INTERNAL ANA TOMY. 



The first two pairs of nerves differ in many points from 

 those which succeed them, and are known as the cranial 

 ncrres. Thus they have no corresponding ventral roots ; 

 they appear to be exclusively sensory, and do not inner- 

 vate any muscles ; their distribution is confined to the 

 snout, and they are above all characterised by the pres- 

 ence of peripheral ganglionic enlargements which occur 

 chiefly on the finer branches of 

 the nerves near their distal ex- 

 tremities. Furthermore they lie 

 in front of the first myotome. 

 The first pair of dorsal spinal 

 nerves (i.e. the third pair alto- 

 gether) belonging to the first 

 myotome passes from the nerve- 

 tube to the skin through the 

 dissepiment which separates the 



Fig. 42 B. Diagram illustrat- 



first myotome from the Second, ing the branching of a dorsal spinal 

 A i ,, n ,1 i- nerve of Amphioxus. (After HAT- 



And so with all the succeeding SCHEK) 



dorsal roots, they lie at the back d -''- Dorsal root - *<* Ramus 



dorsalis. r.v. Ramus ventralis. 

 Of the myotome tO which they r .vi. Ramus visceralis. r.c. Ramus 



1 i i , i ,1 cutaneus ventralis innervating ecto- 



belong, between it and the next derm of metapleun ,. | entral 



followin " Segment. (Cf. Fi"S. or motor root, indicated as if in the 



same plane as the dorsal root. 



2 and 42 A.) 



Shortly after leaving the central nervous system, the 

 dorsal roots divide into two branches, a minus dorsalis 

 and a ram us ventralis (Fig. 42). These two branches run 

 upwards and downwards respectively, in the gelatinous 

 layer of the sub-epidermic cutis ; that is to say, external 

 to the muscles. 



In the Craniota the corresponding branches of the 

 spinal nerves lie for the first part of their course internal 

 to the muscles, between the latter and the notochord. The 



