i GENEEAL PHYSIOLOGY OF MUSCLE 11 



While cold diminishes muscular excitability and renders the 

 muscle less easily fatigued and more resistent, heat, after a brief 

 rise of excitability, leads to easy exhaustion. When the rise of 

 temperature exceeds 40-50 C. the muscle enters into thermal rigor, 

 in which it gives its maximal contraction, and does not relax again. 



(c) The duration and form of the muscle twitch also depend on 

 the degree of fatigue. If a series of twitches from a frog's muscle, 

 uniformly loaded and excited at equal intervals (1-2 sees.), with 

 uniform shocks from make or break induction currents are recorded 

 on the drum of the myograph, a fatigue curve will be obtained which 

 shows a gradual retardation and weakening of muscular activity, 

 preceded by a short phase of augmentation. Fig. 6 shows that in 

 a preliminary period, consisting of some ten twitches, the tracings 

 rise in height, and the duration of both contraction and elongation 

 is lengthened. In a second much longer period the height drops, 



Fio. 6. Curve of fatigue, with direct stimulation of frog's gastrocnemius. (A. D. Waller.) Tracing 

 of 125 maximal contractions at 1 J sees, interval. The experiment was stopped before the muscle 

 became fully exhausted. 



while the duration of both phases increases, but particularly that 

 of relaxation. 



Kronecker (1871) showed that when a frog's muscle, excited 

 at regular intervals with maximal induction shocks, is loaded 

 only at the moment at which it commences its contraction (after 

 loading}, the apex of the twitches forms a straight line, which 

 drops more rapidly towards the abscissa in proportion as the 

 interval between the single stimulations diminishes. In repro- 

 ducing Kronecker's experimental conditions it is necessary first 

 to test the excitability of the muscle in order to find the least 

 stimulus that will produce a maximal effect; next, the single 

 stimuli must succeed each other at long intervals, so that the 

 muscle shall not be excited again before the phase of relaxation is 

 fully completed, which takes longer and longer as the fatigue 

 increases. The apparent rise of activity, often seen at the com- 

 mencement of muscular fatigue, is probably due to the fact that, 

 owing to the lengthening of the phase of relaxation, the muscle 

 receives the next shock before it has completely relaxed. In 

 this case each new excitation summates with the residue of the 

 previous contraction, and the level of the myogram rises in conse- 

 quence (Fr. W. Frohlich, 1905). 



