THE BRAIN 



343 



thalamus of the diencephalon and in development is closely connected with it, 

 although the thalamus forms always a separate structure. The corpus striatum 

 elongates as the cerebral hemisphere lengthens, its caudal portion curving around 

 to the tip of the inferior horn of the lateral ventricle and forming the slender tail 

 of the caudate nucleus (Fig. 331). The thickening of the corpus striatum is due 

 to the active proliferation of cells in the ependymal layer which form a prominent 

 mass of mantle layer cells. Nerve fibers to and from the thalamus to the cere- 

 bral cortex course through the corpus striatum as laminae which are arranged in 



Anterior horn 



Chorioid plexus of 

 lat. ventricle 



Posterior horn 



Lenticular nucleus 

 Ant. columns of 

 fornix 

 Internal capsule 



Thalamus 



Hippocampus 



FIG. 332. Horizontal (coronal) section through the fore-brain of a 16 cm. fetus (His). 



the form of a wide V, open laterally, when seen in horizontal sections. This V-- 

 shaped tract of white fibers is the internal capsule, the cranial limb of which partly 

 separates the corpus striatum into the caudate and lenticular nuclei (Fig. 332). The 

 caudal limb of the capsule extends between the lenticular nucleus and the thalamus. 

 The thalamus and corpus striatum are separated by a deep groove until the 

 end of the third month (Fig. 329). As the structures enlarge, the groove between 

 them disappears and they form one continuous mass (Fig. 332). According to 

 some investigators, there is direct fusion between the two. 



