Home >> Anatomy Of The Brain >> Sensory Nerves And Sensory to White Matter Of The >> The Pons Varolii

The Pons Varolii

surface, fibers, posterior, superior, pontis, lateral, medulla, brachium and cerebellum

THE PONS (VAROLII) The pons and medulla form the ventral part of the rhomben cephalon, the cerebellum being its dorsal portion. By a trans verse indentation of its roof, the posterior brain-vesicle is par tially divided into an upper vesicle, the metencephalon, and a lower vesicle, the myelencephalon; the latter is the embryonic medulla, the former gives rise to the cerebellum and the pons. The pons is developed from the floor of the metencephalon (Fig. 104). It is so named because it forms the connecting link or bridge between the mid-brain above and the cerebellum and medulla oblongata below; between the medulla and cere bellum and between the two cerebellar hemispheres (Fig. III).

In shape the .pons is roughly cylindrical. It has a broad basal or ventral part, the pars basilaris pontis, and a narrower dorsal portion, the pars dorsalis pontis (Fig. I 13).

Size.—The pons is about 2.5 cm. (I inch) long. It is a broader than long, and measures 2.5 cm. dorso-ventrally.

Position.—It rests in the anterior end of the groove which extends from the foramen magnum to the dorsum sellaa, and lies between and ventral to the hemispheres of the cerebellum. Superiorly, it joins the mid-brain; and, below, it is continuous with the medulla oblongata.

Surfaces of the Pons.—The pons has four .surfaces, viz., superior (attached); inferior (attached); anterior (free), and posterior (partially free); and two borders, namely, right and left lateral, continuous with the brachium pontis of the cere bellum.

The superior and inferior surfaces are made by section, and are directly continuous with mid-brain above and the medulla below.

Anterior Surface (tuber annulare).—The anterior surface of the pons (Fig. r) looks forward and slightly downward and rests on the sphenoid bone behind the dorsum sellw. It is divided into lateral halves by the sulcus basilaris, containing the basilar artery; and is bounded latelally by a sagittal plane cutting the root of the trigeminal nerve. Vertically the sur face is slightly convex, and is markedly so from side to side. It shows transverse striations, which converge laterally, due to the fibers that form it and enter the brachia pontis of the cere bellum. The fibers of the anterior surface are not exactly transverse in direction. Those at the superior end of the pons bend downward and form a rounded margin, which covers the lower part of the bases pedunculi of the mid-brain; at the inferior extremity of the pons, the fibers are convex downward and partially conceal the pyramids of the medulla oblongata. Just medial to the root of the trigeminal nerve there is an oblique bundle of fibers called the fasciculus obliquus pontis. This fasciculus rises in the medull, oblongata from the nucleus ponto-bulbaris (the tail end of the nucleus pontis); it runs up ward 4.

across the ponto-medullary groove, between the facial andintermediate nerves; at the level of the trigeminal root it bends sharply toward the median line, mingles with the transverse fibers of the pons and, with them, enters the opposite brachium pontis. Like other fibers of the anterior surface of the pons, those of the fasciculus obliquus terminate in the cerebellar cortex. The two roots of the fifth nerve (trigeminal) are at tached to the lateral border (Henle) of this surface, a little above the middle.

The posterior surface of the pons is concealed by the cere bellum (Fig. 114). It is free in its middle part, where it forms the floor of the superior half of the fourth ventricle (Fig. 11 2). The ventricular area of the posterior surface is completely concealed by the superior medullary velum. If examined, it is found to be divided into lateral halves by a median longitudinal groove. Each half presents in its posterior part a rounded eminence, the colliculus facialis, which flanks the median furrow and is in turn bounded, laterally, by a linear valley, the sulcus limitans, lying near the brachium conjunctivum cerebelli and parallel with it. The inferior end of the valley is called the fovea superior; its upper part has a bluish tint, due to underlying pigmented cells, and is called the locus cceruleus. Attached Area.—Lateral to this ventricular area the posterior surface of the pons is attached to the restiform body and the conjoined arms of the cerebellum. The restiform bodies enter the surface at the lower end of the pons and then bend backward into the cerebellum; while the brachia conjunctiva, in their course up to the cerebrum, partly imbed themselves in the lateral part of the posterior surface and form the walls of the fourth ventricle. The lateral fillet issues from this surface just lateral to the brach ium conjunctivum. It runs obliquely across the upper end of the brachium to the inferior colliculus of the corpora quad rigemina, and produces a flat striated ridge, which may be seen easily in a well-hardened specimen. A second bundle of fibers issues from the attached area of the posterior surface of the pons and winds upward over the brachium conjunctivum: it is the ventral spino-cerebellar fasciculus. Passing over the brachium, it enters the superior medullary velum and terminates in the cortex of the vermis cerebelli.

Structure of the Pons.—The pons is composed of transverse and longitudinal white fibers and of gray matter. The trans verse fibers are found chiefly in the basilar portion of the pons; the longitudinal, in both the basilar and the dorsal parts. The basilar longitudinal intersect the deep transverse fibers of the pars basilaris.