6 52 A MANUAL OF PHYSIOLOGY 



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

 it will be found that when the ' block ' is removed the cor- 

 responding 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 stimula- 

 tion and the muscles may be used to prevent the excitation from 

 passing down (p. 708). Or a dose of curara just sufficient to 

 paralyze the motor inner vation may be given to a rabbit, and the 

 animal kept alive by artificial respiration. The sciatic is now 

 stimulated for many hours. Nevertheless, as soon as the 

 influence of the curara begins to wear off, the muscles of the leg 

 contract. 



The possible seats of fatigue caused by voluntary muscular 

 contraction are (i) the muscle; (2) the nerve-endings (or the recep- 



FIG. 242. FATIGUE CURVE TAKEN ON A SLOWLY MOVING DRUM (REDUCED 

 TO HALF) : FROG'S GASTROCNEMIUS. 



Excited through the sciatic nerve by maximal shocks once in six seconds. 



tive substances in the muscle, p. 634) ; (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. 774), the fibres of the pyramidal 

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

 Although the matter cannot be considered definitely settled, ergo- 

 graphic experiments (Mosso and Maggiora, Lombard, etc.) (p. 709) 

 have been interpreted as showing that fatigue after voluntary 

 effort is due to central changes, and not entirely to changes in the 

 muscles and nerves themselves. Thus, electrical stimulation, 

 either of a ' tired ' muscle or of its nerve, is readily responded to 

 at a time when the weight cannot be raised by voluntary con- 

 traction. Now, it is argued, there is no reason to suppose that 

 the nerve-fibres in the central nervous system differ essentially 

 from those of peripheral nerves, and therefore no reason for 

 placing the seat of the fatigue anywhere in the pyramidal fibres, 



