THE GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM 101 



muscle 



M.2, M.S). On the other hand, in case the stimulus received by the 

 neuron of the first order is distributed to several neurons, all of which dis- 

 charge into the same motor center, the stimulus may be reinforced because 

 each neuron of the second order may discharge its own reserve energy in 

 such a way as to send out a stronger impulse than the one received, so that 

 the total discharge into the motor 

 center is greatly strengthened (Fig. 

 40). Such an impulse may be said 

 to accumulate momentum as it ad- 

 vances like an avalanche on a moun- 

 tain slope, and hence this type of re- 

 action has been termed by Ramon 

 y Cajal "avalanche conduction." 

 In some parts of the brain there are 

 very special mechanisms for this sort 

 of cumulative discharge, as in the 

 cortex of the cerebellum (p. 192) and 

 the olfactory bulb (p. 218). 



The intensity of nervous dis- 

 charge in all of its forms is very 

 dependent upon the general physio- 

 logical state of the body, some con- 



Fig. 40. Diagram of the mechan- 

 ism of reinforcement whereby a 

 single weak afferent nervous im- 

 pulse may be received by several 

 neurons of the second order which 

 discharge their greatly strengthened 

 nervous impulses into a single final 

 common path. 



ditions, such as fatigue and various 

 intoxications, tending to depress the 

 activity, and other conditions tend- 

 ing to facilitate it. The main- 

 tenance of good nervous tone is, 

 therefore, essential to the highest 

 efficiency. Some of these physio- 

 logical agents may also act locally on particular parts of the nervous system 

 and thus determine the selection of one instead of another out of several 

 possible modes of response in the variable type of behavior. 



Fatigue of nerve-cells may be brought about in two ways, 

 which have been clearly distinguished by Verworn: (1) by 

 the consumption of reserve material from which the energy of 

 the cell is derived more rapidly than this material can be re- 

 stored, and (2) by the accumulation of waste-products more 

 rapidly than they can be eliminated from the cell. These forms 

 of fatigue have recently been named by Dolley respectively 

 "fatigue of excitation" and "fatigue of depression." 



In his interesting discussion of neuro-muscular fatigue, Stiles 

 (1914, p. 101) enumerates several particular ways (in addition 

 to the two general methods just mentioned) by which fatigue 

 may be brought about, among which are the following: (1) 

 fatigue of muscle-fibers, (2) fatigue of the junction of the motor 

 nerve with the muscle-fiber at- the motor end-plate (see Fig. 

 5, p. 40), (3) fatigue of the nerve-fibers, (4) fatigue of the motor 



