444 PRINCIPLES OF GENERAL PHYSIOLOGY 



certain surfaces running longitudinally in the muscle. After the maximal tension 

 has been developed, its potential energy can be used for doing external work or 

 may be converted into the equivalent quantity of heat. The initial process of 

 contraction, with which we are at the moment concerned, consists in the develop- 

 ment of this potential energy of tension, which can do work or be converted into 

 heat. Hill has shown that, under optimal conditions, the total amount of heat 

 developed in the actual contractile process is practically identical with that which 

 would be derived from the energy of the tension, so that the "efficiency," in tin- 

 mechanical sense, of this first process is 100 per cent. (1913, 2, p. 463). 



This high efficiency is clearly sufficient to exclude the possibility of the muscular 

 machine being a heat engine; the chemical energy of the food taken by an 

 organism is converted into work by a more efficient mechanism. As we shall set- 

 presently, however, the efficiency of the total muscular process, although high, is 

 only about half that of the first, contractile stage. 



This work on the production of heat obviously requires ^, very perfect experimental 

 technique, details of which will be found in the papers by A. V. Hill referred to, especially 

 that of 1913, No. 4. A sketch of the thermopile used, in its latest form, is given in Fig. 136. 



The essential process in muscular contraction is, then, the development of a 

 certain degree of tension. As this, if unused, is converted into the equivalent 

 amount of heat, we can, by determining the total amount of tension developed in 

 a series of contractions, deduce the amount of heat. The application of this fact 

 will be seen presently. 



The stage in which we have now left the muscle is with a diminished store of 

 potential energy in the complex physico-chemical system and with a certain 

 amount of lactic acid, which has been separated from this system. In order to 

 restore the muscle to its previous state, work must clearly be expended on it, 

 otherwise we should be obtaining work from nothing. What do we know about 

 the way this restoration is brought about 1 



We have seen that, in order to detect the production of lactic acid, the muscle 

 must not have oxygen at its disposal, and it was definitely shown experimentally 

 by Fletcher and Hopkins (1907) that the lactic acid disappears under the action of 

 oxygen. It is natural to suppose that it might be oxidised to carbon dioxide and 

 water, since, although muscular activity, in the absence of oxygen, is not associated 

 with evolution of carbon dioxide, this gas is given off when oxygen is present. 

 It is a remarkable fact that experimental evidence shows that this is not the 

 way in which lactic acid disappears. Fletcher and Hopkins have shown that, 

 when muscle is allowed to enter into a state of spontaneous " rigor," which can 

 be accelerated by raising the temperature to 45, a certain definite amount of lactic 

 acid comes from the decomposition of the " inogen " substance. 



The experiment is done as follows : Muscle is stimulated in absence of oxygen, so that 

 lactic acid is formed ; it is then exposed to an atmosphere of oxygen, and the lactic acid 

 disappears ; the stimulation and exposure to oxygen are repeated nine times. Finally, the 

 muscle is put into heat rigor and the lactic acid estimated. If it had been oxidised, it is 

 clear that the final amount to be obtained would be diminished. In point of fact, it is 

 found to be the same as if the muscle had not been stimulated and exposed to oxygen. It 

 must, therefore, have been replaced, by some means, in the system from which it was 

 split off on stimulation (see the paper by Fletcher and Hopkins, 1907, pp. 292-296, and 

 Fig. 137 below). 



In case any doubt may arise in the mind of the reader as to the origin of the 

 lactic acid in contraction and in rigor being the same, the work of Peters (1913) 

 may be consulted. 



Further evidence as to the fact of the non-oxidation of lactic acid is given in 

 a calculation by A. V. Hill (1914, 2) ; but we must first consider the facts known 

 with regard to the consumption of oxygen and the evolution of carbon dioxide 

 connected with the removal of lactic acid. 



It may be repeated that there is no evolution of carbon dioxide, and obviously 

 no consumption of oxygen, in the absence of this gas, whereas both processes 

 occur in its presence. Further, the difference between the two cases is that, 

 in the first case, the lactic acid remains, while in the second, it disappears. This 

 fact indicates that the oxidation process is a stage secondary to that of 



