The eighth. or auditory nerve, is the nerve of the special sense of hearing and is distributed to the internal ear. The nerve is composed of two divisions, the cochlear division and the vestibular division. The fibres of the cochlear division are axmies of bipolar cells in the ganglion of Corti. The peripheral processes of these cells end among the epithelial cells of Corti's organ within the cochlea. Their central processes enter the me dulla at the junction of the medulla and d terminate in two nuclei known .s as the ventral and dorsal nuclei of the cochlear nerve. By means of neurones whose cell bodies are situated in these nuclei, auditory im pressions are carried to higher centres. The neurones of the vestibular division of the nerve have their cell bodies located in Searpa's ganglion or the ganglion vestihul:n•e. These evils are bipolar and their peripheral processes end among the hair cells of the crista and maseula aenstiea. Their central processes enter the medulla and pass to two nuclei of termination in which are cells whose axones carry the impulses to higher cent res. See also EAR.
The ninth, or glosso-pharyngeal nerve, leaves the upper part of the medulla in the groove be tween the olive and the restiform body. It con sists of a sensory part and a motor part. The nerve cells of the sensory part are situated in t.NVO small ganglia, the jugular ganglion and the petrous ganglion. The central pnwesses of these cells enter the medulla and terminate in a nucleus in the floor of the fourth ventricle, which is CO111111011 to this nerve and the tenth. The cells whose axones make up the motor root are situ ated in a nucleus which is called the widens a mbigumA because of its connection with this m•rve and the tenth, and which lies deep in the forma tio reticularis. The two parts of the nerve unite and leave the skull through the jugular foramen. Branches of this nerve are distributed to the tympanum. to the carotid, to the muscular coat and mucous membrane of the pharynx, to the stylo-pharyngeus, to the tonsil, and to the tongue.
The tenth, vagus, or pneumogastric nerve. leaves the medulla just behind the ninth. It is composed of two portions, a sensory and a motor, which are connected With the already described nuclei common to both ninth and tenth nerves. The nerve leaves the cranial cavity through the jugular foramen. In the foramen it presents a swelling known as the ganglion jug Clare or ganglion of the root. while just outside the fora men is a second ganglion. the inferior ganglion Or ganglion of the trunk. It is in these two ganglia that the cell bodies of the sensory neu rones of the nerve are situated. After its exit from the foramen, the nerve passes vertically down the neck within the sheath of the carotid Vessels.
The eleventh nerve, or spinal accessory, is composed of two parts, a spinal part and a cerebral part. or part accessory to the vagns. The latter is the smaller, and arises from a group of cells situated in the medulla below the nucleus of the vagus. It leaves the medulla as three o• four small filaments just below the exit of the vagus. It passes through the jugular foramen in a sheath common to it and the vagus. The spinal portion of the nerve originates in a Iong nuclear situated in the lateral region and ex tending from the middle of the medulla to the fifth or sixth cervical nerve. The fibres leave the cord in the mill-lateral region and pass through the jugular foramen.
The twelfth. or hypoglossal nerve, serves as the motor nerve for the tongue. lts nucleus is situated in the middle and lower part of the medulla. in the floor of the fourth ventricle, and ventral and lateral to the central canal. Fibres from this nuelens leave the medulla in bundles, from ten to fifteen in number, between the olive and the anterior pyramids. The nerve passes through the anterior condyloid foramen. The 1.E:1114114..4 of 111.4111)11i 1011 rirl• the deseendens noni, the thyro-hyoid muscular branches, and menin gen 1 branches.