STIMULI AND THEIR ACTIONS 



463 



series on, as Marey ('68) did a long time ago. Then it is found 

 that, as Helmholtz discovered, with increasing fatigue not only 

 does the curve decrease in height, but it becomes more extended, 

 its descending limb especially undergoing a lengthening. In other 

 words, the work done by the muscle becomes less while the dura- 

 tion of the contraction increases. The latter phenomenon depends 

 chiefly upon the increasing duration of the stage of expansion. 

 The fatigued muscle needs more time to extend to its complete 

 length. 



The phenomena of fatigue appear, perhaps, still more clearly 

 upon stimulation by the tetanizing current than by single induc- 

 tion-shocks. If the curve of te- 

 tanus of a frog's gastrocnemius 

 muscle, not too strong and 

 weighted, be recorded upon a 

 rotating drum, it is seen that it 

 continues at its original height 

 for a long time, and follows a 

 straight line (Fig. 238). But 

 after some time it begins slowly 

 to fall, and, not rarely at the 

 same time, small irregularities in 

 its course become visible, which 

 are due to the fact that the 

 muscle begins to tremble. The 

 curve continues to fall gradually. 

 If the stimulation be interrupted, 

 the curve usually does not sink at 

 once to the level of its starting- 

 point, but remains some distance 

 above the latter, and only in the 

 course of a considerable time re- 

 turns to it. Hence there is a 

 considerable contraction-remainder 



in the fatigued muscle after the end of stimulation, and the 

 muscle assumes its original length only very slowly. 



It is of great interest that microscopic changes have been 

 observed in fatigued muscle. Of a number of wholly similar 

 blue-bottle flies (Musca vomitoria) H. M. Bernard ('94) kept 

 some in continual motion by constantly exciting them, until 

 they fell to the ground completely exhausted. The fatigued flies 

 were at once killed simultaneously with the others, which, in the 

 meantime, had remained at rest. The two kinds of specimens 

 were then subjected to the same treatment. A marked difference 

 appeared between them. While in the resting flies the muscle- 

 fibrillse showed distinct cross-striation and the various discs of the 

 individual segments showed differences in staining-capacity, in the 



FIG. 237. Curve of fatigue ; decrease of the 

 height of the curves with numerous suc- 

 cessive contractions of the flexor muscles 

 of the fingers. (After Mosso.) 



