78 DISSECTION OF THE BRAIN. 



posterior to the cerebellum, and posteriorly passes into 

 the spinal cord. Anteriorly the medulla widens out to 

 form Vaefotirih ventricle, 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 Vieussens or the anterior medullary 

 velum, 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 lying 

 immediately under the cerebellum and probably re- 

 moved in exposing the ventricle, the posterior medul- 

 lary vchim. 



f. Floor of the Fourth Ventricle. About the 

 middle are seen two transverse bands of fibres — medul- 

 lary or auditory stricB — the origin of the auditory 

 nerve. The posterior end of the ventricle where the 

 lateral walls converge is known as the calamus scrip- 

 torijis. 



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 fasciculus 

 gracilis (posterior pyramid). Just exterior to this on 

 each side is the small fasciculus cuncatus. 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 pedimcles of the 

 ccrebelhim, making a connection between the cere- 

 bellum and cord. 



