208 



PHYSIOLOGY 



It is very often observed that the height of contraction of the warmed muscle is 

 greater than that obtained at ordinary temperatures. It seems that this apparent 

 increase in height is really instrumental in origin, the quicker-moving muscle jerking 

 the lever beyond the real extent of the contraction. If proper means are taken to 

 eliminate this overshooting of the lever, it is found that the height of contraction is 

 unaltered between 5 and 2G C., the only change being in the time -relations of the 

 curves. This is especially well shown in the so-called ' arrest ' curves (Fig. 69). 



FIG. 69. Isotonic and ' arrest ' curves of muscle -twitch : (1) unloaded at 14 C. ; 

 (2) at 25 C. ; (3) at C. ; (4) loaded at 14 C. Note that the arrest curves 

 attain the same height throughout. (KAISER.) 



If a muscle be heated gradually (without stimulation) up to about 45 C., 

 it begins to contract slowly at about 34 C., and this contraction reaches its 

 maximum at 45 C., at which point the muscle has entered into pronounced 

 rigor mortis. 



Cold has the reverse effect. The intra-molecular processes which lie at 

 the root of the muscular activity are slowed, so that the latent period and the 

 contraction period are prolonged. The action of cold on the excitability of 

 muscle is to increase it, so that any form of stimulus is more effective at 5 C. 

 than at 25 C. Moreover, when maximal stimuli are being used, and the 

 muscle is heavily loaded, the first effect of the application of cold may be to 

 increase the height as well as the duration of contraction, for the same 

 reason that a gentle push is more efficacious in closing a door than would be 

 a heavy blow with a hammer. If, however, a muscle be cooled for a short 

 time to zero or a little below, it loses its irritability, which returns if the 

 muscle be gradually warmed again. Prolonged exposure to severe cold 

 irrevocably destroys its irritability. Warming the muscle will now simply 

 bring about rigor mortis. 



FATIGUE. A muscle will not go on contracting indefinitely. If it 

 be repeatedly stimulated, changes soon become apparent in the curve of 

 contraction. The latent period is prolonged, as well as the length of the 

 contractions ; the absolute height and work done are diminished. At 

 the same time the muscle does not return to its original length ; the shorten- 

 ing which remains is spoken of as ' contraction remainder.' After an initial 

 rise during the first few contractions, these diminish uniformly in height 



