192 INTRODUCTION TO NEUROLOGY 



of the afferent fibers of the inferior cerebellar peduncle, and that 

 the ascending fibers are the terminals of the fibers from the 

 middle peduncle (brachium pontis). 



Since each fiber from the inferior peduncle branches exten- 

 sively and reaches many granule cells in widely separated parts 

 of the cerebellum, and since the axon of each granule cell- reaches 

 the dendrites of a very large number of Purkinje cells, a single 

 incoming nervous impulse may excite a very large number of 

 Purkinje cells, and thus its physiological effect may be greatly 

 enhanced. The same result is also secured by the action of the 

 basket cells (Fig. 89, 6) and other forms of neurons with short 

 axons within the cortex (Fig. 89, e, /), each of which may 

 discharge powerful impulses directly upon several Purkinje 

 cells. The axons of the Purkinje cells themselves also give off 

 collateral fibers into the granular layer, whose neurons dis- 

 charge back into the Purkinje cells again. In all of these 

 ways provision is made for the diffusion, summation, and re- 

 inforcement of stimuli during the process of their transmission 

 through the cerebellar cortex, and also for prolongation of motor 

 reactions which would otherwise soon subside, and for the main- 

 tenance of muscular tone. 



This type of reaction has been termed "avalanche conduction" 

 (see p. 101), and its mechanism here is similar to that found in 

 the olfactory bulb (p. 218), but much more complex. It is prob- 

 able that the reciprocal relation between the cerebellum and the 

 cerebral cortex is of a similar sort, all cortical activities exciting 

 also the cerebellum and drawing therefrom additional nervous 

 energy as needed to maintain the tone of the reacting mech- 

 anism; and voluntary movements excited by the cortico-spinal 

 or pyramidal tract from the cerebral cortex (see p. 283) are 

 under especially direct proprioceptive control from this source. 



The relationships of the centers of the brain stem, the cerebral 

 cortex, and the cerebellum may be illustrated somewhat crudely 

 by the analogy of the three chief departments of the national 

 government. The reflex centers of the brain stem correspond 

 to the legislative branch of government, determining in advance 

 by virtue of their innate structure what actions may appropri- 

 ately be performed in each particular type of frequently recur- 

 ring situation. The cerebral cortex is a sort of glorified judicial 



