CEEEBEAL NEEVES OE VEETEBEATA. 521 



clefts iu its walls, and belonged to the pharynx, into a portion of the 

 intestinal tract, serving exclusively for the reception of food. Nor 

 is there anything remarkable in the existence of cardiac branches, 

 when the fact is recognised that the heart was derived from a region 

 which was partly within the area supplied by the vagus. 



The ramus lateralis appeal's to be a sensory branch of the vagus, 

 which was some time in being developed proportionately to the 

 growth of the sensory apparatus of the lateral line, which is inner- 

 vated by it. 



Taking therefore the vagus as a whole, we see that in it, just 

 as — though to a less extent — in other nerves (e.g. the facial 

 and trigeminal of the Amphibia), a number of nerves are united 

 together ; and these nerves show both by their origin and by 

 their peripheral relations that they are the remnants of nerves 

 which were primitively distinct; our view, therefore, of the 

 characters of the vagus agrees very closely with the in- 

 dications given us by the posterior portion of the cranium. 



The phenomenon of the concrescence of separate nerves is 

 carried still farther in the vagus of the Selachii, and destroys all 

 signs of individuality, for in most of them (all the Rays) the 

 separate roots come nearer to one another ; this arrangement is the 

 dominant one in all other Fishes also. 



In the Teleostei some of the relations of the vagus are much 

 changed. A few filaments of the hinder roots fuse with an inferior 

 root, and form a large nerve, which passes out from the cranium, and 

 seems to go to the muscles of the shoulder- girdle. The relations of 

 this nerve require more accurate investigation. 



In other points the peripheral relations of the vagus are the same 

 as those which we have described. A dorsal branch, which is present 

 in some of the Teleostei, must be more particularly mentioned. It is 

 united to a dorsal branch (R. recurrens) of the trigeminal, and goes to 

 the base of the dorsal fin, receiving connecting branches from some 

 spinal nerves as it does so. 



§ 392. 



In the Amphibia the vagus has the same relations as in Fishes, 

 so long as the gills are present ; it- even gives off a lateral branch, 

 which in the Caducibranchiata has the same fate as the branchial 

 branches, when the gills are atrophied. 



In the Amniota the only portion of the vagus which is present is 

 the part which is developed from the anterior portion, of the series 

 of superior roots of the Selachian vagus ; the trunk, which they form, 

 is only distributed as far as the stomach in the intestinal tract, while, 

 owing to the absence of gills, the branchial branches have dis- 

 appeared, or — and this is probably more correct — have been partly 

 converted into pharyngeal branches. In Fishes the air-bladder, 

 which is differentiated from the enteric tube, receives branches from 

 the vagus, and so also the respiratory apparatus of the Amphibia 

 and of the Amniota, which has a similar origin, also receives branches 



