78 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



reduced. This would suggest that the fatigue which finally asserts itself is 

 due to some general rather than local influence. To understand this we must 

 recall the fact that all parts of the body are in communication by means of 

 the circulatory system. The ever-circulating blood as it is thrown out by the 

 heart is divided into minute streams, which, after passing through the many 

 organs of the body, unite again on their return to the heart. If materials be 

 taken from the blood by one part, they are lost to all the rest, and if materials 

 be added to the blood by any part, they are sooner or later carried to all the rest. 

 During the course of a long march, the muscles of the leg take up a great deal 

 of nutriment, and give oif many waste products, and all the organs suffer in con- 

 sequence. Mosso, 1 in his experiments upon soldiers taking long forced marches, 

 found that lack of nutriment is not the only cause of the general fatigue 

 produced by long-continued muscular work. The soldiers, though somewhat 

 refreshed by the taking of food, did not recover completely until after a pro- 

 longed interval of rest. He attributed this to the fatigue-products which he 

 supposed the muscles to have given oif, and concluded ihat they were only 

 gradually eliminated from the blood. To see if there were fatigue-products 

 in the blood of a tired animal capable of lessening the irritability of organs 

 other than those which had been working, he made the following experiment : 

 He drew a certain weight of blood from the veins of a dog, and then put back 

 into the animal an equal amount of blood from another completely rested dog. 

 The dog which was the subject of the experiment appeared to be all right after 

 the operation. On another day he repeated the experiment, but this time the 

 blood which was put back was taken from a dog that was completely tired out 

 by running. The effect of the blood from the fatigued animal was very 

 marked ; the dog receiving it showed all the signs of fatigue, and crept off into 

 a corner to sleep. Mosso concluded from this experiment, that during muscular 

 work fatigue-products are generated in the muscles, pass from thence into the 

 blood, and are conveyed to other muscles, where they produce the lowered 

 irritability and loss of power characteristic of fatigue. Many years before, 

 Von Ranke extracted from the tired muscles of frogs substances which he 

 considered fatigue materials. We know many of the waste products formed 

 by muscles, and have learned that some of them lower the irritability, but 

 what the exact substances are which produce the effects observed in the above 

 experiments is not known. 



Maggiora, in his experiments upon the fatigue of special groups of muscles, 

 likewise found that the taking of food causes only a partial recovery of the 

 tired muscles, and that an interval of rest is essential to complete recovery. 

 In these experiments the irritability of the muscles was tested not only by 

 volitional impulses, but by the strength of the electric current required to 

 cause direct excitation. In the case of vigorous men, one and a half hours 

 suffices to restore the muscles of the forearm which have been completely tired 

 out by raising a heavy weight many times. He also observed that the time 

 required for recovery can be greatly shortened if the circulation of the blood 

 1 Archiv fur Anatomic und Physiologic, 1890 ; physiologische Abtheilung. 



