The terminal or sensory nucleus (n. terminalis or sensibilis) of the trigeminal nerve begins at the middle of the pons and extends to the second segment of • the spinal cord. At its superior end it is ventro-lateral to the motor nucleus and under cover of the brachium conjunctivum of the cerebellum (Figs. 112 and r14). Near the medulla it lies ventro-medial to the resti form body and the vestibular root of the acustic nerve. This part of it is almost in contact with the nucleus of the facial nerve and its distance from the ventricle is greater than it is higher up. The nucleus is gelatinous in character and is con tinuous with the same substance in the posterior columna of the spinal cord. It receives the sensory root of the trigeminal nerve. Just lateral to it runs the spinal tract of the fifth nerve, the fibers of which gradually bend into the nucleus and ter minate in rich arborizations. May and Horsley traced ascending rami from the sensory root of the trigeminal nerve up to the level of the middle of the superior quadrigeminal colliculus and they inferred from this fact that the sensory nucleus of the trigeminal reaches to that level. But those ascending rami may be purely of reflex function, the arc having only two neurones. Axones from the nucleus pursue several different courses: (a) Reflex fibers go directly to the motor nucleus of the fifth and through the medial longitudinal bundle to other motor nuclei. Coordinating reflex fibers run through the restiform body to the cerebellar cortex and are connected with motor nuclei by cortico nuclear, cerebello-tegmental and vestibular fibers. (b) Tactile, pain and temperature fibers are supposed to enter the opposite spino-thalamic tract through which they reach the thalamus; perhaps some run through the ventral spino-cerebellar tract to the cerebellum. (c) Tactile and muscular sense fibers pro ceed to the thalamus, probably through the medial fillet on the opposite side.
Nucleus of the Abducent Nerve (n. nervi abducentis).—This motor nucleus is close to the median plane and is separated from the ependyma of the ventricular floor only by the fibers of the seventh or facial nerve. It is situated in the colliculus facialis and is purely somatic (Figs. 112 and 114). The root fibers of the facial nerve run lateral to the sixth nucleus, de scribe a loop on its dorsal surface and then return lateral to it. Cortical Connection.—The abducent nucleus receives the end tufts of motor fibers from the opposite pyramidal tract and from the cerebro-pontal tracts. It receives reflex impulses through the anterior tecto-spinal and medial longitudinal bundles and the pedicle of the superior olivary nucleus and perhaps also through the brachium conjunctivum from the cerebellum. The axones of the cell-bodies in the abducent nucleus run in two directions: The greater number run ventro lateralward and emerge at the lower part of the pons as ab ducent nerve; a small bundle of axones runs to the oculomotor nucleus on the opposite side by way of the medial longitudinal bundle. The former innervates the lateral rectus muscle of the eye on the same side as the nucleus; the latter through the third nerve innervates the medial rectus of the opposite eye, though that muscle receives independent fibers from the third also.
The nucleus of the facial or seventh nerve (n. nervi facialis) is somatic motor (Fig. r14). It is situated deep in the pons in the lateral part of the formatio reticularis beneath the superior fovea. Medio-ventral to it is the superior olivary nucleus and the substantia gelatinosa (Rolandi) lies dorso-lateral to it. The nucleus is placed midway between the spinal tract of the fifth nerve and the olivary fasciculus. The facial nucleus is prolonged upward somewhat in the pons and the superior part of the nucleus gives origin to the fibers that supply the frontalis, procerus and corrugator supercilii (Harman).
Cortical Connections.—It receives voluntary motor impulses from the cerebral cortex of the opposite hemisphere via the pyramidal tract and probably fibers of the cerebro-pontal tracts terminate in it. These establish its motor connections. The reflex connections of the facial nucleus are established by fibers from the spinal tract of the trigeminal nerve, from the trapezoid body (Cunningham), and from the medial longitudinal bundle. The axones of the cell-bodies in the nucleus facialis all enter the root of the facial nerve. By its direction this root is divided into three parts, viz., two distinct parallel parts joined by a very short ascending portion. (I) The recurrent part, the pars prima, runs dorso-medianward to the colliculus facialis passing lateral and then dorsal to the lower end of the abducent nucleus; (2) it then ascends about one-fifth of an inch (Cunningham) between the ventricular ependyma, dorsally, and the abducent nucleus and medial longitudinal bundle, ventrally, and this part is called the genu internum; and (3) the pars secunda, bending sharply outward over the nucleus of the sixth nerve, then plunges ventrally through the pons; this emergent part of the root runs between the nuclei of the facial and trigeminal nerves. The root of the facial nerve is joined at the genu internum by fibers from the medial longitudinal bundle which rise in the oculomotor and hypoglossal nuclei and supply the facial muscles above the orbit and the orbicularis respectively.
Nucleus Salivarius.—Kohnstamm, Yagita and others have located a salivary nucleus in the dog's and Anthony Felling has found it in the human brain (Brain, Vol. 36). The nucleus salivarius is an elongated nucleus situated chiefly in the reticular formation of the medulla, dorsal to the inferior olive but extending up into the lower sections of the pons where it lies medial and slightly ventral to the facial nucleus. It is made up of cells with scanty cytoplasm (autonomic or sympa thetic cells). Inasmuch as it supplies glands and smooth muscle, it is a visceral nucleus. Its axones, running through the intermediate and glossopharyngeal nerves, terminate in the spheno-palatine, submaxillary and otic ganglia, through which they innervate the cells and blood-vessels of the salivary and other glands. Like other visceral nuclei of the brain, the salivary nucleus belongs to the cranial autonomic or sympa thetic system. Its cortical connection is unknown. It is as sumed to have abundant reflex connections, especially with the nerves of taste and smell.