1 70 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



pears without leaving a trace, unless it contribute to the ciliary ganglion. Its status 

 as a nerve is very uncertain. 



The Eye Muscle Nerves (fig. 137). The III (oculomotorius), 

 IV (trochlearis) and VI (abducens) nerves are distributed to the 

 muscles which control the movements of the eye and hence are treated 

 together. The oculomotor supplies the superior, inferior and internal 

 rectus and inferior oblique muscles; the trochlearis goes to the superior 

 oblique, while the abducens innervates the external rectus muscle. 



FIG. 170. Diagram of branches and components of the fifth or trigeminal nerve in a 

 shark, gg, Gasserian ganglion; /', Jacobson's commissure, connecting with glossopharyn- 

 geal; md, mandibularis nerve; mx, maxillaris nerve; op, os, ophthalmicus profundus and 

 superficialis nerves. 



These peculiarities of distribution are explained by the development 

 of the muscles (p. 128), the derivatives of each somite having a common 

 nerve supply. The oculomotor nerve springs from the ventral surface 

 of the mid-brain, the fourth from the dorsal surface at the hinder margin 

 of the mesencephalon, while the sixth comes from the ventral surface 

 of the myelencephalon. Inside the brain the trochlearis is traced to 

 its nucleus in a ventral position. 



In the majority of vertebrates these nerves are readily traced from 

 the brain to the muscles they supply, but not infrequently the abducens 

 (lacking in Petromyzon) is united proximally with the fifth nerve, 

 while in a few forms the trochlearis has not been recognized, and it is 

 said that in the adult Bdellostoma all eye-muscle nerves are lacking. 

 The ciliary ganglion is closely associated with the oculomotor nerve. 

 All three eye-muscle nerves belong to the somatic motor group. 



V. The Trigeminal, one of the largest of the cranial nerves, arises 



