G54 PHYSIOLOGY. 



The corpora quadrigemina are four small bodies or rounded 

 eminences. They are composed, for the great part, of gray matter, 

 although covered externally by and containing in their interior some 

 white fibers. They lie beneath the pulvinar of the optic thalamus. 



The corpora are arranged in two pairs: one anterior, the other 

 posterior. 



The upper, or anterior, pair is broader, longer, and darker than 

 the posterior pair. Laterally the corpora extend into distinct and 

 prominent tracts of white substance. 



The lou-er, or posterior, corpora are composed almost entirely of 

 gray matter. 



Internal Capsule. The name of internal capsule is given to a 

 thick band of white fibers situated between the optic thalamus and 

 caudate nucleus on one side and the lenticular nucleus on the other. 

 In a frontal section of the brain the tract is seen to follow a course 



Fig. 261. Internal Capsule. ( SHEBRINGTON. ) 



upward and outward in an oblique manner between the preceding 

 nuclei. Downward it is continuous with the cerebral peduncle. 



Where the capsule enters the lenticulo-striate defile it expands 

 like a bundle of stalks to form the corona radiata of Eeil. 



If studied horizontally, the internal capsule is seen to present 

 the shape of an angle opening outward and embracing the lenticular 

 nucleus. The capsule seems to be composed of two parts or segments 

 and a bend, or genu. 



The anterior segment is placed between the lenticular and 

 caudate nuclei; it bears the name of arm, or lenticulo-striate segment. 

 The posterior segment, situated between the optic thalamus anfl 

 lenticular nucleus, for this reason takes the name of knliculo-optic 

 segment. 



The point of union of the two segments is called the Icnee, or 

 genu. Its position is exactly at the center of the three nuclei just 

 mentioned. 



CAPSULAR STRUCTURE. With the naked eye or even a micro- 

 scope the internal capsule presents itself as a homogeneous structure, 



