78 DISSECTION OF THE BRAIN. 



posterior to the cerebellum, and posteriorly passes into 

 the spinal cord. Anteriorly the medulla widens out to 

 form l\\QfoiLrtJivcntricle, which lies immediately under 

 the cerebellum. This lozenge-shaped or oval cavity is 

 covered over anteriorly by a thin sheet of nervous mat- 

 ter, the valve of Vieiisscns or the anterior medullary 

 vehim, easily seen, by pressing backward the cerebel- 

 lum, as a thin membrane running from the posterior 

 border of the corpora quadrigemina over the anterior 

 part of the fourth ventricle. Posteriorly the fourth 

 ventricle is roofed in by a similar membrane lyings 

 immediately under the cerebellum and probably re- 

 moved in exposing the ventricle, the posterior medul- 

 lary velum. 



f. Floor of the Fourth Ventricle. About the 

 middle are seen two transverse bands of fibres — medul- 

 lary or auditojy strict — the origin of the auditory 

 nerve. The posterior end of the ventricle where the 

 lateral walls converge is known as the calai7ius scrip- 

 torius. 



g. Fasciculi of the Medulla Oblongata. Pos- 

 terior to the calamus scriptorius the dorsal surface of 

 the medulla shows a median fissure continuous with 

 the posterior median fissure of the cord. On each 

 side of this lies a rounded eminence, the fascicuhis 

 gracilis (posterior pyramid). Just exterior to this on 

 each side is the small fasciculus cuueatus. If these 

 two fasciculi are followed forward they run into a 

 rounded eminence on each side, forming the lateral 

 boundary of the fourth ventricle, the restiform bodies, 

 and these followed anteriorly are seen to pass up into 

 the cerebellum, forming the posterior peduncles of the 

 cerebellum, making a connection between the cere- 

 bellum and cord. 



