THE NERVOUS SYSTEM. 



515 



FIG. 439. i, 2 and 3, Schematic horizontal sections through human embryonic fore-brains at dif- 

 ferent stages of development; 4, vertical section through fore-brain at about same stage as i. 

 Goldstein. 



a, That part of the lateral ventricle lying between the corpus striatum and the junction of medial 

 hemisphere wall and thalamus (leading into the inferior horn); b, furrow or trough between 

 mesial hemisphere wall and thalamus, produced by backward extension of hemisphere; c. /., 

 internal capsule; P.M., foramen of Monro; &, external surface at junction of mesial hemi- 

 sphere wall and thalamus; Str., corpus striatum; Th., thalamus; U, place where mesial hemi- 

 sphere wall continues into the thalamus wall (junction of hemisphere wall and thalamus) ; 

 U 1 , place where mesial hemisphere wall is continuous with lateral hemisphere wall. 



In I, owing to the thickening of U and growth of the corpus striatum, these two are brought 

 into apposition, as indicated by the dotted lines on the right, and apparently fuse, obliterating 

 a and producing the condition shown in 2 and 3. In 2 and 3 the position of the former 

 space a is indicated by the dotted lines a a' By comparison with 4, it will be seen that this 

 obliteration by apparent fusion is actually produced by a filling up from the bottom of a (in- 

 dicated faintly by dotted lines on the right in 4). The thickening of thfe walls at this region 

 also produces a shallowing of b (indicated by dotted lines on the right in i). The principal 

 cause of this general thickening is the passage of the fibers of the thalamic radiation to the 

 hemispheres and, later, of fibers from hemisphere to pes, forming the internal capsule (4, 2 

 and 3). 



