THE RFISPIRATDRY SYSTEM. 383 



less than this amount. A similar relationship may be considered 

 to hold for the domestic animals, the quantities varying, of course. 



From experiment it has been ascertained that the amount 

 of carbonic dioxide is for the average man 800 grammes (406 

 litres, equivalent to 218 '1 grammes carbon; daily, the oxygen 

 actually used for the same period being 700 grammes. But the 

 variations in such cases are very great, so that these numbers 

 must not be interpreted too rigidly. Experience proves that, 

 while chemists often work in laboratories in which the per- 

 centage of carbonic anhydride (from chemical decompositions) 

 reaches 5 per cent, an ordinary room in which the amount of 

 this gas reaches 1 per cent is entirely unfit for occupation. This 

 is not because of the amount of the carbon dioxide present, but 

 of other impurities which seem to be excreted in proportion to 

 the amount of this gas, so that the latter may be taken as a 

 measure of these poisons. 



What these are is as yet almost entirely unknown, but that 

 they are poisons is beyond doubt. Small effete particles of 

 once living protoplasm are carried out with the breath, but 

 these other substances are got rid of from the blood by a vital 

 process of secretion (exci'etion), we must believe; which shows 

 that the lungs to some degree play the part of glands, and that 

 their whole action is not to be explained as if they were merely 

 moistened bladders acting in accordance with ordinary physi- 

 cal laws. 



An estimation of the amount of atmospheric air required 

 may be calculated from data already given. 



Thus, assuming that a man gives up at each breath 4 per 

 cent of carbon dioxide to the 500 cc. of tidal air he expires, and 

 breathes, say, seventeen times a minute, we get for the amount 

 of air thus charged in one hour to the extent of 1 per cent: 

 500 X 4 X 17 X 60 = 3,040,000 cc., or 2,040 litres. 



But if the air is to be contaminated to the extent of only ^ 

 per cent of carbonic anhydride, the amount should equal at 

 least 2,040 x 10 hourly. A very much larger quantity would, 

 of course, be required for a horse or an ox. 



RESPIRATION IN THE BLOOD. 



It may be noticed that arterial blood kept in a confined space 

 grows gradually darker in color, and that the original bright- 

 scarlet hue may be restored by shaking it up with air. When 



