THE ELASMOBRANCH FISHES 



261 



INNEEVATION 



The ear receives its innervation from the auditory or eighth nerve. This nerve 

 as it leaves the brain stem separates into two main divisions, each of whieli 

 further subdivides. Principal among the branches are the rami to the ampullae 

 of the anterior oblique, the horizontal, and the posterior oblique semicircular 

 canals; and those rami to the utriculus, the sacculus, and to the lagena. 



Sensory Canal System 



The sensory canal system in Heptanchiis consists of cranial canals in the head 

 connected with a canal over the pharynx and a lateral open groove in the 

 body region (see figs. 15, 227, and 228). The lateral canal in Heptanchus is 



Fig. 228. Cephalic canals and ampullae of Lorenzini, Heptanchus maculatus, side 

 view. (G. L. Hanner, orig.) (For explanation see fig. 227.) 



especially simple for even in large specimens the canal as such is closed poste- 

 riorly only to about the fifth cleft, back of which it remains an open groove 

 toward the tip of the tail. 



Anterior to the spiracle and just posterior to the endolymphatic ducts a 

 small transverse or supratemporal canal {cc, figs. 15 and 227a) passes off 

 from the lateral canal toward the median line. This, however, does not meet 

 and fuse with the similar canal from the opposite side. In Heptanchus macu- 

 latus there may l-)e two supratemporal canals on a side, one posterior to the en- 

 dolymphatic duct as just described, the other anterior to it. From the supra- 

 temporal canal the lateral line canal passes slightly outward and forward to 

 join the cranial canals. The cranial canal passing above the eye is the supra- 

 orbital canal (figs. 227 and 228, soc). In front of the eye the supraorbital 

 swerves outward and then sharply inward; it then turns backward and down- 

 ward above the nasal aperture. The infraorbital canal {ioc.) back of the eye 

 drops downward, sends the hyomandibular canal (hmc.) backward, and then 

 continues forward under the eye. Before reaching the nasal aperture this 

 canal is joined by the supraorbital canal (fig. 228). The main infraorbital 

 next bends sharply toward the middle line, without joining the canal of the 



