INTERNAL ANA TOMV. 



85 



The first two pairs of nerves differ in many points from 

 those which succeed them, and are known as the cranial 

 nerves. 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 



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

 AT -.1 n ^1 1- nerve of Amphioxus. (After Hat- 



And so with all the succeedmg „-i„r,- ^ 

 dorsal roots, they lie at the back 

 of the myotome to which they 

 belong, between it and the next 

 following segment. (Cf. Figs. 

 2 and 42 A) 



Shortly after leaving the central nervous system, the 

 dorsal roots divide into two branches, a ramus dorsalis 

 and a ramus 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 



Diagram illustrat- 



d.r. Dorsal root. r.d. Ramus 

 dorsalis. ?-.v. Ramus ventralis. 

 r.tv. Ramus visceralis. r.c. Ramus 

 cutaneus ventralis innervating ecto- 

 derm of metapleur. Z'.r. Ventral 

 or motor root, indicated as if in the 

 same plane as the dorsal root. 



