580 A MANUAL OF PHYSIOLOGY 



14 to *i8 C. in excised frogs' muscles when tetanized for a 

 couple of minutes. 



Heidenhain, with a very delicate pile, found a rise of *ooi 

 to '005 C. for a single contraction of a frog's muscle. On 

 the assumption that the pile had time to take on the tempera- 

 ture of the muscle before there was any appreciable loss of 

 heat, this would be equal to the production by every gramme 

 of muscle of a thousandth to five-thousandths of a small 

 calorie (p. 493) of heat. From Pick's observations we may 

 take about three-thousandths of a small calorie as the 

 maximum production of a gramme of frog's muscle in a 

 single contraction. 



It is certain that when work is done by a muscle an equi- 

 valent amount is subtracted from its sum-total of energy, 

 and we might therefore expect that the heat produced in 

 contraction should diminish as the work increases. But 

 experiment does not fulfil this expectation. The manner 

 and the rate of its expenditure of energy depend upon the 

 conditions under which the muscle is placed. A stretched 

 muscle, when caused to contract, produces more heat than 

 if it had started without tension, and still more heat when 

 it is fixed so that it cannot shorten during stimulation. 

 A muscle, starting without tension, produces more heat 

 when it contracts isometrically than when it contracts 

 isotonically. This fact does not, however, prove that the 

 heat-production is greater when no work is done, because 

 the tension increases during excitation when contraction is 

 prevented, and, as has been said, increase of tension alone 

 causes more heat to be given out by an active muscle. 



When a muscle, excited by maximal stimuli, is made to 

 lift continuously increasing weights, both the work done and 

 the heat given out increase up to a certain limit. The 

 muscle, as it were, burns the candle at both ends. This 

 would be of itself enough to show that there is no fixed 

 relation between the work and the heat-production; although 

 the latter reaches its maximum somewhat sooner than the 

 former. 



We have already seen that when a muscle is cooled, or 

 fatigued, or poisoned with veratria or with suprarenal extract, 



