690 



NERVOUS SYSTEM. (NERVOUS CENTRES. THE ENCEPIIALON.) 



Fig. 390. 



- -.A 



Superior surface of the cerebellum, 



A, the square lobe ; P, the posterior superior lobe ; S, superior layer of the eras cerebri ; 

 q, tubercula quadrigemina ; /, locus niger ; i, inferior layer of the crus cerebri. 



terior laminae form the floor of the posterior 

 notch: the anterior form, by their adhesion 

 to each other, the layer known by the name 

 of valve of Vieussens, which fills up the interval 

 between the processes cerebelli ad testes. 



The lamina which form the superior surface 

 of the median lobe, (or the superior vermiform 

 process,) are considerably fewer than those of 

 the hemispheres. This explains the less depth 

 of the median lobe, when measured from before 

 backwards, than of the hemispheres. Two or 

 more of the lamina? of the latter are united to 

 a single lamina of the former, and thus the 

 superior vermiform process serves as a trans- 

 verse commissure to the superior laminae of the 

 hemispheres. 



The inferior surface of the median lobe, or 

 inferior vermiform process, is likewise com- 

 posed of laminse, which take a transverse di- 

 rection and present a free convex border, with 

 some resemblance to the rings of a worm in 

 action. ( I 1 'ig. 391.) These laminse are not all of 

 equal transverse extent. The middle and pos- 

 terior are the broadest; the anterior gra- 

 dually diminish in size. Hence the body which 

 results from the conjunction of all the lamina? 

 has a triangular form, its apex being anterior 

 and its base posterior, corresponding to the 

 notch between the hemispheres. The laminse 

 which occupy its middle have a greater depth 

 than the rest, and consequently the body is 

 more prominent at this situation. 



Certain deep fissures divide the inferior ver- 

 miform process into segments which evidently 

 correspond with and connect the lobes into 



which the hemispheres are subdivided on their 

 inferior surface. 



These segments may be very readily distin- 

 guished from each other, and the names which 

 the accurate lleil has given them are suffi- 

 ciently appropriate. By separating each seg- 

 ment from the adjacent ones and tracing its 

 lateral relations, the anatomist may form a 

 better idea than by any other means of the way 

 in which this portion of the cerebellum is con- 

 nected with the hemispheres. 



The anterior extremity of the inferior vermi- 

 form process projects into the cavity of the fourth 

 ventricle, and serves to close it at its inferior ex- 

 tremity. It is a pointed process, furrowed trans- 

 versely, continuous by its base with the rest of 

 the vermiform process. Reil has named it the 

 Nodule. From either side of it a valve-like 

 membrane of exquisite delicacy extends forwards 

 and outwards towards a lobule which is at- 

 tached to each crus cerebelli near to the origin 

 of the auditory nerve. These membranes re- 

 semble very much in shape the semilunar 

 valves of the aorta. By their attached margin 

 they adhere to the crus cerebelli, and their free 

 margin projects into the cavity of the fourth 

 ventricle. Their inner extremities adhere to the 

 nodule, and are connected to each other by a 

 thin membrane of precisely similar texture, 

 which is a commissure to them. Reil gives to 

 the two membranes and their intermediate con- 

 necting one the name of posterior medullary 

 velum.* The lateral membranes were first de- 



* The valve of Vieussens is the anterior medul- 

 lary velum. 



