PLATE 23.

Lateral View of the Medulla Oblongata, showing, in Schematic Form, the Positions of the Nuclei and the Course of the Cranial Nerves within the white Substance.

The highest nerve is the oculomotor (III), the nucleus of which is divided into several portions and occupies an area of considerable extent in the central gray matter underneath the aqueduct of Sylvius. Posteriorly is the nucleus of the trochlear nerve (IV); it is the only cranial nerve which emerges on the dorsal surface of the brain, where it makes its appearance behind the posterior corpora quadrigemina and, after crossing that of the other side, passes to the base of the brain.

The trigeminus (V) originally consists of two distinct parts: The anterior motor portion (m) springs from the motor nucleus of the fifth nerve, and is probably joined by the motor nasal descending root (radix superior), which arises laterally from the aqueduct. The posterior sensory portion (V ́s) springs from the Gasserian ganglion, and subdivides into three portions in the pons: one portion ends in the sensory nucleus of V, another passes as the caudal descending root (radix inferior) through the medulla oblongata to the upper portion of the cervical enlargement, and is gradually lost in this region; a third portion goes to the cerebellum. The facial nerve (VII) arises from its long nucleus, which is deeply seated. In the medulla the fibers collect and rise toward the surface to form the knee of the facial, which surrounds the nucleus of the abducens (II) and passes first outward and finally downward to its point of exit at the base of the brain. The course of the abducens is very simple. The auditory nerve (VIII) ends as the cochlear nerve (VIII c) in the ventral, and as the vestibular nerve (III) in the dorsal nucleus of the auditory nerve and its immediate neighborhood. One portion continues for some distance caudad as the descending root of III. The sensory fibers of the glossopharyngeal and vagus arise in the jugular and petrosal ganglia, and end in the sensory nuclei of IX in the posterior extremity of the floor of the fourth ventricle (X, green); one portion continues for some distance caudad as the descending root of X (fasciculus solitarius). The motor fibers arise in part from the nucleus ambiguus (X, blue), in part from the nucleus of the spinal accessory.

The nucleus of the hypoglossus (XII), which is situated in the caudal dorsal segment of the medulla, sends its fibers through the substance of the medulla, on the surface of which they emerge laterally from the pyramids. The nuclei of the spinal accessory (XI) extend far down into the upper portion of the cervical enlargement. The fibers emerge on the surface of the lateral columns; r. p, highest posterior cervical roots from the spinal ganglia; r. a, highest anterior roots of the cervical enlargement.

PLATE 24.

Nuclei of the Motor Peripheral Neurons (carmin medullary sheath stain).

FIGURE 1 shows a segment of the gray anterior horn, with its motor ganglion cells, from the cervical enlargement. The cells give origin to the anterior spinal roots (a. r), which pass through the adjoining white substance of the anterolateral tracts (black medullated fibers seen in transverse section). The fine medullated fibers which surround the cells are in part concerned in the transmission of reflexes (reflex collaterals from the posterior roots).

FIGURE 2 represents a portion of the nucleus of the hypoglossus. The fibers of the nerve spring from the numerous large multipolar ganglion cells. The cells are surrounded by a delicate network of coarse and fine medullated nerve-fibers. Some of these fibers belong to central pathways, others effect a close connection between the two nuclei, thereby facilitating the consentaneous action of the two halves of the tongue, and subserve the transmission of reflexes. The nuclei of the facial, oculomotor, etc., are in every respect similar in structure. Thus, the peripheral motor neuron begins in these two regions, while its cells are surrounded by the terminal fibrils of the central neuron; the latter can not be seen with this stain.