THE MEDULLA OBLONGAIA. 697 



The Fourth Ventricle (Fig. 371). 



The fourth ventricle is lozenge- or diamond-shaped ; that is to say, it is com- 

 posed of two triangles, with their bases in contact. The sides of the lower triangle 

 are formed by the divergence of the funiculi graciles, funiculi cuneati, and resti- 

 form bodies of the medulla on either side. As these columns pass upward in the 

 medulla they turn outward from the median line, and, diverging from each other, 

 form the lateral boundaries of the lower half of the fourth ventricle. In like 

 manner the sides of the upper triangle are formed by the convergence of the 

 superior peduncles of the cerebellum. These peduncles are separated below by a 

 somewhat wide interval, but as they pass upward and forward toward the 

 corpora quadrigemina they gradually converge and ultimately come into contact 

 with each other. This cavity is therefore bounded laterally by the superior 

 peduncles of the cerebellum in its upper half, and by the funiculi graciles, the 

 funiculi cuneati, and the restiform bodies in its lower half. It presents four 

 angles. The upper angle reaches as high as the upper border of the pons, and 

 corresponds with the lower opening of the aqueduct of Sylvius, by which this 

 ventricle communicates with the third ventricle. The lower angle is on a level 

 with the lower border of the olivary body, and is continuous with the central 

 canal of the spinal cord. From the resemblance that it bears to the point of a 

 writing pen it has been named the calamus scriptorius. Its lateral angles extend 

 for some distance between the medulla and the cerebellum, each forming a pointed 

 lateral recess. 



The roof of the fourth ventricle is formed from above downward by the 

 following structures : a part of the superior peduncles of the cerebellum, the 

 superior medullary velum, the inferior medullary velum, the tela choroidea inferior, 

 the obex, and the ligula. 



The superior peduncles of the cerebellum, when they emerge from the medullary 

 substance of its hemispheres, pass upward and forward, forming the lateral 

 boundaries of the upper half of the fourth ventricle, but, converging as they 

 approach the corpora quadrigemina, the mesial portions of the peduncles form a 

 part of the roof of the cavity, in consequence of the ventricle extending to a 

 slight extent underneath the peduncles. 



The Superior Medullary Velum (Valve of Vieussens). In the angular interval 

 left between the two superior peduncles is a thin lamina of white matter, con- 

 tinuous with the white centre of the cerebellum, which bridges across from one 

 peduncle to the other, and so completes the roof of the superior part of the 

 ventricle. This is the superior medullary velum, or valve of Vieussens. Its dorsal 

 surface is covered by the folia of the lingula, already described (page 686). 



The inferior medullary velum is a thin layer of white substance, prolonged 

 from the white centre of the medulla on either side of the nodule, which assists in 

 forming a part of the roof of the fourth ventricle, stretching over it toward its 

 lateral angles. It is continuous with the white substance of the cerebellum by 

 its convex edge, while its thin concave margin is apparently free. In reality, 

 however, it is continuous with the epithelium of the ventricle, which is prolonged 

 downward from the velum to the edge of the ligula. 



The tela choroidea inferior is a layer of pia mater, which covers in the lower 

 part of the fourth ventricle below the inferior medullary velum. Superiorly 

 it is reflected on to the under surface of the cerebellum, while inferiorly it is 

 continued on to the restiform bodies and lower part of the medulla. This part of 

 the roof of the ventricle contains no nervous matter, but consists merely of the 

 ventricular epithelium covered by pia mater. The tela choroidea inferior, like the 

 superior, really consists of two layers, which become more or less adherent, viz., 

 that covering the under surface of the cerebellum and that covering the epithelium. 

 It also possesses a pair of choroid plexuses, which project into the ven- 

 tricular cavity invaginating before them the epithelial lining. Each plexus con- 

 sists of a vertical portion which extends forward, near the middle line, from the 



