PHYSIOLOGICAL CHANGES IN THE CENTRAL SYSTEM. 28 1 



observations, the cause of this divergence being still 

 undetermined. It has been suggested that the rate was 

 not the same in the different portions of the nervous 

 system, but this suggestion is without evidence, though 

 there is reason to think that in passing through the cell- 

 body, or in passing from one cell element to another, 

 the impulse is much delayed. The signs of metabolism 

 following activity in the nerve fibres are so slight that 

 they have not yet been clearly demonstrated, therefore 

 it is possible to assume that either there are metabolic 

 changes which have not yet been detected, or that the 

 nerve impulse is not accompanied by such changes. 

 However this question shall be decided for the fibres, 

 there is the plainest evidence for metabolic changes in 

 the bodies of the nerve cells, but the description of 

 these will be reserved for the chapter on fatigue. If 

 the nerve impulses are normally initiated at the tips 

 of the cell branches, metabolic changes which have not 

 yet been discovered are to be expected at these points. 



To arouse a nerve impulse several conditions must 

 be fulfilled. First a sudden stimulus must be applied. 

 For example, the rapidly interrupted electrical current 

 stimulates, whereas a slow variation in the intensity of 

 this same current does not. Or, again, when, mechanical 

 stimulation is employed a slow increase of pressure fails 

 to elicit a response, whereas a light tapping will do so. 

 Gradual changes, therefore, do not act as stimuli. 

 This, however, is by no means equivalent to the state- 

 ment that they are without effect, although concerning 

 the effect of such inefficient stimuli nothing is known. 

 It has been also shown that the determination of a 

 reaction is more dependent on the rate at which the 

 stimuli occur than upon their strength, and that a single 

 stimulus does not elicit a response. It appears, in con- 

 sequence, that the arrangements in the central nervous 



