion c of the pipe 
 represents the venous trunk 
 by which the blood enters 
 
 the heart ; the pump a represents the auricle, and the raising 
 of its piston enables the fluid to enter and fill it. When its 
 piston is lowered, its fluid is forced through the valve d into 
 the pump b (which represents the ventricle), whose piston 
 rises at the same time to receive it ; and when this piston is 
 lowered in its turn, the fluid (being prevented from returning 
 into a by the closure of the valve d) is propelled through the 
 valve e into the pipe f, which may represent an arterial tube ; 
 whilst at the same time a fresh supply of blood is received 
 into the pump a by the raising of its piston. 
 
 271. The number of contractions of the heart ordinarily 
 taking place in an adult man, is from 60 to 70 per minute. 
 It is usually rather greater in women ; and in children it is 
 far higher, being from 130 to 140 in the new-born infant, and 
 gradually diminishing during the period of infancy and child- 
 hood. It is rather greater in the standing than in the sitting 
 posture, and in sitting than in lying down : it is increased by 
 exercise, especially by ascending a steep hill or going upstairs, 
 and also by any mental emotion. It is important to remember 
 these facts, in reference to the management of those who are 
 suffering under diseases of the heart or of the lungs, which 
 prevent the ready passage of the blood through these organs ; 
 for if more blood be brought to the heart by the great veins, 
 than it can propel through the pulmonary arteries, a feeling of 
 
234 VALVES OF THE HEAET. 
 
 very great distress is experienced; and there may be even 
 danger of rupture of the heart or large vessels, or of sudden 
 cessation of the heart's action, causing instant death. Such 
 persons ought, therefore, carefully to refrain from any violent 
 muscular movement, and also to avoid giving way to strong 
 mental emotions. In syncope or fainting, the heart's action is 
 so weakened as to be scarcely perceptible, though it does not 
 entirely cease ; and this state may be brought on by several 
 causes which make a strong impression on the nervous system, 
 such as violent mental emotion (whether joy, or grief, or terror), 
 sudden loss of blood, and the like. 
 
 272. The blood which has been received by each ventricle 
 from its auricle, is prevented from being driven back into the 
 latter, on the contraction of the former, by a valve that guards 
 the aperture through which it entered. This valve consists of 
 a membranous fold, surrounding the borders of the aperture, 
 and so connected with the neighbouring parts, as to yield when 
 the blood passes from the auricle into the ventricle, but to 
 be tightened so as completely to close the aperture when 
 the blood presses in the contrary direction. The manner 
 in which these valves act will be seen from fig. 127, which 
 is a section of the right auricle with its ventricle. The 
 auricle, a, 'receives its blood from the two venae cavse, e, e' } 
 and transmits it into the ventricle, b, by the orifice, c. On 
 either side of this orifice are seen the membranous folds, 
 which are kept in their places by the tendinous cords, d. 
 a Now when the blood is passing from 
 
 a to 5, these folds yield to the current ; 
 L. / hut when the cavity b is filled and begins 
 
 to contract, the blood presses against 
 ff their under sides, so as to make them 
 
 close against each other, as far as they 
 b are permitted to do by the tendinous 
 
 cords. In this manner the aperture is 
 
 completely shut, and no blood can flow 
 Fig. 127. SECTION OP ONE back. A valve of this kind exists on 
 HEART> each side of the heart; but there is 
 a slight difference between the forms of the two, whence 
 they have received different names. That on the right side 
 has three pointed divisions, to which the tendinous cords 
 are attached, and it is hence called the tricuspid valve; 
 
SEMILUNAB VALVES. 235 
 
 whilst that on the left side has only two, so as to bear some 
 resemblance to a bishop's mitre, whence it is called the mitral 
 valve. 
 
 273. The aorta and pulmonary artery are in like manner 
 furnished with valves, which prevent the blood that has been 
 forced into them by the contraction of the ventricles, from 
 returning into those cavities when they begin to dilate again. 
 These valves, however, are formed upon a different plan, and 
 more resemble those of the veins, which will be presently 
 described. They consist of three little pocket-shaped folds of 
 the lining membrane of these arteries (similar to those at b b, 
 fig. 128), which are pressed flat against the walls of those 
 tubes when the blood is forced into them ; but as soon as they 
 are filled, and the ventricles begin to dilate, so that the blood 
 has a tendency to return, it presses upon the upper side of 
 these pockets, and fills them out against one another, in such 
 a manner as completely to close the entrance into the ven- 
 tricle. The three little pocket-shaped folds, however, would 
 not close the centre of the aperture, were it not that each of 
 them has a little projection from its most prominent part, 
 which meets with those of the others, and effects the requi- 
 site end. The situation of these valves (which are termed 
 semilunar from their half-moon shape) is seen at g, fig. 127, 
 / being the pulmonary artery. 
 
 274. The amount of blood sent-out from either ventricle 
 at each contraction, in a middle-sized man, seldom exceeds 
 3 ounces ; but the whole quantity of blood contained in the 
 body is not less than 181bs. : hence, it would require 96 
 contractions of the heart to propel the whole of this blood 
 through the body, and these (at the ordinary rapidity) would 
 occupy about Ij- minute. It has been calculated, from recent 
 experiments, that the usual force of the heart in man would 
 sustain a column of blood about 7 feet 2 inches high, the 
 weight of which would be about 4 Ibs. 3 oz. on every square 
 inch. The backward pressure of this column upon the walls 
 of the heart, or in other words, the force which they have to 
 overcome in propelling the blood into the aorta, is estimated 
 at about 13 Ibs. 
 
 275. From the mode in which the blood is forced into the 
 arterial system by a series of interrupted impulses, it might 
 be supposed that its course would be a succession of distinct 
 
236 EQUALIZING ACTION OP ARTERIES I PULSE. 
 
 jets ; but this is prevented, so that the current is reduced to 
 an equable stream by the time it reaches the capillaries, 
 through the elasticity of the walls of the arteries. In order 
 to comprehend how this acts, we may suppose a forcing-pump 
 ( 270) to propel its fluid, not into a hard unyielding tube of 
 iron or lead, but into an elastic tube of india-rubber. The 
 effect of each stroke of the pump will be partly expended in 
 distending the tube, so as to make it contain an additional 
 quantity of water ; and the suddenness of the jet at its oppo- 
 site extremity will be diminished. In the interval of the 
 stroke, the elasticity of the wall of the tube will cause it to con- 
 tract again, and to force-out the added portion of its contents ; 
 this it will not have completed by the time that the action of 
 the pump is renewed ; and in this manner, instead of an inter- 
 rupted jet at the mouth of the tube, we shall have a continuous 
 flow, which, if the tube be long enough, will become quite 
 equable.* It is precisely in this manner that the elasticity of 
 the arteries influences the flow of blood through them, by 
 converting the interrupted impulses which the heart com- 
 municates to it, into a continued force of movement. In the 
 large arteries, these impulses are very evident ; in the smaller 
 branches they are less so, but they still manifest themselves 
 by the jerking in the stream of blood proceeding from a 
 wound in one of these vessels ; whilst in the capillaries, the 
 influence of the heart's interrupted impulses cannot usually 
 be seen at all, the streams that pass through them being 
 perfectly equable. 
 
 276. The phenomenon which we call the pulse, is nothing 
 else than the change in the condition of the artery occasioned 
 by the increased pressure of the fluid upon its walls, at the 
 moment when the heart's contraction forces an additional 
 quantity of blood into the arterial system. By the frequency 
 and force of this change, we can judge of the power with 
 which the blood is being propelled. But the pulse can only 
 be well distinguished, when we can compress the artery 
 against some resisting body, so that there is a partial obstruc- 
 tion to the flow of blood through it, which causes the disten- 
 sion to be more powerful ; the most convenient artery for this 
 
 * The same effect is obtained in an ordinary fire- or garden-engine, 
 by the interposition of an air-vessel, in which the elasticity of com- 
 pressed air is substituted for that of the wall of the pipe. 
 
PULSE : WOUNDS OF ARTERIES. 237 
 
 purpose is the radial artery (fig. 124) at the wrist ; but the 
 carotid artery in the neck, and the temporal artery in the 
 temple, may be felt, when it is desired to know the force of 
 the circulation in the head; as may the arteries supplying 
 other parts, when we wish to gain information respecting 
 the organs they supply. An increased action in the organ, 
 whether this be due to inflammation, or to a state of unusual 
 activity of its function, causes an increase of size in the artery 
 which supplies it ; and thus the pulsation may be unusually 
 strong in a particular trunk, when the heart's action and the 
 general circulation are not in a state of excitement. For 
 instance, a whitlow on the thumb will occasion its artery to 
 beat almost as powerfully as the radial artery usually does ; 
 and excessive activity of the mind, prolonged for some hours, 
 greatly increases the force of the pulsations in the carotid 
 arteries, from which the brain is chiefly supplied. 
 
 277. When an artery is wounded, there is often great 
 difficulty in controlling the flow of blood ; for pressure can 
 seldom be effectually applied in the situation of the wound ; 
 and the surgeon is generally obliged to tie the vessel above 
 the orifice. As a temporary expedient, the loss of blood may 
 be prevented by making firm pressure upon the artery above 
 the wounded part, that is, nearer the heart ; and many valu- 
 able lives have been saved by the exercise of presence of 
 mind, guided by a little knowledge. The best means of 
 keeping-up the requisite pressure, until the proper instrument 
 (the tourniquet) can be applied, is to lay over the artery (the 
 place of which may be found by its pulsation) a hard pad, 
 made by tightly rolling or folding a piece of cloth ; this pad 
 and the limb are then to be encircled by a bandage, by which 
 the pressure is maintained ; and this bandage may be tightened 
 to any required degree, by twisting it with a ruler or a piece 
 of stick. Thus a constant pressure may be exercised upon 
 the artery, which will be generally sufficient to control the 
 bleeding from it. But there are, unfortunately, many cases 
 in which pressure of this kind cannot be applied ; as for 
 instance when the femoral artery is wounded high up in the 
 thigh, or the carotid artery in the neck. And nothing else 
 can then be done, but to compress the artery with the thumb, 
 or with some round hard substance (such as the handle of an 
 awl), until proper assistance can be obtained. 
 
238 FLOW OF BLOOD THROUGH THE VEINS. 
 
 278. The impulse of the heart, and the elasticity of the 
 arteries, which together propel the blood through the capillary 
 system, continue to act upon it after it is received into the 
 veins ; and are in fact the chief causes of its movement in 
 them. If we interrupt the current of blood through an artery 
 by making pressure upon it, and open the corresponding vein, 
 the fluid will continue to flow from the latter, so long as the 
 artery contains blood enough to be forced into the vein by its 
 own contraction ; but as soon as it is emptied, the flow from 
 the orifice in the vein will cease, even though the vein itself 
 remains nearly full. If the pressure be then taken off the 
 artery, there is an immediate renewal of the stream from the 
 vein, which may be again checked by pressure on the artery. 
 In the ordinary operation of bleeding, we cause the superficial 
 veins of the arm to be distended, by tying a bandage round 
 them above the point at which we would make the incision ; 
 and when an aperture is made, the blood spouts forth freely, 
 being prevented by the bandage from returning to the heart. 
 But if the bandage be too tight, so that the artery also is 
 compressed, the blood will not flow freely from the vein ; and 
 the loosening of the bandage will then produce the desired 
 effect. When a sufficient quantity of blood has been with- 
 drawn, the bandage is removed ; and the return-flow through 
 the veins being now unobstructed, the stream from the orifice 
 immediately diminishes so as to be very easily checked by 
 pressure upon it, or may even cease altogether. 
 
 a 279. The veins contain a great 
 
 number of valves, which are 
 formed, like the semilunar valves 
 of the aorta ( 273), by 'a 
 doubling of their lining mem- 
 brane. Their situation may be 
 known by the little dilatations 
 which the veins exhibit at the 
 points where they occur ; and 
 which are yery obvious in the 
 arm of a person not too fat, 
 
 -____ ;.jw'j|^ M ' when it is encircled by a bandage 
 that causes distension of the 
 
 Fig. 128,-VEiK LA ID OPEK, TO superficial veins. _ The structure 
 SHOW ITS VALVES. oi these valves is seen at o 0, 
 
FLOW OP BLOOD THROUGH THE VEINS. 239 
 
 fig. 128; they consist of pocket-like folds of the lining mem- 
 brane, which allow the blood free passage as it flows towards 
 the heart, but check its reflux into the arteries. Hence it 
 follows, that every time pressure is made upon the veins, it 
 will force towards the heart a portion of the blood they con- 
 tain, since this cannot be driven in a contrary direction. IsTow, 
 from the manner in which the veins are distributed, some of 
 them must be compressed by almost every muscular move- 
 ment; these will become refilled as soon as the muscles relax; 
 and they will be again pressed-on, when the movement is 
 repeated. Hence a succession of muscular movements will act 
 the part of a diffused heart, over the whole of the venous 
 system, and will very much aid the flow of blood through its 
 tubes. It is partly in this manner, that exercise increases the 
 rapidity of the circulation. If the blood is brought to the heart 
 by the great veins more rapidly than usual, the heart -must go 
 through its operations more rapidly, in order to dispose of the 
 fluid ; and if these actions be impeded, great danger of their 
 entire cessation may exist. Hence the importance of bodily 
 tranquillity to those affected with diseases of the heart or 
 lungs ( 271). 
 
 280. Besides the aid thus afforded to the venous circulation, 
 it is probable that there is another cause of the motion of the 
 blood in them, which is independent of the action of the 
 heart and of the arteries. Many facts lead to the belief that 
 a new force is produced, while the blood is flowing through 
 the capillary vessels, a force which may, in some instances, 
 maintain the circulation by itself alone. Thus in many of 
 the lower animals, it seems as if the power of the heart were 
 so unequal to the maintenance of the circulation, that this 
 must partly depend upon some other influence ; and even in 
 the highest, there is evidence that the movement of blood in 
 the capillaries may continue for a time, after the action of the 
 heart and of the arteries has ceased, to affect it.* This 
 movement seems intimately connected with the changes 
 to which the blood, is subservient in the capillaries; for, 
 if these be checked, not even the heart's action can 
 propel the blood through them, although no mechanical 
 
 * For a full consideration of this question, see the Author's Principles 
 of Comparative Physiology (4th edition), 247-251 ; and Principles 
 of Human Physiology (5th edition), 267-275. 
 
240 CIRCULATION IN MAMMALS AND BIRDS. 
 
 obstruction exists. Thus, when the admission of air to the 
 lungs is prevented, the blood will not pass through the 
 pulmonary capillaries, since it cannot undergo the change 
 which ought to be performed there ; and it therefore accumu- 
 lates in the pulmonary artery, the right side of the heart, and 
 venous system ; and if no relief be afforded by the admission 
 of air into the lungs, the whole circulation is thus brought to 
 a stand. This condition, which is termed Asphyxia, occurs 
 in drowning, hanging, and other forms of suffocation ( 338). 
 
 Course of the Blood in the different Classes of Animals. 
 
 281. The Circulation of the Blood takes place on the 
 same general plan in all other MAMMALS, and in BIRDS, as 
 in MAN. In all the animals included in these groups, the 
 heart is composed of two halves quite distinct from each 
 other ; each possessing an auricle or receiving cavity, and a 
 ventricle or propelling cavity. The course of their blood, 
 which goes through a complete double circulation, is shown by 
 the diagram (fig. 129). The vessels and cavities of the heart 
 which contain venous blood are shaded ; whilst those which 
 convey arterial blood are left white : and this distinction is 
 kept-up in the other figures. The direction of the blood is 
 indicated by the arrows. Every drop of blood which has 
 passed through the capillaries of the system, is transmitted 
 to the lungs before it is allowed again to enter the aorta ; 
 and the whole mass of "the blood passes twice through the 
 heart, before any part of it is transmitted a second time to the 
 vessels from which it was before returned. 
 
 282. The two sides of the heart do not possess, when that 
 organ is perfectly formed, any communication with each other, 
 except through the pulmonary vessels ; and thus they might 
 be regarded as two distinct organs, united for the sake of 
 convenience. The right side of the heart, being placed at 
 the origin of the pulmonary artery, and having for its office 
 to propel the blood through the lungs so as to receive the 
 influence of the air, may be called the respiratory heart : 
 whilst the left side, which is placed at the origin of the 
 aorta, and has to propel the blood to the body in general, may 
 be called the systemic heart. The circulation would be per- 
 formed precisely in the same manner, if these two organs 
 
DIFFERENT FORMS OF CIRCULATING APPARATUS. 241 
 
 Lesser circulation. 
 
 Pulmonary veins 
 
 Pulmonary artery 
 
 Left auricle 
 
 Right ventricle ,' 
 
 Aorta 
 
 Left ventricle 
 
 Greater circulation. 
 Fig. 129. DIAGRAM OF THE CIRCULATION IN MAMMALS AND BIRDS. 
 
 were quite distinct from each other ; and in fact they are 
 almost so in the Dugong, one of the herbivorous Whales- 
 Lesser circulation. 
 
 Heart 
 
 Vena cava 
 
 Aorta 
 
 Single ventricle 
 Greater circulation. 
 
 Fig. 130. DIAGRAM OF THE CIRCULATION IN REPTILES. 
 R 
 
242 DIFFERENT FORMS OF CIRCULATING APPARATUS. 
 
 Lesser circulation. 
 
 ; Heart 
 
 Dorsal artery 
 
 Veins 
 
 Greater circulation. 
 Fig. 131. DIAGRAM OF THE CIRCULATION IN FISHES. 
 
 (ZOOLOGY 305). In the lower tribes of animals we shall 
 
 Lesser Circulation. 
 
 Bran chio- cardiac canals 
 
 :..._ Heart 
 
 Veins 
 
 Arteries 
 
 Greater circulation. 
 Fig. 132. DIAGRAM OF THE CIRCULATION IN CRUSTACEA. 
 
CIKCULATION IN FCETUS AND IN REPTILES. 243 
 
 presently find that there is but a single, instead of a double, 
 heart ; and that the organ which is absent is sometimes the 
 systemic, and sometimes the respiratory heart. 
 
 283. Previously to birth, when the lungs are not yet dis- 
 tended with air, and the aeration of the blood is provided-for 
 in other ways, the circulation takes place on a different plan 
 from that on which it is afterwards performed. There exists 
 at that period an opening in the partition between the two 
 auricles, by which they have a free communication; and 
 there is also a large trunk which passes from the right 
 ventricle into the aorta. By these channels, the blood which 
 is received from the systemic veins can pass at once into the 
 aorta, without going through the pulmonary vessels. But 
 when the young animal begins to breathe, these communi- 
 cations are speedily obliterated; the blood is transmitted 
 through the pulmonary vessels to the lungs ; and the whole 
 circulation takes place upon the plan just described. There 
 are occasional instances, however, in which the communica- 
 tion between the auricles remains open, so that the double 
 circulation is never perfectly established ; for a portion of the 
 blood is allowed to pass from the right to the left side of the 
 heart, without being aerated in the lungs, so that the blood 
 which is sent to the system contains a mixture of venous with 
 the proper arterial fluid, a state which will be presently seen 
 to be natural in the Reptile. Such cases are recognised by 
 the blueness of the skin, the lividity of the lips, and the 
 indisposition to bodily or mental exertion. Persons affected 
 with this malformation seldom reach adult age. 
 
 284. In the class of REPTILES, there is not a complete 
 double circulation ; for a mixture of arterial and venous blood 
 is sent alike to the lungs and to the general system ; and no 
 part is supplied with the pure arterialized fluid. In general 
 the heart contains only three cavities, two auricles and one 
 ventricle (fig. 133). One of the auricles receives the venous 
 blood from the system ; whilst the other receives the arterial- 
 ized blood from the lungs. Both these pour their contents 
 into the same ventricle, where they are mingled together; 
 and this mingled blood is transmitted, by the contraction of 
 the ventricle, partly into the lungs, and partly into the aorta 
 (fig. 130). In some Reptiles there is a partial division of 
 the ventricle, so that the mixture of the arterial and venous 
 
 R2 
 
244 CIRCULATION IN REPTILES. 
 
 blood is not complete ; and whilst the Wood transmitted to 
 the lungs is chiefly that which has returned from the systemic 
 veins, the blood which enters the aorta for the supply of the 
 
 Pulmonary artery ^^jtJ^f^^WjS^^S*^** Pulmonary artery 
 
 fll /^Jsi40fe. fflxmPulmonary vein 
 Pulmonary vein ] 
 
 Right auricle- 
 
 Left auricle 
 
 Single ventricle 
 
 Ventral aorta 
 Fig. 133. HEART OF TORTOISE. 
 
 system is chiefly that which has returned from the lungs in an 
 arterialized state. Hence such animals have a circulation 
 which approaches very closely to that of Mammals and Birds ; 
 and it is among them that we find the greatest vigour and 
 activity in this generally inert and sluggish class. 
 
 285. The general arrangement of the blood-vessels in 
 Eeptiles is shown in fig. 134. It is seen that the aorta, 
 soon after its origin, divides into three arches on either side ; 
 and that these, after sending off branches to the head and to 
 the lungs, reunite into a single trunk, which corresponds 
 exactly with the aorta of the higher animals. These arches 
 are in fact the remains of a set of vessels, which will be 
 found to be of the highest importance in Fishes, being there 
 subservient to the aeration of the blood : in the true Reptiles, 
 however, they are never concerned in this function, but they 
 still remain, as if to show the unity of the plan on which 
 this apparatus is formed. Precisely the same arrangement of 
 the vessels may be seen in Birds and Mammalia, at an early 
 stage of their development; but it afterwards undergoes 
 considerable changes, by the obliteration of several of the 
 arches ; for of the four pairs which may be seen at one period, 
 a single branch only remains on either side ; and one of these 
 becomes the permanent arch of the aorta, whilst the other 
 becomes the permanent pulmonary artery. 
 
CIRCULATION IN REPTILES. 
 
 245 
 
 Arches of aorta 
 
 Left auricle 
 
 \ 
 
 \ 
 
 Super, vena cava _ 
 
 Ventral aorta 
 
 Pulmonary artery 
 
 Inferior vena cava 
 
 Liver and hepatic 
 vein 
 
 Kidneys 
 Ventral Aorta 
 
 Carotid artery 
 
 Arches of Aorta 
 Right Auricle 
 
 Ventricle 
 Pulmonary vein 
 Brachial artery 
 
 Pulmonary artery 
 
 Lungs 
 Stomach 
 
 Gastric vein 
 
 Vena portas 
 Intestines 
 
 Fig. 134. CIRCULATING APPARATUS OB LIZARD. 
 
 286. In the class of FISHES, the circulating apparatus is 
 still more simple. The heart only possesses two cavities, an 
 
246 
 
 CIECULATION IN FISHES. 
 
 auricle and a ventricle. It is placed at the origin of the 
 vessels which are concerned in the aeration of the blood ; and 
 
 Branchial artery 
 
 Arterial bulb , 
 
 
 Ventricle -^ 
 
 Auricle 
 
 Venous sinus 
 
 Vena portae, liver, &c. 
 
 Intestine 
 
 Venacava 
 
 Vessels of the gills 
 
 , Dorsal artery 
 
 .. Kidneys 
 
 . Dorsal artery or aorta 
 
 Fig. 135. CIRCULATING APPARATUS OF FISH. 
 
 it receives and transmits venous blood only; hence it is 
 analogous to the right side, or respiratory heart, of Birds and 
 Mammalia. The venous blood, which is brought to it by the 
 
CIKCULATION IN BATEACHIA. 247 
 
 systemic veins, is transmitted by its ventricle into a trunk, 
 which subdivides into four or five pairs of branches or arches 
 (fig. 135). These branches run along the fringes which form 
 the gills of the fish, and send a minute vessel into every one 
 of their filaments ( 312). Whilst passing through this 
 vessel, the blood is submitted to the influence of the air 
 diffused through the water, to which the gills are freely 
 exposed, and is thus aerated; and it is then collected from 
 the several filaments and fringes, into a single large trunk, 
 analogous to the aorta of the higher animals, by which the 
 whole body is supplied "with arterialized blood. After circu- 
 lating through the system, the blood returns to the heart in a 
 venous condition, and again goes through the same course. 
 This course is represented in a simple form in the diagram, 
 fig. 131 ; and it will be seen, on a little consideration, that 
 it does not differ from that which exists in Animals with a 
 complete double circulation, in any other essential particular 
 than this, that there is no systemic heart to receive the blood 
 from the gills or aerating organs, and to convey it to the body 
 at large. But, though all the blood must necessarily pass 
 through the gills before it can again proceed to the body, it 
 does not follow that the blood should be as completely aerated 
 as in Reptiles, in whose circulation there is a mixture of 
 venous and arterial blood; for the exposure of the blood to 
 the small quantity of air which is diffused through water is 
 not nearly so effectual as its direct exposure to air. 
 
 287. There is a group of animals which forms the transition 
 between Fishes and Eeptiles ; some of them being Fishes at 
 one part of their lives, and Reptiles at another ; whilst others 
 remain, during their whole lives, in a condition intermediate 
 between the two groups. Of this group ( 86), the common 
 Frog is a familiar example. In the Tadpole state, it is essen- 
 tially a Fish, breathing by means of gills, and having its cir- 
 culation upon a corresponding plan; but after it has gone 
 through its metamorphoses, it breathes by lungs, its heart 
 acquires an additional auricle, and the whole plan of the circu- 
 lation is changed, so as to become comformable to that of the 
 true Reptile. This process takes place, not suddenly, but by 
 progressive stages ; and as these are extremely interesting, 
 they will now be briefly described. In fig. 136 we have a 
 representation of the circulating apparatus of the Tadpole in 
 
248 CHANGE OP CIRCULATION IN TADPOLE. 
 
 its fish-like condition. At a is seen the large trunk which 
 issues from the ventricle, forming a bulbous enlargement like 
 that which is seen in the corresponding part of the Fish. 
 From this enlargement proceed three trunks on each side, 
 called the branchial arteries (br\ br 2 , br*), which convey the 
 blood to the gills or branchice; and after being aerated bypassing 
 through their filaments, the blood is collected by the bran- 
 
 ab 
 
 a ap av c ab 2 vb 
 
 Fig. 136. BLOOD-VESSELS OF THE TADPOLE, IN FIRST STATE. 
 
 chial veins (vb, vb). Of these, the first pair transmits its blood 
 by the vessels o, o, t, (which are also formed in part by the 
 "econd pair) to the head and upper extremities ; whilst the 
 greater part of the blood of the second pair, with the whole 
 of that of the third, is discharged into the trunk c on either 
 side. Ey the union of that vessel with its fellow, the trunk 
 a v is formed ; which conveys the blood that has been aerated 
 in the gills, to the general system, and is thus to be evidently 
 regarded as the aorta. But we find here three small vessels 
 (1, 2, and 3), which do not exist in the Fish; and which 
 establish a communication between the branchial arteries and 
 the branchial veins, in such a manner, that the blood may 
 pass from the former into the latter, without going through 
 the filaments of the gills. These communicating vessels are 
 very small in the Tadpole, and scarcely any blood passes 
 through them ; but it is chiefly by their enlargement, that the 
 course of the blood is subsequently altered. There is also a 
 fourth branch, ap, which proceeds to the lungs on either 
 side ; and as these organs are not yet developed, this pulmo- 
 nary artery also is at first of very small size. 
 
 288. As the metamorphosis of the other parts proceeds, 
 
CIRCULATION IN TADPOLE. 249 
 
 however, and the animal is being prepared for its new mode 
 of life, the lungs are gradually developed, and the pulmonary 
 arteries greatly increase in size ; whilst the gills, on the other 
 hand, do not continue to grow with the animal, but rather 
 shrink, from the diminished supply of blood w T hich they 
 receive. For, during this period, the communicating branches 
 
 ap av ap 
 
 Fig. 137. THE SAME, IN A MORE ADVANCED STATE. 
 
 increase in size ; so that a considerable part of the blood which 
 has been transmitted into the branchial arteries passes at once 
 into the veins, and thence into the aorta, without being made 
 to traverse the gills ; its aeration being partly accomplished 
 by the lungs. This state of things is seen in fig. 137 ; where 
 ap, ap, are the enlarged pulmonary arteries ; and where the- 
 communicating branches are seen almost to form the natural 
 continuations of the branchial arteries. A condition of this 
 kind exists permanently in those Batrachia which retain their 
 gills during their whole lives, and have the lungs imperfectly 
 developed ( 87). When the metamorphosis is complete, the 
 branchial vessels altogether disappear, but the arches still 
 remain, as shown in fig. 138. The first of these arches sup- 
 plies the vessels of the head, 1 1 ; which also, however, receive 
 a branch o from the second arch. The second arch, after 
 giving off that branch, unites with its fellow to form the 
 aortic trunk av. The third arch has completely shrivelled 
 up. And the fourth arch or pulmonary artery has now 
 attained its full size, and is become the sole channel through 
 which the aeration of the blood is effected. 
 
250 CIRCULATION IN INVERTEBRATED CLASSES. 
 
 289. Among the Invertebrated classes generally, the condi- 
 tion of the circulating apparatus differs from that which prevails 
 
 br 1 
 
 ap av ap 
 
 Fig. 138. THE SAME, IN THE PERFECT ANIMAL. 
 
 throughout the Vertebrata, in one remarkable feature; 
 namely, that whereas in the latter the blood moves in every 
 part of its course through a set of closed vessels, it meanders 
 in the former through a set of channels or sinuses excavated 
 in the substance of the tissues, and communicating with the 
 " general cavity of the body " in the midst of which the viscera 
 lie. Generally speaking, it is in the venous system that the 
 greatest deficiency exists ; for the heart usually sends forth 
 the blood by definite arterial trunks, which distribute it by 
 its ramifications through the substance of the various parts of 
 the body ; and it is in its course from these to the respiratory 
 organs that it is least restrained within definite boundaries. 
 The degree of this imperfection differs considerably in the 
 several groups of Invertebrata ; for whilst, in the highest Mol- 
 lusca and Articulata, the vascular system is almost as complete 
 as in Vertebrated animals, we find it gradually becoming less 
 and less distinct as we descend, so that in the lower forms of 
 both series it presents itself merely as an extension of the 
 general cavity of the body, and is not furnished with any 
 special organ of impulsion. 
 
 290. In the greater part of the MOLLUSC A, the circulation 
 
CIRCULATION IN MOLLUSCA. 
 
 251 
 
 takes place nearly on the same general plan as in Fishes ; the 
 heart having.two cavities, and the whole of the blood travers- 
 ing "both the respiratory and the systemic vessels, between 
 each time of its leaving the heart and returning to it again. 
 But this heart is systemic, and not pulmonary ; for it receives 
 the arterial blood from the gills, and transmits it to the great 
 systemic artery ; and after the blood has been rendered venous 
 by its passage through the tissues of the body, it enters the 
 channels which distribute it to the gills, before being again 
 subjected to the action of the heart. The accompanying figure 
 (fig. 139) of the circulation in the Doris (a kind of sea slug) 
 
 Fig. 139. CIRCULATING APPARATUS OF DORIS. 
 
 will serve to show the general distribution of the vessels in 
 this group. The heart consists of the ventricle a, whence 
 issues the main artery I ; and of a single or double auricle c, 
 in which terminate the veins, d, of the branchial apparatus e. 
 The aerated blood which these convey to the heart, is trans- 
 mitted by it, through the artery 6, to the system at large ; and 
 from this it is collected, in the* state of venous blood, by the 
 sinuses which terminate in the large trunk //. By this trunk it 
 
252 CIRCULATION IN GASTEROPODA AND CEPHALOPODA. 
 
 is distributed to the gills e; and thence it returns to the heart, 
 after having undergone aeration. Now if a second heart had 
 been placed on the trunk //, just as it is about to subdivide 
 for the distribution of the blood to the gills, the circulation 
 would have been analogous to that of Birds and Mammals. 
 There is a great variety in the position of the gills in Mollus- 
 cous animals, and a corresponding variety in the situation of 
 the heart, which is usually placed near them. In the Doris 
 the gills are arranged in a circular manner, round the termina 
 tion of the intestinal canal ; but in many Mollusca they form 
 straight rows of fringes on the two sides of the body. In 
 these last, the heart not unfrequently has two auricles ; but 
 these are not analogous to the two auricles of Eeptiles ; for 
 each has the same function with the other the reception of 
 the blood from the gills of its own side. 
 
 291. There is a very interesting variety in the conformation 
 of the heart in the CEPHALOPODA, or Cuttle-fish tribe; which 
 
 vb 
 
 .... br 
 
 ab 
 
 \ 
 
 vv av a cs vv 
 
 Fig. 140. CIRCULATING APPARATUS OF CUTTLE-FISH. 
 
 seems to form a connecting link between the plan of the cir- 
 culation that prevails among the Mollusca in general, and that 
 
CIRCULATION IN CEPHALOPODA AND CRUSTACEA. 253 
 
 which we have seen in the class of FISHES. The auricle and 
 ventricle of the heart are separated from each other; and 
 whilst the latter remains in the position just described, the 
 auricle occupies the place which the whole heart possesses in 
 the class above. The course of the blood in these animals is 
 shown in fig. 140; where c represents the ventricle or sys- 
 temic heart, from which arises the aorta a, a, as, av, that 
 supplies the body with arterial blood. The venous blood is 
 returned through the great vein vc, covered with a curious 
 spongy mass cs, the use of which is not known; this also 
 receives the blood from the intestinal veins vv ; and it divides 
 into two trunks which convey the blood to the gills or branchiae 
 (br and br), where it undergoes aeration. On each of these 
 trunks is an enlargement, cb, which has the power of con- 
 tracting and dilating, and thus of assisting the transmission 
 of the blood through the arteries of the gills, a b. The blood 
 is returned to the ventricle by the branchial veins, vb, on 
 each of which there is another dilatation, bu, which might be 
 regarded as analogous to the auricle of the other Mollusca, 
 but that it is not muscular. Thus in the Cuttle-fish, the blood 
 receives an impulse from the systemic heart, by which it is 
 transmitted into the main artery ; and when it returns by the 
 systemic veins, it receives another impulse from the branchial 
 hearts, before it passes through the gills ; an arrangement 
 obviously analogous to that which we meet with in the highest 
 Vertebrata. 
 
 292. In the Crab and Lobster, and other animals of the 
 class CRUSTACEA, the blood for the most part follows the same 
 
 e f i a d b 
 
 Fig. 141. CIRCULATING APPARATUS OF LOBSTER. 
 
 course as in the Mollusca, excepting that the heart contains 
 but a single cavity. The arrangement of the circulating appa- 
 
254 CIRCULATION IN CRUSTACEA. 
 
 ratus of a Lobster is seen in fig. 141, in which, a is the heart ; 
 6 and c, the arteries to the eyes and antennae ; d, the hepatic 
 artery ; and e and f, the arteries which supply the abdomen 
 and thorax. The blood that has been propelled through these 
 by the action of the heart, finds its way into the great venous 
 sinus g g } which receives the fluid collected from all parts of 
 the body ; from this it passes to the gills, h ; and thence it 
 is returned to the heart by the branchial veins, i. Another 
 view of a portion of the circulating apparatus is given in fig. 
 14:2, which represents a transverse section of it in the region 
 
 b ve c f vb 
 
 st ce 
 Fig. 142. BRANCHIAL CIRCULATION OF LOBSTER. 
 
 of the heart, with one pair of gills. The heart is seen at c ; 
 and from its under side proceeds one of the arterial trunks 
 which convey the blood to the system. Returning thence, 
 the blood enters the venous sinus s, which has an enlarge- 
 ment at the base of each gill ; and this seems to act the part 
 of a branchial heart, like the corresponding enlargement on 
 the branchial vessels of the Cuttle-fish. From this cavity, it 
 is carried by the vessel va into the branchiae 5 ; and after it 
 has passed through the capillaries of the gill-filaments, it is 
 collected by the vessels ye, which carry it to the branchial 
 veins, v!>, and thence to the heart. The general plan of the 
 circulation in this class is shown in fig. 132. 
 
 293. In the class of INSECTS we find a still greater incom- 
 pleteness in the system of vessels for the conveyance of blood. 
 Arterial trunks can only be traced to a short distance from 
 the dorsal vessel, which answers the purpose of a heart ; and 
 the nutritive fluid which they convey is delivered into the 
 channels or sinuses that exist among the different organs. 
 
CIRCULATION 
 
 INSECTS. 
 
 255 
 
 Nevertheless, it has a tolerably regular circulation ; and the 
 organ by which this movement is chiefly effected is .a long 
 tube, termed the dorsal vessel, which seems to propel it for- 
 wards, whilst two principal sinuses, one on either side, convey 
 it backwards. The dorsal vessel, seen at a, is a membranous 
 
 Fig. 143.--CiiicuLATioN IN INSECTS. 
 
 tube lying along the back of the insect, and partly divided 
 into several compartments by incomplete valvular partitions, 
 which bear no inconsiderable resemblance to the valves of veins 
 ( 279). By the successive contraction of these different por- 
 tions, the blood which entered at the posterior extremity of 
 the dorsal vessel is gradually propelled forwards ; and when 
 it arrives at the front of the body, it passes out by a series of 
 canals, some of which convey it to the head, whilst others 
 pass sideways and backwards for the supply of the body, with 
 its appendages, the legs and wings. On returning from these 
 parts, it re-enters the posterior end of the dorsal vessel. But, 
 Asides ministering to this general circulation, the several 
 compartments of the dorsal vessel seem to act as independent 
 hearts, each for its own segment ; into which they send forth 
 blood ^by rainute arterial trunks, and from. which they receive 
 it again by minute apertures furnished with valves. It is to 
 be remarked that in Insects no special arrangement of vessels 
 for the aeration of blood is required ; since this aeration is 
 
256 
 
 CIRCULATION IN ARACHNIDA AND ANNELIDA. 
 
 Fig. 144. DORSAL VESSEL 
 OP SPIDER. 
 
 accomplished by the conveyance of air, by means of minute 
 air- tubes, into every part of the body, however small ( 321) ; 
 a mode of respiration different from 
 any that we notice elsewhere. A very 
 similar arrangement of the circulating 
 apparatus is met with in the Spider 
 tribe ; but as the body is not so long, 
 the dorsal vessel is less extended in 
 length, and is of larger diameter. This 
 is seen in fig. 144 ; where a represents 
 the abdomen of the animal; ar, the 
 large dorsal vessel or heart ; c } a trunk 
 passing forwards to the head; and v, 
 vessels communicating with the re- 
 spiratory organs. 
 
 294. In the animals of the Worm tribe, 
 belonging to the class Annelida, there 
 is a general similarity in the course of 
 the blood to that which prevails in Insects ; but as the respi- 
 ration is accomplished by means of special organs, which are 
 sometimes diffused along the entire body, and sometimes 
 restricted to one part of it ( 314), there is considerable variety 
 in the provisions for submitting the blood to the influence of 
 the air. In those which possess red blood ( 226), this fluid 
 can be seen to move in a closed system of vessels ; whilst a 
 colourless fluid containing numerous corpuscles flows through 
 a set of canals prolonged from the general cavity of the body. 
 It may be surmised that the two principal offices to which the 
 circulation of the blood is subservient, are here separately 
 performed; the red non-corpusculated fluid having for its 
 office to aerate the tissues, whilst the colourless but corpus- 
 culated fluid serves for their nutrition. 
 
 295. A very curious departure from the normal type of the 
 circulation presents itself in the class of TUNICATA, the lowest 
 of the Molluscous series ( 114). The heart in these animals 
 is much less perfectly formed than in the higher tribes ; 
 though it still contains two cavities, one for receiving and the 
 other for impelling the blood. The blood may be sometimes 
 seen to flow in the direction customary among Mollusks; 
 coming to the heart from the respiratory surface, and then 
 going forth through an arterial trunk that conveys it into a 
 
CIRCULATION IN LOWER INVERTEBRATA. 257 
 
 system of channels excavated through the tissues, after passing 
 through which it finds its way again to the respiratory sur- 
 face, and thence to the heart. But after a certain duration of 
 its flow in this direction, the current stops, and then re-com- 
 mences in the contrary direction, proceeding first to the 
 respiratory organs, and then to the system in general. It 
 would seem as if in this, one of the lowest forms of animals 
 possessing a distinct circulation, the central power were not 
 yet sufficiently strong to determine the course which the fluid 
 is to take. In the group of JBryozoa, which forms a connecting 
 link between the Tunicata and Zoophytes, we lose all trace 
 of a distinct circulation, which is only represented by the 
 movement of fluid in the general cavity of the body, and in 
 the prolongations of this cavity in the arms that surround 
 the mouth (fig. 64). In the Star-fish, Sea- Urchin, and other 
 animals belonging to the class ECHINODERMATA, there seems 
 to be a regular circulation of nutritious fluid, carried on through 
 distinct vessels, but without any definite heart The only 
 trace, indeed, of anything like a propelling organ, is an en- 
 largement of one of the trunks, which pulsates with tolerable 
 regularity ; but this would not seem to have force enough to 
 propel the fluid through a complex system of vessels ; and 
 the circulation seems to be carried on chiefly by some force 
 produced in the capillaries ( 280). 
 
 296. The circulating apparatus of higher animals is only 
 represented in Zoophytes, Medusae, and the lower Worms, by 
 ramifying prolongations of the digestive cavity, which extend 
 throughout the body, and are specially distributed to the 
 respiratory surface, so as to subject the products of digestion 
 to the aerating process. Thus, in the stony corals which are 
 formed by animals constructed upon the general plan of the 
 Sea Anemone ( 127), the gelatinous flesh that connects the 
 polypes is traversed by a network of canals that open freely 
 into the sides of their visceral cavities, of which they may be 
 regarded as extensions ; whilst in the Campanularia (fig. 72) 
 and other composite Hydraform Zoophytes, a like communi- 
 cation is established by a system of canals passing along the 
 stem and branches, and becoming continuous with the base of 
 each polype. In this system of canals, viewed under a sufficient 
 magnifying power, a granular fluid may be seen to move, the 
 direction of the flow being sometimes from the stem towards 
 
258 CIRCULATION IN ZOOPHYTES AND SPONGES. 
 
 the polypes, and sometimes from the polypes towards the 
 stem ; the rapidity and constancy of these currents depending 
 apparently on the activity of the growth of the parts towards 
 which they are directed. In the Medusae we find the central 
 stomach sending out prolongations towards the margin of the 
 disk, where they frequently inosculate so as to form a net- 
 work, which seems to have for its purpose to expose the 
 product of digestion to the aerating action of the surrounding 
 water ; and in this system of canals, also, a movement of fluid 
 may be observed, which appears to depend upon the action of 
 cilia in their interior. In all these cases, it is to be observed 
 that the circulation of nutritive fluid is really effected by a 
 modification of the digestive apparatus, instead of by an appa- 
 ratus set apart for this sole purpose ; and the blending of the 
 two functions is still more remarkably exhibited in the Sponge, 
 the inosculating canals of which ( 136) may be regarded 
 alike as constituting a ramifying digestive cavity, or as a simple 
 form of circulating apparatus. The most correct method is 
 perhaps to consider it as representing both these systems, 
 which are here blended (as it were) into one ; the simplicity 
 of structure characteristic of this type not admitting of the 
 division of labour which we meet with in higher organisms. 
 
 CHAPTER VI. 
 
 OP RESPIRATION. 
 
 297. WE have seen that arterial blood, by its action on the 
 living tissues, loses those qualities which rendered it fit for 
 the maintenance of life ; and that after having undergone 
 this change, it recovers its original properties by exposure to 
 air. This exposure is necessary, therefore, to the continued 
 existence of Animals in general. If we place an animal 
 under the receiver of an air-pump, and exhaust the air either 
 partially or completely, a great disturbance soon shows itself 
 in its various functions ; shortly afterwards, the several 
 actions of life cease to take place; and a state of apparent 
 death comes on, which speedily becomes real, if air be not 
 re-admitted. The influence of air is not less necessary to 
 
NATURE OP RESPIRATION. 259 
 
 Plants than to Animals; for they also die when excluded 
 from it : and thus its presence may be stated to be a general 
 condition, necessary for the continuance of the life of all 
 organised beings. There is, however, a marked difference in 
 the rapidity with which the deprivation of air occasions 
 death in different animals ( 310). 
 
 298. At first sight it might be thought that Animals which 
 always live beneath the surface of the water, such as Fishes, 
 Zoophytes, and many Mollusca, are removed from the influence 
 of the air ; and that they hence constitute an exception to this 
 general law. But such is not the case ; for the liquid which 
 they inhabit has the power of absorbing, and of holding dis- 
 solved in it, a certain quantity of air, which they can easily 
 separate from it, and which is sufficient for the maintenance 
 of their life. They cannot exist in water which has been 
 deprived of air (as by boiling, or by being placed under the 
 exhausted receiver of an air-pump) ; for they then become 
 insensible and die, just as do Mammalia and Birds when 
 prevented from inhaling air in the ordinary manner. 
 
 299. The changes which result from the exposure of the 
 blood or nutritious fluid of Animals to the air, either in the 
 atmosphere, or through the medium of water, form a very 
 important part of their vital actions ; and the changes them- 
 selves, together with the various mechanical operations by 
 which they are effected, constitute the function of Respiration. 
 The nature of these changes will be first explained ; and the 
 structure and operations of the organs by which they are 
 performed will be afterwards described. 
 
 Nature of the Changes essentially constituting Respiration. 
 
 300. Atmospheric air, it has been stated, is necessary to 
 the continued life of all animals ; but this fluid is not com- 
 posed of one element alone. By the science of Chemistry, 
 it is shown to be a mixture of three gases possessing very 
 different properties. Besides the watery vapour with which 
 the atmosphere is always more or less charged, the air con- 
 tains 21 parts in 100 of oxygen, and 79 parts of nitrogen or 
 azote ; with about l-5000th part of carbonic acid gas. The 
 question immediately presents itself, therefore, whether these 
 gases have the same action on animals ; or, if their actions 
 
 s2 
 
260 CHANGES IN AIR BY RESPIRATION. 
 
 be different, to which, of them specially belongs the property 
 of thus contributing to the maintenance of life. This question 
 may be decided by a few simple experiments. If we place 
 a Bird or small Mammal in a jar filled with air, and cut off 
 all communication with the atmosphere, it perishes by suffo- 
 cation in a longer or shorter time ; and the air in the vessel, 
 which has thus lost the power of maintaining life, is found 
 by chemical analysis to have lost the greater part of its 
 oxygen. If we then place another animal in a jar filled with 
 nitrogen gas, it perishes almost immediately; whilst if we 
 place a third in pure oxygen, it breathes with greater activity 
 than in air, and shows no sign of suffocation. It is then 
 evident, that it is to the presence of oxygen that atmospheric 
 air owes its vivifying properties. 
 
 301. But the change produced in the atmosphere by animal 
 respiration is not limited to this. The oxygen which disap- 
 pears is replaced by carbonic acid ; which, instead of being 
 favourable to the maintenance of life, causes the death of 
 animals which inhale it, even in small quantities. The 
 exhalation of this substance is an action not less general in 
 the Animal kingdom than the absorption of oxygen ; and it 
 is in these two changes that the act of respiration essentially 
 consists. 
 
 302. The quantity of nitrogen or azote in the air that has 
 been respired, varies but very little. There appears, however, 
 to be a continual absorption of nitrogen by the blood, and 
 as continual an exhalation of it. When the quantity exhaled 
 and the amount absorbed are equal, or nearly so, no change 
 manifests itself in the air which has been breathed ; when the 
 quantity absorbed is the greater, there is a diminution in that 
 which the respired air contains; and when the quantity 
 exhaled is the greater, there is a corresponding increase. An 
 exhalation of nitrogen seems to be ordinarily taking place in 
 warm-blooded animals, to an extent varying between l-50th 
 and 1-1 00th of the oxygen consumed; but when the same 
 animals are partially or wholly deprived of food, an absorption 
 of nitrogen usually occurs. 
 
 303. The differences in the character of the blood which 
 are produced by its exposure to the air, have already been 
 noticed ( 227); and we now see that they are essentially due 
 to the absorption of oxygen, and the setting free of carbonic 
 
CHANGES IN BLOOD BY RESPIRATION. 261 
 
 acid. These changes will take place out of the living body as 
 well as in it ; provided that the blood can be exposed as com- 
 pletely to the influence of the atmosphere. When blood is 
 drawn from a vein into a basin or cup, the dark hue of the 
 surface is usually seen to undergo a rapid alteration, so as to pre- 
 sent the arterial tint ; but this is confined to the upper surface, 
 because it alone is exposed to the influence of the atmosphere. 
 The alteration takes place still more rapidly and completely 
 if the blood be exposed to pure oxygen gas ; but even then it 
 is almost confined to the surface. It is not prevented, even 
 though the direct communication between the blood and the 
 gas be cut off by a membranous partition, as it is in the living 
 animal ; for if the "blood be tied up in a bladder, the gas has 
 still the power of penetrating to it, and of effecting the change 
 in it ; and the change is manifested, not only by the alteration 
 in the aspect of the blood, but by the disappearance of a 
 certain quantity of oxygen, and its replacement by carbonic 
 acid. Now if, by spreading out the blood in a very thin layer, 
 we expose a much larger surface to the air, or if, by frequently 
 shaking it, we continually change its surface, we render the 
 change more complete. But still it is accomplished far less 
 effectually than it is in the lungs or gills of a living animal ; 
 for when it is passing through their capillaries, it is divided 
 into an immense number of very minute streams, each of 
 which is completely exposed to the influence of the air, and 
 the combined surface of which is very great. 
 
 304. The question next arises, what becomes of the Oxygen 
 which disappears, and what is the origin of the Carbonic acid 
 which is thus produced by respiration ? This question will 
 now be considered. 
 
 305. When charcoal is burned in a vessel filled with air, 
 the oxygen disappears, and is replaced by an equal bulk of 
 carbonic acid : at the same time there occurs a consider- 
 able disengagement of heat. During respiration, the same 
 phenomena occur : there is always an evident relation between 
 the quantity of oxygen employed by an animal, and the 
 amount of carbonic acid it produces (the latter being usually 
 somewhat less than the former) ; and, as we shall see hereafter 
 (Chap, ix.), there is always a greater or less amount of heat 
 produced. There exists, then, a great analogy between the 
 principal phenomena of respiration, and those of the combus- 
 
262 SOURCE OF CARBONIC ACID EXHALED. 
 
 tion of carbon ; and this agreement in tlie results naturally 
 leads to the belief that the causes of both are the same. It 
 is to be borne in mind, however, that the substitution of 
 carbonic acid for oxygen is not the only change produced by 
 respiration in the air ; for there is nearly always a disap- 
 pearance of oxygen (to an amount sometimes equal to one- 
 third of that exhaled in combination with carbon), which 
 is taken into the system to be applied to other uses 
 ( 343). 
 
 306. It was at one time supposed that the oxygen of the 
 inspired air combines, in the lungs themselves, with the carbon 
 brought there in the blood ; and thus produces the carbonic 
 acid which is expired, occasioning at the same time the 
 development of heat. But this theory is inconsistent with 
 experiment ; for it has been proved that the carbonic acid, is 
 not formed in the lungs, but that it is brought to them in 
 the venous blood of the pulmonary artery ; and that their 
 office is to disengage or get rid of it, whilst they at the same 
 time introduce oxygen into the arterial blood. For in the 
 first place, it can be shown that a considerable quantity of 
 carbonic acid exists in venous blood, from which it may be 
 removed by drawing it into a vessel filled with hydrogen or 
 nitrogen, or by placing it under the vacuum of an air-pump ; it 
 can also be shown that arterial blood contains a consider- 
 able quantity of oxygen. Again, if Frogs, Snails, or other 
 cold-blooded animals, be confined for some time in an atmo- 
 sphere of nitrogen or hydrogen (neither of which gases in itself 
 exerts any injurious effect upon them), they will continue for 
 some tune to give off nearly as much carbonic acid as they 
 would have done in common air; thus proving that the 
 carbonic acid is not formed in the lungs by the union of carbon 
 brought in the venous blood with the oxygen of the air, since 
 here no oxygen was supplied ; and showing that the carbonic 
 acid must have been brought ready-formed. This process, , 
 however, could not be continued for any great length of time, 
 even in cold-blooded animals ; since a supply of oxygen is 
 necessary to the performance of their various functions. And 
 in warm-blooded animals, a constant supply of this element 
 is so much more important, that they will die if cut off from 
 it, even for a short time. 
 
 307. The quantity of oxygen thus taken in, and of carbonic 
 
SOURCE OF CARBONIC ACID EXHALED. 263 
 
 acid thus disengaged, bears a very regular proportion to the 
 amount of exertion which is made during the same time. 
 Hence it is much greater in tribes whose habits are active, 
 than in those which are inert ; and it is also greater in any 
 individual, during a day spent in active exercise, than it is in 
 the same person during a day passed in repose. This obviously 
 results from the fact, now established beyond all doubt, that 
 every muscular contraction or production of muscular-force, 
 and every production of nerve-force by which muscular contrac- 
 tion is usually called forth, involve, as their essential condition, 
 the death and disintegration of proportionate amounts of 
 muscular and nervous substances, which pass from the state of 
 living tissues to that of dead matter ; and for this operation, 
 the presence of the oxygen in arterial blood is required. This 
 oxygen combines with part of the materials thus set free as 
 waste ( 55), and converts them into the products that are 
 thrown off by the various excretions. One of the chief of 
 these products is carbonic acid, which is carried off by the 
 lungs in the manner already described. Thus the presence 
 of oxygen in the blood is essential to the development of 
 nervo-muscular force ; while the elements of the blood 
 itself are required to re-form the tissues which have been thus 
 destroyed. 
 
 308. It is among Birds and Insects that we find the greatest 
 quantity of carbonic acid produced, in proportion to the size 
 of the animals ; and in both these classes we find extraordi- 
 nary provisions for the energetic performance of this function 
 ( 321 and 326). The greater energy of the respiration of 
 Birds than that of Mammals, when compared with the greater 
 number of the red corpuscles in their blood, gives an increased 
 probability to the idea, that the red corpuscles are the chief 
 carriers of oxygen from the lungs to the capillaries of the 
 system, and of carbonic acid from the capillaries of the system 
 to those of the lungs ( 235). The energetic respiration of 
 Insects, though these corpuscles are absent, is fully accounted 
 for by the peculiar manner in which the air enters every part 
 of their bodies (321). In no case do we see the influence 
 of muscular activity, on the amount of carbonic acid thrown 
 off, more strongly manifested than in Insects. A humble-bee, 
 while in a state of great excitement after its capture, made 
 from 110 to 120 respiratory movements in a minute; after 
 
264 DIMINUTION OF RESPIRATION IN TORPID STATE. 
 
 the lapse of an hour, they had sunk to 58 ; and they sub- 
 sequently fell to 46. In the first hour of its confinement it 
 produced about l-3rd of an inch of carbonic acid (a quantity 
 many times greater, in proportion to its size, than that which 
 Man would have set free in the same time) ; and during the 
 whole twenty-four hours of the subsequent day, the insect 
 produced a less amount than that which it then evolved in a 
 single hour. In the Larva state, which is usually one of com- 
 parative inactivity, the respiration is not much above that of 
 the Worm tribes ; and in the Chrysalis state of those which 
 become completely inactive, the amount of respiration is still 
 lower. 
 
 309. This chrysalis state, indeed, bears a strong resemblance 
 to the condition of. torpor in which many animals pass the winter. 
 Reptiles, and most Invertebrata that inhabit the land, become 
 (to all appearance) completely inanimate when the temperature 
 is lowered below a certain point ; yet retain the power of 
 exhibiting all their usual actions when the temperature rises 
 again. In this state, their circulation and respiration appear 
 to cease entirely ; or, if these functions are carried on at all, 
 they are performed with extreme feebleness ; and the animals 
 may be prevented from reviving for a long time, without their 
 vitality being permanently destroyed, if they be surrounded by 
 an atmosphere sufficiently cold. Thus Serpents and Frogs have 
 been kept for three years in an ice-house, and have completely 
 revived at the end of that period. Among Mammals there are 
 several species which pass the winter in a state of torpidity ; 
 but this is less profound than the torpidity of cold-blooded 
 animals, for the circulation and respiration never entirely 
 cease, though they become very slow. There are various 
 gradations between this condition and ordinary deep sleep. 
 Thus some of the animals which hybernate or retire to winter 
 quarters, lay up a supply of food in the autumn, and pass the 
 cold season in a state differing but little from ordinary sleep, 
 from which they occasionally awake, and satisfy their hunger. 
 But others, like the Marmot, are inactive during the whole 
 period, taking no food, and exhibiting scarcely any evidence 
 of life, unless they are aroused. The consumption of oxygen 
 and the production of carbonic acid, under such circumstances, 
 are extremely slight, as might be anticipated from the languor 
 of the circulation and the inactivity of the nervo-muscular 
 
TOLERANCE OP DEPRIVATION OF AIR. 265 
 
 system. But a very slight irritation is sufficient to produce 
 respiratory movements ; the heart's action is quickened ; 
 and the animal for a time shows an increase of its general 
 activity. 
 
 310. Animals will in general bear deprivation of air well 
 or badly, according as the respiration is more or less active. 
 Thus a warm-blooded animal usually dies, if kept beneath 
 water for more than a few minutes ; though there are some 
 which are enabled, by peculiar means, to sustain life much 
 longer ( 265). In cold-blooded animals, however, whose 
 demand for oxygen is much less energetic, this treatment may 
 be continued for a much longer time without the loss of life. 
 Thus the common Water-Newt naturally passes a quarter of 
 an hour or more beneath the surface, without coming up to 
 breathe j and it may be kept down for many times that 
 period without serious injury. And, as we might expect from 
 their peculiar condition, warm-blooded animals, when hyber- 
 nating, may be kept beneath water for an hour or more, 
 without apparent suffering; although the same animals, in 
 their active state, would not survive above three minutes. 
 There is reason to believe that a similar condition may be 
 produced in Man, by the influence of mental emotion, or of a 
 blow on the head, at the moment of falling into the water ; 
 so that recovery is by no means hopeless, even though the 
 individual may have been more than half an hour beneath the 
 surface, 
 
 Structure and Actions of the Respiratory Apparatus. 
 
 311. In animals whose organization is most simple, the act 
 of respiration is not performed by any organ expressly set 
 apart for it ; but it is effected by all the parts of the body that 
 are in contact with the element in which the animal lives, 
 and from which they derive their necessary supply of oxygen. 
 This is the case, for example, in the lower class of Animal- 
 cules, in the Polypes, Jelly-fish, Entozoa, and many other 
 animals. Even in the higher classes, a considerable amount 
 of respiratory action takes place through the skin, especially 
 when it is soft and but little covered with hair, scales, &c., as 
 in Man, and in the Frog tribe ; but we almost invariably find 
 in them a prolongation of this membrane, specially designed to 
 enable the blood and the air to act upon each other, and having 
 
266 STRUCTURE OF RESPIRATORY ORGANS. 
 
 its structure modified for the advantageous performance 
 of this function. This modification consists in the peculiar 
 vascularity of this membrane, that is, in the large number of 
 vessels that traverse its surface ; and also in the thinness of 
 the membrane itself, by which gases are enabled to permeate 
 it the more readily. Moreover, we always find this membrane 
 so arranged, that it exposes a very large surface to the air or 
 water which comes into contact with it ; and this surface may 
 be immensely extended, without any increase in the size of 
 the organ. Thus the small lungs of a Eabbit really expose 
 a much larger respiratory surface to the air, than is afforded 
 by the large air-sacs of a Turtle which are ten times their size. 
 This is effected by the minuteness of the subdivision of the 
 former into small cavities or air-cells, whilst the latter remain 
 as almost undivided bags. 
 
 312. It is desirable to possess a distinct idea of the mode 
 in which the surface is thus extended by subdivision. We 
 may, for the purpose of explanation, compare the lung to a 
 chamber, on the walls of which the blood is distributed, and 
 to the interior of which the air is admitted. This chamber, 
 for the sake of convenience of description, we shall suppose to 
 
 have two long and two short sides, as at A. JSTow if a parti- 
 tion be built-up in the direction of its length, as at B, a new 
 surface will be added, equal to that which the two sides 
 previously exposed ; since both the surfaces of this partition 
 are supplied with blood, and are exposed to the air. Again, 
 if another partition be built-up across the chamber, as at c, a 
 new surface will be added, equal to that which the ends of the 
 chamber previously exposed. And thus, by the subdivision 
 of the first chamber into four smaller ones, the extent of sur- 
 face has been doubled. Now if each of these small ones were 
 divided in the same manner, the surface would again be 
 doubled ; and thus, by a continual process of subdivision, the 
 
STRUCTURE OF RESPIRATORY ORGANS. 267 
 
 surface may be increased to almost any extent compatible with 
 the free access of air to the cavities, and of blood to the walls. 
 In the same manner, where the respiratory membrane is pro- 
 longed outwardly, so as to form gills, which hang from the 
 exterior of the body (as is the case in most aquatic animals), 
 its surface is very much extended by disposing it in folds, and 
 by dividing these folds into fringes of separate filaments. It 
 has been calculated that, by this kind of arrangement, the 
 gills of the Skate present a surface four times as great as that 
 of the Human body. 
 
 313. The structure and arrangement of the Eespiratory 
 organs differ, according as they are destined to come in con- 
 tact with air in the state of gas, or to act upon water in which 
 a certain amount of air is dissolved. In the former case, we 
 usually find the respiratory membrane (which is but a pro- 
 longation of the skin or general envelope) forming the wall of 
 an internal cavity, just in the same manner as the membrane, 
 through which the act of absorption takes place in animals, 
 is prolonged from the skin so as to form the wall of the 
 digestive cavity ( 14). Such a cavity for the reception of 
 air into the interior of the body, exists in all air-breathing 
 animals ; and in the Vertebrata it receives the name of lung. 
 On the other hand, in animals that breathe by means of water, 
 the respiratory surface is prolonged externally, so as to be 
 evidently but an extension of the general surface, -just in the 
 same manner as the roots of plants are prolonged into the soil 
 around them. These prolongations, termed branchice or gills, 
 which may have various forms, carry the blood to meet the 
 surrounding water, and to be acted-on by the air it contains ; 
 and then return it to the body in a purified condition. 
 
 314. The form and arrangement of the gills vary greatly 
 in the different classes of aquatic animals. Sometimes they 
 simply consist of little leaf-like appendages, which have a 
 texture rather more delicate than that of the rest of the skin, 
 and which receive a larger quantity of blood. In other in- 
 stances, they are composed of a number of branching tufts, 
 which are more copiously supplied with vessels. Among 
 the ANNELIDA we observe a great variety in the mode in 
 which these tufts are disposed; and this is connected with 
 the general habits of the animal. Thus in the Serpula (fig. 
 145), whose body is inclosed in a tube, the tufts are disposed 
 
268 
 
 RESPIRATORY ORGANS OP AQUATIC ANIMALS. 
 
 around the head alone, and spread out widely into the sem- 
 blance of a flower. In the Nerds (fig. 52) and its allies, they 
 are set upon nearly every 
 division of the body, and are 
 much smaller. Their usual 
 arrangement in these marine 
 worms may be seen in fig. 
 146, which represents one 
 of the appendages of Eunice. 
 The tuft of gills is shown 
 at b ; at c is seen a bristle- 
 shaped filament, which may 
 perhaps be regarded as the 
 rudiment of a leg ; and the 
 projections to which the 
 letters t and ci point, also 
 seem connected with the 
 movements of the animal. 
 In the Arenicola (the lob- 
 worm of fishermen) we find 
 the respiratory tufts dis- 
 posed on certain segments 
 
 only, and possessing more of an arborescent (tree- 
 like) form (fig. 147). 
 
 315. A somewhat similar 
 arrangement is seen in the 
 larvae of many aquatic IN- 
 SECTS, which breathe by 
 means of gills ; although all 
 perfect Insects breathe air 
 in the manner to be pre- 
 sently described. In fig. 
 148 is represented the larva 
 of the Ephemera (Day-fly), 
 which breathes by means 
 
 of a series of gill-tufts disposed along the abdomen, 
 and also prolonged as a tail. In the CRUSTACEA, we usually 
 find the gills presenting the form of flattened leaves or plates. 
 In the lower tribes of the class, they project from the surface 
 of the body; but in the higher, they are inclosed within 
 a cavicy, through which a stream of water is made con- 
 
 . 145. 
 GILL-TUFTS OF SERPULA. 
 
 Fig. 146. 
 GILL-TUFT OF EUNICE. 
 
 Fig. 147. 
 ARENICOLA. 
 
RESPIRATORY ORGANS OF AQUATIC ANIMALS. 
 
 269 
 
 stantly to flow, by mechanism adapted for the purpose. Their 
 
 form and position in the Crab are shown at b, b', fig. 47 
 
 Although these animals usually reside in the water, 
 
 or only quit it occasionally, there are some species, 
 
 known under the name of land-crabs, which have 
 
 the power of living for some time at a distance 
 
 from water. In order to prevent their gills from 
 
 drying up, which would destroy their power of 
 
 acting on the air, there is a kind of spongy 
 
 structure in the gill-chamber, by which a fluid is 
 
 secreted that keeps them constantly moist. 
 
 316. In theMoLLuscA we find the gills arranged 
 in a great variety of modes. In the lowest class, 
 the TUNICATA, the respiratory membrane is merely 
 the lining of the large chamber formed by the 
 mantle (fig. 63), through which a stream of water 
 is continually made to flow by ciliary action 
 ( 319); and this surface is sometimes extended by the 
 folding or plaiting of the membrane. In most of the CON- 
 CHIFERA, however, we find four lamellae or folds of membrane 
 
 Fig. 148. 
 
 LARVA OF 
 
 EPHEMERA. 
 
 Fig. 149. RESPIRATORY APPARATUS OF THE OYSTER. 
 
 , one of the valves of the shell ; v 1 , its hinge ; m, one of the lobes of the mantle; 
 m', a portion of the other lobe folded back; c, muscles of the shell; br, gills; 
 b, mouth ; t, tentacula, or prolonged lips; /, liver; i, intestine; a, anus- co 
 heart. 
 
270 
 
 RESPIRATORY ORGANS OF AQUATIC ANIMALS. 
 
 (br, fig. 149), lying near the edge of the shell, and copiously 
 supplied with blood-vessels. In the Oyster, these are freely 
 exposed to the surrounding element, the lobes of the mantle 
 being separated along their entire length; but where the 
 
 mantle-lobes are united along 
 their margin, so as to shut-in 
 the gills, there are two ori- 
 fices, often prolonged into 
 tubes (as in the Tellina, fig. 
 150), through one of which 
 the water is drawn-in for the purpose of respiration, whilst 
 through the other it passes out, as in the Tunicata. In the 
 aquatic GASTEROPODA there is scarcely any part of the body to 
 which we do not find the gills attached in some species or other. 
 In the naked marine species, which may be called Sea- slugs, 
 they form fringes which are sometimes disposed along the sides 
 of the body, as in the Tritonia and Glaucus (figs. 151, 152), 
 
 50. TELLINA. 
 
 Fig. 151. TRITONIA. 
 
 Fig. 152. GLAUCUS. 
 
 sometimes arranged in a circle around the end of the intestine, 
 as in the Doris (fig. 153, see also fig. 139); and are some- 
 times covered-in, more or less 
 completely, by a fold of the mantle. 
 In most of the species that pos- 
 sess shells, the gills form comb- 
 like fringes, which are lodged in 
 a cavity inclosed in the last turn 
 Fig. 15S.-DOHU. of the ,3^ shell . and to this 
 
 cavity the water is admitted, sometimes by a large opening, 
 sometimes by a prolonged tube. In the CEPHALOPODA, we find 
 
RESPIRATORY ORGANS OF AQUATIC ANIMALS. 271 
 
 the gills composed of a collection of little leaf-like folds, placed 
 on a stalk (6, fig. 154); they are inclosed in a cavity which is 
 covered-in by the mantle ; and the walls of this cavity have 
 the power of alternately dilating and contracting, so as to 
 draw-in and expel water. It communicates with the exterior 
 by two orifices, one of which, o, a wide slit, is for the entrance 
 
 Fig. 154. GILLS OF POULP. 
 
 of water ; whilst the other, t, is tube-like, and serves not only 
 to carry-off the water that has passed over the gills, but also 
 to convey away the excrements, and the fluid ejected by the 
 ink-bag. This is called the funnel. 
 
 317. In FISHES, the gills are composed of fringes, which 
 are disposed in rows on each side of the throat, and are 
 covered by the skin. The cavity in which they lie has two 
 sets of apertures ; one communicating with the throat, and 
 the other opening on the outside. In the Fishes with a car- 
 tilaginous skeleton, we usually find as many of these external 
 orifices as there are rows of gills ; thus in the Lamprey there 
 are seven, as shown in the succeeding figure (a). But in 
 Fishes with a bony skeleton, there is usually but a single 
 large orifice on either side ; and this is covered with a large 
 valve-like flap, which is termed the operculum or gill-cover. 
 A continual stream of water is made to pass over the gills by 
 the action of the mouth, which takes-in a large quantity of 
 
272 
 
 RESPIRATORY ORGANS OF FISHES. 
 
 fluid, and then forces it through the apertures on each side of 
 the throat, into the gill-cavities ; and from these it passes out 
 by the other orifices just described. Fishes, in common with 
 other animals that breathe by gills, can only respire properly 
 
 Fig. 155. LAMPREY. 
 
 when these are kept moist, and are so spread out as to expose 
 their surface to the surrounding element. The act of respira- 
 tion can take place when they are exposed to air, provided 
 these conditions be fulfilled ; but in general it happens that, 
 when a Fish is taken out of water, its gills clog together and 
 dry up, so that the air cannot exert any action upon them ; 
 and the Fish actually dies of suffocation, under the very cir- 
 cumstances which are necessary to the life of an air-breathing 
 animal. 
 
 318. There are certain Fishes, however, which are provided 
 with an apparatus for keeping the gills moist, somewhat re- 
 sembling that which has been already noticed in the land- 
 crab. The bones of the pharynx are extended and twisted 
 
 in such a remarkable 
 manner (fig. 156), as 
 to form a number of 
 small cavities ; these 
 cavities the Fish can 
 fill with water; and 
 they form a reservoir 
 of fluid, from which 
 the gills may be sup- 
 plied with a sufficient 
 amount to keep them 
 moist during some 
 time. The gill-fila- 
 ments themselves are 
 
 so arranged that they do not clog together ; and by this combi- 
 nation of contrivances, the species of Fish that are furnished 
 with it can live for a long time out of water, so as to be able to 
 journey for a considerable distance on land. Such a provision 
 
 Fig. 156. RESPIRATORY APPARATUS OF ANABAS. 
 
AQUATIC RESPIRATION : USE OF CILIA. 273 
 
 is especially desirable in tropical climates, where shallow 
 lakes are often dried-up by continued drought, so that their 
 inhabitants must perish, if they were not thus enabled to 
 migrate. One of the most curious of these Fishes (most of 
 which are inhabitants of India and China) is the Andbas or 
 climbing-perch of Tranquebar ; which climbs bushes and trees 
 in search of its prey, a species of land-crab, by means of the 
 spines on its back and gill-covers. The gills of the Amphi- 
 bious Reptiles, in their Tadpole state, resemble those of Pishes, 
 and are connected with the mouth in the same manner. 
 
 319. In the respiratory actions of nearly all these animals, 
 a very important part is performed by the cilia ( 45) which 
 cover the surfaces of the gills. Even in such as do not 
 possess any special respiratory organs ( 311), the action of 
 the cilia is very important, in causing a constant change -in 
 the water that is in contact with their surface. Thus in 
 Zoophytes, which are for the most part fixed to one spot, the 
 action of the cilia produces currents in the surrounding water. 
 On the other hand, in the actively-moving Animalcules, the 
 same action propels their bodies rapidly through the water ; 
 though in some of them, which occasionally fix themselves 
 like Polypes, the action of the cilia resembles theirs. In 
 either case there is a continual change in the layer of water 
 which is in immediate contact with the surface ; and thus a 
 constant supply of the air contained in the water is secured. 
 A similar action goes-on, still more energetically, over the gill- 
 tufts of the Annelida; and this action continues after the 
 death of the animal, or after the tuft has been separated from 
 it, producing evident currents in the water in which it is 
 placed. It is by the action of the cilia alone, that the con- 
 tinual current of water is kept-up through the respiratory 
 chamber of the lower Mollusca; but this is superseded in 
 Cephalopods and Fishes by the other means for sustaining 
 this current which have been already noticed. Ciliary action 
 may be well observed in the young Tadpole of the common 
 Water-Newt, whose gills hang freely from the neck on either 
 side; the cilia are themselves so minute that they cannot be 
 readily distinguished ; but the motion of the water produced 
 by them may be at once perceived in a tolerable microscope, 
 especially when small light particles, such as those of 
 powdered charcoal, are diffused through it. 
 
274 
 
 ATMOSPHERIC RESPIRATION. 
 
 320. In animals whose blood is made to act directly upon 
 the air, we usually find a provision of some kind for intro- 
 ducing the air into the interior of the body. The simplest 
 arrangement is that which we meet-with in the Snail and other 
 terrestrial GASTEROPODS ; and it consists merely of a cavity 
 (p, fig. 157), resembling that in which the gills are disposed in 
 the aquatic Mollusca, but having a free communication with 
 
 * d / 
 
 Fig. 157. ANATOMY OF SNAIL. 
 
 /", muscular disc or foot; t, tentacula; d, diaphragm separating the respiratory 
 cavity p from other organs, but here turned back; s, stomach; o, ovary; or, 
 arterial trunk supplying the system; i, r, intestine; I, liver; h, heart ; ap, vascular 
 trunk spreading over the pulmonary cavity p ; cv, canal for excreting the viscid 
 mucus secreted by the gland v. 
 
 the external air, and having the blood minutely distributed by 
 vessels upon its walls. In the MYRIAPODA or Centipede tribe, 
 in conformity with the general plan of Articulated structure 
 ( 93), we find a repetition of similar cavities along the body, 
 one pair usually existing in each segment; 
 and these open externally by small apertures, 
 which are termed spiracles. 
 
 321. In INSECTS, the same general arrange- 
 ment is modified in the most remarkable manner. 
 The spiracles do not open into distinct air-sacs, 
 but into canals, which lead to two large tracheae 
 which run along the sides of the body, and are 
 connected by several tubes that pass across it 
 one usually for each segment. From these 
 tracheae others branch off, which again subdivide into more 
 
 Fig. 158. 
 
 AIB.-TUBE OP IN- 
 SECT. 
 
RESPIRATION OP INSECTS. 
 
 275 
 
 minute tubes ; and, by the ramifications of these, even the 
 minutest parts of the body are penetrated (fig. 159). These 
 tubes are formed upon a similar plan with the air-vessels 
 of Plants, having a spiral fibre winding inside their outer 
 membranous coat (fig. 158) ; by the elasticity of which 
 fibre, the tube is kept from being closed by pressure. In 
 this manner the air is brought into contact with almost 
 
 Head. 
 
 1st Pair of legs 
 
 1st Segment of\ 
 thorax) 
 
 Origin of wing 
 2d Pair of legs 
 3d Pair of legs 
 
 Tracheae 
 
 Stigmata 
 
 Air- sacs. 
 
 Fig. 159. RESPIRATORY APPARATUS OP INSECT (NEPA). 
 
 every portion of the tissue, and is enabled to act most ener- 
 getically upon it ; and thus the feeble circulation of these 
 animals ( 293) is in a great degree counterbalanced by 
 
 T2 
 
276 RESPIRATION OF INSECTS AND SPIDERS. 
 
 the extraordinary activity of their respiration. There are 
 no animals which consume so much oxygen, in proportion 
 to their size, as Insects do when they are in motion ( 308) ; 
 but when they are at rest, their respiration falls to the 
 low standard of the tribes to which they bear 
 the greatest general resemblance. Although, as 
 we have seen, the respiration of aquatic larvae is 
 sometimes accomplished by means of gills, yet 
 many aquatic Iarva3 breathe air by means of 
 tracheae ; and such are consequently obliged, like 
 Whales and other aquatic Mammals, to come 
 occasionally to the surface for the purpose of 
 gaining a fresh supply of air. The larva of the 
 Gnat, which breathes in this manner, has one of 
 the spiracles of its tail-segment prolonged into a 
 tube ; and it may often be seen suspended, as it 
 were, in the water, with its head downwards, the 
 end of this tube (t, fig. 160) being at the surface. 
 
 322. In the greater number of perfect Insects, we find the 
 trachese dilated at certain parts into large air-sacs (fig. 159) ; 
 these are usually largest in Insects that sustain the longest 
 and most powerful flight ; in some of which, as in the 
 common Bee, they occupy a greater portion of the trunk than 
 they do in the insect whose system of air-tubes has been just 
 represented, this insect, the Nepa or water-scorpion, being 
 of aquatic habits, and seldom using its wings for flight. 
 There can be little doubt that one use of these cavities is to 
 diminish the specific gravity of the Insect, and thus to render 
 it more buoyant in the atmosphere ; but it would not seem 
 improbable that they are intended to contain a store of air 
 for its use while on the wing, as at that time a part of 
 the spiracles are closed. We shall find in Birds, the Insects 
 of the Vertebrated division, a structure bearing remarkable 
 analogy to this ( 326). 
 
 323. In some of the ARACHNIDA, such as the Cheese-mite, 
 the respiration is accomplished by tracheae, as in Insects ; but 
 in the Spiders it is performed by a different kind of apparatus. 
 Instead of opening into a system of prolonged tubes, each 
 spiracle leads to a little chamber, the lining membrane of 
 which is arranged in a number of folds that lie together like 
 the leaves of a book; and thus a large surface is exposed 
 
KESPIEATION OF AIR-BREATHING VERTEBRATES. 277 
 
 to the air which is admitted through the spiracles. This 
 arrangement is shown in fig. 46, I. 
 
 324. Hitherto it has been seen that the respiratory appa- 
 ratus is not connected with the mouth, which in the Inverte- 
 brated classes has the reception of food as its sole function. 
 On this account, we cannot regard the air-sacs of Insects as 
 bearing any real analogy to the lungs of Vertebrata. The 
 simplest condition of the true lung is that which constitutes the 
 air-bladder (or " sound ") of Fishes. This we sometimes find in 
 the condition of a closed bag, lying along the spine ; and its use 
 cannot be that of assisting respiration, since air is not ad- 
 mitted to it from without. But in other cases we find it 
 connected with the intestinal tube, by means of a short wide 
 duct ; and since many Fishes, as the Loach, are known to 
 swallow air, which is highly charged with carbonic acid when 
 it is again expelled, it seems probable that their air-bladder 
 effects this change in precisely the same manner as a lung 
 would do the air being transmitted to it from the intestine. 
 There are some Fishes in which the resemblance of the air- 
 bladder to a lung is more decided, and its connexion with the 
 function of Eespiration is evidently more important. The 
 canal by which it communicates with the alimentary canal 
 opens into the latter above the stomach, and even, in some 
 instances, at the back of the mouth; so that a gradual 
 approach is seen to the arrangement which exists in air- 
 breathing animals. In these Fishes, as in the Amphibia that 
 retain their gills ( 87), it would appear that the respiration 
 is accomplished partly by the lungs, and partly by the gills ; 
 this is the case in the curious Lepidosiren (fig. 41), which, as 
 formerly mentioned, is regarded by some naturalists as a Fish, 
 and by others as a Eeptile. 
 
 325. The lungs of EEPTILES are for the most part but little 
 divided ; so that, although they hold a very large quantity of 
 air, this does not act advantageously upon the blood, in con- 
 sequence of the small surface over which the two are brought 
 together ( 312). In Serpents we find but a single lung, that 
 of the other side not being developed (fig. 34) ; and this lung 
 extends through nearly half the length of the body. Eeptiles 
 have no power of filling their lungs by a process resembling 
 our inspiration, or drawing-in of air ; but they are obliged to 
 swallow it, as it were, by the action of the mouth. The skin 
 
278 RESPIRATION OP REPTILES AND BIRDS. 
 
 of Frogs is of great importance in their respiration in fact, 
 of almost as much consequence as their lungs. The necessity 
 for more energetic respiration increases in these animals with 
 the temperature, every rise in which excites them to greater 
 activity. During the winter, which they pass beneath the 
 water in a state of torpidity, the action of the water upon 
 their skin is sufficient to aerate their blood. When the re- 
 turning warmth of spring arouses them from their inaction, 
 they need a larger amount of respiration, and come occa- 
 sionally to the surface to take-in air by their lungs. And 
 when summer comes on, the greater heat increases their need 
 of respiration ; and they quit their ditches and ponds, so as 
 to allow the atmosphere to act upon their skin as well as 
 upon their lungs. If they are prevented from doing so, they 
 will die ; and the same result follows if the skin be smeared 
 with grease, so that the air cannot permeate it. Moreover, if 
 the lungs be removed, and the animal be made to breathe by 
 its skin alone, it may live for some time, if the temperature 
 be not high. These facts show the great importance of the 
 skin as a respiratory organ in Frogs. 
 
 326. The respiration of BIRDS is more active than that of 
 
 Trachea 
 
 Pulmonary vessels 
 
 ___jBronchial tube 
 
 Orifice of bronchial) 
 tube) 
 
 ("Bronchial tube 
 (.opened. 
 
 Fig. 161. AIR-TUBES AND LUNGS OF BIRDS. 
 
 any other Yertebrata ; that is, they consume more oxygen, 
 and form more carbonic acid, in proportion to their size. 
 
RESPIRATION OF BIRDS AND MAMMALS. 279 
 
 Their lungs, however, are not so minutely subdivided as are 
 those of Mammals ; but the surface over which the air can 
 act upon the blood is immensely extended, by a provision 
 which is peculiar to this class. The air introduced by the 
 windpipe passes not only into the lungs properly so called, 
 but into a series of large air-cells, which are disposed in 
 various parts of the body, and which even send prolongations 
 into the bones, especially in Birds of rapid and powerful 
 flight, whose whole skeleton is thus traversed by air. The 
 mode in which some of the bronchial tubes, or subdivisions of 
 the windpipe, pass from the lungs to these air-cells, is shown 
 in fig. 161. Now, by this arrangement, a much largtr quan- 
 tity of air is taken-in at once, and a much more extensive sur- 
 face is exposed to its action, than could otherwise be provided 
 for ; and as the air which is received into the air-cells has to 
 pass through the lungs, not only when it is taken-in, but when 
 it is expelled again, its full influence upon the blood is secured. 
 
 327. Birds, like Eeptiles, are destitute of the peculiar 
 apparatus by which Mammals are enabled to fill their lungs 
 with air ; but it is introduced without any effort on their 
 parts. For the cavity of their trunk is almost surrounded by 
 the ribs and breast-bone ; and the elasticity of the former 
 keeps it generally in a state corresponding to that of our own 
 lungs when we have taken-in a full breath. Thus the state 
 of fullness is natural to the lungs and air-cells of Birds. 
 When the animal wishes to renew their contents, however, it 
 compresses the walls of the trunk, so as to diminish its cavity 
 and to force out some of the air contained in the lungs, &c. ; 
 and when the pressure is removed, the cavity returns to its 
 previous size by the elasticity of its walls, and a fresh supply 
 of air is drawn into the lungs. The air-sacs answer the same 
 purpose in Birds as in Insects, diminishing the specific gravity 
 of the body, by increasing its size without adding to its weight, 
 and thus rendering it more buoyant. 
 
 328. In Man and other MAMMALS, the lungs are confined 
 to the upper portion of the cavity of the trunk, termed 
 the thorax; which is separated from the abdomen by the 
 diaphragm, a muscular partition, whose action in respiration 
 is very important. (An imperfect diaphragm is found in 
 some Birds, which approach most nearly to Mammals in their 
 general structure.) The lungs are suspended, as it were, in 
 
280 
 
 RESPIRATORY APPARATUS OF MAMMALS. 
 
 this cavity, by their summit or apex ; and are covered by a 
 serous membrane termed the pleura, which also lines the 
 thorax, being reflected from one surface to the other precisely 
 in the manner of the pericardium ( 43). Thus the pleura of the 
 outer surface of the lung is continually in contact with that 
 which forms the inner wall of the thorax ; they are both kept 
 moist by fluid secreted from them ; and they are so smooth, 
 as to glide over one another with the least possible friction. 
 
 The lungs themselves are 
 very minutely subdivided ; 
 and thus expose a vast ex- 
 tent of surface in proportion 
 to their size. The air-cells of 
 the human lung, into which 
 the air is conveyed by the 
 branches of the wind-pipe, and 
 on the walls of which the 
 blood is distributed, do not 
 average above the 1-1 00th of 
 an inch in diameter. In the 
 accompanying figure is repre- 
 sented, on one side, the lung, 
 d, presenting its natural ap- 
 pearance ; and on the other, 
 the ramifications of the air- 
 passages or bronchial tubes, 
 c, e, by which air is con- 
 veyed into every part of the 
 
 Fig. ^.-AIR-TUBES AND LUNG OF MAN. lungs. The trachea^ or wind- 
 pipe, 6, opens into the 
 
 pharynx or back of the mouth, by the larynx, a. The con- 
 struction of this is especially destined to produce the voice, 
 and will be explained under that head (Chap, xin.); but 
 it may be here mentioned that the entrance from the 
 pharynx into the larynx consists of a narrow slit, capable of 
 being enlarged or closed by the separation or approximation 
 of its lips, which form what is called the glottis. The aper- 
 ture of the glottis is regulated by the muscular apparatus of 
 the larynx ; the actions of which are not under the direct 
 control of the will, but are automatic, like those concerned in 
 swallowing ( 194) ; and the purpose of this provision is to 
 
RESPIRATORY APPARATUS OP MAMMALS. 281 
 
 prevent the entrance of anything injurious into the windpipe. 
 Thus if we attempt to breathe carbonic acid gas, which would 
 produce an immediately fatal result if introduced into the 
 lungs, the lips of this chink immediately close together, and 
 so prevent its entrance. The contact of liquids or of solid 
 substances, too, usually causes the closure of the aperture, so 
 that they are prevented from finding their way into the wind- 
 pipe ; but this does not always happen, especially when the 
 glottis is widely opened to allow the breath to be drawn- in 
 ( 193). 
 
 329. The larynx, trachea, and bronchial tubes, to their 
 minutest ramifications, in all air-breathing Vertebrata, are 
 lined by a mucous membrane continued from the back of the 
 throat ; and this membrane, like the gills of aquatic animals, 
 is covered with cilia, which are in continual vibration. It is 
 obvious, however, that the purpose of this ciliary movement 
 must be here different from that which is fulfilled by the same 
 action on the surface of the gills ( 319); and it probably 
 serves to get rid of the secretion which is being continually 
 poured out from the surface of the mucous membrane, and 
 which, if allowed to accumulate there, would clog up the air- 
 cells, and in time produce suffocation. The vibration of the 
 cilia is observed to be always in one direction, towards the 
 outlet ; and it is in this direction, therefore, that the fluid is 
 gradually but regularly conveyed. The ciliary movement may 
 be seen to take place on the surface of the mucous membrane 
 of the nose ; but not on that of the pharynx, where it would 
 be continually interrupted by the passage of food. 
 
 330. The constant renewal of the air in the lungs is pro- 
 vided for, in Mammals, by a peculiar mechanism, which accom- 
 plishes this purpose most effectually, though itself of the 
 most simple character. We must recollect that the thorax in 
 this class is an entirely closed cavity. It is bounded above 
 and at the sides by the ribs (the space between which is filled 
 up by muscles, &c.), and below by the diaphragm, which here 
 forms a complete partition between the thorax and abdomen. 
 The whole of this cavity, with the exception of the space 
 occupied by the heart and its large vessels (and also by the 
 gullet, which runs down in front of the spine), is constantly 
 filled-up by the lungs. Now the size of this cavity may 
 be made to vary considerably; in the first place, by the 
 
282 RESPIRATORY MOVEMENTS OF MAMMALS. 
 
 movement of the diaphragm ; and in the second, by that of 
 the ribs. 
 
 331. i. The diaphragm, in a state of rest or relaxation, 
 forms a high arch, which rises into the interior of the chest, 
 as at <7, fig. 163 ; but when it contracts, it becomes much 
 natter (though always retaining some degree of convexity 
 upwards), and thus adds considerably to the capacity of the 
 lower part of the chest. The under side of the diaphragm is 
 in contact with the liver and stomach, which, to a certain 
 
 c a h 
 
 Fig. 163. THORAX OP MAN. 
 
 degree, rise and fall with it. It is obvious that, when the 
 diaphragm descends, these organs, with the abdominal viscera 
 in general, must be pushed downwards ; and as there can be 
 no yielding in that direction, the abdomen is made to bulge 
 forwards when the breath is drawn-in. On the other hand, 
 when the contraction of the diaphragm ceases, the abdominal 
 muscles press back the contents of the abdomen, force up the 
 
RESPIRATORY MOVEMENTS OF MAMMALS. 283 
 
 liver and stomach against the nnder side of the diaphragm, 
 and cause it to rise to its former height. 
 
 332. ii. The play of the ribs is rather more complicated. 
 These bones, c c, and c c' (to the number of twelve on each 
 side in Man), are attached at one end by a moveable joint to 
 the spinal column, a \ whilst at the other they are connected 
 with the sternum (breast-bone) by an elastic cartilage. Now 
 each rib, in the empty state of the chest, curves downwards 
 in a considerable degree ; and it may be elevated by a set of 
 muscles, of which the highest, i, are attached to the vertebrae 
 of the neck and to the first rib, whilst others, e, e, e (termed 
 intercostals), pass between the ribs. The cartilages also curve 
 downwards in the opposite direction, from their points of 
 attachment to the sternum. When the breath is drawn-in, 
 the first rib is raised by the contraction of the muscles, i ; and 
 all the other ribs, which hang, as it were, from it, would of 
 course be raised by this action to the same degree. But each 
 of them is raised a little more than the one above it, by the 
 contraction of its own intercostal muscle ; and thus the lower 
 ribs are raised very much more than the upper ones. Now 
 by the raising of the ribs, they are brought more nearly into 
 a horizontal line, as are also their cartilages ; and since the 
 combined length of the two is greater, the nearer they approach 
 to a straight line, it follows that the raising of the ribs must 
 throw them further out at the sides, and increase the pro- 
 jection of the sternum in front, thus considerably enlarging 
 the capacity of the chest in these directions. When the 
 movement of inspiration is finished, the ribs fall again, partly 
 by their own weight, partly by the elasticity of their carti- 
 lages, and partly by the contraction of the abdominal muscles 
 which are attached to their lower border. For the full under- 
 standing of this description, it is desirable that the reader 
 should examine the movements of his own or another person's 
 chest, by placing his fingers upon different points of the ribs, 
 and watching their changes of position during the drawing-in 
 and the expulsion of the breath. 
 
 333. Now the cavity of the thorax is itself perfectly 
 closed; so that, if it were not for the expansion of the 
 lungs, a void or vacuum would be left when the diaphragm 
 is drawn down and the ribs elevated. The atmosphere 
 around presses to fill that vacuum; but this it can only 
 
284 RESPIRATION IN MAN. 
 
 do by entering the lungs through, the windpipe, and inflating 
 them (or blowing them out), so as to increase their size in 
 proportion to the increase of the space they have to fill. In this 
 manner the lungs are made constantly to fill the cavity of the 
 chest, however great may be the increase in the latter. But 
 if we were to make an aperture through the walls of the chest, 
 the air would rush directly into its cavity, when the move- 
 ments of inspiration are performed, and the lung of that side 1 
 would not be dilated. The same thing would happen if there 
 were an aperture in the lung itself, allowing free communica- 
 tion between one of the larger bronchial tubes and the cavity 
 of the chest ; for the air, although still drawn-in by the wind- 
 pipe, would pass directly into the cavity of the chest, rather 
 than dilate the lung, which would thus become entirely useless. 
 Such an aperture is sometimes formed as the result of disease ; 
 and if the action of both lungs were thus prevented, death 
 must immediately take place from suffocation. 
 
 334. The extent of the respiratory movements varies con- 
 siderably ; but in general it is only such as to change about 
 the seventh part of the air contained in the lungs. (It may 
 be generally noticed, that every fifth or sixth inspiration in 
 Man is longer and fuller than the rest.) Their rate varies 
 according to age, and to the state of the nervous system ; being 
 faster in infants and in young persons than in adults; and 
 more rapid in states of mental excitement, or irritation of the 
 bodily system, than in a tranquil condition. In a state of rest, 
 from 14 to 18 inspirations take place every minute in an 
 adult, and at each about 20 cubic inches of air are drawn-in ; 
 but both the depth and frequency of the inspirations are con- 
 siderably increased by exercise. Taking an average alterna- 
 tion of activity and repose, it appears that about 360 cubic 
 feet of air pass through the lungs every twenty-four hours, 
 or 15 cubic feet every hour; and as the air which has once 
 passed through the lungs contains about 1-2 4th part of 
 carbonic acid, about 15 cubic feet of that gas, containing 
 nearly 8 ounces of solid carbon, are thrown-off in the course 
 of twenty-four hours. 
 
 335. Now carbonic acid, when diffused through the atmo- 
 sphere to any considerable amount, is extremely injurious to 
 
 1 Each lung has its own cavity ; the two being completely separated 
 from each other by the pericardium (43). 
 
IMPOETANCE OF FREE VENTILATION. 285 
 
 animal life ; for it prevents the due excretion by the lungs of 
 that which has been formed within the body ; and the latter 
 consequently accumulates in the blood, and exercises a very 
 depressing influence on the action of the various organs of the 
 body, but particularly on that of the nervous system. The 
 usual proportion is not above 1 part in 5000 ; and when this 
 is increased to 1 part in 100, its injurious effects begin to be 
 felt by Man, in headache, languor, and general oppression. 
 ISTow it is evident, from the statements in the last paragraph, 
 that, as a man produces in twenty-four hours about 15 cubic 
 feet of carbonic acid, if he were inclosed in a space containing 
 1500 cubic feet of air (such as would exist in a room 15 feet 
 by 10, and 10 feet high), he would in twenty-four hours 
 communicate to its atmosphere from his lungs as much as 
 1 part in 100 of carbonic acid, provided that no interchange 
 takes place between the air within and the air outside the 
 chamber. The amount would be further increased by the car- 
 bonic acid thrown off by the skin, the quantity of which has 
 not yet been determined. 
 
 336. In practice, such an occurrence is seldom likely to 
 take place; since in no chamber that is ever constructed, 
 except for the sake of experiment, are the fittings so close as 
 to prevent a certain interchange of the contained air with 
 that on the outside. But the same injurious effect is often 
 produced by the collection of a large number of persons for a 
 shorter time, in a room insufficiently provided with the means 
 of ventilation. It is evident that if twelve persons were to 
 occupy such a chamber for two hours, they would produce the 
 same effect with that occasioned by one person in twenty-four 
 hours. Now we will suppose 1200 persons to remain in a 
 church or assembly-room for two hours ; they will jointly 
 produce 1500 cubic feet of carbonic acid in that time. Let 
 the dimensions of such a building be taken at 100 feet long, 
 50 broad, and 30 high ; then its cubical content will be 
 (100 x 50 x 30) 150,000 cubic feet. And thus an amount 
 of carbonic acid, equal to 1-1 00th part of the whole, will be 
 communicated to the air of such a building, in the short space 
 of two hours, by the presence of 1200 people, if no pro- 
 vision be made for ventilating it. And the quantity will 
 be greatly increased, and the injurious effects will be pro- 
 portionably greater, if there be an additional consumption of 
 
286 IMPORTANCE OF FREE VENTILATION. 
 
 oxygen, produced by the burning of gas-lights, lamps, or 
 candles. 
 
 337. Hence we see the great importance of providing for 
 free ventilation, wherever large assemblages of persons are 
 collected together, even in buildings that seem quite adequate 
 in point of size to receive them ; and much of the weariness 
 which is experienced after attendance on crowded assemblies 
 of any kind, may be traced to this cause. In schools, facto- 
 ries, and other places where a large number of persons remain 
 during a considerable proportion of the twenty-four hours, it 
 is impossible to give too much attention to the subject of 
 ventilation ; and as, the smaller the room, the larger will be 
 the proportion of carbonic acid its atmosphere will contain, 
 after a certain number of persons have been breathing in it 
 for a given time, it is necessary to take the greatest precaution 
 when several persons are collected in those narrow dwellings, 
 in which, unfortunately, the poorer classes are compelled to 
 reside. Even the want of food, of clothing, and of fuel, are 
 less fertile sources of disease than insufficient ventilation; 
 which particularly favours the spread of contagious diseases, 
 on the one hand by keeping-in the poison, and thus concen- 
 trating it upon those who expose themselves to its influence ; 
 and, on the other, by obstructing the elimination of the waste 
 matter from the system, the presence of which in the blood 
 renders it peculiarly liable to be acted-on by all poisons 
 having the nature of " ferments." 
 
 338. When the quantity of carbonic acid in the air accu- 
 mulates beyond a certain point, it speedily produces suffocation 
 and death. This is occasioned by the obstruction to the flow 
 of blood through the capillaries of the lungs, which takes 
 place when it is no longer able to get rid of the carbonic acid 
 with which it is charged, and to absorb oxygen in its stead. 
 The general principle to which this stagnation may be referred 
 has already been noticed ( 280). Now, as all the blood of 
 the system, in warm-blooded animals, is sent through the 
 lungs before it is again transmitted to the body, it follows 
 that any such obstruction in the lungs must bring the whole 
 circulation to a stand. The functions of the nervous system 
 are directly dependent upon a constant supply of arterial 
 blood (Chap, x.) ; and, accordingly, as this supply becomes 
 progressively diminished in quantity and deteriorated in qua- 
 
SUFFOCATION FROM WANT OP RESPIRATION. 287 
 
 lity, its actions first become irregular, producing violent con- 
 vulsive movements, and at last cease altogether, the animal 
 becoming completely insensible. In this condition, which is 
 termed A. sphyxia, the pulmonary arteries, the right side of the 
 heart, and the large veins which empty themselves into it, are 
 gorged with dark blood ; whilst the pulmonary veins, the left 
 side of the heart, and the arteries of the system, are compara- 
 tively empty. Hence the action of the right side of the heart 
 comes to an end, through a loss of power in its walls, occa- 
 sioned by their being over-distended ; whilst that of the left 
 side ceases for want of the stimulus of the contact of blood, 
 by which the muscular fibre is excited. If this state be 
 allowed to continue, death is the consequence ; but if the 
 carbonic acid in the lungs be replaced by pure air, the flow of 
 blood through their capillaries recommences, the right side 
 of the heart is unloaded and begins to act again, arterial 
 blood is sent to the left side, and excites it to renewed motion, 
 and the same being propelled by it to all parts of the body, 
 their several functions are restored, the nervous system re- 
 covers its power of acting, and all goes on as before. These 
 changes occur in exactly the same manner when a warm- 
 blooded animal is made to breathe nitrogen or hydrogen ; 
 since these gases do not perform that which it is the office of 
 oxygen to effect, the removal of carbon from the system, in 
 the form of carbonic acid. And they also take place in a 
 perfectly pure atmosphere, when the individual is prevented 
 from receiving it into its lungs by an obstruction to its passage 
 through the windpipe, such as that produced by hanging, 
 strangulation, drowning, &c. For the air in the lungs, not 
 being renewed, speedily becomes charged with carbonic 
 acid, to an extent that checks the circulation through their 
 capillaries; and all the consequences of this follow as 
 before. 
 
 339. The most efficient remedy in all such cases is evidently 
 that suggested by the facts stated in the last paragraph, 
 the renewal of the air in the lungs. But with this, other 
 means should be combined; and the general directions 1 
 of Dr. Marshall Hall, with the method of producing artificial 
 
 1 The instructions, though specially intended for the resuscitation of 
 persons apparently drowned, are applicable with slight modification to 
 other forms of Asphyxia. 
 
288 TREATMENT OF THE APPARENTLY-DROWNED. 
 
 respiration suggested by Dr. H. E. Silvester, seem most likely 
 to answer in practice : 
 
 Treat the patient instantly, on the spot, in the open air, 
 exposing the face and chest to the breeze, except in severe 
 weather. 
 
 i. To clear the Throat, place the patient gently on the 
 face, with one wrist under the forehead (all fluids and the 
 tongue itself then fall forwards, leaving the entrance into the 
 windpipe free). If there be breathing, wait and watch ; if 
 not, or if it fail, 
 
 ii. To excite Respiration, turn the patient well and in- 
 stantly on his side, and excite the nostrils with snuff, or the 
 throat with a feather, &c., and dash cold water on the face 
 previously rubbed warm. If there be no success, lose not a 
 moment, but instantly, 
 
 in. To imitate Respiration, lay the patient on his back, 
 with the head and shoulders slightly elevated ; then let the 
 arms be raised and steadily extended upwards, by the sides of 
 the head, so as to draw-up the shoulders. In this way, the 
 ribs are drawn-up by the muscles passing to them from the 
 shoulders, and the cavity of the chest is enlarged. If the arms 
 be then carried-down to the sides of the body, the shoulders 
 fall, the ribs are lowered, and the sides of the thorax approach 
 one another, as in natural expiration, an effect which 
 may be increased by moderate pressure on the front and 
 sides of the chest. By an alternation of these movements, 
 an artificial Inspiration and Expiration will be effected, 
 which, though imperfect, may restore life. 
 
 iv. To induce Circulation and Warmth, meantime rub 
 the limbs upwards, with firm grasping pressure and with 
 energy, using handkerchiefs, &c. (by this measure the blood 
 is propelled along the veins towards the heart) ; let the limbs 
 be thus warmed and dried, and then clothed, the bystanders 
 supplying the requisite garments j avoiding the continuous 
 warm bath, and the position on, or inclined-to, the back. 
 
 340. The ordinary movements of respiration belong, like 
 those of swallowing, to the class of reflex actions ( 430). We 
 have seen that, in every such movement, a great number of 
 muscles are called into play simultaneously; and this is 
 effected by means of the stimulus which is sent to them from 
 the spinal cord. But this stimulus does not originate there ; 
 
CAUSE OF RESPIRATORY MOVEMENTS. 289 
 
 for it is the consequence of impressions conveyed to the spinal 
 cord, and especially to its upper end, by several nerves, some 
 originating in the lungs, and others in the general surface. 
 The nerves originating in the lungs convey to the spinal cord 
 the impression produced by the presence of venous blood in 
 their capillaries : of this impression we are not ordinarily 
 conscious ; but if we hold our breath for a few moments, we 
 become aware of it ; and it speedily becomes so distressing as 
 to force us to breathe, even though we may try to resist it by 
 an effort of the will. The impression conveyed by the nerves 
 of the general surface is chiefly that produced by the applica- 
 tion of cold to the skin. It is this which is the cause of the 
 first inspiration in the new-born infant ; which is not unfre- 
 quently prevented by the seclusion of its face (the part most 
 capable of receiving this impression) from the influence of the 
 air. Every one knows that, when the face is dipped into 
 water, an inspiratory movement is strongly excited ; and the 
 same happens when a glass of water is dashed over the face. 
 This simple remedy will often put a stop to hysterical laughter, 
 by producing a long sighing inspiration. A still- stronger 
 tendency to draw-in the breath is experienced in the first 
 dash of water over the body in the shower-bath. The respi- 
 ratory movements, in the higher Animals, are placed under 
 the control of the will, to a certain extent, because on them 
 depend the production of sounds, and in Man the actions of 
 speech ; but that they are quite independent of the will, and 
 even of sensation, is shown by the fact that they will continue 
 after the brain has- been completely removed, provided the 
 spinal cord and its- nerves are left without injury. In most 
 of the Invertebrata they are connected with distinct ganglia, 
 which minister to them alone. (See Chap, x.) 
 
 341. The actions of sighing, yawning, sobbing, laughing, 
 coughing, and sneezing, are nothing else than simple modifica- 
 tions of the ordinary movements of respiration, excited either 
 by mental emotions, or by some stimulus originating in the 
 respiratory organs themselves. Sighing is nothing more than 
 a very long-drawn inspiration, in which a larger quantity of 
 air than usual is made to enter the lungs. This is continually 
 taking place in a moderate degree, as already noticed ( 334) ; 
 and we notice it particularly, when the attention is released 
 after having been fixed upon an object which has excited it 
 
 u 
 
290 VARIOUS MOVEMENTS CONNECTED WITH RESPIRATION. 
 
 strongly, and which, has prevented our feeling the insufficiency 
 of the ordinary respiratory movements. Hence this action is 
 only occasionally connected with mental emotion. Yawning 
 is a still deeper inspiration, which is accompanied by a kind 
 of spasmodic contraction of the muscles of the jaw, and also 
 by a very great elevation of the ribs, in which the shoulders 
 and arms partake. The purely involuntary character of this 
 movement is sometimes seen in a remarkable manner in cases 
 of palsy, in which the patient cannot raise his shoulder by an 
 effort of the will, but does so in the act of yawning. Never- 
 theless the action may be performed by the will, though not 
 completely ; and it is one that is particularly excited by an 
 involuntary tendency to imitation, as every one must have 
 experienced who has ever been in company with a set of 
 yawners. Sobbing is the consequence of a series of short 
 convulsive contractions of the diaphragm ; and it is usually 
 accompanied by a closure of the glottis, so that no air really 
 enters. In Hiccup, the same convulsive inspiratory movement 
 occurs, the glottis closing suddenly in the midst of it ; and 
 the sound is occasioned by the impulse of the column ol 
 air in motion against the glottis. In Laughing, a precisely 
 reverse action takes place ; the muscles of expiration are in 
 convulsive movement, more or less violent, and send out the 
 breath in a series of jerks, the glottis being open. This some- 
 times goes on until the diaphragm is more arched, and the 
 chest more completely emptied of air, than it could be by an 
 ordinary movement of expiration. The act of Crying, though 
 occasioned by a contrary emotion, is, so far as the respiration 
 is concerned, very nearly the same. We all know the effect 
 of mixed emotions in producing something " between a laugh 
 and a cry." 
 
 342. The purposes of the acts of coughing and sneezing are, 
 in both instances, to expel substances from the air-passages, 
 which are sources of irritation there ; and this is accomplished 
 in both by a violent expiratory effort, which sends forth a 
 blast of air from the lungs. Coughing occurs when the source 
 of irritation is situated at the back of the mouth, in the 
 trachea, or bronchial tubes. The irritation may be produced 
 by acrid vapours, or by liquids or solids that have found 
 their way into these passages, or by secretions which have 
 been poured into them in unusual quantity as the result of 
 
COUGHING AND SNEEZING AQUEOUS EXHALATION. 291 
 
 disease ; and the latter will be the more likely to produce the 
 effect, from the irritable state in which the lining membrane 
 of the air-passages already is. The impression made upon 
 this membrane is conveyed by the nerves spread out beneath 
 its surface to the spinal cord; and the motor impulses are 
 sent to the different muscles, which they combine in the act 
 of coughing. This act consists, 1st, in a long inspiration, 
 which fills the lungs ; 2d, in the closure of the glottis at the 
 moment when expiration commences ; and 3d, in the burst- 
 ing-open, as it were, of the glottis, by the violence of the 
 expiratory movement, so that a sudden blast of air is forced 
 up the air-passages, carrying before it anything that may offer 
 an obstruction. Sneezing differs from coughing in this, that 
 the communication between the larynx and the mouth is 
 partly or entirely closed, by the drawing-together of the sides 
 of the veil of the palate over the back of the tongue ; so that 
 the blast of air is directed more or less completely through 
 the nose, in such a way as to carry-off any source of irritation 
 that may be present there. 
 
 343. Every one is aware that the air he breathes-forth con- 
 tains a large quantity of vapour : this is not perceptible in a 
 warm atmosphere, because the watery particles remain dis- 
 solved in it and do not affect its transparency ; but in a cold 
 atmosphere they are no longer held in solution, and conse- 
 quently present the appearance of fog or steam. The quantity 
 of fluid which thus passes off is by no means trifling, 
 probably not less than from 16 to 20 ounces in the twenty- 
 four hours; a portion of it undoubtedly proceeds from the 
 moist lining of the mouth, throat, &c., but the greater part 
 is thrown-off by the lungs themselves. This fluid, when col- 
 lected, is found to contain a good deal of decomposing organic 
 matter, especially in cases in which the respiratory process 
 has not been carried on with perfect freedom; such matter 
 being oxydized and thrown-off under other forms, when the 
 blood is duly aerated. Various substances of an odoriferous 
 character, which have been taken into the blood, manifest 
 their presence in this exhalation : thus turpentine, camphor, 
 and alcohol, communicate their odour to the breath; and 
 when the digestive system is out of order, the breath fre- 
 quently acquires a disagreeable taint, from the reception of 
 putrescent matters into the blood, and their exhalation through 
 
 u2 
 
292 ABSORPTION OF VAPOUR POISONOUS GASES. 
 
 this channel. Of the water of the blood, from which this 
 exhalation is given-off, a small part is most probably formed 
 by the direct union of the hydrogen contained in the food 
 (especially when this is one of its predominating components, 
 153) with the oxygen absorbed. For it has been found by 
 careful experiment, that the proportion of inspired oxygen 
 which disappears (not being contained in the carbonic acid 
 expired, 305), is much greater in animals that are fed on a 
 flesh diet, than in those living on farinaceous food. Another 
 portion of such oxygen probably unites with the sulphur and 
 phosphorus of the food and tissues, to form sulphuric and 
 phosphoric acids, which are excreted through the kidneys in 
 combination with alkaline bases ( 367). 
 
 344. Certain gases act as violent poisons, even when respired 
 in very small proportion. Thus, a Bird is speedily killed by 
 breathing air which contains no more than 1-1 500th part of 
 sulphuretted hydrogen ; and a Dog will not live long in an 
 atmosphere containing 1 -800th part of this gas. The effects 
 of carburetted hydrogen are similar ; but a larger proportion 
 is required to destroy life. Both these gases are given-off by 
 decomposing animal and vegetable matter ; the neighbourhood 
 of which is consequently very injurious to health. Several 
 cases of arsenical poisoning have occurred, from the accidental 
 inhalation of a small quantity of arseniuretted hydrogen, the 
 amount of arsenic contained in which must have been so 
 minute as to be scarcely appreciable. 
 
 CHAPTER YIL 
 
 OF EXCRETION AND SECRETION. 
 
 General Purposes of the Excreting Processes. 
 
 345. WE have seen that the Blood, in the course of its 
 circulation, not only deposits the materials that are converted 
 into the several fabrics of which the body is composed, but 
 also takes-up into itself the products of the decomposition 
 which is continually going-on in its various parts ; and it is 
 to replace this, that the constant Nutrition of the tissues is 
 required. In order that the blood may retain its fitness foi 
 
OF EXCRETION AND SECRETION. 293 
 
 ae purposes to which it is destined, it is requisite that these 
 >roducts should be drawn-off from the current of the circula- 
 don, as constantly as they are received into it ; and this is 
 accomplished by the various processes of Excretion, which are 
 continually taking place in different parts of the body. The 
 uninterrupted performance of these is even more essential to 
 the maintenance of life, than is an uninterrupted supply of 
 nutritive materials ; for an animal may continue to exist for 
 some time without the latter, but it speedily dies if either 
 of the more important excretions be checked. We have a 
 striking instance of this in the case of the Eespiration, which 
 may be regarded as a true function of Excretion, having for 
 its object to set free Carbonic acid from the blood in a gaseous 
 form, thereby contributing to the introduction of Oxygen 
 into the blood, for the various important actions to which 
 that element is subservient, especially the maintenance of 
 Animal Heat. (Chap, ix.) The effects of the suspension of 
 v he respiratory process, even for a few minutes, in a warm- 
 )looded animal, have been shown ( 338) to be certainly and 
 speedily fatal ; and they are as certainly fatal in the end in 
 cold-blooded animals, though a longer time is required to 
 produce them. 
 
 346. The products of excretion are the same, as to their 
 essential characters at least, through the whole Animal king- 
 dom ; and for this it is not difficult to find a reason. It will 
 be remembered that the ultimate elements of the Animal 
 tissues are four in number : oxygen, hydrogen, carbon, and 
 nitrogen ; and that the materials which make up the chief 
 part of the fabric of different classes of animals albumen, 
 gelatin, fatty matter, &c. contain these elements united in 
 constant proportions, from whatever source we obtain them. 
 Hence we should expect to find the products of their decom- 
 position also the same ; and this is, for the most part, the 
 case. Of these four ingredients, oxygen can never be said 
 (in the healthy state at least) to be superfluous in the body ; 
 for a large and constant supply of it is required, to unite with 
 the others and carry them off in their altered conditions. 
 Thus, unless oxygen were continually introduced into the 
 system, for the sake of uniting with the carbon that is to be 
 thrown off by Respiration, that excretion must be checked ; 
 and it is required, in like manner, for uniting with hydrogen 
 
294 NATURE AND OBJECTS OF EXCRETORY ACTIONS. 
 
 to form water, and with compounds of nitrogen to form urea. 
 Hence there is no need of an organ to carry off the super- 
 fluous oxygen ; but an organ to introduce it is rather required ; 
 and this purpose, as we have seen, is answered by the Respi- 
 ratory apparatus. But we find organs of excretion specially 
 destined to carry off the carbon, hydrogen, and nitrogen, which 
 are set free, under various forms, by the decomposition of the 
 tissues. Thus the Respiratory organs, as we have seen, throw 
 off carbon in the form of carbonic acid, and hydrogen which 
 has been in like manner united with oxygen so as to form 
 water. The Liver has for its office partly to separate these 
 same elements from the blood in a different form, throwing 
 them off in the condition of a peculiar fatty matter, which 
 consists almost entirely of carbon and hydrogen. But it has 
 another function of no less importance in animals whose 
 respiration is active ; for by its agency the hydro-carbonaceous 
 matter circulating in the blood is brought into a state in 
 which it readily combines with oxygen to form carbonic acid 
 and water ; and thus the liver may be said to prepare the 
 pabulum for the combustive process. Lastly, the Kidneys 
 have for their chief object to throw off the azotized compounds 
 which result from the decomposition of the tissues ; these 
 contain a very large proportion of azote or nitrogen, which is 
 united with the other elements into the crystalline compounds, 
 urea, and uric or lithic acid, the latter of which is usually 
 thrown off in combination with soda or ammonia And the 
 kidneys further serve as the channel through which soluble 
 matters of various kinds, which have found their way into 
 the current of the circulation, and are foreign to the composi- 
 tion of the blood, are eliminated from it. 
 
 347. It is obvious that, when an animal has retained its 
 usual weight for any length of time without change, the total 
 weight of its excretions must be equivalent to the total weight 
 of the solids and fluids it has taken-in. If these last have been 
 no more in amount than was absolutely necessary for the main- 
 tenance of the body during that period, all the azotized portion 
 of the food was first appropriated to the formation of the 
 azotized tissues ; whilst the non-azotized portion was used-up 
 in maintaining the respiration ( 157), Consequently, no 
 part of the food would pass at once into the biliary and urinary 
 excretions ; and these would have no other function than to 
 
EXCRETION OF SUPERFLUOUS AZOTIZED NUTRIMENT. 295 
 
 separate or strain-off, as it were, the products of the decompo- 
 sition of the tissues formed from it, when their term of life 
 had expired ( 161). But it is certain that Man (as well as 
 other animals which have in some degree learned his habits) 
 frequently consumes much more food than is necessary for 
 the supply of his wants ; and a little consideration will show, 
 that the surplus must pass-off by these excretions, without 
 ever forming part of the living fabric. For new muscular 
 tissue is not formed in proportion to the quantity of aliment 
 supplied, but in proportion to the demand created by the 
 exercise of it ( 587); consequently, if more food be taken-in 
 than is necessary to supply that demand, no use can be 
 made of it. We never find that a Man becomes more fleshy 
 by eating a great deal and taking little exercise ; indeed, the 
 very contrary result happens, his flesh giving place to fat. 
 But let him put his muscles to regular and vigorous exercise, 
 and eat as much as his appetite demands, and they will then 
 increase both in strength and bulk. 
 
 348. Hence, if more azotized food be taken-in, than is 
 required to supply the waste of the muscular and other azotized 
 tissues, the surplus must be carried-off by the organs of 
 excretion chiefly, indeed almost entirely, by the Kidneys. 
 By throwing upon them more than their proper duty, they 
 become disordered and unable to perform their functions ; 
 hence the materials which they ought to separate from the 
 blood accumulate in it, and give rise to various diseases of a 
 more or less serious character, which might have been almost 
 certainly prevented by due regulation of the diet. The most 
 common of these diseases among the higher classes are Gout 
 and Gravel ; both of these may be often traced to the same 
 cause, the accumulation in the blood of lithic acid, which 
 results from the decomposition of the superfluous azotized 
 food, and which the kidneys are not able to throw-off in the 
 proper state, that is, dissolved in water. That these diseases 
 are, comparatively speaking, rare among the lower classes, is 
 at once accounted-for by the fact, that they do not take-in 
 any superfluous azotized food; all that they consume being 
 appropriated to the maintenance of their tissues, and the 
 kidneys having only to discharge their proper function of 
 removing from the blood the products of the decomposition 
 of these. 
 
296 EXCRETION OF SUPERFLUOUS NON-AZOTIZED NUTRIMENT. 
 
 349. Hence the radical cure of these diseases, in most 
 persons who have a sufficiently vigorous constitution and firm 
 resolution to adopt it, is abstinence from all azotized nutri- 
 ment whether contained in animal flesh, bread, or other 
 articles of vegetable diet, save such as is required to supply 
 the wants of the system. If such abstinence be carried too 
 far, however, it will produce injurious instead of beneficial 
 results, weakening the fabric, and impairing the digestive 
 powers ; and if food be employed of a kind which is liable to 
 produce lactic acid (the acid that appears in milk, when it turns 
 sour), much disorder may still remain, which must be avoided by 
 using the kind of diet that is least liable to undergo this change. 
 
 350. Again, if more non-azotized food is taken into the 
 system than can be got rid of by Eespiration, it must either 
 be deposited as fat, or it must be separated from the blood, 
 and carried-off by the excretion of the Liver. But if too 
 much work be thrown upon this organ, its function becomes 
 disordered, from its inability to separate from the blood all 
 that it should draw-off ; the injurious substances accumulate 
 in the blood, therefore, producing various symptoms that are 
 known under the general term of bilious ; and to get rid of 
 these, it becomes necessary to administer medicines (especially 
 those of a mercurial character) which shall excite the liver to 
 increased secretion. The constant use of these medicines has 
 a very pernicious effect upon the constitution; and careful 
 attention to the regulation of the diet, and especially the 
 avoidance of a superfluity of oily or farinaceous matter, will 
 generally answer the same end in a much better manner. 
 
 351. That the materials of the Biliary and Urinary excre- 
 tions pre-exist (like the carbonic acid thrown-off by respiration) 
 in the blood, in forms which, if not identical, are at any rate 
 closely allied to those under which they present themselves 
 in the bile and urine, has now been fully proved. The 
 quantity of them present in the circulating fluid, however, is 
 usually very small ; for the simple and obvious reason that, 
 if the excreting organs are in a state of healthy activity, these 
 substances are drawn-off by them from the blood, as fast as 
 they are introduced into it. But if the excretions be checked, 
 they speedily accumulate in the blood, to such a degree as to 
 be easily detected by the Chemist, and also to make their 
 presence evident by their effects upon the animal functions, 
 
NATURE AND PURPOSES OF ANIMAL SECRETIONS. 297 
 
 especially those of the nervous system. This sometimes 
 happens in consequence of disease, and it may be imitated by 
 experiment ; for when the trunk of the blood-vessel convey- 
 ing the blood to the liver or kidney is tied, the excretion is 
 necessarily checked, and the same results take place as when 
 the stoppage has depended on want of secreting power. The 
 biliary and urinary matters have the effect of narcotic poisons 
 upon the brain ; when they have accumulated in the blood, 
 their symptoms begin to manifest themselves ; and these 
 symptoms increase in intensity, as the amount of the sub- 
 stances becomes augmented, until death takes place. 
 
 352. Besides the Excretions, we find various Secretions 
 elaborated in different parts of the bodies of animals, with a 
 view not so much to the purification of their blood, as to the 
 fulfilment of special purposes in their economy. These vary 
 considerably in the different classes of animals ; though some 
 of them, being concerned in functions almost universally per- 
 formed, are equally general in their range. Thus we find the 
 Salivary and Gastric fluids poured into the mouth and stomach, 
 for the reduction and solution of the food ( 190 and 204); 
 and the Lachrymal secretion poured out upon the surface of 
 the eye, for the purpose of washing it from impurities ( 541) : 
 while the secretion of Milk for the nourishment of the 
 young is limited to Mammals ; and poisonous secretions are 
 formed in Serpents ami Insects, for the destruction of their 
 prey or for means of defence. Any one of these may be 
 checked, without rendering the blood impure by the accumu- 
 lation of any substances that should be drawn-off from it ; 
 but its cessation may produce effects fully as injurious, by 
 disordering the function to which it is subservient. Thus, if 
 the salivary and gastric secretions were to cease, the reduction 
 of the food could not be effected, and the animal must starve, 
 though its stomach were filled with wholesome aliment. It 
 is to be observed, in regard to nearly all these secreted fluids, 
 that they contain but a small quantity of solid matter, and 
 that this matter seems to be formed from the albumen of the 
 blood by a process of incipient decomposition, which gives it 
 the character of a " ferment." 
 
 353. The various acts of Secretion and Excretion which are 
 continually taking place in the living body, are, like those of 
 Nutrition, completely removed from the influence of the will; 
 
298 INFLUENCE OF EMOTIONS UPON SECRETIONS. 
 
 but they are strongly affected by emotions of the mind. This 
 has been already pointed out in regard to the Saliva ( 190); 
 and it is equally evident in the case of the Lachrymal secre- 
 tion ( 541). In these instances, however, the effect of the 
 emotion is manifested upon the quantity only of the secretion ; 
 in the case of the secretion of Milk, not only the quantity "but 
 quality is greatly influenced by the mental state of the nurse. 
 The more even her temper, and the more free from anxiety 
 her mind, the better adapted will be her milk for the nourish- 
 ment of her offspring. There are several instances now on 
 record, in which it has been clearly shown, that the influence 
 of violent passions in the mother has been so strongly exerted 
 upon the secretion of milk, as almost instantaneously to com- 
 municate to it an absolutely poisonous character, which has 
 occasioned the immediate death of the child. 1 The influence of 
 emotional states upon the Secretions is probably communicated 
 by the Sympathetic system of Nerves ( 461), which is very 
 minutely distributed, with the blood-vessels, to the several 
 glands which form them. 
 
 Nature of the Secreting Process. Structure of the Secreting 
 Organs. 
 
 354. Notwithstanding the different characters of the pro- 
 ducts of Secretion and Excretion, and the variety of the pur- 
 poses to which they are destined to be applied, the mode in 
 which they are elaborated or separated from the blood is 
 essentially the same in all. The process is performed, in the 
 Animal, as in the Plant, by the agency of cells; and the 
 variety of the structure of the different Glands, or secreting 
 organs, has reference merely to the manner in which these, 
 their essential parts, are arranged. The simplest condition 
 of a secreting cell, in the animal body, is that in which it 
 exists in Adipose or fatty tissue ; which is composed, as 
 formerly explained ( 46), of a mass of cells, bound together 
 by areolar tissue that allows the blood-vessels to gain access 
 to them. Every one of these cells has the power of secreting 
 or separating fatty matter from the blood ; and the secreted 
 product remains stored-up in its cavity, as in Plants (VEGET. 
 
 1 See the Author's Principles of Human Physiology, chap. xv. ; and 
 Dr. A. Combe on the Management of Infancy, chap. x. 
 
ESSENTIAL STRUCTURE OF SECRETING ORGANS. 2? 9 
 
 PHYS. 324) not being poured forth, as it is in most other 
 cases, by the subsequent bursting of the cell. 
 
 355. But when the secreting cells are disposed on the 
 surface of a membrane, instead of being aggregated in a mass, 
 it is obvious that, if they burst or dissolve-away, their contents 
 will be poured into the cavity bounded by that membrane; 
 and this is the mode in which secretion ordinarily takes 
 place. Thus, the Mucous Membranes ( 39) are covered with 
 epithelium-cells, which are continually being cast-off, and 
 which are replaced as constantly by a fresh crop ; and they 
 form by their dissolution the glairy viscid substance termed 
 mucus, which covers the whole surface of the membrane, 
 and serves for its protection. In parts of the membrane 
 where it is necessary that the secretion should be peculiarly 
 abundant, we find its secreting surface greatly increased, 
 by being prolonged into vast numbers of little pits or bags, 
 termed follicles, which are lined with epithelium-cells, that 
 resemble those of its general surface (see fig. 9). Such 
 follicles are very abundant along the whole alimentary canal 
 of Man ; and the glandulae in which the Gastric and Intes- 
 tinal fluids are elaborated, are almost equally simple in their 
 structure ( 204). 
 
 356. Now although the most 
 important Secretions and Ex- 
 cretions are separated, in Man 
 and the higher animals, by 
 organs of a much more com- 
 plex nature, yet in the lower 
 we find them generated after 
 the same simple fashion. Thus 
 in the little Bowerbankia ( 
 115), the bile is secreted by 
 minute follicles which are 
 lodged in the walls of the 
 stomach (fig. 64, c) and pour 
 their secretion separately into 
 its cavity, having no communi- 
 cation with one another. In 
 more complex forms of glan- 
 dular structure, however, several follicles open together 
 into a tube, which discharges the product of their secretion 
 
 Fig. 164. GLAND THAT SECRETES THE 
 ACRID FLUID DISCHARGED BY THE 
 BOMBARDIER BEETLE. 
 
300 
 
 ESSENTIAL STRUCTURE OF SECRETING ORGANS. 
 
 (fig. 164) j and thus the entire mass may be composed of 
 numerous lobules, each having its own duct. Passing to still 
 higher forms, we find all the ducts coalescing into a common 
 trunk, so that the gland bears a strong resemblance to a bunch 
 of grapes ; as is seen in fig. 1 65, which represents the structure 
 
 Fig. 165. INTIMATE STRUCTURE OF A COMPOSITE GLAND (THE PAROTID). 
 
 of the Parotid (one of the salivary glands) of Man. The 
 main stalk is the duct into which all the others enter ; from 
 this pass off several branches, and these again give off smaller 
 
 twigs, the extremities of which 
 enter the minute follicles in 
 which the secretion is formed. 
 These follicles are lined, as in 
 their simple condition, with cells, 
 which are the essential instru- 
 ments in the production of the 
 secretion; the fluid which they 
 separate is poured, by the 
 giving-way of their walls, into 
 the small canals proceeding 
 from the follicles, thence into 
 the larger branches, and finally 
 into the main trunk, by 
 into the 
 is to be 
 
 employed or from which it is to pass out. The Liver will 
 be seen to possess a structure exactly resembling this, in the 
 
 Fig. 166. PORTION OF ONE OF THE 
 TUBI;LI URINIFERI OF THE 
 HUMAN KIDNEY; 
 
 Showing its lining of flattened epithe- which it is Carried 
 
 Uum ceils. situation where it 
 
ESSENTIAL STRUCTURE OF SECRETING GLANDS. 
 
 301 
 
 Crustacea, by referring to fig. 47, fo ; and in the Mollusca it is 
 nearly the same (figs. 157, /, and 149,/). 
 
 357. The required extent of secreting surface is not unfre- 
 quently given, however, by the prolongation of the follicles 
 into tubes, rather than by a great multiplication in their 
 number. Of this we have a remarkable example in the 
 Kidney of the higher animals ( 368), which is entirely com- 
 posed of such tubes, together with areolar tissue which binds 
 them together, and the blood-vessels distributed amongst them. 
 These tubes, like the follicles, are lined with epithelium-cells 
 (fig. 166), which are the real instruments in the separation of 
 their secreted product. 
 
 358. That there is nothing in the form of any secreting 
 apparatus, however, which determines the peculiar nature of 
 its secretion, is evident from this 
 
 fact, that, in glancing through the 
 Animal series, we find the same secre- 
 tion elaborated by glandular struc- 
 tures of every variety of form. Thus, 
 we have seen that the bile is secreted, 
 in the lowest animals in which we 
 can distinguish it, by a number of 
 distinct follicles, as simple in their 
 structure as are those by which the 
 mucous secretions are formed in the 
 highest. Again, the bile is secreted 
 in Insects, by a small number of long 
 tubes, which open separately into the 
 intestinal canal j ust below the stomach 
 (fig. 112); and these tubes appa- 
 rently differ in no respect from those 
 that form the urinary secretion in the 
 same animals, which open nearer the 
 outlet of the intestinal canal. In 
 fact, the distinct function of the 
 latter was not known, until it was 
 ascertained that uric acid is to be 
 found in them. In fig. 167, which 
 represents the digestive apparatus of the Cockchafer, it is 
 seen that the biliary vessels are only four in number, but 
 are very long ; and that, for a good part of their length, 
 
 Fig. 167. 
 
 ALIMENTARY CANAL AND 
 HEPATIC TUBULES OF COCK- 
 
 CHAFER. 
 
302 ESSENTIAL STRUCTURE OF SECRETING GLANDS: 
 
 they are beset with a series of short tubes opening from 
 them, by which the extent of secreting surface is much in- 
 creased. On the other hand, although the urinary secretion 
 is generally formed by long tubes, yet in the Mollusca it is 
 secreted by follicles, according to the general plan of their 
 glandular structures. 
 
 359. The secreting cells not unfreqnently possess the power 
 of elaborating a peculiar colouring matter, either separately, 
 or along with the substances which seem more characteristic 
 of the secretion. Thus the ink of the Cuttle-fish is in reality 
 its urine, charged with a quantity of black matter formed in 
 the pigment-cells (resembling those of the interior of the eye, 
 533) that line its ink-bag ; and the corresponding secretion 
 in other Mollusca is rendered purple by the same cause. 
 The bile seems to be universally tinged with a yellow or 
 greenish colouring matter, which may be regarded, therefore, 
 as an essential part of the secretion ; and the urine of Mam- 
 mals is also tinged by a yellow pigment, which seems related 
 in its nature to that of the bile. In all these pigments, carbon 
 is the predominating ingredient ; and their amount is increased 
 when the respiratory process is insufficiently performed. 
 
 360. It appears, then, that the different secreting cells have 
 the power of elaborating a great variety of products ; and that 
 no essential differences can be discovered in the structure of 
 the glands into whose composition they enter, which can 
 account for that variety. We are entirely ignorant, therefore, 
 of the reason why one set of cells should secrete biliary matter, 
 another urea, another a colouring substance, and so on ; but 
 we are as ignorant of the reason why, in the parti-coloured 
 petal of a flower, the cells of one portion should secrete a red 
 substance, whilst those in immediate contact with it form a 
 yellow or blue colouring matter ; and we know as little of the 
 cause, which occasions one set of the cells of which the embryo 
 is composed to be converted into muscular tissue, another 
 into cartilage, and so on, 
 
 361. One of the most curious points in the Physiology of 
 Secretion, is the interchange which sometimes occurs in the 
 functions of particular glands. "When the operation of some 
 one gland is checked or impaired by disease, it not unfrequently 
 happens that another gland, or perhaps only a secreting sur- 
 face, will perform its functions more or less perfectly; this 
 
RECEPTACLES FOR SECRETED PRODUCTS. 303 
 
 happens most frequently in regard to the important Excretions, 
 as if Nature had especially provided for their continued sepa- 
 ration from the blood, that its purity may be unceasingly 
 maintained. Thus the urinary secretion has been passed off 
 from the surfaces of the skin, stomach, intestines, and nasal 
 cavity, and also from the mammary gland; the colouring 
 matter of the bile, when it accumulates in the blood (as in 
 jaundice), is separated from it in the skin and conjunctival 
 membrane of the eye ( 537) ; and milk has been poured 
 forth from pustules on the skin, and from the salivary glands, 
 kidneys, &c. Such cases have been regarded as fabulous ; 
 but they rest upon good authority, and they are quite consistent 
 with physiological principles. 
 
 362. Some of the main ducts or channels, through which 
 the glands pour forth their secretions, are provided with 
 enlargements or receptacles, which serve to retain and store 
 up the fluid for a time, until it may be desirable or convenient 
 that it should be discharged. Thus, in most of the higher 
 animals, the duct which conveys into the intestinal tube the 
 bile secreted by the liver, is also connected with a receptacle 
 termed the gall-bladder ; the bile, as it is secreted, passes into 
 this, and is retained there until it is 
 wanted for assisting in the digestive 
 process ( 213); when it is pressed out 
 into the intestinal canal. It is a curious 
 fact, that in most persons who die of 
 starvation, the gall-bladder is found dis- 
 tended with bile; showing that the 
 secretion has continued, although it has 
 not been poured into the intestine for 
 want of the stimulus occasioned by the 
 presence of food in the latter. In many 
 quadrupeds, especially those of the 
 Euminant tribe, the milk-ducts are in 
 a, kidneys; 6, ureters; e , ^ke manner dilated into a large re- 
 hiadder ; d, its canal, the ceptacle, the udder, which retains the 
 
 urethra. , . .!_/ i . -i .-i 
 
 secretion as it is formed, until the 
 period when it is needed. In all Mammals, and in some 
 Eeptiles, Mollusks, and Insects, but not in Birds or Fishes, 
 we find the ureters, which convey away the urinary excretion 
 from the kidneys, dilated at their lower extremity into a 
 
 *. /* 
 
 Fig. 168. URINARY AP- 
 PARATUS. 
 
304 
 
 STRUCTURE OF THE LIVER. 
 
 bladder (fig. 168), which serves to retain all the fluid that is 
 poured forth by the gland during a considerable length of 
 time, and thus prevents the necessity for its being continually 
 passed out of the body. 
 
 Characters of Particular Secretions. 
 
 363. In nearly all animals, the Liver holds the first rank 
 among Glands or secreting organs, in regard both to its size 
 and to the obvious importance of its function. The principal 
 varieties of its plan of structure in the Invertebrated classes 
 having been already noticed ( 356), we shall here limit 
 ourselves to a sketch of that peculiar arrangement of its 
 elementary parts, which presents itself in Man and other 
 Yertebrata. The position of this organ in the abdominal 
 cavity is shown in fig. 30. It is chiefly composed of a mass 
 of cells of a flattened spheroidal form (fig. 169, B), the dia- 
 meter of which is usually from l-800th to 1-1 600th of an inch ; 
 each cell presents a distinct nucleus, which is surrounded by 
 yellow biliary matter in a finely granular condition ; and in 
 the midst of this there are usually one or two large fatty 
 globules, or five or six small ones. The quantity of fat in 
 the liver is very liable to increase, however, when there is a 
 large amount of oily or fatty matter in the food, or when the 
 respiratory function is not performed with sufficient activity. 
 
 The hepatic cells are 
 clustered together into 
 lobules of irregular form, 
 but about the average 
 size of a millet-seed; 
 these lobules are disposed 
 upon the ramifications of 
 the hepatic vein (fig. 
 169, A), like leaves upon 
 the branches of a tree j 
 and they are separated 
 from one another by the 
 peculiar distribution of 
 the " portal " vessels and 
 of the hepatic ducts. 
 The Vena Portce, it will be remembered, collects the blood 
 that has been distributed to the alimentary canal, and conveys 
 
 Fig. 169. PORTION OF THE HUMAN LIVER. 
 
 A, Showing the manner in which the substance 
 of its lobules is disposed around the branches 
 of the hepatic vein a ; B, cells of which the 
 lobules are composed, more highly magnified. 
 
STRUCTURE OF THE LIVER. 
 
 305 
 
 it to the liver, through, which it is distributed by the sub- 
 divisions of this vessel, which acts the part of an artery 
 ( 267). Its branches proceed to the surfaces of the lobules, 
 amidst which they form by mutual inosculation a tolerably 
 regular network (fig. 170, 6, 6, 6); and from these branches a 
 
 Fig. 170. TRANSVERSE SECTION OF THREE LOBULES OF THE LIVER; 
 
 Showing the passage of the ramifications of the portal vessels from the network 
 b b bb, which surrounds the lobules, towards the centre of each lobule, near 
 which they become continuous with the rootlets a a a of the hepatic veins. 
 
 set of capillary twigs proceeds inwards towards the centre of 
 each lobule, traversing in their course its aggregation of 
 secreting cells. These capillaries finally terminate in the 
 rootlets of the hepatic veins, which diverge from the centre of 
 each lobule (fig. 170, a, a, a), and which collect the blood 
 that has traversed its capillary system, to transmit it through 
 larger trunks into the Vena Cava ( 266), and thence to the 
 heart. The liver is also supplied with arterial blood by the 
 Hepatic artery ; but this seems to have for its function rather 
 to nourish the solid tissues of the organ, than to supply the 
 materials for secretion. The bile-ducts, which convey away 
 the fluid that is elaborated by the hepatic cells, appear to form 
 a network which surrounds the lobules, connecting them 
 together and sending branches towards the interior of each 
 (fig. 171). It is still doubtful, however, whether they extend 
 through the entire substance of the lobules, and whether the 
 
306 STBUCTUBE OF THE LIVER ! BILE. 
 
 hepatic cells are really included within their extensions (as 
 they are within the tubes or follicles of the liver of Inverte- 
 brata); or whether the cells lie outside the bile-ducts, in 
 immediate contact with the capillary blood-vessels that tra- 
 verse the lobule, filling up the entire space not occupied by 
 them, and transmitting the products of their secretion from 
 one to another, until these reach the exterior of the lobule, 
 where they find their way into the bile-ducts and are carried 
 
 *& 
 -Jfisini 
 
 
 Fig. 171. TRANSVERSE SECTION op TWO LOBULES OP THE LIVER; 
 
 Showing the bile-ducts distended by injection ; a a, ramifications of the hepatic 
 vein, occupying the centres of the lobules ; b b b, branches of the hepatic 
 ducts, which are largest in the space c, between the lobules, and which pass 
 towards the centre through d d, the substance of the lobules. 
 
 off by them. The bile may flow directly, as it is secreted, 
 into the intestinal tube ( 213); but if digestion be not going 
 on, so that its presence there is not required, it regurgitates 
 into the gall-bladder (fig. 30), which stores it up until it is 
 needed. In this reservoir it undergoes a certain degree of 
 concentration by the removal of its watery part. 
 
 364. Bile is a yellowish (sometimes a greenish-yellow) 
 fluid, somewhat viscid and oily-looking, and having a very 
 bitter taste, followed by a sweetish after-taste. It is readily 
 miscible with water, its solution frothing like one of soap ; 
 and it has the power, in common with soap, of dissolving oily 
 matters ; so that ox-gall is not unfrequently used to remove 
 grease-spots from woollen stuffs. The basis of the principal 
 ingredient of biliary matter, which constitutes about 5 parts 
 in 100 of the secretion, is a fatty or resinoid acid which is 
 termed the Cholic; this consists of 49 Carbon, 39 Hydrogen, 
 and 9 Oxygen ; and it forms, by " conjugation " with glycine (a 
 
SECRETION OF BILE. 307 
 
 sugary substance that is derivable from the decomposition of 
 gelatin and albumen) and withtaurine (a substance distinguished 
 for the large proportion of sulphur it contains, no less than 
 25 per cent), two other acids, the Glycocholic and the Tauro- 
 cholicj which are mingled in different proportions in the bile 
 of different animals, both being combined with soda as a base. 
 Bile also contains a white crystallizable fatty substance 
 resembling spermaceti, which is termed Cholesterin ; this 
 consists of 36 Carbon, 32 Hydrogen, and 1 Oxygen; and 
 though its quantity in healthy bile appears to be very small, 
 yet it occasionally increases to such an extent as to form the 
 concretions known as "gall-stones," which, getting into the 
 bile-duct, are transmitted along it with great pain and diffi- 
 culty, or block it up altogether. The peculiar colouring 
 matter of bile is quite distinct from the preceding substances ; 
 but like them it is extremely rich in carbon and hydrogen. 
 
 365. The bulk of the Liver, and the activity of the Respira- 
 tory apparatus, seem generally to bear an inverse ratio one to 
 the other. Thus we find in Insects, a respiratory system 
 possessing enormous extension and activity of function, and a 
 liver so slightly developed, that for a long time it was not 
 recognised as such. On the other hand, in the Mollusca, we 
 find the respiration carried-on upon a lower plan, and with 
 far less activity; but the liver is of enormous size, often 
 making up a large part of the bulk of the body. Moreover, 
 in the Crustacea, which are formed upon the same general 
 plan with Insects, but which have an aquatic and therefore 
 less energetic respiration, we find the liver very large, as in 
 the Mollusca. In Reptiles and Fishes, again, whose respira- 
 tion and temperature are low, the liver is comparatively larger 
 than in Birds and Mammals, in which classes the respiration 
 is more energetic, and the blood warm. In all these in- 
 stances, however, the bulk of the liver depends in great part 
 upon the accumulation of fat in its cells ; and the secreting 
 activity may be positively less in them, than it is in animals 
 which have a comparatively small biliary apparatus. 
 
 366. The materials of the secretion of Bile are probably 
 derived in part from the disintegration of the tissues, and in 
 part more directly from the food. It is an interesting fact 
 that the composition of bile and urine, taken together, corre- 
 sponds closely with the composition of the blood ; so that it 
 
 x2 
 
308 ASSIMILATING ACTION OF LIVER. 
 
 would appear as if the nutritive materials, in their ultimate 
 metamorphosis, resolved themselves chiefly into these two 
 excretory products. The greater part of the biliary matter 
 poured into the intestinal canal seems to be ordinarily re- 
 absorbed with the fatty matter of the food, and to be, like it, 
 carried out of the system through the lungs in the form of 
 carbonic acid and water; it being only when the bile has 
 either been formed in excessive amount, or has been pro- 
 pelled along the intestinal tube with undue activity, that it is 
 discharged in any quantity from the rectum, as in bilious 
 diarrhoea. The secreting action of the Liver, however, is by 
 no means its sole mode of influencing the composition of the 
 blood ; for it has been shown by the recent researches of 
 M. Bernard, that the blood which leaves the liver by the 
 hepatic vein contains a peculiar substance of a saccharine 
 nature, 1 which does not exist in the blood brought to the 
 organ by the portal vein. This substance appears to be 
 elaborated by the converting power- of the liver, either from 
 materials supplied by the food, or from the products of the 
 waste of the system ; and it seems to be specially destined as 
 a pabulum or fuel for the combustive process, being usually 
 eliminated from the blood in the form of carbonic acid and 
 water during its passage through the lungs, so as not to pass 
 into the systemic circulation unless either its quantity be un- 
 usually great, or its elimination be interfered with by imperfect 
 respiration. The liver seems also to form a peculiar fat, which 
 is usually consumed in the same manner ; but if the respiratory 
 process be feeble, this fat accumulates in the cells of the liver 
 itself. 
 
 367. The Urinary excretion has for its chief purpose to 
 throw off those products, formed in a similar .manner, which 
 are highly charged with azote. The most important of its 
 ingredients, in Man and the Mammalia, is the substance termed 
 Urea, which has a crystalline form, and is very soluble in 
 water. It contains 2 equivalents of Carbon, 4 of Hydrogen, 
 2 of Azote, and 2 of Oxygen ; and it will be seen, by referring 
 to the statement formerly given of the composition of albumen 
 
 1 This substance is spoken of by M. Bernard as sugar : it has been 
 demonstrated, however, by the recent researches of Dr. Pavy, that the 
 liver does not form sugar, but a substance that becomes sugar almost 
 immediately upon contact with albuminous matters. 
 
SECRETION OF URINE. 309 
 
 ( 13) and gelatin ( 19), that the amount of azote in propor- 
 tion to that of the other elements is much greater in urea 
 than it is in these substances, which form the materials of 
 the animal tissues. The quantity of Urea which is daily 
 excreted is very considerable, the average in an adult being 
 about an ounce, and in a child of eight years old about half 
 as much. There is another compound which does not usually 
 exist in large amount in the urine of the Mammalia, but 
 which makes up a considerable part of the solid matter of 
 this secretion in Birds and the lower Vertebrata ; this is uric 
 or lithic acid, which consists of 10 equivalents of Carbon, 4 of 
 Hydrogen, 4 of Azote, and 6 of Oxygen. It is almost entirely 
 insoluble in water, unless it be combined with soda or am- 
 monia ; and in this state it ordinarily exists. When formed 
 in too large quantity, however, it may be deposited in an 
 insoluble form, constituting gravel ( 348) ; and the same 
 effect may result from the presence of some other acid, which, 
 combining with the ammonia, precipitates or sets free the 
 lithic acid. In the urine of herbivorous animals, uric acid is 
 replaced by Hippuric acid, which contains a much larger 
 proportion of carbon, its composition being 18 Carbon,. 8 
 Hydrogen, 1 Nitrogen, and 5 Oxygen. Urine also contains 
 a considerable quantity of Saline matter; part of which 
 consists of what has been introduced into the system in the 
 same form, and has to be got rid of as superfluous; whilst 
 another part results from the conversion of the sulphur and 
 phosphorus of the food into sulphuric and phosphoric acids by 
 union with atmospheric oxygen ( 343), and from the com- 
 bination of these acids with alkaline bases furnished by the 
 food. The amount of alkaline phosphates contained in the 
 urine may be considered as in some degree a measure of the 
 expenditure of nervous tissue ; whilst that of alkaline sulphates 
 has some relation to the expenditure of muscular substance. 
 
 368. The Kidney, by which the secretion of Urine is eli- 
 minated from the blood, is an organ whose structure in the 
 higher animals is very peculiar, although in the lower it is a 
 mere aggregation of tubes or of follicles. If we make a ver- 
 tical section of the kidney of Man or any of the higher Mam- 
 malia (fig. 172, A), we find that it seems composed of two 
 different substances, one surrounding the other ; to the outer; 
 a, the name of conical (bark-like) substance has been given ; 
 
310 
 
 STRUCTURE OF THE KIDNEY. 
 
 whilst the inner, 6, is termed medullary (or pith-like). In 
 the cortical substance, no definite arrangement can be de- 
 tected by the naked eye ; it chiefly 
 consists of a very intricate network 
 of blood-vessels, surrounding the 
 extremities of the tubes. But in 
 the medullary substance we can 
 trace a regular passage of minute 
 tubes, from the circumference to- 
 wards the centre. They commence 
 in the midst of the network of 
 blood-vessels (B, a), and then pass 
 down in clusters, nearly in a 
 straight direction, and slightly con- 
 verging towards each other, until 
 each cluster terminates in a little 
 body, called the calyx or cup, which 
 discharges the fluid it receives into 
 the large cavity of the kidney, 
 termed the pelvis or basin (A, c). 
 From this it is conveyed away by 
 the ureter d, which terminates in 
 the bladder. 
 
 369. One of the most interesting 
 circumstances in reference to the 
 Urinary secretion, is the very large 
 quantity of water which, in the 
 higher animals, is got rid of through 
 this channel, and the means by 
 which it is drawn off. The kidneys 
 seem to form a kind of regulating valve, by which the quan- 
 tity of water in the system is kept to its proper amount. The 
 exhalation from the Skin is liable to sustain great variations 
 in its amount from the temperature of the air around ; for 
 when this is low, the exhalation is very much diminished ; 
 and when it is high, the quantity of fluid that passes off in 
 this manner is increased ( .371). Hence, if there were not 
 some other means of adjusting the quantity of fluid in the 
 blood-vessels, it would be liable to continual and very inju- 
 rious variation. This important function is performed by the 
 kidneys, which allow such a quantity of water to pass into 
 
 Fig. 
 
 172. STRUCTURE OP 
 KIDNEY OF MAN. 
 
 A, vertical section of the kidney ; 
 a, cortical substance ; b, tubular 
 substance ; c, calyx and pelvis ; 
 d, ureter. 
 
 B, portion of the gland enlarged; 
 a, extremity of the uriniferous 
 tubes; b, straight portion; c, 
 their termination in the calyx. 
 
MALPIGHIAN BODIES OF THE KIDNEY; 
 
 311 
 
 Fig. 173. 
 
 -MALPIGHIAK BODIES 
 THE KIDNEY. 
 
 the urinary tubes, as may keep the pressure within the vessels 
 very nearly at a uniform standard ; anl a distinct and very 
 curious provision exists for its separation. The extremity of 
 many of the uriniferous tubes is made to include little knots 
 or bunches of capillary vessels, 
 which have extremely thin 
 walls (fig. 173) ; and a vast 
 number of such knots, which are 
 termed " Malpighian bodies," 
 after the name of their dis- 
 coverer, are scattered through 
 the cortical portion ( 368) of 
 the kidney. To these the blood 
 brought to the organ by the 
 renal artery is first conveyed; 
 and the membranes that sepa- 
 rate the interior of the capil- 
 lary vessels from the cavity of 
 the uriniferous tube, being of 
 extreme thinness, water is 
 readily able to traverse them; and will do so in larger or 
 smaller quantity, according as the pressure upon the walls 
 of the capillaries is greater or less. The blood which has 
 passed through these is next conducted to another set of 
 capillaries, which form a network upon the part of the tube 
 that is lined by the secreting cells ; and it is there subservient 
 to the elaboration of the solid part of the secretion. Hence 
 the quantity of water in the urinary secretion depends in part 
 upon the amount exhaled from the skin, being greatest 
 when this is least, and vice versa, and in part upon th& 
 quantity which has been absorbed by the vessels. The quan- 
 tity of solid matter in the secretion has but little to do with 
 this ; for it depends upon the amount of waste of the muscular 
 and other tissues that has been occasioned by their activity 
 (367); and also upon the quantity of surplus aliment which 
 has to be discharged through this channel, there being no 
 other vent for it ( 348). 
 
 370. Next to the excretions formed by the liver and the 
 kidneys, that of the Skin probably ranks in importance. A 
 large quantity of watery vapour is constantly passing-off from 
 the whole surface of Man and other sofVskin&ed animals; 
 
312 EXHALATION FKOM THE SKIN. 
 
 and this amount is greatly increased under particular circum- 
 stances. A continual evaporation takes place from the surface 
 of the skin, wherever it is not protected by hard scales or 
 plates ; and the amount of it will depend upon the warmth, 
 dryness, and motion of the surrounding air, exactly as if the 
 fluid were being evaporated from a damp cloth. Every one 
 knows that the drying of a cloth is much more rapidly effected 
 in a warm dry atmosphere, than in a cold moist one ; more 
 quickly, too, in a draught of air, than in a situation where 
 there is no current, and where the air is consequently soon 
 charged with moisture. That all these influences affect the 
 evaporation from the bodies of Animals, there is ample evi- 
 dence derived from experiment. 
 
 371. But besides this continual evaporation, a special 
 exhalation of fluid takes place from the vast number of 
 minute perspiratory glands imbedded in the fatty layer just 
 beneath the Skin ( 37). Every one of these glandule, when 
 straightened out, forms a tubule about a quarter of an inch in 
 length ; and as it has been estimated that in a square inch of 
 surface on the palm of the hand there are no fewer than 3528 
 of these glandule, the length of their tubing must be 882 
 inches or 73 J feet. The average number in other parts of the 
 body may be estimated at about 2800 per square inch ; and 
 as the number of square inches of surface on a man of ordinary 
 stature is about 2500, the total number of perspiratory glan- 
 dulse must be not less than seven millions, and the length of 
 their tubing nearly twenty-eight miles. The fluid which these 
 perspiratory glands ordinarily exhale, is dissolved by the atmo- 
 sphere, and carried off in the state of vapour, so as to pass 
 away insensibly ; but they are stimulated to increased action 
 by the exposure of the body to heat, which causes them to 
 pour forth their secretion in greater abundance than the air 
 can carry off, and this consequently accumulates in drops upon 
 the surface of the skin. The amount of perspiration may be 
 considerably increased, without its becoming sensible, if the air 
 be warm and dry, so as to carry off, in the form of vapour, 
 the fluid which is poured out on the skin ; but, on the other 
 hand, a very slight increase in the ordinary amount immedi- 
 ately becomes sensible on a damp day, the air being already 
 too much loaded with moisture to carry off this additional 
 quantity. The distinction between insensible and sensible 
 
COOLING EFFECT OF CUTANEOUS EXHALATION. 313 
 
 perspiration, is not the same, therefore, with the difference 
 between simple evaporation and exhalation from the skin ; for 
 a part of the latter is commonly insensible ; and the degree 
 in which it is so depends upon the amount of fluid exhaled, 
 and the state of the surrounding atmosphere. If the fluid 
 thus poured forth be allowed to remain upon the surface of 
 the skin, it produces a very oppressing effect ; most persons 
 have experienced this, when walking in a mackintosh cloak or 
 coat, on a damp day. The waterproof garment keeps in the 
 perspiration, almost as effectually as it keeps out the rain; 
 and consequently the air within it becomes loaded with fluid, 
 and the skin remains in a most uncomfortable as well as pre- 
 judicial state of dampness. 
 
 372. The purpose of this watery exhalation, and of its 
 increase under a high temperature, is evidently to keep the 
 heat of the body as near as possible to a uniform standard. 
 By the evaporation of fluid from the surface of the skin, a 
 considerable quantity of heat is withdrawn from it, becoming 
 latent (according to ordinary phraseology) in the change from 
 fluid to vapour : of this we make use in applying cooling 
 lotions to inflamed parts. The more rapid the evaporation, 
 the greater is the amount of heat withdrawn in a given time ; 
 hence, if we pour, on separate parts of the back of the hand, 
 small quantities of ether, alcohol, and water, we shall find 
 that the spot from which the ether is evaporating feels the 
 coldest, that which was covered by the alcohol less so, whilst 
 the part moistened with water is comparatively but little 
 chilled. The greater the amount of heat applied to the body, 
 then, the more fluid is poured out by the perspiratory glands ; 
 and as the air can carry it off more readily in proportion to 
 its own heat, the evaporation becomes more rapid, and its 
 cooling effect more powerful. It is in this manner that the 
 body is rendered capable of sustaining very high degrees of 
 external heat, without suffering injury. Many instances are 
 on record, of a heat of from 250 to 280 being endured in 
 dry air for a considerable length of time, even by persons 
 unaccustomed to a peculiarly high temperature ; and indi- 
 viduals whose occupations are such as to require it, can sustain 
 a much higher degree of heat, though perhaps not for any 
 great length of time. Thus, the workmen of the late Sir F. 
 Chantrey were accustomed to enter a furnace in which his 
 
314 IMPOETANCE OF CUTANEOUS EXHALATION. 
 
 moulds were dried, while the floor was red-hot, and a ther- 
 mometer in the air stood at 350 ; and Chabert, the " Fire- 
 king," was in the habit of entering an oven whose temperature 
 was from 400 to 600. It is possible that these feats might 
 be easily matched by many workmen, who are habitually 
 exposed to high temperatures; such as those employed in 
 iron-foundries, glass-houses, and gas-works. 
 
 373. That the power of sustaining a high temperature 
 mainly depends upon the dryness of the atmosphere, is evident 
 from what has just been stated j since, if the perspiration that 
 is poured-forth upon the skin is not carried-off with sufficient 
 rapidity, on account of the previous humidity of the air, the 
 temperature of the body will not be sufficiently kept down. 
 It has been found, from a considerable number of experiments, 
 that when warm-blooded animals are placed in a hot atmos- 
 phere saturated with moisture, the temperature of their bodies 
 is gradually raised 12 or 13 above the natural standard ; and 
 that the consequence is then inevitably fatal. 
 
 374. The amount of fluid exhaled from the skin and lungs 
 ( 343) in twenty-four hours, probably averages about three 
 or four pounds. The largest quantity ever noticed, except 
 under extraordinary circumstances, was 5 Ibs. ; and the smallest, 
 If Ibs. It contains a small quantity of solid animal matter, 
 besides that of the other secretions of the skin which are 
 mingled with it ; and there is good reason to think that this 
 excretion is of much importance, in carrying off certain sub- 
 stances which would be injurious if allowed to remain in the 
 blood. That which is called the Hydrophatic system, proceeds 
 upon the plan of increasing the cutaneous exhalation to a 
 very large amount ; and there seems much evidence, that 
 certain deleterious matters, the presence of which in the blood 
 gives rise to Gout, Rheumatism, &c., are drawn off from it 
 more speedily and certainly in this way, than in any other. 
 
 375. Besides the perspiratory glands, the skin contains 
 Others, which have special functions to perform. Thus in 
 most parts which are liable to rub against each other, we find 
 ft considerable number of sebaceous follicles (fig. 8, a a), which 
 secrete a fatty substance that keeps the skin soft and smooth. 
 Besides these, the skin contains other follicles in particular 
 parts, for secreting peculiar substances ; as, for instance, those 
 which form the cerumen, a bitter waxy substance that is 
 
MAMMAKY GLAND : SECRETION OF MILK. 315 
 
 poured into the canal leading to the internal ear, for the pur- 
 pose (it would seem) of preventing the entrance of insects. 
 
 376. The secretion of Milk is important, not so much to 
 the parent who forms it, as to the offspring for whose nourish- 
 ment it is destined. It does not seem to carry off from the 
 system any injurious product of its decomposition ; for it bears 
 a remarkable analogy to blood in the combination of substances 
 which it contains ; nevertheless it is found that, when this 
 secretion is once fully established, it cannot be suddenly 
 checked, without producing considerable disturbance of the 
 general system. The structure of the Mammary gland closely 
 resembles that of the parotid already described (fig. 165). It 
 consists of a number of lobules, or small divisions, closely 
 bound together by fibrous and areolar tissue ; to each of these 
 proceeds a branch of the milk-ducts, together with numerous 
 blood-vessels ; and the ultimate ramifications of these ducts 
 terminate in a multitude of little follicles, about the size (when 
 distended with milk) of a hole pricked in paper by the point 
 of a very fine pin. 
 
 377. The nature of the composition of Milk is made evident 
 by the processes to which we commonly subject it. When it is 
 allowed to stand for some time, its oleaginous part, forming the 
 cream, rises to the top. This is still combined, however, with 
 a certain quantity of albuminous matter, which forms a kind 
 of envelope round each of the oil-globules ; but in the process 
 of churning, these envelopes are broken, and the oil-globules 
 run together into a mass, forming butter. In ordinary butter 
 a certain quantity of albuminous matter remains, which, from 
 its tendency to decomposition, is liable to render the butter 
 rancid ; this may be got rid of by melting the butter at the 
 temperature of 180, when the albumen will fall to the bottom, 
 leaving the butter pure and much less liable to change. In 
 making cheese, we separate the albuminous portion, or casein, 
 by adding an acid which coagulates it. The buttermilk and 
 whey left behind after the separation of the other ingredients, 
 contain a considerable quantity of sugar, and some saline 
 matter. The proportion of these ingredients varies in different 
 animals ; and also in the same animal, according to the sub- 
 stances upon which it is fed, and the quantity of exercise it 
 takes. The amount of casein seems to be greatest in the milk 
 of the Cow, Goat, and Sheep ; that of oleaginous matter in the 
 
316 GENERAL REVIEW OP NUTRITIVE OPERATIONS. 
 
 milk of the Human female ; and that of sugar in the milk of 
 the Mare. The milk of the Cow, if a portion of its casein 
 were removed, would resemble Human milk more nearly than 
 any other ; and it is therefore hest for the nourishment of 
 Infants, when the latter cannot be obtained. The important 
 influence of Mental emotion on this secretion has already been 
 noticed ( 353) ; and many more instances might be related, 
 were not the ordinary facts in regard to it generally known. 
 
 CHAPTEE VIII. 
 
 GENERAL REVIEW OF THE NUTRITIVE OPERATIONS FORMATION 
 OF THE TISSUES. 
 
 General Review of the Nutritive Operations. 
 
 378. IN the preceding Chapters (HI. to v.) those processes 
 have been described, by which the alimentary materials that 
 constitute the raw material of the tissues, are converted into 
 a fluid adapted for the Nutrition of the body ; and we then 
 (CHAPS, vi. and VH.) considered those functions, by which this 
 fluid is kept free from the impurities it acquires during its 
 circulation through the body, and is maintained in the state 
 which alone can adapt it to the purposes it is destined to fulfil. 
 These purposes may be regarded as fourfold. In the first 
 place, the Blood is destined to supply the materials of the 
 fabric of the body ; which, as it is continually undergoing 
 decay ( 68), requires the means of as constant a renovation. 
 Secondly, the Blood (in most animal& at least) serves to convey 
 to the tissues the supply of oxygen which is required by 
 them, especially by the muscular and nervous tissues, as a 
 necessary stimulus to the performance of their functions. 
 Thirdly, the Blood furnishes to the secreting organs the 
 materials for the elaboration of the various fluids, which have 
 special purposes to serve in the Animal economy, such, for 
 instance, as the Saliva, Gastric juice, Milk, &c. And lastly, 
 the Blood takes up, in the course of its circulation, the pro- 
 ducts of the waste or decomposition of the various tissues, 
 which it conveys to the several organs, the Lungs, Liver, 
 
FORMATION OF THE TISSUES. 317 
 
 Kidneys, &c., destined to throw them off by Excretion. 
 The greater number of these processes have already been 
 treated of in more or less detail. Those included under the 
 first head were considered, in a general form, in Chap. i. of 
 this Treatise. Those which are comprehended under the 
 second head have been dwelt-on in Chaps, v. and vi. ; and 
 will be again noticed, when the actions of the Nervous and 
 Muscular tissues are described. And the varied actions which 
 are included under the third and fourth classes, have been 
 discussed in the two Chapters which precede the present one. 
 We have now to enter, in more detail, into the mode in which 
 the circulating fluid is applied to the Nutrition and Formation 
 of the Tissues. 
 
 Formation of the Tissues. 
 
 379. There is sufficient reason to believe that every living 
 being is developed from a germ; no organized structure being 
 able to take its origin (as some have supposed) in a chance 
 combination of inorganic elements. All the facts relating to 
 the production of Fungi and Animalcules, which have been 
 imagined to favour this doctrine, may be satisfactorily ex- 
 plained in other ways (VEGET. PHYS. 779 ; ZOOL. 1213). 
 Now the first structure developed from this germ, in the 
 Animal as in the Plant, is a simple cell; and the entire fabric 
 subsequently formed, however complex and various in struc- 
 ture, may be considered as having had its origin in this cell. 
 The cells of Animals, like those of Plants, multiply by the 
 development of new cells within them; each of these be- 
 comes in its turn the parent of others ; and thus, by a con- 
 tinuance of the same process, a mass consisting of any number 
 may be produced from a single one. It is in this manner that 
 the first development of the Animal embryo takes place, as 
 will be shown hereafter (Chap. xv.). A globular mass, con- 
 taining a large number of cells, is formed before any diversity 
 of parts shows itself; and it is by the subsequent development, 
 from this mass, of different sets of cells, of which some are 
 changed into cartilage, others into nerve, others into muscle, 
 others into vessels, and so on, that the several parts of the 
 body are ultimately formed. 
 
 380. This process of differentiation is carried to very 
 different degrees in the development of the several classes of 
 
318 DIFFERENTIATION OF STBUCTUEE. 
 
 animals ; for in some it is checked so early, that scarcely any 
 distinction either of organs or of tissues ever shows itself; 
 whilst in others it continues during a large proportion of the 
 earlier period of life. It has no relation to growth, or simple 
 increase of size ; for this may take place by the multiplication 
 of similar parts, giving rise to that "vegetative repetition" 
 which is so characteristic of the lower tribes of Animals, and 
 which gives to many of them so strong a resemblance in 
 general aspect to Plants ; whilst, on the other hand, the de- 
 velopmental process by which higher forms of structure are 
 evolved, sometimes takes place without any increase at all. 
 It is in its degree of such differentiation, that what is called the 
 lower or the higher organization of any living being essentially 
 consists ; for whilst in the simplest forms of Animal struc- 
 ture every part is similar to every other, so that all the 
 functions of life are performed in common by each, we find 
 in Man (whose body may be regarded as presenting the 
 highest type or example of this differentiating process) that 
 no two parts are precisely similar, except those on the 
 opposite sides of the body. This fact is occasionally mani- 
 fested in a very singular manner, in the symmetry of disease ; 
 certain morbid poisons (as those of gout, and of several affec- 
 tions of the skin), which have a tendency to single out par- 
 ticular spots of the tissues whose nutrition they disturb, 
 exhibiting their action in those parts of the two lateral halves 
 of the body which precisely correspond with each other. 
 
 381. Now in the lowest grades of Animal structure, we find 
 that the several tissues of the body can themselves appropriate 
 from the products of digestion the nutrient materials they 
 respectively require ; so that, for their growth and mainte- 
 nance, it is sufficient that these products should be carried 
 into their neighbourhood by extensions of the digestive cavity 
 ( 296). But in all the -more highly-organized animals, it 
 appears requisite that the nutrient material should pass 
 through an intermediate stage of preparation, which is termed 
 assimilation (or making-like); and this is effected by their 
 introduction into the current of the circulation, and their 
 mixture with the pre-existing blood, which, in virtue of its 
 own vital powers, exerts upon them a converting action, which 
 prepares them for being appropriated by the solid tissues. 
 
 382. When once the several forms of tissue have been 
 
MUTUAL DEPENDENCE OF PARTS OP ORGANISM. 319 
 
 developed, their nutrition is kept up by the supply of their 
 respective materials which they derive from the blood. Each 
 tissue draws from the circulating current that which it re- 
 quires ] and it is one of the most wonderful proofs of the skill 
 with which the entire fabric has been planned and constructed, 
 that the composition of the blood should be maintained at a 
 nearly uniform standard, in spite of the continual change which 
 is thus taking place in its actual components. It has been 
 justly remarked, that each part of the body, by taking from 
 the blood the peculiar substances which it needs for its own 
 nutrition, does thereby act as an excretory organ, inasmuch 
 as it removes from the blood that which, if retained in it, 
 would be injurious to the nutrition of the body generally. 
 
 383. Hence it seems that such a mutual dependence must 
 exist among the several parts and organs of the body, as 
 causes the evolution of one to supply the conditions requisite 
 for the production of another ; and this view is borne out by 
 a great number of phenomena of very familiar occurrence, 
 which show that a periodical change in one set of organs 
 governs changes in others which at first sight might seem to 
 have no relation to them. Thus the plumage of Birds, at the 
 commencement of the breeding season, becomes (especially in 
 the male) more highly coloured, besides being augmented by 
 the growth of new feathers ; but when the generative organs 
 pass into their condition of periodical inactivity, the plumage 
 begins at once to assume a paler and more sombre hue, and 
 many of the feathers are usually cast, their nutrition being no 
 longer kept up. So, again, it is no uncommon occurrence 
 among Birds, for the female, after ceasing to lay, to assume 
 the plumage of the male, and even to acquire other character- 
 istic parts, as " spurs " in the fowl tribe. That, in these and 
 similar instances, the development of organs is immediately 
 determined by the presence or absence in the blood of the 
 appropriate pabulum for the parts in question, and that its 
 existence depends upon changes taking place in other parts, 
 has been rendered still more probable by the results of expe- 
 riments, which show that if the ordinary changes in one set 
 of organs be prevented by their removal, those usually taking 
 place in the others do not occur. 
 
 384. Though all the tissues derive the materials of their 
 development from the blood which circulates in the vessels, 
 
320 NUTRITION OF NON-VASCULAR TISSUES. 
 
 yet there is considerable variety in the mode in which the 
 supply is afforded ; some tissues being furnished with blood 
 much more copiously and directly than others, in consequence 
 of the greater minuteness with which the capillaries are dis- 
 tributed through their substance. There are several, indeed, 
 into which no blood-vessels enter, in their natural state ; but 
 which derive their nutriment by absorbing the liquor san- 
 guinis that is brought into their neighbourhood. This is 
 the case, for instance, with the Epidermis and Epithelium 
 ( 38, 40) ; the cells of which are developed at the expense 
 of the fluid which they absorb, through the basement mem- 
 brane on which they lie, from the vessels of the skin or 
 mucous membrane beneath it. In like manner, even the 
 thick layer of Cartilage which covers the ends of most of the 
 long bones, is destitute of blood-vessels ; and the small amount 
 of nourishment it requires, is obtained by absorption from the 
 vessels which surround it ( 47). This tissue undergoes very 
 little change from time to time ; and its growth takes place 
 chiefly by addition of new matter to its surface ; consequently 
 there is no necessity for any active circulation through its 
 interior; and the transmission of nutritive fluid from one 
 cell to another (as in the cellular tissue of Plants) is sufficient 
 for its wants. Even in Bone, the blood-vessels are not very 
 minutely distributed ; for although there is a close network 
 of capillary vessels on the membrane lining the Haversian 
 canals and the cells of the cancellated structure ( 49), yet 
 none of these pass into the actual substance of the bone. 
 The simple Eibrous tissues are, for the most part, sparingly 
 supplied with blood-vessels, as they are but little liable to 
 decay or injury ; though the Areolar tissue serves as the bed 
 for the reception of the vessels which are on their passage to 
 other tissues. Thus it is by its means that blood-vessels are 
 conveyed into the Adipose tissue ; for the ultimate elements 
 of that tissue, namely, the fat-cells, are surrounded by capil- 
 lary vessels, not entered by them. The same important pur- 
 pose is answered by the areolar tissue that lies amongst the 
 tubes which form the essential parts of the Nervous and 
 Muscular tissues ; for these tubes are not perforated by ves- 
 sels, so that their contents must be nourished by fluid absorbed 
 through their walls. 
 
 385. In no instance that we are acquainted with, in the 
 
IMPERFECT NUTRITION : CONSUMPTION. 321 
 
 higher animals at least, do the vessels directly pour the blood 
 into any tissue for the purpose of nourishing it. Unless there 
 have been an actual wound which has artificially opened the 
 blood-vessels, no fluid can escape from them into the substance 
 traversed by the capillaries, except by transuding the walls of 
 the latter ; and hence it would seem impossible that any of 
 the floating cells contained in the blood can be deposited in 
 the tissues and contribute to their development. The Liquor 
 Sanguinis seems, therefore, to furnish all that is wanting for 
 this purpose ; and it readily permeates the walls of the capil- 
 laries, the basement-membrane, and any other of the softer 
 tissues, so as to arrive at the parts where it is to be applied. 
 As it is withdrawn from the blood, it is continually being 
 re-formed from the food ; but if it be not supplied in sufficient 
 quantity by the latter, the tissues are imperfectly nourished, 
 and the strength of the body and the vigour of the mind are 
 consequently alike impaired. 
 
 386. This imperfect nutrition seems to be the essential 
 conditjpn of one of the most destructive diseases to which the 
 human frame is liable, that commonly known as Consump- 
 tion ; which is, however, but one out of several diseases that 
 may result from the same general defect of nutrition. If the 
 liquor sanguinis be imperfectly elaborated, it is less fit to 
 undergo organization ; and, consequently, instead of being 
 converted into living tissue, part of it is deposited, as an 
 imperfectly organized mass, in the state known to the Medical 
 man as Tubercle. Such deposits take place more frequently 
 in the lungs than in any other part ; and besides impeding 
 the circulation and respiration, they produce irritation and 
 inflammation, in the same manner as other substances im- 
 bedded in the tissues would do ; and so far from having, like 
 many other diseases, a natural tendency to cure, this malady, 
 if unchecked, almost certainly leads to a fatal termination. 
 Microscopic examination of tubercular matter shows that it 
 consists of half-formed cells, fibres, &c., together with a granu- 
 lar substance which seems to be little else than coagulated 
 albumen. The only manner in which any curative means can 
 be brought to bear upon this terrible scourge, is by attention 
 to the constitutional state from which it results. This is 
 sometimes hereditary ; and is sometimes induced by insuffi- 
 cient nutrition, obstructed respiration, habitual exposure to 
 
 Y 
 
TUBERCULAR DIATHESIS : ITS TREATMENT. 
 
 cold and damp, long-continued mental depression, &c. The 
 treatment of the Tubercular diathesis (as this state of consti- 
 tution is termed) must be directed to the invigoration of the 
 system by good food, active exercise, pure air, warm clothing, 
 and cheerful occupation; and by the due employment of 
 these means, at a sufficiently early period, many valuable lives 
 may be saved which would have otherwise fallen a sacrifice. 
 The value of cod-liver oil in the treatment of this disease, 
 which is now a well-established fact, seems to depend upon 
 the facility with which it is assimilated as a nutritive material. 
 It is a remarkable fact that the inhabitants of Iceland, the 
 greater part of whom live under conditions that might be 
 expected to favour the development of tubercular disease, are 
 singularly free from it; and the source of this exemption 
 seems to consist in the very oleaginous nature of their diet. 
 Consumption presents itself among the inhabitants of all 
 climates ; and the value of change to a patient who is 
 affected with this malady, chiefly depends upon the oppor- 
 tunity which it affords him for abundant exercise in the open 
 air, without injurious exposure to cold or damp. 
 
 387. From the foregoing facts it is evident, that the opera- 
 tions of Nutrition are due, on the one hand, to the indepen- 
 dent properties of the several Tissues, which draw from the 
 blood the materials of their continued growth and renewal ; 
 and, on the other, to the properties of the Blood, which 
 supplies them with these materials. The blood, left to itself, 
 could form no tissue more complex than a mere fibrous net- 
 work : whilst, conversely, the various tissues of the body 
 could not draw their nourishment directly from the products 
 of digestion, and are consequently dependent upon the blood 
 for their supply. We may illustrate the relation between the 
 three states, that of aliment, blood, and organized tissue, 
 by comparing them with the three principal states which 
 Cotton passes through in the progress of its manufacture, 
 namely, the raw cotton, spun-yarn, and woven fabric. The 
 spun-yarn could not of itself assume that particular arrange- 
 ment which is given to it by the loom ; and the loom could 
 make nothing of the raw cotton, until it has been spun into 
 yarn. 
 
 388. It is also evident, that the blood-vessels have no other 
 purpose in the act of Nutrition, than to convey the circulating 
 
ACTION OF BLOOD-VESSELS IN NUTEITION. 323 
 
 fluid into the neighbourhood of the part where it is to "be 
 employed ; and the blood, or at least its organizable portion 
 the liquor sanguinis must quit the vessels before it can be 
 employed in the development of new tissue. We might illus- 
 strate this by the distribution of water-pipes through a city ; 
 they might pass into every house, nay, into every room, and 
 yet the water must be drawn from the pipes before it can be 
 applied to any required purpose. The spaces untraversed by 
 vessels have been shown to be larger in some tissues, and 
 smaller in others ; the distribution of the capillaries being 
 more minute, in proportion as the nutritive actions of the 
 part go on more energetically. Now in the embryo, even of 
 the most complex and perfect animals, there is a period when 
 no blood-vessels exist, the whole mass being made-up of cells, 
 every one of which lives for itself and by itself, absorbing 
 nutriment from a common source, and not at all dependent 
 upon its brethren. It is only when a diversity of structure 
 begins to show itself, one part undergoing transformation 
 into bone, another into muscle, and so on, and when some 
 portions of the fabric are cut-off from the direct supply of 
 nourishment, that vessels begin to show themselves. These 
 are formed, like the ducts of Plants, by the breaking-down of 
 the partitions between contiguous cells ; they at first seem 
 rather like passages or channels, than tubes with walls of their 
 own ; and this condition they retain in certain cases through 
 life ( 289). 
 
 Repair of Injuries. 
 
 389. Every animal possesses, in a greater or less degree, 
 the power of not merely maintaining its organized fabric in 
 its integrity, by the renewal of the parts which are from time 
 to time passing into decay, but also of reproducing parts of 
 that fabric which have been lost by disease or accident. This 
 power seems greatest among the lowest tribes of Animals ; in 
 many of which the entire organism can be reproduced from a 
 small portion of it, as is the case with the Hydra ( 122), and 
 with some species of Sea-Anemone ( 126). In the Star-fish, 
 a far more highly-organized animal, the regenerative power is 
 more limited, though it is still very remarkably manifested ; 
 for if one, two, or more of the rays be broken or cut off, they 
 are gradually restored, provided the central disc be uninjured. 
 
 Y2 
 
324 REPARATIVE POWERS OP LOWER ANIMALS. 
 
 Of certain kinds of Holothuria (fig. 67), which eject the entire 
 mass of viscera under the influence of alarm, it has been ob- 
 served that they not only continue to move about as if nothing 
 had happened, but that, under favourable circumstances, they 
 regenerate the whole of the digestive and reproductive appa- 
 ratus thus parted-with. Next to Zoophytes, there are no 
 animals in which the regenerative power is known to manifest 
 itself so strongly as the lower members of the Articulated 
 series, such as the inferior Entozoa and the Turbellaria (Zoo- 
 LOGY, 924), among which last we find the Planaria almost 
 rivalling the Hydra, although it is an animal of much more 
 complex structure. The common Earthworm can reproduce 
 either the head or any portion of the body of which it may 
 have been deprived; but it cannot be multiplied by the division 
 of its body into two or more parts (as asserted by some), since 
 these parts, although they continue to move for a time, soon 
 perish. There are Worms allied to it, however, in which the 
 regenerative power is sufficient to produce the whole body 
 from a separated fragment; and no fewer than twenty-six 
 have thus been made to originate by the subdivision of a 
 single individual. In the higher Articulata, such as Crus- 
 tacea, Insects, and Spiders, the reparative capacity is limited 
 to the restoration of limbs ; and even this would seem to be 
 seldom preserved in perfect Insects, being restricted to the 
 larval period of their lives. Little is known of the regene- 
 rative power possessed by the higher Mollusks ; but it has 
 been affirmed that the head of the Snail may be reproduced 
 after being cut-off, provided the cephalic ganglion be not 
 injured, and an adequate amount of heat be supplied. 
 
 390. It is only among the cold-blooded members of the 
 Yertebrated series, that the reparative power extends to the 
 renewal of entire organs ; and this seems limited in Fishes to 
 the reproduction of portions of the fins which have been lost 
 by disease or accident. In JBatrachia, however, it has been 
 found that entire new legs, with perfect bones, nerves, 
 muscles, &c., may be reproduced after severe loss or injury 
 of the original members ; and even a perfect eye has been 
 formed in place of one that had been removed. It is inte- 
 resting to observe that the exercise of this reparative power 
 essentially depends upon the temperature in which the animal 
 is living; the reproduction of entire members apparently 
 
EEPARATIVE POWERS OF HIGHER ANIMALS. 325 
 
 requiring a higher degree of the stimulus of Heat, than does 
 their ordinary nutrition. In Lizards, an imperfect reproduc- 
 tion of the tail takes place when part of it has been broken 
 off; but the newly-developed portion contains no perfect ver- 
 tebrae, its centre being occupied by a cartilaginous column 
 like that of the lowest fishes. In the warm-blooded Verte- 
 brata generally, the power of reproduction after loss or in- 
 jury seems much more limited. We do not find that entire 
 parts or members once destroyed, are completely renewed; 
 though very extensive breaches of substance are often filled 
 up. The tissues most readily reproduced are Bone, the Simple 
 Fibres ( 22), and the Membranes (such as the Skin, the 
 Mucous and Serous membranes), of which these tissues form 
 the basis. As a general rule, losses of substance in Glandular 
 tissue, Muscle, and other parts of comparatively high organi- 
 zation, do not seem to be reproduced ; but there is a curious 
 exception to this in the case of Nervous tissue, which, with 
 Blood-vessels, is very readily re-formed in the new growths by 
 which losses of substance are repaired, as we often see in tha 
 rapid skinning-over of a large superficial wound. One of the 
 most remarkable manifestations of reparative power in the 
 Human body, is the re-formation of an entire bone, when the 
 original one has been destroyed by disease. The new bony 
 matter is thrown-out, sometimes within and sometimes around 
 the dead shaft ; and when the latter has been removed, the 
 new structure gradually assumes the regular form, and all the 
 attachments of muscles, ligaments, &c., become as complete 
 as before. A much greater variety and complexity of actions- 
 are involved in this process, than in the reproduction of whole 
 parts in the simpler animals ; though its effects do not appear 
 so striking. It appears that, in some individuals, this regene- 
 rating power is retained to a much greater degree than it is by 
 the species at large ; thus, there is a well-authenticated instance, 
 in which a supernumerary thumb on a boy's hand was twice re- 
 produced, after having been removed from the joint. In many 
 cases in which the crystalline lens of the eye has been re- 
 moved, in the operation for cataract, it has been afterwards 
 regenerated ; and there is evidence that, during embryonic 
 life, the regeneration of lost parts may take place in a degree 
 to which we have scarcely any parallel after birth ; attempts 
 being sometimes made at the re-formation of entire limbs, in 
 
326 REPAIR OF LOSSES OF SUBSTANCE. 
 
 place of such as are lost during the earlier periods of develop- 
 ment. 
 
 391. When an entirely new structure is to be formed, as 
 for the closure of a wound, the union of a broken bone, or the 
 repair of any other injury, the process is of a kind very much 
 resembling the first development of the entire fabric. The 
 neighbouring vessels pour out their liquor sanguinis, which is 
 known to the Surgeon under the name of coagulable lymph ; 
 this fills up the open space, and forms a connecting medium 
 between the separated parts. If this intervening layer be 
 thin, the two sides of the wound may adhere so closely as to 
 grow together without any perceptible interposition of new 
 substance ; this is what is called " healing by the first inten- 
 tion." But if the loss of substance has been too great to 
 allow of such adhesion, the vacant space is filled by the 
 gradual organization of the coagulable lymph ; and this may 
 take place in one of two very different modes, the determina- 
 tion being chiefly dependent 011 the condition of the wound 
 as to seclusion from air or exposure to it. 
 
 392. The former of these conditions is by far the more 
 favourable of the two ; for the reparative material is usually 
 developed gradually but surely into fibrous tissue, without 
 any loss, and with very little irritation either in the part 
 itself or in the system at large. This process seems to take 
 place naturally in cold-blooded animals, even in open wounds ; 
 the contact of air not having that disturbing influence in 
 them, which it exerts in warm-blooded animals. And Nature 
 frequently endeavours to bring it about in the superficial 
 wounds of warm-blooded animals, by the formation of a large 
 scab, which protects the exposed surface ; but this happens 
 much less frequently in the Human subject than it does 
 among the lower animals, the unnatural conditions in which 
 a large proportion of the so-called civilised races habitually 
 live (especially deficient purity of the air, continual excess in 
 diet, and the frequent abuse of stimulants) being unfavourable 
 to it. The performance of many operations which formerly 
 left open wounds, in such a manner that the air may be 
 effectually excluded by a valvular fold of skin, is one of the 
 greatest improvements in modern Surgery. 
 
 393. In an open wound, on the other hand, which is 
 healing by the process termed " granulation," the reparative 
 
HEALING OF OPEN WOUNDS. 327 
 
 material is rapidly developed into cells, amongst which, blood- 
 vessels speedily extend themselves. The formation of new 
 blood-vessels, in this and other cases, seems to commence in 
 the giving- way of the walls of some of the previously-existing 
 capillary loops, at particular spots, and in the escape of blood 
 corpuscles in rows or files into the soft substance that sur- 
 rounds them ; thus channels or passages are excavated, which 
 come into connexion with each other; and these channels, 
 after a time, acquire proper walls, and become continuous 
 with the vessels from which they originated, to be in their 
 turn the originators of a new series. The vitality of this new 
 " granulation- tissue," however, is very low; and the part ex- 
 posed to the air passes into the condition of pus (the yellow 
 creamy fluid discharged from an open wound), which contains 
 the same materials in a decomposing state. Thus there is a 
 constant waste of organizable substance, the amount of which, 
 in the case of an extensive wound, becomes a serious drain 
 upon the system ; at the same time, there is a much greater 
 irritative disturbance both in the part itself and in the system 
 generally ; and the new tissue that is formed is of such low 
 vitality that it subsequently wastes away, so as by its disap- 
 pearance to leave a contracted cicatrix or scar. The difference 
 between the two modes of reparation now described is often 
 one of life and death, especially in the case of large burns of 
 the body in children. 
 
 CHAPTEE IX. 
 
 ON THE EVOLUTION OP LIGHT, HEAT, AND ELECTRICITY BY ANIMALS. 
 
 Animal Luminousness 
 
 394 A large proportion of the lower classes of aquatic 
 Animals possess, in a greater or less degree, the power of 
 emitting light. The phosphorescence of the sea, which has 
 been observed in every zone, but more remarkably between 
 the tropics, is due to this cause. "When a vessel ploughs the 
 ocean during the night, the waves especially those in her 
 wake, or those which have beaten against her sides exhibit 
 a diffused lustre, interspersed here and there by stars or 
 ribands of more intense brilliancy. The uniform diffused 
 
328 LUMINOSITY OF MAKINE ANIMALS. 
 
 light is chiefly emitted by innumerable minute animals, which 
 abound in the waters of the surface; whilst the stars and 
 ribands are due to larger animals, whose forms are thus bril- 
 liantly displayed. This interesting phenomenon, when it occurs 
 on our own coasts, is chiefly produced by incalculable multi- 
 tudes of a small creature, termed the Noctiluca, having a nearly 
 globular form, and a size about equal to that of the head of 
 a minute pin. When these cover the water, and a boat is 
 rowed among them, every stroke of the oars produces a flash 
 of light ; and the ripple of the water upon the shore is marked 
 by a brilliant line. If a person walk over sands that the tide 
 has left, his footsteps will seem as if they had been impressed 
 by some glowing body. And if a small quantity of the water 
 be taken up and rubbed between the hands, they will remain 
 luminous for some time. The transparency of the little ani- 
 mals to which these beautiful appearances are due, might cause 
 them to be overlooked if they are not attentively sought ; they 
 somewhat resemble grains of boiled sago in their aspect, but 
 are much softer. In the general simplicity of their structure, 
 the Noctilucos appear to correspond rather with the Rhizopoda 
 ( 130) than with any other group; but they are distinguished 
 by some remarkable peculiarities. 
 
 395. Of the larger luminous forms which are seen to float 
 in the ocean-waters, a great proportion belong to the class 
 Acalephce. The light emitted by these appears to be due to 
 the peculiar chemical nature of the mucus secreted from their 
 bodies ; for this, when removed from them, retains its pro- 
 perties for some tune, and may communicate them to water 
 or milk, rendering those fluids luminous for some hours, parti- 
 cularly when they are warmed and agitated. It is probably from 
 this source, that the diffused luminosity of the sea is partly 
 derived. The secretion appears to be increased in amount, by 
 anything that irritates or alarms the animals ; and it is from 
 this cause that the dashing of the waves against each other, 
 the side of a ship, or the shore, or the tread of the foot 
 upon the sand, or the compression of the animals between 
 the fingers, occasions a greater emission of light. But some 
 of these causes may act, by bringing a fresh quantity of the 
 phosphorescent secretion into contact with air, which seems 
 necessary to maintain the kind of slow combustion on which 
 the light depends. 
 
LUMINOSITY OP MARINE ANIMALS AND INSECTS. 329 
 
 396. But the Noctilucae and Acalephae are by no means the 
 only luminous animals which tenant the deep. Many Zoo- 
 phytes appear to have this property in an inferior degree, and 
 also some of the Echinodermata. Of the lowest class of 
 Mollusks, the Tunicata, a very large proportion are luminous, 
 especially those which float freely through the ocean, and 
 which abound in the Mediterranean and tropical seas ; the 
 brilliancy of some of these can scarcely be surpassed. Among 
 some of the Conchifera, also, the phenomenon has been ob- 
 served ; as well as in certain marine Annelida. Other marine 
 animals of higher classes are possessed of similar properties ; 
 thus, many Crustacea, especially the minuter species, are 
 known to emit light in brilliant jets ; and the same may be 
 said of a few Fishes : but it is probable that the luminosity 
 attributed to many of the latter is due to the disturbance they 
 make in the surrounding water, which excites its phospho- 
 rescence in the manner just explained. In all these, the 
 general phenomena are analogous, the luminous matter ap- 
 pearing to be a secretion from the surface of the animals, and 
 to undergo a sort of slow combustion by combination with 
 oxygen. Wherever it is presented by these animals, it is 
 always most brilliant upon the surfaces concerned in respira- 
 tion. The light continues for some days after death ; but 
 ceases at the commencement of putrefaction. 1 
 
 397. In the class of INSECTS, there are several species which 
 have considerable luminous power ; and in these the emission 
 of light is for the most part confined to a small part of the 
 surface of the body, from which it issues with great brilliancy. 
 The luminous Insects are most numerous among the Beetle 
 tribe, and are nearly restricted to two families, the Elateridas, 
 and the Lampyridce. The former contains about 30 luminous 
 species, which are known as fire-flies; these are all natives of 
 the warmer parts of the New World. Their light proceeds 
 from two minute but brilliant points, which are situated one 
 on each side of the front of the thorax ; and from another 
 
 1 There are certain cases, however, in which the production of Light, 
 like that of Electricity ( 423), appears to be a peculiar manifestation 
 of Nervous power. There is strong reason to believe that Nerve-force 
 may be directly metamorphosed (as it were) into these or other forms of 
 physical and vital force, according to the principle of " Correlation * r 
 now generally admitted as regards the Physical Forces. 
 
330 LUMINOSITY OF INSECTS. 
 
 beneath the hinder part of the thorax, which is only seen 
 during flight. The light proceeding from these points is 
 sufficiently intense to allow small print to be read in the pro- 
 foundest darkness, if the insect be held in the fingers and 
 be moved along the lines ; and the natives of the coun- 
 tries where they are found (particularly in St. Domingo, 
 where they are abundant) use them instead of candles in 
 their houses, and tie them to their feet and heads, when 
 travelling at night, to give light to their path through the forest. 
 In all the luminous species of this family, the two sexes are 
 equally phosphorescent. 
 
 398. The family Lampyridce contains about 200 species 
 known to be luminous ; the greater part of these are natives 
 
 of America, whilst others are 
 widely diffused through the 
 Old World. In most of these, 
 the light is most strongly dis- 
 played by the female, which 
 is usually destitute of wings, 
 so that it might be mistaken 
 for a larva. The species of 
 
 Fig. 174. MALE AND FEMALE GLOW- OUT own country is known as 
 
 the Glow-worm (fig. 174). 
 
 399. The light of the Glow-worm issues from the under 
 surface of the last three abdominal rings. The lumiaous 
 matter, which consists of little granules, is contained in 
 minute sacs covered with a transparent horny lid ; and this 
 exhibits a number of flattened surfaces, so contrived as to 
 diffuse the light in the most advantageous manner. The sacs 
 are mostly composed of a close network of finely-divided air- 
 tubes ( 321), which ramify through every part of the granular 
 substance ; and it appears that the access of air through these 
 is a necessary condition of the phosphorescence. For if the 
 aperture of the large trachea which supplies the luminous sac 
 be closed, the light ceases ; but if the sac be lifted from its 
 place, without injuring the trachea, the light is not inter- 
 rupted. All the luminous insects appear to have the power 
 of extinguishing their light ; and this they probably do when 
 alarmed by approaching danger. The sudden extinction of 
 the light is probably due to the animal's power of closing the 
 aperture of the trachea. 
 
LUMINOSITY OF INSECTS. 331 
 
 400. There are a few other Insects not included in these 
 families, which are reputed to possess luminous powers ; and 
 of these the most remarkable are the Fulgorce, or Lantern- 
 flies (fig. 175); of which one species inhabits Guiana, whilst 
 another is a native of 
 
 China. These are in- 
 sects of very remark- 
 able form, having an 
 extraordinary proj ection 
 upon the head ; and 
 this is the part said to 
 be luminous. The au- 
 thority for the assertion, 
 
 however, is doubtful; Fig " 175 - F 
 
 and many Entomologists who have captured the insect, have 
 denied the phosphorescent power imputed to it. But it is not 
 impossible that the female only may possess it, and that it may 
 only be manifested at one part of the year. One of the common 
 English species of Centipede, which is found in dark, damp 
 places, beneath stones, &c., is slightly luminous ; and the 
 common Earthworm is also said to be so at the breeding 
 season. 
 
 401. Of the particular objects of this provision in the 
 Animal economy, little is known, and much has been con- 
 jectured. It is not requisite to suppose that its purposes are 
 always the same ; the circumstances of the diiferent tribes 
 which possess it being so different. The usual idea of its use 
 in Insects, that it enables the sexes of the nocturnal species 
 to seek each other for the perpetuation of the race, is pro- 
 bably the correct one. The light is more brilliant at the 
 season of the exercise of the reproductive functions, than at 
 any other ; and is then exhibited by animals which do not 
 manifest it at any other period. Moreover, it is well known 
 that the male Glow-worm, which ranges the air, whilst the 
 female, being destitute of wings, is confined to the earth, is 
 attracted by any luminous object; as are also the Fire-flies, 
 which may be most easily captured by carrying a torch or 
 lantern into the open air : so that the poetical language 
 in which this phosphorescence is described as " the lamp of 
 love the pharos the telegraph of the night, which marks 
 by its scintillations, in the silence of the night, the spot 
 
332 PHOSPHORESCENCE OF DECAYING ANIMAL MATTER. 
 
 appointed for the lovers' rendezvous," would not seem so incor- 
 rect as the ideas of poets on subjects of Natural History too 
 frequently are. Regarding the uses of the luminosity of the 
 lower marine tribes, it is more difficult to form a definite 
 idea ; since many of them are fixed to one spot during the 
 whole of life, and in many others the sexes do not require to 
 seek each other. 
 
 402. It not unfrequently happens, that an evolution of 
 light takes place from the bodies of animals soon after their 
 death, but before their decomposition has advanced far. 
 This has been most frequently observed to proceed from the 
 bodies of Fishes, Mollusks, and other marine tribes ; but it 
 has been seen also to be evolved from the surface of land 
 animals, and even from the Human body. Indeed, some 
 well-authenticated cases have been put on record, in which a 
 considerable amount of light was given off from the faces of 
 living individuals, who were near their end. All animal 
 bodies contain a considerable quantity of phosphorus ( 166) ; 
 and it is by no means impossible that some peculiar compound 
 of this substance may be formed, during the early stages of 
 decomposition, or even before death, which may, by its slow 
 combustion, give rise to the luminous appearance. It appears 
 that the whole substance of the body of the Fire-flies is phos- 
 phorescent ; for, according to an early historian of the West 
 Indies, "many wanton wilde fellowes" rub their faces with 
 the flesh of a killed fire-fly, "with purpose to meet their 
 neighbours with a flaminge countenance." 
 
 Animal Heat 
 
 403. One of the conditions necessary for the performance 
 of Vital action, is a certain amount of warmth ; and we have 
 seen that the animals which alone are capable of retaining 
 their activity in the coldest extremes of temperature, are those 
 which have the power of generating heat within themselves, 
 and thus of keeping-up the temperature of their bodies to a 
 high standard. Those which do not possess a power of this 
 kind, are either rendered completely inactive, even by a com- 
 paratively moderate cold, or are altogether destroyed by it. 
 Those which ordinarily do possess this power are destroyed 
 even more rapidly by cold, if from any cause the production 
 of heat within their bodies be interrupted ; for they are the 
 
TEMPERATURE OF COLD-BLOODED ANIMALS. 333 
 
 animals whose vital actions are the most varied and energetic, 
 and in which an interruption to any one of them most 
 speedily brings the rest to a stand. The inquiry into the 
 amount of heat generated by different animals, and into the 
 sources of its production, is one, therefore, of great practical 
 importance. 
 
 404. Our knowledge of the heat evolved by the lower In- 
 vertebrated animals is very limited; but it is probable that 
 in most of them the temperature of the body follows that 
 of the element they inhabit, keeping a little above it for a 
 time, if it be much lowered. Thus, when water containing 
 Animalcules is frozen, they are not at once destroyed; but 
 each lives for a time in a small uncongealed space, where the 
 fluid seems to be kept from freezing by the heat liberated 
 from its body. The temperature of Earthworms, Leeches, 
 Snails, and Slugs, ascertained by introducing a thermometer 
 into the midst of a heap of them, is usually about a degree or 
 two above that of the atmosphere ; and they also have the 
 power of resisting for a time the influence of a degree of cold, 
 which would otherwise iTnTnp.rHa.te1y freeze their bodies. 
 
 405. In the cold-blooded Vertebrata, also, the heat of the 
 body is almost entirely dependent upon that of the surround- 
 ing element. Thus most FISHES are incapable of maintaining 
 a temperature more than two or three degrees higher than 
 that of the water in which they live ; and the warmth of 
 their bodies consequently rises and falls with that of the sea, 
 river, or lake they may inhabit. There are, however, a few 
 marine Fishes which have the power of maintaining a tem- 
 perature 10 or 12 degrees higher than that of the sea ; and 
 these are peculiar for the activity of their circulation, and for 
 the deep colour of their blood, which possesses red particles 
 ( 229) enough to give to the muscles a dark red colour, like 
 that of meat. The Thunny, a fish which abounds in the 
 Mediterranean, where there are extensive fisheries for it, is 
 one of this group. It is to be remembered that the animals 
 of this class are less liable to suffer from seasonal alternations of 
 temperature, than are those which inhabit the air. In climates 
 subject to the greatest atmospheric changes, the heat of the 
 sea is comparatively uniform throughout the year, and that of 
 deep lakes and rivers is but little altered. Many have the 
 power of migrating from situations where they might other- 
 
334 TEMPERATURE OF FISHES AND REPTILES. 
 
 wise have suffered from cold, into deep waters ; and those 
 species which are confined to shallow lakes and ponds, and 
 which are thus liable to be frozen during the winter, are fre- 
 quently endowed with sufficient tenacity of life, to enable 
 them to recover after a process which is fatal to animals much 
 lower in the scale. Fishes are occasionally found imbedded 
 in the ice of the Arctic sea.s ; and some of these have been 
 known to revive when thawed. 
 
 406. In REPTILES, the power of maintaining an uniform tem- 
 perature is somewhat greater; being especially shown when 
 the external temperature is reduced very low. Thus when 
 the air is between 60 and 70, the body of a Reptile will 
 be nearly of the same heat; but when the air is between 
 40 and 50, it may be several degrees higher. Frogs and 
 other aquatic Reptiles have a remarkable power of sustaining 
 a temperature above that of freezing, when the water around 
 is not only congealed, but is cooled down far below the 
 freezing-point. Thus in ice of 21, the body of an edible 
 frog has been found to be 37 1 ; and even in ice of 9, the 
 animal has maintained a temperature of 33. In these cases, 
 as in Animalcules, the water in immediate contact with the 
 body remains fluid, so long as the animal can generate heat ; 
 but at last it is congealed, and the body also is completely 
 frozen. But it is certain that Frogs, like Fishes, may be 
 brought to life again, after the fluids of their bodies have 
 been so completely congealed that their limbs become quite 
 brittle ; it is not known, however, whether this may happen 
 with other Reptiles. It would appear that among Reptiles, 
 as among Fishes, some of the more active species have the 
 power of maintaining their bodies at a temperature consider- 
 ably higher than that of the atmosphere ; thus in some of the 
 more agile of the Lizard tribes, the high temperature of 86 
 has been noticed, when the external air was but 71. 
 
 407. The only classes of animals in which a constantly 
 elevated temperature is kept up, are BIRDS and MAMMALS. 
 The bodily heat of the former varies from 100 to 112; the 
 first being that of the Gull, the last that of the Swallow. In 
 general we find that the temperature is the highest in species 
 of rapid and powerful flight ; and least in those which inhabit 
 the earth. Birds that are much in the water have a special 
 provision for retaining within their bodies the heat which 
 
TEMPERATURE OF WARM-BLOODED ANIMALS. 335 
 
 would otherwise be too rapidly conducted away ; their bodies 
 being clothed with a thick and soft down, which is rendered 
 impenetrable to fluid by an oily secretion applied with the 
 bill. The temperature of MAMMALS generally seems to range 
 from 96 to 104; but that of the Bat, and probably of 
 other hybernating species, appears to be frequently much 
 below the lower of these limits, even when the animals are in 
 their ordinary activity. The mean or average heat of the 
 body of Man is about 100; but it has been observed as 
 low as 96^ when the temperature of the air was 60, and as 
 high as 102 when the air was at 82. As a variation of 5J 
 may occur when the range of the external temperature of the 
 air is only from 60 to 82, it is probable that observations 
 made in cold climates will show that the temperature of the 
 body may be still further lowered, when that of the air around 
 is much depressed. But it seems that, in Man, as in other 
 animals, the lower the temperature of the air around, the 
 greater is his power of generating heat within his body, to 
 keep up the necessary standard; and no observations yet 
 made indicate that the temperature of the body ever falls 
 below 95 in health. 
 
 408. The young of warm-blooded animals have usually less 
 power of maintaining an independent heat than adults. The 
 embryo, whether in the egg, or within the body of the parent, 
 is dependent, upon external sources for the heat necessary to 
 its full development. The contents of the egg, when lying 
 under the body of its parent, are so situated, that the germ- 
 spot ( 756) is brought into the nearest neighbourhood of 
 the source of warmth. It is not usually until some weeks 
 after the hatching of Birds, or the birth of Mammals, that 
 the young animals have the power of maintaining an inde- 
 pendent temperature. Thus young Sparrows, taken from the 
 nest a week after they were hatched, were found to have a 
 temperature of from 95 to 97 ; but this fell in one hour to 
 66J, the temperature of the atmosphere being at the same 
 time 621 ; and the rapid cooling was proved not to be due 
 to the want of feathers alone. There are some birds, how- 
 ever, which can run about and pick up their food the moment 
 they are hatched : these come into the world in a more 
 advanced condition than the rest, and can maintain their 
 temperature with little or no assistance. We find the same 
 
336 TEMPERATURE OP WARM-BLOODED ANIMALS. 
 
 to be the case among Mammals. There are some species 
 (such as the Guinea-pig) whose young are able from birth to 
 walk and run, and to take the same food with the mother ; 
 and these have from the first the power of maintaining a 
 steady temperature when the air around is not very cold. 
 But, in general, the young of Mammals are much less advanced 
 at the time of birth, being not unfrequently born blind as 
 well as helpless j and they require considerable assistance, in 
 keeping up their heat, from the parent or nurse. Thus the 
 temperature of new-born puppies, removed from the mother, 
 will rapidly sink to between 2 and 3 above that of the air. 
 
 409. These facts are of extreme practical importance, in 
 regard to the treatment of the Human infant. Though not 
 destitute of sight, at its entrance into the world, like the 
 young of the Cat, Dog, or Rabbit, it is equally helpless, and 
 dependent upon its parent not only for support but for 
 warmth. And as the Human body is longer in arriving at 
 its full development than is any other, so is it necessary that 
 this assistance should be longer afforded. This assistance is 
 the more necessary in the case of infants born prematurely ; 
 and it should be kept up during the years of childhood, 
 gradually diminishing with age. It is too frequently neglected, 
 by those who are well able to afford it, under the erroneous 
 idea of hardening the constitution ; and the want of it, con- 
 sequent upon poverty, is one of the most fertile sources of the 
 great mortality among children of the poorer classes. This 
 is easily proved by the proportional number of deaths which 
 take place in different parts of the year, at different ages. 
 During the first month of infant life, the winter mortality is 
 nearly double that of the summer ; though there is very little 
 difference between the two seasons in the mortality of adults. 
 But in old age the difference again manifests itself to the 
 same amount as in infants ; for old persons are almost equally 
 deficient in the power of maintaining heat; they complain 
 that their " blood is chill," and suffer greatly from exposure 
 to cold. 
 
 410. The class of INSECTS presents us with some very 
 interesting phenomena. In the larva and pupa states, the 
 temperature of the body is never more than from J to 4 
 above that of the surrounding medium ; but, in many tribes, 
 the temperature of the perfect' Insect rises so high, when it is 
 
PRODUCTION OP HEAT BY INSECTS. 337 
 
 in a state of activity, that it might be at such times called a 
 warm-blooded animal ; though in the states of abstinence, 
 sleep, and inactivity, its temperature falls again nearly to that 
 of the atmosphere. A single Humble-bee, inclosed in a phial 
 of the capacity of 3 cubic inches, had its temperature speedily 
 raised by violent excitement, from that of rest (2 or 3 above 
 that of the atmosphere) to 9 above that of the external air ; 
 and communicated to the air in the phial as much as 4 of 
 heat within five minutes. In another similar experiment, the 
 temperature of the air in the phial was raised nearly 6 in 
 eight minutes. It is among the active Butterflies, and the 
 Hymenopterous insects (Bee and Wasp tribe), which pass 
 nearly the whole of their active condition on the wing, that 
 we find the highest temperature ; and next to them are the 
 more active of the Beetle tribe. Those of the latter which 
 seldom leave the ground, have little power of producing heat. 
 411. The greatest manifestation of this power is shown 
 among Insects which live in societies ; most of which belong 
 to the order Hymenoptera. It has been seen that the body 
 of a Humble-bee, in a state of activity, has a temperature of 
 about 9 above that of the atmosphere ; but its nest has been 
 found to have an ordinary temperature of from 14 to 16 abova 
 the air, and from 17 to 19 above that of the chalk bank in. 
 which it was formed. The production of heat is increased 
 to a most extraordinary degree, when the pupae are about to 
 come-forth from their cells as perfect Bees, requiring a higher 
 temperature for their complete development. This is fur- 
 nished by a set of Bees termed Nurse-bees, which are seen 
 crowding upon the cells and clinging to them, for the purpose 
 of communicating to them their warmth; being themselves 
 evidently very much excited, and respiring rapidly, even at 
 the rate of 130 or 140 inspirations per minute. In one 
 instance, the thermometer introduced amongst seven nursing- 
 bees stood at 92|, whilst the temperature of the external air 
 was but 70. In Hive-bees, whose societies are large, this 
 process occasions a still more remarkable elevation of tempe- 
 rature ; for a thermometer introduced into a hive during May 
 has been seen to rise to 9 6 or 98, when the range of atmospheric 
 temperature was between 56 and 58. In September, when 
 the bees are becoming stationary, the temperature of the hive 
 is but a few degrees above that of the air. It was formerly 
 
338 SOURCES OP ANIMAL HEAT. 
 
 supposed that Bees do not become torpid during the winter ; 
 but this is now known to be a mistake. Bees, like other 
 Insects, pass the winter in a state of hybernation ; but their 
 torpidity is never so profound as to prevent their being aroused 
 by moderate excitement. The temperature of a hive is usually 
 from 5 to 20 above that of the atmosphere ; being kept at or 
 above the freezing-point, when the air is far below it. Under 
 such circumstances, their power of generating heat is most 
 remarkable. In one instance, the temperature of a hive, of 
 which the inmates were aroused by tapping on its outside, 
 was raised to 102; whilst a thermometer in a similar hive 
 that had not been disturbed, was only 48^; and the tempe- 
 rature of the air was 34|. 
 
 412. The evolution of Heat in the Animal body may now 
 be stated with tolerable certainty to depend for the most part 
 on the union, by a process resembling ordinary combustion, 
 of the carbon and hydrogen which it contains, with oxygen 
 taken-in from the air in the process of Eespiration. It has 
 been elsewhere shown that, even in Plants, this union, when 
 it takes place with sufficient rapidity, is accompanied by the 
 disengagement of a considerable amount of heat (VEGET. PHYS., 
 381); and in all those Animals which can maintain an 
 elevated temperature, we find a provision for this union, both 
 in regard to the constant supply of carbon and hydrogen from 
 the body, and to the introduction of oxygen from the air. 
 The supply of carbon and hydrogen may be derived (as already 
 shown, 157), either directly from the food, a large proportion 
 of which is thus consumed in many animals without ever 
 forming part of the tissues of the body ; or it may be the 
 result of the waste of the tissues, especially of the muscular, 
 consequent upon their active employment ( 160), and con- 
 verted into a substance peculiarly adapted for combustion by 
 the agency of the liver ( 366). Or, again, it may be derived 
 from the store laid-up in the system in the form of fat ; which 
 seems destined to afford the requisite supply, when other 
 sources fail. Thus, when food is withheld, or when dis- 
 ease prevents its reception, the fat in the body rapidly 
 diminishes ; being burnt off, as it were, to keep up the 
 temperature of the system. This is the case, too, during 
 hibernation; the animals which undergo this change usually 
 accumulating a considerable amount of fat in the autumn, and 
 
SOURCES OF ANIMAL HEAT. 339 
 
 being observed to come forth from their winter quarters, with 
 the return of spring, in a very lean condition. 
 
 413. The consumption of oxygen and the production of 
 carbonic acid are found to bear, in every animal, a very close 
 relation to the amount of heat liberated at the time. Thus in 
 warm-blooded animals, the respiratory function is much more 
 active than in the cold-blooded ; but when the former are 
 reduced to the state of cold-blooded animals, as occurs in 
 hybernation ( 309), their respiration is proportionately low. 
 On the other hand, whenever the temperature of an animal 
 is quickly raised by any extraordinary stimulus, above that 
 which it was previously maintaining, it is always by means of 
 increased activity of the respiratory movements, and augmented 
 consumption of oxygen. Thus during the incubation of Bees 
 ( 411), the insect, by accelerating its respiration, causes the 
 evolution of heat and the consumption of oxygen to take place 
 at least twenty times as rapidly as when in a state of repose. 
 The same takes place when a hybernating animal is roused ; 
 and it is remarkable that even extreme cold will effect this 
 for a time ; but the animal, if exposed for too long a period 
 to a very low temperature, will not be able to resist its 
 influence, and will perish. 
 
 414. Although the combustion of carbon and hydrogen 
 within the Animal body is undoubtedly the chief source of 
 the production of heat, yet it must not be left out of view that 
 there are other chemical changes in the system, which also con- 
 tribute to it, though in a minor degree ( 343). Of this kind 
 are the oxidation of the sulphur and phosphorus which enter 
 the body in the organic compounds used as food, and which, 
 being united by a combustive process with oxygen, pass out 
 of the system in the urine, in the form of sulphuric and phos- 
 phoric acids, combined with alkaline bases ( 367). 
 
 415. Besides all these sources, it seems probable from 
 various considerations, that Heat may occasionally be generated, 
 like light and electricity, by the direct agency of the Nervous 
 system; as one of the modes of force into which nervous 
 power may be metamorphosed. Of course, in any such gene- 
 ration of heat, there must be the same consumption of nervous 
 tissue, as would occur if its equivalent of nerve-force had been 
 manifested. 
 
340 PRODUCTION OF ELECTRICITY BY ANIMA-LS. 
 
 Animal Electricity. 
 
 416. Almost all chemical changes are attended with some 
 alteration in the electric state of the bodies concerned ; and 
 when we consider the number and variety of these changes in 
 the living animal body, it is not surprising that disturbances 
 of its electric equilibrium should be continually occurring. 
 But these, when slight, can only be detected by very refined 
 means of observation ; and it is only when they become con- 
 siderable, that they attract notice. Some individuals exhibit 
 electric phenomena much more frequently and powerfully 
 than others ; and cases are occasionally recorded in the 
 Human subject, in which there has been a most decided pro- 
 duction of electricity, which manifested itself in sparks when- 
 ever the individual was insulated. 
 
 417. The sparks and crackling noise, however, which are 
 occasionally observed on pulling off articles of dress that 
 have been worn next the skin, especially in dry weather, are 
 partly due to the friction of these materials with the surface and 
 with each other ; the production of electricity being greatly influ- 
 enced by their nature. Thus, if a black and a white silk stocking 
 be worn, one over the other, on the same leg, the manifesta- 
 tion of electricity when they are drawn off, especially after a 
 dry frosty day, is most decided ; but this would also be the 
 case if they were simply rubbed together, without any con- 
 nexion with the body. 
 
 418. In most animals with a soft fur, sparks may be pro- 
 duced by rubbing it, especially in dry weather ; this is familiar 
 to most persons in the case of the domestic Cat. But the 
 electricity thus produced seems occasionally to accumulate in 
 the animal, as in the Ley den jar, so as to produce a shock. 
 If a cat be taken into the lap, in dry weather, and the left 
 hand be applied to the breast, whilst with the right the back 
 be stroked, at first only a few sparks are obtained from the 
 hair; but after continuing to stroke for some time, a smart 
 shock is received, which is often felt above the wrists of both 
 the arms. The animal evidently itself experiences the shock, 
 for it runs off with terror, and will seldom submit itself to 
 another experiment. 
 
 419. But there are certain animals which are capable of 
 producing and accumulating electricity in large quantities, by 
 
ELECTRIC FISHES I GYMNOTUS. 
 
 341 
 
 means of organs specially adapted for the purpose; and of 
 discharging it at will, with considerable violence. It is re- 
 markable that all these belong to the class of Fishes ; l and that 
 they should differ alike in their general conformation, and in 
 their geographical distribution. Thus, the two species of 
 Torpedo, belonging to the Eay tribe, are found on most of 
 the coasts of the Atlantic and Mediterranean ; sometimes so 
 abundantly, as to be a staple article of food. The Gfymnotus, 
 or Electric Eel, is confined to the rivers of South America. 
 The Malapterurus (commonly known as the Silurus) which 
 approaches more nearly to the Salmon tribe, occurs in the 
 Niger, the Senegal, and the Nile; and there are two other less 
 known Fishes, said to possess electric properties, which inhabit 
 the Indian seas. 
 
 420. Of all these, the Gymnotus (fig. 176) is the one which 
 possesses the electric power in the most extraordinary degree. 
 It is an eel-like fish, having nothing remarkable in its external 
 appearance ; its usual length is from six to eight feet, but it 
 is said occasionally to attain 
 the length of twenty feet. This 
 fish will attack and paralyse 
 horses, as well as kill small 
 animals ; and the discharges of 
 the larger individuals sometimes 
 prove sufficient to deprive even 
 Men of sense and motion. This 
 power is employed by the fish to 
 defend itself against its enemies; 
 and even, it is said, to destroy 
 its prey (which consists of other 
 fishes) at some distance; the 
 shock being conveyed by water, 
 as a lightning-conductor conveys to the earth the effects of 
 the electric discharge of the clouds. The first shocks are 
 usually feeble ; but as the animal becomes more irritated, 
 their power increases. After a considerable number of power- 
 ful discharges, the energy is exhausted, and is not recovered 
 for some time ; and this circumstance is taken advantage 01 
 in South America, both to obtain the fishes (which afford 
 
 1 Certain Insects and Mollusks have been said to possess electrical 
 properties ; but no special electric organ has been discovered in them. 
 
 Fig. 176. GYMNOTUS. 
 
342 ELECTRIC FISHES I TORPEDO. 
 
 excellent food), and to make the rivers they infest passable 
 to travellers. A number of wild horses are collected in the 
 neighbourhood, and are driven into the water ; the Gymnoti 
 attack these, and speedily stun them, or even destroy their 
 lives by repeated shocks ; but their own powers of defence 
 and injury are exhausted in the same degree, and they then 
 become an easy prey to their captors. 
 
 421. The shock of the Torpedo (fig. 177) is less powerful; 
 but it is sufficient to benumb the hand that touches it. From 
 its proximity to European shores, this fish 
 has been made the subject of observation 
 and experiment more completely than the 
 other ; and some curious results have been 
 attained. It seems essential to the proper 
 reception of the shock, that two parts of the 
 body should be touched at the same time ; 
 and that these two should be in different 
 electrical states. The most energetic dis- 
 charge is procured from the Torpedo, by 
 touching its back and belly simultaneously ; 
 the electricity of the back being posi- 
 :DO> tive, and that of the belly negative. When 
 two parts of the same surface, at an equal distance from the 
 electric organ, are touched, no effect is produced ; but if one 
 be further from it than the other, a discharge occurs. It has 
 been found that, however much a Torpedo is irritated through 
 a single point, no discharge takes place ; but the fish makes 
 an effort to bring the border of the other surface in contact 
 with the offending body, through which a shock is then sent. 
 This, indeed, is probably the usual manner in which its dis- 
 charge is effected. If the fish be placed between two plates 
 of metal, the edges of which are in contact, no shock is per- 
 ceived by the hands placed upon them, since the metal is a 
 better conductor than the human body ; but if the plates be 
 separated, and, while they are still in contact with the opposite 
 sides of the body, the hands be applied to them, the discharge 
 is at once rendered perceptible, and may be passed through a 
 line formed by the moistened hands of two or more persons. 
 In the same manner, also, a visible spark may be produced ; 
 but this is less easily obtained from the Torpedo than from 
 the Gymnotus. As to the uses of the electrical organs to the 
 
STRUCTURE OF ELECTRIC APPARATUS. 
 
 343 
 
 Fishes which possess them, no definite information can be 
 given. It is doubtful to what extent they are employed in 
 obtaining food ; since it is known that the Gymnotus eats 
 very few of the fishes which it kills by its discharge ; and 
 that Torpedos kept 
 in captivity do not 
 seem disposed to ex- 
 ercise their powers 
 on small fishes placed 
 in the water with 
 them. The chief use 
 of their electrical 
 power appears to be, 
 to serve as a means 
 of defence against 
 their enemies. 
 
 422. The electric 
 organs of the Torpedo 
 (fig. 178) are of flat- 
 tened shape, and 
 occupy the front and 
 sides of the body, 
 forming two large 
 masses, which ex- 
 tend backwards and 
 outwards from each 
 side of the head. 
 They are composed 
 of two layers of mem- 
 brane, the space be- 
 tween which is di- 
 vided by Vertical "&* l^S. ELECTRIC APPARATUS OF TORPEDO. 
 
 partitions into hex- c, brain; me, spinal cord; o, eye and optic nerve; e, 
 afOTial fplls ^ lilrp electric organs ; tip, pneumogastric nerve, supplying 
 
 us, e, like electric or * gans '. f ateral n - Jn d ** 
 
 those of a honey- 
 comb, the ends of which are directed towards the two sur- 
 faces of the body. These cells which are filled with a whitish 
 soft pulp, somewhat resembling the substance of the brain, 
 but containing more water are again subdivided horizontally 
 by little membranous partitions ; and all these partitions are 
 profusely supplied with vessels and nerves. The electrical 
 
344 STRUCTURE AND ACTIONS OF ELECTRIC ORGANS. 
 
 organs of the Gymnotus are essentially the same in structure, 
 but differ in shape in accordance with the conformation of the 
 animal; they occupy one-third of its whole bulk, and run 
 nearly along its entire length, being arranged in two distinct 
 pairs, one much larger than the other. In the Malapterurus 
 
 Fig. 179.- ELECTRIC MALAPTKRURUS. 
 
 (fig. 179), there is not any electrical organ so definite as those 
 just described ; but the thick layer of dense areolar tissue 
 which completely surrounds the body, appears to be sub- 
 servient to this function ; being composed of tendinous fibres 
 interwoven together, and containing a gelatinous substance in 
 its interstices, so as to bear a close analogy with the special 
 organs of the Torpedo and Gymnotus. 
 
 423. In all these instances, the electrical organs are sup- 
 plied with nerves of very great size, larger than any others in 
 the same animals, and larger than any nerves in other animals 
 of like bulk. These nerves arise from the top of the spinal 
 cord, and seem analogous to the pneumogastric nerve ( 458) 
 of other animals. The influence of these nerves is essential to 
 the action of the electric organs. If all the trunks on one 
 side be cut, the power of the corresponding organ will be 
 destroyed, but that of the other may remain uninjured. If 
 the nerves be partially destroyed on either or both sides, the 
 power is retained by the portions of the organs which are still 
 connected with the brain by the trunks that remain. Even 
 slices of the organ entirely separated from the body, except by 
 a nervous fibre, may exhibit electrical properties. Discharges 
 may be produced by irritating the part of the nervous centres 
 from which the trunks proceed (so long, at least, as they are 
 entire), or by irritating the trunks themselves. In all these 
 respects, there is a strong analogy between the action of the 
 nerves on the electric organs and on the muscles (Chap, xn.) ; 
 and it may be safely affirmed that the Nervous force develops 
 
ELECTRIC MANIFESTATIONS OF MUSCLE AND NERVE. 345 
 
 Electricity by its action on the electrical organs, just as it pro- 
 duces Motion by its action on the muscles. 
 
 424. It is another interesting point of analogy between the 
 action of Muscles and that of Electrical organs, that the former, 
 like the latter, is attended with a change of electric state. In 
 any fresh vigorous muscle, there is a continual current from the 
 interior to exterior, which appears to depend upon the fact 
 that the actions connected with the nutrition and disintegra- 
 tion of its tissue go on more energetically in the interior of 
 the muscle, than they do near its surface, where the proper 
 muscular fibres are mingled with a large proportion of areolar 
 and tendinous substance. During the contraction of a muscle, 
 this current is diminished in intensity, or is even entirely 
 suspended; but it is renewed again, so soon as the muscle 
 relaxes. An electric current has been found to exist also in 
 Nerves ; and its conditions are in most respects similar to 
 those of the muscular current. 
 
 CHAPTER X. 
 
 FUNCTIONS OF THE NERVOUS SYSTEM. 
 
 425. WE have now completed our consideration of the 
 Functions of Organic or Vegetative Life; those changes, 
 namely, in the Animal body, which are concerned in the 
 maintenance of its own fabric ; and which, although per- 
 formed in a different mode, and having different objects to 
 fulfil, are essentially the same in character with those which 
 take place in Plants. The first and most striking difference of 
 mode results, as we have seen, from the nature of the food of 
 Animals, which requires that they should possess a cavity for 
 its reception, and a chemical and mechanical apparatus for its 
 digestion (or reduction to the fluid form), in order that it may 
 be prepared for absorption into the vessels. In regard to the 
 absorption of the aliment, and its circulation through the 
 system, there is but little essential difference between Plants 
 and the lower Animals ; but in the higher tribes of the latter, 
 we find that a muscular organ having the action of a forcing- 
 pump is appended to the system of tubes in which the fluid 
 
346 FUNCTIONS OP ORGANIC AND ANIMAL LIFE. 
 
 circulates, in order to drive it through them with the requisite 
 certainty and energy. The respiration of Animals, again, is 
 essentially the same with that of Plants j the chief difference 
 being that, in order to secure the active performance of this 
 important function, the higher Animals are provided with 
 a complex apparatus of nerves and muscles, by which the air 
 or water in contact with the aerating surface is continually 
 renewed. And in regard to the functions of secretion and 
 excretion, we have seen that, though there is a wide difference 
 in the form of the organs by which they are executed, they 
 are the same in essential structure ; and that the difference in 
 their mode of operation consists chiefly in this, that their 
 products in the Animal are destined to be carried out of the 
 body, instead of being retained within it, as in Plants. 
 
 426. In regard to the immediate objects of these functions, 
 also, there is but little essential difference ; for in both in- 
 stances it is the conversion of alimentary materials into living 
 organized tissue. But the ultimate purpose of this tissue is 
 far from being the same in the two kingdoms. Nearly all the 
 nourishment taken-in by Plants is applied to the extension of 
 their own fabric ; and hence there is scarcely any limit to the 
 size they may attain. There is very little waste or decay of 
 structure in them, the parts once formed (with the exception 
 of the leaves and flowers) continuing to exist for an indefinite 
 time ; this is a consequence of the simply physical nature of 
 the functions of the woody structure, which has for its chief 
 object to give support to the softer parts, and to serve as the 
 channel for the movement of the fluid that passes towards and 
 from them. The case is very different in regard to Animals. 
 With the exception of those inert tribes which may be com- 
 pared with Plants in their mode of life, we find that the 
 whole structure is formed for motion ; and that every act of 
 motion involves a waste or decay of the fabric which executes 
 it. An energetic performance of the nutritive actions is re- 
 quired, therefore, in the more active Animals, simply to make 
 good the loss which thus takes place ; we find, too, that their 
 size is restrained within certain limits ; so that, instead of the 
 nourishment taken into the body being applied, as in Plants, 
 to the formation of new parts, it is employed for the most part 
 in the simple repair of the old. Thus we may say that, whilst 
 the ultimate object of Vegetable Life is to build up a vast 
 
GENERAL PURPOSES OP NERVOUS SYSTEM. 347 
 
 fabric of organized structure, the highest purpose of the 
 Organic Life of Animals is to construct, and to maintain in a 
 state fit for use, the mechanism which is to serve as the 
 instrument of their Functions of Animal Life, enabling them 
 to receive sensations, and to execute spontaneous movements, 
 in accordance with their instincts, emotions, or will. 
 
 427. This mechanism consists of two kinds of structure, 
 the Nervous and Muscular, which have entirely different 
 offices to perform. The Nervous system is that which is the 
 actual instrument of the Mind. Through its means, the indi- 
 vidual becomes conscious of what is passing around him ; its 
 operations are connected, in a manner we are totally unable to 
 explain, with all his thoughts, feelings, desires, reasonings, 
 and determinations ; and it communicates the influence of 
 these to his muscles, exciting them to the operations which 
 he determines to execute. But of itself it cannot produce 
 any movement, or give rise to any action ; any more than 
 the expansive force of steam could set a mill in motion, 
 without the machinery of the Steam-engine for it to act upon. 
 The Muscular System is the apparatus by which the move- 
 ments of the body are immediately accomplished ; and these 
 it effects by the peculiar power it possesses of contracting 
 upon the application of certain stimuli, of which Nervous 
 agency is the most powerful. 
 
 428. Although the presence of a Nervous System is the 
 most distinguishing attribute of Animals, yet we do not en- 
 counter it by any means universally. For among certain of 
 those classes which possess on other grounds a title to be 
 ranked in the Animal kingdom, it seems beyond a doubt that 
 no nervous system exists; and there are many others in 
 which, if it be present at all, its condition is so rudimentary, 
 that it can take little share in directing the general operations 
 of the organism. The life of such beings, in fact, is chiefly 
 vegetative in its nature ; their movements are not dissimilar in 
 kind to those that we witness in Plants ; and their title to a 
 place in the Animal kingdom chiefly rests upon the nature of 
 their food, and the mode in which they appropriated ( 7, 8). 
 This is the case with the Protozoa generally ( 128137), 
 and in a less degree with Zoophytes ( 121 127). 
 
 429. In proportion as we ascend the Animal series, how- 
 ever, we find the Nervous System presenting a greater and 
 
348 GENERAL PURPOSES OF NERVOUS SYSTEM. 
 
 greater complexity of structure, and obviously acquiring 
 higher and yet higher ' functions ; so that in Vertebrated 
 animals, and more especially in Man, it is evidently that 
 portion of the organism to whose welfare everything else is 
 brought*into subordination ( 73). And we observe this to be 
 the case, not merely in virtue of its direct instrumentality as 
 the organ of Mind, but also in the intimacy of its relation to 
 the Organic functions, which are placed in great degree under 
 its control. Thus we find that the inlets and outlets to the 
 Digestive apparatus, the mechanism by which food is brought 
 to the mouth and conveyed into the stomach, and that by 
 which indigestible matters are voided from the large in- 
 testine, are subjected to its influence ; although the act of 
 digestion itself, and the passage of the aliment from one end 
 of the canal to the other, are performed independently of it. 
 So, again, the movements of Eespiration, by which the air 
 within the lungs is renewed as fast as it becomes vitiated, are 
 not only effected through its instrumentality, but are placed, 
 for the purposes of Vocalization, as far under the control of 
 the Will as would be consistent with a due regard to the 
 safety of life. Yet among many of the lower tribes of Animals, 
 the ingestion of food and the aeration of the circulating fluid 
 are provided-for by ciliary action alone ( 45), in which we 
 have every reason to believe that nervous agency has no par- 
 ticipation whatever. 
 
 430. If, taking the Nervous System of Man as the highest 
 type of this apparatus, we analyse in a general' way the actions 
 to which it is subservient, we find that they may be arranged 
 under several distinct groups, which it is very important to 
 consider apart, whether we are studying his psychical J functions 
 or those of the lower animals. 1. The simplest mode of its 
 action is that in which an impression made upon an afferent 
 nerve excites, through the ganglionic centre in which it ter- 
 minates, an impulse in the motor nerve issuing from it, which, 
 being transmitted by it to the muscular apparatus, calls forth 
 a respondent movement. Of this action, which is called reflex, 
 or " excito-motor," and which may be performed without any 
 consciousness either of the impression or of the motion, we 
 have already seen examples in the movements of Deglutition 
 
 1 This term is used to designate the sensorial and mental endowmenti 
 of Animals, in the most comprehensive sense. 
 
MODES OF ACTION OF NERVOUS SYSTEM. 349 
 
 ( 194) and Eespiration ( 340). 2. If the ganglionic centre 
 to which the impression is conveyed, should be one through 
 which the consciousness is necessarily affected, sensation be- 
 comes a necessary link in the circular chain ; and the action 
 is distinguished as consensual, or " sensori-motor." The closing 
 and opening of the pupil of the eye, in accordance with the 
 amount of light that falls upon the retina, together with other 
 remarkable adjustments which are involuntarily made in the 
 working of that wonderful organ, are characteristic examples 
 of this class. In the foregoing operations no mental change 
 higher than simple consciousness of impressions that is to 
 say, Sensation, with which may be blended the simple feelings 
 of pleasure and pain is involved. Such would appear to be 
 the condition of the Human infant on its first entrance into 
 the world, before the self-education of its higher faculties has 
 commenced ; and such is probably the state of Invertebrated 
 animals generally, whose instinctive actions seem to be refer- 
 able to one or other of the foregoing classes. 
 
 431. But Sensation is the very lowest form of purely 
 Mental action. When the outness or externality of the 
 objects which give rise to our sensations has been recognised 
 by perception, we begin to form ideas respecting their nature, 
 qualities, &c. ; and it is in the various processes of association, 
 comparison, &c., to which these ideas are subjected, that our 
 Reasoning faculty consists. Now these processes may go on 
 in great degree automatically, that is, without any control or 
 guidance on our own part, as happens in the states of Dream- 
 ing, Reverie, and Abstraction ; and they may express them- 
 selves in action, as we see in the movements of a Somnambulist, 
 who may be said to be acting his dreams. This form (3) of 
 Nervous activity, which may be termed ideo-motor, seems to be 
 the ordinary mode in such of the lower animals as are governed 
 by Intelligence rather than by instinct (Chap, xiv.) ; but it is 
 abnormal and exceptional in Man. With ideas are associated 
 feelings of various kinds, which constitute Passions and Emo- 
 tions; and these (4), when strongly excited, may become direct 
 springs of action, so powerful as even to master the control of 
 the Will, producing emotional movements. 
 
 432. In the well-regulated mind of Man, however, the Will 
 (5) possesses supreme direction over the whole current of 
 thought, feeling, and action : regulating the succession of the 
 
350 NERVOUS SYSTEM OP BADIATA. 
 
 ideas ; keeping in check the passions and emotions, or, on the 
 other hand, promoting their healthful activity by directing the 
 attention to the objects of them ; and determining the move- 
 ments which the reason prompts : and the acquirement and 
 right direction of such regulating power is the highest object 
 of all Education. 
 
 433. It will be recollected that every form of Nervous 
 System essentially consists of two kinds of nervous tissue, 
 the tubular or fibrous, whose functions seem to be purely 
 conductive ( 60, 62), and the vesicular or ganglionic, which 
 seems to be the seat of all the changes to which this apparatus 
 ministers, and the source of all its peculiar powers ( 61, 63). 
 The principal forms under which this apparatus presents 
 itself in the several divisions of the Animal Kingdom, and 
 the general nature of the functions to which it is subservient 
 in each, will now be successively described in the ascending 
 series, from Zoophytes up to Man. 
 
 Structure and Actions of the Nervous System in the principal 
 Classes of Animals. 
 
 434. In most of the EADIATED classes, it is difficult to dis- 
 cover any distinct traces of a Nervous System ; the general 
 softness of their tissues being such, that it cannot be certainly 
 distinguished amongst them. It clearly exists, however, in 
 the highest group, the ECHINODERMATA ; and it presents an 
 
 extremely simple form, which par- 
 takes of the general arrangement of 
 parts in these animals. In the 
 Star-fish, for instance, it forms a 
 ring which surrounds the opening 
 into the stomach (fig. 180) ; this 
 ring consists of a nervous cord that 
 forms communications between five 
 ganglia, one of which is placed at 
 the base of each ray. These ganglia 
 appear to be all similar to each 
 
 Fig. ISO.- 
 
 position of the mouth. them sends a large trunk along its 
 own ray ; and two small branches 
 to the organs in the central disk. The rays being all similar 
 to each other in structure, it would appear that no one of these 
 
NERVOUS SYSTEM OF BADIATA AND TUNICATA. 351 . 
 
 ganglia can have any controlling power over the rest. All the 
 rays have at their extremities what seem to be very imperfect 
 eyes ; and so far as these can aid in directing the movements 
 of the animal, it is obvious that they will do so towards all 
 sides alike. Hence there is no one part which corresponds 
 to the head of higher animals ; and the ganglia of the nervous 
 system, like the parts they supply, are but repetitions of one 
 another, and act independently of one another. Each would 
 perform its own individual functions if separated from the 
 rest ; but, in the entire animal, they are brought into mutual 
 relation by the circular cord, which passes from every one of 
 the five ganglia to those on either side of it. In Man, as well 
 as in all the Vertebrated and Articulated animals, and in some 
 of the Mollusca, there is a like repetition of the parts of the 
 Nervous System on the two sides of the central line of the 
 body ; but the organs are only double, instead of being 
 repeated five times. Still the two hemispheres of the brain, 
 and the two halves of the spinal cord, in the Vertebrated 
 animal, and the two halves of the chain of ganglia, in the 
 Articulated animal, are as independent of one another as are 
 the five separate ganglia of the Star-fish ; and they are made 
 to act in mutual harmony by similar uniting bands of nervous 
 fibres, which are termed commissures. 
 
 435. In the nervous system of MOLLUSCA, we do not meet 
 with any such repetition of parts ; the body itself not pre- 
 senting this character. In the lowest and 
 simplest animals of this group, there exists 
 only a single ganglion, which may be 
 regarded as analogous to any one of the 
 ganglia of the Star-fish ; but in the higher, 
 we find the number of ganglia increased, 
 in accordance with the increase of the 
 functions which they have to perform. 
 The simplest form of the nervous system 
 in this class is seen in the accompanying 
 figure (fig. 181), which represents one of the 
 solitary TUNICATA, the A scidia. At a is seen 
 the orifice by which the water enters for sup- Fig. isi. NERVOUS SYS- 
 plying the stomach with food, and for aerat- TEM OF ASCIDIA - 
 ing the blood ( 114); and at b is the orifice by which the current 
 of water passes out again, after it has served these purposes. 
 
352 NERVOUS SYSTEM OF TUNICATA AND CONCHIFEEA. 
 
 Between these orifices is the single ganglion c, which sends 
 filaments to both of them, and other branches which spread 
 over the surface of the mantle d. These animals are for the 
 most part fixed to one spot during nearly the whole of their 
 existence ; and they show but little sign of life, beyond the 
 continual entrance and exit of the currents already adverted 
 to. When any substance is drawn-in by the current, however, 
 the entrance of which would be injurious, it excites a general 
 contraction of the mantle ; and this causes a jet of water to 
 issue from both orifices, which carries the offending body to 
 a distance. And in the same manner, if the exterior of the 
 body be touched, the mantle suddenly and violently contracts. 
 
 436. These are the only actions, which, so far as we know, 
 the nervous system of these animals is destined to perform. 
 They scarcely exhibit any traces of eyes or other organs 
 of special sense; and the only parts that appear peculiarly 
 sensitive, are the small tentacula which guard the orifice a. 
 It would seem as if the irritation caused by the contact of any 
 hard substance with these, or with the general surface of the 
 animal, caused a reflex contraction of the mantle, having for 
 its result the getting-rid of the source of the irritation. Such 
 a movement could only be performed by the aid of a Nervous 
 system, which has the power of receiving impressions, and of 
 immediately exciting even the most distant parts of the body 
 to act in accordance with them. In the Venus' s Fly-trap and 
 Sensitive Plant (YEGET. PHYS., 214, 391), an irritation 
 applied to one part is the occasion of a movement in another ; 
 but this takes place slowly, and in a manner very different 
 from the energetic and immediate contraction of the mantle 
 of the Tunicata. 
 
 437. In the CONCHIFERA, or animals inhabiting bivalve 
 shells, there are invariably at least two ganglia, having differ- 
 ent functions. The larger of these, corresponding to the single 
 ganglion of the Tunicata, is situated towards the posterior end 
 of the body (B, fig. 182), in the neighbourhood of the posterior 
 muscle ; and its branches are distributed to that muscle, the 
 mantle, the gills, and the siphons. But we find another gan- 
 glion, or rather pair of ganglia (A A), situated near the front 
 of the body, either upon or at the sides of the oesophagus, and 
 connected by a commissural band that arches over it ; these 
 ganglia receive nerves from very sensitive tentacula which 
 
NERVOUS SYSTEM OF MOLLUSKS. 353 
 
 guard the mouth; and they evidently correspond, both in 
 position and functions, to the sensory ganglia of higher ani- 
 mals, whilst the posterior gan- 
 glion has for its office to regulate 
 the respiratory movements. In 
 the Pecten, however, as in other 
 Conchifera which possess a 
 foot (fig. 62), we find an addi- 
 tional ganglion (c), the pedal, 
 connected with the cephalic 
 ganglia, and sending nerve- 
 trunks to that organ. There 
 is good reason to believe that, 
 whilst the cephalic ganglia alone 
 are the instruments of sensation, 
 so that they exert a general 
 
 Control and direction over the Fig. m-NBKVous SYSTEM OF PECTEN. 
 movements Of the animal, the A A, cephalic ganglia ; B, branchial 
 
 pedal and branchial ganglia fg? ; c ' pedal ganglion; e ' 
 
 minister to the reflex actions 
 
 ( 433) of the organs which they supply. 
 
 438. A similar arrangement is 
 found in the higher Mollusks, 
 among which the ganglia are more 
 numerous, in accordance with the 
 greater variety of functions to be 
 performed. Of this we have an 
 example in the Aplysia, a sort of 
 sea-slug somewhat resembling those 
 formerly alluded to ( 316). In 
 proportion as we ascend the scale, 
 we find the cephalic ganglia rising 
 higher and higher on the sides of 
 the oesophagus ; and in the Aplysia 
 they meet on the central line above 
 it, forming the single mass (A, fig. 
 183), which receives the nerves of 
 the eyes, tentacula, &c., and sends 
 branches of communication to the 
 other ganglia. The branches which 
 it sends backwards are three on each 
 
 A A 
 
 Fig. 183. NERVOUS SYSTEM OB 
 APLYSIA. 
 
354 NERVOUS SYSTEM OF MOLLUSKS. 
 
 side. Of these, one passes through the ganglionic masses cc, to 
 communicate with the ganglion B, which is the one connected 
 with the respiratory movements. The others are distributed 
 with the branches of the ganglia cc, the function of which is 
 double ; for one set of branches from each is distributed to 
 the mantle in general, every part of which (in these shell-less 
 Mollusks) is capable of contracting and giving motion to the 
 body; whilst another set is distributed to that thick and 
 fleshy part of it which is called its foot, and on which the 
 animal crawls ( 107). There is another ganglion, D, lying 
 in front of the cephalic ganglion, and also receiving branches 
 of communication from it ; this ganglion is specially connected 
 with the actions of mastication and swallowing, and is called 
 the pharyngeal ganglion. 
 
 439. Thus we see that the cephalic ganglion sends branches 
 to all the other ganglia, though these having different functions, 
 do not communicate with each other; and thus every part 
 has two sets of nervous connexions, one with the cephalic 
 ganglia, and the other with its own ganglion. By the former, 
 the animal becomes conscious of impressions made upon it, 
 these impressions being converted in the cephalic ganglia into 
 sensations ; and the influence of its conscious power is exerted 
 through them upon the several parts of its body, causing 
 spontaneous motion. By the latter are produced those reflex 
 actions of the several organs, which do not require sensation, 
 but which depend upon the simple conveyance of an im- 
 pression to the ganglion, and the transmission of the resultant 
 motor impulse from it to the muscles supplied by its nerves. 
 A small part only of the Nervous System of Mollusks 
 ministers to the general movements of the body; and this 
 corresponds with what has been elsewhere stated ( 107) of 
 the inertness which is their general characteristic, and of the 
 small amount of muscular structure which they possess. 
 
 440. On the other hand, in the ARTICULATED classes, in 
 which the apparatus of movement is so highly developed, and 
 whose actions are so energetic, we find the Nervous System 
 almost entirely subservient to this function. Its usual form 
 has been already described ( 94) as a chain of ganglia con- 
 nected by a double cord, which commences in the head, and 
 passes backwards through the body. In general, we find a 
 ganglion (or rather a pair of ganglia united on the middle 
 
NERVOUS SYSTEM OF ARTICULATED ANIMALS. 
 
 355 
 
 Fig. 184. NERVOUS SYSTEM OF 
 AN INSECT. 
 
 line) in each segment; hence in the ANNELIDA and MYRIA- 
 PODA, the ganglia are very numerous ; but they are pro- 
 portionably small. In INSECTS (fig. 184), the number of 
 segments, and consequently of ganglia, never exceeds thirteen ; 
 and the ganglia are larger. What- 
 ever be the number of the ganglia, 
 they are usually but repetitions of 
 one another, the functions of each 
 segment being the same with those 
 of the rest. The nerves proceeding 
 from them are chiefly distributed to 
 the muscles of the legs ; or, where 
 legs do not exist (as in the Leech), 
 to the muscles that give motion to 
 the body. This is the case in the 
 larva of the Insect, as in the Cen- 
 tipede or Nereis ; but in the per- 
 fect Insect the case is different ; for 
 the apparatus of locomotion is con- 
 fined to the thorax ( 97), and 
 the segments of the abdomen have 
 
 no members. We accordingly find that the ganglia of the 
 thorax, from which the legs and wings are supplied, are very 
 much increased in size, and are sometimes concentrated into 
 one mass ; whilst those of the abdomen are very small, one or 
 two of them occasionally disappearing altogether. 
 
 441. A good example of this curious change in the nervous 
 system of Insects, is seen in the Sphinx ligustri, or Privet 
 Hawk-Moth, as shown in the succeeding diagrams. In 
 fig. 185 the nervous system of the Caterpillar is represented; 
 this consists of a pair of cephalic ganglia (1); from which 
 proceeds, on each side, a cord of communication to the first 
 ganglion of the trunk (2), and thence to the other ganglia (3 
 13). No difference is seen in these ganglia, except that 
 the last two are more closely connected than the rest. The 
 cephalic ganglia, with their cords of communication, form a 
 ring, through which the oesophagus passes ; they are situated 
 above it; but the whole chain of ganglia of the trunk is 
 situated beneath the alimentary canal. In fig. 186 is shown 
 the Nervous System of the perfect Insect; in which it is 
 seen that the whole is considerably abbreviated (the body of 
 
 A A2 
 
356 
 
 NERVOUS SYSTEM OF ARTICULATED ANIMALS. 
 
 the Moth being much shorter than that of the Caterpillar), 
 and that great changes have taken place in the relative sizes 
 
 of the ganglia. The cephalic 
 ganglia, being now connected 
 with much more perfect eyes and 
 other organs of sense, are greatly 
 enlarged; the thoracic ganglia, 
 from which the legs and wings 
 are supplied, are enlarged and 
 concentrated ; whilst the abdomi- 
 nal ganglia are relatively dimi- 
 nished in size, the 7th and 8th 
 being entirely wanting. 
 
 442. When the structure of 
 the chain of ganglia is more par- 
 ticularly inquired into, it is found 
 to consist of two distinct tracts; 
 one of which is composed of 
 nervous fibres only, and passes 
 backwards from the cephalic 
 ganglia, over the surface of all 
 the ganglia of the trunk, giving 
 off branches to the nerves that 
 proceed from them; whilst the 
 other connects the ganglia them- 
 selves. Hence, as in Mollusca, 
 every part of the body has two 
 
 vus TSTEOP sets of nervous connexions ; one 
 
 Fig. 185. NER- 
 VOUS SYSTEM OF 
 
 LAR L V iGus F TRi HINX zfioSSw *^ the ce P nalic g^g 1 ^, tne 
 
 other with the ganglion of its 
 
 own segment. Impressions made upon it, being conveyed by 
 the fibrous tract to the cephalic ganglia, become sensations; and 
 by the influence of the conscious power, operating through 
 these same ganglia, the general movements of the body are 
 harmonised and directed. It is obvious that, as the motions 
 of an animal are chiefly guided by its sight, the cephalic 
 ganglia would have a governing influence over the rest, if 
 only from their peculiar connexion with the eyes ; but there 
 is good reason to believe that their functions are still more 
 different from those of the ganglia of the trunk, and that 
 sensation resides in them alone. The motions produced by 
 
EEFLEX MOVEMENTS OF ARTICULATA. 357 
 
 the ganglia of the trunk, when separated from the head, are 
 often very remarkable, and seem at first sight to indicate 
 sensation and a guiding will; but, when they are carefully 
 studied, it is found that striking differences are to be detected, 
 by which their nature is found to be simply reflex, a certain 
 stimulus or irritation producing a certain movement, without 
 any choice or guidance on the part of the animal, and pro- 
 bably without its consciousness. As there are no animals in 
 which these reflex movements are more remarkable than they 
 are in Centipedes and Insects, we shall pause to dwell upon 
 them here in more detail. 
 
 443. If the head of a Centipede be cut off whilst the animal 
 is in motion, the body will continue to move onwards by the 
 action of the legs ; and the same will take place if the body 
 be divided into several distinct portions. After these actions 
 have come to an end, they may be excited again by irritating 
 any part of the nervous centres, or the cut extremity of the 
 nervous cord. The body is moved forwards by the regular 
 and successive action of the legs, as in the natural state ; but 
 its movements are always forwards, never backwards ; and are 
 only directed to one side when the direct movement is 
 checked by an interposed obstacle. There is not the slightest 
 indication of consciousness, either in direction of object, or in 
 avoidance of danger. If the body be opposed in its progress, 
 by an obstacle not more than one half its own height^ it 
 mounts over it and moves directly onwards, as in a natural 
 state ; but if the height of the obstacle be equal to its own, its 
 progress is arrested, and the cut extremity of the body 
 remains opposed to it, with the legs still moving. If, again, 
 the nervous cord of a Centipede be divided in the middle of 
 the trunk, so that the hinder legs are cut off from connexion 
 with the cephalic ganglia, they will continue to move, but not 
 in harmony with those in the fore part of the body, being 
 completely withdrawn from any .control on the part of the 
 animal, though still capable of performing reflex movements 
 by the influence of their own ganglia. Or, again, if the head 
 of a Centipede be cut off, and, while it remains at rest, some 
 irritating vapour (such as that of ammonia or muriatic acid) 
 be caused to enter the air-tubes on one side of the trunk, the 
 body will be immediately bent in the opposite direction, so as 
 to withdraw itself as much as possible from the influence of 
 
358 REFLEX ACTIONS OF ARTICULATA. 
 
 the vapour : if the same irritation be applied on the other 
 side, the reverse movement will take place; and the body 
 may be caused to bend in two or three curves, by bringing 
 the irritating vapour into the neighbourhood of different parts 
 of either side. This movement is evidently a reflex one, and 
 serves to withdraw the entrances of the air-tubes from the 
 source of irritation ; just as the act of sneezing in higher 
 animals causes the expulsion from the air-passages of any 
 irritating matter, whether solid, liquid, or gaseous, which may 
 have found its way into them; and we have no reason to 
 regard the former as more voluntary than the latter, which we 
 know to be purely reflex ( 342). 
 
 444. Among Insects, we meet with reflex actions yet more 
 curious. The Mantis religiosa (fig. 187) is remarkable for the 
 
 Fig. 187. MANTIS RELIGIOSA. 
 
 peculiar conformation of its first pair of legs, which serve as 
 claws for seizing its prey ; and also for the peculiar attitude 
 which it assumes, especially when threatened or attacked. 
 Supporting itself upon its two hinder pairs of legs, it rears up 
 its head upon the long first segment of the thorax, elevating 
 at the same time its large and powerful arms ; and the resem- 
 blance fancied to exist between this attitude and that of prayer, 
 is the cause of the epithet religiosa having been given to it. 
 Now if the first 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 position 
 when disturbed, and performing the same agitated movements 
 of the wings as when the unmutilated animal is excited. But 
 it will remain quite at rest, so long as it is not irritated. On 
 the other hand, the detached portion of the thorax which 
 
REFLEX ACTIONS OF ARTICULATA. 359 
 
 contains a ganglion, will, when separated from the head, set in 
 motion its long arms, and impress their hooks on the fingers 
 which hold it. Again, a specimen of Dytiscus (a water- 
 beetle), from which the cephalic ganglia have been removed, 
 executes the usual swimming motions when cast into water, 
 with great energy and rapidity, striking all its comrades to 
 one side by its violence ; in these it will persist for half an 
 hour, though so long as it lies on a dry surface it remains 
 quiescent. 
 
 445. From these and similar facts, it appears that the ordi- 
 nary movements of the legs and wings of Articulated animals 
 are of a reflex nature, and are dependent upon the ganglia 
 with which these organs are severally connected ; whilst 
 in the perfect animal they are harmonised, controlled, and 
 directed by its conscious power, which acts through the 
 cephalic ganglia and the trunks proceeding from it. When 
 we come to compare the reflex movements of Insects with 
 those of the higher animals, we shall perceive that there is no 
 ground for supposing the ganglia of the trunk to be in them- 
 selves endowed with sensibility ; so that, when the head is cut 
 off, or the cephalic ganglia are removed, or their connexion 
 with any part of the body is interrupted by division of their 
 nervous cord, no sensation is felt, however much the move- 
 ments it performs may seem at first to indicate this. (See 
 467.) 
 
 446. From this account of the structure and uses of the 
 chain of ganglia in the Articulata, it is obvious that these 
 ganglia are so many repetitions of the pedal ganglia (or gang- 
 lion of the foot) of the Mollusca ; and we have not yet had 
 to notice any ganglia appropriated to other functions. In fig. 
 186, however, is seen a small ganglion in front of the cephalic 
 mass, which corresponds to the pharyngeal ganglion of the 
 Aplysia (fig. 183, D) ; and we have now to describe an entirely 
 distinct system of nerves, appropriated to the function of respi- 
 ration. As the respiratory apparatus of Articulata, instead of 
 being confined to one spot, like that of the Mollusca, is dis- 
 persed througk the body ( 315 and 320), the ganglia which 
 minister to its actions are repeated in the several segments. 
 There is, in fact, a chain of minute ganglia lying upon the 
 larger cord, and sending off its nerves between those proceed- 
 ing from the latter, as seen in fig. 185. These respiratory 
 
360 
 
 RESPIRATORY NERVES OF ARTICULATA. 
 
 ganglia and their nerves are best seen in the front of the body, 
 where the cords that pass between the ganglia diverge or 
 separate from each other. This is shown 
 on a larger scale in fig. 188; where AB, 
 A B, are two pairs of ganglia in the thoracic 
 region, connected by two cords which di- 
 verge from one another; and between these 
 are seen the small respiratory ganglion a, and 
 its branches b b. These branches are distri- 
 buted to the air-tubes and other parts of the 
 respiratory apparatus, and communicate 
 with those of the other system. "We shall 
 find that, even in the highest Vertebra ta, 
 there is a portion of the nervous centres 
 which is set apart for the maintenance of the 
 respiratory actions, and which may be regarded as the respi- 
 ratory ganglion ; though it is so closely connected with other 
 parts of the mass as to seem but a part of it ( 450). 
 
 447. In the higher Invertebrata, among both the Articulated 
 and the Molluscous classes, we find a tendency to the concen- 
 tration of the ganglia into one or two masses, carrying to a 
 
 Fig.lSS. PORTION OF 
 THE NERVOUS SYS- 
 TEM OF INSECT; 
 
 Showing the respira- 
 ratory ganglia and 
 nerves. 
 
 Fig. 189. NERVOUS SYSTEM OF CRAB (Maia). 
 
 ca, upper part of the shell laid open ; a, antennae ; y, eyes ; e, stomach ; c, cephalic 
 ganglion; no, optic nerves; co, cesophageal collar; ns, stomato-gastric nerves; 
 t, thoracic ganglionic mass ; np, nerves of the legs , na, abdominal nerve. 
 
CONCENTRATION OF GANGLIA IN HIGHEST INVERTEBEATA. 361 
 
 greater extent that which has been already noticed in the per- 
 fect Insect ( 441). Thus in the Spider, the cephalo-thorax 
 contains a single large ganglion (t, fig. 46), from which all the 
 legs are supplied. The same is the case in the Crab, whose 
 nervous system is represented in fig. 189. Besides this mass, 
 t, however, which is situated beneath the alimentary canal, 
 there is a single or double cephalic ganglion, c, which receives 
 the nerves from the organs of sense, and sends backwards, to 
 communicate with the mass t, a pair of cords that separate to 
 give passage to the oesophagus, round which they form a sort 
 of collar co. And there are other small ganglia and nerves, 
 connected with the operations of mastication and digestion, 
 which are called stomato-gastric (from two Greek words, 
 meaning the mouth and the stomach). 
 
 448. A similar concentration, though with a different 
 arrangement of parts, is seen in the nervous system of the 
 Poulp, one of the Cephalopoda ( 111). There is still a 
 nervous collar through which the oesophagus passes (a, fig. 
 190) ; but the organs of locomotion being the enlarged tenta- 
 cula that surround the mouth, the nerves given off to them 
 arise from ganglia that form part of the cephalic mass, b, b, 
 instead of being located at a distance from it. At o are seen 
 the optic nerves, proceeding from distinct ganglia ; and at c 
 is a heart-shaped ganglionic mass, which seems to bear more 
 resemblance to the proper brain of higher animals, than does 
 any that we elsewhere find in the Invertebrata. In front of 
 this are two ganglia on the middle line, both of which belong 
 to the stomato-gastric system, one supplying the lips and the 
 other the pharynx. From the mass g, situated beneath the 
 oesophagus, there pass backwards two cords m m, each of 
 which has a ganglion e upon its course, and from this are 
 given oif nerves to the general surface of the mantle ; and also 
 other two cords, which run backwards to supply the viscera, 
 and especially the gills, each passing through a long narrow 
 ganglion r, before entering them. It would seem as if the 
 ganglia e and r corresponded with the ganglia c and B in the 
 Aplysia ; but as if, in consequence of the great enlargement 
 of the cephalic mass, they were proportionally reduced in 
 size. 
 
 449. In the nervous system of Vertebrated animals, the 
 ganglia are no longer scattered through the body, but are 
 
362 
 
 NERVOUS SYSTEM OF VERTEBRATA. 
 
 united into one continuous mass ; and this mass, constituting 
 the Brain and Spinal Cord, is inclosed within the bony 
 
 Fig. 190. NERVOUS SYSTEM OF OCTOPUS (PouLP). 
 
 casing formed by the skull and vertebral column, in such a 
 manner as to be protected by it from injuries to which it 
 would otherwise be continually liable ( 72, 73). We have 
 seen that among the Invertebrated classes the nervous system 
 has no such peculiar defence, but lies among the other organs, 
 sharing with them the protection afforded by the general hard 
 envelope of the body. But in the Vertebrata, its development 
 
NERVOUS SYSTEM OP VERTEBRATA. 363 
 
 is so much higher, and its importance so much greater, that 
 special care is taken to guard it from injury. The term brain 
 is commonly applied to the whole mass of nervous matter 
 contained within the cavity of the skull ; but this consists of 
 several distinct parts, which have obviously different charac- 
 ters. The principal mass in Man and the higher Vertebrata 
 is that which is termed the Cerebrum (fig. 195, a) ; this occu- 
 pies all the front and upper part of the cavity of the skull, and 
 is divided into two halves or hemispheres by a membranous 
 partition which passes from back to front along the middle 
 line. Beneath this, at the back part of the skull, is another 
 mass, b, much smaller, but still of considerable size, termed 
 the Cerebellum; and this also is divided into two hemi- 
 spheres. At the base or under side of the cerebrum, and 
 completely covered-in by it, are two pairs of ganglia (1 and g, 
 fig. 196), which belong to the nerves of smell and sight. We 
 shall presently find that these are, relatively speaking, much 
 larger in the lower Vertebrata than in the higher. 
 
 450. The several masses of nervous matter contained in the 
 skull, are connected with each other and with the spinal cord 
 by bands of nerve-fibres and tracts of vesicular substance, 
 which serve to bring the brain into connexion with the nerve- 
 trunks issuing from the spinal cord. But the Spinal Cord 
 has also distinct properties of its own, analogous to those 
 which have been shown to exist in the chain of ganglia in 
 Insects. The upper part of it, which passes-up into the 
 cavity of the skull, is termed the Medulla Oblongata (/', fig. 
 197). This is connected with the nerves of respiration, masti- 
 cation, and deglutition; and may be regarded as combining 
 together the respiratory and the stomato-gastric systems of 
 Invertebrata. The remainder of the spinal cord, which de- 
 scends through the vertebral column, sends its nerves to the 
 limbs and trunk ; and may be regarded as analogous to the 
 chain of ganglia by which the corresponding parts are sup- 
 plied in Insects. 
 
 451. The nerves which issue from the Spinal Cord, all 
 possess two sets of roots ; one from the anterior portion of 
 the cord, the other from its posterior portion (fig. 191). The 
 fibres which come-off by these two sets of roots, soon unite 
 into the trunk of the nerve, which thus possesses the proper- 
 ties common to both. It was the great discovery of Sir 
 
364 NERVOUS SYSTEM OF VERTEBRATA. 
 
 Charles Bell, that the posterior set of roots consists of those 
 fibres that bring impressions from the body in general to the 
 Spinal Cord ; which impressions, if carried-on to the Brain, 
 become sensations. On the other hand, 
 the anterior roots consist of fibres which 
 convey motor influence from the Spinal 
 Cord and Brain, to the muscles of the body. 
 Thus if the spinal cord of an animal be 
 laid bare, and the posterior set of roots be 
 touched, acute pain is obviously produced ; 
 whilst, if the anterior roots be irritated, 
 violent motions of the muscles supplied by 
 that nerve are occasioned. Both these 
 Fig. i9i. PORTION op roots contain fibres that connect them with 
 
 THE SPIKAL CORD, the brain ag well ag with the g -^ CQrd 
 Showing the origin of ,-, , ,, , ,, , .., 
 
 the nerves : a, spinal so that, through the same trunk, either of 
 
 c^n g n fupon Us theS6 CentreS ma J act U P n tlie P ar > We 
 
 course; d, anterior shall presently find that there is good 
 b^ne'unTon^Sht reason to believe the Brain to be the seat 
 /, branch. of sensibility and of voluntary power ; whilst 
 
 the Spinal Cord is the instrument of those reflex actions which 
 take place automatically, as it were, without direction on the 
 part of the animal, and which are concerned in the mainte- 
 nance of the organic functions of the body, and in its preser- 
 vation from injury. 
 
 452. The relative proportions which these different parts 
 present, are very different in the several classes of Vertebrata. 
 We find that among the lower, the Sensory Ganglia, or gan- 
 glionic centres immediately connected with the organs of sense 
 (which are analogous to the cephalic ganglia of thelnvertebrata), 
 are very large, and occupy a considerable part of the cavity of the 
 skull ; whilst the Cerebrum and Cerebellum are comparatively 
 small. The Cerebrum increases, as we ascend the scale, in 
 proportion to the development of the intelligence, and the 
 predominance which it gradually acquires over blind unde- 
 signing instinct (Chap. xiv.). Its greatest development is 
 seen in Man. The Cerebellum seems to be connected with 
 muscular motion, and to bear a proportion in size with the 
 variety and complexity of the movements which the animal 
 performs, serving to harmonise these and blend them together 
 ( 480). On the other hand, the Spinal Cord, and the nerves 
 
NERVOUS CENTRES OF FISHES. 
 
 365 
 
 proceeding from it, are largest in those animals in which the 
 brain is smallest. 
 
 453. It is in FISHES that we find the brain least developed, 
 and the cerebral hemispheres bearing the smallest proportion 
 to the other parts. On opening the skull, we usually observe 
 four nervous masses (three of them in pairs) lying, one in 
 front of the other, nearly in the same line with the spinal 
 cord. Those of the first pair are olfactory ganglia, or the 
 ganglia of the nerves of smell (fig. 192 A, ol). In the Shark, 
 and some other Fishes, these are separated from the rest by 
 peduncles or foot-stalks (B, ol) ; a fact of much interest, as 
 explaining the arrangement 
 
 which we find in Man( 458). 
 Behind these is a pair of gan- 
 glionic masses (c h), of which 
 the relative size varies con- 
 siderably in different fishes e 
 (thus in the Cod they are 
 much smaller than those of. 
 which succeed them, whilst ce 
 in the Shark they are much 
 larger) ; these are the cerebral sp 
 hemispheres. Behind these, 
 again, are two large masses 
 (op), the optic ganglia, in 
 which the optic nerves termi- 
 nate. And at the back of these, overlying the top of the 
 spinal cord, is a single mass, the cerebellum (ce) ; this is seen 
 to be much larger in the active rapacious Shark, the variety 
 of whose movements is very great, than in the less energetic 
 Cod. The spinal cord (sp) is seen to be divided at the top by 
 a fissure, which is most wide and deep beneath the cerebellum, 
 where there is a complete opening between its two halves. 
 This opening corresponds to that through which the oesophagus 
 passes in the Invertebrata ; but, as the whole nervous mass of 
 Vertebrated animals is above the alimentary canal ( 74), it 
 does not serve the same purpose in them ; and in the higher 
 classes the fissure is almost entirely closed by the union of the 
 two halves of the cord on the central line. 
 
 454. In REPTILES we do not observe any considerable 
 advance in the character of the brain, beyond that of Fishes ; 
 
 Fig. 192. BRAINS OF FISHES. 
 A, Cod ; B, Shark. 
 
366 NERVOUS CENTRES OF REPTILES, BIRDS, AND MAMMALS. 
 
 save that the Cerebral hemispheres are usually larger, extend- 
 ing forwards so as to cover-in the Olfactive ganglia (fig. 193). 
 The Cerebellum is generally smaller, as we should expect from 
 the inertness of these animals, and the want of 
 variety in their movements ( 480). The 
 Spinal Cord is still very large, in proportion 
 to the nervous masses contained in the skull ; 
 and, as we shall hereafter see, its power of 
 keeping-up the movements of the body, after 
 it has been cut-off from connexion with the 
 krain, is very considerable. 
 
 455. In BIRDS, however, we find a consi- 
 derable advance in the character of the brain, 
 towards that which it presents in Mammalia. 
 The Cerebral hemispheres (a, fig. 194) are 
 greatly increased in size, and cover-in, not only the olfactory 
 ganglia, but also in great part the optic ganglia, 6. The Cere- 
 bellum, c, also, is much more developed, 
 as we should expect from the number and 
 complexity of the movements performed 
 by the animals of this class ; but it is still 
 undivided into hemispheres. The Spinal 
 Cord, d, is still of considerable size, and is 
 much enlarged at the points from which 
 the nerves of the wings and legs originate ; 
 in the species whose flight is most ener- 
 getic, the enlargement is the greatest in 
 ^ e ne ig nDournoo( i f the wings ; but in 
 those which, like the Ostrich, move 
 principally by running on the ground, the posterior en- 
 largement, from which the legs are supplied with nerves, is 
 the more considerable. 
 
 456. In MAMMALS, we find the size of the Cerebral 
 hemispheres very greatly increased, especially as we rise 
 towards Man ; whilst the olfactive and optic ganglia are pro- 
 portionally diminished, and are completely covered-in by 
 them. The surface of the cerebral hemispheres is no longer 
 smooth, as in most of the lower classes, but is divided by 
 furrows into a series of convolutions (fig. 196) ; by these, the 
 surface over which the blood-vessels come into relation with 
 the nervous matter is very greatly increased ; and we find the 
 
NERVOUS SYSTEM OF MAX. 
 
 367 
 
 Fig. 195.- NERVOUS SYSTEM OF MAN. 
 
368 NERVOUS CENTRES OP MAMMALS. 
 
 convolutions more marked as we rise from the lowest Mam- 
 malia, in which they scarcely exist, towards Man, in whom 
 the furrows are deepest. The two hemispheres are much 
 more closely connected with each other, by means of fibres 
 running across from either side, than they are in the lower 
 tribes ; and in fact, a considerable part of their mass is made 
 up of fibres that pass among their different portions, uniting 
 them with each other. The Cerebellum, also, is divided into 
 two hemispheres (b, fig. 195) ; and the grey matter in its 
 interior has a very complex and beautiful arrangement, which 
 causes it to present a tree-like aspect when it is cut across (d, 
 fig. 196). The Spinal Cord is much reduced in size, when 
 compared with the other parts of the nervous centres ; the 
 motions of the animal now depending more upon its will and 
 being more guided by its sensations, and the simply reflex 
 actions bearing a much smaller proportion to the rest. 
 
 457. The general arrangement of the nervous centres, and 
 distribution of the nervous trunks, of Man, are shown in fig. 
 195. At a are seen the hemispheres of the Cerebrum ; at b 
 those of the Cerebellum ; and at c, the Spinal Cord. The 
 principal motor nerve of the face (the facial) is seen at d; and 
 and at e is seen the brachial plexus, a sort of net- work of 
 nerves, originating by several roots from the spinal cord, and 
 going to supply the arm. From this plexus there proceed the 
 median nerve, // the ulnar nerve, g ; the internal cutaneous 
 nerve, h; and the radial and musculo-cutaneous nerves, t. 
 From the Spinal Cord are given off the intercostal nerves, j, 
 passing between the ribs ; the nerves forming the lumbar 
 plexus, &, from which the front of the leg is supplied ; and 
 those forming the sacral plexus, I, from which the back of the 
 leg is supplied. The latter gives origin to the great sciatic 
 nerve ; which afterwards divides into the tibial nerve, m ; 
 the peroneal oifibular nerve, n ; the external saphenous nerve, 
 o ; and other branches. 
 
 458. We shall now examine the structure of the Brain 
 itself, and the arrangement of the nerves which proceed from 
 it; confining ourselves to the points of most physiological 
 importance, and neglecting those which are interesting only 
 to the professed anatomist. In fig. 196 is represented a per- 
 pendicular section of the Human Brain down its middle ; the 
 two hemispheres forming the Cerebrum having been separated 
 
BRAIN OF MAN. 369 
 
 from each, other by the division of the broad fibrous band^J 
 termed the corpus callosum, which unites them. Each, hemi- 
 sphere is considered as made up of three lobes or divisions, 
 
 7 11 9 10 6 e 
 
 Fig. 106. SECTION OF THE BRAIN op MAN. 
 
 the anterior a, the middle 6, and the posterior c; but these 
 are not by any means distinctly marked-out, either on the 
 external surface or in the internal structure of the organ. The 
 vesicular or ganglionic nerve-substance is disposed for the 
 most part upon the exterior, forming a continuous layer, whose 
 extent is greatly increased by the convoluted folds in which it 
 lies ; and it is very copiously supplied with blood from the 
 pia mater, a membrane which consists almost entirely of blood- 
 vessels and of the areolar tissue that holds them together, and 
 which so closely enfolds the hemispheres as to dip down into all 
 the furrows of their surface. The principal part of the internal 
 substance of each hemisphere is composed of nerve-fibres, of 
 which some pass between its convolutions and the chain of 
 ganglionic masses on which the cerebrum is superposed, others 
 
 B B 
 
370 BRAIN OF MAN. 
 
 pass from each hemisphere to its fellow through the corpus 
 callosum, whilst others again bring the different convolutions 
 of the same hemisphere into mutual connexion. The hemi- 
 spheres are (so to speak) wrapped round the collection of 
 Sensory Ganglia in which the spinal cord may be said to ter- 
 minate at its upper end, in such a manner as to leave two 
 cavities, one on either side, which are called the lateral ven- 
 tricles. 1 The Sensory Ganglia are so small relatively to the 
 Cerebrum, that they would scarcely attract notice as inde- 
 pendent centres, if they were not carefully compared with the 
 ganglionic centres corresponding to them among the lower 
 animals. The olfactory ganglia are mere bulbous enlarge- 
 ments upon the cords (1), which, though commonly termed the 
 olfactory nerves, are really (as in the Shark, 453) footstalks 
 connecting these ganglia with the rest of the series ; it being 
 from these ganglia that the true olfactive nerves are given off 
 ( 506). The optic ganglia, g, only in part represent the 
 optic lobes of Fishes ; the function of the latter being shared 
 by two large masses termed the thalami optici, which form 
 the hinder part of the floor of the lateral ventricles, and 
 which also seem to participate in the sense of touch, as the 
 sensory columns of the spinal cord may be traced up to them. 
 This close connexion of the sensorial centres of Sight and 
 Touch is just what we might anticipate from the continual 
 co-operation of these two senses ( 556, 557). In front of 
 the optic thalami is another pair of large ganglionic masses, 
 termed the corpora striata, which is in the like close relation 
 with the motor columns of the spinal cord ; and it is chiefly 
 from them and from the thalami optici, that the fibres pro- 
 ceeding to the surface of the Cerebral hemispheres radiate. 
 The Cerebellum, which has no direct communication with the 
 Cerebrum, but possesses independent connexions of its own 
 with the upper part of the spinal cord, has its grey or vesicular 
 and its white or fibrous substance so peculiarly disposed, as 
 to present in section the appearance delineated at d, which 
 is termed the arbor vita?, or tree of life. 
 
 459. Of the nerves given off within the skull (figs. 1 9 6, 1 97), 
 
 1 There are other ventricles, which are merely spaces left on the 
 middle plane by the imperfect coalescence of the two lateral columns 
 of the nervous axis, like the openings formed by the divergence of the 
 two halves of the nervous cord in Insects (fig. 188). 
 
CEREBRO-SPINAL NERVES. 371 
 
 the first pair are the olfactive, which proceed from the bulbs 
 (1) of the olfactive peduncles, into the cavity of the nose. 
 Next to these are the optic nerves (2), which may be partly 
 traced to the optic ganglia, and 
 partly to the thalami optici. The 
 third (3), fourth (4), and sixth pairs 
 (6), are nerves of motion only, and 
 are distributed to the muscles of 
 the eye. The^A pair is for the 
 most part a nerve of sensation 
 only. Before leaving the skull, it 
 divides into three great branches ; 
 of which the first (5) passes into 
 the orbit (or cavity in which the 
 eye is lodged), endows the parts 
 contained in it with sensibility, 
 and then comes out beneath the 
 eyebrow, to be distributed to the 
 forehead and temples ; the second 
 (5') passes just beneath the orbit, 
 and makes its way out upon the 
 face, supplying the cheeks, nose, 
 upper lip, &c., which it endows 
 with sensibility; whilst the third 
 (5"), which (like the spinal nerves) 
 possesses a motor root also, supplies 
 the muscles of mastication with 
 the power of moving, and the 
 parts about the mouth with sensi- 
 bility. The seventh pair (7), or 
 facial, is the general motor nerve 
 of the face ; and this does not 
 endow the parts which it supplies 
 with the least sensibility. Beneath 
 the origin of this nerve is seen the 
 cut extremity of another trunk, 
 that of the auditory nerve (8), or 
 nerve of hearing. At 9 is seen the glosso-pharyngeal nerve, 
 which supplies the back of the mouth and pharynx, and is 
 concerned in the act of swallowing. Originating from the 
 upper part of the spinal cord (or medulla bloongata) very near 
 B B 2 
 
 Fig. 197. BRAIN AND SPINAI 
 CORD OF MAN. 
 
372 CEREBRO-SPINAL NERVES. 
 
 this, is the pneumogastric nerve, or par vagum (10), which 
 supplies the lungs and air-passages, and also the heart and 
 stomach. Below this, again, is the hypoglossal nerve (11), 
 which gives motion to the tongue j at 12 is a nerve termed 
 the spinal accessory, which is concerned in the acts of respira- 
 tion; and at 13 and 14 are two of the regular spinal nerves. 
 The termination of all these nerves is either in that prolonga- 
 tion of the Spinal Cord into the cavity of the skull, which is 
 termed the Medulla Oblongata (fig. 197, /'), or in the Sensory 
 Ganglia which are closely connected with the upper part of 
 this prolongation. Although some of them seem to pass 
 directly into the Cerebrum, it is very doubtful if such is really 
 the case. 
 
 460. A general connected view of the Brain and Spinal 
 Cord is given in fig. 197 ; which represents the front of the 
 latter, with the Brain a turned back, so as to expose its 
 under side. At 5 is seen its anterior lobe ; at c its middle 
 lobe ; and its posterior lobe d is almost entirely concealed by 
 the Cerebellum e. At /' is shown the Medulla Oblongata, 
 or upper end of the Spinal Cord ff. The brachial plexus 
 is seen at g, formed by the nerves that originate in the cervical 
 region of the cord ; at h is the lumbar plexus formed by the 
 nerves of the lumbar portion ; and at k is the sacral plexus 
 formed by the sacral nerves. The spinal cord terminates at 
 its lower extremity in a bundle of nerves /, to which the name 
 cauda equina is given, from its resemblance to a horse's tail. 
 The various pairs of nerves from 1 to 14 are the same as in 
 the preceding description; 15 and 16 are nerves from the 
 upper part of the cervical region ; 25, a pair from the dorsal 
 region ; and 33, a pair from the lumbar region. All these 
 spinal nerves find their way out through apertures in the 
 vertebral column, which are formed by a union of two notches, 
 one in each of the adjoining vertebrae. 
 
 461. The system of nerves which has been now described 
 is termed the Cerebro-Spinal ; but it is not the only set of 
 nerves and ganglia contained within the bodies of Vertebrated 
 animals. In front of the vertebral column there is a chain of 
 oblong ganglia, which communicate with two large ganglia 
 that lie among the intestines, and with several small ganglia 
 in the head and other parts. They communicate also with the 
 posterior roots of the spinal nerves, on which are another set 
 
SYMPATHETIC SYSTEM OF NERVES. 373 
 
 of ganglia (c, fig. 191), that seem to belong to the same system. 
 The nerves proceeding from this system, which is called the 
 Sympathetic, are distributed, not like those of the cerebro- 
 spinal, to the skin and muscles, but to the organs of digestion 
 and secretion, and to the heart and blood-vessels. Hence the 
 former system of nerves, being that by which sensations are 
 received and spontaneous motions executed, is called the 
 nervous system of animal life ; whilst the latter, being con- 
 nected with the nutritive processes alone, is termed the nervous 
 system of organic life. 
 
 462. What is the nature of the influence which the Sympa- 
 thetic system exerts over the functions of the parts to which 
 it is distributed, is not yet clearly made out. The sympathetic 
 nerves distributed to the alimentary canal have been ascer- 
 tained to have the power of exciting its peristaltic actions ; 
 and those which are distributed with the blood-vessels (on the 
 coats of which they form a minute net- work) have a direct influ- 
 ence over their calibre, producing changes in the local circulation 
 in obedience to passions and emotions of the mind, as well as 
 to states of other bodily organs. Of this influence we have a 
 familiar example in the acts of blushing and turning pale from 
 agitation of the feelings, and a more decided but less frequent 
 one in the fainting which sometimes occurs from a sudden 
 shock. It is doubtful, however, whether the Sympathetic 
 system really possesses motor filaments of its own ; its motor 
 actions being certainly in part dependent upon filaments de- 
 rived from the cerebro-spinal system. The action of its motor 
 fibres upon the muscular coats of the blood-vessels supplying 
 the glands, serves to regulate the quantity of the fluids secreted 
 by these organs, especially in cases in which the demand for 
 the secretion is intermittent; but as there is evidence that 
 the quality of many secretions may be affected by mental states 
 ( 353), it seems likely that the fibres peculiar to the Sympa- 
 thetic system ( 60) may be the channel of this influence. 
 Although it is still impossible to define precisely the functions 
 of the Sympathetic system, yet it may be stated generally, 
 that in virtue of the two modes of action just explained, it 
 seems to harmonise and blend together the various actions of 
 Nutrition, Secretion, &c., in such a manner as to bring them 
 into conformity with each other, and with the condition of 
 the organs of Animal life. 
 
374 FUNCTIONS OF SPINAL COED : EEFLEX ACTION. 
 
 463. We shall now consider, in more detail, the functions 
 of the different parts of the Cerebro-Spinal System in Man 
 and the higher animals ; referring occasionally to the Inver- 
 tebrated classes for illustrations which they can best afford. 
 We shall commence by examining the functions of the Spinal 
 Cord and Medulla Oblongata, which are the parts concerned 
 in reflex action. 
 
 Functions of the Sjiinal Cord. Reflex Action. 
 
 464. The Spinal Cord of Vertebrated Animals may be con- 
 sidered as a collection of ganglia, analogous to those of which 
 the ganglionic cord of Articulata is composed ; these ganglia 
 being united, however, in an unbroken line, instead of being 
 distinct from one another and brought into communication by 
 coiinecting cords. There is great difficulty in tracing-out the 
 precise course of the nerve-fibres which form the white strands 
 of the Spinal Cord ; and it is doubtful how far any of them 
 form a continuous connexion between the roots of the Spinal 
 Serves and the Brain. But there can be no doubt that such 
 a connexion is established, either by the fibrous tracts or by 
 the grey matter of the Spinal Cord; experiment having 
 unequivocally shown that the latter participates with the 
 former in this conducting power. 
 
 465. When the Cerebro-Spinal system is in full activity, 
 its nerves convey impressions from every part of the body to 
 the Brain, where they are communicated to the mind, that is, 
 the individual becomes conscious of them, or feels them as 
 sensations. And by the fibres of the same system which pass 
 in the contrary direction, the will acts upon the muscles so as 
 to produce voluntary motion. Now the brain is not in con- 
 stant action, even in a healthy person ; it requires rest ; and 
 during profound sleep it is in a state of complete torpor. Yet 
 we still see those movements continuing, which are essential 
 to the maintenance of life ; the breathing goes on uninter- 
 ruptedly, liquid poured into the mouth is swallowed, and the 
 position is changed when the body would be injured by 
 remaining in it. The same is the case in apoplexy, in which 
 the actions of the brain are suspended by pressure upon it. 
 And the same will take place, also, in an animal from which 
 the cerebrum has been removed ; or in which its functions 
 are completely suspended by a severe blow on the head. If 
 
FUNCTIONS OF SPINAL COBD I REFLEX ACTION. 375 
 
 the edge of the eyelid be touched with a straw, the lid imme- 
 diately closes ; if a candle be brought near the eye, the pupil 
 contracts ( 534); if liquid be poured into the mouth, it is 
 swallowed ; if the foot be pinched or be burnt with a lighted 
 taper, it is withdrawn ; and, if the experiment be made upon 
 a Frog, the animal will leap away as if to escape from the 
 source of irritation. The respiratory movements, too, are kept 
 up with regularity; so that there is no impp.diTnp.Tit to the 
 continuance of the circulation, and the organic life of the 
 animal may thus endure for some time. In one of the experi- 
 ments made with the view of ascertaining the degree in which 
 the activity of the Cerebrum is essential to the maintenance 
 of life, a pigeon was kept alive (if alive it could be called) for 
 some months after the removal of its cerebrum, running 
 when it was pushed, flying when it was thrown into the air, 
 drinking when its beak was plunged in water, swallowing 
 food which was put in its mouth, though at all other times, 
 when left to itself, appearing like an animal in profound 
 sleep. 
 
 466. It is evident, therefore, that we are not to regard the 
 Brain (according to the former opinion of Physiologists, and 
 the belief which is still commonly entertained) as the only 
 centre of nervous power, and as essential to the maintenance 
 of the life of the body; and that we must attribute to the 
 Spinal Cord no small amount of independent power. We 
 might be disposed to infer, from the statements in the last 
 paragraph, that an animal whose brain has been removed can 
 still feel and judge and perform voluntary motions by means 
 of the Spinal Cord; but this, again, would be putting a wrong 
 interpretation upon the phenomena in question. It is ob- 
 served that the motions performed by an animal in such 
 circumstances are never spontaneous ; they are always excited 
 by a stimulus of some kind. Thus a decapitated Frog, after 
 the first violent convulsive movements occasioned by the ope- 
 ration have passed away, remains at rest until it is touched ; 
 and then its leg, or even its whole body, will be thrown into 
 sudden action, which immediately subsides again. In the 
 same manner, the action of swallowing is not performed, 
 except when it is excited by the contact of food or liquid 
 ( 195) ; and even the respiratory movements, spontaneous as 
 they seem to be, would not continue long, unless they were 
 
376 FUNCTIONS OF SPINAL CORD I REFLEX ACTION. 
 
 excited by the presence of venous blood in the vessels espe- 
 cially in those of the lungs. These movements are all necessarily 
 linked with the stimulus that excites them; that is, the 
 same stimulus will always produce the same movement, when 
 the condition of the body is the same. Hence it is evident 
 that the judgment and will are not concerned in producing 
 them ; but that they may be rather compared to the move- 
 ments of an automaton, which are calied-forth by touching 
 certain springs. 
 
 467. The next question is, whether these movements can 
 be performed without any feeling or sensation, on the part of 
 the animal, of the cause that produces them. It is difficult 
 to imagine that an animal, executing such regular and 
 various actions, which so strongly resemble those it would 
 execute in its complete state, and which are so perfectly 
 adapted to their obvious purposes, can do so without con- 
 sciousness ; and accordingly some Physiologists have regarded 
 them as furnishing proof that the Spinal Cord possesses the 
 property of sensibility, or, in other words, that an animal 
 whose Brain has been removed can still feel. Eut this in- 
 ference will not bear a close examination. Such movements 
 take place, not only when the Brain has been removed and 
 the Spinal Cord remains entire, but even when the Spinal 
 Cord has been itself cut across into two or more portions. 
 Thus if the head of a Frog be cut off, and its Spinal Cord be 
 divided in the middle of the back, so that its fore-legs remain 
 connected with the upper part, and the hind-legs with the 
 lower, each pair of members may be excited to movement by 
 a stimulus applied to itself; but the two pairs will not 
 execute any consentaneous motions, as they will do when the 
 Spinal Cord is undivided. Or, when the Spinal Cord is cut 
 across without removal of the Brain, the lower limbs may be 
 excited to movement, though completely paralysed to the will; 
 whilst the upper remain under the control of the animal's 
 sensation and conscious power. 
 
 468. Now although the Frog cannot tell us that it has no 
 sensation in its lower limbs, we have very strong evidence to 
 that effect; for cases are of no infrequent occurrence in 
 Man, in which, the Spinal Cord having been injured in 
 the middle of the back by disease or accident, there is not- 
 only loss of voluntary control over the motions of the legs, 
 
REFLEX ACTION WITHOUT SENSATION. 377 
 
 but loss of sensation also. Further, in several cases of this 
 kind, in which the injury was confined to a small portion of 
 the cord, and the part below was not seriously disturbed, it 
 has been noticed that motions may be excited in the limbs by 
 stimuli applied directly to them, as, for instance, by tickling 
 the sole of the foot, pinching the skin, or applying a hot plate 
 to its surface ; and this without the least sensation, on the 
 part of the patient, either of the cause of the movement, or 
 of the movement itself; the nervous communication, which 
 would otherwise have conveyed the impression to the brain 
 and there given rise to sensation, being interrupted in the 
 spinal cord. 
 
 469. By such cases, then, it appears to be clearly proved, 
 that the actions performed by the Spinal Cord, when the 
 Brain has been removed, or its power destroyed, or its com- 
 munication with the part cut-off, do not depend upon Sensa- 
 tion; but upon a property peculiar to the Spinal Cord, by 
 which impressions, made upon certain parts, necessarily excite 
 motions of an automatic character. By other experiments it 
 has been shown to be necessary for the exercise of this Reflex 
 function (as it has been termed), that an impression should be 
 conveyed by one set of nervous fibres, from the point where 
 the stimulus is applied, to the Spinal Cord ; and that a motor 
 impulse, conveyed by another set of filaments, should issue 
 from the Cord to the muscles. The excitor and motor fila- 
 ments distributed to any part are commonly bound up in 
 the same trunk, and are connected with the same part of the 
 Spinal Cord; so that, if this portion or segment be com- 
 pletely separated from the rest, it may still execute the reflex 
 movements of the parts to which its nerves are distributed ; 
 just as a single segment of a Centipede will continue to 
 move its legs, provided its own ganglion be entire ( 443). 
 
 470. But in other instances it happens that we can more 
 clearly distinguish between the excitor and the motor nerves, 
 from their being distributed separately, and being connected 
 with distinct portions of the spinal cord. Thus in the act of 
 deglutition ( 195), the chief excitor nerve is the glosso-pha- 
 ryngeal ( 459) ; this conveys the impression made by the 
 contact of food with the pharynx, to the Medulla Oblongata ; 
 but it does not convey the motor influence to the muscles, 
 this being accomplished by branches of another nerve, the 
 
378 REFLEX ACTIONS OF THE SPINAL CORD. 
 
 pneumogastric. If the trunk of the glosso-pharyngeal nerve 
 be pinched, an act of deglutition is made to take place ; but 
 if it be separated from the Medulla Oblongata, or the pneumo- 
 gastric nerve be divided, or the Medulla Oblongata itself be 
 destroyed, the movement can no longer be thus excited. 
 Hence we see the necessity of the completeness of this 
 nervous chain or circle consisting of the nerve proceeding 
 from the part stimulated to the ganglion, the ganglion itself, 
 and the nerve proceeding from the ganglion to the muscles 
 acted-on in order that any such reflex movements may be 
 produced. 
 
 471. The functions of the Spinal Cord appear to be wholly 
 restricted to the performance of movements of this character. 
 The proportion they bear to the motions which are de- 
 pendent upon sensation and will, varies greatly in different 
 animals ; and it may be judged-of with tolerable accuracy, by 
 comparing the relative sizes of the spinal cord and the brain. 
 Thus in the lowest Fishes, the spinal cord seems the principal 
 organ, and the brain an insignificant appendage to it. In 
 Man, on the contrary, the spinal cord is so small in com- 
 parison with the brain, as to have been regarded (though 
 incorrectly, as we have seen) in the light of a mere bundle 
 of nerves proceeding from it. In the former, the ordinary 
 movements of the body seem principally to depend upon the 
 spinal cord, being only controlled and directed by the brain ; 
 just as those of Articulated animals are chiefly dependent 
 upon the ganglia of the trunk, being only guided by those of . 
 the head ( 442). But in Man, those only are left to the 
 spinal cord which are necessary for the maintenance of life ; 
 the ordinary motions of the body being for the most part 
 voluntary. Still, as we have just now seen ( 468), reflex 
 movements may be excited through the spinal cord, even in 
 Man, when the influence of the will is cut off; and it is 
 curious to observe, that the stimulus is most powerful when 
 it acts upon the soles of the feet, and that it ceases to produce 
 the same effect, when, by the restoration of the functions of 
 the injured part of the cord, the power of the will over the 
 limbs, and also their sensibility, are regained. There is much 
 reason to believe that, when we are walking steadily onwards, 
 and the mind is intently occupied with some train of thought 
 which engrosses its whole attention, the individual movements 
 
REFLEX ACTIONS OF THE SPINAL CORD. 379 
 
 of the limbs may be kept-up by reflex action, while their 
 general direction is guided by visual sensation ( 479). ^And 
 even when the mind is sufficiently on the alert to guide, direct, 
 and control the motions of the limbs, their separate actions 
 appear to be performed without any immediate exertion of the 
 will ; and probably depend, therefore, rather upon the reflex 
 function of the spinal cord, than upon the continual influence 
 of the brain. 
 
 472. Besides the reflex movements of deglutition and re- 
 spiration, which have been formerly considered ( 195 and 
 340), and those of locomotion, on which we have now dwelt 
 sufficiently, there are several others of a similar character, all 
 of which have for their object the supply of the wants of the 
 body, or its preservation from injury. Of these the only one 
 which it is desirable here to notice is that of sucking, as per- 
 formed by the young Mammiferous animal. In this opera- 
 tion there is a very complex union of the actions of different 
 muscles, those of respiration, together with those of the 
 tongue and lips. So beautifully adapted is this combination 
 to its designed purpose, that it could not be better contrived 
 by the longest experience or the most careful study. Yet we 
 find that the young Mammal commences to perform it without 
 any experience or study, the instant that its lips touch the 
 nipple of its parent. And that it is a reflex action, dependent 
 upon the spinal cord alone for its performance, and requiring 
 a stimulus to excite it, is proved by these remarkable facts ; 
 that it has been performed by human infants which have 
 been born destitute of brain, and which have lived for some 
 hours ; and also by puppies whose brain had been removed. 
 These last not only sucked a moistened finger, when it was 
 introduced between their lips, but also pushed out their feet, 
 as the young animal naturally does against the dugs of the 
 parent. 
 
 473. There are many irregular actions of the Spinal Cord, 
 however, the careful study of which is of the highest impor- 
 tance to the Medical Man. It is probable that all convulsive 
 movements are produced through its agency ; these being for 
 the most part of a reflex character, that is, dependent upon 
 some stimulus or irritation which acts through the nervous 
 circle described in 470. Thus, convulsions are not unfre- 
 quent in children during the period of teething ; and are then 
 
380 CONVULSIVE FORMS OF REFLEX ACTION. 
 
 excited by the irritation which results from the pressure of 
 the tooth as it rises against the unyielding gum ( 174). 
 They are often occasioned, too, by the presence of indigestible 
 or injurious substances, or of intestinal worms, in the alimen- 
 tary canal ; and will cease as soon as this is properly cleared 
 out. Again, in Tetanus or "lockjaw" resulting from a lace- 
 rated wound, the irritation of the injured nerve is the first 
 cause of the convulsive action ; and a similar local irritation 
 is often the origin of Epileptic fits, in which the convulsion 
 is accompanied by loss of consciousness. When these com- 
 plaints prove fatal, it is usually by suffocation, the muscles 
 of respiration being fixed by the convulsive action, in such a 
 manner that air cannot pass either in or out. 
 
 474. In other forms of convulsive disorders, however, the 
 cause of irritation may directly affect the Spinal Cord, instead 
 of being conveyed to it by the nerves from a distance. This 
 seems to be the case, for example, in Hydrophobia ; which 
 terrible complaint is probably due to a poison introduced into 
 the blood by the bite of the rabid animal, and conveyed by 
 the circulating current to the nervous centres. So, when the 
 poison termed Strychnia has found its way into the circula- 
 tion, the whole Spinal Cord is thrown into such an excitable 
 state, that the slightest stimulus produces the most violent 
 convulsive movements, which succeed one another in extra- 
 ordinary variety. And the teething-convulsions of infants 
 often depend more upon a peculiar excitable state of the 
 spinal cord, which results from atmospheric impurity, and is 
 removed by change of air, than they do upon the irritation 
 of the gums. By knowing, as he now does, the part of the 
 nervous system on which these convulsive disorders depend, 
 the Physician is enabled to apply his remedies with much 
 greater precision than heretofore, and to form a much more 
 accurate estimate of the danger which attends them. 
 
 Functions of the Ganglia of Special Sense. Consensual Actions. 
 
 475. It has been seen that the nerves of special sense 
 those of smell, sight, and hearing terminate in ganglionic 
 centres peculiar to themselves, which are lodged within the 
 skull, and form part of what is commonly termed the brain, 
 though distinct both from the Cerebrum and the Cerebellum. 
 These Sensory Ganglia are almost the only representatives of 
 
GANGLIA OF SPECIAL SENSE : INSTINCTIVE ACTIONS. 381 
 
 the brain in the Invertebrated animals ; and in Fishes they 
 bear a very large proportion to the other parts, their relative 
 size gradually diminishing as we ascend the scale towards 
 Man. JSTow when we study the actions of these lower tribes 
 of animals, we find that those which evidently depend upon 
 sensation, especially the sense of sight, are very far from 
 being of the same spontaneous or voluntary character as those 
 which we perform. We judge of this by their unvarying 
 nature, the different individuals of the same species execut- 
 ing precisely the same movements, when the circumstances 
 are the same, and this evidently without any choice, or 
 intention to fulfil a given purpose, but in direct respondence 
 to an internal impulse. Of this we cannot have a more 
 remarkable example than is to be found in the operations 
 of Bees, "Wasps, and other social Insects ; which construct 
 habitations for themselves upon plans which the most enlight- 
 ened human intelligence could not surpass ; yet which do so 
 without hesitation, confusion, or interruption, the different 
 individuals of a community all labouring effectively for one 
 common purpose, because their impulses are the same (Chap- 
 ter XIV.) 
 
 476. In higher animals we may often notice the effect of 
 similar promptings, by which the various species are guided 
 in their choice of food, in the construction of their habitations, 
 in their migrations, &c. : but these are frequently modified 
 to a certain degree by the intelligence which they possess. 
 The closure of the pupil when the eye is exposed to a strong 
 light, and its dilatation when the light diminishes ( 534), 
 afford a very marked example of this " consensual" class of 
 movements, which differ from the simply-reflex in requiring the 
 stimulus of sensations, but which are, like them, quite indepen- 
 dent both of the reason and of the will. A still more striking 
 illustration, however, is furnished by the mode in which a 
 little Fish, termed the Chcetodon restrains, obtains its food. 
 Its mouth is prolonged into a kind of beak or snout, through 
 which it shoots drops of liquid at insects that may be hover- 
 ing near the surface of the water, and rarely fails in bringing 
 them down. Now, according to the laws of Optics, the insect, 
 being above the water whilst the eye of the fish is beneath, 
 it, is not seen by it in its proper place ; since the rays do not 
 pass from the insect to the fish's eye in a straight line ( 528). 
 
382 CONSENSUAL ACTIONS IN MAN. 
 
 The insect will in fact appear to the fish a little above the 
 place which it really occupies ; and the difference is not con- 
 stant, but varies with every change in the relative positions 
 of the fish and the insect. Yet the wonderful instinct with 
 which the fish is endowed, leads it to make the due allowance 
 in every case ; doing that at once, for which a long course of 
 experience would be required by the most skilful Human 
 marksman, under similar circumstances. 
 
 477. Though the Intelligence and Will of Man in a great 
 degree supersede his consensual impulses, in the same man- 
 ner as they hold in subordination his reflex movements 
 ( 471), yet we have many indications of the direct operation 
 of sensations in determining respondent movements. Of this 
 kind are the start produced by a loud sound, particularly if 
 unexpected ; the closure of the eyes to a dazzling light, or on 
 the sudden approach of a body that might injure them ; the 
 production of sneezing by a dazzling light ; the provocation 
 of laughter by tickling, or by some sight or sound to which 
 no distinct ludicrous idea or emotion attaches itself ; and the 
 excitement of vomiting by highly disagreeable sensations, as 
 the sight of a loathsome object, an offensive smell, a nauseous 
 taste, or by tickling the back of the mouth with a feather. 1 
 None of these " consensual " movements can be excited with- 
 out the consciousness of the subject of them; and this 
 circumstance marks them out as belonging to a different 
 category from the "reflex" movements performed through 
 the instrumentality of the Spinal Cord. In some convulsive 
 disorders, the attacks are excited by causes that act through 
 the organs of sense : thus, in Hydrophobia we observe the 
 immediate influence of the sight or sound of liquids ; and in 
 many Hysteric subjects, the sight of a paroxysm in another 
 individual is the most certain means of its induction in them- 
 selves. 
 
 478. But we may trace the agency of the Sensory Ganglia 
 
 1 This is the most ready way of exciting vomiting, when it is desired 
 to free the stomach from poisons or unwholesome articles of food ; 
 but care must be taken not to apply the feather so low down as to 
 cause it to be grasped by the muscles concerned in the act of swallow- 
 ing ; for its irritation, instead of producing vomiting, will then occasion 
 an act of deglutition ( 195), which may draw the feather from the 
 hand of the operator, and carry it down into the stomach of the 
 patient. 
 
IMPORTANCE OF GUIDING SENSATIONS. 383 
 
 in Man and the highest Vertebrata, not merely in their direct 
 and independent operation on the Muscles, but also in the 
 manner in which they participate in all voluntary action. 
 For it is now well established, that the Will cannot bring 
 about any definite movement, except under the guidance of 
 sensations, derived either from the muscles themselves, or 
 through some channel of information which indicates what 
 the muscles are doing. It is for want of the guiding sensa- 
 tions afforded by the ear, that persons who are born deaf are 
 also dumb, the will not being able to make use of the muscles 
 concerned in vocalization ; and where, by long training, some 
 imperfect power of speech has been acquired, it has been 
 gained by attention to the sensations arising from the mus- 
 cular exertion of the organs themselves. It is by the guiding 
 influence of the visual sensations, that the movements of the 
 two eye-balls are made to correspond ; and, in children born 
 completely blind, it may be observed that the eyes roll about 
 without any harmony, though a very slight perception of light 
 is sufficient to bring their motions into consent. So, again, 
 if the arm or the leg be so paralysed that its sensibility is 
 lost whilst its muscles are still under the power of the will, 
 that power can only be exerted to occasion movement by the 
 assistance of the sight ; a mother, for example, so affected, 
 being only able to hold her infant upon her arm so long as 
 she looks at it ; and a man, whose legs are thus paralysed, 
 being only able to sustain himself in standing or walking by 
 constantly looking at his legs. 
 
 479. It seems to be obviously through the shorter channel 
 afforded by the Sensory Ganglia, that those actions are per- 
 formed, which, though originally directed by Intelligence and 
 Will, come by frequent repetition to be so completely auto- 
 matic as to resemble the instinctive actions of the lower 
 animals. Thus it is within the experience of almost every 
 one, that he occasionally walks through the streets with his 
 mind intently and continuously engaged on some train of 
 thought, without the least attention to, or even consciousness 
 of, the direction he is taking; yet he avoids obstacles, and 
 follows his accustomed course, obviously under the guidance 
 of his visual sense, whilst the movements of his limbs are 
 kept-up by reflex action (471); and on awaking, as it were, 
 from his reverie, he may find that he has thus been automa- 
 
384 HABITUAL ACTIONS : FUNCTION OF CEREBELLUM. 
 
 tically conducted to a place very different from that to 
 which, he had intended going. So, again, we may read 
 aloud, or play on a musical instrument, without being at all 
 aware of what we are about, the whole attention being ab- 
 sorbed by some engrossing thoughts or feelings within. And 
 it seems to be in this manner that the movements of Som- 
 nambulists are guided; their Cerebrum being, as it were, 
 cut-off from communication with the outer world, and their 
 Sensory Ganglia acting independently of it. 
 
 Function of the Cerebellum. Combination of Muscular Actions. 
 
 480. Much discussion has taken place of late years respect- 
 ing the uses of the Cerebellum ; and many experiments have 
 been made to determine them. That it is in some way con- 
 nected with the powers of motion, is now generally admitted. 
 Its size in the different tribes of Yertebrated animals bears a 
 pretty close correspondence with the variety and energy of 
 the movements performed by them ; being greatest in those 
 animals which require the constant united effort of a large 
 number of muscles to maintain their usual position, whilst it 
 is least in those which require no muscular exertion for this 
 purpose. Thus in animals that habitually rest and move upon 
 four legs, there is but little occasion for any organ to combine 
 and harmonize the actions of their several muscles ; and in 
 these the Cerebellum is small. But among the more active 
 predaceous Fishes (as the Shark), Birds of most powerful 
 and varied flight (as the Swallow, which not only flies rapidly, 
 but executes the most complicated evolutions in pursuit of its 
 Insect prey with the greatest facility), and Mammals which 
 can maintain the erect position and use their extremities for 
 other purposes than support and motion, we find the Cere- 
 bellum of much greater size : whilst in Man, who surpasses 
 all other animals in the number and variety of the combina- 
 tions of muscular movement which he is capable of executing, 
 it attains its largest dimensions and its greatest complexity of 
 structure. 
 
 481. From experiments upon all classes of Vertebrated 
 Animals, it has been found that, when the Cerebellum was 
 removed, the power of walking, springing, flying, standing, 
 or maintaining the equilibrium of the body, was destroyed. 
 It did not seem that the animal had in any degree lost volun- 
 
FUNCTIONS OP THE CEREBELLUM AND CEREBRUM. 385 
 
 tary power over its individual muscles ; but it could not 
 combine their actions for any general movement of the body. 
 The reflex movements, such as those of respiration, remained 
 unimpaired. When an animal in this state was laid on its 
 back, it could not recover its former posture ; but it moved 
 its limbs or fluttered its wings, and evidently was not in a 
 state of stupor. When placed in the erect position, it stag- 
 gered and fell like a drunken man ; not, however, without 
 making efforts to maintain its balance. Phrenologists, who 
 attribute a different function to the Cerebellum, have attempted 
 to put aside these results, on the ground that the severity of 
 the operation was alone sufficient to produce them j but (as 
 we have already seen, 465) after a much more severe opera- 
 tion the removal of the Cerebral Hemispheres, the Cere- 
 bellum being left untouched the animal could stand, walk, 
 fly, maintain its balance, and recover it when disturbed. 
 
 482. The motions of the body in the Invertebrated classes, 
 being simple in their nature, and probably all of a reflex 
 character ( 442), do not require a Cerebellum ; and we do 
 not find in them any nervous mass which clearly represents 
 this organ, 
 
 Functions of the Cerebrum. Intelligence and Will. 
 
 483. From the facts already stated, it is tolerably clear that 
 the Cerebrum is the organ by which we reason upon the ideas 
 that are excited by sensations, by which we judge and de- 
 cide upon our course of action, and by which we put that 
 decision into practice, by issuing a mandate (as it were), which, 
 being conveyed by the nervous trunks proceeding from the 
 brain to the muscles, excites the latter to contract. It is a 
 common, but entirely erroneous idea, that Reason or Intelli- 
 gence is peculiar to Man ; and that the actions of the lower 
 classes of Animals are entirely due to Instinct. There can be 
 no doubt, however, that reasoning processes exactly resem- 
 bling those of Man are performed by many Mammals, such 
 as the Dog, the Horse, and the Elephant ; and it is probable 
 that although we are best acquainted with these animals, 
 on account of their tendency to associate with Man, there 
 are others which have powers yet higher. We must admit 
 that an animal reasons, when it profits by experience, and 
 obviously adapts its actions to the ends it desires to gain, 
 
 
 
386 SUPERIOR INTELLIGENCE OP HIGHER VERTEBRATA. 
 
 especially when it departs from its natural instincts to do 
 this. Such is continually the case with the animals just 
 mentioned, as will appear from some striking examples to be 
 mentioned hereafter (Chap. xiv.). We perceive the presence 
 of Intelligence also in the differences of character which we 
 encounter among the various individuals of the same species ; 
 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 and good- 
 tempered Men. But no one could distinguish between a 
 stupid Bee and a clever Bee, or between a good-tempered 
 Wasp and an ill-tempered Wasp ; simply because all the 
 actions of these animals are prompted by an unvarying instinct. 
 484. Among Birds, too, there are many manifestations of 
 Intelligence, which constitute a remarkable distinction between 
 their actions and those of Insects ; though the instinctive 
 tendencies of the two classes bear a close correspondence with 
 each other. Their mode of life is nearly the same, so that 
 Birds may be called the Insects of the Vertebrated series, 
 whilst 'Insects may be regarded as the Birds of the Arti- 
 culated ; and there are several curious points of analogy in 
 the structure of their bodies. The usual arts which Birds 
 exhibit in the construction of their habitations, in pro- 
 curing their food, and in escaping from danger, must be 
 regarded (like those of Insects) as instinctive; on account 
 of the unrformity with which they are practised by different 
 individuals of the same species, and the perfection with which 
 they are exercised on the very first occasion, independently of 
 all experience. But in the adaptation of their operations to 
 particular circumstances, Birds display a variety and fertility 
 of resource far surpassing that which is manifested by Insects j 
 as for instance, 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 com- 
 forts of their dwelling ; or when they are influenced in the 
 choice of a situation by peculiar conditions, such as in a 
 state of nature can scarcely be supposed to affect them. All 
 these are obvious indications of an Intelligence which Insects 
 do not possess ; that which is most wonderful in the actions 
 of the latter (and there are none more wonderful) being the 
 same in all the individuals of one species, being uninfluenced 
 
LOW INTELLIGENCE OP REPTILES AND FISHES. 387 
 
 by education, and being performed under the direction of 
 an Intelligence much higher than the boasted reasoning 
 power of Man. 
 
 485. In the classes of EEPTILES and FISHES, the manifesta- 
 tions of Intelligence are so slight as to be scarcely distin- 
 guishable. We find them capable of such an amount of 
 education as enables them to recognise individuals from whom 
 they have been accustomed to receive food ; but they seem to 
 have very little further power of profiting by experience ; and 
 we do not find that individuals ever shape-out for themselves 
 a new course which can be regarded as purely rational. This 
 very low grade of Intelligence obviously corresponds with 
 the very rudimentary development of the Cerebrum in these 
 classes ( 453, 454). 
 
 The contrast between Instinct and Intelligence will be more 
 fully displayed in a future Chapter ; in which also a general 
 account will be given of the Mental Operations to which the 
 Cerebrum of Man is subservient. 
 
 CHAPTEE XL 
 
 ON SENSATION, AND THE ORGANS OF THE SENSES. 
 
 486. ALL save the very lowest kinds of Animals possess, 
 there is good reason to believe, a consciousness of their own 
 existence, first derived from & feeling of some of the changes 
 taking place within themselves; and also a greater or less 
 amount of sensibility to the condition of external things. 
 How far any such endowment can be possessed by creatures 
 which are destitute of a nervous system, and which are little 
 else than particles of animated jelly, may be questioned. But 
 there can be no reasonable doubt that where a nervous 
 system exists, whatever consciousness any Animal may pos- 
 sess of that which is taking place within or around itself, is 
 all derived from impressions made upon the extremities of 
 certain of its nervous fibres ; which, being conveyed by them 
 to the central sensorium, are there felt ( 430). Of the mode 
 in which the impression, hitherto a change of a material cha- 
 racter, is there made to act upon the mind, we are absolutely 
 ignorant ; we only know the fact. Hence, although we com- 
 
 c c 2 
 
388 SENSATION IN GENERAL. 
 
 monly refer our various sensations to the parts at which the 
 impressions are made, as, for instance, when we say that we 
 have a pain in the hand, or an ache in the leg, we really 
 use incorrect language ; for, though we may refer our sensa- 
 tions to the points where the impression was made on the 
 nerve, they are really felt in the brain. This is evident from 
 two facts ; first, that if the nervous communication of the 
 part with the brain be interrupted, no impressions, however 
 violent, can make themselves felt; and, second, that if the 
 trunk of the nerve be irritated or pinched anywhere in its 
 course, the pain which is felt is referred, not to the point 
 injured, but to the surface to which these nerves are distri- 
 buted. Hence the well-known fact that, for some time after 
 the amputation of a limb, the patient feels pains which he 
 refers to the fingers or toes that have been removed ; this con- 
 tinues until the irritation of the cut extremities of the nervous 
 trunks has subsided. 
 
 487. Among the lower tribes of Animals, it would seem 
 probable that there is no other kind of sensibility than 
 that which is termed general or common, and which exists, in 
 a greater or less degree, in almost every part of the bodies of 
 the higher. It is by this that we feel those impressions, 
 made upon our bodies by the objects around us, or by actions 
 taking place within, which produce the various modifications 
 of pain, the sense of contact or resistance, the sense of varia- 
 tions of temperature, and others of a similar character. From 
 what was formerly stated ( 63) of the dependence of im- 
 pressions made on the sensory nerves upon the action of the 
 blood-vessels, it is obvious that no parts destitute of the latter 
 can receive such impressions, or (in common language) can 
 possess sensibility. Accordingly we find that the hair, nails, 
 teeth, tendons, ligaments, fibrous membranes, cartilages, and 
 bones, whose substance either contains no vessels, or but very- 
 few, are either completely incapable of receiving painful 
 impressions, or have but very dull sensibility to them. On 
 the other hand, the skin and other parts which usually receive 
 such impressions, are extremely vascular ; and it is interesting 
 to observe that some of the tissues just mentioned, when new 
 vessels form in them in consequence of diseased action, 
 become acutely sensible. It does not necessarily follow, how- 
 ever, that parts should be sensible in a degree proportional to 
 
NERVES OF SPECIAL SENSIBILITY. 389 
 
 the amount of blood they contain ; since this blood may be 
 sent to them for other purposes. Thus, it is a condition 
 necessary to the action of Muscles, that they should be 
 copiously supplied with blood ( 591) ; but they are not 
 acutely sensible : and Glands, also, the substance of which 
 has very little sensibility, receive a large amount of blood for 
 their peculiar purposes. 
 
 488. But besides the general or common sensibility which is 
 diffused over the greater part of the body of most animals, 
 there are certain parts which are endowed with the property 
 of receiving impressions of a peculiar or special kind, such as 
 sounds or odours, which would have no influence upon the 
 rest ; and the sensations which these impressions excite, being of 
 a kind very different from those already mentioned, arouse ideas 
 in our minds such as we should never have formed without 
 them. Thus, although we can gain a knowledge of the shape 
 and position of objects by the touch, we could form no notion 
 of their colour without sight, of their sounds without hearing, 
 or of their odours without smell. 
 
 489. The nerves which convey these special impressions 
 are not able to receive those of a " common " kind : thus, the 
 Eye, however well fitted for seeing, would not feel the touch 
 of the finger, if it were not supplied with branches from the 
 5th pair, as well as by the optic nerve. .Nor can the different 
 nerves of special sensation be affected by impressions that are 
 adapted to operate on others : thus, the ear cannot distinguish 
 the slightest difference between a luminous and a dark object; 
 nor could the eye distinguish a sounding body from a silent 
 one, except by seeing its vibrations. But Electricity possesses 
 the remarkable power, when transmitted along the several 
 nerves of special sense, of exciting the sensations peculiar to 
 each; and thus, by proper management, this single agent 
 may be made to produce flashes of light, distinct sounds, 
 a phosphoric odour, a peculiar taste, and a pricking feeling, 
 in the same individual at one time. The inference which 
 might hence be drawn that Electricity and Nervous agency 
 are identical is nevertheless premature, as will be shown 
 hereafter ( 585). 
 
390 TACTILE SENSIBILITY OP THE SKIN. 
 
 Sense of Touch. 
 
 490. By the sense of Touch is usually understood that 
 modification of the common sensibility ( 487) of the body, of 
 which the surface of the skin is the especial seat. In some 
 animals, as in Man, nearly the whole exterior of the body is 
 endowed with it in no inconsiderable degree ; but in others, 
 as in the larger number of Mammals, most Birds and Eep- 
 tiles, and many Fishes, the greater part of the body is so 
 covered by hairs, scales, or bony plates, as to be nearly insen- 
 sible ; and the faculty is restricted to particular portions of 
 the surface, which often possess it in a very high degree. 
 The sensory impressions, by which we receive the sensation of 
 Touch, are made by the objects themselves upon the nerves 
 which are distributed to the Skin ; the general structure of 
 which has been already described ( 36 38). Of the papillce 
 which are thickly set upon many parts of its surface, some 
 contain looped tufts of blood-vessels without nerves ; and as 
 these are found to be largest where the Epidermis is thickest 
 (as, for example, in the pads on the under side of the Dog's 
 foot), it seems obvious that they minister, not to sensation, 
 but to the nutrition of that protective coating ( 492). But 
 in other papillae the blood-vessels are comparatively scanty, 
 their interior being chiefly occupied by little cushions of con- 
 densed areolar substance to which the sensory nerves proceed ; 
 and as their Epidermic coating is thin, and as the degree 
 of sensibility of any part of the skin bears a close correspond- 
 ence to the number of these papillae which are included 
 within a given area of its surface, it can scarcely be doubted 
 that they are the special instruments of the sense of Touch. 
 
 491. The true skin, or Cutis ( 37), from which alone 
 leather is prepared, is thicker in most Mammals than in 
 Man ' } but the thickness of the skin does not by any means 
 involve (as is commonly supposed) deficient sensibility. Thus, 
 in the Spermaceti Whale it has been observed that the 
 sensory nerves, which are destined to be distributed on the 
 skin, pass through the blubber without giving off any con- 
 siderable branches, but spread out into a network of extreme 
 minuteness as soon as they arrive near the surface. It is 
 a fact well known to Whale-fishers, especially to those who 
 pursue this species, that these animals have the power of 
 
EPIDERMIC PROTECTION. OTHER ORGANS OF TOUCH. 391 
 
 communicating with each other at great distances. It has 
 often been observed, for instance, that, when a straggler is 
 attacked, at the distance of several miles from a " school," a 
 number of its fellows bear down to its assistance in an' almost 
 incredibly short space of time. It can scarcely be doubted 
 that the communication is made through the medium of the 
 vibrations of water, excited by the struggles of the animal, or 
 perhaps by some peculiar movements specially adapted for 
 this purpose, and propagated through the liquid to the 
 immense surface of the skin of the distant Whales. 
 
 492. The sensibility of the true skin would be too great, if 
 it were not protected by the Epidermis ( 38), the thickness 
 of which varies considerably, according as the part is to be 
 endowed with acute sensibility, or to be protected from impres- 
 sions of too strong a nature. Thus it is particularly thin at the 
 ends of the fingers, and on the surface of the lips, which are 
 used for feeling; but is thick on the palm of the hand, which 
 is used for firmly grasping, and which would be continually 
 suffering pain if its sensibility were too acute ; and it is still 
 thicker on the sole of the foot, especially on the heel and the 
 ball of the great toe, where pressure has to be sustained. 
 
 493. Although the fingers of Man and of the Quadrumana, 
 being endowed with peculiar sensibility, are their special organs 
 of touch, yet we find that they cease to be so in most of the 
 other Mammalia, whose extremities are adapted only for sup- 
 port and locomotion, and are terminated by hard claws or 
 hoofs that completely envelop them. In many of these, we 
 find the lips and tongue employed as the chief organs of touch ; 
 in the Elephant, this sense is evidently possessed very acutely 
 by the little finger-like projection at the end of its trunk ; and 
 in several other cases the mbrissce or whiskers are its special 
 instruments, the bulbs of their long stiff hairs being copiously 
 supplied with sensory nerves. 
 
 494. A curious modification of the sense of Touch appears 
 to exist in Bats. It has been found that these animals, 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 through places with which they 
 were previously unacquainted, without striking against the 
 objects near which they passed. The same result happened 
 
392 IMPROVEMENT OF TOUCH BY EXERCISE. 
 
 when threads were stretched in various directions across the 
 apartment. Hence some Naturalists were inclined to attribute 
 to the Bat the possession of a sixth sense unknown to Man ; 
 but Cuvier correctly pointed out that this idea becomes un- 
 necessary, if we attribute to the delicate membrane of the 
 wings (as we are justified in doing) a high degree of tactile 
 sensibility, so as to receive impressions from the pulses of the 
 air that are produced by the action of the wings and modified 
 by the neighbourhood of solid bodies. 
 
 495. The only idea communicated to our minds by the sense 
 of Touch, when this is exercised in its simplest form, is that of 
 resistance; and we cannot form a notion either of the size or 
 shape of an object, or of the nature of its surface, by feeling 
 it, unless we move the object over our own sensory organ, or 
 move the latter over the former. By the various degrees of 
 resistance which we encounter, we estimate the hardness or 
 softness of the body ; and by the impressions made upon the 
 papillae, when they are moved over its surface, we form our 
 idea of its smoothness or roughness. It is by attention to the 
 muscular movements we execute, in passing our hands or 
 fingers over its surface, that we acquire our ideas of its size 
 and figure; and hence we perceive that the sense of touch, 
 without the power of moving the tactile organ over the object, 
 would have been- of comparatively little use. 
 
 496. This sense is capable of improvement to a remarkable 
 degree ; as we see in persons who have become more dependent 
 upon it in consequence of the loss of their sight. This doubt- 
 less results, in part, from the increased attention which is 
 given to the sensations ; and partly from the greater acuteness 
 or impressibility of the organ itself, arising from the frequent 
 use of it. Amongst other remarkable instances of this kind 
 was that of Saunderson, who, though he lost his sight at two 
 years old, acquired such a reputation as a mathematician, that 
 he obtained a Professorship at Cambridge. He exhibited, in 
 several ways, an extraordinary acuteness in his touch ; but 
 one of his most remarkable faculties was the power of distin- 
 guishing genuine .medals from imitations, which he could do 
 more accurately than many connoisseurs in full possession of 
 their senses. 
 
 497. The sense of temperature is of a different character 
 from common tactile sensibility; and either may be lost, 
 
SENSE OF TEMPERATURE. ANTENNA OF INSECTS. 393 
 
 without the other being affected. It is rather of a comparative 
 than of a positive kind ; that is, we form our estimate of tem- 
 perature rather by comparing it with that to which our body 
 (or the part of it employed to test the heat or cold) has been 
 previously exposed, than by any knowledge which we derive 
 through the sensation as to the actual degree of heat or cold 
 to which the organ is exposed. Thus, if we plunge one hand 
 into a basin of hot water, and the other into cold, and then 
 transfer both of, them to a basin of tepid water, this will feel 
 cold to the hand which has been previously accustomed to 
 the heat, and warm to the other. In the same manner, the 
 temperature of Quito, which is situated half-way up a lofty 
 mountain, is felt to be chilly by a person who has ascended 
 from the burning plains at its base, whilst it seems intensely 
 hot to another who has descended from its snow-capped sum- 
 mit ; the residents in the town at the same time regarding it 
 as moderate, neither hot nor cold. It is a curious circum- 
 stance, that a weak impression made on a large surface seems 
 more powerful than a stronger impression made on a small 
 surface ; thus, if the fore-finger 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. 
 
 498. Where any special organs of Touch exist in Inverte- 
 brated Animals, they are for 
 the most part prolongations 
 from the portion of the head 
 near the mouth. This is 
 the case with the arms of 
 the Cuttle-fish, and with the 
 tentacula of the lower Mol- 
 lusca which are similar in 
 position. Among Crustacea 
 and Insects, the antennae or 
 feelers (fig. 198, a, a) appear 
 to be the special organs of 
 touch. These are frequently Fig - ^.-CAPRICORN-BEETLE. 
 very long, and present an extraordinary variety in their forms, 
 of which some are depicted in fig. 199. They contain, 
 for the most part, a large number of joints (in the Mole- 
 
394 
 
 ANTENNAE OF INSECTS. 
 
 Cricket above 100), and are very flexible. This flexibility 
 enables them to be turned towards any object under examina- 
 tion by the Insect ; and when the animal is walking, we see 
 them constantly being applied to the surfaces of the bodies 
 which it approaches, in a manner which leaves little doubt 
 that they are used as organs of touch. It is no objection to 
 this view, to say that, as their surfaces are hard, no delicate 
 sensations can be received through them; for the slightest 
 
 Fig. 199. VARIOUSLY-FORMED ANTENNJE of INSECTS. 
 
 contact of their firmest points with a hard substance, may 
 produce a sense of resistance which will afford to the animal 
 the information which it requires. The stick used by the 
 blind man in feeling his way, serves a very similar purpose. 
 It appears to be by sensations received through their 
 antennae, that Bees and other Insects which naturally work 
 in the dark, are enabled to carry-on their labours without 
 confusion or inaccuracy ; and to be by the same means, that 
 they communicate with each other. When the antennae are 
 cut off, the Bee at once ceases to work, and seems unable to 
 direct its movements in any other way than towards the light. 
 When any important event has happened in a community, 
 such as the loss of the Queen, the spreading of the intelligence 
 through the whole hive may be watched by a close observer. 
 The working bees which were near her are observed to run 
 about restlessly, applying their antennae to those of the others 
 they may meet, crossing them and striking lightly with them ; 
 these in their turn become agitated and do the same ; and 
 thus the intelligence is speedily propagated throughout the 
 hive. In the same manner, when two bees meet each other 
 out of their hives, they seem to reconnoitre one another for 
 some time by the movements of their antennae; and often 
 
SENSE OP TASTE I PAPILLA OF TONGUE. 395 
 
 keep up these movements for a considerable period, as if 
 carrying on a close conversation. That the Antennae are 
 delicate organs of Touch can, therefore, be scarcely questioned. 
 
 Sense of Taste. 
 
 499. The sense of Taste, like that of touch, is excited by 
 the direct contact of particular substances with certain parts 
 of the body ; but it is of a much more refined nature than 
 touch, inasmuch as it communicates to us a knowledge of 
 properties which that sense would not reveal to us. All sub- 
 stances, however, do not make an impression on the organ of 
 taste. Some of them have a strong savour, others a slight 
 one, and others again are altogether insipid. The cause of 
 these differences is not understood ; but it may be remarked 
 that, in general, bodies which cannot be dissolved in water 
 have no savour ; whilst most of those which are soluble have 
 a taste more or less strong. Their solubility, in fact, seems 
 to be one of the conditions requisite for their action on the 
 organ of taste ; for when that organ is completely dry, it does 
 not receive any sensation from solid bodies brought into con- 
 tact with it, which may have the most powerful taste if re- 
 duced to a fluid form ; and there are substances known, which, 
 being perfectly insoluble in water, are insipid if applied to 
 the tongue when it is covered as usual with a watery secre- 
 tion ; but which have a strong taste when they are dissolved 
 in some other liquid, spirit of wine for instance. 
 
 500. The sense of Taste has for its chief purpose, to direct 
 animals in their choice of food ; hence its organ is always 
 placed at the entrance to the digestive canal. In the higher 
 animals, the tongue is the principal seat of it ; but other 
 parts of the mouth are also capable of receiving the impres- 
 sions of certain savours. The mucous membrane which covers 
 the tongue is copiously supplied with blood-vessels ; and is 
 thickly set, especially upon its upper surface, and towards the 
 tip, with papillae, resembling in structure those of the skin, 
 but larger. These papillae, however, are not all sensory ; for 
 some, which are of conical form, are covered with a firm horny 
 epithelium, and their function seems to be chiefly mechanical. 
 These " conical " papillae are very strongly developed in the 
 tongues of many of the lower Mammalia, to which they impart 
 a particular roughness ; thus it is by their means that a Dog 
 
396 PAPILLA OF TONGUE : NATURE OF SENSE OF TASTE. 
 
 cleans of all its flesh the bone he licks, and that the Lion, by 
 a single stroke of his tongue, can take off" the skin from any 
 part of the Human body. The tongue itself is made-up of 
 muscular substance, which accomplishes the varied move- 
 ments that are required in the acts of mastication and in the 
 production of articulate sounds. It is supplied with nerves 
 from the third division of the fifth pair, from the glosso- 
 pharyngeal, and from the hypoglossal ( 459). The last is 
 the motor nerve of the tongue ; the first is the one chiefly 
 concerned in the conveyance of sensory impressions from the 
 front and sides of the tongue ; and the other (the glosso- 
 pharyngeal) seems, to have for its office to convey those im- 
 pressions from the back of .the tongue which excite the muscles 
 of swallowing to action ( 470), as well as those which produce 
 the sensation of nausea and excite the act of vomiting. The 
 gustative papillae, which have a very thin epithelial covering, 
 are for the most part supplied from the fifth pair ; and the 
 branch of this proceeding to the tongue is known as the 
 lingual nerve. When they are called into action by the con- 
 tact of substances having a pleasant savour, they not unfre- 
 quently become very turgid, and rise-up from the surface of 
 the mucous membrane ; in this manner is produced the rough- 
 ness that is felt on the surface of any portion of the tongue or 
 inside of the cheek, against which a piece of barley-sugar or 
 other similar substance has lain for some little time. 
 
 501. A considerable part of the impression produced by 
 many substances, is received through the sense of Smell rather 
 than by that of Taste. Of 4his any one may convince him- 
 self by closing the nostrils and breathing through the mouth 
 only, whilst holding in the mouth, or even rubbing between 
 the tongue and the palate, some aromatic substance ; its taste 
 is then scarcely recognised, although it is immediately per- 
 ceived when the nasal passages are re-opened, and its effluvia 
 are drawn into them. There are many substances, however, 
 whose taste, though not in the least dependent upon the 
 action of the nose, is nevertheless of a powerful character ; 
 such are sugar, salt, quinine, and vinegar. Others, again, 
 by irritating the mucous membrane, produce a sense of pun- 
 gency allied to that which the same substances (strong acids, 
 for instance, pepper, or mustard) will produce when applied 
 to the skin for a sufficient length of time. Such sensations, 
 
USES OF SENSE OF TASTE. 397 
 
 therefore, are evidently of the same kind with those of Touch, 
 differing from them only in the degree of sensibility of the 
 organ through which they are received ; and through these the 
 sense of Taste is more nearly related to that of Touch, than is 
 either of the other forms of special sensibility. 
 
 502. This sense has a very important function in most 
 animals which possess it, that of directing them in their 
 choice of food. Most of the lower animals will instinctively 
 reject the articles of food that would be pernicious to them ; 
 thus even the voracious Monkey will seldom touch fruits of a 
 poisonous character, though their taste may be agreeable ; and 
 animals whose digestive apparatus is adapted to one kind of 
 food, will reject all others. It may be stated, as a general 
 rule, that substances of which the taste is agreeable to us 
 are useful and wholesome articles of food, and vice versd; but 
 there are many signal exceptions to this. It is interesting to 
 remark that in Man, when the reasoning powers are obscured 
 by disease, his instincts in regard to food often manifest them- 
 selves strongly, and frequently constitute the best guide in its 
 administration ; thus, there are many cases of fever in which 
 the physician is in doubt whether wine will be injurious or 
 beneficial, and in which he will usually find the disposition of 
 the patient to reject it, or his readiness to receive it, to be tiis 
 best guide. And in general it may be remarked that, in ill- 
 ness, the desire of the patient for food, or his disposition to 
 take it, pretty certainly indicate the fitness or unfitness of the 
 system to digest and appropriate it. 
 
 503. The tongue presents nearly the same structure among 
 the Mammalia in general, as in Man ; but in Birds it is 
 usually cartilaginous or horny in its texture, and destitute of 
 nervous papillae, so that the sense of taste cannot be very 
 acute in any of those animals. Several of them use their 
 tongues for other purposes, the Woodpecker, for instance, 
 to transfix insects, and the Parrot to keep steady the nut or 
 seed which is being crushed between the jaws. In some 
 Reptiles the tongue is large and fleshy; in others long and 
 slender ; in others, again, it is almost entirely deficient : but 
 in no instance does it seem to minister to any acute sense of 
 taste. In Fishes it is for the most part absent. Many In- 
 vertebrated animals possess a tongue ; but its uses are for the 
 most part mechanical. Thus the tongue of the Limpet is a 
 
398 
 
 SENSE OF SMELL : ODOROUS SUBSTANCES. 
 
 powerful rasp (resembling that in fig. 107), by which it rubs 
 down the sea- weeds on which it feeds ; whilst the tongue of 
 the Bee (fig. 289) forms a channel through which it draws-up 
 the juices of flowers. In most Insects, the palpi, small jointed 
 appendages in the neighbourhood of the mouth ( 172), seem 
 to answer the purpose of an organ of taste ; being observed 
 to be in incessant motion whilst the animal is taking food, 
 touching and examining it before it is introduced into the 
 mouth. 
 
 Sense of Smell. 
 
 504. Certain bodies possess the property of exciting in us 
 sensations of a ^peculiar nature, which cannot be perceived by 
 the organs of taste or touch, and which seem to depend upon 
 emanations that spread from them through the air, pro- 
 ducing what we term odours. It appears probable that odours 
 are, in reality, very finely-divided particles of the odoriferous 
 substance ; and this idea derives support from the fact that 
 most volatile bodies are more or less odorous, whilst those 
 which do not readily transform themselves into vapour, have 
 little or no fragrancy in their natural state, but possess strong 
 odorous properties as soon as they are converted into vapour 
 by the aid of heat, for instance. The most powerful and 
 penetrating odours are for the most part those of bodies 
 already in a gaseous state, such as sulphuretted and carbu- 
 retted hydrogen; or of fluids which readily pass into the 
 state of vapour, as the vegetable essential oils. But there are 
 some solid substances, as musk, which are very strongly 
 odorous ; and which yet do not appear to diffuse themselves 
 through the air in the state of vapour. The odoriferous 
 particles of these must be of extreme minuteness ; for the 
 substances do not seem to lose weight by freely imparting 
 their peculiar scent to an unlimited quantity of air. Thus the 
 experiment has been tried, of keeping a grain of musk freely 
 exposed to the air of a room of which the doors and windows 
 were constantly open, for a period of ten years ; the air, thus 
 continually changed, was completely impregnated with the 
 odour of musk ; and yet at the end of that time, the particle 
 was not found to have suffered any perceptible diminution in 
 weight. 
 
 505. In order that we should become sensible of odours, it 
 
ODOEOUS SUBSTANCES : STRUCTURE OF NOSE. 
 
 399 
 
 seems necessary that the odoriferous particles should come into 
 actual contact with the membrane on which the nerve of smell 
 is spread out. In this respect, the sense of Smell agrees with 
 the senses of taste and touch ; whilst it differs from those of 
 sight and hearing, which take cognisance of changes that are pro- 
 duced by vibrations or undulations in the surrounding medium. 
 It is, moreover, desirable that these odoriferous particles should 
 be conveyed by the air, and not be diffused through fluid ; 
 for though it is necessary to the perfection of the sense of 
 smell that the olfactory membrane should be kept moist, too 
 great a quantity of fluid upon its surface deadens its peculiar 
 sensibility, as we find to be the case when we are suffering 
 under an ordinary " cold." Hence it is only in air-breathing 
 animals, that the sense of Smell can possess any considerable 
 acuteness. 
 
 506. The most advantageous position of this organ is evi- 
 dently at the commencement of the respiratory passages j so 
 that the air which is being re- 
 ceived into the lungs may pass 
 through it and be tested (as it 
 were) by its peculiar sensibility. 
 In all the air-breathing Verte- 
 brata we find a pair of cavi- 
 ties, the nasal fossae (fig. 200), 
 which are situated between the 
 mouth and the orbits. They 
 possess two orifices, the ante- 
 rior nares, or nostrils (6), usually 
 opening upon the front of the 
 face, and the posterior nares, 
 which open into the upper part 
 of the pharynx (c). The two 
 cavities are separated from each 
 other by a vertical partition, 
 which passes backwards and 
 forwards on the middle line ; their sides are formed by the 
 various bones of the face, and by the cartilages of the nose ; 
 their extent is very considerable, especially in animals that 
 have a prolonged muzzle. The interior of these cavities is 
 lined by a delicate mucous membrane, whereon the Olfactory 
 nerves, which enter through a multitude of minute orifices in 
 
 Fig. 200. VERTICAL SECTION OP THE 
 NASAL CAVITY. 
 
 a, mouth; b, nostril; c, posterior open- 
 ing ; d, portion of the base of the 
 skull ; e, forehead ; /, h, passages be- 
 tween the spongy bones, g, i, k; I, 
 frontal sinus; m," sphenoidal sinus; 
 w.opening of Eustachian tube ; o, cur- 
 tain of the palate. 
 
400 SENSE OF SMELL. 
 
 the roof of the cavity, are distributed ; and the extent of its 
 surface is increased, by its being folded over certain projec- 
 tions from the walls of the cavity, which are termed spongy 
 bones. Of these there are three in Man (<jr, i, k). Prolonga- 
 tions of this membrane are carried also into cavities hollowed 
 out in the neighbouring bones, which are termed sinuses. 
 Thus we have the frontal sinuses I, situated just above the 
 nose, between the eyebrows ; and the sphenoidal sinuses m, 
 situated further back. There is also a large cavity hollowed 
 out in the bone of the upper jaw on either side. The mem- 
 brane which lines these is kept moist by its own secretion ; 
 and it is covered with vibratile cilia, the office of which seems 
 to be to prevent that secretion from unduly accumulating, by 
 conveying it over the surface of the membrane to the outlet. 
 
 507. The mechanism of the sense of Smell is very simple. 
 When air charged with odoriferous particles passes over the 
 membrane that lines the nose, some of these particles are 
 delayed by the mucous secretion that covers it, and act upon 
 the delicate sensory extremities of the olfactory nerve with 
 which it is thickly set. The highest part of the nasal cavity 
 appears to be that in which there is the most acute sensibility 
 to odours ; and hence it is that when we snuff the air, so as 
 to direct it into the upper part of the nose, instead of allowing 
 it to pass simply along the lower portion from the anterior to 
 the posterior nares, we perceive delicate odours which would 
 have otherwise escaped us. The acuteness of the sense of 
 smell depends, in no small degree, upon the extent of surface 
 exposed by the membrane lining the nasal cavity; and in this 
 respect Man is far surpassed by many of the lower Mammalia, 
 especially among the Carnivora, Euminantia, and some Pa- 
 chydermata. The extreme delicacy of this sense in Deer, 
 Antelopes, &c., is well known, from its being a source of great 
 difficulty to the hunter, who cannot advance near enough to 
 attack them, except by stealing upon them in the direction 
 contrary to that of the wind. In these animals it serves as 
 the chief means by which they are warned of the proximity 
 of their enemies j in the Carnivora, on the other hand, it 
 serves to direct them to their prey. In general, however, it 
 seems to have for its office to assist in directing animals to 
 their food, and in warning them of the presence of noxious 
 vapours. 
 
SENSE OF SMELL. 401 
 
 508. Besides receiving the Olfactory nerve, the mucous mem- 
 brane of the nose is supplied by branches of the Fifth pair ; 
 this nerve endows it with common sensibility, and also receives 
 the impressions produced by acrid or pungent vapours, which 
 act upon it in the same way as the corresponding fluids do 
 upon the tongue. Such vapours are felt by the irritation they 
 produce, rather than smelt; and the impression they occasion 
 gives rise to the reflex action of sneezing, by which they are 
 driven from the nose ( 342). Hence this action may be 
 excited by an irritating agent (such as snuff) after the olfac- 
 tory nerve has been divided, if the branches of the fifth pair 
 be entire : whilst it does not take place when the fifth pair is 
 paralysed, even though the sense of smell may be retained. 
 This sense loses much of its acuteness, however, when the 
 branches of the fifth pair supplying its organ can no longer 
 discharge their functions ; for the membrane then becomes 
 dry from the want of its proper secretion, and the odoriferous 
 particles are consequently not properly applied to it. 
 
 509. Among animals that live in water, the olfactory organs 
 cannot act to the like advantage ; and we do not find much 
 provision made for this sense. In the Whale tribe, the nostrils 
 serve as the channels by which the water is expelled, that has 
 been drawn-in through the mouth ( 185) ; they are situated 
 at the top of the head, and are known as blow-holes. In 
 Fishes, the nasal cavity has no posterior opening; but the 
 surface of its lining membrane is very much extended by its 
 arrangement in folds, which are sometimes disposed in a 
 radiated manner around a centre, and sometimes parallel like 
 the teeth of a comb. There are many Invertebrated Animals, 
 from whose actions it may be judged that they possess a deli- 
 cate sense of smell, although the precise seat of it cannot be 
 assigned. This is the case especially with Insects, Crustacea, 
 and the higher Mollusca. The lining membrane of the air- 
 tubes of Insects appears to be delicately sensitive to irritating 
 vapours ( 443) ; but we have no evidence that it ministers 
 to the sense of Smell properly so called. 
 
 Sense of Hearing. 
 
 510. By this sense we become acquainted with the Sounds 
 produced by bodies in a certain state of vibration. The vibra- 
 tions which sonorous bodies undergo, are communicated by 
 
 D D 
 
402 
 
 TRANSMISSION OF SONOROUS VIBRATIONS. 
 
 them to the air, producing in it a series of undulations or 
 waves, by which the sound is conveyed to a great distance. 
 These undulations spread more widely as they become more 
 distant from the sounding body, just like the ripples produced 
 on the surface of the water when we throw a stone into it ; 
 and in proportion as they spread, they become less powerful. 
 This is the reason why Sound becomes less intense as the 
 sounding body is more distant. Although: air is the usual 
 conducting medium for the sonorous undulations, liquids or 
 solids may answer the same purpose. Thus if a person hold 
 his head under water, whilst two stones are struck together, 
 also under water, even at a considerable distance, he will hear 
 the sound produced by the blow with extreme distinctness, 
 and even with painful force. Or if the ear be laid against 
 one end of a long piece of timber, whilst a scratch with a pin 
 be made on the other, or a watch be laid upon it, even the 
 faint sounds thus produced will be heard very distinctly. 
 That a medium of some kind is necessary to convey the 
 sonorous vibrations, is proved by the fact, that if a bell be 
 made to ring in the receiver of an air-pump from which the air 
 has been exhausted, no sound is heard, though ..its ringing 
 becomes audible as soon as the air is allowed to re-enter. 
 
 511. It, is a fact of much importance, in regard to the 
 action of the organ of Hearing, that sonorous vibrations which 
 have been excited and are being transmitted in a medium of 
 one kind, are not imparted with the same readiness to others. 
 The following conclusions have been drawn from experi- 
 mental inquiries on this subject. I. Vibrations excited in 
 solid bodies may be transmitted to water without much loss 
 of their intensity, although not with the same readiness that 
 they would be communicated to another solid, n. On the 
 other hand, vibrations excited in water lose something of 
 their intensity in being propagated to solids ; but they are 
 returned, as it were, by these solids to the liquid, so that the 
 sound is more loudly heard in the neighbourhood of those 
 bodies, than it would otherwise have been. in. The sonorous 
 vibrations of solid bodies are much more weakened by trans- 
 mission to air; and those of air make but little, impression 
 on solids, iv. Lastly, sonorous vibrations in water are trans- 
 mitted but feebly to air ; and those which are taking place in 
 air are with difficulty communicated to water ; but the com- 
 
TRANSMISSION OP SONOROUS VIBRATIONS. 403 
 
 munication is rendered muck more easy by the intervention 
 of a membrane extended between them. 
 
 512. The Auditory nerve, or nerve of Hearing, is adapted 
 to receive and transmit to the brain the sonorous undulations 
 produced in the surrounding medium by vibrating bodies. 
 Now, it is obvious that it may be affected by these in various 
 ways, especially in animals that inhabit the water. The 
 vibrations excited in the liquid will be transmitted to the 
 solid parts of the head, and thence to the nerve contained in 
 it, without much interruption ; and this independently of any 
 special apparatus of hearing. Indeed, the simplest form of 
 this apparatus is only designed to give increased effect to the 
 vibrations thus excited in the solid parts of the head ; for it 
 consists merely of a cavity excavated in their thickness, which 
 cavity is filled with fluid, and is lined by a membrane whereon 
 the auditory nerve is minutely distributed. This is the con- 
 dition of the organ of hearing in the Mollusca, where any such 
 exists j and also in many of the Crustacea. In those of the 
 latter class which chiefly inhabit air, however, this cavity is 
 excavated in the surface of the shell covering the head, and is 
 shut-in by a membrane which is exposed to the surrounding 
 medium. According to the principle (iv.) mentioned in the 
 last paragraph, the liquid contained in the chamber will be 
 thrown into undulation by vibrations in air, as well as by 
 those of water; so that those animals which possess this kind 
 of apparatus are able to hear much better in air, than are those 
 in which the cavity is completely shut-in by stony walls. Of 
 the degree in which sonorous vibrations may be communicated 
 to our own auditory nerves through the solid parts of the 
 skull, we may easily satisfy ourselves by closing the ears 
 carefully, and placing any part of the head against a solid 
 body which communicates with the one in vibration. In this 
 manner we may hear the sounds produced by the latter with 
 considerable distinctness, though accompanied by an unpleasant 
 jarring. A deaf gentleman was once agreeably surprised 
 to find that, when smoking his pipe, with the bowl resting on 
 his daughter's pianoforte, he could distinctly hear the music 
 she was producing from it ; and many deaf persons may be 
 made to hear conversation, by holding a piece of stick 
 between their own teeth, and placing it against the teeth of 
 the person speaking. 
 
 DD 2 
 
404 STRUCTURE OF ORGAN OF HEARING. 
 
 513. In animals which have the organ of hearing con- 
 structed upon the simple plan just described, the force of the 
 vibrations of the fluid contained in the cavity is increased by 
 several minute stony concretions suspended in it : these act 
 according to the second principle just stated ( 511). They 
 are termed otolithes, or ear-stones ; and some traces of them 
 may be found even in Man and the higher animals. 1 
 
 514. We see, then, that a cavity excavated in the solid 
 walls of the head, covered-in externally by a membrane, 
 having the auditory nerve distributed upon its walls, and 
 filled with fluid, is the simplest form of the organ of hearing ; 
 and may be regarded as including all that is essential to the 
 exercise of this function. No more complicated apparatus is 
 to be found in any of the Invertebrata ; and even in the 
 lowest Fishes there is but little variation from this type. On 
 the other hand, in Man and the higher Vertebrata we find a 
 very complex structure, adapted to render the faculty much 
 more perfect ; enabling us to receive impressions which make 
 us. aware, not only of the presence of a sounding body, but of 
 its nature, its direction, the pitch and peculiar quality of the 
 sound ; and also, it is probable, taking cognizance of sounds 
 much fainter than those which would be perceptible to the 
 lower animals. Yet even in the most complicated forms of 
 the organ of hearing, we shall find that the essential part is 
 still the same as that which forms the whole organ in the 
 lower tribes ; and also that the faculty is retained, though in 
 an inferior degree, when by disease or injury the accessory 
 parts are prevented from acting. To the structure of the Ear 
 of Man we shall now proceed. 
 
 515. The organ of hearing in Man may be divided into 
 three parts the external, the middle, and the internal ear. The 
 former is the fibro-cartilaginous appendage placed on the out- 
 side of the head, to receive and collect the sounds which are 
 to be transmitted to the interior ; the two latter divisions are 
 excavated in a bone of remarkable solidity, the petrous (stony) 
 portion of the temporal bone. The uses of the different 
 
 1 Vesicles containing otoliths which are kept in rapid movement 
 within them by ciliary action, are found in immediate contiguity with 
 the cephalic ganglia of the lower Mollusks, or are even imbedded in 
 their substance; and these seem to constitute the most rudimentary 
 form of an organ of hearing. 
 
EXTERNAL EAR : TYMPANIC CAVITY. 405 
 
 hollows and elevations on the surface of the external ear of 
 Man are not very apparent; but it is probable that they 
 direct the sonorous undulations towards the entrance of the 
 canal which leads to the middle ear. The form of the external 
 ear in many Quadrupeds evidently adapts it to this purpose ; 
 and there are several which have the power of changing its 
 direction by muscular action, in such a manner as to enable it 
 to catch most advantageously the faintest sounds from any 
 quarter. This is especially the case with animals that are 
 naturally timorous, such as the Hare or the Deer ; these have 
 usually very large external ears. But it is among the Bat 
 tribe whose residence in the dark recesses of caverns and 
 excavations makes their eyes of comparatively little use to 
 them, and causes them to depend greatly for guidance in their 
 movements upon the sense of hearing that we find the 
 greatest development of the external ear (fig. 201). 
 
 Fig. 201. -LONG-EARED BAT. 
 
 516. The canal hollowed -out in the temporal bone (d, 
 fig. 204) into which the external ear collects the sonorous 
 vibrations, passes inwards until it is terminated by a mem- 
 brane stretched across it, which is called the membrana tym- 
 panij or membrane of the drum of the ear (g). This forms 
 the outside wall of a cavity (h) which constitutes the middle 
 ear, and which is bounded on the inside by a bony wall that 
 separates it from the internal ear. The cavity of the tym* 
 panum is not one of the essential parts of the organ ; for 
 nothing analogous to it exists either in Fishes or in the lower 
 Eeptiles. It contains air ; and communicates with the back 
 
406 TYMPANIC CAVITY AND CHAIN OF BONES. 
 
 of the nasal cavity (n, fig. 200) by a canal termed the Eusta- 
 chian tube (k, fig. 204). The partial or complete closure of 
 this tube, occasioned either by swelling of its lining membrane 
 or by the viscid secretion from it, produces the slight deafness 
 common among those who are suffering from " colds." Within 
 the cavity of the tympanum, there is a very curious apparatus 
 of small bones and muscles, which serves to establish a con- 
 nexion between the membrane of the drum and the small 
 membrane covering the entrance to the internal ear. These 
 bones are four in number ; and are termed the malleus or 
 
 Fig. 202. BONES OF Fig. 203. CAVITY OF THE TYMPANUM, WITH 
 
 THE EAR. THE BONES IN THEIR PLACES. 
 
 hammer (a, fig. 202) ; the incus, or anvil (b) ; the os orbicu- 
 lare, a minute globular bone (c); and the stapes, or stirrup- 
 bone (d). These bones are connected together in the manner 
 represented in fig. 203 ; where a a represents the wall of the 
 tympanic cavity ; 5, the membrana tympani ; c, one of the 
 long processes of the malleus, which is attached to the mem- 
 brane ; d, the head of the malleus, which articulates with the 
 incus ; e, the other long process of the malleus, which is 
 acted-on by the minute muscle f, that serves to tighten the 
 tympanum ; g, the incus, of which one leg is in contact with 
 the wall of the cavity, whilst the other is connected with the 
 orbicular bone h ; i, the stapes, of which the upper end is 
 connected with the orbicular bone, whilst the lower (which is 
 of an oval form) is attached to the membrane that covers the 
 entrance to the internal ear ; and Tc is a small muscle which 
 
ACTION OF THE TYMPANIC APPARATUS. 
 
 407 
 
 acts upon this bone in such a manner as to relax the tym- 
 panum. 
 
 517. The us of this apparatus is evidently to receive the 
 sonorous vibrations from the air, and to transmit them to the 
 membrane forming the entrance to the internal or essential 
 part of the organ of hearing ; in such a manner, that the 
 sonorous vibrations excited in the latter may be much more 
 powerful than they would be if the air acted immediately 
 upon it. The usual state of the membrane of the tympanum 
 appears to be rather lax or slack ; and when in this condition, 
 
 Je h g q r f s b 
 
 Fig. 204. VERTICAL SECTION OP THE ORGAN OP HEARING IN MAN. 
 
 The internal portions are proportionately enlarged to make them more evident: 
 a, b, c, the external ear ; d, entrance to the auditory canal /; e, e, petrous portion 
 of the temporal bone, in which the internal ear is excavated ; g, membrane of the 
 tympanum ; h, cavity of the tympanum, the chain of bones being removed ; 
 , openings from the cavity into the cells j excavated in the bone ; on the side 
 opposite themembrana tympani are seen the fenestra ovalis and rotunda;"*, Eusta- 
 chian tube ; I, vestibule ; m, semicircular canals ; n, cochlea ; o, auditory nerve ; 
 p, canal by which the carotid artery enters the skull ; g, part of the glenoid fossa 
 which receives the head of the lower jaw ; r, styloid process of the temporal bone. 
 
 it vibrates in accordance with grave or deep tones. By the 
 action of a small muscle lodged within the Eustachian tube, it 
 
408 TYMPANIC APPARATUS : INTERNAL EAR. 
 
 may be tightened, so as to vibrate in accordance with sharper 
 or higher tones ; but it will then be less able to receive the 
 impressions of deeper sounds. This state we may artificially 
 produce either by holding the breath and forcing air into the 
 Eustachian tube, so as to make the membrane bulge-out by 
 pressure from within ; or by exhausting the cavity by an effort 
 at inspiration with the mouth and nostrils closed, which will 
 cause the membrane to be pressed inwards by the external 
 air. In either case the hearing is immediately found to be 
 imperfect ; but it will be observed that while the experimenter 
 thus renders himself deaf to grave sounds, acute sounds are 
 heard even more distinctly than before. There is a different 
 limit to the acuteness of the sounds of which the ear can 
 naturally take cognizance, in different persons. If the sound 
 be so acute (or high in pitch) that the membrana tympani will 
 not vibrate in unison with it, the individual will not hear it, 
 although it may be loud ; and it has been noticed that some 
 persons cannot hear the very shrill tones produced by par- 
 ticular Insects, or even by Birds, which are distinctly audible 
 to others. There is good reason to think that the two 
 muscles which have been mentioned ( 516) as tightening 
 and relaxing the tympanum, exert a regulative influence upon 
 its tension analogous to that which the contractile fibres of 
 the iris possess in regard to the diameter of the pupil ( 534) ; 
 preparing it to be acted-on by faint sonorous undulations 
 when we are listening, and moderating the concussion of very 
 loud sounds which are anticipated. 
 
 518. The internal ear is composed of various cavities that 
 communicate with each other; of these the vestibule (I, fig. 204) 
 may be regarded as the centre, whilst from it there pass-off 
 on one side the three semicircular canals, m, and on the other 
 the cochlea, n. The vestibule is the part which corresponds 
 with the simple cavity that constitutes the whole organ of 
 hearing in the lower animals ( 512), and the others may be 
 regarded as extensions of it for particular purposes. It com- 
 municates with the cavity of the tympanum by a small orifice 
 in the bony wall that separates them, termed the fenestra ovalis 
 (oval window) ; but this orifice is closed by a membrane, to 
 which the lower end of the stapes is attached. The three 
 semicircular canals are passages excavated in the solid bone, 
 and lined by a continuation of the same membrane as that 
 
STRUCTURE OF THE INTERNAL EAR. 409 
 
 which lines the vestibule ; each passes-off from the vestibule 
 and returns to it again. The cochlea, n, also is a cavity exca- 
 vated in the hard bone, and lined by a continuation of the 
 same membrane ; its form is almost precisely that of the in- 
 terior of a snail-shell (whence its name), being a spiral canal 
 which makes about two turns and a half round a central pillar. 
 This canal is divided into two, however, by a partition that 
 runs along its whole length ; which partition is partly formed 
 by a very thin lamina of bone, and partly (in the living state) 
 by a delicate membrane. The two passages do not communi- 
 cate with each other except at the top or centre; at their 
 lower end (corresponding to the mouth of the snail-shell) they 
 terminate differently; for whilst one freely opens into the 
 vestibule, the other communicates with the cavity of the 
 tympanum, by an aperture termed the fenestra rotunda (round 
 window), which is closed by a membrane. 1 Thus the internal 
 ear communicates with the cavity of the tympanum by two 
 minute orifices only, the fenestra ovalis and the fenestra 
 rotunda, both of them closed by membranes, against the 
 former of which the stapes abuts, whilst the latter is free. 
 
 519. The whole internal ear is lined by a delicate mem- 
 brane, on which the auditory nerve (o, fig. 204) is very 
 minutely distributed, especially on the membranous portion 
 of the partition between the two passages of the cochlea. 
 The cavities are completely filled with fluid, which is set in 
 vibration by the movements of the stapes, communicated 
 through the membrane of the fenestra ovalis; and these vibra- 
 tions are probably rendered more free by the existence of the 
 second aperture the fenestra rotunda. It is by the influence 
 of these undulations upon the expanded fibrils of the auditory 
 nerve, that the sensation of sound is produced ; but in what 
 way the different parts of the labyrinth (as this complex 
 series of cavities is not unaptly called) contribute to the per- 
 formance of this function, is not yet known. In all Fishes 
 but the lowest, the three semicircular canals exist ; they have, 
 however, no vestige of a cochlea. In the true Eeptiles, a 
 rudiment of the cochlea may be generally discovered. In 
 Birds, this cavity is more completely formed, though the 
 passage is not spiral, but is nearly straight; of its real 
 
 1 There is a double spiral staircase constructed exactly on this plaa 
 in Tamworth church. 
 
410 FUNCTIONS OF THE INTERNAL EAR. 
 
 character, however, there can. be no doubt, from its being 
 divided, like the cochlea of Mammals and of Man, by a 
 membranous partition on which the nerve is spread out. 
 
 520. From the circumstance that in almost every instance 
 in which the semicircular canals exist at all, they are three in 
 number, and lie in three different directions, corresponding to 
 those of the bottom and two adjoining sides of 'a cube, it has 
 been supposed (and with much probability) that they assist 
 in producing the idea of the direction of sounds. It has been 
 also supposed that the cochlea is the organ by which we judge 
 of the pitch of sounds ; and this would seem to be not im- 
 probable, especially when we compare the development of the 
 cochlea in different animals, with the variety in the pitch of 
 the sounds which it is important they should hear distinctly, 
 especially the voices of their own kind. The compass of the 
 voice (that is, the distance between its highest and its lowest 
 tones) is much greater in Mammals than in Birds; as is also 
 the length of the cochlea. In Reptiles, which have little true 
 vocal power, the cochlea is reduced to its lowest form ; and in 
 the Amphibia, it disappears altogether. 
 
 521. That the Vestibule, and the passages proceeding from 
 it, constitute even in Man the essential part of the organ of 
 hearing, is evident from the fact, that when (as happens not 
 unfrequently) the membrana tympani has been destroyed by 
 disease, and the chain of bones has been lost, the faculty is 
 not by any means abolished, though it is deadened. In this 
 state, the vibrations of the air must act "at once upon the 
 membrane of the fenestra ovalis, as in the lower animals which 
 possess no external or middle ear; instead of striking the 
 membrane of the tympanum, and being transmitted along the 
 chain of bones. 
 
 522. It- has been stated ( 510) that the sensation of 
 hearing is produced by the successive undulations or vibra- 
 tions communicated to the Ear from the sonorous body, either 
 by the air, or by a liquid or solid medium. This is the case 
 with all continuous sounds or tones; but single momentary 
 sounds, such as those produced by the discharge of a pistol, 
 the blow of a hammer, the ticking of a watch, or the beat of 
 a clock, make their impression on the ear by a single shock. 
 All continuous tones are in fact caused by a succession of 
 such shocks, communicated to the ear with sufficient rapidity 
 
NATURE OP CONTINUOUS TONES. 411 
 
 for the interval between them not to be distinguished. Thus, 
 if we cause a tight string to vibrate by pulling or striking it, 
 we occasion, not one vibration only, but a long succession of 
 vibrations (MEGHAN. PHILOS. 187) ; every one of which 
 gives a new impulse to the air, and produces a new impression 
 on the organ of hearing. These vibrations we can see, when 
 they are sufficiently extensive ; and we can always feel them, 
 by placing the finger on the string. In the same manner, 
 the vibrations of a bell or of a tuning-fork continue long after 
 the first blow ; and these, though we cannot see them, may 
 be readily felt with the finger. It is, in fact, in their power 
 of continuing to vibrate after they have been struck, that the 
 peculiarity of these resonant bodies consists. In other instances 
 in which continuous tones are produced, the vibrations are 
 kept-up by the continued application of the original cause, 
 and cease as soon as it is withdrawn : this is the case, for 
 instance, in the string of the violin when set in vibration by 
 the bow, and in the flute and organ-pipe when caused to 
 sound by the passage of air through them. 
 
 523. In all these cases, then, the continuous tones are due 
 to a succession of impulses given by the sounding body to the 
 air ; and according to the rapidity with which the impulses 
 succeed one another, will be the pitch of the sound. It is 
 not difficult to ascertain by experiment the number of such 
 impulses required to produce particular tones. The lowest 
 note (C) given by any musical instrument (that which is 
 produced by an open organ-pipe of 32 feet long, or by a 
 stopped pipe of 16 feet) requires 16 impulses per second; 1 
 but continuous tones have been produced by impulses occur- 
 ring at the rate of only 7 or 8 per second; so that the 
 impression produced upon the ear by each must have lasted 
 at least l-7th or l-8th of a second. On the other hand, it 
 has been ascertained that the ear can appreciate tones pro- 
 duced by 24,000 impulses in a second; so that the limit 
 already adverted to ( 517) must be above this tone, the 
 pitch of which is about 4 octaves above the highest F of the 
 pianoforte. 
 
 524. The strength or loudness of musical tones depends 
 
 1 A backward as well as forward vibration must take place with 
 every impulse ; consequently the number of the vibrations is twice that 
 of the impulses. 
 
4:12 DIFFERENCES IN TONES, AND IN SENSE OF HEARING. 
 
 upon the force and extent of the vibrations communicated by 
 the sounding body to the air. Thus, when we draw the 
 middle of a tight string far out of the straight line, and then 
 let it go, a loud sound is produced, and we can see that the 
 space through which the string passes from side to , side is 
 considerable. As the extent of the vibrations of the string 
 diminishes^ the sound becomes less powerful ; and when we 
 can no longer see the vibrations, but can only feel them, the 
 sound is faint. The length of the undulations in the air 
 corresponds with that of the vibrations in the sounding 
 body ; and consequently they will strike upon the tympanum 
 with more or less force, according as these are longer or 
 shorter. The cause of the differences in the timbre or quality 
 of musical tones, such, for instance, as those which exist 
 between the tones of a flute, a violin, and a trumpet, all 
 sounding a note of the same pitch, are unknown ; but they 
 probably depend upon the different form of the vibrations. 
 
 525. The faculty of hearing, like that of sight, may be very 
 much increased in acuteness by cultivation ; but this increase 
 depends rather upon the habit of attention to the faintest 
 impressions made upon the organ, than upon any change in 
 the organ itself. This habit may be cultivated in regard to 
 sounds of some one particular class ; all others being heard as 
 by an ordinary person. Thus the watchful North American 
 Indian recognises footsteps, and can even distinguish between 
 the tread of friends or foes, whilst his companion who lives 
 amid the busy hum of cities is unconscious of the slightest 
 sound. Yet the latter may be a Musician, capable of distin- 
 guishing the tones of all the different instruments in a large 
 orchestra, of following any one of them through the part 
 which it performs, and of detecting the least discord in the 
 blended effects of the whole, effects which would be, to his 
 coloured companion, but an indistinct mass of sound. In the 
 same manner, a person who has lived much in the country is 
 able to distinguish the note of every species of bird which 
 lends its voice to the general concert of .Nature ; whilst the 
 inhabitant of a town hears only a confused assemblage of 
 shrill sounds, which may impart to him a disagreeable rather 
 than a pleasurable sensation. Of the direction and distance 
 of sounds, our ideas are for the most part formed by habit. 
 Of the former we probably judge, in great degree, by the 
 
APPEECIATION OF SOUNDS I TKANSMISSION OF LIGHT. 413 
 
 relative intensity of the impressions received by the two 
 ears ; though we may form some notion of it by either 
 singly ( 520). Of the distance we judge by the intensity of 
 the sound, comparing it with that which we know the same 
 body to produce when nearer to us. The Ear may be deceived 
 in this respect as well as the eye ( 566) ; thus the effect of 
 a full band at a distance may be given by the subdued tones 
 of a concealed orchestra close to us ; and the Ventriloquist 
 produces his deceptions by imitating, as closely as possible, 
 not the sounds themselves, but the manner in which they 
 would strike the ear. 
 
 Sense of Sight. 
 
 526. By the faculty of Sight we are made acquainted, in. 
 the first place, with the presence of light ; and by the medium 
 of that agent we take cognizance of the forms of surrounding 
 bodies, their colours, dimensions, and positions. It is desira- 
 ble that a short account should be here given of the laws of 
 the transmission of light; since, without the knowledge of 
 them, the beautiful action of the Eye cannot be understood. 
 
 527. The rays of light uniformly travel in straight lines, 
 so long as they traverse the same medium (air, water, or 
 glass, for instance), without obstruction. When issuing from 
 a single luminous point into space, they diverge or separate, 
 in such a manner as to cover a larger and larger surface as 
 they proceed ; and the intensity of the light diminishes in the 
 same proportion. But when the rays pass from one medium to 
 another either more or less dense, 
 
 they are bent out of their straight 
 course, or refracted; unless they 
 should happen to pass from the one 
 to the other in a direction perpendi- 
 cular to the plane which separates 
 them. This maybe made evident 
 by a very simple experiment. Place 
 a com or any heavy body (a, fig. 205) 
 at the bottom of a basin, and then 
 retreat from it until the coin is hidden by the edge of the basin ; 
 if water be then poured-in, up to the level c, the coin will 
 again become visible, although neither its own place nor that 
 of the observer has undergone any change. The reason of this 
 
414 COURSE OP REFRACTED RAYS. 
 
 is, that the rays of light, as they pass out of the water, are 
 bent downwards, or from the perpendicular ; so that they 
 reach the eye of the observer when situated at a lower point 
 than that at which the rays would have arrived if they had 
 proceeded in a straight line. Thus the eye, situated at the 
 end of the line a c, could not see the coin in a straight line, 
 because rays passing in that line would be interrupted by the 
 opaque sides of the basin ; but it receives the ray which was 
 passing through the water in the direction a d, and which 
 was bent downwards at the moment of quitting it. If the 
 eye had been placed directly over the coin, however, so that 
 the ray passing through the latter to it would have emerged 
 from the water in a direction perpendicular to its surface, no 
 change in the apparent place of the object would have been 
 made by pouring-in the water ; since a ray that passes from 
 one medium to another, however different, in a direction 
 perpendicular to the surface which separates them, is not 
 refracted. Those rays which pass-out most nearly in this 
 direction are refracted least, whilst those which pass-out most 
 nearly in the horizontal direction are refracted most. 
 
 528. The general law of refraction then is, that all rays 
 passing from a dense to a rare medium are refracted from the 
 perpendicular, the degree of change being less as they are 
 near the perpendicular, and greater as they depart from it. 
 On the other hand, when rays pass from a rare medium into 
 a dense one, they are bent towards the perpendicular ; and 
 this in a greater or less degree, according as their direction is 
 more distant from the perpendicular, or nearer to it. Thus, 
 in fig. 205, we will suppose the point a to be the position of 
 the eye of a Fish ; and the end of the line a c (previously 
 occupied by the eye of the observer) to be the position of an 
 Insect in the air. Now this insect will not be seen by the 
 fish in its true place ; for a ray passing from it to c would be 
 so bent out of its course as not to reach the point a. The 
 direction in which it is really seen is a d ; for the ray pro- 
 ceeding from the object to the surface of the water, there 
 undergoes such a refraction that it is bent downwards to a ; 
 and, as we always judge of the place of an object by the 
 direction in which the rays last come to the eye, the insect is 
 seen by the fish at d, that is, considerably above its real place 
 ( 476). 
 
REFRACTION OP RAYS THROUGH CURVED SURFACES. 415 
 
 529. When the surface which separates the two media is 
 not flat, but is either convex or concave (bulging or hollowed- 
 out), a very important alteration is produced in the direction 
 
 of the rays that fall upon it. Thus we shall suppose that 
 three diverging rays, issuing from a point, a (fig. 206), and 
 traversing the air, strike upon a convex surface of glass, bb'. 
 The central ray a c falls upon the glass in a direction perpen- 
 dicular to its surface at that point, and passes-on unchanged 
 in its course. But the ray a d falls upon the surface very 
 obliquely ; and consequently in entering the glass it will be 
 bent towards the line e, which is perpendicular to the surface 
 at the point where it enters, and will pass onwards in the 
 direction / In the same manner, the ray a g will be refracted 
 into the direction i. Hence these rays, now converging, would 
 be found, if prolonged, to meet each other again ; and the 
 point at which they meet is termed the focus. To this point 
 all the other rays which fall upon the convex surface, at a 
 moderate distance from the central ray, will also be conducted. 
 530. On the other hand, if the surface of the glass, instead 
 of being convex, is concave, the diverging rays which fall upon 
 it will be made to diverge still more. Thus in fig. 207, let a 
 be the point whence the rays issue, and b b the surface of the 
 lens ; the central ray a c will pass-on unchanged as before ; 
 but the ray a d will be bent towards the perpendicular e, so 
 as to pass-on in the direction/; and the ray a j will be bent 
 towards the perpendicular h, into the direction i. It is easy 
 to understand that the change of direction will be greater, as 
 
416 REFRACTION BY LENSES: FORMATION OF IMAGES. 
 
 the curvature of the lens is more considerable. Thus a convex 
 lens has a long focus or a short focus (that is, brings rays to a 
 
 Fig. 207. 
 
 focus at a greater or less distance from itself) according as the 
 curvature of its surface is less or more considerable. 
 
 531. The rays issuing from every point in an object, and 
 falling upon a convex lens, are brought to a focus on the 
 other side of the lens ; and thus a distinct image or picture 
 is formed upon any screen placed at the proper distance to 
 receive it (as is seen in a Camera Obscura or a Magic Lantern), 
 every point in that image being the representative of the 
 corresponding point in the object, but this image being 
 inverted. 
 
 532. JSTow, the Eye, in its most perfect form such as it 
 possesses in Man and the higher animals is an optical 
 instrument of wonderful completeness, designed to form an 
 exact picture of surrounding objects upon the expanded sur- 
 face of the optic nerve, by which its impression is conveyed 
 to the brain. As it is in the most perfect form of this instru- 
 ment that we are best able to judge of the uses of its different 
 parts, it will be preferable to consider this in the first instance, 
 and then to advert to the less complete forms which we meet 
 with in the lower animals. 
 
 533. The Eye of Man, like that of all Vertebrata, has a 
 nearly globular form. The walls of the sphere are composed 
 of three coats; whilst in its interior are found three humors 
 of a more or less fluid character. The outer coat, named the 
 Sclerotic (s s, fig. 208), is tough and fibrous, and is destined 
 to support and protect the delicate parts which it contains. 
 It does not cover the whole globe, however ; but gives place 
 in the front of the eye to a transparent lamina of cartilaginous 
 
COATS OP THE EYE. 
 
 417 
 
 Fig. 208. INTERIOR OF THE EYE. 
 
 c, cornea; s, sclerotic; *', portion of 
 the sclerotic turned back to show 
 the parts beneath; cA, choroid; 
 r, retina; n, optic nerve; ca, an- 
 terior chamber; i, iris; p, pupil; 
 cr, crystalline lens ; pc, ciliary pro- 
 cesses ; v, vitreous humor ; bb, con- 
 junctiva. 
 
 structure c, which is termed the Cornea. The manner in which 
 this cornea is set upon the sclerotic coat, so as to serve as the 
 continuation of it, may be compared to that in which a watch- 
 glass is made to serve as the con- 
 tinuation of the watch-case over 
 the dial. The cornea is rather 
 more convex than the rest of 
 the sphere of the eye ; so that 
 the globe makes a slight addi- 
 tional projection in that part. 
 When the sclerotic coat is 
 removed, we come upon the 
 second coat ch, which is termed 
 the Choroid; this is much more 
 delicate in its structure, con- 
 sisting almost entirely of blood- 
 vessels and nerves ; and it has 
 a deep black hue, owing to its 
 being lined with a thick layer 
 of black pigment, which consists 
 of cells that have the power of 
 secreting a black granular matter in their interior. 1 This coat 
 also changes its character in the front of the eye ; being there 
 continuous with the Iris, or coloured portion, i, which forms 
 a sort of curtain that hangs-down behind the cornea. The 
 surface of the iris is flat, or nearly so ; and there is con- 
 sequently a space between it and the cornea, like that which 
 intervenes between the dial-plate and the glass of a watch ; this 
 space is termed the anterior chamber of the eye. The iris is 
 perforated in its middle by an aperture p, termed the Pupil. 
 This aperture is always round in Man ; but in animals whose 
 range of vision is required to extend widely in a horizontal 
 direction (such as the Euminants, and others which feed 
 upon herbage), it is an ellipse with the long diameter hori- 
 zontal ; whilst in animals which rather seek their food above 
 or below them (such as the Cat and other Carnivora which 
 naturally live among trees and high places), the pupil is an 
 ellipse whose long diameter is vertical. 
 
 1 Similar pigment-cells, having great variety in their form, are to be 
 found composing the black spots on the skin of the Frog, Water 
 Newt, &c. 
 
 E E 
 
418 CONTRACTION AND DILATATION OP THE PUPIL : RETINA. 
 
 534. By the contraction and relaxation of certain fibres in 
 the Iris, the size of the Pupil is changed according to the 
 degree of light to which the eye is exposed ; the aperture being 
 made to diminish in a strong light, in such a manner as to 
 exclude the rays that would be superfluous, and to prevent 
 too many from falling upon the expansion of the optic nerve ; 
 whilst it dilates in a faint light, so as to admit as many rays 
 as possible. If we notice the pupil of a Man who is looking 
 towards the mid-day sun, we shall see that it is contracted to 
 a small round speck ; but the pupil of a Sheep would be con- 
 tracted, in similar circumstances, into a horizontal slit ; and 
 the pupil of a Cat into a vertical one. The alteration in the 
 size of the pupil in accordance with the degree of light, may 
 be easily observed by stationing oneself at a window pro- 
 vided with shutters, and holding a looking-glass in the hand : 
 if the light be at first strong, the pupil will be seen in a con- 
 tracted state ; but if the shutters be gradually closed, so as to 
 diminish the amount of light that falls upon the eye, the 
 pupil will be seen to enlarge; and it will diminish again 
 when the shutters are re-opened. The blackness which the 
 pupil always presents, in the healthy state of the eye, is due 
 to our seeing the black lining of the back of the eye through 
 it. In many Quadrupeds, the black pigment is replaced, in 
 one portion of the eye, by a layer of a blueish colour, having 
 an almost metallic lustre ; and from this we see the light 
 brilliantly reflected, when we look at their eyes in certain 
 directions. 
 
 535. On turning back the choroid coat, we come to the third, 
 r, of the layers of which the wall of the eye is composed : this 
 is an extremely delicate film, chiefly consisting of nerve-cells 
 and nerve-fibres that spread-out from the optic-nerve, n ; and 
 it is called the Retina (or net). It advances nearly as far as 
 the iris ; but it is deficient in the front of the eye. The part 
 of the retina which lines the globe at the point exactly 
 opposite to the centre of the pupil, is distinguished from the 
 rest by a peculiar yellow opacity, which causes it to be 
 designated " the yellow spot/' In this spot, visual sensibility 
 is more acute than elsewhere. On the other hand, the part 
 of the retina that covers the entrance of the optic-nerve 
 (which is below " the yellow spot," and nearer to the nose) is 
 so much less sensitive to light than the rest, that under 
 
HUMOES OF THE EYE : CONJUNCTIVA. 419 
 
 certain circumstances the image that falls upon it may not be 
 perceived at all ( 554). 
 
 536. The cavity of the globe is occupied by three humors 
 of different consistence the Aqu^pus, Vitreous, and Crystal- 
 line. The aqueous humor is nearly pure water, being nothing 
 else than the serum of the blood very much diluted : it occu- 
 pies the anterior chamber ca, and a small space behind the 
 iris, in front of the crystalline lens. The vitreous humor v 
 resembles thin jelly in consistence, and occupies the greater 
 part of the globe of the eye behind the iris. The crystal- 
 line humor cr is of much firmer consistence, resembling 
 very thick jelly or soft gristle ; it has the form of a double- 
 convex lens, the posterior surface of which is more convex 
 than its anterior j and hence it is commonly known as the 
 crystalline lens. It is suspended in its place by a set of little 
 bands pc, proceeding from the choroid coat, and known as 
 the ciliary processes. 
 
 537. The cornea is covered externally by a membrane bb, 
 termed the Conjunctiva. This membrane is perfectly transparent 
 where it covers the cornea, and seems like an outer layer of 
 it ; the front of the sclerotic also is covered by it, but it is 
 there semi-opaque, having a whitish colour. The membrane 
 does not pass back over the globe of the eye, however, but 
 bends forward again, as seen at bb, so as to form the lining of 
 the eyelids, at the edge of which it becomes continuous with 
 the skin. Thus the smooth surfaces of the eye and of the 
 under side of the lids are both formed by this membrane ; 
 the mucous secretion from which serves to diminish the 
 friction of one upon the other. But the smallest particle of 
 any hard substance, which may become interposed between 
 these surfaces, produces great irritation. It cannot pass far 
 backwards, however, on account of the bend of the membrane 
 at bb; and thus it may be always removed (if loose) with 
 little difficulty. The lower lid can be easily drawn down, so 
 as to expose the membrane as far as this bend ; and any loose 
 particle that is lying upon its surface may thus be detected 
 and removed. But the upper lid, being longer, cannot be 
 drawn out sufficiently for this purpose ; and it is necessary to 
 evert it, or turn it inside-out. As the knowledge of the mode 
 of performing this very simple operation will often save a 
 good deal of suffering, it will be here described. Nothing 
 
 E E 2 
 
420 
 
 MUSCLES OF THE EYE. 
 
 more is necessary than to close the upper lid not forcibly, 
 however ; next to make pressure upon its upper part with a 
 pencil, bodkin, knitting-needle, or other hard body of small 
 diameter; and then, taking hold of the eyelashes, to draw 
 the lower edge of the lid forwards and upwards. By a dex- 
 terous movement of this kind, the lid may be everted without 
 any pain, a little temporary discomfort being all that the dis- 
 placement occasions ; its lining membrane is then exposed, 
 and any offending particle may be readily removed. 
 
 538. The globe of the eye is moved by six muscles, which 
 are lodged within the bony cavity or orbit, hollowed-out in the 
 skull. All these muscles, except one, originate at the back of 
 
 the orbit, and are inserted 
 into the sclerotic coat, near its 
 front, by broad thin tendons. 
 Four of them are termed recti 
 or straight muscles. One of 
 these, the superior rectus(e, fig. 
 209), is inserted at the upper 
 part of the eye, and conse- 
 quently by its contraction rolls 
 the globe upwards; another, 
 d, the inferior rectus, pro- 
 duces a corresponding move- 
 ment downwards. A third, the 
 
 internal rectm ( which could 
 
 nerve; d, inferior rectus muscle; e, supe- not be shown in this figure), 
 
 rior rectus ;/, cut extremity of the ex- 11 fi o-ln"hp inwarrlc m- 
 
 ternal rectus; g, end of the inferior * inwards, C 
 
 obU^ue ; h, superior oblique ; i, elevator towards the nOS6 I whilst a 
 
 of the upper lid ; A, lachrymal gland. /> ,1 ji /,i 
 
 fourth, the external rectus (the 
 
 cut extremity of which is seen at/), turns it outwards. Besides 
 these, there is a remarkable muscle, h, the superior oblique, 
 which originates at the back of the orbit, comes forwards to 
 the front, where its tendon passes through a pulley, and then 
 turns backwards to be inserted into the sclerotic coat, at a point 
 considerably behind the pulley. The sixth muscle, g, termed 
 the inferior oblique, does not arise, like the rest, from the 
 back of the orbit, but from its lower border. The action of 
 the two oblique muscles (which act in antagonism the one to 
 the other) appears to be to rotate the eyeball upon its axis ; 
 as is done when the eyes are kept steadily fixed upon any 
 
 Fig. 209. VERTICAL SECTION OF THE 
 
MOVEMENTS OF THE EYE : EYELIDS, ETC. 421 
 
 object, whilst the head is inclined to one side or the other. 
 Of these muscles, the superior, inferior, and internal recti, 
 together with the inferior oblique, and also the elevator 
 muscle, i, of the upper eyelid, are supplied with motor nerves 
 by the third pair ( 459) ; whilst the superior oblique has a 
 nerve to itself, the fourth; and the external rectus has another 
 nerve to itself, the sixth. 
 
 539. There is this very peculiar circumstance attending 
 the movements of the two eyes, that although they are 
 harmonious, they are seldom symmetrical. Thus, when we 
 direct our eyes towards an object on one side of us, they move 
 harmoniously, that is, with a common purpose ; but their 
 movement is not symmetrical, for one globe is rolled inwards 
 by the internal rectus, whilst the other is rolled outwards by 
 the external rectus. These two different actions seem to be 
 instinctively connected, and to be guided by the sensations 
 which are received through the two eyes respectively ( 478). 
 They are performed without any consciousness on our own 
 part, when, having fixed our gaze upon any object, we 
 rotate the head from side to side in the horizontal plane, the 
 eyeballs executing a corresponding rotatioin in the opposite 
 direction. 
 
 540. The eyebrows, eyelids, and eyelashes, serve in various 
 ways for the protection of the eyes. In Birds and Reptiles 
 there is a third eyelid, which is drawn across the eye by a 
 muscle that passes through a loop in it. This nictitating 
 membrane, as it is termed, is semi-transparent ; and it serves 
 to protect the eye from the too-powerful rays of light, without 
 destroying the power of vision. The upper and lower eyelids 
 of Mammals, and the nictitating membrane of Birds and 
 Reptiles, are very frequently drawn over the front of the 
 globe during the waking state, for the purpose of sweeping 
 from it dust and other accidental impurities which would 
 otherwise be injurious. 
 
 541. Beneath the upper eyelid, in the upper and outer 
 portion of the orbit, is situated the lachrymal gland (k, fig. 
 209) ; this is continually pouring-out a watery secretion over 
 the globe of the eye, which serves to wash from it these 
 impurities and to keep it moist. It is only, however, when 
 the quantity of this secretion is increased by mental emotion 
 or by irritation in the eye itself, so as to produce a flow of 
 
LACHRYMAL APPARATUS I SENSE OF VISION. 
 
 tears, that we become conscious of its existence. It is ordi- 
 narily drawn-off as fast as it is formed, by a curious apparatus 
 situated at the inner border of the eye. If the edges of the 
 lids be carefully examined, there will be seen upon each of 
 them, close to the inner corner of the eye, a minute spot 
 which is the entrance to a small canal termed the lachrymal 
 duct. The two ducts, one commencing at the corner of the 
 upper lid and the other at that of the lower, soon unite into one 
 canal, which swells into a sort of reservoir, the lachrymal sac, 
 that lies upon the side of the upper part of the nose ; and 
 from this reservoir a canal passes down through the bones of 
 the nose into its cavity. By this apparatus, the fluid which 
 is poured by the lachrymal gland over the exterior of the eye, 
 is drawn-off at the interior after washing its surface ; whence 
 it is carried into the nose, to be got rid of by the current of 
 air that passes through its cavity in- breathing. The edges 
 of the lids meet in such a manner, when they are closed, as 
 to form a sort of gutter or channel, along which the lachrymal 
 secretion flows from their outer to 
 their inner corner during sleep. 
 
 54:2. Having thus described the 
 structure of the Eye, and the general 
 actions of the parts by which it 
 is adapted to the performance of 
 its remarkable function, we shall 
 examine into the details of this 
 function ; in other words, into the 
 nature of vision. 
 
 543. The rays of light which 
 diverge from the several points of 
 any object, and fall upon the front 
 of the cornea, are refracted by its 
 convex surface whilst passing 
 through it to the eye, and are made 
 to converge slightly. They are 
 brought more closely together by 
 the crystalline lens, which they 
 reach after passing through the 
 pupil ; and its refracting influence, 
 
 together with that produced by the vitreous humor, is such 
 as to cause the rays that issued from each point to meet 
 
FORMATION OF IMAGE ON THE RETINA. 423 
 
 in a focus on the retina. As every point is thus repre- 
 sented in its proper position relatively to others (except that 
 those which were above are now below, and vice versd), a 
 complete inverted image or picture of the object is formed 
 upon the retina. This is shown in fig. 210 ; where, for the 
 sake of convenience, three rays only are represented as issuing 
 from the centre and the two extremities of an object a b. These 
 rays cross each other at h, in the middle of the eye ; so that 
 those from a being brought to a focus at c, and those from 
 b at d, and all the other rays being refracted in the same 
 manner, a complete inverted picture of the object is formed at 
 the back of the eye. 
 
 544. That this is really the case, may not only be inferred, 
 but proved. If the eye of a Rabbit be removed from its 
 socket, and cleansed of the muscles, fat, &c., adherent to its 
 back part, and a candle be then brought in front of it, the 
 transparency of the sclerotic coat will allow the image of the 
 candle that is formed upon the retina to be distinctly seen. 
 Or, if we take the eye of a Sheep or an Ox, and after cleans- 
 ing it in the same manner, we cut out from the back of it- a 
 portion of the sclerotic and choroid coats, covering the part 
 of the retina thus left bare with a piece of tissue-paper (for 
 the purpose of keeping -in the vitreous humor, without 
 interrupting our view of the image), a distinct but inverted 
 miniature picture of all the objects in front of the eye will 
 be seen at its back. It is necessary in these experiments 
 that the eyes should be taken from animals recently killed ; 
 as the cornea and humors soon lose their transparency, and 
 the distinctness of the picture is consequently impaired. 
 
 545. The black pigment, which is situated immediately 
 behind the retina, that is, in contact Avith its external 
 surface, is destined to absorb the rays of light immediately 
 that they have passefl through the retina j so as to prevent 
 them from being reflected from one part of the interior of 
 the globe of the eye to another, which would cause a great 
 confusion and indistinctness in the picture. Hence it is that 
 in those individuals (both among Man and the lower animals) 
 in whose eyes this pigment is deficient, vision is extremely 
 imperfect. The eyes of such individuals (termed Albinos) 
 derive from the absence of pigment a very peculiar appear- 
 ance. The iris does not possess its ordinary colour ; but, 
 
424 
 
 SPHERICAL ABERRATION. 
 
 owing to the large quantity of minute blood-vessels which it 
 contains, it presents a bright red hue. The choroid coat, seen 
 through the pupil, has exactly the same aspect ; so that the 
 pupil is not readily distinguished. During the day the vision 
 of these Albinos is very indistinct, and the glare of light is 
 painful to them ; and it is only when twilight comes-on, that 
 they can see clearly and without discomfort. 
 
 546. The eye is much more remarkable for its perfection 
 as an optical instrument, than we might be led to suppose 
 from the cursory view we have hitherto taken of its functions ; 
 for by the peculiarities of its construction certain faults and 
 defects are avoided, to which all ordinary optical instruments 
 are liable. One of these, termed spherical aberration, results 
 from the fact, that rays falling upon the central and the outer 
 portions of an ordinary convex lens, whose surfaces form part of 
 a sphere, are not brought to meet in one point, the focus of 
 the central portion being rather more distant than that of the 
 outer part. This is shown in fig. 211, where L L is the lens, 
 
 Fig. 211. 
 
 R L, R L, are rays falling upon its circumference, and R' L', 
 R' L', are rays falling near its centre. The former set of rays 
 meet in /; whilst the latter pass-on to F, before they meet in 
 a focus. This may be shown by covering the central and 
 the outer portions of the lens, alternately, with some opaque 
 substance, which shall stop all the rays of light proceeding 
 through either. When the central portion is covered, a distinct 
 image will be formed at/ by the rays falling upon the outer 
 portion; and when the outer portion is covered, a distinct 
 image will be formed at F by the rays that have passed 
 through the central portion. But when the whole lens is 
 employed, no distinct image is formed anywhere; for if a 
 screen be held at /, it will receive, not only the rays which 
 are brought to a focus at that point, but also the rays which 
 are going-on to meet at F ; whilst, on the other hand, if the 
 
SPHERICAL AND CHROMATIC ABERRATION. 425 
 
 screen be held jat" r, it will receive, not only the rays which 
 are brought to a focus there, but also those which, having 
 met at/, have crossed and passed-on to G and H. 
 
 547. Now this indistinctness is ordinarily got over in 
 practice, by employing only the central portion of the lens ; 
 so that only those rays which correspond to R' I/, R' i/, shall 
 pass- through it. This we observe in ordinary Microscopes 
 and Telescopes ; a stop or perforated partition being inter- 
 posed behind the lenses, so as to allow the light to pass 
 through only a small aperture in their centre. By this plan 
 a great deal of light is cut off, so that the image is rendered 
 dark. The spherical aberration may be completely corrected, 
 however, by a certain adaptation of two or more lenses whose 
 surfaces have different curvatures ; the effect of which is, to 
 bring all the rays that have passed through every part of this 
 compound lens to the same focus. Now this very adjustment 
 is made in the eye, by the arrangement of the curvatures of 
 the cornea and of the two surfaces of the crystalline lens ; 
 and in the well-formed eye it is so perfect as to produce 
 complete distinctness in the image or picture thrown upon 
 the retina. The only case in which this would not occur, is 
 when an object is brought very near the eye ; for the rays 
 then diverge from each other at so wide an angle, that those 
 which fall upon different parts of the lens would not be all 
 brought to the same focus. This error is corrected by the 
 contraction of the pupil, which takes place involuntarily 
 when an object is brought very near the eye, and thus 
 cuts-off the rays that would otherwise render the picture 
 indistinct. 
 
 548. But there is another imperfection to which ordinary 
 optical instruments are liable, that is completely corrected in 
 the eye. If we look through a common Microscope, especially 
 when a high power is employed, by the light of a lamp or 
 candle, we see that the edges of the image are bordered by 
 coloured fringes, which very much impair its distinctness, 
 and prevent it from being seen in its true aspect. This is 
 the result of what is termed chromatic aberration; and it 
 results from the fact, that the rays of different colours, which 
 are all blended in ordinary colourless light, are refracted by 
 the same lens in different degrees, so as to be brought to a 
 focus at different points. Thus we will suppose that the lens 
 
426 CHROMATIC ABERRATION I ACHROMATISM OP THE EYE. 
 
 L L (fig. 211) has been corrected for spherical aberration ; and 
 that R L, R L, are violet rays falling upon it, whilst R' L', R' L', 
 are red rays. The former are capable of being refracted in a 
 much higher degree than the latter ; so that they are brought 
 to a focus at/, whilst the others do not meet until F. Hence 
 if a screen be placed to receive the image at f y the picture 
 will be formed by the violet rays only, and will be surrounded 
 by red fringes occasioned by the red rays which are passing 
 on to their focus at P ; whilst, on the other hand, if the screen 
 be placed at F, the picture will be chiefly formed by the red 
 rays, and will be surrounded by violet fringes produced by 
 the violet rays, which, having met in /, have crossed and 
 passed-on to G and H. Now as from each point of almost 
 every object proceed rays in which the different colours are 
 blended, the refraction of an ordinary lens produces a sepa- 
 ration of these, and a consequent indistinctness and false 
 colouring in the picture. This is particularly the case with 
 regard to the rays that pass through the outer portion of the 
 lens ; for, as these are subject to greater change in their 
 direction than are those which pass through its centre, the 
 separation of the differently-coloured rays of which they are 
 composed is more considerable. 
 
 549. In practice, this error is got over, like the preceding, 
 by very much contracting the aperture of the lens ; so that 
 only the central rays, in which the colours are but little 
 separated, are allowed to pass. But it may be perfectly cor- 
 rected by combining lenses formed out of different materials, 
 which possess a different refracting power; the errors of 
 these being made to counterbalance one another. Such 
 lenses, which are termed achromatic, are now employed in 
 all superior Telescopes and Microscopes ; but no artificial 
 combination can surpass that which exists in the Eye, the 
 different density of whose humors is adjusted in such a 
 manner as completely to answer this purpose. The contrac- 
 tion of the pupil which takes place when we look at a very 
 near object, prevents the only imperfection which could occur ; 
 and thus the picture on the retina, in a healthy eye, is always 
 rendered free from false colours. It is said that the first idea 
 of uniting glasses of different kinds, so as to produce an 
 achromatic lens, was taken from the Eye ; and this is not at 
 all improbable. In this, as in many other instances, Nature 
 
ADJUSTMENT OF THE EYE FOR VARYING DISTANCES. 427 
 
 has served as a guide to Art ; or, in other words, the Divine 
 Artificer has thus condescended to teach the human workman. 
 
 550. There is another wonderful arrangement in the 
 healthy Eye, which the optician can only imitate in his 
 instruments in a very bungling manner. It is that by which 
 the eye adapts itself to view objects at different distances 
 from it, with equal distinctness. If we look at a near object 
 with a Telescope, adjusting the instrument so as to see it dis- 
 tinctly, and then turn it towards a remote object, we shall 
 not see the latter with equal clearness until the instrument 
 has been again adjusted. If we then turn it back to the 
 nearer object, we shall find that the change in the adjustment 
 occasions the representation of it to be now indistinct ; and 
 in order to bring back the image to its former clearness, it is 
 requisite to re-adjust the instrument to its first condition. 
 This is a necessary consequence of the optical law, that the 
 distance of the image from the lens which forms it, varies 
 with that of the object, being increased as the object is 
 brought nearer, and diminished as it recedes. If the Eye 
 were constructed in the same manner, we should not be able 
 to see distinctly, without the aid of artificial assistance, at any 
 other distance than that for which it is adjusted. Hence if a 
 perfect picture of an object situated at twelve inches' distance 
 from the eye, were formed upon the retina, we should not be 
 able to see it clearly when brought to the distance of six inches, 
 nor when removed to the distance of six feet ; because in the 
 first of these cases the rays would not be brought to a focus 
 upon the retina, but at a point behind it (if they were 
 allowed to pass on unchecked) ; whilst in the second, they 
 would be brought to a focus at a point nearer than the retina, 
 and would consequently begin to separate again before they 
 reach it. 
 
 551. But the healthy eye possesses a power of perfect 
 adjustment to the viewing of objects situated at different 
 distances ; and this without any effort or intention on our 
 parts, but, as it were, by an instinctive operation. That such 
 a change really takes place, we may readily convince ourselves, 
 by looking at a near and at a distant object placed in the 
 same line, a pencil-case, for instance, held up at a few inches 
 from the eye, and a chimney half a mile off. We shall find 
 that no effort of attention will enable us to see them both 
 
428 LONG AND SHORT SIGHT: SPECTACLES. 
 
 distinctly at the same time ; but that, on whichever of the 
 two objects we fix our eyes, we shall see it clearly, whilst the 
 other will become indistinct. Recent observations have con- 
 clusively shown that this adjustment is brought about by an 
 alteration in the curvature of the crystalline lens ; its con- 
 vexity being increased when a near object is looked at, so as 
 to act more powerfully in bringing its diverging rays into 
 convergence ; and being diminished when the gaze is turned 
 towards a distant object. 
 
 552. In advanced life, however, from the diminution in 
 the convexity of the cornea and in the refracting power of the 
 humors, the eye can no longer accommodate itself to near 
 objects ; their rays not being brought to a focus by the time 
 that they reach the retina. But as it is still able to see dis- 
 tant objects clearly, it is said to be long-sighted. By the use 
 of a convex glass, however, adapted to supply that additional 
 amount of refraction which is required, near objects may be 
 distinctly seen. A contrary state of the eye not unfrequently 
 exists, in which the cornea is too convex, and the refracting 
 power of the humors is too high ; from which it happens 
 that the rays proceeding from distant objects are brought to a 
 focus too soon, so as to cross each other before they reach the 
 retina. But as such an eye can form a very distinct picture 
 of a near object, it is said to be near-sighted. This imper- 
 fection is remedied by interposing a concave lens between the 
 object and the eye, by which its refracting power is dimi- 
 nished to the necessary degree. 
 
 553. In the choice of spectacles or eye-glasses for these 
 purposes, particular care should be taken that they are not too 
 powerful ; since great mischief is frequently done to the eye, by 
 the employment of lenses of too great a curvature. A person 
 who in youth and middle age has enjoyed good sight, very 
 commonly finds it necessary to employ spectacles for assist- 
 ance in reading and writing, as his age advances towards fifty 
 years; and he should be very cautious, when first availing 
 himself of their assistance, to employ those of the longest 
 focus. As his age advances, it will be necessary to substitute 
 more powerful lenses for these ; but this should be done very 
 gradually; and in no instance should a glass be employed 
 that produces any apparent enlargement in the object, its 
 proper use being only to render the object distinct. The evil 
 
SPECTACLES : SENSITIVE SPOT OF RETINA. 429 
 
 influence of using spectacles of too high a power, soon mani- 
 fests itself in the strained feeling which the eyes experience 
 for some time ; but this feeling at last subsides, in conse- 
 quence of the eye having adapted itself to the glasses, and 
 having thus undergone a change which it might otherwise 
 take years to produce. In this manner the eyes of a person 
 at sixty may be brought to the state which, under more 
 careful management, might have been deferred ten or fifteen 
 years longer. Similar remarks apply to the use of concave 
 lenses by short-sighted persons. They should never be em- 
 ployed of a higher power than is requisite to see objects with 
 distinctness, when at a moderate distance ; and on no account 
 should any glasses be used that diminish their apparent size. 
 As age advances, the eyes of short-sighted persons usually 
 become more flattened, and are then able to adapt themselves 
 to objects at a variety of distances ; so that persons who have 
 been short-sighted when young, are not unfrequently able to 
 see distinctly at an advanced age, without the assistance of 
 convex glasses. 
 
 554. The power of receiving and transmitting visual im- 
 pressions is by no means uniform over the whole retina. In. 
 the whole field of vision which at any time lies before the 
 eye, we only see with perfect distinctness that part to which 
 its axis (namely, that diameter of the sphere which passes 
 through the centre of the pupil) is directed, and of which the 
 image, therefore, is formed upon "the yellow spot" ( 535) 
 which lies at the posterior pole of the axis. Nevertheless we 
 have a sufficiently distinct perception of the remainder of the 
 field, to enable us to judge of the general relations of its 
 objects to each other and to those which we distinctly see : 
 thus, whilst reading or writing, we can only recognise letters 
 and words at any one moment within a spot which a sixpence 
 or a shilling would cover, but we may distinguish the lines 
 over the whole area of the page, and can plainly see the 
 position of the book or paper upon the desk or table, together 
 with the position of this in the apartment. In the act of 
 reading or writing, as in surveying the different parts of a 
 landscape or a picture, or in examining any solid object that 
 is brought under our notice, we direct the axis of the eye 
 successively to one point after another, until we have satisfied 
 ourselves that we have gained a distinct view of every part, 
 
430 INSENSIBLE SPOT OF RETINA : VISUAL ATTENTION. 
 
 as well as of its relations to the rest. It will be presently 
 shown that when we employ both eyes at once, their axes 
 meet in the object, and that the degree of their convergence 
 affords us a very important means of judgment as to their 
 distance ( 563, 564). The part of the retinal surface which 
 lies over the entrance of the optic nerve, is remarkable for the 
 imperfection of its power of receiving impressions ; as is made 
 apparent by the following 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 dis- 
 appears, though it is clearly seen when the paper is brought 
 nearer or removed further ; and it can be shown that in this 
 position of the eye and the object, the rays from the right- 
 hand spot fall upon the point in question. 
 
 555. The degree in which the attention is directed to them, 
 has a great influence on the readiness with which very minute 
 or distant objects can be perceived; and there is a much 
 greater variation in this respect amongst different individuals, 
 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 otherwise distin- 
 guish them. There must be few who have not experienced 
 this, in regard to a balloon 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 if we withdraw our eyes for a few 
 minutes we search in vain for them, until they are again 
 pointed-out to us by some one who has been watching in the 
 interval. The faculty of rapidly descrying such objects much 
 depends upon the habit of using the eyes in search of them ; 
 thus a seaman will distinguish land, when to the landsman 
 there is no appearance more distinct than that of a faint cloud 
 on the horizon presenting no definite outline ; or he will 
 recognise the course and rig of a distant ship, which to the 
 landsman appears but as a white speck on the ocean : yet the 
 landsman, placed before a piece of delicate machinery, might 
 be able to astonish the seaman by distinguishing the forms 
 and movements of minute parts, which to the latter appear 
 only as a confused mass. 
 
INTERPRETATION OF VISUAL SENSATIONS. 431 
 
 556. The picture formed upon the retina closely resembles 
 that which we see in a camera obscura. It represents the 
 outlines, colours, lights and shades, and relative positions, of 
 the objects before us ; but these do not necessarily convey 
 to the mind the knowledge of their real forms, characters, or 
 distances. It would appear, from the actions of the lower 
 animals, that many of them have the power of intuitively or 
 instinctively determining the latter from the former, from the 
 moment when they come into the world ; thus a Fly-catcher 
 just come out of its egg, has been seen to make a successful 
 stroke with its bill at an insect which was near it. If it were 
 not so, those animals which are thrown from the first upon their 
 own resources, would perish almost inevitably ; being starved 
 by want of food during the time required for them to learn 
 how to obtain it. But this is well known not to be the case 
 in regard to Man. The infant is educating his senses long 
 before any indications of mind present themselves. By the 
 combination, especially, of the sensations of sight and touch, 
 he is learning to judge of the shapes and surfaces of objects, as 
 they would be felt, by the appearance they present, to form 
 an idea of their distance, by the mode in which his eyes are 
 directed towards them ( 563), and to estimate their size, by 
 combining the notions obtained through the picture on the 
 retina, with those he acquires by the movement of his hands 
 over their different parts. A simple illustration will show 
 how closely the ideas excited by the two sets of sensations 
 are blended in our minds. The idea of smoothness is one 
 which has reference to the touch, and yet it constantly occurs 
 to us on looking at a surface which reflects light in a particular 
 manner. On lie other hand, the idea of polish is essentially 
 visual, having reference to the reflection of light from the 
 surface of the object ; and yet it would occur to us from the 
 sensation conveyed through the touch, even in the dark. 
 
 557. That this combination is not formed intuitively in 
 Man, but is the result of experience, is particularly evident 
 from cases in which the sense of sight has been wanting 
 during the first years of life, and has then been obtained by 
 an operation. Several such cases are now on record. The 
 earliest, which still remains the most interesting, is one which 
 occurred to Cheselden, a celebrated surgeon at the beginning 
 of the last century. The youth (about twelve years of age), for 
 
432 INTERPRETATION OP VISUAL SENSATIONS. 
 
 some time after tolerably distinct vision had been obtained, 
 saw everything flat as in a picture, the impression made upon 
 his retina being simply transferred to his mind ; and it was 
 some time before he acquired the power of judging, by his 
 sight, of the real forms, characters, and distances of objects 
 around him. Thus, among other interesting circumstances, 
 it is mentioned that 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, at 
 the same time looking steadfastly at her, 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 facts of so striking a 
 character. One curious circumstance, however, may be men- 
 tioned, as fully bearing out the view here given. The lad had 
 been accustomed to find his way readily about his father's 
 house by the use of his hands, and he continued to do the 
 same for some time after his sight was tolerably clear, being 
 evidently puzzled, rather than assisted, by the impressions 
 conveyed through his new sense ; but, when learning a new 
 locality, he employed his sight, and evidently perceived the 
 increase of facility which he derived from it. Hence, we can 
 have little hesitation in deciding upon the question which has 
 perplexed many able reasoners, 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 
 recognise these bodies by the latter sense. This question was 
 answered in the negative by the celebrated mental philosopher, 
 Locke, and with perfect justice. 
 
 558. We shall now inquire into the mode in which we 
 form our notions of the nature, sizes, distances, &c., of external 
 objects, from their pictures impressed upon our retina. The 
 first question is one on which a vast amount of discussion has 
 taken place, with very little satisfactory result. It is, why 
 are the objects which we see, represented to our minds in 
 their true erect position, their images upon the retina being 
 inverted? Various replies to this question have been pro- 
 posed at different times; and, amongst others, it has been 
 actually assumed that the Infant really does see objects 
 
SENSE OF DIRECTION : SINGLE VISION. 433 
 
 inverted, and that this error is only corrected by experience. 
 The cases alluded-to in the last paragraph, however, satisfac- 
 torily prove this assumption to be incorrect ; since, although 
 the individuals saw everything upon the same plane, as in a 
 picture, the representation was erect from the first. The fact 
 now appears certainly to be, that we have an intuitive sense 
 of direction, which guides us in our appreciation of the actual 
 situations of objects and parts of objects ; so that, when a visual 
 impression is made upon any part of the retina, we see the 
 point from which the rays proceed, in the direction of a line 
 drawn from the affected spot of the retina through the common 
 centre (fig. 210, h) through which all the rays pass, this line 
 serving as a true guide to the actual place of the object. 
 
 559. The same may be said of the cause of single vision, 
 that is, of our seeing but one object, although its picture is 
 double, being formed upon both retinse. In animals which, 
 like Man, look straight forwards, the field of vision of the 
 two eyes is nearly the same ; so that most of the objects seen 
 with one eye will be seen with the other also. The objects 
 at the right and left sides of the field of vision, however, are 
 seen with the right and left eyes singly ; yet we perceive no 
 difference in the aspect of these from that of the objects 
 towards which both our eyes are directed. It is evident, 
 then, that the pictures formed on the two retinae are blended, 
 as it were, by the mind, into a single perception. This seems 
 to be, in part at least, the effect of habit. "When the images 
 do not fall upon parts of the two retinas which are accustomed 
 to act together, double vision is the result. Thus if, when 
 looking steadily at an object, we press one of the eyeballs 
 sideways with the finger, the object is seen double. In the 
 same manner, if an affection of the nerves or muscles of one 
 eye (as happens temporarily in intoxication) prevent it from 
 being directed to the same point with its fellow, double vision 
 of all objects is the result. This, when it does not soon pass 
 away, is a not unfrequent symptom of serious disorder within 
 the brain. If it continue long enough, however, the indivi- 
 dual becomes accustomed to the double images, or rather 
 ceases to perceive that they are double, probably because the 
 mind becomes habituated to receive them, or rather perceives 
 only one of the impressions on the two parts of the retinae 
 which now act together. For if, after the double vision has 
 
 F F 
 
434 COMBINATION OP RETINAL PICTURES : STEREOSCOPE. 
 
 passed away, the conformity of the two eyes be restored (as 
 by the operation for the cure of squinting), there is double 
 vision for some little time, although the two parts of the 
 retinae, which originally acted together, are now brought to 
 do so again. 
 
 560. That the combination of the two images must be 
 effected by an operation of the mind, is evident from another 
 circumstance. It is easy to show that no near object is seen 
 by the two eyes in exactly the same manner. 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 view of it is very different, according 
 to the eye. with which he sees it. With the right eye he will 
 see its back and right side, the latter very much foreshortened, 
 but none of the left side ; whilst with the left eye he will see 
 its back and left side, the latter also foreshortened, but none 
 of the right side. Hence if he were to draw a perspective 
 view of the object as seen by each eye, the two delineations 
 would be very different. But on looking at the object with 
 the two eyes conjointly, there is no confusion between these 
 pictures, nor does the mind dwell upon either of them singly ; 
 the union of the two gives us the idea of a solid projecting 
 body such an idea as we could have only acquired otherwise 
 by the exercise of the sense of touch. 
 
 561. That this is really the case, has been proved by ex- 
 periments with the very ingenious instrument (invented by 
 Professor Wheatstone) known as the Stereoscope. In its 
 original form this consisted of two plane mirrors, inclined with 
 their backs to one another at an angle of 90, the point of 
 meeting being opposite to the middle of the forehead. Two 
 drawings representing the different perspective views of any 
 solid object, as seen by the two eyes, being placed before 
 these mirrors, in such a manner that their images are re- 
 flected to the two eyes respectively, and are made to fall 
 on corresponding parts of the two retinae as the two images 
 formed by the solid object itself would have done, so 
 that their apparent places are the same, the mind perceives 
 not one or other of the single representations 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 
 
MENTAL APPBECIATION OP PROJECTION. 435 
 
 were made. In the small portable instrument which has of 
 late become so extensively popular, the like effect is produced 
 by a particular arrangement of convex lenses, devised by 
 Sir David Brewster, which also has the advantage of magnify- 
 ing the pictures. 
 
 562. It is, then, by the combination which is effected 
 through a mental process, based on the consentaneous percep- 
 tion of the two dissimilar pictures formed on the two retinas, 
 that these are made to blend into one representation, which 
 gives the idea of projection. When we look at a distant 
 object, our judgment is based on less positive data, the two 
 pictures being then almost precisely the same ; and hence it 
 is impossible (without moving the head) to distinguish with 
 certainty between a well-painted picture, in which the pro- 
 portions, lights and shades, &c. are well preserved, and the 
 objects it is intended to represent, if we are prevented from 
 knowing that it is a picture. Some admirable illusions of 
 this kind have been effected in the Diorama. But a slight 
 movement of the head suffices to dispel the doubt; since 
 by this movement a great change would be effected in the 
 perspective view of a solid object, a little of the side of a 
 projecting buttress or column being seen, for instance, where 
 only the front was perceived before, whilst the image formed 
 by a picture is but slightly affected. The same indecision is 
 experienced when we look with a single eye at certain near 
 objects, which the mind can apprehend either as projecting or 
 as receding, with equal, or nearly equal, readiness ; such, for 
 example, as a metal plate stamped-out into a figure which 
 stands-forth in relief on one side and is counter-sunk on the 
 other. And the idea of the object which is the reverse of 
 the reality may present itself most forcibly, if it should 
 happen to be the one most familiar to the mind ; thus if we 
 look with one eye at the interior of a mask that has been 
 coloured to the semblance of a human face, it will seem to 
 rise into the likeness of the exterior ; whilst the actual pro- 
 jecting surface of the mask will never seem to exhibit the 
 concavity of the interior. 1 In looking with a single eye, 
 
 1 In making these and similar experiments, it is necessary to take 
 care that the whole of the projecting or receding surface is equally 
 illuminated; since the presence of any shadow proceeding from a 
 known source of light, destroys the illusion, by forcing the mind to 
 recognise the real figure of the object. 
 
 F P 2 
 
436 ESTIMATION OF DISTANCE. 
 
 moreover, we are deprived of that power of measuring the 
 relative distances of near objects, which we derive from the 
 conjoint use of both eyes ( 563) ; and thus a well-painted 
 picture, still more a photograph, may so strongly suggest the 
 idea of projection, in virtue of its exact perspective and its 
 contrast of light and shadow, that it is difficult to believe it 
 to be a flat surface, even though it be within but arm's 
 length of the eye. 
 
 563. Our idea of the distance of objects is evidently 
 acquired by experience. An infant, when a bright object 
 is held before its eyes, attempts to grasp it with its little 
 hands, but obviously has no certain idea of its situation; 
 and the same is observed in persons who have but recently 
 acquired sight. Here, then, the impressions made upon the 
 eyes have to be corrected by those received through the touch, 
 before the power of judging of distances is acquired; but 
 when it has been once acquired, we can accurately estimate 
 the relative distances of near objects without using our hands. 
 This we do chiefly by the interpretation we have learned to 
 make, of the sensations which are occasioned in the muscles 
 of the eyes by their various actions. When we fix both our 
 eyes upon a distant object, their axes are nearly parallel to 
 each other ; but when we direct them to a near object, the 
 axes of the eyes meet in the point at which we are looking. 
 This is very easily seen by watching the eyes of another 
 person, when fixed upon an object, first held at arm's length, 
 and then brought nearer and nearer to the middle point 
 between the eyes ; the two cornese are at first directed nearly 
 straightforwards ; but they gradually turn inwards as the 
 object is brought nearer, and at last a very decided inward 
 squint is produced, which disappears as soon as the object is 
 removed. Thus, for objects within a moderate distance, the 
 degree of convergence of the axes of the eyes, and the mus- 
 cular sensations thereby produced, afford us sufficient means 
 of judgment. 
 
 564. We perceive this in another, as well as in ourselves ; 
 for by observing his eyes, we can judge, not only of the 
 direction, but of the distance, of the object he is looking at. 
 Thus when a person A sees a friend B looking towards 
 him, he can at once tell, by the appearance of B's eyes, 
 whether he is looking at Mm, or at an object nearer or more 
 
ESTIMATION OP DISTANCE AND SIZE. 437 
 
 remote ; or whether, being in a contemplative mood, his eyes 
 are fixed upon no definite object, but are looking into space. 
 In the latter case, as in the case of blind persons in whose 
 eyes there is no other indication of loss of sight, the peculiar 
 vacant expression is due to the want of any convergence 
 between the axes of the eyes, such as would indicate that 
 they are fixed upon an object. The assistance which the 
 joint use of both eyes affords in the estimation of distance, is 
 evident from the fact, that, if we close one eye, we are unable 
 to execute with certainty many actions which require a 
 precise appreciation of the distance of near objects, such as 
 threading a needle, or snuffing a candle. Instances are not 
 unfrequent in which persons have first become aware, by 
 experiencing this difficulty, that they had lost the sight of' 
 one of their eyes. 
 
 565. In regard to distant objects, our judgment is chiefly 
 founded upon their apparent size, if their actual size be 
 known to us, and also upon the extent of ground which we 
 see to intervene between ourselves and the object. But if 
 we do not know their actual size, and are so situated that we 
 cannot estimate the intervening space, we principally form 
 our judgment from the greater or less distinctness of their 
 colour and outline. Hence, this estimate is liable to be very 
 much affected by varying states of the atmosphere ; a distant 
 ridge of hills, for example, sometimes appearing to be more 
 remote, at other times to be comparatively near, according as 
 the air is hazy or peculiarly clear. 
 
 566. Our notion of the size of an object is closely con- 
 nected with that of its distance. It is founded on the 
 dimensions of the picture formed by the object upon the 
 retina ; but it is corrected by the known or supposed distance 
 of the object itself. Thus, I hold up a book at a certain 
 distance from my eye, and it covers the whole of the opposite 
 window ; the apparent size of both pictures, therefore, is just 
 the same ; but knowing that the book is much nearer than 
 the window, I infer that it is much smaller. When we know 
 their respective distances, the estimate of their real sizes is 
 very easy : but this is not the case when we only guess-at 
 their distances. Hence our estimation of the size of objects 
 even moderately distant, is much influenced by states of the 
 atmosphere. Thus, if we walk across a common in a fog, 
 
438 DURATION OF LUMINOUS IMPRESSIONS. 
 
 a child approaching us appears to have the size of a man, and 
 a man seems like a giant ; for the indistinctness of the outline 
 excites in the mind the idea of distance ; and the same pic- 
 ture, if supposed to be that of a more remote object, will give 
 rise to the idea of greater size. 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 unprepared 
 for such illusions, by a skilfully-painted picture, the view of 
 which is so contrived that its distance from the eye cannot be 
 estimated in the ordinary manner ; the objects it represents 
 being invested by the mind with their real sizes and respective 
 distances, as if their real images were formed upon the 
 retina. This illusion has been extremely complete in some 
 of those who have seen the panoramic view of London in the 
 Colosseum. 
 
 567. When a number of luminous impressions are made 
 upon the retina at short intervals, they become blended into 
 one, the intervals being undistinguishable. Thus, when we 
 rapidly move the end of a lighted stick in a straight line or 
 circle, the impression produced is that of a band or ring of 
 light ; for the impression made by the light, as it passes each 
 point, remains for some time subsequently. If the stick be 
 whirled round with sufficient rapidity for it to reach any 
 point a second time, before the impression made by its pre- 
 vious passage has departed, an unbroken circle of light is 
 produced. By experiments made in this manner, we may 
 determine the longest interval during which visual im- 
 pressions remain on the sensorium ; for if we find that a hot 
 coal, whirling round at the rate of six times in a second, 
 produces a continued circle of light, but that the circle is 
 broken when it turns round only five times in a second, we 
 know that the length of the impression is l-6th of a second. 
 By experiments of this kind, it has been found that the 
 duration varies in different individuals, and in the same 
 individual at different times, from l-4th to 1-1 Oth of a 
 second. On this principle several very ingenious toys have 
 been constructed, in which two or more images are com- 
 bined, by the rapid revolution of a wheel on which they are 
 painted. 
 
 568. Some persons, whose sight is perfectly good for forms, 
 distances, &c., are unable to discriminate colours. This is 
 
COMPLEMENTARY COLOURS I COLOURED SHADOWS. 439 
 
 particularly noticed in regard to the complementary^ colours, 
 especially red and green ; so that such persons are not able to 
 distinguish ripe cherries amongst the leaves of the tree, except 
 by their form. 
 
 569. When the retina has been exposed for some time to a 
 strong impression of some particular kind, it seems less sus- 
 ceptible of feebler impressions of the same kind ; just as the 
 ear, when it has been exposed to a series of very loud sounds 
 (as the discharge of cannon in a naval engagement), is for 
 some time deaf to fainter ones. Hence several curious visual 
 phenomena result. 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 affected 
 by the fainter rays reflected by that part of the paper. If the 
 eye has received a strong impression from a coloured object, 
 the spot afterwards seen 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 then be suddenly turned so 
 as to rest upon the white surface, we see a green image of the 
 spot. The same explanation applies to the curious pheno- 
 menon of coloured shadows. It may be not unfrequently 
 observed at sunset, that, when the light of the sun acquires 
 a bright orange colour from the hue of 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 impression of one colour is made upon the general 
 surface of the retina ; and at any particular spots from which 
 the coloured light is excluded, that particular hue is not per- 
 ceived in the faint light that remains, and its complement 
 only is 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. 
 
 1 White, or colourless light, is spoken of as composed of three primary 
 colours, red, blue, and yellow. By the complementary colour is meant 
 that which would be required to make white light, when mixed with 
 the original. Thus, red is the complement of green (which is composed 
 of yellow and blue) ; blue is the complement of orange (red and 
 yellow); yellow of purple (red and blue); and vice versd, in all 
 instances. 
 
440 COMPLEMENTARY COLOURS. DIRECTION OF MOVEMENTS. 
 
 570. Upon these properties of the eye are founded the laws 
 of harmonious colouring ; a full knowledge of which should 
 be possessed by artists of every kind who are concerned with 
 contrasts of colour, whether in pictures, architectural decora- 
 tions, or even in dress. All complementary colours have an 
 agreeable effect when judiciously disposed in combination ; 
 and all bright colours which are not complementary have 
 a disagreeable effect, if they are predominant : this is espe- 
 cially the case in regard to the simple colours (red, blue, and 
 yellow), strong combinations of any two of which, without 
 any colour that is complementary to either of them, are ex- 
 tremely offensive. Painters who are ignorant of these laws, 
 introduce a large quantity of dull grey into their pictures, in 
 order to diminish the glaring effects which they would other- 
 wise 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 proper attention to physiological 
 principles. 
 
 571. The Eye is endowed with common sensibility ( 487) 
 .by the fifth pair of nerves ; and when this is paralysed, all 
 parts of it are completely insensible to the touch, although 
 the power of vision may remain unimpaired. It seldom pre- 
 
 . serves its healthy condition in this state, however ; for the 
 . lachrymal and mucous secretions which protect its surface, are 
 no longer formed as they should be ; and inflammation, often 
 , terminating in the destruction of the eye, is the result. 
 
 572. The visual sensations obtained through the Eye have 
 numerous and varied purposes among the lower animals. 
 That they chiefly serve to direct their movements, is evident 
 jrom observation of these movements ; and from the fact, 
 .that, when the eyes are covered or destroyed, most animals 
 make little attempt at determinate motions, though they fre- 
 quently exhibit a great deal of restlessness. There are a few 
 Vertebrata, however, which do not possess perfectly-formed 
 eyes, and which are consequently guided in their movements 
 by other senses. This is the case with the Mole, which 
 spends its whole life in burrowing beneath the ground ; and 
 also with the Proteus, and some others of the lower Am- 
 phibia, which inhabit the dark recesses of subterranean lakes 
 and channels. 
 
 73. In the Articulated series of animals, we meet with 
 
COMPOUND EYES OP ARTICULATA. 441 
 
 eyes of a kind entirely different from those which, have been 
 previously described. In most Insects we notice a large black 
 or dark-brown hemispherical body, situated on either side of 
 the head (fig. 212) ; and in Crabs, Lobsters, &c., we find 
 spherical bodies of similar appearance mounted on short 
 footstalks, which are capable of some degree of motion. 
 When these are examined with the microscope, their surface 
 
 Fig. 212. HEAD AND EYES OF THE BEE, SHOWING THE DIVISION INTO FACETS. 
 a, a, antennae; A, facets enlarged ; B, the same with hairs growing between them. 
 
 is seen to be divided into a vast multitude of hexagonal (six- 
 sided) facets. In a species of Beetle (Mordella) upwards of 
 25,000 of these have been counted ; in a Butterfly, above 
 17,000; in a Dragon-fly, more than 12,500; and in the 
 common House-fly, 4,000. Every one of these facets may be 
 regarded as the front of a distinct eye, which, however, instead 
 of being globular, is conical in its form ; the front being the 
 base of the cone, and the apex or point being directed towards 
 the optic nerve, which swells-out into a bulbous expansion 
 that fills a large part of the interior of the hemisphere. Each 
 individual eye consists, therefore, of its facet, which (being- 
 convex on both surfaces) acts as a lens ; of the transparent 
 cone behind this, which may be compared to the vitreous 
 humour; and of the fibre which passes from the bulbous 
 expansion of the optic nerve to the point of this cone. The 
 several fibres are separated from one another by a considerable 
 quantity of black pigment, which also fills up the spaces 
 between the cones ; and it is to this that the black appearance 
 of the whole compound eye is due. 
 
 574. We must thus regard each of the cones, which, united 
 together, constitute the hemispherical or globular mass, in 
 the light of a distinct eye ; but the entire aggregate seems to 
 
442 COMPOUND EYES OF ARTICULATA. 
 
 correspond in function with the single eye of the Vertebrate 
 animal. For no rays except those which correspond in direc- 
 tion with the axis of each cone, can reach the fibre of the 
 optic nerve at its apex ; all others being stopped by the layer 
 of black pigment which surrounds it. Hence it is evident 
 that each separate eye must have an extremely limited range 
 of vision, being adapted to receive but a very small collection 
 of rays proceeding from a single point in any object ; and as 
 these eyes are usually immoveable, animals with but a small 
 number of them would be very insufficiently informed of the 
 position of external things. But by the vast multiplication in 
 the number of the eyes, and the direction of their axes to 
 every point in the hemisphere, their defects are compensated j 
 a separate eye being provided, as it were, for every point to 
 be viewed. And it is quite certain, from observation of the 
 movements of Insects, that their vision must be very perfect 
 and acute. 1 
 
 575. Although these Compound Eyes exist in all Insects 
 and in most Crustaceans, Spiders and Centipedes, they are in 
 general not the only organs of vision which these animals 
 possess. Most of them are also furnished with several simple 
 eyes, analogous in their structure to those of higher animals, 
 but les? complex and perfect in their organization ; these, 
 which are for the most part disposed on the back of the head, 
 are largest in Spiders. The larvas of some Insects possess the 
 simple eyes without the compound; the latter being only 
 developed at the time of the last metamorphosis. The simple 
 eyes of Insects do not appear to be nearly so efficient as 
 instruments of vision, as are their compound ones ; for when 
 the latter are covered, the animals seem almost as perplexed 
 as if they were perfectly blinded. Simple eyes, closely re- 
 sembling those of Insects in structure, are found in most of 
 
 1 It is commonly believed that each of these compound eyes pro- 
 duces its own image of the same external object, as do our two eyes; 
 but from the description here given of their separate directions when 
 united, it is evident that in no two of them can an image of the same 
 object be formed at the same time. The membrane formed of all the 
 lens-like cornese united together, when separated from the other parts 
 of the eye, and flattened-out, has the properties of a multiplying-glass, 
 each lens forming a distinct image of the same object ; but this is not 
 the case when they are arranged in their natural position, because no 
 two of them have the same direction. 
 
EYES OP MOLLUSKS, ETC. ANIMAL MOTION. 443 
 
 the Mollusca which possess a head namely, in the Gastero- 
 pods, Pteropods, and Cephalopods ; those of the last class 
 present an evident approach to the eyes of Fishes, in the 
 greater completeness of their structure, and in their adapta- 
 tion for distinct vision. In many of the lower Mollusca, as in 
 the Eotifera and several Annelida, and also at the end of the 
 arms of the Star-fish, red spots may be seen, which appear to 
 be rudiments of eyes ; but no distinct organs of vision can be 
 seen in the Zoophytes and lowest Mollusca ; although many 
 of them appear very sensible to the action of light. 
 
 CHAPTER XII. 
 
 ANIMAL MOTION, AND ITS INSTRUMENTS. 
 
 576. THE different modifications of the faculty of Sensation 
 which have been described in the preceding chapter, enable 
 Man and other Animals to become acquainted with what is 
 going-on around them. But their connexion with the external 
 world is not confined to this faculty ; for if they possessed it 
 alone, they would be nearly as passive as are Plants, expe- 
 riencing, it is true, pain and pleasure from their sensations, 
 but not having the power of avoiding the one or of procuring 
 the other. They are endowed, however, with another faculty, 
 that of spontaneous movement ; which serves the double 
 purpose of enabling them to act upon the inanimate world 
 around them, and of communicating to each other their feel- 
 ings and ideas. Thus, if we find ourselves scorched by a flame, 
 we either withdraw our bodies from it, under the direction of 
 the instinct which leads us to avoid suffering, or we set about 
 to extinguish the fire by an act of the will, founded upon our 
 rational knowledge of its injurious tendency. The Plant, 
 even if it had sensation (which some naturalists have sup- 
 posed), could do neither of these things. Again, it is entirely 
 by the movements concerned in speech, by those giving 
 expression to the countenance, and by the gestures of the 
 body, that we convey to beings like ourselves a knowledge of 
 what is passing in our own minds ; of this power we know 
 that plants are entirely destitute, and it is possessed in a very 
 limited degree by the lower Animals. 
 
444 MOVEMENTS NOT DEPENDENT ON MENTAL DIRECTION. 
 
 Contractile Tissues : Muscular Contractility. 
 
 577. When we examine into the nature of the movements 
 of the lower tribes of Animals, however, we find that they 
 bear a much closer analogy to those of Plants, than they do 
 to those which are the expressions of the self-determining 
 power of higher Animals. Among the simplest Protozoa, it 
 seems as if the change of form of the single cell of which each 
 individual is composed, were the sole means of movement 
 which it possesses ( 129) ; and this change of form often 
 appears rather to be due to the nutrient actions taking place 
 within the cell, than to occur in respondence to any im- 
 pression made upon its exterior. In such movements it is 
 impossible to suppose with any probability that consciousness 
 can participate. So, again, among Infusory Animalcules, all 
 the movements of the body are effected by the action of cilia 
 ( 133), which we know in our own experience to be entirely 
 removed from any mental direction, and which we see to be 
 exhibited under a no less remarkable aspect by the " zoo- 
 spores " or motile buds of the Algae (BOTANY, 767). 
 
 578. Although the movements of the Hydra ( 121) and 
 other Zoophytes may appear to indicate the existence of a self- 
 determining power, yet it is very doubtful whether such an 
 endowment is really possessed by these animals. For their 
 contractile tissue is of the simplest possible character, resem- 
 bling that which is found in the early state of newly-forming 
 parts of higher Animals ; and when the movements executed 
 by it are carefully compared with our own, it becomes obvious 
 that they are analogous, not to those of the Human body and 
 limbs generally, but to those of the muscular coat of the 
 alimentary canal and of the muscles concerned in deglutition, 
 which not only take place without any voluntary determina- 
 tion on our parts, but may even be performed without our 
 consciousness. In like manner, the rhythmical movements of 
 the umbrella-like discs of the Medusce ( 120), by which many 
 species of them are propelled through the water, bear a much 
 closer analogy to the rhythmical movements of the heart of 
 higher Animals, than they do to any other of their actions ; 
 and are probably performed, like these, without any exercise 
 of will, and even without the guidance of consciousness. 
 
 579. In proportion, however, as we ascend the scale, we 
 
MOVEMENTS PRODUCED BY MUSCULAR CONTRACTION. 445 
 
 find a peculiar tissue, the Muscular ( 55 59), distin- 
 guished from the rest ; in which the general contractility of 
 the body becomes, as it were, concentrated. In proportion to 
 the development and complexity of this muscular apparatus, 
 it supersedes, the more feeble contractility diffused through 
 the fabric of the lower tribes. It now, moreover, becomes in 
 great degree subjected to the Nervous System ; by which all 
 those parts of it which are not connected with the functions 
 of Organic life merely, are rendered subservient to the Will, 
 :md thus become its instruments in determining the place 
 and the various actions of the body. Still we find that the 
 ordinary actions of those portions of the muscular apparatus 
 which are most immediately subservient to the functions of 
 organic life, are essentially independent of nervous influence, 
 and are very little under its control ; as we see in the case of 
 the alternate contraction and relaxation of the heart, and the 
 peristaltic movements of the alimentary canal. 
 
 580. The peculiar contractility of muscular fibre may be 
 called into action by various means. As in certain vegetable 
 tissues (VEGET. PHYS. 390), contraction may be excited by 
 a mechanical stimulus directly applied to the muscle itself 
 Thus, if the heart of an animal recently killed be touched 
 with a pointed instrument, it will contract and then dilate, as 
 if performing its ordinary action ; and this may be repeated 
 several times. In the same manner, if the walls of the intes- 
 tinal canal be pricked or pinched, they will re-commence and 
 continue for a short time their peristaltic movement. And if 
 any part of an ordinary muscle be irritated in the same 
 manner, that particular bundle will contract, but the rest will 
 not be affected. Now these actions are analogous to those 
 performed by the Sensitive Plant, Venus' s Fly-trap, and many 
 other plants, some part of whose tissues contracts in like 
 manner when an irritation is applied to it, causing it may 
 be extensive and important motions. It appears to be in 
 this manner that the contractions of the heart, and of the 
 alimentary tube from the stomach to the rectum, are ordinarily 
 excited in the living body. 
 
 581. But there must be some other cause for the con- 
 tinuance of the rhythmical movements of the heart, as well as 
 of some other organs; for the heart of many cold-blooded 
 animals will continue to contract and dilate many hours after 
 
446 STIMULANTS TO CONTRACTION OF MUSCLES. 
 
 it has been removed from the body, when it neither receives 
 nor propels blood. 1 In the same manner, the peristaltic 
 motions of the intestine continue to propel its contents for 
 some time after the general death of the body; and may even 
 take place when the whole tube has been removed from it, 
 and has been completely emptied. There is strong reason, in 
 fact, for attributing to certain kinds of muscular tissue an 
 inherent motiltiy, in virtue of which it moves of itself without 
 any external stimulation; the changes which are concerned 
 in its nutrition developing a force which must manifest itself 
 in action; just as a Leyden jar, which is receiving a con- 
 tinuous charge from an electrical-machine, discharges itself 
 whenever its electricity attains a certain tension. 
 
 582. But the muscles of the trunk, limbs, &c., are not 
 called into action in this manner; for, as just now stated, 
 a stimulus applied to any one part of these does not excite 
 contraction in the whole muscle (as it does in the case of the 
 heart), but only in the individual bundle of fibres irritated. 
 These muscles are all of them supplied with nerves ( 63) 
 from the Cerebro-spinal system, or the nervous centres that 
 correspond to them in Invertebrated animals ; and it is only 
 by a stimulus transmitted to them along these trunks, that all 
 the bundles of which the muscle is composed can be called 
 into action at once. 
 
 583. When the trunk of a nerve supplying a muscle is 
 irritated by pricking, pinching, &c., in the body of a living 
 animal, or in one recently dead, the whole muscle is thrown 
 into contraction; and this contraction is peculiarly strong 
 when a current of Electricity is transmitted along the nerve. 
 In cold-blooded animals, whose muscular fibre retains its vital 
 properties for a much longer time after death than that of 
 warm-blooded, this contraction may be excited by a very feeble 
 current of electricity, even in a limb which has been separated 
 from the body for twenty-four hours or more ; and it was 
 owing, in fact, to this circumstance, that the peculiar form of 
 electricity which is now termed Galvanic or Voltaic was dis- 
 covered. The wife of Galvani, who was Professor of Medicine 
 
 1 There is an instance on record, in which the heart of a sturgeon, 
 that had been removed from the body and had been inflated with air, 
 continued to beat until the auricle had become so dry as to rustle 
 during its movements. 
 
INFLUENCE OF ELECTRICITY ON MUSCLES. 447 
 
 at Bologna, being about to prepare some soup from frogs, and 
 having taken off their skins, laid them on a table in his study, 
 near the conductor of an electrical machine which had been 
 recently charged ; and she was much surprised, upon touching 
 them with the scalpel (which must have received a spark 
 from the machine), to observe the muscles of the frog strongly 
 convulsed. Her husband, on being informed of the circum- 
 stance, repeated the experiment ; and found that the muscles 
 were called into action by electricity communicated through 
 the metallic substance with which the limb was touched. 
 
 584. The experiment was repeated in various ways by 
 Volta, who was Professor of Natural Philosophy at Pavia; 
 and he found that the effects were much stronger when the 
 connecting medium through which the electricity was sent, 
 consisted of two metals instead of one ; and from this circum- 
 stance he was led to the discovery that electricity is produced 
 by the contact of two different metals a discovery which has 
 since been so fruitful in most important results. The follow- 
 ing simple experiment puts this in a striking point of view. 
 If the skin of the legs of a Frog recently killed be removed, 
 and the body be cut across, above the origin of the great 
 (sciatic) nerve going to the legs, if the spine and nerves be 
 then enveloped in tin-foil, and the legs be laid upon a plate 
 of silver or copper, convulsive movements in the muscles 
 will be excited every time that the metals are made to touch 
 each other so as to complete the electric circuit. 
 
 585. Similar experiments have been tried with the Voltaic 
 battery, upon the dead bodies of criminals recently executed. 
 If one wire be placed upon the muscles which it is desired to 
 call into action, and the other upon the part of the spine from 
 which the nerves proceed, movements of every kind may be 
 produced. No agent more effectually imitates the natural 
 action of the nerves, in exciting the contractility of muscles, 
 than Electricity thus transmitted along their trunks ; and we 
 have already seen ( 489) that Electricity, transmitted along 
 the sensory nerves, excites the peculiar changes in the brain 
 by which sensations are produced. Hence it has been, sup- 
 posed by some philosophers, that electricity is the. real force 
 by which the nerves act upon the muscles ; more especially 
 since it is certain that, in those animals which generate large 
 quantities of electricity, the nerves have a great share in this 
 
448 INFLUENCE OF NERVES ON MUSCLES. 
 
 peculiar operation ( 423). But there are many objections to 
 such a view ; and it appears more correct to regard Electricity 
 and Nerve-force as correlated, that is, as each capable, under 
 certain conditions, of exciting an equivalent measure of the 
 other, than to consider them as identical. 
 
 586. The power, whatever be its nature, by which the 
 Nerves act upon the Muscles in the living body, originates 
 in the central organs, or ganglionic masses, of the nervous 
 system ; and is propagated from these, through the nervous 
 fibres, to the muscles, in a mode precisely analogous to that 
 in which the electric power, called-forth by the action of an 
 electrical machine or galvanic battery, is transmitted to any 
 distance through conducting wires. If the conductor be 
 divided, no action at the centre, however powerful, can pro- 
 duce any change at its extremities ; and in this manner, by 
 division of the nervous trunk, the muscle supplied by it is 
 palsied. The muscle itself does not thereby lose its contrac- 
 tility; for it may still be made to contract by a stimulus 
 transmitted through the part of the trunk that remains 
 attached to it, as, for instance, by pricking or pinching the 
 cut extremity, or by passing an electric current along it ; but 
 it is completely withdrawn from the dominion of the nervous 
 centres under which it previously was ; and cannot be called 
 into action either by the will, by an emotion, or by a reflex 
 impulse. The part of the trunk in connexion with it soon 
 loses its power of conveying irritations ; and the muscle itself 
 being thrown into disuse, in time loses its contractility. 
 
 587. From this last fact it has been supposed that the 
 contractility of muscular fibre depends upon its connexion 
 with the nervous system, and is not an endowment peculiar 
 to itself. But this idea is disproved by a number of circum- 
 stances. Thus the contractility of the heart and intestinal 
 tube is exhibited, long after these parts have been separated 
 from their nerves. The contractility of other muscles may be 
 exhausted by repeated excitement, so that even the stimulus 
 of galvanism will not produce movement in them ; and yet it 
 may be recovered after the nervous trunks have been divided. 
 And it has been ascertained that if the muscles be frequently 
 exercised, as by the application of galvanism once or twice a 
 day, they will retain their contractility for any length of time. 
 This exercise is further found to have the effect of preventing 
 
EFFECTS OF WITHDRAWAL OF NERVOUS POWER. 449 
 
 the wasting-away of the muscles which otherwise takes place ; 
 and thus we see that the preservation of this peculiar property 
 is dependent upon the due nutrition of the muscle, whilst the 
 loss of the property results from its want of nutrition, as we 
 find to be the case in regard to other tissues. Further, the 
 activity of the nutrition of muscles depends in great part 
 upon the use that is made of them ; and thus we find that 
 any set of muscles in continual employment undergoes a great 
 increase in size and vigour ; whilst those that are disused, 
 even though their nervous connexions remain entire, lose 
 their firmness and diminish in bulk, until, if the inaction be 
 continued long enough, almost all trace of proper muscular 
 substance disappears, and the contractility of the part is lost. 
 
 588. But a muscle may be palsied by some change taking 
 place in the central organs, which shall prevent the nervous 
 influence from being excited there. Thus by an effusion of 
 blood in a certain part of the brain, the arm, leg, or the whole 
 of one side may be paralysed to the influence of the will. 
 But the muscles which are thus withdrawn from the power of 
 the will, may sometimes be moved by an emotional or instinctive 
 impulse, or by reflex action ; their connexion with the parts of 
 the nervous centres, in which these actions respectively origi- 
 nate, remaining unimpaired (Chap. x.). Thus a completely 
 paralytic arm has been seen to be violently shaken, when the 
 emotions of the patient were strongly excited by the approach 
 of a friend. The muscles of the shoulder, in a case of com- 
 plete paralysis of one side, were called into contraction in the 
 reflex movement of yawning ( 341). And the muscles of 
 the legs, when their communication with the brain, and 
 consequently the control of the will over them, has been 
 completely cut off", have been made to act energetically when 
 the feet were tickled, although the patient was not conscious 
 either of the irritation or of the motion. When the muscles 
 are thus aroused to occasional activity, their nutrition is not 
 so much impaired, and their contractility does not depart 
 nearly as completely as when they are thrown into entire 
 disuse by division of their nerves. 
 
 589. Muscles are commonly divided into voluntary and 
 involuntary, according as they act in obedience to the will, 
 or are not under its dominion. But this is not a correct 
 division ; since, whilst nearly all the muscles of the body are 
 
 G G 
 
450 VOLUNTARY AND INVOLUNTARY ACTIONS OF MUSCLES. 
 
 more or less under the control of the will, they may all at 
 times have an involuntary action. The heart and the muscular 
 coat of the alimentary canal, with the muscles concerned in 
 swallowing and in one or two other actions of a similar 
 character, are the only muscles which the will cannot either 
 set in action, or control when in action. There are several 
 muscles whose usual movements are of a reflex and therefore 
 involuntary character, and are yet capable of being, to a 
 certain extent, controlled and governed by the will. Such 
 are the movements of respiration ; which will continue to 
 take place after the brain has been removed, and which go on 
 regularly during the profoundest sleep and the most complete 
 withdrawal of the attention from them. In the Invertebrated 
 animals these motions are probably not influenced by the 
 will ; but in the air-breathing Vertebrata they are placed in a 
 certain degree under the dominion of the will, in order that 
 they may be made to contribute to the production of the vocal 
 actions of speaking, singing, &c., which are restricted to these 
 classes. We can hold the breath for a certain time by a 
 voluntary effort, or we can expel or draw it in more quickly 
 tli an usual ; but no voluntary effort can cause the breath to 
 be held for more than a few moments; for the uneasiness 
 which is then felt, and which is continually increasing, causes 
 an involuntary action of the muscles, by which action it is 
 relieved. 
 
 590. But again, there are other muscles, whose ordinary 
 actions are voluntary ; but which are occasionally made to 
 act independently of the will, or even against its direction. 
 Such are those which are excited by the emotions, as in 
 laughing, crying, sobbing, &c. We may have the strongest 
 desire to check these actions, owing to the unfitness of the 
 time and place for their manifestation ; and yet we may be 
 unable to do so. And lastly, muscles whose action is usually 
 voluntary may be occasionally called into powerful contrac- 
 tion, which the will cannot in the least degree control or 
 prevent ; this is the case in cramps, convulsions, &c., of 
 various kinds. All these facts are readily accounted-for by 
 the knowledge we now possess, as to the functions of the 
 different parts of the nervous centres from which the muscles 
 receive their stimulus to action (Chap. x.). 
 
 591. The vigorous action of the muscular structure is de- 
 
CONDITIONS OP MUSCULAR ACTIVITY. 451 
 
 pendent upon several conditions. In the first place, it requires 
 an active nutrition of the muscles themselves. Firm, plump, 
 and high-coloured muscles act with greater force than those 
 which are pale and flabby, even though the size of the latter 
 may be greater. Again, in all those animals whose activity is 
 greatest, a constant supply of oxygen is requisite for muscular 
 vigour. This, like the nutritive material, is conveyed, in 
 Birds and Mammals, by the blood ( 235) ; in Insects, on the 
 other hand, it actually enters the muscular tissue in the state 
 of atmospheric air ( 321). In Eeptiles, again, the blood 
 goes to the tissues very imperfectly oxygenated ; and their 
 movements are comparatively slow and feeble. But it is a 
 remarkable circumstance, that in the dead bodies of the latter, 
 or in parts separated from the living body, the property of 
 contractility does not depart nearly so soon as it does in similar 
 parts of warm-blooded animals. By experiments on Mammals 
 it has been found that the muscles of the trunk cannot be 
 caused to contract by galvanism for more than two or three 
 hours after death, though the auricles of the heart retain their 
 contractility for some hours later. The muscles of Birds 
 (whose respiration is more active, and whose temperature is 
 higher) lose their contractility yet sooner ; but those of Rep- 
 tiles sometimes retain the power of contracting for several 
 days. When venous or imperfectly-aerated blood is made to 
 circulate through the vessels of warm-blooded animals, it acts 
 like a poison upon them, diminishing or even destroying their 
 contractility. 1 
 
 592. Further, the energy of muscular contraction depends 
 in great degree upon the power of the stimulus which is trans- 
 mitted to it through the nervous system. We often have the 
 opportunity of observing this, in the case of persons who are 
 under the excitement of violent passion or of insanity; a 
 delicate female becoming a match for three or four strong 
 men, and even breaking cords and bands that would hold the 
 most powerful man in his ordinary state. The strength in 
 such circumstances seems almost preternatural ; but it is not 
 
 1 Other substances do this with even greater rapidity ; thus a strong 
 solution of nitrate of potass (nitre) injected into the blood-vessels, and 
 conveyed by them to the heart, causes the immediate cessation of its 
 action, the poison finding its way, through the vessels of the organ 
 itself, into the capillaries of its muscular structure. 
 GG2 
 
452 FATIGUE. ENERGY AND KAPIDITY OF MOVEMENT. 
 
 greater than that which we see manifested in convulsive 
 actions, where the movements depend only upon the reflex 
 activity of the spinal cord. Thus a slender girl affected with 
 a spasmodic affection of the muscles of the spine, which threw 
 the back into an arch of which the head and the heels were 
 the two resting-points, has been known to raise a weight of 
 9001bs. laid on the abdomen with the absurd intention of 
 straightening the body. 
 
 593. The sense of fatigue, which comes-on after prolonged 
 muscular exertion, is really dependent upon a change in the 
 brain, though usually referred by us to the muscles that have 
 been exercised. For it is felt after voluntary motions only ; 
 and the very same muscles may be kept in reflex action for 
 a much longer time, without any fatigue being experienced. 
 Thus, we never feel tired of breathing; and yet a forced 
 voluntary action of the muscles of respiration soon causes 
 fatigue. The voluntary use of the muscles of our limbs, in 
 walking or running, soon occasions weariness; but similar 
 muscles are used in Birds and Insects, for very prolonged 
 nights, without apparent fatigue; and as we find that the 
 actions of flight may be performed, after the brain, or the 
 ganglia that correspond to it in Insects, have been removed 
 ( 444, 465), we may regard them as of a reflex character ; 
 and the absence of fatigue is thus accounted-for. 
 
 594. 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 sixty 
 times its own length, and to move as many times its own 
 weight. The short-limbed Beetles that inhabit the ground 
 have an enormous power, which is manifested both in their 
 movement of heavy weights, and in the resistance they 
 overcome with their jaws. Thus the Dung- or " shard-borne " 
 Beetle can support uninjured, and even elevate, a weight equal 
 to at least 500 times that of its body. And the 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 
 rapidity of the movements of Insects is also most extraordi- 
 narily great, and is especially seen in the vibrations of their 
 wings. It would be impossible to form an estimate of the time 
 occupied by these, were it not for the musical tones they pro- 
 duce ; and it may be calculated from these that the wings of 
 
APPLICATIONS OF MUSCULAR POWER. 453 
 
 many Insects strike the air several hundred times, and 
 those of some of the smaller Insects many thousand times, 
 in a second of time. 
 
 Applications of Muscular Power : Bones and Joints. 
 
 595. Muscular contraction performs an important part in 
 nearly every one of the functions of which we have already 
 treated. Thus the reception of the food, and its propulsion 
 along the alimentary canal, forming part of the function of 
 Digestion, are accomplished through its means. The Circula- 
 tion of the blood, again, depends mainly on the agency of a 
 contractile organ, the heart. The Eespiration cannot be kept 
 up, in the higher animals at least, without the aid of certain 
 movements which are accomplished by the muscles. With 
 the processes of Nutrition and Secretion it is not so closely 
 connected ; but the latter is dependent upon it so far as this, 
 that its products are carried out of the body by the aid of 
 muscular contraction. And even in Sensation, the peculiar 
 endowment of muscular tissue comes into use ; by giving to 
 the organs of sense those movements which enable them to 
 take a wider range, and to apply themselves most perfectly to 
 the objects before them. But we have now to study its appli- 
 cations in those general and partial movements of the body, 
 on which depend the locomotion (or change of place) of 
 animals, their attitudes, and a number of other important 
 actions, entirely of a mechanical nature. 
 
 596. The organs by which these are effected, may be con- 
 veniently divided into the active and passive. The active are 
 those which have peculiar vital powers within themselves, and 
 which exert these in giving motion to other parts. To this 
 class, therefore, we refer the Muscles, whose peculiar endow- 
 ments have been just considered. The passive organs, on the- 
 other hand, are those which perform no action of themselves,, 
 which have no power but that of yielding a simply mechanical 
 support, and which consequently perform no movement but 
 such as they are made to do by the muscles. Of this kind 
 are the hard parts which form the skeleton or solid frame- 
 work of the body, whether this be internal or external. 
 
 597. In the lower tribes of animals, the muscles are all 
 inserted in the soft and flexible membrane which covers the 
 body ; and it is by acting upon this, that they can change the 
 
454 APPLICATIONS OF MUSCULAR POWER I SKELETON. 
 
 form of the body in such a manner as to cause it to move, 
 either altogether or in part. This is the case, for example, in 
 the Leech, Earth-worm, and other Annelids ; which are fur- 
 nished with two sets of muscular fibres, one running along 
 the body, and the other passing round it in rings. By the 
 contraction of the former, the two ends are drawn together, 
 so that the body is shortened ; whilst by that of the latter, 
 its diameter is lessened, so that it is necessarily lengthened. 
 By these two movements, which take place alternately, the 
 progression of the animal is accomplished ; and by varying 
 the contractions of one part or another, almost any form and 
 direction can be given to the soft and flexible body. 
 
 598. But in the higher animals we find the apparatus of 
 movement to consist, not only of muscles, but also of a frame- 
 work of solid pieces, which serves to augment the precision, 
 the force, and the extent of the movements ; whilst, at the 
 same time, it determines the general form of the body, and 
 protects the viscera against injury from without. This solid 
 framework, or skeleton, to which the muscles are attached, 
 may be, as we have seen, either internal or external. In the 
 Vertebrated classes, the hard skeleton is internal; in the 
 Articulated series it is external; in the Mollusks it is 
 external, but does not afford fixed attachments to muscles, 
 except to such as draw together its valves, or connect it with 
 the soft body of the animal ; and in the Radiata its position is 
 variable, being sometimes external as in the Echinodermata, 
 and sometimes internal as in the stony Corals. 
 
 599. The skeleton of Vertebrata differs from that of all the 
 Invertebrated classes in the much higher character of its or- 
 ganization, which enables it to grow with the growth of the 
 body generally, not merely in virtue of the additions it 
 receives, but by the successive removal of its previously 
 formed parts as occasion may require ; so that the skeleton of 
 the adult has been entirely substituted for that of the child, 
 probably no part of the latter being contained in the former. 
 The skeletons of the Invertebrata, where they are not formed 
 of horny matter alone, are consolidated by carbonate of lime, 
 which in some instances (as the shells of many Mollusks, 
 and the Stony Corals) bears so large a proportion to the 
 animal basis, that the latter can scarcely be detected. The 
 growth of these skeletons takes place entirely by additions to 
 
CONNEXION OF SEPARATE PIECES OP SKELETON. 455 
 
 the parts already formed ; there is nothing like that " inter- 
 stitial " change which we see in bone, and which is performed 
 by the agency of the blood conveyed through the Haversian 
 canals that traverse its substance j and where the skeleton is 
 external, it must either be adapted by such additions to the 
 augmenting bulk of the body it incloses, or must be cast-off 
 and replaced by another. The latter method is that which 
 is followed in the Crustacea ; of the former we have examples 
 in the shell-bearing Mollusks, whose shells receive successive 
 additions at their free margins, and in the Echinodermata, 
 whose box-like envelopes are made to increase equally in all 
 directions, by additions to the edges of the numerous separate 
 pieces of which they are composed ( 118). 
 
 600. The different portions of the skeleton are articulated, 
 or united by joints to one another, in such a manner that 
 they can move with greater or less freedom. This we see 
 both in the Vertebrated and in the Articulated classes. In the 
 latter, the joints are for the most part very simple in their 
 construction. The different rings or pieces are held together 
 by a flexible membrane passing from one to the other j this 
 seems to be little else than a portion of the integument 
 originally covering the body, which has remained uncon- 
 solidated whilst the rest has been hardened. And sometimes 
 they are made to adhere to each other by a kind of " solder- 
 ing," so as to be altogether immovable. But in the internal 
 skeletons of the Vertebrata we find a more complex mode of 
 union, fitted to afford scope for the greater variety of motions 
 which their parts perform. Here, too, we find some parts 
 immovably united to each other, where support and protec- 
 tion alone are required. These immovable articulations, of 
 which there are several kinds, will be first considered. 
 
 601. All the bones of the head and face (with the excep- 
 tion of the lower jaw), in Man and the higher Vertebrata, 
 have their edges in immediate contact with each other; so 
 that they hold together in the dry skull, as well as during 
 life. Those bones of the skull, which inclose and protect the 
 brain, are very firmly united by what are termed sutures, 1 
 which are mostly formed by the interlocking of the jagged 
 edges of one bone into corresponding notches of the adjoining 
 one : though in some this kind of union is incomplete, while 
 
 1 From the Latin sutura, a seam. 
 
456 SUTUEES : MOVABLE ARTICULATIONS. 
 
 in others it is replaced by a bevelling of the edges that are in 
 contact, or by the reception of a ridge of one bone into a 
 groove in the other. So firmly are the bones united in this 
 manner, that it is difficult to separate them without breaking 
 away some of their projecting parts ; and in the skulls of 
 old persons, the sutures are almost obliterated by the complete 
 union between the adjacent bones. In the infant, on the 
 other hand, the bones of the skull are only united to each 
 other by a membranous substance ; and there is a point at 
 the top of the head, which is not even covered by a bony 
 layer for some time after birth. It is only as the age ad- 
 vances, and ossification becomes more complete ( 52), that 
 firm bony union is effected. 
 
 602. In several other articulations, the bones do not come 
 into direct contact with each other, but are connected by an 
 intervening layer of cartilage, and also by ligaments and 
 other fibrous membranes encircling the articulations. The 
 adjacent surfaces of the bones are flat, and have a slight 
 gliding movement over one another; but the extent of 
 motion permitted is very small. This kind of articulation 
 exists between the bodies of the vertebrae of Man and the 
 higher Vertebrata, between the bones of the pelvis, and some 
 other parts. 
 
 603. The proper movable articulations, by which the limbs 
 are connected with the trunk, and their different divisions to 
 each other, are those to which we commonly give the name of 
 joints. In these, the surfaces of the adjacent bones are not 
 united in any other way than by the ligaments and muscles 
 which surround them ; and they have a free gliding move- 
 ment over each other. They are covered, it is true, by car- 
 tilage; this, however, does not pass from one bone to the 
 other, as in the previous case, but forms a thin layer over the 
 end of each, and presents a very smooth surface, which is 
 secreted from the synovial membrane that envelops the joint 
 lubricated by the fluid ( 44). The beautiful smoothness of the 
 surfaces of the joints, and the manner in which the bones are 
 held together by the muscles and ligaments, is well seen by 
 examining the knuckle-joint at the lower end of a leg of 
 mutton (before being cooked), and the joint which connects 
 it -with the bones of the haunch. These two joints are 
 examples of the two principal varieties of freely-movable 
 
HINGE- AND BALL-AND-SOCKET JOINTS; DISLOCATIONS. 457 
 
 articulations, the hinge-joint, and the ball-and-socket joint. 
 In the first of these, the surfaces of the bones are so formed 
 that the movement, though free as regards its extent, is very 
 limited in its direction; being in fact restricted to a back- 
 ward and forward action in the same line, just like that given 
 by a common hinge. In the second, the end of one bone is 
 formed into a rounded head or ball, and this is received into 
 a corresponding socket or cup in the other, the edge of which 
 is usually deepened by cartilage ; in this manner the bone 
 which carries the ball is enabled to move upon the other in 
 any direction, unless restrained by external checks. Of the 
 hinge-joint we have examples in the elbow, the knee, and the 
 joints of the fingers and toes. Of the perfect ball-and-socket 
 joint we have in Man only two examples, the shoulder, and 
 the hip. In the former the socket is much shallower than in 
 the latter, and the motions of the arm are consequently more 
 extensive than those of the thigh : both, however, are un- 
 checked in regard to their range and direction, except when 
 the limb is brought against the body or against its fellow. 
 The wrist and the ankle-joint are of an intermediate cha- 
 racter; the former more resembling the ball-and-socket, and 
 the latter the hinge-joint. 
 
 604. All these joints are more or less subject to dislocation, 
 by violence of different kinds. This takes place by the 
 slipping-away from each other of the two surfaces, which 
 ought to be in contact. Thus the head of the humerus (or 
 arm-bone) may slip over the edge of its socket, so as to lie 
 entirely on the outside of it ; and this, in consequence of the 
 shallowness of the cup, happens not unfrequently. The head 
 of the thigh-bone, also, may slip out of its socket ; but this 
 accident is more rare, on account of the deepness of its cup. 
 The elbow and knee-joints, as also those of the wrists, ankle, 
 fingers, and toes, may be dislocated by the slipping of one 
 surface on the other, either forwards or backwards, to one side 
 or to the other. One of the most common dislocations is that 
 of the thumb, the lowest articulation of which has rather the 
 character of the ball-and-socket (with a very shallow cup), 
 than of the hinge-joint. But in proportion to the liability of 
 any joint to dislocation, is usually the ease with which it may 
 be brought into place again. 
 
 605. Of the attachments of muscles to the skeleton, one is 
 
458 ORIGIN AND INSERTION OF MUSCLES. 
 
 usually called the origin, and the other the insertion, the 
 origin being in the part that is most fixed, and the insertion 
 in that which moves upon it. Thus the muscle chiefly con- 
 cerned in bending the elbow, has its origin at the shoulder 
 and its insertion in the bones of the fore-arm ; its general 
 dction being to move the latter, while the former is fixed or 
 nearly so. But its attachment to the fore-arm may really 
 become its origin, and its other attachment its insertion ; for, 
 when a person is hanging by his hands from a beam or cord, 
 and raises his body by bending his elbows, the fore-arm is 
 the fixed point, and the shoulder is moved upon it. In like 
 manner, the muscular mass at the bottom of the back, having 
 one attachment to the bones of the pelvis and another to the 
 thigh-bone, serves to draw the latter backwards when the 
 former is made the fixed point, as when we rise-up from the 
 sitting posture ; but it is also continually serving to keep 
 the body upright upon the thighs, the latter being the fixed 
 point, and brings it back into this position when it has been 
 bent forwards as in stooping. Muscles are seldom directly 
 attached to the hard parts, but are united with them by 
 means of the fibrous bands which are called tendons ( 29). 
 Sometimes the tendon is long and round ; this is the case with 
 several of those that move the hand and fingers (fig.^28), which 
 arise from the muscles forming the fleshy part of the fore- 
 arm, and may be felt at the wrists as hard round cords. In 
 other instances, however, the tendon is a broad flat band ; of 
 such we find several within the shell of the body and limbs 
 of the Crab or Lobster, when we have removed the muscle 
 or flesh. 
 
 606. The action of any muscle, in producing a change in 
 the position of a movable bone on which it acts, is deter- 
 mined in the first place by the nature of the movement of 
 which the bone is capable ; and in the second, by the direc- 
 tion in which the power of the muscle is applied to it. 
 Having now considered the former of these conditions, we 
 proceed to the latter. The contraction of a straight muscle, 
 which is attached to a fixed point at one end, and to a 
 movable point at the other, will obviously tend to draw the 
 latter towards the former. Thus the muscles which bend 
 the fingers lie in the palm of the hand and on the correspond- 
 ing side of the fore-arm; whilst those that straighten the 
 
ACTION OF MUSCLES ON BONES. 459 
 
 fingers are situated on the opposite side. But we often find 
 that the direction of a muscle's action is changed, by the 
 passing of its tendon through a pulley-like groove or loop ; 
 so that it draws the movable bone in a direction different 
 from that of its fixed attachment. This is the case, for ex- 
 ample, with some of the muscles that bend the toes ; these 
 being situated in the calf of the leg, would draw the toes 
 upwards, were it not that their tendons are carried beneath 
 the bones of the heel, working in smooth pulley-like channels 
 hollowed-out in them (fig. 233) ; hence, when the muscle con- 
 tracts, the tendons draw the ends of the toes towards the heel, 
 and thus bend them. 
 
 607. We generally find that even movements of a simple 
 character are performed by the combined action of several 
 muscles ; of which some may be considered as the principal, 
 and others as assistants. Those which are principals in one 
 movement may become assistants in another ; and vice versd. 
 Thus, if we wish to bend the wrist directly downwards upon 
 the fore-arm, we put in action, not only certain muscles whose 
 tendency would be to produce this movement, but others 
 which, acting by themselves, would produce a different motion. 
 One of these would draw the wrist towards the thumb-side 
 of the fore-arm, and the other towards the little-finger-side, 
 and they become the principal muscles in these movements 
 respectively ; but when they act together, their several ten- 
 dencies to draw the wrist to opposite sides counterbalance 
 one another, and they simply assist the principal muscles in 
 bending the wrist downwards upon the fore-arm. 
 
 608. Almost every muscle in the Human body has its 
 antagonist, which performs an action precisely opposite to its 
 own. Thus by one set of muscles, termed flexors, the joints 
 are bent ; by a contrary set, the extensors, they are straightened. 
 One set of muscles draws the arm or leg outwards, or away from 
 the central line of the body; another draws the limbs inwards. 
 One set, again, closes the jaws ; and another opens it. But 
 we find an economy of muscular substance in some of the 
 lower animals, where parts are to be usually kept in a parti- 
 cular position, which has only to be changed occasionally and 
 for a short time; the antagonism being then supplied by 
 yellow elastic tissue ( 29). 
 
 609. We commonly find that, in order to preserve the 
 
460 ACTION OP MUSCLES ON BONES. 
 
 necessary form of the animal body, Muscles are applied at a 
 great mechanical disadvantage as regards the exercise of their 
 power ; that is, a much larger force is employed than would 
 suffice, if differently applied, to overcome the resistance. But 
 we generally find that, in this as in other forms of lever action, 
 what is lost in power is gained in time ; and thus a very slight 
 change in the length of a muscle is sufficient to produce a 
 considerable movement. 
 
 610. The first source of disadvantage results from the 
 direction in which the muscle is attached to the bone. This 
 is rarely at right angles to it ; and consequently a considerable 
 part of the power is lost (see MEGHAN. PHILOS., 299). Thus 
 if the muscle m (fig. 213), whose force we shall suppose equal 
 
 to 10, be fixed at right angles 
 to the bone /, whose extremity 
 a is movable upon the point of 
 support r, its force of contrac- 
 tion will be most advantageously 
 applied to overcome the resist- 
 ance, and will draw the bone 
 from the position a b into the 
 direction a c, making it traverse 
 a space which we shall also 
 represent by 10. But if this muscle act obliquely on the 
 bone, in the direction of the line n b for example, it will 
 be quite otherwise ; for it will then tend to draw the bone 
 in the direction b n, and will consequently make it approach 
 the articular surface r. But as this bears upon an immovable 
 socket, and as the bone can move in no other way than by 
 turning upon the point r as upon a pivot, the contraction of 
 the muscle to the same amount as before will carry the bone 
 no further than into the direction a d ; three-quarters of the 
 force employed will thus be lost, and the resulting effect will 
 be no more than one-fourth of that which the same power 
 applied perpendicularly to the bone would have produced. 
 
 611. We usually find that the muscles are inserted so 
 obliquely, that their power is applied at a great disadvantage ; 
 but this disadvantage is rendered much less than it would 
 have otherwise been, by a very simple contrivance, that very 
 enlargement of the bones at the joints which is necessary to 
 give them the required extent of surface for working over 
 
LEVERAGE OP BONES. 461 
 
 each other. Thus, let r and o (fig. 214) be two bones con- 
 nected by a joint ; and let the muscle m, which moves the 
 lower bone upon the upper, be attached to the former at i. 
 Now as this muscle acts almost precisely in the line of the 
 bones themselves, almost all its 
 power will be expended in draw- 
 ing the lower bone against the 
 upper. But by the enlargement 
 of the ends of the bones, as seen 1 
 in fig. 215, the direction of the 
 tendon of the muscle m is so 
 changed, near its insertion i, that 
 
 the contraction of the muscle will cause the lower bone to 
 turn upon the upper one with comparatively little loss of 
 power. In the knee we find a still greater change of direction 
 effected, by the interposition of a movable bone, the patella or 
 knee-pan, in the substance of the tendon. 
 
 612. But the advantage or disadvantage with which the 
 muscles act upon the bones, depends in great degree upon the 
 relative distances of their point of attachment from the fulcrum 
 on which the bone moves, and from the point at which the 
 resistance is applied. Every bone acted-on by muscles may 
 be regarded as a lever, having its fulcrum or point of support 
 in the joint, its power where the muscle is attached to it, and 
 its weight where the resistance is to be overcome ; and the 
 distances of the fulcrum from the power and the weight 
 respectively are termed the two arms of the lever. Now, on 
 the mechanical principles fully explained elsewhere (MEGHAN. 
 PHILOS., 287), the relative length of 
 these two arms determines the force y 
 
 which is necessary to overcome a I ... 1M ...... 
 
 given resistance. Thus in the Steel- j J""" ^^ P 
 
 yard (fig. 216), the beam is divided T 
 into two arms of unequal length O 
 at the point of support or fulcrum a ; 
 
 at the end of the short arm r, hangs the body whose down- 
 ward pressure we wish to determine ; and on the other p 
 there slides a weight, which will balance a greater or less 
 amount of pressure at the opposite extremity r, according as 
 it is made to hang from a point which is more distant from 
 the fulcrum or nearer to it, that is, according as the length 
 
462 
 
 LEVEEAGE OP BONES. 
 
 P 
 
 P 
 
 
 
 
 m 
 
 c 
 
 W* v, 
 
 7 
 
 2 
 
 
 
 Fig. 217. 
 
 of the power-arm of the lever is increased or diminished, that 
 of the weight-arm remaining the same. 
 
 613. Now in order that there may be an equilibrium, or 
 balancing between the power and the weight, it is necessary 
 that they should be inversely proportional to the lengths of 
 their respective arms ; that is, the power multiplied by the 
 length of its arm, should be always equal to the weight 
 multiplied by the length of its arm. Thus, to balance a 
 
 certain resistance r, equal 
 to 10, and applied at the 
 end of a lever a b (fig. 
 217), whose length we 
 shall call 20, it is neces- 
 sary that a force p, ap- 
 plied at the same point, 
 and consequently at the 
 same distance from the 
 fulcrum a, should also be equal to 10 ; but, if the power be 
 applied at the point c, which is at only half the distance from 
 the fulcrum a, it must be doubled in amount, or equal to 20, 
 since it must be sufficient, when multiplied by its distance 10 
 from the fulcrum, to make 200, which is the product of the 
 resistance 10 and its distance from the fulcrum 20 ; and in 
 like manner, if the power be applied at d, where its distance 
 from the fulcrum is only 2, its amount must be 100, in order 
 that its product with the distance at which it is applied may 
 be equal to 200. Hence, when a muscle is applied near the 
 
 fulcrum, while the resist- 
 ance is at a distance from 
 
 rt ^ c t r it, so that the bone be- 
 comes a lever of the 
 "third order," its force 
 must be proportionably 
 
 jr. 
 
 614. But this arrange- 
 ment greatly increases the 
 rapidity of the motion 
 which is the consequence 
 Fig<218< of the muscular action. 
 
 For let us suppose that the muscle p (fig. 218) acts upon the 
 lever a r, in such a manner that its point of insertion c tra- 
 
LEVERAGE OF BOtfES. 463 
 
 verses a space equal to 5 in one second ; the extremity r of 
 the lever will traverse a space equal to 25 in the same time, 
 its distance from the fulcrum a being five times as great as 
 that of the point c from the fulcrum. Hence, although, to 
 raise a given weight at r, a power more than five times its 
 amount must be applied at c, that power will raise the weight 
 through a space five times as great as that through which 
 itself passes in the same time. Thus, what is lost in power is 
 gained in time ; and the shortening of a muscle, small in 
 amount, but effected with sufficient power, causes the raising 
 of a weight through a considerable space. 
 
 615. We shall find that this is the case in regard to most 
 of the muscular actions in the animal economy. Thus, the 
 fore-arm (fig. 219, b, c) is bent upon the arm a by a muscle d, 
 
 Fig. 219. 
 
 which arises from the top of the latter, and which is inserted 
 at e, a short distance from the elbow-joint. Hence its con- 
 traction to a very slight extent will raise the hand through a 
 considerable space ; but a proportional increase in its power 
 will be required to overcome any resisting force in the hand. 
 The arm is straightened again by an antagonist muscle, 
 which lies on the back of the arm, and which is attached to a 
 short projection made by one of the bones of the fore-arm 
 behind the elbow : this muscle also operates at a similar dis- 
 advantage in regard to power, and advantage in point of time, 
 in consequence of its point of attachment being so near to the 
 fulcrum. In responding to its action, however, the bones of 
 the fore-arm constitute a lever of the " first order ;" the elbow- 
 joint, which serves as the fulcrum, being now between the 
 power and the resistance. 
 
464 
 
 BONES OF THE SKULL. 
 
 Motor Apparatus of Man : Skeleton and Muscles. 
 
 616. Before entering upon the examination of the various 
 movements of the lower animals, and of the means by which 
 these are effected, it will be useful to acquire a general know- 
 ledge of the structure of the Human Skeleton, and of the 
 uses of its several parts. The skeleton, which is formed by 
 the union of about 200 bones, is divided like the body into head., 
 trunk, and members. The bones of these parts will now be 
 separately described. 
 
 617. The Head is composed of two parts, the cranium or 
 skull, and the face. The cranium (fig. 220) is a bony case of 
 oval form, occupying the upper and back part of the head, 
 
 and serving for the protection of 
 the brain, which is lodged in its 
 cavity. Its walls are made-up of 
 eight bones : the frontal/ in the 
 region of the forehead; the two 
 parietal bones p, which occupy 
 the top and sides of the skull; 
 the two temporal bones t, which 
 form the walls of the temporal 
 region ; the occipital bone o at the 
 back of the head ; and the sphe- 
 noid s, and the ethmoid, which 
 assist in forming the floor of the 
 cavity. These bones are firmly 
 united to each other by sutures, 
 the character of which varies in 
 different parts of the cranium, 
 so that they are the better able to resist external violence. 
 Thus, a blow upon the top of the arch formed by the parietal 
 bones will tend to separate them from each other and from 
 the frontal bone, and to force asunder their lower borders. 
 Both these effects are resisted by the peculiarity of the suture 
 which unites different parts of the parietal bone to its neigh- 
 bours ; for at the top of the skull the bones are firmly held 
 together by the interlocking of the projections of each, whilst the 
 lower edge of the parietal bone is prevented from being driven 
 outwards by the overlapping edge of the temporal bones, which 
 form, as it were, a buttress to the arch. This same contrivance 
 
 Fig. 220. HUMAN SKULL. 
 
 /, frontal bone ; p, parietal ; t, tem- 
 poral ; o, occipital ; , sphenoid ; 
 n, nasal ; ms, superior maxillary ; 
 j, malar or cheek bone ; mi, in- 
 ferior maxilla; no, anterior open- 
 ing of the nose ; ta, auditory aper- 
 ture; az.zygpmatic arch ; a,b,c,d, 
 lines indicating the facial angle. 
 
BONES OF THE SKULL. 465 
 
 prevents the temporal bone from being driven inwards, as it 
 might have otherwise been, by a blow on the side of the head. 
 
 618. In the base or floor of the cavity of the cranium are 
 seen a number of apertures, which serve for the passage of 
 the blood-vessels that supply the brain, and of the nerves that 
 issue from it. One of these apertures, much larger than the 
 rest, and situated in the occipital bone, gives passage to 
 the Spinal Cord ; and on each side of this aperture there is 
 a large bony projection from the under surface, termed the 
 condyle, by which the skull rests on the vertebral column, and 
 is enabled to move forwards or backwards upon it. The head 
 is nearly balanced upon this pivot ; nevertheless, the portion 
 situated in front of the joint is more heavy than that which 
 is situated behind it, and is consequently not altogether 
 counterpoised by the latter. Hence the muscles which, arising 
 from the back and being attached to the occipital bone, tend 
 to draw the head backwards, and thus to keep it upright, are 
 more numerous and powerful than those which are situated in 
 front of the vertebral column, and which tend to draw the head 
 downwards and forwards ; and when the former are relaxed, 
 as in a person sleeping upright, the head has a tendency to 
 fall forwards upon the chest. In no other animal is this joint 
 situated so far forwards as in Man. As we descend the scale, 
 we find it nearer and nearer to the back of the skull ; and 
 consequently the whole weight of the head bears, not directly 
 upon the spine, but upon the muscles and ligaments by which 
 it is attached to the vertebral column. 
 
 619. On each side of the base of the cranium, we observe 
 a large rounded projection, termed the mastoid. To this pro- 
 jection (which we feel behind the lower part of the ear) is- 
 attached on either side a powerful muscle, the sterno-mastoid 
 ( 28 , fig. 227), which passes downwards and towards the central 
 line ; so that the two muscles nearly meet at the bottom of the 
 neck, where they are attached to the upper edge of the breast- 
 bone. These muscles, acting together, serve to draw the head 
 forwards ; but either of them acting separately will turn it to 
 one side or the other. In front of these two projections of 
 the skull, we notice the opening t a of the external ear ; 
 which, like the different chambers of the internal ear, is 
 excavated in a portion of the temporal bone which is termed 
 petrous from its very dense and stony character. 
 
 H H 
 
466 BONES OP THE FACE. 
 
 620. The face is formed by the union of fourteen bones ; 
 and presents five large cavities, which serve for the lodgment 
 and protection of the organs of sight, smell, and taste. All 
 the bones of the face, with the exception of the lower jaw, are 
 completely immovable, and are firmly united to each other 
 and to the bones of the cranium ( 617). The two principal 
 are the superior maxillary (m s, fig. 220), which, form nearly 
 the whole of the upper jaw, and are connected with the 
 frontal bone in such a manner as to contribute to the forma- 
 tion of the orbital cavities in which the eye is lodged, and of 
 the nasal cavities which form the interior of the nose ; they 
 also constitute the front of the roof of the mouth ; on the 
 sides of the face, they articulate with the malar or cAee^-bones 
 j \ whilst they are united behind with the palate-bones which 
 form the back part of the roof of the mouth, and which in 
 their turn are united to the sphenoid. 
 
 621. The orbits, as we have already seen ( 538), are two 
 deep cavities, of a conical form, the base of the cone being 
 directed forwards, and its apex or point backwards ; the roof 
 of these cavities is formed by a portion of the frontal bone, 
 and their floor chiefly by the superior maxillary. Their 
 inside wall is formed by the ethmoid bone, and by the small 
 bone termed the lachrymal, in which is the canal for the 
 passage of the tears into the nose ( 541) ; and the outside 
 wall is formed partly by the cheek-bone and partly by the 
 sphenoid, the latter also bounding the cavity at its deepest 
 part, and containing the apertures which serve for the passage 
 of the optic and other nerves that enter the orbit from the 
 cranium. In the roof of the orbit, on its outer side, there is 
 a broad shallow pit or depression, in which the lachrymal 
 gland is lodged. 
 
 622. The greater part of the nose is formed by cartilages ; 
 so that, in the bony skull, the anterior opening of the nasal 
 cavity (n a, fig. 220) is very large ; and the bony portion of 
 the nose, formed by the two small bones (n) termed nasal, 
 projects but slightly. The nasal cavity, divided in the middle 
 by a vertical partition into two fossce or excavations, is very 
 extensive ( 506) ; at the upper part it is hollowed-out into 
 the ethmoid bone, the whole interior of which is made-up of 
 large cells ; its floor is formed by the arch of the palate, 
 which separates it from the mouth ; behind it extends as far 
 
BONES OF THE FACE. 467 
 
 as the back of the mouth, and communicates with the 
 pharynx by two apertures termed the posterior nares (fig. 
 200, c). The partition between the fossse is formed at the 
 upper part by a plate that projects downwards from the eth- 
 moid bone, and at the lower by a distinct bone called the 
 vomer (or ploughshare) from its peculiar form ; to the front 
 edge of this last is attached a cartilage, which continues the 
 partition forwards into the soft projecting portion of the nose. 
 It is through the thin horizontal plate of the ethmoid bone, 
 which separates the nasal cavity from that of the skull, that 
 the olfactory nerves make their way out from the former into 
 the latter : they descend in numerous branches, for the passage 
 of which through the roof of the nose this plate is perforated 
 by a number of small apertures, which give it a sieve-like 
 aspect; whence it is called the cribriform^- plate of the 
 ethmoid. 
 
 623. It is in the superior maxillary bone that all the teeth 
 of the upper jaw are implanted in Man ; but in the embryo 
 this bone is composed of several pieces ; and one of these 
 pieces, termed the intermaxillary bone (im, fig. 221), remains 
 
 mi mo c 
 
 Fig. 221. SKULL op HORSE. 
 
 OC, occipital bone ; t, temporal ; /, frontal ; , nasal ; tn, superior maxillary ; im, 
 intermaxillary; mi, inferior maxillary; o, orbit; , incisor teeth; c, canines; mo, 
 molars. 
 
 permanently separate in most of the lower animals. The 
 
 lower jaw of adult Man, also, is composed but of a single 
 
 piece ; though this is divided in the infant on the central line, 
 
 and the two halves remain separate in many of the lower 
 
 animals. This bone has a general resemblance in form to a 
 
 horse-shoe with its extremities turned up considerably ; it is 
 
 1 From the Latin, crilrum, a sieve. 
 
 H H 2 
 
468 BONES AND MUSCLES OF THE FACE. 
 
 articulated with, the temporal bones by a condyle or projecting 
 head with which each of these extremities is furnished ; and 
 this head is received into what is called the glenoid l cavity 
 on the under side of the temporal bone. In front of the 
 condyle is another projection, or process, termed the coronoid 
 (a, fig. 92), which serves for the attachment of one of the 
 principal muscles that raise the jaw. These muscles are all 
 attached near the angle of the jaw (or the point at which it 
 bends upwards), and they consequently act at a small distance 
 from its fulcrum, whilst the resistance is applied at the 
 furthest point ( 180). We are continually reminded of the 
 loss of mechanical power which results -from this, by our in- 
 ability to exercise the same force with our front teeth that we 
 can employ with the back. Thus, when we wish to crack 
 a nut, or to crush any hard substance between the teeth,, we 
 almost instinctively carry it to the back of the jaws, so as to 
 place it nearer the joint, where it may receive more of the 
 power of the muscle. 
 
 624. The general arrangement of the chief muscles of the 
 face is seen in fig. 222. The largest is the temporal muscle, t, 
 the fibres of which arise from an extensive 
 surface of the parietal and temporal bones, 
 and then converge or approach each other, 
 passing under the bony arch or zygoma, z 
 (which is partly formed by a process from 
 the temporal bone, and partly by the malar 
 or cheek bone), to be attached to the 
 j m c a coronoid process of the lower jaw. This 
 Fig. 222. muscle is of extraordinary power in those 
 
 beasts of prey which lift and drag heavy carcases in their 
 jaws; and in those which (like the Hyaena) obtain their 
 support by crushing the bones that others have left. It is 
 assisted by the masseter muscle m, which passes from the 
 zygomatic arch and cheek-bone to the angle of the lower jaw, 
 and also by other muscles. Besides these, the figure shows 
 the ring-like muscle or sphincter o, which surrounds the 
 opening of the eye, and serves by its contraction to close the 
 lids ; and also the similar muscle 6 b, which surrounds the 
 
 1 The term condyle is applied to most of the projecting surfaces of 
 articulation, in different parts of the body; and the term glenoid to 
 the cavities into which these are received. 
 
Frontal bone 
 
 Parietal bone 
 
 Orbit 
 
 Lower Jaw 
 
 Cervical Vertebrae 
 
 Scapula 
 
 Humerus 
 
 LumbarVertebrae 
 
 Ilium 
 
 Ulna 
 Radius 
 
 Carpus 
 Metacarpus 
 
 Fig. 223. SKELETON OF MAN. 
 
470 STRUCTURE OP THE VERTEBRAL COLUMN. 
 
 mouth and draws together the lips. The antagonists to these 
 are several small muscles which form the fleshy part of the 
 face, and produce the various changes "by which its expression 
 is given. These muscles are more numerous in Man and the 
 Monkey tribe than in any other animals. 
 
 625. Besides the twenty-two bones of which the skull is 
 properly composed, we may reckon as belonging to it the four 
 small bones which form part of the apparatus of hearing 
 ( 516); and also the hyoid bone, which lies at the root of 
 the tongue and at the top of the larynx (fig. 107). This last 
 bone, in Man and the Mammalia generally, is connected with 
 other parts of the skeleton by ligaments and muscles only ; 
 but in Birds it is connected with the temporal bone on each 
 side by a set of bony pieces jointed together like links in 
 a chain. 
 
 626. The most important part of the Trunk, and even of 
 the whole skeleton, that which serves to sustain the rest, 
 and which varies the least in the different classes of Verte- 
 brated animals, is the spinal or vertebral column. The 
 general conformation of this has been already described (71). 
 
 In Man it consists of 33 vertebra (fig. 224), 
 which are arranged under five divisions ; r. The 
 Cervical vertebrae c, or vertebrae of the neck, of 
 which there are 7 \ n. The dorsal vertebrae d, or 
 vertebrae of the back, of which there are 12 ; 
 in. The lumbar vertebrae I, or vertebrae of the 
 loins, of which there are 5 ; iv. The sacral ver- 
 tebrae s, of which also there are 5 ; and v. The 
 coccygeal vertebrae co, of which there are 4. All 
 these vertebrae are separate at the time of birth ; 
 but the 5 sacral vertebrae are soon afterwards united 
 into one piece, forming the bone which is termed 
 \ s the sacrum : and the coccygeal vertebrae are also 
 ) commonly united into one piece, the coccyx, which 
 is not unfrequently united in old age to the sacrum. 
 
 Fig. 224. In old persons, too, it is not uncommon for the 
 VERTEBRAL lumbar vertebrae to be united together by bony 
 
 COLUMN. ma ^. e r deposited in their cartilages and ligaments. 
 
 627. The dorsal vertebrae are distinguished from the cervi- 
 cal and lumbar, as being those to which the ribs are attached. 
 It is remarkable that the number of the cervical vertebrae 
 
STRUCTURE OF THE VERTEBRAL COLUMN. 
 
 471 
 
 should be the same in all the Mammalia ; even the long- 
 necked Giraffe having only seven, while the Whale, whose 
 head seems to be joined to its body without the intervention 
 of any neck, also has seven cervical vertebrae, although they 
 are almost as thin as a sheet of paper. It is owing to the 
 small number of joints in its neck, that the movements of the 
 head of the Giraffe are far less graceful than those of the 
 Swan and other long-necked Birds, in which the number of 
 cervical vertebrae is much greater. The following table shows 
 the number of vertebrae in animals of different groups. 
 
 MAMMALIA. 
 
 Cervical. 
 
 Dorsal. 
 
 Lumbar. 
 
 Sacral. 
 
 Coccygeal 
 
 Total. 
 
 Man ... . 
 
 
 
 
 
 
 
 Long-tailed Monkey 
 Lion 
 
 7 
 
 7 
 
 12 
 13 
 
 7 
 
 7 
 
 3 
 3 
 
 31 
 2fi 
 
 60 
 
 Long-tailed Opossum ... 
 Long-tailed Ant- Eater ... 
 Elephant 
 
 7 
 7 
 7 
 
 16 
 16 
 20 
 
 6 
 3 
 3 
 
 2 
 6 
 4 
 
 36 
 40 
 27 
 
 67 
 72 
 61 
 
 Giraffe 
 
 7 
 
 
 
 
 
 48 
 
 Whale 
 
 7 
 
 
 
 
 
 
 BIRDS. 
 Vulture 
 
 15 
 
 7 
 
 
 
 g 
 
 41 
 
 Swallow ... ... 
 
 
 
 
 
 
 
 Turkey 
 
 14 
 
 j 
 
 
 
 Q 
 
 42 
 
 Ostrich 
 
 18 
 
 
 
 
 
 
 Crane . 
 
 17 
 
 
 
 
 
 
 Swan 
 
 
 
 
 
 
 
 REPTILES. 
 Tortoise 
 
 
 
 
 
 
 
 Monitor (Lizard) 
 Python (Boa) 
 
 6 
 
 21 
 
 2 
 
 2 
 
 115 
 
 146 
 
 Rattle-Snake 
 Land Salamander 
 Axolotl . 
 
 1 
 
 171 
 14 
 
 
 
 1 
 
 36 
 26 
 
 207 
 42 
 
 FISHES. 
 Perch 
 
 
 21 
 
 
 
 21 
 
 42 
 
 Mackerel 
 
 
 15 
 
 
 
 16 
 
 31 
 
 Trichiurus 
 
 
 60 
 
 
 
 100 
 
 160 
 
 Salmon . . .. 
 
 
 34 
 
 
 
 22 
 
 56 
 
 Cod 
 
 
 19 
 
 
 
 34 
 
 53 
 
 Conger Eel 
 
 
 60 
 
 
 
 102 
 
 162 
 
 Electric Eel 
 
 
 
 
 
 
 236 
 
 Shark 
 
 
 95 
 
 
 
 270 
 
 365 
 
 
 
 
 
 
 
 
 We see from the above table, that it is by the multiplica- 
 tion of the coccygeal vertebrae, that the tail is prolonged in 
 those animals which possess it. In fact, it is only in Man, 
 and in those of the Ape tribe which approach nearest to him, 
 that the number of these vertebrae is as low as 4. 
 
472 STRUCTURE AND CONNEXIONS OF VERTEBRA. 
 
 628. It has been already noticed (71) that an ordinary 
 character of the vertebra consists in their being perforated by 
 an aperture (fig. 225), which, when several vertebrae are united. 
 
 together, forms a continuous tube or canal for 
 the lodgment of the spinal cord. This cha- 
 racter is usually lost, however, in the coccy- 
 geal vertebrae ; which are so much contracted 
 and simplified as to contain no aperture. 
 The purpose of the division of the spinal 
 column into so large a number of separate 
 Fig. 225. SINGLE bones, is obviously to allow of considerable 
 freedom of motion by a slight shifting amongst 
 the individual parts ; whilst any such sudden bend as would 
 be injurious to the spinal cord, is avoided. Each vertebra con- 
 sists of a solid " body " a, which is situated in front of the 
 spinal canal in Man, but below it in animals whose back has 
 a horizontal position, and which serves to give solidity to the 
 structure, and of "processes" or projections, b and c, that 
 serve to form the spinal canal, and to unite the vertebra to 
 each other. In Man and other warm-blooded animals, the 
 two surfaces of the "body" are nearly flat and are parallel to 
 each other ; and they are united to the corresponding surfaces 
 of the neighbouring vertebrae by a disk of fibro-cartilage (47), 
 which extends through the whole space that intervenes be- 
 tween them, and which, being firmly adherent to both, prevents 
 them from being far separated from each other. 
 
 629. But in Reptiles and Fishes, a different plan is adopted. 
 In the animals of the former class, particularly in Serpents, 
 we find one surface of each vertebra convex or projecting, and 
 the other concave or hollowed-out ; and the convex surface of 
 each vertebra fits into the concave surface of the next, in such 
 a manner that the whole spinal column becomes a series of 
 ball-and-socket joints, and is thus endowed with that flexibi- 
 lity which is essential to the peculiar movements of these 
 animals. In Fishes both surfaces are concave, and between 
 each vertebra there is interposed a bag containing fluid, and 
 having two convex surfaces, over which those of the vertebrae 
 can freely play. Extreme facility of movement is thus given 
 to the spinal column ; but its strength is proportionally dimi- 
 nished. It is to be remembered, however, that strength is 
 not required in the bony framework of animals, whose bodies, 
 
MUTUAL CONNEXIONS OP VERTEBRA. 473 
 
 instead of being supported upon foui fixed points, are buoyed 
 up in every part by a liquid of nearly the same density with 
 themselves. The extreme flexibility of the spine of Fishes, 
 enables them to propel their bodies by the movements of the 
 hinder portion and tail from side to side ; their members, or 
 pectoral and ventral fins (fig. 243), being but little used 
 except for influencing the direction of their motion. And 
 thus we see that in the lowest Vertebrate, as in the lower 
 Articulata (such as the Leech and Earth-worm), the propul- 
 sion of the body being accomplished by the movements of 
 the trunk itself, its skeleton (internal in the one case, external 
 in the other) is left in the soft condition which it has in all 
 at an early period : whilst in the higher classes of both series, 
 Birds and Insects for example, the extremities being so 
 developed, and being furnished with muscles so powerful 
 that the function of locomotion is entirely committed to them, 
 the skeleton of the body undergoes great consolidation, its 
 various pieces being so knit together as to make the trunk 
 almost immovable. 
 
 630. This knitting-together is partly accomplished by 
 means of projections or processes from^the several vertebrae, 
 which are united to one another by muscles and ligaments. 
 Of these processes there are seven in Man from each vertebra. 
 One of these, termed the spinous process (b, fig. 225), projects 
 directly backwards ; and thus is formed the prominent ridge 
 on the back, in which the ends of these projections can be 
 distinguished. The spinous processes serve in Man to give 
 attachment to the muscles, by which the trunk and head are 
 kept erect ; in Animals whose spine is horizontal, they are 
 generally much longer, in order to give firm attachment to the 
 muscles and ligaments which support the head (fig. 229, vc, vd). 
 And in Fishes they are greatly prolonged (fig. 243), so as to in- 
 crease the surface by the stroke of which from side to side the 
 body is propelled through the water. On each side of the 
 vertebra is a process (c, fig. 225) which is called transverse; 
 this serves for the attachment of the ribs to the vertebra. 
 And lastly, from the upper and under side of each vertebra, 
 two articulating processes project, which lock against each 
 other in such a manner as to prevent the movements of the 
 vertebrse from being carried to an injurious extent. These 
 processes are peculiarly long in Birds, where they almost 
 
474 UNION OF VERTEBRAE IXTO SPINAL COLUMN. 
 
 completely check the movements of the dorsal vertebrae ; 
 thereby giving to the trunk that firmness which is required 
 for the attachment of the muscles of the wings. The portions 
 of bone which pass backwards from the body of each vertebra 
 to its transverse processes, and thus form the side-wall of the 
 spinal canal, are called the arches of the vertebrae. These are 
 the parts first formed. On the under edge of each there is 
 a notch which corresponds with one in the upper side of the 
 next, in such a manner that, when two vertebras are placed 
 together, a complete foramen or aperture is formed, which 
 serves for the passage of the nerves that are given-off from 
 the Spinal Cord ( 457). 
 
 631. The vertebral column of Man is disposed in a double 
 curve, as seen in fig. 224 ; the effect of this is to diminish 
 the shock that would be produced by a sudden "jar," such 
 as when a man jumps from a height upon his feet. If the 
 vertebral column had been quite straight, this jar would have 
 been propagated directly upwards from the pelvis to the head, 
 and would have produced very injurious effects upon the 
 brain ; but by means of the double curvature, and the elasti- 
 city of the ligaments % &c. which hold together the vertebrae, 
 it is chiefly expended in increasing for a moment the curves 
 of the spine, which thus acts the part of a spring. The 
 constant pressure of the head and upper part of the trunk 
 has a tendency to increase these curves permanently, and 
 thus to diminish the height of the body. The elasticity of 
 the intervertebral substance, however, causes it to recover, 
 during the time when the body is in the horizontal posture, 
 the form it had lost by pressure in the upright position ; and 
 thus a man is taller by half an inch or more when he rises 
 in the morning, than he was when he lay down the night 
 before. 
 
 632. The first vertebra of the neck, termed the atlas, is 
 much more movable than the rest, and differs considerably 
 from them in its form. It is destitute of body ; but it has a 
 broad smooth surface on either side, on which rest the " con- 
 dyles" of the occipital bone of the skull ( 618), in such a 
 manner that the head is free to nod backwards and forwards. 
 The atlas itself turns upon a sort of pivot, formed by an 
 upward projection from, the next vertebra, which is termed 
 the axis; this projection, called from its form the processus 
 
ARTICULATION OF HEAD WITH SPINAL COLUMN. RIBS. 475 
 
 dentatus (or tooth-like process), occupies the place of the body 
 of the atlas ; and by the rotation of the atlas around it, the 
 movements of the head from side to side are accomplished. 
 Wherever great freedom of motion is permitted, displacement 
 or dislocation is necessarily more easy ; and accordingly we 
 find that the atlas and axis can be more easily separated from 
 each other, than can any other two vertebrae. This dislocation 
 may be produced by violence of different kinds ; thus if the 
 head be suddenly forced forwards while the neck is held back, 
 the tooth of the axis may be caused to press against the 
 spinal cord, and thus to interrupt or completely check its 
 functions. Or, again, if the weight of the body be suspended 
 from the head, and especially if it be thrown upon it with a 
 jerk, the two vertebras are liable to be dragged asunder, and 
 the spinal cord to be stretched or broken. This is sometimes 
 the immediate cause of death in hanging ; and it has not 
 unfrequently occurred when children have been held in the 
 air by the hands applied to the head, a thing often done in 
 play, but of which the extreme danger should prevent its 
 ever being practised. Any serious injury of the spinal cord 
 in this region must be immediately fatal, for the reason for- 
 merly stated ( 470), that it causes the suspension of the 
 motions of respiration. 
 
 633. The number of the ribs which are attached to the 
 bodies and transverse processes of the dorsal vertebra, is, in 
 the Human species, twelve on each side. 1 The number in 
 different animals may be judged-of by that of the dorsal ver- 
 tebras in the table already given ( 627) ; since it is the attach- 
 ment of the ribs that makes the essential difference between 
 the dorsal vertebras and the cervical or lumbar. The other 
 extremity of each rib is connected with a cartilage, which is 
 a sort of continuation of it ; in Birds, the cartilages of the 
 ribs are ossified or converted into bone. The cartilages of 
 the first seven ribs (in Man), which are termed the true ribs, 
 are united to the sternum or breast-bone, which forms the 
 front wall of the thorax (fig. 163). The cartilages of the five 
 lower ribs are not directly connected with this, and they are 
 hence called false ribs ; those of three of them, however, are 
 
 1 It is scarcely necessary here to state, that the common notion 
 respecting the deficiency of a rib on one side of the body of Man is a 
 popular error. 
 
476 RIBS AND STERNUM. BONES OF SHOULDER. 
 
 connected with the cartilage of the seventh rib ; while the 
 other two ribs, being completely unattached at the anterior 
 ends, are termed floating ribs. The sternum or breast-bone 
 is flat and of simple form in Man ; but it is much larger in 
 many other animals. In those which have need of great 
 strength in the upper limbs, such as Birds, Bats, and Moles, 
 it is not only increased in breadth, but is furnished with a 
 projecting keel or ridge for the attachment of powerful muscles 
 (fig. 250). In the Turtle tribe, on the contrary, it is very 
 much extended on the sides, so as to afford, with the ribs, a 
 complete protection to the contained parts ( 83). 
 
 634. We have next to consider the structure of the members 
 or appendages which are attached to this central framework. 
 These are spoken-of as superior and inferior, when we are 
 treating of Man, whose erect posture places one pair above 
 the other : but when the ordinary Quadrupeds are alluded to, 
 they are termed anterior and posterior, one pair being in front 
 of the other. Each member consists of a set of movable 
 bones, which serve as levers ; but the socket in which the 
 first of these works, is formed by a bony framework, which is 
 connected more or less closely with the spinal column. This 
 framework, in the upper extremity, consists of the Scapula 
 or blade-bone, and the Clavicle or collar-bone. In the lower 
 extremity, it is formed by a set of bones, the union of which 
 with the sacrum completes the Pelvis or bason at the bottom 
 of the spinal column (fig. 223). 
 
 635. The Scapula is a large flat bone, which occupies the 
 upper and external 1 part of the back. Its form is somewhat 
 triangular ; and at its upper and outer angle is a broad but 
 shallow cavity, destined to receive the head of the humerus 
 or arm-bone. Above this cavity is a large projection, termed 
 the acromion-process, which is united by ligaments, &c , with 
 the external end of the clavicle, and thus forms the bony 
 eminence that we feel at the top of the shoulder. A little 
 internally to this we find another process, the coracoid, which 
 only serves in Man for the attachment of certain muscles, but 
 which in Birds is developed into a distinct bone ( 668). 
 The hinder surface of the scapula is divided into two by a 
 projecting ridge or keel, which gives a more extensive and 
 
 1 The term external is continually used in Anatomy, to describe the 
 parts furthest removed from the central or median line of the body. 
 
MUSCLES OF THE BACK OF THE TRUNK, 
 
 477 
 
 Fig. 226 MUSCLES OF THE BACK OF THE TRUNK, THE SUPERFICIAL LATER BEING 
 
 SHOWN ON THE LEFT SIDE, THE Mll>DLE LAYER ON THE RIGHT. 
 
 I. Trapezius ; 2, occipital bone; 3, spine of the scapula; 4, acromipn process; 
 5, deltoid; 6, infraspinatus ; 7, teres minor; 8, teres major; 9, latissimus dorsi; 
 10, its aponeurosis ; 11, glutaeus ma^nus ; 12, space between the latissimus dorsi 
 and external oblique; 13, rhomboideus major ; 14, splenius; 15, angularis scapulae ; 
 16, rhomboideus minor; 17, external oblique; 18, supraspinatus ; 19, serratus 
 magnus ; 22, inferior serratus minor; 23, internal ob'ique; 24, crest of the iliac 
 bone; 25, glutasus medius; 26, pyramidalis; 27, gemellus superior; 28, obturator 
 internus; 29, gemellus inferior; 30, quadratus femoris; 31, semi-membranosus ; 
 32, tuberosity of the ischium ; 33, insertion of the glutaeus maximus ; 38, long 
 head of the triceps extensor of the arm. 
 
478 BONES OF SHOULDER I SCAPULA AND CLAVICLE. 
 
 firmer attachment to the muscles that arise from it (fig. 226, 3 ). 
 The scapula is never deficient in animals that possess a superior 
 extremity, though sometimes it is very narrow. The muscles 
 attached to it are chiefly those which draw the arm upwards, 
 and which turn it on its axis. In Man, their actions are 
 very numerous and varied ; but in animals that only use 
 their extremities for giving motion to the body, the muscular 
 apparatus is much simpler, and the scapula is narrower (fig. 
 229, o). This is particularly the case in Birds (fig. 250, o), 
 the raising of whose wings in flight is an action that requires 
 very little power, though for their depression or pulling-down 
 great muscular force is needed. 
 
 636. The Clavicle is a rounded bone, attached at one ex- 
 tremity to the acromion-process of the scapula, and at the 
 other to the top of the sternum. Its principal use is to keep 
 the shoulders separate ; and we accordingly find it strongest 
 in those animals, the actions of whose superior extremities 
 tend to draw them together ; whilst it is comparatively weak 
 or altogether deficient in animals, the actions of whose limbs 
 naturally tend to keep them asunder. In Birds, the violent 
 drawing-down of whose wings in flight would tend to bring 
 the shoulders together if they were not prevented, there is 
 not only a strong clavicle, but usually a second bone having 
 a similar function ( 668). In the Horse and other animals, 
 on the contrary, the bearing of whose weight on their fore- 
 legs tends rather to separate the shoulders than to bring them 
 together, the clavicle is deficient. 
 
 637. The Scapula is connected with the central framework 
 of the skeleton by various muscles (fig. 226), which pass 
 towards it from the spinal column and ribs, and which serve 
 alike to fix it, and to assist in sustaining the weight which it 
 sometimes has to bear. In Man these are numerous, and their 
 actions are various ; since the scapula is left very movable in 
 him, that the actions of the arm may be more free. In 
 Quadrupeds it is generally more fixed ; and the trunk is slung 
 from it, as it were, by a muscle (the serratus magnus, 9 ) of mode- 
 rate thickness in Man, but in these animals of great strength, 
 which passes from the scapula to be attached to the ribs. 
 
 638. The superior or anterior member itself is divided into 
 three principal portions, the arm, fore-arm, and hand. The 
 arm is supported by a single long and cylindrical bone, which 
 
MUSCLES OF THE FRONT OF THE TRUNK. 479 
 
 is called the kumerus ; this has a large rounded head, which 
 is received into the glenoid cavity of the scapula ; whilst its 
 lower end is rather flattened, so as to articulate with the two 
 bones of the fore-arm in the hinge-joint of the elhow. The 
 
 Fig. 227. MUSCLES OF THE FRONT OF THE TRUNK, THE SUPERFICIAL LATER 
 
 BEING SHOWN ON THE LEFT SIDE OF THE FIGURE, THE MIDDLE LAYER ON THE 
 RIGHT. 
 
 1, sternum; 2, sternal portion of the pectoralis major; 3, clavicular portion of the 
 same muscle; 4, space between the deltoid and pectoral muscles; 5, deltoid; 6, 6, 
 clavicles; 7, external oblique; 8, digitations of the serratus magnus ; 9, 9, latissi- 
 musdorsi; 10, aponeurosis of the external oblique; 11, linea alba; 12, umbilicus; 
 13, pectoralis minor; 14, 14, crests of the iliac bone; 15, symphysis of the pubis ; 
 16, crural arches; 17, rectus abdominis; 18, inferior oblique; 20, internal inter- 
 costal; 21, external intercostal; 22, infra-clavicular; 23, coracoid process; 21, in- 
 ferior border of the internal oblique and transversalis muscles; 25, short head 
 of the biceps ; 26, long head of the biceps ; 27, biceps ; 28, sternomastoid ; 32, 
 adductor of the thigh; 33, rectus femoris. 
 
480 BONES AND MUSCLES OF UPPER EXTREMITY. 
 
 muscles which move it are for the most part attached to its upper 
 third ; and the chief of them are the pectoralis major (fig. 227, 
 2 , 3 ) which rises from the sternum and cartilages of the ribs, 
 and consequently draws the arm forwards, inwards, and down- 
 wards, the latissimus dorsi (fig. 226, 9 ), which rises from the 
 spinal column and hinder part of the ribs, and consequently 
 draws the arm backwards, inwards, and downwards, and the 
 deltoid (fig. 226, 5 ), which arises from the upper edge of the 
 clavicle, and from the ridge of the scapula, and is the chief 
 muscle concerned in raising the arm. The first of these forms 
 the principal part of the fleshy mass upon the front of the 
 chest, and is the muscle which is so remarkably developed in 
 Birds. It forms also the front boundary of the axillary space, 
 or hollow of the arm-pit, the hinder boundary of which is 
 formed by the second muscle. This space, of which we can 
 distinctly feel the front and back walls when we raise the 
 arm a little from the side, contains the large vessels and 
 nerves proceeding to the arm, and also a number of lymphatic 
 glands ( 219). The deltoid muscle forms the thick fleshy 
 mass on the top of the shoulder and on the upper part of the 
 outside of the arm. 
 
 639. In the fore-arm of Man there are two long bones, 
 termed the Radius and the Ulna, which lie nearly parallel to 
 each other ; the radius being on the outer or thumb side of the 
 fore-arm, and the ulna on the inner. They are connected 
 with one another, not only by ligaments at their extremities, 
 but by a strong fibrous membrane that passes between their 
 adjacent edges, along their entire length. Nevertheless they 
 have considerable freedom of motion, not only upon the 
 humerus, but upon each other; so as to give to the fore -arm 
 the power of rotation on its own axis, by which either the 
 palm or the back of the hand may be turned upwards. The 
 ulna is connected with the humerus, at the elbow, by means 
 of a hinge-joint, into which the radius does not enter ; but it 
 is the radius with which the hand is connected at the wrist, 
 by a kind of ball-and-socket joint, the ulna having no direct 
 share in this articulation : hence, while both bones move 
 together in bending or straightening the elbow, we can make 
 the radius roll round the ulna, carrying the hand with it. 
 This movement is one of very great importance, in rendering 
 the hand capable of a great variety of uses to which it would 
 
BONES AND MUSCLES OF ARM AND HAND. 481 
 
 be otherwise inapplicable. It is only among the higher 
 orders of Quadrupeds, however, that it can possibly be exe- 
 cuted ; for in the lower, the two bones are united more or less 
 completely into one, or are articulated in such a manner as to 
 be incapable of rotation. 
 
 640. The fore-arm is bent upon the arm, chiefly by muscles 
 that lie upon the front of the latter ; of these the principal is 
 the biceps or two-headed muscle ( 7 , fig. 227), which arises from 
 the coracoid process of the scapula, and from the top of the 
 glenoid cavity, and is inserted into the radius a little in front 
 of the elbow, forming a great part of the fleshy mass in front 
 of the arm (fig. 2 1 9). The arm is straightened again by a large 
 muscle, the triceps or three-headed muscle, which arises from 
 the back of the humerus and scapula, and passes down to be 
 inserted into a projection of the ulna behind the elbow-joint, 
 forming the fleshy mass at the back of the arm. The muscles 
 which rotate the fore-arm arise from the lower end of the 
 humerus, or from one of its own bones, and pass obliquely 
 across to the other. 
 
 641. The Hand is anatomically divided into three portions, 
 the carpus^ metacarpus, and phalanges (fig. 223). The 
 carpus, which is the portion nearest the wrist-joint, is com- 
 posed of eight small short bones, which are firmly united to 
 each other by ligaments, but yet have a certain degree of 
 motion permitted them ; these are arranged in two rows, of 
 which one has a rounded surface, and enters into the forma- 
 tion of the wrist-joint ; whilst the other has a series of shal- 
 low pits, to receive the rounded heads of the metacarpal 
 bones. These last almost precisely resemble the bones of the 
 fingers, and in the skeleton might be mistaken for their first 
 joints ; but with the exception of that of the thumb they are 
 all united to each other by ligaments and muscles, so as to 
 form the compact framework which gives support to the palm 
 of the hand. The metacarpal bone of the thumb is much 
 more free in its movements ; and it is chiefly by an alteration 
 in its direction, that the thumb can be opposed to the fingers. 
 The thumb and fingers are formed by a series of small bones 
 which are termed the phalanges; of these there are only two 
 in the thumb, whilst there are three in the fingers. They are 
 bent on each other chiefly by the action of the muscles that 
 occupy the front of the fore-arm ; and they are extended or 
 
 i i 
 
482 
 
 MUSCLES OF THE HAND. 
 
 straightened by others that lie along its back. These termi- 
 nate in long tendons, which are bound down at the wrist by 
 a fibrous band that stretches between the bony projections on 
 
 either side, and is termed 
 the annular (or ring-like) 
 ligament (fig. 228). The 
 tendons then spread asunder 
 in the hand, and pass-on to 
 be inserted into the bones 
 of the several fingers, being 
 reinforced by a set of small 
 muscles that arise from the 
 hand itself. 
 
 642. When we consider 
 the superior extremity of 
 Man as a whole, we remark 
 that the several levers which 
 are joined end-to-end to form 
 it, diminish progressively in 
 length. Thus the arm is 
 longer than the fore-arm; 
 the latter is longer than the 
 wrist ; and each of the pha- 
 
 Fig. 228. MUSCLES OP THE PALM OF THE , 
 
 HAND (SUPERFICIAL LAYER) lailgCS IS longer than the 
 
 1, anterior annular ligament of the carpus; one which SUCCCeds it. The 
 2, 2, extremities of the short abductor of f ,-1 -> rrn-m n-n4- 
 
 the thumb, the intermediate body of the purpose of this arrangement 
 
 muscle having been removed; 3, opposing ig very evident. The nume- 
 muscle of the thumb; 4, short flexor of . J . . . -,, 
 
 the thumb; 5, adductor of the thumb ; 6, roUS joints, in the neighDOUr- 
 
 lower border of the same muscle; 7, 7, "U^^rl n f oar>Ti nfhpr wliiph 
 
 lumbricales; 8, one of the tendons of the h 0(i Ol eacn Otner > Wm 
 
 deep flexor of the fingers, passing-on to we S66 towards the extremity 
 insert itself in the bone of the third pha- f ,, i- "U ^ ^U Uc, O^TTQT-Q! 
 
 lanx, after perforating the tendon of the of the limb, permit its Several 
 
 superficial flexor ; 9, tendon of the long portions to change their place 
 
 flexor of the thumb; 10, adductor of the f 
 
 little finger; 11, short flexor of the little in VariOUS Ways, SO as to aC- 
 
 themselves to 
 
 the form of the body which 
 it is desired to grasp ; whilst the long levers formed by the 
 arm and fore-arm, allow the place of the entire hand to be 
 rapidly changed to a considerable extent. It is principally 
 by the movements of the humerus upon the scapula, that 
 the direction of the limb is given ; the bending or straight- 
 ening of the limb regulates its length; whilst the move- 
 
PECULIAR ENDOWMENTS OP HUMAN HAND. 483 
 
 ments of the thumb and fingers are concerned in its particular 
 applications. 
 
 643. The hand of Man is distinguished from the extremity 
 of most Quadrupeds by its possession of an opposable thumb, 
 that is, of a finger which can be made to act in a direction 
 opposite to that of the rest. But among the Apes and 
 Monkeys, we find this peculiarity not only in the superior 
 extremity, but also in the inferior ; whence these animals are 
 said to be quadrumanous or four-handed, whilst Man is 
 bimanous, possessing two hands only. It must not be sup- 
 posed, however, that Apes and Monkeys are superior in this 
 respect to Man ; for they possess this distinguishing character 
 in a much less striking degree than he does. All the four 
 extremities of Apes and Monkeys possess the power of grasp- 
 ing, but they are all used also for support ; and we find that 
 in consequence of the shortness of the thumb and great toe, the 
 grasping power is very inferior to that which Man possesses. 
 But of the four extremities of Man, one pair is specially adapted 
 for support, and the other for prehension or grasping ; and this 
 by the length and mobility of the thumb, which is capable of 
 being brought into exact opposition to the extremities of 
 all the fingers, whether singly or in combination. But even 
 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 opposed to each 
 other, and then with only a slight degree of force ; hence, 
 although completely 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 minute 
 objects with that precision, nor support large ones with that 
 firmness, which is essential to the dexterous performance of a 
 variety of actions for which the hand of Man is admirably 
 suited. Hence they may be more appropriately termed 
 claspers than hands. 
 
 644. In many of the inferior Mammalia, whose extremities 
 are adapted for support only, we find each row of phalanges 
 consolidated into two bones, or even into one. This is the 
 case, for example, in the Ruminant Quadrupeds, as the Camel 
 (fig. 229), and in the Horse ( 652). Such an arrangement 
 obviously increases the firmness of the limb, though it 
 altogether deprives it of prehensile power. In other in- 
 
 i I 2 
 
484 
 
 EXTREMITIES OF LOWER ANIMALS. 
 
 stances, we find the number of bones in the hand increased, 
 but all of them enclosed in one envelope, so that the fingers 
 are not separate. This is the case with many aquatic animals 
 
 Fig. 229. SKELETON OF THE CAMEL. 
 
 vc, cervical vertebrae ; v d, dorsal vertebrae ; vl, lumbar vertebras ; vs. sacral vertebrae ; 
 vq, caudal vertebras; s, scapula; h, humerus; cu, ulna; ca, carpus; me, meta- 
 carpus ; ph, phalanges ; fe, femur ; ro, patella ; ti, tibia ; ta, tarsus ; nut, metatarsus. 
 
 such as the Whale tribe among Mammals, Turtles among 
 Reptiles, and Fishes in general, in which the hand is made 
 to serve as a fin or paddle. In most of these, the bones of 
 the arm are very short ; and the movements of the extremity 
 are chiefly confined to the wrist-joint. 
 
 645. The structure of the lower extremities has a very great 
 analogy to that of the upper \ and the principal differences to 
 be remarked between them, are such as are necessary to give 
 to the former more solidity at the expense of freedom of 
 motion, and to make them organs of locomotion instead of 
 organs of prehension. Here, too, we have a bony framework, 
 for the purpose of connecting the limb itself with the spine ; 
 and as the weight of the body is constantly thrown upon the 
 
BONES AND MUSCLES OF LOWER EXTREMITY. 485 
 
 lower extremities, this framework is much more firmly at- 
 tached to that of the trunk, than is the case with that which 
 supports the arms. It consists, on each side, of a bone which 
 in the adult state is single, though at an early age it is com- 
 posed of three distinct pieces ; and this is closely connected 
 with the sacrum behind, while it meets with its fellow in 
 front in such a manner as to form a sort of bason termed the 
 Pelvis. The spreading sides of this, formed by the iliac bones 
 (Fig. 213), afford support above to the viscera contained in 
 the abdomen ; and they give attachment by both surfaces to 
 large muscles by which the thigh-bone is moved, and by their 
 edges to large expanded muscles that pass upwards to the ribs 
 and sternum, and form the walls of the abdomen. Below this 
 spreading portion, we find the articular cavity of the thigh- 
 bone, which is so deep as almost to form a hemispheric cup 
 when it is completed by its cartilaginous border. The move- 
 ments of the thigh-bone are consequently more limited than 
 those of the arm ; but it is much less liable to displacement. 
 
 646. The thigh, like the arm, contains but a single bone, 
 which is named the Femur. Its upper extremity is bent at 
 an angle ; and its rounded head is separated from the rest by 
 a narrow portion which is termed its neck. At the point 
 where this neck joins the shaft of the bone, there are two 
 large projections termed trochanters, one on the outer side and 
 the other on the inner ; these serve to give attachment to the 
 muscles by which the thigh is moved. Of these muscles, one 
 descends from the lumbar vertebrae, and passes-down with 
 another that rises from the upper expanded surface of the 
 pelvis, over the front border of the pelvis, to be attached 
 to the smaller and interior of the projections just mentioned ; 
 these with the assistance of other muscles raise or draw 
 forwards the thigh, an action which does not require in Man 
 to be performed with any great force. The muscles which 
 draw back the thigh, on the other hand, arise from the under 
 surface and back of the pelvis, where they form a very thick 
 fleshy mass ( u , 25 , fig. 226) ; and they pass to the larger and 
 external projection, and to a ridge which runs from it down 
 the thigh-bone. Other muscles which arise from the lower 
 border of the pelvis, serve to rotate the thigh upon its axis. 
 The lower end of the thigh-bone spreads into two large condyles, 
 on which the principal bone of the leg moves backwards and 
 
486 BONES AND MUSCLES OF THE LOWER EXTREMITY. 
 
 forwards. The knee is a good example of a pure hinge-joint, 
 all its movements being restricted to one plane. 
 
 647. The leg, although containing two bones like the fore- 
 arm, does not in Man possess the peculiar movement which 
 characterises it. One of these bones, called the Tibia, is much 
 larger than the other which is called the Fibula ; and it is 
 the former alone on which the thigh-bone rests, and which 
 itself rests upon the foot, so that no movement of rota- 
 tion is permitted in the leg. In fact, the fibula, which is 
 a long slender bone running nearly parallel with the tibia 
 (fig. 223), looks like a mere appendage or rudiment, and 
 serves only for the attachment of muscles. The upper end of 
 the tibia is broad, and has two shallow excavations, in which 
 the condyles of the femur are received. Upon the front of 
 the knee-joint we find a small separate bone, the patella or 
 knee-pan ; the purpose of this is to change the direction of 
 the tendons that come down from the front of the thigh to be 
 attached to the tibia ; in such a manner as to enable them to 
 act more advantageously, upon the principle formerly stated 
 (. 611). In the elbow-joint, this change was not required ; 
 since the ulna projects sufficiently far backwards to afford ad- 
 vantageous attachment to the tendon of the extensor muscle. 
 The very powerful muscles which tend to straighten the 
 knee-joint, arise from the front of the pelvis and from the 
 femur itself j and they form the fleshy mass of the front 
 of the thigh. On the other hand, those which bend the knee 
 arise from the lower border of the pelvis and from the back of 
 the thigh-bone, and pass downwards to be inserted into the 
 sides of the tibia and fibula a little below the knee, then- 
 tendons forming the two strong cords known as the hamstrings. 
 The articulating surface at the lower extremity of the leg, 
 which enters into the ankle-joint, is principally formed by the 
 tibia j but its outer border is formed by the fibula, which 
 there makes a considerable projection that can be felt through 
 the skin. In the Quadrumana, and in a less degree in some 
 other Mammals, the two bones of the leg resemble those 
 of the fore-arm ; and are so articulated as to give to the foot 
 a power of rotation corresponding with that of the hand. 
 
 648. The Foot is composed, like the hand, of three distinct 
 portions, which are called the tarsus, metatarsus, and. phalanges. 
 There are seven bones in the tarsus, all of which are larger 
 
BONES AND MUSCLES OF THE FOOT. 
 
 487 
 
 than those of the carpus, and some of them of considerable 
 size. The articulation with the leg is formed by one of these 
 only, the astragalus, which projects above the rest, and is im- 
 bedded between the projecting extremity of the tibia (which 
 forms the inner boundary of the ankle-joint) and that of the 
 fibula. The astragalus rests on the os calcis or bone of the 
 heel, which projects considera- 
 bly backwards, and is connected 
 in front with the other bones of 
 the tarsus. In front of the 
 tarsus we find the metatarsus, 
 composed of five long bones, 
 which in man are all attached 
 to each other, but of which one 
 is separate in the Quadrumana, 
 in order to give freer play to 
 the great toe, the action of which 
 resembles that of the thumb. 
 The toes, like the fingers, are 
 composed of three phalanges 
 (with the exception of the 
 great toe, which has only two) ; 
 these are in Man much shorter 
 than those of the hand, and 
 are evidently not adapted for 
 prehension but in many of the 
 Quadrumana, their length is 
 nearly equal to that of the 
 fingers, and the great toe is as 
 opposable as the thumb. The 
 foot is far from being thus con- 
 verted, however, into a perfect 
 hand ; but it becomes a very 
 useful instrument for clasping 
 the small branches and twigs 
 of the trees among which these 
 animals live. The foot of Man 
 is distinguished from theirs, by 
 its power of being planted flat upon the ground, and thus 
 affording a firm basis of support. Even the Chimpanzee 
 and the Orang, when they attempt to walk erect, rest upon 
 
 Fig. 230, MUSCLES OF THE SOLE OF 
 
 THE FOOT (MIDDLE LAYER). 
 
 1, accessory of the long flexor of the 
 toes ; 2, tendon of the long flexor 
 issuing from its sheath ; 3, tendon 
 of the long flexor of the great toe; 
 4, first lumbricalis ; 5, tendon of 
 the superficial flexor, divided be- 
 hind its perforation ; 6, short flexor 
 of the little toe ; 7, short flexor of the 
 great toe; 8, portion of the oblique 
 abductor of the great toe ; 9, poste- 
 rior extremity of the fifth metatar- 
 sal bone ; 10, sheath of the long pero- 
 neal; 11, os calcis, or bone of the 
 heel. 
 
488 BONES AND MUSCLES OF THE FOOT. 
 
 the side of the foot ; and the absence of a projecting heel 
 causes them to be very deficient in the power of keeping the 
 leg upright upon it. For it is to this projection that the 
 strong muscles of the calf of the leg are fixed, by which the 
 heel is drawn upwards or the leg drawn back upon it. Other 
 muscles at the side and back of the leg, the direction of whose 
 tendons is changed by a sort of pulley at the ankle-joint, 
 aided by the muscles of the foot itself, serve to bend the toes, 
 an action which gives great assistance in walking, running, 
 leaping, &c. And the toes are straightened by an extensor 
 muscle, which lies on the front of the leg, and of which 
 the tendon runs under an annular ligament that encircles 
 the ankle, and is then divided and spread -out to the 
 toes, over the upper surface of the foot. The great toe is 
 a very important instrument in the act of walking, since 
 much of the spring forwards is given by the bending of 
 its phalanges ; and it is provided with two flexor muscles 
 of its own. 
 
 649. On the internal side of the foot, the bones of the 
 tarsus and metatarsus form a kind of vault or arch, which 
 serves to lodge and protect the vessels and nerves that 
 descend from the leg towards the toes. This arch further 
 serves the important purpose of deadening the shock that 
 would otherwise be experienced every time that the foot is 
 put to the ground ; for, by the elasticity of the ligaments 
 which hold together the bones that compose it, a sort of 
 spring is formed, which yields for a moment to the shock, 
 and then recovers itself. We feel the difference which this 
 makes, when we jump from a height upon our heels ; the jar 
 is then propagated directly upwards from the heel to the leg, 
 thence to the thigh, and thence to the spinal column , and if 
 it were not from the peculiar manner in which this is con- 
 structed ( 631), a severe shock of this kind might produce 
 fatal effects by concussion (or shaking) of the brain. In 
 animals which walk upon four extremities, the difference of 
 direction in which the legs are connected with the spine 
 prevents a jar from being propagated along the latter to a 
 similar degree. But in those which are destined to obtain 
 their food by sudden and extensive leaps, such as the animals 
 of the Cat tribe (the Lion, Tiger, &c.), we find an arrange- 
 ment of the bones of the foot, well adapted to diminish the 
 
STANDING POSTURE : EQUILIBRIUM. 489 
 
 shock produced by the sudden descent of the body upon the 
 ground. 
 
 Of the Attitudes of the body, and the various kinds of Locomotion. 
 
 650. A small number of Vertebrated animals, Serpents, 
 for instance, bear habitually on the whole length of their 
 bodies, which rest entirely on the ground ; and their only 
 movements are effected by undulations of the spinal column. 
 But the rest are supported upon their extremities ; and we 
 give the name of standing to that position in which the 
 animal rests supported by its limbs upon the ground or on 
 any firm horizontal basis. In maintaining this position, the 
 extensor muscles, by which the joints are straightened, must be 
 in continual action, since the limbs would otherwise bend 
 beneath the weight of the body. Now as the sense of 
 fatigue, in any set of muscles, depends in great degree upon 
 the length of time during which they have been in continuous 
 action, the maintenance of the standing posture for a long 
 period is, in most animals, more fatiguing than walking ; 
 since in the latter exercise the action of the flexors alternates 
 with that of the extensors. 
 
 651. But this condition is not the only one essential to 
 steadiness in the standing posture ; for in order that the 
 body may rest firmly upon the members, it must be in equi- 
 librium. It has been shown (MEGHAN. PHILOS. Chap, iv.) 
 that equilibrium exists, or in other words, that a body 
 remains at rest in its position, not only when it bears upon 
 the whole of a broad sur- 
 face, but also when it is 
 
 so placed that the tenden- 
 cies of its different parts 
 to descend or gravitate 
 towards the earth counter- 
 balance each other. This 
 is the case when its centre 
 of gravity is supported, 
 that is, when a line drawn 
 
 perpendicularly from that centre falls within the base. In 
 order, then, that an animal may rest in equilibrium on its legs, 
 it is necessary that the vertical line from its centre of 
 gravity (or line of direction) should fall within the space 
 
490 EQUILIBRIUM OF ANIMALS : BASE OF SUPPORT. 
 
 which its feet cover and inclose between them ; and the 
 wider this space, in proportion to the height of the centre of 
 gravity, the more stable will the equilibrium be, since the 
 body may be more displaced without being upset. Thus in 
 fig. 231 the table a must be upset ; because the line of direc- 
 tion e from the centre of gravity c falls outside the base of 
 support d ; whilst the table b, although equally inclined, will 
 not be upset but will return to its proper place, because the 
 line of direction e from its centre of gravity c falls within its 
 base d. Hence an animal which is supported upon four legs 
 will stand much more firmly than one which rests on two 
 only ; since its real base is the whole space included between 
 its four points of support. And again, an animal is more 
 firm when standing upon two legs, than when resting upon 
 one only. 
 
 652. Moreover when an animal rests upon four legs, the 
 extent of its base is but little influenced by the size of the 
 
 feet ; and thus to render them 
 broad would be to increase 
 their weight without adding 
 much to their use as supports. 
 This is easily understood by 
 comparing a quadruped to a 
 four-legged table ; if the legs 
 are sufficiently strong to support 
 the weight that rests upon 
 them, it matters little in regard 
 to the steadiness of the table, 
 whether they bear upon the 
 ground by mere points or by flat 
 surfaces j since it is the large sur- 
 face that would be enclosed by 
 lines joining them, which consti- 
 tutes the real base. Hence we 
 SE . find that, in most quadrupeds, 
 the limbs only touch the ground 
 by slightly-dilated extremities ; and the number of fingers is 
 reduced more and more, without diminishing their effect as 
 instruments of locomotion. Thus in Ruminant animals, as 
 the Deer, the number of toes is reduced to two in each foot, as 
 seen in fig. 232, where represents the tibia, ta the bones of 
 
 Fig. 232. 
 FOOT OF DEEB 
 
 233 
 
EQUILIBRIUM OF ANIMALS : - BASE OF SUPPORT. 491 
 
 the tarsus, c the bone of the metatarsus termed the canon (in 
 which the trace of a division into two pieces can be seen), 
 and p, pi, pt, the three phalanges of the toes, of which the 
 last is enveloped in the hoof, which is nothing else than a 
 large nail inclosing the whole extremity of the toe. In the 
 Horse this consolidation is carried still further than in the 
 Ruminants, for it has only one toe in each foot (fig. 233); but 
 we see the rudiment of an additional bone in the metatarsus 
 5, which is commonly termed the splint bone. 
 
 653. But when an animal is supported upon two feet only, 
 whatever may be their degree of separation from each other, 
 the base of support cannot have sufficient extent, unless the 
 extremities touch the ground by a considerable surface. This 
 is the case with the foot of Man, and still more with that of 
 many Birds which habitually stand 
 upon one leg (fig. 234). In. order 
 that an animal may hold itself in 
 equilibrium upon a single limb, it 
 is necessary that the foot should 
 be placed vertically beneath the 
 centre of gravity of the body ; 
 and that its muscles should be so 
 arranged as to permit it to keep 
 this limb inflexible and immov- 
 able. Man can accomplish this, 
 for the centre of gravity of his 
 body is at about the middle of the 
 pelvis ; and to place this vertically 
 over one foot, it is sufficient for 
 him to bend himself a little from 
 the side which is not supported. 
 But the greater number of Qua- 
 drupeds are destitute of the power 
 of doing this ; and a large part 
 of them cannot even raise them- 
 selves on their hind legs, on 
 account of the direction of these members relatively to the 
 trunk; or if they can do so for an instant, they cannot 
 maintain themselves in this position. The reason of this is 
 very simple. The base of support, on account of the small- 
 ness of the feet, is very narrpw, and the centre of gravity of 
 
 
492 MUSCULAR EXERTION TO MAINTAIN EQUILIBRIUM. 
 
 the body is placed near the front ; hence the body must be 
 entirely changed in its position by a violent and not sustain- 
 able action of the muscles which connect it with the hind 
 legs ; and, when thus reared up, it cannot rest with firmness 
 on account of the narrowness of the base. 
 
 654. There are some Quadrupeds, however, which are able 
 to raise themselves occasionally into this position ; this is the 
 case, not only with the Quadrumana, but also with the Bear, 
 Squirrel, and other animals whose habits require them to 
 ascend and live among trees, as well as in the Kangaroo, 
 and animals constructed upon the same plan, whose peculiar 
 organisation will be presently considered (661). In standing 
 upright, the muscles of the back part of the neck are kept in 
 a contracted state, to retain the head in equilibrium on the 
 vertebral column ; and the extensor muscles of that column 
 must also be kept in action, to prevent it from bending 
 forwards under the weight of the head, upper extremities, 
 and viscera of the trunk. The whole weight of the upper 
 part of the body is thus transmitted to the sacrum, and thence 
 to the other bones of the pelvis, by which it is brought to 
 bear on the femur. If left to themselves, the thigh-bones 
 would bend beneath the pelvis, and the trunk would fall 
 forwards ; but the contraction of their extensor muscles keeps 
 them firm. In the same manner, the extensor muscles of the 
 knee and ankle keep these joints from yielding beneath the 
 weight of the body, which is thus at last transmitted to the 
 ground. The sitting posture is less fatiguing than the stand- 
 ing position, because the weight of the body is then directly 
 transmitted from the pelvis to the base of support, so that it 
 is not requisite for the extensor muscles of the lower limbs 
 to keep-up a sustained action. But the lying posture is that 
 of the most complete rest ; because the weight of every part 
 of the body is at once transmitted to the surface on which it 
 bears, and no muscular movement is requisite to keep it in its 
 position. 
 
 655. This difference in muscular effort, is the cause of a 
 well-marked variation in the pulse, according to the position 
 in which the body is at the time. From a considerable 
 number of observations it has been found that the average 
 pulse of an adult man is about 81 when standing, 71 when 
 sitting, and 66 when lying ; so, that the difference between 
 
PULSE IN DIFFERENT POSTURES : ACT OF WALKING. 493 
 
 standing and sitting is 10 beats of l-8th of the whole, 
 whilst the difference between sitting and lying is 5 beats or 
 1-1 3th of the whole. In the female, the pulse is quicker in 
 each position by from 10 to 14 beats per minute ; but the 
 differences occasioned by position are nearly the same. It 
 will be observed that the difference between standing and 
 sitting is greater than that between sitting and lying ; and 
 this closely corresponds with the relative amounts of muscular 
 exertion required in these positions respectively. At the 
 moment when the posture is changed, the pulse is considerably 
 quickened, in consequence of the muscular effort required for 
 the purpose, which acts especially on the veins, and forces the 
 blood more rapidly back to the heart ( 279); but this 
 increase in rapidity is temporary only. 
 
 656. All that has been said of the positions of Vertebrated 
 animals applies equally well to those of the Invertebrata, 
 which like them have the body raised from the ground upon 
 extremities. This is the case in the higher Articulata, such 
 as Insects, Crustacea, Arachnida, and Myriapoda. But the 
 lower Articulata crawl, like Serpents, upon the whole length 
 of their bodies ; or, being aquatic, are buoyed-up by the 
 element they inhabit. And among the Mollusca and Eadiata, 
 there are none that have members upon which they can be 
 said to stand. 
 
 657. The progressive movements by which the bodies of 
 Man and other animals are made to change their places, are 
 accomplished by means of the alternate contractions and 
 extensions of those limbs, which we have hitherto considered 
 only as supporting them in a rigid position. It is easy to see 
 that when a joint is straightened after being bent, the two 
 ends of the levers which form it must be separated from each 
 other, and that motion must thus be given to the parts against 
 which one or both of them bear. Now in the ordinary move- 
 ments of progression, one of these levers bears against the 
 ground, which is immovable ; and the whole motion produced 
 by straightening the joint must consequently be communicated 
 to the body. In the ordinary act of walking, one of the feet 
 is planted in front, whilst the other is extended or carried 
 backwards beneath the leg, by the action of the muscles of 
 the calf aided by those of the toes (. 648). Its length is 
 thus increased j and as it bears upon the resisting soil, this 
 
494 ACT OF WALKING : OTHER MODES OF LOCOMOTION. 
 
 elongation acts through the thigh upon the pelvis, and thus 
 carries forward the whole body. At the same time, the pelvis 
 makes a slight turn upon the femur of the other side on 
 which it is resting ; and the limb which was at first behind 
 the other, is now drawn forward by a flexion of its joints, 
 and is planted on the ground in front of the other, so as to 
 serve for the support of the body in its turn ; whilst the 
 other, by extending itself, gives a fresh forward impulse to 
 the body. Thus each limb is alternately made to support the 
 whole weight of the body, just as it would do in standing on 
 one leg ; while at the same time the other is engaged in 
 urging it forwards. Hence the centre, of gravity must vibrate 
 a little from side to side in the act of walking, so that it may 
 be brought alternately over each foot ; and this movement 
 from side to side is the more obvious, in proportion as the 
 pelvis is wider, and the limbs more separated from each other. 
 Hence it is more seen in women than in men, on account of 
 the greater proportional breadth of the hips in the former. 
 
 658. In all the higher animals, as in Man, there are 
 members which serve for locomotion ; but the nature of these 
 movements varies greatly ; and there is a corresponding differ- 
 ence in the structure of the instruments by which they are 
 performed. The manner in which the Creator has made the 
 same organs answer a variety of different purposes, in accord- 
 ance with the habits of the animals to which they belong, is 
 a most interesting object of study ; for we see the most 
 varied results attained, without the least departure from the 
 general plan which has been adopted in the construction of 
 the various species of the same group ; and this solely by 
 slight changes in the forms and proportions of some of the 
 instruments whose union makes-up the entire body. The 
 organs of locomotion in the Mammalia furnish us with 
 obvious examples of this principle. This class includes not 
 only the quadrupeds which run or bound along the surface of 
 the ground, but animals which are destined to live solely in 
 water like fishes, others which sometimes swim through that 
 element and sometimes inhabit the land, others which 
 possess wings that enable them to fly through the air like 
 birds, and others which only employ their anterior members 
 for grasping or feeling ; yet in all these animals, these organs 
 are constructed of the same parts. In the paddles of a Seal 
 
ACTS OF WALKING AND KUNNING. 495 
 
 (fig. 240), the wing of a Bat (fig. 251), and the fore-paw of a 
 Squirrel or a Mole, we find the same bones as in the arm of 
 Man (fig. 223). And even in the fore-legs of the Ruminant 
 Quadrupeds, and in those of the Solidungula, or single-toed 
 animals (such as the Horse), we can usually perceive traces of 
 the existence of three or four toes, whose bones are more or 
 less completely united. 
 
 659. From what has already been stated as to the influence 
 of the length of the levers on the quickness of the movement 
 of the extremities (. 614), it is easy to see that animals 
 which have the most rapid progression must necessarily have 
 long members ; since, the quickness with which the extensor 
 muscles act remaining the same, the change of place in the 
 free extremity of the lever will be greater, in proportion as 
 that extremity is more distant from the point of insertion of 
 the muscles that move it, and from the fulcrum on which the 
 tever works. But in proportion to the elongation of this arm 
 of the lever, must be the increase in the power of the muscles 
 that move it, in order to overcome the same resistance ; 
 according to the general principle that what is gained in 
 velocity is lost in power. Hence, in order to endow an animal 
 with great agility, it is only necessary to lengthen its limbs, 
 and to render its muscles capable of exerting a proportional 
 power. 
 
 660. We have seen that in walking, the body is sustained 
 upon one limb (in quadrupeds, upon one pair of limbs), 
 whilst it is pushed onwards by the other ; so that it never 
 ceases to bear upon the ground. In running r , however, the 
 body of Man momentarily quits its support at intervals ; the 
 foot in advance not being planted on the ground by the time 
 that the hinder one quits it. In this action, the Ostrich and 
 its allies probably surpass all other animals ; as they can out- 
 strip the fleetest horse at full gallop, or the swiftest greyhound 
 at its greatest speed. The amble of Quadrupeds is a pace 
 which resembles the walk or run of bipeds, the two legs on 
 one side being moved together, whilst the body rests upon the 
 other. This pace is peculiar to the Giraffe, and to horses 
 which have been trained to execute it. The trot, however, is 
 a step of a different and much more secure nature. The fore- 
 foot of one side is raised and advanced with the hind foot on 
 the other side ; and when these are set down, the other fore 
 
496 ACTS OF RUNNING AND LEAPING. 
 
 and hind feet are raised and advanced together. Now, if we 
 consider the fore-feet of a horse as constituting the four angles 
 of a parallelogram, it is easy to see that the base of support, 
 when the feet are thus raised, will be one of its diagonals ; 
 and as the feet are alternately advanced, the weight will 
 alternately be thrown upon these two lines. But the centre 
 of gravity in the horse, especially when carrying a rider, is in 
 a point almost exactly above that at which the two diagonals 
 cross ; so that it is always supported either by the one or the 
 other. The gallop of greatest speed is a run performed on 
 the same plan as the trot ; that is, the right fore and left 
 hind feet leave and reach the ground together, and then the 
 left fore and right hind feet are advanced. The canter is a 
 kind of step altogether different. The four legs strike the 
 ground successively, the left hind foot reaching it first, the 
 right hind foot second, the left fore foot third, and the right 
 fore foot fourth. The celebrated race-horse Eclipse, when 
 galloping at liberty and with his greatest speed, passed over 
 the space of 25 feet at each stride or leap ; this he repeated 
 2jL times in a second, so as to pass over 58 feet in that time, 
 which was at the rate of nearly 4 miles in six minutes and 
 two seconds. But this performance was completely surpassed 
 by that of Flying Childers, who was computed to have 
 passed over 82| feet in a second, or nearly a mile in a 
 minute. 
 
 661. In leaping, the body is projected into the air by the 
 sudden extension of the joints, especially those of the hinder 
 part of the body which had been previously bent ; and having 
 traversed a greater or less distance, the body comes again to 
 the ground and may be again projected. This is a kind of 
 motion usually practised by many animals whose structure is 
 expressly adapted to it. Thus among Mammals we find se- 
 veral in which the hind legs are enormously elongated, for 
 the purpose of giving greater quickness to the motion of the 
 body ; and their muscles are developed to an extraordinary 
 degree in order to supply the necessary force. This is the 
 case among most of the animals of the order Rodentia, such 
 as the Hare, Rabbit, Squirrel, &c. ; but particularly in the 
 Jerboa or Jumping Rat, and in the Kangaroo and its allies. 
 In these animals the fore feet, which are little used for pro- 
 gression, are comparatively small j and in the last they are 
 
ACTS OF RUNNING AND LEAPING : KANGAROO. 
 
 497 
 
 less than half the length of the hinder limbs (fig. 235). The, 
 feet, as well as the legs, of the Kangaroo are very long (fig. 
 
 Fig. 235. KANGAROOS. 
 
 236), so as to afford (in conjunction with the tail) a firm sup- 
 port to the animal when preparing to leap. Quadrupeds in 
 
 Fig. 236. SKELETON OF KANGAROO. 
 
 which the length of the posterior extremities greatly predomi- 
 nates over that of the anterior, are observed to descend hills 
 with difficulty at a rapid pace, since the forward inclination 
 
 K K 
 
498 
 
 ACTS OF LEAPING : PLEA : CRICKET. 
 
 of their bodies places them in continual danger of oversetting ; 
 they therefore take a zig-zag course. In ascending a hill, 
 however, their progression is greatly favoured by the length 
 of their posterior extremities (fig. 237). The Rabbit, when 
 
 Fig. 237. HARE ASCENDING A HILL. 
 
 moving slowly, advances the fore-feet two or three steps 
 alternately, the posterior limbs remaining inactive ; and the 
 body having been lengthened by these means, the posterior 
 legs are suddenly extended together, and then drawn for- 
 wards : thus the rabbit walks with the fore and leaps with 
 the hind pair of legs. The Frog moves in a very similar 
 manner. 
 
 662. It is among Insects that we find the most extraordi- 
 nary powers of leaping, considered with reference to the size 
 
 of the animals that 
 possess them. Thus 
 the Flea will spring 
 to a height equal to 
 200 times the length 
 of its body. Let us 
 imagine a Kangaroo 
 or a Tiger doing the 
 same ! In many of 
 
 Fig.238.-C R iCK ET . ^ leaping ingectg> 
 
 the hind legs are of great length, as in the Grasshopper 
 and Cricket tribe (fig. 238); and in one curious family, 
 that of the Poduras or spring-tails, the leap is accomplished 
 by the sudden extension of the tail, which is ordinarily bent 
 under the body (fig. 239). A very remarkable kind of leap is 
 
LEAPING INSECTS. SWIMMING AND PLYING. 499 
 
 executed by the Beetles of the family of Elateridce ; the larva 
 of one species of which devours the roots of wheat, and is 
 known under the name of the 
 "wire -worm;" whilst other 
 species inhabiting tropical cli- 
 mates, and having the power 
 of emitting light, are termed 
 "fire-flies" ( 397). The legs 
 of these insects are very short : 
 so that when they are laid on Fig " 239 - 
 
 their backs they cannot by means of them recover their 
 natural position. This they are enabled to do, however, by 
 their power of jerking backwards the head and upper part 
 of the thorax, which causes the body to be projected verti- 
 cally into the air, whence it usually descends with the feet 
 towards the ground. The leap of the Crickets, Locusts, Frog- 
 hoppers, &c. is executed more in a horizontal direction ; and 
 it is assisted by the wings, which bear-up the body whilst it 
 is moving onwards through the air. In this manner a Locust 
 can traverse 200 times its length, and a Frog-hopper 250 
 times ; which is as if a Man were to take a quarter of a mile 
 at one leap. 
 
 663. Swimming and Flying are movements which have 
 much resemblance to each other ; both being executed in a 
 fluid medium, which to a certain extent buoys-up the body, 
 which offers resistance to its progress, and which also offers 
 something resembling a fixed point against which the mem- 
 bers may act to propel it. The chief differences between 
 them depend upon the nature of the medium ; this being 
 liquid in the one case, and aeriform or gaseous in the other. 
 The liquid medium affords more support to the body, and a 
 firmer surface for the action of its propelling organs ; but at 
 the same time it offers more resistance to its progress. The 
 movement of a body through the atmosphere, as in flight, 
 requires a considerable expenditure of power to keep it up ; 
 and the yielding nature of the element prevents the propelling 
 organs from acting against a firm surface ; but the onward 
 movement, in consequence of the slight resistance, is easily 
 accomplished. 
 
 664. When the feet of a Quadruped are to serve both as 
 walking and swimming organs, the end is accomplished by 
 
 K K 2 
 
500 ADAPTATION OP EXTREMITIES FOR SWIMMING. 
 
 the spreading-out of the fingers, and their union by means of a 
 fold of skin which is stretched over them ; as the web of a 
 swimming Bird is stretched over its toes, so as to make an 
 oar or paddle of sufficiently wide surface. This is the ease, 
 for example, in the Ornithorhyncus of Australia, and in the 
 Otter of our own country. When the members are intended 
 
 Fig. 240. SKELETON OF SEAL. 
 
 vc, cervical vertebrae, vd, dorsal vertebrae; vl, lumbar vertebrae; vs, sacral vertebras; 
 vq, caudal vertebrae ; b, pelvis ; s, sternum; h, humerus; r, radius ; ca, carpus ; 
 me, metacarpus ; ph, phalanges ; o, scapula ; c, ribs ; /, femur ; r, patella ; t , tibia ; 
 ta, tarsus ; mt, metatarsus ; ph, phalanges. 
 
 exclusively for swimming, however, they undergo more con- 
 siderable modifications in structure. The parts corresponding 
 with the arm and fore-arm are very short, and the movements 
 of the hand are thus limited, but they can be accomplished 
 with all the more force. But the bones of the hand are large 
 and spread asunder, and are enclosed in a firm integument 
 which may even cover their extremities. Sometimes the 
 number and arrangement of these bones are precisely the 
 same as in the hand of Man ; this we see in the Seal (fig. 240), 
 where their extremities are furnished with separate claws that 
 project beyond the integument. Sometimes the number of 
 phalanges in the fingers is considerably increased, as in the 
 Whale ; and in other instances, the fingers are replaced by a 
 multitude of small rods of bone, enclosed within a continuous 
 skin, such as we see in the fins of Fishes (fig. 243). 
 
 665. In the Seal, which does not depart widely in its 
 general construction from land quadrupeds, the hind feet are 
 formed upon the same plan as the fore ; but they are carried 
 
ADAPTATION OF EXTREMITIES FOR SWIMMING. 501 
 
 far backwards, so as almost to occupy the position of the tail. 
 In the Whale and its allies, on the other hand, the posterior 
 extremities are almost entirely wanting, and the tail is- greatly 
 prolonged and expanded at its extremity (fig. 241), This 
 
 Fig. 241. SKELETON OF DUGONG. 
 
 expansion, however (which is in the horizontal direction, fig. 
 242), is not supported by bones, except in its centre ; but it 
 consists internally of cartilages and tendons, which last are 
 prolonged from a set of very powerful muscles that are at- 
 tached to the spine, and give to this organ an enormous force 
 
 Fig. 242. TAIL-FIN OF WHALE. 
 
 and great variety of motion. The texture of the portion of it 
 by which the blow is usually given, is such that it can hardly 
 be injured ; it is so tough that it cannot be torn, and so free 
 from feeling, that a stroke of it against a hard substance gives 
 no pain to the animal. If it strike a boat across the middle 
 with its edge, the boat is cut asunder as clean and suddenly 
 as if by one stroke of a giant axe ; whereas, if it strike with 
 
502 PROPULSION OF WHALES AND FISHES BY TAIL. 
 
 the flat surface, the boat is driven to the depth of many fa- 
 thoms with the swiftness of an arrow. Hence this tail is a 
 most efficient instrument for the propulsion of the bulky 
 body of the Whale through the water ; and it is, in fact, its 
 principal organ of locomotion. The paddles formed by the 
 fore-feet are placed near the centre of gravity of the whole 
 mass ; and thus can readily exert their peculiar action, which 
 is that of changing the direction of the movement, and espe- 
 cially of raising and lowering the body. 
 
 666. The propulsion of the body by the stroke of the tail 
 in Whales and Fishes, is effected precisely in the same manner 
 as the urging-forwards a boat through the water, by the 
 
 Fig. 243. SKELETON OF PERCH. 
 
 lateral strokes of an oar at the stern, in the mode commonly 
 termed sculling. The expansion of the Whale's tail-fin being 
 horizontal, its stroke is vertical, and may thus readily bring 
 the animal to the surface of the water for occasional respira- 
 tion, as well as propel it forwards ; but that of the Fish's 
 body and tail being vertical, its stroke is horizontal, and its 
 action will simply be to urge the body through the water. 
 The power of ascending and descending, as well as of changing 
 the direction of the motion, is principally due to the side-fins, 
 which represent the arms and legs. The direction of the 
 surface and stroke of these side-fins varies in different species. 
 In the Cod, Halibut, and others, their action appears to be 
 principally directed towards keeping the body in its right 
 position in the water; since, without such an action, the body 
 would be liable to turn over, in consequence of the position 
 
ACTION OF THE FINS OF FISHES : FLYING FISH. 503 
 
 of its centre of gravity. In other instances, the pectoral and 
 ventral fins move in such a manner as to assist the action of 
 the tail. In the Kays, the pectoral fins are developed to an 
 enormous extent (fig. 244) ; and being 
 directed horizontally, their action is vertical 
 like that of the wings of a bird. They are 
 furnished with a great number of joints, 
 by which they are rendered very flexible \ 
 and their surface may be thus increased 
 during the down-stroke of the fin, and 
 diminished during the -^p-stroke. If this 
 were not done, the action of the fins in 
 elevation would exactly counterbalance the 
 effect of their depression ; and no movement Flgt 244 *~ RAT ' 
 would be produced. The great power of the pectoral fins of 
 these Fishes seems connected with their want of an air- 
 bladder, which causes them to require a constant exercise of 
 force to keep them up in the water. Their propulsion forwards 
 is chiefly accomplished, as in other Fishes, by the action of the 
 tail. But sometimes the Rays change their position and swim 
 sideways, making horizontal strokes with the pectoral fins 
 (whose surface is then vertical), by which they are moved 
 through the water, and sustaining themselves by vertical 
 strokes of the tail, whose surface is then horizontal. 
 
 667. The structure of the organs adapted for movement in 
 air bears great analogy to that of such expanded fins ; and 
 there are instances in which the same instruments may serve 
 both purposes. Thus there are Fishes which are able to quit 
 the water, and execute leaps of considerable length, supported 
 
 Fig. 245. FLYING-FISH. 
 
 upon their wing-like pectoral fins. These are known as 
 Fly ing- Fish (fig. 245) ; but it is not correct to speak of their 
 
504 ORGANS OF FLIGHT : FLYING FISH, PENGUIN, ETC. 
 
 movement as one of flight, since it does not appear that they 
 have any power of propelling themselves in the air ; the 
 impulse being given at the moment of their quitting the 
 water, in the manner of a leap. From 50 to 100 yards, how- 
 ever, are sometimes traversed by the Fish at one leap ; and 
 the height to which it rises from 
 the surface of the water is occa- 
 sionally such as to carry it over the 
 deck of a ship. On the other hand, 
 there are several among the diving 
 Birds which use their wings as 
 instruments of progression beneath 
 the water in other words, as fins. 
 The most remarkably constructed 
 of all these is the Penguin (fig. 
 246), in which the wings are so 
 short as to be incapable of answer- 
 ing any other purpose ; but there 
 are several species in which they 
 Fig. 246.-PENGUIN. ma y fc e use d Qs, organs of flight 
 
 in the air, without losing their fin-like power in the water. 
 There are several animals that can sustain themselves for a 
 short time in the air, by the aid of 
 an expanded surface formed by an 
 extension of the skin and serving 
 as a parachute. This is the case, 
 for instance, with the Galeopithecus, 
 or Flying Lemur (fig. 247), the 
 Flying Squirrel, and the Petaurus, 
 or Flying Phalanger (ZooL. 314), 
 which have the skin stretched out 
 on either side like a cloak, sup- 
 ported by the anterior and pos- 
 terior extremities and by the tail. 
 By this parachute-like surface they 
 are sustained in extensive leaps 
 from bough to bough; though it 
 does not enable them to support 
 
 Fig. 247.-GAoPi.HKCU.. themselves in the air for any len g th 
 
 6f time. In the Draco Volans (fig. 248), a little animal which 
 lives among the trees of tropical forests, the body is furnished 
 
DRACO VOLANS. WINGS OP BIRDS. 505 
 
 with a wing-like appendage on either side, formed by an 
 expansion of the skin over six lengthened ribs. These 
 appendages serve as a kind of parachute, on which this little 
 
 Fig. 248. DRACO VOLANS. 
 
 animal, not more than a few inches long, flutters from branch 
 to branch in search of its insect prey, or shoots, like the flying 
 squirrel, from tree to tree. They cannot be made to strike the 
 air, and therefore are not true wings ; but they can be folded 
 up and extended at the will of the animal. 
 
 668. True wings, or instruments of propulsion as well 
 as of support in the air, are found in some members of all 
 classes of air-breathing Vertebrata; but they are especially 
 characteristic of the class of Birds, in which the absence of 
 them is the exception to the general rule, whilst in Mam- 
 mals and Reptiles it is their presence which constitutes the 
 exception. These wings are universally formed by some 
 modification of the anterior extremities, which renders them 
 unfit to be used as instruments of progression on the ground ; 
 but the nature of this modification varies considerably. In 
 the Bird, the required extent of surface is chiefly given by 
 the feathers ; these are supported upon an anterior member, of 
 which the arm and fore-arm (especially the latter) constitute 
 the largest part, the hand being contracted and consolidated. 
 The general structure of the Bird's skeleton, the whole of 
 which is modified with special reference to the actions of 
 flight, is shown in fig. 249, which represents that of the 
 Vulture. The head is supported upon a very flexible neck, of 
 which the vertebrae vc are often very numerous. The ver- 
 tebrae of the back and loins, however, are usually few in 
 
506 
 
 SKELETON OF BIRDS. 
 
 number, and are connected together very firmly, so as to form 
 a nearly inflexible column ; and this, again, is closely united 
 to the sacrum vs. The vertebrae of the tail v% are few in 
 
 ta 
 
 Fig. 249. SKELETON OF VULTURE. 
 
 vc, cervical vertebrae; vs, sacral vertebrae; vq, caudal vertebrae; cl, clavicle; 
 h, humerus ; o, fore-arm; ca, carpus; ph, phalanges; st, sternum; /, femur; 
 t, tibia ; ta, tarsus. 
 
 number, and possess little motion. The ribs are very strongly 
 connected to each other and to the vertebras, and are united 
 to the sternum st by bony instead of cartilaginous prolonga- 
 tions. Thus the whole bony apparatus of the trunk is very 
 strongly knit together ; and the purpose of this is evidently 
 to give as firm an attachment as possible to the muscles which 
 move the wings. The sternum is raised into a high keel or 
 ridge (as is better seen in fig. 250, 6), for the attachment of 
 the powerful pectoral muscles which draw down the wings ; 
 and the degree of this projection is proportioned to the power 
 of flight which the species possesses, the sternum being flat 
 (as in Mammals) in birds which, like the Ostrich, have the 
 wings undeveloped. The scapula (fig. 250, o), to which are 
 
SKELETON OF BIRDS. 507 
 
 attached the muscles that raise the wings, is very narrow in 
 
 Birds, in accordance with their small demand for muscular 
 
 power in this direction. This narrow scapula forms one part 
 
 of what is known as the " side-bone ;" the other part c of 
 
 which is formed by a bone termed the coracoid, that is only 
 
 represented in Man and other Mammals by the short coracoid 
 
 process of the scapula 
 
 ( 635). The two clavi- 
 
 cles// are united together 
 
 where they join the ster- 
 
 num, to form the fork- 
 
 like bone known as the 
 
 " merry - thought/' the 
 
 strength of which, like 
 
 the projection of the keel 
 
 of the sternum, serves 
 
 to indicate the power of 
 
 flight, by the degree of 
 
 resistance which it is ca- 
 
 ,, ~, ,. Fig. 250. BONES OF THE SHOULDER AND 
 
 pable 01 attorning to the BREAST OF BIRDS. 
 
 drawing-together of the 0;Scapu]a; c , coracoid bone; /? claviclesunited 
 
 Shoulder- JOintS by the at their junction with the summit of the keel 
 
 nf fho vkonfnval b of the sternum s, which is connected with 
 Ot tne ectOial 
 
 the ribs by the ossified costal cartilages co. 
 
 muscles. The bones of 
 
 the pinion consist of the humerus (fig. 249, A), the two bones 
 of the fore-arm o, the bones of the wrist ca (which are here 
 scarcely developed), and the bones of the fingers ph, each 
 joint of which shows indications of being made up of two or 
 three separate bones united together. In no bird are these 
 bones ever separated into distinct fingers, since they are never 
 required for any other purpose than that of supporting the 
 wing-feathers. The leg is connected with the spinal column 
 by a pelvis, of which the iliac bones are greatly lengthened 
 and firmly attached to the spine, but which is not completed 
 into a ring by the junction of the bones in front, as in Mam- 
 mals ; such a completion would have prevented the passage 
 of the bulky eggs deposited by these animals ( 755). In the 
 hinder extremity we find the femur or thigh-bone (principally 
 concealed in the figure by the bones of the wing), the two 
 bones of the leg t, which are commonly united in part of their 
 length, the shank or ancle-bones ta, which are peculiarly 
 
508 
 
 SKELETON OF BATS AND PTERODACTYLS. 
 
 elongated in the wading birds, and the four separate toes, by 
 the spread of which the body is firmly supported, though 
 resting only on two feet. 
 
 Fig. 25!. SKELETON OF BAT. (References as in Fig. 229.) 
 
 669. In the Bat (fig. 251), however, the plan is very dif- 
 ferent. We have here no long stiif feathers, by the projection 
 of which from the limb itself the surface may be increased to 
 almost any extent ; but the wing is formed by an expansion 
 
 Fig. 252. SKELETON OF PTERODACTYLS. 
 
 of soft and delicate skin over a framework of bones, which 
 must consequently be made to support it to its very edge. 
 
PTERODACTYLE. WINGS OF INSECTS. 509 
 
 This is accomplished by the enormous extension of the bones 
 of the hand, especially the metacarpal me, which are here 
 separate ; and the membrane is further sustained by the legs 
 and tail. The thumb po is not included in the wing, but 
 serves as a hook by which the animal can suspend itself. 
 The only true flying Eeptile is (or rather was) the Pterodac- 
 tyle,, a kind of winged lizard, which does not now exist, but 
 of whose character the skeletons that are found imbedded in 
 the earth afford most convincing proof. The structure of its 
 wing differed from that of either Birds or Bats ; for it appears, 
 from the conformation of its anterior member (fig. 252), that 
 the animal could have used it for resting or walking, the 
 framework of the wing being formed by the enormous elonga- 
 tion of one finger only. 
 
 Fig. 253. DRAGON FLY. 
 
 670. The wings of Insects (fig. 253) have no correspon- 
 dence whatever with those of Vertebrata, except in serving 
 for the like use, and in being composed of an expanded sur- 
 face of membrane, stretched upon a firm framework. This 
 framework is not composed of solid pieces jointed together, 
 but is merely an extension of the air- tubes and vessels within 
 the body, which are strengthened by a continuation of its 
 hard envelope. Their only action is a hinge-like movement 
 at the point where they are united to the body ; and this 
 is accomplished by powerful muscles contained within the 
 thorax. 
 
 671. In all instances, the action of the wings must be such, 
 that the air is struck with less force during the up-stroke than 
 
510 POWER OF FLIGHT POSSESSED BY BIRDS. 
 
 during the down-stroke ; otherwise the effect of the former 
 would neutralise that of the latter. This is partly accom- 
 plished by the great velocity of the down- stroke compared 
 with the up-stroke, which causes the resistance of the air to be 
 much greater against the former than against the latter. 1 But 
 it is by the alteration in the surface of the wing, as it acts upon 
 the air, that the chief difference is made in Birds ; the arrange- 
 ment of their great feathers being such, that they strike the air 
 with their flat sides, but present only their edges in rising. 
 What is called " feathering the oar " in rowing, is a similar 
 operation, performed with the same intention, and deriving 
 its name from this resemblance. 
 
 672. The degree in which the wings act in raising the body 
 or in propelling it through the air, varies considerably in 
 different species, according to the way in which they are set. 
 Thus in Birds of Prey, which require a rapid horizontal 
 motion, the surface of the wings is very oblique, so that they 
 strike backwards as well as downwards, and thus impel the 
 body forwards whilst sustaining it in the air. Such birds find 
 a difficulty in rising perpendicularly ; and can in fact only do 
 so by flying against the wind, which then acts upon the 
 inclined surface of the wings just as it does upon that of a 
 kite. On the other hand, the Lark, Quail, and such other 
 birds as rise to great heights in a direction nearly vertical, 
 have the wings so disposed as to strike almost directly dowTi- 
 wards. It has been estimated that a Swallow, when simply 
 sustaining itself in the air, is obliged to use as much force to 
 prevent its fall, as would raise its own weight to a height of 
 about twenty-six feet in a second. Hence, we may form some 
 idea of the enormous expenditure of force which must take 
 place, when the body is not only supported, but raised and 
 propelled through the air. The Eider-duck is said to fly 
 90 miles in an hour, and the Hawk 150. The Swallow 
 and Swift pass nearly the whole of the long summer days 
 upon the wing, in search of food for themselves and their 
 
 1 This resistance varies as the square of the velocity of the stroke. 
 Hence, if the down-stroke be made three times as fast as the up-stroke, 
 the resistance it experiences will be nine times as great. But as this 
 only operates during one-third of the time, it is in effect equal to three 
 times that which operates against the up-stroke, and which would tend 
 to lower the Bird in the air. 
 
IMPOSSIBILITY OF HUMAN FLIGHT. 511 
 
 helpless offspring ; and the rapidity of their flight is such, 
 that they can scarcely traverse less than seven or eight hun- 
 dred miles in that time, although they go but a short distance 
 from home. The flight of Insects is even more remarkable 
 for its velocity in proportion to their size ; thus a Swallow, 
 which is one of the swiftest-flying of Birds, has been seen to 
 chase a Dragon-fly for some time without success ; the Insect 
 always keeping about six feet in advance of the Bird, and 
 turning to one side and the other so instantaneously, that the 
 Swallow, with all its powers of flight, and its tact in chasing 
 Insects, was unable to capture it. 
 
 673. If the preceding estimate of the power expended by a 
 Bird in sustaining itself in the air be correct, it may be easily 
 proved that it would be impossible for a Man to sustain him- 
 self in the air by means of his muscular strength alone, in 
 any manner that he is capable of applying it. It is calculated 
 that a man of ordinary strength can raise 13 J Ibs. to a height 
 of 3^ feet per second; and can continue this exertion for 
 eight hours in the day. He will then exert a force capable of 
 raising (13| X 60 X 60 X 8) 381,600 Ibs. to a height of 
 3J feet ; or one-eighth that amount, namely 47,700 Ibs., to the 
 height of twenty-six feet, which, as we have seen, is that to 
 which the Bird would raise itself in one second, by the force it 
 is obliged to exert in order to sustain itself in the air. Now if 
 we suppose it possible that a Man could by any means concen- 
 trate the whole muscular power required for such a day's 
 labour, into as short a period as the accomplishment of this 
 object requires, we might find the time during which it would 
 support him in the air, by simply dividing this amount by his 
 weight, which we may take to be 150 Ibs. The quotient is 
 318, which is the number of seconds, during which the ex- 
 penditure of a force that would raise 47,700 Ibs. to a height 
 of twenty-six feet, will keep his body supported in the air ; 
 and this is but little more than five minutes. There is no 
 possible means, however, by which a Man could thus concen- 
 trate the force of eight hours' labour, into the short interval in 
 which he would have to expend it while supporting himself 
 in the air. And we have elsewhere seen (MECHANICS, 285), 
 that by no combination of mechanical powers can force be 
 created; as these only enable force to be more advantageously 
 applied. Hence, the problem of human flight will never be 
 
512 USE OF PREHENSILE ORGANS IN LOCOMOTION. 
 
 solved, until some source of power shall be discovered, far 
 surpassing that which his muscular strength affords, and so 
 portable in its nature as not materially to add to his weight. 
 
 674. The only other organs of locomotion which we have 
 to c6nsider, are those of prehension. Of these, the principal 
 have been elsewhere noticed, with reference to their use in 
 laying hold of food and conveying it to the mouth ( 172), 
 and with regard to the differences between the hand of Man 
 and the claspers of the Quadrumana ( 643). The hand of 
 Man is seldom employed to assist in his locomotion, except 
 in swimming (where it serves the purpose of a fin), and in 
 climbing ; neither of which kinds of movement can be said 
 to be natural to him. But the claspers of the Quadrumana 
 
 (fig. 254) are most efficient instru- 
 ments of locomotion ; enabling 
 them not only to grasp the branches 
 of the trees which they climb to 
 despoil them of their fruit, but 
 also to catch hold of them at the 
 end of a long leap. This they do 
 with the most wonderful agility ; 
 as all who have seen Monkeys in 
 circumstances at all like those of 
 their natural habitations, must 
 have observed. The Gibbons, or 
 long-armed Apes of the East Indies, 
 are probably the most remarkable 
 in this respect. The Author has 
 seen the Unglcaputi leap round and 
 round a room of about fifteen feet 
 square, catching at each side by 
 some small support attached to the 
 wall ; and taking its next leap (if 
 such it could be called) by merely 
 
 Fig. 254.-^TiAEE. swinging itself from this, without 
 touching anything solid with, its 
 feet. There are many of the Monkey tribe, however, espe- 
 cially in the New World, whose hands are less efficient as 
 instruments of prehension ; and these are furnished with a 
 prehensile tail ; that is, a tail which can be coiled round the 
 branch of a tree, and by which the animal can suspend itself 
 
PRODUCTION OP SOUNDS BY ANIMALS. 
 
 513 
 
 (fig. 255). A similar tail is possessed by some of the Opos- 
 sum tribe ; and by the Chameleon among Reptiles. 
 
 Fig. 255. WHITE-THROATED SAJOU. 
 
 CHAPTER XIII. 
 
 OF THE PRODUCTION OF SOUNDS : VOICE AND SPEECH. 
 
 675. IT is not by their movements alone, that Animals are 
 enabled to influence one another. Were it so, their commu- 
 nication would be restricted to the small amount which can 
 be effected by signs and gestures. This, however, is necessa- 
 rily the case amongst aquatic animals in general ; since they 
 are prevented by the nature of the medium they breathe 
 from producing sounds through its means. Some of them 
 appear to have the power of communicating with each 
 other by the vibrations which they can excite in the water ; 
 of this we have already noticed an example among the 
 Whale tribe ( 491) ; and there is reason to believe that certain 
 Mollusks possess a similar means of communication. 
 
 L L 
 
514 PRODUCTION OF SOUNDS BY INSECTS. 
 
 676. Many Insects have the power of prodacing a conti- 
 nuous sound, which probably serves the purpose of intimating 
 to each other the neighbourhood of their own kind ; and 
 even, in some instances, of expressing their feelings : some 
 of these sounds are produced only during flight. Of this 
 kind is the sharp hum of the Gnat, Mosquito, Gad-fly, &c., 
 which, though often a source of extreme annoyance to man 
 and beast, serves to give warning of the proximity of these 
 blood-thirsty Insects, and is therefore of real service to the 
 animals they attack. From recent experiments, however, it 
 appears that in Bees and Flies, at least, the sound is not 
 produced so much by the vibrations of the wings (to which it 
 
 is commonly attributed), as 
 by those of a little mem- 
 branous plate, situated in 
 one of the spiracles or stig- 
 mata ( 321) of the thorax; 
 for if the apertures of these 
 be stopped, no sound is heard, 
 though the wings remain in 
 movement. But in Cock- 
 chafers, and other noisy 
 Beetles, Butterflies, &c., no 
 such apparatus can be dis- 
 Fig. 256.-BoMBYi.ius. covered. Other sounds are 
 
 produced while the insect is feeding ; that occasioned by the 
 armies of Locusts, when incalculable millions of powerful jaws 
 are in action at the same time, has been compared to the crack- 
 ling of a flame of fire driven by the wind. Certain two-winged 
 Flies, distinguished by a long proboscis (fig. 256), make a 
 humming sound whilst sucking honey from flowers ; and the 
 same is the case with some of the Hawk-moths. 
 
 677. Some Insects are remarkable for a peculiar mode of 
 calling, commanding, or giving an alarm. The neuters or 
 soldiers among the White Ants make a vibrating sound, 
 rather shriller and quicker than the ticking of a watch, by 
 striking against hard substances with their mandibles ; this 
 seems intended to keep the labourers, who answer it by a 
 hiss, upon the alert and at their work. The well-known 
 sound termed the "death-watch" is produced by a small beetle 
 termed Anobium (fig. 257), that burrows in old timber ; and 
 
PRODUCTION OF SOUNDS BY INSECTS. 
 
 515 
 
 it is occasioned by the striking of its mandibles upon the 
 wood. The sound is evidently intended by the animal as a 
 means of communication with its fellows ; for if it be an- 
 swered it is continually repeated, whilst if no answer be 
 returned the animal repeats the signal in another place. The 
 noise exactly resembles that pro- 
 duced by tapping moderately 
 with the nail upon the table ; 
 and, when familiarised, the insect 
 will very readily answer this imi- 
 tation. The most remarkable 
 example of the production of 
 sounds for the purpose of autho- 
 rity, is that of the Queen-Bee ; which has the power of 
 influencing the whole hive, especially about the time of 
 swarming, by the peculiar notes she produces. 
 
 678. Many Insects have the power of expressing their 
 passions, also, as fear, anger, sorrow, joy, or love, by the 
 sounds they can generate. The most curious of those given 
 out under the influence of alarm is that produced by the 
 Sphinx Atropos or Death's-head Hawk-moth (fig. 258); which 
 
 Fig. 257. ANOBIUM. 
 Natural size and magnified. 
 
 Fig. 258. SPHINX ATHOPOS. 
 
 when confined, or taken into the hand, sends forth a strong 
 and sharp cry, resembling, some say, that of a mouse, but 
 more plaintive and even lamentable. The means by wliich 
 this cry is produced, have not yet been certainly ascertained. 
 The influence of anger, sorrow, and joy, in modifying the tone 
 of the hum of Bees, is well known to those who have studied 
 their habits ; the first is particularly evident in the sharp 
 angry tone which is heard when the hive has been disturbed, 
 especially if some of the Bees have been killed ; the second 
 
 LL2 
 

 516 SOUNDS PRODUCED BY INSECTS. 
 
 is manifested in a low plaintive tone which is given-out when 
 the queen has been taken away ; and the cheerful humming 
 which is immediately heard when the sovereign is restored, is 
 an evident indication of the last. Of all the Insects inha- 
 biting this country, the most noisy are the Crickets; whose 
 
 Fig. 259. HOUSE-CRICKET. 
 
 sound, which seems to be their expression of love, is produced 
 by the rubbing of the elytra or wing-covers one against the 
 other. In several species it may be distinctly seen that a 
 very strong nervure on one of these has a jagged surface like 
 that of a file ; and that this works against a collection of 
 smaller nervures, which resemble so many strings. 
 
 679. The Cicada (fig. 260) was a very favourite insect 
 among the ancient Greeks ; and was frequently mentioned 
 by their poets with the most endearing epithets. Its song 
 was considered particularly musical ; and it was regarded as 
 the happiest as well as the most innocent of animals. The 
 Cicadae of other countries produce an extremely shrill and 
 disagreeable sound, which can be heard at a great distance. 
 In the warmer parts of the United States, there is a species 
 which, in the hotter months of summer, is a very troublesome 
 and impertinent neighbour. The Cicadae of Brazil are said 
 to be audible at the distance of a mile : this is as if a man of 
 ordinary stature, supposing his powers of voice increased in 
 the ratio of his size, could be heard all over the world. The 
 organs by which the sound is produced are placed on the 
 under side of the body, between the base of the hind legs 
 and the abdomen, and consist externally of a pair of large 
 flattened plates of a horny texture, varying in form in the 
 different species. When these are raised, they are found to 
 conceal a large cavity partially covered with a membrane of a 
 
SOUNDS OF INSECTS. VOICE OF VEKTEBRATA. 
 
 517 
 
 much, more delicate nature than the external covering, with a 
 horny plate in the middle, which lies along the bottom. 
 Still more internally are two bun- 
 dles of muscles, which are the real 
 agents in producing the sound ; 
 for, when they are pulled and sud- 
 denly let go, even in a dead speci- 
 men, the sound is produced as 
 well as though the insect were 
 alive. They draw-in and force-out, 
 by their alternate and rapid con- 
 traction, a horny drum or mem- 
 brane, stretched in such a manner 
 as to vibrate readily ; the sound 
 occasioned by the movements of 
 which passes out through an aper- 
 ture resembling the sound-holes 
 of a violin. The Fulgorce, also, 
 have considerable sound-producing 
 powers, but exert them in the 
 night, whilst the Cicadae perform 
 in the day. The Great Lantern- 
 fly of Guiana ( 400, fig. 175) begins regularly at sunset; 
 and its noise, resembling that of a razor-grinder at work, is 
 so loud, that the insect is called " scare-sleep" by the Dutch 
 colonists. 
 
 680. In all air-breathing Vertebrata, the production of 
 sound depends upon the passage of air through a certain, 
 portion of the respiratory tube, which is so constructed as to 
 set the air in vibration. In Reptiles and Mammals, it is at- 
 the point where the windpipe opens into the front of the 
 pharynx, that this vibrating apparatus is situated. Few of 
 the animals of the former class, however, can produce any 
 other sound than a hiss, occasioned by the passage of air 
 through the narrow chink by which the trachea communicates 
 with the pharynx ; but this sound, owing to the great capa- 
 city of their lungs ( 325), is often very much prolonged. 
 Among Mammals, on the other hand, there are few, if any, 
 which have not some vocal sound ; but the variety and 
 expressiveness which can be given to it differ considerably in 
 the several tribes of this class, being by far the greatest in 
 
 Fig. 260. CICADA. 
 
518 
 
 STRUCTURE OF THE LARYNX. 
 
 Man. This sound is produced by the apparatus termed the 
 larynx, which is situated beneath the base of the tongue, and 
 in front of the pharynx ( 192, fig. 107). It is suspended, 
 as it were, from the hyoid bone (h, fig. 261), a bone of a 
 horse-shoe form, detached from the rest of the skeleton; 
 from two projections (I) on the upper side of which, several 
 of the muscles of the tongue originate. The sides of the 
 larynx are formed by two large cartilages (t, fig. 261), which 
 
 VERTICAL SECTION op 
 THE LARYNX. 
 
 ar, arytenoid cartilages; 
 v, ventricle of tlie glot- 
 tis ; e, ep : glottis; the 
 other references as be- 
 fore. 
 
 Fig. 263. 
 
 FRONT VIEW OF THE 
 LARYNX. 
 
 The interior wall is mark- 
 ed by the lines a, a,b,b; 
 li, inferior ligaments 
 of the glottis, or vocal 
 cords ; Is, superior liga- 
 ments; the other re- 
 ferences as before. 
 
 tr 
 
 Fig. 261. 
 
 HUMANLARYNX,VIEWED 
 SIDEWAYS. 
 
 h, hyoid bone ; I, point of 
 origin of muscles of the 
 tongue ; t, thyroid car- 
 tilage ; a, projection in 
 front, commonly called 
 Adam's apple ; c,cricoid 
 cartilage ; tr, trachea ; 
 o, posterior side of the 
 larnyx, in contact with 
 the oesophagus. 
 
 are termed the thyroid cartilages ; where these meet on the 
 middle line a projection is formed, which is particularly 
 prominent in Man, and has received the name of Pomum 
 Adami, or Adam's apple (a). The thyroid cartilages rest 
 upon another, termed the cricoid (c); this has the form of a 
 ring, much deeper behind than in front, and surmounts the 
 trachea, with the upper ring of which its lower edge is con- 
 nected by a membrane. Upon the upper surface of the back 
 of the cricoid cartilage, where there is an open space left 
 between the two thyroid cartilages, are mounted two small 
 cartilaginous bodies, the arytenoid (ar, fig. 262). These are 
 movable to a certain extent ; and their position may be 
 changed in various directions by several muscles which act 
 upon them. 
 
STRUCTUEE OF THE LARYNX. 519 
 
 681. To these arytenoid cartilages are attached two ligaments 
 of elastic fibrous substance ( 23), which pass forwards to be 
 attached to the front of the thyroid cartilage, where they meet 
 in the same point. These are the instruments concerned in the 
 production of sound, and also in the regulation of the aperture 
 by which air passes into the trachea; and they are termed 
 the vocal cords or ligaments (fig. 263, li). By the meeting of 
 these ligaments in front and their separation behind, the usual 
 aperture has the form of a V ; but it may be narrowed by the 
 drawing-together of the arytenoid cartilages, until the two 
 vocal ligaments touch each other along their whole length, 
 and the aperture is completely closed. In this manner, the 
 amount of air permitted to pass through the larynx is regu- 
 lated ; and a protection is afforded against the entrance of 
 solid substances. An additional guard is afforded by the 
 doubling of the lining membrane, in such a manner as to form 
 a second pair of folds (I s, fig. 263), above the preceding ; and 
 over the space between these (which is much wider than that 
 between the vocal cords) there is a valve-like flap, the epi- 
 glottis (e, fig. 262), which is pushed-down upon it in the act 
 of swallowing, so as to prevent the entrance of solid or fluid 
 particles into the space beneath, which is called the glottis. 
 From the causes formerly mentioned ( 193), such particles 
 are occasionally drawn into the glottis ; and they excite, by a 
 reflex action, an involuntary and extremely violent cough, 
 which tends to expel them again. Sometimes, however, solid 
 bodies of no inconsiderable size find a lodgment in the wide 
 spaces (v, fig. 263) between the upper and lower pair of liga- 
 ments, which are termed the ventricles of the larynx ; and 
 occasionally they pass through the opening between the vocal 
 cords, which is termed the rima glottidis or fissure of the 
 glottis, into the wind-pipe. 
 
 682. In the ordinary acts of inspiration and expiration, the 
 arytenoid cartilages are wide apart, so that the aperture is as 
 large as possible ; but for the production of vocal sounds, it 
 is necessary that the aperture should be narrowed, and that 
 the flat sides, rather than the edges, of the vocal ligaments 
 should be opposed to one another. This is accomplished by 
 a peculiar movement of the arytenoid cartilages, occasioned 
 by the contraction of certain muscles. When these ligaments 
 are thus brought into position, the air in passing through the 
 
520 ACTIONS OF THE LARNYX : VOICE. 
 
 larynx sets them in vibration, in a manner very much resem- 
 bling that in which the reed of a Hautboy or Clarionet, or the 
 tongue of an Accordion or Harmoninm, is set in vibration by 
 the current of air that is made to pass beneath them. The 
 rapidity of the vibrations, and consequently the pitch of the 
 sound ( 523), depends on the degree of tension or tightness 
 of the vocal ligaments ; and this is regulated by muscles which 
 act upon the thyroid and arytenoid cartilages. If the thyroid 
 cartilage be drawn forwards, and the arytenoid cartilages back- 
 wards, the two ends of the vocal cords will be further sepa- 
 rated from each other, and they will consequently be tightened ; 
 by the contrary movements they will be relaxed. 
 
 683. It is on account of the greater length of the vocal 
 cords, that the pitch of the voice is much lower in Man than 
 in Woman ; but this difference does not arise until the end 
 of the period of childhood, the size of the larynx being about 
 the same in the Boy and Girl up to the age of 14 or 15 years, 
 but then undergoing a rapid increase in the former, whilst it 
 remains nearly stationary in the latter. Hence it is that Boys, 
 as well as Girls and Women, sing treble ; whilst Men sing 
 tenor which is about an octave lower than the treble, or bass 
 which is lower still. 
 
 684. The cause of the variations of timbre or quality in 
 different voices, is not certainly known ; but it appears to be 
 due, in part, to differences in the degree of flexibility and 
 smoothness in the cartilages of the larynx. In women and 
 children these cartilages are usually soft and flexible, and 
 their voices clear and smooth ; whilst in men, and in women 
 whose voices have a masculine roughness, the cartilages are 
 harder, and are sometimes almost completely ossified. The loud- 
 ness of the voice depends in part upon the force with which the 
 air is expelled from the lungs ; but the variations in strength 
 of voice which exist among different individuals, are in some 
 measure due to the degree in which its resonance is increased 
 by the vibration of the other parts of the larynx and of the 
 neighbouring cavities. In the Howling Monkeys of America, 
 there are several pouches which open from the larynx, and are 
 destined to increase the volume of tone that issues from it ; 
 one of these i excavated in the substance of the hyoid bone 
 itself, which is very greatly enlarged ; and to this bony drum 
 seems due the mournful plaintiveness which characterises the 
 
LARYNX OF BIRDS. 
 
 521 
 
 tone of these animals. Although the largest of the American 
 Monkeys, these Howlers are of inconsiderable size ; yet their 
 voices are louder than the roaring of lions, being distinctly 
 audible at the distance of two miles ; and, when a number are 
 congregated together, the effect is terrific. 
 
 685. In Birds, the situation of the vocal organ is very 
 different. The trachea opens into the pharynx, as in Reptiles, 
 by a mere slit ; the borders of which have no other movement 
 than that of approaching one another, so as to close the aper- 
 ture when necessary. But at the lower extremity of the 
 trachea, just where it subdivides into the bronchial tubes, 
 there is a sort of larynx or vocal organ, which is of very 
 complex construction, especially in the singing-birds. The 
 external surface of this larynx is represented in fig. 264; its 
 muscles, m m', being left i 
 
 in their places on one side, 
 
 and removed on the other. , 
 
 At 1 1, is seen the trachea ; 
 
 at the lower extremity of 
 
 which, ^, is a sort of bony * 
 
 drum, I, divided at its , 
 
 lower part by a partition of 
 
 the same material (o, fig. b 
 
 265), which is surmounted l 
 
 by a semilunar membrane 
 
 (c). This drum communi- c 
 
 cates below with the two Fig. ZBI.-LAHYHX OF 
 
 bronchial tubes, b b' (fig. 
 
 264), each of which has its own glottis and vocal cords; 
 
 the inner lip of one of these is seen at a (fig. 265) ; and at me 
 
 is shown a drum-like membrane, forming the inner wall of 
 
 the bronchial tube, which probably increases the resonance 
 
 of the voice. These parts are acted-on by several muscles, 
 
 the number of which varies according to the compass and 
 
 flexibility of the voice in the different species ; being very 
 
 considerable in the most esteemed of the singing-birds, and 
 
 being reduced to a small amount in those which have no vocal 
 
 powers. Iii some, indeed, they are altogether absent; and 
 
 the state of the glottis can be influenced only by those muscles 
 
 which raise and lower the whole trachea. 
 
 686. The vocal sounds produced by the action of the larynx 
 
 Fig. 265. VERTICAI 
 SECTION OF SAME. 
 
522 DIFFERENT KINDS OF VOICE. 
 
 are of very different characters ; and may be distinguished 
 into the cry, the song, and the ordinary or acquired voice. 
 The cry is generally a sharp sound, having little modulation 
 or accuracy of pitch, and being usually disagreeable in its 
 timbre or quality. It is that by which animals express their 
 unpleasing emotions, especially pain or terror ; and the Hu- 
 man infant, like many of the lower animals, can utter no other 
 sound. In song, by the regulation of the vocal cords, definite 
 and sustained musical tones are produced, which can be 
 changed or modulated at the will of the individual. Different 
 species of Birds have their respective songs ; which are partly 
 instinctive, depending upon the construction of their larynx ; 
 and are partly governed by their education. In Man, the 
 power of song is entirely acquired ; but, when once acquired, 
 it is far more susceptible of variety and expression than that 
 of any other animal. In fact, the larynx of Man may be 
 said to be the most perfect musical instrument ever con- 
 structed. The voice is a sound more resembling the cry, in 
 so far as it does not consist of sustained musical tones ; but 
 it differs from the cry, both in the quality of its tone, and in 
 the modulation of which it is capable by the will. In ordi- 
 nary conversation, the voice passes through a great variety of 
 musical tones, in the course of a single sentence, or even a 
 single word, sliding imperceptibly from one to another; 
 and it is when we attempt to fix it definitely to a certain 
 pitch, that we change it from the speaking to the singing 
 tone. 
 
 687. It is to the wonderful power that Man possesses, of 
 producing articulate sounds, which form a medium whereby 
 he can communicate ideas of any kind to his fellows, that 
 much of his superiority to other animals is due. Neverthe- 
 less, it is not to this alone that we must attribute it ; for many 
 animals, especially Birds, can produce, by imitation, sounds 
 as articulate as those of Man ; but the mind which originates 
 them, and which uses them as expressions of its ideas and 
 desires, is deficient. 
 
 688. All spoken language is made up of a certain number 
 of elementary sounds, which are combined into syllables, words, 
 and sentences. It may be easily shown, upon arithmetical 
 principles, that from 20 or more of these elementary sounds, 
 an almost infinite variety of combinations may be produced ; 
 
ELEMENTARY ARTICULATE SOUNDS. 523 
 
 and from such an inexhaustible store there is no difficulty in 
 deriving new combinations, to represent any new ideas that 
 we may desire to express. These simple or elementary sounds 
 ought to be represented by an equal number of single letters ; 
 this is the case, however, in but few languages. Our own is 
 particularly faulty in this respect ; for there are many simple 
 sounds that can be only represented by a combination of 
 letters, whilst others may be represented by more than one 
 single letter, and in some instances a single letter represents 
 a composite sound. Thus the sounds of au and th are really 
 simple ones, and ought to be represented by single letters. 
 Again, the sound of k is represented also by the hard c, as in 
 the first syllable of concert ; and the sound of s by the soft c, 
 as in the second syllable of the same word, where the c is 
 sounded exactly as the s in consent. And the letter i (as 
 usually pronounced in English) does not represent a simple 
 sound, but a combination of two, as will be presently shown. 
 Most of the Continental languages are superior to the English 
 in this respect. 
 
 689. Vocal sounds are divided into Vowels and Consonants ; 
 the true distinction between which appears to be, that the 
 Vowel sounds are continuous tones, modified by the form of 
 the aperture through which they pass out ; whilst in giving 
 utterance to Consonants, there is a partial or complete inter- 
 ruption to the breath in its passage through the organs in 
 front of the larynx. Hence all true Vowels may be prolonged 
 for any length of time that the breath is supplied from the 
 lungs ; whilst the sound of many Consonants is momentary 
 only. It is easy for any one to convince himself that the 
 Vowel sounds are governed simply by the form of the cavity 
 of the mouth, and by that of the aperture of the lips ; by 
 passing, in one continued tone, from one of the following 
 Vowel sounds to another : 
 
 English a 
 English a 
 English a 
 English e 
 English 
 English oo 
 
 
 as in ah 
 as in all 
 as in name 
 as in theme 
 as in cold 
 as in cool 
 
 . Continental a 
 Diphthong au 
 . Continental e 
 . Continental i 
 . Continental o 
 . Continental u 
 
 The short Vowel sounds, as a in fat, e in met, o in pot, &c., 
 are not capable of being prolonged ; as they are formed in 
 
524 VOWELS AND CONSONANTS STAMMERING. 
 
 the act of preparation for sounding the succeeding consonant. 
 The sound of the English i is a compound one, being formed 
 in the act of transition from that of a in ah, to that of e as in 
 theme ; hence it cannot be prolonged ; and it is the very worst 
 vowel sound upon which to sing a long note, since it is impos- 
 sible to prevent its being heard as one of the sounds of which 
 it is composed. Much discussion has taken place withr efer- 
 ence to the true characters of the letters w and y, when 
 employed to commence words, as wall, yawl, wet, yet. A little 
 attention to the state of the mouth in pronouncing them will 
 show, however, that they are then really vowel sounds, rapidly 
 transformed into the succeeding ones ; for the sound of w in 
 this situation is oo ; and that of y is the long e ; so that wall 
 might be spelt ooall, and yawl eaul. 
 
 690. Consonants are naturally divided into those which 
 require a total stoppage of the breath at the moment previous 
 to their being pronounced, and which cannot therefore be 
 prolonged ; and those in pronouncing which the interruption 
 is partial, and which can be prolonged like the vowels. The 
 former are termed explosive consonants ; the latter continuous. 
 The explosive consonants are 6 and p, d and t, the hard g and 
 k. All the others are continuous ; but the sound is modified 
 by the position of the tongue, palate, lips, and teeth ; and also 
 by the degree in which the air is permitted to pass through 
 the nose. 
 
 691. The study of the mode in which the different conso- 
 nants are produced, is of particular importance to those who 
 labour under defective speech, especially that difficulty which 
 is known as stammering. This very annoying impediment is 
 occasioned by a want of proper control over the muscles con- 
 cerned in articulation ; which are sometimes affected with a 
 kind of spasmodic action. It is in the pronunciation of the 
 consonants of the explosive class, that the stammerer usually 
 experiences the greatest difficulty ; for the total interruption 
 to the breath which they occasion is frequently continued 
 involuntarily, so that either the expiration is entirely checked, 
 or the sound comes out in jerks. Sometimes, on the other 
 hand, in pronouncing vowels and continuous consonants, the 
 stammerer prolongs his expiration, without being able to 
 check it. The best method of curing this defect (where there 
 is no malformation of the organs of speech, but merely a want 
 
REFLEX AND INSTINCTIVE ACTIONS. 525 
 
 of power to use them aright), is to study the particular defect 
 under which the individual suffers ; and then to make him 
 practise systematically the various movements concerned in 
 the production of the sounds in question, at first separately, 
 and afterwards in combination, until he feels that his volun- 
 tary control over them is complete. 
 
 CHAPTER XIV. 
 
 OF DISTINCT AND INTELLIGENCE. 
 
 692. IT will be remembered that, when the Nervous System 
 was described (Chap. XL), it was shown to be the instrument 
 of three classes of operations, each of which seems to be per- 
 formed by a distinct portion of the apparatus. I. The first of 
 these is the class of simply Reflex actions, which are executed 
 only in respondence or answer to impressions made upon the 
 nerves proceeding to a ganglionic centre ; as when a Dytiscus, 
 whose head has been cut off, executes swimming movements 
 immediately that its feet come in contact with water. These 
 movements evidently take place without any choice or direc- 
 tion on the part of the animal, which, in , executing them, 
 seems like a mere machine adapted to perform certain actions 
 when certain springs are touched ; and it has been shown 
 that they may be called-forth even without its consciousness. 
 Of these reflex movements, the Spinal Cord of Vertebrata, and 
 in Invertebrata the ganglia corresponding to it (in regard to 
 their connexions with the organs of locomotion, respiration, 
 &c.), are the instruments. II. The second class comprehends 
 those Instinctive actions, which differ from the preceding in 
 being dependent on the sensations received by the animal, 
 and in being, therefore, never performed without its conscious- 
 ness. Nevertheless, the animal in executing them is not 
 guided by any perception of the object to be attained, or by 
 any choice of the means by which it is to be accomplished ; 
 but acts blindly and involuntarily, in accordance with an 
 irresistible impulse, implanted in it by its Creator for the 
 purpose of causing it to do that, without or even against its 
 Will, which it would not have chosen or devised by its very 
 imperfect intelligence. The actions of this class are most 
 
526 RELATION OF INSTINCTIVE TO INTELLIGENTIAL ACTIONS. 
 
 wonderful in the Invertebrata, which possess the least Intel- 
 ligence ; and, on the contrary, they are fewest and least 
 remarkable in Man, whose Intelligence is highest. From 
 the constant proportion they bear to the size of the ganglia 
 of sensation, which form nearly the whole nervous mass in 
 the head of Insects, &c., and a large part of that of the lower 
 Vertebrata, but which are comparatively small in the Mam- 
 malia and especially so in Man, there seems good reason to 
 regard these organs as their chief instruments. III. The 
 third and highest class of actions, is that in which Intelligence 
 is the guide, and the Will the immediate agent. The animal 
 receives sensations, forms a notion of their cause, reasons 
 upon the ideas thus excited, perceives the end to be attained, 
 chooses or devises the means of accomplishing it, and volun- 
 tarily puts those means into execution. These actions are 
 seen, in their highest and most complete form, in Man ; but 
 they are not confined to him ; for, as will be shown hereafter, 
 true reasoning processes are performed by many of the lower 
 animals. There can be no doubt that the Cerebral Hemi- 
 spheres, which form the Brain properly so called, constitute 
 the instrument by which these actions are executed; for 
 we find that their size and development bear a very regular 
 proportion to the degree of Intelligence which the animal 
 possesses. 
 
 693. It follows, then, that the lower we descend in the 
 scale of Animal life, the larger is the proportion of the move- 
 ments of any particular species which we are to attribute to 
 the Reflex and the Instinctive classes ; whilst the proportion 
 which is due to Intelligence and Will diminishes in a like 
 degree. Thus we have seen that the ordinary movements of 
 locomotion, which Man performs in the first instance by volun- 
 tary effort, are reflex in Insects ( 445) and there can be no 
 reasonable doubt that the movements of the tentacula of the 
 Hydra, by which it entraps its prey and draws it to the entrance 
 of its stomach ( 121), are of a reflex, rather than a voluntary 
 or instinctive character, since they are obviously analogous to 
 those movements of the pharyngeal muscles, by which the food 
 is grasped and carried into the alimentary tube of the highest 
 animals ( 195). There is one curious fact, which would 
 seem to indicate a difference between them, but which is 
 really a strong argument in favour of their analogy. It is 
 
CHARACTERISTICS OF REFLEX AND INSTINCTIVE ACTIONS. 527 
 
 continually observed that when the stomach of the Polype is 
 full, its arms do not make any attempt to seize objects that 
 touch them ; so that small worms, insects, &c., which would 
 at other times be entrapped, may now come near them with 
 impunity. It has been supposed that this results from an act 
 of choice on the part of the animal, and that its choice is 
 influenced by its consciousness that its stomach is supplied 
 with food. It must seem improbable that an Animal which 
 so nearly resembles Plants in its general habits, and in which 
 the nervous system is so obscure that it has not yet been dis- 
 covered, should possess mental endowments of so high a 
 character ; and we may find, in studying our own functions, 
 a circumstance exactly parallel to that just mentioned. For 
 when we commence eating, with a good appetite, we may 
 notice that the muscles of Deglutition act very readily ; but 
 when we are completely satisfied, it is often difficult to excite 
 these muscles to contraction, so as to swallow another morsel, 
 even though, for the gratification of our palate, we may desire 
 to do so. Thus we see how much better a guide we find in 
 Nature, for the amount of food we require, than in our own 
 pampered tastes. 
 
 694. The first class, that of Reflex movements, has been 
 already considered in sufficient detail ; but it is intended, in 
 the present chapter, to offer some examples of those of the 
 second and third classes, those actions, namely, which are 
 guided by Instinct and Intelligence respectively. These actions 
 may be usually distinguished by the two following tests : 
 1. Although, in most 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 dif- 
 ferent actions which result from an Instinctive impulse may 
 follow one another with unerring precision. 2. Instinctive 
 actions are performed by the different individuals of the same 
 species, nearly, if not exactly, in the same manner ; present- 
 ing no such variation of the means applied to the objects in 
 view, and admitting of no such improvements 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, which are for the most part adapted to the attainment 
 of particular ends, by voluntary efforts guided and directed 
 by reason. Where, as in the examples hereafter to be men- 
 
528 DISTINCTIVE CHARACTERS OF INTELLIGENTIAL ACTIONS. 
 
 tioned ( 717), we find individual animals "learning wisdom 
 by experience," and acquiring the power of performing actions 
 which do not correspond with their natural instincts, we 
 cannot do otherwise than regard them as possessed of a certain 
 degree of Intelligence, by which they are rendered susceptible 
 of education. 
 
 695. The amount of Intelligence displayed in such acquire- 
 ments, can only be judged-of, however, by carefully examining 
 the circumstances under which they are made. If the new 
 habits are gained like the talking of a Parrot by imitation 
 simply, no great degree of intelligence is manifested ; but if 
 it spontaneously result from a reasoning process on the part 
 of the animal, our idea of its sagacity is raised. There may 
 be a combination of both these conditions ; as in the following 
 curious circumstance, related to the Author by a friend who 
 has repeatedly witnessed it. Some horses kept in a paddock 
 were supplied with water by a trough, which was occasionally 
 filled from a pump, not, however, as often as the horses 
 seem to have wished ; for one of them learned, of his own 
 accord, to supply himself and his companions, by taking the 
 pump-handle between his teeth, and working it with his 
 head. The others, however, appear to have been less clever, 
 or more lazy ; and finding that this one had the power of sup- 
 plying their wants, they would teaze him, by biting, kicking, 
 &c., until he had pumped for them, and would not allow him 
 to drink until they were satisfied. That this was not a mere 
 act of imitation, appears from the circumstance that the horse 
 did not attempt to imitate the movement of the man, but 
 performed the same action in a different manner, evidently 
 because it had associated in its mind the motion of the pump- 
 handle with the supply of water. 
 
 696. The Instincts of Animals may be shown to have 
 immediate reference, probably in every instance, to the supply 
 of the wants of the individual, or to the continuance of the 
 race. Thus we have Instincts which guide in the selection 
 and acquirement of food ; others which govern the construc- 
 tion of a habitation for the individual, and of a receptacle for 
 the eggs, and these may influence a number at once, in such 
 a manner as to unite them into a society ; and others which 
 direct their migrations, whether in search of food, for the 
 deposit of their eggs, or for other purposes. Of these, some 
 examples will now be given. 
 
INSTINCT OF THE ANT-LION. 
 
 529 
 
 697. Among the instincts which direct animals in the 
 acquirement of their food, few are more remarkable than those 
 possessed by the larva of the Ant-lion (fig. 266), a small insect 
 allied to the Dragon-fly. This larva (fig. 267) is destined to feed 
 
 Fig. 206. ANT LION IN PERFECT STATE. 
 
 upon ants and other small insects, whose juices it sucks ; but 
 it moves slowly and with difficulty, so that it could scarcely 
 have obtained the requisite supply of food, if Nature had not 
 guided it in the construction of a remarkable snare, which 
 entraps the prey it could not acquire by pursuit. It digs in 
 fine sand a little funnel-shaped pit (fig. 268), and conceals 
 
 Fig. 267. LARVA or 
 THE ANT-LION. 
 
 Fig. 268. PITFALL OF THE ANT-LI-. x. 
 
 itself at the bottom of this, until an insect falls over its edge ; 
 and if its victim seeks to escape, or stops in its fall to the 
 bottom, it throws over it, by means of its head and mandibles, 
 a quantity of sand, by which the insect is caused to roll down 
 the steep, within reach of its captor. The manner in which 
 
 M M 
 
530 
 
 PITFALL OF ANT-LION : WEB OF SPIDER. 
 
 the Ant-lion digs this pit is extremely curious. After having 
 examined the spot where it purposes to establish itself, it 
 traces a circle of the dimensions of the mouth of its pit ; then, 
 placing itself within this line, and making use of one of its 
 legs as a spade, it digs out a quantity of sand, which it heaps 
 upon its head, and then, by a sudden jerk, throws this some 
 inches beyond its circle. In this manner it digs a trench, 
 which serves as the border of its intended excavation, moving 
 backwards along the circle, until it comes to the same point 
 again ; it then changes sides, and moves in the contrary 
 direction, and so continues until its work is completed. If, 
 in the course of its labours, it meets with a little stone, the 
 presence of which would injure the perfection of its snare, it 
 neglects it at first, but returns to it after finishing the rest of 
 its work, and uses all its efforts to get this upon its back, and 
 carry it out of the excavation ; but if it cannot succeed in 
 doing so, it abandons its work and commences anew else- 
 where. When the inclination of the walls of the pit has 
 been altered by any slip, as almost always happens when 
 an insect has fallen-in, the Ant-lion hastens to repair the 
 damage. 
 
 698. Snares of a still more singular character are con- 
 structed by many Spiders, which spin webs of the finest silk, 
 for the purpose of entrapping their prey. The arrangement 
 
 Fig. 269. ETEIRA DIATEMA. 
 
 of these toils varies according to the species, and sometimes 
 does ijot present any regularity ; but in several instances it is 
 
BURROWS OF HAMSTER AND MYGALE. 531 
 
 of extreme elegance ; and no one can watch the labours of a 
 common garden spider, as, for instance, the Epeira diadema 
 (fig. 269), without being struck with the marvellous sagacity 
 which it displays in the execution of its work, and the per- 
 fection with which its web is constructed. 
 
 699. An equally curious instinct is often displayed in the 
 construction of the habitations which the animal designs for 
 its abode. Thus the 
 
 Hamster (fig. 270), a 
 small rodent animal 
 allied to the Eat, which 
 is met with in most 
 cultivated districts on 
 the Continent from 
 Alsace to Siberia, and 
 which is very injurious 
 to agriculture, con- Fig. 27o.-iiAMST E n. 
 
 structs a burro win the 
 
 soil which has always two openings, one in an oblique direc- 
 tion, which serves the animal for casting out the earth it has 
 dug away, the other perpendicular, which is the passage by 
 which it enters and makes its exit. These galleries lead to 
 a regular series of circular excavations, which communicate 
 with each other by horizontal passages ; one of these cavities, 
 furnished with a bed of dried herbage, is the abode of the 
 Hamster; while the others serve as magazines for the pro- 
 visions which it collects in large quantities. 
 
 700. There are certain Spiders known to Zoologists under 
 the name of Mygale, which perform operations analogous to 
 those of the Hamster, but still more complicated ; for not 
 only do they excavate in the ground a large and commodious 
 habitation, but they line it with a silken tapestry, and 
 furnish it with a door regularly hung upon a hinge (fig. 271). 
 For this purpose, the Mygale digs, in a clayey soil, >a sort of 
 cylindrical pit, about 3 or 4 inches in length ; and plasters 
 
 -its walls with a kind of very consistent mortar. It then 
 constructs, of alternate layers of earth, and of threads woven 
 into a web, a trap-door exactly adapted to the orifice of its 
 hole, and only capable of opening outwards ; and it attaches 
 this by a hinge of the same thread to the tapestried lining of 
 its chamber. The outside of this trap-door is covered with 
 
532 
 
 HABITATIONS OF SPIDERS AND INSECTS. 
 
 materials resembling the soil around ; and so little does it 
 differ from this, as to be with difficulty distinguished, even 
 
 by a person seeking to discover 
 the Spider's habitation. If an 
 attempt is made to lift it, when 
 the animal is within its excava- 
 tion, the movement is resisted 
 >S?& by the whole force of the Spider, 
 ich holds down the door, by 
 
 ^^^^liirP^^^'Sc*^ fixing its claws into small holes 
 on its under surface at the 
 point most distant from the hinge, 
 r-*p*r where its force may be most ad- 
 *s2a^. vantageously applied. 
 
 Fig. 271. NEST OF MYGALE. 701 ' Am n S IllSects > We find 
 
 a great number of very curious 
 
 processes instinctively performed in the construction of 
 their habitations. Many Caterpillars form for themselves a 
 protection, by rolling together portions of leaves, and attach- 
 ing these by threads. In almost every garden, we may 
 observe (at the proper season) nests of this kind, on the 
 leaves of the Lilac or Gooseberry ; and a similar one, repre- 
 sented in fig. 272, is constructed in the leaves of the oak, by 
 the caterpillar of a small nocturnal Butterfly, the Tortrix viri- 
 dissima. The Larva of the little Clothes-moth, again, forms a 
 sort of tubular sheath, composed of the filaments it detaches 
 
 from the stuff through which 
 it excavates its galleries ; this 
 sheath it is continually prolong- 
 ing at one extremity ; and 
 when, in consequence of the 
 growth of the larva, its tube 
 becomes too small for its com- 
 fortable residence, it slits it 
 down and lets-in a piece. The 
 aquatic Larvae of the Caddice- 
 flies (fig. 273, c), which are 
 commonly known as Caddice- 
 worms, house themselves in straws, pieces of hollow stick, 
 rushes, &c. ; and those of some species glue together a 
 number of minute stones, pieces of stick, small shells, &c., 
 
 Fig. 272. NiiST OF TORTRIX. 
 
HABITATION OP CADDICE-WORM. 
 
 >33 
 
 so as to make a tube (A), in which the animal creeps along 
 the bottom and sides of the brook it inhabits, and sometimes 
 rows itself on the surface 
 of the water. When full- 
 grown, the larva attaches 
 its case by threads to some 
 large stone ; and then 
 covers its mouth with an 
 open net-work of threads 
 (B), sufficiently close to 
 prevent the entrance of 
 insects, but with meshes 
 permitting the water to 
 pass through. In this way 
 
 it Undergoes its metamor- Fig 2 73.-C, PHRYGANF.A OR CADDICE-FLY 
 phosis into the Pupa State ; A, tube formed by its larva ; B, network at 
 
 and a short time before its the entrance of the tube - 
 
 last change it cuts the threads of the network, by means 
 
 of two hooks with which its head is furnished, and creeps 
 
 out of the water ; soon after which it changes into the perfect 
 
 insect. 
 
 702. It is scarcely possible to point to any actions better 
 fitted to give an idea of the nature of Instinct, than those 
 which are performed by various Insects when they deposit 
 their eggs. These animals never behold their progeny, and 
 cannot acquire any notion from experience, therefore, of that 
 which their eggs will produce ; nevertheless they have the 
 remarkable habit of placing, in the neighbourhood of each of 
 these bodies, a supply of aliment fitted for the nourishment 
 of the larva that is to proceed from it ; and this they do, 
 even when they are themselves living on food of an entirely 
 different nature, such as would not be adapted for the larva. 
 They cannot be guided in such actions by anything like 
 reason, since the data on which alone they could reason 
 correctly are wanting to them ; so that they would be led to 
 conclusions altogether erroneous, if they were not prompted 
 by an unerring instinct, to adopt the means best adapted for 
 the attainment of the required end. 
 
 703. Of this kind of instinct, the Necrophorus (fig. 274), a 
 kind of Beetle not uncommon in our fields, offers a good 
 example. When the female is about to lay her eggs, she 
 
534 
 
 PREPARATION OF FOOD FOR INSECT-LARVAE. 
 
 seeks for the dead body of a mole, shrew, or such other 
 quadruped ; and having found one, she excavates beneath it 
 a hole of sufficient dimensions to contain the body, which she 
 gradually drags into it; she then de- 
 posits her eggs in the carcase, so that 
 the larvse, when they come forth, find 
 themselves in the midst of a supply of 
 carrion, on which they feed like their 
 parents. This instinct is still more 
 remarkable, when an Insect whose diet 
 is exclusively vegetable prepares for its 
 larva a supply of animal food. Such is 
 the case with the Pompilus, an Insect 
 allied to the wasp. In its perfect state 
 it lives entirely on the juices of flowers ; but the larvae are 
 carnivorous ; and the mother provides for them the requisite 
 supply of the food they require, by placing in the nest, by 
 the side of the eggs, the body of a spider or caterpillar which 
 she had previously killed by means of her sting. The Xylocopa, 
 
 Fig. 274. NECROPHORUS. 
 
 Fig. 275. XYLOCOPA. 
 
 Fig. 276. NEST OP XYLOCOPA. 
 
 or Carpenter-bee (fig. 275), has very analogous habits ; the 
 female makes long burrows in wood, palings, &c., in which 
 she excavates a series of cells (fig. 276) ; and in every one of 
 these she deposits an egg, with a supply of pollen-paste. 
 
 704. The instinct of support and protection to the young 
 and helpless offspring, is seen in all animals in which it is 
 needed; and it is particularly observable in Birds. The nests 
 
NESTS OF BIRDS. 535 
 
 which they construct are destined much more for the recep- 
 tion of their eggs, and for the protection of the young, than 
 for their own residence ; for there are few Birds which pass 
 much time in their nests, except at night, and during the 
 period of incubation. It is impossible to watch the process 
 of their construction, without admiring the perseverance with 
 which the materials are brought together that are destined for 
 their erection, and the art with which these are arranged. The 
 form and structure of the habitations are always nearly the 
 same among the individuals of the same species ; but there 
 is necessarily a certain latitude in regard to the materials of 
 which they are composed, since the same could not be every- 
 where procured. The nests of different species vary greatly, 
 however, both as to form, structure, and materials; and these 
 are admirably adapted to the particular circumstances in which 
 the young families are respectively destined to live. Some- 
 times these habitations are constructed of earth, the particles 
 of which are united by the viscid saliva of the Bird into a 
 tenacious mortar ; and they are then commonly built against 
 the sides of a rock or wall. But, in general, they are corn- 
 
 Fig. 277. NEST OF GOLDFINCH. 
 
 posed of sticks, straws, and other vegetable substances ; and 
 are placed either on the ground, or among the branches of 
 
536 
 
 NESTS OF BAYA AND TAILOR-BIRD. 
 
 trees. The greater number of them have a somewhat hemi- 
 spherical form, resembling a little round basket; and their 
 interior is lined with moss and down (fig. 277). 
 
 705. But sometimes the arrangement is much more com- 
 plicated, so as to avert some particular danger, or to answer 
 some special purpose. Thus the nest of the Baya, a little 
 Indian bird allied to our Bulfinch, has the form of a bottle ; 
 and it is suspended from a twig of such slenderness and flexi- 
 bility, that neither monkeys, serpents, nor squirrels can reach 
 it (fig. 278). It is rendered still more secure against the 
 attacks of its numerous enemies, by the formation of the 
 entrance of the nest on its under side, so as to be only reached 
 by even the bird itself with the aid of its wings. This curious 
 habitation is constructed of long grass ; and several chambers 
 are found in its interior, of which one serves for the female 
 
 Fig. 278. NEST t.F THE BAYA. Fig. 279. NEST OF THE TAILOR-BIRD. 
 
 to sit on her eggs, whilst another is occupied by the male, 
 who solaces his companion with his song, whilst she is occu- 
 pied in maternal cares. Another curious nest is that of the 
 
BUILDING INSTINCT OF BEAVER. 537 
 
 Sylvia sutoria, or Tailor-bird, a little eastern bird allied to 
 our linnet ; which, by the aid of filaments of cotton drawn 
 from the cotton-plant, sews leaves together with its beak and 
 feet, in such a manner as to conceal the nest which they 
 enclose from the observation of its enemies (fig. 279). 
 
 706. The association of a number of individuals of a certain 
 species, for the performance of labours in which they all unite 
 to one common end, is another most remarkable example of 
 the operation of instinct. Several Mammals exhibit this 
 tendency in a greater or less degree ; but the most interesting 
 of all, in this point of view, is the Beaver (fig. 280), which is 
 
 Fig. 280. BEAVER. 
 
 now chiefly found in Canada, though it formerly abounded 
 on the Continent of Europe. During the summer it lives 
 solitarily in burrows, which it excavates for itself on the 
 borders of lakes and streams ; but as the cold season ap- 
 proaches, it quits its retreat, and unites itself with its fellows, 
 to construct, in common with them, a winter residence. It is 
 only in the most solitary places that their architectural in- 
 stinct fully developes itself. Having associated in troops of 
 from two to three hundred each, they choose a lake or river 
 which is deep enough to prevent its being frozen to the 
 bottom ; and they generally prefer running streams, for the 
 sake of the convenience which these afford in the transporta- 
 tion of the materials of their erection. They begin by 
 constructing a sloping dam, whereby the water is kept-up to 
 
538 BUILDING INSTINCT OF BEAVER. 
 
 a uniform height ; this they form of branches interlaced one 
 with another, the intervals between them being filled with 
 stones and mud, with which materials they give a coat of 
 rough-cast to the exterior also. When the dam passes across 
 a running stream, they make it convex towards the current ; 
 by which it is caused to possess much greater strength than 
 if it were straight. This dam is usually eleven or twelve feet 
 across at its base, and it is enlarged every year ; and it fre- 
 quently becomes covered with vegetation, so as to form a kind 
 of hedge. 
 
 707. When the dam is completed, the community separates 
 into a certain number of families ; and the Beavers then em- 
 ploy themselves in constructing huts, or in repairing those of 
 a preceding year. These cabins are built on the margin of 
 the water ; they have usually an oval form, and an internal 
 diameter of six or seven feet. Their walls are constructed, 
 like the dam, of branches of trees ; and they are covered on 
 two of their sides with a coating of mud. Each has two 
 chambers, one above the other, separated by a floor ; the 
 upper one serves as the habitation of the Beavers ; and the 
 lower one as the magazine for the store of bark which they 
 lay up for their provision. These chambers have no other 
 opening than one by which they pass out into the water. It 
 has been said that the flat oval tail of the Beavers serves them 
 as a trowel, and is used by them in laying-on the mud of 
 which their erections are partly composed ; but it does not 
 appear that they use any other implements than their incisor 
 teeth and fore-feet. With their strong incisors they cut-down 
 the branches and even the trunks of trees which may be 
 suitable ; and by the aid of their mouths and fore-feet they 
 drag these from one place to another. When they establish 
 themselves on the banks of a running stream, they cut-down 
 trees above the point where they intend to construct their 
 dwellings, set them afloat, and, profiting by the current, direct 
 them to the required spot. It is also with their feet that 
 they dig-up the earth they require for mortar, from the banks 
 or from the bottom of the water. These operations are exe- 
 cuted with extraordinary rapidity, although they are only 
 carried-on during the night. When the neighbourhood of 
 Man prevents the Beavers from multiplying to the degree 
 necessary to form such associations, and from possessing the 
 
BUILDING INSTINCT OF BEAVER. 539 
 
 tranquillity which, they require for the construction of the 
 works now described, they no longer build huts, but live in 
 excavations in the banks of the water. 
 
 708. The building instinct shows itself, even when the 
 Beaver is in captivity, and under circumstances in which it 
 could be of no use. A half-domesticated individual, in the 
 possession of Mr. Broderip, began to build as soon as it was 
 let out of its cage and materials were placed in its way. Even 
 when it was only half-grown, it would drag along a large 
 sweeping-brush or warming-pan, grasping the handle with its 
 teeth, so that the load came over its shoulder ; and would 
 endeavour to lay this with other materials, in the mode em- 
 ployed by the Beaver when in a state of nature. " The long 
 and large materials were always taken first ; and two of the 
 longest were generally laid cross-wise, with one of the ends 
 of each touching the wall, and the other ends projecting out 
 into the room. The area formed by the cross-brushes and 
 the wall, he would fill up with hand-brushes, rush-baskets, 
 books, boots, sticks, cloths, dried turf, or anything portable. 
 As the work grew high, he supported himself upon his tail, 
 which propped him up admirably ; and he would often, after 
 laying-on one of his building materials, sit up over against it, 
 appearing to consider his work, or, as the country people say, 
 'judge it.' This pause was sometimes followed by changing 
 the position of the material judged ; and sometimes it was 
 left in its place. After he had piled up his materials in one 
 part of the room (for he generally chose the same place), he 
 proceeded to wall-up the space between the feet of a chest of 
 drawers which stood at a little distance from it, high enough 
 on his legs to make the bottom a roof for him ; using for this 
 purpose dried turf and sticks, which he laid very even, and 
 filling up the interstices with bits of coal, hay, cloth, or any- 
 thing he could pick up. This last place he seemed to appro- 
 priate for his dwelling ; the former work seemed to be intended 
 for a dam. When he had walled-up the space between the 
 feet of the chest of drawers, he proceeded to carry-in sticks, 
 cloths, hay, cotton, &c., and to make a nest; and when he 
 had done, he would sit up under the drawers, and comb him- 
 self with the nails of his hind feet." 
 
 709. "We see, in this account, a very interesting example 
 of the irrational character of Instinct. If the animal were 
 
540 
 
 IRRATIONALITY OF INSTINCT. 
 
 guided in its ordinary building operations by such an amount 
 of intelligence as would lead it to choose and execute its 
 various movements with a design to accomplish certain ends, 
 the same intelligence would direct it to leave these actions 
 unperformed when the purpose no longer required it ; instead 
 of which, we see that the animal is impelled by an internal 
 impulse to construct erections as nearly resembling those 
 which it would build-up on the banks of its native streams, 
 as the materials and circumstances will permit. Other ani- 
 mals are, in like manner, occasionally conducted by their 
 natural instincts to the performance of actions equally irra- 
 tional, and quite incapable of answering the purpose which 
 the particular instinct is destined to serve. Thus the Hen 
 will sit upon an egg-shaped piece of chalk, as readily as upon 
 her own egg, being deceived without difficulty by the general 
 similarity of its appearance ; and the Flesh-fly lays its eggs 
 in the petals of the carrion-flower, whose odour so much 
 resembles that of tainted meat as evidently to furnish the 
 same attraction to the insect. 
 
 Fig. 281. NEST OF REPUBLICAN GKOSBEAK. 
 
 710. Societies like those of the Beaver are rare among 
 Birds, whose associations are usually less perfect. There is 
 
SOCIETIES OP BIRDS AND INSECTS. 
 
 541 
 
 a small species, however, termed the Republican Grosbeak 
 (Loxia Socia), which lives in numerous societies in the neigh- 
 bourhood of the Cape of Good Hope, and constructs its nest 
 tinder a sort of roof which is common to the whole colony 
 (fig. 281). t 
 
 711. It is among Insects that we find the most remarkable 
 examples of this kind of social instinct ; and the structures 
 which are produced by the united labours of a large number, 
 working together in harmony, are extremely interesting. The 
 nests of Wasps are constructed in this manner. In order to 
 
 Fig. 282. NEST OF WASP. 
 
 form the materials for building them, these Insects detach 
 with their mandibles the fibres of old wood, which they convert 
 by mastication into a sort of pulp that hardens into the con- 
 sistence of pasteboard ; of this substance they construct several 
 ranges of hexagonal cells ; and the combs thus formed are 
 
542 SOCIETIES OP INSECTS 1 HIVE-BEE. 
 
 arranged parallel to each other at a regular distance, and are 
 united at intervals by little columns which serve to suspend 
 them (fig. 272). The whole is either hung in the air, lodged 
 in the hollow of a tree, or buried in the ground ; and it is 
 sometimes enclosed in a general envelope, sometimes left un- 
 covered, according to the species. 
 
 712. The same community of labour is observed in the 
 ordinary Hive-Bees, which present to the intelligent observer 
 a source of interesting occupation that scarcely ever fails. 
 The number and variety of instincts, each of them most per- 
 fectly adapted to the end in view, which these Insects exhibit, 
 is most wonderful; and many volumes have been written 
 upon them, without by any means exhausting the subject. 
 Nothing more than a very general sketch of these can be 
 attempted in the present treatise ; but the illustrations they 
 afford of the general remarks heretofore made upon the nature 
 of Instinct, are too valuable to be passed-by. Each Hive 
 contains but a single queen ; and she is the only individual 
 ordinarily capable of laying eggs. There are usually from 6 to 
 800 males or drones ; and from 10,000 to 30,000 neuters or 
 " working-bees " (fig. 283). In their 
 natural condition Bees live in the 
 hollows of trees ; but they appear 
 equally ready to avail themselves of 
 the habitations prepared for them 
 by Man. The cells of which their 
 combs are composed, are built-up of 
 Fig. 283.- WORKING BEE. the material that we- term wax. Of this 
 a part may be obtained direct from Plants, since it is secreted 
 in greater or less abundance by several species ; but there seems 
 to be no doubt, that Bees can elaborate it for themselves from 
 the saccharine materials of their aliment ( 155). The wax is 
 separated in little scales, from between the segments of the 
 abdomen ; these scales are kneaded-together by the mandibles 
 of the Insect, and are then applied to the construction of the 
 cells. It is easy to understand that the hexagonal form is 
 that which enables the cells to be best adapted to the purposes 
 for which they are built, whilst the least amount of material 
 is expended. As they are intended not only to contain a 
 store of honey, but also to serve as the residence for the larvae 
 (fig. 284) and pupce (fig. 285), it is evident that their form. 
 
ARCHITECTURE OF HIVE-BEE. 
 
 543 
 
 must approach near to that of the cylinder, in order that 
 there may be the greatest economy of space ; but it is also 
 evident that if their walls were circular, a large quantity of 
 
 Fig. 284. LARVJE OF BEE. 
 (Natural size and Magnified.) 
 
 Fig. 285. PUPA. OF BEE. 
 (Magnified.) 
 
 Fig. 286. HEXAGONAL CELLS. 
 
 (Showing the manner of their 
 union at the Base.) 
 
 material would be required to fill up the interspaces left 
 between them; whilst, by giving the cells the hexagonal 
 form, their walls everywhere have the same thickness, and 
 their cavity is sufficiently well adapted to the forms of the 
 larva and the pupa. 
 
 713. Every comb contains two sets of cells, one opening on 
 each of its faces. The cells of one side, however, are not 
 exactly opposite to those of the other, for the middle of each 
 cell abuts against the point where 
 the walls of three cells meet on the 
 opposite side ; and thus the partition 
 that separates the cells of the op- 
 posite sides is greatly strengthened. 
 This partition is not flat, but con- 
 sists of three planes, which meet 
 each other at a particular angle, so as to make the centre 
 of the cell its deepest part. Of the three planes which form 
 the bottom of each cell, one forms part of the bottom of each 
 of the three cells against which it abuts on the opposite side, 
 as shown in the accompanying figure. Now it can be proved, 
 by the aid of mathematical calculation of a very high order, 
 that, in order to combine the greatest strength with the least 
 expenditure of material, the edges of these planes should have 
 a certain fixed inclination ; and- the angles formed by them 
 were ascertained by the measurement of Maraldi to be 
 109 28', and 70 32' respectively. By the very intricate 
 mathematical calculations of Koenig, it was determined that 
 the angles should be 109 26', and 70 34', a coincidence 
 between the theory of the Mathematician and the practice of 
 the Bee (untaught, save by its Creator^ which has been ever 
 
544 
 
 ARCHITECTURE OF HIVE-BEE. 
 
 regarded as truly marvellous, and as affording one of the most 
 remarkable examples of the operation of instinct. The very 
 small discrepancy, amounting to only two minutes of a degree 
 (or 1-1 0,800th part of the whole circle), was usually sup- 
 posed to result from a slight error in the observation of the 
 angle employed by the Bees ; until Lord Brougham, not being 
 satisfied with this explanation, applied himself to a fresh 
 mathematical investigation of the question ; and he showed 
 that, owing to the neglect of certain small quantities, the 
 result previously obtained was erroneous to the exact amount 
 of two minutes ; so that the Bees proved to be right, and the 
 Mathematician wrong. 1 
 
 714. The ordinary cells of the comb are of two sizes ; one 
 for the larvee of the working-bees, and the other for those of 
 
 Fig. 287. APIARY. 
 
 the drones. Both of these may be used for layiiig-up a store 
 of food, either for themselves or their progeny ; but it is ob- 
 served that in the breeding season 
 the central portion only of each comb 
 is tenanted by the young Bees, this 
 being the part of the hive where 
 they will most constantly obtain the 
 warmth requisite for their develop- 
 
 Fig. 288.-ROYAL CELLS. ment ^ 4n y The deposition of 
 
 the eggs in these cells only, therefore, is another remark- 
 1 See h : s Supplement to New Edition of Paley's Natural Theology. 
 
COLLECTION OF FOOD BY BEES. 
 
 545 
 
 able instinct on the part of the Queen ; and this is further 
 manifested in the fact, that she never deposits eggs in the 
 comb which fills the glasses that are sometimes placed on 
 the top of a hive, as in fig. 287, the temperature of these 
 glasses being necessarily lower than that of the interior of 
 the hive. The " royal cells," as they are termed, in which 
 the larvae of the young queens are reared, are different in 
 form from the rest (fig. 288); sometimes they lie in the 
 midst of them ; but most commonly they project from the 
 sides or edges of the comb. 
 
 715. The food which the Bees collect is of two kinds, 
 the honey of flowers for themselves, and the pollen for their 
 larvse. The honey, which they suck-up by means of their 
 proboscis-like tongues (fig. 289), seems to undergo some change 
 
 Fig. 289. BEE'S MOUTH. 
 
 Fig. 290. 
 HIND LEG OF WORKER. 
 
 in their digestive cavity ; and the part not required for nourish- 
 ment is afterwards returned from the stomach, and deposited 
 in one of the cells, which, when filled, is sealed with a cover- 
 ing of wax. The pollen is gathered by rubbing the body either 
 against the anthers, or against other parts of the flower over 
 which it may have been scattered by their bursting ; and 
 when the surface of the body has been sufficiently dusted 
 with its fine particles, these are collected from it by little 
 brushes with which the feet of the Bee are furnished, and are 
 worked-up into small pellets, which the Insect carries home 
 in basket-shaped hollows, of which there is one on each 
 hind- thigh (fig. 290). The pollen or farina thus collected is 
 worked-up with honey in a mass, to which the name of 
 " bee bread " has been given ; and with this the larvae are 
 
 N N 
 
546 ARTIFICIAL PRODUCTION OF QUEENS. 
 
 nourished, until the time when they are about to pass into 
 the pupa state. The mouth of the cell is then sealed by a 
 waxen cover ; and the larva spins a delicate silken cocoon, 
 within which it undergoes its metamorphosis. In the 
 chrysalis state it remains quite inactive for some days ; and 
 during the latter part of this period, when it is rapidly ap- 
 proaching the condition of the perfect Insect, its development 
 is aided by the heat supplied by the "nurse-bees," whose 
 remarkable instinct has been already described ( 411). 
 
 716. One of the most curious features in the whole 
 economy of Bees is the manner in which they manufacture 
 new Queens, when from any cause (as by the intentional 
 removal of her from the hive) their sovereign has been lost. 
 In order to understand the process, it is necessary to be 
 aware that the ordinary working-bees may be regarded as 
 females, with the reproductive organs undeveloped ; and it 
 appears to depend on the manner in which they are treated 
 in the larva state, whether the egg shall be made ultimately 
 to produce a queen or a working-bee. For if, when the 
 queen has been removed, the royal cells (which are usually 
 among the last constructed) be not sufficiently forward, and 
 contain no eggs, the bees select one or more worker- eggs or 
 larvae, remove the egg or larva on either side of it, and throw 
 the three cells into one. The larva thus promoted is liberally 
 fed with "royal jelly," a pungent food prepared by the 
 working-bees' for the exclusive nourishment of the queen 
 larvae; and in due time it comes forth a perfect queen. 
 This change is doubtless owing to the peculiar effect of the 
 food ; and it is remarkable that it should operate, not only in 
 developing the reproductive organs, but also in altering the 
 shape of her tongue, jaws, and sting, in depriving her of the 
 power of producing wax, and in obliterating the hollows just 
 referred-to, which would otherwise have been formed upon 
 her thighs. 
 
 Manifestations of Intelligence. 
 
 717. The amount of reasoning power possessed by some 
 among the lower animals, may be considered as very much 
 upon a par with that exhibited by an intelligent child, about 
 the time when it is learning to speak. One of its first exer- 
 cises is in the connexion or association of ideas, which is the 
 
INTELLIGENCE OF LOWER ANIMALS. 547 
 
 source of the faculty of Memory, and thus becomes the 
 foundation of that power of profiting by experience, which is 
 manifested in the actions of animals that are distinguished 
 for Intelligence. Such a power is well shown in the following 
 instance, related to the Author by an eye-witness. A "Wren 
 built its nest in the slate- quarries at Penrhyn, in such a 
 situation as to be liable to great disturbance from the occa- 
 sional explosions. It soon, however, learned to quit its nest 
 and fly to a little distance, on the ringing of the bell which 
 warned the workmen. This action, having been noticed, was 
 frequently shown to visitors, the bell being rung when there 
 was not to be an explosion ; so that the poor bird suffered 
 many needless alarms. It seems gradually to have learned, 
 however, that the first notion it had formed, by the associa- 
 tion of the ringing of the bell with the explosion, was liable 
 to exceptions, and to have formed another more correct ; for 
 it was observed, after a time, that the wren did not leave its 
 nest, unless the ringing of the bell was followed by the 
 moving-away of the workmen. A similar process of associa- 
 tion, carried rather further, but still quite simple enough to be 
 readily believed, is shown in two Dogs, which have been 
 taught by their master to play at dominoes, and which go 
 through the game with another person (under circumstances 
 which render the idea of collusion with their master impos- 
 sible) with the utmost regularity and correctness ; not only 
 playing rightly themselves, but watching to see that their 
 adversary does so too. This, also, is a feat which a very 
 young child might be taught to perform. A third instance 
 has reference to the patient endurance of bodily pain, in 
 opposition to the instinctive tendency to struggle against the 
 infliction of it, and evidently occasioned by a voluntary effort 
 on the part of the animal, made by it in obedience to the 
 dictates of its reason. Dr. Davy mentions having seen an 
 Elephant, in India, that was suffering under a deep abscess in 
 its back, which it was necessary to lay open in order to effect 
 a cure. " He was kneeling down, for the convenience of the 
 operator, not tied ; his keeper was at his head. He did not 
 flinch, but rather inclined towards the surgeon, uttering a low 
 suppressed groan. He seemed conscious that what was doing 
 was intended for his good ; no human being could have be- 
 haved better; and so confident were the natives that he 
 
 NN2 
 
548 RELATION OF INTELLIGENCE TO CEREBRUM. 
 
 would behave as lie did, that they never thought of tying 
 him." It were much to be wished, that all human beings 
 would imitate this docile Elephant's self-control. It is some- 
 times manifested, however, even in Infancy ; the painful 
 operation of lancing the gums being often sustained without 
 a cry, from the consciousness of the benefit derived from it. 
 
 718. It has been stated that the relative amount of Intelli- 
 gence in different animals bears a pretty constant proportion 
 to the size and development of the Cerebral hemispheres 
 ( 452). That size alone, however, does not produce the dif- 
 ference, is evident from a number of facts. As we advance 
 from the lower to the higher Vertebrata, we observe an obvious 
 advance in the complexity of the structure of the brain. In 
 proportion to the increase in the number and depth of the 
 convolutions by which its surface is extended ( 456), do we 
 find an increase in the thickness of the layer of grey or 
 vesicular matter ( 61), which seems to be the real centre of 
 all the operations of the organ. The arrangement of the 
 white or tubular tissue ( 60), which forms the interior of 
 the mass, also increases in complexity ; and as we ascend 
 from the lower Mammalia up to Man, we trace a great in- 
 crease in the number of the fibres which establish communi- 
 cations between different parts of the surface. Still there can 
 be no doubt that the size of the Cerebrum, compared with that 
 of the Spinal Cord and of the Sensory Ganglia at its summit, 
 usually affords a tolerably correct measure of the intelligence 
 of the animal ; and that, even in comparing together different 
 Men, we shall find the same rule to hold good, when due 
 allowance has been made for the comparative activity of their 
 general functions, such as is expressed by the word tempera- 
 ment. Thus, two men, having brains of the same size and 
 general conformation, may differ greatly in mental vigour, 
 because the general system of one performs its functions 
 much more actively and energetically than that of the other. 
 For the same reason, a man of small brain, but whose general 
 habit is active, may have a more powerful mind than another 
 whose brain is much larger, yet whose system is inert, his 
 perceptions dull, and his movements languid. But of two 
 men alike in these respects, and having the same general con- 
 figuration of head, it cannot be doubted that the one with the 
 larger brain will surpass the other. It is a striking fact, that 
 
SIZE OF BRAIN : FACIAL ANGLE. 
 
 549 
 
 Fig. 291. SKULL OF EUROPEAN. 
 
 almost all those persons who have been eminent for the 
 amount of their acquirements, or for the influence they have 
 obtained by their talents for command over their fellow-men, 
 have had large brains : this was the case, for example, with 
 Newton, Cuvier, and Napoleon. 
 
 719. The size of the brain, and especially of its anterior 
 lobes (which seem particularly connected with the higher 
 reasoning powers), as compared with that of the face, may be 
 estimated pretty correctly by the measurement of the facial 
 angle; as proposed by Camper, an eminent Dutch naturalist. 
 This is done by drawing a horizontal line (cd, figs. 291 and 
 292), between the entrance to the 
 
 ear and the floor of the nose, so as 
 to pass in the direction of the base 
 of the skull ; this is met by another 
 line (a, b) which passes from the 
 most prominent part of the forehead 
 to the front of the upper jaw. It is 
 evident that this last will be more 
 inclined to the former, so as to make 
 a more acute angle with it, in pro- 
 portion as the face is more developed and the forehead more 
 retreating ; whilst it will approach more nearly to a right angle, 
 if the forehead be prominent, and the muzzle project but little. 
 Hence this facial angle will indicate, with tolerable correct- 
 ness, the proportion which the brain bears to the face, the 
 instrument of intelligence, to the receptacle of the organs 
 of sense. 
 
 720. Of all animals, there are none in which the facial 
 angle is so open as in Man ; and great variations- exist 
 in this respect, even among the . a 
 
 different human races. Thus, in 
 European heads, the angle is usually 
 about 80 (fig. 291). The ancient 
 Greeks, in those statues of Deities 
 and Heroes to which they wished 
 to give the appearance of the greatest 
 intellectual power, made it 90, or 
 even more, by the projection they 
 gave to the forehead. On the other 
 hand, in the Negro races, it is commonly about 70 (fig. 292) ; 
 
 Fig. 292. SKULL OF NEGRO. 
 
550 FACIAL ANGLE I SPECIAL ENDOWMENTS OF MAN. 
 
 in the different species of the Monkey tribe, it varies from 
 about 65 to 30 (fig. 293) ; and as we descend still lower, we 
 find it still more acute. In the Horse and Boar, for example, 
 it becomes impossible to draw a straight line from the fore- 
 
 Fig. 293. SKULL OP MACACOS. Fig. 294. SKULL OF BOAR. 
 
 head to the upper jaw ; in consequence of the retreating 
 character of the former, and the projection of the nose ; this 
 will be evident from an examination of fig. 294. In Birds, 
 Reptiles, and Fishes, the facial angle, when it can be mea- 
 sured, is found to be still further diminished. 
 
 721. It appears, then, that the mind of Man differs from, 
 that of the lower animals, rather as to the degree in which the 
 reasoning faculties are developed in him, than by anything 
 peculiar in their kind. Among the more sagacious Quadrupeds, 
 it is easy to discover instances of reasoning as close and pro- 
 longed as that which usually takes place in early childhood ; 
 and it is only with the advance of age and the maturity of 
 the powers, that the superiority of Man becomes evident. The 
 foundation of this superiority lies in the power of self-direc- 
 tion and self-improvement which Man possesses. No race 
 among the lower animals ever exhibits a spontaneous ten- 
 dency to the elevation of its mental powers. When placed 
 under new circumstances, and especially when subjected to 
 Human training, the domesticable races acquire new capa- 
 cities ; and individuals frequently display a very extra- 
 ordinary degree of sagacious appreciation of matters quite 
 foreign to their natural habits of life. But neither in races 
 nor in individuals are these powers transmitted from one 
 generation to another, when, left to themselves, they return 
 to anything like a state of nature. In Man, on the other 
 hand, the power which every rightly-constituted and rightly- 
 
SPECIAL ENDOWMENTS OF MAN. 551 
 
 trained individual possesses ( 525) of fixing his attention 
 upon any particular object of consciousness, to the exclusion 
 of all others, becomes the source of the highest and most 
 enduring intellectual advancement, and of all moral improve- 
 ment. It is in virtue of this power, that he is not only 
 enabled to profit largely by the acquired knowledge of others, 
 but that he comes to possess a moral responsibility for the 
 use he makes of his faculties, which cannot be predicated 
 of beings whose succession of ideas is entirely determined 
 by impressions made from without. 
 
 722. There is another attribute, moreover, by which Man 
 seems to be distinguished from all other animals ; namely, 
 that disposition to believe in the existence of an unseen but 
 powerful Being, which seems never to be wanting (under 
 some form or other) in any race or nation, although (like 
 other natural tendencies) it may be defective in individuals. 
 It requires a higher mental cultivation than is commonly to 
 be met with among savage races, to conceive of this Power as 
 having a spiritual existence ; but it appears, from the reports 
 of Missionaries who have laboured to spread Christianity 
 amongst the Heathen, that an aptitude or readiness to receive 
 this idea is rarely wanting ; so that the faculty is obviously 
 present, though it has not been called into operation. Closely 
 connected with this tendency to believe in a Great unseen 
 Power, is the desire to share in His spiritual existence, which 
 seems to have been implanted by the Creator in the mind of 
 Man, and the existence of which is one of the chief natural 
 arguments for the immortality of the soul, since it could 
 scarcely be supposed that such a desire would have been 
 implanted, if it were not in some way to be gratified. Such 
 views tend to show us the true nobility of Man's rational 
 and moral nature, and the mode in which he may most 
 effectually fulfil the ends for which his Creator designed him. 
 "We learn from them the evil of yielding to those merely 
 animal tendencies, those "fleshly lusts which war against 
 the soul," that are characteristic of beings far below him 
 in the scale of existence, and tend to degrade him to their 
 level ; and the dignity of those pursuits, which, by exercising 
 his intellect, and by expanding and strengthening the higher 
 part of his moral nature, tend to raise him towards the per- 
 fection of the Divine Being. 
 
552 TWO PRINCIPAL MODES OF REPRODUCTION. 
 
 CHAPTEE XV. 
 
 OF REPRODUCTION. 
 
 723. THERE is no one of the functions of living beings, that 
 distinguishes them in a more striking and evident manner 
 from the inert bodies which surround them, than the process 
 of Eeproduction. By this function, each race of Plants and 
 Animals is perpetuated ; whilst the individuals composing it 
 successively disappear from the face of the earth, by that 
 death and decay which is the common lot of all. A very un- 
 necessary degree of mystery has been spread around this pro- 
 cess. It has been regarded as one altogether inscrutable, 
 whose real nature could not be unveiled, even by the scientific 
 inquirer, and whose secrets the uninitiated should never seek 
 to comprehend. But so much light has been thrown upon it 
 by recent investigations, that we now know at least as much 
 of this, as of almost any other function ; and the Author's ex- 
 perience has led him to believe that such knowledge may be 
 communicated to the general reader, without the least in- 
 fringement of the purest delicacy of feeling. In his own 
 judgment, indeed, it is far better to afford a legitimate satis- 
 faction to the curiosity which naturally exists upon the sub- 
 ject, than, by refusing all information, to drive the inquirer 
 into objectionable methods of gratifying it. 
 
 724. It has been elsewhere shown (YEGET. PHYS., Chaps, 
 ix., XIL), that, in the Vegetable Kingdom, there are two 
 distinct modes by which the propagation of Plants may 
 take place ; the extension of the parent structure into new 
 portions, which, being independent of it and of each other, 
 can maintain their lives when separated from it ; and the 
 origination of a new being by the concurrent action of two 
 sets of cells set apart for this special function, and desig- 
 nated " sperm-cells " and " germ-cells." The bodies of the 
 first class are known as leaf-buds or gemmae in the Flowering 
 Plants, and sometimes also among Cryptogamia, some of which 
 last, as the Marchantia (VEGET. PHYS. 757), are fur- 
 nished with a peculiar means of producing them ; and it 
 appears from recent investigations that the " spores " of Ferns, 
 
TWO PRINCIPAL MODES OF REPRODUCTION. 
 
 553 
 
 Mosses, and Hepaticae, as well also the " zoospores " of Algae, 
 belong to the same class of reproductive bodies. The gemmae 
 of Phanerogamia may be developed in connexion with the 
 parent .structure, and may continue to form a part of it ; or 
 they may be removed from it (as in the processes of budding, 
 grafting, &c.), and may be developed into new individuals. 
 On the other hand, the bodies of the second class are known 
 as seeds among Flowering Plants ; among the Cryptogamia 
 they present a variety of forms. From the very first, these 
 are destined to produce new individuals ; and although they 
 are often assisted in the early stage of their development by 
 the parent, they are its true offspring, rather than (like 
 gemmae) extensions of itself. Both these modes of Keproduc- 
 tion, namely, gemmation and sexual generation, exist in the 
 Animal Kingdom ; but the former is confined to its lower- 
 tribes, among which we often find it exercised in very remark- 
 able modes. 
 
 Gemmiparous or Non-Sexual Reproduction. 
 
 725. Among Infusoria ( 133) we find the process of gem- 
 mation, or of fission, which is a modification of it, to be almost 
 the only ostensible means of propagation which the beings 
 composing that wonderful group possess. The former may be 
 continually witnessed by the microscopic observer in the 
 common Vorticella, a bell-shaped animalcule attached by a stalk 
 
 Fig. 295. VARIOUS FORMS OF ANIMALCULES, some of them undergoing spon- 
 taneous fission. 
 
 (fig. 295, a, a), and abundant in almost every pool in which 
 aquatic vegetables grow, especially clustering around the stems 
 of Duckweed ; and its various stages closely resemble those 
 
554 GEMM1PAROUS REPRODUCTION OP LOWEST ANIMALS. 
 
 which have been already described (122) in the Hydra. But 
 not unfrequently in this species, and ordinarily in many others, 
 the body divides into two equal parts, in each of which we see 
 a mouth and other parts resembling those of the original. This 
 division is gradual. A narrowing of the body along or across 
 its middle (for the fission or cleavage sometimes takes place 
 lengthways, as at 6, sometimes transversely, as at c), is first 
 seen j the indentation at the edge becomes gradually deeper, 
 and at last the two parts hold together by but a narrow band, 
 which finally breaks, and they become free. The same 
 method of multiplication is observed among the simple 
 Rhizopoda ( 129); but when the gemmae remain connected 
 with each other, as in Zoophytes, we have such composite 
 fabrics as are presented to us in the classes of Foraminifera 
 ( 131) and Sponges ( 136). 
 
 726. Reproduction by Gemmation is most characteristically 
 seen among the Radiated classes j and in none better than in 
 the Hydra already so frequently referred-to. Although this 
 interesting little animal sometimes reproduces itself by true 
 sexual generation ( 734), yet its usual mode of propagation 
 is by buds ( 122), as shown on the left hand side of the ac- 
 companying figure (fig. 296). And, as already explained, it 
 
 is by this same process of gemma- 
 tion that the arborescent struc- 
 tures of the Composite Zoophytes 
 are formed ; the gemmae not de- 
 taching themselves, but remaining 
 as parts of the common stock ( 
 124,127). In some of those, how- 
 ever, which are formed upon the 
 plan of the Sea Anemone ( 126), 
 the multiplication (fig. 297) is ef- 
 fected rather by fission or division 
 into two equal parts (as among In- 
 fusoria), than by the out-growth of 
 buds. We have already had occa- 
 sion to notice ( 125) the very re- 
 markable form of gemmation that 
 takes place among Zoophytes, giv- 
 ing origin to independent beings 
 which seem to belong to a class altogether different, but which 
 
 Fig. 296. HYDIUE (attached to duck- 
 weed): one of them developing buds. 
 
GEMMATION OF EADIATA AND ABTICULATA. 
 
 555 
 
 are in reality the representatives of the flower-buds of Plants, 
 distinguished by their capability, not only of living and en- 
 during, but of obtaining their own nu- 
 triment, after their spontaneous detach- 
 ment from the stock that bore them. 
 Among the Medusae we occasionally 
 meet with instances of propagation 
 by buds that resemble the stock from 
 which they proceed, and that are 
 thrown off in due time so as to lead 
 independent lives ; but this kind of 
 gemmation seems limited to , -ie lower 
 members of the group. In a large 
 proportion of it, however, a very extra- 
 ordinary kind of multiplication by 
 gemmation takes place at an early 
 period of development ( 740). In 
 the highest Eadiata, the class of 
 Echinodermata, we take leave of multiplication by gemmation 
 altogether ; for although the bodies of these creatures possess 
 a very extraordinary reproductive power, so that the result of 
 very severe injuries may be repaired ( 389), we do not find 
 that they either spontaneously produce independent buds, or 
 that they have the capacity for being multiplied by artificial 
 division. 
 
 727. Among several of the lower Articulata, detached 
 segments of the body appear to be capable of reproducing the 
 whole ; and there are some whose ordinary propagation is 
 
 Fig. 297. POLYPES OP 
 
 ASTRJEA, 
 
 Undergoing fission ; a, b, c, 
 d, successive stages. 
 
 Fig. 298. NEREIS PROLIFERA. 
 
 accomplished by an exercise of this power. Thus in the Nais, 
 an aquatic worm allied to the Earth-worm, the last joint of the 
 body gradually extends and increases to the size of the rest of 
 the animal ; and a separation is made by a narrowing of the 
 
556 GEMMATION OF ARTICULATA AND MOLLUSCA. 
 
 preceding joint, which at last divides. Previously to its separa- 
 tion, however, the young one often shoots out another from 
 its own last joint, in a similar manner ; and three successions 
 have thus been seen united. In some species of Nereis, the sepa- 
 ration takes place nearer the middle of the body (fig. 298). In 
 the greater number of cases, however, in which such a detach- 
 ment of the posterior part of the body of Annelids takes place, 
 the separated gemma does not contain the structure of the entire 
 animal, but consists of little else than the generative apparatus, 
 endowed with locomotive organs ; so that this process of mul- 
 tiplication does not so much correspond with the ordinary pro- 
 pagation by buds, as with the peculiar development and throw- 
 ing-off of generative buds to be presently described. Among 
 the higher Articulata, we do not meet with any instances of 
 ordinary gemmation ; but the non-sexual production, which 
 is now known to take place not only in the Aphides ( 746) 
 but in many other Insects, as well as in Rotifera (Wheel- 
 Animalcules) in Entomostracous Crustacea (Water Fleas, &c.), 
 and probably in some higher Crustacea, must be regarded as 
 a peculiar form of the same process ; the offspring being pro- 
 duced from eggs, which have the power of self-development 
 without sexual fertilization, and which must therefore be 
 accounted internal gemmce. 
 
 728. In the Molluscous series, the power of multiplying by 
 gemmation appears to be limited to the Tunicata ( 114) 
 and the Polyzoa ( 115); being restricted in the first of 
 these classes to a section of the group ; whilst in the second, 
 which closely follows the habit of Zoophytes, it seems to 
 be universal. The bud arises in some instances directly 
 from the body ; but in other cases it is put forth by a stolon 
 or creeping stem that connects all the bodies together 
 (fig. 63). Among the Polyzoa the buds usually remain in 
 connexion with the parent-stock, so as to form composite 
 fabrics so closely resembling those of Zoophytes as to be com- 
 monly ranked with them. And the like happens also among 
 the Compound Ascidians. But where gemmation takes place 
 among the solitary Tunicata, the bud becomes detached, and 
 maintains a perfectly independent existence. There is a very 
 curious case of internal gemmation among the Salpce (a tribe 
 of Tunicata which are not attached, but float over the waves) ; 
 for the buds are developed, not from the exterior of the 
 
GEMMATION OF MOLLUSCA AND VERTEBRA! A. 557 
 
 body, but from a kind of stolon within it ; and they 
 differ from their parent-stock in having organs of attach- 
 ment to each other, whereby they hold together in long 
 
 Fig. 299. AGGREGATE SALP^E. Fig. 300. SOLITARY SALPA. 
 
 chains (fig. 299). These, in their turn, being furnished with 
 true generative organs, give origin to the solitary SalpaB (fig. 
 300) by ordinary sexual reproduction ; whilst the solitary 
 Salpa never comes to possess any sexual apparatus, but merely 
 continues the race by gemmation from its internal stolon. 
 
 729. In the Vertebrated classes, as in the higher Mollusca, 
 we lose all trace of propagation by gemmation as an ordinary 
 method of multiplication. Yet there is evidence that this 
 power is not altogether extinguished, even in Man. For we 
 not unfrequently hear of " monstrosities by excess," that is, 
 of cases in which the body possesses a superfluity of some of 
 its parts ; the simplest cases being those of double thumbs or 
 of six fingers on each hand, and the gradation being so conti- 
 nuous from these to cases like that of the Siamese twins (in 
 which there are two complete bodies united only by a cross 
 band), as to make it evident that they are all referable to one 
 common principle. And although it has been commonly 
 believed that monsters with two heads and one body, or with 
 two bodies and one head, or with supernumerary legs or arms, 
 are results of the partial " fusion" of two distinct germs at an 
 early period, yet there is now far stronger reason to believe that 
 they proceed from a kind of attempt at multiplication by 
 fission or gemmation, that is sometimes made by a single germ 
 at a time when its grade of development corresponds with 
 that of a Hydra or Planaria ; which attempt, under peculiarly 
 favourable circumstances, may proceed to the full length of 
 production of two complete bodies. (See 390.) 
 
 Sexual Reproduction, or Generation. 
 
 730. We now have to notice the most important features 
 of the proper Generative process, which differs from the 
 
558 
 
 SEXUAL GENERATION : SPERM-CELLS. 
 
 preceding in exactly the same manner as the flowering and 
 fruiting of Plants differ from their extension and propagation 
 by leaf-buds. In all save the very lowest tribes of Animals, 
 we meet at particular seasons with two peculiar sets of cells, 
 termed sperm-cells and germ-cells ; these are sometimes borne 
 by the same individuals, which then correspond as regards 
 their reproductive apparatus with the generality of Flowering 
 Plants ; but they are more commonly separated, as in dioecious 
 Plants (VEGET. PHYSIOL., 409) ; the individual bearing the 
 "sperm-cells" being then designated as the male, and the 
 individual bearing the "germ-cells" as the female. 
 
 731. The " sperm-cells " very closely resemble those con- 
 tained within the antheridia of Cryptogamia (VEGET. PHYSIOL. 
 399 ; BOTANY, 737, 776). When mature, each cell is 
 
 found to contain one or more spirally 
 coiled filaments (fig. 301), which, 
 when set free by the bursting of 
 the cell, have an active spontaneous 
 movement lasting for some time like 
 ciliary action. These filaments were 
 formerly regarded as true Animal- 
 cules ; but since other examples of 
 independent movement have been 
 discovered in what are certainly 
 nothing else than detached parts of 
 the organism, and more especially 
 since moving filaments of a precisely 
 analogous character have been dis- 
 covered in Plants, all idea of their 
 independent animality has been laid 
 aside, and they are now known as 
 spermatozoids. The use of their 
 motor activity is obviously to bring 
 them into contact with the germ-cells, when both have been 
 set free from the interior of the bodies within which they 
 were formed. When the sperm-cells are developed in a 
 special or distinct organ, as happens in all save the lowest 
 types of Animal structure, this organ usually more or less 
 resembles the ordinary glands in structure ( 356, 357), and 
 is termed the Testis. 
 
 732. The "germ-cells" are not so clearly distinguished 
 
 Fig. 301. SPERMATOZOIDS : 
 e, immature sperm-cells. 
 
GERM-CELLS : FERTILIZATION OF GERM. 559 
 
 from other cells by the nature of their contents, though they are 
 usually recognisable by the peculiar nuclei they present ; each 
 cell is known as the germinal vesicle (fig. 302, d), whilst its 
 nucleus (e) is designated the germinal spot. The act of fertiliza- 
 tion appears to consist in the contact of one or more spermato- 
 zoids with the exterior of the germinal vesicle ; the sperma- 
 tozoids, ceasing to move, undergo a sort of liquefaction ; and 
 the product of their dissolution, being received by absorp- 
 tion into the interior of the germinal vesicle, mingles with its 
 contents, to form with them the basis of the new structure. 
 When, as usually happens, the germ-cells are developed in a 
 
 Fig. 302. SECTION OF OVAB.IUM OF FOWL: 
 
 a, fibrous substance of the ovary; b, yolk; c, yolk-bag; d, germinal vesicle; e, 
 germinal spot. 
 
 special and distinct organ, this organ, which is termed the 
 Ovary, has very commonly among the lower animals a glan- 
 dular character, the mature ova being discharged by the ovi- 
 duct, just as the products of secretion pass-off through the 
 ducts of their respective glands : but among the Vertebrata 
 the ovary has a much more solid texture, and the germ-cells, 
 developed in the very midst of its fibrous tissue (fig. 302, d), 
 have to find their way to its surface, and to burst forth from it ; 
 being then received into an oviduct, whose trumpet-shaped 
 mouth embraces the ovary, so as to prevent the liberated 
 germs from falling (as they would otherwise do) into the 
 visceral cavity of the body. 
 
 733. "With the "germ-cell" there is always associated in 
 Animals, as in Flowering Plants, a store of nutriment that 
 serves for the early development of the germ ; this consists 
 of a mixture of albuminous and oily matter, known as the 
 yolk (fig. 302, b) ; and it is inclosed in a membranous envelope c, 
 
560 SIMPLEST FOKM O7 GENERATIVE PROCESS. 
 
 termed the yolk-bag. The yolk-bag and its contents, namely 
 the yolk and the germinal vesicle, constitute the ovum. The 
 eggs of many animals (as of Birds) contain an additional 
 store of liquid albumen, the " white," enveloping the yolk-bag 
 and destined to be gradually drawn into" it, so as to replace 
 the albumen of the yolk as it is progressively used-up in the 
 development of the embryo ; and the " shell " is a subsequent 
 formation around this ( 755). 
 
 734. The Hydra presents us with a very apposite illustra- 
 tion of the simplest mode in which the generative function is 
 performed. Sperm-cells are developed at certain periods in 
 the substance of its body near the origin of the arms ; whilst 
 ovules are evolved in the wall of the stomach nearer to the 
 foot or base. By the rupture of the sperm-cells, their con- 
 tained spermatozoids are set free in the surrounding water, 
 and they penetrate to the ovules, which are exposed to their 
 influence by the thinning-awa'y of their exterior covering. 
 From what has been observed in higher animals, there seems 
 no reasonable doubt that the spermatozoids make their way 
 through the germinal membrane, and penetrate into absolute 
 contact with the germinal vesicle, which then lies near the 
 surface of the ovule. What is the precise change effected by 
 fertilization, has not yet been fully ascertained ; the germinal 
 vesicle, however, disappears ; and it would seem as if its 
 contents, with the product of the liquefaction of the sperma- 
 tozoids, were diffused through the yolk, which soon begins to 
 undergo changes of a very remarkable nature. 
 
 735. Before going further, it may be well to notice the 
 remarkable antagonism which exists between the processes of 
 Gemmation and Generation, as regards the conditions by which 
 they are respectively favoured. For we see that in the Hydra, 
 as in Plants, the extension of the body into buds is promoted 
 by warmth and a copious supply of food ; so that, as it would 
 appear, if these be afforded, this mode of multiplication may 
 be protracted indefinitely. On the other hand, if the supply 
 of food be limited and the temperature lowered, the pro- 
 duction of buds ceases, and the formation of sperm-cells and 
 of germ-cells begins. The result is that ova or eggs are 
 produced, which have a very firm horny covering, and possess 
 a great power of resistance to cold; and thus provision is 
 made for the continuance of the race through a winter tern- 
 
EARLIEST STAGES OF DEVELOPMENT OF OVUM. 561 
 
 perature that might be fatal to the Hydrse themselves. The 
 same thing is observable among the Rotifer a; for, as has 
 long been known, two kinds of eggs are produced by them, 
 the ordinary and the "winter eggs ;" and it now appears that 
 the ordinary eggs, being evolved without any generative pro- 
 cess, and with a rapidity proportional to the favouring 
 influences of food and warmth, are really to be regarded as 
 internal gemmae ; whilst the " winter eggs," which are pro- 
 duced in the autumn by the concurrent action of males and 
 females, and have a peculiarly dense horny investment, are 
 the only true ova. Among the Aphides ( 746), again, it has 
 been experimentally shown that the non-sexual multiplication 
 may be indefinitely protracted by warmth and food ; whilst a 
 reduction in the temperature and in the supply of nutriment 
 causes this at any time to give place to sexual generation. 
 
 736. The first obvious change that presents itself in the 
 Ovum, after its fertilization, is the " segmentation," or division 
 .of the yolk-mass into two halves, by the formation of a sort 
 of hour-glass contraction, which gradually deepens, until it 
 produces a complete separation. Another segmentation of 
 these two halves soon follows in the opposite direction, so 
 that the yolk-mass becomes divided into four segments ; each 
 of these in its turn undergoes the like subdivision ; and this 
 duplicating process is repeated, forming successively 8, !(> 
 32, 64, &c., segments, until a " mulberry-mass " is produced,, 
 which is composed of an aggregation of an immense number 
 of minute yolk-spherules. Up to this stage, the develop- 
 mental process takes place on essentially the same plan in all 
 animals, save that in some the process of segmentation does 
 not extend to the entire mass of the yolk, but only to a small 
 proportion of it, which is distinguished as the "germ-yolk," 
 whilst the remainder, which is applied to the nourishment of" 
 the more advanced embryo through an entirely different chan- 
 nel, is known as the "food-yolk" ( 754). 
 
 737. It appears, among some of the simplest Worms, as if 
 the " mulberry-mass " gradually shaped itself into the body 
 of the animal, without the intervention of any intermediate 
 structure ; but in almost all Animals, the first stages in deve- 
 lopment tend to the production of a membranous expansion 
 that may be likened to the " cotyledon " of Flowering Plants, 
 with this important difference, however, that whilst the latter 
 
 o o 
 
562 GERMINAL MEMBRANE. DEVELOPMENT OP POLYPES. 
 
 spreads itself out so as to come into contact with the " albu- 
 men" of the seed by its external surface, the "germinal 
 membrane " of the Animal forms itself around the yolk, and 
 thus constitutes as it were a temporary stomach, within which 
 the nutrient material is stored-up, and through the walls of 
 which it is drawn into the embryo. This is accomplished in 
 the following manner. The spherules of the outer layer of 
 the mulberry-mass which are in immediate contact with the 
 yolk-bag become invested with walls of their own, and thus 
 become converted into proper cells ; these are somewhat flat- 
 tened and of a polygonal shape, very much resembling those 
 of the epithelium of serous membrane (fig. 10). Another 
 layer is afterwards formed within this, the cells of which 
 retain more of their original globular form. But the spherules 
 of the internal portion of the mulberry-mass, instead of be- 
 coming converted into cells, undergo dissolution and return 
 to the condition of a liquid yolk ; so that the ovum, in this 
 stage of its development, consists of two layers of cells, con- 
 stituting what are known as the " serous " and the " mucous " 
 layers of the germinal membrane, enclosing a mass of nutritious 
 matter on which a change has been worked that seems to 
 predispose it to become organized. 
 
 738. The development of the Polypes seems to advance 
 but little beyond this point. The covering of the ovum 
 bursts, and the contained embryo is set at liberty as soon as 
 the germinal membrane has been formed around the yolk. 
 In this state it becomes clothed with cilia, and is termed a 
 gemmule ; and it swims about freely in water for some time. 
 Its form gradually becomes more elongated (fig. 303), 
 tapering away at one end, which attaches itself after 
 a time to some solid body \ and its development 
 into the polype-form soon commences. In the group 
 of which the Hydra is an example, this change takes 
 place in the following simple manner. The germinal 
 membrane gradually thins away at the point furthest 
 removed from the attached base, and at last an aper- 
 ture is formed, which becomes the mouth ; from around this 
 aperture the tentacula or arms shoot forth, a single row being 
 first formed, and others being afterwards added in those species 
 in which they are numerous. Thus the two layers of the 
 germinal membrane enter into the permanent structure of 
 
DEVELOPMENT OF POLYPES AND MEDUSAE. 563 
 
 the animal ; the outer one constituting the external integu- 
 ment, and the inner becoming the lining of the stomach. 
 The arborescent fabric of the composite Hydrozoa ( 124) 
 is gradually evolved by continuous gemmation from the original 
 Polype ; and whilst in some of them the sperm-cells and 
 ova are developed within peculiar capsules not ostensibly 
 differing (except in size) from the ordinary polype-cells, 
 there are others in which they are the product of peculiar 
 buds having the form and structure of Medusce, which buds 
 in many instances become detached, and henceforth live as 
 independent zooids, their sexual apparatus being only evolved 
 after they have separated themselves from the parent stock. 
 The sperm-cells and the ova are developed within different 
 Medusa-buds ; but both kinds of buds may (in many cases at 
 least) be put forth from the same Polype-stock, as in moncecious 
 Flowering-Plants. 
 
 739. Although the two layers of the germinal membrane 
 remain united in the Hydra and other Zoophytes formed upon 
 its simple plan, they separate from each other at certain points 
 in the Sea Anemone and its allies, so that a series of chambers 
 is formed between them ; and these chambers are afterwards 
 set apart for the production of sperm-cells and germ-cells 
 ( 126). We do not meet in this group of Anthozoa with 
 any example of that detachment of the sexual apparatus in 
 the form of separate zooids, which is so remarkable a feature 
 of the Hydrozoa. 
 
 740. The development of the Medusce, as elucidated by 
 recent discoveries, presents several features of extraordinary 
 interest. The sexes are distinct in these animals ; sperm- 
 cells being developed in some individuals, and ova in others, 
 within the fxrar chambers that surround the stomach ( 120). 
 When the ova have received the fertilizing influence, their 
 first products are ciliated gemmules resembling those of 
 Hydraform Polypes (fig. 304, a). These, after moving about 
 for some time in the ovarial chambers of their parent, make 
 their exit by the orifices of these, and then swim freely 
 through the water. Gradually, however, they undergo the 
 usual elongation, and fix themselves by one extremity (e) ; at 
 the opposite extremity a depression appears in the middle, 
 which is to become the mouth, as seen at 6, and an elongation 
 of the four corners (c,f) gives origin to the first tentacula, 
 
 o o 2 
 
564 
 
 DEVELOPMENT OF MEDUSAE. 
 
 Fig. 304. DEVELOPMENT OF MEDUSJE. 
 
 which are afterwards increased by the addition of many others 
 (g). In this manner a true polype is formed, which leads the 
 life of a Hydra, and, like it, propagates its kind by the forma- 
 tion of polype-buds, which detach themselves and lead inde- 
 pendent lives ; and thus from a single Medusan egg there may 
 arise a whole colony of polypes multiplied by gemmation. These 
 differ entirely from Hydras, however, in regard to their sexual 
 apparatus, which is detached (as in the" composite Hydrozoa) 
 under an entirely different form, that of a Medusa. The body of 
 
 the polype undergoes a great 
 lengthening, and seems as if 
 divided by transverse bands, 
 which gradually deepen, so as 
 to make the whole body almost 
 resemble a pile of saucers 
 with divided edges (h) ; for 
 beneath the lowest of these 
 saucer-like disks, a new set 
 of polype -arms makes its 
 appearance ; and after the 
 detachment of the whole pile of disks, the polype-body 
 remains at their base, and may continue to lead its former 
 life, and to propagate itself in the polype-form. The disks 
 progressively enlarge, those at the summit of the pile in- 
 creasing most rapidly, and then detaching themselves from 
 the pile (i) ; when thus detached, they swim about freely in 
 the water after the manner of the smaller and simpler Me- 
 dusas, to which they closely correspond in form (d) ; and they 
 gradually enlarge and acquire the structure of their original 
 parents (&). It is not correct to represent (which is commonly 
 done) the pile of Medusa-disks as being formed 1 by the sub- 
 division of the polype-body. The Medusa-disks are in reality 
 sexual buds, resembling those of the composite Hydrozoa 
 ( 125); and the only essential difference between the two 
 cases lies in the fact, that among the latter it is the Zoophytic 
 form which ostensibly constitutes the animal (the Medusan 
 buds being thrown-off only at certain times for a special pur- 
 pose), whilst the former are only known (save to such as 
 search-out the history of their polypoid development) in the 
 Medusan stage of their lives. 
 
 741. The recent researches of Professor Muller and others 
 
DEVELOPMENT OF ECHINODEKMATA. 565 
 
 have brought to light a most remarkable set of facts in regard 
 to the developmental history of the Echinodermata. The 
 details of this history vary so greatly in the different sections 
 of the group, that all which can be here attempted is a 
 general notice of its most important features. The em- 
 bryonic mass of cells, which is produced in the ordinary way 
 from the egg, is usually converted, not (as in Insects) into a 
 larva which is subsequently to attain the perfect form by a 
 process of metamorphosis, but into a peculiar being, destined 
 to a merely temporary existence, whose function seems to be 
 to give origin to the real Echinoderm by internal gemmation, 
 to obtain and prepare for it the materials of its development, 
 and to carry it to a distance from its fellows, so as to prevent 
 the spots inhabited by the several species from being over- 
 crowded by the accumulation of their progeny. These larval 
 zooids present many points of resemblance to the larvae of 
 certain Annelids ; their bodies have a bilateral not a radial 
 symmetry, the two sides being exactly alike ; each side 
 
 Fig. 305. PENTACRI.XOID LARVA OF COMATULA. Fig. 306. COMATULA. 
 
 is furnished with, a ciliated fringe along the whole or the 
 greater part of ifs length ; and the two fringes are united 
 by an upper and a lower transverse ciliated band, between 
 which the, mouth of the zooid is situated. Although the 
 adult Star-fish and Sand-stars have neither intestinal tube nor 
 
566 DEVELOPMENT OF ECHINODERM AT A AND ENTOZOA. 
 
 anal orifice, their larval zooids, like those of other Echino- 
 derms, always possess both. It is from the side of the intes- 
 tinal canal, that the young Echinoderm is usually budded-off. 
 In some instances it separates itself completely from the 
 zooid, when it has attained a certain stage of development, 
 no part of the latter entering into its composition ; this seems 
 to be the case in the Comatula, which, present this further 
 remarkable feature, that the young Echinoderm at first 
 attaches itself to some fixed object by a footstalk (fig. 305), 
 so as to resemble the fossil Encrinites in every essential 
 particular, but afterwards becomes detached, and henceforth 
 remains free (fig. 306). In the Starfish and Echinus, the only 
 part of the larval zooid which is retained in the Echinoderm, 
 is a portion that is (as it were) pinched-off from the stomach 
 and intestines. In the Holothuria (fig. 67), on the other hand, 
 which has a much closer conformity to the type of the Annelids, 
 a much larger part of the larva is retained in the adult, and 
 the process of development more nearly resembles an ordinary 
 metamorphosis. 
 
 742. Passing-on now to the Articulated series, we find 
 that the developmental history of its lower forms presents 
 phenomena not less remarkable than those already noticed, 
 feecent studies on the propagation of the Entozoa have re- 
 moved many of the difficulties previously felt -in regard to 
 their mode of passage from one animal to another ; by 
 showing that the same creature may exist under two or more 
 forms, which may differ so greatly from each other as appa- 
 rently to belong to separate orders. This has now been 
 ascertained to be the case, for example, in regard to the 
 Tcenia or Tape- worm (fig. 53) and the Cysticercus (fig. 307). 
 The segments of which the body of the Tape-worm is composed, 
 are in reality repetitions (like the medusa-buds of a Hydroid 
 zoophyte) of its generative apparatus ; with this difference, 
 however, that each segment contains both kinds of sexual 
 organs, so that the eggs it contains are fertilized without any 
 extraneous assistance. These segments, when mature, detach 
 themselves one by one ; and being voided from the intestine, 
 fall to the ground, over which the eggs they contain become 
 disseminated by various agencies. Being swallowed with 
 the herbage or the water ingested by herbivorous animals, 
 the eggs are conveyed into their stomachs, where* the little 
 
DEVELOPMENT OF ENTOZOA I OLENIA AND CYSTICERCUS. 567 
 
 embryos escape from them. These embryos, which are small 
 vesicles furnished with six minute hooks or spines, make 
 their way through the walls of the stomach into the substance 
 of other viscera ; and by getting into the current of the cir- 
 culation, they are sometimes carried to remote parts of the 
 body. Nourished by the juices which it absorbs, the vesicle 
 swells : and a head resembling that of the Tape-worm (fig. 
 307, a), begins to bud from its wall into its cavity. In this 
 condition the product of the egg of the Tape-worm has long 
 been known as the Cysticercus (its presence in large numbers 
 in the flesh of the Pig giving to it that diseased appearance 
 which is known as " measly "), without its relationship to its 
 parent being in the least suspected ; and it undergoes no 
 further change until the flesh of the animal it inhabits is 
 devoured by some other, so that the Cysticercus is conveyed 
 into the intestinal canal of the latter. The head which was 
 previously turned into the vesicle, now protrudes from its 
 exterior (fig. 307), and attaches 
 itself to the intestine of its new 
 host by means of the hooks and 
 suckers with which it is furnished 
 (a) ; the vesicle is then cast off, 
 and its place is taken by the 
 series of generative segments suc- 
 cessively budded-forth from the 
 head, which constitutes the body 
 of a new Tape-worm ; and from 
 the ova which these produce there 
 springs a new generation, which 
 
 repeats the same curious cycle. Numerous other parasites 
 present a history that resembles the preceding in 
 all its essential features, whilst varying in details 
 (see ZOOLOGY, 925, 926). 
 
 743. The Trematode Entozoa, of which the Dis- 
 torna (known under the name of fluke) that infests 
 the livers of Sheep is a characteristic example, 
 undergo a yet longer succession of changes j these 
 have been especially studied in a species which 
 infests the Lymnceus, one of the Water-snails, and 
 which is represented in fig. 308. From the egg 
 deposited by this Distoma is produced a long flat-bodied embryo 
 
 Fig. 307. CYSTICERCUS ; a, head 
 greatly enlarged. 
 
568 
 
 DEVELOPMENT OF ENTOZOA. 
 
 (fig. 309) having a sac-like body clothed with cilia and capable 
 of motion ; within the anterior part of which is to be observed 
 an elongated stomach s, whilst the posterior portion of the 
 cavity is occupied by a number of bodies a, having a general 
 resemblance to itself, which are produced from it by a process 
 of internal gemmation. These, when mature, burst forth 
 from the containing cyst, and develope themselves into 
 worms (fig. 310) not very unlike the preceding in general 
 form and organization, though differing from them in some 
 important particulars; and in their turn they produce, by 
 internal gemmation, a fresh brood of bodies much more 
 dissimilar to themselves. These, when set free in due time, 
 
 Fig. 309. 
 GRAND-NURSE OF CEHCARIA. 
 
 Fig. 310. 
 NURSE OF CERCARIA. 
 
 develope themselves into a form which has long been known 
 as the Cercaria (fig. 311), and which has a tadpole-like body 
 with a large sucker a in its middle, a triangular head, a long 
 tail by the motion of which it swims, and various viscera c 
 in its interior. The Cercarise attach themselves by means of 
 their sucker to the bodies of the Lymnseus, and then begin to 
 undergo an important metamorphosis. The tail, being now 
 useless, falls off ; the animal becomes invested with a mucous 
 substance, within which it lies encysted like a chrysalis 
 within its cocoon ; and, on its emergence from this it presents 
 itself as a Distoma, which is ready by the performan.ee of the 
 true generative process, to set in renewed operation the whole 
 
DEVELOPMENT OF ENTOZOA AND ANNELIDA. 569 
 
 succession of these changes. Thus between every act of 
 generation there intervene two sets of gemmations, by which 
 a single embryo may produce a multitude of Cercarise ; and 
 the conversion of the Cercaria into the Distoma involves, in 
 addition, a metamorphosis not less complete than that of 
 Insects. The body (fig. 310) within which the Cercariae are 
 developed, and which is the second remove from the Dis- 
 toma, has been called their "nurse ;" and that (fig. 309) from 
 which the nurses themselves are developed, and which is the 
 first remove from the Distoma, has been called their " grand- 
 nurse." 
 
 744. Among the Annelida, or Worms properly so called, 
 there is considerable variety in the history of development ; 
 some of them, as the Leech and Earth-worm, coming forth 
 from the egg in a nearly perfect state ; whilst in most of the 
 marine worms that state is not attained until long after the 
 embryo has begun to lead an independent life. This embryo, 
 on its first emersion from the egg, very commonly has an oval 
 or roundish body, furnished with one or more bands of cilia, 
 by the agency of which it swims freely in the water ; the 
 body then gradually becomes elongated, and additional bands 
 of cilia make their appearance ; and after a time a mouth and 
 intestinal canal are formed, indications of eyes and of a seg- 
 mental division show themselves, and the cilia disappear, their 
 place being usually taken by bristly appendages. The body is 
 gradually elongated by the production of additional segments, 
 sometimes to the number of several hundred ; each new seg- 
 ment being formed between those which were previously the 
 last and the last but one. Thus the formation of the body of 
 the Worm is really accomplished by a process of continuous 
 gemmation ; and it is therefore the less surprising that some 
 of these worms should be capable of producing independent 
 buds ( 727), which buds, however, not unfrequently resem- 
 ble the segments of the Tape-worm, in containing nothing 
 else than the generative apparatus, save locomotive organs for 
 the purpose of dispersing its products. 
 
 745. It was in the class of INSECTS that the phenomena of 
 metamorphosis were first studied ; and notwithstanding the 
 familiarity of the leading facts of the case, it is desirable to 
 recapitulate them here, for the sake of showing their relation- 
 ship to those we have been already considering. There are 
 
570 METAMORPHOSIS OF INSECTS. 
 
 some Insects (such, as the Grasshopper and the Cricket) which 
 come-forth from the egg in a form so nearly resembling that 
 which they are ultimately to present, that the deficiency of 
 wings is their principal difference. Such are said to undergo 
 an incomplete metamorphosis; the fact being, however, not 
 that these finally attain a less elevated condition than other 
 Insects, but that they make a much nearer approach to it in 
 that part of their embryonic state which they pass within the 
 egg. In the tribes of Beetles, Butterflies, Bees, and Flies, on 
 the other hand, the embryo comes-forth from the egg in the 
 condition of a Worm ; and only acquires either the form or 
 structure of an Insect after a complete metamorphosis, in which 
 every part of its organization undergoes important modifica- 
 tions. The larva, sometimes known as a " maggot," sometimes 
 as a " caterpillar " or " grub," is in many instances completely 
 destitute of legs ; and where it does possess feet by which it 
 can crawl, these are not jointed members, but mere fleshy 
 protuberances. The segments are all nearly equal and similar, 
 both externally and internally; they are never more than 
 thirteen in number, counting the head as one and having 
 been all formed in the first instance by the subdivision of the 
 original yolk-mass, they undergo no subsequent augmentation 
 but that of size. The voracity of the larva is its most extra- 
 ordinary characteristic ; and its increase in bulk is propor- 
 tional, the full-sized larva being estimated in some instances 
 to weigh no less than 72,000 times as much as it did when it 
 came-forth from the egg (141). During this rapid increase, its 
 skin is several times thrown off; a new one being first formed 
 within, this, better adapted to its augmented size. Very little 
 change takes place in the structure of the larva, until after 
 the completion of its growth ; it then ceases to eat, and fre- 
 quently forms some protection to itself, either by spinning a 
 silken cocoon, or by gluing bits of stick, straw, &c. into a 
 case, or it may bury itself in the ground. The last larva-skin 
 hardens into a firm case around the body, which, diminishing 
 in size, shrinks away from its interior. The creature, now 
 known as a chrysalis or pupa, remains for some time without 
 food and apparently inert ; important changes, however, are 
 taking place within its body, which tend towards the forma- 
 tion of the organs of the perfect insect ; and these are pro- 
 duced at the expense of the mass of nutrient material that 
 
METAMORPHOSIS OP INSECTS. 571 
 
 had been stored-up within the body of the larva. "When the 
 development of the wings, legs, &c. has been completed, the 
 Imago or perfect insect bursts-forth from' its pupa-case, and 
 enters upon the life of activity for which it is destined. In 
 this condition alone does it possess proper generative organs ; 
 and the business of rearing the larvae, or of preparing a habi- 
 tation in which they shall find a store of food laid-up for 
 them, seems generally one of the principal objects of its 
 existence. In many instances, indeed, as in the Silkworm, 
 the Imago takes no nourishment whatever, and dies as soon 
 as the generative act has been completed, and the fertilized 
 eggs have been deposited. It is scarcely possible to find a 
 greater contrast than that which exists between the footless 
 Maggot, almost destitute of the power of movement, and 
 having no capacity but that of gorging itself with the nou- 
 rishment provided for its sustenance, and the active Bee, 
 almost constantly on the wing, darting from flower to flower 
 in search of the honied sweets which it is now content to 
 sip, and coming home to toil in the construction of that 
 wonderful edifice which human skill could have scarcely 
 rivalled, certainly not surpassed. And it is evident that the 
 true way of looking at the metamorphoses of Insects, is, to 
 consider the chrysalis condition as a continuation of the de- 
 velopmental process which takes place within the egg ; the 
 larva being adapted to come forth into the world for a time 
 in a very immature condition, that it may obtain for itself 
 such a supply of nutrient material as could not have been 
 stored up within the egg, without adding so greatly to its 
 bulk, as to render impossible that enormous multiplication in 
 the number of eggs which is so characteristic a phenomenon 
 in the history of this class. 
 
 746. A remarkable departure from the method of sexual 
 propagation common among Insects, has long been known to 
 occur in the tribe of Aphides, or "plant-lice" (ZooL. 785); 
 which multiply themselves while yet in a state of develop- 
 ment that may be considered as larval, without any proper 
 generative act. No distinction exists between males and 
 females, but every individual is formed upon the ordinary 
 female type ; and eggs are produced from an ovarium, which 
 are hatched within the body, so that the young come forth 
 alive. These in their turn repeat the same process within a 
 
572 AGAMIC REPRODUCTION OF APHIDES, &C. 
 
 very short time ; arid one viviparous brood succeeds another, 
 so long as adequate warmth and food are supplied. Ten or 
 twelve are thus commonly put-forth in a single season ; and 
 as each brood may consist of a hundred individuals, it may 
 be easily calculated that no fewer than ten thousand million 
 million of Aphides may thus be produced in one summer from 
 a single individual. With the advance of autumn, however, 
 the last brood of larval Aphides, instead of continuing to 
 propagate after this fashion, is developed into the perfect 
 sexual form ; distinct males and females present themselves 
 the true generative process is performed ; and, as its result, 
 eggs are deposited, that are capable of resisting the cold of 
 winter, which would be fatal to the viviparous larvae. From 
 these eggs, larval Aphides are hatched in the spring, which 
 repeat the same curious series of phenomena. Recent in- 
 quiries have shown that this method of propagation is by no 
 means confined to the Aphides, but is common to many other 
 Insects. Thus, it appears that the various species of Cynips 
 or gall-fly (ZooL. 755) are for the most part known only 
 under the female type, and that they can propagate without 
 any male ; the eggs which they deposit producing larvas, which 
 are developed into the likeness of their parents without re- 
 ceiving any fertilization. It may be surmised that, as among 
 Aphides, males make their appearance under certain condi- 
 tions, and that a proper generative act occasionally intervenes 
 between the successive productions of non-sexual broods. 
 Comparing these phenomena with those of the gemmation of 
 Salpa3 ( 728), and with other cases of like nature, it might 
 be supposed that the non-sexual or agamic 1 production of 
 Aphides, Cynipidce, &c. is only another case of the same kind. 
 But there is this peculiarity about it, that the young are 
 produced, not from buds, but from bodies having all the 
 characters of ordinary eggs. And it would seem, from the 
 facts next to be mentioned, that in certain cases the same 
 ovum may develope itself either with or without fertilization j 
 its product in the two cases, however, being different. 
 
 747. Among Bees, Wasps, Ants, and other social Insects, 
 
 the generative process is performed in a very peculiar manner. 
 
 By far the larger proportion of their communities are neuters, 
 
 that is, are incapable of reproduction ; the continuance of the 
 
 1 From o, not ; and 7o ( uoy, marriage. 
 
AGAMIC REPRODUCTION OF BEES. 573 
 
 race being effected by a comparatively small number of indi- 
 viduals. Thus, among the common Hive-Bees, the queen is 
 the only perfect female ; the drones are the males ; and the 
 workers are neuters. But these neuters are undeveloped 
 females, as is shown by the curious fact already mentioned 
 with regard to the capacity of their larvae for being developed 
 under certain conditions into queens ( 716). In the case of 
 the " queen-bee," as in the still more remarkable case of the 
 queen Termes (ZooL. 740), which will lay 80,000 eggs in 
 twenty-four hours, and will continue to do so at the same 
 rate for many weeks, a single generative act suffices for the 
 fertilization of a long succession of ova, though a large pro- 
 portion of these may have been undeveloped at the time of 
 its occurrence ; for the spermatic fluid is stored-up in a little 
 receptacle opening-off from the oviduct of the female, so that 
 a minute portion of it may come into contact with the eggs, 
 as they descend one after the other. But it appears from 
 recent inquiries, that the worker-eggs alone undergo this 
 fertilization, and that the drone-eggs are deposited without 
 receiving it ; and there is strong reason to believe that the 
 very same eggs may be developed either into workers or into 
 drones, according as they do or do not receive the influence 
 of the spermatic fluid. When engaged in depositing her 
 eggs, the queen moves over the cells of the comb, apparently 
 without any order, dropping an egg into each ; and it seems 
 to be determined by the size of the cells (those prepared for 
 the drone-eggs being of larger diameter than those destined 
 for the worker-eggs), whether or not this fertilizing act shall 
 be performed as the eggs descend. It has long been observed, 
 that queen-bees will occasionally deposit eggs without having 
 left the hive for the "nuptial flight" with the male ; and the 
 eggs thus deposited always prove to be drones. It has also 
 been observed that eggs are occasionally laid by workers ; 
 and of these also the products are always drones. Hence, it 
 seems certain that the drones are always developed from 
 agamic or unfertilized eggs ; and it would appear that these 
 very eggs, if fertilized by the male spermatic fluid, would 
 produce workers. This is one of the most curious discoveries 
 yet made in the physiology of Bees; and it remains to be 
 determined how far the same thing is true among other tribes 
 of Insects. 
 
574 
 
 METAMORPHOSIS OP CRUSTACEA. 
 
 748. In the class of Crustacea there is great variety as 
 to the degree of metamorphosis undergone by the young after 
 their emersion from the egg ; for whilst there are several in- 
 stances in which either the mature form, or one closely re- 
 sembling it, is presented from the first, the more common fact 
 is that the early or larval condition is extremely dissimilar to 
 that of the parent, and that a succession of changes has to 
 be gone-through before the latter is attained. This is in no 
 instance more remarkable than in that of the common Crab, 
 whose larva (fig. 48) was long known under the name of 
 Zoea, and was supposed to belong to a type altogether dif- 
 ferent. No instance of agamic reproduction by eggs is 
 as yet known among the higher Crustacea ; but in the 
 Entomostracous division there are probably many examples 
 of it. Thus in the little Daphnia (ZooL. 879), one of the 
 commonest of the " water fleas," the ordinary eggs seem to be 
 always " agamic ; " whilst the eggs which are formed within 
 the peculiar case termed the epkippium, and which seem 
 enabled by its protection to endure a degree of cold that is 
 fatal to the ordinary eggs as well as to the parents, are the 
 products of sexual action. The Cyclops, again, has been 
 found, like the Aphis, to produce many 
 successive broods, which broods repeat the 
 like mode of propagation, without the appear- 
 ance of a male. Among the examples pre- 
 sented by this class, of entire change of form 
 in the progress of development, none are 
 more remarkable than those which are met 
 with among the suctorial tribes, which live 
 as parasites upon the exterior of other 
 animals, especially Fishes. As an example 
 of this change we may refer to the Lerncea 
 (fig. 312), an animal which is not unfre- 
 quently found clinging to the eyes and gills 
 of fish, the anterior part of its body being 
 commonly imbedded in the substance of the 
 part to which it attaches itself. This creature 
 is characterized by the size of its large suc- 
 torial trunk a, and by that of its single pair 
 of legs c, which terminate together in the sucker/; and by the 
 immense development of the abdominal portion of its body d, 
 
 Fig. 312. LERNJEA. 
 
METAMOEPHOSES OF CRUSTACEA AND CIRRHIPEDS. 575 
 
 as well as of the two large egg-capsules e, which, are attached to 
 it. Yet in its larval condition (fig. 313) it is an active little 
 creature, resembling in all essential particulars 
 the larvas of Entomostraca generally ; and from 
 this type the males do not depart nearly so much 
 as the females, the former retaining the general 
 plan of structure, as well as the activity of habits, 
 that prevail among the Entomostraca, whilst the 
 latter lose their instruments of movement, acquir- 
 ing the apparatus of suction and prehension. 
 
 749. A still more remarkable example of meta- 
 morphosis is presented by the tribe of Cirrhipeds 
 ( 102), which, notwithstanding that they were 
 long ranked as Mollusks, on account of their shelly invest- 
 ment and their immovable attachment to solid bodies, are 
 now known to be so closely related to Crustacea, as in 
 the opinion of many naturalists to rank merely as a sub- 
 division of that group. The young, alike of the Lepas or 
 " barnacle," and of the Balanus or " acorn-shell," very much re- 
 semble those of the ordinary Entomostraca ; they possess eyes 
 and several pairs of motor appendages in this state ; and they 
 swim freely through the water, after the manner of water-fleas. 
 Before their last change, they are enclosed in a bivalve cara- 
 pace like that of Cypris ; and they then possess a pair of 
 large four-jointed antenna, which are well furnished with 
 muscles ; and it is by these antenna that the animal finally 
 attaches itself, by means of a peculiar cement which is poured 
 out from ducts running up into them from the body. In the 
 anterior of the recently-attached larva, the young Cirrhiped 
 may' be detected with its valves and cirrhi, like the embryo 
 insect in its pupa-case ; and the carapace and integuments of 
 the larva being thrown-off like a pupa-case, the perfect form 
 is disclosed. In most of these animals, as in many mollusks, 
 the sexes are united in the same individuals ; but when they 
 are separate, the males are minute imperfectly-formed crea- 
 tures, which lead a sort of parasitic life upon the surface of the 
 females ; and in some of the hermaphrodite species " supple- 
 mental males" are also provided. 
 
 750. Among the Rotifera or Wheel- Animalcules, the double 
 mode of reproduction by agamic and by sexual eggs seems to 
 be the ordinary rule. The former may go on at a prodigious 
 
576 DEVELOPMENT OF ROTIFERA AND ARACHNIDA. 
 
 rate ; so that from the known rate of propagation in a 
 Hydatina, it is calculated that nearly seventeen millions might 
 be produced from a single individual within twenty-four days,'' 
 although not more than three or four ova are being brought 
 forward at once. The latter takes place only at certain 
 seasons, most frequently before the winter ; the sexual eggs, 
 in these as in other cases, being endowed with a special power 
 of resisting cold. It is only occasionally, therefore, that 
 males are to be met with ; and it is a remarkable circumstance 
 that they are in many instances destined for so brief an exist- " 
 ence, as not even to be furnished with any digestive apparatus ; 
 so that their development is completed, and their reproduc- 
 tive function performed, at the sole expense of the nutriment 
 which was furnished by the egg. 
 
 751. In the class of Arachnida there is nothing that corre- 
 sponds with the metamorphosis of Insects ; for Spiders and 
 Scorpions attain to the full development of their organs within 
 the egg, so that the young come-forth from it differing in 
 scarcely any respect from their parents, except in size ; and 
 among the Acaridce or " mites," the only important difference 
 lies in the deficiency of one of the four pairs of legs, which is 
 supplied after the first moult. This completion of the process 
 of development within the egg could scarcely take place, but 
 for the large supply of nutriment afforded to the embryo 
 by the yolk. The Arachnida deposit a far smaller number of 
 eggs than Insects do ; and thus, as each egg can be made of 
 much greater size, there is no necessity for that early emersion 
 of the embryo in search of the material for its continued de- 
 velopment, which has been shown to be the real purpose of 
 the larva-life of Insects ( 745). 
 
 752. In the Molluscous series the generative function pre- 
 sents few such peculiarities as have been noticed among Articu- 
 lata. Save in the lowest members of the series, the Tunicata 
 and the Polyzoa ( 114, 115), we have no example of repro- 
 duction by gemmation ; and no instance of reproduction by 
 agamic ova is yet known. The union of the two sexes in the 
 same individual is much more common in this series than 
 among Articulata; and thus the fertilization of the eggs is 
 secured without the exercise of locomotive power. It is 
 remarkable, however, that the embryos even of such Mollusks 
 as are destined to remain almost motionless when they attain 
 
DEVELOPMENT OF MOLLUSCA. 577 
 
 their adult condition, are adapted to move actively through, 
 the water : those of the Polyzoa swimming forth as ciliated 
 gemmules ; those of the Tunicata having a tadpole-like tail, 
 formed by an outgrowth from the " mulberry mass," which 
 propels them by its lateral strokes ; and those of the Gastero- 
 pods having two large ciliated lobes, much resembling the 
 "wheels" of Eotifera, placed one on either side of the mouth. 
 It is further remarkable that all Gasteropods possess a shell 
 in their early condition, whether or not they are to possess 
 one ultimately. Among some of the Pectinibranchiata, which 
 constitute the highest order of this class (ZooL. 985), a very 
 remarkable provision has been observed for carrying-on the 
 development of the embryo to a more advanced stage than is 
 attained in other instances within the egg. In addition to the 
 " germ-yolk," which undergoes the usual processes of fission 
 and of conversion into the mulberry-mass ( 736), by the sub- 
 sequent metamorphoses of which the body of the embryo 
 is formed, we find a quantity of " food-yolk " stored-up with 
 each embryo, which may be likened to the supply that is 
 provided by many Insects for the nutrition of their larvas on 
 their first emersion frpm the egg ( 703) ; this store is greedily 
 devoured by the embryos, as soon as they have a mouth to 
 swallow it and a stomach to hold it ; and it is at the expense 
 of this, that all their later development is carried on. 
 
 753. The class of Cephalopods, in which the sexes are 
 always separate, presents us with some extremely curious 
 provisions for bringing the products of the sperm-cells into 
 contact with the eggs. "Whilst passing through the duct that 
 conveys them forth from the glandular organ within which 
 they are formed, the spermatozoids cluster together ; and these 
 clusters become invested with peculiar casings, which, when 
 immersed in water, have a peculiar movement that enables 
 them to advance through it, and causes them, when they meet 
 with an obstacle, to rupture and set free their contents. In 
 this mode it is that the spermatozoids find their way into the 
 midst of the large grape-like clusters of eggs which have been 
 deposited by the females, and which thus receive the fertili- 
 zing influence of the male. In the Argonaut or "paper-nau- 
 tilus " (ZooL. 962) there is a still more remarkable provision 
 for the same end. All the individuals of this species that 
 form the beautiful paper-like shell from which it derives 
 
 p P 
 
578 KEPRODUCTION OF ARGONAUT. 
 
 its name, are females ; and it now seems clear that the 
 essential purpose of the shell is the protection, not of the 
 animal (which is not in any way attached to it), but of the 
 eggs. The male is of such comparatively insignificant size, 
 that, not being provided with a shell, it was not recognised 
 until recently as belonging to the same species. The sper- 
 matic duct passes through one of its arms, which is extended 
 into a long whip-like appendage j and in this duct are found 
 bundles of spermatozoids, all contained within one casing, 
 which does not possess any self-moving power. At a certain 
 epoch, this arm detaches itself entirely from the body, and 
 moves freely through the water by means of the apparatus of 
 nerves and muscles with which it is endowed, until it comes 
 into contact with a female of its own species, whose eggs are 
 fertilized by its contents which are then set free. In this de- 
 tached condition, the arm was long since observed within the 
 shell of the Argonaut, and was supposed to be a parasitic 
 "Worm ; subsequent inquiry showed it to possess, in its suckers 
 and its nervo-muscular apparatus, the characteristic structure 
 of the Cephalopod, and it was at first supposed to be itself the 
 male, destitute (like the male of some Rotifera, 750) of any 
 nutritive apparatus. Its true history, as now elucidated, is 
 one of the most curiously-exceptional phenomena in the whole 
 of this department of physiology. 
 
 754. Having at last arrived at the Vertebrated series, we 
 shall take a general survey of the history of Development as 
 presented to us in the case most familiar to every one, the 
 formation of the Chick, within the egg of the Bird ; pointing 
 out, as occasion arises, the principal points of difference be- 
 tween the mode in which the process is there carried-on, and 
 the corresponding phenomena presented by other classes. 
 The ovum, as formed within the ovary, has neither " white " 
 nor "shell," but consists of the yolk-bag and its contents 
 alone. Under the influence of domestication, which affords 
 a more constant supply of food and warmth than the Fowl 
 would obtain in its natural condition, a much larger number 
 of eggs are produced by the hen than she would produce in 
 her wild state ; so that, instead of laying four or five at a 
 time, with long intervals between each deposit, she is conti- 
 nually evolving them. An enormous quantity of "food- 
 yolk" is prepared, in addition to the "germ -yolk;" and thus 
 the Bird's egg comes to acquire a far larger size in proportion 
 
DEVELOPMENT OP OVA OP BIRD. 579 
 
 to that of the parent, than we see in any other animals. The 
 appearance of the ovary of the Fowl in process of laying is 
 shown in fig. 314 ; its surface is rendered uneven or knobby 
 
 Fig. 314. OVAHY OF THE FOWL, with ova in various stages of development: 
 
 a, mature ovum within its calyx, which is about to rupture along the non-vascular 
 streak bb; c c, less advanced ova ; d, a calyx from which the ovum has escaped ; 
 e, still younger ova. 
 
 by the protrusion of the ova in various stages of enlargement, 
 those which are most mature (a, c) forming pear-shaped 
 projections which only hang-on by a narrow stalk ; but each 
 ovum is still included within an extension of the fibrous sub- 
 stance of the ovary (fig. 302), termed the calyx, through which 
 blood-vessels are conveyed over its surface ; and when the 
 ovum escapes by the rupture of this, along a line (6) from 
 which the vessels have previously retreated, the calyx remains 
 as an open cup (d). The ovarium of such Mammals as pro- 
 duce several young at once, presents a corresponding appear- 
 ance as each set of ova is approaching maturity ; save that, as 
 the mass of yolk is comparatively small (no "food-yolk" being 
 provided), the ova do not project so much from its surface. 
 In Fishes, on the other hand, we find an enormous number 
 of ova produced at once ; the whole ovarium, when they are 
 approaching maturity, being so crowded with them that its 
 p p 2 
 
580 STRUCTURE OF THE BIRD'S EGG. 
 
 own substance is scarcely distinguishable. Not only thousands 
 but tens of thousands of eggs are often produced by a single 
 individual, their aggregate forming what is known as the 
 "hard roe;" whilst the "soft roe" or "milt" is the corre- 
 sponding mass of sperm-cells produced by the male. 
 
 755. After the ovum of the Bird has quitted the ovarium, 
 and is passing through the oviduct towards its outlet, it 
 receives layer after layer of albumen poured out in a viscid 
 condition from the lining membrane of the oviduct, forming 
 the "white" of the egg (fig. 315, g) ; and this is inclosed in 
 
 Fig. 315. SECTION OF FOWL'S EGG : 
 
 o, cicatricula; b, yolk-bag; c, membrane lining shell; d, attachment of chalazag; 
 e, chalazse ; /, air-space ; g, albumen. 
 
 a double membrane composed of a network of fibres, which 
 is formed by the consolidation of a, plastic exudation ( 391), 
 poured out after the albuminous exudation has been com- 
 pleted. The outer layer of this membrane is consolidated by 
 the deposit of calcareous particles in the interspaces of its 
 fibrous matting, so as to form the " shell " of the egg ; an 
 arrangement that gives the necessary protection, without cut- 
 ting-off the contents of the shell from that communication 
 with the atmosphere which is requisite for the development 
 of the embryo. The inner layer, which forms a lining to the 
 shell, separates into two laminae at the large end of the egg ; 
 and, inclosed between these, there is a bubble of air (/), which 
 serves to give the young bird, just before it is hatched, the 
 
EARLIEST STAGES OF EMBRYONIC DEVELOPMENT. 581 
 
 power of filling its lungs with air. The yolk-bag floats within 
 the albumen, and always tends to ta,ke the highest place, being 
 the lighter of the two ; but it is kept nearly to one place by 
 two cords (e, e) termed the chalazce, which seem formed of 
 peculiarly viscid albumen, and connect the yolk-bag with the 
 lining membrane at the two ends of the shell (d, d). In this 
 manner the yolk-bag is always kept at the part of the shell 
 where it can most favourably receive the warmth imparted to 
 it by the mother ; and the cicatricula or germ-spot (which is 
 the mass of cells first developed from the germ-yolk) is made, 
 by a similar contrivance, always to rise to the highest point. 
 In the eggs of Fishes there is no additional albumen ; and 
 in those of Frogs the albumen is common to the general mass 
 of ova, constituting the peculiar "gelatinous envelope," which 
 forms long necklace-like strings of " spawn," within which the 
 black yolk-bags are disposed at tolerably regular intervals. In 
 Mammals, each ovum receives a separate investment of a jelly- 
 like substance in its passage along the oviduct into the uterus ; 
 and around this there is formed a fibrous membrane termed 
 the Chorion, which is destined to take a very important share 
 in the subsequent nutrition of the embryo ( 761). 
 
 756. In the eggs of Frogs, as in those of Mammals, the 
 whole of the yolk undergoes the process of segmentation 
 already described ( 736), and takes a share in the formation 
 of the " mulberry mass." But in the eggs of Fishes, Birds, 
 and the higher Reptiles, this process is limited to that small 
 portion of the yolk which is distinguished as the germ-yolk ; 
 and the formation of the germinal membrane takes place after 
 a different fashion. The mass of cells that immediately results 
 from segmentation, flattens itself out on the surface of the 
 yolk, forming the minute semi-opaque whitish spot, which is 
 known as the cicatricula, germ-spot, or "tread;" and by a 
 further extension it constitutes the "germinal membrane," 
 which gradually spreads itself over the food-yolk ; at the same 
 time dividing itself into two layers, between which a third 
 is afterwards interposed. Thus the " germinal membrane," 
 which may be compared to the seed-leaves or cotyledons of 
 Plants, forms a sort of temporary stomach round the mass of 
 nutriment prepared for the sustenance of the embryo ; the 
 whole of which nutriment, as will be presently seen, is absorbed 
 into the body of the embryo through its instrumentality. 
 
582 PRIMITIVE TRACE : DEVELOPMENT OF VERTEBRAL COLUMN. 
 
 757. The first indication of the permanent fabric in all 
 Vertebrated animals, consists of a delicate longitudinal streak, 
 termed the "primitive trace" (fig. 316, 6), that is observable 
 in the midst of a pellucid area, which is again surrounded by 
 
 a ring of more opaque as- 
 pect (c). This primitive trace 
 is the foundation of the 
 Vertebral Column. It is in 
 the first instance a mere 
 furrow in the outer layer of 
 the germinal membrane ; 
 but the sides of this furrow, 
 known as the dorsal lamince, 
 rise up and arch-over, so as 
 gradually to meet and con- 
 vert the furrow into a canal. 
 The meeting first takes place 
 in what is afterwards to be- 
 come the middle of the back; 
 and here we find the first 
 distinct rudiments of the 
 vertebral column, in the 
 condition of a series of small square plates (figs. 317, c, c, 
 325, I, I) on either side, which are the representatives of the 
 arches of as many vertebras. The furrow widens-out in the 
 situation of the head, so as to form the receptacle (d) for the 
 series of large ganglionic masses that is to constitute the brain 
 (fig. 323, d, e,f) ; and though its sides do not there close-in 
 for some time longer, it receives a special hood-like covering 
 from a peculiar fold of the germinal membrane, the edge of 
 which is seen at e, fig. 317. The cells, of which the parts of these 
 lamines that bound the bottom and sides of this furrow are 
 composed, appear to furnish the rudiments of the nervous 
 centres that are afterwards to occupy the canal ; but beneath 
 its deepest part there lies a continuous rod of peculiar nucleated 
 cells (/), the chorda dorsalis, which marks-out the situation 
 afterwards to be taken by the bodies of the vertebrae. This 
 remains the only representative of the vertebral column in 
 the Lamprey and other Fishes of a low grade, the develop- 
 ment of whose bony skeleton is checked so early that it never 
 advances beyond this simple embryonic type ( 53). 
 
 Fig. 316. YOLK-BAG OF FOWL'S EGG, 
 after twelve hours' incubation : 
 
 a, yolk; b, primitive trace surrounded by 
 pellucid area; c, more opaque ring, the 
 commencement of the vascular area. 
 
DEVELOPMENT OF CIRCULATING APPARATUS. 583 
 
 758. During the progress of this change, another very im- 
 portant one is taking place, which is destined for the nourish- 
 ment of the embryo during its further development. This 
 is the formation of vessels in the substance of the germinal 
 membrane ; which vessels serve to take up the nourishment 
 supplied by the yolk, and to convey it through the tissues 
 of the embryo. The space over which these vessels spread 
 
 Fig. 317. MORE ADVANCED EM- 
 BRYO OF FOWL, much enlarged : 
 
 a, pellucid area; b b, dorsal 
 laminae ; c c, rudiments of ver- 
 tebral arches; d, dilatation for 
 brain ; e e, cephalic hood ; /, 
 chorda dorsalis. 
 
 Fig. 318. YOLK-BAG OF FOWL'S EGG, at 
 the beginning of the third day of incu- 
 bation : 
 
 a, yolk; 6, embryo; c c, arteries of vascu- 
 lar area; d d, veins ; e e, terminal sinus. 
 
 themselves, is called the Vascular Area ; it makes its ap- 
 pearance during the second day of incubation in the Fowl's egg 
 (fig. 316, c), and soon spreads itself over the surface of the yolk 
 (figs. 318, 319). Islets or points of a dark colour first appear 
 in it ; these unite in rows ; and at last continuous vessels are 
 formed. The heart makes its appearance at the twenty-seventh 
 hour of incubation, as a simple dilatation of the trunk into 
 which the blood-vessels unite .(fig. 32Q, h). Its wall is at 
 
584 DEVELOPMENT OF VESSELS AND DIGESTIVE CAVITY. 
 
 first formed by a layer of cells ; and no muscular structure is 
 seen in it, until after its regular pulsations have commenced. 
 It is in these vessels that the first blood is formed ; and the 
 same process appears to be continued through the whole period 
 of incubation, the yolk being progressively converted into 
 
 blood, and this blood being 
 conveyed by the great 
 trunks which collect it into 
 the body of the embryo. 
 Looking at the yolk-bag 
 in the light of a temporary 
 stomach, its vessels may 
 be likened to those which 
 take so large a share in the 
 act of absorption from the 
 digestive cavity of the 
 adult ( 218). 
 
 759. During the same 
 early period of incubation, 
 the layers of the germinal 
 membrane begin to exhibit 
 various folds, which after- 
 wards serve for the forma- 
 tion of the several cavities 
 
 Fig. 319. EMBRYO OF BIRD, WITH THE of the body. The points of 
 VESSELS, i, of the vascular area, after four ., -I--IT v i j/u 
 
 days' incubation. it which lie beyond the 
 
 extremities, and which 
 
 spread-out from the sides of the embryo, are double d-in so 
 as to make a depression upon the yolk ; and their folded edges 
 
 gradually approach one an- 
 other under the abdomen, 
 which lies next the in- 
 terior of the egg. In this 
 manner is formed the per- 
 manent digestive cavity ; 
 which is first a simple 
 pouch communicating with 
 the yolk-bag, by a wide 
 opening, as seen at s, fig. 320 ; but which is gradually separated 
 from it by the narrowing of this orifice (fig. 322), the connecting 
 portion being elongated into a duct (fig. 321, b). Thus we may 
 
 Fig. 320. DIAGRAM OF THE FORMATION 
 OF THE DIGESTIVE CAVITY: 
 
 e, embryo ; /, g, layers of germinal mem- 
 brane ; h, heart; s, stomach. 
 
DEVELOPMENT OF DIGESTIVE CAVITY AND ALLANTOIS. 585 
 
 say that the digestive cavity in Vertebrata is formed by the 
 pinching-off (as it were) of a small portion of the general sac 
 of the yolk. In the Mammalia, the remainder of the yolk-bag- 
 is completely separated from this by the closure of its narrow 
 orifice, and is afterwards thrown off; so that only a very 
 small portion of the germinal membrane is received into the 
 permanent structure. But in 
 Birds and other oviparous 
 animals, the whole of the yolk- 
 bag is ultimately drawn into 
 the abdomen of the embryo ; 
 the former gradually shrinking 
 as its contents are exhausted ; 
 and the latter enlarging, so as 
 to receive it as a little pouch 
 or appendage. In Fishes, the 
 hatching of the egg very com- 
 monly takes place before this 
 process has been completed ; 
 so that the little Fish swims 
 about with the yolk-bag hang- 
 ing from its body. 
 
 760. The embryo, like the 
 adult, has need of Respiration ; 
 partly that its own heat may be 
 kept up; and partly that the 
 carbonic acid liberated in the various processes of nutrition, 
 may be set free. Owing to the peculiar structure of the 
 membrane covering the albumen and forming the basis of the 
 
 Fig. 321. A, MORE ADVANCED EM- 
 BRYO OF FOWL, connected only by 
 the vitelline duct b, with the yolk- 
 bag a a, over which are distributed 
 the blood-vessels cc; B, early form 
 of the anterior extremity a, and of 
 the posterior extremity b. 
 
 Fig 322. DIAGRAM OF THE FORMATION OF THE ALLANTOIS, . 
 (The other references as in fig. 320.) 
 
 shell ( 755), the outer air is enabled to gain access to the 
 interior of the egg; and at first its action upon the blood, 
 
586 RESPIRATION OF THE EMBRYO : ALLANTOIS. 
 
 whilst circulating in the vascular area, is sufficient. In Fishes, 
 no further provision is made for this process ; since, by the 
 time it would be required, the egg is hatched ; the young 
 animal comes forth into the medium it is permanently to 
 inhabit, its own gills come into play, and the air contained in 
 the water can act directly upon the blood circulating in the 
 vascular area. But in the higher oviparous animals, whose 
 
 development proceeds further 
 before they leave the egg, a 
 special provision is made for 
 this purpose. On the third day 
 of incubation, in the Fowl, a 
 bag termed the Allantois (fig. 
 322, i) begins to sprout (as it 
 were) from the lower end of the 
 body ; and gradually enlarges 
 (fig. 323), passing round the 
 y embryo, and beneath its en- 
 Fig. 323.-EMBKTO OP Fowz.Twith the vel P in g membranes, so as 
 
 Allantois, a, over which ramify the almost Completely to inclose 
 
 umbilical vessels, b ; at c is seen the + npi a 11T .fonp n f fhie 
 
 indication of the place of the external lt; - 
 
 ear ; d, e, f, rudiments of the cere- bag is plentifully Supplied 
 bellum, optic ganglia, and cerebrum -ii. ui j i r it 
 
 respectively. with blood-vessels from the 
 
 embryo ; and as one side of 
 
 it lies in close proximity with the membrane of the shell, 
 it is very advantageously situated for receiving the in- 
 fluence of the air. It thus serves as the temporary respi- 
 ratory apparatus of the Chick, up to the time when it is pre- 
 paring to quit the egg. 1 There is reason to believe that the 
 bird then receives air into its lungs, from the air-space formerly 
 mentioned ( 755), which increases in size as the contents of 
 the egg diminish in bulk by the evaporation of their watery 
 part. By the increased vigour which it thus acquires, it is 
 enabled to perform the movements requisite for extricating 
 
 1 If the respiration of the embryo be prevented by rendering the 
 shell impermeable to air, its development is completely checked. No 
 means of accomplishing this is so effectual, as smearing the shell with 
 oil or grease of any kind. Hence the effect of the well-known practice 
 of buttering the surface of the egg, in preventing the chick from being 
 reared ; and the same operation, if performed when the egg is quite 
 fresh, will preserve it for some time fit for eating, its decomposition 
 being prevented by the complete exclusion of the air. 
 
DEVELOPMENT AND NUTKITION OF MAMMALIAN EMBRYO. 587 
 
 itself from its shell ; which it does entirely by its own exer- 
 tions. When it thus becomes independent of the allantois, 
 the circulation through the latter diminishes ; and almost the 
 whole sac is separated from the body by the contraction of 
 the connecting foot-stalk, which at last gives way. 
 
 761. The formation of the yolk-bag and the allantois takes 
 place in Mammals (fig. 324) almost exactly on the same plan 
 
 Fig. 324. EMBRYO OF MOLE : 
 
 A, entire ; B, with the abdomen laid open : a, chorion ; b, footstalk of allantois and 
 umbilical vessels; c, yolk-bag; d, vitelline duct; e, upper portion of intestinal 
 canal; /.lower portion; ff, eye; h, indication of branchial arches; f, auditory 
 vesicle; k, anterior, and I, posterior extremity, m, n, Wolffian bodies or rudi- 
 mentary kidneys ; o, rudimentary lung ; p, trachea ; q, ventricle of heart ; r, 
 atrium of heart. 
 
 as in Birds j but on account of the absence of food-yolk, these 
 sacs are comparatively small ; and the function of both is su- 
 perseded, at an early period of the development of the embryo, 
 by a new and remarkable contrivance. The ovum, in passing 
 through the oviduct, has been already stated to receive a new 
 envelope, analogous to that which forms the membrane of the 
 shell in Birds ; this is termed the Chorion. It is then received 
 into the cavity of the Uterus, a receptacle within which it is 
 delayed for a considerable period, and continually supplied 
 with nourishment drawn from the blood of the parent. From 
 
588 
 
 FORMATION AND USES OP PLACENTA. 
 
 the whole surface of the chorion, a number of little tufts shoot 
 out (fig. 325), which come into contact with the lining mem- 
 brane of the uterus ; and this is furnished with a multitude 
 of glandular follicles, which secrete a nutritious fluid that is 
 absorbed by the tufts of the chorion, and by them communi- 
 cated to the embryo. When the allantois is formed, it serves 
 to carry the blood-vessels of the embryo to the inner surface 
 of one part of the chorion ; and they shoot through this, so as 
 
 Fig. 325. INTERIOR OF HUMAN UTERUS at the seventh week of Pregnancy : 
 b, outlet of the uterus, of which the walls c, c, c, c, laid open by incision, are turned 
 back to display its contents; d d, its lining membrane ; g, tufted surface of the 
 chorion ; g%, its internal aspect ; h, h, amnion ; i, yolk-bag ; k, umbilical cord ; 
 /, embryo. 
 
 to dip-down, as it were, into large expanded vessels that extend 
 outwards from the walls of the uterus. In this manner is 
 formed, in all the higher Mammalia, the important organ 
 termed the Placenta; which essentially consists of the ramifi- 
 cations of the foetal blood-vessels contained in the Umbilical 
 Cord or " navel-string," ensheathed by prolongations of the 
 large vessels of the maternal uterus. In the Marsupials and 
 Monotremes (ZOOLOGY, 309 320), no placenta is ever 
 formed, the embryo coming into the world in a stage scarcely 
 more advanced than that represented in fig. 325. In either 
 case, the vessels of the embryo are enabled to absorb from 
 the blood contained in those of the parent, through the thin 
 
DEVELOPMENT OF CIRCULATING APPARATUS. 
 
 589 
 
 walls of both, the materials requisite for its growth j but there 
 is no direct communication between the two. The same means 
 serve for the aeration of the blood of the embryo ; for this, 
 being brought from its body in the venous condition, is exposed 
 to the influence of the arterial blood of the parent, through 
 the thin walls of its vessels, just as the venous blood of aquatic 
 animals is aerated in their gill-tufts, and passes back to the 
 embryo in the arterial condition, having imparted its carbonic 
 acid to the blood of the parent, 
 and received from it oxygen. 
 Thus all but the very early stages 
 of development are performed in 
 Mammals, by means of which we 
 scarcely find a trace in Oviparous 
 animals ; yet the ova of both are 
 originally formed on the same plan, 
 and the first changes which they 
 undergo are exactly analogous. 
 
 762. It would not be consistent 
 either with the design or with the 
 limits of this work, to enter in 
 much detail into the considera- 
 tion of the processes of develop- 
 ment, although they present many 
 points of the highest interest. 
 The general history of the evolu- 
 tion of the Circulating apparatus 
 and of the Nervous centres may, 
 however, be noticed, as character- 
 istic examples of the mode in 
 which the evolution of the several 
 organs of the body takes place. 
 The Heart, in Man and other 
 Mammals, as in the Bird ( 758), 
 is at first a simple tube, resembling 
 the pulsatile trunk that remains as 
 
 the sole organ of impulsion in the lowest forms of circulating 
 apparatus. After a time this tube is doubled upon itself, and 
 two cavities are formed, an auricle and a ventricle ; in this con- 
 dition, it strongly resembles the heart of the Fish ( 286). The 
 circulation too is, at an early period, that of the Fish ; for the 
 
 Fig. 326. EMBRYO OP THE FOWL, 
 from the Ovum shown in fig. 
 318, greatly enlarged : 
 
 a, b, folds of irerminal membrane, 
 enveloping head and tail; c, la- 
 teral folds; d, e, rudiments of 
 optic ganglia and cerebrum ; f, 
 heart ; g, dilated termination of 
 venous trunk, forming atrium of 
 heart; A, aorta; 1, 2, 3, 4, bran- 
 chial arches; i, i, vessels of 
 vascular area; k, k, dorsal lami- 
 nse; I, I, rudiments of vertebral 
 arches. 
 
590 DEVELOPMENT OF CIRCULATING APPARATUS. 
 
 arterial trunk that springs from the ventricle, divides into a < 
 set of arches on each side (fig. 326), which closely resemble 
 the branchial arches of Fishes and Tadpoles. Although 
 no gills are present, yet there is a series of clefts on 
 each side of the neck, passing through to the pharynx, 
 which are analogous to the branchial apertures of the 
 cartilaginous Fishes ( 317). After a time, however, the 
 auricle and ventricle of the heart are each divided by a 
 vertical partition, so that four cavities are formed, out of 
 the two which previously existed ; and at the same period, 
 the arrangement of the vessels undergoes a change, by the 
 division of some trunks, and the obliteration of others, so 
 that they gradually assume the distribution which is charac- 
 teristic of warm-blooded animals ( 281). But even up to 
 the time of the birth of the Mammalia, there is a communica- 
 tion between the two sides of the heart, and between the 
 pulmonary and systemic vessels, which is closely analogous to 
 that which permanently exists in the Crocodile ( 283). 
 
 763. Again, the space within the head of the embryo, into 
 which the vertebral canal widens-out ( 757), is occupied in 
 the first instance by a succession of vesicles or bags, arranged 
 in a linear series (fig. 323, d, e, /) ; each of which is the rudi- 
 ment of one of those principal ganglionic masses, that col- 
 lectively make-up the brain of the Fish ( 453), in which 
 they present a very similar aspect (fig. 192). As in many 
 Fishes, too, the Cerebrum is inferior in size to the Optic 
 ganglia, and only comes to surpass and finally (as it were) to 
 overpower them ( 455, 456) in the later periods of embry- 
 onic development. 
 
 764. The true representation of these and similar facts is 
 not, as was maintained when they were first brought into view, 
 that the several organs of the higher animals go through a 
 series of forms which remain permanent in the lower; but 
 that the development of all animals formed upon the same 
 general plan commences in the same manner, their special 
 differences manifesting themselves as development proceeds. 
 Thus, as we have seen, the foundation of the Vertebral column 
 is laid in all Vertebrata in precisely the same method ( 757) ; 
 in some of the lowest Fishes, the evolution of this structure 
 is checked at so early a period that it never advances beyond 
 the embryonic type ( 53) ; but the fully-formed spine has a 
 
VON BAER'S LAW OP DEVELOPMENT. 591 
 
 characteristically-different structure in each of the classes of 
 Vertebrata, which is not presented at any period in the history 
 of the others. So, the evolution of the Circulating apparatus 
 commences in all Vertebrata upon the same original plan ; and 
 from this plan there is but little departure in the Fish ; but 
 the circulating apparatus of the early Human embryo, how- 
 ever like that of the adult Fish, differs from it in this essential 
 particular, the absence of gill-tufts receiving capillary 
 vessels from the branchial arches ( 286). The like is true 
 in regard to the Nervous centres ; for although the earliest 
 condition of the Human brain very closely resembles that of 
 the brain of the foetal Fish, it never bears any exact analogy 
 to that of the adult Fish. 
 
 765. Hence the principal facts of Organic Development 
 admit of being stated in this general formula, which we owe 
 to the sagacity of Von Baer, that the more special forms of 
 structure arise progressively out of the more general, a prin- 
 ciple than which there is none more comprehensive or more 
 important in the whole range of Physiological Science. 
 
 766. THE Unity of Plan which is visible through the whole 
 Animal Kingdom, is nowhere more remarkable than in the 
 function of which an outline has now been given. We have 
 seen that, however apparently different, the essential character 
 of the Eeproductive process is the same in the highest Animal 
 as in the lowest. It has been shown that the development of 
 the highly-organized body of Man, though it is to serve as the 
 instrument of those exalted faculties, by the right employment 
 of which he is made " but a little lower than the Angels," 
 commences from the same starting-point with that of the 
 meanest creature living : for even Man, in all the pride of his 
 philosophy, and all the splendour of his luxury, was once but a 
 single cell, undistinguishable, by all human means of observa- 
 tion, from that which constitutes the entire fabric of the 
 simplest Protozoon. And when the Physiologist is inclined 
 to dwell unduly upon his capacity for penetrating the secrets 
 of Nature, it may be salutary for him to reflect that, even 
 when he has attained the furthest limits of his Science, by- 
 advancing to those general principles which tend to place it 
 
592 CONCLUSION. 
 
 on the elevation which others have already reached, he yet 
 knows nothing of those wondrous operations, which are the 
 essential parts of every one of those complicated functions by 
 which the life of the body is sustained. Why one cell should 
 absorb, why another, that seems exactly to resemble it, 
 should assimilate, why a third should secrete, why a fourth 
 should prepare the reproductive germs, and why, of the two 
 germs that seem exactly similar, one should be developed into 
 the simplest Zoophyte, and another into the complex fabric of 
 Man, are questions that Physiology is not likely ever to 
 answer. All our science is but the investigation of the mode 
 or plan on which the Creator acts ; the Power which operates' 
 is Infinite, and therefore inscrutable to our limited eompre- 1 
 hension. But when Man shall have passed through this 
 embryo state, and shall have undergone that metamorphosis 
 by which everything whose purpose was temporary shall be 
 thrown aside, and his permanent or immortal essence shall 
 alone remain, then, we are encouraged to believe, his finite 
 mind shall be raised more nearly to the character of the Infi- 
 nite, all his highest aspirations shall be gratified, and never- 
 ending sources of delightful contemplation shall be continually 
 opening to his view. The Philosopher who has attained the 
 highest summit of mortal wisdom, is he who, if he use his 
 mind aright, has the clearest perception of the limits of human 
 knowledge, and the most earnest desires for the lifting of the 
 veil that separates him from the Unseen. He, then, has the 
 strongest motives for that humility of spirit and purity of 
 heart, without which, we are assured, none shall see God. 
 
INDEX. 
 
 N.B. The numbers refer to the paragraphs. 
 
 A. 
 
 ABERRATION, chromatic, 548, 549; 
 spherical, 546, 547; avoided in the 
 eye, 549. 
 
 ABSORPTION, nutritive, 171 ; in Verte- 
 brata, through lacteals, 217 ; through 
 blood-vessels, 218: in Invertebrata, 
 225; in non-vascular animals, 225; 
 interstitial, performed by lymphatics, 
 219; from general surface, by veins, 
 220. 
 
 Abstinence, power of, 140, 141, 152. 
 
 ACALEPHJE, 120 ; luminousness of, 395 ; 
 (see Medusa). 
 
 Acephalous Mollusca, 111, 113. 
 
 Actinophrys, 130. 
 
 Adipose tissue, 46. 
 
 Aeration of blood, 253, 303. 
 
 Aerating surface, 311, 312. 
 
 Agamic Reproduction, of Insects, 746, 
 747 ; of Entomostraca, 748. 
 
 Air, atmospheric, composition of, 300; 
 need of in animals, 207; change of, by 
 respiration, 300 302; effects of de- 
 privation of, 297, 298, 310, 338. 
 
 bladder of Fishes, 324. 
 
 cells of Birds, 80, 326, 327. 
 
 tubes and sacs of Insects, 320 322. 
 
 Albinos, 545. 
 
 Albumen, chemical composition of, 13, 
 14; use of in blood, 240. 
 
 Albuminous principles of food, 153; des- 
 tination of, 158164. 
 
 Aliment, definition of, 132 (see Food). 
 
 Allantois, 760, 761. 
 
 Ammonite, 111. 
 
 Amphibia, 85, 87; circulation in, 287 
 289 ; (see Batrachia and Frog}. 
 
 Amphioxus, 53. 
 
 Anabas, respiratory organs of, 318. 
 
 Anastomosis, of arteries, 262 265 ; of 
 nerves, 428. 
 
 Anemone, Sea, 126, 127; reproduction 
 of, 726, 739. 
 
 Aneurism, 263. 
 
 Animal Life, functions of, 6, 426, 427 ; 
 nervous system of, 461. 
 
 Animals, distinctive characters of, 6 
 12; dependent on Plants for support, 
 144--147. 
 
 Animalcules, structure of, 13, 136, 137; 
 heat produced by, 404 ; movements of, 
 577; reproduction of, 135, 725. 
 
 ANNELIDA, general characters of, 104; 
 circulation in, 294 ; respiration of, 
 314; luminosity of, 396; nervous sys- 
 tem of, 440; muscular motion of, 597; 
 reproduction of, 727, 744. 
 
 Annular ligament, of wrist, 641 ; of 
 ancle, 648. 
 
 Anobium, sound of, 677. 
 
 Ant-eaters, 186. 
 
 Antennae of Insects, uses of, 498. 
 
 Anthpzoa, 126; development of, 736. 
 
 Ant-lion, instinct of, 697. 
 
 Aorta, 258 ; valves of, 273. 
 
 Aphides, agamic reproduction of, 727, 
 735, 746. 
 
 Aplysia, nervous system of, 438. 
 
 Aquatic animals, respiration in, 298. 
 
 Aqueous humor, 536. 
 
 ARACHNIDA, general characters of, 106; 
 circulation in, 293; respiration in, 
 323 ; development of, 751. 
 
 Arachnoid membrane, 43. 
 
 Area pellucida, 757. 
 
 vasculosa, 758. 
 
 Areas, comparative, of arteries, 247. 
 
 Arenicola, 314. 
 
 Areolar tissue, structure and properties 
 of, 2427. 
 
 Argonaut, reproduction of, 753. 
 
 Arm, bones and muscles of, 638 640. 
 
 Arteries, 246; comparative areas of, 247; 
 walls of, 248; pressure of blood in, 
 249 ; protection given to, 250 ; division 
 of into capillaries, 251 ; general distri- 
 bution of, in Man, 258, 259 ; in Bird, 
 260, 261 ; peculiarities of distribution f, 
 262, 264, 265 ; flow of blood through, 
 274276 ; wounds of, 277. 
 
 ARTICULATA, general structure of, 93 
 96; skeleton of, 597600; nervous 
 system of, 440447; eyes of, 573575. 
 
 Articulate sounds, 687 691. 
 Q Q 
 
594 
 
 INDEX. 
 
 Articulation of bones, different modes 
 of, 600604. 
 
 Arytenoid cartilages, 680, 681. 
 
 Ascidia, 114; nervous system of, 435. 
 
 Asphyxia, 280, 338 ; treatment of, 339. 
 
 Assimilating glands, 223, 224. 
 
 Astraea, fission of, 726. 
 
 Atlas-vertebra, 632. 
 
 Attention, effect of, on sense of touch, 
 494; on hearing, 525; on sight, 555; 
 on mental power, 721. 
 
 Auditory nerve, 512. 
 
 Auricles of heart, 257 ; action of, 270. 
 
 Automatic movements, 479. 
 
 Axis vertebra, 632. 
 
 Axolotl, 87. 
 
 Azotized principles of food, 153, 154; 
 destination of, 158165; effects of ex- 
 cess of, 348. 
 
 Azotized compounds, excretion of, 346 
 348. 
 
 B. 
 
 Balancing of body, 480, 481. 
 
 Balanus, 102; development of, 749. 
 
 Ball-and-socket joint, 603. 
 
 Barnacle, 102; development of, 749. 
 
 Basement-membrane, 31. 
 
 Bat, wings of, 669; ear of, 515; hyber- 
 nation of, 407 ; peculiar sensibility of, 
 494. 
 
 Satrachia, 86, 87; reparative power of, 
 390; development of egg of, 756. 
 
 Baya, nest of, 705. 
 
 Beat of heart, 269, 271. 
 
 Beaumont, Dr., his experiments upon 
 gastric fluid, 207, 208. 
 
 Beaver, operations of, 706 708. 
 
 Bees, formation of wax by, 156; heat 
 produced by, 410, 41 ]; use of antennae 
 in, 498; sounds of, 678; instincts of, 
 712716; reproduction of, 747. 
 
 Beetle, digestive apparatus of, 202. 
 
 Bell, Sir C., his discoveries, 429, 451. 
 
 Bicuspid teeth, 187. 
 
 Bile, chemical characters of, 364 ; use of, 
 in digestion, 213; formed from venous 
 blood, 266, 366 ; purposes of excretion 
 of, 346351, 365 ; effect of suspension 
 of, 351. 
 
 Bilious complaints, 350. 
 
 Binary subdivision of cells, 33. 
 
 BIRDS, general characters of, 78 80; 
 digestive apparatus of, 200 ; digestive 
 powers of, 201 ; blood-discs in, 230 ; 
 arterial system in, 260; circulation in, 
 281, 282; respiration in, 326, 327; heat 
 of, 407, 413; nervous system of, 455 ; 
 intelligence of, 484, 717; wings of, 
 669; flight of, 672; larynx of, 685; 
 nests of, 704, 705; social instinct in, 
 710; ovarium of, 754; structure of 
 eggs of, 755 ; development of embryo 
 of, 757760. 
 
 Bladder, gall, 362 ; urinary, 362. 
 
 Blastema. 33; production of cells from, 34. 
 
 Blood of Vertebrata, general characters 
 of, 226; arterial and venous, 227; ge- 
 neral purposes of, 228, 239; composed 
 of liquor sanguinis and corpuscles, 
 229; coagulation of, 236 239; serum 
 of, 238; buffy coat of, 236 ; flow of, 
 means of checking, 277, 278; changes 
 of, in disease, 233 ; assimilating power 
 of, 242. 
 
 Blood of Invertebrata, character of, 235. 
 
 Blood Corpuscles, colourless, 35, 234. 
 
 red, 35, 229 234; ac- 
 tion of endosmose on, 231 ; composi- 
 tion of, 232 ; variety of proportion of, 
 in different animals, 233; functions 
 of, 235, 241 ; change of, in respiration, 
 310 ; connexion of, with heat libe- 
 rated, 413. 
 
 Blood-vessels, 244; formation of in new 
 tissues, 393; in embryo, 758. 
 
 Bombylius, sound of, 676. 
 
 Bone, structure of, 49, 50; chemical 
 composition of, 51 ; formation of, from 
 cartilage, 52, 53 ; reparation of, 390. 
 
 Bones of head, 617 693; of ear, 516; of 
 spine, 626632; of trunk, 623; of 
 upper extremity, 635 644; of lower 
 extremity, 645648. 
 
 Bowerbankia, 115, 356. 
 
 Brain of Vertebrata, 72; not the only 
 source of action, 465, 466 ; comparative 
 size of, 718,719; development of, 763; 
 (see Cerebrum and Cerebellum). 
 
 Branchial arches, 286289 ; of embryo, 
 762. 
 
 Bronchial tubes, 326, 328. 
 
 Bri/ozoa, 115; (see Polyzod). 
 
 Buds, reproduction by (see Gemmation). 
 
 Buffy coat of blood, 236. 
 
 Bulk required in food, 205. 
 
 Butter, 377. 
 
 C. 
 
 Caddice-worms, 701. 
 
 Calamary, 111. 
 
 Camel, stomach of, 198; skeleton of, 644. 
 
 Campanularia, 124. 
 
 Cancelli of bone, 49 ; formation of, 52. 
 
 Canine teeth, 181, 183. 
 
 Cantering, 660. 
 
 Capillary vessels, 251; movement of 
 blood in, 275, 280. 
 
 Carapace of Turtles, 83. 
 
 Carbon, modes of excretion of, 345 351, 
 365; combustion of, 157, 305, 306, 
 412, 413.. 
 
 Carbonic acid, set free in respiration, 
 301, 303; mode of its production, 305, 
 306 ; quantity of, proportional to ac- 
 tivity of animal, 307309 ; amount of, 
 disengaged by Man, 334 ; deterioration 
 of atmosphere by, 335338. 
 
INDEX. 
 
 595 
 
 Carnivorous tribes of animals, 148 151 ; 
 nutrition of, 161; teeth of, 179, 
 182. 
 
 Carpenter-Bee, 703. 
 
 Cartilage, structure of, 47 ; nutrition of, 
 48 ; transformation of, into bone, 52. 
 
 Casein, 15. 
 
 Cat, electricity of, 418. 
 
 Cauda equina, 460. 
 
 Cells, nature of, 32 ; multiplication of, 
 33; new production of, 34, 393 ; inde- 
 pendent condition of, 35 ; origin of all 
 organisms from, 379 ; differentiation of 
 functions of, 380. 
 
 Cells of Bee, 712, 713; royal, 714, 716. 
 
 Cementum of Teeth, 54. 
 
 Centipede, 93, 103; reflex actions of, 
 443. 
 
 CEPHALOPODA, general characters of, 
 121 ; circulation in, 291 ; respiration 
 in, 316; nervous system in, 448; re- 
 production of, 753. 
 
 Cercaria-larva of distoma, 743. 
 
 Cerebellum, 449; development of, in 
 different classes, 452456; in Man, 
 458; functions of, 480, 481. 
 
 Cerebro-spinal nerves, 458461. 
 
 Cerebrum, 449 ; development of, in 
 different classes of Vertebrata, 452 
 456; in Man, 458; development of, 
 correspondent with intelligence, 452, 
 718 ; functions of, 483485 ; effects 
 of removal of, 465. 
 
 Cerumen of ear, 375. 
 
 Chaetodon rostratus, 476. 
 
 Chalaza? of egg, 755. 
 
 Cheese, 377. 
 
 Chemical constitution of organized 
 bodies, 4; of albumen, 13, 14; of casein, 
 15; of syntonin, 16; of fibrin, 17, 18; 
 of gelatin, 19; of chondrin, 20; of 
 bone, 51; of teeth, 54. 
 
 Cheselden's case, 507. 
 
 Childers, speed of, 660. 
 
 Chimpanzee, 173, 674. 
 
 Cholesterin, 364. 
 
 Cholic acid, 364. 
 
 Chondrin, 20. 
 
 Chorda dorsalis, 53, 757. 
 
 Chorion, 761. 
 
 Choroid coat, 533. 
 
 Chromatic aberration, 548, 549. 
 
 Chrysalis, 97, 309, 745. 
 
 Chyle, 171, 213, 222; change of, in lac- 
 teals, 222, 223 ; delivery of into blood- 
 vessels, 221. 
 
 Chylification, 171, 212215. 
 
 Chyme, 171, 211. 
 
 Cicada, sound of, 679. 
 
 Cicatricula, 755, 756. 
 
 Cilia, 45, 143, 319, 329. 
 
 Ciliary processes, 536. 
 
 CIRCULATION, 253 ; purposes of, 228, 
 253, 254-. complete double, 282; 
 greater, 253; lesser, 253, 268; pecu- 
 
 liarity of, in liver, 267; mechanism of, 
 269, 270. 
 
 Circulation, course of, in warm-blooded 
 animals, 282; in foetus, 283,762; in 
 Reptiles, 284,285; in Fishes, 286; in Am- 
 phibia, 287, 288 ; in Invertebrata, 289 ; 
 in Mollusca, 290 ; in Cephalopoda, 291 ; 
 in Crustacea, 292 : in Insects, 293 ; in 
 Spiders, 293; in Worms, 294; inTuni- 
 cata and Echinodermata, 295 ; in Zoo- 
 phytes and Sponges, 295. 
 
 Cirrhipeda, general structure of, 102; de- 
 velopment of, 74!). 
 
 Claspers of monkeys, 643, 674. 
 
 Clavicle, 634, 636. 
 
 Climbing perch, 318. 
 
 Coagulation of albumen, 14; of fibrin, 
 17 ; of blood, 236-238 ; of chyle, 222. 
 
 Coagulable lymph, 391. 
 
 Cochlea of ear, 5 1 8520. 
 
 Cockchafer, digestive apparatus of, 358. 
 
 Cod, brain of, 453. 
 
 Cod-liver oil, use of, 386. 
 
 Coscum, 214. 
 
 Cold sustained by animals, Introd., 405. 
 
 Cold-blooded animals, temperature of, 
 403406. 
 
 Coloured shadows, 569. 
 
 Colouring matter, formed by cells, 359, 
 360, 533. 
 
 Colourless corpuscles of blood, 234. 
 
 Colours, want of power to distinguish, 
 468; complementary, 568 570. 
 
 Comatula, development of, 741. 
 
 Combustion in animal body, 157, 305, 
 306,412,413. 
 
 Commissures of nervous system, 434, 
 458. 
 
 Complementary colours, 568 570. 
 
 Compound eyes of Articulata, 572575. 
 
 Polypes, 124, 127. 
 
 Tunicata, 114. 
 
 CONCHIFERA, 113; respiration in, 316; 
 luminosity of, 396; nervous system 
 of, 437. 
 
 Conjunctival membrane, 537- 
 
 Consonants, 689, 690. 
 
 Consumption, nature and treatment of, 
 386. 
 
 Contractility of muscular fibre, 579, 
 590 ; dependent on its nutrition, 591. 
 
 Contractions of heart, 269, 271, 581, 587. 
 
 of muscles, energy of, 592 
 
 594; use of, in organic functions, 
 595 ; in locomotion, 595. 
 
 Convex surfaces, influence of, on light. 
 529531. 
 
 Convolutions of brain, 456, 458. 
 
 Convulsive movements, 473, 474; energy 
 of, 592. 
 
 Cooling effects of evaporation, 372, 373. 
 
 Coral-forming animals, 131134. 
 
 Cornea, 533 ; structure of, 46. 
 
 Corpora striata, 458. 
 
 Corpus callosum, 458. 
 
 QQ2 
 
596 
 
 INDEX. 
 
 Corpuscles of blood, 35 (see Blood-cor- 
 puscles). 
 
 chyle, 222. 
 
 Cortical substance, of brain, 430. 
 kidney, 357. 
 
 Coughing, act of, 342. 
 
 Crab, anatomy of, 9!) ; metamorphosis 
 of, 101 ; nervous system of, 447. 
 
 Cranium, bones of, 617 623. 
 
 Crassamentum of blood, 236. 
 
 Cray-fish, 95. 
 
 Cricket, leaping powers of, 662; sound 
 produced by, 678. 
 
 Cricoid cartilage, 680. 
 
 Crinoidea, 118. 
 
 Crocodile, 93. 
 
 Crusta petrosa of teeth, 54. 
 
 CRUSTACEA, general characters of, 99 
 101; formation of shell of, 170; teeth 
 of, in stomach, 202 ; circulation in, 
 292 ; respiration in, 315 ; liver of, 356 ; 
 luminousness of, 396; reproduction of 
 claws in, 389 ; generation of, 748 ; 
 development of, 101, 748. 
 
 Crystalline lens, 536 ; reproduction of, 
 390; change of form of, 551. 
 
 Cutis, structure of, 37 ; sensibility of, 
 490, 491. 
 
 Cuttle-fish, 111; ink of, 359. 
 
 Cynips, agamic reproduction of, 746. 
 
 Cysticercus, development of, 742. 
 
 D. 
 
 Daphnia, reproduction of, 748. 
 
 Death, of the body, 67 ; apparent, 66. 
 
 of parts, continually taking place, 
 
 65, 68. 
 
 Death-watch, 677. 
 
 Death's-head Moth, 678. 
 
 Decay of dead animal matter, 54, 160. 
 
 Decay continually taking place in living 
 body, 65, 68. 
 
 Deer, foot of, 652. 
 
 Defecation, 171, 216. 
 
 Degeneration of tissues, from want of 
 use, 30. 
 
 Deglutition, 171, 192196, 470. 
 
 Dentine, 54. 
 
 Development, first stages of in ovum, 
 736, 737; of Polypes, 738, 739 ; of Me- 
 dusae, 740 ; of Echinodermata, 741 ; of 
 Entozoa, 742, 743 ; of Annelida, 744 ; 
 of Insects, 745747 ; of Crustacea, 748; 
 of Cirrhipeda, 749 ; of Rotifera, 750 ; of 
 Arachnida, 751; of Mollusca, 752; of 
 Vertebrata, 756762. 
 
 Diaphragm, 328; deficiency of, in Birds 
 and Reptiles, 327 ; peculiar to Mam- 
 mals, 330; action of, in respiration, 
 331333. 
 
 Diet, natural, of Man, 163 165. 
 
 Differentiation of structure and function, 
 380. 
 
 DIGESTION, several stages of, 171; gas- 
 tric, 204211 ; intestinal, 212215. 
 
 Digestive cavity, characteristic of Ani- 
 mals, 8; different forms of, 197203; 
 formation of, in embryo, 759. 
 
 Direction of action of muscles, 606 611. 
 visual objects, 558. 
 
 Dislocations, 604. 
 
 Distance, adaptation of the eye to, 550, 
 551 ; mode of estimating, 563565. 
 
 Distoma, development of, 743. 
 
 Division of labour in living organisms, 2. 
 
 Dogs, intelligence of, 717. 
 
 Doris, circulation in, 290; respiration in, 
 316. 
 
 Dorsal vessel of Articulata, 293. 
 
 Double vision, 559. 
 
 Draco volans, 668. 
 
 Drowning, 338 ; treatment of, 339. 
 
 Drum of the ear, 516. 
 
 Duration of luminous impressions, 567. 
 
 Dytiscus, 444. 
 
 Ear, simplest forms of, 512, 514; of Man, 
 external, 515; middle, 516, 517; in- 
 ternal, 518521 ; bones of, 516. 
 
 Earthworm, 104, 142, 389, 400, 404, 597. 
 
 ECHINODERMATA, 118,119; circulation 
 in, 295; luminousness of, 396; deve- 
 lopment of, 741. 
 
 Echinus, 118; teeth of, 189. 
 
 Egg of Birds, structure of, 755 ; shell of, 
 its permeability to gases, 760. 
 
 Elastic fibrous tissue, 23, 29. 
 
 Elasticity of arteries, 274, 275; of foot, 
 649; of vertebral column, 631. 
 
 Elateridce, luminousness of, 397; leap- 
 ing power of, 662. 
 
 Electricity, animal, sources of, 416, 417; 
 in Cat, 418; in Fishes, 419423; of 
 muscle and nerve, 424; analogy of to 
 nervous agency, 488, 585. 
 
 Electric organs, structure of, 421, 422; 
 copiously supplied with nerves, 423. 
 
 Elephant, trunk of, 172, 4f!3; molar 
 teeth of, 102; tusks of, 177; intelli- 
 gence of, 717. 
 
 Embryo, origin of, 737, 757; develop- 
 ment of, 757 762; circulation in, 283, 
 758, 762 ; respiration in, 760. 
 
 Emotions, 477, 478 ; influence of on 
 muscles, 590; on organic functions, 
 461. 
 
 Enamel, structure and composition of, 
 54; arrangement of, 182. 
 
 Encrinites, 118. 
 
 Endosmose, action of, on blood cor- 
 puscles, 232. 
 
 Entomostraca, agamic reproduction of, 
 727, 748. 
 
 Entozoa, 105; development of, 742, 743. 
 
 Ephemera, 315. 
 
 Ephippial eggs of Daphnia, 748. 
 
 Epidermis, structure of, 38; use of, 492. 
 
 Epidermic appendages, 38. 
 
 Epiglottis, 193, 681. 
 
INDEX. 
 
 597 
 
 Epileptic fits, 473. 
 
 Epithelium, structure of, 40 ; action of, 
 in secretion, 41, 42, 355. 
 
 Eunice, 314. 
 
 Eustachian tube, 516, 517. 
 
 Evaporation from surface, 370373; 
 cooling effects of, 372, 373. 
 
 EXCRETION, objects of, 345351 ; mate- 
 rials of, formed in the blood, 351; effects 
 of retention of, 351. 
 
 Exhalation of moisture from the lungs, 
 343 ; from the skin, 371 374; amount 
 of, 374. 
 
 Eye, an optical instrument, 532, 543 
 553; structure of, in Man, 533 536; 
 muscles of, 538 ; motions of, 538, 539 ; 
 aberration corrected in, 547, 549 ; 
 adaptation of to distance, 550, 551; 
 limits of vision by, 554 ; common sen- 
 sibility of, 571 ; long and near-sighted, 
 62, 553; peculiar structure of, in 
 Articulata, 573 575; deficient in seme 
 Vertebrata, 572; rudiments of, in the 
 lower animals, 575. 
 
 Eyelids, uses of, 537. 
 
 Flying Phalanger, 668. 
 
 Squirrel, 668. 
 
 Follicles, of mucous membrane, 41. 
 of glands, 42, 355, 356. 
 
 Food of Animals, 7, 8 ; derived from 
 Plants, 144 147; from Animals, 148 
 150; chemical nature of, 153, 154, 164; 
 mineral ingredients of, 166, 167. 
 
 demand for, 140, 141 ; economy 
 
 of, 165. 
 
 Food-yolk, 736, 752, 754. 
 
 Foot, structure of, 648, 649. 
 
 Foraminifera, 128, 131. 
 
 Freezing of animal bodies, 67, 405. 
 
 Frog tribe, 86, 87 ; blood-discs of, 230 ; 
 metamorphosis of, 86, 87; change of 
 circulating system in, 287 289 ; respi- 
 ration of, 325 ; experiments on nervous 
 system of, 466, 468 ; eggs of, 755 ; de- 
 velopment of, 756. 
 
 FulgoridB, luminousness of, 400; sound 
 emitted by, 679. 
 
 Functions of living beings, 2 ; nutritive, 
 6 ; animal, 6 ; relation of organic and 
 animal, 425427. 
 
 F. 
 
 Face, bones of, 690 623; muscles of, 
 624. 
 
 Facial angle, 719, 720. 
 
 Fat, structure and uses of, 46,412; de- 
 position of, 157, 162; of blood, 232, 
 241. 
 
 Fatigue, sense of, 595. 
 
 Fertilization of ovum, 732, 734, 736. 
 
 Fibre, muscular (see Muscular Fibre). 
 
 Fibres, of nerves (see Nerves). 
 
 Fibrin, composition and properties of, 
 17,18; uses of in blood, 236240. 
 
 Fibrous membranes, 29. 
 
 tissues, general uses of, 12 ; for- 
 mation of, 22 ; general characters of, 
 2230 ; nutrition of, 384, 390. 
 
 Fibro-cartilages, 47. 
 
 Fins of fishes, 666. 
 
 Fire-flies, 397. 
 
 FISHES, general structure of, 88 91; 
 teeth of, 188; circulation in, 286 ; re- 
 spiration in, 317, 318; air-bladder of, 
 324; luminousness of, 396; heat of, 
 405 ; electricity of, 419424; nervous 
 system of, 453 ; organs of smell of, 
 509 ; vertebral column of, 629, 630 ; 
 movements of, 666, 667; reproduction 
 of, 754, 755, 759. 
 
 Fission, multiplication of cells by, 33; 
 of Infusoria by, 135, 725. 
 
 Flea, leaping powers of, 594, 662. 
 
 Flesh-fly, voracity of larva of, 141. 
 
 Flight, action of, 667672; impossible 
 iu Man, 673. 
 
 Flying Fish, 667. 
 
 Lemur, 668. 
 
 G. 
 
 Gall-bladder, 362. 
 
 Galloping, 660. 
 
 Galvanic electricity, discovery of, 583. 
 
 Ganglia, 61 (see Nervous System). 
 
 of special sense, functions of, 
 
 475479. 
 
 olfactive and optic, 453456. 
 
 Gases, poisonous, 335, 344. 
 GASTEROPODA, 107, 108,112; palate of, 
 
 189 ; circulation in, 290; respiration in, 
 316; nervous system of, 438; develop- 
 ment of, 752. 
 Gastric follicles, 204. 
 
 juice, 204; properties of, 207 
 
 210; artificial, 210. 
 
 Gavial, teeth of, 1 86. 
 
 Gelatin, chemical compostion of, 19 ; 
 use of as food, 159 ; present in blood, 
 380. 
 
 Gelatinous nerve-fibres, 60. 
 
 principles of food, 153 ; des- 
 tination of, 159. 
 
 Gemmation, multiplication by, of Plants, 
 724; of Infusoria, 725 ; of Zoophytes, 
 726; of Medusae, 726, 741 ; of Echiro- 
 dermata, 726, 741; of Articulata, 7/.7, 
 744 ; of Mollusca, 728 ; of Vertebrata, 
 729 ; antagonism of, to Generation, 
 735; (see Agamic Reproduction.) 
 
 Gemmules of polypes, 738. 
 
 GENERATION, sexual, essential nature 
 of, 730 733; antagonism of, to gem- 
 mation, 735; simplest form of, 734. 
 
 Germ-cells of plants, 724; of animals, 
 732. 
 
 Germ -yolk, 736, 752, 754. 
 
598 
 
 INDEX. 
 
 Germinal membrane, 737, 756. 
 
 spot, 732. 
 
 vesicle, 732. 
 
 Gizzard, of Birds, 200, 201 ; of Insects, 
 
 202 ; of Bryozoa, 202; of Rotifera, 202. 
 
 Glands, secreting, structure of, 355358. 
 
 mesenteric, 218; lymphatic, 219. 
 
 Glaucus, 316. 
 
 Globules of Blood (see Blood). 
 Globulin, 232. 
 
 Glosso-pharyngeal nerve, 459, 470. 
 Glottis, 193, 681. 
 Glow-worms, 398401. 
 Gluten of bread, composition of, 153. 
 Glycine, 364. 
 Glycocholic acid, 364. 
 Gnat, larva of, 321. 
 Goat-moth, larva of, 141. 
 Goldfinch, nest of, 704. 
 Gout, nature and cure of, 348, 349. 
 Granulation, repair of wounds by, 392. 
 Gravel, 348, 367. 
 Gregory, Dr., case of, 349. 
 Grey substance of nerves, 430, 431. 
 Guiding Sensations, importance of, 478. 
 Gymnotus, electricity of, 419 424. 
 
 H. 
 
 Habitual actions, 479. 
 
 Haematin, 232. 
 
 Hairs, structure of, 38. 
 
 Hall, Dr. Marshall, his treatment of 
 asphyxia, 339. 
 
 Hamster, instinct of, 699. 
 
 Hands, use of, in prehension, 172, 173 ; in 
 locomotion, 674; structure of, 641 
 644. 
 
 Hare, leaps of the, 661. 
 
 Hartz forest, devastated by Insects, 147. 
 
 Haversian canals of bone, 49, 50 ; forma- 
 tion of, 52. 
 
 Head, definition of, 111 ; bones of, 617 
 623 ; muscles of, 624. 
 
 Healing of wounds, 391, 392. 
 
 Hearing, sense of, 510524; improved 
 by cultivation, 525. 
 
 Heart, 245 ; structure of, in Man, &c., 
 256, 257 ; respiratory and systemic, 
 281; action of, 269, 270, 581, 583; 
 valves of, 272, 273 ; number of pulsa- 
 tions of, 271; development of, 753, 
 762. 
 
 structure of, in Reptiles, 284 ; in 
 
 Fishes, 286 ; in Mollusca, 290 ; in 
 Cephalopoda, 291; in Crustacea, 292 ; 
 in Insects, 293. 
 
 Heat, sustainable by Animals, Introd., 
 372, 373. 
 
 generated by Animals, 403 415 ; 
 
 of Invertebrata, 404; of Fishes, 405; 
 of Reptiles, 406; of Birds, 407; of 
 Mammals, 407 ; of Man, 407 ; of young 
 animals, 408, 409 ; of Insects, 410, 
 411. 
 
 Heat, animal, dependent on combustion 
 of carbon and hydrogen, 412, 413; on 
 supply of oxygen, 413 ; maintained by 
 respiration, 414; influence of nervous 
 system on, 415. 
 
 Hemispheres of Brain, 449. 
 
 Herbivorous animals, 144 147; nutri- 
 tion of, 162 ; teeth of, 179, 182. 
 
 Hiccup, 341. 
 
 Hinge-joint, 603. 
 
 Hippuric acid, 367. 
 
 Holothuria, 118, 119; reparative power 
 of, 389; development of, 741. 
 
 Horse, foot of, 652 ; intelligence of, 695. 
 
 Howling Monkeys, 684. 
 
 Humble-bee, heat produced by, 410. 
 
 Hunger, sense of, 140, 205. 
 
 Hyalaea, 112. 
 
 Hybernation, 309. 
 
 Hydatina, multiplication of, 750. 
 
 Hydra, 121; referred to, 131, 296, 577; 
 propagation of, by buds, 122, 726; by 
 artificial division, 122; by eggs, 123, 
 734 ; development of ovum of, 738. 
 
 Hydrogen, combustion of, in animal body, 
 343. 
 
 Hydropathic system, 374. 
 
 Hydrophobia, 474. 
 
 Hydrozoa, 124, 125; development of, 
 726, 738. 
 
 Hyoid bone, 625, 680. 
 
 Hysteric disorder, 474. 
 
 I. 
 
 Iliac bones, 645. 
 
 Images, formation of, by lenses, 531 ; on 
 retina, 543, 544. 
 
 Imago or perfect insect, 745. 
 
 Immortality of the soul of man, 721, 
 722. 
 
 Impressions on nervous system, 432,486. 
 
 Incisor teeth, 181, 183. 
 
 Infants, necessity of warmth to, 408, 409. 
 
 Infusoria, 133 135; multiplication of, 
 725. 
 
 Ink, of cuttle-fish, 359. 
 
 Insalivation, 179, 190, 191. 
 
 INSECTS, general characters of, 97 ; diges- 
 tive apparatus of, 202 ; circulation in, 
 293 ; respiration in, 308, 321, 322 ; repa- 
 rative powers of, 389 ; secreting appa- 
 ratus in, 358 ; luminousness of, 3U7 
 401 ; heat of, 410, 411 ; nervous system 
 of, 440446 ; instincts of, 483, 484, 667 
 716; antennae of, 498. 499; eyes of, 
 573575 ; muscular power of, 594, 662 ; 
 wings of, 670 ; production of sounds by, 
 676, 679 ; reproduction of, 745, 746. 
 
 Instinctive actions, 692 ; predominance 
 of, in Articulata, 96 ; characters of, 
 694; examples of, 696 716; corre- 
 spondence of, with ganglia of special 
 sense, 475479; irrationality of, 709. 
 
INDEX. 
 
 599 
 
 Intelligence of Vertebrata, 73, 483485, 
 694; examples of, 695, 717; corre- 
 spondence of, with development of the 
 Cerebrum, 452, 692, 718720. 
 
 Intervertebral substance, 631. 
 
 Intestinal juice, 213. 
 
 tube, motion of aliment 
 
 through, 215; digestion continued in, 
 213, 214; relative length of, 213. 
 
 Worms, 105 ; development of, 
 
 742, 743. 
 
 Invertebrata, 92 ; absorption in, 225 ; 
 
 nature of circulating fluid in, 225, 235 ; 
 
 heat of, 404 ; skeletons of, 598, 599. 
 Involuntary movements, 589, 590. 
 Iris, 533, 534. 
 Iron, a constituent of animal bodies, 166, 
 
 167; of red corpuscles of blood, 232. 
 Ivory, structure and composition of, 54. 
 lulus, 103. 
 
 J. 
 
 Jaw, motion of, in Quadrupeds, 138 ; in 
 
 Man, 189; articulation of, 623. 
 Jelly-fish, 120. 
 Joints, 603; dislocation of, 604. 
 
 K. 
 
 Kangaroo, leaping powers of, 661 ; skele- 
 
 ton of, 661. 
 Kidneys, structure of, 357, 358, 368, 369 ; 
 
 purposes of their excretion, 346 348, 
 
 367. 
 
 L. 
 
 Labyrinth of ear, 519. 
 Lachrymal apparatus, 540, 
 
 gland, 540. 
 
 sac, 540. 
 
 Lacteals, 217, 218. 
 
 Lactic acid, 349, 367. 
 
 Lacunae of bone, 50. 
 
 Lamprey, 317; chorda dorsalis of, 53, 
 
 757. 
 
 Lampyridae, 397399. 
 Land-crabs, 315. 
 Lantern-flies, 400. 
 Larva, of Cirrhipeds, 749; of Crab, 101 ; 
 
 of Echinodermata, 741 ; of Entozoa, 
 
 742 ; of Insects,97, 141 ,745 ; of Medusae, 
 
 740. 
 Larynx, 192; structure of, 680, 681 ; 
 
 action of, (582684 ; in Birds, 685. 
 Laughing, act of, 341. 
 Leaping, 661, 662. 
 Leech, 104, 105. 
 Leg, bones and muscles of, 647. 
 Lepidosiren, 81; blood-discs of, 230; 
 
 respiration of, 324. 
 Leverage of bones, 612 615. 
 Life, maintained by continual change, 
 
 68. 
 Ligaments, structure of, 29. 
 
 vocal, 681684. 
 
 Light, emitted by living animals, 394 
 402 ; by dead bodies, 402. 
 
 propagation of, 526 ; refraction of 
 
 527532. 
 
 Lirne, amount of, in bones, 49; in teeth, 
 54; in egg-shell, 169; in shells of 
 Mollusks, 169; in shells of Crustacea, 
 170. 
 
 sources of, in animal bodies, 166 
 
 170. 
 
 Limulus, 100. 
 
 Lingual nerve, 500. 
 
 Liquids, reception of, 173. 
 
 Liquor sanguinis, 229, 232, 236 241, 385, 
 387, 391. 
 
 Lithic acid, 346, 348, 367. 
 
 Liver, structure of, 356, 358, 363; circu- 
 lation in, 267, 363 ; assimilating action 
 of, 224 ; secreting action of, 364366 ; 
 formation of sugar by, 366. 
 
 objects of its excretion, 346, 350. 
 
 Living beings, distinctive characters of, 
 
 15. 
 Lizard tribe, 84; reparative powers of, 
 
 390. 
 
 Lobster. 100 ; circulation in, 292. 
 Lock-jaw, 473. 
 Locomotion, reflex movements of, 471 ; 
 
 organs of, 596. 
 Locusts, voracity of, 146 ; multiplication 
 
 of prevented, 149. 
 Long-sighted eyes, 552, 553. 
 Luminousness, animal, 394-400 ; uses of, 
 
 401; from decomposition, 402. 
 Lungs, rudimentary in Fishes, 324 ; in 
 
 Reptiles, 325 ; in Birds, 326, 327 ; in 
 
 Mammals, 326333. 
 Lymnaeus, parasites of, 743. 
 Lymph, coagulable, 391. 
 Lymphatics, 219, 220. 
 
 M. 
 
 Mactra, 113, 
 
 Madrepore, 127. 
 
 Malapterurus, electric, 419, 422. 
 
 Malpighian bodies of kidney, 369. 
 
 MAMMALS, general structure of, 77 ; di- 
 gestive apparatus in, 197 199; blood- 
 discs in, 229; circulation in, 281, 282; 
 respiration in, 328333 ; heat of, 407 ; 
 nervous system of, 456 ; reproduction 
 in, 756, 761. 
 
 Mammary glands, structure of, 376. 
 
 Man, food of, 163 ; stomach of, 197 ; 
 heart of, 256, 257 ; arterial system of, 
 258; quantity of air required by, 334 
 337 ; reproduction of lost parts in, 
 390; repair of injuries in, 391393; 
 heat of, 407; nervous system of, 456 
 462 ; peculiar characters of soul of, 
 721, 722. 
 
 Mantis, actions of, 444. 
 
 Mantle of Mollusca, 107. 
 
 Marmot, hybernation of, 309. 
 
 Marrow of bones, 49. 
 
 Spinal (see Spinal Cord). 
 
600 
 
 INDEX. 
 
 Mastication, 171, 174189. 
 
 Mastodon, teeth of, 182. 
 
 Measles of pork, 742. 
 
 Medulla oblongata, 450, 460. 
 
 Medusa, 120 ; development of, 125, 740 ; 
 circulation in, 296; rhythmical move- 
 ments of, 578; gemmation of, 726. 
 
 Membrana Tympani, 516. 
 
 Membrane, basement or primary, 31. 
 
 Membranes, fibrous, 29 ; serous, 28, 43 ; 
 mucous, 39 41. 
 
 Mesentery, 217. 
 
 Mesenteric glands, 217. 
 
 Metamorphosis of Frog tribe, 86, 87. 
 
 Insects, 97, 745. 
 
 Crustacea, 101. 
 
 Milk, different classes of aliment con- 
 tained in, 158; chemical composition 
 of, 377 ; influence of mind on, 353. 
 
 Milk-teeth, 184. 
 
 Mineral ingredients required by Ani- 
 mals, 166170. 
 
 Mitral valve, 272. 
 
 Molar teeth, 181183. 
 
 MOLLUSCA, general characters of, 106 
 110; circulation in, 290; respiration 
 in, 316, 320 ; structure of liver in, 356; 
 of kidneys, 358 ; luminousness of, 396 ; 
 nervous system of, 435 439 ; develop- 
 ment of, 752. 
 
 Monkey, interior of, 77. 
 
 Monstrosities by excess, 729. 
 
 Mortality under different circumstances, 
 Introd. 
 
 Mucous membranes, general structure 
 of, 3941. 
 
 Mulberry mass of ovum, 736, 737. 
 
 Muscles, general purposes of, 10 ; general 
 mode of action of, 605 615; of eye, 
 538; of face, 624; of trunk, 637; of 
 arm, 638, 640 ; of hand, 641 ; of leg, 
 646,647; of foot, 648. 
 
 Muscular Contraction, 58, 59, 579; con- 
 ditions of, 591; stimuli to, 580586; 
 influence of electricity on, 583 585 ; 
 relation of, to nervous power, 586 
 588, 592, 593; voluntary and involun- 
 tary, 589, 590; energy of, 592594; 
 use of, in organic functions, 595 ; in 
 locomotion, 605615. 
 
 Muscular Fibre, structure of, 55 57 ; 
 contraction of, 58; alternates with re- 
 laxation, 58, 59. 
 
 Musk, odour of, 504. 
 
 Mygale, nest of, 700. 
 
 MYKIAPODA, general structure of, 103; 
 nervous system of, 440. 
 
 N. 
 
 Nails, structure of, 38. 
 Nais, spontaneous fission of, 727. 
 Near-sighted eyes, 552, 553. 
 Necrophorus, instinct of, 703. 
 Negro, skin of, 375. 
 Nemestrina, trunk of, 173. 
 
 Nepa, tracheal system of, 322. 
 
 Nereis, 104, 314, 727. 
 
 Nerita, palate of, 189. 
 
 NERVOUS SYSTEM, general structure of, 
 483; general objects of, 9, 10, 429 
 432; form of, in Vertebrata, 72; in 
 Articulata, 94; in Mollusca, 110; in 
 Radiata, 116. 
 
 particular structure 
 
 and actions of, in Radiata, 434; in 
 Mollusca, 435439 ; in Articulata, 440 
 446; in highest Invertebrata, 447, 
 448; in Vertebrata, 449 452; in Fishes, 
 453; in Reptiles, 454; in Birds, 455; 
 in Mammalia, 456; in Man, 457 462. 
 
 Sympathetic, 461,462. 
 
 the instrument of the 
 
 mind, 427; influence of, on secretion, 
 190, 353 ; on muscular contraction, 
 584, 585 ; on animal heat, 415. 
 
 Nervous Tissue, white or fibrous sub- 
 stance of, 62, 63 ; distribution of, 63 ; 
 grey or vesicular substance of, 61. 
 
 Nests of Insects and Birds, 700705, 
 710714. 
 
 Newt, 87. 
 
 Nictitating membrane, 540. 
 
 Nitrogen, absorption and exhalation of, 
 302. 
 
 Non-azotized constituents of food, 154; 
 destination of, 155157, 162165 ; 
 effects of excess of, 350. 
 
 Nose, structure of, 506, 507; common 
 sensibility of, 508. 
 
 Nurse-bees, 411. 
 
 Nurses of Cercariae, 743. 
 
 NUTRITION of tissues, increased by use, 
 242, 589; dependent on liquor san- 
 guinis, 240, 241, 385; mode of, in dif- 
 ferent tissues, 384, 385 ; share of blood 
 in, 387 ; share of blood-vessels in. 388; 
 share of tissues in, 387 ; imperfect 
 forms of, 386. 
 
 O. 
 
 Oak, caterpillars supported on, 145. 
 
 Octopus, 121. 
 
 Odours, 504, 505. 
 
 (Esophagus, 192. 
 
 Oleaginous principles, 153; destination 
 
 of, 154157, 162. 
 Olfactive ganglia, 453 456, 458. 
 
 nerve, 507. 
 
 Optic ganglia, 453456, 458. 
 
 nerve, 459. 
 Organic Life, 6, 425, 426; nervous system 
 
 of, 461. 
 
 Functions, relation of, to ani- 
 mal, 425 427; influenced by emo- 
 tions, 461. 
 
 Organized bodies, distinctive form of, 
 1 ; structure of, 2 ; consistence of, 3 ; 
 chemical constitution of, 4; actions 
 of, 5. 
 
INDEX. 
 
 601 
 
 Organs of Sense (see Sensation, Organs 
 of). 
 
 Ornithorhyncus, 186, 664. 
 
 Otolithes, 513. 
 
 Ovarium, 732 ; of Bird, 754. 
 
 Ovum, structure of, 732, 733. 
 
 Oxygen, carried by blood-corpuscles, 
 235 ; by liquor sanguinis, 241 ; ab- 
 sorbed in respiration, 300306, 343, 
 346 ; consumption of, dependent on 
 
 muscular action, 307 309. 
 
 Oyster, 113, 316, 437. 
 
 P. 
 
 Palates of Gasteropods, 189. 
 
 Palpi of Insects, 172, 503. 
 
 Palsy of muscles, 586 58S. 
 
 Paludina, 112. 
 
 Pancreatic fluid, use of in digestion, 213. 
 
 Papillae of skin, 37, 490; of tongue, 500. 
 
 Parotid gland, 356. 
 
 Paxy-waxy, 29. 
 
 Pecten, nervous system of, 110, 437. 
 
 Pectinibranchiata, development of, 752. 
 
 Pedal ganglia, of Mollusks, 437, 438; of 
 Articulata, 446. 
 
 Pelvis, 645. 
 
 Penguin, G67. 
 
 Pentacrinoid-larva of Comatula, 741. 
 
 Perch, skeleton of, 666. 
 
 Pericardium, 43. 
 
 Peristaltic movement of intestines, 215, 
 579. 
 
 Peritoneum, 43. 
 
 Perspiration, 371374. 
 
 Pharyngeal ganglia, of Mollusks, 438; 
 of Articulata, 446. 
 
 Pharnyx, 192. 
 
 Phosphorescence of the sea, 394, 395. 
 
 Phosphorus in animal bodies, 166; 
 sources of, 167; light produced by. 
 402. 
 
 Pia mater, 458. 
 
 Pigment, black, of eye, 533; use of, 545 
 
 Pigment-cells, 533. 
 
 Pitch of sound, dependent on number 
 of vibrations, 523, 682. 
 
 Placenta of Mammals, 761. 
 
 Planaria, reparative power of, 389. 
 
 PLANTS, general comparison of with 
 Animals, 6 12 ; afford food to Ani- 
 mals, M4 147; resemblance of their 
 life to organic life in Animals, 425, 
 426. 
 
 Plastron, of Turtles, 83. 
 
 Plethoric state of body, 233. 
 
 Pleura, 328. 
 
 Pneumogastric nerve, 459, 470. 
 
 Podura, leaping power of, 662. 
 
 Poisonous gases, 344. 
 
 Polijcystina, 132. 
 
 Polype, fresh-water (see Hydra). 
 
 POLYPIFEUA, 121125 (see Zoophytes). 
 
 Polyzoa, 115; gizzard of, 202; circula- 
 
 tion of, 295 ; gemmation of, 728 ; de- 
 velopment of, 752. 
 
 Pompilus, nest of, 703. 
 
 PORJFERA, 136, 137; circulation in, 296. 
 
 Portal system of blood-vessels, 267, 366. 
 
 Poulp, 316, 448. 
 
 Primitive trace, 757. 
 
 Prehension, act of, 171173, 643, 674. 
 
 Projection, idea of, to what due, 560 562. 
 
 Proteus, blood-discs of, 230, 231. 
 
 PROTOZOA, 128 ; movements of, 577. 
 
 Pseudopodia, 130, 131. 
 
 Pterodactylus, 669. 
 
 PTEROPODA, 122. 
 
 Pulmonary circulation, 268. 
 
 Pulse, 276; influence of posture on, 655. 
 
 Pupa, of insect, 97. 
 
 Pupil, 553 ; dilatation and contraction 
 of, 534. 
 
 Pus, 393. 
 
 Q. 
 
 Quadrumana, extremities of, 643, 648, 
 
 674. 
 Queen-bee, 712, 714, 716, 747. 
 
 R. 
 
 Rabbit, teeth of, 177; movements of, 661. 
 
 RADIATA, general characters of, 116, 
 117; stomach of, 203 ; nervous system 
 of, 434. 
 
 Radius, 639. 
 
 Ray, pecuuar swimming of, 666. 
 electric, 419. 
 
 Red Corpuscles of blood (see Blood- 
 Corpuscles). 
 
 Reflex actions, 195, 340, 430, 692; in 
 Mollusca, 436. 439 ; in Articulata, 442 
 445, 693; In Vertebrata, 451; the 
 spinal cord their instrument, 464 
 474 ; dependent on stimuli, 466 ; not 
 dependent on sensation, 467 469. 
 
 Refraction of light, 527 532. 
 
 Relief, perception of, 560562. 
 
 Rennet, action of, 15; nature of, 199. 
 
 Repair of injuries, 389393. 
 
 Reproduction (see Development, Gemma- 
 tion, and Generation). 
 
 REPTILES, general characters of, 81 
 87; teeth of, 187; blood-discs in, 430; 
 circulation in, 284, 285 ; respiration in, 
 325; importance'of skin, as respiratory 
 organ in, 325 ; heat of, 406 ; nervous 
 system of, 454; vertebral column of, 
 629 ; reproduction of, 756 760. 
 
 Republican Grosbeak, 710. 
 
 Resistance, sense of, 496. 
 
 RESPIRATION, 299; use of blood-cor- 
 puscles in, 235 ; necessity for, 297 ; in 
 aquatic animals, 298 ; changes pro- 
 duced by, in air, 300302, 334336; 
 in blood, 303306; related to nervo- 
 muscular activity, 307 ; energy of, in 
 
602 
 
 INDEX. 
 
 Birds, Mammalia, and Insects, 308 ; 
 small amount of, in cold-blooded ani- 
 mals, 309, 310; no special provision 
 for in lowest, 311 ; uses of cilia in, 
 319, 329; an excreting process, 345, 
 346 ; subservient to maintenance of 
 heat, 412 414; in embryo, 760, 761. 
 Respiratory apparatus of Annelida, 314 ; 
 of aquatic Insects, 315; of Crustacea, 
 315 ; of Mollusca, 316, 320; of Fishes, 
 317, 318, 324; of Myriapoda, 320; of 
 Insects, 321 322; of Arachnida, 323; 
 of Reptiles, 325; of Birds, 326, 327; 
 of Mammals, 328333; of embryo, 
 760, 761. 
 
 movements, 331334, 340 
 
 342. 
 
 surface, extension of, 312; pro- 
 longation of, externally, into gills, 313; 
 internally into lungs, 313. 
 
 system of nerves, in Articu- 
 
 lata, 446; in Mollusca, 437, 438; in 
 Vertebrata, 450. 
 
 Rete mucosum, 38. 
 
 Retina, 535; yellow spot of, 554; in- 
 sensible spot of, 554. 
 
 Rhizopods, 129 ; substance of, 64. 
 
 Rhythmical movements, 578, 581. 
 
 Ribs, 633. 
 
 Rodentia, teeth of, 177. 
 
 Rooks, benefit of, 148. 
 
 Rotifera, 105; reproduction of, 750; 
 drying up of, 66. 
 
 Ruminating Animals, stomach of, 198; 
 foot of, 652. 
 
 Runningj act of, 66C. 
 
 Saccharine aliments, 153 ; destination of, 
 155 157, 162; conversion of, into olea- 
 ginous, 156. 
 
 Sacrum, 624, 645. 
 
 Salamander, 732. 
 
 Saliva, secretion of, 190; union of with 
 food, 191. 
 
 Salpa, reproduction of, 728. 
 
 Salt, use of, 166, 167. 
 
 Sandhopper, 100. 
 
 Sanguification, 222224. 
 
 Sarcode, 128. 
 
 Saunderson, case of, 496. 
 
 Scapula, 634, 635, 637. 
 
 Sclerotic coat, 533. 
 
 Scurvy, 165. 
 
 Seal, 664, 665. 
 
 Sebaceous follicles, 38, 375. 
 
 SECRETION, general nature of, 245; act 
 of, performed by cells, 42, 354 ; distin- 
 guished from excretion, 352 ; influence 
 of mind upon, 353 ; transference of, 
 361. 
 
 Secreting follicles, 355, 356. 
 
 membranes, 355. 
 
 tubes, 357, 358. 
 
 Segmentation of yolk, 736, 756. 
 
 Semicircular canals., 518, 520. 
 
 Semilunar valves, 273. 
 
 Sensation, 432, 486 ; organs of, 9 ; general, 
 487 ; special, 488, 489 ; dependent on 
 supply of blood, 63, 487; modes of 
 exciting, 487, 488. 
 
 Sensori-motor actions, 430. 
 
 Sensorium, 429, 486. 
 
 Sensory ganglia, 452 ; functions of, 475 
 479. 
 
 Serous membranes, structure of, 28; 
 arrangement of, 43. 
 
 Serpents, 85, 203; lung of, 325; verte- 
 bral column of, 629. 
 
 Serpula, 314. 
 
 Serum of blood, 238. 
 
 Shark, teeth of, 188; brain of, 453. 
 
 Shell of Mollusca, 106 ; of Crustacea, 
 99, 170; of Bird's egg, 755. 
 
 Siamese twins, 729. 
 
 Sighing, 341. 
 
 Sight, sense of, 526575 (see Vision). 
 
 Silk-worm, voracity of larva of, 141. 
 
 Single Vision, 559. 
 
 Sitting posture, 654, 655. 
 
 Size, visual estimate of, 566. 
 
 Skeleton, position of, in different animals, 
 598; internal, of Vertebrata, 71, 599; 
 external, of Articulata, 93, 598; of 
 Mollusca, 106, 598; of Radiata, 118, 
 124, 127, 131, 132, 598; of Man, 616; 
 of Camel, 644; of Bird, 669; of Perch, 
 666 ; of Kangaroo, 661 ; of Seal, 664; 
 of Dugong, 664 ; of Bat, 669 ; of Pteio- 
 dactylus, 669. 
 articulation of pieces of, 601 605. 
 
 Skin, structure of, 36 38; exhalation 
 from, 370 374 ; secretions from, 375 ; 
 sensory papillae of, 37, 490 ; sensibility 
 of, 491495. 
 
 Skull, bones of, 617619. 
 
 Sloth, peculiar arterial distribution in, 
 264. 
 
 Slug, 106, 107. 
 
 Smell, sense of, 504509; concerned in 
 taste, 501. 
 
 Snail, 106, 107; torpidity of, 67; respi- 
 ration of, 320 ; reparative power of, 389. 
 
 Sneezing, 342, 508. 
 
 Sobbing, 341, 
 
 Societies of animals, 706 711. 
 
 Song of animals, 686. 
 
 Soul of Man, 721, 722. 
 
 Sounds, propagated by vibrations, 510 
 512; produced by insects, 676679; by 
 larynx, 682 ; pitch of, dependent on 
 number of vibrations, 523, 682. 
 
 Spatangus, 142. 
 
 Spectacles, choice of, 553. 
 
 Speech, articulate, 686691. 
 
 Spermatozoids, 731. 
 
 Sperm-cells, of plants, 724 ; of animals, 
 730,731. 
 
 Spherical aberration, 546, 547. 
 
 Sphinx atropos, sound produced by, 678. 
 ligustri, nervous system of, 441. 
 
INDEX. 
 
 603 
 
 Spiders, 98; circulation in, 293; respi- 
 ration in, 323 ; nervous system of, 447; 
 instincts of, 698, 700. 
 
 Spinal column, 71, 626632. 
 
 Spinal cord, 72, 451, 460; independent 
 powers of, 464 474; nerves, 451, 457, 
 460. 
 
 Spiracles of Insects, 320322. 
 
 Spleen, uses of, 224. 
 
 Sponge, 136, 137; circulation in, 296. 
 
 Stammering, 691. 
 
 Standing posture, 650 654. 
 
 Star-fish, 116119; reparative power of, 
 389; nervous system of, 434; develop- 
 ment of. 741. 
 
 Sternum, 633. 
 
 Stereoscope, 561. 
 
 Stomach, need of in animals, 8 ; form of, 
 197; in Ruminants, 198, 199; move- 
 ments of, 206. 
 
 Stomato-gastric system of nerves, 447 
 450. 
 
 Stork, 653, 
 
 Strychnia, action of, 474. 
 
 Sturgeon, continued action of heart of, 
 583. 
 
 Sucking, act of, 172,472. 
 
 Suffocation, 338, 339. 
 
 Sugar, formation of by liver, 366. 
 
 Sulphur in animal bodies, 166; sources 
 of, 167. 
 
 Supra-renal capsules, 224. 
 
 Sutures, 602. 
 
 Swallowing, act of, 192196. 
 
 Swimming, act of, 663 666. 
 
 Symmetry of disease, 380. 
 
 Sympathetic system of nerves, 60, 61, 
 461, 462. 
 
 Syncope, 271. 
 
 Synovial membranes, 44. 
 
 Syntonin, 16. 
 
 T. 
 
 Tadpole, 9597; circulation in, 287 
 288. 
 
 Tailor-bird, nest of, 705. 
 
 Tape-worm, 105 ; development of, 742. 
 
 Tardigrada, drying-up of, 66. 
 
 Taste, sense of, 499, 503. 
 
 Taurine, 364. 
 
 Tauro-cholic acid, 368. 
 
 Teeth, structure of, 54 ; development of, 
 174; cutting of, 175; cessation of 
 growth of> 176; continued growth of, 
 177; structure of, 178180; different 
 kinds of, 181183; first set of, 184; 
 motion of, in mastication, 178 180. 
 
 Teething, convulsions of, 174, 473, 474. 
 
 Tellina, 316. 
 
 Temperament, 718 
 
 Temperature, sense of, 497. 
 
 Tendons, structure of, 29; attachment 
 of muscles by, 605. 
 
 Testacella, 106. 
 
 Testis, 731. 
 
 Tetanus, 380. 
 
 Thalami optici, 458. 
 
 Thigh, bone and muscles of, 646. 
 
 Thoracic duct, 221. 
 
 Thorax, 328; movements of, 332. 
 
 Thumb, uses of, 643; reproduction of, 
 390. 
 
 Thunny, temperature of, 405. 
 
 Thymus gland, 224. 
 
 Thyroid cartilage, 680. 
 gland, 224. 
 
 Timbre of Sounds, 524. 
 
 Tissues, of Animals, distinctive pecu- 
 liarities of, 10 12; chemical composi- 
 tion of, 1321; fibrous, 2230; mem- 
 branous, 37 45; osseous, 49 54; cel- 
 lular, 3236, 4648 ; muscular, 55 
 59; nervous, 60 63; degeneration of, 
 from want of use, 58 ; continual decay 
 and renewal of, 67, 68; self-formative 
 power of, 382, 387; reproduction of, 
 390. 
 
 Tongue, nerves of, 501 ; mechanical 
 uses of, 503. 
 
 Torpedo, electricity of, 419, 421. 
 
 Torpidity of animals, 66, 309. 
 
 Tortoise-shell, 92. 
 
 Tortrix, nest of, 701. 
 
 Touch, sense of, 490499. 
 
 Tracheae of Insects, 321, 322. 
 
 Tranference of secretion, 361. 
 
 Transfusion of blood, 239. 
 
 Trematode Entozoa, 743. 
 
 Tricuspid valve, 272. 
 
 Tridacne, 119. 
 
 Tritonia, 316. 
 
 Trunk of Elephant, 172. 
 Insects, 173. 
 
 Tubercle, nature of, 386. 
 
 TUNICATA, 114; circulation in, 295 ; re- 
 spiration in, 316 ; luminousness of, 396 ; 
 nervous system of, 435, 436 ; gemmi- 
 parous reproduction of, 728 ; develop- 
 ment of, 752. 
 
 Turbo, anatomy of, 108. 
 
 Turnip-fly, voracity of, 147. 
 
 Turtle tribe, 83. 
 
 Tympanum, 516, 517. 
 
 U. 
 
 Ulna, 639. 
 
 Ungkaputi, 674. 
 
 Unity of Design, 261, 763. 
 
 Urea, 346, 367. 
 
 Ureters, 362. 
 
 Uric acid, 346, 348, 367. 
 
 Urinary apparatus, 368, 369. 
 
 bladder, 362. 
 
 excretion, purposes of, 346 
 
 348; effects of suspension of, 351; 
 composition of, 367 ; water discharged 
 by, 369. 
 
 Uterus, 761. 
 
604 
 
 INDEX. 
 
 V. 
 
 Valves of heart, 272, 273; of veins, 279. 
 
 Vascular area, 758. 
 
 Vegetative repetition of parts, 2. 
 
 Veins, 246 ; structure of, 248 ; pressure 
 of blood in, 249; arrangement of, 250, 
 266 ; flow of blood through, 277, 278 ; 
 valves in, 279. 
 
 Vena cava, 266. 
 
 Venaportae, 267, 366. 
 
 Ventilation, importance of, 336, 337. 
 
 Ventricles of brain, 458. 
 
 of heart, 257; action of, 270. 
 
 Ventriloquism, 525. 
 
 Vertebrae, structure of, 71, 628; classifi- 
 cation of, 626 ; number of, 627 ; con- 
 nexion of, in Reptiles and Fishes, 
 629 ; in Man, 630; in Birds, 630. 
 
 Vertebral column, 70, 71, 626632 ; first 
 development of, 757. 
 
 VERTEBRATA, general characters of, 70 
 76; nervous system of, 449 452; 
 skeleton of, 599 ; gemmation in, 729 ; 
 embryonic development of, 757 762. 
 
 Vessels, origin of, 393. 
 
 Vestibule of ear, 518, 521. 
 
 Vibrations, sonorous, 510 512; pitch 
 determined by number of, 523, 682. 
 
 Villi of mucous membrane. 41 ; absorp- 
 tion performed through, 41, 217. 
 
 Vision, dependent on light, 526, 542 ; 
 adaptation of eye to distinct, 543 553 ; 
 influence of attention on, 555 ; infe- 
 rences drawn from, 556 566 ; duration 
 of impressions, 567 ; distinction of 
 colours by, 568 570; erect, though 
 picture inverted, 558 ; single, with two 
 eyes; 559; double, 559. 
 
 Vitality, independent, of parts of organ- 
 ism, 65. 
 
 Vitreous humour, 536. 
 
 Vocal cords, 681. 
 
 Voice, confined to Vertebrata, 680 ; how 
 produced in larynx, 682; differences in 
 pitch and quality of, 683, 684. 
 
 Voltaic electricity, discovery of, 583, 584. 
 
 Voluntary movements, 589, 590. 
 Vorticella, reproduction of, 725. 
 Vowel sounds, 689, 690. 
 
 Vulture, skeleton of, 668. 
 
 W. 
 
 Walking, act of, 657. 
 
 Warm-blooded animals, 407409. 
 
 Wasps, nest of, 711. 
 
 Waste of the system, 160, 307, 345. 
 
 Water, passed off" by kidneys, 369 ; ex- 
 haled from lungs, 343, 344 ; from skin, 
 370374. 
 
 Water-Newt, 87. 
 
 Wax formed from sugar only, 155. 
 
 Webs of Spiders, 698. 
 
 Whale, mouth of, 185; peculiar arterial 
 distribution in, 265; sensibility of 
 surface in, 491; blow-holes of, 509; 
 propulsion of, in water, 665, 666. 
 
 Whalebone, 185. 
 
 Wheel-animalcules, (see Rotifer a}. ' 
 
 White fibrous tissue, 2329. 
 
 White of egg, 14, 755. 
 
 Wings, of Birds, 78, 668 ; of Bat, 669 ; 
 of Pterodactylus, 669; of Insects, 670; 
 action of, 667 672. 
 
 Winter eggs of Hydra and Rotifera, 735. 
 
 Wounds, healing of, 391 393; of ar- 
 teries, treatment of, 277. 
 
 Wren, intelligence of, 717. 
 
 X. 
 
 Xylocopa, nest of, 703. 
 
 Y. 
 
 Yawning, 341. 
 
 Yellow Fibrous tissue, 2329. 
 
 Yellow spot of retina, 554. 
 
 Yolk of egg, 733, 736, 754. 
 
 Yolk-bag, 733, 754. 
 
 Young animals, heat of, 408, 409. 
 
 Z. 
 
 Zoea, larva of Crab, 101. 
 
 Zoophytes, 117, 121; tissues of, 64, 577; 
 circulation in, 296; gemmiparous pro- 
 duction of, 726; development of Me- 
 dusae from, 738, 740; generation of, 
 738, 739. 
 
 THE END. 
 
 I. CLAY, PRINTER, BREAD STREET JlILL. 
 
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