78 DISSECTION OF THE BRAIN. 
posterior to the cerebellum, and posteriorly passes into 
the spinal cord. Anteriorly the medulla widens out to 
form the fourth 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 Aosterzor medutl- 
lary velum. 
f. FLoor oF THE FourtTH VENTRICLE. About the 
middle are seen two transverse bands of fibres—szeda/- 
lary or auditory strze—the origin of the auditory 
nerve. The posterior end.of the ventricle where the 
lateral walls converge is known as the calamus scrip- 
torvits. 
eo. FascicuLi oF THE MeEpuLita OsBLoncaTaA. 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 /asceculus 
eracilts (posterior pyramid). Just exterior to this on 
each side is the small fasczculus cuneatus. 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 vestzform bodes, 
and these followed anteriorly are seen to pass up into 
the cerebellum, forming the fosterzor peduncles of the 
cerebellum, making a connection between the cere- 
bellum and cord. 
