144 ANIMAL PHYSIOLOGY. 



the same air is breathed several times, as happens in crowded 

 rooms, and with deficient ventilation, the percentage of 

 carbonic acid in the expired air is increased. But this 

 accumulation of carbonic acid in the air furnishes an im- 

 pediment to breathing, independent of the exhaustion of the 

 oxygen; for however often the same air be passed through 

 the lungs, it never contains more than 10 per cent, carbonic 

 acid. In an artificial atmosphere, animals have lived till the 

 percentage of carbonic acid reached 1 2 and even 1 8 ; but 

 when they are placed at once in an atmosphere containing 

 that amount, they are immediately suffocated, no matter 

 how great the amount of oxygen present. It is plain, there- 

 fore, that carbonic acid inhaled is actively deleterious, and 

 differs altogether from the nitrogen contained in the atmo- 

 sphere; nitrogen being simply negative, acting as a diluent 

 of the oxygen, incapable of taking the place of that sub- 

 stance, but in no way interfering with its action. Animals 

 live without discomfort in an atmosphere in. which hydrogen 

 is substituted for nitrogen. 



The amount of carbonic acid exhaled in a given time goes 

 on increasing in males till thirty years of age, while from 

 forty, or sooner, it diminishes as age advances. 



In females the amount ceases to increase at puberty, and 

 remains stationary till the cessation of reproductive activity, 

 when it again increases for a time. The amount of carbonic 

 acid exhaled from the lungs of an adult man may be esti- 

 mated as sufficient to yield about nine ounces avoirdupois of 

 carbon in twenty-four hours.* But it varies according to 

 a great variety of circumstances, being increased by cold, by 

 food, and most of all by exercise ; while warmth, fasting 3 rest, 

 and sleep dimmish it. 



107. The amount of oxygen taken into the blood in respira- 

 tion does not bear any constant proportion to the amount of 

 carbonic acid given off; but it is generally somewhat greater, 

 and is always so when the period of observation is extended 

 over twenty-four hours. Carbonic acid contains exactly its 

 own volume of oxygen ; and therefore if the oxygen taken in 

 corresponded with the carbonic acid given off in each respira- 



* Eight ounces is the amount generally mentioned in text-books ; 

 but that means troy ounces formerly in use in matters medical. 



