The sense-organs are indubitably among the most important and
interesting parts of the human body; they are the organs by means of
which we obtain our knowledge of objects in the surrounding world.
Nihil est in intellectu quod non prius fuerit in sensu. They are the
first sources of the life of the soul. There is no other part of the
body in which we discover such elaborate anatomical structures,
co-operating with a definite purpose; and there is no other organ in
which the wonderful and purposive structure seems so clearly to compel
us to admit a Creator and a preconceived plan. Hence we find special
efforts made by dualists to draw our attention here to the "wisdom of
the Creator" and the design visible in his works. As a matter of fact,
you will discover, on mature reflection, that on this theory the
Creator is at bottom only playing the part of a clever mechanic or
watch-maker; all these familiar teleological ideas of Creator and
creation are based, in the long run, on a similar childlike
anthropomorphism.
However, we must grant that at the first glance the teleological
theory seems to give the simplest and most satisfactory explanation of
these purposive structures. If we merely examine the structure and
functions of the most advanced sense-organs, it seems impossible to
explain them without postulating a creative act. Yet evolution shows
us quite clearly that this popular idea is totally wrong. With its
assistance we discover that the purposive and remarkable sense-organs
were developed, like all other organs, without any preconceived
design--developed by the same mechanical process of natural selection,
the same constant correlation of adaptation and heredity, by which the
other purposive structures in the animal frame were slowly and
gradually brought forth in the struggle for life.
Like most other Vertebrates, man has six sensory organs, which serve
for eight different classes of sensations. The skin serves for
sensations of pressure and temperature. This is the oldest, lowest,
and vaguest of the sense-organs; it is distributed over the surface of
the body. The other sensory activities are localised. The sexual sense
is bound up with the skin of the external sexual organs, the sense of
taste with the mucous lining of the mouth (tongue and palate), and the
sense of smell with the mucous lining of the nasal cavity. For the two
most advanced and most highly differentiated sensory functions there
are special and very elaborate mechanical structures--the eye for the
sense of sight, and the ear for the sense of hearing and space
(equilibrium).
Comparative anatomy and physiology teach us that there are no
differentiated sense-organs in the lower animals; all their sensations
are received by the surface of the skin. The undifferentiated
skin-layer or ectoderm of the Gastraea is the simple stratum of cells
from which the differentiated sense-organs of all the Metazoa
(including the Vertebrates) have been evolved. Starting from the
assumption that necessarily only the superficial parts of the body,
which are in direct touch with the outer world, could be concerned in
the origin of sensations, we can see at once that the sense-organs
also must have arisen there. This is really the case. The chief part
of all the sense-organs originates from the skin-sense layer, partly
directly from the horny plate, partly from the brain, the foremost
part, of the medullary tube, after it has separated from the horny
plate. If we compare the embryonic development of the various
sense-organs, we see that they all make their appearance in the
simplest conceivable form; the wonderful contrivances that make the
higher sense-organs among the most remarkable and elaborate structures
in the body develop only gradually. In the phylogenetic explanation of
them comparative anatomy and ontogeny achieve their greatest triumphs.
But at first all the sense-organs are merely parts of the skin in
which sensory nerves expand. These nerves themselves were originally
of a homogeneous character. The different functions or specific
energies of the differentiated sense-nerves were only gradually
developed by division of labour. At the same time, their simple
terminal expansions in the skin were converted into extremely complex
organs.
The great instructiveness of these historical facts in connection with
the life of the soul is not difficult to see. The whole philosophy of
the future will be transformed as soon as psychology takes cognisance
of these genetic phenomena and makes them the basis of its
speculations. When we examine impartially the manuals of psychology
that have been published by the most distinguished speculative
philosophers and are still widely distributed, we are astonished at
the naivete with which the authors raise their airy metaphysical
speculations, regardless of the momentous embryological facts that
completely refute them. Yet the science of evolution, in conjunction
with the great advance of the comparative anatomy and physiology of
the sense-organs, provides the one sound empirical basis of a natural
psychology.
(FIGURE 2.305. Head of a shark (Scyllium), from the ventral side. m
mouth, o olfactory pits, r nasal groove, n nasal fold in natural
position, n apostrophe nasal fold drawn up. (The dots are openings of
the mucous canals.) (From Gegenbaur.))
In respect of the terminal expansions of the sensory nerves, we can
distribute the human sense-organs in three groups, which correspond to
three stages of development. The first group comprises those organs
the nerves of which spread out quite simply in the free surface of the
skin itself (organs of the sense of pressure, warmth, and sex). In the
second group the nerves spread out in the mucous coat of cavities
which are at first depressions in or invaginations of the skin (organs
of the sense of smell and taste). The third group is formed of the
very elaborate organs, the nerves of which spread out in an internal
vesicle, separated from the skin (organs of the sense of sight,
hearing, and space).
(FIGURES 2.306 AND 2.307. Head of a chick embryo, three days old:
2.306 front view, 2.307 from the right. n rudimentary nose (olfactory
pits), l rudimentary eyes (optic pits), g rudimentary ear (auscultory
pit), v fore brain, gl eye-cleft, o process of upper jaw, u process of
lower jaw of the first gill-arch.
FIGURE 2.308. Head of a chick embryo, four days old, from below. n
nasal pit, o upper-jaw process of the first gill-arch, u lower-jaw
process of same, k double apostrophe second gill-arch, sp choroid
fissure of eye, s gullet.
FIGURES 2.309 AND 2.310. Heads of chick embryos: 2.309 from the end of
the fourth, 2.310 from the beginning of the fifth week. Letters as in
Figure 2.308, except: in inner, an outer, nasal process, nf nasal
furrow, st frontal process, m mouth. (From Kolliker.) Figures 2.306 to
2.310 are magnified to the same extent.)
There is little to be said of the development of the lower
sense-organs. We have already considered (Chapter 2.24) the organ of
touch and temperature in the skin. I need only add that in the corium
of man and all the higher Vertebrates countless microscopic
sense-organs develop, but the precise relation of these to the
sensations of pressure or resistance, of warmth and cold, has not yet
been explained. Organs of this kind, in or on which sensory cutaneous
nerves terminate, are the "tactile corpuscles" (or the Pacinian
corpuscles) and end-bulbs. We find similar corpuscles in the organs of
the sexual sense, the male penis and the female clitoris; they are
processes of the skin, the development of which we will consider later
(together with the rest of the sexual parts, Chapter 2.29). The
evolution of the organ of taste, the tongue and palate, will also be
treated later, together with that of the alimentary canal to which
these parts belong (Chapter 2.27). I will only point out for the
present that the mucous coat of the tongue and palate, in which the
gustatory nerve ends, originates from a part of the outer skin. As we
have seen, the whole of the mouth-cavity is formed, not as a part of
the gut-tube proper, but as a pit-like fold in the outer skin (Chapter
1.13). Its mucous lining is therefore formed, not from the visceral,
but from the cutaneous layer, and the taste-cells at the surface of
the tongue and palate are not products of the gut-fibre layer, but of
the skin-sense layer.
This applies also to the mucous lining of the olfactory organ, the
nose. However, the development of this organ is much more interesting.
Although the nose seems superficially to be simple and single, it
really consists, in man and all other Gnathostomes, of two completely
separated halves, the right and left cavities. They are divided by a
vertical partition, so that the right nostril leads into the right
cavity alone and the left nostril into the left cavity. They open
internally (and separately) by the posterior nasal apertures into the
pharynx, so that we can get direct into the gullet through the nasal
passages without touching the mouth. This is the way the air usually
passes in respiration; the mouth being closed, it goes through the
nose into the gullet, and through the larynx and bronchial tubes into
the lungs. The nasal cavities are separated from the mouth by the
horizontal bony palate, to which is attached behind (as a dependent
process) the soft palate with the uvula. In the upper and hinder parts
of the nasal cavities the olfactory nerve, the first pair of cerebral
nerves, expands in the mucous coat which clothes them. The terminal
branches of it spread partly over the septum (partition), partly on
the side walls of the internal cavities, to which are attached the
turbinated bones. These bones are much more developed in many of the
higher mammals than in man, but there are three of them in all
mammals. The sensation of smell arises by the passage of a current of
air containing odorous matter over the mucous lining of the cavities,
and stimulating the olfactory cells of the nerve-endings.
Man has all the features which distinguish the olfactory organ of the
mammals from that of the lower Vertebrates. In all essential points
the human nose entirely resembles that of the Catarrhine apes, some of
which have quite a human external nose (compare the face of the
long-nosed apes). However, the first structure of the olfactory organ
in the human embryo gives no indication of the future ample
proportions of our catarrhine nose. It has the form in which we find
it permanently in the fishes--a couple of simple depressions in the
skin at the outer surface of the head. We find these blind olfactory
pits in all the fishes; sometimes they lie near the eyes, sometimes
more forward at the point of the muzzle, sometimes lower down, near
the mouth (Figure 2.249).
(FIGURE 2.311. Frontal section of the mouth and throat of a human
embryo, neck half-inch long. "Invented" by Wilhelm His. The vertical
section (in the frontal plane, from left to right) is so constructed
that we see the nasal pits in the upper third of the figure and the
eyes at the sides: in the middle third the primitive gullet with the
gill-clefts (gill-arches in section); in the lower third the pectoral
cavity with the bronchial tubes and the rudimentary lungs.)
This first rudimentary structure of the double nose is the same in all
the Gnathostomes; it has no connection with the primitive mouth. But
even in a section of the fishes a connection of this kind begins to
make its appearance, a furrow in the surface of the skin running from
each side of the nasal pit to the nearest corner of the mouth. This
furrow, the nasal groove or furrow (Figure 2.305 r), is very
important. In many of the sharks, such as the Scyllium, a special
process of the frontal skin, the nasal fold or internal nasal process,
is formed internally over the groove (n, n apostrophe). In contrast to
this the outer edge of the furrow rises in an "external nasal
process." As the two processes meet and coalesce over the nasal groove
in the Dipneusts and Amphibia, it is converted into a canal, the nasal
canal. Henceforth we can penetrate from the external pits through the
nasal canals direct into the mouth, which has been formed quite
independently. In the Dipneusts and the lower Amphibia the internal
aperture of the nasal canals lies in front (behind the lips); in the
higher Amphibia it is right behind. Finally, in the three higher
classes of Vertebrates the primary mouth-cavity is divided by the
formation of the horizontal palate-roof into two distinct
cavities--the upper (secondary) nasal cavity and the lower (secondary)
mouth-cavity. The nasal cavity in turn is divided by the construction
of the vertical septum into two halves--right and left.
(FIGURE 2.312. Diagrammatic section of the mouth-nose cavity. While
the palate-plates (p) divide the original mouth-cavity into the lower
secondary mouth (m) and the upper nasal cavity, the latter in turn is
divided by the vertical partition (e) into two halves (n, n). (From
Gegenbaur.))
Comparative anatomy shows us to-day, in the series of the double-nosed
Vertebrates, from the fishes up to man, all the different stages in
the development of the nose, which the advanced olfactory organ of the
higher mammals has passed through at various periods in the course of
its phylogeny. It first appears in the embryo of man and the higher
Vertebrates, in which the double fish-nose persists throughout life.
At an early stage, before there is any trace of the characteristic
human face, a pair of small pits are formed in the head over the
original mouth-cavity; these were first discovered by Baer, and
rightly called the "olfactory pits" (Figures 2.306 n and 2.307 n).
These primitive nasal pits are quite separate from the rudimentary
mouth, which also originates as a pit-like depression in the skin, in
front of the blind fore end of the gut. Both the pair of nasal pits
and the single mouth-pit (Figure 2.310 m) are clothed with the horny
plate. The original separation of the former from the latter is,
however, presently abolished, a process forming above the
mouth-pit--the "frontal process" (Figure 2.309 st). Its outer edge
rises to the right and left in the shape of two lateral processes;
these are the inner nasal processes or folds (in). Opposite to these a
parallel ridge is formed on either side between the eye and the nasal
pit; these are the outer nasal processes (an). Thus between the inner
and outer nasal processes a groove-like depression is formed on either
side, which leads from the nasal pit towards the mouth-pit (m); this
groove is, as the reader will guess, the same nasal furrow or groove
that we have already seen in the shark (Figure 2.305 r). As the
parallel edges of the inner and outer nasal processes bend towards
each other and join above the nasal groove, this is converted into a
tube, the primitive nasal canal. Hence the nose of man and all the
other Amniotes consists at this embryonic stage of a couple of narrow
tubes, the nasal canals, which lead from the outer surface of the
forehead into the rudimentary mouth. This transitory condition
resembles that in which we find the nose permanently in the Dipneusts
and Amphibia.
A cone-shaped structure, which grows from below towards the lower ends
of the two nasal processes and joins with them, plays an important
part in the conversion of the open nasal groove into the closed canal.
This is the upper-jaw process (Figures 2.306 to 2.310 o). Below the
mouth-pit are the gill-arches, which are separated by the gill-clefts.
The first of these gill-arches, and the most important for our
purpose, which we may call the maxillary (jaw) arch, forms the
skeleton of the jaws. Above at the basis a small process grows out of
this first gill-arch; this is the upper-jaw process. The first
gill-arch itself develops a cartilage at one of its inner sides, the
"Meckel cartilage" (named after its discoverer), on the outer surface
of which the lower jaw is formed (Figures 2.306 to 2.310 u). The
upper-jaw process forms the chief part of the skeleton of that jaw,
the palate bone, and the pterygoid bone. On its outer side is
afterwards formed the upper-jaw bone, in the narrower sense, while the
middle part of the skeleton of the upper jaw, the intermaxillary,
develops from the foremost part of the frontal process.
The two upper-jaw processes are of great importance in the further
development of the face. From them is formed, growing into the
primitive mouth-cavity, the important horizontal partition (the
palate) that divides the former into two distinct cavities. The upper
cavity, into which the nasal canals open, now develops into the nasal
cavity, the air-passage and the organ of smell. The lower cavity forms
the permanent secondary mouth (Figure 2.312 m), the food-passage and
the organ of taste. Both the upper and lower cavities open behind into
the gullet (pharynx). The hard palate that separates them is formed by
the joining of two lateral halves, the horizontal plates of the two
upper-jaw processes, or the palate-plates (p). When these do not,
sometimes, completely join in the middle, a longitudinal cleft
remains, through which we can penetrate from the mouth straight into
the nasal cavity. This is the malformation known as "wolf's throat."
"Hare-lip" is the lesser form of the same defect. At the same time as
the horizontal partition of the hard palate a vertical partition is
formed by which the single nasal cavity is divided into two
sections--a right and left half (Figure 2.312 n, n).
(FIGURES 2.313 AND 2.314. Upper part of the body of a human embryo,
two-thirds of an inch long, of the sixth week; Figure 2.313 from the
left, Figure 2.314 from the front. The origin of the nose and the
upper lip from two lateral and originally separate halves can be
clearly seen. Nose and upper lip are large in proportion to the rest
of the face, and especially to the lower lip. (From Kollmann.))
The double nose has now acquired the characteristic form that man
shares with the other mammals. Its further development is easy to
follow; it consists of the formation of the inner and outer processes
of the walls of the two cavities. The external nose is not formed
until long after all these essential parts of the internal organ of
smell. The first traces of it in the human embryo are found about the
middle of the second month (Figures 2.313 to 2.316). As can be seen in
any human embryo during the first month, there is at first no trace of
the external nose. It only develops afterwards from the foremost nasal
part of the primitive skull, growing forwards from behind. The
characteristic human nose is formed very late. Much stress is at times
laid on this organ as an exclusive privilege of man. But there are
apes that have similar noses, such as the long-nosed ape.
(FIGURE 2.315. Face of a human embryo, seven weeks old, (From
Kollmann.) Joining of the nasal processes (e outer, i inner) with the
upper-jaw process (o), n nasal wall, a ear-opening.)
The evolution of the eye is not less interesting and instructive than
that of the nose. Although this noblest of the sensory organs is one
of the most elaborate and purposive on account of its optic perfection
and remarkable structure, it nevertheless develops, without
preconceived design, from a simple process of the outer germinal
layer. The fully-formed human eye is a round capsule, the eye-ball
(Figure 2.317). This lies in the bony cavity of the skull, surrounded
by protective fat and motor muscles. The greater part of it is taken
up with a semi-fluid, transparent gelatinous substance, the corpus
vitreum. The crystalline lens is fitted into the anterior surface of
the ball (Figure 2.317 l). It is a lenticular, bi-convex, transparent
body, the most important of the refractive media in the eye. Of this
group we have, besides the corpus vitreum and the lens, the watery
fluid (humor aqueus) that is found in front of the lens (at the letter
m in Figure 2.317). These three transparent refractive media, by which
the rays of light that enter the eye are broken up and re-focussed,
are enclosed in a solid round capsule, composed of several different
coats, something like the concentric layers of an onion. The outermost
and thickest of these envelopes is the white sclerotic coat of the
eye. It consists of tough white connective tissue. In front of the
lens a circular, strongly-curved, transparent plate is fitted into the
sclerotic, like the glass of a watch--the cornea (b). At its outer
surface the cornea is covered with a very thin layer of the epidermis;
this is known as the conjunctiva. It goes from the cornea over the
inner surface of the eye-lids, the upper and lower folds which we draw
over the eye in closing it. At the inner corner of the eye we have a
rudimentary organ in the shape of the relic of a third (inner)
eye-lid, which is greatly developed, as "nictitating (winking)
membrane," in the lower Vertebrates (Chapter 1.5). Underneath the
upper eye-lid are the lachrymal glands, the product of which, the
lachrymal fluid, keeps the outer surface of the eye smooth and clean.
Immediately under the sclerotic we find a very delicate, dark-red
membrane, very rich in blood-vessels--the choroid coat--and inside
this the retina (o), the expansion of the optic nerve (i). The latter
is the second cerebral nerve. It proceeds from the optic thalami (the
second cerebral vesicle) to the eye; penetrates its outer envelopes,
and then spreads out like a net between the choroid and the corpus
vitreum. Between the retina and the choroid there is a very delicate
membrane, which is usually (but wrongly) associated with the latter.
This is the black pigment-membrane (n). It consists of a single
stratum of graceful, hexagonal, regularly-joined cells, full of
granules of black colouring matter. This pigment membrane clothes, not
only the inner surface of the choroid proper, but also the hind
surface of its anterior muscular continuation, which covers the edge
of the lens in front as a circular membrane, and arrests the rays of
light at the sides. This is the well-known iris of the eye (h),
coloured differently in different individuals (blue, grey, brown,
etc.); it forms the anterior border of the choroid. The circular
opening that is left in the middle is the pupil, through which the
rays of light penetrate into the eye. At the point where the iris
leaves the anterior border of the choroid proper the latter is very
thick, and forms a delicate crown of folds (g), which surrounds the
edge of the lens with about seventy large and many smaller rays
(corona ciliaris.)
At a very early stage a couple of pear-shaped vesicles develop from
the foremost part of the first cerebral vesicle in the embryo of man
and the other Craniotes (Figures 1.155 a and 2.297 au). These growths
are the primary optic vesicles. They are at first directed outwards
and forwards, but presently grow downward, so that, after the complete
separation of the five cerebral vesicles, they lie at the base of the
intermediate brain. The inner cavities of these pear-shaped vesicles,
which soon attain a considerable size, are openly connected with the
ventricle of the intermediate brain by their hollow stems. They are
covered externally by the epidermis.
(FIGURE 2.316. Face of a human embryo, eight weeks old (From Ecker.))
At the point where this comes into direct contact with the most curved
part of the primary optic vesicle there is a thickening (l) and also a
depression (o) of the horny plate (Figure 2.318, I). This pit, which
we may call the lens-pit, is converted into a closed sac, the
thick-walled lens-vesicle (2, l), the thick edges of the pit joining
together above it. In the same way in which the medullary tube
separates from the outer germinal layer, we now see this lens-sac
sever itself entirely from the horny plate (h), its source of origin.
The hollow of the sac is afterwards filled with the cells of its thick
walls, and thus we get the solid crystalline lens. This is, therefore,
a purely epidermic structure. Together with the lens the small
underlying piece of corium-plate also separates from the skin.
As the lens separates from the corneous plate and grows inwards, it
necessarily hollows out the contiguous primary optic vesicle (Figure
2.318, 1 to 3). This is done in just the same way as the invagination
of the blastula, which gives rise to the gastrula in the amphioxus
(Figure 2.38 C to F). In both cases the hollowing of the closed
vesicle on one side goes so far that at last the inner, folded part
touches the outer, not folded part, and the cavity disappears. As in
the gastrula the first part is converted into the entoderm and the
latter into the ectoderm, so in the invagination of the primary optic
vesicle the retina (r) is formed from the first (inner) part, and the
black pigment membrane (u) from the latter (outer, non-invaginated)
part. The hollow stem of the primary optic vesicle is converted into
the optic nerve. The lens (l), which has so important a part in this
process, lies at first directly on the invaginated part, or the retina
(r). But they soon separate, a new structure, the corpus vitreum (gl),
growing between them. While the lenticular sac is being detached and
is causing the invagination of the primary optic vesicle, another
invagination is taking place from below; this proceeds from the
superficial part of the skin-fibre layer--the corium of the head.
Behind and under the lens a last-shaped process rises from the
cutis-plate (Figure 2.319 g), hollows out the cup-shaped optic vesicle
from below, and presses between the lens (l) and the retina (i). In
this way the optic vesicle acquires the form of a hood.
(FIGURE 2.317. The human eye in section. a sclerotic coat, b cornea, c
conjunctiva, d circular veins of the iris, e choroid coat, f ciliary
muscle, g corona ciliaris, h iris, i optic nerve, k anterior border of
the retina, l crystalline lens, m inner covering of the cornea
(aqueous membrane), n pigment membrane, o retina, p Petit's canal, q
yellow spot of the retina. (From Helmholtz.))
Finally, a complete fibrous envelope, the fibrous capsule of the
eye-ball, is formed about the secondary optic vesicle and its stem
(the secondary optic nerve). It originates from the part of the
head-plates which immediately encloses the eye. This fibrous envelope
takes the form of a closed round vesicle, surrounding the whole of the
ball and pushing between the lens and the horny plate at its outer
side. The round wall of the capsule soon divides into two different
membranes by surface-cleavage. The inner membrane becomes the choroid
or vascular coat, and in front the ciliary corona and iris. The outer
membrane is converted into the white protective or sclerotic coat--in
front, the transparent cornea. The eye is now formed in all its
essential parts. The further development--the complicated
differentiation and composition of the various parts--is a matter of
detail.
(FIGURE 2.318. Eye of the chick embryo in longitudinal section (1.
from an embryo sixty-five hours old; 2. from a somewhat older embryo;
3. from an embryo four days old). h horny plate, o lens-pit, l lens
(in 1. still part of the epidermis, in 2. and 3. separated from it), x
thickening of the horny plate at the point where the lens has severed
itself, gl corpus vitreum, r retina, u pigment membrane. (From
Remak.))
The chief point in this remarkable evolution of the eye is the
circumstance that the optic nerve, the retina, and the pigment
membrane originate really from a part of the brain--an outgrowth of
the intermediate brain--while the lens, the chief refractive body,
develops from the outer skin. From the skin--the horny plate--also
arises the delicate conjunctiva, which afterwards covers the outer
surface of the eyeball. The lachrymal glands are ramified growths from
the conjunctiva (Figure 2.286). All these important parts of the eye
are products of the outer germinal layer. The remaining parts--the
corpus vitreum (with the vascular capsule of the lens), the choroid
(with the iris), and the sclerotic (with the cornea)--are formed from
the middle germinal layer.
The outer protection of the eye, the eye-lids, are merely folds of the
skin, which are formed in the third month of human embryonic life. In
the fourth month the upper eye-lid reaches the lower, and the eye
remains covered with them until birth. As a rule, they open wide
shortly before birth (sometimes only after birth). Our craniote
ancestors had a third eye-lid, the nictitating membrane, which was
drawn over the eye from its inner angle. It is still found in many of
the Selachii and Amniotes. In the apes and man it has degenerated, and
there is now only a small relic of it at the inner corner of the eye,
the semi-lunar fold, a useless rudimentary organ (Chapter 1.5). The
apes and man have also lost the Harderian gland that opened under the
nictitating membrane; we find this in the rest of the mammals, and the
birds, reptiles, and amphibia.
The peculiar embryonic development of the vertebrate eye does not
enable us to draw any definite conclusions as to its obscure
phylogeny; it is clearly cenogenetic to a great extent, or obscured by
the reduction and curtailment of its original features. It is probable
that many of the earlier stages of its phylogeny have disappeared
without leaving a trace. It can only be said positively that the
peculiar ontogeny of the complicated optic apparatus in man follows
just the same laws as in all the other Vertebrates. Their eye is a
part of the fore brain, which has grown forward towards the skin, not
an original cutaneous sense-organ, as in the Invertebrates.
(FIGURE 2.319. Horizontal transverse section of the eye of a human
embryo, four weeks old (magnified one hundred times). (From Kolliker.)
t lens (the dark wall of which is as thick as the diameter of the
central cavity), g corpus vitreum (connected by a stem, g, with the
corium), v vascular loop (pressing behind the lens inside the corpus
vitreum by means of this stem g), i retina (inner thicker, invaginated
layer of the primary optic vesicle), a pigment membrane (outer, thin,
non-invaginated layer of same), h space between retina and pigment
membrane (remainder of the cavity of the primary optic vesicle).
FIGURE 2.320. The human ear (left ear, seen from the front, natural
size), a shell of ear, b external passage, c tympanum, d tympanic
cavity, e Eustachian tube, f, g, h the three bones of the ear (f
hammer, g anvil, h stirrup), i utricle, k the three semi-circular
canals, l the sacculus, m cochlea, n auscultory nerve.)
The vertebrate ear resembles the eye and nose in many important
respects, but is different in others, in its development. The
auscultory organ in the fully-developed man is like that of the other
mammals, and especially the apes, in the main features. As in them, it
consists of two chief parts--an apparatus for conducting sound
(external and middle ear) and an apparatus for the sensation of sound
(internal ear). The external ear opens in the shell at the side of the
head (Figure 2.320 a). From this point the external passage (b), about
an inch in length, leads into the head. The inner end of it is closed
by the tympanum, a vertical, but not quite upright, thin membrane of
an oval shape (c). This tympanum separates the external passage from
the tympanic cavity (d). This is a small cavity, filled with air, in
the temporal bone; it is connected with the mouth by a special tube.
This tube is rather longer, but much narrower, than the outer passage,
leads inwards obliquely from the anterior wall of the tympanic cavity,
and opens in the throat below, behind the nasal openings. It is called
the Eustachian tube (e); it serves to equalise the pressure of the air
within the tympanic cavity and the outer atmosphere that enters by the
external passage. Both the Eustachian tube and the tympanic cavity are
lined with a thin mucous coat, which is a direct continuation of the
mucous lining of the throat. Inside the tympanic cavity there are
three small bones which are known (from their shape) as the hammer,
anvil, and stirrup (Figure 2.320, f, g, h). The hammer (f) is the
outermost, next to the tympanum. The anvil (g) fits between the other
two, above and inside the hammer. The stirrup (h) lies inside the
anvil, and touches with its base the outer wall of the internal ear,
or auscultory vesicle. All these parts of the external and middle ear
belong to the apparatus for conducting sound. Their chief task is to
convey the waves of sound through the thick wall of the head to the
inner-lying auscultory vesicle. They are not found at all in the
fishes. In these the waves of sound are conveyed directly by the wall
of the head to the auscultory vesicle.
The internal apparatus for the sensation of sound, which receives the
waves of sound from the conducting apparatus, consists in man and all
other mammals of a closed auscultory vesicle filled with fluid and an
auditory nerve, the ends of which expand over the wall of this
vesicle. The vibrations of the sound-waves are conveyed by these media
to the nerve-endings. In the labyrinthic water that fills the
auscultory vesicle there are small stones at the points of entry of
the acoustic nerves, which are composed of groups of microscopic
calcareous crystals (otoliths). The auscultory organ of most of the
Invertebrates has substantially the same composition. It usually
consists of a closed vesicle, filled with fluid, and containing
otoliths, with the acoustic nerve expanding on its wall. But, while
the auditory vesicle is usually of a simple round or oval shape in the
Invertebrates, it has in the Vertebrates a special and curious
structure, the labyrinth. This thin-membraned labyrinth is enclosed in
a bony capsule of the same shape, the osseous labyrinth (Figure
2.321), and this lies in the middle of the petrous bone of the skull.
The labyrinth is divided into two vesicles in all the Gnathostomes.
The larger one is called the utriculus, and has three arched
appendages, called the "semi-circular canals" (c, d, e). The smaller
vesicle is called the sacculus, and is connected with a peculiar
appendage, with (in man and the higher mammals) a spiral form
something like a snail's shell, and therefore called the cochlea (=
snail, b). On the thin wall of this delicate labyrinth the acoustic
nerve, which comes from the after-brain, spreads out in most elaborate
fashion. It divides into two main branches--a cochlear nerve (for the
cochlea) and a vestibular nerve (for the rest of the labyrinth). The
former seems to have more to do with the quality, the latter with the
quantity, of the acoustic sensations. Through the cochlear nerves we
learn the height and timbre, through the vestibular nerves the
intensity, of tones.
(FIGURE 2.321. The bony labyrinth of the human ear (left side). a
vestibulum, b cochlea, c upper canal, d posterior canal, e outer
canal, f oval fenestra, g round fenestra. (From Meyer.)
FIGURE 2.322. Development of the auscultory labyrinth of the chick, in
five successive stages (A to E). (Vertical transverse sections of the
skull.) fl auscultory pits, lv auscultory vesicles, lr labyrinthic
appendage, c rudimentary cochlea, csp posterior canal, cse external
canal, jv jugular vein. (From Reissner.))
The first structure of this highly elaborate organ is very simple in
the embryo of man and all the other Craniotes; it is a pit-like
depression in the skin. At the back part of the head at both sides,
near the after brain, a small thickening of the horny plate is formed
at the upper end of the second gill-cleft (Figure 2.322 A fl). This
sinks into a sort of pit, and severs from the epidermis, just as the
lens of the eye does. In this way is formed at each side, directly
under the horny plate of the back part of the head, a small vesicle
filled with fluid, the primitive auscultory vesicle, or the primary
labyrinth. As it separates from its source, the horny plate, and
presses inwards and backwards into the skull, it changes from round to
pear-shaped (Figures 2.322 B lv and 2.323 o). The outer part of it is
lengthened into a thin stem, which at first still opens outwards by a
narrow canal. This is the labyrinthic appendage (Figure 2.322 lr). In
the lower Vertebrates it develops into a special cavity filled with
calcareous crystals, which remains open permanently in some of the
primitive fishes, and opens outwards in the upper part of the skull.
But in the mammals the labyrinthic appendage degenerates. In these it
has only a phylogenetic interest as a rudimentary organ, with no
actual physiological significance. The useless relic of it passes
through the wall of the petrous bone in the shape of a narrow canal,
and is called the vestibular aqueduct.
It is only the inner and lower bulbous part of the separated
auscultory vesicle that develops into the highly complex and
differentiated structure that is afterwards known as the secondary
labyrinth. This vesicle divides at an early stage into an upper and
larger and a lower and smaller section. From the one we get the
utriculus with the semi-circular canals; from the other the sacculus
and the cochlea (Figure 2.320 c). The canals are formed in the shape
of simple pouch-like involutions of the utricle (cse and csp). The
edges join together in the middle part of each fold, and separate from
the utricle, the two ends remaining in open connection with its
cavity. All the Gnathostomes have these three canals like man, whereas
among the Cyclostomes the lampreys have only two and the hag-fishes
only one. The very complex structure of the cochlea, one of the most
elaborate and wonderful outcomes of adaptation in the mammal body,
develops originally in very simple fashion as a flask-like projection
from the sacculus. As Hasse and Retzius have pointed out, we find the
successive ontogenetic stages of its growth represented permanently in
the series of the higher Vertebrates. The cochlea is wanting even in
the Monotremes, and is restricted to the rest of the mammals and man.
The auditory nerve, or eighth cerebral nerve, expands with one branch
in the cochlea, and with the other in the remaining parts of the
labyrinth. This nerve is, as Gegenbaur has shown, the sensory dorsal
branch of a cerebro-spinal nerve, the motor ventral branch of which
acts for the muscles of the face (nervus facialis). It has therefore
originated phylogenetically from an ordinary cutaneous nerve, and so
is of quite different origin from the optic and olfactory nerves,
which both represent direct outgrowths of the brain. In this respect
the auscultory organ is essentially different from the organs of sight
and smell. The acoustic nerve is formed from ectodermic cells of the
hind brain, and develops from the nervous structure that appears at
its dorsal limit. On the other hand, all the membranous,
cartilaginous, and osseous coverings of the labyrinth are formed from
the mesodermic head-plates.
(FIGURE 2.323. Primitive skull of the human embryo, four weeks old,
vertical section, left half seen internally. v, z, m, h, n the five
pits of the cranial cavity, in which the five cerebral vesicles lie
(fore, intermediate, middle, hind, and after brains), o pear-shaped
primary auscultory vesicle (appearing through), a eye (appearing
through), no optic nerve, p canal of the hypophysis, t central
prominence of the skull. (From Kolliker.))
The apparatus for conducting sound which we find in the external and
middle ear of mammals develops quite separately from the apparatus for
the sensation of sound. It is both phylogenetically and
ontogenetically an independent secondary formation, a later accession
to the primary internal ear. Nevertheless, its development is not less
interesting, and is explained with the same ease by comparative
anatomy. In all the fishes and in the lowest Vertebrates there is no
special apparatus for conducting sound, no external or middle ear;
they have only a labyrinth, an internal ear, which lies within the
skull. They are without the tympanum and tympanic cavity, and all its
appendages. From many observations made in the last few decades it
seems that many of the fishes (if not all) cannot distinguish tones;
their labyrinth seems to be chiefly (if not exclusively) an organ for
the sense of space (or equilibrium). If it is destroyed, the fishes
lose their balance and fall. In the opinion of recent physiologists
this applies also to many of the Invertebrates (including the nearer
ancestors of the Vertebrates). The round vesicles which are considered
to be their auscultory vesicles, and which contain an otolith, are
supposed to be merely organs of the sense of space ("static vesicles
or statocysts").
The middle ear makes its first appearance in the amphibian class,
where we find a tympanum, tympanic cavity, and Eustachian tube; these
animals, and all terrestrial Vertebrates, certainly have the faculty
of hearing. All these essential parts of the middle ear originate from
the first gill-cleft and its surrounding part; in the Selachii this
remains throughout life an open squirting-hole, and lies between the
first and second gill-arch. In the embryo of the higher Vertebrates it
closes up in the centre, and thus forms the tympanic membrane. The
outlying remainder of the first gill-cleft is the rudiment of the
external meatus. From its inner part we get the tympanic cavity, and,
further inward still, the Eustachian tube. Connected with this is the
development of the three bones of the mammal ear from the first two
gill-arches; the hammer and anvil are formed from the first, the
stirrup from the upper end of the second, gill-arch.
(FIGURE 2.324. The rudimentary muscles of the ear in the human skull.
a raising muscle (M. attollens), b drawing muscle (M. attrahens), c
withdrawing muscle (M. retrahens), d large muscle of the helix (M.
helicis major), e small muscle of the helix (M. helicis minor), f
muscle of the angle of the ear (M. tragicus), g anti-angular muscle
(M. antitragicus). (From H. Meyer.))
Finally, the shell (pinna or concha) and external meatus (passage to
the tympanum) of the outer ear are developed in a very simple fashion
from the skin that borders the external aperture of the first
gill-cleft. The shell rises in the shape of a circular fold of the
skin, in which cartilage and muscles are afterwards formed (Figures
2.313 and 2.315). This organ is only found in the mammalian class. It
is very rudimentary in the lowest section, the Monotremes. In the
others it is found at very different stages of development, and
sometimes of degeneration. It is degenerate in most of the aquatic
mammals. The majority of them have lost it altogether--for instance,
the walruses and whales and most of the seals. On the other hand, the
pinna is well developed in the great majority of the Marsupials and
Placentals; it receives and collects the waves of sound, and is
equipped with a very elaborate muscular apparatus, by means of which
the pinna can be turned freely in any direction and its shape be
altered. It is well known how readily domestic animals--horses, cows,
dogs, hares, etc.--point their ears and move them in different
directions. Most of the apes do the same, and our earlier ape
ancestors were also able to do it. But our later simian ancestors,
which we have in common with the anthropoid apes, abandoned the use of
these muscles, and they gradually became rudimentary and useless.
However, we possess them still (Figure 2.324). In fact, some men can
still move their ears a little backward and forward by means of the
drawing and withdrawing muscles (b and c); with practice this faculty
can be much improved. But no man can now lift up his ears by the
raising muscle (a), or change the shape of them by the small inner
muscles (d, e, f, g). These muscles were very useful to our ancestors,
but are of no consequence to us. This applies to most of the
anthropoid apes as well.
We also share with the higher anthropoid apes (gorilla, chimpanzee,
and orang) the characteristic form of the human outer ear, especially
the folded border, the helix and the lobe. The lower apes have pointed
ears, without folded border or lobe, like the other mammals. But
Darwin has shown that at the upper part of the folded border there is
in many men a small pointed process, which most of us do not possess.
In some individuals this process is well developed. It can only be
explained as the relic of the original point of the ear, which has
been turned inwards in consequence of the curving of the edge. If we
compare the pinna of man and the various apes in this respect, we find
that they present a connected series of degenerate structures. In the
common catarrhine ancestors of the anthropoids and man the
degeneration set in with the folding together of the pinna. This
brought about the helix of the ear, in which we find the significant
angle which represents the relic of the salient point of the ear in
our earlier simian ancestors. Here again, therefore, comparative
anatomy enables us to trace with certainty the human ear to the
similar, but more developed, organ of the lower mammals. At the same
time, comparative physiology shows that it was a more or less useful
implement in the latter, but it is quite useless in the anthropoids
and man. The conducting of the sound has scarcely been affected by the
loss of the pinna. We have also in this the explanation of the
extraordinary variety in the shape and size of the shell of the ear in
different men; in this it resembles other rudimentary organs.