OUTPUT OF WASTE MATTER 129 



lung the oxygen passes through the thin 

 wall between the blood and the air possibly 

 physically, inasmuch as the tension of the 

 oxygen in the air cells, especially at the 

 end of an inspiration, is greater than the 

 tension of the oxygen in the blood, but again 

 various considerations lead us to suppose that 

 the taking up of oxygen may be a vital 

 process due to the activity of the cells lining 

 the air cells and also lining the vessels. The 

 air bladder of a fish is the representative of the 

 lung ; in many cases it contains a large 

 percentage of oxygen, secreted from the 

 blood of the fish by the epithelium lining 

 the bladder. (It is remarkable, however, 

 that in shallow water fishes the gas in the 

 air bladder is chiefly nitrogen.) This oxygen 

 was in the first instance separated from the 

 water by the blood vessels of the gills as the 

 water flowed over them. In the tissues, 

 again, internal respiration may not be entirely 

 a physical process, as we are dealing with 

 living tissues. Here, again, there may be 

 selection of living gases by living cells. 

 What we usually think of as respiration 



