48 MORPHOLOGY OF THE ORGANS OF VERJ^EBRATES. 



tral rami interlace to form a plexus (cervical and brachial in 

 front, lumbar and sacral for the hind limb), from which nerves 

 are distributed to the appendage. 1 The spinal 

 cord is enlarged where the spinal nerves forming 

 these plexuses are given off. 



In the early stages the spinal cord is 

 as long as the region of the body sup- 

 plied by it, but with increase in size the 

 other tissues grow faster than the cord, 

 As a result, the more posterior spinal 

 nerves take a very oblique course, while 

 the hinder end of the cord is drawn out 

 into a very slender thread, the filum 

 terminale. The large bundle of nerves 

 which consequently extends be- 

 hind the cord forms what is 

 known as a cauda equina. 

 The brain is an enlarged 

 and immensely 

 complicated ante- 

 rior end of the 

 central nervous 

 system, and yet 

 we can trace in 

 it some of the 

 constituents we 

 have come to rec- 

 ognize in the 

 spinal cord. Very 

 soon after the clo- 

 sure of the neural 

 tube the region 

 which is to form 

 the brain becomes 

 differentiated into three hollow enlargements or vesicles, which 

 have received the names, according to position, of fore, mid, 



1 A brachial plexus is formed in snakes and footless lizards. None is found in the 

 caecilians. Siren lacks a sacral plexus. 



HI 



FIG. 49. Right human cervical and brachial plex- 

 uses (from Martin) showing the interlacing of nerve 

 trunks. C, I- VII, roots of cervical nerves ; D, I-III, 

 three anterior dorsal roots; 1-4, nerves of cervical 

 plexus; 4', phrenic nerve (to diaphragm) ; r, circumflex ; 

 r, musculo-cutaneus ; ic, internal cutaneus; m, median; 

 2, intercostals; u, ulnar. 



