350 H. W. NORRIS AND SALLY P. HUGHES 



lateralis elements have been added secondarily. In Squalus, 

 Mustelus, and Raja, and presumably in all elasmobranchs, it is 

 wholly lateralis. 



6. The truncus hyomandibularis VII 



There are three chief constituents of the hyomandibular 

 trunk: a) lateral line, ramus mandibularis externus, whose root, 

 as already described, enters the lateral-line lobe and the acusti- 

 cum together with the buccalis root, and whose ganglion occurs 

 mostly distal to the geniculate ganglion as an elongated column 

 which scarcely affects the size or shape of the hyomandibular 

 trunk (figs. 17, 20, 24, 48, 51 to 53, mde., gmde) ; b) visceral sen- 

 sory, ramus mandibularis interims, whose fibers arise in the geni- 

 culate ganglion (figs. 48, 51 to 53), mdi.); c) visceral motor, 

 ramus hyoideus, the motor constituent of the facial nerve (figs. 

 48, 51 to 53, %.). 



- Emerging from the hyomandibular canal, the hyomandibular 

 trunk passes posterodorsally without branching, at the lateral 

 border of the ear capsule, around the mesial dorsal border of the 

 spiracle to the lateral border of the anterior part of the second 

 dorsal constrictor muscle and at the lateral border of the hyo- 

 mandibular cartilage. Thence it passes posteroventrally along 

 the anterior lateral border of the same muscle and posterior 

 border of the spiracle (figs. 48, 51 to 53, tr. hmd.). 



In Mustelus the lateralis component of the hyomandibularis 

 is relatively smaller than in Squalus, due probably to the lack 

 of the hyoidean group of ampullae of Lorenzini in the former. 



The ramus mandibularis externus VII. The first branch of 

 considerable size given off from the hyomandibular trunk is the 

 anterior division of the ramus mandibularis externus. It leaves 

 the main trunk at the horizontal level of the ventral end of the 

 hyomandibular cartilage and runs anteroventrally, dividing 

 into a dorsal branch that supplies the jugular or hyomandibular 

 canal and a ventral branch that innervates the mandibular canal 

 (fig. 50). After giving off the last twig to the mandibular canal 

 organs, this ventral branch sends a twig into a tubular structure 



