THE ELASMOBRANCH FISHES 239 



of the seventh nerve. At the anterior end of the orbit the 

 two nerves pass through the ophthalmic foramen to the surface 

 of the snout, where they break up into branches supplying 

 the mucous canals of that region. The main branch of the 

 fifth nerve passes through a large foramen in the posterior 

 angle of the orbit and runs over the orbital floor as a broad 

 flat band. About half-way across it divides into two branches. 

 The anterior and larger of these is the maxillary nerve; it 

 passes over the edge of the upper jaw to the anterior end 

 of the orbit, and there breaks up into branches, supplying the 

 skin and mucous canals on the under side of the snout. The 

 smaller posterior branch is the mandibular nerve. It runs 

 outwards over the upper edge of the palato-quadrate cartilage, 

 and turning downwards, passes between the levator labii 

 superioris and adductor mandibuli muscles, supplying both 

 with nerves, and is continued forward towards the mandibular 

 symphysis. It will be noticed that the main branch of the 

 fifth nerve forks over the mouth, one branch, the maxillary, 

 running in front of, and the other, the mandibular, behind, 

 the mouth opening. The significance of this relation will be 

 better understood after consideration of the other cranial- 

 nerves. 



The seventh nerve, like the fifth, has three branches. The 

 anterior, or ophthalmic branch, originates from the dorsal edge 

 of the medulla behind the cerebellum, and runs forward, 

 leaving the skull by an aperture above and behind the 

 aperture for the ophthalmic division of the fifth. In the orbit 

 it joins the latter nerve and runs close alongside of it for 

 the remainder of its course. The main branch of the seventh 

 leaves the skull in company with the main branch of the fifth, 

 and runs outwards and backwards along the hinder wall of 

 the orbit and in front of the auditory capsule. Shortly after 

 leaving the skull it gives off a palatine branch, which traverses 

 the floor of the orbit behind and parallel to the main branch 

 of the fifth. On reaching the upper jaw, the palatine nerve 

 divides into two branches, the anterior running forward along 

 the jaw, the posterior turning downwards and inwards between 

 the jaw and the skull, and innervating the mucous membrane 

 of the roof of the mouth. Just to the outside of the palatine 

 branch the main branch of the seventh gives off one or two 

 slender nerves to the prespiracular muscle (musculus levator 



