60 THE HUMAN MECHANISM 



Again, the mere exposure of a resting muscle to blood 

 containing lactic acid or to blood heavily charged with 

 carbon dioxide (CO 2 ) produces the condition of fatigue. 

 Now in the last chapter it has been shown that both lactic 

 acid and carbon dioxide are waste products of muscular 

 activity; and these and other facts have led to the view, 

 now generally received, that the waste products of the 

 active organ interfere with the work of the organ and so 

 constitute one of the main causes of fatigue. It is ap- 

 parently for this reason that the injection of an extract of 

 worked muscle fatigues fresh muscle, for the extract con- 

 tains waste products. It is for the same reason that wash- 

 ing out a fatigued muscle with salt solution produces partial 

 recovery, for the waste products of activity are in this way 

 partially removed. We can also understand why fatigue 

 always accompanies vigorous work. Waste products then 

 necessarily accumulate and clog the living mechanism be- 

 cause they cannot be removed by the blood as fast as they 

 are formed by the muscle cells. No fatigue occurs with 

 only a single contraction every ten seconds or more be- 

 cause between contractions sufficient time is given to insure 

 the complete removal of wastes. 



3. Loss of fuel in the working muscle as a cause of fatigue. 

 The blood, however, not only removes the wastes but also 

 brings new food and oxygen with which the muscle makes 

 good the loss of fuel; and it may well be although it is 

 not absolutely proved that recovery from fatigue depends 

 upon both of these good offices of the blood. We have 

 certainly one well-established cause of fatigue, namely, the 

 presence of the waste products of activity ; and we recog- 

 nize the probability that the depletion of fuel may also 

 contribute to the result. But whether the first of these 

 causes alone is sufficient to explain it, or whether both 

 work together, we can understand that the maintenance 

 of a good blood supply is of the first necessity and that 



