Section 15. the Dog-Fish Has Ten Pairs of Cranial Nerves,

corresponding to the anterior ten of the rabbit very closely, when we allow for the modification the latter has suffered through the conversion of some part of the spiracular cleft to an eardrum, and the obliteration of the post-hyoid branchial slits.

The first and second nerves are really brain lobes, and nerves of the special senses of smell and sight respectively.

The third (oculomotor), the fourth (patheticus), and the sixth (abducens) are distributed to exactly the same muscles of the eyeball as they are in the rabbit.

The fifth nerve, has, in the dog-fish, as in the rabbit, three chief branches. V.2 and V.3 fork over the mouth just as they do in the mammal; V.1 passes out of the cranium by a separate and more dorsal opening, and runs along a groove along the dorsal internal wall of the orbit, immediately beneath a similar branch of VII., which is not distinct in the rabbit. The grooves are shown in the figure of the cranium, Sheet 18; the joint nerve thus compounded of V. and VII. is called the ophthalmic (oph.). It is distributed to the skin above the nose and orbit. When the student commences to dissect the head of a dog-fish he notices over the dorsal surface of the snout an exudation of a yellowish jelly-like substance, and on removing the tough skin over this region and over the centre of the skull he finds, lying beneath it, a quantity of coiling simple tubuli full of such yellowish matter. These tubuli open on the surface by small pores, and the nerves terminate in hair-like extremities in their lining. These sense tubes are peculiar to aquatic forms; allied structures are found over the head and along a lateral line (see below) in the tadpole, but when the frog emerges from the water they are lost. They, doubtless, indicate some unknown sense entirely beyond our experience, and either only possible or only necessary when the animal is submerged.

In addition to the ophthalmic moiety mentioned above, the seventh nerve has a vidian branch (vid.) running over the roof of the mouth, and besides this its main branches fork over the spiracle, just as V. forks over the mouth, and as IX. and X. fork over gill clefts. This nerve in the rabbit is evidently considerably modified from this more primitive condition.

The eighth is the auditory nerve, as in the rabbit.

The ninth nerve forks over the first branchial cleft.

The tenth nerve is easily exposed by cutting down through the body wall muscles over the gill clefts, into the anterior cardinal sinus (A.C.S.). It gives off (a) branches forking over the posterior four gill slits, (b) a great lateral nerve running inward, and back through the body-wall muscle, and connected with a line of sense organs similar to those in the head, the lateral line, and (c) a visceral nerve curving round to the oesophagus and stomach. In dissection it becomes very evident that the tenth nerve is really a leash of nerves, each one equivalent to the ninth.

We may here call the attention of the reader to the fact of the singular resemblance of V., VII., IX., and the factors of X. That each has a ventral fork, we have already noticed. Each also (?IX.) has a dorsal constituent connected with the sense organs of the skin. The vidian branch of VII., however, is not evidently represented in the others.