770 



DEVELOPMENT OF THE NERVOUS SYSTEM. 



thalami. The third vesicle also separates into two portions, of which 

 the anterior becomes the cerebellum, the posterior the medulla oblongata. 



Fig. 281. 



Fig. 282. 



* 



FOJTAL PlG, 1 centimetre long, showing the condition 

 of the brain and spinal cord. 1. Hemispheres. 2. Tubercula 

 quadrigemina. 3. Cerebellum. 4. Medulla oblongata. 



There are, therefore, at this time five cerebral 

 vesicles, all of which communicate with each 

 other and with the central cavity of the spinal 

 cord. The entire cerebro-spinal axis also be- 

 comes strongly curved in an anterior direction, 

 corresponding with the anterior curvature of 

 the body of the embryo (Fig. 282) ; so that the 

 middle vesicle, or that of the tubercula quadri- 

 2. vesicle of the tubercula gemina, occupies a prominent angle at the upper 



quadrigemina. 3. Vesicle 



Formation of the CERK- 



of the medulla oblongata. 



part of the encephalon, while the hemispheres and 

 the medulla oblongata are situated below it, ante- 

 riorly and posteriorly. At first the relative size of the various parts 

 of the encephalon is very different from that presented in the adult 

 condition. The hemispheres are hardly larger than the tubercula quad- 

 rigemina; and the cerebellum is inferior in size to the medulla oblongata. 

 Soon afterward, the relative position and volume of the parts begin to 

 alter. The hemispheres and tubercula quadrigemina grow faster than 

 the posterior portions of the encephalon ; and the cerebellum becomes 

 doubled backward over the medulla oblongata. (Fig. 283.) Subse- 



Fig. 283. 



Fig. 284. 



FCETAL P i o, three centimetres long. 

 1. Hemispheres. 2. Tubercula quadri- 

 gemina. 3. Cerebellum. 4 Medulla ob- 

 longata. 



HEAD OF FCETAL PIG, nine centimetres 

 long. 1 Hemispheres. 3. Cerebellum. 4. Me- 

 dulla oblongata. 



quently, the hemispheres enlarge more rapidly, growing upward and 

 backward, so as to cover both the optic thalami and the tubercula 



