506 THE NERVOUS SYSTEM 



The important bundle thus inserted between the basal nuclei is 

 known as the internal capsule. 



As seen in horizontal section the internal capsule, conform- 

 ing to the convex surface of the thalamus, makes a sharp bend. 

 Based upon this fact, the tract is arbitrarily divided into a 

 knee, a posterior limb, and an anterior limb. In transverse, viz., 

 frontal, section this bend is not seen, the white fibres are con- 

 tinued directly into the medullary white substance of the cere- 

 bral hemisphere in a radiating manner, thus forming the corona 

 radial a. 



The lenticular nucleus forms the ventro-lateral boundary of 

 the internal capsule. It is connected with its fellow of the oppo- 

 site side by a compact bundle, the anterior commissure. This 

 nucleus is divisible into a lateral portion, the putamen, and a 

 mesial portion, the globus pallidus. 



Lateral from the lenticular nucleus, and separated from it by 

 the thin external capsule of white matter, is a flattened mass of 

 grey matter, the claustrum. This nucleus lies just mesial to the 

 island of Reil (gyrus seu lobus insulce}. 



At a somewhat lower level, in the angle between the ventral 

 surface of the pyramids as they pass lateralward into the internal 

 capsule, and the mesial border of the lenticular nucleus, lies the 

 optic tract on its way to the posterior border of the thalamus, 

 where it enters the lateral geniculate body. This small ovoid body 

 lies just ventro-lateral to the pulvinar, the medial geniculate body 

 occupying the angle between the pulvinar and the corpora quadri- 

 gemina. 



In the interior of the brain-stem, in an area bounded ventrally 

 by the pyramids and dorsally by the red nuclei and optic thalami, 

 is the diencephalic continuation of the long fibre paths of the 

 mesencephalic tegmentum. In front of this area is the cephalic 

 end of the substantia nigra Someringi, and more lateralward the 

 Jiypothalmic nucleus. This nucleus in transection presents an oval 

 mass of small nerve cells. The zona incerta, the cephalic continua- 

 tion of the formatio reticularis, is interposed between the red 

 nucleus, on its dorsal side, and the substantia nigra and hypothal- 

 mic nucleus, which form its ventral boundary. 



From the lateral surface of the nucleus ruber, a heavy bundle 

 of fibres, the probable continuation of the path of the superior 

 cerebellar peduncles by means of secondary neurones whose cells 

 lie in the red nucleus, is directed lateralward toward the ventro- 



