Fatigue of Excitation and Fatigue of Depression etc. 57 



highly differentiated cell of cerebrum or cerebellum may un- 

 doubtedly run its course in a matter of hours under continuous 

 stimulation. 



In the second place, physiological experimentation deals with the 

 collective action of groups of cells. The exhaustion of even many 

 units which appears anatomically need not hinder the response of 

 those less affected. In such an organ as the cerebellum, it takes an 

 extraordinary effort to call all cells into play. Indeed it is very 

 definite that progressive degrees of intensity and duration of work 

 call more and more resting types into activity, in such a way that 

 there is something very suggestive of a relay action. Reserve nervous 

 power depends upon cells in reserve as well as the reserve power in 

 the individual unit. A glance at the widely varying degrees of 

 activity in any so-called nerve centre gives this a general application. 

 It receives a side-light also from another point of view. Without 

 saying that we possess more nerve cells than we actually need — 

 which by analogy with other tissues is not impossible — the organism 

 can function fairly well with a diminished number. Hodge's (1894) 

 claim of a reduction of 25°/ in his man of ninety- two does not 

 appear in the writer's experience an extreme exception, so that it is 

 questionable if functional capacity fails in direct ratio to the decrease 

 in cells. It appears then that in ordinary effort we do not use all 

 the cells we possess. 



The reserve power in the individual unit is the third consider- 

 ation, in respect to the relative nervous inexhaustibility, and the 

 predominant factor. Sufficient has been said to indicate in terms of 

 time how enormous this power is. It will only be added that the 

 cell goes through all grades of functional hypertrophy to accomplish 

 its purpose. What makes it doubly efficient is its continuity of 

 reaction. No one following the cytological pictures can doubt that 

 it is the characteristic of the cell to continue to respond until ex- 

 haustion calls a halt. So long as excitation reaches it, response will 

 follow. Such is its differentiation. Incidentally this explains why 

 we can drive ourselves through fatigue until we can go no farther, 

 until collapse. 



