BIOLOGY LIBRARY G THE Physiology of the Senses** BY JOHN GRAY M'KENDRICK, M.D., LL.D., F.R.SS.L. AND E. PROFESSOR OF PHYSIOLOGY IN THE UNIVERSITY OF GLASGOW ANp WILLIAM SNODGRASS, M.A., M.B., C.M. MUIRHEAD DEMONSTRATOR OF PHYSIOLOGY IN THE UNIVERSITY OF GLASGOW THIRD IMPRESSION (FOURTH THOUSAND) WITH 127 ILLUSTRATIONS LONDON JOHN MURRAY, ALBEMARLE STREET 1898 A II rights reserved GENERM GULIELMO TENNANT GAIRDNER MEDICINE ' APUD UNIVERSITATEM GLASGUENSEM PROFESSORI HUNG LIBELLUM COLLEGA COLLEGE DISCIPULUS MAGISTRO AMANTISSIMO UTERQUE ANIMO DEDICAVERUNT 107487 AUTHORS' PREFACE IT is the aim of this book to give a succinct account of the functions of the organs of sense as these are found in man and the higher animals. The Authors have refrained from discussing with fulness of detail either the comparative physiology of the senses or the numerous interesting questions of a psychological character that inevitably arise in connection with the study of the mechanism of sensory perceptions. Each of these aspects of the subject would require a volume for itself. On the other hand, a perusal of this volume, which has been written so as to be readily understood even by those who have not made physiology a special subject of study, will be a suitable preparation for entering upon the more recondite questions that underlie physiological psychology. The Authors have endeavoured to treat the physiology of the senses as fully as space would allow, and have also suggested comparatively simple experiments by which any one interested in the subject may test some of the statements for himself. They would also direct attention to the last chapter, in which an attempt is made to elucidate the nature of the physiological basis of sensa- viii Physiology of tJie Senses tion, in the hope that it may be found to be a contribution to speculative thought on this problem. While every page has been subjected to the careful consideration and revision of both Authors, it may be mentioned that the Introduction and the sections on Sight and Hearing have been mainly written by Dr. Snodgrass. J. G. M. W. S. UNIVERSITY OF GLASGOW, March 1893. CONTENTS GENERAL INTRODUCTION SENSORY MECHANISM PAGE Terminal organs of sensory nerves ..... 2 Nerve matter and nerves ....... 3 Nature of nerve current . . . . . . . 5 Origin of nervous system . . . . . . 7 Structure of nerves and nerve cells ..... 10 PATHS OF NERVOUS IMPULSES The spinal cord . . . . . . . . .12 The medulla oblongata . . . . . . .19 The cerebellum . . . . . . . . .19 The pons 21 The cerebrum ......... 22 SENSORY CENTRES IN THE CORTEX OF THE BRAIN The centre for vision ........ 30 The centre for hearing ........ 32 The centres for taste and smell ...... 34 The centre for touch ........ 34 The muscular sense ........ 36 Physiology of the Senses RELATION OF STIMULUS AND SENSATION PAGE Quality of sensation ........ 36 Quantity of sensation . . . . . . . -37 Sensations and perceptions ....... 39 THE SENSE OF TOUCH Structure of the skin 41 (1) The true skin ........ 42 (2) The epidermis . . . . . . . .42 Structure of tactile organs ....... 45 (1) Free nerve-endings 45 (2) Nerve-endings in corpuscles ..... 45 (3) Nerve-endings in connection with tactile hairs . . 50 Nature of the tactile mechanism ...... 52 Sensitiveness of the skin ....... 54 Sense of locality 56 Absolute sensitiveness ........ 56 Fusion of tactile impressions 58 .After-tactile impressions ....... 58 Information from tactile impressions ..... 59 Theories as to touch ........ 62 Sensations of temperature . . . . . . .64 Sensation of pain . . . . . . . . .67 The muscular sense ........ 68 THE SENSE OF TASTE The organs of taste 70 Minute structure of the gustatory organ . . . 71 Physical causes of taste . . ' . . . . -73 Physiological conditions of taste . ... . . .74 Differentiation of tastes . . . . . .76 General sensibility of the tongue ..... 78 Subjective tastes ........ 78 Nerves of the tongue ....... 78 Contents xi THE SENSE OF SMELL PAGE The organs of smell . . . . . . . . 80 Physiological anatomy of the nose . . . . .81 Physical causes of smell ....... 86 Chemical nature of odorous substances .... 87 Flowers and odours ....... 89 Odour and heat absorption ...... 89 Odours and ozone . . . . . .90 Odours and surface tension ...;.. 90 Special physiology of smell . . . . . . -91 Mode of excitation of the olfactory nerves 93 THE SENSE OF SIGHT I. STRUCTURE OF THE EYE Coats of the eyeball 97 Contents of the eyeball ....... 105 The optic nerve . . . . . . . . .109 Movements of the pupil . . . . . . 1 1 1 II. PHYSIOLOGY OF VISION i. Laivs of Dioptrics The physical nature of light . . . . . . 115 Reflection and refraction . . . . . . .116 Action of lenses . . . . . . . . .119 Formation of images by biconvex lenses . . . 1 20 Spherical aberration . . . . . . . .122 Chromatic aberration . . . . . . . .124 Optical properties of a system of lenses . . . . 125 2. The Dioptric System of the Eye Focal points . . . ... . . . .128 Principal points . . . . . . . . .129 Nodal points . . . . . . . . .129 Physiology of the Senses 3. Anomalies in the Eye as an Optical Instrument PAGE 1. Divergence of optic from visual axis . . . -131 2. Divergence of line of regard from line of vision . -131 3. Chromatic aberration . . . . . . -131 4. Spherical aberration . . . . . . .132 5. Astigmatism . . . . . . . . .132 4. Adjustment of the Eye for different Distances The near point of vision . . , . . . 137 Irradiation . . . . . . . . . . 140 Entoptic phenomena ........ 141 Examination of the interior of the eye . . . . 143 The visual angle ... ...... 145 The size of the retinal image ...... 148 The blind spot ......... 149 Action of light on the retina . . . . . . .150 Amount of light required to excite the retina . . .152 Persistence of retinal impressions ...... 152 5. Sensation of Colour Complementary colours . . . . . . .158 Colour as dependent on the retina . '. . . .158 Colour blindness . . . . . . . . -159 Coloured after-images . . . . . . . .161 Theories of colour vision . . . . . .161 6. Binocular Vision Movements of the eye . . . . . . .170 The ocular muscles . . . . . . . .172 How an object is seen as one with two eyes . . . 175 Perception of solidity . . . . . . . .180 The stereoscope . . . . . . . . .181 The telestereoscope . . . . . . . .184 Contents xiii PAGE Estimation of distance 187 Estimation of size . . . . . . .190 Illusions of vision . . . . . . . .192 SOUND AND HEARING The external ear 200 External meatus . . . .'.'.". . . 202 The middle ear 204 The Eustachian tube ........ 207 The chain of bones ........ 209 Movements of the bones . . . . . . .211 Response of the tympanic membrane to sound waves . . 214 Transmission of vibration by the auditory ossicles . . .218 THE INTERNAL EAR The osseous labyrinth 223 The auditory nerve 225 The membranous labyrinth ....... 225 The cochlea 228 The cochlear canal ........ 230 The organ of Corti . . . . . . . .231 The inner hair-cells . . . . . . . 233 The outer hair-cells ........ 235 Innervation of the cochlea ....... 237 AUDITORY SENSATIONS Physiological characters of sounds ..... 240 (1) Pitch 242 (2) Intensity or loudness ... . . 246 (3) Quality, timbre, or klang . . . . . -247 Resonators . . . . . . . . . -251 Analysis of compound tones by resonators . . . .252 Noise ........... 262 General mode of action of the ear . . . . . 263 Analytic power of the ear ....... 269 xiv Physiology of the Senses THE PSYCHICAL ELEMENTS IN AUDITORY SENSATIONS PAGE Externality of sound . . . . . . . .277 Direction of sound ........ 280 Distance of the source of sound . . . . . .281 Memory of sound . . . . . . . . 282 Mental receptivity for sound ..... . 283 Binaural audition . . . . ... . . 283 THE PHYSIOLOGICAL CONDITIONS OF SENSATION APPENDIX I The action of light on the retina ...... 299 APPENDIX II Derivations of scientific terms 302 INDEX 311 LIST OF ILLUSTRATIONS FIG. PAGE 1 . Cells of various Forms . " 7 2. Neuro-epithelial Cell 8 3. Section of Spinal Cord . . . . . . 10 4. Nerve Fibres ........ n 5. Multipolar Nerve Cells . . . . . . 12 6. Pyramidal Nerve Cells 12 7. Section of Spinal Cord . . . . . 16 8. Base of the Brain 21 9. Diagram of Encephalon ...... 23 10. Diagram of Side of Brain ...... 25 11. Median Aspect of Cerebral Hemisphere ... 27 12. Section of Skin 41 13. Grandry's Corpuscles ...... 46 14. Wagner's Corpuscle 47 15. Krause's End-bulb . 48 1 6. Large End-bulb ........ 48 17. Nerves with Pacinian Corpuscles .... 49 1 8. Pacinian Corpuscle 50 xvi Physiology of the Senses FIG. PAGE 19. Weber's Compasses . . . . . . 54 20. Sieveking's ^Esthesiometer . . . . . 55 21. Aristotle's Experiment ...... 61 22. Goldscheider's Cold and Hot Spots .... 64 23. Papilla Foliata 71 24. Taste bud 73^ 25. Section of Nasal Cavities . . . . . . So 26. Outer side of Nares 82 27. Olfactory Region of Rabbit . . . . . 84 28. Olfactory Cells . 85 29. Section of Eyelid ....... '96 30. Diagram of Eyeball . . . . . . . 98 31. Section of Cornea . . . ,. , . . . -99 32. Section of Conjunctiva ...... 100 33. Ciliary Region of Eye . . . . . . 101 34. Vessels of Choroid and Iris . . . . . 102" 35. Retina . 103 36. Retina 104 37. Rods and Cones ....... 105 38. Ends of Rods and Cones ...... 106 39. Pigment Cells . 106 40. Fibres of Lens ..... . . . 107 41. Diagram of Lens ....... io& 42. Structure of Lens . . . . . . . 109 43. Nerve Fibres in Retina . . . . . . 109 44. Optic Decussation . . . .. . . no 45. Reflection of Light 117 List of Illustrations ' xvii t FIG. PAGE 46. Refraction of Light . . . . . . . 118 47. Prism . . . . . . . . 119 48. Lenses . . . .... . . 119 49. Biconvex Lens . . . . . . . . 120 50. Conjugate Foci . . . . . . . 120 51. Virtual Focus . . . . . . . . 120 52. Formation of Image . . . . . . . 121 53. Effect of Absence of Lens from the Eye . . . 122 54. Use of Lens in Formation of Image in the Eye . . 122 55. Spherical Aberration . . . . . . 123 56. Chromatic Aberration . . . . . . 124 57. Achromatic Lens . . . . . . . 125 58. Course of a Ray through a Dioptric System . . 126 59. Image of a Point ....... 127 60. Schematic Eye . . . . . . . . 130 61. Astigmatism ........ 132 62. Cylindrical Lens for Astigmatism . . . . 133 63. Adjustment of Eye for Distance .... 134 64. Mechanism of Accommodation . . . . . 135 65. Reflected Images in Eye . . . . . . 136 66. Phakoscope . . . . . . . . 136 67. Scheiner's Experiment . . . . . 137 68. Different Forms of Eye . . . . . . 1 39 69. Irradiation . . . . . . . . 140 70. Formation of Purkinje's Figures .... 142 71. Principle of the Ophthalmoscope . . . . 144 72. The Visual Angle . . . . . . . 146 xviii Physiology of the Senses PAGE 73. Small Retinal Images . . . . . 147 74. The Blind Spot . ... ... 150 75. Fusion of Retinal Impressions . . . . . 153 76. Lambert's Method of studying Combinations of Colours . . . . . . . . 156 77. Diagram to illustrate the Young-Helmholtz Theory of Colour Vision . . . . . . . 162 78. Diagram to illustrate Hering's Theory of Colour Vision ......... 166 79. The Visual Field 171 80. Diagram of Ocular Muscles . . . . . 172 81. Section through the Orbit and its Contents . . 174 82. Binocular Visual Field . . . . . . 176 83. The Horopter .. 177 84. Formation of Homonomous Images . . . . 178 85- Formation of Heteronomous Images .... 179 86. Truncated Cone seen from above . . . . 181 87. Wheatstone's Stereoscope . . . . . . 182 88. Brewster's Stereoscope . . . . . . 183 89. Telestereoscope . . . . . . . 185 90. Causation of Luminosity . . . . . . 187 91. Estimation of Distance . . . . . . 189 92. Estimation of Space . . . . . . . 189 93. Visual Angle in Estimation of Size . . . . 190 94. Estimation of Size . . . . . . . 191 95. Error of Judgment in Estimation of Size . . . 192 96. Zollner's Lines . . . . . . . . 192 List of Illustrations xix PAGE 97. Illusion of Vision . . . . . . 193 98. Perception of Solidity . . . ... 195 99. Auditory Vesicle of Phialidium . . . . . 199 100. Right Auricle . . . . . . . 201 101. Diagram of the Ear . . . . . . . 203 1 02. Left Tympanic Membrane ..... 205 103. Horizontal Section through Ear .... 206 104. Incus and Malleus in Tympanum .... 207 105. Malleus . . . . . . . . 209 106. Incus . . . . . . . . . 210 107. Stapes . . . . . . . . . 211 108. Leverage Action of Malleus and Incus " . . 213 109. Vibrating Strings . . . . . . . 215 1 10. Wave-forms ........ 220 in. Osseous Labyrinth ....... 223 112. Formation of Semicircular Canals .... 224 113. Membranous Labyrinth ...... 225 114. Section of Macula Acustica ..... 226 115. Epithelium of Macula ...... 227 1 1 6. Otoconia or Otoliths 227 117. Osseous Cochlea ....... 228 1 1 8. Section through Coil of Cochlea .... 229 119. Section through Cochlear Duct ..... 231 1 20. RodsofCorti ........ 232 121. Surface View of Corti's Organ ..... 233 122. Section of Corti's Organ ..... . 234 123. Diagram of Change in Breadth of the Basilar Membrane 239 Physiology of the Senses PAGE 124. Double Syren . . . ;.'." 2 43 125. Pendular Vibrational Curves . . 249 126. Resonator . . . . . . . 251 127. Konig's Apparatus for studying Vibration of Air in Organ Pipes . . . 254 GENERAL INTRODUCTION THE senses are called into play when the condition of the body has been affected to a certain degree by external or internal agencies. A flash of light, a piercing sound, a gentle touch, may so act upon the bodily organism as to be followed by a sensation or mental state, by the conscious- ness of an alteration that has taken place in the body or in its environment. Sensitiveness is a property of all animals, and possibly of not a few plants. Some animals, indeed, are so low in the scale of organisation as to have no special parts set aside for the reception of sensory impressions, but every part of their body seems alike fitted to recognise varia- tions in its surroundings. As soon, however, as we pass to the higher grades of animal life we find certain parts or organs of sense whose duty is to keep the body in touch with its surroundings, and a nervous system which receives impressions and ensures the co-operation of all the individual elements of the body one with another. In order that sensations may be felt, we are provided with a central nervous system, or sensorium, from which nerve fibres pass outwards to all parts of the body, and at the ends of the nerve fibres certain structures or terminal organs may be found, which are so formed as to be capable of responding to some special variety of impression. Thus the terminal organ of the nerve of vision is insensitive to B 2 Physiology of the Senses the vibrations which, by acting upon the ear, originate changes leading to the sensation of sound. But, as will be shown in greater detail hereafter, this receptivity is largely conditioned by the special function of each sensory nerve centre. For the sensorium does not act as a whole, but is differentiated so that one part is devoted to one sense, another to another ; and when the nerves which lead to these nerve centres have been stimulated, it matters not what the nature of the stimulus to the nerve has been, the sensation experienced is always for each centre of one and the same kind. Thus the optical centre always gives rise to the sensation of seeing something, the auditory centre to that of hearing, the olfactory centre to sensations of smell, the gustatory centre to those of taste, and the tactile centre to touch. But, over and above these special forms of sensation, there are many vague or general sensations, such as those of heat or cold, of pain or fatigue, of pressure, resistance, and the like, which may seem to be felt in almost every part of the body ; and although each of these has in all probability its special nerve centre, yet no special terminal organ seems to be necessary. Special terminal organs, then, are developed for the senses of sight, hearing, smell, taste, and touch : their structure will, be described when we consider these senses separately. While we may readily distinguish these organs from one another by examination, either with the naked eye or the microscope, it is quite otherwise when we come to study the nerve fibres or nerve centres. So far as we can as yet determine, the nerve fibres which transmit the various sensory impressions are all of exactly the same composition and structure ; and though in recent times it has been found possible to localise with considerable accuracy the centres which are related to special sensations, still it has not been possible to fix upon the exact microscopical General Introduction 3 elements concerned ; in other words, physiologists cannot define the particular structure which alone is concerned in a given special sensation. We have no means of observing directly the minute molecular changes which go on in nervous substance ; we know only that this substance is very complex, and that during life it undergoes continual change, and is being constantly built up and broken down ; but neither the microscope nor chemical analysis has hitherto enabled us to determine why one centre should respond to one form of physical change, and another to another ; or why, when one part is stimulated, we have one kind of sensation, and when another part acts we have a different kind. A brief consideration of the composition and structure of nerve fibres and of nerve centres will enable us, however, to understand better the mechanism required for the trans- mission and recognition of a sensory impression* Nerve matter consists mainly of a variety of the sub- stance called protoplasm^ which is composed of a network of exceedingly fine fibres, the meshes of which are filled up with a fluid or semi-fluid substance. The exact chemical nature of protoplasm cannot be stated, for, in the first place, it is constantly varying during life by taking up nutrient matter of different kinds, and by throwing off certain waste substances, the product of vital action ; and in the second place, whenever we try to subject it to chemical analysis, it dies and is broken up into simpler chemical compounds. The most important chemical elements found in protoplasm are Carbon, Oxygen, Hydrogen, Nitrogen, Sulphur, and Phosphorus, and they are combined in such quantities and proportions as to form molecules of a highly complex nature. Now the more complex a chemical compound is, the more unstable it is ; or, in other words, the more easily may it be broken up, and resolved into simpler substances ; and hence we have in nervous tissues, which are largely com- 4 Physiology of the Senses posed of protoplasm, a material which may be very readily changed when acted upon by external forces. That a change does take place in nerve matter, when in action, has been inferred, although we cannot tell what the exact chemical constitution of nervous matter is, nor how it is changed. We know that for the efficient working of the nervous system there must be a full and unrestricted blood supply, bringing fresh nutrient matter to make up for waste, and oxygen, to promote chemical changes. The blood, again, must be free from impurities, or nerve action will be disordered. Surrounding nerve fibres we find a system of fine spaces or channels into which waste products of nerve action are poured, so as to secure their ready removal. When a nerve is acting we can also detect electrical changes corresponding in all probability to chemical trans- formations of nerve substance, but it must be admitted that no proof has yet been given of chemical changes in a nerve. Nerves. When a nerve has been kept in action for some time it apparently becomes fatigued that is to say, the irritation of the nerve ceases to be followed by the usual result. Thus, if we irritate a nerve passing to a muscle, the muscle at first responds by contracting, but by and by the stimulations of the nerve fail to call forth contraction. We then say the nerve is fatigued, and we may suppose that its vital activity is diminished from lack of time to build up its wasted substance, or from the ac- cumulation of waste products which prevent free action. Of late, however, physiologists are gradually coming to the opinion that there is no direct evidence -of fatigue in the nerve itself, and that the phenomena on which fatigue depends really occur in the apparatus or structure at the end of the nerve. From this point of view, nerve fibres may be regarded as not subject to much tear and wear, and they may act more like metallic conductors conveying General Introduction 5 currents of electricity, in which the current does not produce what are usually called chemical phenomena. Nature of Nerve Current. Structures known as nerve cells maintain the nutrition of nerve fibres. If a fibre is cut off from the cell with which it is connected it soon degenerates, and can no longer transmit a nerve current. But in a healthy nerve fibre a change known as a " nerve current " passes along it in both directions of its length from the point of stimulation. This change may be of a chemical kind, although, as already pointed out, there is no proof of this, and certain facts point the other way. We may imagine, on the chemical hypothesis, the fine nerve fibre as containing very complex and unstable molecules, which are readily broken up when acted upon by some external force. And just as when a match is set to one end of a train of gunpowder, the chemical change in the first granules of powder liberates energy, which gives rise to action in adjoining granules with disintegration of their substance and the formation of simpler compounds, so in nerve the change in one part or molecule may give rise to changes in adjoining molecules, and a so-called current will pass along the fibre. The fact that one current may follow another with great rapidity shows that the nerve substance is altered only in part and is quickly regenerated ; but, on the other hand, the too frequent or prolonged application of a stimulus is followed by diminished power of conductivity by a nerve, or of receptivity in the nerve centres. It was at one time sup- posed that the nerve current might be a purely electrical change, and that it travelled with the lightning velocity of the electric current. And no doubt in our ordinary experience this seems to be the "case. If the skin be touched with a red-hot iron wire, we seem at the same instant to feel the heat and pain. But by means of ap- paratus for registering minute intervals of time, and by 6 Physiology of the Senses stimulating a nerve in different parts of its length, we have ascertained that the rate of the nerve current is much slower than it would be were it purely electric ; and while there may be electric disturbance due to chemical change of the substance of the nerve fibre, that disturbance is probably only a minor part of the phenomenon. The electric flash passes at the rate of thousands of miles, the nerve current never faster than 200 feet, per second. 1 This rate of transmission of a nerve impulse must how- ever be carefully distinguished from the time occupied by nerve centres in undergoing those changes which may or may not lead to consciousness or the perception of the sensation. Thus if it be arranged that a person shall make a signal as quickly as possible after seeing a flash of light, it is found that the time which elapses between the two events will be greater than would be required for the sensory impulse to pass to the sensory centre, and thence by efferent nerves to the muscles of the limb by which the movement is effected. There is time required for the supervention of the conscious state, and for the generation of the volition which leads to the movement. This interval has been called the psycho-physical time, because we have here to do not merely with changes in nerve matter, but also with mental conditions and acts. The psycho-physical time varies considerably under different circumstances. Thus, for example, less time will be required if the observer has merely to make a prearranged signal that he has become conscious of some given sensory stimulus the so-called perception time than if he be asked to decide between two sensations, as of a low and high sound, 1 Recently it has been suggested that the nervous impulse is elec- trical, and that its velocity is slow compared with the velocity of elec- tricity, because great delay occurs at certain points along the fibre, known as the nodes of Ranvier. No positive proof has yet been offered of this somewhat fascinating theory. General Introduction or a bright or dull colour. This latter task requires nearly half a second of time. Even longer time is involved when the observer has to make a choice as to which of two stimuli he shall signal somewhat more than half a second being usually required. On the other hand, it is possible that a stimulus to a sensory nerve may give rise to movement quite indepen- dently of consciousness and volition. In this case the sensory impulse affects certain nerve centres, either in the spinal cord or the base of the brain, which are able so to respond as to cause an efferent current to bring about some /! 6 c FIG. i. Various forms of cells, a, cylindrical or columnar ; /, caudate or tailed ; c, fusiform or spindle-shaped ; d, ciliated, having fine filaments pro- jecting from their free surface ; e, stellate or branched. muscular action. In this case the time occupied in the nerve centre is less than when volition is involved, but is, however, greater than would be required for the simple passage of the nerve current along a nerve. It amounts to about .05 of a second. Origin of Nervous System. We have said that the nerves are largely composed of protoplasm. But this substance exists in all parts of the body, at least in early life. W T hen we examine microscopically the tissues of the. body during the earliest periods of its existence, we find that it is composed of minute vital elements to which the name of cells or corpuscles has been given. 8 Physiology of the Senses These cells are composed of protoplasm, and usually contain an exceedingly minute body, called the nucleus, whose composition is in certain respects different from that of protoplasm, and the cells may, moreover, be surrounded by a cell wall of less actively vital matter. At first the various cells of the body closely resemble one another, but as growth advances they become differentiated in form (Fig. i ) and structure in order to perform special functions, some cells g'oing to build up the skin, some the muscles, some the nervous tissues and the like. In low forms of animal life, however, these cells are often not so highly differ- entiated as in man. Thus in the sea-anemone (Actinia), among the cells which go to form the outer covering or skin, we find certain cells from the free surface of which a hair -like filament projects, while from their attached border a number of processes pass inwards and join with like process'es from other similar cells. These hair cells form rudi- mentary sense organs (Fig. 2). FIG. 2. N euro-epithelial cell from the upper nerve ring , , , of Carmina hastata. c, sense hair passing to the " Ultner, in the surface ; the two long thin processes join a ring of network formed nerve fibres containing ganglion cells. (Hertwig.) IT r by the union of the processes just mentioned may be found cells which seem to have sunk inwards from the surface showing like processes, and regarded by Balfotir a as an elementary sensory nervous apparatus. In general, it may be said that a study of the facts of development shows us that nerve cells appear at first upon the surface of the body, but that during the growth of the organism the cells become shut off from the surface; and in order to maintain their connection with the 1 F. M. Balfour, Comparative Embryology, vol. ii. p. 332. General Introduction 9 periphery, long processes called nerve fibres pass from the cells thus deeply embedded to the surface. Nerve cells may occur singly, or more commonly they are found gathered together in groups called ganglia, the individual cells being known as ganglionic nerve cells. These ganglionic cells are more or less closely connected with one another by means of nerve fibres, and thus community of action is established. In insects, for example, we find two rows of ganglia, the cells of which are united by nerve fibres both longitudinally and transversely. Sensory impressions pass by nerve fibres to these ganglia, and again, by other fibres passing out from these 'ganglia and ending in muscular tissue the move- ments of the body are regulated. In insects, too, it may be noted that the ganglia connected with organs of special sense, such as the eye or ear, are larger than the others. A further development of the nervous system arises through the fusion of ganglia with each other, so that the brain and spinal cord of vertebrate animals may be regarded as a vast number of ganglionic cells and nerve fibres bound into one consistent whole by a fine network of a connective tissue, and by an interlacing of nerve fibres. The nerve fibres connected with the brain and spinal cord may be divided, according to their function, into two sets those which transmit sensory impressions inwards, the afferent nerves, and those which have to do with the regulation of such changes in the body as lead to motion or secretion, and known as efferent nerves. Thus the sensa- tiojt of pain, as, for example, toothache, originates from stimulation of a sensory or afferent nerve ; and the move- ments involved, say, in swallowing, from stimulation of efferent nerves passing outwards from the brain or cord. Structure of Nerves and Nerve Cells. The progress of research tends to show that fibres of varying function io Physiology of the Senses always occupy a similar relative position in the central nervous system. As long ago as 1822, Majendie showed that the afferent or sensory fibres always pass into the spinal cord by what is known as the posterior root of a spinal nerve, while efferent or motor fibres emerge from its anterior aspect. See Fig. 3. But it has been found a matter of the greatest difficulty to determine accurately the course of fibres in the cord itself. When we look with the naked eye at a cross section of the spinal cord, we can see at a glance that it is made up apparently of two kinds of material, the outer part being whiter than the inner, which is i FIG. 3. Portion of the spinal cord from the region of the neck, with roots of the nerves (slightly enlarged), i, i, The anterior median fissure ; 2, the posterior median fissure ; 3, the anterior lateral groove, from which the anterior roots of the nerves are seen emerging ; 4, posterior lateral groove where the pos- terior nerve roots enter the cord ; 5, anterior roots, to the right passing the ganglion ; 5', anterior root cut across ; 6, posterior root with ganglion at 6' ; 7, the nerve made up of anterior and posterior fibres ; 7', the first branches from the compound nerves. (Allen Thomson.) of a gray colour. This whiteness is due to the fact that the protoplasmic substance of the nerve fibre, the part which conveys the nerve current, the so-called axis-cylinder of the nerve, is, in the greater part of its length, surrounded by a sheath of fatty material, known as the white substance of Schwann (Fig. 4), which in bulk gives a creamy white appearance to a group of nerve fibres. This, in turn, is enclosed by a thin transparent covering known as Schwamrs sheath, or the primitive sheath. But in the central parts of the cord the white substance is to a large extent absent, and we here find among the fibres great numbers of ganglionic General Introduction nerve cells. These cells vary much in shape, but are mostly of the form called multipolar, on account of the large number of poles or nerve fibres which spring from them (Fig. 5), while others, and more especially the cells in the posterior part of the gray matter, are often spindle-shaped or pyramidal (Fig. 6). These cells are in direct connection, for the most part, with efferent motor nerves ; and if they are destroyed by disease or otherwise, the nerve fibres with which they are connected quickly degenerate, and the parts supplied by them are paralysed. These are the cells which may be roused to action by the sensory nerves quite apart from any conscious sensation. If the foot of a person in profound sleep be lightly tickled, it will be drawn away without the sleeper being dis- turbed. If the middle or upper parts of the spinal cord be destroyed with- out injury to the lower part of the cord, while sensory impressions can pass to this lower part, and can set up changes in the nerve cells which lead to the movement of the lower f . FIG. 4. Nerve fibres. B, part of the body or legs, these move- T he axis - cylinder sur- ments are performed unconsciously, and therefore cannot be controlled or restrained by an act of will, since the impression is not transmitted to the brain. Man is only conscious when certain parts of his brain have been affected. Unless sensory impressions are transmitted to these parts, or unless these parts have been called into action by some variation in their chemical composition, there will be no consciousness. If these parts are ill- rounded by the white substance of Schwann, which is interrupted at A, a node of Ranvier, and contains a nucleus at C. The external line represents the primitive sheath or neurilemma. 12 Physiology of the Senses developed and ill -nourished, sensation will be feeble or perverted ; and if they are destroyed, the possibility of consciousness will be permanently lost. FIG. 5. Multipolar nerve cells in the anterior part of the gray matter of the spinal cord, ar, anterior roots of emergent nerve fibres coming from the nerve cells, gc \ nf^ nerve fibres cut across. PATHS OF NERVOUS IMPULSES i. The Spinal Cord. When we seek the exact paths, how- FlG. 6. Pyramidal nerve cells found principally in the brain. General Introduction 13 ever, along which sensory impulses pass up the cord to the brain, we are met by many difficulties. We can only infer that an animal feels some sensation ; we cannot enter into its , consciousness of it. When the foot of an animal is pinched we believe that it feels pain because of some movement it makes, or some sound it utters, and because we know that a similar pinch to our own feet would cause a sensation of pain in us. But if, by careful and gradual operation, the greater part of the brain has been removed and the animal has survived, we find that the application of the stimulus may still educe movements or cries, while we cannot suppose the animal to be conscious of what it does. Another difficulty in the determination of the sensory path is that of isolating or destroying a certain part of the cord without injury to other parts, and without setting up irrita- tion or shock which may lead to erroneous inferences. It is impossible to reach the deeper parts of the cord without injuring the more superficial, and the individual fibres are so small that it is very much a matter of guess-work whether we have cut the parts we wish or not. We know that sensory fibres enter at the posterior part of the cord, that- some of these fibres pass directly into the gray, some into the white, matter ; but hitherto it has not been possible to trace these fibres to any extent, on account of their bending away from the plane of section. It has been observed that at different stages of development certain strands of fibres are superposed, as it were, on others ; and by examining sections of cords of animals at different ages the connections of special tracts have been traced. Another method of study which has afforded valuable results is based upon the observation that when nerve fibres have been cut off from the nerve cells with which they are connected, the fibres quickly degenerate ; and thus it has been found possible to trace the line of de- 1 4 Physiology of the Senses generation for some distance. Similarly, in cases of loss of sensation in disease, it may be possible to discover, by post-mortem examination, the part which has suffered; but it will readily be seen that this, and the above- mentioned methods of research, can only afford rough and inaccurate results. One interesting fact we can con- clusively settle from cases of disease in the human being is, that different kinds of sensations travel by different paths in the cord. A lesion which may cut off the pos- sibility of feeling pain in a given part of the body, may leave it still susceptible to sensations of heat and cold ; or the sensation of touch may be present while the sensa- tion of pain cannot be aroused. From this we see that nerve impulses giving rise to sensations of touch, of pain, of temperature, of the muscular sense, must pass upwards to the sensorium by different paths, one of which may be cut off while the others remain. We may also learn from such cases that the sensory fibres, after passing up the cord, terminate in the opposite side of the brain from that' in which we seem to have the sensation. Where the sensory fibres cross from one side to the other is not known. The experiments of the older physio- logists, and more especially those of the French observer, Brown -Sequard, seemed to show that the sensory fibres cross to the other side almost immediately after their entrance into the cord ; but later workers in this field of research maintain that the majority of the sensory fibres do not cross at once, but pass up almost to the base of the brain before they change sides. In some parts of the cord, however, the fibres do cross from the right to the left side, and vice versa, or decussate, as it is called ; so that sensory fibres from the right side of the body pass to the left side of the brain, and from the left side of the body to the right side of the brain. It is probable that they do not extend General Introduction 15 continuously, however, as single threads, from the peri- phery to the sensorium. We have seen that the stimulation of a sensory nerve, say in the right foot, may give rise to changes in the lower part of the cord, and hence to involuntary movements of which we are totally unconscious ; or it may cause a sensation by stimulation of the brain. Now we do not find nerve fibres branching except at their endings. Hence we are led to conjecture that the majority of the sensory fibres pass immediately into the gray matter of the cord and there become connected with nerve cells. From these some fibres may pass to the cells in the cord connected with efferent nerves, while other fibres pass upwards to the brain. To givea slightly more definite idea of the paths pur- sued by the different sensory fibres, we may refer to Fig. 7, in which we have a diagrammatic representation of a transverse section of the spinal cord divided into tracts or areas, which are to be understood as indicating bundles or columns of fibres running side by side and communicating freely with one another, but each containing, in the main, fibres of special origin and function. Thus, for example, the nerve fibres which convey painful impressions appar- ently pass into the gray matter of the cord, for if the gray matter be completely divided at any given level of the cord, there will no longer be a sensation of pain when the parts are injured which send nerve fibres to the cord below the level of section. From the gray matter fibres prob- ably pass outward and upward in the anterior root zone (ar, ar, Fig. 7). Suppose the gray matter were divided close above the region where sensory fibres from the legs pass into the cord. Then we might lacerate the foot, and though we might feel that it was being touched, we would have no sensation of pain from the operation. We distin- guish, therefore, between analgesia, or that condition in which painful sensations cannot be excited, and ancesthesia. i6 Physiology of the Senses or the state in which we are insensitive to tactile sensa- tions. It will readily be understood that analgesia of any part of the body might lead to disastrous consequences. Thus among paralytics we find patients who feel no pain in, and are unable to move, the lower limbs. They will allow some part, such as the heel, to remain motionless on dc FIG. 7. Transverse section of human spinal cord, ah, ah', anterior horns of gray matter ; ph, pk' , posterior horns of gray matter ; ar> ar' ', anterior root zones ; pr,p^ ', posterior root zones ; P, P', pyramidal fibres of lateral columns (mainly motor in function) ; T, columns of Tiirck (motor in function) ; G, columns of Goll ; dc, dc 1 , direct cerebellar tract ; c, anterior commissure ; below r, central canal of cord lined with columnar epithelium. (Ross and Young.) a couch so long that the circulation of blood in it ceases, and its vitality may be seriously impaired. Similarly where the front of the eyeball has become insensitive to pain, the presence of small foreign bodies in the eye being no longer felt, such bodies accumulate in the eye, interfere with its well-being, and give rise to ulceration and de- struction of the ball. To the healthy body pain is nature's indicator of danger ; the burnt child dreads the fire. General Introduction 1 7 Tactile impressions in man pass upward, for the most part, in those columns of the cord which lie between the posterior roots of the spinal nerves. In this part, besides the paths for the stimuli which give rise to the sense of touch, we have probably also those which excite the sensa- tions of heat and cold, of pressure and resistance, and of tickling. That this is so is most distinctly shown by the study of changes in the cord during the progress of the disease known as locomotor ataxia a disease, one promi- nent symptom of which is disorder of the power of walking. Patients subject to this disease usually suffer, in the earlier stages, from severe pains shooting apparently into the legs, and due to -inflammatory changes in the posterior horns of the gray matter. Then the areas immediately adjoining these (pr, prj Fig. 7) become diseased, and the muscular sense is impaired, so that there is not the accustomed guide to the muscles as to the amount of force required for movement, and the patient tends to lift his feet too high and to set them down with a stamp. He is not able to judge accurately as to the weight of his limbs, nor of heavy masses attached to them. Then the delicacy of his sense of touch becoming impaired, he has the feeling, even when walking on rough ground, as if he were treading on velvet. No longer receiving the wonted guiding* im- pressions from his feet, he must watch with 'his eyes his movements in walking, directing his steps by his sense of sight, and if he shuts his eyes he staggers and falls. His muscles act spasmodically, independently of each other, without due co-ordination. At first the motor power re- mains, but eventually it too may become involved, and the patient is paralysed for motion as well as sensation. In some animals, such as rabbits, it has been supposed that the tract for tactile sensations is in the lateral columns ; but all experiments on the sensory tracts are very apt to C 1 8 Physiology of the Senses be deceptive from the difficulty of interpreting the resulting phenomena. As the sensory tracts pass upward in the spinal cord they are somewhat modified in size and in relative position, owing to intercommunication and the entrance of fresh fibres, but on the whole the strands preserve the same general relationship. But just as the cord enters the cavity of the skull it enlarges, to form a portion about an inch and a quarter long, known as the bulb or medulla oblongata. Here the arrangement of the white and gray matter is much modified, and mixed with the fibres con- ducting nerve impulses to and from the brain we find several ganglionic centres which are of vital importance. Here, for example, we find centres which preside over the great functions of respiration and the circulation of the blood, besides such as regulate the acts of mastication and of swallowing, vocal utterance, the secretion of saliva and of sweat. To these centres come efferent impulses from all parts of the body, impulses which may never indeed give rise to conscious sensation, but which, acting on the nerve centres of the medulla, so stimulate and affect them as to keep them constantly ready to respond to the needs of the organism. Under all the ordinary circum- stances of life, whether we be sleeping or waking, these centres pursue the even tenor of their way. Influenced by some great emotion, at some great crisis, when all the energy of our being is centred upon one thought or one swift effort, these centres may stand in abeyance for the moment ; nay, the pang may be so great that the vital chain is for ever broken, but as a rule we are unconscious even of the results of their activity. All the great vital functions go on unheeded, unless when some cause arises to interfere with their free and unimpeded action. But their influence over conscious life is none the less potent ; without their General Introduction 19 action the great receptive centres of the brain would be powerless. The freedom we have from the necessity of consciously watching over these things alone renders a higher life possible. \ 2. The Medulla. The difficulties experienced in ascer- taining the paths of sensory influences in the cord are great, but they are vastly increased when we come to examine the medulla. We have, in fact, to depend mainly upon anatomical and pathological research for what little we know, and it is only possible to separate certain fibres which we can positively affirm to be associated with motor functions. The upward bound fibres passing through the medulla may either go to the ganglia at the base of the brain, to the cerebellum (Fig. 9, B), or to the cerebral hemispheres. A complete description of the structure and functions even of the parts of the brain devoted in the main to the sensory activities, is beyond the scope of the present work. We can only attempt to give a mere outline of the cerebral mechanism. 3. The Cerebellum. The cerebelhtm, or little brain, is connected by strands of nerve fibres both with the cord and with the brain proper ; and though in all likelihood it acts as a co-ordinating or arranging centre f r the nerve currents that induce complicated movements, we have no evidence that it contains any sensory centres. No pain is felt when its sub- stance is injured, nor can we detect any alteration in general or special sensitivity. Some physiologists have advanced the view that it may be connected with the muscular sense. The staggering gait and irregular movements characteristic of an animal whose cerebellum has been destroyed, indicate a loss of a regulating centre which normally is at work. We may understand this if we reflect for a moment upon the complicated nature of the movements we habitually 2o Physiology of the Senses perform. Walking, for example, involves the co-ordinated action of many groups of muscles, each of which must act exactly at the proper time and with most delicately adjusted force. The acquirement of the power is only gained after many attempts, and the mere preservation of the upright attitude of the body is only possible when the sensory impressions from the feet and limbs are duly transmitted and take their place in the complex sum of afferent impulses. Of the means or methods by which the multifarious peri- pheral impressions are correlated, and after the nerve centres are excited, the adjustment is carried out and the different muscles set in regulated motion, we know nothing. We do not even think how a movement is to be made. We simply will something to be done, and it is done ; but of the intervening causal chain we are quite unconscious. We think of the end and not of the means. In that sense our movements are automatic ; and it is interesting to note that the more any given movements are practised, the more auto- matic they become ; and the more purely automatic they are, the more accurately are they adapted to their aim. Illustra- tions of this are afforded us in all employments where a certain small piece of work is done to the exclusion of all else. The hands will work busily while the thoughts are far away. In such a case we have the same sensory im- pression travelling to the same centre, giving rise to the same outflow of energy, and along the same efferent channels, and an unconscious memory of what has been required in the past enables us to determine without effort the neces- sities of the present. But vary the surroundings a little, and new conscious efforts must again be made, and the work requires longer time and conscious effort and attention. It is possible that the necessary fusion of impressions takes place in the ganglia at the base of the brain, and messages to the cerebellum act through its cells and fibres as through General Introduction 21 distributing centres to the muscles ; but of this we cannot at present speak with certainty. 4. The Pons. The medulla, as we have seen, is con- FIG. 8. Base of the brain, i, i, The longitudinal fissure dividing the hemi- spheres ; 2, 2', 2", the anterior lobe of the brain ; 3, fissure of Sylvius ; 4, 4', 4", the middle lobe of the brain ; 5, 5', posterior lobe ; 6, bulb or medulla oblon- gata ; 7, 8, 9, 10, the inferior surface of the cerebellum. . The figures I to IX indicate cerebral nerves : thus I is the olfactory bulb removed on the right side ; II is the optic nerve with decussation ; V, the sensory nerve of the face and part of the scalp; VII, the auditory nerve; VIII, the glosso- pharyngeal with sensory fibres from mouth and throat ; III is on a crus cerebri ; VI and VII are placed on the Pons Varolii ; X, the first nerve emergent on the neck. nected with the cerebellum ; the rest of the fibres passing upwards from it enter a structure known as the pom Varoliij 22 Physiology of the Senses or bridge of Varolius (Fig. 8, VI, VII ; Fig. 9, C), so called because numerous fibres pass through it from one side of the cerebellum to the other, and these form a transverse prominence like a bridge across the main course of the nerve fibres which pass up and down. In the pons, as in the medulla, we find many nerve centres mixed with the fibres. Here, for example, among others are situated the centres of origin of the great nerve the fifth cranial (Fig. 8, V), or main path for general sensory impressions from the face and scalp, of the auditory nerve (Fig. 8, VII) coming from the ear, and of the nerves which control the movements of the muscles of the face. Fibres carrying painful, thermal, and tactile impressions probably pass up through the centre of the pons, where also some of them decussate. The motor fibres are mainly in front of, and the nerve centres behind, these thermal and tactile paths. 5. The Cerebrum. Fibres from the pons and cerebelhim pass to the cerebrum, or brain proper, by the connecting strands known as the cerebral peduncles. These slope upwards and forwards, and the anterior and lower fibres branching outward as they enter each side of the brain are known as the legs of the brain, or crura cerebri. The upper and back part of the peduncles is composed mainly of gray matter, and when seen from above shows four slight elevations known as the corpora quadrige- mina. It is of interest to note that the corpora quadrigemina receive nerve fibres from the eyes through the optic tracts, and are concerned in the mechanism of vision. Destruction of one side causes blindness in the eye of the opposite side, with loss of power of accommoda- tion of the pupil of the eye. Whether they are the seat of conscious sensation is, however, very dubious. They are small, and hidden away under the superposed cerebral mass in man, but the corresponding structures General Introduction 23 known as the optic lobes in birds, reptiles, and fishes are large and important relatively to the rest of the brain. The most attractive hypothesis is that they act in man as centres for the fusion of impressions coming from the eyes by the separate nerve fibres, and for the regulation of bodily or ocular movements dependent upon visual impressions, but FIG. 9. Plan in outline of the encephalon, or central nerve system within the skull, as seen from the right side. A, Cerebrum ; B, Cerebellum ; C, Pons Varolii ; D, Medulla oblongata ; a, cms cerebri or cerebral peduncle ; b superior, c middle, d inferior cerebellar peduncles ; b is placed just in front of the corpora quadrigemina ; e, fissure of Sylvius ; f anterior, g middle, h posterior lobes of cerebrum. that for conscious vision the gray matter of the cerebrum must be likewise affected. In front of the corpora quadrigemina, and lying at the base of the brain, lie two large ganglionic masses on each side of the middle line the thalami optici and the corpora striata between which passes an important set of fibres from the crura, known as the internal capsule. Many 24 Physiology of 'the Senses sensory fibres are believed to enter the optic thalami, coming either by way of the corpora quadrigemina, the crura, or the internal capsule, while other fibres join the thalami with the cerebral hemispheres. From their con- nection with the corpora quadrigemina we find, as might have been expected, that injury to the optic thalami, more especially in their hinder parts, causes visual disturbance, but the thalami are probably connected with many other sensory fibres besides those of vision. The human brain, when stripped of its investing mem- branes and viewed from above, is seen to consist of two masses or hemispheres of a grayish colour externally, a deep furrow running between the hemispheres from before backward, at the bottom of which is a broad band of white nerve fibres, the corpits callosum, joining the two masses. The surface is not smooth, but thrown into numerous folds, convolutions, or gyri, between which lie depressions of vafy- ing depth called sulci, or fissures. Such convolutions are absent from the brains of many of the lower forms of animals, and even in man, in the earliest periods of life, and they are present in the adult brain in order to allow for increased area of the cerebral surface or cortex. At a first glance these ridges and furrows seem to be quite irregular and devoid of arrangement, but a study of the comparative appearances of many human brains leads us to see that though there may be slight divergencies in the number, depth, and regularity of the convolutions, these are largely formed on the same plan. We see that the brain may be regarded as made up of several lobes (Fig. 10), which are named according to the part of the cranium in which they lie, and that each lobe has a definite number of ridges and furrows, the names of which are given in the explana- tion of Fig. 10. So long as it was supposed that the brain acted as a whole, and that no special function was associated General Introduction 2 5 with any particular area, the relationship of the convolutions was deemed of comparatively little importance. Now, F FIG. 10. Semi-diagrammatic view of the left side of the brain. F, Frontal lobe ; P. Parietal lobe ; O, Occipital lobe ; T, Temporo-sphenoidal lobe ; S, fissure of Sylvius ; S' horizontal, S" ascending branch of the same ; c, central sulcus or fissure of Rolando ; A, ascending frontal ; B, ascending parietal convolu- tion ; Fj, Fo, F,3, superior, middle, and inferior frontal convolutions \f\,f-i, superior and inferior frontal sulcus ; /-j, praecentral sulcus ; PI, superior parietal lobule ; Po supra-marginal gyrus, and P% angular gyms, parts of inferior parietal lobule ; ip, intra-parietal sulcus ; on, end of calloso-marginal fissure (see Fig. n); Oi, Oo, O^, first, second, and third occipital convolu- tions ; po< parieto-occipital fissure ; o, transverse occipital fissure ; 0o> inferior occipital fissure ; TI, To, T$, first, second, and third occipital convolutions ; II, fa, first and second temporo-sphenoidal fissures. (Ecker.) however, it is well to know the names and positions of the various lobes, convolutions, and furrows, so as to be able to 26 Physiology of tJie Senses understand descriptions of special areas of the surface. The lobes are named from the special bones of the skull with which they come into contact, and are known respec- tively as the Frontal,- F, the Parietal, P, the Occipital, O, and the Temporo-sphenoidal, T, lobes. It will be seen by reference to Fig. 10 that there are two specially deep and well-marked fissures, those of Rolando (r, Fig. 10) and of Sylvius (S, S', Fig. 10), the latter of which is branched, S". To the front are three well-marked and constant ridges, the frontal gyri (Fp F 2 , F 3 ), separated by two furrows, / p f^ In front of the fissure of Rolando we have the ascending frontal convolution, and behind it the ascending parietal, behind which again, and separated by the intra-parietal furrow, lie two other parietal convolutions, P 1 and P 2 . The second parietal convolution becomes continuous with the superior of three temporal convolutions, T v T 2 , and T 3 , by a bend round the end of the Sylvian fissure immediately below P 2 , known as the supra-marginal convolution, and the superior and middle temporal convolutions are connected posteriorly by a small angular convolution at P 2 ', commonly known as the angular gyrus. Parts of three occipital con- volutions, O 1 , O 2 , O 3 , are seen. Of the various fissures that of Sylvius is much the most marked, the others being merely furrows. The Sylvian fissure really indicates that the posterior part of the hemi- sphere has in the process of development been bent round and packed away under the frontal and parietal regions. When the Sylvian fissure is opened up there is seen a small pyramidal mass of gray matter the island of Reil the convolutions of whose surface, being hidden when the brain is in its natural state, are known as the gyri operti. The letter S lies external to the spot in which these convolu- tions are to be found. When the two hemispheres are separated by an antero- General Introduction 27 posterior section in the median plane of the body, each internal surface is seen to present certain fissures and con- volutions, the principal of which are (i) the marginal gyrus F 1 , which is really the internal aspect of the superior and ascending frontal convolutions and ends posteriorly at the fissure of Rolando ; (2) the gyrus fornicatus^ Qf, _po FIG. ii. Semi-diagrammatic view of the right cerebral hemisphere in its median aspect. CC, corpus callosum divided vertically; Gf, gyrus fornicatus ; H, gyrus hippocampi ; 7i, sulcus hippocampi ; U, uncinate gyrus ; cm, calloso- marginal fissure ; F, first frontal convolution ; c, end of central sulcus (fissure of Rolando) ; A, ascending frontal ; B, ascending parietal convolu- tions ; PI', praecuneus ; po> parieto-occipital fissure ; .:*v * o" 64 Physiology of the Senses Sensations of Temperature. The skin is also the organ by which we appreciate temperature, and it is not improbable that there are thermal nerves and thermal end-organs. Sensations of heat and cold can only be felt by the skin. Direct irritation of a nerve does not give rise to these sensations. Thus if we plunge the elbow into very hot water, or into ice-cold water, we do not experience heat or cold by thus irritating the ulnar nerve, which lies here just below the skin, but there is a painful sen- sation referred to the extremities of the nerve. The ex- posed pulp of a diseased tooth, when irritated by hot or cold fluids, gives rise to pain, not to sensations of tem- perature. Recent obser- ,* vations show that there % are minute areas on the > . skin in which sensations ,\ of heat and cold may be ,e" {* more acutely felt than in adjoining areas. Some of these areas are more FIG. 22.-C Cold spots ; H, hot spots, sensitive to cold, and (Goldscheider.) hence are called cold spots, and others, more sensitive to heat, have received the name of hot spots, and they appear to be, or to con- tain, end-organs, arranged in points, subservient to a temperature sense. A topographical view of such spots on the radial half of the dorsal surface of the wrist, as depicted by Goldscheider, is shown in Fig. 22. A simple method of demonstrating this curious phenomenon is to use a solid cylinder of copper, eight inches in length, by J. inch in thickness, and sharpened at one end to a fine pencil-like point. Dip the pointed end into hot water, close the eyes and touch parts of the skin. When a hot spot is touched there is an acute sensation of burning. The Sense of Touch 65 Such a spot is often near a hair. Again, in another set of experiments, dip the copper pencil into ice-cold water and search for the cold spots. When one of these is touched, a curious sensation of cold, as if gathered to a point, is experienced. It will be found, in this way, that in a given area of skin there may be hot spots, cold spots, and tactile spots. Cold spots are more abundant than hot spots. The spots are arranged in curved lines, but the curve uniting a number of cold spots does not coincide with the curve forming a chain of hot spots. Both spots may be perceived as double, by the Weberian method, but we can discriminate cold spots at a shorter distance than hot spots. Thus on the forehead cold spots have a minimum dis- tance of .8 mm. and hot spots 4 mm. ; on the skin of the breast, cold spots 2 mm. and hot spots 5 mm. ; on the back, cold spots 1.5 mm. and hot spots 4 to 6 mm. ; on the back of the hand, cold spots 3 mm. and hot spots 4 mm. ; on the palm, cold spots .8 mm. and hot spots 2 mm. ; and on the thigh and leg, cold spots 3 mm. and hot spots 3.5 mm. No terminal organ for this sense has yet been found. Electrical and mechanical stimulation of the hot or cold spots call forth the corresponding sensa- tion. This indicates that a special terminal organ probably exists. It is highly probable that there are nerve filaments specially devoted to conveying to the nerve centres what may be termed thermal impressions, and possibly there may be parts of the brain specially connected with the translation of such impressions into sensations of tempera- ture. A leg sent to "sleep" by pressure on the sciatic nerve will be found to be less sensitive to heat, but dis- tinctly sensitive to cold. In some cases of disease it has been noticed that the skin is sensitive to a tem- perature above that of the limb, but insensitive to cold. F 66 Physiology of the Senses Tactile and thermal sensations affect each other. Thus a weight is always felt to be heavier when it is cold than when it is hot, and the minimum distance at which two compass points are felt is diminished when one point is warmer than the other. Not unfrequently in diseases of the nervous system tactile sensibility may be diminished or increased without the sense of temperature being affected, and the reverse condition also occurs. The skin, as an organ for the appreciation of tempera- ture, may be considered from another point of view. In a warm-blooded animal (that is an animal possessing a heat- regulating mechanism by which the mean temperature of its body is maintained fairly constant although the tempera- ture of the surrounding medium may vary within wide limits) the mean temperature of the skin is regulated by the amount of blood passing through it in a given time, and by the degree of activity of the sweat glands. Heat is 'lost from the skin both by radiation and conduction. If a man stands before a thermal pile connected with a sensitive galvanometer, the radiant heat from his body is at once detected by the movement of the needle of the galvano- meter. In this case heat leaves his body by radiation, and also reaches the thermal pile by convection through the air. Again, when he stands before a fire he becomes warm, heat entering the body. When he touches anything it feels hot or cold, according as it conducts heat out of or into the skin. In this way the amount of heat entering or issuing from the skin is constantly varying, and the skin appreciates these variations. When any part of the skin is above its normal mean temperature, warmth is felt ; in the opposite case, cold. The following are the chief points that have been ascertained regarding the appreciation of variations of temperature. (i) With a skin temperature of from 15. 5 C. to 35 C, The Sense of Touch 67 the tips of the fingers can distinguish a difference of .2 C. Temperatures below that of the blood (33 C. to 27 C.) are distinguished by the more sensitive parts even to .05 C. (2) Parts having the thermal sense acute occur in the following order : Tip of tongue, eyelids, cheeks, lips, neck, belly. The smallest difference of temperature, in degrees centigrade, appreciated by the skin of the breast is .4 ; back, .9 ; back of hand, .3 ; palm, .4; arm, .2 ; back of foot, .4; thigh, .5; leg, .6 to .2; cheek, .4; temple, .3, giving a mean of about .3 that is, -^ of a degree centigrade. (3) Sensations of heat and cold may alternate. Thus, if we dip the hands into water at 10 C. we feel cold ; then transfer them to water the temperature of which is 16 C. and there will be a feeling first of warmth and then of cold. (4) The extent of the area subjected to heat or cold influences the sensation. For example, the whole hand dipped into water at 2 9. 5 C. feels warmer than when the finger is dipped into water having a temperature of 32 C. (5) Great sensibility to differences of temperature is noticed after removal, alteration by vesicants, like can- tharides, mustard, or strong acetic acid, or destruction of the epidermis, and in the skin affection (known to be of nervous origin) termed herpes zoster. On the other hand, removal of the epidermis increases tactile sensibility. Pain. The sensation termed pain is often referred to the skin, and is due to direct irritation of sensory nerves. Ordinary sensory nerves convey impressions from all parts of the body to the nerve centres, and these impressions give rise to sensations, often of a vague and evanescent charac- ter, such as a feeling of general bodily comfort, free 01 obstructed breathing, hunger, thirst, fatigue, etc. If such nerves are more strongly irritated the sensation becomes one of pain, and, in accordance with the law of the peripheral 68 Physiology of the Senses reference of sensation, the sensation may be referred to the origin of the nerve in the skin. Sometimes this pain is distinctly located, but in other cases it may be irradiated in the nerve centres, and then referred to areas of skin or regions of the body which are not really the seat of the irritation. The acuteness or intensity of pain depends partly on the intensity of the irritation, and partly on the degree of excitability of the sensory nerves at the time. Sometimes, for example, the excitability of sensory nerves may be so high that a whiff of air may cause acute distress. If only a few nerves are affected the pain is acute and piercing, but if many nerves are involved it may be more massive and diffuse in character. The quality of pain whether it is piercing, cutting, throbbing, gnawing, dull, or boring depends on the nature of the irritation, and on whether the irritation is constant or intermittent. Lastly, in many nervous diseases involving the centres of sensation, disordered sensations may be referred to the skin, such as abnormal feelings of heat or cold, creeping, itching, burn- ing, or a sensation of insects crawling in the skin, all giving rise to great distress. The Muscular Sense. As a rule, we do not judge of the weight of a body by the sense of pressure on the skin alone, but we lift the body and come to a conclusion as to its weight by a sense of the muscular tension necessary to support it against gravity. This is the so-called mus- cular sense. It depends on sensory nerves originating in the muscles, and carrying impressions from these to the nerve centres. Weber made some ingenious experiments on the delicacy of the muscular sense. Thus he placed certain weights in a cloth, and held it suspended by the four corners, so as thus to remove the effect of pressure or fric- tion, and then he endeavoured to form a judgment as to The Sense of Touch 69 the weights by the sensations of muscular resistance referred to the muscles of the forearm. He found that he was unable to form a correct estimate of the amount of the weight either by the muscular sense or by the tactile sense, but he found the muscular sense more discriminating than the tactile sense as to estimation of differences of weight. Thus, by the muscular sense he was able to distinguish weights the ratio of which was as 39 : 40, while by the tactile sense (sense of pressure) he could only distinguish weights the ratio of which was as 29 : 30. There is not so accurate a perception of locality in connection with muscular as there is in the case of tactile impressions that is to say, there is no well-defined muscular field like the tactile field. In actual experience, tactile and muscular impressions are blended so as to give a sharp representa- tion of the position at any time of the parts of the body, as well as of any change in such position brought about even by a passive movement. Thus, if we place the arm of a blindfolded person across the chest, he is immediately con- scious of the position of the limb, although he has made no muscular effort. Finally, when active movements are made by which the limb is placed in a certain position in space, we have contributing to the mental representation of this position, not only tactile and sensory muscular impressions, but also the sense of effort necessary to cause the muscles actively to perform the requisite movement. This sense of effort may be called a sense of innervation^ and is distinct both from the muscular sense, properly so called, and from the tactile sense. THE SENSE OF TASTE THIS sense is located chiefly in the tongue, but sensations of taste may also be referred to the soft palate and even to the region of the fauces. The tongue is a muscular organ covered with mucous membrane. By means of it's complicated movements it plays an important part in chewing, in swallowing, and in articulate speech. The mucous surface of the organ is covered with minute prominences or papilla, of which there are three kinds Most abundant are the filiform papilla, small cylindrical bodies, about one-twelfth of an inch in length. Inter- spersed with these are the fungiform papilla, so called because each consists of a narrow stem supporting a flattened top, something like the shape of a mushroom. They are shorter than the filiform papillae, varying from one-fiftieth to one-twelfth of an inch in length, and they may often be detected by their bright red colour, caused by their great vascularity. Towards the root of the tongue we find the third kind of papillae, the circumvallate, eight to fifteen in number, arranged in the form of a V, with the apex directed backwards. Each papilla, surrounded by a deep circular furrow hence the name consists of connective tissue clothed with epithelial cells, and its height varies from one-twenty-fifth to one-fifth of an inch,, and its breadth from one-twenty-fifth to one-eighth of an The Sense of Taste inch. It is in connection with the fungiform and circum- vallate papillae that we find the terminal organs of taste. Minute Structure of Gustatory Organ. In many of the fungiform and in all the circumvallate papillae are the structures called taste buds or taste goblets. They also occur to a small extent on the soft palate, and even on the surface of the epiglottis. They are most conveniently studied in the tongue of the rabbit. Two oval patches papilla foliates may be seen with the naked eye near the root of the tongue of this animal, one on each side and placed obliquely. Each patch consists of about twenty laminae or folds of mucous membrane, running parallel, like the leaves of a book, and each fold is composed of three, ridges of the derma. Thus a transverse section gives the appearance seen in Fig. 23. It Will ^ be ^ Seen that the FlG . 23 ._ Ve rtical section through a epithelium is thick Over the top portion of the pafilla foliate of a j i ^ ^i i r 1 r i j rabbit X 80 d. Each fold, /, has and thin at the sides of the fold, secondary folds, t \ g, taste goblets ; , medullated nerve fibres ; d, a serous gland ; M, muscular fibres of the tongue. (Stohr.) and that, in section, the space between two folds has the appearance of a deep groove. About the middle of the depth of this groove we find a row of minute oval bodies, from three to five in number these are the taste buds, or taste goblets. They exist in immense numbers. In the papillae foliatae of the rabbit there are from 14,000 to 15,000, while the tongues of the sheep and pig have yielded 9500, and that of the ox 30,000 72 Physiology of the Senses taste buds. As many as 1760 have been counted on one circumvallate papilla of an ox. The taste buds are oval bodies, one-three-hundredth oi an inch in length by about one-six-hundredth of an inch in breadth, embedded in the epithelial layer. The base rests on the derma, while the other and somewhat narrower end is directed towards the sides of the papilla or folds already described, and shows a minute funnel-shaped opening, called the taste pore. Each taste bud is formed of three kinds of epithelial cells : an outer set, of almost uniform breadth throughout, and shaped somewhat like the staves of a cask, and an inner of two varieties, smaller and pointed at each end. The outer cells protecting cells forming the outer part, are evidently structures that serve the purpose of protecting the more delicate cells in the interior of the little flask. There appear to be two kinds of inner cells. First, we find cells that are narrow and slightly thickened in the middle, where there is a nucleus, surrounded by only a very small amount of cell substance. The outer half of the cell is first cylindrical, then conical, and ends in a fine point, while the inner half runs deeply, sometimes divides into two roots, and is lost in the under- lying tissue. Such cells have been termed rod cells, and they probably support the true sensory cells that are found in the middle of the flask. These the true taste cells are similar in appearance to the rod cells, but more delicate ; and their external portions, in the form of fine threads, converge so as to form a tuft at the taste pore. Both the rod cells and the true taste cells stain with chloride of gold, and behave, to chemical reagents, like sensory cells. Terminations of Gustatory Nerves. As to the way in which the nerve fibres terminate there is still consider- able doubt. The fibres of the glosso-pharyngeal nerve ramify in the derma, or tissue underlying the taste buds, The Sense of Taste 73 forming plexuses or networks from which minute twigs pass into the taste buds. Many of these fibres are non- medullated. Efforts to trace them into connection with the true taste cells, or with the rod cells, have failed, but there is little doubt that this is their mode of termina- tion. Probably some fibres may not enter the taste buds at all, but may end by fine processes among the epithelium on the top or sides of the papilla. The proofs that the taste buds are the end organs of taste maybe shortly stated as follows: (i) The sense of taste is weakened or absent in those areas of mucous mem- brane on the tongue from which they are absent or exist only in small numbers ; (2) the sense is most acute where they are found in large numbers ; (3) section of the glosso-pharyngeal nerve, which is distributed tO FlG 24. -Taste bud seen in the the area of mucous membrane where taste is present, is fol- lowed by degeneration of the rod and taste cells, and ultimately by the entire disappear- ance of the taste bud. Physical Causes of Taste. All substances that give rise to taste are soluble in the fluids of the mouth. In- soluble substances are tasteless. Thus, if we touch the surface of a crystal of quartz with the tongue, we have a sensation of smooth contact, or touch, and a sensation of cold, because the crystal conducts heat out of the tongue, but there is no sense of taste. Contrast this with the sensations of saline taste, contact, and coolness experienced when we bring the tongue into contact with the surface of a crystal of rock salt. As solution is a necessary condi- papilla fohata of a rabbit X 560 d. ^ Taste bud, showing outer sup- porting ceil* ;*, fine ends of taste cells;/, taste pore. (Stohr.)' 74 Physiology of the Senses tion of taste we find near the taste organs numerous small serous or albuminous glands (see Fig. 23), the secretions of which assist in dissolving sapid substances. No con- nection has yet been traced between the chemical composi- tion of sapid substances and the different kinds of tastes to which they give rise. Substances of very different chemical composition may give rise to similar tastes. For example, sugar, acetate of lead, and chloroform have all a sweetish taste, although their chemical composition is as diverse as can well be imagined. Acids are usually sour ; alkalies have a peculiar soapy taste ; salts vary much, from the sweetness of sugar of lead to the bitterness of sulphate of magnesia ; the soluble alkaloids, such as quinine, strych- nine, etc., are usually bitter ; and the higher alcohols are more or less sweet. Physiological Conditions of Taste. The tongue, as already pointed out, is the seat of sensations that are quite unlike each other. Thus, there are tactile sensations, as when we touch the organ with a pin, sensations of pressure, sensations of heat and of cold, burning or acrid sensations, peculiar sensations excited by the application to the tongue of an interrupted electrical current, and, lastly, sensations of true tastes. We must also distinguish from these, sensa- tions that are called flavours, experienced when we bring the tongue into contact with an onion or a savoury bit of cooked meat or fish. These are in reality sensations compounded of smells and tastes, and the sensation of tasting an onion is thus quite changed when we hold the nose and avoid breathing. True tastes may be classified as sweet, bitter, salt, sour, alkaline, and, perhaps, metallic. All of these are specifically distinct sensations, and they are no doubt due to some kind of action, probably chemical, which they excite in the taste cells. If we assume that the taste cells are connected with the ends of the nerves, The Sense of Taste 75 then we can imagine that the chemical changes thus excited in the taste cells set up nerve currents which, propagated to specific centres of taste in the brain, give rise there to molecular changes that in turn are related to consciousness. While, however, chemical action probably lies at the root of the mechanism of taste, it is remarkable that true- tastes may be excited by causes that are not strictly chemical. Thus a smart tap on the tongue may excite a taste ; and Siilzer demonstrated, so long ago as 1752, that a constant current causes (more especially at the moments of opening and of closing the current) a sensation of acidity at the anode (positive pole) and of alkalinity at the kathode (negative pole). No doubt it is possible that the mechanical irritation, in the one case, and the electrical current, by electrolysis, in the other, may set free chemical stimuli ; but of this there is no proof. On the other hand, it has been found that sensations of taste may be excited by rapid induction currents currents too rapid to produce electrolytic action. The extent of surface acted on increases the massiveness of the sensation of taste, while the intensity is influenced by the degree of concentration of the solution of the sapid substance. Suppose we gradually dilute solutions with water, tasting from time to time, until no taste is experienced, some common substances may be classed in the following order : syrup, sugar, common salt, aloes, quinine, sulphuric- acid. That is to say, the sweetness of syrup disappears ' first, and the sourness of sulphuric acid last. Again, it has been found that the taste of quinine continues until diluted with twenty times more water than common salt. It is evident, then, that smaller quantities of some substances, as compared with others, excite taste, or, in other words, the taste cells are more susceptible to the chemical action of some substances than of others. Attempts have been made 7 6 Physiology of the Senses to measure the time required to excite tastes. Thus, from the moment of contact with the tongue, saline matters are tasted more rapidly (.17 second) than sweet, acid, and bitter (.258 second) the difference being probably due to the activity of diffusion of the substance. After a taste has been developed, it appears to last for relatively a long time, but it is not easy to say whether this is due to a per- sistent change in the taste cells, after removal of the exciting cause, or to the continued action of the exciting substance. It is well known that a temperature of about 40 F. is most favourable to the development of tastes, fluids much above or below this temperature either masking or tem- porarily paralysing the taste cells. Thus, if the mouth be rinsed with either very hot or very cold water, a solution of sulphate of quinine, distinctly bitter at a temperature of 40 F., will scarcely be perceived. As one would expect from the anatomical distribution Df the taste buds, the surface of the tongue is not uniformly sensitive as regards taste. The sense is most acute in or near the circumvallate papillae. The middle of the tongue is scarcely sensitive to taste, while the edges and the tip are, as a rule, highly sensitive, although it is said that the sensitiveness of the edges varies much in different ipdivi- duals. Taste is feebly developed on the soft palate and on the pillars of the fauces, so that after complete extirpation of the tongue, including the part bearing the circumvallate papillae, feeble sensations may still remain. Differentiation of Tastes. Recent observations by Shore 1 have thrown light on the question whether there may be in the tongue different end organs appropriated to special tastes. If all the taste buds are the same, it is difficult to explain why, in the majority of persons, the back part of the tongue is most sensitive to bitters and the tip to sweets, 1 Shore, //. of Physiology, 1891. The Sense of Taste 77 tvhy saline matters are perceived most distinctly at the tip and acid substances at the sides, and why there should be individual variations, as undoubtedly is the case. Assuming that there are different kinds of taste cells, it might be possible to paralyse some without affecting others, and thus different sensations of tastes might be discriminated. This has been done by the use of the leaves of a common Indian plant, Gyjnnema sylvestre. If some of these be chewed, it has been found that bitters and sweets are paralysed, while acids and salines are unaffected. Again, certain strengths of decoctions of the leaves appear to paralyse sweets sooner than bitters. These interesting observations indicate the existence*- of different taste cells for sweets, bitters, acids, and salines ; and it is clear that the region of the tongue most richly supplied with taste cells sensitive to sweets will respond best to sweet substances, while another region, supplied by taste cells sensitive to bitters, will respond best to bitter substances. In like manner the argument may be applied to other tastes. Suppose, again, a set of taste cells sensitive to bitter substances : it is conceivable that in whatever way these were irritated, a bitter taste would result. If so, a substance which applied to one part of the tongue would cause a sweet sensation, might cause a bitter if applied to a part of the tongue richly supplied with taste cells sensitive to bitters. This may explain why sul- phate of magnesia excites at the root of the tongue a bitter taste, while applied to the tip it causes a sweet or acid taste. Saccharine, in like manner, is sweet to the tip and bitter to the back of the tongue. Again, it has been found that if the " sweet " and " bitter " taste cells are paralysed by gymnema, electrical irritation of the tip does not give rise to an acid taste mixed with sweet, but to sensations somewhat different, and described as " metallic," or " salt," or "acid." 78 Physiology of the Senses General Sensibility of the Tongue. As already said, the tongue is endowed with acute general sensibility. It is evident, then, that a sensation caused by dropping a little vinegar on the tongue is due partly to stimulation of the tactile organs, and partly to stimulation of the true taste cells. Cocaine, the active alkaloid of the coca plant, paralyses tactile sensibility ; and it is said that if the surface of the tongue be painted with a solution of this substance, that .acid tastes become more clear and marked. The drug, how- ever, ultimately affects all the end organs, so that lingual sensations disappear in the following order : general sensi- bility and pain, bitters, sweets, salines, acids, and tactile sensibility. Siibjective Tastes. Disease of the tongue causing un- natural dryness may affect taste. Substances circulating in the blood sometimes give rise to subjective sensations of taste. Thus santonine, morphia, and biliary products, as in jaundice, cause a bitter sensation, while in diabetes there is often a persistent sweetish taste. The insane occasionally suffer from distressing subjective tastes. In such cases the sensation is caused by irritation of the gustatory nerve, or by changes in the taste centres of the brain. There is, how- ever, no evidence showing that direct irritation of gustatory nerves is followed by sensations of taste. Nerves of the Tongue. The distribution of nerves to the tongue is remarkably complicated, and the whole subject presents numerous difficulties. The motor nerve, that is, the nerve that excites and governs the movement of the tongue, is the ninth cranial nerve, known to anatomists as the hypo-glossal. The sensory nerves are usually described as two in number, the anterior two-thirds of the tongue being supplied by the gustatory or lingual branch of the fifth cranial nerve, and the posterior third the situation of the circumvallate papillae by the glosso-pharyngeal nerve. The Sense of Taste 79 Afc^CA! , ^-K The lingual branch of the fifth" nerve-comains both ordinary sensory and gustatory filaments, and the glosso-pharyngeal supplies the circumvallate papillae and taste buds. Another nerve, however, has to be considered, namely, the chorda tympani, a branch given off by the facial nerve during the passage of the latter through a canal in the petrous portion of the temporal bone known as the aqueduct of Fallopius. Loss of taste on one side of the tongue has been observed in cases of disease of the ear involving the chorda nerve. This, however, is not conclusive evidence that the chorda contains gustatory filaments, as the loss of taste following its injury may be due to the removal of its influence over the nutrition of the mucous membrane of the organ. On the other hand, there are good grounds for the view that the gustatory filaments, both of the lingual branch of the fifth, and of the glosso-pharyngeal itself, come primarily from the roots of the fifth nerve. Disease of this nerve within the cranial cavity causes loss of taste in one lateral half of the tongue, both tip and back, but no case has been recorded of disease of the glosso-pharyngeal being followed by this result. THE SENSE OF SMELL THE seat of the structures concerned in the sense of smell is in the nasal cavities^ situated between the base of the FIG. 25. Transverse vertical section across the nasal cavities, opposite to the middle of the hard palate ; the anterior part of the section seen from behind, i, Part of inner surface of cranium ; 2, projection between the two cribri- form plates of the ethmoid bone ; 3, median septum or partition in the ethmoid bone ; 4, 4, cells in the lateral masses of the ethmoid bone ; 5, 5, the middle turbinated portion of the ethmoid bone ; 6, 6, the two turbin- ated bones ; 7, the vomer, or bony septum or partition, of the nose ; 8, section of the malar or cheek-bone ; 9, a large sinus or space in the superior maxillary bone sometimes called the maxillary sinus, or antrum of High- more ; it communicates with the nasal cavity, at 10, and there is a corre- sponding space on the other side. (Arnold.) cranium and the roof of the mouth, at the upper and fore part of the face. The floor, sides, and roof of these cavities TJie Sense of Smell 8 1 are formed by certain of the bones of the cranium and face (see Fig. 25). Physiological Anatomy of the Nose. The ethmoid bone, which also forms part of the floor of the cranial cavity, is concerned in the formation of the olfactory region. Thus its cribriform plates form the roof; its sides, which contain numerous cavities or cells formed of bone, constitute the convoluted sides of the upper part of the cavity ; and a median plate of bone, forming a septum or partition, assists in dividing the one nasal cavity from the other. The anterior part of the nasal cavities is completed at the sides and in the middle by plates of cartilage or gristle, called the nasal cartilages. These cartilages are firmly attached to the margin of the nasal aperture seen in a skull, and they give form and firmness to the visible part of the nose. The nostrils open anteriorly by apertures called the anterior nares, and they are lined by an infolding of skin, bearing short stiff hairs, vibrissce, which, to some extent, prevent the entrance of foreign bodies. Posteriorly, the nostrils open into the pharynx by two apertures, fo& posterior nares (see Fig. 26). The middle wall of each nostril is formed by the septum or partition between the two, and presents a smooth surface. The outer wall, on the other hand, is more or less convoluted from the presence of three delicate scroll-like bones, namely, the upper and middle tur- binated parts of the ethmoid, and the lower turbinated bones (see Fig. 25, 5, 6). There are thus three spaces, or recesses, called the superior, middle, and inferior meatus, and these meatuses communicate with cavities, called sinuses, in the ethmoid, sphenoid, frontal, and upper jaw-bones. These spaces, along with the cavity of the nose itself, being full of air, act as resonators, and affect the quality of the voice. The cavity of the nose is lined by a membrane, called the nasal mucous membrane, or Schneiderian membrane, G 82 Physiology of the Senses which secretes a peculiar kind of mucus known as pituita. The lining membrane is continuous with that of the sinuses already mentioned, and with the lining of the pharynx and Eustachian tube, while it is prolonged on each side, through a small canal, into the lachrymal sac, thus also merging into the conjunctiva, the mucous membrane of the eye-lids. FIG. 26. Outer side of left naris. i, Sinus or hollow in the frontal bone ; 2, free border of the nasal bone ; 3, lamina cribrosa or perforated plate of ethmoid bone, through which pass the twigs of the olfactory nerve ; 4, an- trum or hollow of the sphenoid bone ; 5, hairs in the vestibule of the nose ; 6, 6', vestibule of the nose separated by a prominence, 7, from 8, the entrance to the middle meat us or passage of the nose ; 9, agger or mound of the nose, the rudiment of a muscle; 10, concha or shell of Santorini ; n, entrance to 4; 12, superior spongy bone; 13, upper meatus ; 14, middle spongy bone; its inferior free border from b to c ; 15, inferior spongy bone; i^, naso-pharyngeal fold; 17, naso-pharyngeal duct; 18, pharyngcal opc-niii- of the Eustachian tube ; 19, fold between 18 and pharynx ; 20, depression of Rosenmiiller ; 21, the incisor canal. (Schwalbe.) The nerves supplying the nasal mucous membrane come from three sources. First, it is supplied by the nasal and anterior dental branches of the fifth pair of cranial nerves ; second, branches are distributed to it from the vidian, naso- palatine, descending palatine, and spheno-palatine nerves, in which run fibres of the sympathetic ; and, third, we find The Sense of Smell 83 in the upper part of the nasal cavities branches of the first pair of cranial nerves, the olfactory nerves. The first two groups of nerves endow the nose with general sensibility, / and supply its blood-vessels and glands. The olfactory nerves are the true nerves of smell, and their branches end in the special terminal organs devoted to that sense. The olfactory lobes (see Fig. 8, p. 21) lie within the cranium on the cribriform, or sieve -like, plates of the ethmoid bone, and about twenty small branches, the olfactory nerves, issue from their under surface, pass through minute canals in the ethmoid bone, and thus gain the upper part, or roof, of the nasal cavities. There they divide into three groups, one supplying the roof, a second the membrane covering the cellular part of the ethmoid bone, while the third pass as low as the middle turbinated bone. Some fibres also reach and are distributed to the upper third of the nasal septum. The nasal mucous membrane is richly supplied with blood, a dense capillary network lying below the epithelial layer. The veins converge to the posterior part of the lower meatus, where they form a thick dense plexus. The existence of so many vessels accounts for the nasal haemor- rhage often observed, and as the bleeding not unfrequently proceeds from the venous plexus situated far back in the cavities, it is sometimes staunched with difficulty. Minute Structure of the Olfactory Organ. As already mentioned, the membrane lining the movable (anterior) part of the nose is developed from an infolding of the skin, and in structure it resembles skin, showing a layer of stratified squamous epithelium covering papillae. In it we find numerous sebaceous glands and hair follicles, from which vibrissce spring. This part of the nose, the vesti- bular portion, is at the entrance of the respiratory passage. The respiratory portion forms the lower part of the nasal Physiology of the Senses Epithelium. passage. It is lined by a stratified cylindrical epithelium, the cells of which bear cilia, short vibratile processes, by the movements of which currents are established in the fluid bathing the surface. In this portion, the membrane of which is about one-sixth of an inch in thickness, are numerous minute race- mose glands secreting a fluid, thus keeping the surface moist, and it is noticeable that in the sinuses already mentioned the membrane is much thinner, and only very few glands exist. The upper, or olfactory portion^ is the part spe- cially connected with the sense of smell. It is narrow from side to side, and clothed with a thick mucous membrane, often of a yellowish - brown FIG. 27. Vertical section through the olfac- Colour, that Contrasts with tory region of a rabbit, magnified 560 dia- th reddish hue o f that meters, s, Border ; zo, zone of the oval nuclei; zr, zone of the round nuclei; b, lining the VCStiblllar and basal cells; dr t portions of Bowman's respiratory reg ions. A glands. The lower part of the duct is seen on the right. , branch of olfactory nerve, vertical Section of this membrane is seen in Fig. 27. It is formed of an epithelial layer, olfactory epithelium , resting on a basement membrane. Two forms of cells are found. The one (Fig. 28, sf] has the upper half cylindrical, and the free border sometimes shows minute stiff cilia, while the lower half is narrowed, shows indentations, and finally ends in long, sometimes double, processes, which Mucous membrane The Sense of Smell apparently join with those of adjoining cells. These knife- handle-like cells, called supporting cells^ show each an oval nucleus, and the rows of such nuclei, seen in a section, as in Fig. 27, form a zone, known as the zone of oval nuclei. The second cells have a round nucleus surrounded by only a small quantity of protoplasm, and from this there passes to the surface a narrow round filament, bearing a single cilium on its free end, while another slender filament passes in the oppo- site direction, and terminates in filaments of the olfactory nerve. These are the olfac- tory cells. The juxtaposition of the round nuclei forms a zone, called the zone of round nuclei. At the boundary of the epithelial layer with the connective tissue, peculiar, somewhat flattened or irregularly cubical cells are found, termed basal cells (Fig. 27, b). FIG. 28. Isolated The layer on which the epithelium rests is a loose felt work of connective tissue, con- taining elastic fibres, and the latter may be so close together as to form an elastic layer. Numerous simple or branched glands exist in the olfactory region, named after their discoverer the glands of Bowman. They secrete mucus, but their special function is unknown. As to the mode of termination of the olfactory nerves there is still considerable difference of opinion, some holding that they end only in the true olfactory cells (Fig. 28, r\ while others maintain that they also end in the basal cells (Fig. 27, ), and even in the supporting cells (Fig. 27, j, and Fig. 28, st}. The evidence is clear that they end in the olfactory cells, but doubtful as regards the others, and, from the analogy of cells from the ol- factory region of a rabbit, magnified 560 diameters, st, Supporting cells ; s, short, stiff cilia, or, according to some, cones of mucus resemb- ling cilia ; r, r, ol- factory cells. The nerve process has been torn off the lower cell marked r. (Stohr.) 86 Physiology of the Senses other end organs, it is probable that the basal and support- ing cells have only indirectly to do with the action of odori- ferous substances on the nerve-endings. Physical Causes of Smell. Substances that excite the sense of smell must exist in the atmosphere in a state of fine subdivision, and even vapours and gases may be supposed to consist of minute molecules of matter. If air conveying an odour be passed through a long glass tube packed firmly with cotton wool, it will still be odorous, although this proceeding will remove all particles larger than the one - hundred - thousandth of an inch. Again, a grain of musk will for years communicate its odour to the air of a room, and at the end of the time it will not have appreciably diminished in weight. Odoriferous particles will mix with the air either in accordance with the laws of diffusion of gases or by virtue of their volatility, that is, the rapidity with which they evaporate. In the case of odorous gases, no doubt mixture takes place by diffusion, but an odorous essential oil will give off particles by a kind of evaporation. The volatility of a substance may be expressed by the weight that evaporates from a unit of surface in a unit of time. By means of a specially- contrived instrument Ch. Henry has measured the volatility of various odorous substances, and, as might be expected, it is very great. Thus, taking unity as the one-thousandth of a milligramme * evaporating from one square millimetre in one second, the following values were obtained : ether, .7 ; ylang-ylang, .0176; rosemary, .0446; caraway, .0315; mint, .0354; winter-green, .0165 ; bergamot, .0331 ; and lavender, .0292. These minute quantities are readily appreciated by the sense of smell, if the nose is held near the evaporating surface. 1 The one-thousandth of a milligramme = one twenty-five-millionth of a grain. The Sense of Smell 87 Chemical nature of odorous substances. Attempts have been made, notably by Ramsay and Haycraft, to dis- cover a relation that might exist between odours and the chemical composition of substances emitting them. Certain gases excite smell, while others are odourless. Thus the following having no smell : hydrogen, oxygen, nitrogen, water gas, marsh gas, olefiant gas, carbon monoxide, hydro- chloric acid, formic acid, nitrous oxide, and ammonia. It is necessary, of course, to distinguish between the irritant action of such gases as ammonia and hydrochloric acid, and the true odour. On the other hand, the following gases have an odour : chlorine, bromine, and iodine, the compounds of chlorine and bromine with oxygen and water, peroxide of nitrogen, the vapours of sulphur and phosphorus, arsenic, antimony, sulphurous acid, carbonic acid, some compounds of selenium and tellurium, the compounds of chlorine, bromine, and iodine, with the above-named metals, and many of the volatile compounds of carbon. Substances of low molecular weight either simply irritate the nose, or have no odour. Ramsay states that in the carbon com- pounds increase of specific gravity as a gas is related (up to a certain point) to smell. Thus, if we take the methane or marsh gas series (the paraffins), the first two have no smell, ethane (fifteen times as heavy as hydrogen) has a faint odour, and it is not till we reach butane (thirty times heavier than hydrogen) that a distinct odour is noticeable. Again, methyl alcohol has no smell ; ethyl, or ordinary alcohol, has a true alcoholic smell, " and the odour rapidly becomes more marked as we rise in the series, till the limit of volatility is reached, and we arrive at solids with such a low vapour tension that they give off no appreciable amount of vapour at the ordinary temperature." x Again, acids increase in odour with an increase in density as a 1 Ramsay, Nature, vol. xxvi. p. 187. 88 Physiology of the Senses gas. Formic acid, for example, has no smell ; acetic acid has its well-known odour of vinegar ; and propionic, butyric, and valerianic acids increase in odour as we ascend the series. Groups of chemical substances have sometimes characteristic smells. Thus many compounds of chlorine, sulphur, selenium, tellurium, the paraffins, alcohols, nitrites, amines, the pyridenes, and the benzene group have each a characteristic odour. Again, substances not related, but similar in chemical structure, may have somewhat similar odours. Thus the compounds of hydrogen with sulphur, selenium, and tellurium, and the compounds of these with methyl or ethyl, have all a disagreeable odour, something like that of garlic. The odours of chloroform and iodoform are not unlike. On the other hand, many substances have odours that are very similar, and yet there is no resemblance in chemical constitution. Why, for example, should arsenical com- pounds have the odour of garlic ? Why have nitro-benzene, benzoic aldehyde, and prussic acid almost the same odour ? Mix sulphuric acid with water, and an odour like that of musk may be given out. It is said that emeralds, rubies, and pearls if triturated for a long time give out an odour like that of violets. Again, the disease called favus, ringworm of the scalp, the body of a patient sick of typhus, and mice have similar odours. It is well known that perfumes from very different sources may be classed under certain types. Thus, the rose type includes geranium, eglantine, and violet-ebony ; the jasmine type, lily of the valley and ylang-ylang ; the orange type, acacia, seringa, and orange- flower ; the vanilla type, balsam of Peru, benzoin, storax, tonka bean, and heliotrope ; the lavender type, thyme and marjoram ; the mint type, peppermint, balsam, and sage ; the musk type, musk and amber seed ; and the fruity type, pear, apple, pine-apple, and quince. The Sense of Smell 89 Flowers and odours. Attempts have also been made to discover a relation between the colours of flowers and the intensity of their perfumes. White flowers manifest the greatest variety of odours, and then follow reds, yellows, greens, and blues. The ratio of the number of odorous species to the number of species in each colour, is as follows : whites, I to 6.37 ; reds, I to ip.8 ; yellows, I to 12.6; greens, I to 12.7; and blues I to 19. .It is also noticeable that flowers which by their colour emit most heat will volatilise the greatest amount of perfume, and that the more refrangible the rays reflected from the flower the smaller is the amount of perfume. Coloured substances have also different powers of absorbing odours. Whites, yellows, reds, greens, and blues absorb odours on a decreas- ing scale. The more intense the colour the more likely is it to emit a strong odour, because no doubt the light acts on the essential oil on which the odour depends. Heat more than light favours the volatilisation of perfumes. Hence the odours of a flower-bed in a garden are often most apparent, not in bright sunshine, but in the shade. Some essential oils require a higher temperature than others to bring out their characteristic perfumes. An air of moderately high temperature and the presence of moisture favour the diffusion of the odours of most flowers. Odours and heat absorption. Tyndall showed that many odorous vapours have a considerable power of absorbing heat. Taking the absorptive capacity of air as unity, the absorption per cent, for certain odorous matters was as follows: patchouli, 30; sandal-wood, 32; geranium, 33; oil of cloves, 33.5 ; otto of roses, 36.5 ; bergamot, 44 ; neroli, 47 ; lavender, 60 ; lemon, 65 ; portugal, 67 ; thyme, 68 ; rosemary, 74; oil of laurel, 80; and cassia, 109. In comparison with the air introduced in the experiments the weight of the odours was extremely small. " Still we find ijo Physiology of the Senses that the least energetic in the list produces thirty times the effect of air, while the most energetic produces one hundred and nine times the same effect." a These results, although interesting, are not of the value they would have possessed if the tensions of the odorous vapours had also at the same time been determined because the tension of the vapour would influence its capacity for absorbing radiant heat. Odours and ozone. It is remarkable that on the one hand ozone (condensed, or allotropic oxygen, O 3 ), as pro- duced by electricity, develops the odours of the essential oils, and on the other, that these oils produce ozone by their action on the oxygen of the air. Thus, slow oxidation of oil of turpentine, or of one of the essential oils, produces ozone. Ozone, again, exists in the air of the sea-side when the grassy banks are clothed with wild thyme and other scent-giving plants, and it abounds on the heather-clad hills, more especially when the heather is in bloom. This suggests that the atmosphere of our cities might be ozonised and made more healthy by the free use of odorous substances like oil of turpentine or the perfumes. Odours and surface tension. Some of the physical characters of odorous bodies have been studied by Venturi, Prevost, and Lie*geois. It is well known that if minute fragments of camphor or succinic acid are placed on the surface of pure water, they move with extreme rapidity, owing to changes in the surface tension. If odorous particles are placed on a glass plate, the surface having been previously moistened with water, the particles at once fly from each other, it may be to a distance of several inches. This simple method constitutes the odoroscope of PreVost. Liegeois has pointed out that the movements of camphor in water are arrested when an odorous substance is brought into contact with the water. The odorous oil or essence at 1 Tyndall, Contributions to Molecular Physics, p. 99. The Sense of Smell 9 1 once forms a pellicle on the surface of the water, and this pellicle consists of minute particles, not broader than from .001 to .003 of a millimetre (that is, from YT^OIT to __ 1 __ of an inch). 1 This shows how the dissemination of odours is favoured by moist surfaces. Flowers give off odours most powerfully after a shower of rain. No doubt also when the odoriferous substance falls on the moist olfactory membrane it is rapidly disintegrated into ex- tremely minute particles, which are thus more readily brought into close relation with the olfactory nerve endings. These figures, given by Lie'geois, are probably far too high, and consequently the particles are much smaller. Calculation shows that the thickness of the layer of oil which is necessary to stop the, movement of small pieces of camphor over a definite area surface of water amounts to only 1.5 millionth of a millimetre 2 (that is, about one- sixteen-millionth of an inch). Special Physiology of Smell. The air containing the odour must be driven against the membrane. The nostrils may be filled with an odoriferous substance like eau-de- cologne, or air impregnated with sulphuretted hydrogen, and no smell will be experienced if no inspiration is made. When we make a sniff, the air in the nasal passages is rarefied, and as the odour-bearing air rushes in to equili- brate the pressure, it is forcibly driven against the olfactory surface. Odorous air passing from the posterior nares also gives rise to a sensation of smell, although not so intense as when it passes in the normal direction. An odour may be perceived even although the nostrils are full of fluid. Weber stated that no odour was noticeable if the nostrils were full of water, but Arensohn has shown that this was because the water injured the olfactory surface, 1 Ltegeois, Archiv. de Physiologie, 1868. 2 Lord Rayleigh, Proc. Roy, Soc. t 28th March 1890. 92 Physiology of the Senses and that if the water was replaced by a weak solution of common salt (.07 per cent an inert fluid), odours were readily perceived. It is well known, also, that fishes possess a sense of smell. Fragments of bait cast into the water soon attract fishes to a fishing-ground, and that at depths into which little or no light can penetrate. The fish must smell the odoriferous morsels. The intensity of an odour depends ( I ) on the number of olfactive particles, and (2) on the extent of olfactory surface affected, or, in other words, on the number of nerve-endings stimulated. It is remarkable that sensations of odours are very evanescent. Hence to maintain the sensation fresh particles must be brought to act on the olfactory surface, and when we wish to maintain the sensa- tion experienced in sniffing the delicate odour of a flower, we sniff and sniff again. The delicacy of the sense varies much in different individuals and in different animals. It is highly developed both in carnivora and herbivora. The dog, for example, appears to depend on the sense of smell almost to as great an extent as on the sense of sight, and olfactory impressions probably are to him both more vivid and more permanent than to a man. Attempts have been made to combine odours, but with' out success. Thus, if we fill each nasal passage with a different odour, we do not experience a mixture of two sensations, but the odours come alternately, and we smell only one at a time, There is usually a difference as regards olfactive sensibility between the two nasal cavities, when they are tested with the same odour. Beaunis, 1 by noting exactly the moment that an odour is experienced after it has been presented to the nose, has discovered that this time is not the same for all odours. 1 Beaunis, Recherches exptrim. , 1884. The Sense of Smell 93; Some have greater power of penetration than others,. the maximum being reached by ammonia, and the* minimum by musk, and odours analogous to it. This power of penetration is in the inverse ratio to the divisibility of the odorous substance. He divides odours into (a) piire odours, like musk, which he terms scents or perfumes, and () mixed' odours, like that of peppermint, in which there is a com- bination of odour with a vague tactile sensibility referred to the mucous membrane. To these we may add (c) substances like acetic acid, that act at the same time on the olfactory nerves and on the tactile nerves of the- mucous surface the latter action being stronger and more irritating than in the case of b, the mixed odours, and (d). substances that act only on the tactile nerves, like carbonic acid. Mode of Excitation of the Olfactory Nerves. No- satisfactory theory of smell has yet been offered. Graham suggested that the odorous substance was probably oxidised on the olfactory surface, but this view was founded only on the observation that odorous substances are readily oxidisable. Ramsay has offered the theory that smell may be excited by vibrations the period of vibration of the lighter mole- cules being too rapid to affect the sense then a number of vibrations is reached capable of exciting the sense organ, while beyond an upper limit the vibrations again are not attuned to the sense organ and the odour dis- appears. All this is merely speculative, and has no founda- tion on experiment. Schultze was inclined to the view that the action might be mechanical, because he found minute stiff cilia on the olfactory surface, but this mechanism is. far too coarse for the appreciation of the almost in- finitesimal amount of odorous substances capable of exciting the sense. Stimulation by electricity has thrown no light on the question. The opening and closing of a 94 Physiology of the Senses continuous current, led to the olfactory surface through a solution of common salt at a temperature of 38 C., cause a sensation of an odour like that of phosphorus. The action of odours is not through the medium of the ether, the movements of which account for the phenomena of light. Odours have to do with the grosser forms of matter, and all the evidence is in favour of some kind of chemical action, the nature of which, however, is quite unknown. Loss of the sense of smell is termed anosmia. This is a rare condition, usually congenital. In such cases all tactile sensations referred to the mucous membrane of the nose, and all tactile and gustatory sensations referred to the tongue, may exist. The sense of smell alone is absent. Subjective sensations of odour are rare, but they have been found in the insane, and are due to excitation of the part of the brain connected with the sense of smell. The sense of odour, termed by Kant taste at a distance^ gives us information as to the quality of food and drink, and more especially as to the quality of the air we breathe. Hence we find the organ placed at the opening of the respira- tory passage and in close proximity to the organs devoted to taste. Taste is at the gateway of the alimentary canal, just as smell is the sentinel of the respiratory tract ; and just as taste, when combined with smell to give the sensa- tion we call flavour, influences the digestive process, and is influenced by it, so smell influences the respiratory process. This has recently been shown by Ch. Henry. 1 He has. recorded the entrance and exit of air by the nose, with and without odours (the quantity of odoriferous substance being noted), and he finds that the presence of odours influences both the amplitude and the number of the 1 Ch. Henry, Revue Scientifique, 1892, p. 73. The Sense of Smell 95 respiratory movements. Thus the smell of winter- green notably increased the respiratory work ; next came ylang- ylang ; and last rosemary. The breathing of a fine odour is therefore not only a pleasure, but it increases the amplitude of the respiratory movements. Just as taste and flavour influence nutrition by affecting the digestive process, and as the sight of agreeable or beautiful objects, and the hearing of melodious and harmonious sounds, react on the body and help physiological well-being, so the odours of the country, or even those of the perfumer, play a beneficent role in the economy of life. THE SENSE OF SIGHT FIG. 29. Antero-posterior section through upper eyelid, X jd. i, Outer skin E, epidermis ; C, corium; Sc, subcutaneous tissue; H,fmehairs; K, M, sweat glands ; W, eyelash ; W, W", roots of eyelashes ; E/z, reserve hair ; 2, muscles for closing eye O, muscular bundles cut trans- versely ; M<:R, ciliary muscle of Riolanus ; 3, tendon of muscle elevating the eyelid, mps ; 4, conjunctival region ; tp, tunica propria ; e, con- junctival epithelium ; at, gland ; /, tarsus ; in, Meibomian gland ; a, a', arteries ; 5, corner of eyelid. (Stohr.) THE sense of sight differs from the senses of taste and smell in this im- portant particular, that through it we seem to be- come aware of the exist- ence of things which are entirely apart from us," and have no direct or material link connecting them with our bodies. Yet physi- cists tell us that in vision the eye must be affected by a something which is as certainly material as a sapid or an odorous sub- stance, and which, per- meating the universe, transmits by its vibrations movements that affect the eye, and give rise to the sensation of light, or to the perception of even the most distant objects. This medium for the transmission of light is The Sense of Sight 97 called the luminife,rous ether, and our eyes are so constituted as to respond to its vibrations ; changes are set up in the optic nerve and in the brain, and we see. That the eye may be sufficiently sensitive to the ray of light, its sensory surface must be carefully protected from all hurtful influences. Accordingly, we find that the eyeball, embedded in soft fat, is placed in a socket whose margins are formed of strong bone which can withstand heavy blows ; it is also protected from drying by the action of the lachrymal gland which secretes a watery fluid, and from dust and foreign bodies by the lids with their long eyelashes. The watery fluid which bathes the eyes passes away by two fine pores at the inner angles of the eyelids into a passage to the nose, and is prevented from overflow- ing and running down the cheeks by an oily secretion coming from glands in the upper eyelid (Fig. 29, m) which anoints the edges of the eyelids (Fig. 29). Furthermore, the eyebrows protect the eyes from perspiration trickling from the forehead. The eye may be moved in various directions by muscles which will be described later. I STRUCTURE OF THE EYE Coats of the Eyeball. The eyeball is nearly spherical in shape, but is slightly elongated from before backwards, for the front part, which is clear and transparent, to allow the entrance of the rays of light, bulges forward somewhat prominently, The ball is elastic but firm, and is enclosed by a covering which may be divided into three layers, each of which has important functions to discharge. (For the relative position of the various parts of the eyeball see 'Fig. 30.) i. The outermost coating is composed of a layer of firmly H 98 Physiology of the Senses felted fibrous tissue, which, being very tough, preserves the form, and prevents rupture of the eyeball. To it the muscles that move the eyeball are attached. It is called the sclerotic (Greek, scleros, hard), and the part of it seen when the eye is open is known as the " white of the --77 FIG. 30. Diagrammatic section of the eyeball, i, Sclerotic ; 2, junction of sclerotic and cornea ; 3, cornea ; 4, 5, conjunctiva ; 6, posterior elastic lamina ; 7, junction of iris with choroid ; 8, canal of Schlemm, a lymph space ; 9, pigmented tissue uniting sclerotic to choroid; 10, choroid; n, 12, 13, ciliary processes ; 14, iris touching, but not connected with lens posteriorly ; 15, retina lined by hyaloid membrane ; 16, optic nerve ; 17, central artery of the retina ; 18, yellow spot with central groove ; 19, 20, anterior portion of retina ; 21, junction of choroid and ciliary processes ; 23, free border of ciliary process resting on anterior suspensory ligament of lens ; 22, canal of Petit ; 24, hyaloid membrane ; 25, fibres to posterior surface of lens ; 26, 27, 28, lens ; 29, vitreous humour ; 30, anterior chamber containing aqueous humour ; 31, posterior chamber communicating with 30. eye." In early childhood the white of the eye, being thin, appears bluish in tint from the pigment seen through it, while in old age it becomes yellowish by a deposit of fat. The clear transparent circular disc in the front of the eye, the cornea^ is a modification of this external coat. The TJie Sense of Sight 99 fibres of the cornea are united by a cement substance into transparent sheets or membranes, which lie parallel to one another like the coats of an onion, but connected together by many intercommunicating fibres (Fig. 31). In the flat spaces between the fibrous sheets lie numerous corpuscles, flattened, transparent, and branching so as to join with one another. The fibrous sub- stance of the cornea is lined in front and behind by a homogeneous elastic layer, that at the back of the cornea being the thicker and called the posterior elastic lamina of Bowman, or the membrane of Descemet. This lamina is itself covered on its posterior aspect by a layer of flattened cells lying side by side as in a tesselated pavement. There are no blood-vessels in the cornea, nutrition being effected through the branching cells. The whole of the exposed part of the eye is covered with a trans- FlG - 3*. Antero-postcrior sec . . . tionof cornea, e, Conjunctiva ; parent epithelium or skin called the n> nerve send i n g branches to conjunctiva, which is Continuous all cornea arid conjunctiva ; /, . . . . ... . ... fibres of cornea between which round with that lining the eyelids, and which, closely adherent to the cornea, and more loosely joined to the sclerotic, forms a sensitive pro- tective covering for the open eye. 2. The middle coat, the choroid, is largely composed of blood-vessels which branch frequently in its outer part, are flattened spaces containing corpuscles ; d, layer of cells covering posterior surface of cornea, and separated from the fibrous part by the posterior elastic membrane. (Schofield.) 100 Physiology of the Senses and form a very fine network of capillaries to the inside. The blood-vessels of the choroid coat are known as the ciliary arteries and veins. The veins as they emerge join together in a stellate fashion, forming groups, the vena voriicosce, from the union of which single veins pass out- wards through the sclerotic. The spaces between the vessels are occupied by elastic fibrous tissue, and by cells loaded with granules of very dark brown pigment, the whole being bound together by cement substance. The colouring matter renders the choroid opaque, and absorbs the rays of light pass- ing into the eye, thus preventing their reflec- tion to and fro in the interior of the eyeball, and the confused vision that would ensue there- from. FIG. 32. Antero- posterior section through , . . . . conjunctiva and fore part of human cornea, L ne ChorOlCl IS Closely X24od. i, Conjunctiva ; a, nerve fibres in united to the Sclerotic conjunctiva ; s, network of nerve fibres be- , r tween conjunctiva and cornea; 2, anterior b Y me anS of Connective elastic membrane; 3, substance of cornea tissue, but JUSt where with n, a nerve passing through i. (S.ohr.) ^ sderotic merges ; nto the cornea an interesting and important alteration occurs, Were the choroid to line the cornea as it does the sclerotic, light could not enter the eye. Accordingly this coloured layer hangs separate from the cornea as a curtain or ring of variable size called the iris (iris, a rainbow), and is pierced by an aperture known as the pupil, through which light may enter. The space between the iris and the cornea, the anterior chamber, is filled with a watery fluid, the aqueous humour. The back of the iris is lined with dark pigment, and according as the substance of the iris con- tains less or more pigment, the eye has a blue, gray, or 1 lie Sense of Sight 101 brown colour. The central aperture is usually black, from the pigment absorbing most of the light that enters the eye, so that almost none is reflected out again ; but sometimes, as in albinos, the pigment is awanting, and then the pupil is pink, as may be seen in white rabbits. In many of the lower animals the pupil is often seen of a greenish lustre owing to partial reflection of light from the back of the eye. In herbivora this iridescent gleam is due to the arrange- ment of the fibres to the outside of the capillary layer in a FIG. 33. Meridional section through ciliary region of human eye, X 20 d. i, 2, Epithelium and loose connective tissue of conjunctiva ; 3, sclerotic ; 4 meri- dional, 5 radiating, and 6 circular fibres of ciliary muscle ; 7, ciliary process ; 8, ciliary part of retina ; 9, pigmentary layer on the posterior surface of the iris; 10, the iris; ir, the posterior elastic lamina; 12, the cornea; 13, con- junctiva ; 14, canal of Schlemm ; 15, in the anterior chamber points to junc- tion of iris with sclerotic. (Stohr.) structure called the tapctum, while in carnivora and birds of prey it is brought about by reflection from cells which contain minute crystals and act like prisms. The amount of light, moreover, which enters the eye is regulated by variation in the size of the pupil. There are con- tractile fibres radiating in the iris like the spokes of a wheel, and when these contract the pupil dilates. On the other hand, if too much light is entering the eye, a circular band of muscle fibre in the iris, near the margin of the pupil, Physiology of the Senses contracts, and the pupil is lessened in size. The iris is joined to the sclerotic by muscular as well as by connective tissue. The muscular fibres are disposed, partly so as to radiate from the junction of the cornea and sclerotic to that of the iris and choroid, and partly to form a ring round the outer border of the iris, as seen in Fig. 33. Together they form what is called the ciliary muscle, and this assists largely in accommodating the eye for the perception of objects at different distances. Just behind the ciliary muscle lies a curious modifica- tion of the choroid, consisting of a ring of tooth-like tufts of capillary blood- vessels, bound together by connective tissue, and pointing towards the pupil. These are the ciliary processes. The choroid and ciliary processes are lined internally by a thin transparent mem- brane, known as the membrane of Bruch. 3. The innermost coat, the retina, the human eye seen from is the terminal organ of vision, and within, a, Capillary ves- -, - 1 i i sels of the choroid ; b, 1S almost transparent, with a pinkish serrated line of union of tinge, except at a point in the visual choroid with ciliary pro- ax j g ca j led the ve ll ow sfiot of which cesses ; c, veins of ciliary + * ring; d, capillaries of more anon. The retina contains the ciliary processes ; ,, radi- terminal branches of the Optic nerve, atmg veins of ciliary part of iris; /vessels of pupil- which, piercing the sclerotic and laryzoneof iris. (Arnold.) choroid in the human eye at a point about y 1 ^- of an inch nearer the nose than the antero-posterior axis of the eye, and forming an oval area known as the optic pore, spreads out in nerve fibres ramifying over all the FIG. 34. Blood-vessels of the choroid and iris of The Sense of Sight interior of the eye as far forward as the ciliary processes. These nerve fibres are the more transparent as theyare simply axis cylinders, devoid in the retina of the white substance of Schwann. They are supported by connective tissue which is found in most parts of the retina as fibres passing radi- ally, \htfibres of M filler. The connective tissue also forms external and internal limiting membranes and a fine net- work through the substance of the retina, keeping the various elements in their proper places. Small blood- vessels are also found in the inner layers of the retina. Pigmentary layer not seen. ~ 2 Layer of rods and cones. ^,* External limiting membrane. -4' Outer nuclear layer. ;5\ Outer reticular layer. i. Inner nuclear layer. --, 7 Inner reticular layer. Ganglion cell layer. 9. Nerve fibre layer. b k FlG. 35. Vertical section of human retina, X 240 d. b, Blood-vessel ; k, conical base of radiating sustentacular fibre of Miiller. The base of several fibres uniting gives rise to the appearance of an internal limiting membrane. (Stohr.) After spreading over the fundus or concavity of the retina, the nerve fibrils turn outwards and become con- nected with a set of ganglionic cells (see Fig. 36), from which, again, fibres may be traced outwards for a certain distance. These fibres are believed to become connected with nuclei, which are found in two layers to the outside of the ganglionic cells, and from the outer layer of nuclei fibres pass to the true terminal sensory organ, the so-called Jacoffs membrane or layer of rods and cones. This layer lies outside of and upon the external limiting membrane. 104 Physiology of the Senses The rods and cones consist alike of an inner and an outer part. In the cones, the inner part is thick and conical, and exhibits a longitudinal striation (Fig. 37) ; in the rods it is thinner : both are connected with nucleated fibres, internal to the outer limiting membrane. The outer part of the rods FIG. 36. Diagram showing retinal elements. Two fibres of Miiller with expanded bases at , pass outwards as fine cylindrical processes, giving off slender lateral twigs (not shown in diagram) in the reticular layers d andyj and forming meshworks in the layers e and g. The spaces of the meshwork are occupied by nuclei. The fibres terminate in the external limiting membrane h. Opposite c two ganglionic cells are seen, their inner processes continuous with optic nerve fibres in b, their outer processes breaking up into numerous twigs in d. The nuclei of the layer e belong partly to the fibres of Miiller, partly to cells which send many branching processes to the outer and inner reti- cular layers, and probably establish functional continuity between the ganglion cells and the rods and cones. The nuclei of g are surrounded by a thin layer of protoplasm, and are connected externally with the rods and cones by processes perforating the external limiting membrane, and internally by fine fibres known respectively as rod and cone fibres, with the network of the outer reticular layer. The nuclei connected with the rods show one or two transverse dark bands. The rods and cones of the layer z show the differentiation into an outer and inner limb. The outer limb of the cone is shorter than that of the rods. (Zehender.) is of a pink colour, and considerably longer than that of the cones, but both exhibit a transverse striation, and, under the influence of macerating reagents, tend to break up into highly refractile discs. The rods are much more numerous than the cones, but the fore part of the retina has cones The Sense of Sight I0 5 only, while the part of the retina lining the iris has neither rods nor cones. On the other hand, in the yellow spot above mentioned we find cones but no rods. Here, too, we find the layer of ganglion cells at first thickened, but soon thinning, and there is formed in the centre of the yellow spot a short groove or depression, the foyeajgnttaliS) where the various layers of the retina above described disappear, and we find only a layer of cones with the fine terminations of the nerves. This spot is the seat of most distinct vision. Outside of, and in apposition with, Jacob's membrane lies a layer of hexagonal ceils, containing, more espe- cially on their inner side, a vast number of pigment granules of a brown colouring matter called fuscin or melanin. Under the action of light, the cells send pro- cesses carrying the pigment inwards between the outer segments of the rods and cones, and thus absorb the rays of light after they have passed through the retina. If the eye is kept in darkness for some time, these processes are with- i ^i -i j- < i 11 FIG. 37. Diagram of drawn into the main bodies of the cells, rods and coneSj show . and the layer of pigmented epithelium may then be easily detached from the adjoining layer of the retina (Fig. 39). Contents of the Eyeball. Inside of, and closely adherent to, the retina we find a perfectly transparent, highly elastic bag called the hyaloid membrane (Jiyalos, glass), which might be compared to the membrane lining the shell of an egg. This bag is filled with a transparent ing faint longitudinal striation of inner limbs of rods and cones, and varicosities of the rod- fibres. (Max Schultze.) io6 Physiology of the Senses FIG. 38. Rods and cones seen from without on glassy-like jelly, like white of egg, called the vitreous humour (Fig. 30, p. 98), and composed of fluid, penetrated in all direc- tions by fine fibres and a few connect- ive tissue cells. In front, the hyaloid membrane closely adheres to the circle of ciliary processes but not to the iris, and it splits into two layers or suspen- sory ligaments, which are attached to a capsule in which lies the crystalline lens. The suspensory ligament forms a ring called the zonule of Zinn, and removal of pigmentary layer. The larger circles bounded by the two layers and the lens represent the inner limb i s a triangular space containing fluid, of the cones ; the smaller , . , , . central circles, the outer and Called the Canal f Petlt > The limb of the cones, in 2 ligament, it may be noted, is much and -5, the cones are sur- , . j 1 r -n i i rounded by rods. i,From P llCated b Y following the Convolutions the yellow spot ; 2, from of the ciliary processes, and the pos- terior layer is perforated with numerous apertures (Fig. 30). The lens is composed of fine flattened fibres hexagonal in cross section, and with serrated edges which fit exactly into one another, and are bound together by a kind of cement substance. The fibres run in an obliquely meridional direction (see Fig. 41, C), not forming complete semicircles from pole to pole, but fixed at their ends to a tri-radiate mass of cement substance, whose rays form angles of 120 with one another, and, as they pass through the sub- stance of the lens, are rotated like a wheel in motion border of yellow spot ; 3, from middle of retina. a T> FIG. 39. Hexagonal pigmented cells covering Jacob's membrane, a, Surface- view ; 3, cells seen from the side, sending fine processes between rods and cones. The lighter portion in the centre of the cells in a, indicates the non-pigmented nucleus. (Max Schultze.) The Sense of Sight 107 through an angle of 60. The lens, like the capsule which holds it, is perfectly clear and transparent. Should it become opaque, we have the disorder known as cataract. It has a bi- convex form, its front sur- face being somewhat more flattened than that behind, but it is highly elastic, and the curves are constantly changing as the eye is accommodated for near and distant objects. The capsule surrounding the lens is very thin and elastic, and, by the tension of the anterior suspensory liga- ment, the surface of the lens is kept slightly flat- tened. In its earliest stages of development, the lens is formed by an invagina- tion or growth inwards of a process of the deepest layer of the epidermis, which is cut off as a closed sac. The ce,ntral cavity is obliterated by the elonga- tion of the cells at the back of the sac, the cells in front remaining small and cubical, and forming the FIG. 40. Lens fibres. A, From eye of ox showing serrated edges ; B, cross section of lens fibres from human eye ; C, fibres from the equatorial region of the human eye. The fibres are seen edgewise except in A and at C, 2. Near i, nuclei of lens fibres. (Schwalbe, after Kdlliker and Henle.) io8 Physiology of the Senses anterior epithelium of the lens. The lens may be artificially broken up into a set of concentric layers (Fig. 42), in which the fibres run in a meridional direction, and the outer layers are softer and more gelatinous than those towards the centre. The lens from the eye of a lightly boiled fish affords con- venient material for the study of the structure of the lens. It appears as an opaque white ball, but when the outer part is detached with a knife an inner translucent core is found, from which thin transparent sheets may be readily peeled FIG. 41. Diagram of arrangement of lens fibres. A Posterior, B anterior, and C lateral view, c, in each figure, indicates the centre of the tri-radiate cement substance. The numbers i to 6 indicate the same six lens fibres, the course they take being seen by comparison of the figures. (Allen Thomson.) off and broken up into fibres. The iris, to have perfect mobility, hangs free, not only of the cornea in front, but also of the lens and its suspensory ligament behind, except in its central part round the pupil, where it rests lightly on the lens. The space behind the iris and in front of the lens and suspensory ligament is called the posterior chamber. This is filled with fluid, which is similar to, and in com- munication with, the aqueous humour in the anterior chamber. We thus see that the contents of the eyeball are all transparent, and light traversing the eye must pass first The Sense of Sight 109 through the conjunctiva and cornea in front, then through the aqueous humour, thereafter through the lens with its capsule, and finally through the vitreous humour and the hyaloid mem- brane. The Optic Nerve. The nerve fibres converge from all parts of the retina to the optic pore, and there passing through , . 1-1 FIG. 42. Laminated structure of a membrane in which are many the crystalline lens . The laminaj are split up after hardening in alcohol. i, The denser central part ; 2, 2, 2, concentric outer layers. (Arnold.) fine openings for their passage, the lamina cribrosa^ they are grouped together into a bundle forming the optic nerve. The optic nerve from each eye passes backwards, and entering the hollow of the cranium by a passage at jjy. the back of the ^ m orbit, joins with its fellow in a ; union called the jjjj:; optic commissure. \ At the commis- ftg sure some of the > fibres pass directly upwards into the brain, but in the |||| v human eye the most of the fibres 2 3 1 from the inner or FIG. 43. Course of nerve fibres in posterior part of nasal half of each retina, i, Optic pore ; 2, yellow spot (macula /) ; ^^ decussate 3, fibres to yellow spot. (Schwalbe.) or in other words cross over, and pass backwards to the half of the brain no Physiology of the Senses opposite to the eye from which they have come, while fibres from the outer or temporal (next the temples) side of each retina pass back to the brain on the same side as the eye from which they have sprung. Hence it will be seen that almost all the fibres affected by rays of light which come from objects on the left side of the body (a, Fig. 44) will transmit impressions to the right side of the brain, while luminous impressions from the right side of the eyes will be transmitted to the left half of the brain. The bundles of nerve fibres continued behind the optic commissure are known as the optic tracts, and they pass to certain ganglia at the base of the brain, from which again fibres pass to the #v^ occipital or posterior part of \ the cerebral hemispheres, the stimulation of which gives^rise to a sensation of light. But the eye is in connec- tion with other nerve fibres besides those of the optic nerve. FIG. 44 .-Diagrammatic representa- We a11 knOW how Sensitive tion of decussation of fibres of the the eye is to touch, and llOW opticnerves< acutely painful is any lesion of the eyeball. Impulses giving rise to tactile or painful sensations are sent to the brain through the medium of branches of a nerve known as the ophthalmic division of the fifth cranial, or great sensory, nerve of the head, from which there also pass to the iris several branches known as the long ciliary nerves, to whose function reference will shortly be made. Again, the eye, as a whole, and certain parts within the eye, can be moved under the influence of muscular contrac- tion, and to effect these movements we have the oculo-motor or third cranial nerve, and the fourth and sixth cranial nerves. The fibres of the third cranial which supply the The Sense of Sight 1 1 1 sphincter of the iris pass through a ganglion known as the ciliary ganglion, where they meet with fibres from the sympathetic system, and a branch from the ophthalmic nerve. From the ganglion a large number of twigs, the short ciliary nerves, pass to the back of the eyeball, where, having pierced the sclerotic coat, they run forward between the sclerotic and choroid coats to the ciliary muscle, the iris, and the cornea. Stimuli pass by the short ciliary nerves, as a result of which the pupil may vary in diameter, or the eye be accommodated for the perception of objects at vary- ing distances. Movements of the Pupil. Various influences may cause change in the size of the pupil. The brighter the light entering the eye, the nearer the object we look at, or the more we converge the two eyes, the more the pupil contracts. In certain stages of poisoning by opium, tobacco, alcohol, chloroform, and physostigmin, in sleep, or in unconscious states as during an epileptic fit, the pupil may be contracted to a mere pin-hole aperture. Dilation of the pupil occurs when the light is dim, when the eye is looking at distant objects, when respira- tion is obstructed, or the body strongly stimulated ; under the effect of certain drugs, such as belladonna, or its active principle atropin, by Indian hemp or hyoscyamin ; in the later stages of poisoning by alcohol, chloroform, and other substances ; and under the influence of mental emotions, such as fear. This change in size of the pupil is an involuntary move- ment, and goes on without consciousness upon our part, unless we are directly observing it in a mirror. It is of the nature of a reflex act. The usual exciting cause of the movement is a variation in the amount of light entering the eye, and a consequent variation of the amount of stimulus to the optic nerve. If the optic nerve is cut, or if the 1 1 2 PJiysiology of the Senses centre to which it passes in the brain is destroyed, the pupil no longer contracts when light fails on the retina, although the oculo-motor or short ciliary nerves may still be directly stimulated by electricity or mechanical irritation, so as to cause contraction. Moreover, the third nerve con- tains at least two sets of fibres, stimulation of one of which causes contraction of the pupil, of the other, movements of accommodation, and, as might be expected, these fibres originate in different centres in the brain. These centres are situated close to each other in the basal ganglia, and on a lower level than the cortical centres involved in conscious vision. The pupil is caused to dilate by stimulation of the sym- pathetic nerve which, coming from a ganglionic centre situated in the neck, and having entered the cranial cavity, becomes apposed to the ophthalmic nerve, and is given off to the eye from its nasal branch as the long ciliary nerves. There has been much discussion as to its mode of action, but apparently it supplies the dilating muscular fibres of the iris. The oculo-motor to the sphincter of the iris, and sympathetic to the dilating fibres of the iris, would thus seem to act as antagonists to each other. Moreover, they seem to keep up a constant balancing tonic action, because if one is injured the other immediately shows its power. For instance, if the sympathetic fibres be cut, the pupil will at once contract, and vice versd. But this is merely a particular instance of the general law which regulates the condition of the muscles of the body, so long as their nerve supply is normal and in healthy action. Another point of interest in regard to the human eye is that a strong stimulus to one eye will cause contraction of both pupils. This is probably due to the incomplete decussation of the optic nerves, the fibres from one eye passing, as we have seen, to centres on both sides of the brain ; for in animals that The Sense of Sight 113 have a complete decussation, and want the power of binocular vision, this phenomenon is absent. We should note in passing that the foregoing explanation of the mechanism of contraction and dilation of the pupil has been called in question by some physiologists. They deny that the so-called dilator of the iris consists of true muscular tissue at all, and maintain that the sphincter action of contraction is the only really muscular act. Dilation is attributed to elastic recoil, the sphincter being held to be inhibited or thrown out of action by stimulation of the sympathetic. When the pupil contracts, the elastic radiat- ing fibres are stretched ; when the muscle ceases to act, elasticity comes into play, and the pupil dilates. Recent observations seem to show that changes in the calibre of the blood-vessels of the iris, brought about by nervous action, are not the cause of variations in the diameter of the pupil. The iris of birds contains specially developed striated mus- cular fibres, and a more careful examination of such eyes may yet throw light upon this problem. Drugs may act either directly upon the muscles of the iris, or indirectly through the nerve centres. Thus, even in an eye removed from the body, and cut off from all central control, atropin will cause dilation, physostigmin contrac- tion of the pupil. The explanation of this is difficult, if we suppose that two antagonistic muscles are at work in the eye, for we would expect the poison to act on each alike, and that the pupil would remain unchanged in size. On the other hand, if there is only one muscle at work, we would say that atropin paralyses it, while physostigmin excites it to continuous and prolonged activity. The varia- tion in size of the pupil from emotion, obstructed respira- tion, and the like, is, on the other hand, of a central kind that is to say, in such conditions the activity of the central nervous system is augmented or diminished I ii4 Physiology of the Senses with a corresponding effect upon the innervation of the eyes. Tiie observation has been made that the pupil of the eye of a cat isolated after death, and with even the posterior segment of the eye cut off, will slowly contract on continued exposure to light. This appears to indicate that the iris is susceptible to the action of light even without the presence of a nervous mechanism. II. PHYSIOLOGY OF VISION The optic nerves are the nerves of vision. When stimu- lated or injured no pain is caused, but only a luminous sensation is aroused. Nor are the nerve fibres sensible to light, except in and through the retina. Light falling upon the exposed optic nerve will cause no sensation, but if the nerve be now affected by mechanical, electrical, or chemical means, a sensation of a flash of light is ex- perienced. The sensation, however, is one of mere luminosity ; it is not accompanied by the perception of any object. In order that an object may be perceived, an image of it must be formed on the retina, and hence we note the double function of the eye,, the power of responding to light, due to the structure of the retina, and the power of perceiving objects due to the nature of the transparent media in front of the retina. In many of the lower forms of animals we find nerves ending in coloured spots in the skin, and through these it may be the animal experiences a sensation of a special kind of light ; but, in the absence of a lens or other refractive media, images cannot be formed on these spots, and such animals can have no visual perception of external objects. It will conduce, therefore, to a clear understanding of this The Sense of Sight 115 matter, if we consider briefly the nature of the stimulus light and the laws of its transmission through various media, that is to say, the laws of dioptrics. i. LAWS OF DIOPTRICS The Physical Nature of Light. It was once held that a luminous body shoots out from itself minute particles, which, passing to the observer's eye, give rise upon impact to the sensation of light. This corpuscular theory has now been entirely disproved, and it is generally held by physicists that the undulatory theory, first enunciated by Thomas Young, affords a satisfactory explanation of all the pheno- mena of light. According to this view, light, objectively con- sidered, is simply a mode of motion of a substance called the luminiferous ether which pervades, not only what is commonly regarded as space, but also all translucent sub- stances. By the molecular movements of luminous bodies this ether is set vibrating in series of waves. The com- ponent particles of these waves may be conceived to move at right angles to the direction of the ray of light, just as waves rise and fall while spreading outwards when the sur- face of calm water has been agitated by a stone. Thus a cork floating on the water, traversed by a wave, oscillates up and down nearly at right angles to the direction of the wave. These wave-like movements of the ether impinging on the retina set up in it changes which result in the sensation of light, but the sensation in no way resembles its physical cause, although it varies with variation of the stimulus. The intensity of the sensation varies with the amplitude of the waves. Large waves give rise to a sensa- tion of bright light, small waves to a sensation of dim light. Again, the sensation of colour depends upon the rapidity with which the waves follow one another. This rapidity, 1 1 6 PJiysiology of the Senses though inconceivably great, may still be accurately deter- mined.' Ordinary sunlight, as Newton showed, is composed of a series of colours blended together, but yet separable one from another, because each colour is due to a series of waves differing in rate of succession from the others. Thus the waves of red light follow each other at the rate of about 435 millions of million times per second, while those of violet light succeed each other at about 764 millions of million times per second. Between these, we have an infinite number of series of waves, each giving rise to a special colour sensation, and so between the red and the violet of the spectrum we have a gradation of colour roughly described as orange, green, blue, and indigo, but each of these is itself made up of countless shades, which melt as gradually and imperceptibly into one another as the colours in a sunset sky. The eye is not sensitive to vibrations of the ether succeeding each other more slowly than those of red light, although it may be demonstrated that these exist and originate electrical and thermal phenomena ; nor to those which come more quickly, although these have marked chemical activity, and give rise to fluorescence. Reflection and Refraction. Light waves are propa- gated through the ether at about 190,000 miles per second, but the rate varies according to the medium through which the light is passing. When the medium is homogeneous the ray passes in a straight line. When it meets a polished surface it is reflected ; and the angle which the reflected ray makes with a perpendicular to the surface is equal to that which the ray meeting the surface, or, as it is called, the incident ray, makes with the same perpendicular. Further, the incident ray, the perpendicular, and the reflected ray will all be in the same plane. Few surfaces, however, are so highly polished as to conform entirely to the above laws. A certain part of the ray is usually The Sense of Sight 117 irregularly reflected or scattered, and it is owing to this fact that objects become visible, for it can be easily under- stood that if the rays were reflected entirely to the eye we would only be aware of the luminous body, and not of that which reflects the light. When a ray of light passing through one transparent medium, such as air, meets another, such as water, per- pendicularly, part of it is reflected upon itself, and part passes on in the same straight line through the water. If, on the other hand, the ray meets the surface of the water obliquely, the part which passes through the water continues in the same plane as before, but no longer passes in the same straight line. It is bent or refracted out of its course. Some crystals have a power of double refraction that is to say, the ray of light O B entering them is broken into two rays. _.. ... ' J FIG. 45. Diagram illus- each of which is deflected from the original tracing the law of the course; but as in explaining the pheno- reflection of light from ' a plane surface. zO, mena of vision we do not have to deal incident ray; O, re- with such substances, let it be understood flected ray< that what we have to say with regard to refraction refers merely to simple refraction or bending of the ray. The laws for single refraction have been thus stated x 1. Whatever the obliquity of the incident ray, the ratio which the sine of the incident angle bears to the sine of the angle of refraction is constant for the same two media but varies with different media. 2. The incident ray and the refracted ray are in the same plane, which is perpendicular to the surface separating the media. This ratio of the sines of the incident and refractive angles is known as the index of refraction ; and if the ray 1 Ganot's Physics, p. 466. 1 1 8 Physiology of the Senses be supposed to pass from a vacuum through any transparent substance, this ratio is known as the principal mdex of refraction for that substance, and is commonly represented by the letter /x,. Knowing the index of refraction for any two media, we can calculate the direction which the ray of light will take as it passes through them. Each singly refractive substance, then, has always the same bending power due to its special elasticity and con- sequent interference with the velocity of the ray of light. Water interferes more than air, glass than water ; the diamond bends the ray of light more than any other known substance, or, in other words, is the most refractive sub- stance known. Effect of refraction on a ray passing through glass with parallel surfaces. Suppose the ray EF (Fig. 46) passing through air meets obliquely the upper surface AB of a plate of glass hav- ing parallel surfaces. Part of the light will be reflected in the direction FK, part will pass through the plate, N^Z but not in the original direction FL ; TT it will be bent towards XY, the per- pendicular to the surface, and will FIG. 46. Diagram illustrating . , . . . the refraction of a ray of take the P ath FG ' Meeting the SUr- Hght. For description, see face CD, it now passes out into the air, where it immediately regains its former velocity, or in other words, is bent back again to its former direction, so that it now emerges as GH, not indeed in the same straight line as before, but in a parallel direc- tion to its former course. Effect of refraction when light passes from air through a prism. When light falls obliquely on the sides of a prism it is doubly bent, as may be seen from the accompanying TJic Sense of SigJit 119 B C FIG. 47. Diagram illustrating re- fractive power of a prism. figure. The ray GH (Fig. 47) meeting the surface AB at II, is bent towards DE in the direction HK, and emerging through the surface AC is bent away from EF in the direction KL, that is to say, it is bent away from its original course, and deflected towards the base of the prism. The amount of deflection de- pends upon the shape and material of the prism, and on the angle at which the ray of light impinges on its surface. Action of Lenses. A similar deflecting action is exer- cised by lenses, which may be looked upon as resembling two prisms in apposition by their bases or edges. Thus in Fig. 48, A and B represent pairs of prisms set respec- tively base to base, and edge to edge ; C, D, and E are convex lenses, or, in other words, are thicker at their centre than at their circum- ference, and would exercise a deflecting power upon rays H FIG. 48. Diagram showing comparison of lenses to prisms set base to base or edge to edge. C, Biconvex ; D, plano-convex ; E, concavo-convex ; F, biconcave ; G, plano-concave ; H, convexo-concave lens. of light similar to that of A ; F, G, and H are concave lenses, being thinner at their centres than their circum- ference, and would deflect rays of light in the same way as B. The biconvex lens is of most interest for our present purpose, for, like the transparent media of the eye, it has the property of condensing or focussing rays of light. The common burning-glass or biconvex lens has, as a rule, spherical surfaces. If AB (Fig. 49) represent a biconvex lens, and the line CF its principal axis, i.e. the straight line Physiology of the Senses A j\ through the centre of curvature of its two surfaces, all rays parallel to CF meeting the surface ADB, will be brought to a focus at very nearly the point F, which is called the principal focus ; and, con- versely, rays spreading from F will pass through the lens, and emerge in a parallel direction. ,., ,.. ... . If rays diverge from a FIG. 49. Diagram illustrating course taken by parallel rays of light refracted by bicon- point/ (Fig. 50) in the vexlens - axis of the lens outside of the principal focus, they will be brought to a focus at a point /' on the other side of the lens known as its conjugate focus. FIG. 50. Diagram illustrating the law of conjugate foci. If, as in Fig. 51, the rays diverged from /"to the inside of F, they would still diverge on the other side of the lens ; FIG. 51. Diagram illustrating position of virtual focus. but now if produced backwards, would form a virtual focus at/. Formation of Images by Biconvex Lenses. Any ob- ject at which we look may be regarded as made up of an aggregation of points, each of which sends a pencil of rays of light to the eye, and the main value of the lens for purposes of vision is its power of forming images of objects The Sense of Sight 121 by combining again the scattered rays. Thus all the rays from A falling on CD (Fig. 52) may be collected at the point A', all the rays from B at B', and rays from all intervening points of AB will meet at points along the line A'B', and thus an image of AB is formed, but upside down or inverted. The size and position of the image depend on the position of the object with regard to the principal focus of the lens, and can be calculated by simple mathematical formulae. In Fig. 52, for example, the rays from the point A of the object AB may be supposed to be brought to a focus by the lens CD at the point A'. Those from B FIG. 52. Formation of an image by a biconvex lens. at B', and all intermediate points in AB, at corresponding points in A'B'. We are now in a position to understand why a lens is required for vision. Were light simply to pass through the pupil and fall on the retina without refraction, from each point in the field of vision a cone or pencil of rays would pass to the retina and form a circle of light upon it, and these circles overlapping one another, as in Fig. 53, would simply give a sense of diffused light, and not the perception of each point separate one from another. But suppose the pupil were narrowed to the finest point, so that only one ray of light would pass in from each point of the object, as in 122 Physiology of the Senses Fig. 54, the amount of light admitted would be so infinitesimally small as to be unable to affect the retina. In avoiding overlapping, the amount of light admitted has become infinitely little ; or, in other words, as the pupil diminished in size FIG. S3 .-Diagram showing overlapping of rays ^ object would appear in the absence of a lens. dimmer and dimmer, until it ceased to be seen altogether, for the amount of the stimulus would be too small to excite the sensation of vision. But the refractive media of the eye acting like a lens con- dense the rays which have entered the pupil so as to form an image which, in the normal eye, falls upon the retina ; and each point of the image, being the focus- or meeting- point of a vast number of rays coming from the correspond- ing point of the object, is sufficiently bright to stimu- late the retina to action. We may easily prove that such is the case. If an eye removed from its FIG. S 4.-For explanation, see text. socket be stripped posteriorly of the sclerotic coat, an inverted image of the field of view will be seen on the retina ; but if the lens or other part of the refractive media, be removed, the image will become blurred or disappear altogether. There are, however, two defects in ordinary spherical lenses which, as they affect the eye, deserve our notice. Spherical Aberration. Any one who has attempted with a burning-glass to focus the rays of the sun upon a sheet of paper must have noticed that the circle of light, at first large and dim, gets smaller and brighter for a time The Sense of Sight 123 and then enlarges again, but the image of the sun thus formed is never reduced to a mathematical point. This is due to what is called the spherical aberration of the lens, and a glance at Fig. 5 5 will enable us to understand it. The ray of light CD, which passes through the centre of the lens AB, in Fig. 55, is not refracted at all, but passes on in a straight line. Rays near CD, such as E, E, are slightly bent and intersect CD at a considerable distance from the lens. Rays meeting the surface of the lens at points nearer its circumference than E, E, such as G, G, or K, K, are* more refracted, and intersect CD at points nearer the lens. Thus, as we pass towards the circumfer- ence, the rays are more and more refracted, and do not K- G- E- C- E- G- K- B FIG. 55. Spherical aberration. meet all at one point. Accordingly, when we interpose a screen in the path of the rays, while a few may be accurately brought to a focus upon the screen, the great majority are either still converging or now diverging, and they form concentric rings of light which blend with one another, or diffusion circles, as they are sometimes called, and these blur the image formed by the accurately focussed rays. By interposing a diaphragm, with a central aperture, the outer rays may be cut off and only those rays which pass near the centre will be brought to a focus, and thus the image will be made sharper. If the central part of the lens be more refrangible than the circumference, a similar 124 Physiology of the Senses result will be obtained, for rays passing through the former will be more refracted, and thus be brought to a focus nearer those that have passed through the circumference. Such a provision as this exists in the human eye, the centre of the crystalline lens being more refrangible than the outer parts. Chromatic Aberration. The other defect in ordinary simple lenses is that when sunlight passes through them, owing to the different refrangibilities of the various coloured rays which go to make up white light, the sun's ray. is broken up into its component parts, and some of these are bent more than others. This separation of the coloured rays is known as dispersion. The red rays being least FIG. 56. Chromatic aberration. refrangible are less refracted than the orange, the orange than the yellow, and so on, the violet rays being most refracted of all. Thus, if rays pass through the lens AB (Fig. 56), we may suppose the red rays to intersect the main axis at R, the violet at V. If a screen be interposed in the position aa^ there will be a coloured circular spectrum having the red to the outside and the violet to the inside ; but if the screen be placed at bb> the violet rays will now be outermost and the red rays to the inside. It was formerly supposed that the dispersive power of all bodies was alike, but it is now known that this is not so ; and by combining lenses of opposing action it has been found possible to do away, to a very great extent, with the disper- The Sense of Sight 125 sion of the light, although it is still refracted. Such a lens is usually composed of a concave flint-glass (A, Fig. 57), and a biconvex crown-glass lens (B, Fig. 57), and is said to be achromatic, or in other words, not colour-producing. Optical Properties of a System of Lenses. , r . - .. , . - FIG. 57. Achro- If the rays of light emanating from an mat ; c j enSe A object pass through a series of lenses, differ- Plano-concave ing in shape and refractive power, but having gjass . Bj bi _ their centres in one axis, the position and convex lens of size of the resulting image might be found by crown -s lass - calculating and combining the effect of each lens in turn. This would, however, frequently lead to very elaborate cal- culations, and the researches of Gauss, Mcebius, Listing, and others have shown that for any system of centred spherical surfaces there exist six points known as cardinal points, through four of which pass planes perpendicular to the axis, and that if the position of these has been determined the direction of all rays of light through the system may be readily traced. The cardinal points are the first and second focal, first a?id second principal, and first and second nodal points, and the planes pass through the two first pairs. 1. The first focal point is so placed with regard to the system that all rays passing from it through the system, emerge in a direction parallel to the axis of the system, while all rays parallel to the axis - before entering the system are, having passed through it, gathered at the second principal focal point. This also holds good for all points in the planes through the foci perpendicular to the axis. 2. Theyfrj/ and second principal points are so situated that in the planes passing through them perpendicular to the axis the principal planes there are correspondent 126 Physiology of the Senses points on the same side of, and at the same distance from, the principal axis of the system, through which the refracted rays must pass. Thus each principal plane is the optical image of the other. 3. The first and second nodal points are such that all rays which before being refracted pass through one of them, seem after refraction to emerge from the other and in a direction parallel to what they had at first. 4. The first principal focal length is the distance between the first focal point and the first principal point. 5. The second principal focal length is the distance between the second focal point and the second principal point. 6. The principal points are at the same distance from B C FIG. 58. Diagram illustrating course of ray through a dioptric system. each other as the nodal points, and the distance between the first focus and the first nodal point is equal to that between the second focus and the second principal point. Then the distance between the first principal and first nodal points equals the difference between the first and second principal focal lengths. Given the cardinal points we may, then, trace the course of a ray through the system or calculate the position and size of the image of an object. Suppose in Fig. 58 F 1 F 2 , P 1 P 2 , N 1 N 2 represent re- spectively the first and second focal, principal, and nodal points. Any ray AB from the first focal plane incident upon the first principal plane passes parallel to the main axis to The Sense of SigJit 127 C, and thence in a direction parallel to AN, the line joining A to the first nodal point. To find the position of the image of any point A, we must trace the course of at least two rays from the point through the system till they meet. Thus, in Fig. 59, with the same N f N 2 FIG. 59. Image of a point. letters as above, the ray AB parallel to the main axis passes through C, and thence through the second focus F , while from N., emerges a ray parallel to AN L which meets CF produced, at A r 2. THE DIOPTRIC SYSTEM OF THE EYE It was stated (p. 109) that light, before falling on the retina, passes through a series of transparent refractive sub- stances, viz. the cornea, aqueous humour, crystalline lens, and vitreous humour, and, with certain exceptions, which will be pointed out later, the eye may practically be con- sidered as composed of a centred system, composed of a convex refractive surface, the cornea, and of a biconvex lens, the crystalline lens. The cornea in reality has a double surface, but the outer and inner surfaces are so nearly parallel that the two may be regarded as one ; and although the lens differs much in the refrangibility of its different parts, its action as a whole may be taken as that of a homogeneous substance. The surface which exercises the greatest refractive influence is the anterior surface of the cornea, since the refractive powers of air and the 128 Physiology of the Senses substance of the cornea differ in a marked degree. On the other hand, the aqueous humour approximates so nearly in re- fractive power to the substance of the cornea that the refrac- tion in it may be neglected ; and, again, the refractive power of the vitreous is the same as that of the aqueous humour. Many careful investigations have been made as to the form of the various refracting surfaces of the eye, their relative distances from one another, and of the refractive powers of the different media concerned, and while it is found that the eyes of different persons, and even of the same person, differ to a considerable extent in all these respects, yet certain measurements have been obtained which may be regarded as representing those of an average normal eye. These being known, we can determine the position of the cardinal points, and thus calculate, the course of rays of light in the eye. The following figures represent the latest and most accurate determinations : * Index of refraction of the air . . . . . n I. Index of refraction of the aqueous humour and vitreous body ....... n' 1=1.3365. Total index of refraction of the crystalline . .7*" =1-4371. Radius of curvature of the cornea .... r= 7-829111111. Radius of the anterior surface of the crystalline lens r' = lomm. Radius of the posterior surface of the crystalline lens r" = 6mm. Distance from the anterior surface of the cornea to the anterior surface of the crystalline = 3'6mm. Distance from the anterior surface of the cornea to the posterior surface of the crystalline . . =7-2111111. Hence, thickness of the crystalline . . . . e = 3.6mm. From these data, the following results have been calculated : (A) Focal Points. I. Surface of cornea. First focal distance/o' = - = _ll2_ = 23 -266mm. K'-I i-3365-i 1 Landolt, The Refraction and Accommodation of the Eye, p. 79. The Sense of Sight 129 Second focal distance / " = J^L = r '33 6 5 x 7*829 = n'-i i-3365-i II. Anterior surface of crystalline. First focal distance A' = ;/V = l '3tf$ xl =I ^ 2 .8^mm n'-n' I-437I-I-3365 Second focal distance f\ =. -. '~^L = 142 -8 ^mm. n'-n' I-437I-I-3365 III. Posterior surface of the crystalline. . First focal distance / a '= j^_ i-437i*6 = 85. 7 ii 7 mm. tf-9f I -3365-1 -4371 Second focal distance f-{' = r , = iP-^lJ? = 70.711 ^mm. n'-n" I-3365-I-437I (B) Principal Points. I. The principal points of the cornea coincide with its summit. II. The first and second principal points of the crystalline are at a distance of 2-i2597mm. and I -2756mm. respectively from the anterior and posterior surfaces of the lens. (C) The Nodal Points of the crystalline coincide with its principal points. Hence it is deduced that (1) The first principal focus of the eye is situated 13 -7451 mm. in front of the cornea. The remaining cardinal points of the eye are behind the cornea, and measuring from its anterior surfaces lie at the following distances. (2) The second principal focus of the eye is situated 22-823/111111. behind the cornea. This distance, in other words, is the length in the normal eye between the cornea and the retina. (3) The first principal point, i '7532mm. (4) The second principal point, 2-1101 mm. (5) The first nodal point, 6-9685mm. (6) The second nodal point, 7-325401111. From Fig. 60 (p. 130) it will be seen that the prin- cipal points lie in the anterior chamber, the first nodal K 130 Physiology of the Senses point in the lens, the second nodal point slightly behind it, the first principal focus in front of the eye, and the second principal focus at the posterior surface of the retina. The diagram represents what has been called by Listing the schematic eye. By its aid we may easily trace the course of all rays of light entering the eye. The principal points and the nodal points are seen to be respectively very near each other, and if each pair be FIG. 60. Schematic eye. A, Anterior surface of cornea ; //, i//', first and second principal focus ; H', H", first and second principal points ; K', K", first and second nodal points j F \.,fovea centralis of yellow spot. (Landolt.) regarded as combined into one point, we simplify the con- ception of the eye very much, reducing it to a system having a single spherical surface separating the air from the more refractive media of the eye behind. The prin- cipal point is then at the surface, and the nodal point at the centre of the sphere, the focal points being situated as before. 'Such a conception is known as the reduced eye of Listing. The Sense of Sight 131 3. ANOMALIES IN THE EYE AS AN OPTICAL INSTRUMENT While we may then form a conception of a mathematically correct eye, it must be borne in mind that all eyes present certain variations from the ideal form. 1. Thus the various refractive surfaces are not, as a rule, centred so that the optic axis or line joining their centres coincides with the line of vision, that is to say, with the line from the point viewed to the fovea centralis of the retina. The angle of the one axis to the other, where they meet at the nodal point, may be as great as 12. This divergence of the optic from the visual axis is represented in Fig. 60, where it will be noted that the posterior end of the optic axis does not go to the fovea centralis. 2. Again, the centre around which the eye rotates is usually in the optic and not the visual axis, and, con- sequently, the line joining the point viewed with the centre of rotation of the eye, or, as it is called, the line of regard, does not usually coincide with the line of vision. 3. Further, we have seen (p. 1 24) that in ordinary lenses, white light is broken up into coloured rays which are not focussed at the same point, and we saw how we can correct this by combining lenses of different forms and dispersive powers. Similarly, in the eye, the rays of light are broken up into their constituent colours, but this is done only to a very slight extent, and does not interfere with ordinary vision. In fact, its existence can only be determined by careful experimentation. When we look at red letters on a violet ground, the eye is soon fatigued by the effort to focus both colours on the retina at once, and we experience an unpleasant jarring effect ; or in looking at a violet flame which gives forth red and blue rays, we 132 Physiology of the Senses may either see a red flame with a blue halo, or a blue flame with a red halo, according as the eye is accommodated for red or blue. This may be called the defect of chromatic aberration. 4. The blurring of the image caused by spherical aberration (p. 122) is almost entirely corrected in the eye by the varying refractive powers of the media, especially of the FIG. 61. Astigmatism. The lens ACDEF has greater refractive power in the plane ACD than in the plane AEF ; rays in the vertical plane ACD will be brought to a focus at the point G, while those in the horizontal plane AEF are still converging to meet at the point B. If a screen be held at the point G, a horizontal line of light ad will be seen ; if at the point B, a perpendicular line be ; and if at intermediate points, ellipses of varying shapes as above. lens, by the influence of the iris in cutting off the outer rays, and by the shape of the refracting surfaces, which are not spherical, but of forms known as ellipsoids of revolution, that is to say, surfaces formed by the rotation of an ellipse upon one of its axes. 5. Astigmatism. But these surfaces, while better adapted for vision than spherical surfaces, are themselves The Sense of Sight 133 usually somewhat irregular in this respect, that their curva- tures vary in different planes. In the vertical meridian the curve is in most eyes more convex than that in the horizontal ; and, as a result, rays in a vertical plane are brought to a focus nearer than those passing through the horizontal. Thus all rays diverging from a point cannot be exactly recombined to a point after passing through the eye, and a line is seen either in a horizontal or vertical direc- tion according to the position of the retina, or there is a diffusion ellipse for intermediate, positions. Hence the name astigmatism given by Whewell, from a, without, and stigma, a point. That most eyes are more or less astigmatic is shown by the fact that to almost every man the fixed stars seem to twinkle or send out scintillations radiating from a centre. Were our eyes perfect, the stars would appear as luminous points, not FIG. 6 2 .-Cylindrical lens to cor- rect astigmatism in the eye. " star-shaped." Similarly, in look- Rays In two horizontal planes ing at the bars of a window, the are br ught to a focus, but do not approximate in a vertical astigmatic eye cannot see both direction. vertical and horizontal bars at the same time with the same distinctness, one or other must be blurred by diffusion circles. Astigmatism may be regular, as above described, or irregular, the latter more especially being due to irregularities of the lens, while the former arises most commonly from the shape of the cornea. The effect is so slight in most eyes as to go unobserved, but it may be so great as to require the use of a lens consisting of the longitudinal segment of a cylinder, in which the convexity is greater in one plane than in another to compensate for the deficient convexity of curvature in one meridian as compared with the other (Fig. 62). 134 Physiology of the Senses 4. ADJUSTMENT OF THE EYE FOR DIFFERENT DISTANCES When parallel rays, such as come, for example, from a star, fall upon the normal eye in a state of rest they are brought to a focus on the retina. If, however, the rays emanate from a point within a distance of about 65 metres (71 yards), they are sensibly divergent, and can only be brought to a focus upon the retina by an effort, and the nearer the object viewed is to the eye the greater must be the effort, until at last the eye becomes unable to gather the rays to a point at the retina, and the object is no longer distinctly seen. If, shutting one eye, we hold up a pencil in line with an object at some distance it will be found that both cannot be seen distinctly at the same time. If we see the distant object FIG. 63. For description, see text. distinctly the outline of the pencil is blurred, and vice versa. The eye has the power of adjusting itself so that all rays from beyond a certain near point may be focussed on the retina. Thus if the rays from a point p (Fig. 63) are re- fracted so to meet at r the retina,/ will be seen distinctly, but if the point p be now moved to the point /', unless the eye be adjusted for the change, the rays from p' will be focussed behind the retina, and the point p would be seen indis- tinctly. Now, there are two ways in which this adjustment might be effected. The length of the eye might be varied to meet the varying distance of the focal point, just as a-i photographer moves the sensitive plate of his camera back- r wards or forwards to bring it into focus. But, as a matter-/ The Sense of Sight 135 of fact, another process takes place in the eye. The retina is not moved backwards or forwards, but the refractive power of the crystalline lens is changed by an alteration of its thickness. The more curved the surfaces of a lens are, the greater is its refractive power. Now, when we look at distant objects, and no effort at accommodation is required, the anterior surface of the lens is kept flattened by the pressure of its capsule and by the elastic pull upon it of the anterior suspensory ligament an elastic pull which involves no muscular strain, and consequently no fatigue. But when \ve wish to look at a near object, the ciliary muscle (see p. 101) contracting pulls forward the suspensory ligament FIG. 64. Mechanism of accommodation. A, The lens during accommodation with its anterior surface advanced ; B, the lens at rest ; C, position of the ciliary muscle ; ^, the vitreous humour ; a, the anterior elastic lamina of cornea ; c, corneal substance proper ; b, posterior elastic lamina. and diminishes its circle of attachment, its tension is lessened, the pull on the capsule of the lens diminishes, and the lens, by its own elasticity, assumes a more spherical shape, its anterior surface moving forward, and its power of converging rays being increased. The nearer the object the greater the effort required, and when long sustained the greater is the fatigue experienced. As a rule, however, we are unconscious of the effort, although, as will be seen, the feeling gives us valuable aid in judgment as to the distances of objects. The accompanying diagram (Fig. 64) repre- sents the change, the right side B showing the condition of rest, the left A the state when the eye is adjusted for near 136 Physiology of the Senses FIG. 65. Reflected images in the eye. A, for distant ; B, for near vision. sight. The change in the curvature of the anterior surface of the lens may be demonstrated as follows : Let the observer in a dark room, looking at the side of the eye to be examined, note the reflections of a candle flame held to the other side, and in front of the eye observed. Two bright points can be readily seen one the reflection of the flame from the surface of the cornea, and one from the anterior surface of the lens and, with care, a third, much fainter, from the posterior surface of the lens. When the person whose eye is being examined is directed to look as at an object at a great distance, the three points of light will have the position shown in A (Fig. 65); and now on adjusting the eye so as to see an object close at hand the middle point of light moves forward, nearer to the corneal reflection, and becomes smaller as in B. This is due to the bulging forward of the lens, and the consequent reflection of the light from a surface nearer the cornea, and more curved than before. The experiment can be readily performed in daylight by means of the phakoscope invented by von Helmholtz, which FIG. 66. Phakoscope. The ob- server looking through the aper- ture a sees images of the slits bb' reflected from the observed eye situated at the distant side of the phakoscope, and accom- modated first for distance, and second for near vision, the re- gard in the latter case being fixed on the needle-point in the window c. The Sense of Sight 137 consists of a darkened box applied to the eye, with aper- tures at convenient positions for the light, for the eyes of the experimenter and of the person observed, and with an opening through which the eye to be observed may look. Careful measurements of the sizes of the reflected images have shown that the image on the anterior surface of the lens becomes smaller when we look at a near object, another proof that the lens becomes more convex anteriorly. There is also a slight increase in the posterior convexity of the lens. The Near Point of Vision. The range of accommoda- tion is limited. It begins for objects at about 65 metres (71 yards) from the eye, and for normal eyes reaches to x y FIG. 67. Schemer's experiment. For description, see text. within 20 centimetres (8 inches). The position of the near point of any eye may be readily determined by the classical experiment of Scheiner. It is performed as follows : In a thick card make two small holes with a needle at a distance not greater than the diameter of the pupil, and holding the paper closely to the eye look at the needle through the holes. If the needle be held 4 or 5 inches from the eye two points will be seen, but as the needle is gradually moved farther away the two points will be seen to coalesce into one point, and they do so at the near point of vision, namely, 8 inches from the eye. The meaning of this will be understood from the diagram in Fig. 67. If the needle is at the nearest point at which 138 Physiology of the Senses the rays coming from it to all parts of the pupil can be collected to one point on the retina, the cones of rays passing through the apertures will be collected at r, and we see the needle single, but on bringing the needle nearer to the eye we are unable to adjust the eye for the divergent rays, and it is as if the retina were situated at zz, and two points a and b will be seen ; but as these are due to circles of diffusion and not to rays brought to a point, the image on the retina is blurred, and not so bright as before, owing to the lessened quantity of light admitted by the single hole. A*s the image is projected outward through the nodal point N, the image of b will be seen in the line bb\ and that of a in the line aa ', in other words, the real point seems to be split into two, one on each side of the true position. The distances given above for the far and near points are those for a normal eye at rest, in which the optic axis is of such a length that parallel rays are brought to a focus on the yellow spot (Fig. 68, i). Such an eye is called emme- tropic^ or an eye in measure. But many eyes are not so adapted ; they have the retina either before or behind the focal point, and are then said to be ametropic, or not in measure. The axis may be too long, and parallel rays are focussed before they reach the retina (Fig. 68, 4), as in the short-sighted, myopic, or hypometropic eye ; or the axis may be too short, as in the long-sighted or hypermetropic eye, and the rays are brought to a focus behind the retina (Fig. 68, 3). A short-sighted person, who desires to see distant objects, wears spectacles with concave lenses to make the parallel rays diverge, so that on passing through the eye they will be brought to a focus farther back than usual, and so upon the retina ; while in viewing near objects, as in reading, the book is held nearer the eyes to give greater divergence to the rays. The long-sighted person, on the other hand, The Sense of Sight 139 wears convex lenses, so that the rays may be brought more quickly to a focus, and in reading he holds the book at FIG. 68. i, Emmetropic eye ; 2, normal eye accommodated for near vision by increased curvature of the anterior surface of the lens ; 3, hypermetropic eye ; 4, myopic eye. arm's length for a similar reason. Further, an eye of normal length may gradually lose its power of adjustment for near objects, a condition common in old age, and we have 140 Physiology of the Senses what is known as the presbyopic eye. In the eye of an old person the parts are deficient in elasticity, and the fibres of the ciliary muscle are probably less powerful than in early life. The anterior surface of the lens cannot therefore become sufficiently convex for objects viewed a little beyond the near point of distinct vision. In other words, the near point in a presbyopic eye is farther back than normal, and hence, in reading, the head is thrown back and the news- paper held as far away as possible. In this case, too, convex lenses are used to compensate for the lost power of adjustment for near objects. Irradiation. A minor result of defective power of FIG. 69. Irradiation. accommodation is to be found in the phenomenon known as irradiation. When we look at a bright object on a dark ground it seems larger than when a dark object of similar size is seen on a light ground. People dressed in white look larger than when in black. Note also the two small squares in Fig. 69. The white seems larger than the black, although they are of exactly the same size. This is probably due in part to the formation of circles of diffusion, the more powerful stimulus of the rays from the white surface annulling the less intense rays from the dark border. An interesting example of this is the effect produced on the eye by the glowing filament of the electric lamp. The The Sense of Sight 141 filament may form a loop, but this is not seen when the full light of the lamp meets the eye. We see only a brilliant light. But if we cut off some of the rays by the intervention of a plate of smoked glass, or by winking the eyes rapidly, the filament is distinctly seen, although apparently broader than it really is on account of the intensity of its luminosity. Entoptic Phenomena. In describing the effects of refraction on the rays passing through the eye, we have hitherto spoken as if the transmitting media were perfectly transparent in all parts. It has now to be observed that in almost every eye there are small opaque bodies which intercept the light as it enters, and throw shadows on the retina. These shadows projected out- wards give the impression of rounded or filamentous bodies floating in space. They may be well observed by looking with half-shut eyes at a white cloud, when they will be seen floating away and eluding our efforts to keep them at rest. They have been called on this account muscce volitantes^ and their fleeting character is due to the fact that they are not as a rule directly in the line of distinct vision, and in our attempt to gain a direct view of them we move the eye and with it the substance which gives rise to the appearances. The opaque particles may be either in front of the retina or in the retina itself, and one of the latter phenomena, namely, the shadows of the retinal vessels, is of especial interest, not only from its peculiar appearance, but also from the proof which it affords that the layer of rods and cones is the part of the retina sensitive to light. It may be studied as follows. In a dark room cast a bright ray of light sideways upon the cornea. This pene- trating to the retina forms there a luminous image which itself is reflected to other parts of the interior of the retina. One of these reflected rays may in its course impinge upon 142 Physiology of the Senses a retinal vessel which casts its shadow on the outer corre- sponding part of the retina. The part of the retina upon which the shadow falls, refers this outwards through the nodal point of the eye. The path described is traced in Fig. 70, A. The ray b passing to c' and reflected thence, falls on a vessel x in the retina, and a shadow is cast at d which is referred outwards in the direction da '. If now the source of light be moved to b' the ray will pass to c, be reflected in the direction cd', and intercepted at .r, FIG. 70. Diagram to illustrate the formation of Purkinje's figures. with consequently a shadow on d' which is referred out- wards in the direction d'a. If the ray of light cannot enter the eye by the pupil, but merely passes through the sclerotic, we will have the result depicted in Fig. 70, B. A ray of light entering at a is intercepted by a vessel <:, and the shadow at a is projected outwards to A. If we now move the source of light so that the ray enters at ", the shadow of c will be formed at b' and projected outwards to B", or, in other words, we will see a dark line apparently moving from A to B". The Sense of Sight 143 As a result, then, of this play of light and shadow, there is seen dimly outlined on a darkly luminous ground, and moving as the light moves, an arborescent figure, the shadow of the arteries and veins of the retina. We do not see this under or-dinary circumstances, because light enters the pupil from all parts of the field of vision, and no distinct shadows are cast upon the retina. H. M tiller has proved, by a study of the mathematical conditions of this phenomenon, that the shadows of the vessels must fall upon the layer of rods and cones in order to give the result obtained, or, in other words, that light must penetrate the various internal layers of the retina and affect the outer layer before it can give rise to a sensation of luminosity. Examination of the Interior of the Eye. The pupil of a normal eye is black in appearance, and we cannot study by unaided vision the interior of another eye in situ. Does the eye merely absorb rays and reflect none out- wards ? Von Helmholtz, who has done so much in advancing the science of physiological optics, was the first to show that the eye does reflect rays outwards, and that with proper arrangements we may cause the eye to reflect so much light that its interior can be easily examined. When walking in the street we can scarcely see into the interior of houses through the windows, because the amount of light emerging from within is so much less than the diffused light outside, and the difficulty is increased by the reflection of light from the glass. But we can see into the room better if the window is open, or if the room is lit up within. Similarly with the eye, the light entering is partially reflected outwards by the retina, but most of it is absorbed ; and, further, the part reflected emerges in the same path as it entered, and by the refracting action of the eye is brought to a focus at the original luminous point. If, then, we place a light between our eye and that 144 Physiology of the Senses of the person observed we cannot see into the other's eye, because the emergent rays are focussed at the flame and do not form an image in our eyes. If we bring our eye near to the observed eye, our own head intercepts the rays from without, and we cannot see the interior. But if a light (in Fig. 71) be placed to one side of the observed eye C, and its rays reflected into the eye by a piece of transparent glass, or better still, by a small concave mirror with a central aperture, these rays will illuminate the eye. Then part of the rays again reflected outwards will pass a through the glass to meet and ^^p^^ form an image at a, but being intercepted by the ob- server's eye B, the image is formed on his retina, and thus | the interior of the eye C may be examined. It will be seen that this only holds good if both eyes are emmetropic. If one eye be myopic, the other must be hypermetropic to a FIG. 71. Principle of the ophthalmo- corresponding degree, and scope. (Fick.) . n the ophthalmoscop e__the instrument invented by von Helmholtz for the examination of the interior of the eye there are usually convex and concave lenses by which the observer is able to counteract the effect of any degree of ametropia in the observed eye. In other words, if the observer's eye be emmetropic, the nature and curvature of the lens which must be interposed give an indication of the nature and amount of the ametropia of the observed eye. Thus, by the ophthalmoscope, we can see the interior of the eye, examine all its parts, and judge if it be healthy, while at the same time we determine any short or long-sightedness present. Tke Sense of Sight 145 The retina presents to the observer's eye the appearance of a red-coloured concave disk, with a whitish oval spot to its inner side where the optic nerve enters, from which are seen branching the retinal vessels, the veins being darker in colour than the arteries, and in the visual axis lies the yellow spot already described. The vessels of the fovea centralis are so fine as to be invisible to the naked eye, but they form a very close and fine network at this part of the eye. The retina being concave, all images formed on it larger than points must share in its concavity. This, however, is an advantage, for if the retina were flat, all the outer parts of any image formed upon it, not being exactly focussed, would be distorted, as on the plate of a camera, but on account of the retina's concavity each part of the image is focussed in its proper position, and distor- tion and blurring thus largely avoided. While this is so, it is always to be borne in mind that although the whole posterior part of the retina may have formed upon it a fairly clear and distinct image of all the parts of the visual field, and although by an act of will we may without moving our eyes pay attention to the outlying parts, still the only part of the retinal image which gives rise to distinct vision is that formed upon the nerve termin- ations in the central depression in the yellow spot. In other words, if the rays of light from an object at which we are looking converge towards the optic centre, so as to form an open angle, and then diverging, are brought to a focus on the retina, to form a large image, we will not be able to see the whole object distinctly without moving the eye, so that a series of images of different parts of the object is formed consecutively upon the area of acute vision. The Visual Angle. The angle formed by the rays from the extreme limits of the object of vision at their point of convergence (the nodal point) in the eye is L 146 Physiology of the Senses known as the visual angle, and the visual angle which any object subtends depends upon the size and the distance of the object from the eye. A small visual angle is there- fore a condition of distinct vision. But there is a limit to this, for with most people, if the visual angle subtended by the object be less than 60", the area of the retina stimulated will be so small that all separate points in the object seem to be fused into one in the mental picture obtained by the retinal stimulation. Some carefully -trained observers with acute eyes may possibly distinguish from one another as separate points the ends of a line which subtends an angle of only 50", the image of which in the average nor- mal eye would have a length of -00365 mm. or 3-65 fj.. 1 The diameter of a retinal ? ir i i TU ^T~ j cone is 3-2 a, but as IG. 72. Visual angles. The objects c, d, e, J ' ' though of different sizes, subtend the same the COnCS do IlOt prCSS ^^^^^^^ Qr ^^^ c ^ hom against one another each cone corresponds to an area having a diameter of 4 //,. If the image is so small as to fall entirely upon one cone all points in it will be fused together, but it is conceivable that an image not more than i ^ in length might stimulate adjacent sides of two cones. In such a case, however, there must be a mental fusion of the effect, for images of less diameter than 3-65 /x are always seen as one, and not more than one, point, at least so far as observations have yet been made. It matters not how large the object may be, if it is only far enough away to 1 The Greek letter fj, is used to denote the thousandth of a mm. , and is the unit of measurement for objects of microscopical size. A mm. =-fc of an inch : hence a micromillimctre, i JUL= .^ 5 oou of an inch, and 3.65 /* = TOVi> of an inch. The Sense of Sight 147 O subtend the angle of 50" it must appear as a point. The fixed stars we know to be vast suns, but they appear to us as mere points of light because their dis- tance is so great that they subtend a very small visual angle. Nay more, many stars long supposed to be single have, by the FlG ' 73 Diagram J showing how an aid of powerful telescopes, been shown to image smaller be double, triple, quadruple, or even mul- than ^ e dia ' meter of a cone tiple stars, at vast distances from one another, may affect one, or and vet appearing as one to the naked more than one ' cone at the same eye. time. The image For distinctness of vision the eye must affecting two cones is actually have what we may call resolving power, the smaller than that power of keeping each point of the image clear and distirfct from its neighbour, and this power we have said is greatest in the yellow spot. For example, the two dots below are easily recognised as two, if we look directly at them ; but if we look a little to one side, the two will apparently fuse into one whenever their images are displaced from the yellow spot and fall upon an adjoining part of the retina. By means of a pencil we can map out on the page an area of irregularly oval shape corresponding to the oval shape of the yellow spot, an area in which the two dots are seen as double and not fused. The greater the distance between the dots, the further, cceteris paribus, from the yellow spot of the retina may they be distinguished as such, or in other words, the further we pass on the retina from the yellow spot the less resolving power does the retina possess. We have indicated above the shortest distance between two points which will allow of their being seen as two. A much smaller area of stimulation of the retina is sufficient 148 Physiology of the Senses to give rise to distinct vision. A luminous point or line may be seen as such which gives rise to an image that occupies only a very small part of a cone or row of cones. An object -04 mm. (-\^ of an inch) in breadth at a distance of 25 mm. (i inch) from the eye gives a retinal image of about -002 mm. (-.^^ of an inch) in breadth, and yet it is distinctly visible. This is, however, by no means the minimum, visibile. Objects as small as the TooVoU" f an mc ^ i n diameter (about one -tenth of the length of a wave of light) may be seen with the highest powers of the modern microscope. It is hardly necessary 'to state that even these minute objects are many thousands of times larger than the molecules or atoms of matter dealt with by the physicist. The Size of the Retinal Image. The size of the image of an object upon the retina may be calculated by a simple formula if we know the size of the object, its distance from the nodal point, and that of the nodal point from the retina. In the average normal human eye the distance of the nodal point from the retina is approximately 16 mm., and from the nodal point to the anterior surface of the cornea 7 mm. Let the size of the object be represented by X, its distance in mm. from the anterior surface of the cornea by /, and therefore from the nodal point by p + 7. Then p H- 7 : 1 6 : : X : x, the size of the image ; or x = . Suppose, for example, the object looked at be the page of this book, which is nearly 182 mm. long, and that the book is held half a metre (500 mm.) from the eye. Then the length of the retinal image of the page will be 182 x 16 x = = 5*7 mm., or a little less than one quarter of an inch. Again the length of any small letter on the page is approximately I mm., hence the height of its The Sense of Sight 149 retinal image, the book being held as before, will be i x 16 16 = = -03 mm., or about ^^ of an inch. The above-mentioned formula, however, gives only the length of any diameter of the object in a plane perpendicular to the line of vision. To calculate the area of the image on the retina we have only to remember that the area of the image is to the area of the visual field occupied by the object as the square of the distance of the image from the nodal point is to the square of the distance from the nodal point to the object. The flat retinal image cannot, of course, correspond in area to the superficial area of a solid body, but only to a part of the field of vision cut off by a plane projection of the object upon it. It is as if the visual field were a canvas, every point of which is filled by the representation of some external object, and the retinal image is an exact copy, but reduced in size, of nature's picture. The full moon and a ball held in the hand give alike a flat circular retinal image, but in the "mind's eye" each may be seen as a sphere, although the play of light and shade on the nearer object renders the effort of imagin- ation easier with it than in the case of the more remote. The Blind Spot. It is interesting to note that near the area of greatest sensitivity to light we have a spot in the retina which is devoid of rods and cones, and hence is quite unaffected by images formed upon it. This is the optic papilla^ or place of entrance of the optic nerve, and its diameter being about, i -8 mm., it subtends a visual angle of about 6 degrees. Lines drawn from the border of the optic pore to the nodal point and produced outwards will enclose a flattened cone whose base is contained within the visual field, and within which all objects will be invisible to the unmoving eye. Suppose, for example, the left eye being shut, the right eye be fixed upon the cross in Fig. 74. 150 Physiology of the Senses When the book is held at arm's length, both cross and round spot will be visible ; but if the book be approximated to about 8 inches from the eye, the regard being kept steadily upon the cross, the round spot will at first dis- appear, but as the book is brought still nearer both cross and spot will again be seen. It may also be noted in this ex- periment, that there is no consciousness of a break of continuity in the visual field, no sensation as we might imagine there would be of darkness ; to put it generally, there being no stimulation, there is not consciousness of a lack, but a lack of consciousness. An attempt has been made to determine the rate of decrease of acuteness of vision as we pass outwards from the yellow spot, and Volkmann holds that it diminishes proportionally to the square of the distance from the yellow FIG. 74. spot, but the determination is, in its nature, very hard to make, and much depends on individual peculiarities. Action of Light on Retina. This will be the more readily understood if we consider for a moment the intimate nature of the action of light on the retina. It has been experimentally observed that if the eye be kept in the dark for a time, and if light then be allowed to fall full on the retina, there is a change in its electrical condition. This phenomenon is evidence of change in the condition of the molecules of the sensitive parts of the retina, which might be merely a change of rate of molecular motion such as results from a variation of temperature of a body, or it might be due to a chemical transformation or rearrangement of the molecules so as to form new chemical substances. The Sense of Sight 151 That the latter is more probably the case may be held upon various grounds. If heat rays be substituted for light in the foregoing experiment the electrical change will not occur. Further, it has been observed in the frog's eye (the retina of which contains only rods, and which is also well adapted for the observation of the electrical change produced by light) that in the outer part of the rods of quiescent eyes there is a pigment of a purple colour derived from the pigmented layer outside of Jacob's membrane, and on exposure of the eye to ordinary light this purple * changes to yellow and then to white. On removal of the light the pigment slowly reappears in the rods. This pigment is not found in the cones of the retina of other animals, and hence is absent in the yellow spot. As the yellow spot is the seat of acute' vision in daylight we must infer that the purple pigment is not essential to vision, but we must not conclude from this that it has no visual function. For if we pass from darkness to bright light, the eye at first is dazzled until possibly the visual purple is bleached, or in other words, until the eye's sensibility to light is diminished ; and, on the other hand, if the eye has been exhausted by bright light we do not see objects well in a dim light until the visual purple is restored. In a dim light, the pupil of the eye is dilated, and rays affect the retina round the yellow spot. It would thus appear that visual purple assists vision in dim light while it is not necessary in bright light ; but as we have a chemical change in the purple pigment, so we may have in the yellow spot substances which undergo chemical change, although this be not manifest to the observer. The yellow spot is thus better adapted for acuteness of vision, for concentration of the attention upon minute detail, while the surrounding parts of the retina are more sensitive to the action of light and more fitted for observ- 152 Physiology of the Senses ing bodies emitting or refj ....*mg but a small quantity of light. Amount of Light required to excite the Retina. The smallest amount of light that will excite the retina cannot be stated, as so much depends upon the part of the eye affected, its state of vigour or exhaustion, its previous education, and the like. Thus the sailor will see land in the distance which is imperceptible to the landsman ; the Oriental will distinguish shades of colour more accurately than the European ; and the artist will differentiate where the untrained eye sees but one tint. Again, the exhausted eye will fail to see what is readily perceptible to the fresh eye of one newly wakened from sleep ; and the star, whose faint light is unseen by direct vision, may be seen when its ray meets the retina a little to the outside of the yellow spot. Nay more, even when we are enveloped by the deepest darkness, and when the eyes are shut, the ordinary field of vision seems still irradiated by a faint pervading glow, known as the specific light of the retina, which upon slight pressure by the hands may be broken up into a mosaic of fleeting patterns. The sensations thus excited by pressure are called phosgenes. The retinal light is caused by changes in the retina due to variations in the blood supply. Persistence of Retinal Impressions. The substance of the retina is more or less affected according to the brilliancy of the light and the length of time during which it acts upon the eye. A feeble light acting for a short time will leave but a transient effect, while a strong light, such as that of the sun or of the electric spark acting for an instant only, may give rise to impressions lasting many minutes, or, if the exposure be prolonged, even to permanent damage to the eyesight. If we look directly at the sun and then turn our eyes to the ground, or towards a darkened cloud, The Sense, of Sight 153 the image of the sun forruc^ ipon the retina has been as it were so deeply graven, the retinal structure has been so changed, that for several moments we fail to see the object towards which the eyes are turned, .and we see a round red spot, or several red spots, if the eyes were not steady when the sun was in view. This spot is a spectrum or after-image of the sun projected outwards upon the visual field, moving with every move- ment of the eye, and seen even when the eyes are closed. If a piece of burning wood be shaken rapidly to and fro, we see a line of 'light, because adjacent points on the retina are consecutively stimulated, and the fusion of the after-images gives the sensation of continuity. A disk with alternate lines or sectors of black and white radiating from the centre will, when rotated rapidly, seem to have a uni- form gray colour due to the fusion of the black and white FlG> ?5 _ The disk A having Hack and Spectra; but if seen by the white sectors, when rotated rapidly light Of the instantaneous S"es an even gray tint as in B. electric spark, each black and white line or sector will be visible because the time of illumination and consequent stimu- lation of the retina is so short that there is no time for the superposition of the images one upon the other. Similarly, if various simple colours be painted on the disk, their spectra will, on rotation of the disk, be fused together, giving rise to a sensation of the colour due to their combination. If a series of twenty or thirty instantaneous photographs be taken at short but equal intervals of time of an animal performing some movement, as, for example, a horse leaping over a gate, the pictures fixed to a disk will, when rotating quickly, seem to coalesce each with its predecessor so as to give the impression of the i54 Physiology of the Senses horse in actual movement. This is the principle of the toy known as the Thaumatrope or Wheel of Life. Since the after-image in the instances above mentioned has an appearance similar to that of the object viewed, it is called a positive after-image. But there is another kind of after- image, the negative, which is due to a slightly different cause. Suppose we look fixedly at an object for thirty or forty seconds, so that the eye becomes fatigued, and then turn our eyes to a surface of uniform tint, we will see an image floating on the wall in which the lights will be reversed what was dark will be light, what was bright will be dim. In this case the rays of light reflected from the wall have most effect upon those parts of the retina which are least exhausted, while those parts formerly much stimu- lated will now look dark, not being so easily excited to action. The persistence of retinal impressions is probably in part the cause of the phenomenon known as irradiation (see p. 140). The eye moving rapidly over the white surface, and being more affected by its light, the dark area seems the smaller. It may also be that there is a slight dispersion of light from the retinal elements directly affected to those immediately adjoining, which makes the image larger, and so leads to an erroneous judgment as to the size of the white object. A further and most interesting illustration of the per- sistence of the retinal state may be studied as follows : Look steadily for about half a minute at a disk with alternate black and white sectors which is being slowly rotated. Then turn the eyes to a sheet of paper upon which a number of dark spots may be seen. These will seem to rotate in a direction contrary to that in which the disk was turning. The effect here is of the same nature as the phenomenon often seen on the deck of a steamer. If we lean over the side of the vessel, and watch the water as TJie Sense of Sight 155 the vessel glides along, it soon seems as if the ship were stationary and the water near us in rapid motion in the direction opposite to that in which we are moving the apparent rapidity gradually diminishing as we look at more remote parts of the water. If we now gaze at the deck, the part near us will seem to move towards the bow of the ship, the rest of the deck remaining fixed. Different parts of the retina have been stimulated by rays from different parts of the surface of the water apparently moving at different rates. But when the whole visual field is occupied by the deck, the various parts of which are fixed relatively to each other, the persistence of the retinal impression of greater movement in one part of the visual field than in the rest of it causes us to imagine that parts of the deck, which rela- tively to the rest of the deck are stationary, are actually in motion. 5. SENSATION OF COLOUR In considering the physical nature of light (p. 1 1 5), we saw that the shade of colour, according to the most likely hypo- thesis, depends on the rate of vibration of the luminiferous ether, and that solar or white light is a compound of all the colours in definite proportion. A body which reflects solar light to the eye without changing this proportion appears to be white ; if it absorbs all the ligtit so as to reflect no light to the eye, it appears to be black. If a body held between the eye and the sun transmits light unchanged and is transparent, it is colourless ; but if translucent, it is white. If it transmits or reflects some rays and absorbs others, it is coloured. If, for example, it absorbs all the rays of the solar spectrum but those which give rise to the sensation of greenness, we say that the body is green in colour. But this greenness can only be perceived if the rays of light falling on the body contain rays having the special 156 Physiology of the Senses vibratory rate that is required for this special colour. For, if we use as our light any other pure coloured ray of the spectrum, say the red, its rays being absorbed the body appears to us to be black. A white surface seen in a red light seems to be red, in a green light, green, as it reflects all colours alike, absorbing none. To the normal eye the colour depends, then, on the nature of the body and of the light falling upon it, and the sensation of coloiir only arises when the body reflects or transmits the special rays to the eye. If two rays of FIG. 76. Lambert's method for studying combinations of colour. The rays, e.g:, from the red wafer d reflected by the glass plate a to the eye E are pro- jected outwards and superposed on the blue wafer , which appears of a rose colour. different colour affect one part of the retina at the same time, they are fused together, and we have the sensation of a third colour different from its cause. Thus, if red be removed from the solar spectrum, all the others com- bined will give a sensation of a greenish yellow, although we cannot, with the unaided eye, analyse this into its com- ponents. Fig. 76 shows a method by which different -coloured rays may be made to converge from two bodies on the same part of the retina. Von Helmholtz gives the follow- The Sense of Sight ing table as the result of mixing the pure colours of the spectrum : V. B. G, Y. R. R. Purple. Dull Kose ' Yellow. 1 Orange. Red. Y. Rose. White. Yellow Green. Yellow. G. Pale Blue. Blue Green. Green. Ey Indigo. Blue. V. V. Thus a mixture of red and violet gives purple, of yellow and blue, white. Here we must guard against a possible error. The effect of say yellow and blue light acting at once on the eye is to cause a sensation of white light ; but if we mix blue and yellow pigments the mixture looks green, because the one pigment cuts off the rays at the red end, the other those at the violet end of the spectrum, and the only rays reflected are those of the green or middle part of the spectrum. In the one case we have a com- bination of colours, in the other each absorbs a part of the spectrum previously seen when the pigments were unmixed. Similarly, if the colours of the spectrum be painted upon 158 Physiology of the Senses a disk, in due proportion and in proper series, the disk will, when quickly rotated, look white. This is due to a fusion of colour effects, not to a mixture of the pigments. Complementary Colours. When one colour is separated from the spectral series, the rest, as we have said, may be combined in the retina to give a sensation of one colour, and this colour will, if recom- bined with the one originally separated, give the sensa- tion of white light. These two colours, then, are said to be complementary to each other, and every colour in the spectrum may thus be said to be the complement of all the others. By combining colours at opposite ends of the spectrum, the effect of the intermediate colours may be produced ; but the lowest and highest of the series, the red and the violet, cannot be thus formed. They may be regarded, therefore, as primary colours colours which cannot be produced by the fusion of others. If to red and violet we add the colour whose vibratory rate is about midway intermediate, viz. green^ we may, by their combination, give rise to a sensation approaching that of white light. Consequently these three colours have been designated the fundamental colours. Colour as dependent on the Retina. Our per- ception of colour depends, however, not only on the physical stimulus of light, but also on the part of the retina affected. In and around the yellow spot where the cones are most numerous, the power of distinguishing shades of colour is greatest. Instead of seven colours in the spectrum more than two hundred different tints may be distinguished. Outside of this central area lies a middle zone in which much fewer tints are seen, these being con- fined, indeed, to shades of blue and yellow ; while in the front part of the retina all colour tints are lost, and objects give rise simply to the sensation of dark shadowy The Sense of Sight 159 bodies without colour. Moreover, the range of spectral colours varies with the individual. Colour Blindness. Every colour has three qualities : (1) hue, or tint, as when we speak of red, green, or violet ; (2) purity ', or degree of saturation (due to a greater or less admixture with white), as when we designate a red or green as deep or pale ; and (3) brightness^ or intensity, or lumin- osity, as when we describe the tint of a red rose as dark or bright. On comparing two colours we say they are identical when .they agree as to these three qualities. Observation has shown that in thus assorting colours, about ninety-six out of every hundred men will agree as to identity or difference of colour, and may be said to have normal colour vision^ while the remaining four men will show a defective perception of colour, and are called colour blind. It is curious that colour blindness is about ten times less frequent in the female sex. This condition is congenital and incurable. It is due to some unknown peculiarity of the retina, or nerve centres, or both, and it is to be dis- tinguished from transient colour blindness, sometimes caused by the excessive use of tobacco and by disease. There is probably no such condition as absolute colour blindness, in the sense of total insensibility to colour ; a few rare cases have been noticed in which there was apparently only one colour sensation ; a few cases occur of failure to distinguish blue from green, and insensibility to violet is rare. The common form of defective colour vision is Daltonism or red-green blindness, of which there are two varieties the red-blind +&&& the green-blind. In each variety there are many gradations of sensibility. To the red-blind red appears as a dark green or greenish yellow, yellow and orange appear as dirty green, while green is green and brighter than the green of the yellow and orange. A green-blind person, on the other hand, would call red 160 Physiology of the Senses dark yellow, yellow would be yellow except a little lighter than the red he calls dark yellow, and green would be described as pale yellow. When asked to lo'ok through a spectroscope at the spectrum, the extreme or low red is absent to the red blind, and the brightest part of the spectrum appears to him to be the green, while to the normal eye and to the green-blind eye the spectrum is most luminous in the yellow. Seeing that green lights imply safety, and red lights danger, on our railways, and that in navigation a green or red light on the port or starboard side shows the course a vessel is taking, it is evident that no one who is red- or green - blind . should be employed in the services, and accordingly various tests are now in use for the detection of such defects. The most efficient is the wool-test of Holm- gren, which consists of three skeins of wool dyed with standard test colours, namely, a light green, a pale purple or pink, and a bright red. Other 'skeins of reds, oranges, yellows, yellowish greens, pure greens, blue greens, violets, purples, pinks, browns, and grays, all called confusion colours, are provided, and the examinee is requested to select one and match it with one of the test colours. Suppose the light green skein is shown first. If the examinee matches grays, brownish grays, yellows, orange, or faint pink with this, he is colour blind. Then he is shown the purple skein. If he matches with this blue or violet he is red- blind, but if he selects only gray or green he is green-blind. Finally, he may be shown the red skein, having a bright red colour, like the red flag used on railways. A red-blind person will then match with this green or shades of brown, which to a normal eye seem darker than red ; while if he be green-blind he will select shades of these colours which look lighter than red. Violet blindness is recognised by the examinee confusing red and orange with purple. The Sense of Sight 161 Coloured after-images. The power of the retina in distinguishing colours depends also upon its freedom from fatigue. As there may be after-images of form, so there may be after-images of colour, and these after-images may be negative or positive. If positive, we see with the eyes shut the same colour as we have just been looking at ; if negative, we see the complementary colour, and as we continue examining it we find the colour changing and fading away, the lighter tints merging into the darker. The eye fatigued by gazing at a red square, will, when turned to a white surface, seem to see a blukh-green square on the white ground, for the fatigued eye responds more readily to the stimulus of the other colours of the spectrum ; and these give, when fused, the complementary colour (p. 158). Similarly, a white square seen against a bluish-green background will have a reddish tint, probably because the eye moving quickly over the coloured field, and becoming thereby fatigued, responds more readily to the red rays in the white light than to its other component parts. This is known as the phenomenon of contrast. Theories of Colour Vision. How comes it that we can perceive differences in colour ? This question has never been satisfactorily answered, because the changes caused in the retina by the action of light are too minute to allow of direct observation. Many hypotheses have been framed, but none of them meets all the requirements of the case. We may look for the cause in various direc- tions. We might suppose a molecular vibration to be set up in the nerve-endings synchronous with the undulations of the luminiferous ether, without any change in the chemical constitution of the sensory surface ; and we might suppose that where various series of waves corresponding to different colours act together, these are fused together, or interfere with each other in such a way as to give a M 162 Physiology of the Senses vibration of modified form or rate corresponding somehow to the sensation arising in consciousness. Or again, we might suppose that the effect of different - coloured rays is to promote or retard chemical changes in the sensory surface, which again so affect the sensory nerves as to give rise to differing states in the nerves and nerve centres with differing concomitant sensations. The former of these lines of thought guided Thomas Young, the great expounder of the H Violet. B Red. G F E D Indigo. Blue. Green. Orange yellow FIG. 77. Diagram to illustrate the Young-Helmholtz theory of colour vision. The lines with the letters B, C, D, etc., below the curves indicate certain fixed lines in the solar spectrum, whose wave-length has been determined. Take D, the height of the two curves above it indicates the degrees of stimu- lation of the two sensations red and green that produce orange-yellow. Again, at E we see a mixture of the three sensations that produce spectral green. (Report of the Committee of the Royal Society on Colour Vision.) undulatory theory of light, in his attempt at explaining colour perception ; and his theory adopted and worked out by von Helmholtz has been received with much favour. He supposed that there are three fundamental colour sensa- tions red, green, and violet by the combination of which all other colours may be formed, and that there are in the retina three kinds of nerve elements, each of which is specially responsive to the stimulus of one colour, and much The Sense of Sight 163 less so to the others. If a pure red colour alone act on the retina, only the corresponding nerve element for red sensa- tion would be excited, and so with green and violet. But suppose the colour be mixed, then the nerve elements will be set in action in proportion to the amount of constituent excitant rays in the colour. Thus, if all the nerve elements be set in action, we shall have white light ; if that corre- sponding to the red and green, the resultant sensation will be orange or yellow ; if mainly the green and violet, the sensation will be blue or indigo, and the like. Von Helm- holtz succinctly puts it as follows : (1) Red excites strongly the fibres sensitive to red, and feebly the other two sensation, red. (2) Yellow excites moderately the fibres sensitive to red and green, feebly the violet sensation, yellow. (3) Green excites strongly the green, feebly the other two sensation, green. (4) Blue excites moderately the fibres sensitive to green and violet, and feebly the red sensation, blue. (5) Violet excites strongly the fibres sensitive to violet, and feebly the other two sensation, violet. (6) When the excitation is nearly equal for the three kinds of fibres, then the sensation is white. Another mode of expressing the theory is to say that each primary sensation of red, green, and violet is excited in some degree by almost every ray of the spectrum, but the maxima of excitation occur at different places, while the strength of stimulation in each case diminishes in both directions from the maximum point. Thus, when the three sensations are equally excited, white light is the result ; green is caused by a very weak violet sensation, a stronger red, and a still stronger green sensation. At each end ot the spectrum we have only the simple sensations of red and violet, and all the intermediate colour sensations are compounds of varying proportions of the three primaries. 164 Physiology of the Senses According to this theory, red blindness is attributable to the absence of the red sensation, and green blindness to the absence of the green sensation. When the green and violet sensations are equal in amount, a red -blind person sees what is to him white, and when the red and violet are equal a green-blind person will have a sensation of what in turn is to him white, although to the normal eye these parts are bluish green in the one case and green in the other, as the green sensation is in each added to the sensations of red and blue. But while this theory explains certain phenomena of colour blindness, of after-images, and of colour contrast, it is yet open to serious objections. There is no proof, one way or other, of the existence of three kinds of nerve elements corresponding to the three fundamental colour sensations. Again, it does not explain how red should have to the colour-blind person a similar appearance to green, or how it should give rise to a sensation of colour at all, any more than heat rays which are invisible. Further, if red rays are a necessary constituent of white light, the colour blind should not be able to see white as we do, nor to distinguish white from bluish green^ the complementary colour of red. And yet such distinctions can be made, although it may be argued that a colour-blind person does not see white in the same sense as white is white to a person having normal colour vision. A strong objection to the Young-Helmholtz theory is that in cases of colour blindness following injury to the eye, only the blue of the spectrum is seen, all the rest appearing as white. Here it is impossible to under- stand how a sensation of white can be experienced if the sensations of red and green are lost, for the theory is that white can only be experienced when the sensations of red, green, and violet are all three present. Stanley Hall likewise adopts an anatomical basis for his The Sense of Sight 1 65 theory of colour perception. He holds that only the cones are sensitive to colours, and that these may be regarded as built up of a series of disks like a pile of coins, the lowest of which is the largest. Different disks respond to different colour tones, and give rise to different excitations of the nerve centres. While the disk formation of the cones is undoubted, this theory is open to the same objections, on subjective grounds, as that of Young and von Helmholtz. Other theories of colour perception proceed upon the assumption of chemical changes in the retina under the influence of light. That light does play an important part in physiological action is a well-known fact. Green plants, for instance, can only grow healthily when exposed to the light ; if kept in a dark chamber they quickly blanch, and use up only the reserve material stored up in themselves, because they have no longer the power of obtaining carbon from the carbonic acid of the air. And yet, though this is so, it is also known that direct rays of light have a retarding influence on the growth of certain parts of plants. If a plant is placed in a window, it bends outwards towards the light, because the side of the stem away from the light grows the faster ; similarly leaves of plants grown in the dark, like rhubarb, have long thin stalks which have derived their nourishment from the root, and have not been affected by light. So Hering holds with regard to the retina. According to Hering's theory certain fundamental sensa- tions are excited by light or by the absence of light. These are white, black, red, yellow, green, and blue, and they may be arranged in three pairs, the one colour in each pair being complementary to the other, thus white to black, red to green, and yellow to blue. Hering further supposes that when rays of a certain wave-length fall on visual substances existing in the retina destructive changes occur, while rays having other wave - lengths cause constructive changes. i66 Physiology of tJie Senses Thus, suppose a red-green visual substance exists of such a nature that when destructive and constructive changes occur no sensation is experienced, then when destructive changes are in excess by the action of light of a certain wave-length there is a sensation of red, and when constructive changes occur by the action of shorter waves the sensation is green.l In like manner a yellow-blue visual substance by destruc- tive changes gives a sensation of yellow, and by construc- Yb W gr I. V B G Y O R FIG. 78. Diagram to illustrate Hering's theory of colour vision. The vertical shading represents the red and green, and the horizontal shading the yellow and blue, antagonistic pairs of sensations. The thick line indicates the curve of the white sensation. All above the line X X indicates destructive changes in the retinal substances, and all below constructive changes. See text. (Report of the Committee of the Royal Society on Colour Vision.) tive changes a sensation of blue ; and a white-black visual substance by destructive changes gives white, and by constructive changes black. The member of each pair is thus antagonistic as well as complementary. The red-green and yellow-blue substances are tuned, as it were, to rays of different wave-length. Thus, in the red end of the spectrum, the rays cause great destruction of the red-green substance, while they have no effect on the yellow-blue substance. Hence the sensation is red. Again, the shorter waves The Sense of Sight 167 which correspond to the yellow of the spectrum cause great destruction of the yellow -blue substance, while their de- structive and constructive effects on the red-green substance neutralise each other. Hence the sensation is yellow. Still shorter waves, corresponding to green, cause construc- tion of the red-green substance, while their influence on the yellow-blue substance is neutral, and hence the sensation is green. Again, the shorter blue waves cause construction of the yellow-blue substance, while their action on the red- green substance is neutral, and hence the sensation is blue. At the blue end the short waves are supposed to cause destruction of the red-green substance, and thus give violet by adding red to blue. Orange is caused by excess of destructive changes, and greenish-blue by excess of con- structive changes in both substances. Finally, when all the rays of the spectrum fall on the retina, the constructive and destructive changes in the red-green and yellow-blue substances neutralise each other, but the destructive changes are great in the white-black substance, and we call the effect white. Colour blindness, in the form of red-green blindness, is, according to this theory, due to the absence of the red -green substance, the other two substances remaining. The phenomena of coloured after-images are thus accounted for : Suppose the retina to be acted on by red light, destruc- tion of material takes place the effect continuing, it may be for a time, after withdrawal of the red light giving the positive after-image. Then comes the upbuilding of the material under the influence of nutrition, assisted by the action of light of shorter wave-lengths, and the negative after-image green is perceived. So with yellow and blue, and white and black. That such differences of chemical action are possible, or probable, we may well believe from a consideration of the variation in the actinic effect of different 1 68 Physiology of the Senses rays of the spectrum, and from the action of light upon the pigments of the retina. This theory is also in harmony with what has been observed in connection with many other processes in the body, such as secretion, innervation, and the like, in which tissues, having reached their highest point of vitality through nutrition, disintegrate during functional activity. Serious objections to the theory have, however, been raised. One is thus stated by Ladd 1 : " A light composed of red and green may be made to seem to the eye the same as a light composed of yellow and blue. If, then, the eye is fatigued to red, instead of the red-green mixture appearing greenish, and so distinguishable from the yellow- blue mixture, they both appear the same to the fatigued eye." It has also been pointed out that the two sensations of each pair do not always coexist. One may be present and the other absent. Thus, when the intensity of the light of the spectrum has been much reduced, the green persists long after the red has disappeared ; and after the excessive use of tobacco, yellow may disappear, and blue is the only sensation left. One is also at a loss to understand how colour sensations, so different from one another as red and yellow, can be alike due to destructive changes of retinal substances, or how yellow and green, whose periods of vibration are so nearly alike, can give such antagonistic physiological effects. Such considerations demand the existence not of one but of three visual substances. On the whole, however, speculative as it is, Hering's theory accounts for a larger number of the phenomena of colour vision than that of Young and von Helmholtz. In a new edition of his great work, Handbuch der Physio- logischen Optik, now appearing in parts, von Helmholtz reviews the subject of colour vision, and materially modifies the theory as first announced by him about 1856, and since 1 Ladd, Outlines of Physiological Psychology, p. 268. The Sense of Sight 169 then termed the Young-Helmholtz theory. He now states that luminosity or brightness plays a more important part in our perceptions of colour than has been supposed. He also, by analysing the colours of the spectrum with great care,Tias been able, from these data, to determine three fundamental colour sensations, the first red (a), which is a highly saturated carmine-red, the second green (b) like the green of vegeta- tion, and the third blue (c) like ultramarine. Each spectral colour is made up of certain proportions of these funda- mental colours, or a combination of two of them added to a certain amount of white. Thus 100 parts of green are composed of 15 of a, 51 of b, and 34 of c \ or, to take other examples, spectral red will contain, per cent., 42 of