Page:EB1911 - Volume 19.djvu/413

 The second or optic nerve consists of the optic tract, the optic commissure or chiasma, and the optic nerve proper. The optic tract begins at the lower visual centres or internal and external geniculate bodies, the superior quadrigeminal body and the pulvinar (see fig. 1), but these again are connected with the higher visual centre in the occipital lobe by the optic radiations (see fig. 2). In the chiasma some of the fibres cross and some do not, so that the right optic tract forms the right half of both the right and left optic nerves. In addition to this the fibres coming from the internal geniculate body of one side cross in the chiasma to the same body of the opposite side, forming Gudden’s commissure. The optic nerve passes through the optic foramen in the skull into the orbit, where it is penetrated by the central artery of the retina, and eventually pierces the scelerotic just internal to the posterior pole of the eyeball. Its final distribution is treated in the article.

The third or oculomotor nerve rises from a nucleus in the floor of the aqueduct of Sylvius (see, fig. 8), and comes to the surface in a groove on the inner side of the crus cerebri (fig. 1); it soon pierces the dura mater, and lies in the outer wall of the cavernous sinus, where it divides into an upper and lower branch. Both these enter the orbit through the sphenoidal fissure, the upper branch supplying the superior rectus and levator palpebrae superioris muscles, the lower the inferior and internal rectus and the inferior oblique, so that it supplies five of the seven orbital muscles.

The fourth or trochlear nerve is very small, and comes from a nucleus a little lower than that of the third nerve. It is specially remarkable in that it crosses to the opposite side in the substance of the valve of Vieussens of the fourth ventricle, after which it winds round the outer side of the crus cerebri (fig. 1) and enters the outer wall of the cavernous sinus to reach the orbit through the sphenoidal fissure. Here it enters the superior oblique muscle on its orbital surface.

The fifth or trigeminal nerve consists of motor and sensory roots. The motor root rises from a nucleus in the upper lateral part of the floor of the fourth ventricle, as well as by a descending (mesencephalic) tract from the neighbourhood of the Sylvian aqueduct (see fig. 3). The large sensory root goes to a sensory nucleus a little external to the motor one, and also, by a spinal or descending root, to the substantia gelatinosa Rolandi as low as the second spinal nerve (see fig. 3). The superficial origin of the fifth nerve is from the side of the pons (see fig. 1), and the two roots at once

pass into a small compartment of the dura mater, in front of the apex of the petrous bone, known as Meckel’s cave; here the large crescentic Gasserian ganglion is formed upon the sensory root, and from this the three branches come off, earning the nerve its name of trigeminal. The first of these divisions is the ophthalmic, the second the maxillary, and the third the mandibular, while the motor root only joins the last of these. The first or ophthalmic division of the fifth runs in the outer wall of the cavernous sinus, where it divides into frontal, lachrymal and nasal branches. They all enter the orbit through the sphenoidal fissure. The frontal nerve divides into supraorbital and supratrochlear, which pass out of the upper part of the anterior opening of the orbit and supply the skin of the forehead and upper part of the scalp as well as the inner part of the eyelids. The lachrymal nerve supplies that gland and the outer part of the upper eyelid. The nasal nerve gives off a branch to the ciliary or lenticular ganglion, which lies in the outer part of the orbit, and through which, as well as through its own long ciliary branches, it supplies the eyeball with sensation. It leaves the orbit through the anterior ethmoidal canal, and lies for a short distance on the cribriform plate of the ethmoid; it then enters the nasal cavity through the nasal slit and supplies this cavity, as well as the surface of the nose as far as the tip, with ordinary sensation. The second or maxillary division of the fifth nerve leaves the skull through the foramen rotundum, and then runs across the roof of the spheno-maxillary fossa; here the spheno-maxillary or Meckel’s ganglion hangs from it by two roots. The nerve then runs in the floor of the orbit, giving off superior dental branches, until it emerges on to the face at the infraorbital foramen, where it divides into palpebral, nasal and labial branches, the names of which indicate their distribution. The third or mandibular division of the fifth leaves the skull through the foramen ovale, and at once gives off a set of motor branches for the muscles of mastication; these are derived from the motor root of the fifth, except that for the buccinator, which really supplies only the skin and mucous membrane in contact with the muscle. After the motor branch is given off, the third division of the fifth divides into lingual, inferior dental and auriculo-temporal. The lingual is joined by the chorda tympani branch of the facial nerve, and then passes to the anterior two-thirds of the tongue. In its course it passes deep to the submaxillary gland, and here the small submaxillary ganglion is connected with it by two roots. The inferior dental nerve gives off a small motor branch to the mylohyoid and posterior belly of the digastric muscles, and then enters a canal in the lower jaw, where it gives off twigs to all the lower teeth. A mental branch comes out through the mental foramen to supply the skin of the chin. The auriculo temporal nerve rises by two roots, which embrace the middle meningeal artery, and runs backward and then upward close to the lower jaw joint to supply the parotid gland, the skin on the outer side of the ear, and the side of the scalp. At its beginning it communicates with the otic ganglion, which lies just internal to it below the foramen ovale, and also receives a communication from the nerve to the internal pterygoid muscle.

The sixth or abducent nerve rises from a nucleus in the floor of the fourth ventricle deep to the eminentia teres (see fig. 3). It appears on the surface of the brain just below the pons and close to the middle line (see fig. 1), soon after which it pierces the dura mater and runs in the floor of the cavernous sinus to the sphenoidal fissure. Entering the orbit through this, it quickly 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 (see 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, and having the pars