MUSCLE 



649 



the muscle is benefited by contraction and its excitability 

 increased for a new stimulus. Soon, however, in an isolated 

 preparation, the contractions 

 begin to decline in height, till 

 the muscle is at length utterly 

 exhausted, and reacts no longer 

 to even the strongest stimulation 

 (Figs. 236, 241, 242). 



A conspicuous feature of the 

 contraction-curves of fatigued 

 muscle is the progressive length- 

 ening, which is much more 

 marked in the descending than 

 in the ascending periods ; in other 

 words, relaxation becomes more 

 and more difficult and imperfect 

 (Fig. 241). In smooth muscle 

 (cat's bladder or ring from frog's 

 stomach) fatigue can be very 

 easily demonstrated in the same 

 way, and the curves present 



similar features, with the exception that, instead of becoming 

 longer in fatigue, the successive contractions become shorter. 



It is by no means so easy to fatigue a muscle still in connec- 

 tion with the circulation as an isolated muscle. But even the 

 latter, if left to itself, will to some extent recover, and be again 



FIG. 238. ' STAIRCASE ' IN SKELE- 

 TAL MUSCLE : FROG. 



Stimulation by arrangement shown 

 in Fig. 237. 



\ 



FIG. 239. ' STAIRCASE ' IN CARDIAC MUSCLE. 



. Contractions recorded on a much more quickly moving drum than in Fig. 238. 

 The contractions were caused by stimulating a heart reduced to standstill^ by the 

 first Stannius' ligature (p. 151). The contractions gradually increase in height. 



able to contract, although exhaustion is now more readily 

 induced than at first. 



In man, muscular fatigue can be studied by means of an 

 arrangement called an ergograph (Fig. 240). A record of suc- 

 cessive contractions, say, of one of the flexor muscles of a finger, 

 in raising a weight (iso-tonic method) or in deforming a spring 

 (iso-metric method) is taken on a drum. When the contractions 



