3. _Mesencephalon._--The mesencephalon or midbrain includes the =corpora
quadrigemina= (Fig. 141) and the =pedunculi cerebri= (Fig. 142). In the
primary midbrain there is a pronounced thickening of the walls
accompanied by a reduction of the central canal. The midbrain does not
thus become very large and is concealed in the dorsal view by the
cerebellum and cerebrum, though its floor appears in the ventral view
just craniad of the pons (Fig. 138, _g_). Its narrow canal is the
=aqueductus cerebri= (=aqueduct of Sylvius=) (Fig. 143, _j_). Its roof
forms the =corpora quadrigemina=, and its floor the =pedunculi cerebri=.
In a dorsal view (Fig. 141) the roof is seen to be marked by two pairs
of elevations, the =corpora quadrigemina= (_p_ and _q_). The cranial
pair (_q_) (known as the colliculi superiores) are circular in outline,
surrounded on all sides except the cranial one by a deep groove. From
the cranial side a tract of fibres (=brachium quadrigeminum superius=,
or arm of the cranial corpus) extends craniad and disappears beneath the
thalamus (_t_). Between the anterior or cranial corpora quadrigemina
lies the =pineal body= or epiphysis (corpus pineale) (Fig. 143, _y_;
Fig. 141, 2), a portion of the roof of the ’tween-brain. The caudal
corpora (Fig. 141, _p_) are larger than the cranial ones, and ovoid in
shape with the long axis vertical. They are united in the median line,
and the velum medullare anterius (_m_) stretches between their caudal
borders. The brachium of the caudal corpus quadrigeminum (brachium
quadrigeminum inferius) (_r_) extends craniad and disappears beneath a
considerable elevation, the =corpus geniculatum mediale= (_s_).
Crossing this brachium is seen a small tract of fibres which extends
ventrad, crosses the pedunculus cerebri, and reaches the medial border
of the latter. It is the =tractus transversus peduncularis= (Fig. 142,
_b_). Ventrad of the caudal corpus quadrigeminum is seen a triangular
area of oblique fibres which corresponds in position to the human
lemniscus.
The =pedunculi cerebri= (peduncles of the cerebrum) form the ventral
part of the midbrain. They appear in a ventral view of the entire brain
(Fig. 138) as two broad tracts of fibres (_g_) emerging from beneath the
pons and diverging from one another as they pass craniad, finally
disappearing beneath the cerebral hemispheres. Each is made up of many
fibre-bundles, which are apparent in surface view (Fig. 142). The
peduncles are separated by a small triangular space, which is marked by
a median longitudinal sulcus. In this space, just caudad of the
mammillary bodies, is a small area through which a number of
blood-vessels pass into the brain. This is known as the =posterior
perforated area= (or substance) (Fig. 142, _j_). The cerebral
peduncles (_a_) are crossed by the tractus transversus peduncularis
(_b_) (see above).
The third cranial nerve (Fig. 138, _III_) (=N. oculomotorius=) leaves
the brain at the medial border of the cerebral peduncle (_g_), just
caudad of the tractus transversus peduncularis.
The =aqueductus cerebri= (Fig. 143, _j_; Fig. 153, _d_) (or aqueduct of
Sylvius) is the continuation craniad of the fourth ventricle. It is a
narrow passage, one or two millimeters in diameter, lying dorsad of the
pedunculi cerebri and ventrad of the corpora quadrigemina.
[Illustration: FIG. 142.--VENTRAL SURFACE OF THE MIDBRAIN AND
’TWEEN-BRAIN, WITH THE PONS.
_a_, pedunculi cerebri; _b_, tractus transversus peduncularis; _c_,
corpus geniculatum mediale; _d_, optic tract; _e_, optic chiasma; _f_,
optic nerve; _g_, mammillary bodies; _h_, tuber cinereum; _i_, opening
for infundibulum (which has been removed); _j_, posterior perforated
area; _k_, pons. _III_, _V_, _VII_, _VIII_, third, fifth, seventh, and
eighth cranial nerves.]
C. PROSENCEPHALON.--The prosencephalon or primitive forebrain includes
the =diencephalon= or ’tween-brain and the =telencephalon= or cerebral
hemispheres.
4. _Diencephalon._--The diencephalon or ’tween-brain includes the
=thalami= and the other parts bounding the third ventricle. The
diencephalon is seen in entire brains only in ventral view (Fig. 138,
_c_, _d_, _e_, etc.)
The diencephalon may be considered as forming almost or quite the most
cranial portion of the median nervous tube,--the cranial wall of the
third ventricle (the =lamina terminalis=) (Fig. 143, _d_), ending in the
median line in the deep fissure between the hemispheres of the cerebrum.
Parts of the brain which extend farther craniad than this are lateral
portions, due to the forward growth of the lateral hemispheres. The two
hemispheres of the cerebrum may be considered as lateral outgrowths of
the central ’tween-brain; these outgrowths have extended dorsad,
laterad, craniad, and caudad, so as to cover almost completely the
’tween-brain.
In early stages the cerebral hemispheres are projections from the
_cranial end_ of the ’tween-brain, so that the plane of junction was
nearly transverse, the cranial end of the ’tween-brain joining the
caudal end of the hemispheres. With the increasing size and backward
growth of the latter, the attachment to the ’tween-brain has been
shifted from a cranial to nearly a lateral position, and at the same
time the originally lateral surface of the ’tween-brain has become
nearly caudal. This is shown in Fig. 141, the deep fissure at 1 marking
the line of attachment between the ’tween-brain and the hemispheres. The
dorsoventral plane of junction of ’tween-brain and hemispheres is (as
Fig. 141 shows) not wholly lateral, but oblique, passing from its
cranial end near the middle line caudolaterad.
A second peculiarity of the ’tween-brain lies in the thinness of its
roof. The roof is exceedingly thin and is so intimately connected with
the pia mater that they cannot be removed separately. The ventral thick
floor of the ’tween-brain is directly continuous with the similar floor
of the cerebrum; but where the roof of the ’tween-brain joins the roof
of the cerebrum along the oblique plane already indicated, the roof is
very thin and is intimately connected to the pia mater, and is at the
same time folded into the lateral ventricles to form, together with the
pia mater, the =choroid plexuses= of the lateral ventricles (Fig. 148,
_e_). When the pia mater is removed the thin roof of the brain along the
line of junction of the ’tween-brain and the cerebrum is brought away
and there _appears_ to be a direct communication between lateral
ventricles and the exterior.
We may now take up the parts of the diencephalon in detail.
a. =Thalamus= (Fig. 141, _t_).--The two thalami are large oblique
rounded ridges, forming the sides of the ’tween-brain. They lie just
craniad of the cranial corpora quadrigemina (_q_), but separated from
them by a broad groove, and are completely covered by the caudally
projecting part of the cerebral hemispheres. The medial larger end of
each thalamus is near the middle line; thence the thalamus extends
caudolaterad, and rises at its lateral extremity into a sharply rounded
projection, the =corpus geniculatum laterale= (_x_). From this
projection the thalamus is continued ventrad and then craniomediad as a
large band of fibres, the =optic tract= (=tractus opticus=) (Fig. 142,
_d_), which extends to the optic chiasma (_e_), where it passes into the
optic nerves (_f_). On the caudal border of the thalamus, near the
median line, is a very faint projection, the =pulvinar= (Fig. 141, _w_);
this lies just craniad of the lateral border of the cranial corpus
quadrigeminum (_q_). Just ventrad of the corpus geniculatum laterale
(_x_) is the prominent rounded =corpus geniculatum mediale= (_s_); this
is connected by a prominent ridge, the =brachium quadrigeminum inferius=
(_r_), with the caudal corpus quadrigeminum (_p_). In a similar manner
the =brachium quadrigeminum superius= passes from the cranial corpus
quadrigeminum (_q_) into the thalamus itself.
Between the two thalami there exists a groove, the =sulcus
hypothalamicus=. Over this groove lies the roof of the third ventricle,
forming the =choroid plexus= of the third ventricle (Fig. 143, _w_). The
medial surface of the two thalami are flat and extend directly ventrad,
forming part of the lateral boundary of the third ventricle (Fig. 143,
_h_). The two medial surfaces meet over a considerable area across the
narrow cavity of the third ventricle, and unite, forming the =massa
intermedia= (Fig. 143, _f_) or intermediate mass of the thalamus
(“middle commissure”). This connection of the thalami of the two sides
is thus not a primitive one, forming no part of the roof or floor of the
central cavity, but is a secondary connection due to a growing together
of a part of the two sides of the ventricle across its cavity. Along
the dorsal edge of the medial border of each thalamus passes a distinct
white strand, the =stria medullaris= (Fig. 141, _u_); the two striæ meet
in an arch caudad, lying beneath the pineal body (2).
The thalamus is separated craniolaterad by a groove (1) from the corpus
striatum (_y_), on the floor of the cerebral hemisphere.
_b._ The =roof= of the third ventricle is thin and united with the pia
mater, as already stated. The pia mater bears many blood-vessels, and
the two are folded into the groove between the optic thalami, forming
the =lamina chorioidea epithelialis=, or choroid plexus of the third
ventricle (Fig. 143, _w_). The roof is attached to the dorsomedial
borders of the thalami and becomes continuous with the choroid plexus of
the lateral ventricles (Fig. 148, _e_) at the craniolateral borders of
the thalami.
The =pineal body= (=corpus pineale=) or epiphysis (Fig. 141, 2; Fig.
143, _y_) is a small conical body which is formed as an outgrowth of the
caudal part of the roof of the third ventricle. It lies on the roof of
the brain between the two cranial corpora quadrigemina (Fig. 141, _q_).
It is hollow, containing an extension of the third ventricle. From its
craniolateral angles two white strands, the =habenulæ=, extend into the
striæ medullares (Fig. 141, _u_) of the thalami.
Just ventrad of the pineal body is a transverse band of white fibres,
lying in the caudal part of the roof of the third ventricle. This
interconnects the two thalami, and forms the =posterior commissure=
(Fig. 143, _z′_) (commissura posterior). From this commissure a thin
sheet of tissue extends to the pineal body.
_c._ The =floor= of the third ventricle appears in a ventral view of the
brain (Fig. 138, Fig. 142) as a somewhat diamond-shaped space craniad of
the pedunculi cerebri (Fig. 142, _a_) and bounded along its cranial
margin by the =optic tracts= (Fig. 142, _d_). The optic tracts come from
the thalami, as already described; they converge and unite to form the
optic chiasma (Fig. 142, _e_), from which the optic nerves (_f_)
diverge.
Immediately caudad of the optic chiasma lies a considerable rounded gray
elevation, the =tuber cinereum= (Fig. 142, _h_). This bears on its
ventral surface in the natural condition the =infundibulum= (Fig. 138,
_d_) with the =hypophysis= (Fig. 138, _e_); in cases where the two
latter structures have been removed (Fig. 142) the tuber cinereum (_h_)
bears a small longitudinal opening (_i_) for attachment of the
infundibulum. The infundibulum (Fig. 138, _d_) is a hollow extension of
the floor of the third ventricle, and is attached to the middle of the
ventral surface of the tuber cinereum. It bears at its ventral end the
hypophysis (pituitary body) (Fig. 138, _e_), a vascular non-nervous body
of unknown function. The hypophysis is lodged in the sella turcica of
the sphenoid bone.
At its caudal border the tuber cinereum bears two white elevations, the
=mammillary bodies= (corpora mammillaria) (Fig. 142, _g_).
[Illustration: FIG. 143.--LONGITUDINAL MEDIAN SECTION OF BRAIN.
_I_, olfactory bulb; _II_, cerebrum; _III_, section of cerebellum,
showing “arbor vitæ”; _IV_, medulla oblongata. _a_, tractus
olfactorius; _b_, substantia perforata anterior; _c_, anterior
commissure; _d_, lamina terminalis; _e_, optic chiasma; _f_, massa
intermedia of thalamus; _g′_, infundibulum; _g_, hypophysis; _h_,
third ventricle; _i_, mammillary body; _j_, aqueductus cerebri (or
aqueduct of Sylvius); _k_, pons; _l_, velum medullare anterius; _m_,
fourth ventricle; _n_, velum medullare posterius; _o_, continuation of
canal of spinal cord; _p_, corpus callosum (body); _q_, genu of corpus
callosum; _r_, rostrum; _s_, splenium; _t_, septum pellucidum; _u_,
fornix; _v_, pillars of fornix; _w_, choroid plexus of third
ventricle; _x_, stria medullaris; _y_, corpus pineale; _z_, cranial
corpus quadrigeminum; _z′_, posterior commissure. 1, sulcus falcialis;
2, sulcus cruciatus; 3, sulcus splenialis; 4, sulcus marginalis; 5,
sulcus supracallosalis (or callosalis).]
The =third ventricle= (Fig. 143, _h_; Fig. 141, _v_) is a very narrow
slit-like space, of considerable extent dorsoventrally, but less than a
millimeter in width; it lies between the medial ends of the thalami
(Fig. 141), and extends ventrad into the tuber cinereum, and thence into
the infundibulum (Fig. 143, _g′_). Dorsad it extends into the corpus
pineale (Fig. 143, _y_). The third ventricle communicates caudally near
its dorsal border with the aqueductus cerebri (_j_); craniolaterally
with the lateral ventricles, through the =interventricular foramen=
(foramen of Monroe). Its cranial boundary forms in the middle line a
thin plate, the =lamina terminalis= (_d_), which is, morphologically,
the cranial termination of the cerebrospinal axis; it lies at the bottom
of the deep fissure between the cerebral hemispheres. At the dorsal
border of the lamina terminalis is a strong transverse band of fibres
connecting the two sides of the brain; this is the =anterior commissure=
(_c_). The cavity of the third ventricle is much encroached upon by the
meeting and secondary union of the two thalami across the middle line,
forming the massa intermedia (_f_).
The boundaries of the third ventricle, in order, are as follows,
beginning dorsocraniad of the communication with the aqueductus cerebri:
the posterior commissure (Fig. 143, _z′_), the pineal body (_y_), the
choroid plexus of the third ventricle (_w_), the columns of the fornix
(_v_), the anterior commissure (_c_), the lamina terminalis (_d_), the
tuber cinereum with the infundibulum (_g′_), the substantia perforata
posterior, and the midbrain. The lateral boundaries are formed by the
thalami (Fig. 141, _t_).
5. _Telencephalon._--The telencephalon includes the two cerebral
hemispheres. The name =cerebrum= is also applied to this portion of the
brain; frequently, however, the name cerebrum is used as signifying the
entire mass of the brain craniad of the rhombencephalon,--therefore
including mesencephalon, diencephalon, and telencephalon. The term will
here be used as synonymous with telencephalon.
The =cerebral hemispheres= arise as two lateral outgrowths from the
cranial end of the primitive forebrain. They have undergone great
increase in size in the course of evolution, so as to form the larger
part of the brain; at the same time important modifications of structure
have taken place. In the original condition the medial faces of the two
hemispheres are not connected, the two being separate outgrowths of the
’tween-brain, and connected only through the latter. But secondary
connections have been formed across the fissure between the two
hemispheres, resulting in the production of the =corpus callosum= (Fig.
143, _p_; Fig. 147; Figs. 149-152, _a_), a broad transverse band of
white fibres connecting the two hemispheres dorsad of the ’tween-brain.
Ventrad of the corpus callosum another secondary union has resulted in
the production of the =fornix= (Fig. 143, _u_).
[Illustration: FIG. 144.--LATERAL SURFACE OF THE BRAIN.
_a_, olfactory bulb; _b_, cerebral hemisphere; _c_, cerebellum; _d_,
medulla oblongata; _e_, tractus olfactorius; _f_, lobus pyriformis or
tractus postrhinalis; _g_, pons; _h_, trapezium; _i_, pyramid; _j_,
area elliptica; _k_, area ovalis. _II_, N. opticus; _V_, N.
trigeminus; _VII_, N. facialis; _VIII_, N. acusticus; _IX_, N.
glossopharyngeus; _X_, N. vagus; _XI_, N. accessorius. 1. sulcus
præsylvius; 2, sulcus cruciatus; 3, sulcus ansatus; 4, sulcus
lateralis; 5, sulcus suprasylvius; 6, sulcus anterior; 7, sulcus
posterior; 8, sulcus rhinalis; 9. fissura Sylvii; 10. sulcus rhinalis
posterior.]
=External Features.=--With increase in size the mass of the cerebrum
shows externally a tendency to divide into three lobes, one craniad, the
=frontal= (Fig. 145, _A_); one caudoventrad, the =temporal= (_B_); and
one caudodorsad, the =occipital= (_C_). The two latter are not
distinctly marked off from one another. The limit between the temporal
and frontal lobes is marked by a short deep fissure, the =lateral
fissure= (=fissura cerebri lateralis=), or fissure of Sylvius (Fig.
144, 9; Fig. 145, _a_). Each lobe is thrown up into elevations or
=gyri=, which are separated by grooves or =sulci=; these are described
below. The homology of the cerebral gyri and sulci of the cat with those
of man is in most cases uncertain.
The sulci and gyri of the cerebral hemispheres may be described briefly
in their main features as follows: On the lateral surface of the
hemisphere (Fig. 145) the lateral fissure (fissura cerebri lateralis),
or fissure of Sylvius, separating temporal and frontal lobes, forms the
most convenient point of departure for an understanding of the fissures.
The fissure of Sylvius is short but deep, and is formed early during
development. In man it covers an area, the =insula=, at its bottom,
which may be seen by separating the sides of the fissure. In the cat the
insula is rudimentary and can be demonstrated only with difficulty.
The lateral surface of the hemisphere is marked by fissures which form
three concentric arches (_b_-_c_, _d_, and _e_), irregular and
incomplete, about the fissure of Sylvius. These arches all open ventrad.
The first arch (_b_-_c_) (that next to the fissure of Sylvius) lacks the
central part, the keystone, so that a complete arch is not formed, but
only the two sides of the arch, as two separate sulci (_b_ and _c_). The
one of these craniad of the fissure of Sylvius is the =sulcus anterior=
(_b_); the other is the =sulcus posterior= (_c_). In the dog this arch
is usually complete and the sulcus forming it is frequently known as the
ectosylvian sulcus (=sulcus ectosylvius=).
[Illustration: FIG. 145.--DIAGRAM OF THE SULCI AND GYRI ON THE LATERAL
SURFACE OF THE HEMISPHERE.
_A_, frontal lobe; _B_, temporal lobe; _C_, occipital lobe. _a_,
lateral fissure or fissure of Sylvius; _b_, sulcus anterior; _c_,
sulcus posterior (the broken line connecting the ends of _a_ and _b_
serves to show how the first arch is completed to form the ectosylvian
sulcus, as in the dog); _d_, sulcus suprasylvius; _e_, sulcus
lateralis; _f_, sulcus cruciatus; _g_, sulcus coronalis; _h_, sulcus
ansatus; _i_, sulcus rhinalis; _j_, sulcus præsylvius; _k_, sulcus
rhinalis posterior. 1, anterior Sylvian gyrus; 2, posterior Sylvian
gyrus; 3, gyrus ectosylvius; 4, gyrus suprasylvius; 5, gyrus
marginalis; 6, gyrus compositus posterior; 7, sigmoid gyrus; 8,
olfactory tract; 9, orbital gyrus; 10, lobus pyriformis; 11, olfactory
bulb.]
The gyri included between the fissure of Sylvius on the one hand and the
anterior and posterior sulci on the other are known as the =anterior=
(1) and =posterior= (2) =Sylvian gyri=.
The second arch is formed by the =suprasylvian sulcus= (=sulcus
suprasylvius=) (_d_). The gyrus between the anterior and posterior sulci
ventrad and the suprasylvian sulcus dorsad is the =ectosylvian gyrus=
(3) (because dorsad of the ectosylvian sulcus).
The third arched sulcus, next to the medial margin of the hemisphere, is
variable: it forms the =lateral sulcus= (_e_). The gyrus lying between
the suprasylvian and lateral sulci is the =suprasylvian gyrus= (4)
(because dorsad of the suprasylvian sulcus). The lateral sulcus is
sometimes broken into two (as in Fig. 137, _h_), the two ends
overlapping so as to leave a small gyrus between the two ends.
The gyrus dorsomediad of the lateral sulcus, extending onto the medial
surface of the hemisphere as far as the splenial sulcus, is the
=marginal gyrus= (5).
The four sets of gyri included between these three sets of arched sulci
and the fissure of Sylvius unite caudad and craniad to form single gyri.
The caudal one is the =gyrus compositus posterior= (6). The cranial one
(less regular than the caudal one) is the =sigmoid gyrus= (7). The
latter curves about the cruciate sulcus (_f_) (described below).
In the cranial part of the lateral surface of the hemisphere are certain
sulci and gyri which do not belong to the system above described. The
=cruciate sulcus= (_f_) is a short transverse sulcus passing from the
lateral surface of the hemisphere onto its medial surface, where it
extends caudad about a centimeter. Curving around the lateral end of
this, separated from it by part of the sigmoid gyrus, is the short
=coronal sulcus= (_g_). Connected usually with the end of the lateral
sulcus (_e_) and running nearly parallel with the cruciate sulcus is the
short =sulcus ansatus= (_h_).
At its ventral end the fissure of Sylvius (_a_) joins a longitudinal
groove passing craniad and caudad from the point of junction. That part
which passes craniad is the =sulcus rhinalis= (_i_); it forms the
dorsolateral boundary of the olfactory tract (8). Passing dorsocraniad
from the sulcus rhinalis is a sulcus which separates a very small
cranial lobe from the rest of the hemisphere; this is the =supraorbital=
or =præsylvian sulcus= (_j_). The portion of the hemisphere craniad of
this sulcus is the =orbital gyrus= (9).
The sulcus rhinalis (_i_) is continuous caudad with the =sulcus rhinalis
posterior= (or =sulcus postrhinalis=) (_k_). This extends caudad onto
that portion of the hemisphere which faces the cerebellum. It forms the
lateral boundary of a large elongated oval lobe lying at the side of the
ventral floor of the midbrain and ’tween-brain. This lobe has been
called =tractus postrhinalis= or =lobus pyriformis= (Fig. 145, 10; Fig.
138, _f_).
In addition to the sulci and gyri above mentioned, any given specimen
will usually show a number of small inconstant sulci and gyri in various
regions; these inconstant structures will not be here described.
[Illustration: FIG. 146.--DIAGRAM OF THE SULCI AND GYRI ON THE MEDIAL
SURFACE OF THE HEMISPHERE.
_a_, sulcus splenialis; _b_, sulcus marginalis; _c_, sulcus cruciatus;
_d_, sulcus falcialis; _e_, sulcus supracallosalis; _f_, sulcus
rhinalis posterior; _g_, hippocampal sulcus; _h_, corpus callosum. 1,
gyrus marginalis; 2, gyrus fornicatus; 3, gyrus compositus posterior.]
On the medial surface of the hemisphere (Fig. 146), the following
arrangement of sulci and gyri seems to be typical. Some distance from
the dorsal margin a long sulcus runs parallel with the margin; this is
the =sulcus splenialis= (_a_). The =marginal gyrus= (1) is dorsad of the
splenial sulcus, passing onto the lateral surface of the hemisphere,
where it is bounded by the lateral sulcus. The splenial sulcus extends
onto the caudal surface of the hemisphere. A shallow =sulcus marginalis=
(_b_) occurs frequently between the sulcus splenialis and the dorsal
margin of the hemisphere. Cranioventrad or the cranial end of the
splenial sulcus is that portion of the =sulcus cruciatus= (_c_) that
lies on the medial surface of the hemisphere. Ventrad of this is a short
shallow furrow which has been called the =sulcus falcialis= (_d_).
Immediately dorsad of the corpus callosum (_h_), separating its dorsal
surface from a part of the hemisphere, is the =supracallosal= or
=callosal= sulcus (_e_). Between the supracallosal (_e_) and splenial
(_a_) sulci is the =gyrus fornicatus= (2).
On the caudal surface of the hemisphere (that facing the cerebellum)
appear the ends of the sulcus splenialis (_a_) and the sulcus rhinalis
posterior (_f_). Hidden in the natural condition by the thalami and
optic tract is the broad =hippocampal= sulcus (_g_), marking externally
the course of the hippocampus.
A further extension of the surface of the cerebrum has taken place in
connection with the sense of smell. A mass of gray matter, the
=olfactory bulb= (Fig. 144, _a_), reckoned as a part of the cerebrum, is
separated from the latter and lies against the cribriform plate of the
ethmoid bone. From it the olfactory fibres pass through the perforations
of the plate to the olfactory mucous membrane. The olfactory bulb
contains a cavity, a part of the lateral ventricle.
The bulb lies against the ventral surface of the frontal lobe and
projects craniad of it. It is connected to the cerebrum by a tract of
fibres, the =olfactory tract= (Fig. 138, _a_), which is divisible into
two roots, medial and lateral. The medial root comes from the medial
surface of the frontal lobe, where it is continuous with a tract
extending to the cranial end of the corpus callosum. The lateral root is
traceable from an elevated gyrus-like portion of the cerebrum which lies
at the side of the infundibulum and is known as the lobus pyriformis or
tractus postrhinalis (Fig. 138, _f_). The lateral root is divisible into
a medial white strand and a lateral gray strand.
That part of the brain comprising the olfactory bulb and the parts
intimately related to it are frequently included under the term
=rhinencephalon=.
In the triangular area between the two olfactory tracts and craniad of
the optic chiasma appears a mass of gray matter, subdivided by a
longitudinal fissure. This possesses numerous openings through which
blood-vessels pass to the brain substance, and is thence known as the
=anterior perforated substance= (=substantia perforata anterior=) (Fig.
138, _b_).
=Internal Structures of the Cerebrum.=--The cavity of each of the
cerebral hemispheres is known as a =lateral ventricle=. The two lateral
ventricles constitute the _first_ and _second_ of the ventricles of the
brain, whence the application of the names _third_ and _fourth_
ventricles to the cavities of the ’tween-brain and hindbrain. The
lateral ventricles do not grow at the same rate as the walls of the
hemispheres, so that they remain comparatively small. The cavity of
each ventricle is further reduced in size by the development on its
floor of a large ridge-like thickening, the =corpus striatum= (Fig. 148,
_f_).
The dorsal wall or roof of the lateral ventricle joins the thin roof of
the third ventricle on each side along an oblique curved line (Fig. 141,
1) which follows the cranial or lateral border of the thalamus (Fig.
141, _t_). Along this line the thin roof of the brain is folded in
together with the pia mater to form the =choroid plexus= of the =lateral
ventricles= (=lamina chorioidea epithelialis=) (Fig. 148, _e_). When
this is pulled out there is left a fissure, the “great transverse
fissure of the cerebrum” (Fig. 141, 1). Just dorsad of the groove
between the thalami there runs a tract of white fibres known as the
=fornix= (Fig. 148, _a_). The two halves of the fornix separate at the
cranial ends of the thalami and pass ventrad, forming thus the =pillars=
of the =fornix= (Fig. 148, _b_; Fig. 143, _v_). Dorsad of the cranial
end of the fornix the =corpus callosum= (Fig. 143, _p_) passes from one
hemisphere to the other. Caudad of the pillars of the fornix, the
lateral ventricles communicate with the third ventricle by way of the
=interventricular foramen= (foramen of Monroe).
The parts of the cerebrum may now be taken up in detail.
[Illustration: FIG. 147.--THE CORPUS CALLOSUM.
The dorsal portion of the hemispheres has been dissected away, then
sliced off, showing the plate of transverse fibres forming the corpus
callosum. _a_, splenium; _b_, genu; _c_, line marking the medial edge
of the hemispheres; _d_, line marking the lateral boundary of the
supracallosal sulcus; laterad of this line the corpus callosum lies in
the substance of the hemispheres, which have been dissected away; _e_,
line marking medial limit of cut surface.]
The =corpus callosum= (Fig. 147; Fig. 143, _p_; Figs. 149-152, _a_) is a
broad transverse band of fibres forming a secondary connection between
the medial walls of the two hemispheres, dorsad of the roof of the third
ventricle. Its outer surface (Fig. 147) is exposed at the bottom of the
fissure which separates the hemispheres. On each side it passes laterad,
forming the roof of the lateral ventricle. Its cranial part lies dorsad
of the corpus striatum, and its caudal part dorsad of the thalamus.
Laterally its fibres radiate into the substance of the hemispheres. At
its cranial end the corpus callosum bends ventrad and then caudad (Fig.
143). The part which turns to pass ventrad is the =genu= (Fig. 143, _q_)
or knee, while the part which projects caudad is the =rostrum= (_r_).
The caudal border of the corpus callosum is also thickened and turned
ventrad and is called the =splenium= (Fig. 143, _s_); it lies dorsad of
the cranial corpora quadrigemina (_z_). The caudal half of the ventral
surface of the corpus callosum is united with the fornix (Fig. 143,
_u_).
[Illustration: FIG. 148.--FORNIX, HIPPOCAMPUS, AND CORPUS STRIATUM.
The dorsal portion of the hemispheres has been dissected away and the
corpus callosum removed. _a_, fornix; _b_, columns or pillars of the
fornix; _c_, crura of the fornix; _d_, hippocampus; _e_, choroid
plexus of the lateral ventricles overlying the fimbria (the choroid
plexus shows an artery); _f_, corpus striatum; _g_, corpora
quadrigemina; _h_, position of the interventricular foramen (foramen
of Monroe).]
The =fornix= (Fig. 148, _a_; Fig. 143, _u_, _v_; Figs. 150-152, _b_)
consists of an arched tract of longitudinal fibres near the medial
border of each hemisphere, ventrad of the corpus callosum. Each tract
begins in the mammillary bodies, and passes dorsad, the two converging
until they run side by side, forming a cylindrical dorsoventral bundle
known as the columns or pillars of the fornix (Fig. 143, _v_; Fig. 150,
_e_), which cross the anterior commissure (Fig. 143, _c_; Fig. 150, _f_)
caudad of the latter. Caudad of the pillars of the fornix, between these
and the thalamus, lies on each side the interventricular foramen or
foramen of Monroe, a small opening which connects the lateral ventricles
with the third ventricle. Dorsad of the foramen the fornix turns caudad,
the two fibre-tracts of each hemisphere lying side by side and closely
connected (Fig. 148, _a_), forming thus another secondary union between
the medial surfaces of the two hemispheres. This portion of the fornix
is the =corpus= or =body= (Fig. 148, _a_); it lies dorsad of the roof of
the third ventricle and passes to the splenium (Fig. 143, _s_) of the
corpus callosum, and its dorsal surface unites with the ventral surface
of the latter (Fig. 143). Caudad the two halves of the fornix diverge,
forming the =crura= of the fornix (Fig. 148, _c_); these and the body
are continuous laterally with the =hippocampus= (Fig. 148, _d_) and the
=fimbria= (Fig. 148, beneath _e_).
The =anterior commissure= (Fig. 143, _c_; Fig. 150, _f_) is a transverse
band of white fibres which stretches from one hemisphere to the other
about half way between the interventricular foramen or foramen of Monroe
and the floor of the third ventricle, and just craniad of the pillars of
the fornix. This tract of fibres is developed in the original wall of
the third ventricle, so that it does not form a _secondary_ connection
between the halves of the cerebrum, as do the fornix and corpus
callosum. It lies dorsad of the lamina terminalis (Fig. 143, _d_) and
is continuous with it.
The =septum pellucidum= (Fig. 143, _t_) is a vertical partition which
separates the lateral ventricles and fills the interval between the
corpus callosum dorsad and the fornix ventrad. It is triangular and
translucent. It is formed from the medial walls of the two hemispheres
and therefore is made up of two laminæ which embrace between them a
space which originally was a part of the fissure separating the
hemispheres. This space has been called the fifth ventricle.
[Illustration: FIG. 149.--CAUDAL SURFACE OF TRANSVERSE SECTION OF
BRAIN THROUGH THE GENU OF THE CORPUS CALLOSUM.
FIG. 150.--CAUDAL SURFACE OF TRANSVERSE SECTION OF BRAIN THROUGH THE
OPTIC CHIASMA AND THE REGION OF THE INTERVENTRICULAR FORAMEN (FORAMEN
OF MONROE).
Fig. 149.--Gray matter dotted; white matter with a few lines. 1, right
hemisphere; 2, left hemisphere. _a_, corpus callosum, section through
genu and rostrum; _b_, cavity of lateral ventricles.
Fig. 150.--1, right hemisphere; 2, left hemisphere. _a_, corpus
callosum; _b_, fornix (cut); _c_, cavity of lateral ventricles; _d_,
fimbria (_d′_, cut surface; _d_, caudal uncut surface of the part that
curves ventrad); _e_, pillars of fornix; _f_, anterior commissure;
_g_, parts of third ventricle; _h_, optic chiasma; _i_, parts of
corpus striatum.]
[Illustration: FIG. 151.--CAUDAL SURFACE OF TRANSVERSE SECTION OF THE
BRAIN THROUGH THE TUBER CINEREUM AND INFUNDIBULUM, CRANIAD OF THE
HYPOPHYSIS.
_a_, corpus callosum; _b_, fornix (continuous at the sides with the
hippocampus); _c_, cavity of lateral ventricles; _d_, hippocampus;
_e_, choroid plexus of the third ventricle (roof of the diencephalon);
_f_, dorsal part of cavity of the third ventricle; _g_, section of
thalamus; _h_, massa intermedia of the thalami, dividing the third
ventricle into a dorsal (_f_) and a ventral (_i_) portion; _i_,
ventral part of third ventricle; _j_, part of optic tracts; _k_, thin
wall of infundibulum, with part of cavity of third ventricle; _l_,
line separating roof of thalamus (diencephalon) from floor of
hemispheres (telencephalon).]
The =hippocampus= (Fig. 148, _d_; Fig. 152, _d_) is an elongated rounded
elevation of the floor of the lateral ventricle. It is continuous mediad
with the fornix (Fig. 148, _a_) and extends thence along the inferior
horn of the lateral ventricle to its end in the temporal lobe. It is
somewhat narrower at its lateral end. It is thus curved into a
semicircle in conformity with the inferior horn of the ventricle. Its
dorsal surface is convex and looks into the lateral ventricle; its
ventral surface is concave and rests upon the thalamus and the optic
tract. Along the craniolateral edge of the hippocampus is a broad,
clearly marked fibre-tract, the =fimbria=; this runs parallel to the
choroid plexus of the lateral ventricle and beneath it, and is
continuous at its medial end with the fornix.
The =corpus striatum= (Fig. 148, _f_; Fig. 141, _y_; Fig. 150, _i_) is a
fusiform or ovoid elevation of the floor of the lateral ventricle. One
of its narrow ends lies about opposite the middle of the thalamus (Fig.
141, _t_) and it extends thence obliquely mediad nearly parallel with
the thalamus. In the groove between it and the thalamus is the choroid
plexus of the lateral ventricles (Fig. 148, _e_). Internally the corpus
striatum is made up of several layers of different texture (Fig. 150,
_i_), whence its name.
[Illustration: FIG. 152.--CAUDAL SURFACE OF TRANSVERSE SECTION OF THE
BRAIN THROUGH THE PINEAL BODY AND THALAMI.
FIG. 153.--CAUDAL SURFACE OF TRANSVERSE SECTION OF THE BRAIN THROUGH
THE CRANIAL PAIR OF CORPORA QUADRIGEMINA.
Fig. 152.--_a_, corpus callosum; _b_, fornix; _c_, inferior horn of
lateral ventricles; _d_, hippocampus; _e_, thalamus; _f_, third
ventricle; _g_, pineal body; _h_, space between the floor of the
hemisphere (hippocampus) and roof of the ’tween-brain (thalamus).
Fig. 153.--_a_, cranial corpora quadrigemina; _b_, hippocampus; _c_,
part of inferior horn of lateral ventricle; _d_, aqueductus cerebri
(aqueduct of Sylvius); _e_, space between the outer surface of the
midbrain and the lower surface of the hemisphere; _f_, cross-section
of midbrain; _g_, pedunculi cerebri.]
The =choroid plexus= of the lateral ventricles (=lamina chorioidea
epithelialis=) (Fig. 148, _e_).--The line of junction of the roof of the
third ventricle (i.e., the choroid plexus of the third ventricle) and
the cerebral hemispheres is along the lateral (or cranial) border of the
thalamus (Fig. 141, _t_). Along this line (Fig. 141, 1) the brain-wall
remains very thin and becomes closely associated with the pia mater, so
that on the removal of the pia there is left the “transverse fissure”
which leads into the lateral ventricle. In an early stage, before the
cerebrum has grown far caudad, the fissure is exposed in dorsal view,
but after the hemispheres have covered the thalami the fissure appears
to be in the lower face of the hemisphere. Through this fissure the pia
mater extends upward into the lateral ventricles as a fold separated
from the ventricle by the thin brain-wall. This fold is vascular and is
known as the choroid plexus of the lateral ventricle (Fig. 148, _e_). It
is a fringe-like fold lying on the fimbria and extending from the
foramen of Monroe (at _h_) nearly to the end of the inferior horn of the
lateral ventricle.
The =lateral ventricles= are the extensions into the hemispheres of the
originally unpaired cavity of the forebrain. Each consists of a portion
parallel to the basis cranii and lying at the side of the septum
pellucidum between the corpus callosum dorsad and the corpus striatum,
hippocampus, and fornix ventrad, and of two horns, an =anterior= and an
=inferior= horn. The inferior horn (Fig. 152, _c_) is a narrow cleft,
crescent-shaped in cross-section, which follows the dorsal surface of
the hippocampus (Fig. 148, _d_; Fig. 152, _d_) along the temporal lobe
to its end almost against the tuber cinereum. It thus passes first
caudad and then ventrad and finally mediad. The choroid plexus and
hippocampus project into it from its floor. The =anterior= horn extends
ventrad and then slightly caudad in the frontal lobe, following the
corpus striatum (Fig. 148, _f_), on which it lies.
The =interventricular foramen= (or =foramen of Monroe=).--If the choroid
plexus of the lateral ventricle (Fig. 148, _e_) is followed to its
medial end, it is found to pass through a foramen (at _h_) in which it
becomes continuous with the opposite plexus or roof of the third
ventricle. This foramen leads from one lateral ventricle to the other
and is connected ventrally by a median opening with the third ventricle.
It is thus Y-shaped or T-shaped and is the interventricular foramen (or
foramen of Monroe). The foramen lies opposite the middle of the corpus
striatum and caudad of the pillars of the fornix.
=Membranes of the Brain.=--The membranes of the brain are the =dura
mater=, the =arachnoid=, and the =pia mater=.
The dura mater is a strong fibrous membrane lining the cranial cavity
and covering the brain. It is strongly attached to the projections of
the base of the skull and to the tentorium. It dips between the cerebral
hemispheres and olfactory bulbs, forming the =falx cerebri=. It likewise
dips between the cerebrum and the cerebellum, covering both surfaces of
the bony tentorium. The dura mater does not pass into the fissures or
sulci of the surface of the brain. It becomes continuous with the sheath
of the cranial nerves as they leave the skull. It is continuous with the
dura mater of the spinal cord.
The arachnoid and pia mater are essentially similar to the same
structures on the spinal cord. The pia mater dips into the fissures and
sulci of the brain.