The previous chapters have taught us how the human body as a whole
develops from the first simple rudiment, a single layer of cells. The
whole human race owes its origin, like the individual man, to a simple
cell. The unicellular stem-form of the race is reproduced daily in the
unicellular embryonic stage of the individual. We have now to consider
in detail the evolution of the various parts that make up the human
frame. I must, naturally, confine myself to the most general and
principal outlines; to make a special study of the evolution of each
organ and tissue is both beyond the scope of this work, and probably
beyond the anatomic capacity of most of my readers to appreciate. In
tracing the evolution of the various organs we shall follow the method
that has hitherto guided us, except that we shall now have to consider
the ontogeny and phylogeny of the organs together. We have seen, in
studying the evolution of the body as a whole, that phylogeny casts a
light over the darker paths of ontogeny, and that we should be almost
unable to find our way in it without the aid of the former. We shall
have the same experience in the study of the organs in detail, and I
shall be compelled to give simultaneously their ontogenetic and
phylogenetic origin. The more we go into the details of organic
development, and the more closely we follow the rise of the various
parts, the more we see the inseparable connection of embryology and
stem-history. The ontogeny of the organs can only be understood in the
light of their phylogeny, just as we found of the embryology of the
whole body. Each embryonic form is determined by a corresponding
stem-form. This is true of details as well as of the whole.
We will consider first the animal and then the vegetal systems of
organs of the body. The first group consists of the psychic and the
motor apparatus. To the former belong the skin, the nervous system,
and the sense-organs. The motor apparatus is composed of the passive
and the active organs of movement (the skeleton and the muscles). The
second or vegetal group consists of the nutritive and the reproductive
apparatus. To the nutritive apparatus belong the alimentary canal with
all its appendages, the vascular system, and the renal (kidney)
system. The reproductive apparatus comprises the different organs of
sex (embryonic glands, sexual ducts, and copulative organs).
As we know from previous chapters (1.11 to 1.13), the animal systems
of organs (the organs of sensation and presentation) develop for the
most part out of the OUTER primary germ-layer, or the cutaneous (skin)
layer. On the other hand, the vegetal systems of organs arise for the
most part from the INNER primary germ-layer, the visceral layer. It is
true that this antithesis of the animal and vegetal spheres of the
body in man and all the higher animals is by no means rigid; several
parts of the animal apparatus (for instance, the greater part of the
muscles) are formed from cells that come originally from the entoderm;
and a great part of the vegetative apparatus (for instance, the
mouth-cavity and the gonoducts) are composed of cells that come from
the ectoderm.
In the more advanced animal body there is so much interlacing and
displacement of the various parts that it is often very difficult to
indicate the sources of them. But, broadly speaking, we may take it as
a positive and important fact that in man and the higher animals the
chief part of the animal organs comes from the ectoderm, and the
greater part of the vegetative organs from the entoderm. It was for
this reason that Carl Ernst von Baer called the one the animal and the
other the vegetative layer (see Chapter 1.3).
The solid foundation of this important thesis is the gastrula, the
most instructive embryonic form in the animal world, which we still
find in the same shape in the most diverse classes of animals. This
form points demonstrably to a common stem-form of all the Metazoa, the
Gastraea; in this long-extinct stem-form the whole body consisted
throughout life of the two primary germinal layers, as is now the case
temporarily in the gastrula; in the Gastraea the simple cutaneous
(skin) layer ACTUALLY represented all the animal organs and functions,
and the simple visceral (gut) layer all the vegetal organs and
functions. This is the case with the modern Gastraeads (Figure 2.233);
and it is also the case potentially with the gastrula.
We shall easily see that the gastraea theory is thus able to throw a
good deal of light, both morphologically and physiologically, on some
of the chief features of embryonic development, if we take up first
the consideration of the chief element in the animal sphere, the
psychic apparatus or sensorium and its evolution. This apparatus
consists of two very different parts, which seem at first to have very
little connection with each other--the outer skin, with all its hairs,
nails, sweat-glands, etc., and the nervous system. The latter
comprises the central nervous system (brain and spinal cord), the
peripheral, cerebral, and spinal nerves, and the sense-organs. In the
fully-formed vertebrate body these two chief elements of the sensorium
lie far apart, the skin being external to, and the central nervous
system in the very centre of, the body. The one is only connected with
the other by a section of the peripheral nervous system and the
sense-organs. Nevertheless, as we know from human embryology, the
medullary tube is formed from the cutaneous layer. The organs that
discharge the most advanced functions of the animal body--the organs
of the soul, or of psychic life--develop from the external skin. This
is a perfectly natural and necessary process. If we reflect on the
historical evolution of the psychic and sensory functions, we are
forced to conclude that the cells which accomplish them must
originally have been located on the outer surface of the body. Only
elementary organs in this superficial position could directly receive
the influences of the environment. Afterwards, under the influence of
natural selection, the cellular group in the skin which was
specifically "sensitive" withdrew into the inner and more protected
part of the body, and formed there the foundation of a central nervous
organ. As a result of increased differentiation, the skin and the
central nervous system became further and further separated, and in
the end the two were only permanently connected by the afferent
peripheral sensory nerves.
(FIGURE 2.284. The human skin in vertical section (from Ecker), highly
magnified, a horny layer of the epidermis, b mucous layer of the
epidermis, c papillae of the corium, d blood-vessels of same, ef ducts
of the sweat-glands (g), h fat-glands in the corium, i nerve, passing
into a tactile corpuscle above.)
The observations of the comparative anatomist are in complete accord
with this view. He tells us that large numbers of the lower animals
have no nervous system, though they exercise the functions of
sensation and will like the higher animals. In the unicellular
Protozoa, which do not form germinal layers, there is, of course,
neither nervous system nor skin. But in the second division of the
animal kingdom also, the Metazoa, there is at first no nervous system.
Its functions are represented by the simple cell-layer of the
ectoderm, which the lower Metazoa have inherited from the Gastraea
(Figure 1.30 e). We find this in the lowest Zoophytes--the Gastraeads,
Physemaria, and Sponges (Figures 2.233 to 2.238). The lowest Cnidaria
(the hydroid polyps) also are little superior to the Gastraeads in
structure. Their vegetative functions are accomplished by the simple
visceral layer, and their animal functions by the simple cutaneous
layer. In these cases the simple cell-layer of the ectoderm is at once
skin, locomotive apparatus, and nervous system.
(FIGURE 2.285. Epidermic cells of a human embryo of two months. (From
Kolliker.))
When we come to the higher Metazoa, in which the sensory functions and
their organs are more advanced, we find a division of labour among the
ectodermic cells. Groups of sensitive nerve cells separate from the
ordinary epidermic cells; they retire into the more protected tissue
of the mesodermic under-skin, and form special neural ganglia there.
Even in the Platodes, especially the Turbellaria, we find an
independent nervous system, which has separated from the outer skin.
This is the "upper pharyngeal ganglion," or acroganglion, situated
above the gullet (Figure 2.241 g). From this rudimentary structure has
been developed the elaborate central nervous system of the higher
animals. In some of the higher worms, such as the earth-worm, the
first rudiment of the central nervous system (Figure 1.74 n) is a
local thickening of the skin-sense layer (hs), which afterwards
separates altogether from the horny plate. In the earliest Platodes
(Cryptocoela) and Vermalia (Gastrotricha) the acroganglion remains in
the epidermis. But the medullary tube of the Vertebrates originates in
the same way. Our embryology has taught us that this first structure
of the central nervous system also develops originally from the outer
germinal layer.
Let us now examine more closely the evolution of the human skin, with
its various appendages, the hairs and glands. This external covering
has, physiologically, a double and important part to play. It is, in
the first place, the common integument that covers the whole surface
of the body, and forms a protective envelope for the other organs. As
such it also effects a certain exchange of matter between the body and
the surrounding atmosphere (exhalation, perspiration). In the second
place, it is the earliest and original sense organ, the common organ
of feeling that experiences the sensation of the temperature of the
environment and the pressure or resistance of bodies that come into
contact.
The human skin (like that of all the higher animals) is composed of
two layers, the outer and the inner or underlying skin. The outer skin
or epidermis, consists of simple ectodermic cells, and contains no
blood-vessels (Figure 2.284 a, b). It develops from the outer germinal
layer, or skin-sense layer. The underlying skin (corium or hypodermis)
consists chiefly of connective tissue, contains numerous blood-vessels
and nerves, and has a totally different origin. It comes from the
outermost parietal stratum of the middle germinal layer, or the
skin-fibre layer. The corium is much thicker than the epidermis. In
its deeper strata (the subcutis) there are clusters of fat-cells
(Figure 2.284 h). Its uppermost stratum (the cutis proper, or the
papillary stratum) forms, over almost the whole surface of the body, a
number of conical microscopic papillae (something like warts), which
push into the overlying epidermis (c). These tactile or sensory
particles contain the finest sensory organs of the skin, the touch
corpuscles. Others contain merely end-loops of the blood-vessels that
nourish the skin (c, d). The various parts of the corium arise by
division of labour from the originally homogeneous cells of the
cutis-plate, the outermost lamina of the mesodermic skin-fibre layer
(Figure 1.145 hpr, and Figures 1.161 and 1.162 cp).
In the same way, all the parts and appendages of the epidermis develop
by differentiation from the homogeneous cells of this horny plate
(Figure 2.285). At an early stage the simple cellular layer of this
horny plate divides into two. The inner and softer stratum (Figure
2.284 b) is known as the mucous stratum, the outer and harder (a) as
the horny (corneous) stratum. This horny layer is being constantly
used up and rubbed away at the surface; new layers of cells grow up in
their place out of the underlying mucous stratum. At first the
epidermis is a simple covering of the surface of the body. Afterwards
various appendages develop from it, some internally, others
externally. The internal appendages are the cutaneous glands--sweat,
fat, etc. The external appendages are the hairs and nails.
The cutaneous glands are originally merely solid cone-shaped growths
of the epidermis, which sink into the underlying corium (Figure 2.286
1). Afterwards a canal (2, 3) is formed inside them, either by the
softening and dissolution of the central cells or by the secretion of
fluid internally. Some of the glands, such as the sudoriferous, do not
ramify (Figure 2.284 efg). These glands, which secrete the
perspiration, are very long, and have a spiral coil at the end, but
they never ramify; so also the wax-glands of the ears. Most of the
other cutaneous glands give out buds and ramify; thus, for instance,
the lachrymal glands of the upper eye-lid that secrete tears (Figure
2.286), and the sebaceous glands which secrete the fat in the skin and
generally open into the hair-follicles. Sudoriferous and sebaceous
glands are found only in mammals. But we find lachrymal glands in all
the three classes of Amniotes--reptiles, birds, and mammals. They are
wanting in the lower aquatic vertebrates.
(FIGURE 2.286. Rudimentary lachrymal glands from a human embryo of
four months. (From Kolliker.) 1 earliest structure, in the shape of a
simple solid cone, 2 and 3 more advanced structures, ramifying and
hollowing out. a solid buds, e cellular coat of the hollow buds, f
structure of the fibrous envelope, which afterwards forms the corium
about the glands.)
The mammary glands (Figures 2.287 and 2.288) are very remarkable; they
are found in all mammals, and in these alone. They secrete the milk
for the feeding of the new-born mammal. In spite of their unusual size
these structures are nothing more than large sebaceous glands in the
skin. The milk is formed by the liquefaction of the fatty milk-cells
inside the branching mammary-gland tubes (Figure 2.287 c), in the same
way as the skin-grease or hair-fat, by the solution of fatty cells
inside the sebaceous glands. The outlets of the mammary glands enlarge
and form sac-like mammary ducts (b); these narrow again (a), and open
in the teats or nipples of the breast by sixteen to twenty-four fine
apertures. The first structure of this large and elaborate gland is a
very simple cone in the epidermis, which penetrates into the corium
and ramifies. In the new-born infant it consists of twelve to eighteen
radiating lobes (Figure 2.288). These gradually ramify, their ducts
become hollow and larger, and rich masses of fat accumulate between
the lobes. Thus is formed the prominent female breast (mamma), on the
top of which rises the teat or nipple (mammilla). The latter is only
developed later on, when the mammary gland is fully-formed; and this
ontogenetic phenomenon is extremely interesting, because the earlier
mammals (the stem-forms of the whole class) have no teats. In them the
milk comes out through a flat portion of the ventral skin that is
pierced like a sieve, as we still find in the lowest living mammals,
the oviparous Monotremes of Australia. The young animal licks the milk
from the mother instead of sucking it. In many of the lower mammals we
find a number of milk-glands at different parts of the ventral
surface. In the human female there is usually only one pair of glands,
at the breast; and it is the same with the apes, bats, elephants, and
several other mammals. Sometimes, however, we find two successive
pairs of glands (or even more) in the human female. Some women have
four or five pairs of breasts, like pigs and hedgehogs (Figure 1.103).
This polymastism points back to an older stem-form. We often find
these accessory breasts in the male also (Figure 1.103 D). Sometimes,
moreover, the normal mammary glands are fully developed and can suckle
in the male; but as a rule they are merely rudimentary organs without
functions in the male. We have already (Chapter 1.11) dealt with this
remarkable and interesting instance of atavism.
(FIGURE 2.287. The female breast (mamma) in vertical section. c
racemose glandular lobes, b enlarged milk-ducts, a narrower outlets,
which open into the nipple. (From H. Meyer.))
While the cutaneous glands are inner growths of the epidermis, the
appendages which we call hairs and nails are external local growths in
it. The nails (Ungues) which form important protective structures on
the back of the most sensitive parts of our limbs, the tips of the
fingers and toes, are horny growths of the epidermis, which we share
with the apes. The lower mammals usually have claws instead of them;
the ungulates, hoofs. The stem-form of the mammals certainly had
claws; we find them in a rudimentary form even in the salamander. The
horny claws are highly developed in most of the reptiles (Figure
2.264), and the mammals have inherited them from the earliest
representatives of this class, the stem-reptiles (Tocosauria). Like
the hoofs (ungulae) of the Ungulates, the nails of apes and men have
been evolved from the claws of the older mammals. In the human embryo
the first rudiment of the nails is found (between the horny and the
mucous stratum of the epidermis) in the fourth month. But their edges
do not penetrate through until the end of the sixth month.
The most interesting and important appendages of the epidermis are the
hairs; on account of their peculiar composition and origin we must
regard them as highly characteristic of the whole mammalian class. It
is true that we also find hairs in many of the lower animals, such as
insects and worms. But these hairs, like the hairs of plants, are
thread-like appendages of the surface, and differ entirely from the
hairs of the mammals in the details of their structure and
development.
The embryology of the hairs is known in all its details, but there are
two different views as to their phylogeny. On the older view the hairs
of the mammals are equivalent or homologous to the feathers of the
bird or the horny scales of the reptile. As we deduce all three
classes of Amniotes from a common stem-group, we must assume that
these Permian stem-reptiles had a complete scaly coat, inherited from
their Carboniferous ancestors, the mailed amphibia (Stegocephala); the
bony scales of their corium were covered with horny scales. In passing
from aquatic to terrestrial life the horny scales were further
developed, and the bony scales degenerated in most of the reptiles. As
regards the bird's feathers, it is certain that they are modifications
of the horny scales of their reptilian ancestors. But it is otherwise
with the hairs of the mammals. In their case the hypothesis has lately
been advanced on the strength of very extensive research, especially
by Friedrich Maurer, that they have been evolved from the cutaneous
sense-organs of amphibian ancestors by modification of functions; the
epidermic structure is very similar in both in its embryonic
rudiments. This modern view, which had the support of the greatest
expert on the vertebrates, Carl Gegenbaur, can be harmonised with the
older theory to an extent, in the sense that both formations, scales
and hairs, were very closely connected originally. Probably the
conical budding of the skin-sense layer grew up UNDER THE PROTECTION
OF THE HORNY SCALE, and became an organ of touch subsequently by the
cornification of the hairs; many hairs are still sensory organs
(tactile hairs on the muzzle and cheeks of many mammals: pubic hairs).
This middle position of the genetic connection of scales and hairs was
advanced in my Systematic Phylogeny of the Vertebrates (page 433). It
is confirmed by the similar arrangement of the two cutaneous
formations. As Maurer pointed out, the hairs, as well as the cutaneous
sense-organs and the scales, are at first arranged in regular
longitudinal series, and they afterwards break into alternate groups.
In the embryo of a bear two inches long, which I owe to the kindness
of Herr von Schmertzing (of Arva Varallia, Hungary), the back is
covered with sixteen to twenty alternating longitudinal rows of scaly
protuberances (Figure 2.289). They are at the same time arranged in
regular transverse rows, which converge at an acute angle from both
sides towards the middle of the back. The tip of the scale-like wart
is turned inwards. Between these larger hard scales (or groups of
hairs) we find numbers of rudimentary smaller hairs.
The human embryo is, as a rule, entirely clothed with a thick coat of
fine wool during the last three or four weeks of gestation. This
embryonic woollen coat (Lanugo) generally disappears in part during
the last weeks of foetal life but in any case, as a rule, it is lost
immediately after birth, and is replaced by the thinner coat of the
permanent hair. These permanent hairs grow out of hair-follicles,
which are formed from the root-sheaths of the disappearing
wool-fibres. The embryonic wool-coat usually, in the case of the human
embryo, covers the whole body, with the exception of the palms of the
hands and soles of the feet. These parts are always bare, as in the
case of apes and of most other mammals. Sometimes the wool-coat of the
embryo has a striking effect, by its colour, on the later permanent
hair-coat. Hence it happens occasionally, for instance, among our
Indo-Germanic races, that children of blond parents seem--to the
dismay of the latter--to be covered at birth with a dark brown or even
a black woolly coat. Not until this has disappeared do we see the
permanent blond hair which the child has inherited. Sometimes the
darker coat remains for weeks, and even months, after birth. This
remarkable woolly coat of the human embryo is a legacy from the apes,
our ancient long-haired ancestors.
(FIGURE 2.288. Mammary gland of a new-born infant, a original central
gland, b small and c large buds of same. (From Langer.))
It is not less noteworthy that many of the higher apes approach man in
the thinness of the hair on various parts of the body. With most of
the apes, especially the higher Catarrhines (or narrow-nosed apes),
the face is mostly, or entirely, bare, or at least it has hair no
longer or thicker than that of man. In their case, too, the back of
the head is usually provided with a thicker growth of hair; this is
lacking, however, in the case of the bald-headed chimpanzee
(Anthropithecus calvus). The males of many species of apes have a
considerable beard on the cheeks and chin; this sign of the masculine
sex has been acquired by sexual selection. Many species of apes have a
very thin covering of hair on the breast and the upper side of the
limbs--much thinner than on the back or the under side of the limbs.
On the other hand, we are often astonished to find tufts of hair on
the shoulders, back, and extremities of members of our Indo-Germanic
and of the Semitic races. Exceptional hair on the face, as on the
whole body, is hereditary in certain families of hairy men. The
quantity and the quality of the hair on head and chin are also
conspicuously transmitted in families. These extraordinary variations
in the total and partial hairy coat of the body, which are so
noticeable, not only in comparing different races of men, but also in
comparing different families of the same race, can only be explained
on the assumption that in man the hairy coat is, on the whole, a
rudimentary organ, a useless inheritance from the more thickly-coated
apes. In this man resembles the elephant, rhinoceros, hippopotamus,
whale, and other mammals of various orders, which have also, almost
entirely or for the most part, lost their hairy coats by adaptation.
(FIGURE 2.289. Embryo of a bear (Ursus arctos), twice natural size. A
seen from ventral side, B from the left.)
The particular process of adaptation by which man lost the growth of
hair on most parts of his body, and retained or augmented it at some
points, was most probably sexual selection. As Darwin luminously
showed in his Descent of Man, sexual selection has been very active in
this respect. As the male anthropoid apes chose the females with the
least hair, and the females favoured the males with the finest growths
on chin and head, the general coating of the body gradually
degenerated, and the hair of the beard and head was more strongly
developed. The growth of hair at other parts of the body (arm-pit,
pubic region) was also probably due to sexual selection. Moreover,
changes of climate, or habits, and other adaptations unknown to us,
may have assisted the disappearance of the hairy coat.
The fact that our coat of hair is inherited directly from the
anthropoid apes is proved in an interesting way, according to Darwin,
by the direction of the rudimentary hairs on our arms, which cannot be
explained in any other way. Both on the upper and the lower part of
the arm they point towards the elbow. Here they meet at an obtuse
angle. This curious arrangement is found only in the anthropoid
apes--gorilla, chimpanzee, orang, and several species of
gibbons--besides man (Figures 1.203 and 1.207). In other species of
gibbon the hairs are pointed towards the hand both in the upper and
lower arm, as in the rest of the mammals. We can easily explain this
remarkable peculiarity of the anthropoids and man on the theory that
our common ancestors were accustomed (as the anthropoid apes are
to-day) to place their hands over their heads, or across a branch
above their heads, during rain. In this position, the fact that the
hairs point downwards helps the rain to run off. Thus the direction of
the hair on the lower part of our arm reminds us to-day of that useful
custom of our anthropoid ancestors.
The nervous system in man and all the other Vertebrates is, when fully
formed, an extremely complex apparatus, that we may compare, in
anatomic structure and physiological function, with an extensive
telegraphic system. The chief station of the system is the central
marrow or central nervous system, the innumerable ganglionic cells or
neurona (Figure 1.9) of which are connected by branching processes
with each other and with numbers of very fine conducting wires. The
latter are the peripheral and ubiquitous nerve-fibres; with their
terminal apparatus, the sense-organs, etc., they constitute the
conducting marrow or peripheral nervous system. Some of them--the
sensory nerve-fibres--conduct the impressions from the skin and other
sense-organs to the central marrow; others--the motor
nerve-fibres--convey the commands of the will to the muscles.
The central nervous system or central marrow (medulla centralis) is
the real organ of psychic action in the narrower sense. However we
conceive the intimate connection of this organ and its functions, it
is certain that its characteristic actions, which we call sensation,
will, and thought, are inseparably dependent on the normal development
of the material organ in man and all the higher animals. We must,
therefore, pay particular attention to the evolution of the latter. As
it can give us most important information regarding the nature of the
"soul," it should be full of interest. If the central marrow develops
in just the same way in the human embryo as in the embryo of the other
mammals, the evolution of the human psychic organ from the central
organ of the other mammals, and through them from the lower
vertebrates, must be beyond question. No one can doubt the momentous
bearing of these embryonic phenomena.
(FIGURE 2.290. Human embryo, three months old, natural size, from the
dorsal side: brain and spinal cord exposed. (From Kolliker.) h
cerebral hemispheres (fore brain), m corpora quadrigemina (middle
brain), c cerebellum (hind brain): under the latter is the triangular
medulla oblongata (after brain).
FIGURE 2.291. Central marrow of a human embryo, four months old,
natural size, from the back. (From Kolliker.) h large hemispheres, v
quadrigemina, c cerebellum, mo medulla oblongata: underneath it the
spinal cord.)
In order to understand them fully we must first say a word or two of
the general form and the anatomic composition of the mature human
central marrow. Like the central nervous system of all the other
Craniotes, it consists of two parts, the head-marrow or brain (medulla
capitis or encephalon) and the spinal-marrow (medulla spinalis or
notomyelon). The one is enclosed in the bony skull, the other in the
bony vertebral column. Twelve pairs of cerebral nerves proceed from
the brain, and thirty-one pairs of spinal nerves from the spinal cord,
to the rest of the body (Figure 1.171). On general anatomic
investigation the spinal marrow is found to be a cylindrical cord,
with a spindle-shaped bulb both in the region of the neck above (at
the last cervical vertebra) and the region of the loins (at the first
lumbar vertebra) below (Figure 2.291). At the cervical bulb the strong
nerves of the upper limbs, and at the lumbar bulb those of the lower
limbs, proceed from the spinal cord. Above, the latter passes into the
brain through the medulla oblongata (Figure 2.291 mo). The spinal cord
seems to be a thick mass of nervous matter, but it has a narrow canal
at its axis, which passes into the further cerebral ventricles above,
and is filled, like these, with a clear fluid.
The brain is a large nerve-mass, occupying the greater part of the
skull, of most elaborate structure. On general examination it divides
into two parts, the cerebrum and cerebellum. The cerebrum lies in
front and above, and has the familiar characteristic convolutions and
furrows on its surface (Figures 2.292 and 2.293). On the upper side it
is divided by a deep longitudinal fissure into two halves, the
cerebral hemispheres; these are connected by the corpus callosum. The
large cerebrum is separated from the small cerebellum by a deep
transverse furrow. The latter lies behind and below, and has also
numbers of furrows, but much finer and more regular, with convolutions
between, at its surface. The cerebellum also is divided by a
longitudinal fissure into two halves, the "small hemispheres"; these
are connected by a worm-shaped piece, the vermis cerebelli, above, and
by the broad pons Varolii below (Figure 2.292 VI).
(FIGURE 2.292. The human brain, seen from below. (From H. Meyer.)
Above (in front) is the cerebrum with its extensive branching furrows;
below (behind) the cerebellum with its narrow parallel furrows. The
Roman numbers I to XII indicate the roots of the twelve pairs of
cerebral nerves in a series towards the rear.)
But comparative anatomy and ontogeny teach us that in man and all the
other Craniotes the brain is at first composed, not of these two, but
of three, and afterwards five, consecutive parts. These are found in
just the same form--as five consecutive vesicles--in the embryo of all
the Craniotes, from the Cyclostoma and fishes to man. But, however
much they agree in their rudimentary condition, they differ
considerably afterwards. In man and the higher mammals the first of
these ventricles, the cerebrum, grows so much that in its mature
condition it is by far the largest and heaviest part of the brain. To
it belong not only the large hemispheres, but also the corpus callosum
that unites them, the olfactory lobes, from which the olfactory nerves
start, and most of the structures that are found at the roof and
bottom of the large lateral ventricles inside the two hemispheres,
such as the corpora striata. On the other hand, the optic thalami,
which lie between the latter, belong to the second division, which
develops from the "intermediate brain "; to the same section belong
the single third cerebral ventricle and the structures that are known
as the corpora geniculata, the infundibulum, and the pineal gland.
Behind these parts we find, between the cerebrum and cerebellum, a
small ganglion composed of two prominences, which is called the corpus
quadrigeminum on account of a superficial transverse fissure cutting
across (Figures 2.290 m and 2.291 v). Although this quadrigeminum is
very insignificant in man and the higher mammals, it forms a special
third section, greatly developed in the lower vertebrates, the "middle
brain." The fourth section is the "hind-brain" or little brain
(cerebellum) in the narrower sense, with the single median part, the
vermis, and the pair of lateral parts, the "small hemispheres" (Figure
2.291 c). Finally, we have the fifth and last section, the medulla
oblongata (Figure 2.291 mo), which contains the single fourth cerebral
cavity and the contiguous parts (pyramids, olivary bodies, corpora
restiformia). The medulla oblongata passes straight into the medulla
spinalis (spinal cord). The narrow central canal of the spinal cord
continues above into the quadrangular fourth cerebral cavity of the
medulla oblongata, the floor of which is the quadrangular depression.
From here a narrow duct, called "the aqueduct of Sylvius," passes
through the corpus quadrigeminum to the third cerebral ventricle,
which lies between the two optic thalami; and this in turn is
connected with the pairs of lateral ventricles which lie to the right
and left in the large hemispheres. Thus all the cavities of the
central marrow are directly interconnected. All these parts of the
brain have an infinitely complex structure in detail, but we cannot go
into this. Although it is much more elaborate in man and the higher
Vertebrates than in the lower classes, it develops in them all from
the same rudimentary structure, the five simple cerebral vesicles of
the embryonic brain.
But before we consider the development of the complicated structure of
the brain from this simple series of vesicles, let us glance for a
moment at the lower animals, which have no brain. Even in the
skull-less vertebrate, the Amphioxus, we find no independent brain, as
we have seen. The whole central marrow is merely a simple cylindrical
cord which runs the length of the body, and ends equally simply at
both extremities--a plain medullary tube. All that we can discover is
a small vesicular bulb at the foremost part of the tube, a degenerate
rudiment of a primitive brain. We meet the same simple medullary tube
in the first structure of the ascidia larva, in the same
characteristic position, above the chorda. On closer examination we
find here also a small vesicular swelling at the fore end of the tube,
the first trace of a differentiation of it into brain and spinal cord.
It is probable that this differentiation was more advanced in the
extinct Provertebrates, and the brain-bulb more pronounced (Figures
1.98 to 1.102). The brain is phylogenetically older than the spinal
cord, as the trunk was not developed until after the head. If we
consider the undeniable affinity of the Ascidiae to the Vermalia, and
remember that we can trace all the Chordonia to lower Vermalia, it
seems probable that the simple central marrow of the former is
equivalent to the simple nervous ganglion, which lies above the gullet
in the lower worms, and has long been known as the "upper pharyngeal
ganglion" (ganglion pharyngeum superius); it would be better to call
it the primitive or vertical brain (acroganglion).
Probably this upper pharyngeal ganglion of the lower worms is the
structure from which the complex central marrow of the higher animals
has been evolved. The medullary tube of the Chordonia has been formed
by the lengthening of the vertical brain on the dorsal side. In all
the other animals the central nervous system has been developed in a
totally different way from the upper pharyngeal ganglion; in the
Articulates, especially, a pharyngeal ring, with ventral marrow, has
been added. The Molluscs also have a pharyngeal ring, but it is not
found in the Vertebrates. In these the central marrow has been
prolonged down the dorsal side; in the Articulates down the ventral
side. This fact proves of itself that there is no direct relationship
between the Vertebrates and the Articulates. The unfortunate attempts
to derive the dorsal marrow of the former from the ventral marrow of
the latter have totally failed (cf. Chapter 2.20).
(FIGURE 2.293. The human brain, seen from the left. (From H. Meyer.)
The furrows of the cerebrum are indicated by thick, and those of the
cerebellum by finer lines. Under the latter we can see the medulla
oblongata. f1 to f2 frontal convolutions, C central convolutions, S
fissure of Sylvius, T temporal furrow, Pa parietal lobes, An angular
gyrus, Po parieto-occipital fissure.)
When we examine the embryology of the human nervous system, we must
start from the important fact, which we have already seen, that the
first structure of it in man and all the higher Vertebrates is the
simple medullary tube, and that this separates from the outer germinal
layer in the middle line of the sole-shaped embryonic shield. As the
reader will remember, the straight medullary furrow first appears in
the middle of the sandal-shaped embryonic shield. At each side of it
the parallel borders curve over in the form of dorsal or medullary
swellings. These bend together with their free borders, and thus form
the closed medullary tube (Figures 1.133 to 1.137). At first this tube
lies directly underneath the horny plate; but it afterwards travels
inwards, the upper edges of the provertebral plates growing together
between the horny plate and the tube, joining above the latter, and
forming a completely closed canal. As Gegenbaur very properly
observes, "this gradual imbedding in the inner part of the body is a
process acquired with the progressive differentiation and the higher
potentiality that this secures; by this process the organ of greater
value to the organism is buried within the frame." (Cf. Figures 1.143
to 1.146).
(FIGURES 2.294 TO 2.296. Central marrow of the human embryo from the
seventh week, 4/5 inch long. (From Kolliker.)
FIGURE 2.294. The brain from above, v fore brain, z intermediate
brain, m middle brain, h hind brain, n after brain.
FIGURE 2.295. The brain with the uppermost part of the cord, from the
left.
FIGURE 2.296. Back view of the whole embryo: brain and spinal cord
exposed.)
In the Cyclostoma--a stage above the Acrania--the fore end of the
cylindrical medullary tube begins early to expand into a pear-shaped
vesicle; this is the first outline of an independent brain. In this
way the central marrow of the Vertebrates divides clearly into its two
chief sections, brain and spinal cord. The simple vesicular form of
the brain, which persists for some time in the Cyclostoma, is found
also at first in all the higher Vertebrates (Figure 1.153 hb). But in
these it soon passes away, the one vesicle being divided into several
successive parts by transverse constrictions. There are first two of
these constrictions, dividing the brain into three consecutive
vesicles (fore brain, middle brain, and hind brain, Figure 1.154 v, m,
h). Then the first and third are sub-divided by fresh constrictions,
and thus we get five successive sections (Figure 1.155).
In all the Craniotes, from the Cyclostoma up to man, the same parts
develop from these five original cerebral vesicles, though in very
different ways. The first vesicle, the fore brain (Figure 1.155 v),
forms by far the largest part of the cerebrum--namely, the large
hemispheres, the olfactory lobes, the corpora striata, the callosum,
and the fornix. From the second vesicle, the intermediate brain (z),
originate especially the optic thalami, the other parts that surround
the third cerebral ventricle, and the infundibulum and pineal gland.
The third vesicle, the middle brain (m), produces the corpora
quadrigemina and the aqueduct of Sylvius. From the fourth vesicle, the
hind brain (h), develops the greater part of the cerebellum--namely,
the vermis and the two small hemispheres. Finally, the fifth vesicle,
the after brain (n), forms the medulla oblongata, with the
quadrangular pit (the floor of the fourth ventricle), the pyramids,
olivary bodies, etc.
We must certainly regard it as a comparative-anatomical and
ontogenetic fact of the greatest significance that in all the
Craniotes, from the lowest Cyclostomes and fishes up to the apes and
man, the brain develops in just the same way in the embryo. The first
rudiment of it is always a simple vesicular enlargement of the fore
end of the medullary tube. In every case, first three, then five,
vesicles develop from this bulb, and the permanent brain with all its
complex anatomic structures, of so great a variety in the various
classes of Vertebrates, is formed from the five primitive vesicles.
When we compare the mature brain of a fish, an amphibian, a reptile, a
bird, and a mammal, it seems incredible that we can trace the various
parts of these organs, that differ so much internally and externally,
to common types. Yet all these different Craniote brains have started
with the same rudimentary structure. To convince ourselves of this we
have only to compare the corresponding stages of development of the
embryos of these different animals.
(FIGURE 2.297. Head of a chick embryo (hatched fifty-eight hours),
from the back, magnified forty times. (From Mihalkovics.) vw anterior
wall of the fore brain. vh its ventricle. au optic vesicles, mh middle
brain, kh hind brain, nh after brain, hz heart (seen from below), vw
vitelline veins, us primitive segment, rm spinal cord.)
This comparison is extremely instructive. If we extend it through the
whole series of the Craniotes, we soon discover this interesting fact:
In the Cyclostomes (the Myxinoida and Petromyzonta), which we have
recognised as the lowest and earliest Craniotes, the whole brain
remains throughout life at a very low stage, which is very brief and
passing in the embryos of the higher Craniotes; they retain the five
original sections of the brain unchanged. In the fishes we find an
essential and considerable modification of the five vesicles; it is
clearly the brain of the Selachii in the first place, and subsequently
the brain of the Ganoids, from which the brain of the rest of the
fishes on the one hand and of the Dipneusts and Amphibia, and through
these of the higher Vertebrates, on the other hand, must be derived.
In the fishes and Amphibia (Figure 2.300) there is a preponderant
development of the middle brain, and also the after brain, the first,
second, and fourth sections remaining very primitive. It is just the
reverse in the higher Vertebrates, in which the first and third
sections, the cerebrum and cerebellum, are exceptionally developed;
while the middle brain and after brain remain small. The corpora
quadrigemina are mostly covered by the cerebrum, and the oblongata by
the cerebellum. But we find a number of stages of development within
the higher Vertebrates themselves. From the Amphibia upwards the brain
(and with it the psychic life) develops in two different directions;
one of these is followed by the reptiles and birds, and the other by
the mammals. The development of the first section, the fore brain, is
particularly characteristic of the mammals. It is only in them that
the cerebrum becomes so large as to cover all the other parts of the
brain (Figures 2.293 and 2.301 to 2.304).
There are also notable variations in the relative position of the
cerebral vesicles. In the lower Craniotes they lie originally almost
in the same plane. When we examine the brain laterally, we can cut
through all five vesicles with a straight line. But in the Amniotes
there is a considerable curve in the brain along with the bending of
the head and neck; the whole of the upper dorsal surface of the brain
develops much more than the under ventral surface. This causes a
curve, so that the parts come to lie as follows: The fore brain is
right in front and below, the intermediate brain a little higher, and
the middle brain highest of all; the hind brain lies a little lower,
and the after brain lower still. We find this only in the
Amniotes--the reptiles, birds, and mammals.
(FIGURE 2.298. Brain of three craniote embryos in vertical section. A
of a shark (Heptarchus), B of a serpent (Coluber), C of a goat
(Capra). a fore brain, b intermediate brain, c middle brain, d hind
brain, e after brain, s primitive cleft. (From Gegenbaur.)
FIGURE 2.299. Brain of a shark (Scyllium), back view. g fore-brain, h
olfactory lobes, which send the large olfactory nerves to the nasal
capsule (o), d intermediate brain, b middle brain; behind this the
insignificant structure of the hind brain, a after brain. (From
Gegenbaur.)
FIGURE 2.300. Brain and spinal cord of the frog. A from the dorsal, B
from the ventral side. a olfactory lobes before the (b) fore brain, i
infundibulum at the base of the intermediate brain, c middle brain, d
hind brain, s quadrangular pit in the after brain, m spinal cord (very
short in the frog), m apostrophe roots of the spinal nerves, t
terminal fibres of the spinal cord. (From Gegenbaur.)
FIGURE 2.301. Brain of an ox-embryo, two inches in length. (From
Mihalkovics, magnified three times.) Left view; the lateral wall of
the left hemisphere has been removed, st corpora striata, ml
Monro-foramen, ag arterial plexus, ah Ammon's horn, mh middle brain,
kh cerebellum. dv roof of the fourth ventricle, bb pons Varolii, na
medulla oblongata.)
Thus, while the brain of the mammals agrees a good deal in general
growth with that of the birds and reptiles, there are some striking
differences between the two. In the Sauropsids (birds and reptiles)
the middle brain and the middle part of the hind brain are well
developed. In the mammals these parts do not grow, and the fore-brain
develops so much that it overlies the other vesicles. As it continues
to grow towards the rear, it at last covers the whole of the rest of
the brain, and also encloses the middle parts from the sides (Figures
2.301 to 2.303). This process is of great importance, because the fore
brain is the organ of the higher psychic life, and in it those
functions of the nerve-cells are discharged which we sum up in the
word "soul." The highest achievements of the animal body--the
wonderful manifestations of consciousness and the complex molecular
processes of thought--have their seat in the fore brain. We can remove
the large hemispheres, piece by piece, from the mammal without killing
it, and we then see how the higher functions of consciousness,
thought, will, and sensation, are gradually destroyed, and in the end
completely extinguished. If the animal is fed artificially, it may be
kept alive for a long time, as the destruction of the psychic organs
by no means involves the extinction of the faculties of digestion,
respiration, circulation, urination--in a word, the vegetative
functions. It is only conscious sensation, voluntary movement,
thought, and the combination of various higher psychic functions that
are affected.
(FIGURE 2.302. Brain of a human embryo, twelve weeks old. (From
Mihalkovics, natural size.) Seen from behind and above. ms
mantle-furrow, mh corpora quadrigemina (middle brain), vs anterior
medullary ala, kh cerebellum, vv fourth ventricle, na medulla
oblongata.)
The fore brain, the organ of these functions, only attains this high
level of development in the more advanced Placentals, and thus we have
the simple explanation of the intellectual superiority of the higher
mammals. The soul of most of the lower Placentals is not much above
that of the reptiles, but among the higher Placentals we find an
uninterrupted gradation of mental power up to the apes and man. In
harmony with this we find an astonishing variation in the degree of
development of their fore brain, not only qualitatively, but also
quantitatively. The mass and weight of the brain are much greater in
modern mammals, and the differentiation of its various parts more
important, than in their extinct Tertiary ancestors. This can be shown
paleontologically in any particular order. The brains of the living
ungulates are (relatively to the size of the body) four to six times
(in the highest groups even eight times) as large as those of their
earlier Tertiary ancestors, the well-preserved skulls of which enable
us to determine the size and weight of the brain.
(FIGURE 2.303. Brain of a human embryo, twenty-four weeks old, halved
in the median plane: right hemisphere seen from inside. (From
Mihalkovics, natural size.) rn olfactory nerve. tr funnel of the
intermediate brain, vc anterior commissure, ml Monro-foramen, gw
fornix, ds transparent sheath, bl corpus callosum, br fissure at its
border, hs occipital fissure, zh cuneus, sf occipital transverse
fissure, zb pineal gland, mh corpora quadrigemina, kh cerebellum.
In the lower mammals the surface of the cerebral hemispheres is quite
smooth and level, as in the rabbit (Figure 2.304). Moreover, the fore
brain remains so small that it does not cover the middle brain. At a
stage higher the middle brain is covered, but the hind brain remains
free. Finally, in the apes and man, the latter also is covered by the
fore brain. We can trace a similar gradual development in the fissures
and convolutions that are found on the surface of the cerebrum of the
higher mammals (Figures 2.292 and 2.293). If we compare different
groups of mammals in regard to these fissures and convolutions, we
find that their development proceeds step by step with the advance of
mental life.
Of late years great attention has been paid to this special branch of
cerebral anatomy, and very striking individual differences have been
detected within the limits of the human race. In all human beings of
special gifts and high intelligence the convolutions and fissures are
much more developed than in the average man; and they are more
developed in the latter than in idiots and others of low mental
capacity. There is a similar gradation among the mammals in the
internal structure of the fore brain. In particular the corpus
callosum, that unites the two cerebral hemispheres, is only developed
in the Placentals. Other structures--for instance, in the lateral
ventricles--that seem at first to be peculiar to man, are also found
in the higher apes, and these alone. It was long thought that man had
certain distinctive organs in his cerebrum which were not found in any
other animal. But careful examination has discovered that this is not
the case, but that the characteristic features of the human brain are
found in a rudimentary form in the lower apes, and are more or less
fully developed in the higher apes. Huxley has convincingly shown, in
his Man's Place in Nature (1863), that the differences in the
formation of the brain within the ape-group constitute a deeper gulf
between the lower and higher apes than between the higher apes and
man.
The comparative anatomy and physiology of the brain of the higher and
lower mammals are very instructive, and give important information in
connection with the chief questions of psychology.
(FIGURE 2.304. Brain of the rabbit. A from the dorsal, B from the
ventral side, lo olfactory lobes, I fore brain, h hypophysis at the
base of the intermediate brain, III middle brain, IV hind brain, V
after brain, 2 optic nerve, 3 oculo-motor nerve, 5 to 8 cerebral
nerves. In A the roof of the right hemisphere (I) is removed, so that
we can see the corpora striata in the lateral ventricle. (From
Gegenbaur.))
The central marrow (brain and spinal cord) develops from the medullary
tube in man just as in all the other mammals, and the same applies to
the conducting marrow or "peripheral nervous system." It consists of
the SENSORY nerves, which conduct centripetally the impressions from
the skin and the sense-organs to the central marrow, and of the MOTOR
nerves, which convey centrifugally the movements of the will from the
central marrow to the muscles. All these peripheral nerves grow out of
the medullary tube (Figure 1.171), and are, like it, products of the
skin-sense layer.
The complete agreement in the structure and development of the psychic
organs which we find between man and the highest mammals, and which
can only be explained by their common origin, is of profound
importance in the monistic psychology. This is only seen in its full
light when we compare these morphological facts with the corresponding
physiological phenomena, and remember that every psychic action
requires the complete and normal condition of the correlative brain
structure for its full and normal exercise. The very complex molecular
movements inside the neural cells, which we describe comprehensively
as "the life of the soul," can no more exist in the vertebrate, and
therefore in man, without their organs than the circulation without
the heart and blood. And as the central marrow develops in man from
the same medullary tube as that of the other vertebrates, and as man
shares the characteristic structure of his cerebrum (the organ of
thought) with the anthropoid apes, his psychic life also must have the
same origin as theirs.
If we appreciate the full weight of these morphological and
physiological facts, and put a proper phylogenetic interpretation on
the observations of embryology, we see that the older idea of the
personal immortality of the human soul is scientifically untenable.
Death puts an end, in man as in any other vertebrate, to the
physiological function of the cerebral neurona, the countless
microscopic ganglionic cells, the collective activity of which is
known as "the soul." I have shown this fully in the eleventh chapter
of my Riddle of the Universe.