[Illustration: From _Cat. R.C.S. England_.
FIG. 2l.--Mesial and Lateral Views of the Brain of the Tasmanian Devil
(_Sarcophilus_).]
In the Eutheria or mammals above the marsupials, the cerebellum
gradually becomes more complex, owing to the appearance of lateral
lobes between the flocculus and the vermis, as well as the
paraflocculus on the outer side of the flocculus. The corpus callosum
now first appears as a bridge between the neopallia, and its
development leads to the stretching of the hippocampal formation, so
that in the higher mammals the hippocampus is only found in the lower
and back part of the ventricle, while the rudiments of the dorsal part
remain as the _striae longitudinals_ on the corpus callosum. The
dorsal part of the original anterior commissure becomes the fornix,
and the paraterminal area is modified to form the septum lucidum. The
first appearance of the fissure of Rolando is probably in some of the
Carnivora, in which, as the _sulcus crucialis_, it forms the posterior
boundary of the "ursine lozenge" described by Mivart (_Journ. Linn.
Soc_. vol. xix., 1886) (see fig. 22, _Sulc. Cru_.). In the higher apes
or Anthropoidea the human fissures and sulci are largely recognizable,
so that a gibbon's brain, apart from all question of comparative
anatomy, forms a useful means of demonstrating to a junior class the
main gyri and sulci of Man in a simple and diagrammatic way. The main
points of difference, apart from greater simplicity, are that the
central lobe or island of Reil is exposed on the surface of the brain,
as it is in the human foetus, and that the anterior part of the
occipital lobe has a well-marked vertical sulcus, called the simian
sulcus or _Affenspalte_; this often has a semilunar shape with its
convexity forward, and is then called the _sulcus lunatus_. It is
usually concealed in European brains by the overgrowth of the
surrounding gyri, but it occasionally remains, though less frequently
than in the brains of Egyptian fellaheen. Its relation to the _white
stria of Gennari_ is especially interesting, and is recorded by Elliot
Smith in the _Anatomischer Anzeiger_, Bd. xxiv., 1904, p. 436. The
rhinal fissure, which is so characteristic a feature of the lower
mammals, almost disappears in Man, and is only represented by the
_incisura temporalis_ (see fig. 11, _i.t_). The hippocampal fissure
persists with little modification all through the mammalian class. The
calcarine fissure remains with many modifications from the marsupials
to man, and in view of the famous controversy of 1864, in which Owen,
Huxley and the then bishop of Oxford took part, it is interesting to
note that its hippocampus minor can now be clearly demonstrated, even
in the Marsupialia. Another very ancient and stable sulcus is the
_orbital_, which is a simple antero-posterior line until Man is
reached (see fig. 23, _Sulc. Orb._). The great point of importance,
however, in the evolution of the mammalian brain is the gradual
suppression of the olfactory region, and the development of the
neopallium, a development which takes a sudden stride between the
Anthropoid apes and Man. (For further particulars of this and other
points in the comparative anatomy of the brain, see _Catalogue of the
Physiological Series_ of the Museum of the Royal College of Surgeons
of England, vol. ii. 2nd ed., by R.H. Burne and G. Elliot Smith,
London, 1902.)
[Illustration: From _Cat. R.C.S. England_.
FIG. 22.--Dorsal and Lateral Views of the Brain of a Ratel (_Mellivora
indica_).]
_Embryology._
The brain, like the rest of the nervous system, is developed from the
ectoderm or outer layer of the embryo by the formation of a groove in
the mid-dorsal line. The lips of this _medullary groove_ unite to form
a canal beginning at the place where the neck of the embryo is to be.
The part of the neural canal in front of the earliest union forms the
brain and very early becomes constricted into three vesicles, to which
the names of _prosencephalon_, _mesencephalon_ and _rhombencephalon_
are now usually given. The simple tubular brain we have seen as a
permanent arrangement in Amphioxus, but the stage of the three
vesicles is a transitory one, and is not found in the adult of any
existing animal. From the sides of the prosencephalon, the optic
vesicles grow out before the neural tube is completely closed, and
eventually form the optic nerves and retinae, while, soon after this,
the cerebral hemispheres bulge from the antero-dorsal part of the
first primary vesicle, their points of evagination being the _foramina
of Munro_. From the ventral parts of these cerebral hemispheres the
olfactory lobes are constricted off, while just behind the openings
of the foramina of Munro a constriction occurs which divides the
prosencephalon into two secondary vesicles, the anterior of which,
containing the foramina of Munro, is called the _telencephalon_, while
the posterior is the _thalamencephalon_ or _diencephalon_. A
constriction also occurs in the hind vesicle or _rhombencephalon_,
dividing it into an anterior part, the _metencephalon_, from which the
cerebellum is developed, and a posterior or _myelencephalon_, the
primitive _medulla oblongata_. At this stage the general resemblance
of the brain to that of the lamprey is striking.
Before the secondary constrictions occur three vertical flexures begin
to form. The first is known as the _cephalic_, and is caused by the
prosencephalon bending sharply downward, below and in front of the
mesencephalon. The second is the _cervical_, and marks the place where
the brain ends and the spinal cord begins; the concavity of this
flexure is ventral. The third to appear has a ventral convexity and is
known as the _pontine_, since it marks the site of the future _pons
Varolii_; it resembles the permanent flexure in the reptilian brain.
[Illustration: From _Cat. R.C.S. England_.
FIG. 23.--Lateral view of cerebral hemisphere of Gorilla
(_Anthropopithecus gorilla_).]
It will now be seen that the original neural canal, which is lined by
ciliated epithelium, forms the ventricles of the brain, while
superficial to this epithelium (_ependyma_) the grey and white matter
is subsequently formed. It has been shown by His that the whole neural
tube may be divided into _dorsal_ or _alar_, and _ventral_ or _basal_
laminae, and, as the cerebral hemispheres bud out from the dorsal part
of the anterior primary vesicle, they consist entirely of alar
laminae. The most characteristic feature of the human and anthropoid
brain is the rapid and great expansion of these hemispheres,
especially in a backward direction, so that the mesencephalon and
metencephalon are hidden by them from above at the seventh month of
intra-uterine life. At first the foramina of Munro form a
communication not only between the third and lateral ventricles, but
between the two lateral ventricles, so that the cavity of each
hemisphere is continuous with that of the other; soon, however, a
median longitudinal fissure forms, into which the mesoderm grows to
form the falx, and so the foramina of Munro are constricted into a
V-shaped canal. In the floor of the hemispheres the corpora striata
are developed at an early date by a multiplication of nerve cells, and
on the external surface a depression, called the _Sylvian fossa_,
marks the position of the future central lobe, which is afterwards
hidden as the lips of the fossa (_opercula_) gradually close in on it
to form the Sylvian fissure. The real fissures are complete infoldings
of the whole thickness of the vesicular wall and produce swellings in
the cavity. Some of them, like the choroidal on the mesial surface,
are developed very early, while the vesicle is little more than
epithelial, and contain between their walls an inpushing of mesoderm
to form the choroid plexus. Others, like the hippocampal and
calcarine, appear in the second and third months and correspond to
invaginations of the nervous tissue, the hippocampus major and minor.
The sulci appear later than the fissures and do not affect the
internal cavity; they are due to the rapid growth of the cortex in
certain areas. The corpus callosum and fornix appear about the third
month and their development is somewhat doubtful; they are probably
modifications of the lamina terminalis, but they may be secondary
adhesions between the adjacent surfaces of the cerebral hemispheres
where the cortical grey matter has not covered the white. They begin
at their antero-ventral part near the genu of the corpus callosum and
the anterior pillars of the fornix, and these are the parts which
first appear in the lower mammals. The original anterior vesicle from
which the hemispheres evaginate is composed, as already shown, of an
anterior part or telencephalon and a posterior or thalamencephalon;
the whole forming the third ventricle in the adult. Here the alar and
basal laminae are both found, but the former is the more important;
from it the optic thalami are derived, and more posteriorly the
geniculate bodies. The anterior wall, of course, is the lamina
terminalis, and from it are formed the _lamina cinerea_, the _corpus
callosum_, _fornix_ and _septum lucidum_. The roof largely remains
epithelial and is invaginated into the ventricle by the mesoderm to
form the _choroid plexuses_ of the third ventricle, but at the
posterior part it develops the _ganglia habenulae_ and the pineal
body, from a structure just in front of which both a lens and retinal
elements are derived in the lower forms. This is one great difference
between the development of this organ and that of the true eyes;
indeed it has been suggested that the pineal is an organ of thermal
sense and not the remains of a median eye at all. The floor of the
third ventricle is developed from the basal laminae, which here are
not very important and from which the _tuber cinereum_ and, until the
fourth month, single _corpus mammillare_ are developed. The
_infundibulum_ or stalk of the posterior part of the pituitary body at
first grows down in front of the _tuber cinereum_ and, according to
Gaskel's theory, represents an ancestral mouth to which the ventricles
of the brain and the central canal of the cord acted as the stomach
and intestine (_Quart. Journ. of Mic. Sci._ 31, p. 379; and _Journ. of
Phys._ v. 10, p. 153). The reason why the basal lamina is here small
is because it contains the nuclei of no cranial nerves. The anterior
and posterior commissures appear before the middle and the middle
before the _corpus callosum_, as they do in phylogeny. In connexion
with the thalamencephalon, though not really belonging to it, may be
mentioned the anterior lobes of the pituitary body; these begin as an
upward _diverticulum_ from the posterior wall of the primitive pharynx
or _stomatodaeum_ about the fourth week. This _pouch of Rathke_, as it
is called, becomes nipped off by the developing base of the skull, and
its bifid blind end meets and becomes applied to the posterior part of
the body, which comes down from the brain. In the mesencephalon the
alar laminae form the _corpora quadrigemina_; these at first are
bigeminal and hollow as they are in the lower vertebrates. The basal
laminae thicken to form the _crura cerebri_. In the rhombencephalon
the division into basal and alar laminae is better marked than in any
other part; there is a definite groove inside the fourth ventricle,
which remains in the adult as the superior and inferior _fovea_ and
which marks the separation between the two laminae. In the basal
laminae are found the deep origins of most of the motor cranial
nerves, while those of the sensory are situated in the alar laminae.
The roof of the fourth ventricle widens out very much and remains
largely epithelial as the superior and inferior medullary vela. The
cerebellum develops in the anterior part of the roof of the
rhombencephalon as two lateral rudiments which unite in the mid line
and so form a transverse bar similar to that seen in the adult
lamprey; at the end of the second month the flocculus and
paraflocculus become marked, and later on a series of transverse
fissures occur dividing the various lobes. Of the cerebellar peduncles
the inferior develops first (third month), then the middle forming the
_pons_ (fourth month), and lastly the _superior_ (fifth month) (Elliot
Smith, _Review of Neurology and Psychiatry_, October 1903; W. Kuithan,
"Die Entwicklung des Kleinhirns bei Saugetieren," _Munchener Med.
Abhandl._, 1895; B. Stroud, "Mammalian cerebellum," _Journ. of Comp.
Neurology_, 1895). Much of our knowledge of the tracts of fibres in
the brain is due to the fact that they acquire their white sheaths at
different stages of development, some long after birth.
For further details and references see Quain's _Anat._ vol. i. (1908);
Minot's _Human Embryology_ (New York); W. His, _Anat. menschlicher
Embryonen_ (Leipzig, 1881); Marshall's _Vertebrate Embryology_;
Kolliker, _Grundriss der Entwickelungsgeschichte_ (Leipzig, 1880); A.
Keith, _Human Embryology and Morphology_ (London, 1904); O. Hertwig,
_Handbuch der vergleichenden und experimentellen Entwickelungslehre
der Wirbeltiere_, Bd. 2, part 3 (Jena, 1902-1906); _Development of the
Human Body_, J.P. McMurrich (1906). (F. G. P.)