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 ; <?, transverse occipital 
 sulcus ; Oz, cuneus ; oc, calcarine sulcus ; oc', oc", its superior and inferior 
 branches ; D, descending gyrus ; T4, lateral occipito-temporal gyrus ; T%, 
 median occipito-temporal gyrus. (Ecker.) 
 
 separated from the marginal convolution by (3) <:;;/, the 
 calloso -marginal fissure ', and continuous posteriorly with 
 (4) H, the gyrus hippocampi^ so called from the peculiar 
 appearance of the gray matter at this part. It is con- 
 tinued into (5) U, the uncinate or hook-shaped gyrus. Pj 
 marks the internal aspect of the parietal convolution, or 
 
28 Physiology of the Senses 
 
 prcecunens, and it is separated by po, the parieto-occipital 
 fissure, from (6) the cuneus or wedge-shaped convolution, 
 whose lower surface is separated by the calcarine fissure, 
 oc, from (7) the temporo - occipital convolutions , three in 
 number, which lie at the base. 
 
 Each cerebral hemisphere has a central cavity or 
 ventricle, which is continuous with a small canal that passes 
 through nearly the entire length of the spinal cord. The 
 internal substance of each side of the brain consists of 
 nerve fibres joining the surface of the brain with the lower 
 nerve centres, or one part of the brain with another. The 
 nerve fibres in the brain have only the axis -cylinder and 
 surrounding white substance of Schwann, but no neuri- 
 lemma, and hence the difficulty of dissecting out special 
 strands of nerve fibres and of tracing the course they run, 
 is considerably increased. The fibres which have been 
 most definitely traced are (i) those from the internal 
 capsule (p. 23) as they pass outwards to the cortex, the 
 group called the corona radiata ; (2) the decussating fibres 
 of the corpus callosum ; and (3) the longitudinal or col- 
 lateral fibres connecting different parts of the same side of 
 the brain. 
 
 The gray matter of the cortex consists of nerve cells 
 and fibres embedded in a connective tissue material, the 
 neuroglia, and well supplied with blood-vessels and 
 lymphatic channels. The cells differ slightly from each 
 other in appearance at different depths from the surface 
 and in different areas, and may be of a pyriform, conical, 
 spindle-shaped, or quite irregular shape, but on the whole 
 they present the form of a pyramid whose apex points 
 towards the surface and from which a long thin pole or 
 fibre, the apex process, can be traced outwards for some 
 distance, but whose ultimate destination is unknown. The 
 base of the pyramid is connected with a nerve fibre coming 
 
General Introduction 29 
 
 from the subjacent white matter, and from the angles at 
 the base of the pyramid, or even from the sides, we find 
 numerous branching processes in some cases as many 
 as fifteen to eighteen the number of which seems to 
 depend upon the size and age of the cell. These processes 
 are short and quickly break up into a fine plexus of fibres, 
 and it is probable that these act as internuncial fibres 
 bringing the different cells into relationship with each 
 other. 
 
 The general arrangement of the structures in the cortex 
 is as follows : On the surface we find a layer of nerve 
 fibres supported by fine connective tissue and vessels pass- 
 ing straight inwards to reach the deeper layers. Next to 
 this comes a layer of small oval or angular cells with large 
 nuclei and giving off numerous fine processes. Deeper 
 down comes a layer containing more distinctly pyramidal 
 cells, and in the posterior or sensory regions we find many 
 small conical cells packed together. Below these again, 
 we find, more especially in the motor areas (p. 30), very 
 large pyramidal cells of the form described above. Below 
 these again comes a layer of spindle cells with numerous 
 nerve fibres passing between them to the outer cells. 
 
 The more carefully the gray matter is examined the 
 more clearly do we find that each area has its own special 
 groups of cells a rule that we would expect to hold con- 
 sidering the varying functions of the different areas ; never- 
 theless the transition from one set of forms to another is 
 never very abrupt. That different areas of the brain have 
 different functions, though often conjectured, was not 
 experimentally proved till 1870, when Fritsch and Hitzig 
 performed their celebrated experiments ; and this subject 
 has since been carefully studied by many observers, among 
 whom we may mention Ferrier, Horsley, and Hitzig. 
 Thus it has been established that the convolutions adjoin- 
 
30 Physiology of the Senses 
 
 ing the fissure of Rolando have to do with the initiation, 
 under due stimulation, of movements throughout the body, 
 and, generally speaking, the broad distinction may be drawn 
 that the frontal and front part of the parietal lobes are 
 associated either with the exercise of the more purely mental 
 powers, or with movement, while the posterior parietal con- 
 volutions and the occipital and temporo-sphenoidal lobes 
 have to do with sensation. 
 
 The sensory fibres to the occipital and temporo- 
 sphenoidal lobes corne in the main from the posterior third 
 of the internal capsule, spreading outward thence in a fan- 
 shaped manner as the radiation of Gratiolet. 
 
 The precise position of the different centres cannot be 
 precisely stated, but by localised electrical excitation, or 
 by the destruction of certain areas accidentally, experi- 
 mentally, or by disease, and by careful observation of the 
 variation in the normal phenomena thus caused, the fol- 
 lowing tentative conclusions have been arrived at as to the 
 sensory centres of the cortex. Our information as to the 
 centre of vision is more definite than with regard to the 
 other sensory centres, for it will readily be understood 
 that blindness is much more easily detected in an animal 
 than the loss of any other of the senses. 
 
 SENSORY CENTRES IN THE CORTEX 
 
 i . The Centre for Vision. This is believed to lie in the 
 convex outer surface of the occipital lobe and the angular 
 convolutions (Fig. 10, p. 25, P 2 , P 2 '). It has been found that 
 electrical stimulation of the angular gyrus causes the 
 animal to turn its eyes to the side opposite to that stimu- 
 lated, and upwards or downwards according as the front 
 or back part of the gyrus is excited. Further, the eyelids 
 are closed and the pupil contracts. What is the meaning 
 
General Introduction 31 
 
 of these movements ? As we shall see in dealing with 
 vision, the distribution of the fibres of the optic nerve 
 is such that we would expect that the occipital lobes of 
 say the left side of the brain would take cognisance of 
 everything visible to the right side of a plane passing fore 
 and aft through the body when the eyes are looking 
 straight forward. The left brain has to do with fibres from 
 the left side of each eye, viz. the part that sees objects 
 to the right. If, then, on stimulating the left occipital or 
 " occipito-angular " area the eyes turn to the right, we may 
 with reason interpret the movement as meaning that the 
 stimulus has given rise to the sensation of something 
 being visible in the right half of the field of vision for the 
 better view of which the head is turned to the right, while 
 the contraction of the pupil may indicate that the sensation 
 is of something near at hand, and the closure of the eyelid 
 that the eye is shut for protection from contact with a 
 near object or the shutting out of a too brilliant flash of 
 light. 
 
 Destruction of the central part of the occipito-angular area 
 causes disturbance of vision or blindness of the same side 
 of each retina, or, in other words, for the opposite side 
 of each visual field. But an important and delicate dis- 
 tinction must be drawn. The blindness is, according to 
 Munk, one of mind " a psychical blindness or inability to 
 form an intelligent comprehension of the visual impressions 
 received." The eye performs its function correctly ; the 
 basal ganglia may fuse the sensations into a coherent 
 whole, the animal may act in a reflex way avoiding 
 obstacles in its path, but the object thus seen awakes no 
 mental activity. An example will illustrate our meaning. 
 It can see and avoid as it walks a plate containing food, 
 but it does not recognise food as such, as something to eat, 
 nor does it show signs of fear when threatened with a whip. 
 
32 Physiology of the Senses 
 
 It has been suggested that the removal of the central part 
 of the occipital lobe merely removes that part of the cortex 
 which is associated with the area of distinct vision of 
 the retina, that the animal has conscious but not distinct 
 vision. This would be in agreement with the fact that 
 \\hen only injured upon one side the animal within a few 
 days recovers to some extent the sense of sight on the 
 side affected. The improvement might be due to acquisi- 
 tion by practice of powers of vision not usually possessed 
 by the peripheral parts of the retina, but much has still to 
 be learned on this difficult subject. Complete destruction 
 of the occipito-angular areas of both sides, the cuneus (Fig. 
 n, O^) being included, causes total and permanent blind- 
 ness without any other perceptible loss of sensory or motor 
 power. 
 
 The power of distinct vision, then, depends in man 
 upon the normal working of a terminal organ, the eye, of 
 the optic nerve partially decussating at the optic com- 
 missure, the nerve strands passing thence backwards by 
 the optic tract to the corpora quadrigemina and optic 
 thalami, and thence, by the radiation of Gratiolet, to the 
 cortex of the posterior part of the brain. We have seen it 
 to be the law that when a nerve fibre is cut off from its 
 ganglionic nerve cell the fibre degenerates. In the case of 
 the optic mechanism, these ganglionic cells are situated in 
 the retina, which the study of development has shown to 
 be really a part of the brain, and when the retina is 
 destroyed the optic fibres passing from it undergo de- 
 generation. 
 
 2. The centre of hearing for each ear seems to be 
 situated in the superior temporal convolution (Fig. 10, Tj) 
 of the opposite side. The fibres of the auditory nerve 
 after entering the medulla pass upwards through the pons, 
 decussate there, and thence go through the posterior part 
 
General Introduction 33 
 
 of the internal capsule to the temporal region. Our most 
 valuable evidence as to the auditory centre comes from 
 cases where the brains of deaf or epileptic patients have 
 been examined post-mortem. Thus in certain instances 
 the cause of deafness has been found to be disease of the 
 above - mentioned area ; and in cases of epilepsy where 
 the fit has been preceded by the sensation of a noise the 
 seat of disease has been in the neighbourhood of this part, 
 and the irritation thus arising has determined the onset 
 first of auditory, and then of motor, disturbance in the 
 adjoining motor areas. The study of peculiar sensory 
 disturbances which often precede a convulsive attack, the 
 aura^ as it is called, is of great interest in this connection 
 as showing the part of the brain first affected by the dis- 
 turbing force. Most commonly it is an indescribable 
 sensation seeming to originate in the limbs or body and 
 passing upwards to the head, and that in many cases so 
 slowly as to be capable of being arrested by pressure. In 
 such cases it is most probably due to the disturbance of 
 the muscular sense, but sometimes the aura takes the form 
 of a flash of light, a noise, a disagreeable odour or peculiar 
 taste, in which case the centres of special sense are the parts 
 more directly affected. Fortunately for man, epileptic 
 attacks, are seldom directly, and in the earliest stages of 
 the disease, fatal, and our knowledge of the intimate 
 structure of the brain has been so recently acquired, that 
 pathological investigation has not been of so much service 
 as might be supposed. There is undoubtedly reason to 
 believe that this branch of study will yield fruitful results 
 in future. 
 
 Electrical stimulation of the corresponding area in the 
 dog causes pricking up of the opposite ear, turning of the 
 eyes and head to the opposite side, with the pupils of the 
 eyes dilated, movements such as the dog would make were 
 
 D 
 
34 Physiology of the Senses 
 
 it to hear a sudden sound from the side opposite to that 
 stimulated. 
 
 - Destruction of the superior temporal convolution causes, 
 according to Ferrier and others, deafness in the opposite 
 ear, but this is denied by Schafer and Horsley, who urge 
 the difficulty of determining the presence of deafness, and 
 maintain that in one case where both temporal lobes were 
 completely destroyed there was no perceptible loss of the 
 power of hearing. In the case of human beings it is 
 believed that there may be only a partial decussation of the 
 nerves of hearing, just as in the case of sight, so that injury 
 to one side of the brain may not cause complete loss of 
 hearing on either side, but where both sides have been 
 affected the loss of hearing is complete. With hearing as 
 with sight Munk believes there may be a psychical as 
 opposed to a complete loss of sensation, and he affirms that 
 destruction of the middle part of the convolution causes 
 psychical deafness. 
 
 3 and 4. The centres for taste and smell are supposed 
 by Ferrier to be situated in the anterior part of the hippo- 
 campal or uncinate gyri (Fig. n, H, U), as indicated by 
 movements of the nose and lips on stimulation of these 
 areas. The nerves of smell pass upwards from the nose to 
 the olfactory lobes, which lie in man below, and covered 
 completely by, the frontal lobes, though in many of the lower 
 animals they are prominent bodies projecting forward 
 beyond the rest of the brain. Fibres from the olfactory 
 lobes have been traced to the region above indicated, but 
 with regard to their ultimate distribution, and still more to 
 that of the nerves of taste, there is much to be yet learned. 
 A case is recorded of an epileptic patient whose aura was 
 of an olfactory kind, and the seat of disease was found to be 
 in the right uncinate gyrus. 
 
 5. The centre for touch is believed by Ferrier to be 
 
General Introduction 35 
 
 situated in part at least in the gyrus hippocampi, as shown 
 by loss of tactile sensibility when this area is destroyed. 
 
 Schafer and Horsley found temporary loss of sensation 
 on the opposite side of the body hemi-anassthesia when 
 this part was destroyed, but the loss was more marked and 
 persistent when the greater part of the gyrus fornicatus (Fig. 
 1 1, G/J p. 27) was destroyed. It has not been possible to find 
 separate centres for painful and tactile impressions, although 
 from considerations advanced when speaking of the sensory 
 tracts in the cord it is quite probable that such do really 
 exist. 
 
 It is only of late years that an attempt has been made 
 to distinguish between sensory terminations for the per- 
 ception of heat and cold, and no observation has yet been 
 made as to the localisation of corresponding sensory areas 
 in the brain. It has been found that injury to the basal 
 ganglia, and more especially to the corpus striatum, is 
 followed by a prolonged rise in temperature, as if a centre 
 which had normally to do with the regulation of temperature 
 had been affected ; but this is not known to be associated 
 with any sensory effects, and indeed it would be hard to 
 distinguish experimentally, except upon one's self, between 
 sensations of touch, of pain, and of variation in. temperature. 
 
 In addition to the special forms of sensation we have 
 just considered, there are many sensations of a general kind 
 common sensations arising from the internal conditions 
 of the body, such as hunger, thirst, lassitude, the feelings 
 due to distension of the viscera, and many peculiar sensa- 
 tions due to disturbance of the nervous system, such as those 
 felt when a limb is said to be asleep, or formication, the 
 condition in which it seems as if ants were creeping about 
 under the skin. Again these and even vaguer conditions 
 arising from varying general nutrition, such as the feeling of 
 
36 Physiology of the Senses 
 
 general well-being, and its opposite, discomfort, general 
 depression, or melancholy, or the restless condition caused 
 in many by the disturbance of the electrical condition of the 
 atmosphere usually preceding a thunderstorm. For all 
 these no special cerebral centres have been found. 
 
 The Muscular Sense. Some at least of these may 
 probably be regarded as special forms of the muscular sense, 
 that is to say of that sensation by which we are aware of 
 the position and state of relaxation or contraction of the 
 muscular system of the body, and by which we are guided 
 in our unconscious estimate as to the amount of force 
 necessary for movement. Through it, too, we can estimate 
 the relative -weights of bodies. 
 
 RELATION OF STIMULUS AND SENSATION 
 
 We have now to consider in general terms the effect upon 
 the sensorium which any given change in our environment 
 or in the body itself will bring about. This may be viewed 
 from two aspects, the qualitative and the quantitative. 
 
 Qualitatively, the effect will depend upon whether a 
 special or a common sensory mechanism is affected. If the 
 stimulus be one fitted to excite the sense of taste, the 
 sensation it causes is in no way comparable qualitatively to 
 that caused by excitation of the sense of vision. The 
 variation of quality within the limits of any one of the 
 senses varies with the peculiar nature of the excitant. The 
 quality of colour, e.g., varies with the wave-length of light, 
 or, in other words, with its rapidity ; that of sound with the 
 form of wave, or more accurately with the momentum of 
 impact or pressure on the sensory apparatus; that of taste 
 and smell with the molecular constitution of the body, but 
 whether through the rate of motion of the molecule, or the . 
 form of the path in which it moves, cannot be said. Special 
 
General Introduction 37 
 
 illustrations of this will be found in the chapters upon the 
 special senses. Quantitatively, the character of the sensa- 
 tion depends upon the receptivity of the organism and the 
 amount or strength of the stimulus. The stimulus may be 
 so feeble that it fails to arouse any sensation whatever, a 
 light may be so small or so far removed from the eye as to 
 be invisible, a sound may be so faint as to be inaudible. 
 But when the energy of the physical disturbance reaches a 
 certain degree, supposing that the receptivity of the sensory 
 organ is always the same, a sensation is felt. Other things 
 being equal, the amount of energy required for the stimu- 
 lation of any given sense may be regarded as a constant 
 quantity, and the smallest perceptible amount is known as 
 the lower limit of excitation. This excitant acting on the 
 sensory organ brings us, as it were, to the threshold of 
 sensation. In estimating the comparative intensities or 
 strengths of sensations it is commonly assumed that the 
 difference between zero or absence of excitant and the lower 
 limit of excitation may be regarded as the unit of measure- 
 ment. 
 
 We say, for example, that lights from various sources, as 
 a candle, an oil lamp, an electric light, the sun, have 
 different degrees of brilliancy or intensity. We may 
 diminish the brightness till we reach a point beyond which 
 the light is no longer seen, and yet there is a certain 
 amount of energy being exerted of which our senses fail to 
 take cognisance. In stating the relative brilliancies or 
 intensities of the light we would use as a unit of comparison 
 the amount of light just sufficient to give a sense of lumin- 
 osity. Then so many times this unit would give the measure 
 of the luminosity of the candle, so many more of the lamp- 
 light, so many more of the electric light or of sunlight. 
 We may say that the intensity of one sensation is double, 
 treble, quadruple that of another, and so on ; or, on the 
 
38 Physiology of the Senses 
 
 other hand, we may say that a j.;iven amount of sensation 
 always bears a certain ratio to the least perceptible differ- 
 ence from it, either in the way of increase or diminution. 
 This ratio, again, corresponds with that between the in- 
 tensities of the excitant and the sensation. An endeavour 
 has been made to put this latter ratio upon an absolute 
 basis for each of the senses, but this can only be stated as 
 an average of a number of determinations made by different 
 individuals or by the same individual at different times. 
 Thus, for example, it is stated that the least possible differ- 
 ence in the intensity of light which will allow of a sense 
 of different luminosity is y^-. Given 100 lights, a difference 
 of luminosity would be noted if one were added or with- 
 drawn ; but, given a thousand, no difference would be 
 observed unless at least 10 were added or removed. The 
 least perceptible difference of pressure is caused by the 
 increase or diminution by J of the original amount. If a 
 person is holding three pounds in his hand he will not feel 
 any increase or diminution of their original weight unless 
 as much as one pound is added or subtracted. For the 
 pressure sense the ratio 1:3 is a constant, whatever be 
 the original unit. Similarly for the other senses, the ratio 
 for the sensation of temperature is i : 3, for auditory sensa- 
 tions i : 3, for muscular sensation 6 : 100, and for visual 
 sensation i : 100. 
 
 In the next place, we must note that with variation in 
 the amount of the stimulus there is variation in the intensity 
 of the stimulation, but these do not vary pari passu 
 in the same numerical ratio. We have seen, for ex- 
 ample, that to have any change at all in the sense 
 of pressure there must be an increase or diminution by 
 J of the original pressure, but we do not necessarily 
 recognise directly that the pressure is J more or less. The 
 law only holds that there will be an equal perceptible varia- 
 
Introduction 39 
 
 tion when the stimulus varies in constant proportion. 
 There is the same perceptible variation when 3 Ibs. are 
 im ivased to 4 Ibs., as when 6 are changed to 8, or 12 to 
 1 6. Fechner points out that the strength of a sensation 
 does not increase in the same numerical ratio with the 
 strength of the stimulus, but as the logarithm of the 
 strength of the stimulus, for logarithms of numbers increase 
 by equal increments according to the relative increase of 
 the numbers themselves. Thus i, 2, 3, etc., are the 
 logarithms of 10, 100, 1000, and similarly, the increase in 
 sensation when the excitant is increased from 10 to 100 
 will be the same as when the 100 are increased to 1000. 
 Or, putting it in another way, the strength of the sensation 
 i ^cs in numerical progression as the strength of the 
 stimulus increases in geometrical progression. This law, 
 however, only holds within certain limits between the 
 threshold of sensation on the one hand, and an upper limit 
 on the other. With all sensations there comes a time when 
 an increase in the strength of the stimulus no longer 
 increases the intensity of the sensation, but gives rise to a 
 change in quality. Thus beyond a certain degree of 
 brilliancy the eye will be blinded or rendered insensitive to 
 light, with sounds too loud the ear will be deafened, with 
 too ^re;it pressure the tissues will be crushed, and with 
 injury to the sensory organ the sense of pain arises. 
 Fechner's law, again, fails in its applicability to the senses 
 of taste and smell, and, except within narrow limits, to the 
 sense of temperature, while it holds best perhaps in regard 
 to the sensation of light, where, owing to the delicacy of 
 discrimination of the sense of vision, it is possible to judge 
 of differences over a wide range of sensibility. 
 
 Sensations and Perceptions. There is still one point 
 in which we may note a difference in the mental effect of 
 the action of the different senses, viz. the extent to which 
 
40 Physiology of the Senses 
 
 they are attended by the idea of externality. With both 
 sight and hearing we very early acquire the power of pro- 
 jecting our sensations outwards, so that objects seen are 
 referred to their relative positions in space, while by the 
 aid of other senses we are able to refer the sound to the 
 sounding body. Similarly we refer odours to the body from 
 which they come, and the senses of touch and taste give 
 us information which we interpret as due to objects in con- 
 tact with our body, but external to it. The common senses, 
 such as fatigue, pain, etc., give us no impression of an 
 external body in relation to ours, they are purely feelings 
 devoid of a sense of an underlying objective reality. This 
 aspect of the subject will be better understood, however, 
 when we consider the senses in detail 
 
THE SENSE OF TOUCH 
 
 THE sense of touch is located in the skin. The structure of 
 this organ, which acts as a protective covering, and is also 
 concerned in the excretion of sweat, oily or sebaceous 
 matter, and gases, and in the regulation of the heat of the 
 body, will be readily understood by studying the section 
 seen in Fig. 12. 
 
 ( Horny layer . 
 
 Epi- . 
 clermis. 
 
 j Clear layer -- 
 ^Mucous layer .... 
 
 i Papillary layer 
 
 -5=?^. 
 
 ^~ Y ^ 
 
 True 
 
 Duct of sweat gland . 
 
 "H 
 
 Skin. 
 
 Reticulated layer ... 
 Sweat gland 
 
 ~ll 
 
 
 
 
 Subcutaneous tissue 
 
 Fig. 12. Perpendicular section of the skin of the finger of an adult man. 
 Magnified 15 diameters. (Stohr.) 
 
 Structure of the Skin. It consists of two layers, a 
 deeper, formed of connective tissue, and called the derma, 
 cutis vera (true skin), or coriuw, and a superficial, known 
 as the epidermis, which is composed of epithelium. 
 
42 Physiology of the Senses 
 
 (1) The true skin. If we look at the surface of the 
 skin we see it shows delicate furrows or grooves crossing 
 each other, so as to form small lozenge-shaped areas, or 
 the grooves may run parallel for a considerable distance. 
 The lozenge-shaped arrangement is seen on the surface of 
 the skin of the arm, and that with the grooves forms a 
 marked feature on the skin of the palm or covering the 
 tips of the fingers. On the summits of the ridges, on each 
 side of a groove, or enclosing a lozenge-shaped area, we 
 find small prominences termed papilla, the number and 
 size of which vary much in different parts of the skin. 
 They are most numerous, and attain the greatest size 
 (about the -g- of an inch in length), in the palm of the 
 hand and sole of the foot, while they are much smaller and 
 fewer in number on the skin of the cheeks or forehead. 
 The true skin is formed of a felt-work of connective tissue, 
 mixed with elastic fibres, and having also a considerable 
 number of smooth muscular fibres distributed here and 
 there. In the upper layers the connective tissue is con- 
 densed so as to form a firm stratum, but in the deeper 
 layers the bands of connective tissue run in all directions 
 so as to form an irregular mesh-work, in the spaces of 
 which we find numerous fat-cells. Thus the skin is toler- 
 ably firm in its upper layers, and these may be supposed to 
 rest on an elastic cushion, a condition that favours, as we 
 will find, the mechanism of touch. 
 
 (2) The Epidermis. This, the outermost layer, is formed 
 of more or less flattened epithelial cells, arranged in layers 
 or strata. Two such strata are readily seen when we 
 examine a perpendicular section (Fig. 12): a deeper 
 stratum, of soft consistence, filling up the spaces between 
 the papillae, and termed the stratum mucosum^ or stratum 
 of Malpighi (after the Italian anatomist who first described 
 it), and a superficial and denser stratum, known as the 
 
The Sense of Touch 43 
 
 horny layer or stratum corneum. Both strata are built up 
 of epithelial cells, which change in appearance as we pass 
 from below upwards. Those in the mucous stratum are 
 cylindrical, and have a long nucleus ; and above these 
 we find rounded cells, having little spines projecting from 
 their borders, and hence called prickle cells. The spines of 
 adjoining cells unite, and thus there is a reticulated space 
 round each cell. Above these the cells become more 
 flattened, and contain bright refractive granules. The cells in 
 the mucous layer of the skin rapidly multiply, the youngest 
 cells being next the papillae of the true skin, and each 
 layer is gradually pushed towards the surface by a layer of 
 younger cells below it. The horny layer is formed of flat 
 polygonal cells that have lost their nucleus, and the cells 
 of the most superficial layer are gradually being shed by 
 abrasion or rubbing. Thus thousands of hard dry epithelial 
 cells are being rubbed off daily from the surface of the 
 epidermis. In some parts of the skin where the epidermis 
 is very thick, as on the sole of the foot, a clear stripe is 
 seen between the mucous and horny layers. This, called 
 the clear stratum (stratum luciditm), is formed of cells that 
 refract light strongly, and hence have a translucent appear- 
 ance. The colour of the skin depends partly on granules 
 of pigment found in the cells of the mucous layer, and 
 partly on the blood circulating through it, and the thickness 
 of the layer of tissue between the vessels and the surface. 
 Thus when the vessels of the skin are moderately dilated, 
 and when the vessels lie near the surface, there may be the 
 delicate rosy hue of health, while the reverse conditions 
 may produce a pale or swarthy, or even a yellowish tint of 
 skin. 
 
 It is foreign to the purpose of this work to describe 
 all the so-called appendages of the skin, such as nails, 
 hairs, horn, hoof, quills, feathers, and scales. And yet all 
 
44 Physiology of the Senses 
 
 these may be, to some extent, concerned in the sense of 
 touch. They are all modifications of epidermis, and they 
 are all developed or moulded upon papillae which are 
 similar in character and origin (although often much 
 greater in size) to the papillae on the surface of the true 
 skin already described. The following general statements 
 regarding these appendages are of physiological interest : 
 
 (1) Each epidermic structure may be regarded as a 
 permanent excretion. They are separated from the blood, 
 and thus modify the constitution of that fluid. Thus the 
 nutrition of other organs of the body may be influenced, 
 and in this way we may establish a physiological connec- 
 tion -between the development of hairs, horns, wattles, 
 combs, brilliantly-coloured feathers, etc., and the changes 
 at certain periods of life in the sexual organs. 
 
 (2) Each epidermic structure has an individual exist- 
 ence ; it is developed, grows, reaches maturity, declines, 
 dies, and is removed from the body, to be replaced by 
 another of a similar kind. Thus hairs, nails, feathers, etc., 
 have each a limited duration of life. 
 
 (3) Epidermic structures, similar in origin, but, in their 
 mature condition, very different in structure, may serve 
 purposes of beauty, as the hairs of the seal or ermine, the 
 feathers of the humming-bird or kingfisher, or the scales of 
 the gold-fish or mackerel ; of warmth, as the hair of the 
 polar bear, the wool of sheep, and the feathers of many 
 birds ; of defence, as the horns of the stag, the spines of 
 the hedgehog, or the quills of the porcupine ; as protect- 
 ive and resistant structures, covering delicate parts of the 
 foot, as the hoofs of the horse, etc. ; and as aids to the sense 
 of touch, as the whiskers of the cat, or the hairs on the ears 
 of many nocturnal mammals. It is remarkable that when 
 epidermis is modified for purposes requiring great powers of 
 resistance, it assumes in structure a concentric arrangement 
 
The Sense of ToncJi 45 
 
 of epidermic cells, simulating bone, as may be seen by 
 comparing a section of bone with that of hoof, whalebone, 
 or of rhinoceros horn. Lastly, epidermic structures, by 
 containing pigment, confer brilliant colours on many 
 animals, and even where pigment is absent, beautiful 
 iridescent tints may be produced by fine markings on the 
 surfaces of epidermic structures. These markings, or 
 grooves, form diffraction spectra when the light falls on 
 them, and thus we have many humming-birds flashing 
 a variety of tints as the animals flit to and fro in the sun- 
 light. 
 
 Structure of Tactile Organs. As already explained, 
 sensory nerves are those that convey nervous impulses to 
 the brain, and there give rise to sensations. Such sensory 
 nerves abound in the skin, but if one of these be gently 
 touched, the result will not be a sensation of touch in the 
 proper sense of the word, but a more or less painful and 
 disagreeable impression. The direct contact of any foreign 
 body with a naked sensory nerve is too rude a form of 
 stimulation, and hence we find, as a rule, that the fine fila- 
 ments at the origins of such nerves in the skin are brought 
 into relation with special tactile structures or terminal organs 
 of touch, of which there are several varieties. 
 
 (1) Free nerve -endings. In a few situations, single 
 nerve fibres pass up to the under surface of the epidermis, 
 lose their medullary sheaths, and then the axis -cylinder 
 subdivides into fine filaments, which either lose themselves 
 among the cells of the epidermis, or come into contact with 
 cells having branched processes, called the cells of Langer- 
 hans. This is the simplest form of nerve-ending, and the 
 only form in epidermis. It has been found in the cornea 
 of the eye, the nose of the mole, the nose of the pig, and 
 the skin of the frog and tadpole. 
 
 (2) Nerve -endings in corpuscles. The nerve filaments 
 
46 Physiology of the Senses 
 
 may terminate in various forms of corpuscles, which, how- 
 ever, are (with one exception) situated in the true skin, or 
 in the subcutaneous tissue. Thus we may have (a) simple 
 tactile cells ; (b) groups of tactile cells ; (c) touch cor- 
 puscles, (a) simple, and (/3) compound ; (d) end- bulbs ; 
 and (e) a more complicated structure called a Pacinian 
 corpuscle. 
 
 (a and b) Simple tactile cells. These are oval nucleated 
 cells, about yo^o" f an mc ^ m diameter, found in the 
 deeper layers of the epidermis, or in the true skin close 
 
 to the epidermis. Minute 
 nerve filaments terminate 
 in these by apparently 
 blending with their sub- 
 stance. Sometimes these 
 cells may form a group 
 ^ which takes the form o"f a 
 -''2'"' little cup, like a wine-glass 
 
 FIG. 13. Vertical section through the skin with the bottom broken off, 
 covering the attached end of the upper } ^ . h 
 
 mandible of a goose. Magnified 240 
 
 diameters. Shows two touch corpuscles Stem of the glass, 
 divided transversely to the plane of en- ( a) ^^ ^^ 
 trance of the nerves. i, Tactile cor- 
 puscle consisting of four tactile cells; pUSCleS. These, SOlYie- 
 
 2, twin tactile cells, ts ; , tactile disc ; , times termed the cor p usdes 
 
 (to the left) nerve filament ; n, (to the 
 
 right) medullated nerve fibre ; c, true of Grandry, or the COrpus- 
 
 skin. (Stohr.) des Q j Merkel ^ are formed 
 
 of two or more flattened cells, each cell being about 
 TrVo" f an mc ^ m ^ en ' t h by ~-Q of an inch in breadth. 
 A medullated nerve fibre, on approaching the corpuscle, 
 first loses the white substance of Schwann, and then the 
 axis-cylinder ends in a flat disc placed between two of the 
 tactile cells. This comparatively simple form of touch cor- 
 puscle is found in the skin of the bills, and in the tongues, 
 of birds, especially those of aquatic habits. 
 
The Sense of ToucJi 
 
 47 
 
 (c, P) Compound touch corpuscles. These, termed the 
 corpuscles of Wagner, or the corpuscles of Meissner, are oval 
 bodies, from -%\^ to yl-g- of an inch in length, and ^ to -^\-^ 
 of an inch in breadth, found in the papillae of the true skin, 
 especially in the palm of the hand and sole of the foot. 
 The number of these bodies is very considerable. About 
 fifty in each square millimetre have been counted on the 
 tip of the forefinger. A like area 
 over the second joint contained 
 twenty, and over the first joint 
 seven or eight. About fifteen per 
 square millimetre have been found 
 in the skin of the last joint of the 
 great toe, and three or four in the 
 like area of the sole of the foot. 
 Each tactile corpuscle has one or 
 two medullated nerve fibres twisted 
 spirally round it (Fig. 14), and 
 near the upper pole of the corpuscle 
 the white substance of Schwann 
 
 v j ,1 v j FIG. 14. Touch corpuscle, a. 
 
 disappears, and the axis-cylinder Laye 4 rs of connecti v e tissue of 
 ends in little excrescences or thick- 
 enings. The corpuscle is built up 
 of flattened cells, the edges of 
 which, often seen in section, give 
 it a peculiar striated appearance. 
 These bodies are evidently con- 
 structed on the same plan as the 
 more simple corpuscles in the bills of birds, above described, 
 each consisting of a number of tactile cells. 
 
 (d) End-bulbs. These, sometimes called the end-bulbs, 
 or end-knobs of Krause, occur in the conjunctiva of the eye, 
 the mucous membrane of the mouth, in some of the liga- 
 ments of joints, occasionally in tendons, and in the sexual 
 
 the true skin ; b, body of 
 corpuscle ; d, d, nerve fibres 
 twisted spirally round the cor- 
 puscle ; c, nerve fibres at the 
 lower end of the corpuscle ; 
 e, nerve fibre ending in little 
 thickenings near the upper end 
 of the corpuscle. Magnified 
 50 diameters. 
 
4 8 
 
 Physiology of the Senses 
 
 organs. They have also been found on the under surface 
 of the toes of the guinea-pig, in the ear and body of 
 the mouse, and in the wing of the bat. Varying from 
 ylhr to T yy- of an inch in length, each consists of a deli- 
 cate wall of connective tissue, sometimes forming a little 
 sac, in the interior of which we find granular matter and 
 nuclei (Fig. 16). Sometimes, also, the granular matter 
 takes the form of a knob. The nerve may apparently end 
 at the lower extremity of the bulb (Fig. 15, 2) 3 or it may 
 
 FIG. 15. Various forms of end- 
 bulbs. (Krause.) 
 
 FIG. 16. End -bulb, a, nerve; b, 
 connective tissue wall, (Krause.) 
 
 penetrate it and form a number of loops (Fig. 15, i), or 
 it may end in a long ribbon or rod (Fig. 15, 3). 
 
 (e) Padnian bodies. These, sometimes termed the cor- 
 puscles of Pacini, or the corpuscles of Vater, from ^ to 
 YTT of an inch in breadth, and from T V to J- of an inch in 
 length, are found in the subcutaneous connective tissue of 
 the palm of the hand (including the fingers) and sole of the 
 foot, in the sexual organs, in the deeper layers of connect- 
 ive tissue below the skin near joints, in the mesentery (the 
 fold of peritoneum holding the intestine in position), and in 
 
TJie Sense of Touch 
 
 49 
 
 the connective tissue around the pancreas. They have also 
 been found in the skin of the elephant and of the bat. 
 About 600 exist on the palmar surface 
 of each hand, and as many on each foot. 
 Each Pacinian corpuscle consists of from 
 forty to fifty lamellas or capsules (Fig. 
 1 8) concentrically arranged. The space 
 between each pair of lamellae is lined by 
 a layer of flattened ceils, and is filled 
 with fluid. Each capsule is smaller as 
 we approach the centre, and the capsules 
 are all connected at the pole opposite 
 the entrance -of the nerve by a thicken- 
 ing. A small artery enters the corpuscle. 
 The nerve of the medullated variety 
 enters the corpuscle at one pole, and may 
 be regarded as forming its stem or stalk. 
 The fibre perforates the capsules, and the 
 axis-cylinder runs up into the clear mass 
 in the centre of the corpuscle, the medul- 
 lary sheath and the white substance of 
 Schwann terminating at the entrance of 
 the filament into the clear mass. Near 
 the farthest end the axis-cylinder often 
 divides into two or more branches, and FIG. 17. Diagrammatic 
 these, in turn, end in a little pear-shaped 
 mass, called the terminal bud. Each 
 bud is composed of a dense network of 
 minute fibrils. Surrounding the axis- 
 cylinder we find a transparent or slightly 
 striated substance, with sometimes rows 
 of nuclei, especially near the farther end. 
 Smaller and simpler bodies, but con- 
 structed on the same plan, have been found in the bills and 
 
 view of the under sur- 
 face of the index finger 
 with Pacinian corpus- 
 cles. # , Nerve ; b, c, 
 lateral and terminal 
 branches of the nerves ; 
 d, d, d y Pacinian cor- 
 puscles ; i first, 2 
 second, and 3 third 
 phalanx of the finger. 
 (Schwalbe.) 
 
Physiology of the Senses 
 
 tongues of birds (distinct from Grandry's corpuscles), and 
 
 are termed the corpuscles of Herb st. 
 
 (3) Nerve- endings in connection with tactile hairs. 
 
 A hair grows from a follicle or pit in the substance of the 
 
 true skin. A layer of epidermis 
 dips down into the follicle, lining 
 it, and covering a papilla in the 
 bottom of the follicle. From 
 the surface of the papilla, which 
 is in reality one of the papillae 
 of the true skin, the hair is 
 developed, and as it passes up 
 to the mouth of the follicle, it 
 is covered by a sheath, com- 
 posed of layers similar to those 
 of the epidermis. Each papilla 
 on which a hair grows is richly 
 supplied with capillary blood- 
 vessels. The papillae of the 
 special tactile hairs, like those 
 near the mouth of a cat, are 
 larger and more vascular than 
 those of ordinary hairs. It 
 would appear that each ordinary 
 hair follicle is supplied with fine 
 nerves. Fine medullated nerve 
 fibres form a network in the outer 
 coat of the hair follicle, and they 
 
 then lose the white substance of 
 
 %. 
 
 FIG. i8.-A Pacinian corpuscle. N, Schwann, and run more in a 
 nerve ; v, V, vessel ; T, nerve- longitudinal direction, parallel to 
 
 ending. (Klein and Noble Smith.) 
 
 the hair. They then penetrate 
 
 the wall of the follicle and end in the inner layer of the sheath 
 of the hair, but their exact mode of termination is yet un- 
 
The Sense of Touch 51 
 
 known. The number of nerve filaments brought into close 
 relation with a true tactile hair is very great, dense net- 
 works being formed both in the inner and the outer sheaths 
 of the hair, and they end in small knob -like swellings be- 
 tween the columnar cells of the outer sheath of the hair. In 
 some cases a special plexus of minute nerve fibrils has been 
 found surrounding, like a ring, the neck of the hair follicle. 
 
 It is well known that tactile hairs can be voluntarily 
 caused to stand out stiff and rigid. This is owing to the 
 fact that such hairs possess a special arrangement for so 
 erecting the hairs. Surrounding the neck of the hair follicle 
 we find sinuses and spaces of erectile tissue, controlled by 
 bands of elastic and unstriped muscular fibre. When the 
 spaces are full of blood the hair projects from the centre of 
 a highly elastic cushion, thus, no doubt, giving greater sen- 
 sitiveness to the apparatus. 
 
 The small woolly hairs on the skin of many animals 
 appear to be organs of touch, and experiment has shown that 
 they are more sensitive than the areas of skin between 
 them. In many animals the proper tactile hairs acquire 
 great length, thickness, and stiffness. These vibrissce, or 
 whiskers or mustaches, in marine carnivora, plunging into 
 depths of the sea where there is little or no light, serve, 
 according to Owen, "as a staff, in a way analogous to 
 that held and applied by the hand of a blind man." 
 The night -prowling felines and the nocturnal monkeys, 
 like the aye-aye, have hairs of this kind developed on 
 the eyebrows, lips, and cheeks. Other epidermic append- 
 ages serve useful purposes in connection with the sense 
 of touch. The broad massive hoof of the horse is not 
 adapted for delicate discriminations of tactile sensations, 
 but clothing, as it does, highly vascular and sensitive 
 lamellae, gives broad and massive sensations, which 
 enable the animals to appreciate the solidity of the 
 
5 2 Physiology of the Senses 
 
 ground on which they tread. Animals living in the sea 
 sometimes have touch organs developed which enable 
 them to detect pressures or movements, often at a con- 
 siderable distance from them. Thus whales have large 
 papillae in the skin, richly supplied with nerves, and some- 
 times the skin, bearing these papillae, is thrown into plaits 
 or folds, so as to give a greater extent of sensitive surface. 
 It is said by Owen that this arrangement "is peculiar 
 to the swifter swimming whales that pursue mackerel 
 and herring, and may serve to warn them of shoals, 
 by appreciation of an impulse of the water rebounding 
 therefrom, and so conveying a sense of the propinquity 
 of sunken rocks or sand -banks." The nose -leaves and 
 sensitive ears of some of the bats often show vibratile 
 movements, trembling, like the antennae of insects, as the 
 animal gathers information as to its environment, and thus 
 act as delicate organs of touch. The nose and feet of 
 burrowers in the earth, like the mole, have always delicate 
 organs of touch, by which the animals feel their way in 
 their subterranean galleries. 
 
 Nature of the Tactile Mechanism. Touch is a sensa- 
 tion of pressure referred to the surface of the body. When 
 we touch anything there is always a certain amount of 
 pressure between the sensitive surface and the body 
 touched. What we call contact is gentle pressure ; a 
 greater amount of force or pressure makes the sensation 
 of touch more acute ; by and by, there is a feeling of 
 resistance to pressure, still referred to the skin ; when a 
 weight is supported on the palm of the hand there is then a 
 sensation of muscular resistance, a sensation referred not 
 only to the skin, but also to the muscles, and by which we 
 become aware that the muscles are contracted ; and, 
 finally, the pressure may be so great as to cause a sensa- 
 tion of pain which, however, may be confused with simul- 
 
TJie Sense of Touch 53 
 
 taneous sensations of contact and of muscular resistance. 
 The force, however, that gives rise to touch, in its various 
 degrees, may not act vertically on a sensory surface, but in 
 the opposite direction, as when we pull a hair. Touch is, 
 therefore, in its essence, the appreciation of mechanical 
 force, and in this way it presents a strong resemblance to 
 hearing, which is a more delicate kind of touch, being due 
 to variations of pressure on the auditory organ. In addi- 
 tion, however, to sensations of touch, contact with a foreign 
 body may give rise to sensations of heat or cold that is 
 to say, to sensations of temperature. Thus when we place 
 something on the palm of the hand, the resulting sensation 
 may be of a complex character, involving sensations of 
 gentle pressure (contact), of more severe pressure, and of 
 temperature. True tactile impressions are absent from 
 internal mucous and serous surfaces, as has been proved in 
 men having fistulous openings into the stomach, intestine, 
 bladder, or pleural cavities. In such cases pressure does 
 not cause a sensation of touch, but of pain. 
 
 A consideration of the structure of the terminal organs 
 of touch, above described, shows that they must serve 
 (i) for protecting the extremities of the sensory nerves 
 from direct pressure ; (2) for communicating slight varia- 
 tions of pressure to the nerve-ending; and (3) for so 
 modifying external pressures as to give them more or less 
 of a rhythmic character. Thus if we consider the nerve- 
 ending in an end-bulb, or in a Pacinian corpuscle, lying 
 in a fluid or semi-fluid substance, surrounded by one or 
 more envelopes of connective tissue, we see that most 
 delicate pressures must be communicated to it, and also 
 that a wave -like movement may be set up in the fluid 
 matter, thus subjecting the nerve-ending to a number of 
 intermittent pressures or vibrations. In the case of the 
 touch corpuscles, either simple or compound, the arrange- 
 
54 
 
 Physiology of the Senses 
 
 ment is evidently that of an elastic cushion against which, 
 the nerve filament is pressed, thus again making variable 
 pressures or vibrations possible. In like manner, move- 
 ments communicated to a hair, the follicle of which is sur- 
 rounded by elastic structures and nerve-endings, must give 
 rise to impulses in these nerves, probably of an intermittent 
 or vibratory character. No part of the body, when touching 
 anything, can be regarded as absolutely motionless, and 
 the slight oscillations of the sensory 
 surface, and, in many cases, of the 
 body touched, produce those varia- 
 tions of pressure on which touch de- 
 pends. 
 
 Sensitiveness of the Skin. It is 
 a familiar observation that all parts of 
 the skin are not equally sensitive. The 
 method of determining the degree of 
 sensitiveness, first employed by Weber, 
 consists in finding the smallest dis- 
 tance at which the two points of a pair 
 of compasses can be felt. Two in- 
 struments suitable for such observa- 
 tions are shown in Figs. 19 and 20, and the results in 
 millimetres * are given in the following table : 
 
 FIG. 19. Compasses of 
 Weber. 
 
 Tip of -tongue . 
 
 i.i 
 
 Centre of palm 
 
 8-9 
 
 Under surface of 
 
 third 
 
 Under surface of third 
 
 
 phalanx of finger . 
 
 . 2-2.3 
 
 phalanx of great toe 
 
 ii3 
 
 Red part of the lip . 
 
 . 4-5 
 
 Upper surface of second 
 
 
 Under surface of second 
 
 phalanx of finger 
 
 11.3 
 
 phalanx of finger . 
 
 . 4-4-5 
 
 Back . . 
 
 H.3 
 
 Upper surface of 
 
 third 
 
 Eyelid .... 
 
 ii.3 
 
 phalanx of finger 
 
 . 6.8 
 
 Under surface of lower 
 
 
 Tip of nose 
 
 . 6.8 
 
 third of forearm . 
 
 15.0 
 
 Ball of thumb 
 
 6.5-7 
 
 Cheek .... 
 
 15.8 
 
 i 
 
 i millimetre = 
 
 -J T of an inch. 
 
 
The Sense of Toucli 55 
 
 Temples . . . 22.6 Forearm and leg . .45.1 
 
 Forehead . . .22.6 Neck . . . . 54.1 
 
 Back of head . . . 27.1 Back, opposite fifth dorsal 
 
 Back of hand. . . 31.6 vertebra . . .54-1 
 
 Knee . . . . 36. 1 Upper arm, thigh, centre 
 
 Gluteal region . . 44. 6 of back . . . 67. i 
 
 Numerous investigations made since the time of Weber 
 have shown considerable variations in different individuals. 
 The method is employed by physicians in the diagnosis of 
 nervous diseases affecting the sensitiveness of the skin. 
 The general result of Weber's method is to show that 
 in a healthy person those parts are most sensitive as 
 regards the power of discriminating two points at a certain 
 
 FIG. 20. ./Esthesiometer of Sieveking. 
 
 distance from each other, which we use habitually as organs 
 of touch. Thus the tips of the fingers on their under 
 surface, the palms of the hands, the margins of the lips, are 
 more sensitive than the dorsal surfaces of the limbs or the 
 skin covering the back. Sensitiveness is great in parts of 
 the body that are habitually moved, and it increases from 
 the joints towards the extremities. Again, sensitiveness is 
 finer if we proceed a given distance along the transverse 
 axis of a limb than if we pass the same distance along the 
 long axis. 
 
 Moistening the skin, stretching it, or taking baths in water 
 containing common salt or carbonic acid, increases sensi- 
 tiveness, especially as regards the power of discriminating 
 points. An anaemic condition, venous congestion, cold, 
 
56 Physiology of the Senses 
 
 and the use of solutions of atropine, daturine, morphine, 
 strychnine, alcohol, bromide of potassium, cannabine, and 
 hydrate of chloral, blunt sensibility. Moistening the skin 
 with a solution of caffeine is said to increase sensibility. 
 
 Sense of Locality. Not only is the skin sensitive, but 
 one is able, with great precision, to determine the part 
 that has been touched. This power may be termed the 
 sense of locality. The general law is that the greater the 
 number of sensory nerves in a given area of skin, the greater 
 is the degree of accuracy in distinguishing different points, 
 and in determining locality. Contrast, for example, the tip 
 of the finger with the back of the hand. 
 
 One would imagine that the habitual use of these parts 
 would so educate the mind as to enable us to identify 
 particular parts touched, even although these parts might 
 not be much more sensitive than other parts. It is doubt- 
 ful, however, if exercise improves sensitiveness. Thus 
 Galton found that the performances of blind boys, when 
 examined by the Weberian method, were not superior to 
 those of other boys, and he says " that the guidance of the 
 blind mainly depends on the multitude of collateral indi- 
 cations, to which they give much heed, and not their 
 superiority in any of them." 
 
 Absolute Sensitiveness. Hitherto we have been dis- 
 cussing the power of discriminating points, but this is 
 different from the absolute sensitiveness of any part of the 
 skin. What is the smallest pressure that can give rise to a 
 sensation, and what is the smallest difference that can be 
 observed between two sensations ? Many attempts have 
 been made to answer these questions. Thus small weights 
 have been allowed to press on the skin, and the smallest 
 weight causing a sensation, and the smallest difference 
 between two weights, have been noted. Again, an ordinary 
 balance has been used, and from the under surface of 
 
The Sense of Touch 57 
 
 one scale-pan a fine needle projected which pressed on the 
 skin, while weights were placed in one scale -pan or the 
 other according to the nature of the experiment. In this 
 way accurate measurements were obtained. To avoid the 
 interfering effects of sudden shocks, the skin has been 
 pressed against a fine tube containing water, so that rhythmic 
 waves, like those of the pulse, were caused to impinge on the 
 skin. The general results of these methods may be briefly 
 summarised thus : 
 
 (1) The greatest acuteness was observed on the fore- 
 head, temples, and back of the hand and forearm, which 
 detected a pressure of .002 gramme. 1 The skin of the 
 fingers detected .005 to .015 gramme, and the chin, abdo- 
 men, and nose .04 to .05 gramme. 
 
 (2) One gramme was placed on the skin, and the 
 least additional weight, in grammes or fractions of a 
 gramme, that could be appreciated was then determined, 
 with the following result : Third phalanx of finger, 
 .499; back of the foot, .5 ; second phalanx, .771 ; first 
 phalanx, .82 ; leg, i ; back of hand, 1.156 ; palm, 1.108 ; 
 patella, 1.5; forearm, 1.99; umbilicus, 3.5; back, 3.8. 
 The greatest absolute sensitiveness to a single pressure 
 was on the back of the hand, while the greatest power of 
 discriminating differences of pressure (and also of discrim- 
 inating points) was on the palmar surface. Eulenberg puts 
 the matter thus : the skin of the forehead, lips, cheeks, and 
 temples appreciated differences of pressure to the extent 
 of from JQ- to -g 1 ^ of the first pressure ; the back of the 
 last phalanx of the fingers, the forearm, hand, first and 
 second phalanges, the palmar surface of the hand, fore- 
 arm, and upper arm, distinguished differences of -^~ to -^ ; 
 and then follow the back of the foot and toes, the sole 
 of the foot, and the back of the leg and thigh, all of 
 
 1 A gramme =15. 432 grains. 
 
58 Physiology of the Senses 
 
 which require a greater difference than -^ of the original 
 weight. 
 
 (3) In passing from light to heavier weights, the acute- 
 ness at once increases, a maximum is reached, and then, 
 with still heavier weights, the power of distinguishing differ- 
 ences gradually diminishes and finally disappears. 
 
 Fusion of Tactile Impressions. If the finger is held 
 against a blunt toothed wheel, and the wheel is rapidly 
 rotated, a smooth margin is felt when the intervals of time 
 between the contacts of successive teeth are less than ^-g-g- 
 to -g-J-^- of a second. The same experiment may be made 
 by pressing the finger gently over the holes in one of the 
 outermost circles of a large syren rotating quickly; the sensa- 
 tion of touching individual holes disappears, and there is a 
 feeling of touching a slit. The meaning of these experi- 
 ments is that the individual impressions, if they follow each 
 other with sufficient rapidity, are fused together in conscious- 
 ness, so that we experience one continuous sensation. By 
 attaching light bristles to the prongs of rapidly vibrating 
 tuning-forks, and bringing the bristles into gentle contact 
 with the tips of the fingers, and especially with the margins 
 of the lips, curious observations may be made. If the 
 forks are vibrating at rates of from 600 to 1500 vibrations 
 per second, sensations of an acute and almost unbearable 
 character are experienced, but above this limit, sensation, 
 other than that of mere contact, almost or wholly disappears, 
 although the fork is in active vibration. 
 
 After-tactile Impressions. If the weight be consider- 
 able, and if it be allowed to press on the skin for a few 
 minutes and be then removed, a faint sensation of pressure 
 may continue for a few seconds. This is termed an after- 
 effect. It shows that the influence on the nerves or nerve 
 centres does not disappear the instant the exciting cause is 
 removed. Thus we may compare impressions, and thus 
 
The Sense of Touch 59 
 
 the effect of one impression is more easily fused with the 
 effect of impressions following quickly after it. 
 
 Information from Tactile Impressions. When the 
 skin comes into contact with the surface of any external 
 body, we become aware of the existence of something 
 touching the sensory surface, and from the intensity of the 
 sensation we form a judgment as to the intensity of the 
 pressure. As already pointed out, we, in the first instance, 
 refer the sensation to the skin, but after the pressure 
 has reached a certain intensity, so as to call forth mus- 
 cular action to resist it, the sensation of touch (pressure) 
 is commingled with that of the so-called muscular sense. 
 The number bf points on the surface of the foreign body that 
 individually touch the skin enables us to judge of its smooth- 
 ness or roughness. Thus, if uniformly smooth it gives rise 
 to a sensation like that of touching a billiard ball, and if 
 we move the hand over a considerable distance of smooth 
 surface there is a sensation of massiveness, as when we 
 touch a marble slab. On the other hand, a body having 
 points irregular in size and number in a given area is 
 rough ; and if the points are very close together, like those 
 of the pile of velvet, a peculiar sensation of roughness may 
 be experienced, almost intolerable to some individuals. 
 If a large area of skin be uniformly pressed upon, the 
 sensation of pressure may disappear after a few minutes, 
 and there will be sensation only when there is a variation 
 of pressure. Again, if one part of the body is subjected to 
 one pressure, and an adjacent part to another pressure, 
 the sensation of pressure may be limited to the line 
 dividing the one area from the other. Thus if we plunge 
 the finger into a dish of mercury, a ring of constriction 
 may be felt just at the junction of the surface of the mer- 
 cury with the air. The same is experienced when the body 
 is immersed in a bath. No feeling of pressure is felt in 
 
60 Physiology of the Senses 
 
 the immersed parts, but if the arm or leg be lifted into the 
 air, a sense of pressure may be experienced on the strip of 
 skin where the limb passes from the water into the air. 
 
 The tactile field. As already pointed out, we can deter- 
 mine, with great accuracy, the part touched, and from this 
 the probable position of the touching body. If a point 
 of the skin is touched certain tactile corpuscles are irritated ; 
 these, in turn, set up impulses in sensory nerve fibres, and 
 these impulses are carried by the fibre, first to the spinal 
 cord, and then to the brain, where the fibres end in gan- 
 glionic masses in the gray matter of the cerebral cortex. 
 There are thus, projected, as it were, on the cortex of the 
 brain, tactile centres for the hind-leg, fore-leg, neck, eye, 
 ear, trunk, etc., and it follows that each point of the skin 
 has a corresponding point in the cerebral cortex. Thus for 
 each stimulation of a point of the cerebral cortex there is a 
 local sign, and thus we localise tactile impressions. There is 
 thus in consciousness, and in the brain, a tactile field, to 
 which all points of the skin surface may be referred, point 
 for point. This is comparable to the visual field to which 
 all retinal impressions are related, and which will be after- 
 wards discussed. We have, as it were, a tactile picture of 
 the part touched, and when we pass the hand over any 
 external object '(supposing the eyes to be shut) we touch it 
 at various points, and from the differences of pressure, and 
 from a comparison of the positions of the various points in 
 the tactile field, we judge of the configuration of the body. 
 We obtain a number of tactile pictures, and these are fused 
 together so as to give a conception of the whole object. If 
 the object be large, we do not depend, however, on tactile 
 pictures only. It may be necessary to move the limb, or 
 even the body itself, so as to examine the whole of the 
 external object, and the sensations arising from, or connected 
 with, the muscular movement are, in turn, fused with the 
 
77/6* Sense of Toucli 
 
 61 
 
 tactile pictures. We then judge of the form, size, and 
 nature of surface of the body touched. If there is an 
 abnormal displacement of position of the body touched, or 
 if we touch it with parts of the body that we are not in the 
 habit of using for this purpose, a false conception may arise 
 as to the shape of the body. Thus, in the old experiment 
 of Aristotle, shown in Fig. 21, if a pea be placed between 
 the index and middle finger, so as to touch the outer side of 
 the index finger and the inner side of the middle finger, 
 a sensation of touching one round body is experienced ; but 
 if the fingers be crossed, so that the pea 
 touches the inner side of the index finger 
 and the oute*r side of the middle finger, 
 there will be a sensation of two round 
 bodies, because, in these circumstances, 
 there is added to the feelings of contact 
 a feeling of distortion (or of muscular 
 action) like what would arise if the fingers, 
 for purposes of touch, were placed in 
 that unnatural position. 
 
 The knowledge of the tactile field is 
 usually precise and definite. This is illus- 
 trated by the well-known fact that when FlG - 2i.-Experiment of 
 
 ..... . . Aristotle. 
 
 a piece of skin has been transplanted from 
 the forehead to the nose, in the operation for removing a de- 
 formity of the nose caused by ulcerative disease, the patient 
 may feel the new nasal part as if it were in his forehead, and 
 he may have a headache in his nose. The mind receives the 
 messages thus transmitted to definite points in the cortex, 
 and assumes that these messages come from the locality from 
 which similar messages have come over and over again. 
 Thus it is that a man may feel pain in the toes of an am- 
 putated limb ; and a medical man, who had the misfortune 
 to lose his leg by amputation, told the writer that for years 
 
62 Physiology of the Senses 
 
 he sometimes felt pain in a troublesome corn that once 
 existed in the amputated member. There can be no doubt, 
 however, that our knowledge of the tactile field depends 
 largely on the education of the sense, not merely in the 
 individual, but in the race. Even in the individual much 
 may be done to improve it. Few, for example, have any 
 knowledge of touching anything with the third toe, because 
 this part of the body is not used in collecting tactile in- 
 formation, but a little practice will soon show any one that 
 sensations may be referred to this part with almost as great 
 ease as to the ball of the great toe, which is in habitual use. 
 Theories as to Touch. Various theories have been pro- 
 pounded to explain the phenomena of tactile sensibility, 
 but it cannot be said that any one is wholly satisfactory. The 
 oldest, first put forth by Weber, and modified by various 
 psychologists, states that while we refer every tactile sensa- 
 tion to a certain position in the tactile field, we do not refer 
 it merely to a point, but to a minute area of skin, which has 
 been termed a circle of sensibility. It is also assumed that 
 we can refer a sensation to each circle, as when we 
 touch the skin with the point of one leg of the compasses 
 in Weber's experiment, above described. If, however, we 
 bring both points within one circle, we still have a sensation 
 of one contact, not of two contacts. Even if the point of 
 the second leg be placed on another circle immediately- 
 adjoining, there is still a sensation of only one contact, and 
 to secure a sensation of two contacts it is necessary, 
 according to this theory, to have always one or more circles 
 intervening, or, to put the matter in another form, there 
 must always be "a non-irritated sensory element" between 
 the two points touched. It is also supposed that each 
 circle has its own nerve fibre. There is no proof, however, 
 that this is the case. The extent of such hypothetical 
 circles can be altered by practice and efforts of attention. 
 
The Sense of Touch 63 
 
 We may therefore assume either that the circles overlap, or 
 that even the same circle may be innervated by several 
 nerve filaments, so that when any part of the circle is 
 touched, various nerve filaments may be excited. One can 
 conceive, however, that the nerve filaments in one circle 
 may not be excited to an equal degree, and that the result- 
 ing sensation may thus be variously modified. The sug- 
 gestion of Krause, that the sensitiveness depends on the 
 number of tactile corpuscles in a given area, is worthy of 
 special notice. He states that the distance of the two 
 points of the compasses at which two points are felt in- 
 cludes, in the mean, twelve tactile corpuscles. It is no 
 doubt true that tactile corpuscles are not absolutely essential 
 to touch. The cornea is sensitive, and yet it contains no 
 such bodies, and when portions of the skin which, by 
 experiment, were found sensitive to touch, were extirpated 
 and microscopically examined, no touch bodies were found. 
 Still, on the other hand, we know that where the sense of 
 touch, and especially the power of discrimination of points, 
 is more acute, there touch corpuscles abound ; so we are en- 
 titled to conclude that they act as accessory mechanisms to 
 the sense. Further, it must not be forgotten that processes 
 occur in the nerve centres, and that we must not look to 
 the skin alone for an anatomical explanation. When a 
 nerve cell in the brain receives a nervous impulse by a nerve 
 originating in a given area of skin, the impression may be 
 diffused, by irradiation, to neighbouring nerve cells, which 
 are connected by nerve fibres with adjoining areas of skin. 
 If this be so, then the effect on these cells in accordance 
 with the law that sensations in nerve centres are referred 
 to the origins, in the periphery, of the sensory nerve fibres 
 reaching them will be referred to the adjoining areas of 
 skin, or, in other words, to adjoining points in the tactile 
 field. 
 
Z o ' 
 . e e o e 
 
 >.:*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 <z, 
 i of , and 57 of white ; yellow, n of <z, 14 of <, and 75 of 
 white ; and blue, 2 of #, 1 1 of c, and 87 of white. The 
 white gives the element of brightness. According to this 
 view, it is not necessary to suppose that in the red-blind 
 the red-perceiving elements are awanting, or that in the 
 green-blind the green-perceiving elements are absent, but 
 that these elements may be stimulated with intensities 
 different from those affecting the normal eye. Suppose 
 that in the eye of a colour-blind person the curves of inten- 
 sity representing the red and green coincided, or, in other 
 words, that the elements responsive to red and green in the 
 abnormal eye were stimulated with intensities equal to that of 
 red in a normal eye, the sensation would be yellow, as we 
 find to be the case in so-called green blindness. Again, if 
 in a similar way the red curve coincided as regards inten- 
 sity with the green, the general effect would be that of a 
 red-blind person, the red end of the spectrum would appear 
 to be green, and no red would be visible. This theory does 
 not profess to state what may occur in the retina in the 
 way of chemical change, as is attempted in the theory of 
 Hering. 
 
1 70 Physiology of the Senses 
 
 Captain Abney and Major-General Testing have also 
 investigated the question of colour sensation by photo- 
 metrical methods, and have been able to mark out the 
 curves of luminosity both of normal and of colour-blind 
 eyes. Their observations support the Young-Helmholtz 
 theory, and indicate clearly that the peculiar sensations of 
 colour experienced by colour-blind people are due either 
 to the different intensities of the three primary colour sensa- 
 tions, or to the absence of one or more of those sensa- 
 tions. 1 
 
 6. BINOCULAR VISION 
 
 Having considered the eye as an optical instrument, 
 we have next to inquire how the two eyes act together, 
 and what are the advantages of binocular over monocular 
 vision. 
 
 Movements of the Eye. When we wish to change our 
 field of view, we may do so either by moving the head as a 
 whole, or the eyes alone. The eyes move very freely in 
 their sockets, but, as we shall see, their movements have 
 certain limitations. The orbits the cavities of the skull in 
 which the eyes are set contain the muscles by which the 
 eyes are moved, nerves, vessels, glands, connective tissue, 
 and, lastly, a considerable quantity of fat, which forms an 
 elastic cushion on which the eyeballs rest. The depth of 
 setting of the eyes in the orbits varies in different people, 
 and in the same person from time to time ; but, as a general 
 rule, the eyes are so situated that one may, without moving 
 the head, look outwards and slightly backwards to either 
 side. We may readily prove this by standing erect with 
 the back of the head against a wall. If some bright object 
 on a level with the eyes, and touching the wall, be moved 
 gradually outwards from the head, it will, at a certain point 
 1 Philosophical Transactions, 1886, 1888, 1892. 
 
The Sense of Sight 171 
 
 (about 8 inches to i foot), become visible. The head being 
 kept fixed, a similar point may be determined for the other 
 side of the head ; and a straight line drawn from these 
 points through the outer angles of the orbits will be found 
 to meet at an angle of about 90 ; or, in other words, if the 
 head be considered as placed within a circle, only one 
 quadrant of the circle is shut off from the visual field, 
 namely, that in which the head lies. 
 
 The movements of protrusion and retraction of the eye- 
 balls are involuntary, and of little importance for. vision, but 
 rotatory movements of the eyeball require careful considera- 
 tion. These take place round ^ ~.^ 
 
 a centre of rotation which, /'' v \ 
 
 according to Bonders, lies 
 
 i -77 mm. behind the centre of 
 
 the visual axis, or 16-05 mm - 
 
 from the vertex of the cornea. 
 
 We may conceive of three 
 
 axes passing through this 
 
 centre, an antero-posterior, a 
 
 FIG. 79. Diagram to illustrate the fact 
 
 transverse, and a vertical that we can see objects in a p i ane 
 
 axis, and each of these axes behind a transverse vertical plane 
 , . . through the two eyes. 
 
 may be regarded as lying in 
 
 planes which, passing through the coats of the eyeball, divide 
 the ball into two nearly equal parts, an upper and lower, an 
 outer and inner, and an anterior and posterior. These axes 
 and planes have a certain fixed position, \hzpriinary position ^ 
 with reference to the orbit when the eye is at rest. If the 
 eyeballs rotate on the antero- posterior or visual axes from 
 the primary position, either vertically or horizontally, 
 the eyes are said to have assumed a secondary position, 
 and a tertiary position if they move in an oblique plane, 
 so as to look inwards, and at the same time upwards 
 or downwards. In the secondary position, there can 
 
172 
 
 Physiology of the Senses 
 
 be no rotation of the eye around the antero- posterior 
 axis, but in the tertiary position there is always more 
 or less rotation upon all three fundamental axes on 
 the antero-posterior, for example, it may be even more 
 than 10. Such circular rotation^ or rolling of the eyes, 
 takes place when the head leans towards either shoulder. 
 In this' case the direction of rolling is such as tends to 
 counteract the deviation of the head. 
 
 The Ocular Muscles. The movements of the eye are 
 caused by the action of six muscles. Four of these, the 
 
 direct muscles or reef z (Fig. 
 80), pass forwards from 
 the back part of the orbit 
 to be inserted severally on 
 the upper, lower, inner, 
 and outer sides of the eye- 
 ball, and their action is 
 easily understood. When 
 
 FIG. 80. Diagram of muscles of right eye. the inner muscle Contracts, 
 i Elevator of the eyelid ; a , superior 
 oblique muscle ; 3, superior direct muscle ; 
 4, 4', external direct muscle cut in order tical 
 to show part of the optic nerve, and 
 7, the internal direct muscle ; 5, inferior 
 oblique muscle ; 6, inferior direct muscle, acts, Outwards. When the 
 
 (Schwalbe.) u pp er contractSj t he eyeball 
 
 rotates upon its transverse or horizontal axis and the eye 
 looks upwards ; when the lower contracts, the eye looks 
 down. 
 
 It must be borne in mind, however, that as the upper 
 and lower recti pass somewhat obliquely outwards to their 
 places of insertion in the eyeball, there is a slight inward 
 direction given by them to the line of vision in addition to 
 the deviation up or down. To correct the inward devia- 
 tion, and, in general, to give circular rotation to the eye, 
 two oblique muscles exist. The upper (superior oblique), 
 
 axis 
 
 Qn 
 and looks 
 
The Sense of Sight 173 
 
 passing forwards along the inner wall of the orbit, passes 
 through a small fibrous ring attached to the bone, and turns 
 like a rope on a pulley backwards and outwards to be 
 inserted into the upper surface of the eyeball. The other 
 (inferior oblique), arising from the front part of the inner 
 wall of the orbit, passes backwards and outwards under the 
 eyeball, and is inserted into its outer part. The upper 
 oblique muscle rotates the eye downwards and outwards, the 
 lower upwards and outwards. The outer or inner direct 
 muscle (external or internal rectus) alone suffices to rotate 
 the eyes outwards or inwards in a horizontal plane. To 
 cause upward or downward rotation vertically, the upper 
 rectus and the lower oblique, or the lower rectus and upper 
 oblique, come into play. For oblique movements, the two 
 recti adjoining the quadrant, into which the fore part of the 
 visual axis moves, together with one of the oblique muscles, 
 act simultaneously. Further, since we habitually use both 
 eyes in looking at an object, it will be readily understood 
 how delicate and accurate the co-ordination of the muscular 
 action must be. In looking upwards or downwards similar 
 sets of muscles will of course come into play ; but in look- 
 ing sideways the outer set of one orbit acts at the same 
 time as the inner of the other, and, in converging the eyes 
 upon a near object, the two inner sets will co-operate. The 
 ocular muscles in all voluntary movements tend to render 
 the view of the object we wish to look at distinct, by the 
 formation of its image on the yellow spot, and they cannot 
 act so as to lead to the formation of images on non-corre- 
 sponding points of the retina (see p. 177). We cannot look 
 upwards with one eye while the other eye is turned down- 
 wards, nor can we look with the right eye to the right and 
 the left eye to the left at the same moment. It has been 
 pointed out by Le Conte that in drowsiness, intoxication, 
 and death, when the eyes are in a purely passive state, the 
 
174 Physiology of the Senses 
 
 visual axes diverge slightly, and for this reason the intoxi- 
 cated man sees double. Le Conte attributes this to the 
 divergence of the axes of the orbits of the human skull, and 
 holds it probable that " in a state of perfect relaxation or 
 paralysis of the ocular muscles the optic axes coincide with 
 the axes of the conical eye-sockets, and that it requires 
 
 FIG. Si. Vertical section through the left orbit and its contents in the orbital 
 axis and with eyelids open, a, Frontal bone above orbit ; b, upper jaw-bone 
 below orbit ; c, thickened bone for eyebrow ; d upper, d' under eyelid with 
 eyelashes ; e, e, meeting of conjunctivas of eyelid and eyeball ; /, muscle that 
 elevates upper eyelid ; g; superior direct muscle ; g' , inferior direct muscle ; 
 /i, cross section through inferior oblique muscle ; i, optic nerve ; 2, cornea ; 
 3, anterior chamber ; 4, lens; 5, vitreous humour. (Allen Thomson.) 
 
 some degree of muscular contraction to bring the optic 
 axes to a state of parallelism, and still more to one of con- 
 vergence, as in every voluntary act of sight." T The 
 doubling of the image caused by external deviation of the 
 fore part of the visual axes may be studied if we press upon 
 the outer border of each eyeball with the fingers. All 
 1 Le Conte, Sight, p. 255. 
 
The Sense of Sight 175 
 
 objects in view are now seen double, and if the right eye be 
 shut the left image disappears, and vice versa. 
 
 How an Object is seen as One with Two Eyes. 
 When we look at an object in the far distance the antero- 
 posterior axes of the eyes are parallel, and an image of the 
 object will be formed upon the spot of distinct vision in 
 each eye. Again, when the object viewed is near at hand, the 
 visual axes converge, so that the image is still formed upon 
 the yellow spot of both eyes, and the object is seen as 
 single. This sensation of oneness arises from the habitual 
 use of these 4 areas of the retinae for the observation of one 
 and the same point, and from the attention given to that 
 point alone as distinguished from all others in the visual 
 field. But if we displace one of the visual axes by pressing 
 with the finger upon the corresponding eye we will seem to 
 see all objects doubled, one image being stationary, the 
 other moving as we vary the pressure. The reason for this 
 is as follows : under ordinary circumstances the mind pro- 
 jects the image formed in the eye outwards in the direc- 
 tion of the visual axis, and this being now mechanically 
 displaced the object seems to be in motion. 
 
 But, further, since the whole field of normal vision seems 
 single when seen with both eyes, it follows that the retinae, 
 as a whole, act in combination, and give a single image of 
 that which is focussed upon them. Now, suppose we hold 
 two pencils upright in the middle plane of the body, but at 
 different distances, we can voluntarily fix our attention upon 
 one or other, and the one upon which we concentrate our 
 regard will appear single, while the other will be indistinctly 
 seen and will seem double. The image of that one to 
 which we specially attend is single because the visual axes 
 converge upon it, but the other is indistinct and double 
 because its images on the two retinae are not in the 
 line of regard, and not upon points which habitually act 
 
i 7 6 
 
 Physiology of the Senses 
 
 together. For each person there is always a certain visual 
 field) determined in shape by the outlines of the eyebrows, 
 nose, and cheeks, and by the position of the eyes in regard 
 to them, a field from each point of which rays entering the 
 eyes always fall upon corresponding points in the two eyes. 
 
 FIG. 82. Binocular visual field. If a sheet of paper be held so as to touch the 
 brow and prominence of the nose, the binocular visual field will be seen as in 
 the space in I, bounded by the lines L and R. If the paper be held a few 
 inches from the face the area visible to both eyes will have the shape seen 
 in II. 
 
 If, the head being fixed and both eyes open, the extent of 
 the whole visual field be noted, and if the right and left 
 eyes be alternately closed and opened, it will be found that 
 the projection of the eyebrows and nose cuts off from each 
 eye a certain part of the visual field which is visible to the 
 
The Sense of Sight 
 
 177 
 
 other eye, and that there is a central area common to both 
 eyes, or a binocular visual field^ shaped as in Fig. 82. 
 This area bears a fixed form and magnitude, and from it 
 alone can rays of light enter both eyes. From each point 
 in this field the rays of light entering the eyes must, for a 
 given state of accommodation, fall upon the same points of 
 the retinas. To each point, then, in the binocular visual 
 field there is a corresponding point in each retina ; and, 
 again, the right side of the right retina corresponds point 
 for point with the right side of the left retina, and, similarly, 
 the left side of the right retina 
 corresponds with the left side 
 of the left retina. Thus it 
 follows that the upper halves 
 correspond, and likewise the 
 lower. The yellow spots 
 form corresponding areas, 
 I and when the images of a 
 small object formed upon 
 these, and projected outwards 
 by the mind upon the visual 
 field, coincide in position the 
 object is seen single. 
 
 I f, for example, the eyes are 
 so directed that the images 
 upon them of the point A (Fig. 83) are projected outwards 
 so that the lines of projection meet at A, we will see A as 
 one point, and any other point in its near vicinity, such as 
 B, will likewise be seen single, because its images are 
 formed upon corresponding points of the retinas. If we 
 describe a circle whose circumference passes through the 
 point of sight and the two optic centres, it may be mathe- 
 matically shown that rays from all points of this circle fall 
 upon corresponding points, and objects on it are seen 
 
 x 
 
 FIG. 83. Diagram of one form of 
 horopter. (Miiller.) 
 
I7 8 
 
 Physiology of the Senses 
 
 single. Muller called this circle the horopter \ and, for 
 different positions of the eyes, the horopter may assume 
 complicated forms, but in any horopter all points are seen 
 single. 
 
 We are now able to understand how a double image is 
 seen when objects not in the horopter are seen double. 
 Suppose in the case of looking at the pencils we represent 
 the nearer one by p (Fig. 84), the farther by p' . Then, 
 when the eyes are converged on p, the images of p' are not 
 
 FIG. 84. Diagram to illustrate formation of homonomous double images. 
 
 formed on corresponding points of the retinas, but are each 
 to the inner side of the yellow spot at bb\ and two faint 
 images of p' are seen, one on each side of, and at the same 
 distance from, the eyes as /, viz. for the left eye at <z, for the 
 right eye at a'. On shutting one or other eye, the image 
 on the same side disappears, and it is said to be homo- 
 nomous. But if the gaze be fixed upon p' (Fig. 85) a 
 double image of p, formed external to the yellow spot on 
 both eyes, is mentally projected outwards to the distance of 
 the plane da through /, and now on shutting one or other 
 
The Sense of Sight 
 
 179 
 
 eye the image on the opposite side disappears, and it is 
 hence said to be heteronomous. 
 
 Now, as a rule, we are not conscious of the formation on 
 the retina, nor does the mind project outwards this double 
 image. It is only by special attention to the action of 
 both eyes that we become conscious of it ; and, at a first 
 attempt, it is sometimes difficult to convince a person that a 
 double image is, as in the above experiment, visible. The 
 reason of this is, that attention is paid to the object directly 
 looked at and not to the fainter double images ; and also 
 
 FIG. 85. Diagram to illustrate formation of heteronomous double images. 
 
 because where we do try to see two objects at different 
 distances at one and the same time, the minds of most 
 people attend only to the image formed by the right eye 
 and disregard that of the left. Thus, if you tell a person 
 to point with the finger at a distant object, both eyes being 
 open, and then ask him, while holding the hand steadily, to 
 shut the right eye, he will seem to be pointing to the right 
 of the object, and not directly at it ; but if he shuts his left 
 eye he will seem to be pointing correctly. This applies 
 more especially to right-handed persons, the reverse being 
 
180 Physiology of the Senses 
 
 the case with those who are left-handed. By careful 
 observation, we can note the two images of the finger 
 pointing, and may bring the more distant object between 
 the images, and then, whether the right or left eye be shut, 
 the finger will not seem to be pointing directly at the dis- 
 tant point. Still another reason why we neglect double 
 images is that these are often so large as to overlap one 
 another, so as to be practically indistinguishable ; and the 
 effect of the two combined in a psychical process by the 
 mind is to lead to the perception of the third dimension in 
 space, or in other words, the perception of solidify. 
 
 Perception of Solidity. When we look at a solid 
 body the images formed in the two eyes are not exactly 
 the same, because the right and left eyes view it from 
 different standpoints. This can be best appreciated ^by 
 viewing some small object at no great distance from the 
 eye, e.g. a book. If we alternately examine the book with 
 the right and left eye, the other being meanwhile closed, 
 and compare mentally the appearances presented to the 
 two eyes, we observe that the right eye sees more of the 
 right side of the book, the left more of the left. If we then 
 note what area of background is hidden by the two images, 
 we find that the part hidden from the right eye by the book 
 is different from that for the left. Now, with both eyes 
 open, let vision be accommodated for the background, but 
 examine the effect produced by the interposition of the 
 book. We are then conscious of a solid opaque body 
 obscuring part of the background completely, while to 
 either side of this is a spectral transparent image of the 
 sides of the book through which the wall seems to be seen. 
 On shutting the left eye the solid body seems to move 
 to the left, rendering the left spectral part opaque, because 
 the part of the wall formerly seen by the left eye is no 
 longer visible, and similarly for the right. It will further 
 
The Sense of Sight 
 
 181 
 
 be noted, as we converge the eyes on the book, that the 
 spectral parts disappear, and we see the one solid body 
 only. Lastly, if we look at the book fixedly for some time, 
 one eye being shut, and then if we look with both eyes, it 
 is at once seen that the book stands out in much bolder 
 relief, the various sides and borders taking their natural 
 inclination in reference to space. A suitable object for the 
 study of this phenomenon is a truncated pyramid upon which 
 we look vertically downward. With both eyes open the 
 appearance presented is that seen in B (Fig. 86). Keeping 
 the head in the same position, but looking with the left eye 
 
 B 
 
 L 
 
 Z f 
 
 R 
 
 FIG. 86. Appearance, of a truncated pyramid seen from above with B, both eyes, 
 L, left eye, or R, right eye. 
 
 only, we will see the pyramid as in L, or with the right eye 
 only, as in R. 
 
 The Stereoscope. The combination of L and R, so 
 as to present the appearance of solidity to the eye, may be 
 made by the stereoscope, an instrument invented by Wheat- 
 stone, who first noticed that the perception of solidity was 
 due to the dissimilarity of the images presented to the 
 retinas. In its simplest form the reflecting stereoscope 
 consists of two mirrors placed at right angles to each other, 
 as in Fig. 87. The eyes, looking into these obliquely, see 
 reflections of the dissimilar figures R and L representing 
 the appearances as seen by each eye individually ; and 
 
1 82 Physiology of the Senses 
 
 the images, mentally projected backwards in the line of 
 vision, are combined at the point of intersection of the 
 optic axes, and we seem to see the single solid object as 
 we would if we were looking at it with both eyes. 
 
 Brewster's refracting stereoscope is the one in common 
 use. In this instrument the optical effect is obtained by 
 means of two lenses so arranged that rays of light passing 
 from the stereoscopic pictures impinge on the retina, and 
 are projected backwards so as to converge and meet at points 
 
 FIG. 87. Wheatstone's stereoscope. 
 
 behind the plane of the pictures, as in Fig. 88. Each eye 
 thus sees its own picture, but corresponding points are 
 brought to a focus, and in the union of all we have one 
 picture in relief. 
 
 The apparently differing distances from the eye of 
 different parts of the combined picture are due to the 
 differing distances between corresponding points of the 
 constituent pictures. Those pairs of points which are 
 nearest together stand out in highest relief, or in other 
 words, require the greatest convergence of the optic axes, 
 while those which are most distant from one another seem 
 
The Sense of Sight . 183 
 
 most remote in the combined picture. In Fig. 86, p. 181, 
 
 the points v ,' V/ J , are respectively at equal distances from 
 A , Y , L 
 
 one another, and consequently seem to be in the same 
 plane in B. Similarly 'f' -^ 2 fJ are at equal distances from 
 one another, and seem to be all in one plane, but the dis- 
 
 FIG. 88. Diagram illustrating the principle of Brewster's stereoscope. The 
 points x, x forming images x' ) x 1 are projected outwards and coincide at X ; 
 the points y, y, being nearer to one another than x, x, appear to coincide at a 
 point Y in a plane nearer to the eyes than X. (After Landois and Stirling.) 
 
 tance between any pair of these being less than the distance 
 
 XYZ 
 
 between any pair of the set ,C, ' , ,/ J, the plane xyz 
 
 A, Y , /,, 
 
 seems nearer than the plane XYZ. Hence the trun- 
 cated apex of the pyramid seems nearer the eye than the 
 base. But if we transpose R and L so that R is opposite 
 the left eye and L opposite the right, then the points 
 
 ',] , will respectively be farther from each other than 
 */, * 
 
 XYZ 
 
 Y ,' v ,' ', and we seem to be looking into a hollow pyramid, 
 
 * i * i *- i 
 
184 Physiology of the Senses 
 
 whose apex is directed away from us. In Fig. 88 the points 
 X, x, being farther apart than _y, j, are combined at X in a 
 plane behind that through Y, the point of combination of 
 
 JV>- 
 
 It is indeed unnecessary to have a stereoscope to get 
 
 the combined effect. If we merely fix the eyes upon the 
 diagram, but accommodate the vision for distance, we will 
 see the two diagrams apparently moving towards each 
 other and overlapping until they seem to coincide, when 
 suddenly the effect of a solid body between two faintly 
 visible flat diagrams is perceived. Ordinary stereoscopic 
 pictures are obtained by taking photographs of the same 
 scene from slightly different standpoints, corresponding to 
 the distance between the right and left eyes. These are 
 fixed to a card in their proper relationship to the right and 
 left eye ; and if reversed, they give an inverted picture, 
 all solid bodies seeming to be hollow. Even with the 
 pictures properly placed it is possible, by a simple arrange- 
 ment of lenses, as in the instrument called the pseudoscope, 
 to displace the picture so that our judgment of the size of 
 objects is disturbed by the apparent alteration in their 
 distance from us. 
 
 The Telestereoscope. The stereoscopic effect depend- 
 ing upon the distance between the eyes, it will naturally 
 be greater, the greater the distance. We cannot, indeed, 
 increase the distance between the eyes, although a small 
 solid body stands out in higher relief when near the eyes 
 than when far away, because the visual axes are more 
 convergent. But von Helmholtz has invented an ingenious 
 instrument by which the eyes are virtually separated and 
 a more powerful stereoscopic effect obtained. It is known 
 as the pseudoscope or telestereoscope, and the principle of its 
 construction is as follows. Two mirrors are placed parallel 
 and a little to the side of the mirrors used in Wheatstone's 
 
The Sense of Sight 185 
 
 stereoscope (Fig. 89). The rays from the object to the 
 outer mirrors are reflected to the inner mirrors, and thence 
 to the eyes. It thus happens that rays falling on mirrors 
 much more distant from each other than the eyes, enter 
 the eyes as if coming directly to them from the object. 
 We are thus able to see, as it were, more of the sides of 
 the body than we could under ordinary circumstances ; 
 distant objects seem to be brought nearer, judging by their 
 greater relief, and all parts of the field likewise stand out 
 in a more marked manner than usual. 
 
 In viewjng the different parts of a solid body, or the 
 apparently nearer and more remote parts of a stereoscopic 
 picture, there is a constant 
 movement of convergence or 
 divergence of the eyes, and 
 hence it was maintained that 
 a prime factor in the percep- 
 tion of solidity is the sense 
 of muscular effort required in 
 moving the eyes from point 
 
 to point. This theory, how- FlG * 89.-Telestereoscope. For 
 
 * ' explanation, see text. 
 
 ever, is negatived by the fact 
 
 that we have quite a correct perception of the spatial 
 relations of objects when seen by the instantaneous flash 
 of lightning, a flash which takes place so rapidly that 
 there is no time for all the complicated processes involved 
 in muscular action. Similarly, the stereoscopic effect is 
 seen when the picture is seen by the light of the electric 
 spark ; that is to say, in a time not exceeding the ^-\^$ 
 part of a second. But though the time of stimulation of 
 the retina is momentary, there is an appreciable time lost 
 in the physical change of the condition of the retina, in the 
 passage of the nerve current, in the arousing of sensation 
 and the fusion of the stimuli. Wheatstone held that, 
 
1 86 Physiology of the Senses 
 
 in the fusion of two images not mathematically similar, 
 the mind superadds the perception of solidity. If the 
 points in the two pictures are so far apart that the con- 
 verging apparatus is unable to bring them to a focus, we 
 only see two flat pictures. If the two pictures are exactly 
 similar, and their points may be exactly fused, the result 
 is a flat picture. The mental fusion is the cause of the 
 new sensation. The fusion in ordinary circumstances is to 
 all intents and purposes complete. The external world 
 presents itself to us with each object clearly single and 
 defined. It is only when we pay close attention and 
 carefully analyse our visual sensations that we can detect 
 the fact of incomplete fusion. 
 
 We have, for example, the sensation of luminosity. 
 When carefully examined this is found to be due to .the 
 irregular reflection of rays of light from the uneven surface 
 of a body ; calm water is non -luminous, rippling water 
 sparkles with light, but the amount of light going from the 
 broken surface to one eye differs from that going to the 
 other, and the effort at fusion of the darker and the lighter 
 gives rise to the sensation of luminosity. The combined 
 stereoscopic picture is luminous from the superposition of 
 darker and lighter spots in the one picture, or the reverse 
 in the other. And yet the fusion is incomplete when we 
 look into the matter closely. By an effort of will we can 
 allow the dark or the light to preponderate. Suppose 
 we have two stereoscopic pictures, as in Fig. 90, one 
 of which is light on a dark ground, the other dark on a 
 light ground, we can, by a voluntary effort, superpose the 
 one over the other and give rise to the impression of a 
 luminous solid body ; but we can also easily alter the 
 depth of the grayish luminosity by paying attention to the 
 dark or the light picture at will. 
 
 We have here, indeed, an analogy to the detection 
 
The Sense of Sight 187 
 
 by the ear of the elements of a compound tone. The 
 practised ear is able to separate and attend to any one 
 elementary tone, or, on a larger scale, to any individual 
 instrument in an orchestra ; and the mind may dwell only 
 on the harmonious fusion, experiencing a pleasure from the 
 combination, or it may give itself up at will to the effect of 
 one or of all. The process is easier with the ear than with 
 the eye. The optical fusion is more complete, more diffi- 
 cult to analyse. But it may be made easier if we endeavour 
 to fuse two surfaces of different colours in the stereoscope. 
 Here there js not complete mixing of the colours, but the 
 colour sensation is now that of one, now that of the other 
 
 FIG. go. Diagram to illustrate causation of sensation of luminosity. 
 
 colour, the varying effect being probably due to changes in 
 the activity of the two retinae. 
 
 Estimation of Distance. The foregoing considerations 
 on the perception of solidity will assist us in answering 
 the more general question as to the estimation of space or 
 distance. We have seen that the muscular effort at con- 
 vergence is greater for near than for remote objects, and 
 the greater the effort experienced the nearer do we judge 
 the object to be. But accompanying the effort at converg- 
 ence there is usually a muscular action of accommodation. 
 The pupil contracts to shut off divergent rays of light which 
 would cause blurring of the image,' and the ciliary muscle 
 contracts in order to lengthen the focal distance of the eye 
 
1 88 Physiology of the Senses 
 
 for the nearer object. Each of these muscular efforts must 
 add its quantum to the general sum of muscular sensation. 
 Objects at the point of sight are seen in clear detail, while 
 those which are nearer or farther off are seen indistinctly, 
 and we unconsciously judge of differing distances by varying 
 efforts of accommodation. The dimness of bodies within 
 the near point of vision is due to the impossibility of focuss- 
 ing the object. Far-distant objects are dimly seen because 
 of the aerial perspective. The atmosphere not being per- 
 fectly transparent and colourless, small details are blotted 
 out, and variety of colour lost in a bluish' haze. The dis- 
 tant parts of a landscape are conceived to be nearer and 
 smaller when seen in wet weather than in dry, for dust- 
 laden air gives a more marked aerial perspective than that 
 which has been washed by rain ; and again, in misty 
 weather the half-hidden forms of men may seem far away 
 and of supernatural size. 
 
 Again, varying convergence assists our estimation of 
 distance, not only through the muscular effort involved, 
 but also by variation of the angle of convergence of the 
 visual axes upon the object. For objects of similar size 
 it is evident that the angle of convergence must be greater 
 for near than for remote objects. We learn through the 
 other senses, as well as through sight, to know the com- 
 parative sizes of objects, and by noting and comparing the 
 apparent size of objects we arrive at a judgment as to their 
 distance, the seemingly smaller, of course, being considered 
 the more distant. Persons who have lost the use of one 
 eye, and therefore the valuable aid of convergence, cannot 
 judge accurately of the distance of near objects. If asked 
 to touch an object quickly they are apt to fall short, as ex- 
 perience tells them they may misjudge and strike it roughly 
 if they attempt to reach the full apparent distance. 
 
 Estimation of distance is likewise assisted by observation 
 
The Sense of Sight 189 
 
 of the distance of the background over which a body near 
 to the eye seems to move when the relative positions of the 
 eye and the body are changed. 
 
 In Fig. 91, I. the eye E moves while the body B is 
 stationary, in II. the body moves from B to B' while the 
 eye is stationary. The apparent distance moved by B upon 
 XY is only ab, while upon X'Y' it is the much larger 
 distance db'. The distance over which the body seems to 
 pass gives an indication of the relative distances of the 
 planes XY, X'Y' from the observer. 
 
 We are, also able to give a more accurate estimate 61 
 
 - ___ Y 1 
 
 lb 
 
 II 
 
 FIG. 91. Estimation of distance from change in relative position of the eye and 
 of an object observed. 
 
 the distance between two points when several objects 
 intervene. We take a series of mental leaps, as it were, 
 from point to point, the effort of which is greater than that 
 of passing over the whole distance at one effort. The dis- 
 tance between A and 13 (Fig. 92) seems greater than that 
 between B and C on N account 
 
 of the intervening dots, but it * * * * * o 
 
 is the same. Children often FlG 
 
 amuse themselves with the 
 
 following experiment. A boy, after looking at a landscape 
 in an erect posture, will turn, stoop down, and view it 
 between his legs, and all objects will seem farther off, as,, 
 from the unaccustomed posture and the proximity of the 
 
190 
 
 Physiology of the Senses 
 
 head to the ground, objects in the foreground, formerly dis- 
 regarded, are now more dwelt upon. Similarly, the sky 
 seems nearer us at the zenith than at the horizon, and a 
 landsman has great difficulty of judging distances at sea. 
 The eye projects the image of the object viewed outwards, 
 but if it be at any great distance, the lines of projection 
 from the two eyes are practically parallel, and judgments 
 as to size guide the judgment as to distance. It is interest- 
 ing to note, in this regard, that persons who have been born 
 blind and have by an operation gained the power of vision, 
 seem at first to see all objects close to the eye or almost 
 touching it they " see men as trees walking" and it is 
 only after a process of education in which the sense of 
 
 FIG. 93. a and b are of the same length, but b subtends a greater visual angle, 
 being nearer to the eye. 
 
 touch has much to do that they are able to form a proper 
 estimate of externality or distance through vision. 
 
 Estimation of Size. Closely connected with our esti- 
 mate of distance is that of size. This primarily depends 
 on the size of the retinal image, or in other words, of the 
 vistial angle subtended by the object. In Fig. 72, p. 146, 
 x is the visual angle subtended by the lines c, d, and e, and 
 since these objects make a retinal image of the same size it 
 is evident that, in estimating size, it is necessary to have at 
 least an approximate idea of the distance of the object from 
 the eye. The moon subtends a larger visual angle than the 
 stars because it is so much nearer to us, not because of its- 
 greater size. 
 
 We learn by experience, more especially by the com- 
 

 The $etl& of Sight 
 v 
 
 191 
 
 bination of touch and vision, > tifat-4f-t f oobjects of different 
 sizes subtend the same visual angle, the nearer of the two is 
 the smaller ; and the young artist measures the comparative 
 length and breadth of distant objects by holding his pencil 
 at arm's length between his eye and the thing sketched. 
 
 The degree of convergence of the visual axes is also of much 
 importance in the estimation of size. For by experience we 
 know that an object of known size will subtend a certain 
 visual angle at a given distance, and that the nearer the 
 object is to the eye the greater will be the angle subtended, 
 as in Fig. 93. Then, of all bodies which subtend the same 
 
 L 
 
 ABC 
 
 FIG. 94. 
 
 visual angle, that one must be the largest which requires the 
 least convergence. 
 
 Thus, too, the intervention of bodies of known size gives 
 an idea as to the size of the more remote object. The sun 
 seen on the horizon behind trees seems larger than when in 
 mid-heaven, because we have a better estimate of its dis- 
 tance and of the visual angle it should thus subtend. Few 
 people agree in their estimate of the apparent diameter of 
 the full moon, and in Fig. 94, B seems to have the greatest 
 height from a mental summation of the horizontal spaces, 
 A the greatest breadth, and C to be the smallest. Yet all 
 are of the same area. In this case the three figures are of 
 the same size, and must give rise to retinal images of the 
 same size, but the basis on which we form our judgment as 
 
192 
 
 Physiology of the Senses 
 
 to the area of each being different, we judge them of 
 different size. The judgment errs, not the organs of 
 vision. 
 
 This error of judgment is perhaps even more marked in 
 the case of Fig. 95, where the line A seems longer than B, 
 although in reality of the same length. In A there is 
 
 B 
 
 
 FIG. ( 
 
 insensibly divergence of the optic axes, in B there is con- 
 vergence, owing to the oblique lines. 
 
 The illusion is somewhat different, but it is also marked 
 in Fig. 96, known as Zb'llner's lines. Here the oblique 
 lines seem to converge oniHcnauHmBpEn!raHB2pM| 
 towards one another, 
 though really parallel. 
 The unconscious ten- 
 dency to follow the 
 short lines till they 
 would intersect leads 
 to an impression that 
 the oblique lines would 
 meet if produced in 
 the opposite direction. 
 
 Allied to this illu- 
 sion of vision is that 
 
 FIG. 96. Zollner's lines. 
 
 In Fig. 97, 
 
 produced by drawing 
 
 a thin line to intersect a broad line obliquely. 
 
 EF, not CD, is in the same straight line as AB. 
 
 Illusions of Vision also arise when we look for a short 
 time at a body in motion and then turn our eyes upon 
 one at rest. It seems to move in the opposite direction, 
 
The Sense of Sight 193 
 
 whether that has been one of rotation or of movement in a 
 straight line. Thus if we gaze for about a minute at a wheel 
 revolving rapidly on a fixed axis, and then turn our eyes 
 to the ground, a similar area seems to rotate in an opposite 
 direction round the centre of vision. Similarly, as stated 
 on p. 155, when upon the deck of a ship in motion, if we 
 look for a time on the water and then at the deck, some 
 of the boards seem to creep forwards relatively to those 
 adjoining them. In looking at the water we instinctively 
 try to fix our eyes upon points in the seemingly moving 
 surface, and so the eyes have a backward movement. Owing 
 to the persistence on 
 the retina of visual 
 impressions, we con- 
 tinue unconsciously to \ ^ 
 
 seek back towards the j 
 previously vanishing 
 
 point ; and in doing 
 
 \ s \. E 
 so the new image ZA 
 
 created by the body, FlG> 97 ._ For description, see text, 
 
 stationary with regard 
 
 to ourselves, seems to be that of a body in motion in the 
 opposite direction. 
 
 Vision assists in fo& -perception of motion mainly by the 
 change of position of the retinal image of the moving body, 
 relatively to the fixed position of the image of the rest of 
 the visual field. If the eyes follow the moving body, then 
 its image is fixed on the retina, while the rest of the visual 
 field changes its position.- By the rapidity of movement of 
 eye, head, or body, we judge as to the rate of movement of 
 the object. We can form no idea, through vision, as to the 
 direction of motion, unless we have this relative movement 
 of the various parts of the field. Sitting in a railway train 
 in motion, there is a change of position of near objects as 
 
 O 
 
194 Physiology of 'the Senses 
 
 regards ourselves and the background, which is so rapid 
 that we almost imagine them to be in motion. If another 
 train passes us going in the opposite direction, it seems to 
 be going with great velocity, because we assume the com- 
 partment in which we sit to be stationary, and the velocity 
 of our own movement is added to that of the other train. 
 Similarly, if two trains are standing side by side at a station, 
 and the one adjoining us begins to move, we imagine that 
 it is the train in which we sit that is moving in the opposite 
 direction, because we are by habit led to believe that the 
 station with all its contents is fixed, while our train is the 
 only movable body. We can thus enjoy the sensation of 
 somewhat rapid motion without the jarring that usually 
 accompanies railway travelling, until, the other train having 
 swept past, we see the sides of the station beyond silent 
 and motionless ; and immediately we are brought to rest 
 by a more smoothly working brake than has yet come into 
 general use. 
 
 Our notions of the form of objects are based partly on 
 the fusion of stimuli of different parts of the retina, giving 
 rise to a sense of continuity, and partly from movements of 
 the eyes from point to point. The body may be a plane 
 figure in which, owing to the mode of construction, we may 
 at will imagine different shapes to be represented. Fig. 98, 
 for example, may be conceived at will to represent either 
 "a staircase against a wall, or an overhanging portion of a 
 wall, the lower part of which has been removed, and whose 
 under surface has taken the form of steps." J In the former 
 case, we regard ab as running backwards from a, the nearer 
 point ; in the latter, we suppose b to be the nearer point, 
 and a the more remote, and run the eye along ab in the 
 direction from b to a. 
 
 In the perception of solidity of bodies, the possession, as 
 1 Bernstein, The Five Senses, p. 160. 
 
The Sense of Sight 195 
 
 we have seen, of binocular vision is of marked advantage. 
 The movements of accommodation and convergence, the 
 wider movements of the whole 
 eye from point to point and 
 from plane to plane, the play 
 of light and shade, the relation 
 to surrounding bodies all 
 these are factors which in- 
 fluence the mind in its judg- 
 ment as to solidity. Nay, 
 further, in .certain disordered 
 conditions of the brain, old 
 impressions may be renewed 
 and recombined, and the 
 surrounding space becomes 
 peopled with fantastic forms, 
 lovely or terrible, according 
 to mood forms as real and 
 substantial to the disturbed 
 
 FIG. 98. For description, see text. 
 (After Bernstein.) 
 
 mind as those which appear in ordinary vision. How 
 forcibly has this been painted in the dagger scene in 
 Macbeth 
 
 Is this a dagger which I see before me, 
 
 The handle toward my hand ? Come, let me clutch thee. 
 
 I have thee not, and yet I see thee still. 
 
 Art thou not, fatal vision, sensible 
 
 To feeling as to sight ? or art ikon but 
 
 A dagger of the mind, a false creation, 
 
 Proceeding from the heat -oppressed brain ? 
 
 I see thee yet, in form as palpable 
 
 As this which now I draw. 
 
 Thou marshall' st me the way that I was going ; 
 
 And such an instrument I was to use. 
 
 Mine eyes are made the fools o* the other senses, 
 
 Or else worth all the rest ; I see thee still, 
 
196 Physiology of the Senses 
 
 And on thy blade and dudgeon gouts of blood, 
 Which was not so before. There's no such thing ; 
 It is the bloody business which informs 
 Thus to mine eyes. 1 
 
 And as the perturbed mind may wander in an illusory 
 world of its own, so the abstracted mind may disregard the 
 promptings of sense. The eye is open, the image is painted 
 on the retina, and the nerve currents pass to the visual 
 centre ; but the centre is preoccupied, the mind goes on 
 its own way, the vision is unheeded. Such is the con- 
 dition with the somnambulist. He rises and walks in his 
 sleep ; his eyes are open, but he sees only that which fits 
 in with his dream. So it is with the mesmerised man. His 
 mind, otherwise a blank, is moved this way and that "at the 
 suggestion of the operator, and in a stick he sees a hissing 
 serpent, or an empty table becomes covered with choicest 
 viands. 
 
 Again, as vision is only possible so far as the visual 
 apparatus is perfect, and since we find the organ of vision 
 in every stage of advancement, from the colour spot of the 
 invertebrate up to the complete binocular vision of man, so 
 we may infer that the higher intelligence of man is intimately 
 associated with the perfection of the eye. Crystalline in its 
 transparency, sensitive in receptivity, delicate in its adjust- 
 ments, quick in its motions, the eye is a fitting servant for 
 the eager soul, and, at times, the truest interpreter between 
 man and man of the spirit's inmost workings. The rain- 
 bow's vivid hues and the pallor of the lily, the fair crea- 
 tions of art and the glance of mutual affection, all are 
 
 1 Macbeth, Act II. Scene i. In this scene, also, the great dramatist 
 pictures, with profound psychological insight, the struggle between the 
 delusions of the mind, as projected into space, and their correct appre- 
 ciation by the reasoning faculties. The words indicating the applica- 
 tion of the reason are printed in italics. 
 
The Sense of Sight 197 
 
 pictured in its translucent depths, and transformed and 
 glorified by the mind within. Banish vision, and the 
 material universe shrinks for us to that which we may 
 touch ; sight alone sets us free to pierce the limitless abyss 
 of space. 
 
SOUND AND HEARING 
 
 THE organ of hearing is the ear ; but the human ear is a 
 much more complicated apparatus than most people suppose. 
 The really sensitive part of the ear, the part in which the 
 auditory nerve terminates, and where physical give rise to 
 physiological changes, is buried deep out of sight in the 
 bones of the cranium, and the external ear, that which is 
 seen upon the outside of the head, forms a part only of an 
 elaborate arrangement whereby sound waves may be trans- 
 mitted inwards to the true end organ of hearing. But while 
 this is the case in man, in many of the lower organisms we 
 find an ear which closely resembles the human ear in prin- 
 ciple, though much simplified in detail, and situated upon or 
 immediately below the surface. In its simplest form the 
 ear consists of a set of cells to which we find attached 
 delicate hairs or rod-like structures, which are thrown into 
 vibration by sound waves. These cells are connected, or 
 are in apposition, with the terminal fibrils of the auditory 
 nerve ; and when agitated by sound they produce a nerve 
 impulse, which in turn excites the central nerve cells, 
 and sound is heard. The first step in complexity of organ- 
 isation of the ear is that the hair-cells are no longer on the 
 free surface, but line in part a membranous sac containing 
 fluid, the cells having sunk down into the substance of the 
 animal's body, and being thus better protected from injury 
 
Sound and Hearing 
 
 199 
 
 (Fig. 99). The sac may be of a simple globular shape, or, 
 in highly developed animals, it may assume a very com- 
 plicated form ; so much is this the case in man, that it is 
 known as the membranous labyrinth. The structure of the 
 labyrinth is, as we shall see, of a most delicate and elaborate 
 nature, and though in the embryonic condition it is near 
 the surface of the head, in the adult it is at least i^ inch 
 from the surface, and enclosed in bone so hard that it is 
 called the petrous or stony bone. The osseous covering 
 coincides to a great extent with the membranous bag inside, 
 but a small amount of 
 
 fluid separates the sac j J/^^V ---d r 
 
 from its walls, and 
 protects it from rude 
 shocks transmitted 
 through the bone. 
 The auditory cells are 
 situated in certain 
 parts of this sac, and 
 the auditory nerve 
 passes to them through 
 channels in the bone. 
 There are also two 
 
 openings by which changes of pressure may be transmitted 
 from without to the fluid surrounding, and that contained by, 
 the membranous labyrinth. But these openings cannot be 
 seen from the outside. They communicate with a chamber 
 known as the middle part of the ear, or simply the middle ear, 
 or tympanum, or drum, a chamber containing air and opening 
 by a tube passing forwards and inwards into the throat 
 the Eustachian tube. The middle ear is separated from the 
 passage leading to the auricle, or visible ear, by a mem- 
 brane, known as the membrana tympani (or drtim-head), 
 which vibrates in response to sounds, and whose move- 
 
 FIG. 99. Auditory vesicle of Phiahdium. d\, d^ 
 Epithelium covering the sac ; h, auditory cells, 
 with hh auditory hairs ; #/, nervous cushion for 
 the auditory cells, connected with nr\, the lower 
 nerve ring. (Hertwig and Lankester.) 
 
200 Physiology of the Senses 
 
 ments are communicated to a chain of bones, and by this 
 chain to the inner ear. The membrana tympani closes the 
 passage leading inwards from the outer ear or auricle. There 
 are thus an outer and middle ear for the collection and 
 transmission of sounds, and an inner ear for their reception as 
 stimuli of sensation. By this arrangement the ear becomes 
 more sensitive, for the middle ear acts as a drum giving 
 resonance and strength to delicate sounds (Fig. 101). 
 
 In order to obtain a complete understanding of the 
 manner in which sound affects the ear, we must consider 
 carefully the structure of the ear, and how it is fitted to 
 respond to sonorous vibrations. 
 
 i. External Ear. The shape of the external ear varies 
 to a remarkable degree, and in some cf the lower forms of 
 vertebrates if may be entirely absent. In the frog, for 
 example, there is no external ear, the tympanic membrane 
 being seen as a disc on a level, and continuous with the 
 skin of the head. In birds, again, the auricle is absent, 
 but there is an external auditory canal or meatus leading 
 down to a membrana tympani. The middle and internal 
 ears are more highly developed in birds than in reptiles, 
 but still fall far short of the human ear in complexity. 
 In mammals, the auricle is of very varied size and shape, 
 and it may be either stiff and erect from the presence of an 
 elastic cartilage, as in the ear of the horse or man, or it 
 may be soft and yielding, as in the elephant. The surface is 
 usually convoluted and funnel or trumpet shaped, so as to 
 gather the waves of sound to the best advantage, and many 
 animals have the power of moving the opening of the auricle, 
 by means of voluntary muscles, in the direction from which 
 the sound comes. Thus the horse pricks up its ears when 
 it hears a sound, and no doubt its appreciation of the direc- 
 tion of sounds is thereby assisted. In the human ear there 
 are similar voluntary muscles, but man has, for the most 
 
Sound and Hearing 
 
 part, ceased to have the power of moving" the auricle in 
 response to sounds from varying sources apart from move- 
 ments of the head as a whole. No doubt, by attention and 
 practice, a man may -acquire the power of moving the 
 auricle slightly, and the great German physiologist, Miiller, 
 was proud of being able to do so. But, at best, these 
 movements are small as compared with those of the lower 
 animals. Special names have been given to the various 
 depressions and protuberances of the 
 auricle (for which see description of 
 Fig. 100). . 
 
 If we pass the ringer round the 
 border of the ear we will feel near 
 the upper part a small nodule, which 
 is interesting, according to the com- 
 parative anatomists, as being homo- 
 logous with the tip of the pointed ear 
 of many animals. 
 
 The general effect of the con- 
 volutions of the surface of the auricle 
 
 is to collect and transmit to the ex- FIG. 100. Outer surface of the 
 ternal auditory canal, and that to the ri s ht auricle, i, Helix ; 2 , 
 
 fossa of helix ; 3, antihelix ; 
 
 best advantage, sound waves falling 4i fossa of the ^heiix ; 
 upon the surface of the ear. For 5, antitragus ; 6, tragus ; 7 , 
 
 ... ... concha ; 8, lobule. (Arnold.) 
 
 just as waves of light falling upon a 
 
 transparent body are partly reflected and partly trans- 
 mitted, so sound waves striking the auricle are partly 
 concerned in giving rise to corresponding vibrations in 
 the substance of the auricle, and partly reflected, and the 
 more the waves are sent to the inner ear the more intense 
 will be the sound. The phenomenon familiar to every 
 one, of the echo, is an example of this reflection of sound 
 on a large scale in nature. We hear first the sounds trans- 
 mitted directly to the ear, then those reflected from more 
 
202 Physiology of the Senses 
 
 or less distant bodies. In the whispering gallery of St. 
 Paul's Cathedral in London, or in the ducal mausoleum at 
 Hamilton, faint sounds can be heard at a considerable dis- 
 tance from the point at which they originate, as they are 
 reflected in such a way as to be focussed at a special point. 
 So the shape of the auricle, by focussing sounds, helps the ear 
 to hear sounds of low intensity. It would appear also that 
 the form and size of the depressions of the concha strengthen 
 tones of very high pitch, such as occur in hissing sounds, 
 like the noise of waves breaking on a shingle beach, or that 
 of a waterfall. Thus a very slight change in these depres- 
 sions will affect the musical quality of tones. If the irregu- 
 larities of the surface are filled with wax, sounds are not 
 heard so loudly, and, conversely, we increase our receptivity 
 by putting the hand to the ear, and turning the head side- 
 ways to the sound. If the auricle is entirely removed, 
 hearing is, however, but little diminished. The collecting 
 power of the auricles assists in the determination of the 
 direction from which a sound comes ; the sound being more 
 loudly heard in one ear than the other, we conclude that it 
 comes towards that side of the head on which the louder 
 sound is heard. 
 
 2. Meatus or Passage. From the pinna or auricle, the 
 external auditory meatus^ or passage to the middle ear, 
 passes inwards and slightly forwards, being inclined at 
 first upwards and then bending downwards. The passage 
 is almost circular in cross section, but the outer end is 
 flattened a little from before backwards, while the inner 
 part is broadest in the horizontal plane. The meatus is 
 closed internally by the tympanic membrane^ or drum-head 
 (see Fig. 101, 17), which lies obliquely to the direction of 
 the lumen of the tube, the lower margin being farther in 
 than the upper, and the floor of the passage is thus longer 
 than the roof. 
 
Sound and Hearing 
 
 203 
 
 The wall of the outer part of the meatus consists of 
 cartilage which is continuous with that of the auricle, but 
 round the deeper part of the tube the cartilage is absent, 
 and the lining of skin which passes inwards from without 
 is in close contact with the bone through which the tube 
 
 FIG. 101. Diagram of the ear ; natural size, i, Auditory nerve ; 2, internal audi- 
 tory meatus closed by the cribriform plate of bone through the perforations of 
 which the branches of the auditory nerve pass to the ear ; 3-8, membranous 
 labyrinth composed of 3, utricle, 4, semicircular canals, 5, saccule, 6, duct 
 of the cochlea (the coils not entirely shown), 7, endolymphatic duct with, 8, its 
 saccule lying inside of the cranial cavity ; 9, lymphatic space surrounding 
 the membranous labyrinth ; 10, osseous labyrinth of compact bone lying in 
 the more spongy substance of the petrous bone, n, n ; 12, the oval window, 
 filled by the foot-plate of the stirrup-bone ; 13, the round window, across 
 which is stretched the internal tympanic membrane ; 14, auricle ; 15, 16, 
 external auditory meatus ; 15, its cartilaginous, and, 16, its bony part ; 17, 
 tympanic membrane; 18-20, auditory ossicles; 18, hammer; 19, anvil; 20, 
 stirrup ; 21, middle ear ; 22, osseous, and, 23, cartilaginous portion of the 
 Eustachian tube ; 24, cartilages of external auditory meatus. (Schwalbe.) 
 
 passes. Towards the inner part of the meatus the skin 
 is very thin, and this is especially the case where it is con- 
 tinued as an epidermic covering over the fibrous tympanic 
 membrane. At the outer part the skin is thicker, and from 
 it spring fine hairs slanting outwards. It is well lubricated 
 
204 Physiology of the Senses 
 
 by numerous small glands, of the nature of sweat glands 
 much modified, which secrete a waxy substance known as 
 cerumen. This material has a brownish colour and a bitter 
 taste. The form of the canal is such as to facilitate the 
 passage outwards of the wax, but sometimes it may accumu- 
 late in such quantity -as to diminish the power of hearing to 
 a considerable extent. If this should happen, a sharp hard 
 instrument should not be employed for its removal, as much 
 injury might thereby be inflicted upon the tympanic mem- 
 brane. It is better to soften the wax with an alkaline or 
 oily fluid, and then to syringe the meatus gently to remove 
 the debris. The outward -pointing hairs and the bitter 
 adhesive wax form together a valuable guard against the 
 entrance of foreign bodies, animate or inanimate, into the 
 cavity of the meatus, a provision similar to what we find in 
 many flowers to prevent the store of honey from being 
 plundered by marauding insects. 
 
 3. The Middle Ear. The middle ear, drum^ or tym- 
 panum is, in the adult, about an inch and a quarter from 
 the free surface, and is thus embedded deeply in the sub- 
 stance of the temporal bone. Across this space passes the 
 chain of bones from the drum-head to the internal ear, by 
 means of which the movements of the membrane are trans- 
 mitted to the labyrinth and variations of pressure effected. 
 It receives air at atmospheric pressure through the Eus- 
 tachian tube. The cavity is irregularly wedge-shaped, being 
 wider at the top than at the bottom, and is larger from 
 before backwards than from side to side. It is separated 
 from the cranial cavity above by a thin layer of hard bone, 
 and communicates behind with a set of spaces, which also 
 contain air, lying in the part of the bone which can be felt 
 as a prominence behind the external ear, and known as the 
 mastoid process. The outer boundary of the middle ear is 
 largelv composed of the tympanic membrane, although it is 
 
Sound and Hearing 
 
 205 
 
 1 
 
 to be noted that the cavity extends upwards into the bone 
 above the membrane, while in front of the membrane is a 
 fissure in the bone, known as thefosure of Glascr, from its 
 discoverer, through which pass a nerve (the chorda tym- 
 pant) and a muscle (the laxator tympani), and in which, 
 as in a socket, is fixed one of the processes by which the 
 chain of bones is suspended. The membrane itself is 
 firmly fixed in a groove, 
 which can be readily seen 
 in a macerated bone with 
 the naked eye, and, though 
 very thin and semi-trans- 
 parent, it consists of firm 
 fibrous tissue lined on one 
 side by skin, on the other by 
 mucous membrane. Fig. 
 102 represents the appear- 
 ance of the tympanic mem- 
 brane of the left ear as seen 
 from without, and Fig. 101 
 shows how it is inclined 
 obliquely to the axis of the 
 meatus, both transversely, 
 and from above downwards. 
 The fibres of the mem- 
 brane consist of ordinary 
 connective, and a very 
 small amount of elastic, tissue, and are disposed in a two- 
 fold manner, some of them radiating from a point, the 
 iimbo, slightly below the centre of the membrane to the 
 circumference, while others are arranged concentrically 
 around the same point. The outer surface of the membrane 
 is covered by a very thin layer of skin, while its inner tym- 
 panic surface is lined by ciliated epithelium. The first of 
 
 FIG. 102. Left tympanic membrane show- 
 ing the arrangement of its fibres, a 
 anterior, b posterior border; i, flaccid 
 part of the membrane ; 2, short process 
 of the malleus ; 3, umbo of the mem- 
 brane ; between 2 and 3, the handle of 
 the malleus ; 4, anterior and, 5, posterior 
 end of the tympanic groove, between 
 which are seen circular fibres attached 
 to the short process, 2. (Schwalbe.) 
 
206 
 
 Physiology of the Senses 
 
 the chain of bones is firmly attached to the fibrous part of 
 the membrane in such a way that the central part of the 
 membrane is drawn inwards towards the tympanum, form- 
 ing the umbo (or boss of a shield), 
 and thus the disc is not flat, but 
 slightly conical, and, owing to the 
 circular fibres, the surface towards 
 the meatus is convex. This cur- 
 vature of the membrane, though 
 slight, is of considerable import- 
 ance in connection with the re- 
 sponse of the membrane to sonorous 
 vibrations. The sound waves fall 
 FIG. 103. Horizontal section on the convex surfaces of the 
 
 through the labyrinth, tym- .. . _ 
 
 panum |a ndpartoftheexternal radiating fibres. These keep .the 
 auditory meatus of the left ear. membrane stretched tightly, except 
 
 Between ^and e, the tympanic , f 
 
 membrane, in the centre of at the f re and u PP er P art > where 
 
 which is seen the handle of the the grOOVC of attachment is de- 
 
 malleus cut across ; e, anterior r j ,1 i i 
 
 wall of the tympanum;/, ficient, and the membrane is looser, 
 in the tympanum above the thicker, and more freely supplied 
 
 .> j i i j i 
 
 with nerves and blood-vessels. 
 
 stapes, whose base is inserted 
 
 7 . 
 
 into the fenestra ovahs ; g, 
 
 the stapcdius muscle ; A, por- 
 tion of facial nerve ; /, tensor 
 
 tympani muscle ; X,*, vestibula 
 
 The inner wall of the tympanum, 
 Q [iG ihe mem brane, is irregular 
 
 x 
 
 division, and, /, cochlear divi- in shape, and perforated by two 
 
 sion of the auditory nerve apertures> The upper of these the 
 lying in the internal auditory 
 
 meatus ; vi, cochlea ; n, nerve fenestra ovalis^ or oval window, is 
 
 going to ampullae of ^serni- of an Qvoid Qr kidney shape and 
 circular canals ; o, section of 
 
 utricle ; /, section of sac- has the inner end of the ossicles 
 
 Q f the ear f aste ned into it by 
 
 means of a ligamentous tissue. 
 The fenestra ovalis opens from the middle ear into the 
 vestibule of the labyrinth. Lower down there is a smaller 
 and more rounded aperture, the fenestra rotunda, or round 
 window^ leading into the front part of the labyrinth, 
 
 culejr, section of semicircular 
 canals. (Riidmger.) 
 
Sound and Hearing 
 
 207 
 
 known as the cochlea, but closed during life by a thin mem- 
 brane like the membrana tympani that is to say, com- 
 posed of fibrous structure, with an epithelial lining lipon 
 either side, and having a slight concavity towards the 
 tympanum (Fig. 101, 13). 
 
 Between and in front of the above-mentioned apertures 
 is a rounded elevation called the promontory, which corre- 
 sponds to the first 
 turn of the cochlea 
 (p. 228). Behind the 
 oval window is a 
 very small process 
 of bone perforated 
 to allow the passage 
 of a minute tendon, 
 which gives attach- 
 ment to the stapes 
 (p. 2 1 1) of a small 
 muscle, the stape- 
 dius, the belly of 
 which lies in a space 
 behind the tym- 
 panum (Fig. 103,?). 
 
 The passage 
 leading away from 
 the front of the tym- 
 panum is divided into two parts by a little ledge of bone, 
 known as the processes cochleariformis, the upper part con- 
 taining the fleshy part of a muscle, the tensor tympani, 
 whose tendon crosses the tympanum to be inserted into 
 the malleus, the lower going forwards as the Eustachian 
 tube (Fig. 103, z). 
 
 The Eustachian Tube. The mucous membrane of the 
 Eustachian tube is continuous behind with that of the 
 
 FIG. 104. Incus and malleus of the right side seen 
 in their natural position in the tympanum, i, 
 Tympanic membrane ; 2, Eustachian tube ; 3, 
 tensor tympani muscle seen attached to the 
 malleus ; 4, anterior ligament of the malleus 
 attached to the processus gracilis ; 5, superior 
 ligament of the malleus ; 6, chorda tympani nerve ; 
 a, b, c, sinuses or spaces connected with the 
 tympanum in which the ossicles move freely. 
 (Schwalbe.) 
 
208 Physiology of the Senses 
 
 tympanum, in front with the pharynx or upper part of the 
 throat. When, under certain conditions, this mucous mem- 
 brane becomes swollen, the lumen of the tube may be 
 blocked, and air does not pass readily to and fro between 
 the throat and the middle ear. Then the pressures upon 
 opposite sides of the membrane becoming different, the 
 membrane is too much stretched, does not respond so well 
 as usual to sonorous vibrations, and one becomes slightly 
 deaf. It is commonly held that the Eustachian tube is 
 open only during swallowing, and the positive and negative 
 experiments of Valsalva are brought forward in proof of 
 this. The positive experiment is performed as follows : 
 Close the mouth and nostrils, and then, while making the 
 movements of a forced expiration, swallow. The air in the 
 pharynx is at more than atmospheric pressure, but does 
 not force its way into the tympanum until the tube is 
 opened during swallowing. Then the condensed air pene- 
 trates into the middle ear, raises the pressure there, and 
 the drum-head is forced slightly outwards ami made more 
 taut. The tightening of the membrane gives rise to a peculiar 
 sensation referred to the region of the ears, and similar to 
 what is sometimes felt after yawning. 
 
 We may directly observe this movement by inspection 
 of the membrane during the act. The principle of the 
 negative experiment is much the same. Instead, however, 
 of making a forced expiration, we close the mouth and 
 nostrils, raise the chest as in forced inspiration, and swallow. 
 The air in the throat being at less than atmospheric press- 
 ure, when the Eustachian tube is opened the pressure in 
 the middle ear is reduced, and the tympanic membrane 
 moves inwards by the atmospheric pressure in the meatus. 
 We have also met with a gentleman who had the voluntary 
 control of trie tube, so that he could open or close it at 
 pleasure. The advantage of having the tube closed at all 
 
Sound and Hearing 
 
 209 
 
 times, except when we swallow, lies in this, that were it 
 always open there would be too much reverberation caused 
 in our ears by the sound of our own voice. This, however, 
 cannot affect the ears during swallowing, because then the 
 lower part of the pharynx is cut off from the openings to 
 the nose and ears by the meeting of opposite muscles, and 
 the lifting of the uvula and soft palate. From all this it 
 follows that one, and probably the most 
 important, function of the Eustachian 
 tube is to equalise atmospheric pressure 
 on the two sides of the drum-head. 
 
 The Chain of Bones. Across the 
 cavity of the tympanum stretches the 
 chain of little bones or ossicles (Fig. 
 104), to which frequent reference has 
 already been made. This corresponds 
 to the single bone in the frog's ear, which 
 stretches from the tympanic membrane 
 to the entrance to the inner ear, but, as 
 we shall see, the chain confers consider- 
 able mechanical advantage. It consists 
 from without inwards of the malleus or 
 hammer bone, the incus or anvil bone, 
 and the stapes or stirrup bone. 
 
 The body or head of the malleus 
 (Fig. 105) is situated above the level of 
 the tympanic membrane, and it gives off 
 downwards a comparatively strong process, the handle of 
 the hammer, which is firmly affixed to the fibrous layer of 
 the membrane. And just as a flattened beam will bear 
 a greater downward pressure when placed edgewise than 
 when laid flat, so the handle of the malleus, being flattened, 
 is placed edgewise towards the tympanic membrane, thus 
 combining lightness with power. Another process, the pro- 
 
 P 
 
 FIG. 105. The malleus 
 or hammer bone seen 
 from in front, i, The 
 head ; 2, the processus 
 gracilis foreshortened ; 
 
 3, the short process 
 
 4, the manubrium in - 
 serted into the tym- 
 panic membrane. The 
 surface of the joint 
 with the incus is not 
 seen, as it faces back- 
 wards. (Schwalbe.) 
 
210 Physiology of the Senses 
 
 cessus gracilis more slender and elongated than the handle, 
 passes forwards from the junction of the head with the 
 handle, and is firmly fixed by ligaments to the little fissure 
 in the bone in front of the tympanic membrane. This pro- 
 cess is of interest as constituting one end of the axis upon 
 which the chain of bones rotates. The head of the malleus 
 is rounded, and attached to the roof of the tympanum by a 
 small ligament. It bears upon its pos- 
 terior aspect a smooth surface for arti- 
 culation with the incus. The head is 
 connected with the handle by a con- 
 stricted neck, immediately below which 
 we find, on the inner side of the handle, 
 the point of attachment of the tensor 
 tympani muscle, and on the outer part 
 FIG. io6.-*ight incus a sma11 bon Y prominence which, "im- 
 or anvil bone, x 4. i, pinging upon the tympanic membrane, 
 
 Body ; 2, joint surface . . 
 
 for malleus; 3 , projec- causes a projection outwards of the 
 tion that locks with membrane at that point. The laxator 
 
 malleus to prevent over . , , . , r 
 
 movement ; 4 , short tympani muscle passes backwards from 
 
 process for posterior the fissure of Glaser, to be attached to 
 
 b"" ;TelHpic the neck of the malleus, just above the 
 
 on median side of short origin of the processus gracilis. 
 process ; 6, long pro- Th . anvil-shaped bone (Fig. 
 
 cess ending in lenti- 
 cular knob ; 7, entrance 1 06), lies behind the malleus, and is 
 of nutrient blood- j ointed t it b saddle-shaped surface. 
 
 vessel. (Schwalbe.) J ' 
 
 A short process, pointing backwards, and 
 fixed to the posterior wall of the tympanum by ligaments, 
 forms the posterior end of the axis of rotation of the chain of 
 bones. A longer process, corresponding to the conical pro- 
 jection of an anvil, points almost vertically downwards, but, 
 at its lower extremity, bends inwards and ends as a little 
 flattened knob, the lenticular process, which in early life is 
 a separate bone, the lenticular bone. A small eminence, 
 
Sound and Hearing 211 
 
 immediately below the surface of articulation with the 
 malleus, should be noted, as it fits into a corresponding 
 depression in the malleus and prevents undue rotation. 
 
 The stapes^ or stirrup-shaped bone (Fig. 107), is fixed in 
 a horizontal plane, and at right angles to the descending- 
 process of the incus. The head of the stirrup is jointed to 
 the lenticular process of the incus. Inwards from the head 
 is a slight constriction, the neck, and from this arise the 
 two arms of the stirrup. These are fixed at their inner 
 end into an oval -shaped plate of bone, the base of the 
 stirrup, which again fits into the oval 
 window. The stirrup could move out- 
 wards and inwards freely but for the firm 
 short fibres which unite its base to the 
 margins of the aperture. The space be- 
 tween the arms is filled during life by a 
 
 FIG. 107. Stapes or 
 
 thin membrane, the arms being grooved stirrup bone seen 
 to receive it. By this arrangement, light- from above > x 4- *, 
 
 Base ; 2 anterior, 
 
 ness and strength are secured in the same 3 posterior limb; 
 way as we make wheels with spokes instead 4 head ; 5, neck ; 
 
 6, groove into which 
 
 of solid discs. The tendon of the stape- memb rane is fixed 
 
 dius muscle is attached to the back part of which fills the open- 
 ing. (Schwalbe.) 
 the neck of the stapes. 
 
 Movements of the Bones. The malleus and incus 
 rotate almost as one bone on a horizontal axis, passing fore 
 and aft between the attachments of the slender process of 
 the malleus in front, and the short process of "the incus 
 behind. The plane of rotation is consequently at right 
 angles to that of the tympanic membrane, or across the 
 cavity of the tympanum. When, then, the handle of the 
 malleus is pushed inwards towards the mesial plane of the 
 head, the head of the malleus moves outwards, carrying 
 with it the body of the incus, any excess of movement being 
 prevented by the suspensory ligament of the malleus. The 
 
212 Physiology of the Senses 
 
 body of the incus rotating outwards, its descending process 
 moves inwards synchronously with, and parallel to, the 
 handle of the malleus, and the tip of the process is thus 
 moved inwards and slightly upwards, and pushes the base 
 of the stapes into the fenestra ovalis. There is also a slight 
 rotation of the stapes in a vertical plane, and the upper 
 border of the base of the stapes has a somewhat greater 
 movement than the under side. We see, then, that when, 
 by compression of the air in the external meatus, the tym- 
 panic membrane is forced inwards^ the base of the stapes 
 will also be forced inwards, and the pressure on the internal 
 ear will be increased. 
 
 Again, when the air of the external meatus is rarefied, 
 and the pressure on the inner side of the membrane becomes 
 greater than on the outside, the membrane is forced out- 
 wards, carrying with it the handle of the malleus. Then 
 the head of the malleus above the axis rotates inwards, 
 carrying with it the body of the incus, and the long process 
 of the incus, moving away from the mesial plane, carries the 
 stapes with it, and pressure on the internal ear is diminished. 
 The distance through which the base of the stapes can 
 move is very small, and hence it might happen that a very 
 loud sound, causing the tympanic membrane to vibrate 
 through a comparatively large distance, might tear the 
 stapes from its attachments. This, however, is guarded 
 against in several ways. In the first place, a somewhat 
 dense ligament passes from the upper part of the external 
 wall of the tympanum to the head of the malleus, and this 
 receives the impact of the head of the malleus as upon an 
 elastic cushion, and may, when the head of the malleus 
 tends to move too far inwards, restrain it from moving too 
 freely. Secondly, the process below the upper joint of the 
 incus fits into a depression in the malleus, and when the 
 handle of the malleus tends to move too far inwards, this 
 
Sound and Hearing 213 
 
 projection locks into the opposing socket like the tooth of a 
 cog-wheel, and prevents too great movement inwards. On 
 the other hand, if the handle of the malleus rotates outwards 
 excessively the tooth is withdrawn, and the saddle-shaped 
 joint coming into play, the lower part of the joint tends to 
 gape, and the incus does not move so far outwards as the 
 malleus. 
 
 Further, the chain of bones acts like a bent lever, the 
 arm of the incus being only two-thirds of the length of the 
 malleus. When the lower end of the handle of the malleus, 
 fixed in the^umbo of the tympanic membrane, moves through 
 a given distance, the stapes fixed to the 
 lower end of the process of the incus will 
 only move through two-thirds of this dis- 
 tance. But while the excursion distance 
 is diminished, we know from the principle 
 of the lever that the force with which it FlG - 108. Diagram 
 
 . , . . .. ^. illustrating the 
 
 moves must be increased by one-half. There leverage action of 
 is thus diminished amplitude of movement, the malleus and 
 
 . .. ........ incus; ;//, handle 
 
 but increase of power. This is a distinct of malleus . , ; Iong 
 advantage, considering the small power that process of the 
 sound waves have of moving the tympanic 
 membrane, and the firmness with which the base of the stapes 
 is fixed. This increase of power is augmented by the fact 
 that the tympanic membrane has roughly an area twenty 
 times as great as the base of the stapes. Thus the tym- 
 panic membrane concentrates its power upon an area only 
 one-twentieth of its size, and this, increased by the shorter 
 arm of the lever (of the incus), must give a force at least 
 thirty times as great as that with which the handle of the 
 malleus is moved at the umbo of the tympanum. Another 
 reason why the stapes cannot move far is found in the firm- 
 ness of the fibres of the membrana tympani, and of its 
 attachment to the handle of the malleus ; extensive move- 
 
214 Physiology of the Senses 
 
 ment of the membrane is thus prevented. Lastly, where 
 the membrane might move too freely, we have the action of 
 the tensor tympani muscle coming into play. By the pull 
 inwards of this muscle upon the handle of the malleus, the 
 tension of the membrane is increased, and its extent of 
 vibration correspondingly diminished. But this brings us 
 to a consideration of the manner in which membranes 
 respond to sonorous vibrations. 
 
 Response of the Tympanic Membrane to Sound Waves. 
 The physical cause of the sensation of sound is the rapid 
 vibration to and fro of the molecules of an elastic medium 
 when these have been set in motion by a sudden shock. 
 The particles, when disturbed, vibrate to and fro till they 
 regain their former equilibrium. Such vibration may be 
 transmitted from molecule to molecule through solids, 
 liquids, or gases. Thus the arm of a tuning-fork, when 
 set in vibration, causes an alternate condensation and rare- 
 faction of the air in the space through which it moves. 
 With each successive to -and -fro movement of the fork 
 another alternation of change of density is set up, and this 
 is propagated outwards in all directions from the fork as a 
 centre. The direction of movement of the particles in a 
 sound wave is not transverse to the direction in which the 
 wave is moving, but in the same direction. Hence they 
 are said to be longitudinal waves, as distinguished from the 
 transverse movements characteristic of waves of light, or of 
 waves moving on the surface of water. Such longitudinal 
 waves can readily be set up in solids, as, for example, in a 
 wooden rod by friction, and on account of the closeness to 
 one another of the molecules in solids such vibrations are 
 transmitted with great rapidity. But rods, strings, or mem- 
 branes may be caused to vibrate transversely to their length 
 or plane, as when a violin string is pulled aside by the bow, 
 or a drum is beaten. If these vibrations be in quick 
 
Sound and Hearing 
 
 215 
 
 succession, they will give rise to sound waves in air. In 
 this case it will be noticed that while the particles of the 
 solid body are moving transversely to the length of the rod 
 or string, or the plane of the membrane, their direction of 
 vibration is still longitudinal in so far as the direction of 
 the transmission of sound is concerned. 
 
 The impulses given to the air by a vibrating string are of 
 a complex type, for while it may vibrate as a whole, and give 
 forth a series of waves, which combin- 
 ing excite the sensation of a sound or 
 tone, this fundamental tone is always 
 modified by the presence of overtones 
 produced by the simultaneous vibra- 
 tion of segments of the string (Fig. 109). 
 In the case of a rod or string these seg- 
 ments are respectively a half, a third, 
 a fourth, and so on, of the length of 
 the whole rod or string, and the num- 
 bers of vibrations given forth by these 
 segments are respectively twice, three 
 times, four times, and so on, that of 
 the fundamental tone. In the case of A B 
 
 plates or membranes, the number and FlG - 109. Diagram of 
 
 string vibrating so as to 
 
 character of the overtones are more g i ve forth its fundamental 
 difficult to determine, being dependent tone < A )> and its first - 
 
 . . upper partial tone or 
 
 on the form and elasticity of the octave (B). 
 plates, the manner in which they are 
 
 set vibrating, and the number of vibrations. The smaller 
 and the more tightly stretched a membrane is, the faster 
 will be its rate of vibration and the higher the pitch of the 
 sound thereby caused. On a large vibrating membrane the 
 surface is, as it were, subdivided into many portions of vary- 
 ing sizes, some small, some large, each vibrating at a rate 
 peculiar to itself, and thus giving rise to a complicated set 
 
216 Physiology of the Senses 
 
 of aerial vibrations. Conversely, if the air is vibrating at 
 any of the rates at which the membrane, or parts of it, may 
 vibrate, the membrane will begin to vibrate in response. 
 Suppose two violin strings, or two tuning-forks, are tuned 
 to the same pitch and placed close to one another ; if one 
 of these be set vibrating the other will also begin to vibrate 
 at the same rate ; but strings or rods will not respond so 
 readily as membranes to a variety of tones. Membranes 
 respond more readily to aerial vibrations than plates do, 
 because of the smaller mass of matter requiring to be 
 moved, and the consequently greater flexibility of the 
 surface. A drum-head will move freely to and fro under a 
 blow which will cause almost no apparent result upon a 
 thick plate. 
 
 The application of these facts to the action of the tympanic 
 membrane in hearing is not far to seek. In the first place, 
 the membrane is small, very thin, its fibres are inelastic, 
 and it is firmly but not evenly stretched in all its parts. 
 From its thinness it can respond to aerial impulses of very 
 faint kinetic energy. 
 
 This receptivity we have seen may be interfered with by 
 the accumulation of hardened wax upon the membrane. 
 
 The peculiar arrangement of the fibres of the membrane 
 makes it respond to sounds of widely-varying pitch. The 
 fibres radiating from the umbo to their varying points of 
 attachment in the tympanic groove constitute, as it were, a 
 vast number of strings of varying lengths, each of which 
 will respond most readily to its own particular tone. Again, 
 the concentric circular fibres may be regarded as surround- 
 ing a series of nearly circular discs of gradually increasing 
 size, and therefore of different vibratile capacities. Further, 
 von Helmholtz has shown that the shallow conical form of 
 the membrane, the slight outward convexity of its fibres, 
 renders it less liable to have a fundamental tone only, and 
 
Sound and Hearing 2 1 7 
 
 increases its receptivity for all varieties of sounds. It has 
 been found that if a handle be attached to a flat disc, and 
 the disc be then curved like the tympanic membrane, it 
 ceases to have a fundamental tone. This property of the 
 drum-head is of paramount importance in hearing, as it 
 leaves the ear free from the disadvantage of having all tones 
 but one overburdened by a preponderating fundamental 
 tone. Almost every ear will respond to tones having as 
 low a frequency as 30 vibrations per second, while certain 
 acute ears may hear tones caused by 40,000 vibrations per 
 second. 
 
 The receptivity of the tympanic membrane for sounds of 
 high pitch, that is to say, sounds due to a large number of 
 vibrations per second, is enhanced by the action of the tensor 
 tympani muscle (Fig. 104). When this muscle contracts it 
 pulls the handle of the malleus, and with it the tympanic 
 membrane, inwards, and thus tightens the membrane just as 
 a drum-head is made more tense when it is braced up. 
 The fibres being tighter, their play is diminished, and they 
 respond more readily to vibrations following in quick 
 succession. On the other hand, by the action of the 
 laxator tympani, the membrane becomes more flaccid and 
 responds better to sounds of low pitch. 
 
 It has been suggested that the power which many trained 
 musicians have of recognising the absolute pitch of a note 
 may depend to some extent upon the sense of muscular effort 
 arising from varying degrees of contraction of the tensor 
 tympani. In such cases long practice in the determination 
 of the pitch of notes gives rise to such delicacy of judgment 
 that there seems to be an intuitive and direct recognition of 
 pitch, and not only may the pitch of a sound heard by the 
 musician be named by him, but he may sing a note of any 
 given pitch that he desires without the aid of tuning-fork 
 or instrument. For the performance of this latter act, it is 
 
2 1 8 Physiology of the Senses 
 
 not unlikely that the parts unconsciously assume the neces- 
 sary degree of tension before the sound is uttered, just as 
 we are apt to make involuntary contortions of the facial 
 and other muscles when performing complicated or difficult 
 actions. 
 
 One important factor in the regulation of the tympanic 
 membrane .has still to be mentioned. If we strike the keys 
 of a piano and hold them down so as to prevent the 
 dampers touching the strings, the vibration of the strings 
 will go on for a considerable time ; but when we release 
 the keys, and the dampers touch the strings, the vibration 
 stops. In the ear the handle of the malleus attached to 
 the tympanic membrane acts as a damper. If the mem- 
 brane went on vibrating after the sound wave had ceased, 
 there might be interference with other succeeding sounds, 
 but the duration of the vibration is cut short by the resist- 
 ance offered by the chain of bones. The development of 
 overtones in the membrane is likewise prevented, and the 
 ear is rendered more acute in the discrimination of different 
 sounds following one another in rapid succession, and each 
 tone is heard pure, and not interfered with by those which 
 have immediately preceded it. There is a further provision 
 in the structure of the internal ear for differentiation of sounds, 
 but this we will refer to afterwards. 
 
 Transmission of Vibration by the Auditory Ossicles. 
 We have next to consider how auditory vibrations are 
 conveyed to the internal ear. It has been experimentally 
 determined that sound is mainly transmitted through the 
 middle ear by the movement, as a whole, of the chain of 
 bones. No doubt where these are absent, or have been 
 rendered immovable by disease, a person may still be able to 
 hear, but the acuteness of hearing will be largely interfered 
 with. As to the nature of the movement of the bones 
 there is a common consensus of opinion. It will be readily 
 
Sound and Hearing 2 1 9 
 
 understood that the movement of a solid body may be the 
 resultant of many constituent elements. The earth rotates 
 upon its axis whilst it moves round the sun. In a red-hot 
 cannon ball projected through the air, the molecules of the 
 metal are in a state of extremely rapid movement with 
 reference to each other, as well as in transmission through 
 space. In a tense string set into transverse vibration there 
 must be a continual lengthening and shortening of the 
 string, or in other words, a change in position of the mole- 
 cules relatively to one another and in the direction of the 
 length of the string as well as the transverse movement of 
 the string as a whole. The longitudinal movement of the 
 particles is invisible, the transverse movement is visible, to 
 the naked eye. The former we call molecular, the latter 
 molar movement. Probably there is some molecular move- 
 ment of the ossicles of the ear, but the presence of joints 
 must largely interfere with this, and the movement is mainly 
 of the bones as a whole, that is to say, a molar move- 
 ment, a movement that may be seen with the eye. While 
 this is so, we must be careful to distinguish between the 
 amount of movement of the bones and the length of the 
 sound wave. The length of a sound wave is dependent not 
 upon the amplitude of movement of the sounding body 
 that determines the intensity or loudness of the sound 
 but upon the number of vibrations made in a given time 
 by the sounding body. In Fig. no, p. 220, A represents 
 a long wave of small amplitude of movement, B short 
 waves with greater amplitude. The length of the wave is 
 measured by the interval between two successive points in 
 like phase relatively to one another. Thus in A, we must 
 move from a to c in order to get two particles in like 
 condition of velocity and direction of movement, so we say 
 that ac is the length of the wave. Now the distance 
 through which a sound wave will pass in any medium in a 
 
220 Physiology of the Senses 
 
 given time depends upon the elasticity and density of the 
 body in question. Through air, sound waves pass, on an 
 average, at the rate of 1120 feet per second. If, then, a 
 body makes a complete to-and-fro vibration only once each 
 second, the first movement must have passed 1020 feet 
 before the second begins, or in other words, the wave- 
 length is 1 1 20 feet. If the body performs a complete 
 vibration twice in a second the distance between two 
 points of like condensation and rarefaction will only be one- 
 half of 1 1 20 feet, or 560 feet. The more rapid the rate 
 of vibration, the faster will wave succeed wave, and the 
 shorter will the wave be. The ear can readily distinguish 
 as a musical tone sounds due to vibrations following each 
 
 FIG. no. Diagram illustrating (I.) long waves of small amplitude, and (II.) 
 short waves of greater proportional amplitude. 
 
 other thirty times in a second. The wave-length in such a 
 case would be 1120^30 = 37 feet approximately, while 
 certain ears can hear a sound due to 40,000 vibrations per 
 second, in which case the wave-length will be 1120 feet -r- 
 40,000, or approximately J of an inch. But in either 
 case it will be seen that the bones of the ear cannot move 
 through the " length of the wave," but rather that the time 
 of recurrence of like condition of condensation or rare- 
 faction at the drum-head gives rise to our appreciation of 
 differences of pitch. Regularly succeeding stimuli going to 
 the auditory nerve at the rate of say thirty times a second 
 will give rise to a sensation of a sound of low pitch, and if 
 at the rate of say 4000, to a sensation of a sound of high 
 pitch. The length of the wave is of importance in regulat- 
 
Sound and Hearing 221 
 
 ing the number of times per second the drum-head will 
 vibrate, taking into account the rate of the transmission of 
 sound waves through air ; but the breadth of the ear, and 
 even of the whole head, may only form a very small part of 
 the length of the wave. A tuning-fork bowed gently will 
 give a sound of the same pitch as the same fork bowed 
 strongly. In the one case we cannot see any movement in 
 the limbs of the fork ; in the latter the sharp outline of the 
 limbs is lost, and we can see at once that the limbs are in 
 motion. Similarly in the ear. With weak sounds the 
 drum-head hardly moves, and the ossicles seem to be at 
 rest, but if the sound is loud, the drum-head and the bones 
 may be seen in motion. 1 With very loud sounds, when 
 many molecules of air have been suddenly compressed into 
 a small space, the pressure upon, and consequent move- 
 ment of, the tympanic membrane is very great, and the 
 force may even be so excessive as to cause rupture of the 
 membrane, just as windows are sometimes shattered by a 
 violent and consequently loud explosion. 
 
 While in ordinary circumstances the tympanic mem- 
 brane is usually thrown into vibration through the medium 
 of the air in the external meatus, it should be borne in 
 mind that it may be set in motion also by transmission 
 of vibrations through the bones of the skull. 
 
 If a tuning-fork is struck, and its handle pressed against 
 the teeth, a molecular movement is transmitted to the 
 membrane with such energy as to set the membrane and 
 ossicles into visible molar movement. We can illustrate 
 
 1 A preparation can be made of the ear of a dead cat. The middle 
 ear is laid open by removing a small portion of its wall. After lightly 
 dusting the interior with lycopodium powder, it is strongly illuminated 
 and examined with a microscope of moderate power. When the 
 vibrations of an organ pipe, sounding loudly, are directed into the 
 external ear, little brilliant specks of lycopodium powder may be seen 
 to vibrate. 
 
222 PJiysiology of the Senses 
 
 this by placing a number of marbles in a row, and touching 
 one another. If a smart tap be given -to the marble at one 
 end of the row, it will not apparently move, nor will the 
 intervening members of the series, but the last marble of 
 the row will fly off as if directly struck. The energy of the 
 blow is, in this case, transmitted through the molecules of 
 the marbles, and is sufficient to give rise to visible move- 
 ment in the last member of the series. So the movement 
 transmitted through the bones of the skull gives rise to free, 
 movement of the tympanic membrane, and through it to 
 the internal ear. Trial, however, will show that the tym- 
 panic membrane responds better to the vibrations of the 
 air in the meatus than to those transmitted through the 
 head. If a tuning-fork be struck, and its handle held 
 between the teeth till the sound has apparently ceased, and 
 if then the fork be held opposite the ear, the sound will be 
 distinctly heard again. We may attribute this to the 
 greater mobility of the molecules of air in the meatus than 
 that of the molecules of the bones of the head. They 
 move more freely to and fro, and under a feebler stimulus, 
 than the molecules of the bones, and thus the membrane 
 responds more readily to the tuning-fork held to the ear. 
 Still, although both membranes be absent, the ear is quite 
 capable of hearing and of distinguishing musical sounds by 
 the direct stimulation of the internal ear, and its apprecia- 
 tion of pitch cannot be affected, inasmuch as this is due to 
 the physical fact of a recurrence of stimuli at definite 
 intervals of time. The intensity of the sound will, how- 
 ever, be diminished, because, as we have seen, the arrange- 
 ment of membrane and ossicles gives a mechanical advan- 
 tage in the way of increased power. 
 
Sound and Hearing 
 
 223 
 
 THE INTERNAL EAR 
 
 We have already said that the internal ear consists of a 
 closed sac formed by an invagination of part of the skin at 
 a very early period of life, and that the nerve of hearing 
 ends in this sac. We have now to consider the form of 
 the internal ear, the mode 
 of ending of the auditory 
 nerve in it, and the manner 
 in which its structure is 
 adapted to the function of 
 hearing. And, in the first 
 place, let it be noted that 
 modern research tends to 
 confirm a conjecture made 
 long ago that the front part 
 of the internal ear, the 
 cochlea, has to perform an 
 entirely different function 
 from the posterior part. In 
 correspondence with this, 
 the auditory nerve has been 
 
 shown to consist of two 
 nerves (Fig. 103,^, /) which, 
 arising in different parts of 
 the brain, are united by 
 connective tissue in the 
 greater part of their course, 
 
 FIG. in. Right bony labyrinth viewed 
 from the outside (x 2?,, and natural 
 size). The more spongy material of the 
 petrous bone has been separated from 
 the dense bony wall of the labyrinth, 
 i, The vestibule ; 2, fenestra ovalis or 
 oval window ; 3, superior semicircular 
 canal ; 4, horizontal or external semi- 
 circular canal ; 5, posterior semicircular 
 canal ; * * ampullae or dilatations of 
 semicircular canals ; 6, first coil of the 
 cochlea ; 7, second coil ; 8, apex ; 9, 
 fenestra rotunda or round window. 
 (Sommerring. ) 
 
 but separate again as they 
 
 approach their termination, and end in organs which differ 
 
 widely in appearance from each other. 
 
 The posterior portion of the sac is contained in .the 
 bony cavity known as the vestibule and semicircular canals. 
 
224 
 
 Physiology of the Senses 
 
 We may imagine the canals as having been cut off from 
 the main body of the sac by the meeting and agglutination 
 of opposite parts of the original cavity, just as if, were we to 
 press together between thumb and finger the opposite sides 
 of a bag near one of its corners, we would form a canal or 
 passage communicating at each end with the main cavity of 
 the bag (Fig. 112). This main cavity in the ear is known as 
 the utricle (Fig. 113); it is oblong in shape, being about one- 
 fourth of an inch long, and communicates behind and above 
 with three semicircular canals (Fig. ill, 3, 4, 5) which lie 
 respectively in three planes, one horizontal and two vertical, 
 and all exactly at right angles to each other like three 
 
 FIG. 112. Diagrammatic representation of the manner in which the semicircular 
 canals are formed from a primary cavity. (See text.) 
 
 adjacent sides of a cube. From the direction in which the 
 curves are inclined, the canals are named respectively the 
 horizontal or external, the antero- posterior, or simply the 
 posterior and the transverse or superior canals. Each 
 canal has one of its openings into the utricle dilated to 
 form what is known as an ampulla (Fig. 111), the other 
 end passing into the utricle without enlargement, and the 
 undilated ends of the canals in the vertical planes unite 
 with one another before passing to the utricle, so that there 
 are only five openings for the canals into the utricle, three 
 of which are provided with ampullae. 
 
 The utricle lies in the vestibule. Below, and in close 
 apposition to, the utricle, and, like it, contained in the 
 
Sound and Hearing 
 
 225 
 
 vestibule, we have \hs-saccule (Fig. 113), a smaller and 
 more rounded space than the utricle. These two cavities are 
 formed by a constriction of the primary vesicle, and even 
 in adult life are in connection with each other by a long 
 narrow tube of a Y shape, the ductus endolymphaticus 
 (Fig. 113), one part of which actually penetrates through 
 the bone into the cavity of the skull, and lies enclosed by 
 the membranes surrounding the brain. The saccule, by a 
 narrow tube, the canalis reuniens (Fig. 113), communicates 
 with the long finger-like projection, the canal of the cochlea^ 
 which is packed away in 
 small space by being wound 
 two and a half times round a 
 central supporting pillar of 
 bone, the modiolus(\g. 117). 
 The auditory nerve, enter- 
 ing the bone containing the 
 internal ear by a passage 
 
 called the internal auditory FIG. 113. Membranous labyrinth 
 
 mcatltS, divides, as it enters (d^ammatic). c Cochlea; , sac- 
 
 cule united by/, the ductus endolym- 
 the bony labyrinth, into tWO phaticus, with , the utricle, arising 
 
 main divisions, one going to 
 
 the cochlea, and the other to 
 
 the vestibular pyt of the membranous labyrinth, the latter 
 
 branch quickly dividing further so as to supply a terminal 
 
 branch to the utricle, the saccule, and the ampullae of the 
 
 semicircular canals, and to these parts alone. 
 
 The membranous labyrinth has for its outer coating a 
 layer of connective tissue from which numerous processes 
 pass to the fibrous lining of the bone. The spaces 
 between the processes, similar to other lymph spaces 
 throughout the body, are lined with flat cells and filled with 
 a somewhat viscous fluid. The connective tissue is homo- 
 logous with the true skin, and like it contains blood-vessels. 
 
 
 
 from which are seen the three semi- 
 circular canals. 
 
226 
 
 Physiology of the Senses 
 
 The inner lining of the sac, except where the nerves end, 
 consists of a single layer of flattened cells. In one portion 
 
 of the utricle and of the saccule lies a small oval spot, or 
 macula, and in the ampulla of each canal a ridge or crista 
 
Sound and Hearing 227 
 
 which, since they contain the termination of the vestibular 
 nerves, are known respectively as a macula or crista acustica. 
 Over these the epithelium is stratified, being mainly made 
 up of thread-like columnar cells (Fig. 115), having a well- 
 marked nucleus, and supporting another 
 set of nucleated cylindrical cells, whose free 
 surfaces bear bunches of stiff rod-like hairs 
 which are often adherent one to another, 
 and are known as the auditory hairs. Some 
 observers have described the hairs as pass- 
 ing through a membrane similar to that 
 found in the cochlea (p. 236) ; but this has 
 been disputed. The terminal twigs of the 
 auditory nerve, passing through the con- _ 
 
 J m FIG. 115. Epithelial 
 
 nective tissue which forms the main sub- cells from macula 
 stance of the prominence or ridge, lose acustica of the 
 
 utricle. 
 
 their outer sheaths and pass as naked axis- 
 cylinders into the epithelium, where their mode of termina- 
 tion is not definitely known. Some suppose that they end 
 in the cells, others that they simply surround them with a 
 nest of fine fibrils ; but, from analogy with the other sense 
 organs, we may conjecture that they are at 
 least stimulated by the. agitation of the 
 hair -cells. The free ends of the auditory 
 hairs are embedded in a soft mucous mate- 
 rial, the cupula, in which are often found 
 F from^thf^upuia sma11 cr ystals consisting largely of carbonate 
 above the human of lime, called otoconia, or otoliths (f\<g. 1 1 6). 
 macula acustica. The function of this cov ering is unknown, 
 
 though it has been supposed to act as a damper to the 
 vibration of the auditory hairs. It may possibly be driven 
 mechanically against the points of the hairs by vibrations of 
 sound, and thus increase the sensitiveness of the hairs to 
 such vibrations. 
 
228 
 
 Physiology of the Senses 
 
 The Cochlea. We come now to consider the struc- 
 ture of the cochlea (Gr. cochlias, a snail with spiral shell), 
 which is a tubular cavity coiled in a spiral manner round 
 a central pillar called the modiolus. The part of the mem- 
 branous labyrinth which it contains is much smaller in 
 cross section than the bony space, and is known as the 
 canalis cochlearis. It is fixed in the whole of its course, 
 except at its closed end, to either side of the cochlea, having 
 a broad surface of attachment on the outside, but a very 
 narrow one towards the median column. Indeed, we find 
 
 here that the cochlear canal 
 is only attached on its inner 
 aspect to the free edge of 
 a shelf which winds round 
 the central pillar, projects 
 outwards into the lumen of 
 the cochlea, and is known 
 ,, vi r -q j as the lamina spiralis ossea, 
 
 FIG. 117. The osseous cochlea divided f 
 
 through the middle, X 5. i, Central or Spiral plate of bone. It 
 
 canal of the modiolus in which lies the consists of a double 
 
 cochlear nerve ; 2, the spiral osseous ' 
 
 lamina; 3, scala tympani ; 4, scala plate of bone, between the 
 
 vestibuli ; s, spongy bone of modiolus surfaces o f which the nerves 
 near the spiral canal, 8. (Arnold.) 
 
 pass out from the central 
 
 column to enter the cochlear canal. In a section made 
 transversely through one of the whorls of the cochlea, 
 we see then three spaces represented in Fig. 118. The 
 upper space, containing perilymph, is in connection, at 
 its beginning, with the vestibule, and, as it winds round 
 towards the apex of the cochlea, it is known as the stair- 
 way from the vestibule or scala vestibuli. At the summit 
 of the cone it bends round the closed end of the cochlear 
 canal and the free hook-like end or hamulus of the lamina 
 spiralis, by a little passage called the helicotrema^ and 
 communicates with a descending space which, winding 
 
Sound and Hearing 229 
 
 round the modiolus, ends at the fenestra rotunda, whose 
 membrane closes the opening into the middle ear. This 
 lower space is known as the scala tympani. The two 
 scalae are lined with a connective tissue membrane which 
 is thickened on the outer wall to form the spiral ligament, 
 first described by Bowman, and the free surface of the 
 membrane is covered with a single layer of flattened cells. 
 The scalas being in connection with each other at the top 
 
 FIG. 118. Section through one of the coils of the cochlea (diagrammatic). SV, 
 Scala vestibuli ; ST, scala tympani ; CC, canal of the cochlea ; Iso, lamina 
 spiralis ossea, or spiral plate of bone ; Us, limbus of the spiral lamina ; R, 
 Reissner's membrane ; ss, spiral sulcus or groove ; /, tectorial membrane ; 
 CO, organ of Corti ; b, basilar membrane ; Isp, spiral ligament ; <:, cochlear 
 nerve ; gs, spiral ganglion in course of cochlear nerve. (After Henle.) 
 
 of the whorl, and being filled with perilymph, the pressure 
 of the fluid in the two spaces must be the same when the 
 ear is at rest. If, by the movement of the stapes, the 
 pressure of the fluid in the vestibule be increased or 
 diminished, there must be a corresponding change of 
 pressure transmitted from the scala vestibuli to the scala 
 tympani, and this may be effected either directly through 
 the cochlear canal or through the helicotrema. The fluids 
 of the ear being practically incompressible there must be a 
 
230 Physiology of the Senses 
 
 corresponding movement of the membrane closing the 
 fenestra rotunda. 
 
 Upon the upper surface of the spiral bony shelf, and 
 near its free border, is a thickening of the connective tissue 
 known as the limbus. This thins away as it- covers the 
 free edge of the shelf, and a groove is formed the sulcus 
 spiralis (Fig. 1 1 8) whose free borders are known respec- 
 tively as the vestibular and tympanic lips. 
 
 The Cochlear Canal. In cross section, the canal of the 
 cochlea is roughly triangular in shape, the apex being 
 attached to the spiral plate of bone, the base to the outer 
 wall of the cochlea. That part of the wall of the canal 
 which looks towards the scala vestibuli arises from the upper 
 surface of the spiral shelf a little nearer the modiolus than 
 the limbus, and stretches as a thin fibrous membrane, 
 known as Reissner's membrane^ to the outer wall. It is 
 lined on its vestibular side by flattened cells, while the 
 internal surface is clothed with more cubical cells, some of 
 which have probably a secretory function. 
 
 The wall of the cochlear canal, which takes part in the 
 formation of the scala tympani, stretches from the tympanic 
 lip of the spiral lamina to the spiral ligament, and is known 
 as the lamina spiralis membranacea^ or basilar membrane. 
 It is indistinctly fibrous towards its inner attachment, but 
 in its outer two-thirds shows a radial fibrillation as of rod- 
 like fibres embedded in a homogeneous matrix. This part 
 of the structure is, as we shall see, probably of considerable 
 importance in the appreciation of the pitch of sounds. 
 
 The tympanic surface is lined with cells, often of a 
 spindle shape, which lie transversely to the fibres above 
 them, and, at one part immediately below the organ of 
 Corti about to be described, we find a small blood-vessel, 
 the vas spirale, which ensures a good blood supply to the 
 superjacent structures. 
 
Sound and Hearing 
 
 231 
 
 The" Organ of Corti. The epithelium upon the upper, or, 
 with reference to its position in the head, anterior surface of 
 
 FIG. 119. Cross section of the human cochlear duct at the junction of the first 
 and second turns of the cochlea, X 100. i, Outer wall (part of the spiral 
 ligament) reaching from b to c ; 2, vestibular wall, or Reissner's membrane, 
 from, a to c ; tympanic wall from a to b ; 3, lamina of bone ; 4, its vestibular 
 lip ; 5, its tympanic lip ; 6, nerves of hearing passing to epithelium at 7 ; 8, 
 internal spiral groove with flattened epithelium ; 9, basilar membrane ; 10, 
 its tympanic covering; n, basilar crest of spiral ligament; 12, prominence 
 of spiral ligament with blood-vessel ; between n and 12, the external spiral 
 groove ; 13, vascular layer ; 14, spiral papilla (epithelium of Corti's organ) ; 
 near 14, the outer hair-cells and Deiter's cells ; further inwards the rods of 
 Corti covering the tunnel ; internal to this the inner row of hair-cells ; 15, the 
 tectorial membrane. (After Retzius. ) 
 
 the basilar membrane is of a highly specialised type, and 
 more especially that part which rests upon the inner half of 
 the membrane. This part is commonly known as the organ 
 
232 Physiology of the Senses 
 
 of Corti, from the Italian Marquis of that name who first 
 gave a detailed description of it. When we examine sections 
 made transversely to the length of the canal, we find a 
 peculiar structure resting upon the basilar membrane 
 immediately adjoining its inner line of attachment. This 
 consists of a set of elongated rod-like cells arranged in two 
 rows throughout almost the whole length of the cochlear 
 canal, and known as the outer and inner rods of CortL 
 These rod-cells, rising from the membrane, meet at their 
 upper ends like the beams of a sloping roof, and, together 
 
 with the membrane, enclose a 
 space called the tunnel. The 
 individual rods have a cylin- 
 drical form and an expanded 
 base, by which they are fixed 
 to the basilar membrane. The 
 , , upper ends of the rods are 
 
 1 ( IG. 120. Inner and outer rods of - rr 
 
 Corti from the cochlea of a guinea- enlarged, but flattened at the 
 
 pig, X 275. A, Inner rod-cell ; B, ^ h , ^ contact 
 
 outer rod-cell. In both are seen ' 
 
 i, the foot piece; 2, the body; and, with adjoining rods, and the 
 
 3, upper end of rods ; 4, nucleus inner heads haye n ^^ 
 
 and protoplasm. (Schwalbe.) 
 
 outer aspect a socket into 
 
 which fit the rounded heads of the outer row of rods. 
 From the head of each rod there projects outwards a 
 flattened process, those of the inner row overlapping 
 those of the outer. The inner rods are about a half more 
 numerous than the outer, so that two outer rods fit into 
 three of the inner row. At the base of each rod we 
 find a nucleus and granular protoplasmic material, while 
 the main substance of the rod exhibits no structure, or 
 merely a faint longitudinal striation. The rods being 
 placed in line, and all the head-plates being similar in 
 size and appearance, they present, when seen from above, 
 a remarkable resemblance to the key-board of a piano. 
 
Sound and Hearing 
 
 233 
 
 Fibres of the auditory nerve pass between the rods and 
 across the tunnel, which, during life, contains also a colour- 
 less jelly-like intercellular substance (Fig. 121). 
 
 FIG. i2i. Surface view of the spiral papilla of Corti's organ from the topmost coil 
 of a rabbit's cochlea, from the inner hair-cells to the cells of Deiter. (After 
 Retzius.) Highly magnified, i, Inner row of hair-cells ; 2, boundary line of 
 their surface ; 3, cuticle of the inner hair-cells, each showing eight hairs ; to the 
 left an extra inner cell is present; 4, flattened tops of the inner rods of. 
 Corti ; 5, outer border of these plates ; these completely cover the tops of 
 the outer row of rods, seen between 6 and 7 ; at 6 is seen the inner border 
 line of attachment of the heads of the outer rods. From the tops of the outer 
 rods are seen at 7 the processes to the phalangae, narrow at 8, and widening 
 at 9 to form part of the lamina reticularis. 10, Phalangae of the first row. 
 it, Phalangse of the second row. 10-12 are the cuticular end plates of the 
 three rows of Deiter's cells. In the interspaces between these appear three 
 rows of outer hair-cells, each showing eight hairs, arranged "in horse-shoe 
 shape, projecting from their free cuticular surface. 
 
 The Inner Hair-Cells. Just to the inner side of the 
 rods of Corti we find a row of columnar cells whose free 
 
234 
 
 Physiology of the Senses 
 
 surface is on a level with the head of the inner rods upon 
 which they rest. Each of these columnar cells has project- 
 
 FIG. 122. Radial section through the tympanic wall of the middle of the cochlear 
 duct of the guinea-pig, X 212. i and 2, Upper and lower plates of the 
 osseous spiral lamina ; 3, spiral ganglion ; 4, spiral bundle of medullated 
 nerve fibres ; 5, medullated nerve fibres radiating outwards between the bony 
 plates of the spiral lamina ; 6, thin connective tissue lining bone (periosteum) ; 
 7, limbus of the spiral lamina ; 8, its vestibular lip ; 9, its tympanic lip, 
 through which at 10 the nerve fibres, losing their medullary sheath, pass to 
 the epithelium ; n, beginning of Reissner^s membrane ; 12, union of tympanic 
 lip with basilar membrane ; 13, nucleated transparent layer of the basilar 
 membrane ; 14, layer of basilar fibres ; 15, cellular lining of basilar mem- 
 brane ; 1 6, epithelium of internal spiral groove ; 17, inner supporting cells, 
 below which the nerves emerge ; 18, inner hair-cells ; 19, inner rod of Corti, 
 a, nucleus and protoplasm ; 20, outer rod of Corti with, 3, its nucleus and pro- 
 toplasm ; c, cross section of spiral bundle of nerve fibres winding up with the 
 tunnel ; from it the nerve fibres, d, pass outwards between the outer rods of 
 Corti to the outer hair-cells ; 21, outer hair-cells in three rows alternating 
 with phalangar processes, 22, of Deiter's cells, 23 ; 24, supporting fibres 
 of Deiter's cells ; 25, cells of Hensen ; 26, cells of Claudius ; 27, membrana 
 tectoria ; 28, its marginal thickening. (Schwalbe.) 
 
 ing from its free surface from fifteen to twenty short stiff 
 hairs arranged in a crescentic line, whose convexity faces 
 outwards. The attached ends of the hair-cells are conical 
 
Sound and Hearing 235 
 
 in shape, and do not come down to the basilar membrane, 
 but are connected with, or closely invested by, terminal 
 fibrils of the auditory nerve. There may also be seen 
 around and below the lower ends of the hair-cells a number 
 of nuclei. These belong to elongated filamentous cells, 
 which, arising from the beginning of the basilar membrane, 
 pass to the surface between, and to the inside of, the hair- 
 cells, and, in all probability, act like the rods of Corti as 
 supporting structures. From the inner row of hair-cells 
 epithelial cells, at first columnar, then more cubical or even 
 flattened, line^the spiral groove already referred to, but the 
 overhanging part of the vestibular lip of the limbus is devoid 
 of epithelium, and is broken up by slight radial markings 
 into a set of projections known as the auditory teeth. 
 
 Outer Hair-Cells. To the outer side of the rods of 
 Corti we find rows of hair-cells and supporting cells similar 
 in many ways to the row found to the inside of the rods. 
 In the human ear there are usually four rows of hair-cells, 
 but there may be only three, or as many as five, rows in 
 certain parts of the canal. In the ears of lower mammals 
 there are seldom so many rows as in man. 
 
 The hair-cells of the outer row are likewise columnar, 
 have short stiff hairs arranged in a semicircular or horse- 
 shoe shape convexity outwards on their free surface, a 
 nucleus surrounded by granular protoplasm, and nearer 
 their free border a dark pigmented spot known as Henserfs 
 spot. The lower ends of the hair-cells do not pass down to 
 the basilar membrane, but, like the inner row of hair-cells, 
 are in contact with the terminal fibrils of the auditory nerve. 
 Closely apposed to the outside of each of the hair-cells in 
 the outer rows is a supporting structure, known as Deiter's 
 cell (see Fig. 122), which, arising by a thicker nucleated 
 part from the basilar membrane, gradually becomes nar- 
 rower and passes, as a small cylindrical process, to the free 
 
236 Physiology of the Senses 
 
 surface. Here the Deiterian cells are fixed to fiddle-shaped 
 plates phalangce which, uniting with adjoining plates, 
 and with the processes from the heads of the rods of Cord, 
 form a fenestrated or reticulated membrane, in the meshes 
 of which lie the free ends of the hair-cells. Each hair-cell 
 is thus fixed to and supported by a structure, which is itself 
 inserted at either end into a membrane, and thus the com- 
 ponent cells are firmly held in their respective places, and 
 we can see that any movement of the basilar membrane 
 must be at once communicated to the hair-cells through the 
 medium of Deiter's cells. 
 
 Outside of the rows of hair-cells we find, for a short dis- 
 tance, a row of columnar cells, devoid of hairs, and having 
 no direct connection with the auditory nerve. They are 
 known as Henserts cells, and they soon merge into a layer 
 of cubical cells, the cells of Claudius, which cover the outer 
 third of the basilar membrane, and are continued over the 
 spiral ligament and that part of the cochlear canal which 
 is in contact with the outer cochlear wall. 
 
 The spiral ligament into which the basilar membrane is 
 fixed, consists in the main of connective tissue, but spindle- 
 shaped cells have been described as existing in it, which, as 
 first suggested by Bowman, are supposed to be muscular, 
 and whose function would be to tighten the basilar mem- 
 brane, and adapt it for variations of pitch. The spiral 
 ligament is vascular, and at one part a slight elevation (vas 
 prominens) is made by a vein (Fig. 1 19). 
 
 It will be seen that the neuro-epithelium of the cochlea 
 resembles, in many respects, that found in the vestibular 
 part of the internal ear. This likeness is further increased 
 by the fact that we find, lying in the cochlear canal, fixed 
 at one end to the vestibular lip of the limbus, and at the 
 other free or attached to the outer part of the organ of 
 
Sound and Hearing 237 
 
 Corti, a thickish layer of fibrous tissue known as the mem- 
 brana tectoria. This may, as conjectured in the case of 
 the cupula, act as a damper when resting on the hair-cells, 
 but its action is not known. 
 
 Innervation of the Cochlea. The cochlea is supplied 
 by a branch of the auditory nerve. The modiolus or cen- 
 tral column, round which the cochlea is coiled, is hollowed 
 out in a conical fashion, the space being filled by the coch- 
 lear nerve, which, comparatively thick at first, soon lessens 
 in diameter by giving off numerous branches which pass 
 out into the bony spiral shelf. Before reaching their ulti- 
 mate destination, however, the fibres pass into a mass of 
 ganglionic nerve-cells of a spindle or bi-polar form, which 
 form a continuous spiral from the base to nearly the apex 
 of the cochlea, known as the spiral ganglion (Fig. 122). 
 From this the fibres emerge in bundles which coalesce to 
 form finer bundles. These passing radially outwards, be- 
 tween the opposing surfaces of the spiral lamina, emerge in 
 little furrows or canals at the tympanic lip, called foramina 
 nervina, and, losing here their primitive sheath and white 
 medullary substance, pass as bare axis-cylinders into the 
 neuro-epithelium of Corti's organ. 
 
 The nerve fibres do not seem to pass directly after 
 emerging from the bony plate to the hair-cells opposite. 
 They seem rather to bend round and run in the direction 
 of the cochlear spiral, some below the inner row of hair- 
 cells, some, after entering the tunnel, through interstices 
 between the rods of Corti, and some in spaces between each 
 row of the Deiter's cells supporting the outer row of hair- 
 cells. There are thus an inner spiral strand, a spiral strand 
 of the tunnel, and three or four outer spiral strands. From 
 these spirals are given off the ultimate fibrils which proceed 
 to the hair-cells. Whether they pass into these, or simply 
 into contact with them, is not definitely known. We may, 
 
238 Physiology of the Senses 
 
 however, feel assured, both from analogy and from careful 
 study of the structure, that the hair-cells are the true ter- 
 minal organs of the auditory nerve, that they alone can 
 respond to auditory vibrations, and set up sensory impulses 
 in the auditory nerve, and that the other cells of Corti's 
 organ are merely accessory in function. In birds, for 
 instance, the cochlea is very rudimentary, consisting of a 
 small protuberance from the saccule, and containing only 
 hair-cells on a basilar membrane and no rods of Corti. It 
 may seem strange that in birds, even in the sweetest song- 
 sters, the part of the ear which seems specially devoted to 
 the appreciation of musical tones should be ill developed ; 
 but it must be remembered that the quality and variety of 
 tones of the bird's song are vastly inferior to those of the 
 human voice, nor has the brain of the bird the development 
 necessary for the due recognition of the variety of sounds 
 which the human brain can differentiate. In the human 
 ear itself, the structure of Corti's organ varies as we pass 
 from the beginning to the end of the canal. At first, where 
 it unites with the canalis reuniens (p. 225), it is lined with 
 ordinary epithelium. Then the organ of Corti has at first 
 only three rows of hair-cells ; farther on, four rows appear, 
 and in some ears five. At the closed end of the canal, the 
 neuro-epithelium is again awanting, and gives place to a 
 simple squamous epithelium. 
 
 Observations are still required with regard to the com- 
 parative powers of ears as regards the appreciation of vary- 
 ing sounds according to the number of hair-cells which may 
 be present. While the general principle of formation of 
 Corti's organ remains the same throughout the whole length 
 of the cochlea, the grouping of the supporting cells, and 
 more especially those of Hensen, gives different appearances 
 at different levels of the spiral. It is also noteworthy that 
 the basilar membrane varies in breadth, not, as was at one 
 
Sound and Hearing 239 
 
 time supposed, narrowing from base to apex, but actually in- 
 creasing from .2 i mm. (y-o-g inch) to .36 mm. (nearly ~j inch) 
 (Retzius) in breadth as it ascends. Thus, if we regard its 
 radial fibres as corresponding to the strings of a musical 
 instrument, such as the harp, those fibres which lie at the 
 base of the cochlea, and consequently nearest the vestibule, 
 would compare with the short strings of the harp, which 
 vibrate rapidly, and give forth sounds of high pitch, while 
 those at the apex of the cochlea correspond to the long 
 strings which emit a bass note. If, as has been supposed, 
 this analogy is not a merely fanciful one, it is manifest that 
 we have in this arrangement the greatest mechanical advan- 
 tage, tones of short wave - length obtaining immediate 
 response, while those of greater wave-length must travel 
 
 FIG. 123. Diagram illustrating change in breadth of the basilar membrane from 
 base to apex of cochlea ; the length of the diagram is about twice, the breadth 
 about ten times, the actual dimensions ; the numbers in the diagram indicate 
 in millimetres the size of the structure in the ear, not the lengths of the lines. 
 
 farther. The basilar membrane being, according to Retzius, 
 about 35 mm. (i-fth inch) in length, the accompanying 
 diagram (Fig. 123) represents on an enlarged scale the com- 
 parative breadth of the membrane in different parts in 
 relationship to each other, and to the length of the canal. 
 The actual difference in the length of the fibres is, as will be 
 seen, very little, and it should further be noticed that the 
 distinct fibrillation of the membrane is well marked only in 
 the outer side of the membrane, between the outer rows 
 of hair-cells and the attachment of the membrane to the 
 spiral ligament. If this part alone be considered, we find 
 that the ratio is somewhat altered namely, from .075 mm. 
 at the base to . 1 26 mm. at the apex, or nearly I : 2 instead of 
 3:5. The difference in absolute size may seem very little > 
 
240 Physiology of the Senses 
 
 but we must always bear in mind the exceeding minuteness 
 of all the parts involved, and the extreme delicacy with 
 which so small an organ must be constructed in order to 
 give such complex and varied results as does the human 
 ear. The presence of what seem to be contractile cells 
 in the spiral ligament lends colour to the supposition that, 
 in the length and tension of the fibres of the basilar mem- 
 brane, we are to look for the mechanism for the appreciation 
 of pitch. We have said that possibly, in the cultivated 
 musical ear, the training of the muscles attached to the 
 drum-head, or rather the recognition of the muscular sensa- 
 tion caused by varying degrees of contraction of these 
 muscles, may play a large part. It may now be added 
 that this sensation may be strengthened by the feeling of 
 tension in the spiral ligament ; but at present this is merely 
 a conjecture. 
 
 AUDITORY SENSATIONS 
 
 Physiological Characters of Sounds. We have already 
 referred briefly to the physical causation of sound, and we 
 shall now consider how the physiological variations arise in 
 connection therewith. When we seek to analyse the effect 
 produced in consciousness by the stimulation of the auditory 
 mechanism, we find that all sounds may be roughly divided, 
 in the first place, into such as we designate noises, and 
 those recognised as musical tones. The sounds of a peal 
 of thunder, of the rending of silk, of the creaking of a door 
 on dry hinges these we call noises ; but when a tuning- 
 fork vibrates, or a note on the piano is sounded, we call the 
 effect produced upon the ear musical. The difference, how- 
 ever, between a noise and a musical sound is not of a hard 
 and fast kind. One may merge insensibly into the other. 
 The tuning of musical instruments by an orchestra gives us 
 
Sound and Hearing 241 
 
 a noise as result, but the noise is made up of musical tones, 
 and many sounds usually dismissed as noises, such as 
 street calls, the barking of dogs, or the blast of a fog-horn, 
 contain a distinctly musical element. When aerial vibra- 
 tions agitate the ear in regular recurrence, when equal 
 periods of time elapse between each stimulation, the sound 
 produced is musical ; but in the example mentioned above, 
 of the sound produced when an orchestra tunes its instru- 
 ments, the musical tones from the different players come at 
 irregular intervals, and at rates which interfere with one 
 another in such a way as to produce a harsh or unmusical 
 sound. On the other hand, sounds professedly musical are 
 sometimes noises of the most disagreeable nature. As a 
 combination of musical tones may produce a noise, we will 
 best arrive at a clear comprehension of auditory sensations 
 in general by the study in the main of musical sounds. 
 
 Apart from the emotional feelings which may be aroused 
 by music, there are certain sensations produced in the 
 mind on hearing a musical tone. These sensations may 
 be divided under three heads first, of pitch ; second, of 
 intensity ; and third, a sensation of a special quality of the 
 sound, dependent upon whether it is one simple sound, or 
 a combination of simple sounds. In practice, we seldom 
 hear simple musical tones, such as are produced by a 
 tuning-fork. The sounds produced by such musical instru- 
 ments as the piano, violin, or flute, are not simple tones, 
 but sounds in which many simple tones are blended into 
 one so as to give a sound with a special quality, timbre, or 
 klang, by which we can recognise the kind of instrument 
 that has given it forth. But, given the pitch, intensity, 
 and quality of a sound, we can, with proper instruments, 
 reproduce any variety of tone we please. We shall con- 
 sider, then, in the first place, the nature of pitch and of 
 intensity or loudness, and then how tones of varying pitch 
 
 R 
 
242 Physiology of tJie Senses 
 
 and intensity combine to give rise to a sensation of quality 
 in a musical tone. 
 
 i. Pitch. The pitch of a tone depends upo*n the fre- 
 quency of the vibrations in a given time ; or, to put it in 
 another way, since the wave-length is shorter in direct pro- 
 portion to the rapidity of recurrence, the pitch depends 
 upon the length of the waves which go to produce the 
 sound. If the vibrations come too slowly or too rapidly, 
 no musical sound is perceived, and while ears may hear 
 musical tones produced by vibrations at rates varying from 
 about 30 to 40,000 per second, the range of the tones 
 employed in music lies between 30 and 4000 per second. 
 
 The fact that pitch depends upon frequency of vibration 
 can be easily demonstrated by means of an instrument 
 called the syren. This, in its simplest form, is a thin metal 
 plate revolving upon an axle at a rate which can be exactly 
 regulated. The plate is perforated by a set of holes at 
 equal distances from the axle and from one another. The 
 wheel is first caused to rotate slowly, and a current of air is 
 blown against: the plate, so that it will pass through the 
 holes when they pass a certain point. At first a series of 
 puffs is heard, but, as the speed of rotation is gradually 
 increased, the puffs begin to coalesce, and when they recur 
 at from 20 to 30 times a second, a low buzzing or droning 
 sound is heard. The faster the plate revolves, the more 
 numerous the puffs become, and the higher will be the 
 pitch, until at last the sound grows faint and ceases to be 
 audible. When the pitch of a sound is very high, the 
 effect produced upon the listener is unpleasant. It is as if a 
 thin metallic blade or needle were piercing the ears, or it 
 may be compared to the shimmering effect of sunlight re- 
 flected by the ripplets on the surface of water agitated by a 
 light breeze. If the plate be made to rotate qurckly and at 
 constant speed, the pitch of the note will remain the same. 
 
Sound and Hearing 
 
 243 
 
 Von Helmholtz has devised a double syren, with which many 
 interesting experiments can be performed as to the nature 
 
 FIG. 124. Double Syren of von Helmholtz. a^, a\, Brass wind-chests com- 
 municating by tubes, g, g\, with bellows ; the opposite ends of the cylinders 
 are closed by brass plates perforated with holes corresponding to those seen 
 in the disk, c ; the disks, CQ, Q, rotate on a common axis, <, provided with a 
 screw for the counting apparatus, which is omitted here. The upper cylinder, 
 a, can be rotated on a vertical axis in either direction by toothed wheel, e, 
 with handle, d; the four rows of holes may be opened or shut by means of 
 studs, /", /; there are 8, 10, 12, and 18 holes respectively in the four rows of 
 holes in the lower disk, and 9, 12, 15, and 16 in the upper (not seen in 
 diagram). 
 
 of pitch. It consists (Fig. 124) of two boxes, supplied by 
 bellows with air, which, emerging through the lids of the 
 
244 Physiology of the Senses 
 
 boxes by holes, the number of which can be varied, causes 
 a plate close to, and in a parallel plane with, the lid of each 
 box to rotate. The rotation of the parallel plates allows 
 the air to escape through several series of holes in them, 
 just as in the simple syren. The beauty of the mechanism 
 lies in the power it gives us of regulating exactly the num- 
 ber of impulses per second, of reading off the number upon 
 a dial, and of permitting us to note the effects produced 
 when the two syrens are emitting tones of different pitch. 
 It is thus most valuable in studying concords, discords, and 
 beats, the nature of which will be described shortly. One 
 point which invariably arrests the attention when the syren is 
 heard for the first time is the peculiar effect of the gradual 
 rise in pitch as the velocity of rotation is accelerated. We 
 may say that at one moment it is giving forth many im- 
 pulses, say, 200 per second ; at another a different num- 
 ber, say, 20 1 ; but the change from 200 to 201 is through 
 an infinite fractional series ; and so vrith regard to the sound ; 
 it does not rise by leaps and bounds, but glides up in con- 
 tinuous transition. Just as the colours of the spectrum vary 
 through an infinite series, in passing from one colour to 
 another, so do the sounds in changing from one pitch to 
 another. The same effect can be produced on the violin 
 by sliding the finger up the string while it is being bowed. 
 And, further, as has been mentioned with regard to per- 
 ception of colour, as some eyes are insensible to the red, 
 and others to the violet end of the spectrum, so some ears 
 are insensitive to sounds of low pitch, others to those of 
 high pitch. As might naturally be expected, the sensibility 
 to pitch varies more in the higher than in the lower parts 
 of the scale, and we find people who suppose their powers 
 of hearing to be perfectly normal, who yet fail to hear 
 sounds due to more than 6000 vibrations. Test of power 
 in this respect may be made by means of a set of short- 
 
Sound and Hearing 245 
 
 steel cylinders, made by Konig, which, when suspended 
 by threads to a wooden frame, and struck with a metallic 
 instrument, emit tones to upwards of 40,000 vibrations per 
 second. The same result may be attained by using short- 
 limbed or heavy tuning-forks. 
 
 Within the range of musical pitch, too, we find that 
 people vary much in their capability of distinguishing a tone 
 of one pitch from another nearly the same. This likewise 
 holds good in respect of colour. Orientals distinguish 
 many shades of colours which seem the same to us. 
 While most people can detect a difference of a semitone in 
 two notes sounding together when of medium pitch, some 
 acute ears can detect as small a difference as -g^th of a 
 semitone. It becomes more and more difficult to detect 
 the difference as we pass to the upper or lower limits of 
 hearing a fact one may readily prove for oneself by striking 
 adjoining keys, now in the centre, now at either end of the 
 key-board of a piano. We have already indicated that the 
 power of detecting variations in pitch can be increased by 
 exercise and training, and have suggested a possible 
 explanation as to how this is co. On the other hand, there 
 are some people who are unable to discriminate more than 
 a very few tones, and who find it utterly impossible to sing 
 any complicated tune. The pitch of the ordinary human 
 voice in singing, it may be mentioned in passing, may be 
 as low as f^ (87 vibrations per second), or as high in a 
 good soprano as so! 4 (768 vibrations per second) ; or, in 
 other words, it is comprised within a range of a little more 
 than three octaves. There have been a few exceptional 
 singers who have been able to sing pure musical notes be- 
 yond these limits. Thus Gaspard Forster, a basso, passed 
 from fa - 1 (42 vibrations) to Ia 3 (435 vibrations) ; it is said 
 that Nilsson, in // Flauto Magico 3 can take fa 5 (1365 vibra- 
 tions) ; and Mozart states that in Parma, in 1770, a soprano, 
 
246 Physiology of the Senses 
 
 Lucrezia Ajugari, ranged from so! 2 (192 vibrations) to do 6 
 (2048 vibrations). The latter is the most highly pitched 
 voice in musical literature, an octave and a half above the 
 highest ordinary soprano. The extreme range of the 
 human voice, then, taking into account the extraordinary 
 voices above alluded to, is from fa- 1 (42 vibrations) to 
 do 6 (2048 vibrations), or about six octaves, while the range 
 of the human ear for musical tones is from do - 1 (32 vibra- 
 tions) to do 10 (nearly 40,000 vibrations), or about eleven 
 octaves. 
 
 2. Intensity or Loudness. The second character of 
 a musical tone which we notice is its intensity or loud- 
 ness. This varies with the amplitude of vibration of the 
 sounding body. Thus a tuning-fork bowed gently will give 
 out a faint sound, while the same fork bowed strongly will 
 give a note of the same pitch as the former, but sounding 
 much louder. 
 
 In the case where the particles of the wave move at 
 right angles to the direction in which the wave is advancing, 
 as, for instance, a wave on the surface of water, one can 
 readily understand what is meant by the height or amplitude 
 of the wave. But this is not so easy in connection with a 
 wave of sound where the particles are moving in the same 
 direction as the wave, and we are apt to confuse the ampli- 
 tude with the length of the wave, which, as we have seen, 
 is invariable in any given medium for any given note, and 
 determines pitch, not intensity. We can probably realise 
 the meaning of amplitude best in connection with sound 
 waves by thinking of what happens when a large tuning- 
 fork is vibrating feebly or strongly. In the one case, the 
 excursion of the limbs is so small that, to the unaided eye, 
 the fork seems to be motionless ; in the other, there is a 
 perceptible movement through space, and though the pitch 
 of the note remains the same, it has a louder, stronger 
 
Sound and Hearing 247 
 
 effect upon the ear. The fork makes exactly the same 
 number of vibrations in each case, but in the latter its 
 limbs move through a greater distance. Hence more 
 molecules of air must at one moment be crowded into a 
 ^iven space, at another there must be a more complete 
 rarefaction of the air. There must then be a greater 
 difference in the degree of pressure upon the drum-head of 
 the ear; at one time a greater increase, at the next a 
 greater diminution. Corresponding to this, there will be 
 greater movement of the tympanic ossicles, and more 
 variation in the pressure on the internal ear, and disturb- 
 ance of the nervous arrangements. The contrast of loud 
 and faint sounds can be readily made by holding to the 
 ear a vibrating tuning-fork, and turning it round between 
 finger and thumb, now this way, now that. It will be 
 found and this bears out the statement just made as to 
 amplitude that the sound is loudest when the plane in 
 which the limbs are vibrating is at right angles to the side 
 of the head, for here the air is disturbed with the greatest 
 energy. The same experiment also shows the gradual 
 transition in intensities just as in the case of pitch. The 
 more the energy of vibration, or, in other words, the 
 greater the number of molecules packed into a given space 
 in a given time, the greater will be the loudness a pheno- 
 menon comparable to the sensation of varying brightness 
 of light. 
 
 3. Quality, Timbre, Klang. The quality of a musical 
 sound enables us, after a due amount of training, to know, 
 from the effect produced upon the ear, what is the instru- 
 ment by which the sound has been produced. We readily 
 distinguish, for example, a musical note produced upon the 
 piano from that of the violin, or either of these from the 
 tones of the human voice, or of a wind instrument such as 
 the flute. Each kind of instrument produces a set of 
 
248 Physiology of the Senses 
 
 characteristic wave-forms, and the musician can tell by the 
 effect produced what kind of instrument is sounding. 
 
 The simplest form of vibration which gives rise to the 
 sensation of a musical tone is that of a body vibrating in 
 simple harmonic motion. Suppose a disturbance to be 
 made in the perfectly smooth and level surface of a sheet 
 of water. A concentric series of waves will spread out- 
 wards from the point of disturbance in ever-widening 
 circles. But while the wave -forms move outwards, the 
 particles which go to form the waves have only a vertical 
 motion, up to the crest of the wave above, or down into 
 the trough below, the ordinary water-level ; and after a 
 series of gradually diminishing oscillations, they come to 
 rest exactly in the position from which they started. If 
 the waves were all of equal size the particles would move 
 up and down in simple harmonic motion. Similarly, when 
 a tuning-fork is vibrating so as to give forth a pure tone, its 
 various parts move in approximately simple harmonic 
 motion. 1 
 
 If we attach a stylet to the limb of a tuning-fork, set the 
 fork vibrating, and allow the stylet to write upon a sheet of 
 paper drawn in the direction of the length of the fork, a 
 curved line will be traced upon the paper similar to the 
 curve from d^ to S in Fig. 125. The shape of the tracing 
 will depend upon the rate at which the paper moves. If 
 the paper moves slowly the waves will be short and steep ; 
 if quickly, they will be elongated. Such a series of vibra- 
 tions reaching the ear* gives rise to a sensation which, lacking 
 
 1 A simple harmonic motion is thus mathematically defined by 
 Thomson and Tait, Elements of Nat. Phil. Part I. p. 19 : " When a 
 point Q moves uniformly in a circle, the perpendicular QP drawn from 
 its position at any instant to a fixed diameter AAofthe circle, intersects 
 the diameter in a point P, whose position changes by a simple harmonic 
 motion." 
 
Sound and Hearing 
 
 249 
 
 brilliancy and variety, soon palls on the ear. The one 
 continuous tone has a dull uniformity ; it is monotonous in 
 every sense of the word. 
 
 In the next place, suppose we have two tuning-forks 
 vibrating at the same time but at different rates, and for 
 the sake of simplicity let one of them vibrate twice as 
 quickly as the other. We can now attend at will to the 
 tone given forth by either fork, or to a new third sensation 
 
 FIG. 125. Pendular vibrational curves A and B. C, Vibrational curve obtained 
 by superimposing B on A, so that the point e is on d '> D, vibrational curve 
 obtained by superimposing B on A, with the point e on d\ of A. (Von 
 Helmholtz.) 
 
 produced by the combination of the two tones. If the 
 waves of condensation begin at exactly the same instant, the 
 combined effect may be graphically represented by the 
 continuous line in C, Fig. 125. When both forks produce 
 condensation or rarefaction of the air at the drum-head at 
 the same time, the effect will be that of the sum of the two. 
 If one tend to produce condensation, while the other 
 causes rarefaction, the combined effect will be equal to the 
 
250 Physiology of the Senses 
 
 difference of the two. Thus the height of the continuous 
 curve C (Fig. 1 25) at the perpendicular ^ is equal to the sum 
 of the height a^ d^ of wave A, and of the height of the crest 
 at b^ in curve B. At d^ no effect is produced by B as the 
 crest is changing to the trough. At a 2 d^ A is still pro- 
 ducing condensation, while B is producing rarefaction, the 
 resultant effect being that at this phase the continuous 
 line c falls below the dotted line between c^ c 2 , and so on. 
 If the crests do not occur at the same moment, but at 
 different times, as in D, the resultant form of wave 
 will be different from that of C. Similarly in the case 
 of the smooth sheet of water, if the surface be disturbed 
 at two points the waves meeting and intersecting will 
 have increased height or depth when crest meets crest 
 or when trough meets trough, but if the crest of the .one 
 coincide with the trough of the other, the measure of 
 the amplitude of the resultant wave will be the difference 
 between the two. If the waves be of the same size and 
 meet so that the crest of one exactly coincides with the 
 trough of the other, they will counterbalance or neutralise 
 each other, and the result will* be a level surface for the 
 water, or in the case of sonorous vibrations rest of the 
 molecules and silence. And now let us suppose that we 
 have an indefinite number of sets of vibrations, whose 
 period or time of vibration is such that the primary or 
 fundamental series is always a multiple r of the smaller or 
 more rapid sets, then the resultant curves, as graphically 
 represented, may assume an infinite variety of forms, but 
 these being repeated at regular intervals, the effect upon 
 the ear will be that of a musical note. Wfyat complicated 
 forms the wave may take can be readily imagined if we 
 think of the effect produced on the surface of the sea by a 
 gale of wind. The great rollers have their crests buffeted 
 and broken by conflicting gusts, their surfaces roughened 
 
Sound and Hearing . 251 
 
 by a thousand waves and ripplets. No two great waves 
 seem exactly alike. Such a disturbance of the atmosphere 
 affecting the ear would give rise simply to a noise, but let 
 the great waves, irregular as they may be, succeed each 
 other as exact copies one of the other, then we will have the 
 musical tone, whose pitch or fundamental tone is that of 
 the largest waves, but whose quality is determined by the 
 combination of waves and wavelets into one. 
 
 Resonators. We can easily prove that the musical 
 notes of most instruments are compounded of a fundamental 
 and upper partial or overtones by using the resonators 
 of von Helmholtz. These are hollow spheres of brass or 
 glass with apertures to either side, as seen in Fig 126, or 
 tubes shaped somewhat like a bottle with- 
 out a bottom. The air in these instru- 
 ments vibrates at a given rate, or in 
 other words, with a certain pitch deter- 
 mined by the size of the resonator (the 
 larger the resonator the lower the pitch), 
 
 FIG. 126. Resonator of 
 
 and most loudly when a note of the von Helmholtz. 
 same pitch is sounded in the vicinity 
 of the resonator. When the smaller aperture is inserted 
 into the external ear the special tone is heard to the exclu- 
 sion of all others, the amplitude of the vibration being 
 largely increased in the resonator. The principle by which 
 this is brought about is the same as that which comes 
 into play when any periodic motion is increased in amplitude 
 by slight successive increments. For instance, suppose we 
 wish to cause a person sitting on* a swing to rise to a con- 
 siderable height, or, in scientific terms, to cause the swing 
 to move in vibrations of large amplitude. We first push 
 the swing from the vertical, and thereby cause it to rise 
 a slight distance above its lowest position. Under the 
 influence of gravity the swing falls back to its position of 
 
252 Physiology of the Senses 
 
 rest, but acquiring momentum as it falls it passes the 
 vertical line and rises on the other side until stopped by 
 gravity, the friction of the rope, and the resistance offered 
 by the air to the movement of the body through it. If, 
 further, we ourselves interpose, we can readily prevent the 
 rise and bring the swing to rest. But suppose we wait 
 till the swing, having risen as high as possible, stops and 
 begins to fall again and now give another slight push in 
 the same direction as formerly. The new force added to 
 the old, which has not yet entirely died away, causes the 
 swing to rise a little higher than at first, and the return rise 
 is also higher. Again, when it begins to fall we give a slight 
 push, and so on, till at last the swing sweeps to and fro in 
 wide oscillations and with great momentum. The periodic 
 application of a slight force has given rise by summation of 
 effect to a great force and extensive movement. So is it 
 with the resonator. Vibrations of small amplitude in the 
 external air set the molecules of air in the resonator into 
 oscillation, and the successive impulses are given just at the 
 moment when they will increase the amplitude of vibration. 
 Thus atmospheric vibrations which, when diffused freely 
 through the air, have insufficient energy to give rise to a 
 sensation, will, acting upon the air in the resonator, set up a 
 sympathetic resonance, which enables the ear to detect 
 their presence even amid a multitude of louder sounds. But 
 if the pitch of the external note is sharpened or flattened, 
 the vibrations clash, and the resonator is silent. 
 
 Analysis of Compound Tones by Resonators. To 
 satisfy ourselves that the sound produced by most musical 
 instruments is compounded of many simple tones, we have 
 simply to sound a note upon the instrument in question, and 
 listen with a series of resonators. We will have, firstly, 
 resonance for the fundamental tone, and then for a set of 
 tones of higher pitch whose vibrational numbers are 
 
Sound and Hearing 253 
 
 multiples of that of the fundamental tone. We might have, 
 
 for example, a set of overtones or partials or harmonics of 
 the following relationship : 
 
 Fundamental 
 
 Upper Partials or 
 
 Harmonics. 
 
 Note . . do 1 
 
 do 2 
 
 sol 2 
 
 do 3 
 
 mi 3 soi 3 
 
 sib 3 
 
 do 4 
 
 re 4 
 
 mi 4 
 
 Partial tones 
 
 I 
 
 2 
 
 3 
 
 4 
 
 5 6 
 
 7 
 
 8 
 
 9 
 
 10 
 
 Number of \ 
 vibrations J 
 
 33 
 
 66 
 
 99 
 
 132 
 
 165 198 
 
 231 
 
 264 
 
 297 
 
 330 
 
 Instead of applying a series of resonators to the ear, 
 and so detecting the presence of various simple tones by 
 hearing, we may analyse the compound note, and demon- 
 strate optically the presence of the partial tones by means 
 of an apparatus devised by Konig. This consists of a 
 series of resonators mounted on a frame. The apertures of 
 the resonators, which are usually inserted into the ear, are 
 connected by elastic tubing with a set of small boxes. 
 Coal-gas is led into the boxes, but prevented from passing 
 to the resonators by closure of the entrance to the tubes 
 with a thin india-rubber membrane. The gas passes from 
 the boxes to a corresponding set of small burners, which 
 give long pointed flames. When the air in one of the 
 resonators is set in vibration, the membrane shutting off the 
 resonator from the gas-box vibrates in sympathy, causing 
 a variation in the pressure of the gas, and of the size of the 
 flame. With all musical tones, however, the number of 
 vibrations per second is so great that, from persistence of 
 the retinal impression, we are unable with the naked eye to 
 see the change in size of the flame. To obviate this diffi- 
 culty, the rays of light from the flame are reflected to the 
 eye from the surface of a cubical mirror rotating upon an 
 upright axis. If the flame is burning steadily, the series of 
 reflections of the light sent from the rotating mirror are 
 blended into one smooth edged band of light ; but if the 
 
254 
 
 Physiology of the S&nses 
 
 resonator is in action, the smooth band gives place to one 
 with teeth on its upper border. Each tooth represents an 
 increase of pressure from the resonator, each notch a diminu- 
 tion. When a note containing the overtones to which these 
 resonators respond is sounded, the flame picture in the 
 mirror will declare their presence. The adaptation to organ 
 
 FIG. 127. Konig's apparatus for studying optically the vibration of air in 
 organ pipes. 
 
 pipes of the same kind of apparatus, viz. the gas-box, and 
 the light of the flame reflected from a rotating mirror, is 
 shown in Fig. 127, where we have the means of studying 
 the vibration of air in organ pipes. By such an arrange- 
 ment, for example, we may see that with two organ pipes 
 sounding, the one an octave higher than the other, the 
 
Sound and Hearing 255 
 
 flame picture on the mirror for the upper note will have 
 twice as many elevations as that of the lower. 
 
 In the absence of von Helmholtz's resonators, a simple 
 means of analysing a compound note, or at least of detect- 
 ing its most important partial tones, is to cause the note to 
 sound beside a piano. If we gently depress the key corre- 
 sponding in pitch to that of the note sounded, so as to 
 remove the damper, we will hear quite distinctly the sound 
 of the piano-string vibrating in sympathetic resonance. Next 
 depress the key of the octave above, and we will hear it 
 sounding, but more faintly than the fundamental note. 
 Again, if we press down the key of the fifth (sol) in the 
 second octave, and so on with the various harmonic over- 
 tones, we will hear the resonance, but always becoming 
 weaker. It will, as a rule, be found that the sound obtained 
 from any note other than those in the harmonic series is 
 by no means so distinctly heard, although we may have in- 
 harmonic upper tones due to a note being not purely musical 
 in character, but accompanied in its production or propaga- 
 tion by noises. 
 
 For the notes sounded by almost all musical instruments, 
 then, we may conclude that each note is compounded of a 
 series of simple tones, each of which may be made to pro- 
 duce its effect upon the ear as if the others were absent, 
 and the total effect is due to a summation of the effects and 
 a combination thereof to give a new sensation. 
 
 We can imitate the notes of instruments having special 
 overtones by combining pure partial tones, and in the organ 
 some of the stops are so designed as to make sets of pipes 
 sound together whose pitch is such as to give the effect 
 of some other instrument, such as the flute, the clarinet, or 
 even the human voice (the vox humand). 
 
 As a result of a careful series of observations on the 
 quality of different musical tones, the particulars of which 
 
256 Physiology of the Senses 
 
 are detailed in his book On the Sensations of Tone?- von 
 Helmholtz arrives at the following conclusions : 
 
 " i. Simple tones, like those of tuning-forks applied to 
 resonance chambers, and wide stopped organ pipes, have a 
 very soft pleasant sound, free from roughness, but wanting 
 in power, and dull at low pitches. 
 
 "2. Musical tones, which are accompanied by a moder- 
 ately loud series of the lower upper partial tones up to 
 about the sixth partial, are more harmonious and musical. 
 Compared with simple tones they are rich and splendid, 
 while they are at the same time perfectly sweet and soft 
 if the higher upper partials are absent. To these belong 
 the musical tones produced by the pianoforte, open organ 
 pipes, the softer piano tones of the human voice, and of the 
 French horn. The last-named tones form the transition to 
 musical tones with high upper partials ; while the tones of 
 flutes, and of pipes on the flute stops of organs, with a low 
 pressure of wind, approach to simple tones. 
 
 "3. If only the uneven partials are present (as in narrow 
 stopped organ pipes, pianoforte strings struck in their middle 
 points, and clarinets) the quality of tone is hollow, and, 
 when a large number of such upper partials is present, nasal. 
 When the prime tone predominates, the quality of tone is 
 rich and full ; but when the prime tone is not sufficiently 
 superior in strength to the upper partials, the quality of 
 tone is poor or empty. Thus the quality of tone in the 
 wider open organ pipes is fuller than that in the narrower ; 
 strings struck with pianoforte hammers give tones of a 
 fuller quality than when struck by a stick, or pulled by the 
 finger ; the tones of reed pipes, with suitable resonance 
 chambers, have a fuller quality than those without resonance 
 chambers. 
 
 " 4. When partial tones higher than the sixth or seventh 
 1 Von Helmholtz, Sensations of Tone, pp. 172, 173. 
 
Sound and Hearing 257 
 
 are very distinct, the quality of tone is cutting and rough. 
 The reason for this lies in the dissonances which they form 
 with one another. The degree of harshness may be very 
 different. When their force is inconsiderable, the higher 
 upper partials do not essentially detract from the musical 
 applicability of the compound tones ; on the contrary, they 
 are useful in giving character and expression to the music. 
 The most important musical tones of this description are 
 those of bowed instruments, and of most reed pipes, oboe 
 (hautbois), bassoon (fagot), physharmonica (harmonium, 
 concertina, accordion), and the human voice. The rough 
 braying tones of brass instruments are extremely penetrat- 
 ing, and hence are better adapted to give the impression of 
 great power than similar tones of a softer quality. They 
 are consequently little suitable for artistic music when used 
 alone, but produce great effect in an orchestra." 
 
 It has been stated that the quality of a tone is dependent 
 upon the form of the wave which produces it. We have 
 seen that the graphic representation of a complex tone 
 reveals a series of very different forms of waves, according 
 to the phase or period of combination of the partial tones. 
 The question then arises : Does the ear appreciate these 
 differences of phase in the combinations of partial tones ? 
 For a given set of combined partial tones, do the different 
 resultant wave-forms give rise to sensations of different 
 quality ? To this question conflicting answers have been 
 given. On the one hand, it is maintained by von Helm- 
 holtz that u the quality of the musical portion of a compound 
 tone depends solely on the number and relative strength 
 of its partial simple tones, and in no respect on their differ- 
 ences of phase." The difference of wave-forms C and D in 
 Fig. 125, according to von Helmholtz, makes no difference in 
 the sensation of the quality of the resultant complex tone. 
 The ear has the power of resolving the complex vibrations 
 
 s 
 
258 Physiology of the Senses 
 
 into series of simple vibrations, and of hearing the pure 
 tones corresponding to these sets of vibrations. As accord- 
 ing to mathematical demonstration, however different the 
 wave -forms for any given combinational tone may be, 
 varying with phase of combination, these forms can only 
 be resolved into one definite set of partial tones, the ear 
 must always recognise the same set of partials, and we com- 
 bine them again to give rise to a tone of the same quality. 
 On the other hand, it is asserted that the different forms, 
 representing as they do real differences in pressure on the 
 drum-head of the ear, give rise to sensations of different 
 quality. The curve D, for example, in Fig. 125, may be 
 taken as representing short periods of increased pressure 
 and long periods of diminished pressure upon the tym- 
 panic membrane, while, by slightly altering the phase ot 
 the component parts, we could give rise to alternate long 
 periods of increased pressure and short periods of dimin- 
 ished pressure. In the one case, the general condition 
 is one of diminished pressure on the sensory apparatus 
 with brief change to high pressure ; in the other, the sen- 
 sory apparatus is subject in the main to higher pressure 
 than usual, but with short periods of low pressure interven- 
 ing. The pitch and intensity are, of course, unaffected, 
 because the rate of vibration and amplitude of the waves 
 are the same. The decision between the opposing opinions 
 can be made only by personal trial, for, theoretically, we 
 have no knowledge as to the way in which variations oi 
 pressure in the internal ear affect the sensory apparatus, 
 nor, again, how changes in the end organ are transmuted 
 into conscious sensation. As a matter of fact, the differ- 
 ences of quality, if any do arise, are very slight, and only 
 to be appreciated by a highly-trained ear, and with simple 
 binary compounds. For the notes of ordinary musical 
 instruments, or for combinations of numerous partials into 
 
Sound and Hearing 259 
 
 complex tones, it is practically impossible to detect differ- 
 ences of phase, so that the statement holds good in the 
 main that the quality depends, as von Helmholtz asserts, 
 upon the number and relative strength of the partial tones. 
 This holds for all perfect harmonies, at least those in which 
 the vibrations are strictly periodic and resolvable into series 
 of partial tones the period si the fundamental tone being a 
 multiple of those of the partial tones. 
 
 y~ Beats. When two simple tones of exactly the same 
 pitch are sounded together, if some arrangement be made 
 by which tfye phase of vibration of each coincides, the 
 result of their combination will be increased amplitude 
 of vibration of the drum-head, and increased intensity of 
 sound, but if the phase of one series of vibrations differ by 
 a half wave-length from the other, the one will neutralise 
 or interfere with the other, and there will be silence. Sup- 
 pose, now, that we have two simple tones sounding together 
 of the same intensity, and of nearly the same pitch say, 
 for example, that one is due to 200 the other to 201 vibra- 
 tions per second and suppose that the vibrations are in 
 the same phase to begin with, it is evident that, since one 
 falls behind the other to the extent of one wave-length in a 
 second, it must fall one-half of a wave-length behind in half 
 a second ; near the beginning and near the end of the 
 second the vibrations are nearly in the same phase, and 
 combine to intensity the effect ; but in the middle of the 
 second, being in opposite phases, they tend to counteract 
 each other, and there will be a diminution of intensity even 
 to momentary silence. There will thus be an increase of 
 volume followed by a diminution of volume of the sound 
 every second, and we have an unevenness in the sound, or 
 a succession of what have been called beats. The number 
 of beats per second will depend upon, and be equal to, the 
 difference of rate of vibration of the two partial tones. We 
 
260 Physiology of the Senses 
 
 have seen that a difference of one vibration per second gives 
 one beat per second. If the simple tones differ by two 
 vibrations per second, there must be two beats per second ; 
 for, since the one set falls two wave-lengths behind the 
 other in a second, they must be one wave-length behind in 
 half a second, and a half wave-length behind in a quarter 
 of a second. There is increase of sound about the begin- 
 ning of the first and third quarters, and diminution about 
 the beginning of the second and fourth quarters, or, as we 
 have said, two beats per second. Beats, then, can arise only 
 when the vibrational number of one set is not a multiple of 
 the other ; if the period of one is a multiple of the period 
 of the other, there can be no beat. When there are not 
 more than five or six beats per second, the ear can easily 
 note the gradual rise and fall in intensity, and the effect is 
 not unpleasant. When the beats come more quickly we 
 lose the power of paying attention to the rise and fall of 
 each beat, although we can still for a time recognise the 
 beats as arising and differing from the continuous tones. 
 The effect is that of a whirring harsh sound ; it is called dis- 
 sonance. According to von Helmholtz, by gradually increas- 
 ing the frequency of the beats, we may have as many as 132 
 per minute, and yet recognise the dissonant character of the 
 sound and the presence of beats. Beyond this number the 
 regular recurrence of the beats leads to a secondary fusion, 
 and the starting of a new tone arising from the beats a beat- 
 tone. The ear fails to recognise a strictly musical character 
 in beat-tones even when the beats are much more numerous 
 than the vibrations required for an ordinary musical tone. 
 This we may possibly explain by the fact that the develop- 
 ment of beats is due not so much to a variation of pitch as 
 of intensity. The higher tone continues to sound at exactly 
 the same pitch as before, and there is merely a periodical 
 variation in the amplitude of the vibrations which give rise 
 
Sound and Hearing 261 
 
 to it. We have, then, in the production of beats, a condi- 
 tion analogous to the variations of pressure experienced in 
 the sense of touch, in which, as stated (p. 58), we are able 
 to discriminate the individual stimuli much longer than we 
 can either with visual or ordinary auditory stimuli. There 
 may be no fusion by the sense of touch of as many as 500 
 stimuli per second ; whereas, if the stimuli to the eye come 
 faster than 10 per second, or by the ear 30 per second, 
 there is a fusion in sensation. In the phenomena of beats, 
 then, we seem to find a link between the sensation of touch 
 and that of 'hearing, the tactile element (variation of ampli- 
 tude) being- superposed upon the auditory element (con- 
 stancy of pitch). The unpleasantness of the, sensation 
 excited when the beats come at about 3 5 per second, when 
 carefully investigated, is found to be similar- in kind to tRat 
 experienced when the senses of sight and touch are stimu- 
 lated too rapidly for the bestowal of attention on each 
 stimulus, and yet too slowly to give rise to central sensory 
 fusion. A flickering light has a similar effect. The mind 
 seeks, as it were, to maintain order in the reception of the 
 messages of sense, to give to each sensation its due recogni- 
 tion, and yet to subordinate it to general relationships and 
 conscious sequence. But the stimuli come on the border- 
 line between what may be grasped and what may not. 
 Before the sensorium has had time to give full effect to one 
 stimulus another has come upon it, and finds it partly ready 
 but not quite, or, from the physical point of view, the sen- 
 sory centre has not had time to recover completely from the 
 disintegrating effect of one shock before it has to endure 
 another. Something is being impressed upon the receptive 
 centres which tends to force the mind from the path in 
 which it seeks to move, and which is itself followed by 
 another and another claimant for notice, till we become 
 irritated at the disturbance and weary of the repeated dis- 
 
262 Physiology of the Senses 
 
 traction. All this, of course, takes place in a semi-uncon- 
 scious way, since it is not, as a rule, the beat in the sound 
 or the flicker in the light to which we wish to pay attention : 
 the pure musical sound with which the beat interferes, or 
 the thing seen, now clearly, now dimly, in the changing light, 
 is the object of mental effort. Without analysing the nature 
 of the disturbing element, we feel that it is there, and to 
 this must in the main be attributed the disagreeable effect 
 produced. 
 
 Yet while this holds true of long-prolonged tones roughened 
 by fast-repeated beats, it must be remembered that in ordi- 
 nary orchestral music we rarely hear notes entirely free from 
 beats. While the various notes of a chord struck upon a 
 piano may be of such pitch as not to generate beats, the 
 overtones of these interacting on one another most prob- 
 ably will. Certain chords, no doubt, are freer from such 
 roughness, and it is no uncommon thing to heighten the 
 effect of a pure harmonious note by causing it to be preceded 
 by a discord. Contrast in sound, as in colour, heightens 
 the effect on the sensorium. The eye fatigued by looking 
 at a red colour will, when turned to a green surface, see it 
 of intenser hue ; the ear has a keener appreciation of pure 
 harmony when the harsh note has ceased to jar. 
 
 Noise. When auditory stimuli are non-periodic in char- 
 acter the resultant sensation is that of a noise. A single 
 variation of pressure upon the tympanum might be sufficient 
 to set the mechanism of hearing in action, but the resultant 
 sound could not be musical in character. It has been held 
 by soriie that two impulses exactly alike, and the one quickly 
 following the other, may give rise to a musical sensation, 
 but the probability is that the musical effect is in this 
 instance due to overtones, and to such a sound it is not 
 possible to assign a definite pitch. The ear can easily distin- 
 guish as separate noises the effect upon it of impulses coming 
 
Sound and Hearing 263 
 
 at the rate of less than 1 6 per second. When the noise is 
 due to vibrations coming at the rate of more than about 1 6 
 per second, there is a certain amount of fusion in sensation, 
 and the noise has for us a certain pitch. Where there is 
 an initial shock, as in a thunder-peal, with echoing and re- 
 echoing at somewhat prolonged intervals, we have a deep, 
 rumbling sound ; if the vibrations succeed one another very 
 quickly we have sounds or noises of high pitch, which we 
 describe as crackling, whistling, rustling, shrieking, creak- 
 ing, and so on. The wind sweeping through a forest sets 
 up an infinite number of intermittent variations of aerial 
 pressure as it sways branches and leaves to and fro, and a 
 low rustling sound is heard ; but when it agitates tense 
 structures, such as the cordage of a ship's rigging or the 
 strings of an yEolian harp, the sound becomes more dis- 
 tinctly musical, and especially if the wind blows with a fairly 
 constant force. The harsh nature of the sound educed from 
 a violin by an unskilled performer is due to inequalities of 
 pressure upon the strings with the bow, while the master 
 hand, by maintaining steady continuous pressure for longer 
 or shorter intervals, and thus eliminating discordant over- 
 tones, will draw forth pure melodious sounds. 
 
 General Mode of Action of the Ear. Having con- 
 sidered the structure of the ear and the physical nature of 
 sound, we have next to see how the one is adapted to the 
 other, how the ear responds to auditory stimuli. Much 
 may be learned from the study of pure physics as to the 
 beauty of the mechanical adaptations, but this merely"brings 
 us to the threshold of sensation. The changes in the audi- 
 tory nerves and nerve centres which accompany or give 
 rise to the sensation of sound are almost entirely unknown. 
 Even with regard to the mode of action of the internal ear 
 there is still much uncertainty. 
 
264 Physiology of the Senses 
 
 The external ear, we have seen, acts mainly as a collector 
 of sound waves, and the external meatus, closed internally 
 by the drum-head, helps, like von Helmholtz's resonators, to 
 increase the energy with which the membrane is agitated. 
 
 The middle ear is so constructed as to diminish as little 
 as possible the power of the aerial vibrations in their trans- 
 mission to the sensory terminals. When vibrations pass 
 directly from air to solids or liquids, much of their energy is 
 lost. If a membrane intervenes between the air and a 
 liquid, the energy is not lost to so great an extent. There is, 
 therefore, mechanical advantage in the separation of the fluids 
 of the internal ear from the air by the membranes closing the 
 round and oval windows. But these membranes are small 
 of size, tense in texture, and in apposition upon one side 
 with fluid in an enclosed space. They have thus little 
 amplitude of movement. This is compensated for by the 
 drum-head. Being larger than the membrane of the oval 
 window, and having air upon both sides, it vibrates freely, 
 and being firmly attached to the tympanic ring and tense in 
 the greater part of it, its vibrations are readily transmitted 
 to the attached chain of bones, and by them, with little if 
 any loss of power, to the foot of the stirrup-bone with its 
 membranous attachment to the circumference of the oval 
 window, and so to the perilymph. Nay, there may be an 
 actual gain from the lever action of the chain of bones and 
 the greater size of the drum-head (p. 213). The chain of 
 bones, working freely in the middle ear, gives, as we have 
 seen, a greater amplitude of movement than would be avail- 
 able if the internal ear were simply buried deeply in the 
 cranial bones. Still, the ligamentous connection of the bones 
 with the membranes and the walls of the tympanum hinders 
 over-movement, and enables them to act as dampers, pre- 
 venting unnecessary oscillation of the drum-head. The 
 tenseness of the membrane and, consequently, its power of 
 
Sound and Hearing 265 
 
 responding to sounds of different pitch and intensity are 
 likewise regulated by the intrinsic muscles of the middle 
 ear, and more especially by the tensor tympani muscle, 
 while the entrance of air by the Eustachian tube maintains 
 equality of atmospheric pressure upon the two sides of the 
 drum-head. 
 
 Vibrations then may reach the internal ear either through 
 its osseous walls or through the membranes of the oval and 
 round windows. In the vestibule and semicircular canals 
 these vibrations are further transmitted to the membranous 
 labyrinth through the perilymph, for the connection of this 
 part of the auditory sac, with its surrounding walls, is by no 
 means so close as in the case of the cochlear canal. Through 
 the membranous sac the vibrations reach the endolymph, and 
 so come to the terminations of the vestibular portion of the 
 auditory nerve in the maculce of the utricle and saccule, and 
 in the cristce of the ampullae of the semicircular canals. 
 The effect may be enhanced by the otoconia (p. 227) in the 
 endolymph, and by the rods projecting from the auditory 
 epithelial cells ; for, as has been pointed out, the hand 
 thrust into water may be incapable of detecting the presence 
 of sound waves passing through the water, but will easily do 
 so if grasping a rod. This will be readily understood if we 
 consider that the rod will act as a lever, and so increase the 
 effect of the sound waves on the hand. 
 
 That the auditory hairs do actually sway to and fro under 
 the influence of sonorous vibrations may be taken as proved, 
 for Hensen has seen with low microscopic powers the audi- 
 tory hairs of Mysis (the opossum shrimp) vibrating in 
 response to the notes of a keyed horn. The auditory hair- 
 cells are either the terminations of the auditory nerve fibres, 
 or are in close apposition with them, and, on receipt of the 
 vibrational stimulus, an impulse is given to the nerve ; but 
 at this point we are arrested, for we do not know whether 
 
266 Physiology of the Senses 
 
 or not the nerve current corresponds in rate of intermission 
 with the variation of pressure due to sound, whether vibra- 
 tions are transmitted along the nerve, or whether we have 
 to do with an entire change of physiological phenomena in 
 the development of the nerve current. 
 
 In the case of the cochlea, the vibrations may be trans- 
 mitted by the perilymph, and through the membrane of 
 Reissner and the cochlear endolymph, or through the 
 basilar membrane to the endings of the cochlear branch of 
 the auditory nerve in Corti's organ, or sonorous vibrations 
 of the bones of the skull may, through the medium of the 
 spiral osseous lamina and Bowman's spiral ligament, be 
 directly transmitted to the basilar membrane and its super- 
 jacent structures. 
 
 From noting the mode of termination of the cochlear 
 nerve in or round the hair-cells of Corti's organ, and from 
 the analogy of the nerve-endings in hair-cells in the case of 
 the other special senses, we cannot but infer that the hair- 
 cells in the organ of Corti form the peripheral sensory ter- 
 minals, while the rods of Corti and the supporting cells of 
 Deiter, with their phalangeal connections, serve mainly to 
 transmit to the hair-cells the vibrations set up in the basilar 
 membrane. 
 
 In all parts of the fluid of the internal ear changes of 
 pressure due to movements of the chain of bones must be 
 experienced, and as the fluid is incompressible, there must 
 be an outward or inward movement of the membrane of the 
 round window corresponding respectively to every inward or 
 outward movement of the stapes. The question therefore 
 arises : Do all parts of the internal ear, or at least, do all 
 the terminations of the auditory nerve, respond alike to the 
 sound ; or does each nerve -ending have a special duty to 
 perform, have a special response to a special element of the 
 sound, be it pitch, intensity, or quality ? 
 
Sound and ff ear ing 267 
 
 ' 
 
 The semicircular canals in relation to movements. Con- 
 sidered merely from an anatomical point of view, we should 
 expect a difference in function corresponding to the struc- 
 tural differences between the macula^ cristcs^ and organ of 
 Corti, between the vestibular and cochlear divisions of the 
 auditory nerve, and the different nerve centres to which they 
 pass. It has even been suggested that the vestibular nerve 
 and its terminals have nothing to do with the sense of hear- 
 ing, but have to do with the sense of equilibrium or of the 
 position of the head in space, while the appreciation of 
 sound is relegated to the cochlea alone. In support of this 
 view it has been pointed out that the semicircular canals, with 
 their cristce acustica, may be destroyed without impairment 
 of the sense of hearing. At the same time, the animal be- 
 gins to perform peculiar movements which vary according 
 to the canal destroyed. If either of the canals in the 
 vertical plane is injured, the animal rotates its head round 
 a horizontal axis at right angles to the plane of the canal ; 
 and, if the horizontal canal be injured, rotation takes place 
 round a vertical axis. 
 
 These rotary movements being similar to those produced 
 by lesions of the cerebellum, and being apparently asso- 
 ciated with a disturbance of the power of co-ordinating 
 muscular movement a power which depends largely upon 
 the sense of equilibrium it was held that the canals have 
 to do with this sense, or, as suggested by Cyon in 1872, with 
 sensation as to the position of the head in space. As Crum 
 Brown has shown, the canals of the opposite sides of the 
 head may be divided into three sets of two each in nearly 
 identical planes, and so related as to be nearly at right 
 angles to each other. When the head is moved in any 
 direction, the fluid in the canals tends to move in the 
 opposite direction, or at least to- lag behind the moving 
 walls of the canals, just as when we rotate a vessel contain- 
 
268 Physiology of the Senses 
 
 ing water the inertia of the water prevents its moving so 
 quickly as the vessel at first, and of stopping so quickly 
 when once set in motion. As the volume of fluid in the 
 canals is constant, the fluid must, however, move with the 
 head. It cannot lag behind, but there will be variation of 
 pressure due to inertia. Thus, according to Crum Brown, 
 " in each of the three pairs of canals (right and left hori- 
 zontal, right superior and left posterior, right posterior and 
 left superior) the two canals are so placed that when rota- 
 tion takes place about the axis to which they are perpen- 
 dicular, one of the two canals moves with its ampulla 
 preceding the canal, so that the flow or tendency to flow 
 (or pressure) is from ampulla to canal, while in the other the 
 ampulla follows the canal, and the flow or tendency to flow 
 (or pressure) is from canal to ampulla. If, then, we sup 
 pose that flow from ampulla to canal or adopting Mach's 
 view, increase of pressure in the ampulla alone stimulates 
 the hair-cells, while no effect is produced by flow in the 
 opposite direction or by diminution of pressure in the 
 ampulla we have in the six canals a mechanical system 
 capable of giving us an accurate notion of the axis about 
 which rotation of the head takes place and of the sense of 
 rotation." 1 It has been further urged that the macula of 
 the utricle and saccule have to do respectively with the 
 sense of movement in a vertical or horizontal straight line, 
 just as the cushions of the ampullae respond to rotation. 
 
 On the other hand, it is alleged that even when the 
 auditory nerve is destroyed and the body rotated, a sensa- 
 tion of rotation comes on as usual. If this be so, the canals 
 cannot be essential to the sense of position. Again, it is 
 held that we cannot dissociate the vestibular nerve from 
 
 1 A. Crum Brown, " Cyon's Researches on the Ear," Nature, 
 1878. See also M'Kendrick's Text -Book of Physiology, vol. ii. 
 p. 694. 
 
Sound and Hearing 269 
 
 auditory sensation, since animals which can undoubtedly 
 hear well may have a very rudimentary cochlea. 
 
 On the whole, it seems probable that the vestibular 
 nerve can respond to auditory stimuli. It may act under 
 the stimulus of sound, and it may respond to differences of 
 intensity of sound, but can it lead to the appreciation of 
 differences in the pitch of sound ? To this question we 
 must probably give a negative answer. No doubt, in the 
 case of crustaceans, Hensen has found that auditory hairs 
 of different lengths respond to certain notes better than to 
 others, but <no such difference of length in the auditory 
 hairs of the maculce or cristce can be seen in the human ear, 
 nor any difference that could lead us to imagine that one 
 cell should respond differently from another. The hairs on 
 the hair-cells of Cord's organ are still shorter, so that we 
 cannot conceive that they have any differentiating action 
 as regard the appreciation of pitch. They seem to act 
 rather, as suggested above, as minute levers by means of 
 which the auditory cells are rendered sensitive to even 
 the slightest movements in the fluid that bathes their free 
 surfaces. 
 
 Analytic Power of the Ear. Has the ear, then, any 
 mechanism which enables it to appreciate differences of 
 pitch, or to analyse a compound tone into its constituent 
 partial tones ? There is a fusion of all partial series of 
 vibrations in the air of the external ear. The tympanic 
 membrane vibrates as a whole, and responds to the com- 
 pound summational wave, however complex its form may 
 be that is to say, however quickly it changes, and propor- 
 tionally in extent to the variations of atmospheric pressure. 
 With the drum-head moves the chain of bones, and with it 
 again the perilymph and the endolymph. Yet, in the sen- 
 sorium, we can appreciate either the quality of the complex 
 tone, or we can attend to its constituent parts. Wherein 
 
270 Physiology of the Senses 
 
 comes the power of analysis ? Is it the case, as Ruther- 
 ford holds, that the hairs of all the auditory cells vibrate 
 to every tone, just as the drum of the ear does, and that 
 there is no analysis of complex vibrations in the coch- 
 lea or elsewhere in the peripheral mechanism of the ear ; 
 that the hair- cells transform sound vibrations into nerve 
 vibrations, similar in frequency and amplitude to the sound 
 vibrations ; that simple and complex vibrations of nerve 
 molecules arrive in the sensory cells of the brain, and there 
 produce not sound again, of course, but the sensation of 
 sound, the nature of which depends, not upon the stimula- 
 tion of different sensory cells, but on the frequency, ampli- 
 tude, and form of the vibrations coming into the cells, 
 probably through all the fibres of the auditory nerve ? x 
 
 Upon this theory the whole internal ear vibrates in unison 
 with the drum-head, and the auditory nerve in unison with 
 both, just as the receiving plate of a telephone moves in 
 unison with the transmitting plate. Analysis must then be 
 a mental act dependent upon the powers of the central nerve 
 cells, but how it is to be exercised we are not informed. 
 
 Or does the power of analysis lie with the cochlea ? This 
 is the theory which von Helmholtz first stated and explained 
 with consummate skill. We have seen (p. 255) that when 
 a compound tone is sounded before a piano with uplifted 
 dampers, the strings of the piano which are in tune with the 
 partial tones of the compound tone will vibrate. Similarly, 
 von Helmholtz conceived that the cochlea has the power of 
 analysing compound tones into simple pendular vibrations, 
 and that different parts of the cochlea respond each to the 
 particular partial to which it is attuned. At first, he sup- 
 posed the rods of Corti's organ were the structures which, 
 varying in size and shape, took up each its own tone, and, 
 
 1 Rutherford, "On the Sense of Hearing," The Lancet, January 
 1887. 
 
Sound and Hearing 271 
 
 by striking upon or otherwise exciting the hair-cells with 
 which they were connected by means of the phalangse, caused 
 sensory stimuli to be sent by the nerve fibres attached to 
 the hair-cells to corresponding nerve cells in the sensorium. 
 He did not, however, suppose that the nerve current re- 
 sembled physically in any way the vibration which roused 
 the auditory cell. The resulting sensation was simply due 
 to the specific power of the cell in the brain, to give rise to 
 a sensation of a sound of a certain pitch when stimulated 
 by its proper tone. 
 
 Various considerations, however, induced him to modify 
 his theory. In the first place, the rods of Corti vary very 
 little in form and size, as we pass from the base to the apex 
 of the cochlea. Again, there are only about 3000 of them 
 altogether, and yet we can distinguish differences of pitch 
 in sounds varying in their number of vibrations from 30 to 
 40,000 per second. Further, we have good grounds to 
 believe that birds can distinguish the pitch of tones, and 
 yet the rods of Corti are entirely absent from their cochleas 
 which have the hair-cells in contact with the basilar mem- 
 brane, and are very rudimentary in other respects. For 
 these and similar reasons, von Helmholtz supposed that 
 the real analysers, in respect of pitch, are the fibrils in the 
 outer part of the basilar membrane, and that the rods of 
 Corti simply serve to pick up and transmit their vibrations 
 to the hair-cells. This view is supported by the fact that 
 the basilar membrane is stretched firmly in the direction of 
 these fibrils, but is loose in the direction of the canal. The 
 fibres are easily separated from one another, but are not 
 readily torn across. The membrane will not vibrate, as a 
 whole, like one in which the tension is alike in all directions, 
 but it is made up of strings or fibres, each of which may 
 vibrate independently of the other. 
 
 There are about 24,000 of these fibrils in the basilar 
 

 272 Physiology of the Senses 
 
 membrane a number much larger than that of the rods of 
 Corti, although less than the number of sounds between 
 which we can make a distinction of pitch. Von Helmholtz 
 supposed, then, that these^fibrils, varying in length and 
 possibly in tension, may respond in sympathetic vibration 
 each to its proper tone, and that these vibrations are trans- 
 mitted to the hair-cells by their supporting structures. If a 
 tone falls upon the ear which does not correspond exactly 
 in vibrational frequency with that of any of the fibrils, von 
 Helmholtz suggested that two or more adjacent fibrils might 
 respond in various degrees, that being strongest which 
 approximated most nearly to the stimulus, the others more 
 feebly. By a mental combination and comparison of the 
 different stimuli the true pitch of the note would be arrived 
 at. Thus each fibril has, according to him, one proper 
 tone to which it answers strongly, while to all others it is 
 less responsive. Similarly, in the case of the stimulation 
 of the auditory hairs of Mysis, it was found that different 
 hairs responded strongly to different tones. One, for 
 example, vibrated strongly to d and d'> more weakly to 
 g, and very weakly to G. Another hair answered strongly 
 to a and adjacent tones, more weakly to d and A J. For 
 some tones, then, the cerebral cells are directly tuned, but 
 not for others ; for all others there must be a comparison 
 of several tones and appreciation of pitch through the means 
 of an average. As von Helmholtz does not suppose that the 
 nerve current in any way corresponds in number of vibrations 
 to that of the exciting cause, each nerve cell depends on its 
 own inherent power of response in giving rise to a sensation 
 of a special pitch. But, further, it has been computed that 
 there are only about 15,000 hair-cells, and if it be the case 
 that each of these is connected with one nerve fibre and its 
 special brain cell, and that each hair-cell corresponds only to 
 one tone, the number of special tones to be directly recognised 
 
Sound and Hearing 273 
 
 in the brain is considerably less than the number of fibrils 
 of the basilar membrane would lead us to expect. On the 
 other hand, if the cell may respond to more than one tone, 
 and give rise to sensations of different tones in the sen- 
 sorium, we must have some difference in the nerve currents 
 transmitted at different times from periphery to centre by 
 the same nerve, and this would probably correspond to 
 different rates of vibration of the basilar fibrils. 
 
 Now, it is just possible that there may be a greater 
 power of response in the basilar membrane to sounds of 
 varying pitch than von Helmholtz supposes. If at any 
 particular moment there is no fibril attuned to the pitch of 
 the incoming sound, it may be that the tension of part of the 
 membrane may be varied to suit the exigencies of the case. 
 We have seen that Bowman's ligament, by which the basilar 
 membrane is attached to the outer wall, contains spindle 
 cells which may be regarded as muscular, and by the con- 
 traction of which the pull upon the fibrils may be varied, 
 and their tension increased or diminished. A similar result 
 might follow a change in the amount of blood circulating 
 in the spiral ligament, giving more or less turgidity to this 
 structure. Thus if each fibril of the basilar membrane in its 
 normal condition of length and tension is tuned approxi- 
 mately to a special tone, and if by variation of its length or 
 tension it may be rendered responsive to tones of slightly 
 higher or lower pitch, as we may tune a violin by tightening 
 or slackening the strings, we have in the ear a complete 
 analysing mechanism for the pitch of all musical sounds. 
 Such an hypothesis renders it possible likewise that we may 
 have a complete series of tones from the lowest to the 
 highest, melting one into the other by imperceptible change 
 an ear, in fact, that can appreciate the pitch of any possible 
 tone between the lowest and the highest limits, a capacity 
 which experience shows to be possible in the human ear, 
 
 T 
 
274 Physiology of the Senses 
 
 and that directly for all tones, and not indirectly for some, 
 as von Helmholtz holds. 
 
 If, further, it is the case, as Rutherford suggests, that 
 the sensation varies in the central cell according to the rate 
 at which the peripheral end of the nerve fibre or the hair- cell 
 is stimulated, we arrive at a view which is free from objec- 
 tions that may be urged to the theories both of Rutherford 
 and von Helmholtz. Rutherford's theory is unsatisfactory 
 in so far as it entirely disregards the elaborate structure and 
 wonderful complexity of the cochlea, deprives the ear of any 
 analysing power, and relegates that function to the brain, 
 among whose cells we can find nothing in any way suitable, 
 from a morphological point of view, to lead to a perception of 
 variation of pitch. The physical basis for analysis must be 
 either in the ear or the brain ; but if all parts of the ear, and 
 all the fibres of the auditory nerve, and all the auditory nerve 
 cells, respond together and vibrate alike, we have no such 
 basis. To have the power of selecting one or other partial 
 tone, and of devoting attention to it alone while others 
 are still affecting the sensory mechanism, it seems to us that 
 there must be several structures in vibration or molecular 
 change at different rates. If the auditory centre is in vibra- 
 tion or molecular action as a whole, and similarly in all 
 its parts, it is impossible to understand how a mere effort of 
 will can enable us to note constituent parts of a complex 
 tone. We can pay attention to one or other partial tone in 
 a complex sound, just as we can fix our regard upon one 
 part of the field of vision to the exclusion of all the rest, but 
 how can this be done if all parts of the auditory centre are 
 affected alike ? To each part of the retina there is a cor- 
 responding part in the cortex of the brain ; there is probably 
 a similar relationship between different paits of the cochlea 
 and the auditory centre. 
 
 On the other hand, the main objections to von Helm- 
 
Sound and Hearing 275 
 
 
 
 holtz's theory are the limited number of structures compared 
 with the known capacity of the ear and the supposition that 
 each brain cell is concerned only with the perception of one 
 tone in different degrees of power. All are agreed that the 
 cerebral centres can appreciate variations in strength of 
 stimulus. In all the special senses the strength of the 
 sensation varies with the strength of the stimulus. Now, 
 this does not necessarily imply in regard to the auditory 
 nerve that the actual vibration of the endolymph is trans- 
 mitted as a vibration that might be seen passing along 
 the auditory nerve as we might see a wave of vibration 
 passing along a tensely-stretched rope when it is struck, but 
 it does imply a greater molecular movement in one case 
 than in another, and a greater or less effect upon the proto- 
 plasm of the receptive nerve centre. There may be no real 
 to-and-fro vibration of the nerve corresponding to that of the 
 internal ear, but there must be a variation in the nerve 
 current in respect of amount of movement. If the nerve 
 cell can respond to variations in intensity, there is no greater 
 difficulty in supposing that a cell whose function is to give 
 rise to a sensation of pitch may give slightly different sensa- 
 tions corresponding to slight variations in the rate of stimu- 
 lation. 1 If it be urged that this again relegates distinction 
 of pitch to the brain, and that we might as well suppose 
 each auditory cell to have the power of discriminating 
 between all degrees of pitch, we would answer that the 
 multiplication of centres, each having slightly different 
 receptive powers, affords an anatomical basis for the simul- 
 taneous reception of many stimuli differing from one another 
 
 1 See also the remarks on the modified theory of colour vision 
 recently propounded by von Helmholtz (p. 169). This distinctly 
 favours the view that terminal organs, such as the rods and cones of 
 the eye (and why not the delicate mechanism of the internal ear?), may 
 respond to different rates of vibration. 
 
276 Physiology of the Senses 
 
 only it may be in the matter of pitch, while by allowing 
 that each little centre may give slightly different pitch- 
 sensation with variation in the rate of stimulus we avoid the 
 difficulty into which von Helmholtz's theory plunges us. But, 
 it may be asked, can a nerve fibre respond in this way to 
 different numbers of stimuli per second ? There is not the 
 least doubt that it can. The number of stimuli sent along 
 a nerve to a muscle may be largely varied with varying 
 effect on the muscle in the way of contraction. In the case 
 of insects, for example, the wings may vibrate as often as 
 352 times per second (Rutherford), and each movement 
 must be due to at least one separate nerve impulse. A 
 nerve removed from the body may be inserted in a tele- 
 phonic circuit, and it will conduct the electric current and 
 transmit the delicate variations of electrical intensity neces- 
 sary for telephonic communication. We do not assert that 
 the ordinary nerve current is electrical in character, but if 
 the nerve can transmit variations so delicate as those of the 
 telephone must be, they may as readily be deemed capable 
 of responding in rate to their normal auditory stimuli. 
 Moreover, it must be borne in mind that the sensation of 
 pitch is in no way comparable qualitatively with the phy- 
 sical changes which give rise to it. We have no sensation 
 of each individual variation in the stimulus. The sensorium 
 fuses the impulses so as to give rise to a continuous tone. 
 And again, we do not, as a rule, note the partial tones 
 separately and respectively : indeed, until the time of Tar- 
 tini they were not known to exist, and until the time of von 
 Helmholtz were deemed of small importance. Their com- 
 bination and appreciation, as a sound of determinate quality, 
 is a purely mental act, combined, that is to say, by a 
 mechanism higher than and different from the initial recep- 
 tive auditory centres. It is only when, by conscious effort 
 and using special aids, such as resonators, we pay attention 
 
Sound and Hearing 277 
 
 to the sensory effect that we note the constituent parts. 
 There must be higher mental centres in which fusion occurs, 
 or a unity of mind in which a synthesis of the partial sen- 
 sations is brought about. 
 
 The Psychical Elements in Auditory Sensations. 
 
 When the auditory centres have been stimulated and the sen- 
 sation of sound receives due attention, certain mental effects 
 are produced which are superadded to the simple sensation 
 of sound. We judge, for example, that the sound has been 
 produced outside or inside of the body, that it comes in a 
 certain direction and from a certain distance, or we may 
 recognise that it is purely of a subjective character, and 
 exists only in imagination. In arriving at a decision upon 
 such points as these we are aided by the other senses 
 and by knowledge previously acquired. Thus, when we 
 see a man at a distance from us lifting a gun to his shoulder 
 and a puff of smoke issuing from the muzzle, we know from 
 experience that we will shortly hear the sound of the detona- 
 tion. We infer from the character of the sound, its loudness, 
 and the time that elapses before the report is heard, that 
 it comes from the gun and from no other source. 
 
 Externality of Sound. The power which the mind 
 possesses of determining whether a sound originates out- 
 side or inside of the body seems to be in large measure 
 dependent upon whether the sonorous vibrations are com- 
 municated to the ear through the auditory meatus, the 
 drum-head, and the chain of bones, or directly through the 
 bones of the head. We mentally project the source of the 
 sound outwards when the vibrations act mainly through the 
 meatus on the tympanum, but if the sounding body is 
 touching the head we may have the impression as if the 
 sound came from within the head. Weber has pointed out 
 that if the meatus is filled with water the idea of externality is 
 
278 Physiology of the Senses 
 
 destroyed, and that the sound seems to originate in the head 
 Even when the air in the meatus is vibrating freely in re- 
 sponse to sonorous undulations, if the body emitting the 
 sound touches the head, the idea of externality may dis- 
 appear. Suppose two bodies giving out exactly similar 
 sounds, as when two telephones, connected in one circuit, are 
 held to the two ears and made to respond to one and the 
 same sound. If the telephone to the right side be tightly 
 applied, while the one to the left be held at some little dis- 
 tance from the ear, the sound will seem to originate in the 
 right side of the head. If the one to the left is now pressed 
 closely and that to the right withdrawn a little, the sound is 
 heard in the left side of the head, but if both instruments 
 are held tightly to the ears, the sound seems to originate 
 inside of the head and towards the middle line, so -that 
 it will be described by one observer as seeming to be in 
 the mouth, by another at the top of the head, and by a 
 third at the nape of the neck. Lastly, by slight variations 
 in the pressure on the head we can apparently make the 
 sound move from side to side at pleasure. The sound of 
 our own voice is heard as originating within the head, and 
 certain disorders may give rise to sensations of sounds re- 
 ferred to the ears. Thus when the intracranial circulation 
 has been disturbed, we may have a ringing in the ears, 
 or may hear the throbbing of the pulse. An accumu- 
 lation of cerumen or wax in the external meatus may give 
 rise to unpleasant sounds by interfering with the vibration 
 of the drum-head. Drugs, such as quinine or salicin, may 
 cause hissing or whistling sounds, or even a sensation of 
 deafness, by interfering with the nutrition of the auditory 
 centres, and the insane often think they hear voices and 
 sounds on account of disordered and abnormal stimuli in 
 the diseased brain. So strong, indeed, is the power of 
 imagination in the hallucinations of the insane that nothing 
 
Sound and Hearing 279 
 
 will persuade them that the voices are not actually coming 
 from 'an external source, and it is to be remembered that 
 the sensations are at least real to them, latent impressions 
 being developed or obscure memories recalled by cerebral 
 irritation. Nay more, we may ourselves under certain cir- 
 cumstances by an effort of the mind give rise to auditory 
 hallucinations. Much pleasure may often be derived from 
 the following experiment. If when in bed, lying perfectly 
 quiet, and with no sounds breaking the stillness of the 
 night, we think the music of a song, fixing our attention 
 upon the music but not humming it, we may sometimes 
 seem to hear it being sung an octave higher by a voice ex- 
 ternal to ourselves a female voice apparently, from its 
 delicacy, tenuity, and high pitch and, strange to say, not 
 exactly synchronous with but very slightly behind our own 
 imaginary singing. When the hallucination is thoroughly 
 established and we resign ourselves completely to it, the 
 two voices may seem to go on without effort on our part, 
 and we ourselves to be merely passive listeners. The least 
 movement, however, or wandering of the thoughts to 
 another subject, immediately dispels the illusion. In per- 
 forming this experiment, it is most probable when the mind 
 has all its faculties concentrated upon the endeavour to hear 
 the faint sound that, in thinking the music, we actually give 
 rise to slight variations in the tension of the auditory 
 structures, and possibly stimulate the auditory centre 
 through the auditory nerve, but to so small an extent as to 
 be hardly perceptible to the senses, or it may be that with 
 the concentration of the mind upon the expected sound the 
 nutrition of the auditory centre is involved. It might even 
 be that the auditory centre is stimulated from the parts 
 which subserve volition, but this is mere conjecture, for 
 which no experimental data can be adduced beyond the 
 well-established fact that lower centres maybe inhibited or 
 
280 Physiology of the Senses 
 
 excited by influences coming from higher cerebral centres. 
 As a monarch may summon his ministers and invoke their 
 aid or dismiss them from his presence, so the conscious 
 mind may call upon the senses for their testimony, or may 
 bid them be silent, and the obsequious senses do some- 
 times seem to give that answer which their master desires, 
 although they have no true warrant for so doing. 
 
 Direction of Sound. We have seen (p. 200) that the 
 determination of the direction in which a sound has come 
 is largely due to the greater intensity of the sound in one 
 ear than in the other owing to the sound waves striking 
 more fully and directly upon one ear than the other. If, 
 however, the source of sound is in a plane passing forward 
 through the middle of the body it is impossible by means 
 of this alone to say whether the sound comes from behind 
 or in front. Judgment as to direction is made more 
 accurate by moving the head so that the sound falls more 
 intensely now on one side now on the other. If the apex 
 of a hollow cone or the ear-piece of an ear-trumpet be 
 inserted into the meatus and the instrument be moved for- 
 wards and backwards, the apparent direction of the sound 
 may be largely modified, and we have a similar change if 
 the auricle be flattened out backwards against the side of 
 the head or brought forward with the hand. 
 
 In many cases, we judge the sound to come in a certain 
 direction from knowing where it probably originates, as 
 when we hear a bell rung in a steeple with whose position 
 relatively to ourselves we are acquainted. It is easier to 
 judge the direction of noises than of musical sounds, and 
 that mainly because there is a slight difference in the quality 
 of the sounds coming to the two ears, and noises having 
 generally more partial tones than musical sounds, the differ- 
 ence is more easily noted and the judgment as to direction 
 assisted. 
 
Sound and Hearing 281 
 
 Distance of the Source of Sound. The ear has no direct 
 power of estimating the distance from which a sound 
 comes, since it only becomes cognisant of the sound when 
 it reaches the ear. We can only form a rough estimate 
 from knowing by previous experience that a given sound 
 will presumably have a certain intensity when produced 
 at a certain distance from us, and that, other things 
 being equal, it will diminish to a certain extent the farther 
 it is from the ear. Experimentally, it has been proved 
 that when sound is transmitted through a fairly homo- 
 geneous medium, as through air or water, the intensity of 
 the sound varies inversely as the square of the distance. 
 For twice the distance, the intensity will be one-fourth ; for 
 three times the distance, one-ninth, and so on. But if we 
 modify the conditions for the transmission of sound, our 
 power of judgment soon fails us. If, for example, when 
 sitting at a table we scratch it gently with the finger-nail, 
 the arm being outstretched, we hear a sound of faint inten- 
 sity, the distance of which we can estimate fairly well ; but 
 if the ear be applied to the table, the sound seems to be 
 made at the ear, its intensity not having been materially 
 diminished by transmission through the wood. Similarly, 
 if the sound is transmitted through tubes, the law of diminu- 
 tion of intensity, according to the square of the distance, 
 does not apply, and we hear people speaking through a long 
 tube, as from top to basement of a house, as if they were 
 close beside us. By gradually diminishing the intensity of 
 a sound, it may be made to seem to come from a consider- 
 able distance when really being produced close at hand. 
 Thus, when the operatic chorus leaves the stage, and dis- 
 appears from view behind the scenes, by singing more and 
 more softly, the performers can convey the impression that 
 they have retired to a great distance. So the art of the 
 ventriloquist lies in his power of speaking with almost no 
 
282 Physiology of t lie Senses 
 
 facial movement, of changing rapidly the strength of his 
 voice so as to give the impression of varying distance, 
 and of conveying by gestures that the sound seems to 
 come from a certain spot, whence he seems to hear it 
 coming, just as we do ourselves. A slight variation in the 
 quality of a sound likewise takes place as it recedes from 
 us, certain partial tones becoming inaudible sooner than 
 others ; this too may help our judgment as to distance. 
 
 Memory of Sound. It is sometimes difficult for us to 
 judge by the power of hearing when a sound has ceased to 
 stimulate the ear. When, for example, a bell has been 
 ringing for some time and then stops, the sound gradually 
 dies away, and it is almost impossible for us to tell the 
 exact moment when it has ceased. It may seem to have 
 died away entirely, and we cease to strain the ear to catch 
 its faint tones, but if we listen again we seem to hear it 
 faintly. This may be due to different causes. It may be 
 that the ear has become fatigued for the special sound, 
 and that the momentary withdrawal of the attention has 
 rested the ear, so that it can respond to tones previously 
 inaudible. On the other hand, it may be due to a vivid 
 form of memory. We cannot doubt that there is some 
 physical change in the auditory centre when the sensation 
 of sound is excited, and when the centre has once acted in 
 a particular way it does so more easily when similar circum- 
 stances again arise, or even as the result of a mental effort. 
 Sometimes it may require repeated attempts before we are 
 able to recollect a sound, as, when after hearing a new song, 
 we fail for a day or so to remember the music of it, but 
 gradually note by note, and line by line, it returns, often 
 without conscious effort, until we are able to piece it all 
 together again, more or less correctly, according to acute- 
 ness of ear and receptivity for musical impressions. 
 
 Mental Receptivity for Sound. This is a power which 
 
Sound and Hearing 283 
 
 varies much with the state of the mind and the nature of 
 our environment. As a rule, we pay no attention to, and 
 do not consciously hear, such customary sounds as the tick- 
 ing of a clock, the noise of street traffic, and the like, 
 although they must be constantly acting upon the ear. They, 
 indeed, constitute for us our basis of silence, so to speak, for 
 if the clock should stop, or if we pass to the solitude of the 
 country, we seem to hear the silence which ensues. Again, 
 just as some people are colour blind, so others may be deaf 
 to the pitch of sounds. Some ears are adapted only for 
 sounds of comparatively low pitch, others for high pitch ; 
 they are deaf to all others. If we take the lowest limit for 
 pitch at 1 6 vibrations per second, and the highest at about 
 40,000, we have in all a range of about 1 1 octaves. The 
 ear has thus a much wider range for pitch than the eye for 
 colour, for it will be remembered that the lowest red rays of 
 the spectrum have a vibrational frequency of 435 millions 
 of millions per second, while those of the ultra violet are 
 about 764 millions of millions that is to say, less than twice 
 the number at the lower end of the spectrum, or less than 
 one complete octave. 
 
 But the power of distinguishing tones of varying pitch 
 is, with some, so slight that they are quite unable to distin- 
 guish one tune from another, and others who can recognise 
 the difference are unable to sing more than one or two 
 notes of different pitch. 
 
 Binaural Audition. Some persons have been found 
 who seemed to have the two ears differently tuned, so that 
 the same sound seemed to be of higher pitch to one ear 
 than to the other. Under normal conditions, although 
 from the position and shape of the ears the sound waves 
 which fall upon the drum -head cannot be exactly the 
 same in form nor in time of excitation, yet the resultant 
 sensations in the auditory centre are mentally united, and 
 
284 Physiology of the Senses 
 
 we hear one sound, not two. This is mainly to be accounted 
 for by the fact that the sensation lasts for a short time 
 after cessation of the stimulus, and the two sounds are so 
 slightly separate in time as to blend readily with one 
 another. Inasmuch as the two ears enable us to a cer- 
 tain extent to judge the distance of the sounding body, 
 binaural audition is, in a way, comparable to binocular 
 vision, which assists in the perception of solidity or distance 
 in space, 
 
THE PHYSIOLOGICAL CONDITIONS OF 
 SENSATION 
 
 IN the preceding sections we have given, in the first place, 
 a general view of the mode of action of the nervous system, 
 and then we have described each of the five senses in 
 detail. We have seen that external agents, such as light 
 or sound, act on special terminal organs, and that from 
 these, nervous impulses are carried by the nerves of sense 
 to the central nervous organs. In these central nervous 
 organs molecular changes occur, which are related in some 
 way to conscious states or sensations, and we then refer 
 these sensations to the outer world, and to the agent which 
 we believe to be their primary exciting cause. Further, we 
 know that these sensations may give rise either to voluntary 
 or involuntary movements, and that they may influence 
 many organs of the body, causing, for example, the 
 voluntary movement, the involuntary start, the blush of 
 modesty, or the pallor of fear, the more rapid action of the 
 heart, or the quickening or slowing of respiration. The 
 functions of the central nervous organs and of the organs 
 of sense are so closely related as to make it no easy 
 matter to form a conception of the system working as 
 a whole. The progress of discovery naturally tends to 
 differentiation, and to attaching undue importance to one 
 organ as compared with others, so that we are in danger 
 
286 Physiology of the Senses 
 
 of losing sight of the solidarity of the whole nervous 
 system. 
 
 During the profound unconsciousness of coma, or of deep 
 sleep, the mind is at rest. There are no thoughts and no 
 interpretation of messages from the sense organs. The 
 higher centres of the brain are inactive, but lower centres, 
 such as those governing the circulatory and respiratory 
 mechanisms, may still be active, the heart continues to 
 beat, and an onlooker sees the movements of respiration. 
 During the waking and conscious state, however, the higher 
 centres are active. They are not only the seat of molecular 
 phenomena related to the conscious state, giving rise to the 
 revivications of memory, the play of ideas, the rise of desires 
 and impulses, and efforts of volition, but they now are 
 momentarily receiving messages from the various sense 
 organs. These messages affect the higher centres them- 
 selves, and, through them, lower centres and the body 
 generally. Probably every nervous action, however deli- 
 cate and evanescent, affects more or less the entire system, 
 and thus, in addition to the impulses coming from the 
 various organs of sense, there may be an undercurrent 
 streaming into and out of the nerve-centres. This under- 
 current may never give rise to distinctly conscious states, 
 but, along with numerous interactions in the centres them- 
 selves, it contributes to, and partly accounts for, the appa- 
 rent continuity of conscious experience. 
 
 No one doubts that consciousness has a material sub- 
 stratum, but the problem of the relation between the 
 mental state and the molecular movements in nervous 
 matter is as far from solution as in the days when little 
 was known of the physiology of the nervous system. Con- 
 sciousness has been driven step by step upwards until it 
 now takes refuge in a few thousand nerve-cells in a portion 
 of the gray matter in the cortex of the brain, or it may be 
 
Physiological Conditions of Sensation 287 
 
 in the dense network of fine fibrils that abounds in gray 
 matter. The ancients believed that the body participated 
 in the feelings of the mind, and that the heart, liver, and 
 reins (kidneys) were connected with the emotions, a view 
 quite consistent with the familiar experience that these 
 organs are often influenced by such mental states. As 
 science advanced, consciousness was relegated to the brain, 
 first to the medulla, and lastly to the cortex. But sup- 
 posing we were able to understand all the phenomena 
 chemical, physical, physiological of this intricate gan- 
 glionic mechanism, we would be no nearer a solution of the 
 problem of the connection between the objective and sub- 
 jective aspects of the phenomena. It is no solution to 
 resolve a statement of the phenomena into mental terms 
 or expressions, and to be content with an exclusively 
 idealistic theory of cognition. Nor is it more satisfactory 
 to translate all the phenomena of mind into terms describ- 
 ing physical conditions, as is done by those who support a 
 purely materialistic hypothesis. A philosophy that recog- 
 nises both sets of phenomena, mutually adjusted and ever 
 interacting, recognises the facts of the case, and does not 
 delude the mind by offering a solution which is in reality 
 no solution at all. The difficulty is somewhat lessened if 
 we assume that behind all physical and mental phenomena 
 there is a metaphysical essence, conscious or unconscious, 
 and that the phenomena we term physical and mental are 
 only different sides of the same thing. Such an essence 
 can never be known to science, and the discussion of the 
 possibility of its existence and of its properties belongs to 
 the province of philosophy. 1 
 
 Apart from the ultimate question, however, there is the 
 important one whether physiologists are right in relegating 
 consciousness entirely to the gray matter of the brain. The 
 1 Von Hartmann, Philosophy of the Unconscious, especially vol. iii. 
 
288 Physiology of the Senses 
 
 facts of comparative physiology are against a view so exclu- 
 sive, because we cannot deny consciousness to many animals 
 having rudimentary nervous systems, or none at all. As 
 already said, research in anatomy and physiology, and the 
 observation of disease, have obliged physiologists to adopt 
 the view that the brain is the seat of sensation, or, in other 
 words, of consciousness. This is no doubt true in the sense 
 that it receives all those nervous impulses that result in con- 
 sciousness, but parts acted on by external physical agents 
 (like the retina) and the parts transmitting the nervous 
 impulse (like the optic nerve) are, in a sense, as much con- 
 cerned in the production of conscious states as the brain 
 itself. This view of the matter was urged by Cleland in 
 I Syo, 1 and is consistent with the facts of nervous physio- 
 logy. It presents fewer difficulties than the one generally 
 held which drives consciousness into the recesses of the 
 nerve-cells in the cortex of the cerebral hemispheres. It 
 keeps clear of the prevailing error in the philosophy of 
 modern physiology, that of regarding the body, and even 
 the nervous system, as a vast collection of almost inde- 
 pendent organs, losing sight of community of function and 
 interdependence of parts. At the same time it must be 
 admitted that it approaches no nearer a final solution of 
 the problem of the origin of consciousness ; it only states 
 the conditions of consciousness with greater precision. 
 
 Let us now approach the question from another point of 
 view. The simplest structural nervous unit is a Cell, 
 which we may call A, with a fibre passing to it from a 
 specialised cell, B, on the surface of the body, and another 
 fibre passing from it to a contractile cell, C. A stimulus 
 applied to B causes molecular changes in it, which result in 
 the transmission of an impulse to A, in which molecular 
 changes again occur, resulting in the transmission of an 
 1 Cleland on Evolution, Expression, and Sensation, 1870. 
 
Physiological Conditions of Sensation 289 
 
 impulse to C. This is the simplest form of a so-called 
 reflex mechanism. Suppose the same kind and degree of 
 stimulus be applied to A many thousand times in succes- 
 sion, and repeated not only in an individual, but in a line of 
 individuals genealogically connected as parent and offspring, 
 we can imagine that its molecular structure will become so 
 modified that it will gradually become more and more 
 responsive to stimuli of this kind, the simple mechanism 
 having become attuned to the movements of the outer 
 world. Here, then, we have a molecular condition associated 
 with the dawn of consciousness, and the attuned condition 
 of the structure may be regarded as the beginning of 
 memory. No doubt it is impossible here, just as in dealing 
 with a complex brain, to form any conception of the genesis 
 of consciousness. It evidently cannot be the result, in any 
 physical sense, of the molecular changes in the cell, because 
 even although we were cognisant of all the molecular changes 
 we could not detect a conscious state. So far as an out- 
 sider is concerned, the conscious state of the cell can only 
 be recognised by some outward manifestation in the form of 
 movement, and it is conceivable that the cell might be 
 conscious, and yet not make any movement. Suppose a, b, 
 c, d, e, etc., to represent links in the chain of physical phe- 
 nomena between the. irritation of the cell B and the move- 
 ment of 0, and that consciousness is an attribute of A, 
 which we may denominate x, it will be impossible to find a 
 place for x in the chain, in the same sense as the movement 
 of C is the last link of the chain. It cannot come in be- 
 tween a and , as a is the physical antecedent of , nor, for 
 a similar reason, between b and c, nor between c and d, 
 d and ^, etc. The condition x is therefore outside the 
 physical chain ; and yet it is related to it so intimately as 
 to lead to the illusion that x forms one of the links. 
 This appears to prove that consciousness, x^ is outside 
 
 U 
 
290 Physiology of the Senses 
 
 any chain of related physical phenomena conceivable in the 
 simplest nervous mechanism. 
 
 Nor do we get any farther towards clearing up the 
 mystery if we suppose, as some have done, that even dead 
 matter has in some way associated with it units of con- 
 sciousness, 1 because it is equally impossible in this case to 
 understand the nexus between the material particles and 
 consciousness. The condition of the conscious state may 
 therefore be represented by two parallel curves infinitely 
 close together, the one representing the chain of physical 
 phenomena, linked together as cause and effect, and the 
 other the chain of conscious states. Any variation in the 
 one coincides with a variation in the other, but no explana 
 tion can be given as to how the one influences the other. 
 To assert that one is the cause of the other is simply to 
 beg the question. If we say that the chain of physical 
 phenomena is the cause of the conscious states, in the 
 same sense as the physical phenomena in a cell of the 
 liver is the cause of the secretion of bile, we introduce into 
 the chain an 'immaterial something, and break the physical 
 continuity of the various links ; and, on the other hand, if 
 we try to escape the difficulty by translating the physical 
 links themselves into states of consciousness, and deny any 
 knowledge of the physical substratum,, we are deceived by 
 words and reach no solution. 
 
 Again, to regard consciousness as a mode of energy is 
 unsatisfactory. Energy, in the physical sense, is nothing 
 more than the power any material system has of doing 
 work, owing to the relative position of its component parts. 
 If the relative position of these parts be altered, the distri- 
 bution of energy in the system will also be altered. It 
 follows from this that energy may be manifested by various 
 kinds of movements heat, light, gravitation, etc. and 
 1 W. K. Clifford, Lectures and Essays, vol. ii. p. 31. 
 
Physiological Conditions of Sensation 291 
 
 one form of energy may be resolved into another. But 
 when motion produces heat, there is a quantitative con- 
 version of energy from motion to heat, which is, in turn, 
 another mode of motion. If we now assume molecular 
 changes to be the cause of consciousness, these molecular 
 changes also produce heat, molecular movements associated 
 with chemical action, and perhaps movements on a larger 
 scale ; but the sum of these resultant forms of energy is 
 equal to the energy at first existent in the physical system, 
 which we assume to be also the seat of consciousness. Con- 
 sequently consciousness does not come into the dynamical 
 chain. It cannot be measured ; it cannot be derived from 
 the physical energies, nor can it be resolved into them. It is 
 outside the chain. Movements of matter, therefore, cannot 
 be resolved into consciousness, or, in other words, conscious- 
 ness is not a form of energy. 
 
 We are thus face to face with an insoluble problem, even 
 when we discuss it in its simplest form, and it becomes 
 infinitely more complicated when we consider the manifold 
 phases of consciousness connected with the mechanism of 
 the brain. If, however, we begin with the structural unit 
 of a simple reflex mechanism, along with its associated 
 conscious state, we find that the complex functions of the 
 fully - developed brain are aggregations of the simple 
 mechanism we have considered, and that what we term 
 consciousness is a condition which is the sum of the 
 conscious states of the individual nerve cells, or aggrega- 
 tions of nervous matter, constituting the brain. We can 
 form no conception of the nature of the consciousness of 
 a nerve cell any more than we can of the consciousness of 
 a sea -anemone or of a worm ; but we must assume the 
 existence of consciousness in a nerve cell, otherwise it is 
 impossible to understand how consciousness is associated 
 with an aggregation of such cells in a brain. To deny 
 
292 Physiology of the Senses 
 
 consciousness to such a cell would be equivalent to deny- 
 ing consciousness to the brain, which would be absurd. 
 Whilst, therefore, we give up the explanation of the genesis 
 of consciousness as an insoluble problem, it is possible to 
 gain some insight into the general mode of action of brain 
 as the recipient of sensory impressions. 
 
 Suppose, for example, we irritate the skin of the sole of 
 the foot, an impulse is carried by nerves to cells in the 
 posterior horns of gray matter in the spinal cord (see Fig. 7, 
 p. 1 6), in which molecular processes are excited. From 
 these, impulses are carried by fibres in the cord to cells in 
 the anterior horn ; in which, again, molecular processes 
 occur, resulting in the transmission of nervous impulses 
 along motor nerves to the muscles of the limb, and the 
 limb will be drawn away by a sudden contraction of the 
 muscles. This is a reflex movement, not in obedience to 
 a volitional impulse, not associated with consciousness in the 
 usual sense of the term (as implying activity of the brain), 
 but, from the arguments already led, we may assume 
 that these molecular changes in the cells of the cord are 
 associated with a lower mode of consciousness, such as 
 presumably exists in animals having a nervous system of 
 this simple type. But the cells in the gray matter of the 
 cord are connected with cells in the masses of gray matter 
 in the upper centres, and, in particular, we have reason to 
 believe that each unit area of sensitive surface of the body 
 has a corresponding unit area in the cerebral cortex, that is 
 to say, from each unit area (the size of which varies much in 
 the different sense organs, from a minute area of retina to a 
 much larger area of skin surface) nerve filaments pass which 
 carry impressions to a corresponding unit area in the cortex 
 (see remarks on the tactile field, p. 60, and on the visual 
 field, p. 30 and p. 176). This does not mean that individual 
 nerve fibres necessarily pass from unit area of sensory 
 
Physiological Conditions of Sensation 293 
 
 surface to unit area of cortex, but that impressions are so 
 related. If so, the irritation of the skin of the foot, in the 
 experiment we are considering, may cause impressions to 
 pass, not merely to the cord, but also to the higher centres 
 in the brain, and the result may be a feeling of pain. This 
 may be also explained by supposing that the refleix centre 
 in the cord is intimately connected by fibres with the 
 conscious centres in the cortex, a supposition strongly 
 supported by the increasing mass of evidence as to the 
 paths of transmission between the cord and the brain. The 
 sensation of pain must be associated with molecular changes 
 in the cells of the cortex, and, as a rule, these changes 
 cause, by a kind of irradiation, the transmission of impulses 
 outwards to other nerve centres, which in turn call forth 
 various more or less complicated movements. Thus, for 
 example, they may be carried to the cells in the gray 
 matter of the medulla, which is the origin of the nerves 
 governing the movements involved in crying, in the ex- 
 pression of pain by the muscles of the face, or they may 
 reach the cells in the gray matter of the cord, calling forth 
 the movements of the limb requisite for drawing the limb 
 away from the irritation, or for defending it from further 
 attack. Again, the irritation may call forth involuntary 
 exclamations, in the form of words, expressive of pain, and 
 in this case the centre for articulate speech has been in- 
 volved. Impressions may also be carried from the sensory 
 centre in the cortex to the parts of the brain concerned in 
 volition, and the reflex and involuntary movements we have 
 considered will be added to, or supplanted by, direct 
 voluntary movements. Even voluntary movements, how- 
 ever, are essentially reflex in character, inasmuch as they 
 are called forth by stimulations which have been applied to 
 a sensory surface either immediately before the voluntary 
 act, or which have been applied, it may be, long before. 
 
294 Physiology of the Senses 
 
 In the latter case, the effects of the stimulation still remain 
 in certain groups of nerve cells, as a kind of memory, so 
 that when they are roused into activity, the voluntary act 
 will follow, as it probably did on the first occasion when 
 the stimulus was applied. Finally, the irritating body may 
 be seen^ and the effects of the image formed optically on 
 the retina are carried by the optic nerve to the corpora 
 quadrigemina, and from these to the visual centres in the 
 cortex. Again, a memory of this impression may remain, 
 and may be called into action by nervous influences coming 
 from other parts of the brain, so that a vision of the irritat- 
 ing body may afterwards arise into consciousness, so 
 vividly as to call forth movements similar in character, 
 although, probably, not so intense, as those which occurred 
 in the first instance. This revivication of old impressions 
 is most likely to occur when the upper centres are some- 
 what in abeyance, as in the phenomena of hypnotism and 
 somnambulism. 
 
 Sensory impressions, however, are not only carried to 
 the cerebral cortex, there awakening consciousness, but 
 they are also conveyed, and many of them in the first 
 instance, to the cerebellum, and in this organ they set in 
 action the physiological mechanism that results in co- 
 ordinated movements. It is not improbable that the 
 sensory areas of the body have corresponding areas in the 
 gray matter on the surface of the cerebellar convolutions. 
 Thus the cerebellum is the organ that gives a rhythmic 
 character to certain movements of the body, as those of 
 walking, flying, swimming, etc., and probably it is only when 
 these movements become associated with sensation, or are 
 voluntary, that the centres in the cerebral cortex come into 
 play. 
 
 Again, if an external object acts at the same time on 
 different organs of sense, as when we hold a rose in the 
 
Physiological Conditions of Sensation 295 
 
 hand, admire its colour, and enjoy its delicious perfume, 
 the various sensations thus related to molecular movements 
 in different parts of the cortex are combined by the action 
 of the numerous fibres passing from centre to centre, and 
 the result is a conscious perception of the thing as a whole. 
 These fibres may be called fibres of association, because 
 they combine impressions that have reached various sen- 
 sory cortical centres. It is evident that such a combination 
 of impressions may also give rise to various movements of 
 the limb, or of the muscles of expression, and that the 
 impressions will be more or less vivid as the exciting 
 causes are strong or weak. If they are vivid, or, in other 
 words, if the molecular changes in the nerve cells of parti- 
 cular parts of the cortex of the brain are intense, they will 
 have both a tendency to last after the exciting cause has 
 been removed, and a tendency to be renewed by a slighter 
 stimulus than was at first necessary to produce them. This 
 is the physiological, or organic, foundation for memory, and 
 also for the mental process known as the association oj 
 ideas. Further, if such molecular processes, by frequent 
 repetition, stamp a certain character on particular parts of 
 the cerebral cortex, so as to be transmitted according to the 
 laws of heredity, then we have a physiological basis for 
 innate tendencies or intuitions. The brain of one man 
 differs from another in this respect. The greater the num- 
 ber and variety of impressions made on an individual, the 
 greater will be the number and variety of the molecular 
 movements in the cells of the cortex, and the greater the 
 number and variety of resulting mental and reflex pheno- 
 mena. So intense may these processes be that they may 
 be called into action by a stimulus from another part of the 
 brain, as when irritation of the corpora quadrigemina by 
 Indian hemp awakens in the cells of the visual centres of 
 the cortex those changes which are associated in the mind 
 
296 Physiology of the Senses 
 
 with long-forgotten visual impressions, and the person sees 
 passing before him a phantasmagoria of brilliantly-coloured 
 images. These may also arise spontaneously, but the 
 apparent spontaneity, however, is dependent on a stimulus 
 so feeble as to escape notice, as when the sight of an object 
 suddenly and almost unconsciously awakens memories of 
 past events, and brings before the mind's eye forms and 
 colours that long before produced impressions on the organs 
 of sense. 
 
 Many nervous phenomena are at first in a sense volun- 
 tary, and by and by they become more and more of a reflex 
 character, and are less and less associated with the higher 
 consciousness. Thus a child acquires powers of walking 
 by repeated efforts involving volition, judgment, and per- 
 ception of different impressions, but the same movements 
 of locomotion may be unconsciously performed by an adult. 
 Familiar examples also are seen in the unconscious dexterity 
 of movement of a skilful performer on a musical instrument, 
 or in the deft movements of a cunning artificer. So is it 
 even with psychical operations involving the action of the 
 brain, and the brain cortex may, as in unconscious cerebra- 
 tion^ pass through molecular processes which result in the 
 unconscious performance of actions that would be regarded 
 as the result of mental processes, if the person were con- 
 scious. Many instinctive actions are probably in this sense 
 of an unconscious character. There can be no doubt that 
 even in men the brain may work unconsciously, and the 
 product may suddenly start out into consciousness. 
 
 Facility of mental acquirement means a certain receptive- 
 ness for particular kinds of molecular action. Other per- 
 sonal factors come into operation, such as the power of 
 choice of particular impressions, the degree of attention paid 
 to them at the time (depending largely on strength of will), 
 the degree of stability of the results of the molecular move- 
 
Physiological Conditions of Sensation 297 
 
 ments that have been excited, and the power of associa- 
 tion of different impressions. Each of these factors has a 
 physiological basis peculiar to each individual. They are 
 susceptible of being extended and strengthened by exercise, 
 and just as muscular exercise causes an increased growth 
 of muscular fibre, so regulated mental exercise must develop 
 and strengthen the tissue of the brain. Thus one man 
 differs from another in the primitive constitution of his 
 nerve centres. This determines his degree of intelligence, 
 power of accurate judgment, and aptitude for special kinds 
 of work. These qualities are determined chiefly by inherit- 
 ance from ancestors who have thus given their descendant 
 a groundwork of mental character. In the next place, the 
 influence of a man's environment develops to a greater or 
 less extent this and that faculty. This is the rational basis 
 of all educative processes. Again, the degree of excitability 
 of the nerve centres varies considerably among individuals, 
 and it also may be influenced by exercise. On this depends 
 the aptitude for reflex acts of all kinds. Lastly, there may 
 be a greater or less influence exerted by the higher over the 
 lower centres, or, in other words, a greater or less degree 
 of inhibitory power. The power of the w///, which may 
 also be strengthened by exercise, or weakened by yielding 
 to disease, or by tame compliance, depends on this factor. 
 Thus by a study of nervous actions, as connected with and 
 stimulated by impressions on the organs of sense, we have 
 constructed a physiological basis of character, and that with- 
 out admitting the truth of an exclusively materialistic hypo- 
 thesis. Behind all brain action, although closely connected 
 with it, there is the strongest probability of the existence of 
 an immaterial agent of which Spenser wrote in his Hymn 
 in Honour of Beauty : 
 
 * * For of the soul the body form doth take, 
 For soul is form, and doth the body make." 
 
APPENDIX I 
 
 THE ACTION OF LIGHT ON THE RETINA 
 
 AT p. 150 reference is made to the electrical change that occurs 
 when light falls on the living retina. A full description of this 
 remarkable phenomenon was out of place at that part of the book, 
 but inasmuch as it is the only example we have of a known physical 
 process occurring in a terminal organ of sense, it merits here a 
 further notice. For the detection of electrical currents in living 
 tissues a sensitive galvanometer of high resistance must be employed. 
 The currents are led off the living tissues by electrodes that are so 
 constructed as to be unpolarisable that is to say, they do not them- 
 selves generate any current, nor are they altered by the passage of 
 even a feeble current through them, so as to give rise to any electrical 
 action. They simply lead off to the galvanometer any current that 
 may exist. Such electrodes are variously constructed ; but a con- 
 venient form is a trough of zinc, resting on insulating plates of 
 vulcanite, amalgamated on the inner surface, and filled with a 
 saturated solution of sulphate of zinc. A pad of blotting-paper, wet 
 with the sulphate of zinc solution, is placed into each trough, and 
 on the pad a bit of clay, moistened with saliva, is laid, so as to pro- 
 tect any animal tissues placed on the clay from the irritant action of 
 the sulphate of zinc. The electrodes, so prepared, are connected 
 with the galvanometer. A frog's eye is dissected out (after the 
 animal has been decapitated, and all sensation has been lost), and is 
 so placed on the pads of clay that one pad touches the middle of the 
 surface of the cornea, and the other the posterior surface of the eye- 
 ball and the transverse section of the optic nerve. A current, which 
 we may call the "resting-eye current," is shown by a deflection of 
 the needle of the galvanometer. It can be shown that this current 
 
300 Physiology of the Senses 
 
 passes from the corneal surface through the galvanometer and back 
 to the posterior surface of the eyeball that is to say, the eyeball 
 acts like a little galvanic element, the positive pole of which is the 
 cornea and the negative pole the transverse section of the optic 
 nerve. The eye is now covered with a blackened box so as to keep 
 it in the dark, and the box is provided with a shutter by which the 
 light may be shut off or admitted at pleasure. When we open the 
 shutter, and allow light to fall upon the eye, the needle of the gal- 
 vanometer will be seen to swing in the direction that indicates an 
 increase in the current. If light is allowed to act on the eye for 
 a few minutes, the current diminishes, falls off in strength as the 
 retina becomes fatigued, and soon becomes less than it was when 
 light was allowed to fall on the eye. If the light is allowed to act 
 sufficiently long, the current becomes less and less until it reaches 
 zero. If, however, we remove the light by closing the shutter before 
 the retina has become too fatigued, there is at once a second in- 
 crease in the strength of the current again indicated by a swing of 
 the galvanometer needle, then a rapid diminution, and soon. the 
 needle becomes almost stationary. These are the details of a single 
 experiment ; and they show that light alters the electrical condition 
 of the eye, the impact of light causing an increase, its continued 
 action a diminution, and its removal another increase in the " resting- 
 eye current." 
 
 It can be shown that the effect is due to the action of light on the 
 retina, because if this structure be removed, light will produce no 
 variation in any current that may be got from other structures. The 
 effect is due to light and not to heat, because it is easy to absorb the 
 heat rays, and still allow the light to pass, and vice versa. In both 
 cases it is only when light rays reach the retina that the effect is 
 obtained. These variations have been seen in the eyes of inverte- 
 brates and vertebrates, and even in the eye of man himself. Further, 
 by allowing the different rays of the spectrum to fall on the eye, we 
 can show that the luminous yellow rays produce more effect than 
 the less luminous green, red, blue, or violet rays, and that the sum 
 of the effects of the different rays is almost that of white light. It can 
 also be demonstrated that the effects of varying intensities of light 
 agree with the laws formulating the relation between the strength of 
 the stimulus and the strength of the resulting sensation referred to 
 on p. 39. The importance of this observation is due to the indica- 
 tion it gives that the stimulus-sensation-ratio may be a function of 
 the terminal organ as well as of the brain. 
 
Appendix I 301 
 
 The electrical variations above described may be physical 
 indications of chemical phenomena known to occur in the retina. 
 This, however, has not been proved. It is conceivable, as an alter- 
 native hypothesis, that the rods and cones act as transforming struc- 
 tures, changing the waves of light into electrical variations that pass 
 along the fibres of the optic nerve. Electrical variations are the 
 only phenomena that have yet been demonstrated in a nerve fibre 
 during the passage along it of a nervous impulse ; and if, as the 
 physicists assert, light waves are only short electrical waves, the 
 hypothesis suggested is not unreasonable. 
 
 These electrical changes in the retina, caused by the action of 
 light, were independently discovered by Holmgren in Upsala, and 
 by Dewar and M'Kendrick in Edinburgh, between 1870 and I873. 1 
 
 1 Dewar and M'Kendrick, Proceedings of Royal Society of Edin- 
 burgh, 1874. Also M'Kendrick's Text -Book of Physiology, vol. ii. 
 p. 627. 
 
APPENDIX II 
 
 DERIVATIONS OF SCIENTIFIC TERMS 
 
 ABERRATION, L. ab, away ; erro, erratum, to wander 
 
 Actinic, Gr. aktis, a sunbeam 
 
 Acustica, Gr. akozio, to hear 
 
 JEsthesiometer, Gr. asthesis, feeling ; metron, a measure 
 
 Afferent, L. ad, to ; fero, I carry 
 
 Alkaloid, Arab, alkali ; Gr. eidos, likeness 
 
 Allotropic, Gr. allotropos, of a different nature 
 
 Ametropia, Gr. a, not ; metron, measure ; ops, the eye 
 
 Amplitude, L. amplitude, largeness 
 
 Ampulla, L. ampztlla, a bottle 
 
 Anaesthesia, Gr. a, without ; cesthesis, perception 
 
 Analgesia, Gr. a, without ; algos, pain 
 
 Anode, Gr. ana, up ; hodos, a way 
 
 Anosmia, Gr. a, without ; osme, smell 
 
 Aqueous, L. aqua, water 
 
 Arborescent, L. arboresco, to become a tree 
 
 Astigmatism, Gr. a, without ; stigma, a point 
 
 Ataxia, Gr. a, without ; taxis, arrangement 
 
 Auditory, L. audio, auditum, to hear 
 
 Aura, Gr. ao, to breathe 
 
 Auricle, L. auriculus, dim. of auris, an ear 
 
 Automatic, Gr. automates, of one's own accord 
 
 BASSOON, Gr. basis, base ; a wind instrument giving a low note 
 Biconvex, L. bis, twice ; con, together ; who, vectum^ to carry 
 Binary, L. bina, a pair 
 
Appendix II 303 
 
 Binaural, L. bis, twice ; audio, I hear 
 Binocular, L. bis, twice ; oatlus, the eye 
 
 CALLOSUM, L. callosus, thick-skinned 
 
 Camera, L. camera, a chamber 
 
 Capillary, L. capillus, a hair 
 
 Cardinal, L. cardo, a hinge 
 
 Cataract, Gr. kata, down ; arasso, to fall 
 
 Cerebellum, L. cerebellum, dim. of cerebrum, the little brain 
 
 Cerebrum, L. cerebrum, the brain 
 
 Cerumen, L. cera, wax 
 
 Choroid, Gr. chorion, skin ; eidos, likeness 
 
 Chromatic, Gr. chroma, colour 
 
 Ciliary, L. eilium, an eyelash 
 
 Cilium (//. cilia), L. eilium, an eyelash 
 
 Circumvallate, L. circum, around ; vallum, a wall 
 
 Cochlea, Gr. kochlias, a snail with a shell 
 
 Coma, Gr. koma, drowsiness 
 
 Commissure, L. com, together ; mitlo, missum, to send 
 
 Complementary, L. com, together ; pleo, to fill 
 
 Congenital, L. congenitus, born together with 
 
 Conjugate, L. con, together ; jugum, a yoke 
 
 Conjunctiva, L. con, together ; jungo, junctum, to join 
 
 Consciousness, L. con, together ; scio, I know 
 
 Convergence, L. con, together ; vergo, to bend 
 
 Convolution, L. convolve, convolutum, to roll 
 
 Corium, Gr. chorion, skin 
 
 Cornea, L. cornu, a horn 
 
 Corona, L. corona, a crown 
 
 Corpus (pi. corpora), L. corpus, a body 
 
 Corpuscle, L. corpusculus, dim. of corpus, a body 
 
 Cortex, L. cortex, bark 
 
 Cranium, Gr. kranion, the skull 
 
 Cribriform, L. cribrum, a sieve ; forma, likeness 
 
 Crista, L. crista, a crest 
 
 Cuneus, L. cuneus, a wedge 
 
 Cupula, L. cupula, a small cup 
 
 DALTONISM, Dalton, a celebrated chemist who was colour-blind 
 Decussation, L. decusso, to place crosswise in the form of an X 
 Dental, L. dens, dentis, a tooth 
 
304 Physiology of the Senses 
 
 Derma, Gr. derma, the skin 
 Diabetes, Gr. dia, through ; baino, to go 
 Diaphragm, Gr. dia, across ; phrasso, to fence 
 Dioptrics, Gr. di, through ; horao, I see 
 Dispersion, L. dis, asunder ; spargo, to scatter 
 Dissonance, L. dis, asunder ; sonans, sounding 
 Dynamical, Gr. dynamis, power 
 
 EFFERENT, L. ex, out ; fero, I carry 
 
 Electrode, Gr. elektron, amber ; hodos, a way 
 
 Electrolysis, Gr. elektron, amber ; lysis, a softening 
 
 Emmetropic, Gr. en, in ; metron, measure ; ops, the eye 
 
 Endolymph, Gr. endon, within ; lympha, water 
 
 Entoptic, Gr. entos, within ; ops, the eye 
 
 Epidermis, Gr. epi, upon ; derma, skin 
 
 Epiglottis, Gr. epi, upon ; glotta, a tongue 
 
 Erectile, L. e, out ; recto, to make straight 
 
 Ether, Gr. aither, the upper air 
 
 Ethmoid, Gr. ethmos, a sieve ; eidos, likeness 
 
 FAUCES, L. fauces, the gullet 
 
 Fenestra, L. fenestra, a window 
 
 Fibril, L. fibra, a filament 
 
 Filament, L. filum, a thread 
 
 Filiform, L. filum^ a thread ; forma, form 
 
 Fluorescence, \^.fluo, I flow 
 
 Focus, L. foctts, a fireplace 
 
 Foliata, L. folium, a leaf 
 
 Follicle, L. follicuhis, dim. offollis, a wind ball or bag 
 
 Foramina, L. foro, to bore 
 
 Formication, L. formica, an ant 
 
 Fornicatus, L. fornicatus, arched 
 
 Fovea, L. _/&ztf0, a small pit 
 
 Function, L. fungor, fiinctum, to discharge an office 
 
 Fundus, L. fundus, the bottom 
 
 Fungiform, L. fungus, a mushroom ; forma, form 
 
 Fuscin, L. fuscus, tawny 
 
 GALVANOMETER, Galvani, the discoverer of certain electrical pheno- 
 mena ; metron, a measure 
 
 Ganglion (pi. ganglia), Gr. ganglion, a tumour under the skin 
 Glosso-pharyngeal, Gr. glossa, the tongue ; pharynx, the throat 
 
Appendix II 305 
 
 Gustatory, L. gustatus, tasted 
 Gyri, Gr. gyros ; a circuit 
 
 HAEMORRHAGE, Gr. haima, blood ; rheo, to flow 
 
 Hamulus, L. dim. of hamus, a hook 
 
 Helicotrema, Gr. helix, a spiral ; trema, a perforation 
 
 Hemianaesthesia, Gr. hemi, half ; a, without ; asthesis, feeling 
 
 Heteronomous, Gr. heteros, another ; onoma, a name 
 
 Hippocampus, Gr. hippos, a horse ; kampos, a sea- monster 
 
 Homologous, Gr. homos 9 the same ; logos, a discourse 
 
 Homonomous, Gr. homos, the same ; onoma, a name 
 
 Horopter, Gr. horos, a boundary ; opter, a spectator 
 
 Hyaloid, Gr. hyalos, glass ; eidos, a likeness 
 
 Hypermetropia, Gr. hyper, beyond ; metron, measure ; ops, the 
 
 eye 
 
 Hypnotism, Gr. hypnos, sleep 
 Hypoglossal, Gr. hypo, under ; glossa, the tongue 
 Hypometropia, Gr. hypo, under ; metron, measure ; ops, the eye 
 
 ILLUSION, L. in, in ; ludo, htsum, to play 
 
 Incus, L. incus, an anvil 
 
 Index, L. in, in ; dico, to proclaim 
 
 Internuncial, L. inter, between ; nuntius, a messenger 
 
 Intuition, L. inttis, within ; itum, to go 
 
 Iris, Gr. iris, the rainbow 
 
 JAUNDICE, Yr.jazme, yellow 
 
 KATHODE, Gr. kata, down ; hodos, a way 
 Klang, Ger. klang, the quality of a sound 
 
 LABYRINTH, Gr. labyrinthos, a labyrinth 
 Lachrymal, L. lachryma, a tear 
 Lamella, L. lamella, dim. of lamina, a small plate 
 Lamina, L. lamina, a small plate 
 Laxator, L. laxo, to loosen 
 Lens, L. lens, a lentil 
 Lenticular, L. dim. of lens, a small bean 
 Limbus, I., limbits, a border 
 Lingual, L. lingua, a tongue 
 Logarithm, Gr. logos, ratio ; arithmos, number 
 
 X 
 
306 Physiology of the Senses 
 
 Lumen, L. lumen, light 
 Luminosity, L. lumen, light 
 
 MACERATE, L. macero, to waste away 
 
 Macula, L. macula, a spot 
 
 Malleus, L. malleus, a hammer 
 
 Mastoid, Gr. mastos, the breast 
 
 Meatus, L. meo, meatum, to pass 
 
 Medulla, L. medulla, the marrow ; medius, the middle 
 
 Melanin, Gr. melan, black 
 
 Meridional, L. mer'idies, midday 
 
 Mesentery, Gr. mesos, middle ; enteros, intestines 
 
 Minimum visibile, L. minimum, the least ; visibile, able to be 
 
 seen 
 
 Modiolus, L. dim. of modus, a measure 
 Molecular, L. dim. of moles, a mass 
 Momentum, L. moveo, to move 
 Morphological, Gr. morphe, form ; logos, a discourse 
 Motor, L. moveo, motum, to move 
 Mucus, Gr. muoca, the mucus of the nostrils 
 Muscae volitantes, L. musca, a fly ; volitans, flying 
 Myopia, Gr. muo, to close ; ops, the eye 
 
 NARES, L. nares, the nostrils 
 
 Neurilemma, Gr. neuron, a nerve ; lejnma, a coat 
 
 Neuro-epithelium, Gr. neuron, a nerve ; epi, upon ; tithemi to 
 
 place 
 
 Neuroglia, Gr. neuron, a nerve ; glia, glue 
 Nexus, L. necto, to twine 
 Nodal, L. nodus, a knot 
 Nucleus, L. nucleus^ the kernel 
 
 OCCIPITAL, L. ob, against ; caput, the head 
 
 Odoroscope, L. odor, odour ; Gr. skopeo, I see 
 
 Olfactory, L. olfacio, to smell 
 
 Operti, L. opertus, opened 
 
 Ophthalmic, Gr. ophthalmos, the eye 
 
 Ophthalmoscope, Gr. ophthalmos, the eye ; skopeo, I see 
 
 Orbit, L. orbita, an orbit 
 
 Organ, Gr. organon, an instrument 
 
 Organism, Gr. organon, an instrument 
 
Appendix II 307 
 
 Ossicle, L. dim. of os, a bone 
 Otoconia, Gr. ous, otos, the ear ; konis, dust 
 Otolith, Gr. ous, otos, the ear ; lithos, a stone 
 Ozone, Gr. ozo, to smell 
 
 Pancreas, Gr. pan, all ; kreas, flesh 
 
 Papilla, L. papilla, a nipple 
 
 Parietal, L. paries, a wall 
 
 Pari passu, L. par, equal ; passus, step 
 
 Pathological, Gr. pathos, suffering ; logos, a discourse 
 
 Peduncle, L. pedo, having broad feet 
 
 Pellicle, L. pellicula, dim. oipellis, a skin 
 
 Period, Gr. 'periodos, a going round 
 
 Peripheral, Gr. periphereia, a periphery 
 
 Peritoneum, Gr. peritonaios, stretched over 
 
 Petrous, Gr. petra, a rock 
 
 Phakoscope, Gr. phakos, a lentil, the lens ; skopeo, I see 
 
 Phalangse, Gr. phalanx, a block 
 
 Phantasmagoria, Gr. phantazo, to make appear ; agora, an assembly 
 
 Pharynx, Gr. pharynx, the throat 
 
 Phase, Gr. phasis, phaino, to show 
 
 Phenomenon, Gr. phainomenon, appearing 
 
 Photometrical, Gr. phos, light ; metron, a measure 
 
 Physharmonica, Gr. physao, to blow ; harmonikos, musical 
 
 Pigment, L. pingo, to paint 
 
 Pitch, A.S. pycan, to pick or strike with a pike 
 
 Pituita, L. pittiita, phlegm 
 
 Plane, L. planus, smooth 
 
 Plexus, L. plexus, a network 
 
 Pons, L. pons, a bridge 
 
 Prsecuneus, L. prce, before ; cuneus, a wedge 
 
 Presbyopia, Gr. presbys, old ; ops, the eye 
 
 Prism, Gr. prisvia, from prio, to saw 
 
 Protoplasm, Gr. protos, first ; plasma, anything formed 
 
 Pseudoscope, Gr. pseudos, false ; skopeo, I see 
 
 Psychical, Gr. psyche, the soul 
 
 Pupil, L. pupilla, dim. of pupa, a puppet 
 
 QUADRIGEMINA, L. quatuor, four; gemini, double 
 Quantum, L. quantum, how much 
 
308 Physiology of the Senses 
 
 RECTUS, L. rectus, straight 
 
 Refraction, L. re, back ; frango, fractum, to break 
 
 Refrangible, L. re, back ; frango, to break 
 
 Resonator, L. re, again ; sono, to sound 
 
 Reticulated, L. rete, a net 
 
 Retina, L. rete, a net 
 
 SACCHARINE, L. saccharum, sugar 
 
 Saccule, L. dim. of saccus, a bag 
 
 Schematic, Gr. schema, form 
 
 Sclerotic, Gr. skleros, hard 
 
 Sebaceous, L. sebum, suet 
 
 Section, L. seco, sectum, to cut 
 
 Segment, L. seco, to cut 
 
 Sensorium, L. sentio, sensum, to feei 
 
 Septum, L. sepes, a hedge 
 
 Serous, L. serum, a watery fluid 
 
 Sine, L. sinus, a curve 
 
 Spectrum, L. specio, I see 
 
 Sphenoid, Gr. sphen, a wedge ; eidos, likeness 
 
 Sphincter, Gr. sphingo, I contract 
 
 Squamous, L. squama, the scale of a fish 
 
 Stapes, L. stapes, a stirrup 
 
 Stereoscope, Gr. stereos, solid ; skopeo, I see 
 
 Stimulus, L. stimulus, a goad 
 
 Striata, L. striatttm, grooved 
 
 Stylet, Gr. stylos, a style or pencil 
 
 Sulcus, L. sulcus, a groove 
 
 Synchronous, Gr. syn, together ; chronos, time 
 
 Syren, L. siren, a singer of sweet music 
 
 TAPETUM, Gr. tapes, tapestry 
 
 Telestereoscope, Gr. tele, at a distance ; stereos, solid ; skopeo, 1 
 
 see 
 Temporo-sphenoidal, L. tempora, the temples ; Gr. sphen, a wedge ; 
 
 eidos ) likeness 
 
 Thalamus, Gr. thalamos, a couch 
 Thaumatrope, Gr. thauma, wonder ; tropos, a turning 
 Timbre, Fr. timbre, the sound of a bell, the voice 
 Translucent, L. trans, through ; luceo, to shine 
 Triturate, L. tritus, rubbed 
 
Appendix II 309 
 
 Turbinated, L. turbinatus, pointed 
 Tympanum, Gr. tympanon, a drum 
 
 UMBO, L. umbo, the boss of a shield 
 Uncinate, L. uncus, a hook 
 Undulatory, L. unda, a wave 
 Utricle, L. dim. of uter, a leathern bag 
 Uvula, L. dim. of ztva, a grape 
 
 VAS, L. vas, a vessel 
 
 Vertebrate, I>. verto, I turn 
 
 Vestibule, L. vestibulum, a threshold 
 
 Vibration, L. z;^;v, to quiver 
 
 Vibrissae, L. vibro, to quiver 
 
 Vidian, after Vidius, who described the Vidian nerve 
 
 Viscera, L. viscera, the bowels 
 
 Vitreous, L. vitrum, glass 
 
 Volatility, L. w/0, volatum, to fly 
 
 Vorticosa, L. verto, to turn 
 
 ZERO, Arab, tsaphara, empty 
 Zonule, L. dim. of zona, a belt 
 
INDEX 
 
 ABERRATION, spherical, 122 ; 
 chromatic, 124 ; chromatic, of 
 eye, 131 ; spherical, of eye, 
 132 
 
 ABNEY on colour vision, 170 
 
 Absolute sensitiveness, 56 
 
 Accommodation of eye for dis- 
 tance, 135 
 
 Aerial perspective, 188 
 
 ,/Esthesiometer, 55 
 
 After-image, 153 ; positive, 154 ; 
 negative, 154 ; coloured, 161 
 
 After-tactile impressions, 58 
 
 AjUGARI, LUCREZIA, voice of, 
 246 
 
 Albinos, 101 
 
 Ametropic eye, 138 
 
 Ampulla, 224 
 
 Anaesthesia, 16 
 
 Analgesia, 16 
 
 Analogy between touch and hear- 
 ing. 53 
 
 Angle of convergence, 188 
 
 Anosmia, 94 
 
 Antennae of insects, 52 
 
 Apex-process, 28 
 
 Appendages of the skin, 43 
 
 Aqueous humour, 100 
 
 Area of distinct vision, 145 
 
 ARENSOHN on odours, 91, 92 
 
 ARISTOTLE'S experiment, 61 
 
 Aromatic bodies, 87 
 
 Association, fibres of, 295 ; of 
 ideas, 295 
 
 Astigmatism, 132 
 
 Auditory hairs, 227 ; nerve, 223 ; 
 
 teeth, 235 
 
 Aura of epilepsy, 33 
 Auricle, 200 ; its function, 201 
 Automatic movements, 20 
 
 BALFOUR, F. M. , on sensory 
 apparatus, 8 
 
 Beats, 259 
 
 Beat-tones, 260 
 
 BEAUNIS on odours, 92 
 
 Binaural audition, 283 
 
 Binocular vision, 170; visual field, 
 177 
 
 Birds, cochlea of, 271 
 
 Blindness, psychical, 31 
 
 Blind spot, 149 
 
 BOWMAN, glands of, in nose, 85 ; 
 spiral ligament of, 229 ; ante- 
 rior and posterior elastic lamina 
 of, 99 
 
 Brightness of colour, 159 
 
 BROWN, A. CRUM, on semicircular 
 canals, 268 
 
 BRUCH, membrane of, 102 
 
 Bulb, 1 8 
 
 CALLOSO-MARGINAL fissure, 27 
 
 Canalis reuniens, 225 
 
 Canals, semicircular, their de- 
 velopment, 224; in equilibrium, 
 267 
 
 Cardinal points, 125 
 
312 
 
 Physiology of the Senses 
 
 Cataract, 107 
 
 Cells of cortex of brain, 29 
 
 Centre for hearing, 32 ; for per- 
 ception of heat and cold, 35 ; 
 of rotation of eyes, 171 ; for 
 taste and smell, 34 ; for touch, 
 34; for vision in cortex cerebri, 30 
 
 Cerebellum, 19 
 
 Cerebral peduncles, 22 
 
 Cerebration, unconscious, 296 
 
 Cerebrum, 22 
 
 Cerumen, 204 
 
 Chain of bones, 209 ; movements 
 of, 211 ; transmission of vibra- 
 tions by, 218 
 
 Chamber, anterior, 100 ; pos- 
 terior, 108 
 
 Choice, power of, 296 
 
 Chorda tympani, 205 
 
 Choroid, 99 
 
 Ciliary arteries and veins, 100 ; 
 ganglion, in; muscle, 102; 
 nerves, in; processes, the, 
 102 
 
 Circle of sensibility, 62 
 
 CLAUDIUS'S cells, 236 
 
 CLELAND, theory as to seat of 
 consciousness, 288 
 
 Cochlea, 228 ; its function, 273 
 
 Cochlear canal, 225, 228, 230 
 
 Cold spots, 64 
 
 Colour blindness, 159 
 
 Colour, sensation of, 155 
 
 Colour of the skin, 43 
 
 Colour vision theories, 161 
 
 Coma, 286 
 
 Common sensations, 35 
 
 Compasses for touch, 54 
 
 Cones of retina, 103, 104 
 
 Confusion colours, 160 
 
 Conjunctiva, 99 
 
 Consciousness, 286 ; seat of, 288 ; 
 not a form of energy, 291 
 
 Contrast of colours, 161 , 
 
 Convolutions of brain, 24 
 
 Co-ordinated movements, 294 
 
 Corium, 41 
 
 Cornea, 98 
 
 Corona radiata, 28 
 Corpora quadrigemina, 22 
 Corpus callosum, 24 ; striatum, 
 
 23. 35 
 
 CORTI, organ of, 232 
 Cribriform plate of ethmoid bone, 
 
 83 
 
 Crista acustica, 227 
 Crystalline lens, 106, 107 
 Cuneus, 28 
 Cupula, 227 
 Cutis vera, 41 
 CYON on semicircular canals, 267 
 
 DALTONISM, 159 
 Deafness resulting from destruc- 
 tion of cortical centre, 34 
 Decussation of nerve fibres, 15 
 Degeneration of nerve fibre, 1 3 
 DEITER'S cells, 235 ; their func- 
 tion, 266 
 
 Delicacy of sense of smell, 92 
 Derma, 41, 42 
 
 DESCEMET, membrane of, 99 
 Dewar, observations on physio- 
 logical action of light, 301 
 Dioptrics, laws of, 115 
 Dissonance, 260 
 Distance, estimation of, 187 
 Distinct vision, 175 
 DONDERS on the eye, 171 
 Drum, drum-head, 199, 202 
 Ductus endolymphaticus, 225 
 
 EAR, external, 200 ; middle, 204 ; 
 internal, 223 ; their functions, 
 264, 265 
 
 Emmetropic eye, 138 
 
 End-bulbs, 47 
 
 Endolymph, 266 
 
 Entoptic phenomena, 141 
 
 Epidermic structures, their func- 
 tions, 43, 51 
 
 Epidermis, 41, 42 
 
 Epithelium, olfactory, 84 
 
 Ethmoid bone, 81 
 
 EULENBERG, sensitiveness of skin, 
 57 
 
Index 
 
 Eustachian tube, 82, 199, 207 
 Externality in sensation, 40 
 Eye, adjustment for different dis- 
 tances, 134 ; examination of 
 interior of, 143 ; dioptric sys- 
 tem of, 127 ; its defects as an 
 optical instrument, 131 
 Eyeball, structure of, 97 ; con- 
 tents of, 105 
 
 FATIGUE of nerve, 4 
 FECHNER'S law of sensation, 39 
 Fenestra ovalis, 206 ; rotunda, 
 
 207, 229 
 
 Fenestrated membrane, 236 
 FERRIER on' brain, 29, 34 
 FESTING on colour vision, 170 
 Fissure of ROLANDO, 26 ; of 
 
 SYLVIUS, 26 
 Flavour, 74 
 
 Flowers and odours, 89 
 Fluorescence, 116 
 Focal points, 125, 128 
 Focus, principal and conjugate, 
 
 120 
 
 Follicle of hair, 50 
 Foramina nervina, 237 
 Form, judgment as to, 194 
 Formication, 35 
 
 FORSTER, GASPARD, voice of, 245 
 Fovea centralis, 105 
 FRITSCH on brain, 29 
 Frontal lobe of brain, 26 
 Fundamental colours, 158 ; tone, 
 
 250, 251, 253 
 Fuscin, 105 
 Fusion of tactile impressions, 58 
 
 GALTON'S observations on the 
 
 blind, 56 
 
 Ganglia, 9 ; spiral ganglion, 237 
 GASPARD FORSTER, basso, 245 
 GAUSS, cardinal points of, 125 
 GLASER, fissure of, 205 
 Glosso-pharyngeal nerve, 72, 73 
 GOLDSCHEIDER, hot and cold 
 
 spots, 64 
 GRAHAM on odours, 93 
 
 GRANDRY'S corpuscles, 46 
 
 GRATIOLET, radiation of, 30, 32 
 
 Gustatory nerves, 72 
 
 Gymnema sylvestre, 77 
 
 Gyri operti, 26 
 
 Gyrus, a convolution of brain, 24 ; 
 fornicatus, 27, 35 ; hippo- 
 campi, 27 ; a centre for 
 touch, 35 ; uncinatus, a centre 
 for smell, 34 
 
 HAIR-CELLS, inner, 233 ; outer, 
 235 ; their function, 266 
 
 HALL, STANLEY, theory of colour 
 vision, 164 
 
 Hallucinations, auditory, 279 
 
 Hamulus, 228 
 
 Harmonic motion, 248 
 
 Harmonics, 253 
 
 HARTMANN, Von, 287 
 
 Hearing, 198, centre in cerebrum 
 for, 32 ; range of, 245 
 
 Hearing affected by drugs, 278 
 
 Helicotrema, 228 
 
 HELM HOLTZ, Von , theory of colour 
 vision, 162, 169 ; ophthalmo- 
 scope of, 143 ; telestereoscope 
 of, 184; resonators of, 251; 
 syren of,' 242 ; on quality of 
 sounds, 256, 257, 260 ; theory 
 as to function of cochlea, 271 
 
 Hemianassthesia, 35 
 
 HENRY, CH. , on odours, 86, 94 
 
 HENSEN'S spot, 235 ; cells, 236 ; 
 on Mysis, 265 
 
 HERBST'S corpuscles, 50 
 
 HERING, theory of colour vision, 
 165 
 
 Heteronomous images, 179 
 
 HITZIG on brain, 29 
 
 HOLMGREN, observations on phy- 
 siological action of light, 301 
 
 Homonomous images, 178 
 
 Horopter, 178 
 
 HORSLEY, areasof brain, 29, 34,35 
 
 Hot spots, 64 
 
 Hue of colour, 159 
 
 Hyaloid membrane, 105 
 
Physiology of the Senses 
 
 Hypermetropic eye, 138 
 Hypometropic eye, 138 
 
 ILLUSIONS of vision, 192 
 Images formed by lenses, 120, 
 
 121 
 
 Incus, 210 
 
 Intensity, 246 ; of odours, 92 ; of 
 sensation, 37, 38 ; of taste, 
 76 
 
 Internal capsule, 23 
 
 Intuitions, 295 
 
 Iris, the, 100 
 
 Iridescence of epidermic struc- 
 tures, 45 
 
 Irradiation, 140, 154 
 
 Island of REIL, 26 
 
 JACOB'S membrane, 103 
 
 KLANG of musical tone, 247 
 KOENIG, analysis of compound 
 
 tones, 254 
 KRAUSE'S end-bulbs, 47 ; theory 
 
 as to touch, 63 
 
 LABYRINTH, membranous, 199, 
 
 225 ; osseous, 223 
 Lachrymal gland, 97 
 LADD on colour sense, 168 
 LAMBERT on colours, 156 
 Lamina cribrosa of the eye, 109 ; 
 
 spiralis ossea, 228 ; mem- 
 
 branacea, 230 
 LANGERHANS, cells of, 45 
 Laxator tympani, 205 
 LE CONTE, divergence of visual 
 
 axes, 174 
 
 Lens, biconvex, 119 
 Lenticular process of incus, 210 
 LIEGEOIS on odours, 90 
 Light, physiological action of, 
 
 299; physical nature of, 115; 
 
 reflection and refraction of, 116 
 Limbus, 230 
 Line of regard, 131 ; vision, 
 
 13* 
 LISTING, cardinal points of, 125 ; 
 
 schematic and reduced eye of, 
 
 130 
 
 Lobes of the brain, 26 
 Locomotor ataxia, 17 
 Loudness of sound, 246 
 Lower limit of excitation, 37 
 LUCREZIA AJUGARI, soprano, 
 
 246 
 Luminiferous ether, 97, 115 
 
 MACH, action of semicircular 
 
 canals, 268 
 Macula acustica, 226 
 MAJENDIE, paths of sensory 
 
 fibres, 10 
 Malleus, 209 
 
 MALPIGHI, stratum of, 42 
 Marginal gyrus, 27 
 Massiveness of taste, 75 
 Manubrium of malleus, 209 
 M'KENDRICK, observations on 
 
 physiological action of light, 301 
 Meatus, external auditory, 202 ; 
 
 internal auditory, 225 
 Meatuses of nose, 81 
 Medulla oblongata, 18, 19 
 MEISSNER'S touch corpuscles, 47 
 Melanin, 105 
 Membrana basilaris, 230 ; tec- 
 
 toria, 237 ; tympani, 199, 202, 
 
 205 ; response to sound-waves, 
 
 214 
 
 Memory, 294 ; of sounds, 282 
 MERKEL'S corpuscles, 46 
 Minimum visibile, 148 
 Modiolus, 225, 228 
 MOEBIUS on cardinal points, 125 
 Molar movement, 219, 221 
 Molecular movements, 219 
 Motion, perception of, 193 
 MUELLER, H., sensitive layer of 
 
 retina, 143 
 MUELLER'S sustentacular fibres, 
 
 103 
 
 MUNK, sensory centres, 31, 34 
 Muscse volitantes, 141 
 Muscles of the eye, 172 
 Muscular sense, 36, 68 
 
Index 
 
 Musical tones, 240 
 
 Myopic eye, 138 
 
 Mysis, experiment on, 265, 272 
 
 NARES, anterior and posterior, 
 
 81 
 Nasal cartilages, 81 ; mucous 
 
 membrane, 81 ; cavities, 80 
 Near point of vision, 137 
 Nerves, afferent and efferent, 10 ; 
 
 their structure, 1 1 
 Nerve current, 5 ; rate of, 6 ; 
 
 cells, their origin, 9 ; matter, 
 
 3 
 Nerves, fatigue of nerve, 4 ; of 
 
 the nose, 82 ; of the tongue, 
 
 78 
 Nerve-endings in the tactile hairs, 
 
 50 ; free, 45 ; in corpuscles, 
 
 45 
 
 Neuro-epithelium, 8 
 Neuroglia, 28 
 NEWTON, analysis of light, 
 
 116 
 
 NILSSON, voice of, 245 
 Nodal points, 126, 129 
 Noises, 240, 262 
 Normal eye, average, 128 
 Nose, vestibular portion of, 83 ; 
 
 respiratory, 83 ; olfactory, 84 ; 
 Nose-leaves of bats, 52 
 
 OBLIQUE muscles, 172 
 
 Occipital lobe of brain, 26 
 
 Occipito- angular area, a visual 
 centre, 31 ; blindness resulting 
 from destruction of, 32 
 
 Odoroscope, 90 
 
 Odorous substances, their chemi- 
 cal nature, 87 
 
 Odours, their influence on respira- 
 tion, 94 ; and heat absorption, 
 89 ; pure and mixed, 93 ; and 
 surface tension, 90 
 
 Olfactory cells, 85 ; epithelium, 
 84 ; lobes, 83 ; nerves, 83 
 
 Ophthalmoscope, its principle, 
 143 
 
 Optic commissure, 109 ; lobes, 
 23 ; nerve, 109 ; papilla, 149 ; 
 pore, 102 ; thalami, 23 ; tracts, 
 22, no 
 
 Orbits, 170 
 
 Organ of CORTI, 231 
 
 Otoconia, 227 
 
 Otoliths, 227 
 
 Overtones, 253 
 
 OWEN on tactile hairs, 52 
 
 Ozone and odours, 90 
 
 PACINI'S corpuscles, 46, 48, 49 ; 
 
 their function, 53 
 Pain, 67 ; its quality, 68 
 Papillae, 42 ; filiform, fungiform, 
 
 circumvallate, 70 ; foliatae, 71 
 Parietal lobe of brain, 26 
 Partial tones, 253 
 Peduncles, cerebral, 22 
 Perception time, 6 
 Perilymph, 228 
 Perspective, aerial, 188 
 PETIT, canal of, 106 
 Phakoscope, 136 
 Phalangae, 236 
 Phase of vibration, affecting 
 
 quality of tone, 257 
 Phosgenes, 152 
 Pigments, 157 
 Pitch of musical tones, 242 
 Pituita, 82 
 Points, remote and near, of vision, 
 
 137 
 
 Pons VAROLII, 21 
 Position, primary, secondary, 
 . tertiary, of eyeball, 171 
 Presbyopic eye, 140 
 PREVOST on odours, 90 
 Prickle cells in skin, 43 
 Principal points, 129 ; planes, 
 
 125 
 
 Prisms, 118 
 Processus cochleariformis, 207 ; 
 
 gracilis of malleus, 210 
 Promontory, 207 
 Protecting cells of taste bud, 
 
 72 * 
 
316 
 
 Physiology of the Senses 
 
 Protoplasm, its chemical consti- 
 tution, 3 ; its instability, 4 
 Pseudoscope, 184 
 Psychical blindness, 31 ; deafness, 
 
 34 
 
 Psycho-physical time, 6 
 Pupil, 100 ; movements of, in 
 Purity of colour, 159 
 PURKINJE'S figures, 142 
 Purple of retina, 151 
 
 QUALITY of musical tones, 247, 
 256 ; of sensation, 36 
 
 Quantitative character of sensa- 
 tion, 37 
 
 RAMSAY on odours, 87, 93 
 
 Ray, course of, in dioptric system, 
 126 
 
 Rectus muscle, 172 
 
 Reduced eye, 130 
 
 Reflection of rays of light, 116 
 
 Reflex mechanism, 289 
 
 Refraction of rays of light, 117 ; 
 index of, 118 
 
 Registers of voice, 245 
 
 REIL, island of, 26 
 
 REISSNER'S membrane, 230 
 
 Resolving power of the eye, 
 147 
 
 Resonance, sympathetic, 255 
 
 Resonators, 251 ; analysis of 
 tones by, 252 
 
 Retina, 103 ; appreciation of 
 colour, 151 ; fundus of, 103 ; 
 retinal impressions, 154; action 
 of light on, 150 ; correspond- 
 ing points of, 177 ; electric 
 current of, 299 ; rods of, 103, 
 104 ; examination of, 143 
 
 Rod cells of taste bud, 72 
 
 Rods of CORTI, 232 
 
 RUTHERFORD, theory as to func- 
 tion of cochlea, 270 
 
 SACCULE, 225 
 
 Scala tympani, 229 ; vestibuli, 
 228 
 
 SCHAEFER on sensory centres, 
 
 34. 35 
 
 SCHEINER'S experiment, 137 
 
 Schematic eye, 130 
 
 SCHLEMM, canal of, 101 
 
 Schneiderian membrane, 81 
 
 SCHULTZE on odours, 93 
 
 SCHWANN, white substance cf, 
 1 1 ; primitive sheath of, 1 1 
 
 Sclerotic, 98 
 
 Semicircular canals, their forma- 
 tion, 224 ; their function, 
 267 
 
 Sensorium, i 
 
 Sense of equilibrium, 270 ; of 
 hearing, 198 ; of innervation, 
 69 ; of locality, 56 ; of sight, 
 96 ; of smell, 80 ; of smell, 
 its delicacy, 92 ; of taste, 70 ; 
 of temperature, 64 ; of touch, 
 
 4i 
 
 Sensibility of the tongue, 78 
 
 Sensitiveness, absolute, 56 ; of 
 the skin, 54 
 
 Sensory paths in spinal cord, 
 13 ; impressions, objectivity of, 
 40 ; time in, 6 ; mechanism 
 of, i 
 
 SHORE on tastes, 76 
 
 SIEVEKING, 55 
 
 Size of the retinal image, 148 ; 
 estimation, 190 
 
 Smell, cerebral centres for, 34 ; 
 physical cause of, 86 ; physio- 
 logy of, 91 ; sense of, 80 
 
 Skin as excretory organ, 43 ; 
 structure of, 41 ; true skin, 
 42 ; sensitiveness of, 54 
 
 Solidity, perception of, 180 
 
 Somnambulism, 196 
 
 Sound, 198 ; its externality, 
 277 ; its direction, 280 ; its 
 distance, 281 ; its velocity, 
 220 
 
 Specific light pf the retina, 152 
 
 Spectrum, solar, 116 
 
 SPENSER, relation of soul and 
 body, 297 
 
Index 
 
 Spinal coid, 13 
 
 Spiral ganglion, 237 ; ligament, 
 229, 236 
 
 Stapes, 2ii 
 
 Stapedius muscle, 207 
 
 Stereoscope, 181 
 
 Stimulus and sensation, 36 
 
 Stirrup-bone, 211 
 
 Stratum corneum, of the skin, 
 42 ; lucidum of the skin, 43 ; 
 mucosum of skin, 42 
 
 Structure of the skin, 41 ; of cortex 
 cerebri, 29 
 
 Subjective sensations of odour, 
 94 ; tastes, 78 
 
 SUELZER on taste, 75 
 
 Sulci of the brain, 24 
 
 Sulcus spiralis, 230 
 
 Supporting cells of olfactory epi- 
 thelium, 85 
 
 Suspensory ligament of lens, 106 
 
 Syren, 242 
 
 TACTILE cells, simple, 46 ; com- 
 pound, 47 ; field, 60 ; hairs, 
 51 ; impressions, information 
 from, 59 ; organs, their struc- 
 ture, 45 
 
 TAIT, simple harmonic motion, 
 248 
 
 Tapetum, the, 101 
 
 TARTINI on overtones, 276 
 
 Taste, physical causes of, 73 ; 
 solubility a condition of, 73 ; 
 physiological conditions of, 74 ; 
 classification of, 74 ; excitants 
 of, 75 ; differentiation of, 76 ; 
 massiveness of, 75 ; intensity 
 of, 75 
 
 Taste buds or goblets, 71 
 
 Taste pore, 72 
 
 Telestereoscope, 184 
 
 Temperature, sense of, 64 
 
 Temporo - occipital convolution, 
 28 
 
 Temporo-sphenoidal lobe of brain, 
 26 
 
 Tensor tympani, 207 ; its func- 
 tion, 265 
 
 Terminal organs, 2 
 
 Test colours, 160 
 
 Thalami optici, 23 
 
 Thaumatrope, 154 
 
 THOMSON (Lord Kelvin), simple 
 harmonic motion, 248 
 
 Threshold of sensation, 37 
 
 Timbre of musical tone, 247 
 
 Tone, 240 
 
 Tongue, 70 
 
 Touch, sense of, 41 ; corpuscles, 
 simple, 46 ; compound, 47 ; 
 mechanism of, 52 ; theories as 
 to, 62 
 
 Transmission of sound by cra- 
 nium, 222 ; laws of, 281 
 
 Tuning-fork, 248 
 
 Tunnel of CORTI'S organ, 232 
 
 Turbinated bones of nose, 8 1 
 
 Tympanic groove, 205 
 
 Tympanum, 199, 204 
 
 TYNDALL on odours, 89 
 
 UMBO of tympanic membrane, 
 
 206 
 
 Uncinate gyrus, 27 
 Unconscious cerebration, 296 
 Utricle, 224 
 
 VALSALVA, experiments of, 208 
 Vas spirale, 230 
 VATER'S corpuscles, 48 
 Venae vorticosoe, 100 
 VENTURI on odours, 90 
 Vestibule, 223 
 Vibrations of strings, 215 
 Vibrissae, tactile hairs, 51, 8 1 
 Visual angle, 145, 190 ; field, 
 
 176 
 
 Vitreous humour, 106 
 VOLKMANN, variation of acuteness 
 
 of vision, 150 
 
 WAGNER'S touch corpuscles, 47 
 
 Wave-length, 220 
 
 WEBER on sensitiveness of the 
 
Physiology of the Senses 
 
 skin, 54, 55 ; theory as to 
 touch, 62 ; on odours, 91 
 
 Wheel of life, 154 
 
 WHEWELL on astigmatism, 133 
 
 YELLOW spot, 102 
 
 YOUNG, THOMAS, theory of colour 
 
 vision, 162 ; undulatory theory 
 of light, 115 
 
 ZOELLNER'S lines, 192 
 
 Zone of oval nuclei, 85 ; of round 
 
 nuclei, 85 
 Zonule of ZINN, 106 
 
 THE END 
 
 Printed by R. & R. CLARK, LIMITED, Edinburgh. 
 
 V 
 
T> ' \7 
 
IQiOGY 
 
 LIBRARY 
 
 6 
 
 THE UNIVERSITY OF CALIFORNIA LIBRARY