THE ELASMOBRANCn FISHES 243 



The external iiiaiulil)ularis is in relation to a ganglion by the same name. It 

 leaves the cranium through the facial foramen (see fig. 47, f.VIP, facing 

 p. 44), and passes upward around the spiracle as a part of the ramus hyo- 

 mandibularis. In Squalns acanthias the external mandibularis (md.e.VII, fig. 

 220) divides into an anterior and a posterior division. The anterior division 

 goes to the hyomandibular and mandibular canals. The posterior division 

 supplies the hyoidean ampullae and pit organs. 



The first divisions of the facial which are not connected with lateral line or 

 ampullary organs are certain branches of the hyomandibularis. 



The hj'omaiidibular trunk after leaving the brain stem passes backward, 

 the main stem arching posteriorly around the spiracle as was described above 

 for its external branch. Before reaching the spiracle, however, the palatine 

 nerve separates from the hyomandibular trunk. It arises in the geniculate 

 ganglion (g., fig. 221) and passes as a sensory nerve to the roof of the mouth 

 where, as in Heptanchus, some of its fibers pass forward and others extend 

 over the posterior part of the roof to supply "taste buds." After having given 

 off the palatine, the hyomandibularis supplies a small pretrematicus to the 

 anterior side of the spiracle. 



At about the place where the external mandibular VII, previously de- 

 scribed, is given off from the ramus hyomandibularis the internal mandibular 

 (md.i.VII) separates from the main trunk. This, like the palatine nerve, is 

 sensory and goes to the mucous membrane along the inner side of the man- 

 dibular arch. 



The ramus hyoidius VII (fig. 220) is a motor branch which in Squalus acan- 

 thias divides into an anterior and a posterior division. The anterior branch 

 supplies the second, and the posterior, a part of the first ventral constrictor 

 muscles. 



The chorda tympani is variously interpreted for the Elasmobranchs. In 

 Acanihias {ct., fig. 221) it is apparently a continuation from the palatine 

 branch (pl.VII) and is therefore in front of the spiracle. In Hexanchus it has 

 been considered by Ruge (1897) to be a direct continuation of the hyoman- 

 dibularis and as such is therefore a post-trematicus. In both it is a sensory 

 nerve and supplies the mucous lining in the floor of the mouth just mediad 

 of the teeth. 



The hyomandibularis, in Torpedo, gives rise to a first electric nerve which 

 goes to the anterior and inner part of the electric organ. 



The auditory nerve {VIII, fig. 220) has a large ganglion just back of the 

 ganglia of the seventh nerve. From this fibers extend to the ear in two general 

 groups, an anterior vestibular and a posterior saccular group. Root fibers join 

 the ganglion to the medulla just back of the seventh nerve and terminate 

 around cells in the tuberculum acusticum of the medulla. The distribution of 

 the fibers to the ear will be discussed further in a study of the ear. 



The glossopharyngeal, as described for Heptanchus, may be taken as a typ- 

 ical "mixed" nerve. It is made up of both motor and sensory fibers, the former 

 arising in its visceromotor nucleus of the medulla and the latter springing 



