THE BKAIX IX THE EDENTATA. 379 



From the pallium a large mass of projection-fibres arises, and in its downward path 

 invades the ventral part of the mesencephalon, where it is known as the crus cerebri. 

 The caudal remnant of the same series of fibres in the medulla oblongata forms the 

 pyramid. These series of fibres are lacking in the subraammalia, and they increase in 

 size with the groAvth and perfection of the pallium. On the ventral surface of the hind- 

 brain a new series of fibres, the pons Varolii, makes its appearance. This is part of the 

 association-system between the pallium and the cerebellum. 



This leads us to the consideration of the cerebellum. This organ shows most marked 

 signs of advance in the Mammalia, and, next to the cerebral hemisphere, indicates 

 most strikingly the mammalian superiox'ity of type. It is not improbable that the 

 important changes in the cerebellum are to a large extent the rcult, j^artly direct and 

 partly indirect, of the development of the pallium. 



In the immediate ancestors of the mammal the cerebellum vvas in all probability a 

 most insignificant and simple organ, hut pari passu with the development of the pallium 

 the organ rapidly increases -in size and complexity and becomes intimately connected 

 •with the new pallium. It is in the highest degree probalile that the sudden and con- 

 temporaneous progression of tliese two parts of the brain is a single correlated pheno- 

 menon, and not two coincident but independent acts. Tliere can be little doubt that the 

 increased activity of cerebellar evolution is one of the expressions of the all-powerful 

 pallia! influence. 



We have already briefly and somew^hat crudely indicated how the possession of a 

 pallium may permit the mammal to execute movements of greater variety, of increased 

 nimbleness and immeasurably greater delicacy : in a word, of infinitely gi-eater complexity 

 than is possible in an animal without a pallium. In the execution of such complex 

 movements it becomes of the highest importance that the relative activity of the 

 diff'erent muscles which perform the act should be delicately adjusted and nicely 

 balanced in order that the complex movement may be " co-ordinate " and purposeful. 

 It seems not unlikely, taking into consideration the two-fold connections of the cerebellum 

 on the one hand with the incoming nerves from the sensory surfaces and the muscles, 

 and on the other with the pallium, that this organ may be a complex executive machine 

 which in some way controls and renders possible the delicately-adjusted co-operation of 

 wide groups of muscles. 



If such be the case, we might expect the cerebellum to be modified by two, among 

 in all probability many other, factors. 



In the first place afferent fibres which enter the cerebellum from sensory areas or 

 muscular apparatiis would presumably need a co-ordinating machine proportionate to 

 their number — or, in other words, the size of an animal would influence the dimensions 

 of tlie cerebellum. 



In the second place, the pallial connection would undoubtedly influence the dimensions 

 of the cerebellum. For a large and highly diflcrentiated pallium, which betokens complex 

 potential movements, would presumably need a much richer cerebellar associate than a 

 small and simple one. 



There are unquestionably many other factors at work, such as the possible influence of 



