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 

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

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

 to 0-C05C. 



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 COOL 



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 f ) j BISMUTH 



at different temperatures, a current of electricity 

 generally flows through the circuit. This current can 

 be measured by means of a galvanometer, and is WARM 



proportional to the difference of temperature between J?IG. 73 



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 frogs' 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 galvano- 

 meter, 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 



action 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 met 

 by the difficulty that every junction in the circuit tends to become the seat of an electro- 

 motive force in consequence of slight changes of temperature due to currents of air, &c. 

 It is therefore advisable to use a plan adopted by Blix, of placing all the apparatus, 

 the muscle included, within the galvanometer case. The arrangements of such an 

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



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