Chapter V: Muscles of the Leg and Foot (18)

In the medulla the fiber tracts of the spinal cord either terminate in
the nuclei of the gray matter or undergo changes in their relative
position, and new tracts appear. The gray matter is highly modified
and forms masses which have no homologues in the cord. The central
canal of the closed part of the medulla is surrounded by a thicker
layer of gray matter than is the case in the spinal cord. As the
medulla opens out this gray matter is naturally spread in the floor of
the fourth ventricle. The dorsal horns of gray matter become wide,
spread apart, and are broken up to a great extent in the formatio
reticularis. Two elongated masses of cells appear above the central
gray matter; these are the =nucleus gracilis= and =nucleus cuneatus=,
and in them the fasciculi of like name gradually end. In front of
these are the =terminal nuclei= of the afferent or sensory cranial
nerves and of the sensory portions of the mixed nerves. The ventral
horns are succeeded by the nuclei of =origin= of the efferent or motor
cranial nerves and the motor root-fibers of the mixed nerves. Of the
twelve pairs of cranial nerves, the last eight are connected with
nuclei in the medulla and pons. The =posterior olivary nucleus=
(Nucleus olivaris caudalis) is a conspicuous gray mass which lies
above the pyramid on each side. On cross-sections it appears as a wavy
layer of gray matter which is folded on itself so as to inclose a mass
of white matter. It is about 2 cm. long in the horse, but is smaller
in circumference than in man and hence does not cause any very
distinct external enlargement (olivary eminence), as in the latter. It
is connected with the opposite side of the cerebellum by the
cerebello-olivary fibers. At its inner side are two small accessory
olivary nuclei. The =pyramidal tracts=, which are small in ungulates,
send most of their fibers across to the opposite side in the posterior
part of the medulla, forming the =pyramidal decussation=. Some fibers,
however, continue in the ventral column of the same side of the cord,
and others are connected with the nuclei of the motor nerve-roots.
From the nucleus gracilis and nucleus cuneatus fibers arise which are
traceable forward to the thalamus. These are the =internal arcuate
fibers= (Fibræ arcuatæ internæ), which curve across the median plane
below the central gray matter and form with those of the opposite side
the =decussation of the fillet= (Decussatio lemniscorum). Beyond the
decussation the fibers form an important longitudinal tract which
extends forward in the mid-brain. This is the =fillet= or =lemniscus=,
the chief continuation of the sensory conducting path from the dorsal
roots of the spinal nerves. The =external arcuate fibers=, some of
which were seen on the lateral aspect of the medulla, have a similar
origin. Part of them (Fibræ dorsales) pass directly to the dorsal
aspect of the restiform body of the same side; others (Fibræ
ventrales) cross to the opposite side, descend close to the ventral
fissure, and then curve upward and forward to the restiform body. The
decussation of the arcuate fibers forms the distinct median =raphé=
seen on cross-sections of the medulla anterior to the pyramidal
decussation. The restiform body, situated dorso-laterally, contains,
in addition to the arcuate fibers, the =cerebello-olivary fasciculus=
before mentioned, and the =cerebellospinal fasciculus= or direct
cerebellar tract. The =dorsal longitudinal fasciculus= corresponds to
the ventral ground-bundle of the spinal cord, displaced dorsally by
the decussation of the pyramids and fillet. In the posterior part of
the medulla it is not marked off from the fillet, along the dorsal
edge of which it lies. From the level of the hypoglossal nucleus
forward it is distinct and can be traced as a conspicuous tract in the
ventral margin of the gray matter of the floor of the fourth ventricle
and of the central gray matter of the mid-brain. Below the restiform
body and related externally to the external arcuate fibers, there is a
considerable bundle of longitudinal fibers, the =spinal root= (Tractus
spinalis) =of the fifth nerve=; internal to it is the terminal nucleus
of the sensory root of the nerve.

FIG. 501.—BRAIN OF HORSE, LEFT LATERAL VIEW.

The larger arteries are shown. The nerve-roots are indicated by Roman
numerals. _P.c._, Chorioid plexus of fourth ventricle; _B.p._,
middle peduncle of cerebellum; _P_, pons; _H_, pituitary body;
_L.p._, pyriform lobe; _A.p._, trigonum olfactorium; _B.o._,
olfactory bulb; _S.r.a._, sulcus rhinalis anterior; _S.r.p._, sulcus
rhinalis posterior; _F.l._, lateral fissure (of Sylvius); _F.l.a._,
anterior branch of lateral fissure; _S.p._, presylvian fissure;
_S.p.n._, anterior presylvian fissure; _S.s._, suprasylvian fissure;
_S.e._, ectomarginal fissure.
]

THE PONS

FIG. 502.—CROSS-SECTION OF MEDULLA OBLONGATA OF HORSE, PASSING THROUGH
FACIAL NUCLEUS.

_Cr_, Corpus restiforme; _D_, dorsal longitudinal fasciculus; _Fa_,
ascending part of facial nerve; _L_, fillet; _N7_, nucleus of facial
nerve; _N8_, triangular nucleus of vestibular root of eighth nerve;
_N8′_, spinal root of eighth nerve; _Py_, pyramid; _Ra_, raphé;
_R7_, radicular part of facial nerve; _R8_, vestibular root of
eighth nerve; _Sg_, substantia gelatinosa; _Ta_, posterior end of
tuberculum acusticum; _V_, spinal root of trigeminus.
(Ellenberger-Baum, Anat. d. Haustiere.)
]

FIG. 503.—CROSS-SECTION OF MEDULLA OBLONGATA OF HORSE; SECTION PASSES
THROUGH MIDDLE OF CORPUS TRAPEZOIDEUM.

_Cr_, Corpus restiforme; _D_, dorsal longitudinal bundle; _Fa_,
ascending part of facial nerve; _Fa′_, emergent or descending part
of facial nerve; _R7_, root of facial nerve; _L_, fillet; _M_,
central white matter of cerebellum; _N6_, abducens nucleus; _N8_,
triangular nucleus; _N8′_, nucleus of Deiters; _N8″_, tuberculum
acusticum; _Oo_, anterior olive; _Py_, pyramid; _R6_, root of
abducens nerve; _R7_, root of facial nerve; _Rc_, cochlear nerve;
_Rv_, vestibular nerve; _Sg_, substantia gelatinosa; _T_, corpus
trapezoideum; _V_, spinal root of trigeminus. (Ellenberger-Baum,
Anat. d. Haustiere.)
]

The =pons= (Varolii) is that part of the brain stem which lies between the medulla and the cerebral peduncles; it is marked off from these ventrally by anterior and posterior grooves. Viewed from below it is elongated transversely, convex in both directions, and presents a wide shallow median groove (Sulcus basilaris), which lodges the basilar artery. Laterally a large part of its mass curves upward and backward into the base of the cerebellum, forming the =middle cerebellar peduncle= (Brachium pontis). The superficial origin of the trigeminal (fifth) nerve is at the lateral limit of the ventral surface. Transverse striations indicate the course of its superficial (ventral) fibers which connect the two sides of the cerebellum. The dorsal surface is blended on either side with the overlying anterior peduncles of the cerebellum; the central free portion forms the anterior part of the floor of the fourth ventricle, and will be considered in the account of that cavity.

On cross-section the pons is seen to be composed of dorsal and ventral
parts. The =dorsal part= (Pars dorsalis pontis) consists superficially
of a layer of gray matter covered by the ependyma of the fourth
ventricle. Beneath this the median raphé of the medulla is continued
into the pons, dividing it into similar halves. In the anterior part
of the pons the fillet divides into an inner and outer part, the
=medial= and =lateral fillets= (Lemniscus medialis, lateralis); the
latter arches outward to reach the outer side of the anterior
cerebellar peduncle. The =dorsal longitudinal fasciculus= becomes
sharply defined into a round bundle which lies close to the raphé
under the gray matter of the floor of the fourth ventricle. In
cross-section the =formatio reticularis= forms a large area below the
superficial gray matter and the longitudinal bundles. Dorso-laterally
is the large rounded section of the =anterior cerebellar peduncle=.
Lower down is a large bundle, the =sensory root of the fifth nerve=.
In front of this is the =motor nucleus= of the same nerve, external to
which is its =motor root=. The =ventral part= of the pons (Pars
basilaris pontis) is composed of transverse and longitudinal fibers,
and a large amount of gray matter which is broken up into small masses
(Nuclei pontis) by the intersection of the fibers. The =transverse
fibers= are gathered laterally into a compact mass which turns upward
and backward and enters the central white matter of the cerebellum,
forming the =middle cerebellar peduncle=. Centrally the fibers are
arranged in bundles which intercross. The transverse fibers are
chiefly of two kinds. Some arise from the Purkinje cells of the
cerebellar cortex and pass either to the opposite side of the
cerebellum or turn at the raphé and run forward and backward in the
brain stem. Others are axones of cells of the nuclei pontis, and pass
to the hemispheres of the cerebellum. The corpus trapezoideum is
mainly the central continuation of the cochlear division of the
auditory nerve. Above it is the small =anterior olivary nucleus=
(Nucleus olivaris nasalis). The =longitudinal fibers= of the ventral
part of the pons consist chiefly of the =cerebrospinal= or =pyramidal
fasciculi=. These come from the ventral part (pes) of the cerebral
peduncles and are situated laterally, interspersed among the deep
transverse fibers in the anterior part of the pons. Toward the
posterior part the bundles incline toward the median plane and become
collected into a compact mass which appears superficially at the
posterior border. Many fibers come from the cerebral cortex and
terminate in the nuclei of the gray matter of the pons; they may be
designated =corticopontile fibers=.

THE CEREBELLUM

The =cerebellum= is situated in the posterior fossa of the cranium, and is separated from the cerebral hemispheres by the transverse fissure and the tentorium cerebelli which occupies it. It overlies the pons and the greater part of the medulla, from which it is separated by the fourth ventricle. Its average weight is about two ounces (ca. 60 gm.) or about 9 per cent. of the weight of the entire brain. Its shape is approximately globular but very irregular. It is somewhat compressed dorso-ventrally and its transverse diameter is the greatest.

The =anterior surface= faces upward and forward and is covered partially by the tentorium cerebelli. The =posterior surface= is almost vertical. The =ventral surface= or =base= lies over the fourth ventricle, and is connected by three pairs of peduncles with the medulla, pons, and mid-brain.

It is customary to recognize three gross divisions of the cerebellum, viz., the median vermis and two lateral hemispheres. The =vermis= (cerebelli) is curved in a circular manner so that its two extremities are close together or even in contact on the ventral surface. The anterior extremity is termed the =lingula=; it lies between the cerebellar peduncles and gives attachment to the =anterior medullary velum=, a thin lamina which forms the anterior part of the roof of the fourth ventricle. The posterior extremity, the =nodulus=, gives attachment to the =posterior medullary velum= which covers the posterior recess of the fourth ventricle. The =hemispheres= (Hemisphæria cerebelli) are clearly separated from the vermis by two deep paramedian fissures. They lie in the lateral depressions of the cerebellar compartment of the cranium.

In tracing the fissures from behind forward it will be noticed that
they are nearly sagittal as far as the anterior surface, where they
diverge widely, so that the vermis forms all of the fore part of the
cerebellum.

The surface of the cerebellum is further cut up into numerous =gyri= or =folia= by narrow and relatively deep =sulci=, many of which approach a transverse direction. Certain of the sulci are more pronounced than the others, and by means of them it is possible to define groups of gyri. Such groups are termed =lobes=, and have received specific names, derived chiefly from the systematic descriptions of the human cerebellum.

The lobes of the vermis are readily distinguished on a median section.
Enumerated from the anterior to the posterior extremity they are: (1)
lingula, (2) lobus centralis, (3) lobus ascendens, (4) lobus culminis,
(5) lobus clivi, (6) tuber vermis, (7) pyramis, (8) uvula, (9)
nodulus. Each hemisphere is cut into laterally by two sulci which mark
off two sagittal discoid masses, termed by Ziehen tabulations. The
external tabulation consists of four or five lobules, the lowest of
which is regarded as the flocculus. The inner part of the hemisphere
is divided into three or four lobes. In the absence of a satisfactory
morphological basis it seems undesirable to deal with the lobation of
the cerebellum in further detail.

The =cerebellar peduncles=, three on each side, join the central white matter of the cerebellum at the base. The =posterior peduncle= is the restiform body of the medulla, a large rounded tract derived from the lateral and ventral columns of the cord. Near the middle of the medulla it inclines outward, forms the lateral wall of the fourth ventricle, and ends by entering the central white matter of the cerebellum. The =middle peduncle= is formed, as previously seen, by the brachium pontis. The =anterior peduncles= (Brachia conjunctiva) pass forward on either side on the dorsal surface of the pons, forming the lateral boundary of the fore part of the fourth ventricle. They disappear under the corpora quadrigemina into the substance of the mid-brain. At the point of disappearance the =trochlear= (fourth) =nerve= emerges from the mid-brain. In some cases two or three bundles of fibers (Fila lateralia pontis) arise in the angle between the middle and anterior peduncle, curve obliquely forward and downward over the outer aspect of the latter, and spread out on the ventral face of the cerebral peduncle just in front of the pons.

On sagittal section the cerebellum is seen to consist of a layer of =cortical gray matter= (Substantia corticalis) and the =medullary white matter=. The white matter consists of a large =basal mass= (Corpus medullare), which is joined by the peduncles, and gives off primary laminæ to the lobules; from these secondary and tertiary laminæ arise, the latter entering the gyri. The arrangement on sagittal section is tree-like, hence the classical term “arbor vitæ” which is applied to it. The central gray matter consists of groups of cells which form small =nuclei= embedded in the central white substance.

As noted above the central gray matter does not form a large nucleus,
the corpus dentatum, which is so conspicuous an object on sagittal
sections of the cerebellar hemisphere in man.

The principal connections established by the peduncular fibers of the
cerebellum are as follows: The posterior peduncle (Corpus restiforme)
is composed of afferent and efferent fibers which connect the
cerebellum with the medulla and spinal cord. The =cerebellospinal
fasciculus= or =direct cerebellar tract=, which arises from the cells
of the nucleus dorsalis (Clarke’s column) of the cord, ends in the
cortex of the vermis; many of its fibers cross to the opposite side.
Numerous =arcuate fibers= from the nucleus gracilis and nucleus
cuneatus of the same and opposite sides establish connections with
=cells= of the cerebellar cortex. =Olivocerebellar= fibers (chiefly
afferent) connect with the olivary nucleus of the same and of the
opposite side of the medulla oblongata. The =nucleo-cerebellar
fasciculus= comprises fibers derived from the nuclei of the fifth,
eighth, and tenth cranial nerves (Edinger). The =descending
cerebellospinal fasciculus= consists of fibers which terminate in
relation with cells of the ventral horns of the spinal cord. The chief
facts concerning the middle peduncle have been mentioned in the
description of the pons. The anterior peduncle is essentially an
efferent tract, the fibers of which pass forward to the tegmentum of
the cerebral peduncle, the subthalamic region, and the thalamus. After
the peduncles disappear under the corpora quadrigemina, they converge
and many of their fibers intercross, forming the =decussation of the
superior peduncle=. A considerable number of fibers end in the nucleus
ruber. Thence impulses are transmitted in two directions: first, by
=thalamo-cortical fibers= to the cerebral cortex; second, by the
=rubrospinal tract= through the brain stem and lateral columns of the
cord to the ventral horn cells. The ventro-lateral cerebellospinal
fasciculus (Gowers’ tract) is an ill-defined tract which connects the
spinal cord with the cerebellum. Its fibers appear to be axones of
cells of the posterior horns of the cord; they pass in the lateral
column of the cord, become scattered in passing through the reticular
formation of the medulla and pons, and enter the cerebellum by way of
the anterior medullary velum.

THE FOURTH VENTRICLE

The =fourth ventricle= (Ventriculus quartus) is the cavity of the rhombencephalon; it communicates with the central canal of the spinal cord behind, and through the aqueduct with the third ventricle in front. It is somewhat rhomboid in outline, elongated from before backward and narrowest behind. It is lined completely by an epithelium (Ependyma) and contains a small amount of fluid.

FIG. 504.—BRAIN STEM AND BASAL GANGLIA OF HORSE, DORSAL VIEW.

The cranial nerve-roots are designated by Roman numerals.
]

Its =floor= (Fossa rhomboidea) is formed by the =medulla= and =pons= and is marked by three longitudinal furrows which converge behind. It is widest and deepest a little in front of its middle. The posterior part narrows to a point at the opening of the central canal, and on account of its appearance in man it has been termed the calamus scriptorius. The =median sulcus= (Sulcus medianus) extends the entire length of the floor and is deepest toward the ends. The =limiting sulci= (Sulci limitantes) begin on either side of the opening of the central canal and extend forward as the lateral limits of the rhomboid fossa. Just beyond the middle of the fossa they expand into a shallow depression, the =anterior fovea= (Fovea nasalis). On either side of the median sulcus and margined by the limiting sulcus is a slightly rounded column, the =eminentia medialis=. Opposite the fovea this presents an elongated prominence, the =colliculus facialis=, so named because it overlies the bend formed by the fibers of origin of the facial nerve. External to the limiting sulcus is a long fusiform elevation, the =area acustica=, from which a band of fibers (Striæ acusticæ) winds over the anterior end of the restiform body to the superficial origin of the cochlear nerve.

The =lateral wall= is formed by the restiform body and the anterior peduncle of the cerebellum.

The =roof= is formed in its middle part by the vermis of the cerebellum, covered by the epithelium before mentioned. There is commonly a dorsal recess (Recessus tecti s. fastigium) between the extremities of the vermis. The anterior part of the roof is formed by a thin lamina of white substance, the =anterior medullary velum= (or valve of Vieussens), which extends backward from the corpora quadrigemina, and is attached on either side to the anterior peduncles of the cerebellum. Its anterior part is relatively thick and contains the decussation of the fibers of the trochlear nerves. Posteriorly it blends with the white matter of the cerebellum. A thin lamina of white matter, the =posterior medullary velum=, backed by pia mater, completes the roof posteriorly. After removal of the cerebellum the line of attachment (Tænia ventriculi quarti) to the medulla is seen; it begins centrally over the opening of the central canal, runs forward on the inner face of the restiform body, and turns outward behind the brachium pontis. The thick part which stretches over the posterior angle of the ventricle is termed the =obex=. The posterior part of the ventricle forms three recesses, of which two are lateral and the third median and posterior. The =lateral recesses= communicate with the subarachnoid space by distinct openings (Aperturæ laterales). The layer of pia which strengthens the roof here is named the =tela chorioidea= of the fourth ventricle. It is triangular in outline and closely adherent to the velum. It forms three fringed masses which contain vascular convolutions and are designated the =median= and =lateral chorioid plexuses= of the fourth ventricle. They appear to lie within the ventricle, but are really excluded from the cavity by the epithelial lining, which they invaginate.

FIG. 505.—BRAIN STEM AND BASAL GANGLIA OF HORSE, RIGHT VIEW.

_F.a._, External arcuate fibers; _C.r._, corpus restiforme; _P_,
pyramid; _T_, corpus trapezoideum; _B.p._, middle peduncle of
cerebellum; _P.c._, cerebral peduncle; _S_, sulcus lateralis;
_T.t._, tractus transversus; _L._, trigonum lemnisci; _C.a._, corpus
quad, ant.; _C.p._, corpus quad, post.; _B_, commissure of _C.p._;
_G_, corpus geniculatum internum; _T.o._, olfactory peduncle;
_B.o._, olfactory bulb.
]

THE MESENCEPHALON

The =mesencephalon= or =mid-brain= connects the rhombencephalon with the fore-brain. In the undissected brain it is covered dorsally by the cerebral hemispheres. It consists of a dorsal part, the corpora quadrigemina, and a larger ventral part, the cerebral peduncles, which are visible on the base of the brain. It is traversed longitudinally by a narrow canal, the cerebral aqueduct, which connects the fourth ventricle with the third.

The =corpora quadrigemina=[191] are four rounded eminences which lie under the posterior part of the cerebral hemispheres. They consist of two pairs, separated by a transverse groove. The =anterior pair= (Colliculi nasales) are larger and much higher than the posterior pair. They are gray in color, almost hemispherical, and are separated by a narrow furrow which leads forward to the subpineal fovea. A wide groove intervenes between them and the optic thalami. The =posterior pair= (Colliculi caudales) are relatively small and are paler than the anterior pair. They are marked by a wide median depression, and are limited below by a transverse furrow (Sulcus postquadrigeminus), at either side of which the =trochlear= (fourth) =nerve= emerges. Laterally each is prolonged to the inner geniculate body by a band of white matter termed the inferior brachium.[192]

The =cerebral peduncles= or crura cerebri (Pedunculi cerebri) appear on the base of the brain as two large rope-like stalks which emerge from the pons close together and diverge as they extend forward to enter the cerebrum. At the point of disappearance the =optic tract= winds obliquely across the peduncle. About half an inch further back a small tract (Tractus peduncularis transversus) curves across the peduncles, and behind this near the median line is the superficial origin of the =oculomotor= (third) =nerve=. The triangular depression between the diverging peduncles is the =interpeduncular space= (Fossa interpeduncularis). It is covered to a large extent by the =pituitary body=, a discoid brown mass which is connected with the base of the brain by a hollow stalk, the =infundibulum=. The posterior part of the space is pierced by numerous minute openings which transmit blood-vessels, and is therefore termed the =locus perforatus posticus=. The objects in the space belong to the diencephalon, and will be described later. The lateral aspect of the peduncle is marked by a groove (Sulcus lateralis mesencephali) which indicates the division into a dorsal part, the =tegmentum=, and a ventral part the =basis pedunculi=; these are separated by a layer of dark gray matter, the =substantia nigra=. The triangular area (Trigonum lemnisci) above the lateral groove is faintly marked by fibers passing obliquely upward and backward to the anterior cerebellar peduncle; these belong to the fillet or lemniscus, an important tract that connects the thalamus and corpora quadrigemina with the sensory reception nuclei of the opposite side of the medulla.

The =aqueduct of the cerebrum= or of Sylvius (Aquæductus cerebri) is the canal which extends through the mid-brain from the fourth to the third ventricle. It is surrounded by a layer of gray matter (Stratum griseum centrale), in the ventral part of which are the nuclei of origin of the oculomotor and trochlear nerves, and laterally nuclei of the mesencephalic roots of the trigeminal nerves.

THE DIENCEPHALON

The =diencephalon= or =inter-brain= comprises the thalamus and a number of other structures grouped about the third ventricle, the cavity of this division of the brain.[193] To expose its dorsal aspect, the greater part of the cerebral hemispheres, the corpus callosum, the fornix, the hippocampus, and the tela chorioidea of the third ventricle must be removed.

The =thalamus= (or optic thalamus) is the principal body in this part of the brain. It is a large ovoid gray mass placed obliquely across the dorsal face of each cerebral peduncle, so that the long axes of the two thalami would meet in front about at a right angle. Medially they are fused to a large extent, and around the area of adhesion they are separated by a sagittal circular space, the third ventricle. The dorsal surface is convex in both directions, and is separated from the overlying hippocampus by the tela chorioidea (or velum interpositum). Laterally it is separated from the nucleus caudatus by an oblique groove in which there is a band of white matter termed the =stria terminalis= or tænia semicircularis. Internally it is bounded by a narrow band, the =stria medullaris=, on which is a delicate ridge of ependyma termed the =tænia thalami=. The striæ unite posteriorly and blend with the stalk of the pituitary body. Near this point they present a small enlargement caused by the nucleus habenulæ. Anteriorly there is a small eminence, the =anterior tubercle=. The posterior part of the thalamus has the form of a rounded ridge which is continuous laterally with the optic tract.[194] Behind the point of origin of the tract, in the angle between the thalamus and the cerebral peduncle, is the =internal geniculate body= (Corpus geniculatum mediale), a well defined oval prominence.

The outer surface is separated from the lenticular nucleus by the
=internal capsule=, an important mass of white matter composed of
fibers passing to and from the cerebral cortex. These fibers go to
form a large part of the ventral portion (basis) of the cerebral
peduncle. From the entire external surface of the thalamus fibers pass
into the internal capsule and radiate to reach the cerebral cortex;
similarly fibers coming from the cortex converge in the internal
capsule to enter the thalamus. This arrangement is termed the thalamic
radiation. Ventral to the thalamus proper is the =subthalamic
tegmental region=. This is the continuation of the tegmental part of
the cerebral peduncle into the diencephalon. It contains the =red
nucleus= (Nucleus ruber) an important ganglion on the course of the
motor tracts. It receives numerous fibers from the cerebral cortex and
the corpus striatum. From it fibers proceed to the thalamus and to the
spinal cord; the fibers to the cord (Tractus rubro-spinalis of
Monakow) cross to the opposite side and extend back in the tegmentum
to the lateral columns of the cord. Lateral to the red nucleus a
conspicuous lenticular area of dark gray matter is visible on
cross-sections of the subthalamic region; this is the =subthalamic
nucleus= (Nucleus hypothalamicus s. corpus Luysi), which consists of
pigmented nerve-cells scattered through a dense network of fine
medullated fibers, and is richly supplied with capillary
blood-vessels. The two nuclei are connected by a transverse commissure
(Commissura hypothalamica), which crosses the floor of the third
ventricle above the mammillary body.

The =pineal body= or =epiphysis= is a small ovoid or fusiform red-brown mass situated in a deep central depression between the thalami and corpora quadrigemina. It is variable in size, but is commonly about 10 to 12 mm. long and 6 mm. wide. It is attached at the postero-superior quadrant of the third ventricle by a short stalk, in which is a small recess of that cavity. Its base blends in front with the junction of the striæ medullares of the thalamus. Immediately under the posterior part of the stalk is a short transverse band of white matter, the =posterior commissure=.

The pineal body is inclosed in a fibrous capsule from which numerous
trabeculæ pass inward, dividing the organ into spaces occupied by
round epithelial cells of the same origin as the ependyma of the
ventricle.

The =mammillary body= (Corpus mammillare) is a white round elevation a little larger than a pea which projects ventrally at the anterior end of the median furrow of the interpeduncular space. While it is a single body in external form in the horse, sections show that it is double in structure and contains a nucleus of gray matter on either side.

Three sets of fibers are connected with the mammillary body. The
anterior pillar of the fornix curves down in the lateral wall of the
third ventricle to the body and many of the fornix fibers end in it. A
bundle (Fasciculus thalamo-mammillaris) passes upward and backward
from it into the anterior part of the thalamus, and a tract
(Fasciculus pedunculo-mammillaris) extends back in the floor of the
third ventricle to the tegmentum of the mid-brain.

The =pituitary body= or =hypophysis= was mentioned as covering part of the interpeduncular space. It is oval in outline, flattened dorso-ventrally, and nearly an inch (ca. 2 cm.) in width. It is attached by a delicate tubular stalk, the =infundibulum=, to the =tuber cinereum=, a small gray prominence situated between the optic chiasm in front and the mammillary body behind.

The pituitary body consists of two parts which can be distinguished
readily on sections by their color. The glandular (or anterior) lobe
is brown in color and is inclosed in a fibrous capsule. It is
glandular in character and there is good ground for the view that it
is an organ of internal secretion. Besides the chief cells which stain
lightly, it contains large deeply staining chromophile cells. It
arises as an outgrowth from the primitive mouth cavity. The cerebral
(or posterior) lobe is pale and is connected with the infundibulum so
as to form a rather flask-shaped arrangement. It is almost entirely
inclosed by the glandular part. It arises as an outgrowth from the
primitive diencephalon, but loses most of its earlier nervous
character.

The optic chiasm and tracts form the anterior boundary of the interpeduncular space. The =optic chiasm= or =commissure= (Chiasma opticum) is formed by the convergence of the optic nerves and the crossing of the major part of the fibers of the nerve of one side to the tract of the opposite side. From the chiasm each =optic tract= (Tractus opticus) curves over the cerebral peduncle outward, backward, and upward to the posterior part of the thalamus and the internal geniculate body; some fibers reach the anterior quadrigeminal body.

FIG. 506.—CROSS-SECTION OF BRAIN OF HORSE, NATURAL SIZE.

Section passes through posterior part of third ventricle and is viewed
from behind. _1_, Longitudinal fissure; _2_, hippocampus; _2′_,
fimbria; _3_, septum pellucidum; _4_, lateral ventricle; _5_,
thalamus; _6_, habenula; _7_, third ventricle; _8_, cerebral
peduncle; _8′_, hypothalamus; _9_, mammillary body; _10_, pituitary
body; _11_, pyriform lobe; _12_, ventral end of hippocampus; _13_,
amygdaloid nucleus. Between the upper parts of the tæniæ thalami is
the chorioid plexus of the third ventricle, and above this are the
internal cerebral veins.
]

All the fibers in the chiasm are not derived from the optic nerves.
The posterior part contains fibers which pass from one tract to the
other and are connected with the internal geniculate bodies; this
bundle is called Gudden’s commissure (Commissura inferior). Above it
is Meynert’s commissure (Commissura superior), the fibers of which
enter the subthalamic body.

The =third ventricle= (Ventriculus tertius) is the narrow annular space between the thalami. It communicates by means of the aqueduct with the fourth ventricle behind, and in front it is continuous with the lateral ventricle on each side through the interventricular foramen. Its floor is formed by the structures of the interpeduncular space and to a small extent by the tegmentum of the cerebral peduncles. The roof is formed in the strict sense only by the ependyma, above which is a fold of pia mater, termed the =tela chorioidea= of the third ventricle or =velum interpositum=. The roof is invaginated by two delicate =chorioid plexuses= which appear to lie within the ventricle, although they are excluded from the cavity by the epithelium. When the tela is removed, the delicate ependyma of the roof is torn away with it, leaving the line of attachment to the stria medullaris to constitute the tænia thalami. The =anterior wall= is formed by the =lamina terminalis= (s. cinerea), a thin layer of gray matter which extends upward from the optic chiasm to the corpus callosum. A distinct rounded band of white matter extends across its posterior face, bulging into the ventricle. This is the =anterior commissure= (Commissura nasalis) of the cerebrum; its fibers extend to the olfactory bulb and to the pyriform lobe. A similar but more slender =posterior commissure= (Commissura caudalis) crosses the posterior wall above the entrance to the aqueduct; the connections of its fibers are not yet clearly known. The =interventricular foramen= (of Monroe) is situated on either side of the anterior part of the ventricle and leads outward and slightly upward between the anterior pillar of the fornix and the anterior tubercle of the thalamus. The cavity presents three =recesses= or diverticula, of which two are ventral and the third is supero-posterior. The =optic recess= (Recessus opticus) lies above the optic chiasm. Just behind it is the =infundibular recess= (Recessus infundibuli) which extends through the infundibulum to the pituitary body. The =pineal recess= (Recessus pinealis) is in the stalk of the pineal body.

THE TELENCEPHALON

The =telencephalon= or =end-brain= comprises two principal parts, the cerebral hemispheres and the optic part of the hypothalamus. The latter has been considered as a matter of convenience in the description of the diencephalon.

THE CEREBRAL HEMISPHERES

The =cerebral hemispheres= (Hemisphæria) form the greater part of the fully developed brain. Viewed from above (Fig. 498) they form an ovoid mass, of which the broader end is posterior, and the greatest transverse diameter is a little behind the middle. The two hemispheres are separated by a deep median cleft, the =longitudinal fissure= of the cerebrum, which is occupied by a sickle-shaped fold of dura mater, the falx cerebri. In front the separation is complete, and it appears to be behind also, but here the two hemispheres are attached to each other over a small area by the pia mater. When the hemispheres are gently drawn apart, it is seen that the fissure is interrupted in its middle part at a depth of a little more than an inch (ca. 3 cm.) by a white commissural mass, the =corpus callosum=; this connects the hemispheres for about half of their length. The =transverse fissure= separates the hemispheres from the cerebellum, and contains the tentorium cerebelli.

The =convex= or =dorso-lateral surface=[195] conforms closely to the cranial wall. The =medial= or =internal surface= is flat and sagittal and bounds the longitudinal fissure; to a large extent it is in contact with the falx cerebri, but behind the great cerebral vein the two hemispheres are in contact and are attached to each other over a small area as noted above. In well hardened specimens there is usually an impression for the vein in front of the area of adhesion. The =base= or =ventral surface= (Fig. 499) is irregular. Its anterior two-thirds is adapted to the cerebral fossa of the cranial floor. Crossing this area in front of the optic tract is a depression, the =transverse fossa= (vallecula or fossa of Sylvius), which leads outward to the =lateral fissure= (fissure of Sylvius), and lodges the middle cerebral artery. In front of the fossa there is a considerable rounded elevation known as the =trigonum olfactorium=. The trigonum and the inner part of the fossa are pierced by numerous openings for the passage of small blood-vessels and are equivalent to the locus perforatus anticus of man. Behind the outer part of the fossa is the rounded anterior end of the =pyriform lobe=. Traced backward the lobe curves upward and inward over the optic tract and the thalamus to the tentorial aspect of the hemisphere; its continuation, the hippocampus, forms part of the floor of the lateral ventricle, and will be examined later.

The posterior part or =tentorial area= is flattened, faces inward and backward as well as downward, and rests largely on the tentorium cerebelli; on its anterior part there is a shallow depression adapted to the corpora quadrigemina and the pineal body.

The =frontal pole= or anterior extremity (exclusive of the olfactory bulb) is compressed laterally, and the =occipital pole= or posterior extremity forms a blunt point.

FIG. 507.—LEFT CEREBRAL HEMISPHERE OF HORSE, LATERAL VIEW. THE
OLFACTORY BULB IS CUT OFF.

_1_, Lateral fissure (of Sylvius); _2_, _3_, _4_, middle, posterior,
and anterior branches of _1_; _5_, presylvian fissure; _6_, _6′_,
sulcus rhinalis, anterior et posterior; _7_, suprasylvian fissure;
_8_, ectomarginal fissure; _9_, _9′_, ectosylvian fissure.
]

The hemisphere comprises: (1) the =pallium= or =mantle=, which consists of an outer layer of gray matter, the =cortex= (Substantia corticalis), covering a large mass of white matter (Centrum semiovale); (2) the =rhinencephalon= or olfactory portion of the brain; (3) the =corpus callosum= and =fornix=, the great commissural white masses; (4) the =lateral ventricle= and certain important structures associated therewith.

The =pallium= is thrown into numerous folds, the =gyri cerebri=, which are separated by =sulci= or =fissures= of varying depth. The general pattern of the gyri and sulci is similar in normal brains of the same species, but the details are very variable and are never alike on the two hemispheres of the same brain. In the horse the arrangement is complicated by the existence of numerous short accessory fissures which cut into the gyri at right angles and tend to confuse the observer. The principal fissures and sulci of the convex surface (Figs. 497, 507) are as follows:

1. The =lateral fissure= (Fissura lateralis Sylvii) ascends on the lateral surface of the hemisphere as the continuation of the fossa transversa in front of the pyriform lobe. After crossing the external olfactory tract it divides into three branches; of these one passes upward, one runs obliquely forward and upward, and the third is directed upward and backward.

2. The =suprasylvian fissure= (F. suprasylvia)[196] is long and divides a large part of the convex surface of the hemisphere into dorsal and lateral portions. It begins on the dorso-medial border near its anterior end and, inclining gradually downward, passes back to end on reaching the tentorial surface. It is continuous internally with the transverse fissure usually and in front with the presylvian fissure.

3. The =presylvian fissure= (F. præsylvia) is the continuation forward of the preceding. It passes forward, outward, and downward almost to the frontal pole, and then inclines backward to end at the groove which marks the upper limit of the rhinencephalon (Sulcus rhinalis).

4. The =marginal fissure= (F. marginalis) extends along the dorso-medial border. It begins a little in front of the middle of the border and turns around the occipital pole to end on its tentorial aspect.

5. The =entomarginal fissure= (F. entomarginalis) lies internal to the dorso-medial border. It does not extend quite as far forward as the marginal fissure, from which it is separated by a narrow gyrus.

6. The =ectomarginal fissure= (F. ectomarginalis) lies about midway between the marginal fissure and the posterior part of the suprasylvian fissure.

7. The =sulcus rhinalis= is a very distinct furrow on the lower part of the lateral surface which marks off the olfactory part of the brain (rhinencephalon) from the rest of the hemisphere. It is undulating and is highest where it is crossed by the lateral fissure (of Sylvius).[197]

On the medial surface (Fig. 508) the main fissures and sulci are as follows:

1. The =calloso-marginal fissure= (F. calloso-marginalis) is extensive and well defined. It is approximately parallel to the dorso-medial border of the hemisphere, from which it is about half an inch distant. It begins in front a short distance below and in front of the anterior bend (genu) of the corpus callosum and forms a =C=-shaped curve, its posterior part extending on the tentorial surface to a point behind the depression for the corpora quadrigemina. It separates the marginal gyri above from the gyrus fornicatus, which extends down to the corpus callosum.

2. The =transverse fissure= (F. transversa) begins a little behind the middle of the calloso-marginal fissure, passes obliquely upward and forward to the dorso-medial border—into which it cuts deeply—and usually joins the suprasylvian fissure.[198]

3. The =sublimbic fissure= (F. sublimbica) curves over the gyrus fornicatus a short distance above the corpus callosum. Its middle part is commonly indistinct, and it is often divided into anterior and posterior parts.

4. The =callosal sulcus= (Sulcus corporis callosi) separates the corpus callosum from the gyrus fornicatus.

The =hippocampus= (or cornu Ammonis) is a gyrus which curves from the deep face of the pyriform lobe around the thalamus and forms the posterior part of the floor of the lateral ventricle. It can be displayed by cutting away the brain stem up to the optic tract and the interventricular foramen. Viewed from below the hippocampal gyrus is seen to form a semicircular curve from the apex of the pyriform lobe to the angle of divergence of the posterior pillars of the fornix, _i. e._, to a point under the central part of the corpus callosum. It is separated deeply by the hippocampal fissure from the =gyrus dentatus=. Along the concave margin of the latter is a band of white matter, termed the =fimbria=, which is the prolongation of the greater part of the posterior pillar of the fornix into this region. The ventricular surface of the hippocampus (Fig. 511) is covered with a thin layer of white matter, the =alveus=, which is also derived from the posterior pillar of the fornix, and is therefore continuous with the fimbria. The two hippocampi are connected at their highest parts by transverse fibers which constitute the =hippocampal commissure=.

The interval between the hippocampus and fimbria on the one hand and the brain stem on the other is a lateral continuation of the great transverse fissure of the brain, and is termed the =chorioid fissure=. It is occupied by a fold of pia mater, the =tela chorioidea= of the third ventricle, or =velum interpositum=. This fold is triangular in outline and its apex reaches to the interventricular foramen. Its base is continuous at the transverse fissure with the pia which covers the surface of the brain. Its middle part lies over the epithelial roof of the third ventricle, as has been seen (Fig. 506). The lateral borders will be seen on the floor of the lateral ventricles, where they form thick rounded bands containing convolutions of blood-vessels, known as the =chorioid plexuses= of the lateral ventricles (Fig. 511).

FIG. 508.—MEDIAN SECTION OF BRAIN OF HORSE.

The membranes and vessels are removed. _C_, Central white matter
(corpus medullare) of cerebellum; _P.c._, cerebral peduncle; _C.q._,
corpora quadrigemina; _P_, pineal body; _Th._, thalamus; _V.III._,
third ventricle; _r_, optic recess; _r′_, infundibular recess;
_C.m._, mammillary body; _s_, subcallosal gyrus; _A.p._, area
parolfactoria; _G_, genu of corpus callosum; _S_, splenium of same;
_1_, callosal sulcus; _2_, interventricular foramen; _3_, olfactory
bulb. The cerebral lobe of the pituitary body is distinguished by
its lighter color.
]

When the tela is pulled out one may easily get the impression that the
lateral ventricle communicates with the exterior by means of the
chorioid fissure. Such is not the case, since the chorioid plexus is
covered by the epithelial lining of the ventricle, which has been torn
away.

The =rhinencephalon= or olfactory part of the brain comprises the olfactory bulb, peduncle and tracts or roots, the trigonum olfactorium, the area parolfactoria, and the pyriform lobe.[199]

The =olfactory bulb= (Bulbus olfactorius) is an oval enlargement which curves upward in front of the frontal pole of the hemisphere. Its convex superficial face fits into the ethmoidal fossa and receives numerous olfactory nerve-fibers through the cribriform plate; hence it is very difficult to remove the bulb intact. It contains a considerable =cavity= (Ventriculus bulbi olfactorii) which is connected with the lateral ventricle by a small canal in the middle olfactory tract. The deep face is largely in contact with the frontal pole of the hemisphere and is connected with the olfactory peduncle.

The gray matter of the bulb is external and is thickest on the convex
anterior surface. The posterior surface consists to a large extent of
fibers which are the axones of the mitral cells of the deep layer of
the gray substance and go to form the peduncle and striæ.

The =olfactory peduncle= (Tractus olfactorius) is a very short but wide band of white substance which arises in the olfactory bulb and extends back to be continued by the olfactory tracts.

FIG. 509.—MEDIO-VENTRAL ASPECT OF RIGHT CEREBRAL HEMISPHERE OF HORSE.

The olfactory bulb is cut off. _T.ol._, Olfactory peduncle; _A.p._,
area parolfactoria; _Tr.O._, trigonum olfactorium; _T.op._, optic
tract; _C.o._, chiasma opticum; _G.s._, subcallosal gyrus; _C.c._,
corpus callosum; _S.p._, septum pellucidum; _F_, fornix; _G.c._,
callosal gyrus; _T_, cut surface of thalamus; _F′_, fimbria; _G.d._,
gyrus dentatus; _L.p._, pyriform lobe; _G.h._, hippocampal gyrus.
]

The =olfactory tracts= or =striæ= (Striæ olfactorii) are three in number. The =external tract= (Stria lateralis) is much the largest and most distinct. It passes backward, upward, and outward, widens out and joins the pyriform lobe. It is clearly defined dorsally by the sulcus rhinalis and is marked off from the trigonum olfactorium by the sulcus arcuatus. The =intermediate tract= (Stria intermedia) is short, ill defined, and flat; it contains the canal which opens into the anterior horn of the lateral ventricle. The =internal tract= (Stria medialis) is small, short, and not well defined; it bends over to the parolfactory area (of Broca) on the inner face of the hemisphere below the genu of the corpus callosum.

The =trigonum olfactorium= is the prominent gray area situated in the angle of divergence of the internal and external olfactory striæ. It is bounded externally by the external olfactory stria, from which it is defined by the arcuate sulcus. It is continuous with the =area parolfactoria= on the medial surface; behind the latter a band descends from the rostrum of the corpus callosum and is continuous below with the anterior perforated space; it is termed the =subcallosal gyrus= or peduncle of the corpus callosum.

The =pyriform lobe= (Lobus piriformis) is the well-marked prominence on the base lateral to the optic tract and cerebral peduncle, from which it is separated by a deep fissure. Its nipple-like apex lies behind the fossa transversa and covers the optic tract. The external surface is marked by one or two sulci (lobi piriformis). The lobe contains a cavity, the ventral horn of the lateral ventricle.

The fibers of the olfactory tracts go to the pyriform lobe and
hippocampus, the trigonum olfactorium, the area parolfactoria, the
subcallosal gyrus, and part of the gyrus fornicatus. The central
connections of the olfactory apparatus are complex and are not yet
fully understood. The anterior cerebral commissure contains fibers
which pass from the olfactory bulb of one side by way of the inner
tracts to the bulb of the opposite side; also fibers which cross in it
from the inner tract of one side to the pyriform lobe of the opposite
side. Many fibers pass by way of the septum pellucidum, fornix, and
fimbria to the hippocampus. Other fibers pass in the anterior pillar
of the fornix to the mammillary body and thence to the thalamus by the
thalamo-mammillary bundle.

FIG. 510.—LATERAL VIEW OF CAST OF CAVITIES OF BRAIN OF HORSE.

_Bo_, Cavity of olfactory, which communicates through the canal _I_
with a lateral ventricle; _Ca_, anterior horn, _Cm_, body, _Ci_,
ventral horn of lateral ventricle; _M_, isthmus which connects
lateral and third ventricles _Ep_, suprapineal recess, below which
is the small infrapineal recess; _Ro_, optic recess; _Rh_,
infundibular and pituitary recess; _S_, aqueduct; _Fl_, ridge
corresponding to sulcus limitans; _Rl_, lateral recess, _Rm_,
posterior recess, of fourth ventricle; _Cc_, beginning of central
canal of spinal cord. (Dexler.)
]

The =corpus callosum= is the great transverse commissure which connects the two cerebral hemispheres through about half of their length. On median section (Fig. 508) it is seen to be arched from before backward, white in color, and composed substantially of transverse fibers. The middle part or =truncus= (Truncus corporis callosi) slopes downward and forward and is thinner than the ends. The anterior thickened end, the =genu=, bends downward and backward and thins out to form the =rostrum=; the latter is continuous with the lamina terminalis. The posterior end, the =splenium=, also thick, lies at a considerably higher level than the genu. The =upper surface= is convex in its length, concave transversely. Its central part forms the floor of the longitudinal fissure. It is covered by a thin layer of gray matter (Induseum griseum), in which are strands of longitudinal fibers (Striæ longitudinales); the latter are arranged in median and lateral bundles (Stria medialis, striæ laterales).[200] The =lower surface= has the reverse configuration, and forms the roof of the lateral ventricles. Along the median line the septum pellucidum is attached to it. The fibers of the corpus callosum (with the exception of the longitudinal striæ) run transversely and spread out laterally in all directions in the central white matter of the hemispheres to the cortex, forming the =radiation of the corpus callosum= (Radiatio corporis callosi).

The =fornix= is a bilateral structure composed of white fibers which arch chiefly over the thalamus and the third ventricle. It is described as consisting of a body and two pairs of pillars. The =body= (Corpus fornicis) is formed by the fusion of the two arches of which the fornix is composed. It is triangular and overlies the anterior parts of the thalami and the third ventricle. The upper surface gives attachment to the septum pellucidum and on either side forms part of the floor of the lateral ventricle. The lateral border is related to the chorioid plexus of the lateral ventricle and forms the inner boundary of the interventricular foramen. The =anterior pillars= or =columns= (Columnæ fornicis) (Fig. 512) are two slender round bundles which emerge from the body in front of the interventricular foramen and diverge slightly as they curve downward and backward to the mammillary body.

FIG. 511.—BRAIN OF HORSE, WITH LATERAL VENTRICLES OPENED BY REMOVAL OF
UPPER PART OF CEREBRAL HEMISPHERES.
]

From the mammillary body the greater part of these fornix fibers are
continued to the thalamus by the thalamo-mammillary fasciculus (or
bundle of Vicq d’Azyr). Others pass to the cerebral peduncle. A
portion of the fibers cross to the opposite thalamus and cerebral
peduncle.

The =posterior pillars= (Crura fornicis) are much larger bands which diverge widely from the posterior angles of the body. Each curves outward and backward over the thalamus (from which it is separated by the tela chorioidea) and is chiefly continued as the =fimbria= along the concave border of the hippocampus. The pillars give off fibers to form the alveus or white matter which covers the ventricular face of the hippocampi, and between them are transverse fibers which constitute the hippocampal commissure.

The =septum pellucidum= is the median partition between the two lateral ventricles. Its convex upper border blends with the corpus callosum and its concave lower border joins the fornix. Its anterior part is received into the genu of the corpus callosum. Traced backward it diminishes in height and the two edges meet at an acute angle at the splenium.

The septum consists of two layers (Laminæ septi pellucidi) which are
in direct contact with each other. They consist of nerve-fibers and
gray matter. The latter exists in considerable amount in the thicker
part of the septum adjacent to the anterior pillars of the fornix.
Many of the fibers of the septum pass up through the corpus callosum
to the gyrus fornicatus. Others are connected with the subcallosal
gyrus and the parolfactory area.

FIG. 512.—CROSS-SECTION OF BRAIN OF HORSE, ABOUT NATURAL SIZE.

Section passes through chiasma opticum, and is viewed from in front.
_1_, Longitudinal fissure; _2_, septum pellucidum; _3_, columns
(anterior pillars) of fornix; _4_, lateral ventricle; _5_, chorioid
plexus; _6_, corpus medullare (central white matter) of hemisphere;
_7_, caudate nucleus; _8_, lenticular nucleus; _9_, internal
capsule; _10_, external capsule; _11_, insula.
]

The =lateral ventricle= (Ventriculus lateralis) is the irregular cavity in the interior of each cerebral hemisphere.[201] Each communicates with the third ventricle through the =interventricular foramen= (of Monro), and by a small canal with the cavity of the olfactory bulb. It is usual to describe the ventricle as consisting of three parts, viz., the central part or body, and anterior and inferior horns. The =anterior horn= (Cornu nasale) is the part in front of the interventricular foramen; it communicates ventrally with the olfactory cavity. The =central part= (Pars centralis) extends back to the splenium of the corpus callosum. It opens into the third ventricle through the interventricular foramen, which is situated between the fornix and the anterior part of the thalamus. The =inferior horn= (Cornu ventrale) curves downward and forward into the pyriform lobe. The roof of the ventricle is formed by the corpus callosum and the inner wall is the septum pellucidum. After removal of the roof the floor is seen to be formed chiefly by two bodies. The anterior one is the =caudate nucleus= (Nucleus caudatus), a somewhat pear-shaped gray eminence, the long axis of which is directed obliquely upward, backward, and outward. Its anterior large end is termed the head, and the posterior long tapering end the tail. The posterior body, the =hippocampus=, is white on its ventricular surface, which is strongly convex. It curves outward and backward and then turns downward and forward to join the pyriform lobe. The two bodies are separated by an oblique groove which is occupied by the =chorioid plexus= of the lateral ventricle. This is the thickened edge of a fold of pia mater, the tela chorioidea (or velum interpositum), which lies between the hippocampus and the thalamus. It contains convolutions of small blood-vessels and in old subjects there may be calcareous concretions in it. The plexuses of the two sides are continuous through the interventricular foramen. On drawing the chorioid plexus backward a narrow white band, the =stria terminalis= (or tænia semicircularis), is seen along the margin of the caudate nucleus, where it bounds the intermediate groove. The plexus partially covers a wider white band which is blended with the white substance of the hippocampus; this is the =posterior pillar of the fornix= and its continuation, the =fimbria=.

FIG. 513.—SAGITTAL SECTION OF BRAIN OF HORSE.