242 PHYSIOLOGY 



exercise, lactic acid is certainly formed and may escape from the body. 

 Normal urine has been shown by RyfM to contain about 3-4 mg. 

 of lactic acid per hour. In one case the urine passed thirty minutes 

 after running one-third of a mile with the production of severe dyspnoea 

 contained 454 mg. of lactic acid. The lactic acid under these 

 conditions can be shown also to be increased in the blood. Thus, in 

 one experiment, the blood obtained before running contained 12'5mg. 

 per 100 c.c., that obtained immediately after running one-third 

 of a mile contained 70 '8 mg. per 100 c.c. On the other hand, the 

 examination of the urines of competitors in a twenty-four hours' 

 track walking race showed no increase in the output of lactic acid 

 above the normal 4 mg. per hour. The excretion of lactic acid was 

 also observed many years ago by Araki in cases where the oxidation 

 processes of the body were interfered with in consequence of CO 

 poisoning. 



The second substance, carbon dioxide, is continually being formed 

 by all living tissues, and is the end-product of practically all the carbon 

 metabolism of the body. If a muscle be hung up in a confined space, 

 it will be found to take up oxygen and give off C0 2 ; and these inter- 

 changes are quickened by causing the muscle to contract. It has 

 been shown by Fletcher that the effect of activity is dependent on 

 the composition of the gas surrounding the muscle. If it be hung 

 up in a vacuum or in an atmosphere of nitrogen or hydrogen, there 

 is a slow evolution of C0 2 , which is not appreciably quickened during 

 contraction, and seems to be conditioned by a gradual driving off of 

 C0 2 from the alkaline carbonates in the muscle, as a result of the 

 steady production of lactic acid which precedes the onset of rigor. 

 If, however, the muscle be suspended in an atmosphere of pure oxygen, 

 the formation of acid is diminished or abolished ; but now each con- 

 traction of the muscle is followed by an increased evolution of carbon 

 dioxide. 



We see therefore that, according to the environment of the muscle, 

 its activity is attended by the formation either of lactic acid or of 

 carbon dioxide, the latter substance being the sole product if sufficient 

 oxygen be supplied to the muscle. If the supply of oxygen be 

 inadequate, both substances are produced, the proportion of lactic acid 

 varying according to the relative inadequacy of the oxygen supply. 

 This relation holds good both in rest and activity, the effect of activity 

 being merely to increase the chemical changes which are going on 

 spontaneously in the surviving resting muscle. 



It is an interesting point to determine whether we have here really 

 two alternative chemical mechanisms for the production of energy. 

 We know that sugar can be utilised by muscle as a food and source 



