174 RESPIRATION. 



into the air breathed by a healthy adult man amounts to 

 1346 cubic inches, or about 636 grains per hour. According 

 to this estimate, which corresponds very closely with the one 

 furnished by Sir H. Davy, and does not widely differ from 

 those obtained by Allen and Pepys, Lavoisier, and Dr. Ed. 

 Smith, the weight of carbon excreted from the lungs is about 

 173 grains per hour, or rather more than 8 ounces in the course 

 of twenty-four hours. Discrepancies in the results obtained 

 by different experimenters may be due to the variations to 

 which the exhalation of carbonic acid is liable in different 

 circumstances ; for even in health the quantity varies accord- 

 ing to age, sex, diversities in the respiratory movements, ex- 

 ternal temperature, the degree of purity of the respired air, 

 and other circumstances. Each of these deserves a brief notice, 

 because it affords evidence concerning either the sources of 

 carbonic acid exhaled, or the mode in which it is separated 

 from the blood. 



a. Influence of Age and Sex. According to Andral and 

 Gavarret the quantity of carbonic acid exhaled into the air 

 breathed by males, regularly increases from eight to thirty 

 years of age ; from thirty to forty it is stationary or diminishes 

 a little; from forty to fifty the diminution is greater ; and from 

 fifty to extreme age it goes on diminishing, till it scarcely 

 exceeds the quantity exhaled at ten years old. In females 

 (in whom the quantity exhaled is always less than in males 

 of the same age) the same regular increase in quantity goes 

 on from the eighth year to the age of puberty, when the quan- 

 tity abruptly ceases to increase, and remains stationary so 

 long as they continue to menstruate. When, however, men- 

 struation has ceased, either in advancing years or in preg- 

 nancy or morbid amenorrhoea, the exhalation of carbonic acid 

 again augments ; but when menstruation ceases naturally, it 

 soon decreases again at the same rate that it does in old men. 



b. Influence of Respiratory Movements. According to Vier- 

 ordt, the more quickly the movements of respiration are per- 

 formed, the smaller is the proportionate quantity of carbonic 

 acid contained in each volume of the expired air. Thus he 

 found that, with six respirations per minute, the quantity of 

 expired carbonic acid was 5.528 per cent. ; with twelve respi- 

 rations, 4.262 per cent. ; with twenty-four, 3.355 ; with forty- 

 eight, 2.984 ; and with ninety-six, 2.662. Although, however, 

 the proportionate quantity of carbonic acid is thus diminished 

 during frequent respiration, yet the absolute amount exhaled 

 into the air within a given time is increased thereby, owing 

 to the larger quantity of air which is breathed in the time. 

 This is the case, whether the respiration be voluntarily accel- 



