THE DIENCEPHALON 781 



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.i 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 (Fig. 635). 



The thalamus is the principal body in this part of the brain. It is a large 

 ovoid 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, and is separated from the overlying hippocampus by the tela chorioidea. 

 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. When the tela chori- 

 oidea of the lateral ventricle has been detached from this band it leaves a torn edge, 

 the taenia chorioidea. Medially it is bounded by a narrow white band, the stria 

 medullaris ; to this the tela chorioidea of the third ventricle is attached, and when 

 the latter is removed in dissection, there remains a thin irregular edge termed the 

 taenia thalami. The striae unite posteriorly and blend with the stalk of the pineal 

 body. Near this point they present a small enlargement caused by the nucleus 

 habenulae. Anteriorly there is a small eminence, the anterior tubercle of the 

 thalamus (Tuberculum orale thalami). The posterior part of the thalamus has the 

 form of a rounded ridge which is continuous laterally with the optic tract. ^ Behind 

 the point of origin of the tract, in the angle between the thalamus and the cerebral 

 peduncle, is the medial geniculate body (Corpus geniculatum mediale), a well- 

 defined oval prominence. 



The lateral 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 lateral 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 hypothalamic 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, which constitute the rubro-spinal tract (Tractus rubro-spinalis), 

 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 hypothalamic region; this is the hypothalamic nucleus (Nucleus hypothalamicus 

 Luysi), which consists of pigmented nerve-cells scattered through a dense network of fine medul- 

 lated fibers, and is richly supphed with capillary blood-vessels. The two nuclei are connected 

 by a transverse commissure (Commissura hypothalamioa) , which crosses the floor of the third 

 ventricle above the mammiUary body. 



The pineal body or epiphysis cerebri (Corpus pineale) is a small ovoid or fusi- 

 form 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 



1 On a strictly embryological basis the optic part of the hypothalamus, comprising the 

 anterior part of the third ventricle and the structures associated with it, belongs to the telen- 

 cephalon, but wiU be considered here as a matter of convenience. 



2 This backward projection of the thalamus is equivalent to the pulvinar 'and lateral genicu- 

 late body of man, which are not superficially divided in the horse. 



