2. Optic Nerve (Nereus opticus).—The second nerve, the nerve of sight, is really a brain tract rather than a nerve, and its fibers are imbedded in neuroglia and do not possess a neu rolemma. It rises in the ganglionar layer of the retina. Passing through the chorioid and sclera of the eyeball and the optic foramen of the sphenoid bone, it enters into the optic chiasma where the "nerve" is said to end; but the fibers of the nerve continue without interruption through the optic tracts and their lateral roots to the inter-brain and the mid-brain, whose surfaces they pierce; they end in the lateral geniculate body, in the pulvinar of the thalamus, and in the superior colliculus of the quadrigeminal bodies, where the three terminal nuclei are located.
3. The oculomotor nerve (n. oculomotorius) is the great motor nerve to the eye (Fig. 21). It issues from the mid-brain at the medial border of the basis pedunculi, but its origin is in a mass of gray substance, the genetic nucleus (n. originis), situated within the depths of the mid-brain.
4. Trochlear Nerve (N. trochlearis).—The fourth is a motor nerve to the eye and is the smallest of the cerebral nerves. It may be seen winding forward over the basis pedunculi (Fig. 21). Its point of exit is from the dorsal surface of the brain stem at the junction of the mid-brain with the hind-brain (the isth mus, Fig. 44); this emergence cannot be seen in the complete brain. The genetic nucleus of the fourth nerve is located below that of the third in the mid-brain.
5. Trigeminal Nerve (N. trigeminus).—The trigeminal nerve is a mixed nerve, motor and sensory (Fig. 21). It is attached to the ventral surface of the pons a little above the middle of its lateral border. The small anterior motor root, the masticator nerve (Bean) emerges from this point; but this is the entrance of the large sensory root, which rises in the semilunar ganglion (Gasseri) and enters the pons close to the emergence of the motor root.
6. The abducent nerve (n. abducens) is a motor nerve to the eye. It issues from the pons at its inferior border, or from the transverse groove between the pons and the medulla, just above the pyramid of the medulla and nearly in line with the anterior lateral sulcus (Fig. 2r).
In the transverse groove between the pons and the medulla, lateralward from the root of the sixth nerve, are the roots of the seventh, intermediate and eighth. The seventh is smaller in
diameter than the eighth and medial to it in position; the inter mediate is between these two, separated from the facial by the fasciculus obliquus pontis (Figs. 21 and 57).
7. The facial nerve (n. facialis) is the motor nerve to the muscles of expression (Figs. 21 and 57). Rising from a nucleus in the pons, it emerges from the transverse groove between the medulla and pons. The intermediate nerve (n. intermedius) is so closely associated with the facial nerve that many regard it as the sensory root of that nerve; but the intermediate nerve is in reality a mixed nerve with efferent fibers of vasodilator, secretory and trophic functions and afferent fibers whose function is taste. It may well be called the glossopalatine nerve, as suggested by Robert Bennet Bean (Anat. Rec.). The efferent fibers rise from the salivary nucleus (the visceral or splanchnic part of the facial nucleus) in the pons. They issue from the transverse ponto-medullary groove between the facial and auditory nerves at the point where the afferent fibers enter the brain. The sensory part of the intermediate nerve, which is the nerve of taste to the anterior part of the tongue, takes its origin in the ganglion geniculi situated within the canalis facialis (Fallopii); it enters the brain through the ponto-medullary groove.
8. The acustic nerve (n. acusticus) is a sensory nerve, having two parts, the cochlear and the vestibular nerves, and the double function of hearing and equilibrium (Figs. 21, 56 and 57). It rises from the spiral and vestibular ganglia situated in the petrous bone, and it enters the brain at the bottom of the transverse groove separating the pons from the medulla. The roots of both the seventh and eighth nerves are near the upper end of the posterior lateral sulcus of the medulla ob longata. The acustic nerve fibers, like the optic, are peculiar in that they possess no neurolemma.
9. Glossopharyngeal Nerve (N. glossopharyngeus).—This is a complex mixed nerve, containing efferent fibers (motor, vaso dilator, secretory and trophic) and afferent fibers, which are both common sensory and gustatory. It is joined to the medulla in the bottom of the superior end of the posterior lateral sulcus, where the efferent fibers emerge and the afferent fibers enter the brain (Figs. 21 and 57).