STIMULI AND THEIR ACTIONS 469 



produce an excitation, which results in a powerful increase of the 

 respiration and the activity of the heart, but finally with too great 

 exertion cause a depression, which leads to standstill of the heart 

 and death. The history of the runner of Marathon is a classic 

 example of this course of phenomena. 



But in seeking the origin of muscle-fatigue, we ought not to 

 attach too much importance to the appearance and accumulation 

 of fatigue-substances in the muscle, as is not rarely done. Al- 

 though it is beyond doubt that the phenomena of fatigue can be 

 produced by the accumulation of fatigue-substances, this is not 

 the sole cause. The chief factor in the production of fatigue is 

 the progressive consumption of substances that are necessary to 

 activity. Accordingly, in muscle and probably in all living sub- 

 stance, two different causes of fatigue may be present. Phenomena 

 of fatigue are observed, on the one hand, when certain substances 

 that are necessary to life are consumed during exhaustive activity 

 more rapidly than they are introduced or reformed ; and, on the 

 other, when certain substances that arise as decomposition-pro- 

 ducts during activity accumulate in such quantity that they 

 produce a depressing effect. On account of this fundamental 

 difference in the genesis of the phenomena in question, it seems 

 advantageous to distinguish between the two causes by the use of 

 different terms, and to call the phenomena of depression that 

 result from the consumption of the necessary substances, exhaus- 

 tion, and those that result from the accumulation of and poisoning 

 by decomposition-products, fatigue. The end-result of the two 

 series of phenomena arising from such different causes is the 

 same. Both are characterised by depression of the irritability 

 and the activity of living substance. 



2. Excitation and Depression 



Let us first bear in mind that excitation and depression are 

 merely quantitative opposites. The two are merely different 

 degrees of one and the same phenomenon, namely, life, excitation 

 being an increase, depression a decrease of the normal intensity of 

 vital phenomena. 



It has been seen in a previous section that phenomena of 

 depression can be called out by over-stimulation. This fact is 

 important, for it shows that the same stimuli which with slight 

 intensity or short duration produce excitation, with increased 

 intensity or long duration can produce precisely the opposite 

 effect, namely, depression. 



This relation between excitation and depression is very wide- 

 spread. The phenomena of fatigue are a single example of it. 

 In this respect the effects of anaesthetics form a complete analogy 

 to the phenomena of fatigue. It appears to be a general pro- 



