258 THE NERVOUS SYSTEM 



The Corpora Striata, Optic Thalami and Internal Capsule 

 are closely related and are best considered together. 



Each corpus striatum is pear-shaped with its large end 

 forward and near the median line ; the posterior small ex- 

 tremities are divergent from each other and embrace the two 

 optic thalami. Externally they are white; internally white 

 and gray elements are mixed. Each is separated by the an- 

 terior limb of the internal capsule into two divisions, exter- 

 nal and internal, known respectively as the lenticular and 

 caudate nuclei. (See Fig. 80.) 



The optic thalami, one on either side, have an oval shape 

 and rest upon the crura cerebri between the posterior ex- 

 tremities of the two corpora striata. Most of their external 

 surface is white ; internally each possesses six gray nuclei. 



Separating the two nuclei of the corpus striatum anteri- 

 orly, and the lenticular nucleus from the optic thalamus pos- 

 teriorly, is a band of white fibers known as the internal cap- 

 sule. The part between the two nuclei is the interior limb ; 

 that between the lenticular nucleus and the optic thalamus is 

 the posterior limb. These limbs, joining at an obtuse angle, 

 constitute a bend in the internal capsule which is called the 

 genu, or knee. The fibers of the capsule pass to the frontal, 

 parietal and occipital lobes of the cortex, and in their course 

 to these parts they diverge to form the corona radiata. 



External to the lenticular nucleus is a band of white fibers 

 known as the external capsule. In it is a longitudinal mass 

 of gray matter, the claustrum. Fig. 76 shows the relations 

 of these parts. 



Functions. The exact function of the corpora striata is 

 a matter of some doubt. They have been considered the 

 great motor ganglia of the base of the brain ; but, although 

 lesions here are followed by paralysis on the opposite side of 

 the body, it is held that this phenomenon is due to the prox- 

 imity of the internal capsule. The further fact that irrita- 

 tion of this organ is followed by muscular contraction does 

 not prove that it ordinarily generates motor force, for many 



