2. The fibers of the medulla are found in five situations: (a) In the median raphe; (b) in either half of the me dulla between the anterior and lateral columns, running in a linear series of ten or twelve fascicles toward the anterior lateral sulcus, and (c) in several bundles not exactly in linear series, which run inward or outward through the posterior lateral sulcus between the lateral and posterior columns of each side.
(a) The dorso-ventral fibers of the raphe are the anterior external arcuate fibers (Fig. 123). These can be traced to the cortex of the cerebellum through the restiform body. Their origin is in the nuclei funiculi gracilis and funiculi cuneati (Fig. 125). As they wind outward over the surface of the medulla they are augmented by the axones of the nucleus arcuatus which lies on the surface and among the fibers of the pyramid.
The root-bundles of the eighth to the twelfth cerebral nerves constitute the remaining groups of dorso-ventral fibers. By them the medulla is divided into areas.
(b) The root-bundles of the hypoglossal nerve (Fig. 124) run from the ventricular gray matter, near the median line, ventro lateralward to the anterior lateral sulcus where they emerge. Inclosing between them and the raphe the anterior column they also separate it from the lateral column. A corresponding part of the sheet of gelatinous gray substance in the ventricular floor is included with each column.
(c) The vestibular root of the auditory nerve, the roots of the glossopharyngeal and vagus and the cerebral root of the accessory nerve form the third group of dorso-ventral fibers (Fig. 124).
The motor fibers of the ninth, tenth and cerebral part of the eleventh nerves take their origin in nuclei of the medulla and emerge from the posterior lateral sulcus; while the sensory fibers of the vagus, glossopharyngeal and vestibular nerves enter that sulcus from without and run through the medulla to their terminal nuclei in the ventricular gray matter. These nuclei both genetic and terminal, are located lateral to the hypoglossal nucleus. The nerve roots rising or terminating in them separate the lateral from the posterior column. The posterior column comprises everything dorsal to the above roots of the eighth to eleventh cerebral nerves. It thus includes the lateral gray matter in the floor of the fourth ventricle.
3. Longitudinal Fib ers.—In the medulla, the longitudinal fibers are chiefly continuations of the same in the pons and the restiform bodies; they are also continuous with the tracts of the spinal cord. They can be best located by reference to the three columns bounded by the above dorso-ventral fibers, namely, anterior, lateral and posterior columns, which are distinctly outlined in the upper half of the medulla (Figs. 123 and 124).
Longitudinal Fibers of the Anterior Column.—The anterior column of the medulla lies between the raphe and the roots of the hypoglossal nerve, and between the anterior surface and the floor of the fourth ventricle (Figs. 123 and 124). It con tains the pyramid, the medial fillet, the medial longitudinal bundle, the anterior tecto-spinal tract, the substantia reticularis alba and two nuclei, the arcuate nucleus and the medial accessory olivary nucleus. Excepting that part forming the lateral pyramidal tract and the medial fillet, this column is continued in the anterior column of the spinal cord. It is naturally divided into a ventral and a dorsal part, or the region of the pyramid and the region of the substantia reticularis alba.
The pyramid (pyramis) with the arcuate nucleus imbedded in it and the arcuate fibers winding over it, occupies the ventral portion of the anterior column (Figs. z r z and 123). It is the continuation of the pyramidal tract and is composed of axones of the Betz cells in the anterior central gyms of the cerebrum. The pyramidal tract diminishes in size as it descends through the pons and medulla, because some of its fibers terminate in ramifications about the cells of cerebral nerve nuclei. In the lower part of the medulla the pyramid breaks up into the anterior (direct) pyramidal tract (io per cent. of the pyra mid), which descends along the anterior median fissure in the same side of the spinal cord and the lateral (crossed) pyramidal tract (8o per cent. of the pyramid) which after decussating with its fellow through the anterior median fissure, runs down in the lateral column of the opposite side of the cord (Fig. 126). The fibers of the anterior tract cross in succession to the oppo site side of the cord, through the white anterior commissure and there, with the fibers of the lateral pyramidal tract, ter minate in fibrillar end-tufts about the cell-bodies in the gray matter. Thus the pyramid forms a crossed cerebral tract for motor cerebral and spinal nerves. A small number of pyrami dal fibers (ro per cent.) diverge lateralward from the pyramid in the medulla and descend in the lateral funiculus of the cord without decussation. They account for the weakness on the well side, and for slight motion on the paralyzed side, which are commonly observed in hemiplegia. If, as E. A. Schafer claims, the fibers of the pyramidal tracts end in the posterior columna of gray matter, then at least one neurone intervenes between them and the motor neurones of the spinal nerves; but without doubt, they enter into either direct or indirect relations with those neurones.