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 
Fics. 256 and 257,—Ideal fore and hind limb.—After Gegenbaur. 
H., Humerus; R., radius; U., ulna; ~., radiale; w’., ulnare; ¢., inter- 
medium; ¢., centrale; 1-5, carpalia bearing the corresponding 
digits with metacarpals (zc.) and phalanges (£%.). 
SJ, Femur ; 72., tibia ; 72., fibula ; z., intermedium ; 4., tibiale (astragalus); 
JS» fibulare (os calcis); ¢., centrale; 1-5, tarsalia bearing the corre- 
sponding digits with metatarsals (#¢.) and phalanges (f/.). 
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 neural 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-in 
