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The Fourth Ventricle

floor, lateral, inferior, superior, medulla and roof

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THE FOURTH VENTRICLE The common cavity of the rhombencephalon is the fourth ventricle (ventriculus quartus) (Fig. rr2). The fourth ventricle is dorsal to the pons and medulla, and is ventral to the cere bellum (Fig. 104). It is broadest at the junction of the pons and medulla (Figs. II 2 and 12 2). Above and below that junction, it gradually contracts to the size of the cerebral aqueduct and central canal of the spinal cord, with which it is continuous. Inferiorly it communicates through its roof with the subarach noid space via three apertures, a median and two lateral. The fourth ventricle is a gable-roofed chamber with a diamond-shaped floor. The gables (Fig. 1o4.) are directed lateralward and are prolonged in tunnel-like extensions around the restiform body forming the lateral recess. The long axis of the ventricular floor (Fig. x12 and 133) is parallel with the spinal cord, and extends from the superior extremity of the pons to the middle of the medulla. The transverse axis coincides with the junction of the pons and medulla. Thus the superior triangle of the floor is formed by the pons; the inferior, by the medulla oblongata. The fourth ventricle is lined with ependyma, which is complete throughout, except in the roof of the inferior part, where below the inferior medullary velum only the epithelial layer is present.

floor is formed by the pons and medulla. The lateral wall (superior triangle) is formed by the brachium conjunctivum of the cerebellum; and (inferior triangle) by the tmnia of the fourth ventricle winding across the restiform body, funiculus cuneatus and funiculus gracilis to the obex. The roof is formed by the superior medullary velum (valve of Vieussens) superiorly; and by the inferior medullary velum and roof epithe lium, inferiorly (Fig. 122). The superior and inferior halves of the roof meet at an acute angle, the fastigium, and form the peak of the fourth ventricle (Fig. 104). On either side the gable is pushed out over the restiform body and thus is formed the lateral recess. The lateral recess is a tunnel-like extension of

the ventricular cavity, reaching almost to the posterior lateral sulcus. The recess is bounded superiorly and ventrally by the restiform body; dorsally by the inferior medullary velum and inferiorly by the roof epithelium. The chorioid plexuses of the fourth ventricle invaginate the roof epithelium and hang from the roof into the lateral recesses and the inferior part of the cavity (Fig. 2 2) .

Floor of the Fourth Ventricle (fossa it contains the nuclei of one or more roots of the posterior eight (fifth to twelfth) cerebral nerves, the floor of the fourth ven tricle is a very important area (Figs. 112 and 133). A median groove bounded by the eminentim mediales forms the long axis of the diamond-shaped floor and divides it into two lateral halves; the medial eminences form prominent features of the ventricular floor. They are broadest in the middle; they taper to a point in the lower angle of the ventricle like the nibs of a pen and they are bounded laterally by an important sulcus, the sulcus limitans, which widens out at two points into small fossm, the fovea superior in the pons and the fovea inferior in the medulla. The sulcus limitans separates the ventral zone (efferent) from the dorsal zone (afferent) in the embryo; and in the adult it intervenes between the two regions containing genetic and terminal nuclei. The ventricular floor is bisected transversely by a number of lines, the medullary striae (striae medullares). The striae are produced by bundles of fibers which rise from the lateral cochlear nucleus of the auditory nerve. Diverging somewhat and plunging into the medulla and pons at the median groove the fibers of the strim enter the opposite trapezoid body and lateral fillet. The medullary striae divide each lateral half of the floor into a superior and an inferior triangle.

The superior triangle of the floor presents the colliculus facialis, superior fovea, locus cmruleus and a part of the acustic area.

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