intermedia between (see fig. 1). Entering the internal auditory
meatus with these structures the facial nerve soon passes into a
canal in the petrous bone known as the aqueductus Fallopii, and in
this it makes a sudden
bend and forms the
geniculate ganglion,
from which the great
superficial petrosal
branch to Meckel’s
ganglion is given off.
| From D. J. Cunningham, in Cunningham’s Text-Book of Anatomy. |
| Fig. 2.—Diagram of the Central Connexions of the Optic Nerve and Optic Tract. |
The canal ends at the stylo-mastoid foramen on the base of the skull, and here the nerve 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. In the aqueduct the pars intermedia joins the seventh, and, beyond the geniculate ganglion, leaves it as the chorda tympani, which runs through the tympanum (see Ear) to join the lingual branch of the fifth. It is probable that the pars intermedia, geniculate ganglion, and chorda tympani, represent the sensory root of the facial nerve. Just outside the stylo-mastoid foramen the facial gives off the posterior auricular branch to the occipitalis and posterior auricular muscles, as well as a branch of supply to the stylohyoid and posterior belly of the digastric muscles.
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 (see fig. 3), while 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, while, as is pointed out in the article Brain, the cochlear nuclei are connected with the inferior quadrigeminal body by the lateral fillet as well as with the internal geniculate body, while this body again is connected with the higher auditory centre in the grey cortex of the temporo-sphenoidal lobe by the auditory radiations. The vestibular root passes in front of the restiform body (see fig. 3), and the cochlear behind that body. Together they enter the internal auditory meatus, and, at the end of it, pierce the lamina cribrosa, the vestibular nerve supplying the utricle and superior and external semicircular 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 (See fig. 3. and Brain, fig. 4). It appears on the surface between the olive and restiform body (see fig. 1), and leaves the skull through the posterior lacerated foramen; as it does so two ganglia, the jugular and petrous, are formed on it, after which it runs downward and forward, between the internal and external carotid arteries, and eventually reaches the back of the tongue (see Tongue). On its way it supplies the tympanum, the stylopharyngeus muscle, though there is grave doubt as to whether these fibres are not really derived from the facial nerve, contributions to the pharyngeal plexus, the tonsil and part of the epiglottis.
The tenth nerve or vagus has sensory and motor fibres; the former go to the solitary bundle mentioned in the description of the last nerve (see 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 below the ninth (see fig. 1). It leaves the skull through the posterior lacerated foramen, and, like the glossopharyngeal, has two ganglia developed on it; the upper of these is the ganglion of the root, and the lower the ganglion of the trunk (see fig. 4). From the former the auricular branch or Arnold’s nerve (see Ear) comes off, while from the latter are given off the pharyngeal branches to the pharyngeal plexus (fig. 4, Ph.) and the superior laryngeal branch which is the sensory nerve of the larynx (fig. 4, S.L.). Between the two ganglia the accessory part of the eleventh nerve joins the tenth, and it is from this communication that the motor twigs to the pharynx, larynx, alimentary and respiratory tracts are derived, as well as the inhibitory fibres of the heart. In the neck the vagus accompanies the carotid artery and internal jugular vein, and here it gives off superior and inferior cardiac branches. The left inferior cardiac branch passes to the superficial, while the three others go to the deep cardiac plexus. The nerve, now enters the thorax, passing between the subclavian artery and vein. On the right side its recurrent laryngeal branch loops under the subclavian artery (fig. 4, R.), 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 posterior pulmonary plexus after giving off some branches for the much smaller anterior pulmonary plexus; they then reach the oesophagus, where they again break up into an oesophageal plexus or plexus gulae. As the diaphragm 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.
| From D. J. Cunningham, in Cunningham’s Text-Book of Anatomy. |
| Fig. 3.—Deep Origins of Cranial Nerves from the Fourth Ventricle. |
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 accessory part, the deep origin of which is the lower part of the nucleus ambiguus. The accessory part, as has been noticed, joins the vagus, while the spinal part pierces the sterno-mastoid muscle and runs obliquely downward
