THE INTIMATE NATURE OF MUSCULAR CONTRACTION 237 



active surfaces. The muscle reacts at the end of the excitatory stage not 

 by any active process of lengthening, but by neutralisation, or simply physical 

 diffusion of the active chemical bodies away from the interfaces or mem- 

 branes. Later on, lactic acid is removed or replaced by its previously 

 unstable precursor under the influence of oxygen with the production of some 

 carbon dioxide and a certain amount of heat. We have seen already that 

 the efficiency of the initial chemical change in which lactic acid is set free 

 may approximate 100 per cent. 



It must be noted that although the oxidative processes are responsible 

 ultimately for all the energies of the higher animal, no oxidative change 

 is involved in the production of lactic acid from e.g. glucose, nor is the 

 presence of oxygen necessary for the contraction of muscle to take place. 

 On the other hand, if we wish to obtain the maximum amount of work 

 from a muscle, we must supply it richly with oxygen, the presence of 

 which seems essential not to the contractile process but to the stage of 

 recovery. In this stage a certain amount of heat is evolved, set free by 

 the oxidation of the lactic acid, and we must assume that part of the 

 energy so available is utilised for building up the precursor from which 

 the lactic acid is derived. It is as if the process of oxidation furnished 

 the energy for winding up a spring, whereas excitation removed a catch and 

 allowed the spring to run down, setting free this energy for the performance 

 of work or for conversion into heat.* 



For many years it was imagined, as a result of experiments by Hermann, Pfliiger, 

 and others, that the oxygen supplied to a muscle was built up with its other constituents, 

 especially carbohydrates, into a complex 'inogen' molecule. On stimulation this mole- 

 cule underwent an explosive rearrangement, the carbohydrate and oxygen parts of 

 the molecule combining to form carbonic acid, another product of the decomposition 

 being lactic acid. The careful experiments of Fletcher have shown, however, that 

 in the absence of oxygen there is no evidence of the formation of carbonic acid during 

 contraction, and therefore no reason to assume the presence of oxygen in the muscle 

 in an intramolecular form. Everything points to oxygen being taken in and applied 

 forthwith to the purposes of oxidation, so that the output of carbon dioxide and water 

 keeps pace with the intake of oxygen. 



It is at present quite impossible to come to any conclusion as to the nature of the 

 precursor from which the lactic acid is derived. The immediate precursor cannot be 

 glucose or glycogen since the heat evolved in the initial stage of contraction is two or 



* Peters has shown that if a muscle be stimulated to exhaustion under anaerobic 

 conditions, about 0-2 per cent, lactic acid is formed with the evolution of -9 calories 

 per gramme of muscle substance. The production of 1 gm. of lactic acid is therefore 

 accompanied by the evolution of 450 calories. According to A. V. Hill the ' recovery 

 heat production ' in oxygen is of about the same order as the initial heat production, 

 so that in the oxidative removal of 1 gm. of lactic acid there would also be an evolution 

 of about 450 calories. The oxidation of 1 gm. of lactic acid produces 3700 calories, 

 about eight times as much as the quantity observed. Hill considers this amo 

 far too large to have escaped detection in his experiments, and therefore conclude 

 that the lactic acid is not oxidised but replaced in its previous position under t 

 influence and with the energy of the oxidation either ; (a) of a small part of the h 

 acid itself, or (6) some other body. He regards the latter alternative as the mor< 

 probable, and concludes therefore, that the lactic acid is part of the machim 

 part of the fuel of the muscle. 



