738 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES 



Heidenhain, with a very delicate pile, found a rise of 0-001 to 

 0-005 C. for a single contraction of a frog's muscle. On the assump- 

 tion that the pile had time to take on the temperature 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 thou- 

 sandth to five-thousandths of a gramme-calorie (p. 653) of heat. 

 From Fick's observations we may take about three-thousandths 

 of a gramme-calorie as the maximum production of a gramme of 

 frog's muscle in a single contraction. 



Hill has shown that in the case of the single contraction or twitch 

 the evolution of heat may be so rapid as to be practically instan- 

 taneous, indicating that it depends upon some sudden ' explosive ' 

 chemical reaction; or, on the other hand, under certain conditions 



it may last as long as two 

 seconds that is, from four 

 to ten times as long as the 

 contraction itself. In the 

 absence of oxygen, when 

 the muscle is left, for in- 

 stance, in an atmosphere of 

 hydrogen, the heat produc- 

 tion becomes markedly pro- 

 longed. When abundant 

 oxygen is supplied, the dura- 

 tion of the discharge of heat 

 is decreased. In a tetanus 



B 



Fig. 264. A, a single copper-iron thermo- 

 electric couple ; B.two pairs, one inserted into 



the tissue 6, the other dipping into water in a 

 beaker a. The temperature of the water 

 may be adjusted so that the galvanometer 

 shows no deflection. The temperature of the 

 tissue is then the same as that of the water. 



tne evolution OI heat lags 



behind the excitation, and 

 the discharge associated 

 with each stimulus is not 

 complete till 0-5 to 2-5 

 seconds after the stimulus. In a prolonged complete tetanus 

 the heat production corresponding to the first tenth of a second 

 excitation is far greater than that corresponding to the second 

 tenth of a second, and so on, until eventually a uniform dis- 

 arge of heat, at a rate much smaller than the initial rate, is 

 reached When frogs' muscles are rapidly stimulated indirectly 

 (through the nerves) till fatigue has occurred, the maximum value 

 heat evolved approximates to o-o gramme - calorie per 

 gramme of muscle, about 70 or 80 per cent, being liberated in the 

 first two minutes (Peters). 



A fact of great significance in regard to the relation of the reaction 



upon which the heat production depends and the mechanical 



ions in the active muscle is that the production of heat is 



determined by the length of muscular fibres existing at the time 



n the heat is being evolved. From this it has been assumed 



that the production of heat in active muscle is a surface effect and 



