270 INNERVATION. [CHAP. X. 



angle. These planes are in number about ten, counting those on the 

 upper as well as the under surface. Those situated in front are de- 

 tached at a right angle, the posterior ones at an acute angle. Each 

 plane forms the centre of a lobule, and as it proceeds outwards 

 secondary planes are detached from it, and from these again others 

 separate. These secondary and tertiary planes are clothed by a 

 layer of vesicular matter, which also invests the primary planes at 

 the angles of separation from the principal central column. 



We have described each primary plane as forming the central 

 portion, or stem of a lobule. Each lobule is circumscribed and 

 separated from those in immediate relation to it, by a fissure which 

 extends to the principal column. The lobules are composed of 

 laminae which derive their fibrous matter from the central stem. 



Thus the substance of each hemisphere of the cerebellum is 

 penetrated by a number of fissures, easily traced by following the 

 pia mater, which lines them. These fissures are divisible into two 

 classes, primary and secondary. The primary penetrate to the 

 principal central column, and isolate the lobules; the secondary 

 separate the lamellae of which each lobule is composed. The deep- 

 est and most remarkable of the former corresponds to the posterior 

 margin of each hemisphere, passing in the horizontal plane for- 

 wards, and separating the posterior laminae into a superior and 

 inferior set. 



The structure of the median lobe is essentially the same as that 

 of the hemispheres. A stem of fibrous matter, continuous with the 

 processus cerebelli ad testes, constitutes the central column, and 

 planes radiate from it in the same manner as in the hemispheres. 

 Lobules are formed around these planes, and the aggregate of 

 those on the superior surface of the median lobe constitutes what is 

 called the superior vermiform process ; and that of the inferior ones, 

 the inferior vermiform process. The lobules of the median lobe 

 have a distinct continuity of substance with those of the hemis- 

 pheres on each side, and thus the entire lobe becomes a medium of 

 connexion, or a commissure between the hemispheres ; neverthe- 

 less, the similarity of its structure to that of the hemispheres, 

 and its existence in the animal series without the lateral portions, 

 denote that it exercises an independent function. 



Within the central stem of each hemisphere of the cerebellum, 

 the fibrous matter is partially interrupted by a peculiar arrangement 

 of the vesicular substance, called by Vicq d'Azyr corpus dentatum 

 (fig. 71, d). This is only found in the inner half of the stem, at 

 about a quarter of an inch from the origin of the crus. It may be 



