brief statement of some of the main regults arrived at. The
outgrowth for the vagus nerve supplies in the embryo the fourth
and succeeding visceral arches, and from what we know of it
in the lower vertebrate types, we may conclude that it is a
compound nerve, composed of as many primitively distinct
nerves as there are branches to the visceral arches.
The glossopharyngeal nerve is the nerve supplying the third
visceral arch, the homologue of the first branchial arch of Fishes.
The development of the hypoglossal nerve is not known, but it is
perhaps the anterior root of a spinal nerve. The spinal accessory
nerve has still smaller claims than the hypoglossal to be regarded
as a true cranial nerve. The primitively single root of the
seventh auditory nerves divides almost at once into two branches.
The anterior of these pursues a straight course to the hyoid arch
and forms the rudiment of the facial nerve, Fig. 67, vil; the second
of the two, which is the rudiment of the auditory nerve, develops
a ganglionic enlargement, and, turning backwards, closely hugs
the ventral wall of the auditory involution. The sixth nerve
appears to arise later than the seventh nerve from the ventral
part of the hind-brain, and has no ganglion near its root.
Shortly after its development the root of the fifth nerve shifts
so as to be attached about half-way down the side of the brain.
A large ganglion is developed close to the root, which becomes
the Gasserian ganglion. The main branch of the nerve grows
into the mandibular arch (Fig. 67), maintaining towards it similar
relations to those of the nerves behind it to their respective
arches.
