Part 202

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= 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) 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).

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.

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.