98 THE HUMAN BODY 



are more rapid at higher temperatures than at lower; a law which 

 applies to muscular contraction because the mechanical act of 

 shortening is based on a preceding chemical process. In nature 

 the influence of temperature on muscular contraction is seen only 

 in the lower (cold-blooded) animals, whose bodies are at sub- 

 stantially the temperature of the surroundings and which, as can 

 easily be observed, are sluggish in cold weather and active in warm. 

 In man and the higher (warm-blooded) animals, and in birds, 

 which are also warm-blooded, the body temperature is high and 

 relatively constant, and the muscles are not subjected, therefore, 

 to such temperature variations as occur in lower forms. One ad- 

 vantage of the warm-blooded state is that it insures for the muscles 

 a favorable temperature for effective operation in cold weather 

 as well as in warm. 



Heat Rigor. If an isolated muscle is heated above 40-45 C. 

 (104-113 F.) it is killed by the heat and undergoes a marked con- 

 traction known as heat rigor. Heat rigor like the death stiffening 

 (rigor mortis, p. 92) is accompanied by, and probably caused by, 

 a great production of lactic acid within the muscle. 



The Measure of Muscular Work. The work done by a muscle 

 in a given contraction, when it lifts a weight vertically against 

 gravity, is measured by the weight moved, multiplied by the dis- 

 tance through which it is moved. When a muscle contracts carry- 

 ing no load it does very little work, lifting only its own weight; 

 when loaded with one gram and lifting it five millimeters it does 

 five gram-millimeters of work, just as an engineer would say an 

 engine had done so many kilogrammeters or foot-pounds. If 

 loaded with ten grams and lifting it six millimeters it would do 

 sixty gram-millimeters of work. Even after the weight becomes so 

 great that it is lifted through a less distance, the work done by the 

 muscle goes on increasing, for the heavier weight lifted more than 

 compensates for the less distance through which it is raised. For 

 example, if the above muscle were loaded with fifty grams it would 

 maybe lift that weight only 1.5 millimeters, but it would then do 

 75 gram-millimeters of work, which is more than when it lifted 

 ten grams six millimeters. A load is, however, at last reached 

 with which the muscle does less work, the lift becoming very little 

 indeed, until at last the weight becomes so great that the muscle 

 cannot lift it at all and so does no work when stimulated. Starting 



