THE CEREBELLUM. 



233 



number and variety of functional relationships into which the 

 cerebellum enters. The cerebellum has become in man a great 

 center concerned with correlated movements in response to impulses 

 from many sources. A brief review of the human and mammalian 

 cerebellum will illustrate this. 



In order to understand the mammalian cerebellum it is necessary 

 first to discard the cumbersome and meaningless description of 

 lobes and surface divisions based upon the adult appearances, which 

 is found in text-books of anatomy. The simpler method of dividing 

 the cerebellum based upon the developmental history is much to 

 be preferred, but even this gains its significance only when it is 

 combined with a consideration of the centers and tracts within. 

 First of all must be pointed out the fallacy of the common state- 

 ment that the cerebellar hemispheres of mammals are new form- 

 ations, not found in sub-mammalian classes. The hemispheres 

 are formed first. The outline of the brain of a young marsupial 

 given in Fig. 118 shows that in these forms the cerebellar hemis- 

 pheres correspond closely to those of reptiles, amphibia or even 

 of fishes (compare Figs. 2, 3, 7). The tuberculum acusticum 

 occupies the same position as in lower vertebrates, at the dorsal 

 border of the medulla oblongata immediately behind the cere- 

 bellum, and in front of it the border of the cerebellar hemisphere 

 is formed of the flocculus and paraflocculus and these are con- 

 tinued upward by the middle lobe as the arch of the cerebellum. 

 In the early human embryo after the cerebellar arch has been 

 formed by the massive walls fusing in the roof, the lateral lobes 

 are more prominent than the middle portion and the whole cere- 

 bellum has the same form as in lower vertebrates. The vermis is 

 formed later than the hemispheres, not earlier. So in the phylo- 

 genetic history: the lateral lobes are constant in the vertebrate 

 series, the vermis is formed only hi birds and mammals. What 

 does happen in the higher mammals to produce a cerebellum 

 which seems to differ greatly from that of lower vertebrates is: 

 (i) a great growth of the median or keystone region of the arched 

 cerebellum to form the vermis; (2) a great growth of that part 

 of the hemispheres which lies in front of the floccular lobe, to form 

 what is commonly known as the hemispheres; and (3) an arrest 



