86 ELECTRO-PHYSIOLOGY CHAP. 



tops of the single twitches (recorded at equal distances upon a 

 stationary surface), is in this case a straight line (Fig. 43, A and 

 B} with direct excitation of the muscle. 



Excised muscle becomes exhausted after a certain number of 

 twitches. According to Tiegel, the same thing occurs in curarised 

 muscle that has been freed from blood, on rhythmical excitation 

 with sub -maximal induction currents. The first (20 to 30) 

 twitches of a series are the only exception to the rule that 

 the extreme upper point of equidistant contractions lies in a 

 straight line ; in these the curve, instead of falling, rises in a 

 staircase (si^ra). In the case of curarised muscle with normal 

 circulation, this rise may extend over several hundred twitches, 

 of which over a thousand may remain at the same magnitude, 

 while the rest sink slowly but continuously (Tiegel). Accordingly, 

 as might be presumed, fatigue proceeds much more slowly in 

 muscle with normal circulation and nutrition than in excised 

 bloodless preparations, and the period of " staircase " increment 

 in the twitches is much shorter in the second than in the first 

 case. It is remarkable that, according to Tiegel (I.e. p. 18), the 

 curve of fatigue (i.e. straight line) sinks much more rapidly to 

 the abscissa (i.e. makes a greater angle with it) with sub-maximal 

 than with maximal stimuli ; i.e. the muscle is more quickly 

 fatigued by sub-maximal than by maximal excitation, provided 

 the two kinds of stimulus alternate regularly at short intervals. 

 When a muscle has yielded a series of twitches at maximal, or 

 sub-maximal, excitation, and the rhythm is changed to a weaker 

 stimulus, returning immediately (after twenty or more twitches) 

 to the original excitation, the first contractions of the last series 

 are invariably higher than the last of the first series (Tiegel). 

 The muscle apparently recovers during sub-maximal excitation 

 from the stronger (maximal or sub-maximal) stimuli. The height 

 of twitch diminishes more rapidly in proportion as the excitation 

 interval is shorter (Fig. 43, A}, and this law holds both for 

 maximal and sub -maximal stimuli in curarised muscle. In 

 muscle with normal circulation and nutrition, there is always an 

 interval between each pair of stimuli, in which the height of 

 twitch does not diminish even after protracted excitation, and no 

 fatigue appears (e.g. the beating heart). Hence we may assume, 

 from the previous observation, that during each pause in stimula- 

 tion the " down " change caused by each D-stimulus in the 



