28 LECTURES ON THE CENTRAL NERVOUS SYSTEM. 



of a t'hin membrane (velum medullare posticum), passes into the 

 posterior part of the spinal cord, contains the rudiments of the 

 cerebellum. If you look at the sections shown in Figs. 12 to 17, 

 it will strike you that with, perhaps, the exception of the fore- 

 ' brain, no part of the brain shows so many changes in develop- 

 ment as this. But the cerebellum is not, like the cerebrum, 

 more developed in the higher classes of animals than in the 

 lower. We find very remarkable differences in animals very 

 nearly related to each other, and, on the other hand, in the lower 

 selacians, for instance, an extremely good development of the 

 organ. In the amphibians we meet with the cerebellum in its 

 simplest form. The side of the roof of the hind-brain which 

 faces the mid-brain is thickened into the form of a plate lying 

 across the ventricle. Reptiles do not possess this organ in a very 

 high state of development, but in those of them that swim 

 (alligators) this ydate is twice as thick as ordinary, and extends as 

 far backward as the caudal side of the roof. Large swimming 

 animals, the bony fishes and the selacians, possess a cerebellar 

 organ which is so enormously developed that it must lie in huge 

 transverse folds (Fig. 12), and even at times pushes itself forward 

 under the roof of the mid-brain into the aqueduct of Sylvius 

 (Fig. 13). Fishes living in mud (dipnoi) have a smaller cere- 

 bellum. 



In fishes, amphibians, and reptiles, bundles of fibres pass 

 from the inter- and mid- brain to the cerebellum, and from the 

 spinal cord. We find these same bundles in birds and mammals, 

 but in the former there are added very small, and in the latter 

 very large, bundles of fibres from the fore-brain. These termi- 

 nate in structures which we now meet for 1 the first time, devel- 

 oping on each side of the middle portion of the cerebellum the 

 hemispheres of the cerebellum. In birds these are still small, 

 but in mammals they increase along with the development of 

 the middle portion (from now on called the worm, vermis) until 

 they far exceed the latter in size. The vermis, however, even 

 in the human being, retains the transverse foldings which have 



