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NERVE

395

ferior dental nerve gives off

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 interna 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 hbres 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 Guddenlrcommissure. The optic nerve passes through the optic foramen in the skull into the orbit, where 1t is penetrated by the central artery of the retina, and eventually pierces the sclerotic just internal to the posterior pole of the eyeball. Its final distribution is treated in the article EYE. The third or oculomotor nerve rises from a nucleus in the floor of the aqueduct of Sylvius (see BRAIN, fig. 8), and comes to the surface Olfactory buiu §

Olfactory tract

Olfactory tuberd

Optic

Optic

Oculo -motor

Trigeminal

Abduceut

Facial

Pars

Auditory

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 spheroidal fissure. The frontal nerve divides into snpraorbital and supratrochlear, which pass out of the upper Fart of the anterior opening of the orbit and supply the skin o the orehead 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 maxilbulb B1'°¢3'S area lary division of the jifth nerve leaves 01f21C1101'y fubemle the skull through the foramen Of 0lfa¢f0ry rctundum, and then runs across the roof of the spheno-maxillary

fossa; here the spheno-maxillary

chiasma

perforated spot

Temporal lobe (cut)

tract

  • Ar nerve

I]€l'V€

body

nerve

body

g

§

nerve

inter media

Auditory nerve

Lateral ventricle

Mid. cerebellar peduncle

Glosso-pharyngeal nerve
Vagus nerve
Spinal accessory nerve

(accessory)

Spinal accessory nerve

(spinal)

Occipital lobe (cut)

Glosso-pharyngeal nerve

Vagus

Spinal accessory nerve nerve

Spinal accessory

nerve Vermis (cut)

Fmm D. J. Cunningham, in Cunningham's T ext-book of Anatomy. Fig. I.-View of the Under Surface of the Brain, with the lower portion of the temporal and occipital lobes, and the cerebellum on the left side removed, to show the origins of the cranial nerves. in a groove on the inner side of the crus cerebri (fig. I); 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 spheroidal fissure, the upper branch supplying the superior rectus and levator palpebrae sutperioris muscles, the lower the inferior and internal rectus and the in error obli ue, so that it supplies five of the seven orbital muscles. The fourlh or trochlear nerve is very small, and comes from a nucleus a little lower than that of the third nerve. It is s cially remarkable in that it crosses to the opposite side in the suhgtance of the valve of Vieussens of the fourth ventricle, after which it winds round the outer side of the crus cerebri (fig. I) and enters the outer wall of the cavernous sinus to reach the orbit through the spheroidal 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 (mes encephalic) 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 descend in root, to the substantia gelatinosa Rolandi as low as the second spinal nerve (see fi 3). The su rficial origin of the fifth nerve is from the side of the pons (see I), and the two roots at once 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 infra orbital 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 cf 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 intof 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 sub maxillary gland, and here the small sub maxillary ganglion is connected

with it by two roots. The inmotor 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 'aw 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 cornmunicates 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. I), soon after which it pierces the dura rnater and runs in the floor of the cavernous sinus to the spheroidal 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

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