THE MECHANICAL RESPONSE OF MUSCLE 233 



TEMPERATURE. Speaking generally, the effect of warming a 

 muscle is to quicken all its processes. The latent period becomes 

 shorter and the muscle curve steeper and shorter. 



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 



FIG. 68. 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.) 



height of contraction is unaltered between 5 and 20 C., the only change being 

 in the time-relations of the curves. This is especially well shown in the so-called 

 ' arrest ' curves (Fig. 68). 



If a muscle be heated gradually (without stimulation) up to about 

 45 C., it begins to contract sloAvly at about 34 C., and this contrac- 

 tion 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 



