650 



A MANUAL OF PHYSIOLOGY 



are repeated every second, or every half-second, distinct evidence 

 of fatigue is seen on the tracing after a longer or shorter period, 

 according to the conditions. 



What is the cause of muscular fatigue ? An exact answer is 

 not possible in the present state of our knowledge, but we may 

 fairly conclude that in an isolated preparation it is twofold : 

 (i) The material necessary for contraction is used up more 

 quickly than it can be reproduced or brought to the place where 

 it is required ; (2) waste products are formed by the active 

 muscle faster than they can be removed. That even an isolated 

 muscle has a certain store of the materials needed for contraction 

 which cannot be all exhausted at once, or which can to a certain 

 extent be replenished by processes going on in the muscle, is 

 shown by the beneficial effect of mere rest. Among these 



FIG. 240. ERGOGRAPH (Mosso's, MODIFIED BY LOMBARD). 



materials oxygen holds a conspicuous place. An isolated muscle 

 is necessarily an asphyxiated muscle, and the favourable action 

 of an atmosphere of oxygen on restoration of its contractile power 

 after exhaustion (Fig. no, p. 261) shows that asphyxia is itself 

 an important condition in the onset of fatigue. Injection of 

 arterial blood, or even of an oxidizing agent like potassium 

 permanganate, into the vessels of an exhausted muscle also 

 causes restoration (Kronecker). The depletion of the available 

 store of carbo-hydrate in the form of glycogen (and dextrose) 

 seems to be another factor. That the accumulation of fatigue 

 products has something to do with the exhaustion is shown by 

 the fact that the muscles of a frog, exhausted in spite of the 

 continuance of the circulation, can be restored by bleeding the 

 animal, or washing out the vessels with physiological salt solution, 



