THE BRAIN 



345 



The corpus striatum is developed as a thickening in the floor of each cerebral 

 hemisphere. It is already prominent in embryos of six weeks (13.6 mm.) bulg- 

 ing into the lateral ventricle (Figs. 345 and 347). It is in hne caudally with the 

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

 although the thalamus always forms 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. 349). The thickening of the corpus striatum is due 



Anterior horn 



Nucleus caudatus 



Interiientricidar 

 foramen 



Third ventricle 



Chorioid plexus of 

 lat. ventricle 



'lU'4 V '■ 



Posterior horn ™- '* ' 



Loiticuhir nucleus 

 [ I Aiil. folumns of 



forn ix 

 Jiilcnidl capsule 



Thalamus 



wJTippocanipus 



Fig. 350. — Horizontal (coronal) section through the fore-brain of a 160 mm. fetus (His). 



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 

 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 Kmb of which partly 

 separates the corpus striatum into the caudate and lenticular nuclei (Fig. 350). 

 The caudal Hmb of the capsule extends between the lenticular nucleus and the 

 thalamus. 



