120 HUMAN PHYSIOLOGY. 



and between it and the convolutions of the island of Reil, a thin band of 

 gray matter, the daustrum; the corpora striata are grayish in color, and 

 when divided present transverse striations, from the intermingling of white 

 fibres and gray cells. 



The Optic Thalami are two oblong masses situated in the ventricles 

 posterior to the corpora striata, and resting upon the posterior portion of 

 the crura cerebri. The internal surface projecting into the lateral ven- 

 tricles is white, but the interior is grayish, from a commingling of both 

 white fibres and gray cells. Separating the lenticular nucleus from the 

 caudate nucleus and the optic thalamus, is a band of white tissue, the 

 internal capsule. 



The internal capsule is a narrow, bent tract of white matter, and is, for 

 the most part, an expansion of the motor tract of the crura cerebri. It 

 consists of two segments, an anterior, situated between the caudate 

 nucleus and the anterior surface of the lenticular nucleus, and a posterior, 

 situated between the optic thalamus and the posterior surface of the len- 

 ticular nucleus. These two segments unite at an obtuse angle, which is 

 directed toward the median line. Pathological observation has shown 

 that the nerve fibres of the direct and crossed pyramidal tracts can be 

 traced upward through the anterior two-thirds of the posterior segment, 

 into the centrum ovale, where, for the most part, they are lost ; a portion, 

 however, remaining united, ascend higher and terminate in the paracentral 

 lobule, the superior extremity of the ascending frontal and parietal convo- 

 lutions. The sensory tract can be traced upward, through the posterior 

 third, into the cerebrum, where they probably terminate in the hippo- 

 campus major and unciate convolution. 



Functions. The Corpora striata are the centres in which terminate 

 some of the fibres of the superficial or motor tract of the crura cerebri ; 

 others pass upward through the internal capsule, to be distributed to the 

 cerebrum. It might be inferred, from their anatomical relations, that they 

 are motor centres. Irritation by a weak galvanic current produces mus- 

 cular movements of the opposite side of the body; destruction of their 

 substance by a hemorrhage, as in apoplexy, is followed by a paralysis of 

 motion of the opposite side of the body, but there is no loss of sensation. 

 When the hemorrhagic destruction involves the fibres of the anterior two- 

 thirds of the posterior segment of the internal capsule, and thus separates 

 them from their trophic centres in the cortical motor region, a descending 

 degeneration is established, which involves the direct pyramidal tract of 

 the same side and the crossed pyramidal tract of the opposite side. 



Destruction of the posterior one-third of the posterior segment of the 



