THE CEEEBELLUM. 



103 



The cerebellum is, without doubt, then, one of the most important 

 parts of the brain, for a study of which one is well repaid. 



Its structure is remarkably simple, and similar in all animals, the whole 

 organ being a repetition of a simple histological type. 



As previously shown, the cerebellum is ontogenetically developed from 

 a simple cell-plate; and the figures just presented make it evident that the 

 same is true of its phylogenetic development. All of the manifold forms 

 of the cerebellum arise through a folding of the primitive cerebellar plate, 

 the object of the folding being evidently an increase of surface. Whether 



Fig. 56. — Sagittal section through the cerebellum of the lizard, Yaranus 

 griseus, showing the external lamina (fac. front.) and the internal lamina 

 (fao. caud.]. 



the plate extends upward or downward, whether it remains small or pro- 

 gresses to a high development, it is always built on the same type. Let us 

 take, as our starting-point, the further study of the reptilian cerebellum, 

 because it is a simple thin plate which, lying transverse to the ventricle and 

 transverse to the long axis of the brain, extends toward the crown. One 

 may differentiate a frontal face, turned toward the midbrain, and a pos- 

 terior face. A section shows at once that these two laminae have a different 

 structure. The posterior (internal) one is rich in ganglion-cells, while the 



