72 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



waste products of contraction, so that vigorous muscles can be employed 

 many hours before any marked fatigue is experienced. Sooner or later, 

 however, the vigor of the muscle begins to decrease. The reason for 

 this is nut wholly clear. It is noticeable, however, that not only the 

 muscles employed in the work, hut other muscles, such as those of the 

 arms for instance, even when purposely kept quiet, have their irritability 

 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 lung march, the muscles of the leg take up a great deal 

 of nutriment, and give off 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 off, and concluded that 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 dug 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 mus- 

 cular work fatigue-products are generated in the muscles, pass 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. Lee 2 would draw a sharp distinction between 

 fatigue and exhaustion. lb' considers the former to be a transient change 

 in the capacity for work induced by the presence of waste products, while 

 the latter is a far more serious condition and is due to a lack of nutritive 

 energy-giving substance. He considers that fatigue, by lessening the irri- 



1 Arcliir fur Anatomic mul Physiologie, 1890; phvsiolmrische Abtheilung. 



2 Proceedings of the American Physiological Society, Dec, 1898, published in American 

 Journal of Physiology, 1899, vol. ii. p. 11. 



