The sixth or abducent nerve rises from a nucleus in the floor of the fourth ventricle deep to the eminentia teres (fig. 3). It appears on the surface of the brain just below the pons and close to the middle line (fig. 1), pierces the dura mater and runs in the floor of the cavernous sinus to the sphenoidal fissure. Entering the orbit it supplies the external rectus muscle.
The seventh or facial nerve begins in a nucleus which is about the same level as that for the sixth, but much deeper from the floor of the fourth ventricle as well as farther from the middle line (fig. 3). The fibres of the facial loop round the nucleus of the sixth, and then emerge in the triangular interval between the medulla, pons and cerebellum, close to the eighth nerve (fig. 1). Entering the internal auditory meatus the facial nerve passes into a canal in the petrous bone (aqueductus Fallopii), emerges at the stylo-mastoid foramen on the base of the skull, and enters the parotid gland, in which it forms a plexus called the pes anserinus. From this, branches pass to all the muscles of the face except those of mastication.
The eighth or auditory nerve is in two bundles, cochlear and vestibular. The former comes from the cochlear nuclei which lie deep to the acoustic tubercle in the floor of the fourth ventricle (fig. 3) ; the latter rises from the dorsal nucleus, nucleus of Deiters and the nucleus of the descending root, which are more deeply placed. The nucleus of Deiters is connected with the cerebellum, and is concerned in maintaining the equilibrium (q.v.) of the body; the cochlear nuclei are connected with the inferior quadrigeminal body by the lateral fillet as well as with the internal geniculate body, and thus with the higher auditory centre in the temporo-sphenoidal lobe by the auditory radiations. The vestibular root passes in front of the restiform body (fig. 3), and the cochlear behind that body. Together they enter the internal auditory meatus, pierce the lamina cribrosa, the vestibu lar nerve supplying the utricle and superior and external semi circular canals, the cochlear nerve, the posterior canal, the saccule and the cochlea (see EAR).
The ninth or glossopharyngeal nerve is chiefly, if not entirely, sensory, and its deep termination in the brain is the solitary bundle (fig. 3). It appears on the surface between the olive and restiform body (fig. I), leaves the skull through the posterior lacerated foramen, runs downward and for ward, between the internal and external carotid arteries, and is approached the two nerves become distinct again, but the left one now lies in front and the right behind the food tube, so that, when the stomach is reached, the left vagus supplies the front of the organ and communicates with the hepatic plexus, while the right goes to the back and communicates with the coeliac, splenic and renal plexuses.
The eleventh or spinal accessory nerve is entirely motor, and consists of a spinal and an accessory part. The
former rises from the anterior horn of the grey matter of the spinal cord as low as the fifth cervical nerve. Its fibres come to the surface mid-way between the anterior and posterior nerve roots, and run up through the foramen magnum to join the acces sory part, the deep origin of which is the lower part of the nucleus ambiguus. The accessory part joins the vagus, while the spinal part pierces the sterno-mastoid muscle and runs downward and backward across the posterior triangle of the neck to enter the trapezius; both these muscles are in part supplied by the nerve.
The twelfth or hypoglossal nerve is motor, and rises from a nucleus in the floor of the fourth ventricle. It emerges from the brain between the anterior pyramid and the olive (fig. 1), and leaves the skull in two bundles through the eventually reaches the back of the tongue (see TONGUE).
Vagus.—The tenth nerve or vagus has sensory and motor fi bres; the former go to the solitary bundle mentioned in the de scription of the last nerve (fig. 3), while the latter come from the dorsal nucleus and nucleus ambiguus, both of which are found deep to the lower half of the fourth ventricle. The nerve appears on the surface between the olive and restiform body and just be low the ninth (fig. I). It leaves the skull through the posterior lacerated foramen, gives off auricular and pharyngeal branches and the superior laryngeal branch which is the sensory nerve of the larynx (fig. 4). The accessory part of the eleventh nerve joins the tenth, and from this communication motor twigs to the pharynx, larynx, alimentary and respiratory tracts are derived, as well as inhibitory fibres of the heart. In the neck the vagus ac companies the carotid artery and internal jugular vein, and gives off cardiac branches. It enters the thorax between the subclavian artery and vein. On the right side its recurrent laryngeal branch loops under the subclavian artery (fig. 4), and runs up to supply all the muscles of the larynx except one (see RESPIRATORY SYSTEM). In the thorax the left vagus passes in front of the arch of the aorta, under which the left recurrent laryngeal loops, and on both sides a thoracic cardiac branch is given to the deep cardiac plexus. Both vagi pass behind the root of their own lung, and break up to form the pulmonary plexus; they then reach the oesophagus, where they again break up into an oesophageal plexus. As the diaphragm anterior condylar foramen. Soon after it is closely bound to the vagus, and receives contributions from the loops between the first and second and the second and third cervical nerves. The nerve then passes downward and runs forward on the surface of the hyo-glossus to the muscles of the tongue. It gives off branches to many of the depressor muscles of the hyoid bone and larynx. All the true muscles of the tongue are supplied by the medullary part of the nerve.