CHEMICAL CHANGES IN MUSCLE 2 17 



The appearance of lactic acid thus seems to be attendant on a rHativ 

 deficiency in the oxygen supply to the contracting muscle. Tl, 

 concision may be drawn from experiments made many years ago by A 

 in which lactic acid was observed in quantities in the urine in cases whe, 

 the oxidative processes of the body were interfered with by CO poisoning 



Similar results are obtained when we investigate the chemical changes 

 accompanying the contraction of excised muscles of the frog If frogs' 

 muscle be hung up in an atmosphere of nitrogen and stimulated repeatedly 

 with single shocks, it will give a series of contractions gradually diminishing 

 in size (v. p. 209). After a time the muscle is completely fatigued and 

 no further response can be elicited on stimulation. On now examining it, 

 it is found to be acid in reaction and to contain about -2 per cent, lactic 

 acid. There is no evidence that under these conditions any carbonic acid 

 is produced, though a certain amount may be liberated in consequence of the 

 acidification of the muscle. Almost the same results are obtained when 

 the muscle is stimulated in ordinary atmospheric air. The penetration 

 of oxygen from the air through the body of the muscle is so slow that all 

 the muscle except the thin layer on the surface may be regarded as cut off 

 from the action of oxygen. By hanging the muscle, especially a thin muscle 

 such as the sartorius, in an atmosphere of pure oxygen, the results are quite 

 different. In the first place the muscle does not fatigue so soon. More- 

 over, a muscle which has been stimulated to exhaustion in an atmosphere of 

 nitrogen, if restored to one of pure oxygen, will rapidly recover its power 

 of contraction. In pure oxygen no lactic acid is produced, and a muscle 

 stimulated to exhaustion contains very little more lactic acid than does 

 resting muscle. On the other hand, the intake of oxygen and the output 

 of carbonic acid by the muscle is increased at each contraction. We thus 

 find that a muscle during contraction may produce lactic acid or carbonic 

 acid according as oxygen is absent or present. In both cases contraction 

 takes place apparently normally, but fatigue supervenes much more rapidly 

 in the absence of oxygen. The question arises whether we should regard 

 the formation of lactic acid and carbonic acid as alternative processes, or 

 whether lactic acid is first formed and is then removed under the action of 

 oxygen, undergoing partial or complete oxidation to carbonic acid in thf 

 process. The evidence is distinctly in favour of the second hypothesis. 

 Thus Hopkins and Fletcher have found that muscle possesses in itself a 

 chemical mechanism for the removal of lactic acid. If a fatigued 

 muscle be exposed to pure oxygen, 30 per cent, of the lactic acid present in 

 the muscle may disappear within two hours and 50 per cent, within six to 

 ten hours. Thus, even apart from the circulation which of course would 

 remove large quantities of any lactic acid which might be produced 

 in the muscles, these can deal with this metabolite locally. It has 

 been found that a muscle may be fatigued several times and then placed 

 in oxygen to recover, so that lactic acid is produced and removed also several 

 times. If at the end the muscle be allowed to undergo rigor, it is found to 

 contain 4 per cent, lactic acid, i.e. exactly the same amount as if it had 



