UNIVERSITY MANUALS
EDITED BY PROFESSOR KNIGHT
THE
PHYSIOLOGY OF THE SENSES
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
AND WILLIAM SNODGRASS,
M.A., M.B., C.M.
MU1RHEAD DEMONSTRATOR OF PHYSIOLOGY IN THE UNIVERSITY OF GLASGOW
FOURTH IMPRESSION
(FIFTH THOUSAND)
WITH 127 ILLUSTRATIONS
LONDON
JOHN MURRAY, ALBEMARLE STREET
1902 «i • 10.
A II rights reserved »
GULIELMO • TENNANT • GAIRDNER
MEDICINE • APUD • UNIVERSITATEM • GLASGUENSEM • P ROFESSOR1
HUNC • LIBELLUM
COLLEGA • COLLEGE
DISCIPULUS • MAGISTRO
AMANTISSIMO • UTERQUE • ANIMO
DEDICAVERUNT
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 the 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
PAGB
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
PACK
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 . . . . . . .in
II. — PHYSIOLOGY OF VISION
i. — Laws 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
xii 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 !7O
The ocular muscles . . . . . . . .172
How an object is seen as one with two eyes . . . • J75
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 in
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 ........ II
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.
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 ...... 80
26. Outer side of Nares . ... • - • • • 82
27. Olfactory Region of Rabbit .. . . . . 84
28. Olfactory Cells . . . .... . 85
29. Section of Eyelid ..... V . 96
30. Diagram of Eyeball . . . - . . • 98
31. Section of Cornea . . . V . . . 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 ...... 108
42. Structure of Lens . . . . . •. 109
43. Nerve Fibres in Retina . . . . . 109
44. Optic Decussation . . . . . . . no
45. Reflection of Light 117
List of Illustrations xvii
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. Schemer's Experiment . . . . . . 137
68. Different Forms of Eye . . . . . . 139
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 . . f 192
List of Illustrations xix
FIG. 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
102. 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
no. Wave-forms ........ 220
in. Osseous Labyrinth ....... 223
112. Formation of Semicircular Canals . . . . 224
113. Membranous Labyrinth ...... 225
1 14. Section of Macula Acustica ..... 226
115. Epithelium of Macula ...... 227
116. Otoconia or Otoliths 227
117. Osseous Cochlea ....... 228
118. Section through Coil of Cochlea .... 229
119. Section through Cochlear Duct . . . . . 231
120. 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
xx Physiology of the Senses
PAGE
124. Double Syren ........ 243
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 ta 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 studv
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 V
FIG. i. — Various forms of cells, a, cylindrical or columnar ; <5, 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. When 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 going 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 processes
from other similar
cells. These hair
cells form rudi-
mentary sense
organs (Fig. 2).
FIG. 2. — Neuro-epithelial cell from the upper nerve ring ,_, . . ,
of Carmina hastata. c, sense hair passing to the * U^^er, in the
surface ; the two long thin processes join a ring of network formed
nerve fibres containing ganglion cells. (Hertwig.) , ,
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 Balfour J 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-
tion 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
l
FIG. ^. — 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 Schwann's
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
ii
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 jjyramidal (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
FIG. 4. — Nerve fibres. B,
part Of the body Or legs, these move- The 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 ', «/] nerve fibres cut across.
PATHS OF NERVOUS IMPULSES
i. The Spinal Cord. — When we seek the exact paths, how-
FIG. 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-
14 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 give a 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 anaesthesia^
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
FIG. 7. — Transverse section of human spinal cord, ah, ah', anterior horns of
gray matter ; ph, pti ', posterior horns of gray matter ; ar, ar1 ', anterior root
zones ; pr,pr ', 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, dd ' , direct cerebellar tract ; c, anterior commissure ;
below c, 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 17
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 i§ 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 cerebellum, 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 for 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
20 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 pons Varolii j
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 cerebellum
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 ; «, crus 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, k 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 corpus callosum, joining the two masses.
The surface is not smooth, but thrown into numerous folds,
convolutions, or gyri^ between which lie depressions of vary-
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 functicn was associated
General Introdttction 25
with any particular area, the relationship of the convolutions
was deemed of comparatively little importance. Now,
F
P
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 ; FI, FZ, FS, superior, middle, and inferior frontal convolutions;^,^,
superior and inferior frontal sulcus ; f%, prsecentral sulcus ; Pj, superior
parietal lobule ; P% supra-marginal gyrus, and Pg' angular gyrus, parts of
inferior parietal lobule ; ifi, intra-parietal sulcus ; cm, end of calloso-marginal
fissure (see Fig. n); Oj, Os, OB, first, second, and third occipital convolu-
tions ; po, parieto-occipital fissure ; o, transverse occipital fissure ; 0%, inferior
occipital fissure ; TI, T%, T$, first, second, and third occipital convolutions ;
tl, t2, 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 the 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 (c, 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 (F1? F2, F3), separated by two furrows, fv fy
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, Pj and P2. The
second parietal convolution becomes continuous with the
superior of three temporal convolutions, Tp T2, and T3, by a
bend round the end of the Sylvian fissure immediately
below P2, known as the supra-marginal convolution, and the
superior and middle temporal convolutions are connected
posteriorly by a small angular convolution at P2 , commonly
known as the angular gyrus. Parts of three occipital con-
volutions, O15 O2, O3, 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.p 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^ Gf,
<po
FIG. ii. — Semi-diagrammatic view of the right cerebral hemisphere in its median
aspect. CC, corpus callosum divided vertically; G/j gyrus fornicatus ; H,
gyrus hippocampi ; k, 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', prsecuneus ; po, parieto-occipital fissure ; o, transverse occipital
sulcus ; O.z, cuneus ; oc, calcarine sulcus ; oc', oc", its superior and inferior
branches; D, descending gyrus; T4, lateral occipito-temporal gyrus; T5,
median occipito-temporal gyrus. (Ecker.)
separated from the marginal convolution by (3) cm, the
calloso -marginal fisstire, 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
pracuneus, 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 thd 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 come 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
Dther 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, P2, Pg'). 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
when 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.
u, Qz) 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-anaesthesia — 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 given 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 YJJQ-. Given i oo 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 I : 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-
General Introduction 39
tion when the stimulus varies in constant proportion.
There is the same perceptible variation when 3 Ibs. are
increased 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
increases 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 great 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- I
dermis. j Clear layer .
I. Mucous layer
True
Skin.
r Papillary layer . .
Duct of sweat gland
•Reticulated layer
Sweat gland
Subcutaneous tissue . . •- J
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 corium, 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 y-^ 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 papillas 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 luctdum), 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 Touch 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 ; (£) 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 -^-3 of an inch in 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 wifli their sub-
stance. Sometimes these
cells may form a group
which takes the form of a
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 ^ ending hi the
mandible of a goose. Magnified 240
diameters. Shows two touch corpuscles Stem of the glaSS.
divided transversely to the plane of en- ( ^ ^ f ^ Q
trance of the nerves. i, Tactile cor- v ' '
puscle consisting of four tactile cells; pliscles. These, SOITIC-
2, twin tactile cells, is ; «, tactile disc ; n, times termed h corpusdes
(to the left) nerve filament; n, (to the
right) medullated nerve fibre; c, true of Grandry, Or the COrpHS-
skin. (Stohr.) des Oy Merkel^ are formed
of two or more flattened cells, each cell being about
TTiFTF °f an mc^ m length by -^ ^ 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 Touch
47
(c, /5) Compound touch co^iscles. — These, termed the
corpuscles of Wag7ier^ or the corpuscles of Meissner, are oval
bodies, from -g4T to T-|-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
disappears, and the axis-cylinder
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 ; 6, 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.
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
lfJ7r to T^o 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), or it may
I-..
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) Pacinian bodies. — These, sometimes termed the cor-
puscles of Pacini, or the corpuscles of Vater^ from ~- to
-j1^. of an inch in breadth, and from T^ 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
T/ic Sense of Touch
49
They have also
and of the bat.
the connective tissue around the pancreas,
been found in the skin of the elephant
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 lamellae or capsules (Fig.
1 8) concentrically arranged. The space
between each pair of lamellae is lined by
a layer of flattened cells, 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
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
E
^
FIG. 17. — Diagrammatic
view of the under sur-
face of the index finger
with Pacinian corpus-
cles, a, Nerve • b, c,
lateral and terminal
branches of the nerves ;
d, d, d, Pacinian cor-
puscles ; i first, 2x
second, and 3 third
phalanx of the finger.
(Schwalbe.)
50 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
Schwann, and run more in a
longitudinal direction, parallel to
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-
FIG. 18. — A Pacinian corpuscle. N,
nerve ; V, V, vessel ; T, nerve-
ending. (Klein and Noble Smith.)
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 mbrissce, 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
52 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-
The 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.T
Centre of palm
8-9
Under surface of third
Under surface of third
phalanx of finger .
2-2.3
phalanx of great toe
ii. 3
Red part of the lip .
4-5
Upper surface of second
Under surface of second
phalanx of finger
ii-3
phalanx of finger .
4-4-5
Back . .
ii. 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 millimetre — ^g- of an inch.
The Sense of Touch
55
Temples
Forehead
Back of head .
Back of hand .
Knee .
Gluteal region
22.6 Forearm and leg . .45.1
22.6 Neck . . . .54.1
27. 1 Back, opposite fifth dorsal
31.6 vertebra . . .54.1
36. 1 Upper arm, thigh, centre
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.
I
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 pf
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 ^ to -^Q 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 TJTr
to -^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 of 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
The Sense of Touch
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 outer 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
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
FIG. 21.— Experiment of
Aristotle.
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.
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
more acutely felt than in
adjoining areas. Some
of these areas are more
— "- to cold, and
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 \ 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 i . 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°. t; 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 i o° 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 its
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 papillce, 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
epithelium is thick over the top
and thin at the sides of the fold,
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 foliatse 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
FIG. 23. — Vertical section through a
portion of the papilla foliata of a
rabbit X 80 d. Each fold, /, has
secondary folds, / ; g, taste goblets ;
«, medullated nerve fibres ; </, a
serous gland ; M, muscular fibres
of the tongue. (Stohr.)
72 Physiology of the StnseS
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 of
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- ;*•- Taste bud seen in the
papilla fohata of a rabbit X 560 d.
the area of mucous membrane fj Taste bud, showing outer sup-
where taste is present, is fol- porting cells ;,, fine ends of taste
cells ; /, taste pore. (Stohr.)
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-
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
76 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
of 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 indivi-
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 Shore1
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
why 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,
Gymnema 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.
Subjective 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
The lingual branch of the fifth nerve contains 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 tore
part of the face. The floor, sides, and roof of these cavities
The Sense of Smell 81
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, vibrissa, which, to some extent,
prevent the entrance of foreign bodies. Posteriorly, the
nostrils open into the pharynx by two apertures, the 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,
O
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 meatus or passage of the nose ; 9, agger or mound of the nose,
the rudiment of a muscle; 10, concha or shell of Santorini ; u, 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 ; 16,
naso-pharyngeal fold ; 17, naso-pharyngeal duct ; 18, pharyngeal opening
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.
Tj^e respiratory portion forms the lower part of the nasal
84
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- the reddish hue of that
meters, s, Border ; zo, zone of the oval
nuclei; zr, zone of the round nuclei; b, lining the VCStibular and
basal cells; ^portions of Bowman's respiratory regions. A
glands. The lower part of the duct is seen r °
on the right, », branch of olfactory nerve, vertical SCCtlOn 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, sf). 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,
tvater gas, marsh gas, olefiant gas, carbon monoxidel 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." * 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 10.8 ; 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
90 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." 1 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, O3), 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 Lidgeois. 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 91
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 25^OQ to
eg-'ao 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 i . 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 Li^geois, Archiv. de Physiologic, 1868.
8 Lord Rayleigh, Proc. Roy. Soc., 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) pure odours, like
musk, which he terms scents or perfumes, and (b) 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
™ Eh
FIG. 29. — Antero-posterior section through upper
eyelid, X 7 d. i, Outer skin — E, epidermis ; C,
corium; Sc, subcutaneous tissue; H<5, fine hairs;
K, M, sweat glands ;,W, eyelash ; W',W", roots
of eyelashes ; E^, reserve hair ; 2, muscles for
closing eye — O, muscular bundles cut trans-
versely ; McR, ciliary muscle of Riolanus ; 3,
tendon of muscle elevating the eyelid, mps ; 4,
conjunctival region ; tp, tunica propria ; e, con-
junctival epithelium ; at, gland ; t, tarsus ; m,
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 luminiferous 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 laye 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
--17
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
The Sense of Sight
99
jy~ n
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 ryqh3W*i<ivffl?t*'ffiFJR #
of the eye is covered with a trans- FlG- 31-— Antero-posterior sec-
. , ,. . 11 j i tion of cornea, e, Conjunctiva ;
parent epithelium or skin called the n> nerve sending branches to
Conjunctiva, which is COntinUOUS all cornea and conjunctiva ; f,
. , , ... , .., fibres of cornea between which
round with that lining the eyelids, are flattened spaces containing
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,
corpuscles ; d, layer of cells
covering posterior surface of
cornea, and separated from the
fibrous part by the posterior
elastic membrane. (Schofield.)
ioo 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
vorticosce, 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-
-a 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, rhe choroid IS closely
X24od. i, Conjunctiva ; a, nerve fibres in united tO the Sclerotic
conjunctiva ; s, network of nerve fibres be- , -
tween conjunctiva and cornea; 2, anterior bY meanS °f Connective.
elastic membrane ; 3, substance of cornea tissue, but just where
with n, a nerve passing through it. (Stohr.) the sclerotic merges into
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
The 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
10
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; u, 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 tapetum, 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 ether
hand, if too much light is entering the eye, a circular band
of muscle fibre in the iris, near the margin of the pupil,
102
Physiology of the Senses
contracts, and the pupil is lessened in size. The iris if
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
c tissue, and pointing towards the pupil.
These are the ciliary processes. The
, choroid and ciliary processes are lined
internally by a thin transparent mem-
a brane, known as the membrane of
Bruch.
3. The innermost coat, the retina^
FIG. 34. — Blood-vessels of
the choroid and iris of
the human eye seen from is the terminal organ of vision, and
within, a, Capillary ves-
sels of the choroid : b.
is almost transparent, with a pinkish
serrated line of union of tinge, except at a point in the visual
choroid with ciliary pro- • IIJ-L /r r i •
cesses;,, veins of dlLy EX1S Called thc.Jjrffow SJ0t, of which
ring; d, capillaries of more anon. The retina contains the
iT^rn^'pt ^rminal branches of the optic nerve,
of iris ;f, vessels of pupil- which, piercing the sclerotic and
laryzoneof iris. (Arnold.) choroid jn the human £ye ^ & ^
about y1^ 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
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, fa& fibres of Milller. 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.
J. Pigmentary layer not seen.
Layer of rods and cones.
• External limiting membrane.
• Outer nuclear layer.
/ ^~ 5. Outer reticular layer.
" ,1
6. Inner nuclear layer.
i "7- Inner reticular layer.
Ganglion cell layer.
, Nerve fibre layer.
FIG. 35. — Vertical section of human retina, X 240 d. £, 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
Jacobs membrane or layer of rods and cones. This layer
lies outside of and upon the external limiting membrane.
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
a, pass outwards as fine cylindrical processes,
giving off slender lateral twigs (not shown in'
diagram) in the reticular layers d and./J and
forming meshworks in the layers e and g. The
spaces of the mesh work 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
cuter 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
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
fovea centra/is, 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 cells, 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-
drawn into the main bodies of the cells,
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
(hyalosy glass), which might be compared to the membrane
lining the shell of an egg. This bag is filled with a transparent
FIG. 37. — Diagram of
rods and cones, show-
ing faint longitudinal
striation of inner limbs
of rods and cones, and
varicosities of the rod-
fibres. (Max Schultze.)
io6
Physiology of the Senses
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
FIG. 38.— Rods and cones lens- The suspensory ligament forms
seen from without on a rjng 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 is a triangular space containing fluid,
of the cones ; the smaller , , , , ,
central circles, the outer and called the canal °f P'M- The
limb of the cones, in 2 ligament, it may be noted, is much
and 3, the cones are sur- r.jurii • T- » •
rounded by rods. i.From PllCated bY following the Convolutions
the yellow spot ; 2, from of the ciliary processes, and the pos-
3' 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 a
Complete Semicircles from FlG' 39- - Hexagonal pigmented cells
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
covering J acob s membrane, a, Surface-
view ; b, cells seen from the side, sending
^ Processes between rods and cones.
1 he lighter portion in the centre of the
cells in a, indicates the non-pigmented
nudeus> (Max Schullze-)
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,
bu't 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 central 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 Kolliker 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 Q/ Sight
109
i, The denser central
part ; 2, 2, 2, concentric outer
layers. (Arnold.)
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
, . , FIG. 42. — Laminated structure of
a membrane in which are many the crystalline lens. Thelamin*
fine Openings for their passage, are split up after hardening in
alcohol.
the lamina cnbrosa, they are
grouped together into a bundle
forming the optic nerve. The \jptic nerve from each eye
passes bac!:v.T.rds, and entering the hollow of the cranium
by a passage at
the back of the
orbit, joins with
its fellow in a
union called the
optic commtssitre.
At the commis-
sure some of the
fibres pass directly
upwards into the
brain, but in the
human eye the
most of the fibres
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 lutea) ; rgfjna 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
occipital or posterior part of
the cerebral hemispheres, the
stimulation of which gives rise
7,>>~r7fl to a sensation of light.
But the eye is in connec-
tion with other nerve fibres
•/•// \v\^\ besides those of the optic nerve.
FIG. 44.-Diagrammatic 'represents We a11 kn°W h°W Sensitive
tion of decussation of fibres of the the eye is to touch, and how
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 in
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 Physiology of the Senses
centre to which it passes in the brain is destroyed, the
pupil no longer contracts when light falls 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 j 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.
Trie 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.
IL— 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
T/ie 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,
n6 Physiology 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 dottble
refraction — that is to say, the ray of light —
entering them is broken into two rays. .
3 ' FIG. 45.— Diagram illus-
each of which is deflected from the original trating 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 ; Or, 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 stated1 —
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.
n8 Physiology of the Senses
be supposed to pass from a vacuum through any transparent
substance, this ratio is known as the principal index 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-
T^-
NV >/ ing parallel surfaces. Part of the
light will be reflected in the direction
FK, part will pass through the plate,
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 Path FG« Meeting the SUr-
light. For description, see face CD, it nOW paSSCS 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
C
G
The Sense of SigJit
119
figure. The ray GH (Fig. 47) meeting the surface AB at
H, is bent towards DE in the direction HK, and emerging
through the surface AC is bent
away from EF in the direction D
KL, that is to say, it is bent
away from its original course,
and deflected towards the base
of the prism.
B C
FIG. 47. — Diagram illustrating re-
D
The amount of deflection de- ctive power of a prism'
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
FIG. 48.— Diagram showing comparison centre than at their circum-
of lenses to prisms set base to base fgrenCC, and WOUld exercise a
or edge to edge. C, Biconvex ; D,
plano-convex ; E, concavo-convex ; deflecting power Upon rays
similar to that of A ;
and H are concave
F, biconcave ; G, plano-concave ; H,
convexo-concave lens.
F, G,
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
I2O
Physiology of the Senses
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 " diver€ fr°m *
F,G. 49-Diagram illustrating course taken .
by parallel rays of light refracted by bicon- point f (Fig. 50) in the
vexlens> axis of the lens outside
of the principal focus, they will be brought to a focus at a point
f 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 SigJit
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
formulas. 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
PJiysiology 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. 53.— Diagram showing overlapping of rays the 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. 54- -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. 55 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
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.
T- . r .. , . - FIG. 57.— Achro-
— If the rays of light emanating from an matic lens. A,
object pass through a series of lenses, differ- Plano-concave
lens of flint-
ing in shape and refractive power, but having glass . B> bi_
their centres in one axis, the position and convex lens of
size of the resulting image might be found by crown-glass-
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 and 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,
eme.'ge 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. The first 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
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 FjF^ PjP^ NXN2 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 Sight
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
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 F2,
while from N2 emerges a ray parallel to ANj which meets
CF2 produced, at Ar
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 : x —
Index of refraction of the air . . . . . » = i.
Index of refraction of the aqueous humour and
vitreous body «'= 1-3365.
Total index of refraction of the crystalline . .«"= 1-437 1.
Radius of curvature of the cornea .... ?* = 7«829mm.
Radius of the anterior surface of the crystalline lens r1 = lomm.
Radius of the posterior surface of the crystalline lens /' = 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 -2mm.
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 /0' = -L- = _Z^i_ = 23. 266mm.
n'-i 1-3365-1
1 Landolt, The Refraction and Accommodation of the Eye, p. 79.
The Sense of Sight 129
Second focal distance/0"= J^L = r'3365 * 7'^9 =
ri-i I-3365-I
II. Anterior surface of crystalline.
First foca, distance/^-^ .^fj
Second focaldistance/i"= - = -±-= ,42.853mm.
III. Posterior surface of the crystalline.
First focal distance /,'= ^_/ = _I^371x^_ = 8
n'-n" 1-3365-1-4371
Secondfocal distance// = -^-fl= — I-3365x6 = 79.7 r I3mm.
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- 125970101. and 1-27560101. 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 22-82370101.
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, 1-75320101.
""(4) The second principal point, 2-iioimm.
(5) The first nodal point, 6-96850101.
(6) The second nodal point, 7-32540101.
From Fig. 60 (p. 130) it will be seen that the prin-
cipal points lie in the anterior chamber, the first nocjaj
1 3o
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 ; t/r', \}i", first and second
principal focus ; H', H", first and second principal points ; K.', K", first and
second nodal points ; F.c.,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
I32
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
C
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 aa! 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 FlG- 62.-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 brought 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 fequire 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
photographer moves the sensitive plate of his camera back-
wards or forwards to bring it into focus. But, as a matter
The Sense of Sight
'35
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
we 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 ; D, the vitreous humour ; a, the anterior elastic lamina of
cornea ; c, corneal substance proper ; 6, 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
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
IG. 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. — Scheiner'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. As 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't 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
I40
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 for 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. 6>— 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 th?n 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 hi 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 musca voKtantes,
and their fleeting character is due to the fact that they are
not as a rule directly in the line of distinct vision, and hi
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.
Qne 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 <:,
be reflected in the direction «/', and intercepted at x}
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 <r, and the shadow
at a' is projected outwards to A. If we now move the source
of light so that the ray enters at b' ', 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 ordinary circumstances, because light
enters the pupil from all parts of the field of vision, and
no distinct shadows are cast upon the retina. H. Miiller
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 tha.t
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
through the glass to meet and
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
corresponding degree, and
in the ophthalmoscope — the
FIG. 71.— Principle of the ophthalmo-
scope. (Fick.)
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,
The 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 an
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 /x.1 The
diameter of a retinal
cone is 3-2 u, but as
FIG. 72. — Visual angles. The objects c, a, e, •_
though of different sizes, subtend the same the COnCS do not prCSS
visual angle, being at different distances from affamst one another
the eye.
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 /x 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 ju, are always
seen as one, and not more than one, point, at least so far as
observations have yet been made. It matter's not how
large the object may be, if it is only far enough away to
1 The Greek letter p is used to denote the thousandth of a mm. ,
and is the unit of measurement for objects of microscopical size. A
mm. = 3*5- of an inch : hence a micromillimctre, i AC— asooir °f an mcn>
and 3.65 /* = TOVi> of an inch.
The Sense of Sight 147
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
aid of powerful telescopes, been shown to image smaller
be double, triple, quadruple, or even mul- than the dia-
meter of a cone
tiple stars, at vast distances from one another, may affectone, or
and yet 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 resolvmg power, the smaller than that
power of keeping each point of the image affecting one
clear and distinct 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. fa 2 \ 0 0 of an inch) in breadth,
and yet it is distinctly visible. This is, however, by no
means the minimum msibile. Objects as small as the
4 o 0*0 o o °f an mc^ m 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 + 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
i82x 16
*•=— — — =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
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 Vdlkmann 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 reflecting 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 clo?id,
The Sense of Sight 153
the image of the sun formed upon 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< ^ _The disk A having bkck 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
154 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
The 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 light 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 colour 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 b, 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
'57
ing table as the result of mixing the pure colours of the
spectrum : —
V.
B.
G.
Y.
R.
R.
Purple.
Rose.
Dull
Yellow.
Orange.
Red.
Y.
Rose.
White.
Yellow
Green.
Yellow.
G.
Pale
Blue.
Blue
Green.
Green.
Bl.
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 i ) 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 and 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
1 60 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 look 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 bluish-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
B
Red.
H. G ^ F E D
Violet. 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.
Het 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
direction's 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 of
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 165
.heory 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 the 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.
In like manner a yellow-blue visual substance by destruc-
tive changes gives a sensation of yellow, and by construc-
Yb \V gr
V B G Y O
FIG. 78. — Diagram to illustrate Bering'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 : : — " 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 J^add, Outlines of Physiological Psychology, p. 263.
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, lias 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 £, and 34 of c ; or, to take
other examples, spectral red will contain, per cent., 42 of a,
i of b, and 57 of white ; yellow, 1 1 of a, 14 of £, and 75 of
white ; and blue, 2 of a, 1 1 of ^, 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.
170 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,
according to Bonders, lies
1-77 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 antcro-posterior, a
FIG. 79. — Diagram to illustrate the fact
transverse, and a vertical that we can see objects in a plane
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, \\\^. primary 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 rectt(¥\g.
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 ; 2, superior fa Qn .
oblique muscle ; 3, superior direct muscle ;
4, 4', external direct muscle cut in order tical axis and looks in-
to show part of the optic nerve and ward and wh£n ^ ^^
7, the internal direct muscle ; 5, inferior
oblique muscle ; 6, inferior direct muscle, acts, Outwards. When the
(Schwalbe.) upper contractSj the 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),
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 conjunctivae of eyelid and eyeball ; f, muscle that
elevates upper eyelid ; g, superior direct muscle ; g', inferior direct muscle ;
/z, 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." * 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 versd.
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 areas of the retinas 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 retinas are not in the
line of regard, and not upon points which habitually act
176 Physiology of the Senses
together. For each person there is always a certain visual
Jield^ 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 retinae. 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,
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.
If, 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 retinae. 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
N
FIG. 83. — Diagram of one form of
horopter. (Miiller.)
178
Physiology of the Senses
single. Miiller 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 #'. Then,
when the eyes are converged on^, the images of p' are not
f
FIG. 84. — Diagram to illustrate formation of homonomous double images.
formed on corresponding points of the retinae, 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 a, 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.* 8 5) a
double image of />, formed external to the yellow spot on
both eyes, is mentally projected outwards to the distance of
the plane a' a through p, 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
linage. 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 solidity.
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
Z
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
retinae. In its simplest form the reflecting stereoscope
consists of two mirrors placed at right angles to each other,
as in P"ig. 87. The eyes, looking into these obliquely, see
reflections of the dissimilar figures R and L representing
the appearances as1 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,
XYZ
the points , ' ' are respectively at equal distances from
one another, and consequently seem to be in the same
plane in B. Similarly f' yj ZfJ are at equal distances from
one another, and seem to be all in one plane, but the dis-
..X..
* y
FIG. 88. — Diagram illustrating the principle of Brewster's stereoscope. The
points x, x forming images x1 , x' 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
•y- . -y 7
between any pair of the set ', ' v,' ', the plane xys
A , Y , L ,
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
•'•"> y i z
XYZ
,' ' ' and we seem to be looking into a hollow pyramid,
A, Y , Z,
184 Physiology of the Senses
whose apex is directed away from us. In Fig. 88 the points
#•, .r, being farther apart than y, y, are combined at X in a
plane behind that through Y, the point of combination of
y,y>
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 psetidoscope 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 viewing 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-
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 24^00
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 is not complete mixing of the colours, but the
colour sensation is now that of one, now that of the other
FIG. 90.— 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 ab '. 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 ot
X'— £ -£ Y' X'-
II
Fio. 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 B (Fig. 92) seems greater than that
between B and C on account
of the intervening dots, but it » « « « « * • o
is the same. Children often FlG 92
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
visual angle subtended by the object. In Fig. 72, p. 146,
X is the visual angle subtended by the lines c, d, and <?, 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 Sense of Sight
191
bination of touch and vision, that if two objects 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
r
A B C
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 a§
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
FIG. 95.
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 Zollner's lines. Here the oblique
lines seem to converge
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
previously vanishing
point ; and in doing
so the new image
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 the 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,
Q
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
mind as those which appear in ordinary vision. How
forcibly has this been painted in the dagger scene in
Macbeth —
FIG. 98. — For description, see text.
(After Bernstein.)
Is this a dagger which I see before me,
The handle toward my hand ? Come, let me clutch thce.
I have thee not, and yet I see thee still.
Art thou not, fatal vision, sensible
To feeling as to sight ? or art thou bttt
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 o1 the other senses,
Qr 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}-
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 p^art 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 mayt>e 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
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
FIG. 99. — Auditory vesicle of Phiahdium. d\, «o>
the auditory nerve Epithelium covering the sac ; h, auditory cells,
passes tO them through with hh auditory hairs ; «/, nervous cushion for
the auditory cells, connected with nr\, the lower
Channels in the bone. nerve r;ng. (Hertwig and Lankester.)
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 drum-head},
which vibrates in response to sounds, and whose move-
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 of the lower forms of
vertebrates it 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
201
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, M tiller,
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 finger 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. «oa— Outer sorfeoe of the
ternal auditory canal, and that to the risht auride- i, Helix ; 2>
fossa of helix ; 3, antihelix ;
best advantage, sound waves falling 4i fossa of the antihelix ;
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 meattis, 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
2O4 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 e'xtent. 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
largely composed of the tympanic membrane, although it is
Sound and Hearing
205
a
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 the fissure of Glaser, from its
discoverer, through which pass a nerve (the chorda tym-
pani) 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
umbo, 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
ilSt&S&S "dating fibres- These k-P <he
auditory meatus of the left ear. membrane stretched tightly, except
2S±Jt«£t3?- at the fore and uppfir part> where
which is seen the handle of the the grOOVC of attachment is de-
malleuscK across ; ,, anterior fi . d he membrane js loose
wall of the tympanum ; f,
in the tympanum above the thicker, and more freely supplied
stapes whose base is inserted wj h neryes &nd blood.vessels.
into the fenestra ovahs ; q,
the stapedins muscle ; h, por- The inner wall of the tympanum,
tion of facial nerve ; «, tensor opposite the membrane, is irregular
tynipam muscle ; k, vestibular
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 ; **, cochlea ; «, nerve fenestra OVdllS, Or OVOl Window, IS
going to ampuihe of semi- of an ovojd or kidney shape, and
circular canals ; o, section of
utricle ; /, section of sac- has the inner end of the ossicles
cule; r, section of semicircular Qf the ear fastened into it by
canals. (Riidinger.)
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,
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 upon
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 proccssus 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, 6, 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 and 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 the 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 T
FIG. 105.— The malleus
to the entrance to the inner ear, but, as or hammer bone seen
we shall see, the chain confers consider- I""0"} in front- x» The
head ; 2, the processus
able mechanical advantage. It consists gracilis foreshortened ;
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-
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
4 small ligament. It bears upon its pos-
4 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.-~Right incus a small bony prominence which, im-
or anvil bone, x 4. i, pinging upon the tympanic membrane,
Body ; 2, joint surface . j r i
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
attachment of the , •, c ,-, ,-, • •, .-,
bone; 5, elliptic area the neck °f the malleus, JUSt above the
on median side of short origin of the processus gracilis.
Th<= -««• <"• anvil-shaped bone (Fig.
cular knob ; 7, entrance 106), lies behind the malleus, and is
Jointed to k by a saddle-shaped surface.
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 lentictdar 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 2-
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 4i head ; 5, neck ;
6, groove into which
Of SOlld dlSCS. The tendon Of the Stape- membrane 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; w, handle
but increase of power. This is a distinct Of malleus ;/, long
advantage, considering the small power that process of the
r . , . incus.
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
raent 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- I09- — Diagram of
string vibrating so as to
character of the overtones are more give 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
2i6 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 217
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
218 Physiology of the Senses
not unlikely that the parts unconsciously assume the neces-
sary degree of tension before the sound fs 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 t"he 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
A A A A A A A
11 " VJ/ W \J \J \J W V W
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 -j-
40,000, or approximately ^ 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 Physiology 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 invagihation 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
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. 1 1 2). 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. n i, 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. in), 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 the 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. 1 1 3), 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
meatus, divides, as it enters
the bony labyrinth, into two
main divisions, one going to
the cochlea, and the other to
the vestibular part 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,
Q
FIG. 113. — Membranous labyrinth
(diagrammatic), c, Cochlea ; s, sac-
cule united by/, the ductus endolym-
phaticus, with «, the utricle, arising
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 crisiq
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-
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^puia sma11 cr>7stals consisting largely of carbonate
above the human oflime, called otoconia, or otoliths(¥\g. 1 1 6).
macula acustica. The function of this covering 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 modiohts. 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
FIG. ,i7.-The osseous cochlea divided as the lamina SPiralis °SS™,
through the middle, X 5. i, Central or Spiral plate of bone. It
canal of the modiolus in which lies the ^ . f d ^
cochlear nerve ; 2, the spiral osseous
lamina; 3, scala tympani ; 4, scala plate of bone, between the
vestibuli ; 5, spongy bone of modiolus surfaces of 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
scalas 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 scalae 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 ; /so, lamina
spiralis ossea, or spiral plate of bone ; Us, limbus of the spiral lamina ; R,
Reissner's membrane ; ss, spiral sulcus or groove ; t, tectorial membrane ;
CO, organ of Corti ; b, basilar membrane ; Isp, spiral ligament ; nc, 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
7 A 9 I1\
F
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 Corti.
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
e upper ends of the rods are
FIG. 120.— Inner and outer rods of
Corti from the cochlea of a guinea- enlarged, but flattened at the
pig, X 275. A, Inner rod-cell ; B, ^ ^^ ^ ^ • t t
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 - h d h th j
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).
~ -1
FIG. 121. — 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 j the processes to the phalange, narrow at 8, and widening
at 9 to form part of the lamina reticularis. 10, Phalanga? of the first row.
n, Phalangae 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 ; 16, 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, b, 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 Hcnserts
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 — phalange — which, uniting with adjoining plates,
and with the processes from the heads of the rods of Corti,
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 Henserfs 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. 119).
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 Belter'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 j
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 pf 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 1 mm. (j-J-g inch) to .36 mm. (nearly y-^-g- 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. (ifth 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 sojnewhat 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 the Senses
and intensity combine to give rise to a sensation of quality
in a musical tone.
i. Pitch. — The pitch of a tone depends upon 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 quickly 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. #o> a\i 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, CQ ; the disks, CQ, ci, rotate on a common axis, k, 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, 13, 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, 201 ; 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 trailing, and have suggested a possible
explanation as to how this is GO. 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 faj (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-! (42 vibrations) to Ia3 (435 vibrations); it is said
that Nilsson, in // Flauto Magico, can take fa5 (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 dop
(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-j (42 vibrations) to
do6 (2048 vibrations), or about six octaves, while the range
of the human ear for musical tones is from do — 1 (32 vibra-
tions) to do10 (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, wh;ch, 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 iri 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 dQ to 8 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 harmonir
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
contiguous 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 c^ 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^ 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^ 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 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. What 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 . . do1
do2
sol2
do3
mi3
sol3
sib3
do4
re4
mi4
Partial tones
I
2
3
4
5
6
7
8
9
10
Number of\
vibrations /
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 Senses
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 humana).
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^1 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 " 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
8
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 of
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 of the fundamental tone being a
multiple of those of the partial tones.
Beats. — When two simple tones of exactly the same
pitch are sounded together, if some arrangement be made
by which the 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 1 32
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 that
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 some 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 ^Eolian 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 macula 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 My sis (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 Hearing 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 maculce, cristce, and organ oj
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 cristcz 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." * 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 b£ so, the canals
cannot be essential to the sense of position. Again, it is
held that we cannot dissociate the vestibulas 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 macula 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 Corti'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 ? J
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 phalangae, 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 My sis, 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 Ajf. 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, ^ve 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 parts 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 tRe 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 may be 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 the 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 cei-
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 respiratipn.
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. iti,
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
1870,! 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, 6,
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 0 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
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 it's 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
2 92 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 reflex 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 th^t>rain cortex may, as in unconscious cerebra-
tion, pass thqfcRh molecular processes which result in the
unconscious perrormance 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 oj
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 great(ayDr less degree
of inhibitory power. The power of theWp, which may
also be strengthened by exercise, or weallned 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 basif 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 versd. 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 1873. l
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. aJ), away ; erro, erratum, to wander
Actinic, Gr. aktis, a sunbeam
Acustica, Gr. akouo, to hear
-SIsthesiometer, 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. ampulla, a bottle
Anaesthesia, Gr. -a, without ; asthesis, 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, auditiim, 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 ; veho, vectum, to carry
Binary, L. bina, a pair
Appendix II 303
Binaural, L. bis, twice ; audio, I hear
Binocular, L. bis, twice ; oculus, 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 ; ^zifoy, likeness
Chromatic, Gr. chroma, colour
Ciliary, L. cilium, an eyelash
Cilium (//. cilia), L. cilium, an eyelash
Circumvallate, L. circum, around ; vallum, a wall
Cochlea, Gr. kochlias, a snail with a shell
Coma, Gr. koma, drowsiness
Commissure, L. «w*, together ; mitto, missum, to send
Complementary, L. f0w, together ; pleo, to fill
Congenital, L. congenitus, born together with
Conjugate, L. w», together ; jugum, a yoke
Conjunctiva, L. con, together ; jungo, junctum, to join
Consciousness, L. c0», together ; scio, I know
Convergence, L. #?», together ; vergo, to bend
Convolution, L. convolve, convolution, to roll
Corium, Gr. chorion, skin
Cornea, L. cornu, a horn
Corona, L. corona, a crown
Corpus (//. 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 ; >rwa, likeness
Crista, L. ;mto, 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 ; fcro, I carry
Electrode, G r. elektron, amber ; hodos, a way
Electrolysis, Gr. elektron, amber ; lysis, a softening
Emmetropic, Gr. en, in ; metron, measure ; ops, the eyt
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, "L.Jiuo, I flow
Focus, L. focus, £ fireplace
Foliata, L. fotium, 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. fovea, a small pit
Function, L. fungor, functum, to discharge an office
Fundus, L. fundus, the bottom
Fungiform, L. fungus, a mushroom ; forma, form
Fuscin, L.fusats, 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
HEMORRHAGE, Gr. haima, blood ; rheo, to flow
Hamulus, L. dim. of hamus, a hook
Helicotrema, Gr. helix, a spiral ; trema, a perforation
Hemiansesthesia. 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, 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, lusum, to play
Incus, L. incus, an anvil
Index, L. in, in ; dico, to proclaim
Intermmcial, L. inter, between ; nuntius, a messenger
Intuition, L. intus, within ; itum, to go
Iris, Gr. iris, the rainbow
JAUNDICE, Yr.jaune, 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. &#.y, a lentil
Lenticular, L. dim. of /<?«.$•, a small bean
Limbus, L. limbus, a border
Lingual, L. lingua, a tongue
Logarithm, Gr. /^j, ratio ; arithmos, number
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 : mcdms, the middle
Melanin, Gr. melan, black
Meridional, L. meridies, midday
Mesentery, Gr. mesos, middle ; enteros, intestines
Minimum visibile, L. minimum, the least ; visibile, able to he
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. muxa, the mucus of the nostrils
MuscsB volitantes, L. musca, a fly ; volitans, flying
Myopia, Gr. muo, to close ; ops, the eye
NABES, L. nares, the nostrils
Neurilemma, Gr. neuron, a nerve ; lemma, a coat
Neuro- epithelium, Gr. neuron, a nerve; epi, upon; tithemi to
place
Neuroglia, Gr. neuron, a nerve j 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 s,cv;
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 stonq
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. pcllicula, dim. otpellis, 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
Phalangffl, 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. pituita, phlegm
Plane, L. planus, smooth
Plexus, L. plexus, a network
Pons, L. pons, a bridge
Pracuneus, L. pros, before ; cuneus, a wedge
Presbyopia, Gr. presbys, old ; ops, the eye
Prism, Gr. prisma, from prio, to saw
Protoplasm, Gr.protos, first ; plasma, anything forme!
Pseudoscope, Gr. pseudos, false ; skopeo, I see
Psychical, Gr. psyche, the soul
Pupil, L. pupilla, dim. of pupa, a puppet
QUADRIGEMINA, L. quatuor, four ; gtmini, double
Quantum, L. quantum , how much
308 Physiology of the Senses
REGIUS, 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 feel
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, likenc**
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. striatum, grooved
Stylet, Gr. stylos, a style or pencil
Sulcus, L. sulcus, a groove
Synchronous, Gr. syn, together ; chronos, time
Syren, L. «>*«, a singer of sweet muyic
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 ;
«!o5«, 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
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 uva, a grape
VAS, L. vas, a vessel
Vertebrate, L. verto, I turn
Vestibule, L. vestibulum, a threshold
Vibration, L. vibro, to quiver
Vibrissae, L. vibro, to quiver
Vidian, after Vidius, who described the Vidian nervr
Viscera, L. viscera, the bowels
Vitreous, L. vitrum, glass
Volatility, L. volo, 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, S3
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 reurnens, 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, 1 08
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, 13
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
313
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
G ALTON'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
HELMHOLTZ, 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
Hemianaesthesia, 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, areasofbrain, 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,
131
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 gyms, 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
MCEBIUS 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
Muscae 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, n
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,
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,
I2S
Prisms, 118
Processus cochleariformis, 207 ;
gracilis of malleus, 210
Promontory, 207
Protecting cells of taste bud,
73
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, 1 1 1
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 of,
ii ; primitive sheath of, n
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,
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 of the retina, 152
Spectrum, solar, 116
SPENSER, relation of soul and
body, 297
Index
Spinal coid, 13
Spiral ganglion, 237 ; ligament,
229, 236
Stapes, 211
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, 81
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
Venas vorticosae, 100
VENTURI on odours, 90
Vestibule, 223
Vibrations of strings, 215
Vibrissae, tactile hairs, 51, 8 1
Visual angle, 145, 190; field,
I76
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 oflife, 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 rouno
nuclei, 85
Zonule of ZINN, 106
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