INTERNAL ANATOMY. 



83 



examined from above, its general appearance will be as 

 shown in Fig. 41. In front there is a pair of nerves 

 which proceed symmetrically from the sides of the nerve- 

 tube. Farther back there is 

 another pair of nerves which 

 arise more dorsally than the 

 anterior pair, but are likewise 

 placed symmetrically one oppo- 

 site the other. Behind this 

 second pair of nerves the spinal 

 nerve-roots are no longer dis- 

 posed symmetrically, but alter- 

 nate with one another, in cor- 

 respondence with a similar 

 alternation of the myotomes, 

 the alternation becoming more 

 and more pronounced as we 

 proceed backwards. Again, be- 

 hind the second pair of nerves 

 there are two kinds of spinal 

 nerve-roots, dorsal and ventral. 

 The former leave the nerve-cord 

 from its dorsal surface, and the 

 latter from the margins of its 

 ventral side. 



Fig. 41. — Anterior portion of 

 spinal cord of Amptiioxus ; seen 

 In the dorsal roots f™"' ^bove. (After Schneider.) 

 Between the first pair of cranial 

 the nerve-fibrils are collected nerves is seen the eye-spot; one of 

 , , , . r -1 the branches of the second pair of 



together to form a Smgle com- ^^^j^, ^^^^^ sometimes arises 



pact nerve round which the directly from the spinal cord as 



shown on the right; farther back 

 sheath of the nerve-cord is con- are seen the pigment spots of the 



tinued, while in the ventral roots "'^"''^"'=°''' ■ 

 the nerve-fibres emerge separately in loose bundles unsur- 

 rounded by a sheath, from the spinal cord. A pair of 

 dorsal roots and a pair of ventral root-bundles go to each 



