518 COMPAKATIVE ANATOMY. 



its stapedial, digastric, and stylohyoid, as also its auricular branches, 

 belong to the primitive hyoidean area. Its visceral branch appears 

 to be the palatine of Fishes ; in the Mammalia this is represented 

 by the petrosus superficialis major, and passes to the muscles of the 

 velum palati through the spheno-palatine ganglion. The chorda 

 tympani forms a connecting twig between the facial and the third 

 branch of the trigeminal ; this is found even in Fishes. 



One of the nerves for the optic muscles, the abducens, must also 

 be regarded as part of the facial nerve, as is clear from its area of 

 origin. It supplies, as a rule, the rectus externus, and in Petromyzon 

 the rectus inferior also. The position of the rectus externus explains 

 how it is that it belongs to a different nervous area from that of the 

 other optic muscles. 



§ 390. 



The first nerve of the vagus group, the glossopharyngeal, is 

 the simplest in character. In the Selachii it is distinct, as it is also 

 in the greater number of the Teleostei; in Ohimaera, however, it 

 leaves the cranial cavity in company with the vagus, with which 

 nerve it is united in the Cyclostomata and in Lepidosiren. It has 

 the same relations in the Amphibia, but in the Amniota it is 

 ordinarily distinct. 



In Fishes (many Sharks) it has a dorsal branch, which takes an 

 upward course within the cranium, and there ramifies on the surface. 

 The chief trunk (Fig. 290, Gp) is the ventral one; this is distri- 

 buted all along the first branchial arch ; it gives off a pharyngeal 

 branch to the wall of the pharynx, which represents its visceral 

 branch. When the first branchial arch is metamorphosed, this 

 arrangement is so far modified that the pharyngeal branch and 

 the lingual, which ends in the mucous membrane of the tongue, form 

 the chief portion of the nerve. 



The vagus is closely attached to the glossopharyngeal at the 

 point where it leaves the myelencephalon ; to fully understand this 

 nerve it is necessary to have a knowledge of its simplest characters, 

 as they are best seen in Sharks (cf . Fig. 290). The vagus is here made 

 up of a large number of separate roots, which have their origin in 

 the myelencephalon, nearly as far back as the fourth ventricle ; the 

 anterior ones, which have their origin just behind the glossopharyn- 

 geal, are the larger. The posterior ones become gradually smaller 

 and smaller than those in front of them. The last of the series are 

 collected into a small trunk, which passes forwards, and is attached 

 to one of the more anterior ones. The common trunk thus formed 

 passes through the wall of the cranium obliquely backwards and 

 outwards ; on its course it gives off a small dorsal branch for the 

 occipital region. u 



When it has passed out of the cranium the vagus trunk gives off 

 a number of branchial branches, in correspondence with the number 



