446 PRINCIPLES OF GENERAL PHYSIOLOGY 



As Hill puts it, " the lactic acid is part of the machine and not part of the 

 fuel." If we may compare the muscle to a gas engine, the lactic acid corresponds 

 to some essential moving part, say the piston, which merely undergoes change 

 of position. Its change of position, however, leads to the liberation of energy. 



The energy required to put back the lactic acid and restore the high potential 

 energy of the resting, unfatigued muscle must, accordingly, come from some 

 independent reaction, involving the oxidation of a non-nitrogenous carbon 

 compound. But, if we accept Ostwald's position (see page 30), a "coupled 

 reaction," in order to give chemical energy to another reaction, must have 

 components in common with it, and it is difficult to see what these can be in 

 the case of muscle, if we look upon the potential energy of the " inogen " as 

 chemical energy. Moreover, this inogen is not analogous to a combustible 

 substance containing an excess of oxygen, such as nitro-cellulose, for example. 

 There is direct evidence, as we shall see later, that oxygen is not taken up 

 in this " intra-molecular " form, and, even if it were, the products of contraction 

 in anaerobic conditions would contain considerable amounts of carbon dioxide, 

 which is not the case. The formation of lactic acid from glucose or similar 

 substance is only associated with the giving off of minimal amounts of energy. 

 Again, as Hill points out (1913, 1, p. 77), if the lactic acid precursor could be 

 restored in the presence of oxygen without the evolution of heat, we should be 

 justified in concluding that the oxygen is built up into the precursor, and that the 

 breakdown of the precursor is, in fact, an oxidation with the liberation of heat. 

 But this is not so, "at least as much heat is used in the restoration of the 

 contractile tissues to their previous condition as in their breakdown, so that the 

 oxygen cannot be merely built up as intra-molecular oxygen, but must be utilised 

 in some way in oxidation processes." In fact, one cannot imagine a chemical 

 compound of high potential energy which would satisfy the conditions required. 

 It would be rash to deny its existence, nevertheless, since there are such substances 

 as nitrogen iodide. On the whole, we are driven back to the assumption of a 

 system whose energy is more of the nature of surface energy, a view confirmed by 

 the relation of heat and lactic acid to length of fibres. And, if this be so, the 

 difficulty with regard to the coupled reaction vanishes. 



With the data at our disposal, we can obtain some further idea of the nature 

 of this secondary oxidation process. A. V. Hill (1914, 2) has determined the 

 total energy that can be afforded by isolated muscles stimulated in oxygen. He 

 had previously shown (1913, 2, p. 462) how the heat production in relation to the 

 tension developed can be estimated, and found it to be 5 x 10"" 8 calories (5 micro- 

 calories) per gram-weight of tension developed per centimetre of muscle length. This 

 is in the absence of oxygen, and refers, therefore, to the heat set free from the actual 

 contractile process only. If the heat of the recovery process in oxygen is added, 

 the value becomes 10 micro-calories. If we obtain a record of the tension 

 produced in a muscle in a series of isometric twitches, we can estimate, therefore, 

 the heat produced. Hill has compared the total heat produced when sartorius 

 muscles were stimulated to exhaustion in air, on the one hand (that is, in 

 insufficient oxygen), and in oxygenated Ringer solution, on the other hand. In 

 this latter, they can be stimulated five times a minute for nearly two days, giving, 

 on an average, 30 calories of heat per gram, whereas, as we saw, Peters found only 

 0'9 calorie in absence of oxygen. In Hill's experiments in air, or nearly complete 

 absence of oxygen, the value l - 4 was obtained. Now, to. afford these 30 calories, 

 0-008 g. of lactic acid would have to be burned. The amount that actually 

 disappears is - 002 g., at the most. About the same amount of carbohydrate 

 would be required, and muscle is stated to contain about 0'004 to O'Ol g. per gram 

 of its weight. The higher figure is more than sufficient to account for the energy 

 production. We shall find presently more evidence as to the nature of the 

 substance burned, but it would be interesting to have actual values of the carbon 

 dioxide produced and oxygen consumed under the conditions of Hill's experiments. 



Comparing again the muscle system to a gas engine, it is as if the energy of 

 the combustion of the fuel were not used at once to drive machinery, such as drills, 

 hammers, and so on, by means of shafting and belts, but as if an air compressor 



