726 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES 



same effects on its power of contraction as are produced by fatigue, 

 and there is some reasqn to suppose that lactic acid is the most 

 influential of the fatigue substances.- In smaller concentration, 

 on the contrary, they increase the excitability of the muscle, and, 

 according to Lee, the phenomenon of the ' staircase ' is due to the 

 augmenting action of these, and perhaps other fatigue substances, 

 before they have accumulated sufficiently to cause fatigue. 



The lack of oxygen holds a conspicuous place among the con- 

 ditions which permit an excessive accumulation of fatigue substances, 

 and may contribute also to the failure of the processes normally 

 going on in the muscle which replenish the store of materials 

 needed for contraction. 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. 255. Ergograph (Mosso's, modified by Lombard). 



(Fig. 123, p. 265) shows that asphyxia is itself an important factor 

 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 causes restoration (Kronecker). The 

 depletion of the available store of carbo-hydrate in the form of 

 glycogen (and dextrose) seems to be another factor in fatigue, 

 although not the chief direct cause of the phenomena associated 

 with that condition. 



Seat of Exhaustion in Fatigue. When a fatigued muscle responds 

 no longer to indirect stimulation, it can still be directly excited. 

 The seat of exhaustion must therefore be either the nerve-trunk 

 or the nerve-endings. It is not the nerve-trunk which is first 

 fatigued, for this still shows the negative variation (p. 797) on being 

 excited. And if the two sciatic nerves of a frog or rabbit be stimu- 



