SECTION VI 

 THE PRODUCTION OF HEAT IN MUSCLE 



THE experience of everyday life teaches us that muscular exercise is asso- 

 ciated 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 tem- 

 perature rises from 0-14 to 0-18 C., and for each single twitch from 0-001 

 toO -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 employed in measuring 

 temperature by electrical methods. The thermo- 

 electrical method depends on the fact that, when 

 the junctions of a circuit made of two metals are ANTIMONY I 

 at 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. 73) 

 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 thermo- 

 pile (containing four or six elements composed of iron and German silver, or copper 

 and 'constantan') 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 galva- 

 nometer, the slightest difference of temperature, such as that caused by the contract 

 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 produce* 



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

 by the difficulty that every junction in the circuit tends to become the seat o 

 motive force in consequence of slight changes of temperature due to curren 

 It is therefore advisable to use a plan adopted by Blix, of placing al 

 the muscle included, within the galvanometer case. The arrangements 

 experiment as employed by A. V. Hill are shown in the diagram (Fig. 74). 



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