CHAP, ii.] RESPIRATION. 327 



ration, it is found that, both being dried and measured at the 

 same pressure, the expired air is less in volume than the inspired 

 air, the difference amounting to about ^th or ^th of the volume 

 of the latter. Hence, when an animal is made to breathe in 

 a confined space, the atmosphere is absolutely diminished, as 

 was observed so long ago as 1674 by Mayow. The approximate 

 equivalence in volume between inspired and expired air arises 

 from the fact that the volume of any given quantity of carbonic 

 acid is equal to the volume of the oxygen consumed to produce it; 

 the slight falling short of the expired air is due to the circum- 

 stance that all the oxygen inspired does not reappear in the 

 carbonic acid expired, some having formed other combinations. 



4. The expired air contains about 4 or 5 p.c. less oxygen, and 

 about 4 p.c. more carbonic acid than the inspired air, the quantity 

 of nitrogen suffering but little change. Thus 



oxygen. nitrogen. carbonic acid. 



Inspired air contains 20*81 7915 '04 



Expired 16*033 79'587 4'380 



The quantity of nitrogen in the expired air is sometimes found 

 to be slightly greater, as in the table above, but sometimes less, 

 than that of the inspired air. 



In a single breath the air is richer in carbonic acid (and poorer 

 in oxygen), at the end than at the beginning. Hence the longer 

 the breath is held, the greater the pause between inspiration and 

 expiration, the higher the percentage of carbonic acid in the 

 expired air. Thus by increasing the interval between two ex- 

 pirations to 100 seconds, the percentage may be raised to 7*5. When 

 the rate of breathing remains the same, by increasing the depth 

 of the breathing the percentage of carbonic acid in each breath is 

 lowered, but the total quantity of carbonic acid expired in a given 

 time is increased. Similarly, when the depth of breath remains 

 the same, by quickening the rate the percentage of carbonic acid 

 in each breath is lowered, but the quantity expired in a given 

 time is increased. 



Taking, as we have done, at 500 c.c. the amount of tidal air 

 passing in and out of the chest of an average man, such a person 

 will expire about 22 c.c. of carbonic acid at each breath; this, 

 reckoning the rate of breathing at 17 a minute, would give over 

 500 litres of carbonic acid for the day's production. By actual 

 experiment, however, Pettenkofer and Voit, of whose researches we 

 shall have to speak hereafter, determined the total daily excretion 

 of carbonic acid in an average man to be 800 grins., i.e. rather 

 more than 400 litres (406), containing 218*1 grms. carbon, and 

 581*9 grms. oxygen, the oxygen actually consumed at the same 

 time being about 700 grms. This amount represents the gases 

 given out and taken in, not by the lungs only, but by the whole 



