CHANGES OF THE AIR IN RESPIRATION. 345 



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- 

 ing 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 breath- 

 ing at 17 a minute, would give over 500 litres of carbonic acid for the day's 

 production. Actual determinations, 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 grins, oxygen, the oxygen which actually dis- 

 appeared 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 

 carbonic 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. 



283. 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 amount- 

 ing to about ^th to ^ O th 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 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 circumstances that all 

 the oxygen inspired does not reappear in the carbonic acid expired, some 

 having formed within the body other combinations. 



284. Besides carbonic acid, expired air contains various substances 

 which may be spoken of as impurities, many of an unknown nature, and 

 all in small amounts. Traces of ammonia have been detected in expired 

 air, even in that taken directly from the trachea, in which case its presence 

 could not be due to decomposing food lingering in the mouth. When 

 the expired air is condensed by being conveyed into a cooled receiver, 

 the aqueous product is found to contain organic matter, which from the 

 presence of microorganisms introduced in the inspired air, is very apt 

 rapidly to putrefy. The organic substances thus shown to be present in the 

 expired air are the cause in part of the odor of breath. It is probable that 

 some of them are of a poisonous nature, either poisonous in themselves as 

 coming direct from and produced in some way or other in the pulmonary 

 apparatus, or poisonous as being the products of putrefactive decomposition ; 

 for various animal substances and fluids give rise by decomposition to dis- 

 tinct poisonous products known as ptomaines, and it is possible that some of 

 the constituents of expired air are of an allied nature. In any case the 



