258 THE NERVOUS SYSTEM 



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 anteriorly, 

 and the lenticular nucleus from the optic thalamus posteriorly, 

 is a band of white fibers known as the internal capsule. The 

 part between the two nuclei is the anterior 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 proximity of the 

 internal capsule. The further fact that irritation of this organ 

 is followed by muscular contractions does not prove that it 

 ordinarily generates motor force, for many of the fibers from the 

 motor cortical zone pass to or through the corpus striatum. 

 This may be only a relay station, and the corpus may be quite 

 subsidiary. It undoubtedly, however, is connected with motion 

 in some way. 



The precise function of the optic thalami is equally obscure. 

 The relation of these organs to the tegmenta would suggest that 

 they have something to do with the sensory fibers on their way 

 to the cortex. It cannot be denied that they are concerned in 

 sensation, since their removal is followed by crossed anesthesia. 

 They may likewise be relay stations. Each sends fibers to the 



