CHAP, ii.] RESPIRATION. 551 



The quantity of nitrogen in the expired air is sometimes found 

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

 equal to, and 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 of the breath. 

 Hence the longer the breath is held, the greater the (artificial) 

 pause between inspiration and expiration, the higher the per- 

 centage of carbonic acid in the expired air. Thus by increasing 

 the interval between two expirations to 100 seconds, the per- 

 centage 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, the amount of tidal air passing in 

 and out of the chest of an average man at 500 c.c., 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. Actual de- 

 terminations however give a rather smaller total than this ; thus 

 in a series of experiments of which we shall have to speak 

 hereafter, the total daily excretion of carbonic acid in an average 

 man was found to be 800 grms., i.e. rather more than 400 litres 

 (406), containing 218*1 grms. carbon, and 581*9 grms. oxygen, the 

 oxygen which actually disappeared from the inspired air at the 

 same time being about 700 grms. This amount it should be 

 said represents, owing to the manner in which the experiment 

 was conducted, the gases given out and taken in, not by the 

 lungs only, but by the whole body ; but the amount of carbonic 

 acid given out by other channels than the lungs is, as we shall see, 

 very slight (10 grms. or even less), so that 800 grms. may be taken 

 as the average production of carbonic acid by an average man. 

 The quantity however, both of oxygen consumed and of carbonic 

 acid given out, is subject to very wide variations ; thus in the 

 observations of which we are speaking the daily quantity of car- 

 bonic acid varied from 686 to 1285 grms., and that of the oxygen 

 from 594 to 1072 grms. These variations and their causes will be 

 discussed when we come to deal with the problems of nutrition. 



340. When the total quantity of tidal air given out at any 

 expiration is compared with that taken in at the corresponding 

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

 same temperature and pressure, the expired air is less in volume 

 than the inspired air, the difference amounting to about -^th or 

 sVth of the volume of the latter. Hence, when an animal is made 

 to breathe in a confined space, the air is absolutely diminished in 

 volume. The approximate equivalence in volume between inspired 



