THE CONTROL OF THE RESPIRATION 



367 



an excess of C0 2 was being expired during the work and immediately 

 after it, but that this was subsequently followed by a much lower quo- 

 tient, indicating that C0 2 was being retained. Such a result would be 

 in conformity with the view that an acid -such as lactic is discharged 

 into the blood, on the carbonates of which it would act as explained on 

 page 355. Breathing in and out of a small rubber bag causes the same 

 alterations in the respiratory quotient (see page 358). 



That lactic acid is actually produced by contracting muscle could not, 

 however, be shown by all investigators, and it was not until some years 

 later that Fletcher and Hopkins 29 clearly demonstrated the conditions 

 under w r hich it may appear in active isolated muscle. These observers 

 found that lactic acid is produced in excised muscles only w 7 hen the 

 muscular contraction occurs in a deficiency of 2 . When it occurs in an 

 adequate supply of 2 , C0 2 instead of lactic acid is produced. 



Taking these facts together with what w r e already know concerning 

 the conditions under which the respiratory center reacts to conditions 

 which presumably cause a change in the C H of the blood, we may formu- 

 late the hypothesis that respiratory activity during muscular exercise 

 is due to a slight increase in the C H of the blood, and that this increase 

 is owing partly to an actual increase in C0 2 production by the acting 

 muscles and partly to the production of lactic acid. Such an hypothesis 

 would satisfactorily explain why the actual amount of C0 2 in the blood 

 might be below the normal during muscular exercise, for the C0 2 would 

 be "washed out" from the blood by the.hyperpnea induced by the in- 

 crease in C H . 



The obvious method of putting this hypothesis to the test is to ex- 

 amine the alveolar C0 2 tension and the respiratory quotient under various 

 conditions of muscular activity. The results of such observations are 

 given in the accompanying table. 



