THE NERVOUS SYSTEM 



1379 



usually referred to as the roof, and it is 'divisible into two parts — 

 upper and lower. 



The upper part is formed chiefly by the superior or anterior 

 medullary velum, or valve of Vieussens, which extends between 

 the inner margins of the dorsal aspects of the superior peduncles of 

 the cerebellum. It is also formed to a certain extent by these 

 peduncles as they converge and slightly overhang the angular 

 space between them. The lower part of the roof is formed, from 

 above downwards, by (i) the inferior or posterior medullary velum, 

 and (2) the epend\Tnal epithelium of the ventricle, covered by pia 

 mater. The inferior medullary velima succeeds to the superior 

 medullary velum and terminates in a free margin. Beyond this 



Middle Commissure 



Pineal Peduncle 

 (Habenula) \ 



Third Ventricle 



Median Furrow 



Eminentia Teres_ - ^ , 



Superior Fovea ^3L~~^7^k^ 

 Striae Acnstics .li " 



Inferior Fovea 



Ciueate Tubercle— - 



«^- Optic Thalamus 



.'fl^... Upper Quadrigeminal Body 

 \J\^ Lower Quadrigeminal Body 

 -il^ Crus Cerebri 



~--. Frenulum Veli 

 "---Valve of Vieussens 



^-.Superior Peduncle of 

 Cerebellum 



■-'-Do., do. (cut) 



~~ Striae Acusticae 

 ^ ""»>iTrigonum Acosticum (base) 

 ^-^Trigonum Hypoglossi 

 f/^^ ^^Trigonum Vagi 



"~^,Tubercleof Rolando 

 "-.Clava 

 -- Funiculus Gracilis 



Funiculus Cuncatus 



Fig. 577. — The Floor of the Fourth Ventricle and Adjacent Parts. 



free margin there is the ependymal epithelium of the ventricle, 

 covered, as stated, by pia mater. This portion of pia mater is 

 called the tela choroidea inferior. 



The epithelial part of the roof presents superficially three laminse 

 of white nervous matter — namely, the obex and the ligulae. The 

 obex is a thin triangular lamina which is situated at the point of the 

 calamus scriptorius, being attached laterally to the diverging 

 clavse. The ligulce are two in number, right and left. Each ligula 

 is a narrow band which is continuous inferiorly with the obex. It 

 is attached inferiorly to the clava and the cuneate tubercle. It 

 then passes transversely outwards over the dorsal aspect of the 

 restiform body. The transverse part of the ligula forms the lower 

 boundary of the lateral recess of the ventricle. 



