NERVE, originally a sinew or tendon (cf. "strain every nerve") but now used for the conducting fibres of the nervous system in anatomy and secondarily as a psychical term for courage or firmness and in "nervousness" for the opposite quality. Here the anatomy of the nerves is dealt with; see also NERVOUS SYSTEM, MUSCLE AND NERVE, NEUROPATHOLOGY, etc.
The 12 pairs of cranial nerves rise directly from the brain. With one exception they all contain medullated fibres (see NERVOUS SYSTEM). The following is a list :— (I ) Olfactory; (2) Optic; (3) Oculo-motor or Motor oculi; (4) Trochlearis or Patheticus; (5) Trigeminal or Trifacial; (6) Abducens; (7) Facial; (8) Auditory; (9) Glosso-pharyngeal; (io) Vagus or Pneumogastric; (II) Spinal accessory; (12) Hypoglossal.
first, or olfactory nerve, consists of the olfactory bulb and tract, which are a modified lobe of the brain and lie beneath the sulcus rectus on the frontal lobe of the brain (fig. I). At its posterior end the tract becomes continuous with the brain ; anteriorly is the bulb from which some twenty small non-medullated nerves pass through the cribriform plate of the ethmoid to supply the sensory organs in the olfactory mucous membrane (see OLFACTORY SYSTEM).
nd or optic nerve consists of the optic tract, the optic commissure or
and the optic nerve proper. The optic tract begins at the lower visual centres or internal and exter nal geniculate bodies, the superior quadrigeminal body and the pulvinar (fig. I), but these again are connected with the higher visual centre in the occipital lobe by the optic radiations (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, the fibres from the internal genicu late 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 into the orbit, where it is pene trated by the central artery of the retina, and pierces the sclerotic internal to the posterior pole of the eyeball. See EYE.
Oculomotor.—The third or oculomotor nerve rises from a nucleus in the floor of the aqueduct of Sylvius and comes to the surface on the inner side of the crus cerebri (fig. I), 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 sup plying the superior rectus and levator palpebrae superioris muscles, the lower the inferior and internal rectus and the inferior oblique, thus supplying 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 crosses to the opposite side in the fourth ventricle, 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 sphe noidal 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, and by a descend ing (mesencephalic) tract near the Sylvian aqueduct (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 sub stantia gelatinosa Rolandi as low as the second spinal nerve (fig. 3). The superficial origin of the fifth nerve is from the side of the pons (fig. 1), and the two roots at once pass into a small compartment of the dura mater, where the large crescentic Gas serian ganglion is formed upon the sensory root, and from this the three branches, ophthalmic, maxillary, mandibular, come off. The motor root only joins the mandibular branch. The ophthalmic division 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 maxillary division 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 fora men, where it divides into palpebral, nasal and labial branches. The mandibular division leaves the skull through the foramen ovale, and at once gives off motor branches for the muscles of mastica tion; 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 nerve divides into lingual, inferior dental and auriculo-temporal. The lingual is joined by the chorda tym pani branch of the facial nerve.