MUSCLE 



651 



while injection of a watery extract of exhausted muscle into the 

 bloodvessels of a curarized muscle renders it less excitable 

 (Ranke). This observer supposed that it was specially the 

 removal of the acid products of contraction which restored the 

 muscle. Such acid products as carbon dioxide and lactic acid, 

 when they act on muscle in more than a certain concentration, 

 produce the same effects on its power of contraction as are pro- 

 duced by fatigue. 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 



FIG. 241. FATIGUE CURVE OF SKELETAL MUSCLE : GASTROCNEMIUS OF FROG. 



Indirect stimulation ; taken with arrangement shown in Fig. 261 (p. 707). 

 Time tracing, -^ of a second. 



action of these, and perhaps other fatigue substances, before they 

 have accumulated sufficiently to cause fatigue (p. 649). 



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. 719) on being excited. And if the two sciatic nerves of a frog 

 or rabbit be stimulated continuously with interrupted currents of 

 equal strength, while the excitation is prevented from reaching 



