222 STUDIES IN ADVANCED PHYSIOLOGY. 



presence of any other gases. We have in every case to do 

 only with the pressure of the one gas in question. Now, 

 one-fifth of the atmosphere is oxygen, four-fifths nitrogen, 

 disregarding for the moment the other ingredients. As the 

 pressure of the atmosphere is fifteen pounds to the square 

 inch, a fifth of that is due to oxygen; that is, the pressure 

 of oxygen to the square inch is three pounds, while the 

 pressure of the nitrogen, the remaining four-fifths of the 

 atmosphere, is of course four-fifths of fifteen pounds, or 

 twelve pounds. In studying the action of the oxygen in 

 the lung, we need only to take into account the fact that 

 the oxygen presses on the blood in the capillaries with a 

 pressure equal to three pounds to the square inch, and we 

 may, without fear of any complications, entirely disregard 

 the presence of the other gases. For if the blood should be 

 put into a closed vessel and all the nitrogen of the air in 

 that vessel be removed chemically, no difference would re- 

 sult. In conformity with the law of Dal ton, oxygen will 

 be absorbed by the plasma until the pressure of the oxygen 

 in the plasma becomes the same as the pressure of oxygen 

 above it; that is, three pounds; and the nitrogen will be 

 absorbed by the plasma until the pressure of nitrogen in it 

 will be equal to the pressure of nitrogen immediately 

 above it. 



So far there would be not one iota of difference between 

 what happens to the venous blood in the lung and what 

 happens to the water in a river. Both subjected to the 

 same atmosphere would dissolve the same proportion of 

 oxygen and nitrogen. Animals which have colorless blood 

 only, for instance, the oyster or the crayfish, get their 

 supply of oxygen from the amount of this gas which the 

 water of their blood will dissolve. As every one knows, 

 however, ordinary water does not dissolve very much 

 oxygen, and fishes which are put in an aquarium must have 

 the water renewed at very frequent intervals, as the oxygen 

 supply of the water is rapidly exhausted. For this reason 

 animals that can get no other oxygen except that carried 



