BASAL GANGLIA. 



475 



smaller portion projects into the anterior part of the lateral ventricle. 

 A transection of the corpus striatum shows that it is divided by a 

 band of white matter into two portions, viz. : 



1. The caudate nucleus, the intra- ventricular portion, convex in 



shape with its base directed forward, its apex or tail directed 

 backward and downward. 



2. The lenticular nucleus, the extra- ventricular portion, somewhat 



biconvex in shape and embedded largely in the white matter. 



Each lenticular nucleus is sub- 

 divided by two lamina of white 



matter into three portions. The two 



inner, from their pale yellow color, 



form the globus pallidus; the outer, 



somewhat darker in color, is the 



putamen. 



The Internal Capsule. The band 

 of white matter separating the caudate 

 from the lenticular nucleus has been 

 ^termed the internal capsule from the 

 manner in which it embraces the inner 

 surface of the lenticular nucleus. It 

 consists of nerve-fibers which associate 

 histologically and physiologically all por- 

 tions of the cerebral cortex with the optic 

 thalamus, pons, medulla, spinal cord, and 

 cerebellum. The relation of the capsule 

 to the nuclei through which it passes is 

 readily shown on cross-section (Fig. 216). 

 The appearance which it presents, how- 

 ever, varies considerably at different levels. 

 At a given level it may be said to con- 

 sist of two segments or limbs, an anterior, 

 situated between the caudate nucleus and 

 the anterior extremity of the lenticular 

 nucleus, and a posterior, situated between 

 the optic thalamus and the posterior 

 extremity of the lenticular nucleus. The 



two segments unite at an obtuse angle, termed the knee, which is 

 directed toward the median line. 



The optic thalami are two oblong masses of gray matter situated 

 upon the crura cerebri and behind the corpora striata. The anterior 

 and posterior extremities of each thalamus present enlargements 

 known respectively as the anterior tubercle and the posterior tubercle 

 or pulvinar. The mesial surface of the thalamus forms the lateral 

 wall of the third ventricle and is covered by epithelium resting on a 

 thin layer of ependyma. 



FIG. 216. HORIZONTAL SEC- 

 TION OP THE INTERNAL 

 CAPSULE SHOWING ITS 

 RELATIONS TO THE CAU- 

 DATE NUCLEUS, OPTIC 

 THALAMUS, AND THE 

 LENTICULAR NUCLEUS. * i. 

 Caudate nucleus. 2. An- 

 terior segment of the in- 

 ternal capsule. 3. Exter- 

 nal capsule. 4. Lenticu- 

 lar nucleus. 5. Claus- 

 trum. 6. Posterior seg- 

 ment of internal capsule. 

 7. Optic thalamus. 

 (Modified from Landois.} 



