The nucleus lateralis inferior (Figs. r23 and 125) is situated in the reticular substance of the lateral column. Though it is made up of large cell-bodies, they are so scattered among the fibers of this region that the nucleus is invisible to the naked eye. It lies subjacent to the postero-lateral sulcus, medial to Gowers' tract and is pierced by the more superficial fibers of the olivo-cerebellar fasciculus. It contributes fibers to the anterior and lateral reticulo-spinal tracts, which, upon enter ing the tract, divide T-like and furnish both ascending and descending fibers to them. It is known to receive fibers from the ventral spino-cerebellar tract and from the nuclei funiculi gracilis and funiculi cuneati and it gives rise to the ascending tract already traced to the cerebellum, the reticulo-cerebellar fasciculus, which probably transmits tactile, muscular, pain and temperature impressions to the cerebellar cortex.
The nucleus ambiguus (Fig. 124) forms an irregular sheet of gray substance which extends longitudinally through two thirds of the medulla. It lies near the outer limit of the lateral column dorso-medial to the olivo-cerebellar fasciculus. It is a somatic nucleus, being made up of large stellate cell-bodies with abundant, tigrous cytoplasm. Its axones furnish all the voluntary motor fibers of the glossopharyngeal and vagus nerves and make up the entire bulbar root of the accessory nerve. They emerge from the posterior lateral sulcus of the medulla, those of the glossopharyngeal and vagus between the res tiform body and the olive, the accessory fibers emerge below the level of the olive.
The nucleus of the ala cinerea (Fig. 124) is in part derived from the base of the anterior columna; this part is efferent in func tion and its neurones resemble those of the lateral columna in the cord. It belongs to the tenth nerve. It is situated above close to the ependyma of the fourth ventricle under the ala cinerea and it extends inferiorly into the closed medulla nearly as far as the hypoglossal nucleus. Because of its relation to the nucleus ambiguus it is also called the dorsal nucleus of the vagus. It is a visceral nucleus, belonging to the cranial autonomic (or sympathetic) system. Molhant discovered that all vagus fibers supplying smooth and heart muscle arise in the dorsal nucleus of the vagus, the nucleus of the ala cinerea. In harmony with the principle of differentiation according to func tion, the nucleus should contain two varieties of cells—one for smooth muscle and the other for striated involuntary (heart) muscle. This is found to be the case. In the central nervous
system Malone describes three varieties of efferent neurones: the somatic, which supply voluntary striated muscle; the cardiac visceral, which innervate involuntary striated muscle (heart); and the common visceral, which innervate smooth muscle (and glands, probably). Both kinds of visceral neurones are found in the nucleus of the ala cinerea. The nucleus is com posed of three parts: the superior and inferior parts are common visceral and innervate the smooth muscle of the respiratory tract and of the alimentary tract down to the left colic flexure; the middle part is the nucleus cardiacus, the inhibitory nucleus of the heart. The cell-bodies of the nucleus cardiacus possess a medium amount of the tigrous cytoplasm occupying an inter mediate position between the somatic and the common visceral neurones (Am. Jour. Anat., Vol. 15). All efferent visceral nuclei send their axones out to sympathetic ganglia through which the actual involuntary muscles and glands are supplied. From the nucleus of the ala cinerea the axones run in a curve convex toward the median plane between the restiform body and the olive. They are joined medially by those axones of the nucleus ambiguus which enter the roots of the tenth nerve. By this nucleus and the nucleus ambiguus many distinct fascicles 'are formed, belonging to the roots of the ninth, tenth and the cere bral part of the eleventh nerves. They run in slightly different planes but all of them emerge in the region of the posterior lateral sulcus of the medulla (Fig. III). Intermingled with the motor cells of the nucleus alm cineree, there are the small spindle cells of the terminal nucleus of the vagus and glossopharyngeal nerves which represent neurones of the pos terior columna of gray matter. They receive the end-tufts of the sensory root fibers of the vagus nerve and possibly of a small number from the glossopharyngeal nerve.
The hypoglossal nucleus and the nucleus ambiguus receive many fibers from the opposite pyramidal tract and probably from the cerebropontal tracts which bring to them voluntary motor and inhibitory impulses from the cerebral cortex; their reflex connection is established (1) by fibers of the medial longi tudinal bundle which rise in sensory nuclei, and (2) by cere bello-tegmental fibers of the brachium conjunctivum and corpus restiforme, the latter being assisted by axones of the nucleus of Deiters.