MODIFICATIONS OF RESPIRATION. 843 



gases is said to be proportionally less, when compared with the quan- 

 tity of air ; but the total amount of interchange is greater, because 

 the volume of air inspired and expired, is so much larger. The last 

 portion of air expired, in all cases, contains less oxygen and more 

 carbonic acid than the first portion ; the former, no doubt, containing 

 air coming from the finest air-tubes, close to the air-cells, in which 

 the actual absorption and exhalation occur. 



The relative purity or impurity of the air likewise affects the re- 

 sult; as when the same air is breathed over and over again, and so 

 becomes more or less charged with carbonic acid. Thus, in an experi- 

 ment in which 300 cubic inches of air were repeatedly breathed for a 

 period of three minutes, only 9.5 per cent, of carbonic acid was found 

 in it; the total quantity being 28.5 cubic inches, or 9.5 cubic inches 

 per minute. In the same person, with fresh air at each inspiration, 

 the quantity was 32 cubic inches per minute. However often the 

 same air was respired, it was never found to contain more than 10 per 

 cent, of carbonic acid. (Allen and Pepys.) These results are caused 

 by the increasing difficulty offered by the accumulation of carbonic 

 acid in the air of the air-passages, to the escape by moist diffusion, of 

 the carbonic acid from the blood into the air-cells, and to its simple 

 diffusion through the air of the air-passages. 



The effect of an increased temperature of the air is to diminish, and 

 that of a lower temperature is to increase, the quantity of carbonic 

 acid exhaled by the lungs. Between the temperature of 47 and 67, 

 i. e., with a difference of 20 in the external temperature, a variation 

 has been observed in the quantity of carbonic acid exhaled, of 2.5 

 cubic inches per minute. (Vierordt.) At very low temperatures, the 

 quantity of carbonic acid exhaled may even be more than twice as 

 great as that given off at very high temperatures. A sudden increase 

 of temperature produces a marked immediate effect, viz., a decrease 

 of 2.75 cubic inches per minute, for 16 of elevation of temperature, 

 but this is not subsequently so regularly maintained. (Dr. E. Smith.) 

 The absorption of oxygen is, of course, inversely affected. 



The density of the air also influences the chemical changes depend- 

 ent on respiration, their activity being increased when the density of 

 the air is diminished. 



A moist atmosphere, the temperature being the same, greatly favors, 

 in animals, the exhalation of carbonic acid ; moreover, the influence 

 of moisture is so great as to neutralize, at high temperatures, the ef- 

 fect of such temperatures in diminishing the exhalation of that gas. 

 (Lehmann.) The exact hygrometric state of the air ought always, 

 therefore, to be taken into account, in experiments on the composi- 

 tion of expired air. The great influence of moisture may account for 

 some of the discrepancies between the results of different observers. 



As regards age, the quantity of oxygen absorbed and of carbonic 

 acid exhaled, increases generally, in both sexes, to about the thirtieth, 

 and then remains stationary to the fortieth year, after which it dimin- 

 ishes, so that at seventy, the amount only slightly exceeds that proper 

 to the age of ten years. The influence of sex, as might be expected 

 from the greater size and activity of men, is, after the eighth year, 



