The _auditory nerve_ arises close behind the ventral division of the trigemino-facial root. The root of the auditory nerve encloses a large _auditory ganglion_. A _vestibular nerve_ arises from the anterior end of the auditory ganglion and passes into the ear capsule, innervating the upper part of the utriculo-sacculus and the ampullae of the anterior and horizontal canals. From the posterior part of the ganglion nerves pass to the ventral part of the sacculus and the ampulla of the posterior canal. Trace these branches as thoroughly as possible. The _abducens nerve_ emerges from the cranium under and close to the origin of the posterior rectus muscle, into which muscle it enters. To expose this nerve the trigeminal, facial and auditory nerves must be lifted and cut as they pass through the wall of the cranium. It can be traced obliquely backward and inward, through a long canal, to its origin near the midline of the ventral surface of the medulla. The _glossopharyngeal nerve_ passes through the base of the ear capsule from the side of the medulla to the upper end of the first branchial pouch. A ganglionic enlargement is found near where it emerges from the cartilage. Outside the cranium the glossopharyngeal divides into a _pretrematic branch_, passing down in front of the first gill pouch, and a _posttrematic branch_ running behind the pouch. The pretrematic branch quickly sends off a _pharyngeal nerve_ which runs antero-ventrally to the roof of the pharynx. The pretrematic and posttrematic branches can be followed along the gill arch to the ventral side of the pharynx. A fourth branch of the glossopharyngeal, the _supratemporal_, springs from the dorsal side of the ganglion; passing through the ear capsule it runs to the dorsal surface of the head, where it is distributed to the sense organs of a short section of the lateral line canal. This small nerve can be demonstrated by carefully separating the muscles and perichondrium from the posterior surface of the auditory capsule. The _vagus nerve_ (or _pneumogastric_) arises by an extensive series of roots from the side of the medulla. An easily distinguished ribbon-like portion of the root, the _lateral line root_, runs forward as far as the root of the glossopharyngeal. Note the canal by which the vagus leaves the cranium, and trace the nerve along the inner side of the anterior cardinal vein. The principal branches of the vagus are: 1. The _supratemporal_ branch, a small nerve running dorsal through the posterior part of the ear capsule to the lateral line canal and other sense organs of the head. It will be found near the supratemporal branch of the glossopharyngeal. 2. The _lateral line branch_, a large nerve which separates from the trunk of the vagus just outside the cranium and runs backward through the muscles, parallel to the vertebral column on a level with the lateral line. It sends off numerous small twigs to the sense organs of the lateral line canal.