CRANIAL NERVES 



Among the former may be reckoned the separate course of the 

 ventral motor roots (for these do not mix with the dorsal roots), 

 and the more superficial course of the dorsal ganglionated roots. 

 The cranial nerves of the Craniate, in fact, more closely resemble the 

 segmental nerves of Amphioxm than do the spinal nerves (Fig. 1). 

 The existence of a large number of motor fibres in the dorsal 

 nerves is correlated with the great development of the visceral 

 muscles in the head. This, the chief difference between the two 

 types of nerves, is one of degree. Likewise with regard to the 



no 



FIG. 3. 



Diagrams of the anterior region of a Cyclostome, A, ami of a Gnathostome, B, showing'the 

 position ami development of the myotomes (numbered 1-18). In the Gnathostome the first few 

 metaotic myotomes (4-0) are more or less vestigial. Tho sense-capsules are indicated by dotted 

 lines, a, auditory, n, nasal, and o, optic capsule ; g.s I-VII, gill-slits ; h.p, hypoglossal processes 

 of myotomes ; m, mouth ; s, spiracular slit. 



position they occupy, the cranial nerves seem to lie outside the 

 muscles, while the spinal nerves lie mostly on the inner side of the 

 muscles ; this is explained by the fact that the myotomes are well 

 developed in the trunk and tail, but scarcely at all in the head 

 of Gnathostomes (Fig. 1). The position of the post - auditory 

 cranial nerves is normal in the Cyclostomes, where the myotomes 

 are present. 



Setting aside the special olfactory and optic nerves, we find 

 that the 5th cranial nerve consists of a pair of segmental nerves 

 (Fig. 2). The ventral root of the first (Illrd nerve or motor oculi) 

 supplies eye-muscles derived from the first or premandibular somite 



