. Kirkes' handbook of physiology . too high (400 C. for frog, 500 C. for mammal), themuscle enters into a condition of heat rigor and its irritability is forever cooling, unless the cold be too severe and prolonged, the irritability re-turns as the temperature is raised. A series of vertical records of simple contrac-tions beginning at room temperature and decreasing with a contraction at eachfall of one degree reveals the fact that the amplitude falls off slowly until atemperature of 120 to io° C. is reached, then as gradually increases until 40 to20 C, after which the amplitude dr
. Kirkes' handbook of physiology . too high (400 C. for frog, 500 C. for mammal), themuscle enters into a condition of heat rigor and its irritability is forever cooling, unless the cold be too severe and prolonged, the irritability re-turns as the temperature is raised. A series of vertical records of simple contrac-tions beginning at room temperature and decreasing with a contraction at eachfall of one degree reveals the fact that the amplitude falls off slowly until atemperature of 120 to io° C. is reached, then as gradually increases until 40 to20 C, after which the amplitude drops off sharply to about —1° C. However,this phenomenon is partly one of irritability, since a very strong stimulus willproduce a vigorous contraction until the muscle begins to freeze. If at thefreezing temperature the muscle be slowly and carefully increased in tem-perature it will recover from the effects of the cooling without apparent injury,and will give a reverse series to the one obtained by decreasing the Fig. 329.—Influence of Temperature on the Duration of the Contraction of the Frogs Gastrocnemius. As the increase of temperature is continued above room temperature theamplitude of the contractions very greatly increases (also the elasticity),reaching a maximum in the frogs gastrocnemius at about 350 to 360 C. Theamplitude sharply decreases above 350 C. up to 370 to 380 C, where heatrigor begins and the muscle permanently shortens. Heat rigor is usually 458 MUSCLE-NERVE PHYSIOLOGY complete at 400 to 410 C. A muscle cannot recover its irritability after heatrigor has set in strongly. If the time of the contraction is measured at different temperatures it willbe found to be greatly delayed at 20 to 40 C, and very much quicker than nor-mal at 330 to 350 C. As in fatigue, the effect falls chiefly on the contraction andrelaxation phases and only slightly on the latent period. The latent periodis more sharply influenced by temperature than by fatigue. In
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