ISU8 HANDBOOK OF PHYSIOLOGY. 



is almost from the first widened out and divided into three vesicles. 

 From the anterior vesicle (thalamencephalon) the two primary optic 

 vesicles are budded off laterally : their further history will be traced in 

 the next section. Somewhat later, from the same vesicle the rudiments 

 of the hemispheres appear in the form of two outgrowths at a higher 

 level, which grow upward and backward. These form the prosen- 

 ccphalon. 



In the walls of the posterior (third) cerebral vesicle, a thickening 

 appears (rudimentary cerebellum) which becomes separated from the 

 rest of the vesicle by a deep inflection. 



At this time there are two chief curvatures of the brain (fig. 502). 

 (1.) A sharp bend of the whole cerebral mass downward round the end 



771 



Fig-. 502. Early stages iu development of human brain (magnified). 1, 2. 3. are from an 

 embryo about seven weeks old ; 4, about three months old. m, middle cerebral vesicle (meseu- 

 cephalon) : c, cerebellum ; mo, medulla oblongata: /. thalameiicephalon; h. hemispheres; i', in- 

 fundibulum; Fig. 3 shows the several curves which occur in the course of development; Fig. 

 4 is a lateral view, showing the great enlargement of the cerebral hemispheres which have 

 covered in the thalami, leaving the optic lobes, m, uncovered. (Kolliker.) 



N. B. In Fig. 2 the line ? terminates in the right hemisphere; it ought to be continued into 

 the thalamencephalon. 



of the notochord, by which the anterior vesicle, which was the highest 

 of the three, is bent downward, and the middle one conies to occupy 

 the highest position. (2.) A sharp bend, with the convexity forward, 

 which runs in from behind beneath the rudimentary cerebellum sepa- 

 rating it from the medulla. 



Thus, five fundamental parts of the fo3tal brain may be distinguished, 

 which, together with the parts developed from them, may be presented 

 in the following tabular view : 



