420 LINING MEMBRANE OF THE VENTRICLES. 



floor of the fourth ventricle is formed by two slightly convex bodies, 

 processus teretes or posterior pyramids, separated by a longitudinal groove 

 which is continuous inferiorly with the sulcus longitudinalis posterior 

 of the spinal cord. The processus teretes are crossed transversely by 

 several white and grey fasciculi (lineee transverse} the origin of the 

 auditory nerves. And upon the lower part of the floor of this ventricle 

 is an impression resembling the point of a pen and hence named calamus 

 scriptorius; the lateral boundaries of the calamus are the processus 

 clavati of the posterior median columns of the spinal cord. Above, the 

 fourth ventricle is bounded by the corpora quadrigemina and aqueduct 

 of Sylvius ; and below by a layer of pia mater and arachnoid, called 

 the valve of the arachnoid. It is by rupture of this latter that a com- 

 munication is established between the ventricles of the brain and the 

 sub-arachnoidean space. Within the fourth ventricle and lying against 

 the uvula and tonsils are two small vascular fringes formed by the pia 

 mater, the choroid plexuses of the fourth ventricle. The fourth ven- 

 tricle is lined by grey matter derived from the interior of the spinal 

 cord, the grey matter being partly concealed by a thin expansion of 

 white substance. 



LINING MEMBRANE OP THE VENTRICLES. 



The lining membrane of the ventricles is a serous layer distinct 

 from the arachnoid ; it lines the \vhole of the interior of the lateral 

 ventricles, and is connected above and below with the attached border 

 of the choroid plexus, so as to exclude all communication between the 

 ventricles and the exterior of the brain. From the lateral ventricles 

 it is reflected through the foramen of Monro on each side, into the 

 third ventricle, which it invests throughout. From the third it is con- 

 ducted into the fourth ventricle, through the iter a tertio ad quartum 

 ventriculum, and lines its interior, together with the layer of pia mater 

 which forms its inferior boundary. In this manner a perfect communi- 

 cation is established between all the ventricles, with the exception of 

 the fifth, which has its own proper membrane. It is this membrane 

 which gives them their polished surface, and transudes the secretion 

 which moistens their interior. When the fluid accumulates to an un- 

 natural degree, it may then break down this layer and the layer of 

 pia mater at the bottom of the fourth ventricle, and thus make its way 

 into the sub-arachnoidean space ; but in the normal condition it is 

 doubtful whether a communication exists between the interior of the 

 ventricles and the serous cavity of the sub-arachnoidean space. 



CEREBELLUM. 



The Cerebellum, seven times smaller than the cerebrum, is situated 

 beneath the posterior lobes of the latter, being lodged in the posterior 

 fossa of the base of the cranium and protected from the superincum- 



