The second cranial, o• optic nerve, is the nerve concerned in the special sense of sight. It is distributed entirely to the eyeball. The nerves from tile two eyes, passing backward and inward, unite to form the optic commissure. From the euniinksure two large hIllIdleti of fibres —the optic tracts—pass backward and outward to the brain. The eye differs from all other sense organs in that its nervous apparatus is developed by a direct outgrowth from the embry onic neural canal. later. the ingrowth of IlleS0 tissue separates the external or retinal part from the central nervous system. This retinal part is analogous to the posterior root ganglion of the spinal nerves, and, as is the ease with the latter, its subsequent connection with the central nervous system is made by the growth of axones from the peripherally situated cells hack into the central system. For structure of retina, see EYE.
The third, or oculo-motor nerve, is a purely motor nerve. Its fibres are the axones of a the pons, and, passing along the outer wall of the eavernus sinus, enters the orbit through the sphenoidal fissure. It supplies the superior oblique muscle.
The fifth, trifacial, or trig.eminus, is the largest of the cranial nerves. It resembles the spinal nerves in being composed of a motor part. and a sensory part. The fibres of the smaller gro'up of cells situated just to one side of the median line in the floor of the itcr beneath the corpora quadrigemina. Passing internally to the red nucleus, the nerve reaches the verd nd surface of the mid-brain at the inner margin of the cere bral peduncle just in front of the pons. 'flue nerve runs forward along the inner Nvall of the envernus sinus, and, dividing into two parts. en ters the orbit through the sphenoidal fissure. The superior branch of the nerve supplies the superior rectos and levat or pipeline. The inferior branch supplies the internal rectos, the inferior rectus, and the inferior oblique.
The fourth, or pathetie nerve, is the smallest cranial nerve. Like the preceding, it is a purely motor nerve. Its fibres originate in a group of cells in the floor of the Per just below the nuelens of the third nerve. The nerve winds around the outer side of the emus cerebri just in front of motor root mow from cells situated in the 111(411111a. The filmes of the sensory root come from cells in the Gasserian ganglion. The cen tral proeesses of these cells enter the pens and pass to two nuclei in the floor of the fourth yen triele. The Gasserian ganglion lies in a depres sion in the petrous portion of the temporal bone. From the anterior aspect of the ganglion three branches are given off—the ophthalmic, the su perior maxillary, and the inferior maxillary. The oplithahnie divides into three brandies, the lachrymal, frontal. and nasal, which enter the orbit through the sphenoidal fissure and supply the eyeball, lachrymal gland, mucous membrane of the eye and nose, and the skin of the eyebrow, forehead, and nose. Connected with the ophthalmic branch is a small ganglion called the ophthalmic or eiliary ganglion. The superior maxillary is the middle of the branches of the Gasserian ganglion. It crosses the spheno-maxillary fossa,
in which it gives off three branches, the orbital, the spheno-palatine. and the posterior dental. The nerve next enters the infra-orbital canal, where it gives off the anterior dental branch. The nerve then passes to the face through the infra-orbital foramen and terminates in palpehral, nasal, and labial branches. The spheno-palatine or \leckel's ganglion is connected with this branch of the fifth nerve. It sends off four main sets of branches: ascending. to the orbit; descending. to the palate; posterior, to the pharynx; and intern al. to the nose. The inferior maxillary is the largest branch of the Gasserian ganglion and receives the entire motor portion of the nerve. Just beneath the base of the skull the nerve divides into two branches. an anterior branch and a posterior branch. The anterior branch contains nearly all the motor fibres, and. splitting up into masseterie, deep temporal, buccal, and pterygoid branches, supplies the muscles of mastication. The posterior division is mainly sensory and is distributed as three branches—the auriculo-temporal, gustatory, and inferior dental. Two small ganglia, the otie or Arnold's, and the submaxillary, are connected with this branch of the fifth nerve.
The sixth, n abducens nerve. is a small motor nerve whose fibres ale 1:vales of a group of cells situated in the floor of the fourth ventricle. The nerve reaches the under surface of the medulla just behind the polls between the pyramid and the olive. It passes through the sphenoidal fis sure, enters the orbit, and is distributed entirely to the external reetus muscle.
The seventh, or facial nerve, is a motor nerve. Its nucleus of origin emisists of a group of cells situated deep in the formatio reticularis of the upper part of the medulla. The nerve leaves the surface of the brain just behind the between the olive and the restiforni body. thus lying to the outer side of the sixth nerve. The nerve passes forward and enters the internal auditory meatus with the sixth nerve. At the bottom of the meatus it passes into the Fallopian aqueduct, by means of which it traverses the petrons por tion of the temporal bone. finding exit through the stylonmstoid foramen. It then enters the parotid gland, and just behind the jaw divides into two main branches, the tempero-facial and eervieo-facial. During its passage through the temporal bone it is connected with a small gray mass known as the genieulate ganglion. Its branches of distribution are as follows: Within the Fallopian aqueduct a branch to the stapedius muscle, and the cords tympani to the muscle of the tongue (lingtmlis) ; at its exit from the styloid foramen, the posterior auricular branch to the retrahens aurem and part of the oceipito-frontalis, the stylo-hyoid to the muscle of that name, and the digastrie to the digastrie; on the face, the tempero-faeial. dividing into tem poral branches and malar branches, and the cer vico-facial, distributed to the muscles of the head and face.