STIMULI AND THEIR ACTIONS 461 



integration, beginning with great energy, becomes slower and 

 less extensive the longer the current flows, and after some time 

 is at a complete standstill. This means that the living substance 

 of the ActinospTiccrium becomes fatigued in the course of the 

 continual stimulation, and decreases in irritability; hence the 

 stimulus, which at first induced pronounced phenomena of 

 disintegration, later produces no reaction at all. Pelomyxa 

 is fatigued still more rapidly than Actinosphcerium. Stimula- 

 tion for a few seconds is sufficient to make individuals of this 

 genus wholly non-irritable to currents of equal intensity; 

 a much greater intensity is then required to call out the same 

 reaction. 



In contrast to these forms of living substance which become 

 fatigued very rapidly, nerves seem to be incapable of fatigue : 

 thus far it has been impossible by continual stimulation to 

 demonstrate in them fatigue phenomena. That nerve is really 

 incapable of fatigue is in the highest degree improbable. Since, 

 like all living substance, it has a metabolism so long as it lives, 

 and since its irritability is extinguished with its life, it must be 

 supposed that its irritability is associated with its metabolism, 

 and that every excitation produces a change in its metabolism. 

 Possibly these changes are so slight that fatigue cannot be 

 demonstrated at all by the methods that have been used hereto- 

 fore. To conclude, therefore, from the apparent incapability of 

 fatigue that the function of nerve is entirely independent of 

 metabolism, and is like the capacity of copper wire to conduct 

 galvanic currents, is quite unjustified. Nevertheless, it would 

 be important to investigate the question, whether in nerves the 

 changes of metabolism produced by stimulation are not perhaps 

 compensated by the metabolism as soon as they appear, so that 

 within a limited time no phenomena of fatigue become noticeable 

 externally. That such a condition is very easily possible is shown 

 by the behaviour of another object viz., cardiac muscle. Although 

 from long before birth up to death the heart-muscle labours 

 uninterruptedly, under normal conditions it does not become 

 fatigued, because the changes resulting from its activity become 

 compensated in its metabolism. Nevertheless, it is capable of 

 fatigue, when for any reason it is obliged to make excessive efforts. 

 This is the case in certain diseases. The phenomena of fatigue 

 become then apparent, not at once, but in the course of long 

 spaces of time, and even the substance of the muscle changes 

 profoundly, until its movements wholly cease. Then death by 

 paralysis of the heart results. 



While cardiac muscle is thus capable of fatigue only exception- 

 ally, in the tissue of skeletal muscles fatigue phenomena are very 

 easily induced. Fatigue has been studied most fully and most 



