

THE HIND-BRAIN OR RHOMBENCEPHALON 797 



The Fourth Ventricle (tentrieuku quarlus). The fourth ventricle, or cavity 

 of the hind-brain, is situated in front of the cerebellum and behind the pons 

 and upper half of the medulla oblongata. Developmentally considered, the fourth 

 ventricle consists of three parts : a superior belonging to the isthmus rhombencephali, 

 an intermediate, to the metencephalon, and an inferior, to the myelencephalon. 

 It is lined by ciliated epithelium, and is continuous below with the central canal 

 of the medulla oblongata; 1 above, it communicates, by means of a passage termed 

 the cerebral aqueduct, with the cavity of the third ventricle. It presents four 

 angles, and possesses a roof or dorsal wall, a floor or ventral wall, and lateral 

 boundaries. 



Angles. The superior angle is on a level with the upper border of the pons, 

 and is continuous with the lower end of the cerebral aqueduct. The inferior angle 

 is on a level with the lower end of the olive, and opens into the central canal of the 

 medulla oblongata. Each lateral angle corresponds with the point of meeting 

 of the brachia and inferior peduncle. A little below the lateral angles, on a level 

 with the striae medullares, the ventricular cavity is prolonged outward in the form 

 of two narrow lateral recesses, one on either side; these are situated between the 

 inferior peduncles and the flocculi, and reach as far as the attachments of the glosso- 

 pharyngeal and vagus nerves. 



Lateral Boundaries. The lower part of each lateral boundary is constituted 

 by the clava, the fasciculus cuneatus, and the inferior peduncle; the upper part by 

 the middle and the superior peduncle. 



Roof or Dorsal Wall (Fig. 708). The upper portion of the roof is formed by 

 the superior peduncle and the anterior medullary velum; the lower portion, 

 by the posterior medullary velum, the epithelial lining of the ventricle covered 

 by the tela chorioidea inferior, the tseniae of the fourth ventricle, and the obex. 



The superior peduncle (page 792), on emerging from the central white sub- 

 stance of the cerebellum, pass upward and forward, forming at first the lateral 

 boundaries of the upper part of the cavity; on approaching the inferior colliculi, 

 they converge, and their medial portions overlap the cavity and form part of its 

 roof. 



The anterior medullary velum (page 793) fills in the angular interval between 

 the superior peduncle, and is continuous behind with the central white sub- 

 stance of the cerebellum; it is covered on its dorsal surface by the lingula of the 

 superior vermis. 



The posterior medullary velum (page 794) is continued downward and forward 

 from the central white substance of the cerebellum in front of the nodule and 

 tonsils, and ends inferiorly in a thin, concave, somewhat ragged margin. Below 

 this margin the roof is devoid of nervous matter except in the immediate vicinity 

 of the lower lateral boundaries of the ventricle, where two narrow white bands, the 

 taeniae of the fourth ventricle (ligulce), appear; these bands meet over the inferior 

 angle of the ventricle in a thin triangular lamina, the obex. The non-nervous part 

 of the roof is formed by the epithelial lining of the ventricle, which is'prolonged 

 downward as a thin membrane, from the deep surface of the posterior medullary 

 velum to the corresponding surface of the obex and taenia?, and thence on to the 

 floor of the ventricular cavity; it is covered and strengthened by a portion of the 

 pia mater, which is named the tela chorioidea of the fourth ventricle. 



The taeniae of the fourth ventricle (tasnia ventriculi quarti; ligula) are two narrow 

 bands of white matter, one on either side, which complete the lower part of the roof 

 of the cavity. Each consists of a vertical and a horizontal part. The vertical part 

 is continuous below the obex with the clava, to which it is adherent by its lateral 



1 J. T. Wilson (Journal of Anatomy and Physiology, vol. xl) has pointed out that the central cana Jof the medulla 

 oblongata, immediately below its entrance into the fourth ventricle, retains the cleft-like form presented by the fetal 

 medulla spinalia, and that it is marked by dorso- and ventro-lateral sulci. 



