Chap. XXIV.] PRINCIPAL PARTS OF THE BRAIN. 507 



The fact that the Cerebellum does co-opera^^e with the 

 Cerebrum in some way is clear, because it has been 

 proved that atrophy of one Cerebral Hemisphere leads 

 to atrophy of the opposite half of the Cerebellum.* And 

 that the Cerebellum responds to stimuli from the Cere- 

 brum, rather than vice versa, seems shown by the fact 

 that atrophy of one half of the Cerebellum has, on the 

 other hand, no tendency to cause atrophy of the opposite 

 hemisphere of the Cerebrum. 



The notion that the Middle Peduncles are the parts by which the 

 relation between the Cerebrum and the Cerebellum is principally 

 brought about in Yolitional Action, is strongly supported by two 

 sets of facts; first, by the later development of these Middle 

 Peduncles and of the lateral lobes of the Cerebellum with which 

 they are principally connected in the animal series, as well as by 

 their progressive increase in still higher animals, and by their 

 maximum size in Man;t secondly, the view is also borne out by 

 what we know of their anatomical relations. Broadbent's, as well 

 as Meynert's, descriptions give us some warrant for believing that 

 fibres pass from each Middle Peduncle of the Cerebellam to the 

 opposite half of the ' pons Varolii,' and thence (by way of the Crus 

 Cc-rebri) in part direct to the cortex of the Hemisphere, and in part 

 to the Corpus Striatum only. Other of its fibres may, perhaps, 

 pass down to motor centres in the Pons itself, or to similar centres 

 in the Medulla.;]; 



* That is when the atrophic process of the Hemisphere in- 

 volves such parts as to entail a Hemiplegia — or paralysis of the 

 opposite side of the body. (ISee p. o93.) 



t Meynert (Strieker's "Histology," Eng. Trans., ii. p. 456) calls 

 attention to the fact that as the Cerebral Hemispheres increase in 

 size, so do the motor divisions of the Crus, and so also do the 

 Middle Peduncles and 'lateral lobes' of the Cerebellum. (See 

 p. 278 for some remarks touching this kind of correlation.) 



;j: From the cells of the Corpus Striatum, according to Meynert, 

 there descend " two subsequently diverging tracts, one running 

 into the Spinal Cord, the other into the Cerebellum." The latter 

 ascends as a thick fasciculus in the Middle Peduncle (loc. cit. p. 

 875). This fasciculus may contain ascending (' afferent ') cerebellar 



