738 THE NERVOUS SYSTEM. 



The floor has already been described in detail. It is flat and lozenge-shaped, its 

 upper half being on the dorsal surface of the pons, its lower half lying between 

 the restiform bodies on the upper portion of the dorsal surface of the medulla. 

 Its widest or central portion is at the junction between pons and medulla. Like 

 the floor, the ventricle itself is divided into an upper, a lower, and a middle portion. 



Boundaries of the Fourth Ventricle. The upper portion has for its floor the cen- 

 tral part of the dorsal surface of the pons ; for its lateral boundaries, the inner 

 surfaces of the superior cerebellar peduncles ; for its roof, the superior medullary 

 velum. 



The middle portion has for its roof the tent, or space between central part of 

 inferior medullary velum, below ; and that part of superior medullary velum which 

 is below the lingula, above ; and their line of junction dorsally. The tent, on section, 

 appears pointed, the angle projecting dorsally from pons and medulla into the 

 worm, between lingula above and nodulus below. In the complete ventricle the 

 tent lies dorsal to the widest part of the floor ; and the lateral boundaries of this par- 

 ticular region of the ventricle are the lateral angles (see page 723), each lateral 

 angle being the point of contact of the lower end of the inner surface of the 

 superior peduncle with the upper end of the inner surface of the inferior peduncle, 

 just as each peduncle bends dorsally to enter the hemisphere. 



The lower portion has for its floor that part of the dorsal surface of the 

 medulla which is between the restiform bodies, and for its lateral boundaries the 

 clavcetf the funiculi graciles and the inner surf aces of the restiform bodies. The 

 inner surface of the restiform body is merely the inner aspect of its generally 

 rounded, elevated surface. Hence it is not so marked as that of the flattened 

 superior peduncle, which also has an inner margin, to which is attached the supe- 

 rior medullary velum ; while the inner margin of the rounded inferior peduncle 

 would be merely the line drawn between its inner and dorsal aspects. The roof 

 of the lower portion of the fourth ventricle is the tela choroidea inferior, which 

 will now be described, together with the lateral recesses of the fourth ventricle. 



Roof of Lower Portion of Fourth Ventricle ; Lateral Recess ; Tela Choroidea 

 Inferior. In the description of the white matter of the cerebellum, as it lies 

 between the edges of the great horizontal fissure in the anterior notch, it was 

 stated that this white matter was split into two layers, the lower of which is the 

 inferior medullary velum. An important difference between these two layers must 

 now be noted, in addition to the others already given. This difference is that, in 

 the complete specimen, the inferior medullary velum, as such, has a free edge, 

 while the upper layer is continued directly into the prolongations of its com- 

 ponent parts, superior medullary velum and the peduncles of the cerebellum. 



This/ree edge of the inferior velum is directed, in the natural position, down- 

 ward and forward. The free edge of the mesial part lies over the nodulus, which 

 projects somewhat beyond it. Being very thin, it cannot be made out distinctly 

 except on antero-posterior section. The free edges of the lateral portions, how- 

 ever, are well seen on removal of cerebellum from pons and medulla and after 

 separating the edges of the great horizontal fissure. In the complete condition 

 each of these lateral free edges of the velum lies just dorsal to the upper extrem- 

 ity of the corresponding restiform body just before it bends backward into the 

 cerebellum, and curves around it, as it were, reaching out laterally to the floc- 

 culus, which, in the complete specimen, lies just external to the lateral aspect of 

 the restiform body. 



Having thus located the free edge of the entire inferior medullary velum, it 

 now remains to establish its connection with the subjacent parts. This connec- 

 tion is effected by a layer of epithelial cells prolonged from the general epithelial 

 lining of the ventricle. It is understood, of course, that all the ventricles of the 

 brain, as well as the central canal of the cord, are lined with epithelium. There- 

 fore in the fourth ventricle this epithelium lines the under surface of the superior 

 velum ; the inner surface of the superior cerebellar peduncle ; covers the entire 

 floor, and is also found in the tent and its lateral prolongations. Therefore it 



