THE CEEEBELLUM. 



105 



fibers begin; and through the branches of the cells located there is furnished 

 a wide range of possibilities in the co-ordination of the processes which go 

 on there. 



From the simple type — that of Varanus, for example — ^we may readily 

 derive most of the other types of cerebellum. "We have to deal with only 

 two factors: with the development of the cortex and of the medullary 

 center. If the cortex increases in size it presents folds. Fig. 57 shows how 

 the simple lizard type is doubled in the swimming alligator and turtle, 

 and how, through a farther folding of the same cerebellar plate, the avian or 

 mammalian type may be derived. In teleosts the surface is relatively greater 

 than in amphibians and reptiles and it arises from the facts (1) that the 

 molecular, or inner, layer is thicker; and (2) that an unusual number of 

 afferent bundles pass into the cerebellum, much increasing the medullary 

 center over the ventricle. Thus arises an apparently massive body (see 



Fig. 58. — Sagittal section a little to one side of the median line 

 through the cerebellum of a small ray. 



Fig. 44), in which the part that lies under the roof of the midbrain is 

 designated the Yalvula cerebelli. It has already been mentioned above that 

 at the posterior end of the cerebellum there are associated parts which stand 

 in special relation to the nuclei of the Aqusticus and the Trigeminus. The 

 separation of this region from the cerebellum is not yet to be sharply made 

 in most vertebrates, but in selachians they are separately marked trans- 

 verse folds; so that one may speak of a Lobus cerebellar is Acustici and a 

 Lobus cerebellaris Trigemini (see Pig. 58). 



In teleosts and even in selachians the cerebellum cortex extends some 

 distance posteriorly and beyond these accessory structures. In birds and 

 mammals they are completely included in the formation of the cerebellum, 

 where they lie in the median portion, — in the Vermis. 



Certain cell-groups easily demonstrable in mammals and birds, as yet 

 hardly known in reptiles and amphibians, and easily found — ^to the extent 

 of one cell-group — in fishes, may be called the special cerebellar nuclei. The 



