LECTURE XVIII. 

 OXYGEN. 1 



ALL the foodstuffs which we have considered up to this point are intro- 

 duced into the animal organism by way of the alimentary canal. There 

 is one substance required to nourish the body which differs from all the 

 other organic and inorganic foods, not only in the form in which it is 

 taken up, but also in the manner of its introduction. We refer to oxygen, 

 which is taken up as a gas into the animal organism through the lungs 

 and carried by the blood to the tissues. With the oxygen, as such, there 

 is no available energy introduced into the organism. It possesses no 

 chemical kinetic energy, so that it belongs to the same class of substances 

 in this respect as the salts and water. In every way, however, oxygen 

 occupies an exceptional position. Plants set it free by the aid of chloro- 

 phyll and the influence of the sun's rays in the assimilation of carbon 

 dioxide and water. Energy is required for this process and becomes 

 stored up as chemical energy. Conversely, in the animal cells oxygen 

 again combines with the substances formed in the plants, energy is set 

 free, and we find as the final end-products, water and carbon dioxide, both 

 of which can again take part in the cycle. 2 



The first one to clearly recognize the importance of oxygen for the life 

 process was Lavoisier. He sharply outlined the important role which 

 this substance plays in the combustion processes taking place within the 

 animal organism. With this knowledge, there was laid one of the most 

 important foundation stones for the entire physiology upon which, in the 

 period following, stone after stone was piled in rapid succession until 

 finally the structure was established, the particulars of which we are now 

 studying. No discovery in the whole field of physiology was so decisive 

 for further investigation as this. Lavoisier himself, it is true, believed 

 that the lungs were the seat of all the oxidation processes taking place in 

 the animal organism. The oxygen taken from the air was supposed to 

 oxidize the substances brought to the lungs by means of the blood. Such 

 an assumption was a priori hardly plausible, for in this combustion 



1 Cf. Christian Bohr: Handbuch der Physiologie der Menschen, Vol. I, p. 54, 1905. 



2 Fundamentally, there is no such sharp distinction between plant and animal cells. 

 Plants also utilize chemical energy, but in them the reduction processes far exceed those 

 of oxidation in the daytime, though in the absence of light (night-time) the latter pro- 

 cesses are more prominent. 



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