28 Respiration. 



when exposed to the atmosphere. There are other substances 

 such as pyrogallic acid which do the same, but the red substance 

 of the blood differs from pyrogallic acid in the fact that when it 

 is exposed to an atmosphere devoid of oxygen it disgorges all 

 the oxygen which it has previously absorbed. 



The lung is an apparatus for exposing an immense surface of 

 blood to the air. While thus exposed each corpuscle takes up 

 its cargo of oxygen, and then gets propelled with extreme 

 rapidity in the blood stream to some indigent muscle or nerve 

 which has used up all the oxygen that it possesses. Here, not 

 being surrounded by oxygen, the corpuscle gives up its store of 

 that gas just as a wet sponge-rag would yield up its store of 

 water when removed from a damp to a dry atmosphere. 



It was formerly a matter of great labour, involving the use 

 of large quantities of blood, to demonstrate the relative amounts 

 of oxygen in blood going to and coming from the lungs, but 

 recent researches had made it so simple that he could easily 

 show them how much oxygen the blood loses at its ports of 

 call. 



He had compared the colourless fluid portion of the blood 

 to a waterway, and he could press the comparison a stage 

 further. The clear fluid part is more than a medium for carry- 

 ing the corpuscles — it serves to flush out every piece of muscle 

 and nerve and bone. Each of these accumlates its little store 

 of carbonic acid as it does its work, but this gas si exceedingly 

 soluble in water, and so as fast as it is produced it gets caught 

 up in the colourless part of the blood and carried to the lungs. 

 A pint of water would absorb about a pint of carbonic acid 

 gas. If the solution be shaken up with air the water would 

 lose carbonic acid till both the air and the water contained the 

 same percentage of the carbonic acid. He had almost said 

 that that was an illustration of how the blood lost its carbonic 

 acid in the lung ; that an immense surface of blood was con- 

 tinually circulating through the lung separated only by the 

 thinnest of membranes from the air in that organ ; that it 

 tended always to share its carbonic acid equally with the air, 



