LINING MEMBRANE OF THE VENTRICLES. 375 



and the subarachnoidean space, as imagined by Magendie, is very 

 questionable. 



LINING MEMBRANE OF THE VENTRICLES. 



The lining membrane of the ventricles is a serous layer, quite 

 distinct from the arachnoid, and having no communication with it. 

 This membrane lines the whole of the interior of the lateral ventri- 

 cles, and is connected above and below to the attached border of 

 the choroid plexus, so as to exclude completely all communication 

 between the ventricles and the exterior of the brain. It is reflected 

 through the foramen of JVIunro, on each side, into the third ventricle, 

 which it invests throughout. From the third it is conducted into 

 the fourth ventricle, through the iter e tertio ad quartum ventri- 

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

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

 munication is established between all the ventricles. It is this 

 membrane which gives them their polished surface, and transudes 

 the secretion which moistens their interior. When the fluid accu- 

 mulates to an unnatural 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 subarachnoidean cellular tissue ; 

 but in the normal condition it is doubtful whether a communication 

 exists between the interior of the ventricles and the cavity of the 

 subarachnoidean space. 



CEREBELLUM. 



The Cerebellum, according to Cruveilhier, is seven times smaller 

 than thev cerebrum. Like that organ, it is composed of white and 

 gray substance, whereof the gray is larger in proportion than the 

 white. Its surface is formed by parallel lamella, separated by 

 fissures ; and at intervals deeper fissures exist, which divide it into 

 larger segments termed lobules. The cerebellum is divided into 

 two lateral hemispheres or lobes, two minor lobes called superior 

 and inferior vermiform processes, and some small lobules. 



The Lateral lobes are separated from each other posteriorly by 

 a depression which lodges the falx cerebelli, and, above and below 

 by the projection of the vermiform processes. 



The Superior vermiform process, a slightly elevated ridge along 

 the middle of the upper surface of "the cerebellum, is all that exists 

 of that organ in birds, and it constitutes the largest proportion of 

 the cerebellum in many mammalia. It is situated along the middle 

 line, and serves to connect the lateral lobes superiorly. 



The Inferior vermiform process forms a projection inferiorly, and 

 is the means of connexion between the lateral lobes below. 



The principal lobules are the pneumogastric, the tonsils, uvula, 

 and linguetta laminosa. 



