THE BRA IX AXD IT* MEM-BRACES. 739 



must also cover the upper surface of the entire inferior medullary velum ; and it 

 is the prolongation of this particular layer which was just referred to. 



The epithelium covering the central part of the inferior velum on arriving at 

 its free edge is continued over the projecting portion of the upper surface of the 

 nodulus. and then bends sharply downward and backward around the anterior 

 extremity of the nodulus. and is continued on down to the calamus scriptorius 

 angle of divergence of the clavae where it dips into the upper (medullary) 

 part of the central canal of the cord, and becomes continuous with its dorsal lin- 

 ing epithelium. This layer of epithelium, thus traced from above downward, has, 

 of course, lateral attachments, and on each side this attachment is mainly the line, 

 already referred to, which marks off the inner from the dorsal aspect of the resti- 

 form body. Below, this line of attachment is continued down on the clava, and 

 at the calamus scriptorius meets the line from the opposite side. Along this line 

 for its entire extent the layer of epithelium is continuous with the epithelium 

 lining the slightly elevated inner aspect of the restiform body and that covering 

 the floor, but is not prolonged over the dorsal aspect of the restiform body, which 

 is closely invested with pia mater. Owing to the divergence of the restiform 

 bodies and clava?. it is evident that this layer of epithelium is triangular in shape, 

 with its apex at the calamus scriptorius. This triangular layer of epithelium is 

 the /"// /-"of of the lower portion of the fourth ventricle. 



Lateral Recess. The epithelium covering each lateral portion of the inferior 

 medullary velum on arriving at its free edge is prolonged directly on to the upper 

 extremity of the restiform body, close to which the free edge lies, and is then 

 reflected upward for a very short distance i. e. to where the restiform body bends 

 backward to enter the hemisphere. The epithelium then bends backward also, 

 covering the (now) under surface of the inferior cerebellar peduncle, and thus 

 enters the Jaf<-ral portion of the cleft (see page 734) between the " laminae " of 

 the cerebellar white matter : arriving at the bottom of this cleft, it is reflected 

 back again over the upper surface of the lateral part of the inferior medullary 

 velum to its free edge, at which point its tracing was commenced. 



The line of attachment of the epithelium to the upper end of the restiform 

 body just after its reflection from the edge of the inferior velum is, of course, 

 directed transversely. Its inner end bends downward and becomes continuous 

 with the line already mentioned on the restiform body, along which the '"roof" 

 epithelium blends with that of the side and floor. 



Thus is formed the complete lateral recess, which, when all the parts are con- 

 nected, is really a triangular-shaped, tunnel-like prolongation of the cavity of 

 the fourth ventricle, curving around the extreme upper end of the restiform body 

 just before that body bends backward into the hemisphere of the cerebellum. 



The outer extremity of the lateral recess may be regarded as a pointed cul-de- 

 sac. At its in. ii> r sir- /t't>/ is an aperture through which its lining epithelium is 

 continuous with that of the fourth ventricle. This aperture is situated just at the 

 // '//>// (see above) of the ventricle. The cul-de-sac is situated just between 

 the flocculus externally and the outer aspect of the restiform body internally. 



Tela Choroidea Inferior. The pia mater which invests the inferior worm is 

 continued upward and forward, dipping in and out of the various fissures, until 

 it reaches the nodulus. It now closely invests the nodulus and bends sharply 

 around it. forward, upward, and a little backward, following exactly the course 

 of, but lying posterior to, the epithelial roof of the ventricle, already described, 

 until it reaches the free edge of the inferior velum lying on the nodulus. At this 

 point it is reflected at an acute angle right back on itself, and now follows the 

 "epithelial roof" down to the calamus scriptorius. In its course this reflected 

 lui/'ji- of pia mater lies, naturally, dorsal to the epithelial roof, and so closely adhe- 

 to it that the two form one structure. This structure is the tela choroidea 

 inferior. It is the practical mof of the lower portion of the fourth ventricle, 

 because when the pia mater is pulled away the epithelial layer comes with it, 

 and thus this portion of the ventricle is exposed. 



