34 PKACTICAL PHYSIOLOGY 



relatively less shortened, although muscle does become more extensible 

 as its temperature rises from to 30 C. (Fig. 38). 



The relation between temperature and the height of the contraction 

 is not quite so simple. Between and about 15 C. the actual height 

 of the contraction may fall slightly, and for two reasons : as the tem- 

 perature increases, the irritability of the muscle decreases ; further, 

 other things being equal, the more slowly a muscle contracts, the 

 more time it has to shorten up as much as it will in response to a 

 given stimulus. From 15 to 25 the height of the curve rapidly 

 increases; this is largely, if not entirely, instrumental in origin, and is- 



FIG. 39. Curve of the shortening of the gastrocnemius muscle during heat-riyor. 

 (Pembrey and Phillips.) 



due to the fact that, as the liberation of energy becomes more rapid> 

 the lever receives a considerable jerk from the rapidly contracting 

 muscle. In other words, the increased height of the contraction is due, 

 not to a greater liberation of energy, but to the greater rate at which 

 the same quantity of energy is liberated. From 25 to 35 C. the 

 irritability of muscle and its height of contraction rapidly fall. 



Now pour on some solution warmed to 50 C. When the muscle- 

 fibres reach a temperature of about 40 C. they undergo a rapid 

 shortening (Fig. 39), which, as the temperature of the muscle rises,, 

 passes into the permanent shortening of * heat-rigor.' This condition 

 is due to coagulation of some of the muscle proteids, and in consequence 

 the muscle becomes hard, opaqur, inelastic, and has permanently lost 

 its irritability. 



