688 



NERVOUS SYSTEM. (NERVOUS CENTRES. THE ENCEPHALON.) 



Fig. 389. 

 I 



Front view of the cerebellum, with medulla oblonr/ata and mesncephale. (After Foville.) 



in, m, medulla oblongata. h, h, semilunar fissure. 



n, the flock on the right side, or lobule of the vagus. 



5, fifth nerve. On the left side a layer appears to be extended from this nerve which contributes to 

 form the crus cerebelli. 



rated by the valve of Vieussens, and, beneath 

 these, the fibres of the restiform columns, and 

 the right and left extremities of the pons 

 Varolii, all of which combine to form the crus 

 cerebelli or central stem of each lateral lobe. 



The posterior margin is interrupted in its 

 middle by a vertical notch, which divides it 

 into a right and left portion. This notch is 

 wider in front than behind, whence Reil called 

 it the purse-like fissure. The term posterior 

 notch is preferable. It receives the falx cere- 

 belli, and at its bottom we observe a continuity 

 between the superior and inferior laminae of 

 the median lobe of the cerebellum. 



The superior surface of the cerebellum is 

 slightly convex, inclined backwards and down- 

 wards. It terminates in front by a concave 

 margin, which overlaps the parts contained in 

 the semilunar fissure. This surface is more 

 convex along the middle than on either side. 

 In the latter situations it is inclined and nearly 

 plane ; but in the former it resembles more the 

 surface of a cylinder. This middle portion 

 corresponds to what is commonly called the 

 superior vermiform process : it is in fact the 

 upper surface of the median lobe of the cere- 

 bellum. 



On its inferior surface the subdivision of the 



cerebellum into two symmetrical portions is 

 very apparent, by reason of the existence of a 

 deep fissure which proceeds from before back- 

 wards along the median line, and is continuous 

 behind with the posterior notch. This fissure 

 is called the valley (vallecula, Haller ; grunde 

 scissure mediane du cervelet). It separates the 

 hemispheres of the cerebellum, each of which 

 presents a very convex surface, corresponding 

 to each occipital fossa. The arachnoid mem- 

 brane is extended from one to the other, towards 

 the posterior part of the fissure, leaving a con- 

 siderable space between it and the pia mater, 

 which is traversed by some fine bundles of 

 fibrous tissue and occupied by subarachnoid 

 fluid. This space has already been referred 

 to as the posterior conflux of Majendie. 



The anterior part of this fissure receives the 

 upper and posterior portion of the medulla 

 oblongata. The remainder of it is occupied 

 by the inferior surface of the median lobe of 

 the cerebellum, presenting a remarkable cru- 

 ciform arrangement, which will be presently 

 described. 



Another very remarkable fissure requires a 

 special notice. It is horizontal, and passes 

 into the substance of the cerebellum, dividing 

 it into an upper and an inferior portion. By 



