FATIGUE AND RECOVERY OF MUSCLES AND NERVES 



441 



partial contraction. There can be no doubt that such partial contractions occur 

 also normally under the influence of the nervous system, although probably with 

 still nicer gradations. In this way the activity of the muscle is adapted to the 

 work to be performed. If no great degree of tension is called for, only a few 

 muscle fibers contract, the others remain quiet and do not become fatigued. 

 Since on the other hand the extent of the contraction does not depend upon 

 the cross section, but upon the length of the muscle, we may get just as much 

 shortening with a partial contraction as when the whole muscle is active. 



6. FATIGUE AND RECOVERY OF MUSCLES AND NERVES 

 A. GENERAL PHENOMENA 



If a frog's muscle be stimulated repeatedly with single shocks given every 

 one to two seconds, at first its contractions increase in size, even if the stimulus 

 remain of the same strength (" treppe " of Bowditch and Buckmaster), and then 

 they gradually decrease until 

 complete exhaustion is reached. 

 From the first of the series the 

 contractions become more pro- 

 longed, since both the ascending 

 and the descending limbs of the- 

 curve, but especially the latter, 

 occupy more time. As fatigue 

 progresses and the longer stimu- 

 lation is kept up, there gradu- 

 ally develops a new condition of 

 the muscle: at the end of the 

 contraction it does not return to 

 its resting position but remains 

 more and more shortened. 1 The 

 muscle finally becomes a slug- 

 gish, stubborn mass yielding to 

 the traction which strives to re- 

 store it to its original form, with 

 extreme slowness (Funke). In 

 the series represented in Fig. 

 177 a muscle kept perfused with 

 blood was stimulated every 1.5 

 seconds. Only the first ten of FIG. 177. Changes in the character of the contraction 

 every fifty contractions are here produced by fatigue, after Rollet. 



reproduced, the six series repre- 

 senting all told some three hundred separate movements. When the interval 

 between stimuli is made still shorter, say 0.5 second, as fatigue continues the 

 descending limb of the curve does not reach the base line, before it is met by 

 the following stimulus, and the curve becomes much like an incomplete tetanus. 

 With a longer interval, say six seconds, the contraction is not prolonged, or only 

 slightly so, and the reduction in the height of the curve is the only expression 

 of fatigue. 



The muscle of warm-flooded animals, kept perfused with blood, show, accord- 

 ing to Rollet, only this latter form of fatigue, with no material increase in 

 the duration of the contraction even when the interval between stimuli is very 



Th\s condition is called contracture. ED. 



