The olive (oliva) is an elongated eminence, 13 mm. (12 in.) in length, situated just below the pons (Fig. III). It is pro duced by the inferior olivary nucleus in the lateral column of the medulla and, superficially, is composed of fibers continuous with the fasciculus proprius of the lateral column in the spinal cord (Fig. 124).
Lateral Column (funiculus lateralis, Fig. r r r).—It is made up of three great bundles of fibers (Figs. 124, r25 and '26): The lateral fasciculus proprius, which, splitting into a superficial and a deep lamina, incloses the inferior olivary nucleus; the vestibulo spinal tract, running down the anterior lateral sulcus; and the ventral spino-cerebellar and spino-thalamic tract which runs up the posterior lateral groove. At the junction of the medulla with the spinal cord the' dorsal spino-cerebellar fasciculus (direct cerebellar tract) passes from the lateral to the posterior surface. The anterior external arcuate fibers, running from the anterior surface backward to the restiform body, may be so numerous as to conceal the lateral column and lower part of the olive.
The posterior surface (facies posterior) of the medulla com prises all the surface inclosed between the diverging posterior lateral sulci (Fig. 112). It embraces the surfaces of the two posterior columns of the medulla.
Inferiorly, it is divided into lateral halves by the posterior median fissure and presents four bundles of fibers in each half (Figs. 122, 125 and 133). From the fissure outward they are as follows: The funiculus gracilis, funiculus cuneatus, tractus spi nalis n. trigemini, and the dorsal spino-cerebellar fasciculus. The funiculus gracilis (Fig. 133) is a continuation of the medial tract of the posterior column of the spinal cord, and the funiculus cuneatus is in direct continuity with the lateral tract in the same column of the cord. These two bundles leave the surface and end in the nuclei of these columns in the medulla. The spinal tract of the trigeminal nerve is here situated on the surface; its fibers end in the underlying gelatinous substance. The dorsal spino-cerebellar fasciculus (direct cerebellar tract) is continued up from the lateral column of the spinal cord. Re maining on the surface it runs up to the cerebellum through the restiform body.
Superiorly, the posterior surface on either side is formed by a large rounded band of fibers, the restiform body (Figs. 122 and 133). The restiform body (corpus restiforme) is continued up ward to the pons and then bends backward into the corpus medullare of the cerebellum in connection with which it has already been studied. It enters the corpus medullare just lateral to the origin of the brachium conjunctivum and radi ates through the medullary lamina to the cortex. The resti form body contains the following tracts: The dorsal spino cerebellar fasciculus, external arcuate fibers, reticulo-cerebellar fasciculus from the lateral nucleus of the medulla, olivo-cere bellar fasciculus, direct sensory cerebellar fibers from the roots of the trigeminal and vestibular nerves, and nucleo-cerebellar fibers from terminal nuclei of cranial nerves—all of which are ascending in direction; and the fastigio-bulbar part of the cere bello-tegmental tract, which is a descending tract (see p. 269
for description of restiform body). A single layer of flattened epithelial cells stretches between the two restiform bodies and roofs over the inferior part of the fourth ventricle. That is the roof epithelium. It is continuous with the inferior medullary velum of the cerebellum; and, as it forms a part of the dorsal boundary or roof of the fourth ventricle, it really belongs to the cerebellum. It conceals the ventricular surface of the medulla. The slight crest marking the line of attachment of the roof epithelium to the medulla is called the tcenia of the fourth ventricle.
The roof epithelium (Figs. 120 and 122) seen in the mid-dorsal surface of the medulla, is of triangular shape; its base is attached to the inferior medullary velum of the cerebellum; its apex, which is directed downward, terminates at the obex and covers the inferior angle of the fourth ventricle; and, laterally, it is attached to the clava, the cuneate funiculus and the restiform body. The line of attachment to the restiform body runs first obliquely upward and outward and then transversely outward inferior to the lateral recess. The borders of the epi thelial lamina become thickened by the addition of neuroglia and are in continuity with the ependyma of the ventricle. The thickened apex of the epithelial lamina is called the obex.
With the pia mater investing it, termed the chorioid tela of the fourth ventricle, this roof epithelium is perforated in the median line near the obex by a foramen, the median aperture (apertura mediana ventriculi quarti, Magendii) and over each lateral recess by the lateral aperture (apertura lateralis ventriculi quarti of Luschka). The lateral apertures are so small as to be seen with difficulty but the median aperture measures 7 mm. in width and 5 mm. in length. These foramina establish com munication between the subarachnoid space and the ventricle. On either side of the median line there is a longitudinal in vagination of the epithelial lamina into the ventricle and a similar transverse one just below the inferior medullary velum, both of which are occupied by a vascular fold of pia mater. This fold constitutes the chorioid plexus of the fourth ventricle (plexus chorioideus ventriculi quarti). If the roof epithelium be torn away, as it usually is with the pia, a rough line of separa tion is seen winding over the restiform body: That line is the tcenia of the fourth ventricle. Two layers of ependyma form it. It represents the attenuated edge of the rhombic lip, which is so fertile in the embryonic period.
When the roof epithelium is removed, the lower triangle of the floor of the fourth ventricle is brought into view (Figs. II 2 and 133). Notice the median longitudinal furrow bounded by the eminentio mediales, which 'form the calamus scrip torius; then the little fossa in the sulcus limitans, called the fovea in ferior, situated lateral to the middle of the pen, and the ala cinerea (trigonum vagi), whose superior angle is formed by the fovea inferior; and, last, the large lateral area, located above the ala cinerea lateral to the eminentia medialis and crossed by the nearly transverse medullary stria. This region is called the area acustica.