232 A MANUAL OF ANATOMY. 



pineal gland and the pulvinar, is a shallow depression 

 termed the trigxmum habenulae. 



The optic thalami are joined by two connecting bundles 

 of fibres, the middle, gray or soft commissure, which 

 unites them at the middle of their opposing surfaces, and 

 the posterior commissure, which unites these surfaces be- 

 neath the pineal gland and over the beginning of the aque- 

 duct of Sylvius. 



The Development of the Corpus Striatum, Optic Thal- 

 amus, and Internal Capsule. 



The optic thalamus and the corpus striatum are devel- 

 oped from the floor (the former) and the floor and side 

 wall (the latter) of the fore and interbrains opposite and 

 posterior to the narrowing orifice which connects the cavity 

 of the fore and interbrains (i. e., the future foramen of 

 Monro). They first appear as separate swellings distinct 

 from each other and from the wall of the vesicle, but later 

 the corpus striatum becomes fused along its line of con- 

 tact with the vesicular wall and also to the optic thalamus 

 along their opposing surfaces. See page 173. 



At this early stage the corpus striatum does not show 

 the nuclei that distinguish it in the adult They (the cau- 

 date and lenticular nuclei) are developed later as masses 

 of gray cells which are bunched together in the supero- 

 internal and infero-external regions of the corpus striatum. 

 From these cells and from the cortical wall (with which 

 the corpus striatum has now become fused), white fibres 

 develop which pass downward and inward, separating the 

 two portions (caudate and lenticular) from each other 

 (except at their extreme anterior and inferior parts), and 

 also separating the corpus striatum from the optic thala- 

 mus. This white layer of fibres is the internal capsule. 



