NERVOUS SYSTEM BRAIN. 



483 



pheral system, consisting of spinal and cranial nerves ; and 

 (c) the sympathetic nervous system. 



The central nervous system first appears as a superficial 

 groove along the mid-dorsal line of the embryo. The sides 

 of this ectodermic groove meet, and, uniting, convert the 

 medullary groove into the medullary canal. The greater 



FIGS. 256 and 257. Ideal fore and hind limb. After Gegenbaur. 



H., Humerus ; R., radius ; U., ulna ; /., radiale ; '., ulnare ; /., inter- 

 medium ; c., centrale ; 1-5, carpalia bearing the corresponding 

 digits with metacarpals (me.) and phalanges (ph.). 



/., Femur ; #., tibia ; fi.> fibula ; i. , intermedium ; /., tibiale (astragalus) ; 

 f.) fibulare (0s calcis) \ c. t centrale ; 1-5, tarsalia bearing the corre- 

 sponding digits with metatarsals (tnt.) and phalanges (/>//.). 



part of this canal forms the spinal cord ; the anterior 

 portion of it is specialised as the brain. There is at first 

 a posterior connection between the n'eural canal and the 

 primitive gut of the embryo ; when this is lost the cavity of 

 the neural tube still persists as a little ciliated canal in the 

 centre of the cord, and as the internal cavity of the brain. 

 Brain. At an early stage, even before the closing-m 



