10 COMPARATIVE PHYSIOLOGY 



Thus part of the heat generated in muscular contraction arises 

 in the process which results in tension, part in the process by 

 which the substance liberated at excitation is reinstated or 

 removed. 



When the muscle is stimulated isometrically no work is 

 done. All the energy must disappear as heat. By subtracting 

 the heat-production of isometric contraction in nitrogen from 

 the heat-production of isometric contraction in oxygen, that 

 of the oxidative recovery process is obtained. The heat- 

 production in pure nitrogen represents, on the hypothesis 

 advanced above, at least two processes : the liberation of lactic 

 acid and its subsequent neutralisation during relaxation. 

 The formation of one gram of lactic acid from glycogen in 

 vitro is accompanied by the liberation of 190 calories. The 

 appearance of one gram of lactic acid in muscle is associated 

 with the evolution of total energy equivalent to nearly 400 cals. 

 The heat of neutralisation by bicarbonates or phosphates is far 

 too small to account for the excess. Meyerhof suggests that 

 the remainder may be due to the heat of dissociation of the 

 muscle proteins. This has recently received some experimental 

 confirmation from the work of Hartree and Hill, who have 

 shown that to keep the hydrogen ion concentration inside the 

 muscle within reasonable limits, we must assume the existence 

 of some buffer in it more effective than a bicarbonate or 

 phosphate solution. As in the case of blood, this is presum- 

 ably brought about by alkali proteinates capable of forming 

 neutral salts and undissociated protein. 



The heat-production of oxidative recovery, on the other 

 hand, only accounts for a small fraction of the lactic acid which 

 disappears. Hartree and Hill estimated that in oxygen the 

 delayed heat-production is one and a half times the initial 

 heat-production, which represents about 400 cals., as stated, 

 per I grm. lactic acid. The combustion of i grm. of lactic acid 

 is accompanied by an evolution of 3661 calories in vitro. Since 

 now, according to Meyerhof, the disappearance of lactic acid 

 is associated with increase in glycogen in the muscle, it is 

 suggested that the lactic acid disappears partly by oxidation, 

 the energy so liberated appearing in part as heat and being 



