THE FOURTH VENTRICLE. 781 



pposite side. The fibres are then connected with the red 

 ucleus of the tegmentuni, and are continued with the longi- 

 udinal tegmental fibres to the optic thalamus. 



The valve of f^ieussens, or superior medullary velum, is a thin Valve of 

 ranslucent white layer which enters into the roof of the upper Al ens ' 

 >art of the fourth ventricle (fig. 277, k). It is narrow in front, 

 mt widens behind, where it is continued into the medullary centre 

 f the worm. On each side it joins the superior peduncle. Near 

 the lamina quadrigemina the fourth nerve is attached to the valve : covered by 

 and its upper surface is covered by four or five small transverse lm " la - 

 -f grey ridges, constituting the lingula. 



The middle peduncle (processus ad pontem), commonly named Middle 

 the crus cerebelli, is the largest of the three processes. Its fibres P^ 011016 - 

 begin in the lateral part of the hemisphere, and are directed 

 forward to the pons, of which they form the transverse fibres. 



The inferior peduncle (processus ad medullam) passes downwards inferior 

 to the medulla oblongata, where it is known as the restiform body. J^JIhJ 6 to 

 Its fibres begin chiefly in the laminae of the upper surface of the 

 hemisphere. It will be better seen when the fourth ventricle has 

 been opened. 



Dissection. One other section (fig. 277) must be made to Dissection, 

 expose the fourth ventricle. The cerebellum still resting on its 

 under surface, let the knife be carried vertically through the centre 

 of the vermiform processes ; and then the structure of the worm, 

 as well as the boundaries of the fourth ventricle, may be observed 

 on separating the lateral portions of the cerebellum. 



Structure of the JTorm (fig. 274). The upper and lower vermi- Vermiform 

 form processes of the cerebellum are united in one central piece [jijTother 

 which connects together the hemispheres. The structure of this parts, 

 connecting piece is the same as that of the rest of the cerebellum, 

 viz., a central white portion and investing laminae. Here the 

 branching appearance of a tree (arbor vitas) is best seen, in con- 

 sequence of the laminae being more divided, and the white central 

 stalk being thinner and more ramified. 



The FOURTH VENTRICLE (fossa rhomboidalis) is a space between Fourth 

 the cerebellum and the posterior surface of the medulla oblongata ventncle: 

 and pons (fig. 274). It has the form of a lozenge, with the points 

 placed upwards and downwards. The upper angle reaches as high form and 

 as the upper border of the pons ; and the lower, nearly to the level extent ; 

 of the lower end of the olivary body. Its greatest breadth is breadth : 

 opposite the lower edge of the pons ; and a transverse line in this 

 situation would divide the hollow into two triangular portions 

 upper and lower. The lower half has been named calamus scrip- calamus 

 torius from its resemblance to a writing pen. 



The lateral boundaries are more marked above than below. For Boundaries 

 about half way down the cavity is limited on each side by the OE 

 superior peduncle of the cerebellum, which, projecting over it, 

 forms part of the roof ; and along the lower half lies the eminence 

 of the restiform body, with the clava of the funiculus gracilis 

 (fig. 286, d) at the inferior extremity. At the lower border of the lateral 



