THE BRAIX AXD ITS MEMBRANES. 



747 



immediately beneath the pulvinar (Figs. 439 and 443). Its anterior extremity, 

 which is rounded and smaller than the posterior, forms the posterior boundary of 

 the foramen of Monro. 



Structure of the Optic Thalamus. The optic thalamus is chiefly formed. of gray 

 matter, covered over by a superficial layer of white, which on the outer side 



Genu of the 

 corp. call. ' 



Anterior 

 eoMiisntre 



Chiasma opt 



Infund. Corp. Middle Fed. Epi- Aqux- Lamina 

 mam- comm.ofpin.phy- duct, quad- 

 millare gl. sis Sylvii rigcm. 



FIG. 442. The right side of an antero-posterior median section of the parts immediately around the third 



ventricle. (Gegenbaur.) 



(external medullary lamina) separates it from the internal capsule. This layer 

 on the upper surface is known as the stratum zonale, and is prolonged downward 

 on the internal surface. 



The gray matter is arranged in two masses, the outer and inner nuclei, par- 

 tially divided by a vertical white septum, S-shaped on section, the internal medul- 

 lary lamina. The thalamus is traversed by numerous nerve-fibres, which for the 



Corp. 

 gcnicul 



Tractus options 



Optic chiasma 



FIG. 443. Geniculate bodies seen from below. The cut-surface is through the plane of junction between 

 pons and mid-brain, X { (Gegenbanr.) 



most part have no definite direction : some, however, converge and form various 

 bundles which pass out of the optic thalamus to blend with the white matter of the 

 hemispheres, as follows : (1) from its anterior extremity to the frontal lobes (anterior 

 stalk of thalamus) : (2) from its lower part (subthalamic region) to (a) lenticular 

 nucleus of corpus striatum (ansa lenticulan's) and (/>) internal capsule (ansapedun- 

 cularis), the fibres of which pass below the lenticular nucleus and into the 

 "external capsule" of the hemisphere; (3) from its outer surface through inter- 

 nal capsule to parietal and temporal lobes : (4) from pulvinar (outer aspect) to 

 occipital lobe. These last are long and curve backward, and radiate toward the 

 cortex. They are called the optic radiations. They connect, through the pulvinar, 

 with the outer root of the optic tract. The gray matter contains comparatively 

 large nerve-cells, both multipolar and fusiform. The inner nucleus is connected 



