312 THE MAINTENANCE OF THE INDIVIDUAL 



in animals. It was Priestley (1733-1804), however, who discovered 

 oxygen and recognized its great importance to all living matter. The 

 name of one more important early worker, Lavoisier (1743-1794), 

 should remain in our memory as he was the first man to attempt a 

 quantitative scientific study of the phenomenon of respiration. It 

 was he who first stated ''life is a chemical action" and who realized 

 that animal heat was the result of an oxidation process involving 

 substances of the body. Both he and LaPlace (1749-1827) carried 

 on numerous experiments on respiration and its relation to the pro- 

 duction of animal heat. Out of this humble beginning has come all 

 the later fascinating studies upon respiration by such workers as 

 Liebig, Voit, Rubner, Pettenkofer, Atwater, Rosa, Benedict, and 

 others. 



The Protein, Hemoglobin 



Before turning attention to the various devices developed to meet 

 the problem of respiration -one mechanism that is universally present 

 in the vertebrates should be mentioned, namely, the respiratory 

 pigment hemoglobin. This is a protein compound found in the red 

 corpuscles of vertebrates. It has the ability of combining readily 

 with oxygen to form oxyhemoglobin, thus enabling the blood stream 

 to carry much more oxygen than it could possibly do by saturating 

 the plasma. 



The interchange of oxygen and carbon dioxide may be explained by 

 physical laws. It is known that a gas tends to pass in the direction of 

 the least pressure. Even when a moist, permeable membrane, or a se- 

 lectively permeable membrane, such as the epithelium of the lungs and 

 capillaries, is placed between different gases the molecules pass freely 

 back and forth. In the event of a difference in pressure between the 

 two sides of the membrane, the gases pass through from the region of 

 greater pressure to that of the lower pressure until it is equalized. 

 Oxygen constitutes nearly 21 per cent of the atmosphere and is pres- 

 ent in sufficient amounts to furnish enough pressure to transfer it 

 to regions of lower pressure. If we keep in mind the fact that the 

 pressure of oxygen outside the body must always be greater than that 

 in the blood stream in the lungs, we can readily understand why 

 oxygen must pass through the moist permeable membranes and into 

 the blood stream, thus giving us the explanation of external respiration. 

 On the same basis internal respiration may be explained. The first 

 step involves the liberation of oxygen from the blood to the lymph. 



