Part 203

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.

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). 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. 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=.