SOME NEW VIEW POINTS IN NUTRITION 387 



yiew, however, was slow in gaining ground, and for years it was gen- 

 erally held that oxidation as it occurs in the animal body takes place 

 mainly in the lungs. Not until 1837 when Magnus, through use of the 

 air pump, showed conclusively that both arterial and venous blood con- 

 tain the two gases oxygen and carbon dioxide, though in different pro- 

 portions, did the theory of respiration and its connection with oxidation 

 take on its present form. Then, gradually, physiologists began to 

 perceive that pulmonary or external respiration had to do primarily 

 with the exchange of gaseous materials and that oxidation did not 

 occur in the lungs, neither to any degree in the circulating blood, but 

 rather in the different tissues and organs of the body where activity of 

 various sorts prevails. 



The processes of life, the processes of nutrition, soon came to be 

 looked upon as essentially processes of oxidation. The work of 

 Lavoisier in 1780, indicating as it did that animal heat is the result of 

 a process of slow combustion in which oxygen is used up and carbon 

 dioxide produced, a process analogous to that of the burning candle, 

 naturally emphasized the idea of oxidation. As to the nature of the 

 substance or substances undergoing combustion in the animal body, 

 knowledge at that time was somewhat vague and indefinite. Later, 

 when Chevreul had made his classical studies of fats and Mulder had 

 essayed a description of protein, Liebig came forward with his theories 

 of nutrition, among which was the view so long upheld, that the fats 

 and carbohydrates of the food are burned up directly by the inspired 

 oxygen, while the protein is used to replace the protein of the tissues, 

 the latter being oxidized to furnish energy for muscle work. Liebig 

 conceived that oxygen was the cause of oxidation, a view shown to be 

 incorrect by the well-established fact that animals produce no more 

 carbon dioxide in an atmosphere rich in oxygen than under ordinary 

 atmospheric conditions, i. e., unlike the processes of combustion out- 

 side the organism a forced draft is without effect on the rate of burn- 

 ing. Moreover, it was found that oxidation would take place in a 

 tissue independent of an intake of free oxygen, viz., that a contracting 

 muscle, for example, would give out carbon dioxide even when made to 

 contract in a vacuum, thus implying a decomposition or disassociation 

 in which combined oxygen must have been made use of. Further, it 

 was evident that in the tissues and organs of the body, oxidation pro- 

 ceeded gradually through a series of successive steps ; the large, complex 

 molecules of the food and tissues being slowly transformed into simpler 

 molecules with ultimate formation of carbon dioxide and water, plus 

 some nitrogen-containing compounds of a relatively simple nature. 



During all these years, since the time of Mayow and the later dis- 

 covery of oxygen, oxidation has been the key-note to which all the 

 varied changes characteristic of life have been adjusted. In ultimate 



