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Page:Popular Science Monthly Volume 72.djvu/391

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387
SOME NEW VIEW POINTS IN NUTRITION

view, however, was slow in gaining ground, and for years it was generally 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 contain the two gases oxygen and carbon dioxide, though in different proportions, 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 outside the organism a forced draft is without effect on the rate of burning. 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 proceeded 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 discovery of oxygen, oxidation has been the key-note to which all the varied changes characteristic of life have been adjusted. In ultimate