CRANIAL NERVES.] ANATOMY 881. The Trochlearis or fourth, the smallest cranial nerve, lies at the outer side of the cms cerebri. When traced backwards to its origin it is seen to sink into the valve of Vieussens, where its fibres divide into three roots : one decussates across the valve with a root of the corresponding nerve on the opposite side; another passes backwards to the locus cseruleus; the third sinks into the corpora quadri- gemina and reaches the Sylvian group of nerve cells, from which the third nerve also arises. The fourth nerve runs forward in the outer wall of the cavernous sinus, enters the orbit through the sphenoidal fissure, and ends in the superior oblique muscle. It also communicates with the cavernous plexus of the sympathetic. The Abducent or sixth nerve springs out of the groove between the lower border of the pons and the anterior pyramid of the medulla oblongata. Its roots sink deeply into the pons, and arise from a nucleus of grey matter at the floor of the fourth ventricle, common to it and the portio dura. The sixth nerve runs forward in the inner wall of the cavernous sinus, enters the orbit through the sphenoidal fissure, and ends in the external rectus muscle. It com municates with the carotid plexus of the sympathetic. The Portio dura or motor facial portion of the seventh nerve springs out of the groove between the lower border of the pons and the restiform body. Its roots sink deeply into the pons, and whilst some of its fibres arise from a grey nucleus, at the floor of the fourth ventricle, common to it and the sixth nerve, others ascend from a nucleus which, according to Meynert, lies just on the outer side of the superior olivary body, and others again decussate across the median raphe of the pons. An accessory por tion, called portio intermedia, which is said to arise from the lateral columns of the cord, joins the portio dura. The portio dura enters the internal auditory meatus in the petrous-temporal bone along with the auditory nerve; but at the bottom of the meatus it leaves that nerve and enters the aqueduct of Fallopius, along which it is conducted through the bone to emerge at the stylo-mastoid foramen. When in the aqueduct it forms a knee-shaped lend, and expands into a small ganglion, which is joined by the great, small, and external petrosal nerves, and through the external petrosal it communicates with the sympathetic. The portio dura gives off a, a minute branch to the sta- pedius muscle ; b, the chorda tympani, which, entering the tympanum, passes across that cavity, emerges through the Glaserian fissure, and joins the lingual branch of the fifth nerve, which it accompanies as far as the submaxillary ganglion ; it gives a branch to the ganglion, and one to the lingualis muscle. After the portio dura has passed through the stylo-mastoid foramen it gives off c, the poste rior auricular branch to the occipital belly of the occipito- frontalis and to the retrahens aurem muscle, and d, the digastric branch to the posterior belly of the digastric and stylo-hyoid muscles ; and then runs forwards through the parotid gland to the face, where it breaks up into numerous (e) facial branches to supply the facial muscles of expres sion and the buccinator muscle. The facial is also the secretory nerve for the salivary glands. Through the chorda tympani it influences the secretion of the submaxil lary and sublingual glands, and through the connection between its lesser petrosal nerve and the auriculo-temporal in the otic ganglion it influences the parotid gland. The Spinal Accessory is the lowest division of the eighth Bertie. It springs out of the side of the medulla oblongata, and from the lateral column of the cervical part of the epinal cord as low as the fifth cervical nerve : its roots arise from the intermedio-lateral group of nerve cells in the cord, and from a nucleus of grey matter in the floor of the fourth ventricle. The spinal fibres of origin enter the skill! through the foramen rnagum, join tlis fibres from the medulla, and leave the cranial cavity through the jugular foramen. This nerve, purely motor in function, is sub divided into two parts, an internal and an external. The external passes obliquely outwards across the side of the neck, pierces the sterno-mastoid, and ends in the trapezius, both of which muscles it supplies. The internal joins the pneumogastric nerve, of which it forms the motor or acces sory root, and is distributed along with it. The Hypoglossal or ninth nerve springs out of the groove between the anterior pyramid and olivary body of the medulla oblongata, in series with the anterior roots of the spinal nerves. Its roots pass through the medulla to the floor of the fourth ventricle, to arise from the nerve cells in two nuclei of grey matter situated close to the median furrow. This grey matter is in series with the anterior cornua in the spinal cord. The nerve passes out of the skull through the anterior condyloid foramen, and arches across the side of the neck to the tongue, to end in glossal branches for the supply of the intrinsic and extrinsic muscles of the tongue. It also gives off a, the descendens noni branch, which, after been joined by the communicantes noni from the cervical plexus, supplies the omo-hyoid, sterno-hyoid, and sterno-thyroid muscles ; b, the thyro-hyoid branch to the thyro-hyoid muscle ; c, the genio-hyoid branch to the genio-hyoid muscle. It communicates in the neck with the sympathetic, vagus, lingual branch of the fifth, and cervical plexus. The group of mixed nerves will now be considered. The Trifacial or fifth is the largest cranial nerve. It Mixed springs by two distinct roots out of the side of the pons. cranial The smaller or motor root arises from the nerve cells of a ne nucleus of grey matter situated in the back of the pons, near the floor of the upper part of the fourth ventricle. The larger or sensory root has, according to Meynert, a complex origin a, from a nucleus of grey matter in the pons to the outer side of the origin of the motor root ; &, by descending fibres which arise from nerve cells in the substance of the corpora quadrigeinina, from the grey matter of the locus cseruleus, and from the longitudinal fibres of the pons ; c, by ascending fibres which apparently arise from the grey tubercle of Rolando ; d, probably by fibres which traverse and embrace the superior peduncle of the cerebellum. As the large sensory root of the fifth lies on the petrous bone it expands into the Gasserian ganglion, which resembles in structure the ganglion on the posterior root of a spinal nerve. From this ganglion three large branches arise, named respectively the 1st, 2d, and 3d divisions of the ganglion. The 1st or Ophthalmic division is the upper sensory nerve of the face, and divides into three branches, which pass out of the cranial cavity through the sphenoidal fissure. By its lachrymal branch it supplies the lachrymal gland, and the outer part of the skin and conjunctiva of the upper eye lid ; by its frontal branch, the inner part of the skin and conjunctiva of the upper lid, and the skin of the forehead; by its oculo-nasal branch, it gives long ciliary nerves to the eyeball, and a nasal nerve to the mucous membrane of the nose, and the skin of the side of the nose. From the oculo-nasal nerve arises the long or sensory root of the ciliary ganglion, which lies in the cavity of the orbit, and which receives also a motor root from the third nerve, and a root from the sympathetic. This ganglion gives origin to the short ciliary nerves for the eyeball. The 2d or Superior Maxillary division is the sensory nerve for the middle part of the face. It leaves the skull by the foramen rotundum, passes across the spheno-maxillary fissure, then lies iu the canal in the floor of the orbit, from which it emerges on the face through the infra-orb ital foramen as the infra-orbital nerve. It gives off a small orbital branch to a small part of the skin of the temple,
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