566 A MANUAL OF PHYSIOLOGY 



lated by a series of equal shocks thrown in at regular 

 intervals, and the contractions recorded, it is seen that 



at first each curve overtops its 

 predecessor by a small amount. 

 This phenomenon, which is 

 regularly seen in fresh skeletal 

 muscle, although it was at one 

 time supposed to be peculiarly 

 a property of the muscle of the 

 heart, is called the ' staircase,' 

 and seems to indicate that 

 within limits the muscle is 

 benefited by contraction and 

 its excitability increased for a 



FIG. 173.-' STAIRCASE 'IN SKELE- new stimulus. Soon, however, 

 TAL MUSCLE (FROG). in an isolated preparation, the 



F . Stimulation by arrangement shown in contractions begin to decline 



in height, till the muscle is at 



length utterly exhausted, and reacts no longer to even the 

 strongest stimulation. 



A conspicuous feature of the contraction-curves of fatigued 

 muscle is the progressive lengthening, which is much more 



FIG. 174. ' STAIRCASE ' IN CARDIAC MUSCLE. 



Contractions recorded on a much more quickly moving drum than in Fig. 173. The 

 contractions were caused by stimulating a heart reduced to standstill by the first 

 Stannius 1 ligature (p. 175). The contractions gradually increase in height. 



marked in the descending than in the ascending period ; 

 in other words, relaxation becomes more and more 

 difficult and imperfect. It is by no means so easy to fatigue 

 a muscle still in connection with the circulation as an 

 isolated muscle. But even the latter, if left to itself, will to 



