8o 



Supplefnent to '' Nature,'' July 14, 1923 



rapidly at a higher pressure of oxygen. This agrees 

 with what we know of recovery from exertion, or 

 exhaustion, in man : breathing pure oxygen, instead 

 of air, enormously increases its speed and complete- 

 ness. Moreover, the magnitude of the recovery heat- 

 production told one what happened to the -lactic acid 

 in recovery. One knew how much lactic acid was 

 produced in a given contraction ; one knew, therefore, 

 how much lactic acid was removed in the complete 

 recovery from that contraction : if it were all oxidised 

 the heat evolved could be calculated : actually the 

 amount observed is only about i/6th of the amount 

 calculated : hence the lactic acid removed in recovery, 

 or at any rate its chief part, is not removed by oxidation, 

 but in some other way. 



Apart from this delayed heat-production associated 

 with recovery, one might have expected the rest of 

 the heat to be given out rapidly, more or less ex- 



sm/MG 



ENM4Y T/Mll>J fKtr\ B»rt|;«^^ 

 or r\^.(,N%r>c rvix. 



COrJlbl«.Tio<v , 





Fig. I. — Electromagnetic analogy to the working of muscle. 



plosively, at the commencement of contraction. Con- 

 traction has been likened to the explosion of a cartridge: 

 the muscle suddenly gives out heat and develops force. 

 This force, however, represents a state of elastic 

 potential energy in the muscle, and when the muscle 

 relaxes this potential energy disappears, and we should 

 expect it to reappear as heat. Actually the analysis 

 of the heat-production in the single twitch shows that 

 about 60 per cent, of it is evolved in the initial process 

 of setting up the contraction, 40 per cent, of it in the 

 final stage of relaxation. If the contraction be pro- 

 longed, there is in addition a prolonged evolution of 

 heat, lasting as long as the contraction, the rate of heat- 

 production being proportional to the force maintained. 

 There are, therefore, four phases in the heat-produc- 

 tion of muscle, corresponding (i) to the development, 

 (2) to the maintenance, and (3) to the disappearance of 

 the response, and finally (4) to recovery therefrom. A 

 simple physical picture of the system is given (Fig. i) by 

 an electromagnet, pulling on a piece of iron attached to 

 a spring: a key: a battery: and a dynamo (driven by a 



combustion engine of some kind) to recharge the latter. | 

 Energy is consumed in setting up the pull of th^- 1 

 electromagnet, energy is being consumed all the tin 

 in maintaining the pull, energy — the potential energy 

 of the magnetic field and the spring — is liberated when 

 the current is broken, and energy is used in rechargii 

 the battery. 



This picture has recently been given a more concrete 

 chemical form. In contraction the lactic acid com< 

 from glycogen ; in recovery the lactic acid is restored 

 as the glycogen from which it came, apart from a small 

 proportion — about |th — which is oxidised to provide 

 energy for the restoration. In the setting up of the 

 contraction, therefore, lactic acid is liberated ; in 

 relaxation it is neutralised : it somehow produces the 

 mechanical response by the action of its acidic part 

 upon the structural protein elements of the muscle 

 fibre. Protein is a weak acid at the hydrogen ii 

 concentration of the body, and the structural elemenio 

 of the muscle are in effect highly ionised sodium (or 

 potassium) salts of protein. These structures therefore 

 have a negative electric charge, all along their length, 

 each element of the structure repelling every other 

 element. The localised production of lactic acid 

 causes the formation of sodium (or potassium) lactate, 

 and of undissociated protein acid : the protein structure 

 is discharged electrically : its elements cease to repel 

 each other, and shortening occurs. It is well known 

 that if the surface charge of mercury, in contact with 

 sulphuric acid, be changed by conduction from outside, 

 there results a change of surface tension, and so a 

 movement of the mercury. This principle is utilised 

 in the capillary electrometer, and would seem to have 

 been employed by Natur*^ in the muscle. The heat 

 associated with contraction is due to the chemical 

 formation of lactic acid from glycogen. As soon, 

 however, as the lactic acid is free it is neutralised 

 by the alkalies of the muscle, and relaxation set 

 in, the heat produced in relaxation being due to the 

 chemical process of neutralisation. To maintain a 

 contraction therefore requires a balance between the 

 rate at which lactic acid is produced and the rate at 

 which it is neutralised. Finally, in recovery, the 

 neutralised lactic acid is slowly removed and restored, 

 by the working of some unknown recover}' mechanism, 

 by which 5 parts of it are restored, and i part oxidised 

 to supply the necessary energy. 



Exercise in Man. — Our knowledge of the nature of 

 muscular work in man has been derived largely from 

 a study of the amount of oxygen used, and the varioi: 

 characteristics and time-relations of the oxygen suppl\ . 

 The subject of the experiment carries a large bag on 

 his back (Fig. 2) and by means of a mouthpiece con- 

 taining two valves, and a pipe and tap, he can breathe 



