SECTION VI 

 THE PRODUCTION OF HEAT IN MUSCLE 



THE experience of everyday life teaches us that muscular exercise 

 is associated with increased production of heat. Thus a man walks 

 fast on a frosty day to keep himself warm. In large animals the 

 production of heat in muscular contraction can be easily shown by 

 inserting the bulb of a thermometer between the thigh muscles, 

 and stimulating the spinal cord. The rise of temperature produced 

 in this way may amount to several degrees. This observation is 

 confirmed when we investigate the contraction of an isolated muscle 

 outside the body. If a frog's muscle is tetanised, its tempera- 

 ture rises from 0'14 to 0'18 C., and for each single twitch from 

 0-001 to 0-005 C. 



It is evident that such small changes in temperature as 0-001 cannot be 

 estimated by ordinary thermometric methods. By converting a heat change 



into an electrical change, however, we can 

 estimate differences of temperature with much 

 greater accuracy and fineness than by the use 



of a thermometer. Two main principles are 

 ANTIMONY BISMUTH , , . 



employed in measuring temperature by elec- 

 trical methods. The thermo-electrical method 

 depends on the fact that, when the junc- 

 tions of a circuit made of two metals are at 



FIG. 72. different temperatures, a current of electricity 



generally flows through the circuit. This 



current can be measured by means of a galvanometer, and is proportional to 

 the difference of temperature between the two junctions. Thus in the circuit 

 (Fig. 72) composed of two metals, antimony and bismuth, if the upper junction 

 be cooled, there will be a current flowing from antimony to bismuth in the 

 direction of the arrow, and this current will within limits be proportional to 

 the difference of temperature. 



To measure the production of heat during muscular contraction, a small flat 

 thermopile (containing four or six elements composed of iron and German 

 silver) is fixed with one of its ends between two frog's gastrocnemii. Another 

 exactly similar pile, but reversed, is placed between two other gastrocnemii, 

 which are kept resting and at a perfectly constant temperature. So long as the 

 two piles are at the same temperature no current flows ; but, with a sensitive 

 galvanometer, the slightest difference of temperature, such as that caused by the 

 contraction of one pair of muscles, at once causes a deflection of the galvanometer, 

 the extent and direction of which enable us to estimate exactly the seat and 

 amount of heat produced. 



When we are using such delicate detectors of temperature difference, we are 



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