The entire nervous system is of ectodermal origin, and its first rudiment
is seen in the neural groove which extends along the dorsal aspect of the
embryo (Figs. 90017). By the elevation
and ultimate fusion of the neural folds, the groove is converted into the
neural tube (Figs. 90019). The anterior
end of the neural tube becomes expanded to form the three primary brain-vesicles;
the cavity of the tube is subsequently modified to form the ventricular
cavities of the brain, and the central canal of the medulla spinalis; from
the wall the nervous elements and the neuroglia of the brain and medulla
spinalis are developed.

FIG. 90640– Section of
medulla spinalis of a four weeks’ embryo. (His.)

FIG. 90641– Transverse
section of the medulla spinalis of a human embryo at the beginning of
the fourth week. The left edge of the figure corresponds to the lining
of the central canal. (His.)

FIG. 90642– aged about
four and a half weeks.

FIG. 90643– aged about
three months.
The Medulla Spinalis.—At first the wall of the neural tube
is composed of a single layer of columnar ectodermal cells. Soon the side-walls
become thickened, while the dorsal and ventral parts remain thin, and are
named the roof- and floor-plates (Figs.
90640, 90642, 90643).
A transverse section of the tube at this stage presents an oval outline,
while its lumen has the appearance of a slit. The cells which constitute
the wall of the tube proliferate rapidly, lose their cell-boundaries and
form a syncytium. This syncytium consists at first of dense protoplasm with
closely packed nuclei, but later it opens out and forms a looser meshwork
with the cellular strands arranged in a radiating manner from the central
canal. Three layers may now be defined—an internal or ependymal, an
intermediate or mantle, and an external or marginal. The ependymal layer
is ultimately converted into the ependyma of the central canal; the processes
of its cells pass outward toward the periphery of the medulla spinalis.
The marginal layer is devoid of nuclei, and later forms the supporting
framework for the white funiculi of the medulla spinalis. The mantle
represents the whole of the future gray columns of the medulla
spinalis; in it the cells are differentiated into two sets, viz., (a)
spongioblasts or young neuroglia cells, and (b) germinal
which are the parents of the neuroblasts

or young nerve cells (Fig. 90641).
The spongioblasts are at first connected to one another by filaments of
the syncytium; in these, fibrils are developed, so that as the neuroglial
cells become defined they exhibit their characteristic mature appearance
with multiple processes proceeding from each cell. The germinal cells are
large, round or oval, and first make their appearance between the ependymal
cells on the sides of the central canal. They increase rapidly in number,
so that by the fourth week they form an almost continuous layer on each
side of the tube. No germinal cells are found in the roof- or floor-plates;
the roof-plate retains, in certain regions of the brain, its epithelial
character; elsewhere, its cells become spongioblasts. By subdivision the
germinal cells give rise to the neuroblasts or young nerve cells, which
migrate outward from the sides of the central canal into the mantle layer
and neural crest, and at the same time become pear-shaped; the

tapering part of the cell undergoes still further elongation, and forms
the axiscylinder of the cell.
  The lateral walls of the medulla spinalis continue to increase
in thickness, and the canal widens out near its dorsal extremity, and assumes
a somewhat lozengeshaped appearance. The widest part of the canal serves
to subdivide the lateral wall of the neural tube into a dorsal or
alar, and a ventral or basal lamina (Figs.
90642, 90643), a subdivision which
extends forward into the brain. At a later stage the ventral part of the
canal widens out, while the dorsal part is first reduced to a mere slit
and then becomes obliterated by the approximation and fusion of its walls;
the ventral part of the canal persists and forms the central canal of the
adult medulla spinalis. The caudal end of the canal exhibits a conical expansion
which is known as the terminal ventricle.
  The ventral part of the mantle layer becomes thickened, and
on cross-section appears as a triangular patch between the marginal and
ependymal layers. This thickening is the rudiment of the anterior column
of gray substance, and contains many neuroblasts, the axis-cylinders of
which pass out through the marginal layer and form the anterior roots of
the spinal nerves (Figs. 90640, 90642,
90643). The thickening of the mantle
layer gradually extends in a dorsal direction, and forms the posterior column
of gray substance. The axons of many of the neuroblasts in the alar lamina
run forward, and cross in the floor-plate to the opposite side of the medulla
spinalis; these form the rudiment of the anterior white commissure.
  About the end of the fourth week nerve fibers begin to appear
in the marginal layer. The first to develop are the short intersegmental
fibers from the neuroblasts in the mantle zone, and the fibers of the dorsal
nerve roots which grow into the medulla spinalis from the cells of the spinal
ganglia. By the sixth week these dorsal root fibers form a well-defined
oval bundle in the peripheral part of the alar lamina; this ??
gradually increases in size, and spreading toward the middle line forms
the rudiment of the posterior funiculus. The long intersegmental fibers
begin to appear about the third month and the cerebrospinal fibers about
the fifth month. All nerve fibers are at first destitute of medullary sheaths.
Different groups of fibers receive their sheaths at different times—the
dorsal and ventral nerve roots about the fifth month, the cerebrospinal
fibers after the ninth month.
  By the growth of the anterior columns of gray substance, and
by the increase in size of the anterior funiculi, a furrow is formed between
the lateral halves of the cord anteriorly; this gradually deepens to form
the anterior median fissure. The mode of formation of the posterior septum
is somewhat uncertain. Many believe that it is produced by the growing together
of the walls of the posterior part of the central canal and by the development
from its ependymal cells of a septum of fibrillated tissue which separates
the future funiculi graciles.
  Up to the third month of fetal life the medulla spinalis occupies
the entire length of the vertebral canal, and the spinal nerves pass outward
at right angles to the medulla spinalis. From this time onward, the vertebral
column grows more rapidly than the medulla spinalis, and the latter, being
fixed above through its continuity with the brain, gradually assumes a higher
position within the canal. By the sixth month its lower end reaches only
as far as the upper end of the sacrum; at birth it is on a level with the
third lumbar vertebra, and in the adult with the lower border of the first
or upper border of the second lumbar vertebra. A delicate filament, the
filum terminale, extends from its lower end as far as the coccyx.
The Spinal Nerves.—Each spinal nerve is attached to the medulla
spinalis by an anterior or ventral and a posterior or dorsal root.
  The fibers of the anterior roots are formed by the axons of
the neuroblasts which lie in the ventral part of the mantle layer; these
axons grow out through the overlying marginal layer and become grouped to
form the anterior nerve root (Fig. 90641).
  The fibers of the posterior roots are developed from the cells
of the spinal ganglia. Before the neural groove is closed to form the neural
tube a ridge of ectodermal cells, the ganglion ridge or neural
(Fig. 90644), appears along
the prominent margin of each neural fold. When the folds meet in the middle
line the two ganglion ridges fuse and form a wedge-shaped area along the
line of closure of the tube. The cells of this area proliferate rapidly
opposite the primitive segments and then migrate in a lateral and ventral
direction to the sides of the neural tube, where they ultimately form a
series of oval-shaped masses, the future spinal ganglia. These ganglia are
arranged symmetrically on the two sides of the neural tube and, except in
the region of the tail, are equal in number to the primitive segments. The
cells of the ganglia, like the cells of the mantle layer, are of two kinds,
viz., spongioblasts and neuroblasts. The spongioblasts develop
into the neuroglial cells of the ganglia. The neuroblasts are at first round
or oval in shape, but soon assume the form of spindles the extremities of
which gradually elongate into central and peripheral processes. The central
processes grow medialward and, becoming connected with the neural tube,
constitute the fibers of the posterior nerve roots, while the peripheral
processes grow lateralward to mingle with the fibers of the anterior root
in the spinal nerve. As development proceeds the original bipolar form of
the cells changes; the two processes become approximated until they ultimately
arise from a single stem in a T-shaped manner. Only in the ganglia of the
acoustic nerve is the bipolar form retained. More recent observers hold,
however, that the T-form is derived from the branching of a single process
which grows out from the cell.

FIG. 90644– Two stages
in the development of the neural crest in the human embryo. (Lenhossèk.)
  The anterior or ventral and the posterior or dorsal nerve
roots join immediately beyond the spinal ganglion to form the spinal
which then divides into anterior, posterior, and visceral divisions.
The anterior and posterior divisions proceed directly to their areas of
distribution without further association with ganglion cells (Fig. 90645). The visceral divisions are distributed to the thoracic, abdominal,
and pelvic viscera, to reach which they pass through the sympathetic trunk,
and many of the fibers form arborizations around the ganglion cells of this
trunk. Visceral branches are not given off from all the spinal nerves; they
form two groups, viz., (a) thoracico-lumbar, from the first
or second thoracic, to the second or third lumbar nerves; and (b)
pelvic, from the second and third, or third and fourth sacral nerves.

FIG. 90645– Reconstruction
of periphera nerves of a human embryo of 10.2 mm. (After His.) The abducent
nerve is not labelled, but is seen passing forward to the eye under the
mandibular and maxillary nerves.
The Brain.—The brain is developed from the anterior end of
the neural tube, which at an early period becomes expanded into three vesicles,
the primary cerebral vesicles (Figs. 90018).
These are marked off from each other by intervening constrictions, and are
named the fore-brain or prosencephalon, the mid-brain
or mesencephalon, and the hind-brain or rhombencephalon—the
last being continuous with the medulla spinalis. As the result of unequal
growth of these different parts three flexures are formed and the embryonic
brain becomes bent on itself in a somewhat zigzag fashion; the two earliest
flexures are concave ventrally and are associated with corresponding flexures
of the whole head. The first flexure

appears in the region of the mid-brain, and is named the ventral cephalic
(Fig. 90650). By means of
it the fore-brain is bent in a ventral direction around the anterior end
of the notochord and fore-gut, with the result that the floor of the fore-brain
comes to lie almost parallel with that of the hind-brain. This flexure causes
the mid-brain to become, for a time, the most prominent part of the brain,
since its dorsal surface corresponds with the convexity of the curve. The
second bend appears at the junction of the hind-brain and medulla spinalis.
This is termed the cervical flexure (Fig. 90652), and increases from the third to the end of the fifth week, when
the hind-brain forms nearly a right angle with the medulla spinalis; after
the fifth week erection of the head takes place and the cervical flexure
diminishes and disappears. The third bend is named the pontine flexure
(Fig. 90652), because it is found in
the region of the future pons Varoli. It differs from the other two in that
(a) its convexity is forward, and (b) it does not affect

the head. The lateral walls of the brain-tube, like those of the medulla
spinalis, are divided by internal furrows into alar or dorsal and basal
or ventral laminæ (Fig. 90646).

FIG. 90646– Diagram to
illustrate the alar and basal laminæ of brain vesicles. (His.)

FIG. 90647– Transverse
section of medulla oblongata of human embryo. X 32. (Kollmann.)

FIG. 90648– Transverse
section of medulla oblongata of human embryo. (After His.)
The Hind-brain or Rhombencephalon.—The cavity of the hind-brain
becomes the fourth ventricle. At the time when the ventral cephalic flexure
makes its appearance, the length of the hind-brain exceeds the combined
lengths of the other two vesicles. Immediately behind the mid-brain it exhibits
a marked constriction, the isthmus rhombencephali (Fig. 90650, Isthmus), which is best seen when the brain is viewed from
the dorsal aspect. From the isthmus the anterior medullary velum and the
superior peduncle of the cerebellum are formed. It is customary to divide
the rest of the hind-brain into two parts, viz., an upper, called the metencephalon,
and a lower, the myelencephalon. The cerebellum is developed by a
thickening of the roof, and the pons by a thickening in the floor and lateral
walls of the metencephalon. The floor and lateral walls of the myelencephalon
are thickened to form the medulla oblongata; its roof remains thin, and,
retaining to

a great extent its epithelial nature, is expanded in a lateral direction.
Later, by the growth and backward extension of the cerebellum, the roof
is folded inward toward the cavity of the fourth ventricle; it assists in
completing the dorsal wall of this cavity, and is also invaginated to form
the ependymal covering of its choroid plexuses. Above it is continuous with
the posterior medullary velum; below, with the obex and ligulæ.

FIG. 90649– Hind-brain
of a human embryo of three months—viewed from behind and partly from
left side. (From model by His.)

FIG. 90650– Exterior
of brain of human embryo of four and a half weeks. (From model by His.)
  The development of the medulla oblongata resembles
that of the medulla spinalis, but at the same time exhibits one or two interesting
modifications. On transverse section the myelencephalon at an early stage
is seen to consist of two lateral walls, connected across the middle line
by floor- and roof-plates (Figs. 90647
and 90648). Each lateral wall consists
of an alar and a basal lamina, separated by an internal furrow, the remains
of which are represented in the adult brain by the sulcus limitans on the
rhomboid fossa. The contained cavity is more or less triangular in outline,
the base being formed by the roof-plate, which is thin and greatly expanded
transversely. Pear-shaped neuroblasts are developed in the alar and basal
laminæ. and their narrow stalks are elongated to form the axis-cylinders
of the nerve fibers. Opposite the furrow or boundary between the alar and
basal laminæ a bundle of nerve fibers attaches itself to the outer
surface of the alar lamina. This is named the tractus solitarius
(Fig. 90648), and is formed by the sensory
fibers of the glossopharyngeal and vagus nerves. It is the homologue of
the oval bundle seen in the medulla spinalis, and, like it, is developed
by an ingrowth of fibers from the ganglia of the neural crest. At first
it is applied to the outer surface of the alar lamina, but it soon becomes
buried, owing to the growth over it of the neighboring parts. By the fifth
week the dorsal part of the alar lamina bends in a lateral direction along
its entire length, to form what is termed the rhombic lip (Figs.
90648, 90649). Within a few days this
lip becomes applied to, and unites

with, the outer surface of the main part of the alar lamina, and so covers
in the tractus solitarius and also the spinal root of the trigeminal nerve;
the nodulus and flocculus of the cerebellum are developed from the rhombic
  Neuroblasts accumulate in the mantle layer; those in the basal
lamina correspond with the cells in the anterior gray column of the medulla
spinalis, and, like them, give origin to motor nerve fibers; in the medulla
oblongata they are, however, arranged in groups or nuclei, instead of forming
a continuous column. From the alar lamina and its rhombic lip, neuroblasts
migrate into the basal lamina, and become aggregated to form the olivary
nuclei, while many send their axis-cylinders through the floor-plate to
the opposite side, and thus constitute the rudiment of the raphé of
the medulla oblongata. By means of this thickening of the ventral portion,
the motor nuclei are buried deeply in the interior, and, in the adult, are
found close to the rhomboid fossa. This is still further accentuated: (a)
by the development of the pyramids, which are formed about the fourth month
by the downward growth of the motor fibers from the cerebral cortex; and
(b) by the fibers which pass to and from the cerebellum. On the rhomboid
fossa a series of six temporary furrows appears; these are termed the rhombic
They bear a definite relationship to certain of the cranial
nerves; thus, from before backward the first and second grooves overlie
the nucleus of the trigeminal; the third, the nucleus of the facial; the
fourth, that of the abducent; the fifth, that of the glossopharyngeal; and
the sixth, that of the vagus.
  The pons is developed from the ventro-lateral wall
of the metencephalon by a process similar to that which has been described
for the medulla oblongata.

FIG. 90651– Brain of
human embryo of four and a half weeks, showing interior of fore-brain.
(From model by His.)
  The cerebellum is developed in the roof of the anterior
part of the hind-brain (Figs. 90649
to 90654). The alar laminæ of this
region become thickened to form two lateral plates which soon fuse in the
middle line and produce a thick lamina which roofs in the upper part of
the cavity of the hind-brain vesicle; this constitutes the rudiment of the
cerebellum, the outer surface of which is originally smooth and convex.
The fissures of the cerebellum appear first in the vermis and floccular
region, and traces of them are found during the third month; the fissures
on the cerebellar hemispheres do not appear until the fifth month. The primitive
fissures are not developed in the order of their relative size in the adult—thus
the horizontal sulcus in the fifth month is merely a shallow groove. The
best marked of the early fissures are: (a) the fissura prima
between the developing culmen and declive, and (b) the fissura
between the future pyramid and uvula. The flocculus and nodule
are developed from the rhombic lip, and are therefore recognizable as separate
portions before any of the other cerebellar lobules. The groove produced
by the bending over of the rhombic lip is here known as the

floccular fissure; when the two lateral walls fuse, the right and
left floccular fissures join in the middle line and their central part becomes
the post-nodular fissure.
  On the ventricular surface of the cerebellar lamina a transverse
furrow, the incisura fastigii, appears, and deepens to form the tent-like
recess of the roof of the fourth ventricle. The rudiment of the cerebellum
at first projects in a dorsal direction; but, by the backward growth of
the cerebrum, it is folded downward and somewhat flattened, and the thin
roof-plate of the fourth ventricle, originally continuous with the posterior
border of the cerebellum, is projected inward toward the cavity of the ventricle.
The Mid-brain or Mesencephalon.—The mid-brain (Figs.
90650 to 90654) exists for a time
as a thin-walled cavity of some size, and is separated from the isthmus
rhombencephali behind, and from the fore-brain in front, by slight constrictions.
Its cavity becomes relatively reduced in diameter, and forms the cerebral
aqueduct of the adult brain. Its basal laminæ increase in thickness
to form the cerebral peduncles, which are at first of small size, but rapidly
enlarge after the fourth month. The neuroblasts of these laminæ are
grouped in relation to the sides and floor of the cerebral aqueduct, and
constitute the nuclei of the oculomotor and trochlear nerves, and of the
mesencephalic root of the trigeminal nerve. By a similar thickening process
its alar laminæ are developed into the quadrigeminal lamina. The dorsal
part of the wall for a time undergoes expansion, and presents an internal
median furrow and a corresponding external ridge; these, however, disappear,
and the latter is replaced by a groove. Subsequently two oblique furrows
extend medialward and backward, and the thickened lamina is thus subdivided
into the superior and inferior colliculi.

FIG. 90652– Exterior
of brain of human embryo of five weeks. (From model by His.)
The Fore-brain or Prosencephalon.—A transverse section of
the early fore-brain shows the same parts as are displayed in similar sections
of the medulla spinalis and medulla oblongata, viz., a pair of thick lateral
walls connected by thin floor-and roof-plates. Moreover, each lateral wall
exhibits a division into a dorsal or alar and a ventral or basal lamina
separated internally by a furrow termed the sulcus

of Monro.
This sulcus ends anteriorly at the medial end of the optic
stalk, and in the adult brain is retained as a slight groove extending backward
from the interventricular foramen to the cerebral aqueduct.
  At a very early period—in some animals before the closure
of the cranial part of the neural tube—two lateral diverticula, the
optic vesicles, appear, one on either side of the fore-brain; for
a time they communicate with the cavity of the fore-brain by relatively
wide openings. The peripheral parts of the vesicles expand, while the proximal
parts are reduced to tubular stalks, the optic stalks. The optic
vesicle gives rise to the retina and the epithelium on the back of the ciliary
body and iris; the optic stalk is invaded by nerve fibers to form the optic
nerve. The fore-brain then grows forward, and from the alar laminæ
of this front portion the cerebral hemispheres originate as diverticula
which rapidly expand to form two large pouches, one on either side. The
cavities of these diverticula are the rudiments of the lateral ventricles;
they communicate with the median part of the fore-brain cavity by relatively
wide openings, which ultimately form the interventricular foramen. The median
portion of the wall of the fore-brain vesicle consists of a thin lamina,
the lamina terminalis (Figs. 90654,
90657), which stretches from the interventricular
foramen to the recess at the base of the optic stalk. The anterior part
of the fore-brain, including the rudiments of the cerebral hemispheres,
is named the telencephalon, and its posterior portion is termed the
diencephalon; both of these contribute to the formation of the third

FIG. 90653– Interior
of brain of human embryo of five weeks. (From model by His.)
The Diencephalon.—From the alar lamina of the diencephalon,
the thalamus, metathalamus, and epithalamus are developed. The thalamus
(Figs. 90650 to 90654)
arises as a thickening which involves the anterior two-thirds of the alar
lamina. The two thalami are visible, for a time, on the surface of the brain,
but are subsequently hidden by the cerebral hemispheres which grow backward
over them. The thalami extend medialward and gradually narrow the cavity
between them into a slit-like aperture which forms the greater part of the
third ventricle; their medial surfaces ultimately adhere, in part, to each
other, and the intermediate

of the ventricle is developed across the area of contact. The metathalamus
comprises the geniculate bodies which originate as slight outward bulgings
of the alar lamina. In the adult the lateral geniculate body appears as
an eminence on the lateral part of the posterior end of the thalamus, while
the medial is situated on the lateral aspect of the mid-brain. The epithalamus
includes the pineal body, the posterior commissure, and the trigonum habenulæ.
The pineal body arises as an upward the evagination of roof-plate immediately
in front of the midbrian; this evagination becomes solid with the exception
of its proximal part, which persists as the recessus pinealis. In lizards
the pineal evagination is elongated into a stalk, and its peripheral extremity
is expanded into a vesicle, in which a rudimentary lens and retina are formed;
the stalk becomes solid and nerve fibers make their appearance in it, so
that in these animals the pineal body forms a rudimentary eye. The posterior
commissure is formed by the ingrowth of fibers into the depression behind
and below the pineal evagination, and the trigonum habenulæ is developed
in front of the pineal recess.

FIG. 90654– Median sagittal
section of brain of human embryo of three months. (From model by His.)
  From the basal laminæ of the diencephalon the pars
mamillaris hypothalami
is developed; this comprises the corpora mamillaria
and the posterior part of the tuber cinereum. The corpora mamillaria arise
as a single thickening, which becomes divided into two by a median furrow
during the third month.
  The roof-plate of the diencephalon, in front of the pineal
body, remains thin and epithelial in character, and is subsequently invaginated
by the choroid plexuses of the third ventricle.
The Telencephalon.—This consists of a median portion and two
lateral diverticula. The median portion forms the anterior part of the cavity
of the third ventricle, and is closed below and in front by the lamina terminalis.
The lateral diverticula consist of outward pouchings of the alar laminæ;
the cavities represent the lateral ventricles, and their walls become thickened
to form the nervous

matter of the cerebral hemispheres. The roof-plate of the telencephalon
remains thin, and is continuous in front with the lamina terminalis and
behind with the roof-plate of the diencephalon. In the basal laminæ
and floor-plate the pars optica hypothalami is developed; this comprises
the anterior part of the tuber cinereum, the infundibulum and posterior
lobe of the hypophysis, and the optic chiasma. The anterior part of the
tuber cinereum is derived from the posterior part of the floor of the telencephalon;
the infundibulum and posterior lobe of the hypophysis arise as a downward
diverticulum from the floor. The most dependent part of the diverticulum
becomes solid and forms the posterior lobe of the hypophysis; the anterior
lobe of the hypophysis is developed from a diverticulum of the ectodermal
lining of the stomodeum. The optic chiasma is formed by the meeting and
partial decussation of the optic nerves, which subsequently grow backward
as the optic tracts and end in the diencephalon.
  The cerebral hemispheres arise as diverticula of the
alar laminæ of the telencephalon (Figs.
90650 to 90654); they increase rapidly
in size and ultimately overlap the structures developed from the mid- and
hind-brains. This great expansion of the hemispheres is a characteristic
feature of the brains of mammals, and attains its maximum development in
the brain of man. Elliott-Smith divides each cerebral hemisphere into three
fundamental parts, viz., the rhinencephalon, the corpus striatum,
and the neopallium.

FIG. 90655– Inferior
surface of brain of embryo at beginning of fourth month. (From Kollmann.)
  The rhinencephalon (Fig. 90655) represents the oldest part of the telencephalon, and forms almost
the whole of the hemisphere in fishes, amphibians, and reptiles. In man
it is feebly developed in comparison with the rest of the hemisphere, and
comprises the following parts, viz., the olfactory lobe (consisting of the
olfactory tract and bulb and the trigonum olfactorium), the anterior perforated
substance, the septum pellucidum, the subcallosal, supracallosal, and dentate
gyri, the fornix, the hippocampus, and the uncus. The rhinencephalon appears
as a longitudinal elevation, with a corresponding internal furrow, on the
under surface of the hemisphere close to the lamina terminalis; it is separated
from the lateral surface of the hemisphere by a furrow, the external
rhinal fissure,
and is continuous behind with that part of the hemisphere,
which will ultimately form the anterior end of the temporal lobe. The elevation
becomes divided by a groove into an anterior and a posterior part. The anterior
grows forward as a hollow stalk the lumen of which is continuous with the
anterior part of the ventricular cavity. During the third month the stalk
becomes solid and forms the rudiment of the olfactory bulb and tract; a
strand of gelatinous tissue in the interior of the bulb indicates the position
of the original cavity. From the posterior part the anterior perforated
substance and the pyriform lobe are developed; at the beginning of the fourth
month the latter forms a curved elevation continuous behind with the medial
surface of the temporal lobe, and consisting, from before backward, of the
gyrus olfactorius lateralis, gyrus ambiens, and gyrus semilunaris, parts

in the adult brain are represented by the lateral root of the olfactory
tract and the uncus. The position and connections of the remaining portions
of the rhinencephalon are described with the anatomy of the brain.
  The corpus striatum (Figs.
90651 and 90653) appears in the fourth
week as a triangular thickening of the floor of the telencephalon between
the optic recess and the interventricular foramen, and continuous behind
with the thalamic part of the diencephalon. It increases in size, and by
the second month is seen as a swelling in the floor of the future lateral
ventricle; this swelling reaches as far as the posterior end of the primitive
hemisphere, and when this part of the hemisphere grows backward and downward
to form the temporal lobe, the posterior part of the corpus striatum is
carried into the roof of the inferior horn of the ventricle, where it is
seen as the tail of the caudate nucleus in the adult brain. During the fourth
and fifth months the corpus striatum becomes incompletely subdivided by
the fibers of the internal capsule into two masses, an inner, the caudate
and an outer, the lentiform nucleus. In front, the corpus
striatum is continuous with the anterior perforated substance; laterally
it is confluent for a time with that portion of the wall of the vesicle
which is developed into the insula, but this continuity is subsequently
interrupted by the fibers of the external capsule.

FIG. 90656– Diagrammatic
coronal section of brain to show relations of neopallium. (After His.)
Cs. Corpus striatum. Th. Thalamus.
  The neopallium (Fig. 90656) forms the remaining, and by far the greater, part of the cerebral
hemisphere. It consists, at an early stage, of a relatively large, more
or less hemispherical cavity—the primitive lateral ventricle—enclosed
by a thin wall from which the cortex of the hemisphere is developed. The
vesicle expands in all directions, but more especially upward and backward,
so that by the third month the hemispheres cover the diencephalon, by the
sixth they overlap the mid-brain, and by the eighth the hind-brain.
  The median lamina uniting the two hemispheres does not share
in their expansion, and thus the hemispheres are separated by a deep cleft,
the forerunner of the longitudinal fissure, and this cleft is occupied by
a septum of mesodermal tissue which constitutes the primitive falx cerebri.
Coincidently with the expansion of the vesicle, its cavity is drawn out
into three prolongations which represent

the horns of the future lateral ventricle; the hinder end of the vesicle
is carried downward and forward and forms the inferior horn; the posterior
horn is produced somewhat later, in association with the backward growth
of the occipital lobe of the hemisphere. The roof-plate of the primitive
fore-brain remains thin and of an epithelial character; it is invaginated
into the lateral ventricle along the medial wall of the hemisphere. This
invagination constitutes the choroidal fissure, and extends from the interventricular
foramen to the posterior end of the vesicle. Mesodermal tissue, continuous
with that of the primitive falx cerebri, and carrying bloodvessels with
it, spreads between the two layers of the invaginated fold and forms the
rudiment of the tela choroidea; the margins of the tela become highly vascular
and form the choroid plexuses which for some months almost completely fill
the ventricular cavities; the tela at the same time invaginates the epithelial
roof of the diencephalon to form the choroid plexuses of the third ventricle.
By the downward and forward growth of the posterior end of the vesicle to
form the temporal lobe the choroidal fissure finally reaches from the interventricular
foramen to the extremity of the inferior horn of the ventricle.

FIG. 90657– Median sagittal
section of brain of human embryo of four months. (Marchand.)
  Parallel with but above and in front of the choroidal fissure
the medial wall of the cerebral vesicle becomes folded outward and gives
rise to the hippocampal fissure on the medial surface and to a corresponding
elevation, the hippocampus, within the ventricular cavity. The gray
or ganglionic covering of the wall of the vesicle ends at the inferior margin
of the fissure is a thickened edge; beneath this the marginal or reticular
layer (future white substance) is exposed and its lower thinned edge is
continuous with the epithelial invagination covering the choroid plexus
(Fig. 90656). As a result of the later
downward and forward growth of the temporal lobe the hippocampal fissure
and the parts associated with it extend from the interventricular foramen
to the end of the inferior horn of the ventricle. The thickened edge of
gray substance becomes the gyrus dentatus, the fasciola cinerea and the
supra- and subcallosal gyri, while the free edge of the white substance
forms the fimbria hippocampi and the body and crus of the fornix. The corpus
callosum is developed within the arch of the hippocampal fissure, and the
upper part of the fissure forms, in the adult brain, the callosal fissure
on the medial surface of the hemisphere.
The Commissures (Fig. 90657).—The
development of the posterior commissure has already been referred to (page
743). The great commissures of the hemispheres,

viz., the corpus callosum, the fornix, and anterior commissures,
arise from the lamina terminalis. About the fourth month a small thickening
appears in this lamina, immediately in front of the interventricular foramen.
The lower part of this thickening is soon constricted off, and fibers appear
in it to form the anterior commissure. The upper part continues to grow
with the hemispheres, and is invaded by two sets of fibers. Transverse fibers,
extending between the hemispheres, pass into its dorsal part, which is now
differentiated as the corpus callosum (in rare cases the corpus callosum
is not developed). Into the ventral part longitudinal fibers from the hippocampus
pass to the lamina terminalis, and through that structure to the corpora
mamillaria; these fibers constitute the fornix. A small portion, lying antero-inferiorly
between the corpus callosum and fornix, is not invaded by the commissural
fibers; it remains thin, and later a cavity, the cavity of the septum
forms in its interior.
Fissures and Sulci.—The outer surface of the cerebral hemisphere
is at first smooth, but later it exhibits a number of elevations or convolutions,
separated from each other by fissures and sulci, most of which make their
appearance during the sixth or seventh months of fetal life. The term fissure
is applied to such grooves as involve the entire thickness of the cerebral
wall, and thus produce corresponding eminences in the ventricular cavity,
while the sulci affect only the superficial part of the wall, and
therefore leave no impressions in the ventricle. The fissures comprise the
choroidal and hippocampal already described, and two others,
viz., the calcarine and collateral, which produce the swellings
known respectively as the calcar avis and the collateral eminence
in the ventricular cavity. Of the sulci the following may be referred to,
viz., the central sulcus (fissure of Rolando), which is developed
in two parts; the intraparietal sulcus in four parts; and the cingulate
in two or three parts. The lateral cerebral or Sylvian
differs from all the other fissures in its mode of development.
It appears about the third month as a depression, the Sylvian fossa,
on the lateral surface of the hemisphere (Fig. 90658); this fossa corresponds with the position of the corpus striatum,
and its floor is moulded to form the insula. The intimate connection which
exists between the cortex of the insula and the subjacent corpus striatum
prevents this part of the hemisphere wall from expanding at the same rate
as the portions which surround it. The neighboring parts of the hemisphere
therefore gradually grow over and cover in the insula, and constitute the
temporal, parietal, frontal, and orbital opercula of the adult brain. The
frontal and orbital opercula are the last to form, but by the end of the
first year after birth the insula is completely submerged by the approximation
of the opercula. The fissures separating the opposed margins of the opercula
constitute the composite lateral cerebral fissure.

FIG. 90658– Outer surface
of cerebral hemisphere of human embryo of about five months.
  If a section across the wall of the hemisphere about the sixth
week be examined microscopically it will be found to consist of a thin marginal
or reticular layer, a thick ependymal layer, and a thin intervening mantle
layer. Neuroblasts from the ependymal and mantle layers migrate into the
deep part of the marginal layer and form the cells of the cerebral cortex.
The nerve fibers which form the underlying white substance of the hemispheres
consist at first of outgrowths from the cells of

the corpora striata and thalami; later the fibers from the cells of the
cortex are added. Medullation of these fibers begins about the time of birth
and continues until puberty. A summary of the parts derived from the brain
vesicles is given in the following table:
Hind-brain or Rhombencephalon 1. Myelencephalon Medulla oblongata
Lower part of fourth ventricle.
2. Metencephalon Pons
Intermediate part of fourth ventricle.
3. Isthmus rhombencephali Anterior medullary velum
Brachia conjunctiva cerebelli.
Upper part of fourth ventricle.
Cerebral peduncles
Mid-brain or Mesencephalon………………………… Lamina quadrigemina
Cerebral aqueduct.
Fore-brain or Prosencephalon 1. Diencephalon Thalamus
Pars mamillaris hypothalami
Posterior part of third ventricle.
2. Telencephalon Anterior part of third ventricle
Pars optica hypothalami
Cerebral hemispheres
Lateral ventricles
Interventricular foramen.
The Cranial Nerves.—With the exception of the olfactory, optic,
and acoustic nerves, which will be especially considered, the cranial nerves
are developed in a similar manner to the spinal nerves (see page 735). The
sensory or afferent nerves are derived from the cells of the ganglion rudiments
of the neural crest. The central processes of these cells grow into the
brain and form the roots of the nerves, while the peripheral processes extend
outward and constitute their fibers of distribution (Fig. 90645). It has been seen, in considering the development of the medulla
oblongata (page 739), that the tractus solitarius (Fig. 90660), derived from the fibers which grow inward from the ganglion rudiments
of the glossopharyngeal and vagus nerves, is the homologue of the oval
in the cord which had

its origin in the posterior nerve roots. The motor or efferent nerves arise
as outgrowths of the neuroblasts situated in the basal laminæ of the
mid- and hindbrain. While, however, the spinal motor nerve roots arise in
one series from the basal lamina, the cranial motor nerves are grouped into
two sets, according as they spring from the medial or lateral parts of the
basal lamina. To the former set belong the oculomotor, trochlear, abducent,
and hypoglossal nerves; to the latter, the accessory and the motor fibers
of the trigeminal, facial, glossopharyngeal, vagus nerves (Figs.
90659, 90660).

FIG. 90659– Transverse
section of medulla oblongata of human embryo. X 32. (Kollmann)

FIG. 90660– Transverse
section of medulla oblongata of human embryo. (After His.)


Previous | Next