568 A MANUAL OF PHYSIOLOGY 



ing the animal, or washing out the vessels with normal saline 

 solution, 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 

 (sarcolactic acid and acid potassium phosphate) which 

 restored the muscle. Injection of arterial blood, or even 

 of an oxidizing agent like potassium permanganate, into 

 the vessels of an exhausted muscle also causes restoration 

 (Kronecker). 



When a fatigued muscle responds no longer to indirect stimula- 

 tion, 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 on being excited. And if the two sciatic nerves of a frog 



FIG. 176 FATIGUE CURVE TAKEN ov A SLOWLY MOVING DRUM (reduced 



to half). 



Frog's gastrocnemius excited through the sciatic nerve by maximal shocks once in 

 sixeconds. 



or rabbit be stimulated continuously with interrupted currents of 

 equal strength, while the excitation is prevented from reaching the 

 muscles of one limb till those of the other cease to contract, it will 

 be found that when the ' block ' is removed the corresponding 

 muscles contract vigorously on stimulation of their nerve. The 

 passage of a constant current through a portion of the nerve or the 

 application of ether between the point of stimulation and the muscles 

 may be used to prevent the excitation from passing down (p. 620). 



The possible seats of fatigue caused by voluntary muscular con- 

 traction are (i) the muscle, (2) the nerve-endings, (3) the nerve- 

 trunk, and (4) the path of the voluntary motor impulses in the central 

 nervous system, which includes the pyramidal cells in the motor 

 region of the cerebral cortex (p. 663), the fibres of the pyramidal 

 tract, and the motor cells in the anterior horn of the spinal cord. 

 Actual experiments (with an arrangement called an ergograph, by 



