Part 74

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.

[Illustration:

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.

[Illustration:

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).

[Illustration:

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.

[Illustration:

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.

[Illustration:

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.) ]