371) CRANIAL NERVES. 



This nerve is connected by a commissure with those behind, but this 

 fact may for the moment be left out of consideration. Springing at 

 first from the dorsal line of the hind-brain immediately behind the 

 level of the auditory capsule, it apparently loses this primitive attach- 

 ment and acquires a secondary attachment about half-way down the 

 side of the hind-brain. The primitive undifferentiated rudiment soon 

 becomes divided, exactly like a true posterior root of a spinal nerve, 

 into a root, a ganglion and a nerve. The main branch of the nerve 

 passes ventralwards, and supplies the first branchial arch (fig. 271 yl). 

 Shortly afterwards it sends forwards a smaller branch, which passes 

 to the hyoid arch in front ; so that the nerve forks over the hyo- 

 branchial cleft. A typical cranial nerve appears therefore, except as 

 concerns its relations to the clefts, to develop precisely like the 

 posterior root of the spinal nerve. 



Most of the cranial nerves of the above group, in correlation with 

 the highly differentiated character of the head, acquire secondary 

 differentiations, and render necessary a brief description of what is 

 known with reference to their individual development. 



The Glossopharyngeal and Vagus Nerves. Behind the ear there 



are formed, in Scyllium, a series of five nerves which pass down to respect- 

 ively the first, second, third, fourth and fifth branchial arches. 



For each arch there is thus one nerve, whose course lies close to the 

 posterior margin of the preceding cleft ; a second anterior branch, forking 

 over the cleft and passing to the arch in front, being developed later. These 

 nerves are connected with the brain by roots at first attached to the dorsal 

 summit, but eventually situated about half-way down the sides. The 

 foremost of them is the glossophaiyngeal. The next four are, as has been 

 shewn by Gegenbaur 1 , equivalent to four independent nerves, but form 

 together a compound nerve, which we may briefly call the vagus. 



This compound nerve together with the glossopharyngeal soon at- 

 tains a very complicated structure, and presents several remarkable 

 features. There are present five branches (tig. 271 B), viz. the glossopha- 

 ryngeal ((jl) and four branches of the vagus, the latter probably arising 

 by a considerably greater number of strands from the brain 1 . All the 

 strands from the brain are united together by a thin commissure (tig. 

 271 B, vg), continuous with the commissure of the posterior roots of the 

 spinal nerves, and from this commissure the five branches are continued 

 obliquely ventralwards and backwards, and each of them dilates into a 

 yanglionic swelling. They all become again united together by a second 

 thick commissure, which is continued backwards as the intestinal branch 

 of the vairus nerve. The nerves, however, are continued ventralwards each 



o ' 



to its respective arch. From the lower commissure springs the lateral 

 nerve, at a point whose relations to the branches of the vagus I have not 

 certainly determined. 



With reference to the dorsal commissure, which is almost certainly 

 derived from the original neural ci'est, it is to be noted that there is a 

 longish stretch of it between the last branch of the vagus and the first spinal 



1 "Ueber d. Kopfnerven von Hexanchus," etc., Jenaische Zeitschrift, Vol. vi. 1871. 



