10 



CRANIAL NERVES 



certain portions) would avoid the assumption that new central 

 connections have been established with the brain (Johnston [248a]). 

 The intestinal branch of the vagus may perhaps have developed in 

 somewhat the same manner from the visceral branches of the more 

 posterior nerves. The sympathetic system of nerves, scarcely 

 recognisable as such in the Cyclostomes, would appear to be a 

 specialisation in the general plexus supplying the unsegmented 

 splanchnic structures, and is developed from the dorsal roots and 

 ganglia in the main. The ' dorsal ' cranial nerves are provided 



mt>. 



br. 1 



br? 



FIG. 5. 



Reconstruction of the head of an embryo of Acn.nthins, enlarged. (After Sewertzoff.) .<, 

 cartilage of auditory capsule; al, alisphenoid cartilage; an, auditory capsule; br 1 - 5 , tirst to 

 fifth branchial arches ; cp, epiphysis ; /.ft, fore-brain ; g, spinal ganglion ; It, hyoiil arch ; A. ft, 

 hind-brain ; in, mandibular arch ; rn.b, mid-brain ; w, nasal pit ; p, parachordal plate ; s 8 , eighth 

 scleromere ; tr, trabecula ; v.r, ventral spinal root ; 5, 7, 0, 10, roots of the trigeminal, facial, 

 glossopharyngeal, and vagus nerves. 



with ganglia ; these generally sink deep down, and may even enter 

 the cranial cavity. 



The ventral roots of the segments in the branchial region are 

 variously affected by the fate of their corresponding myotomes. 

 In the Cyclostomes (Petromyzon) they are normally developed 

 (p. 5). Since the myotomes behind the vagus root have a tendency 

 to disappear from before backwards in the Gnathostomes (p. 5), 

 the ventral roots dwindle also. They survive, however, in so far 

 as they supply the epibranchial and hypobranchial muscles (Fig. 5). 

 These are developed, as already mentioned, as ventral downgrowths 

 from somites of the posterior branchial segments and of a varying 

 number of segments farther back (from about the 8th to the 12th 



