VENTRICULUS QUARTUS.
Isthmus, metencephalon and myelencephalon together surround the fourth ventricle, a cavity filled with a small amount of cerebrospinal fluid, which below passes into the central canal of the spinal cord and above is continuous with the Sylvian aqueduct.
Three segments are distinguished, the pars inferior, the pars intermedia and the pars superior ventriculi quarti.
The pars inferior belongs to the medulla oblongata and is embraced by the cor pora restiformia.
The pars intermedia forms the middle and broadest portion and continues above into the region between the pontile crura.
The pars superior belongs to the isthmus rhombencephali, its dorsal boundary being formed by the brachia conjunctiva cerebelli and the velum medullare anterius.
The floor of the fourth ventricle is formed by the fossa rhomboidea and its roof by the anterior medullary velum, the superior cerebellar peduncles or brachia conjunctiva, the posterior medullary velum and the tela chorioidea. The posterior medullary velum and the tela chorioidea together constitute the tegmen fossae rhomboideae, the roof in the limited sense. The edge along which the anterior and posterior medullary vela meet is known as the fastigium; at this place the fourth ventricle projects into the medullary substance of the cerebellum, forming the tent-like recessus tecti. The pars intermedia extends laterally on each side into the recessus lateralis ventriculi quarti. Originally the fourth ventricle is a closed cavity, except above where it communicates with the third ventricle by means of the aquaeductus cerebri and below where it is continuous with the central canal of the spinal cord. Its floor and roof are clothed with epithelium, the Oen dyma. On the roof this epithelium lines the anterior and posterior medullary vela and then continues as the thin lamina chorioidea efiithelialis, which is attached to the tela chorioidea ventriculi quarti and thence is prolonged onto the borders of the abutting parts of the brain. If the ventricle be forcibly opened behind from above, as when the tela chorioidea is removed, the thin epithelial lamina is likewise torn. The separation takes place where the lamina passed onto the more robust surrounding parts of the brain, only a thin white edge, the taenia ventriculi quark remaining along the borders of the tear.
The taenia of the fourth ventricle begins at the obex, thence passes onto the corpus resti forme, there forms the posterior border of the recessus lateralis and continues along the peduncle of the flocculus and the posterior medullary velum. The tela chorioidea of the fourth ventricle represents that part of the pia mater cerebri that projects between the ven tral surface of the cerebellum, more particularly the uvula and the tonsilla, and the dorsal surface of the medulla oblongata (Fig. 84). The two pial sheets are united by subarachnoidal tissue. The tela chorioidea has the form of an equilateral triangle, whose anteriorly directed base is attached in the middle to the nodulus and at the sides along the posterior medullary velum and the flocculus, and whose apex is directed posteriorly toward the hind end of the fourth ventricle. It pushes into the ventricle villiform proc esses that constitute the plexus chorioideus ventriculi quarti, subdivided into medial and lateral portions. The medial plexus consists of two thin stripes that pass in the mid-line, close together, from behind forward to the nodulus. From the latter, the lateral plexus continues, on each side, outward into the recessus lateralis ventriculi quarti. In the early condition, the tela chorioidea, with the lamina chorioidea epithelialis, completely closes the posterior part of the fourth ventricle. Later, however, openings are formed at those places, at which the tela chorioidea and the lamina epithelialis are broken through. Such an opening is the apertura medialis ventriculi quarti or the foramen of Magendi, situated in the posterior part of the tela chorioidea immediately in front of the obex. At the sides, in each lateral recess, is found the apertura lateralis ventriculi quarti (Key-Retzii) or the foramen of Luschka. Through these three openings the ends of the medial and lateral parts of the choroid plexus of the fourth ventricle pass and project into the sub arachnoid space, communication between the ventricle and the subarachnoid space being in this manner established. The villi which protrude through the apertura lateralis are readily found, since they lie medial to the flocculus, between the latter, the lobules biventer and the tonsilla.