224 



THE ELASMOBRANCH FISHES 



main stems of this complex and out from the cranium through its own fora- 

 men. It enters the base of the external rectus muscle. 



The facial or seventh cranial nerve like the fifth is composed of four im- 

 portant branches. These are first the superficial ophthalmic nerve {os.VII, 

 figs. 200a and 200b) which runs above all the eye muscles through the orbit, 

 gives branches dorsally to the supraorbital sensory canal, and then leaves the 

 orbit by the large anterodorsal ophthalmic foramen (f.o.VII, fig. 47). Out- 

 side of the orbit it supplies branches to the supraorbital canal and to certain 

 groups of the ampullae of Lorenzini. The second or buccal division of the 





Fig. 202. Branches of the facial nerve, HeptancMis maculatus. (W. R. Dennes, orig.) 



bu.VII, buccalis of facial; c.t., chorda tympani; limd., hyomandibular (postspiracular) 

 division of seventh nerve; md.e., superior and inferior branches of external mandibularis of 

 seventh; md.i., internal division of mandibularis of seventh; os.VII, ophthalmicus super- 

 ficialis of seventh nerve; pl.VII, palatinus; po.s., postspiracular twigs; pr.s., prespiracular 

 nerve ; sp., spiracle. 



facial nerve {hu.VII) passes from the brain stem just dorsal to the maxillary 

 division of the fifth (mx.V). In the orbit it divides much like the maxillary 

 division of the fifth. It goes to supply the infraorbital canal and the ophthal- 

 mic and buccal groups of ampullae. The palatine division of the seventh 

 (pl.VII, fig. 202) leaves the main stem of the hyomandibular nerve and passes 

 ventralward, dividing into an anterior and a posterior branch, to the palate 

 of the mouth. The most posterior division of the facial, the hyomandibular 

 {hmd., figs. 200a and 202), after giving off the palatine branch, passes 

 sharply backward around the spiracle, downward around the angle of the 

 jaw and forward along the mandible. It first gives a prespiracular branch 

 (pr.s., fig. 202) to the anterior wall of the spiracle; other twigs (po.s.) are 

 next given off to the posterior wall of the spiracle. Back of the angle of the 

 jaw a superficial branch runs forward toward the angle of the jaw, and two 

 branches (md.e.) pass along the external side of the mandible to the hyoman- 

 dibular canal and mandibular groove (hmc. and mg., fig. 228) . A deep branch 

 (md.i., fig. 202) given off at the angle runs along the body of the hyoid, and 

 another branch, the chorda tympani (c.t.), passes forward between the hyoid 

 and the mandible. 



The auditory or eighth nerve in Heptanchus (VIII, figs. 200a and 200b) is 

 more or less clearly separated from the seventh. It has a large ganglion from 



