Oxidation, however, is the keynote in any explanation of the processes of metabolism, whether nitrogenous or non-nitrogenous matter is involved. Both alike undergo oxidation, but it is not simple oxidation or combustion that we have to deal with. In the time of Lavoisier, as already stated, it was thought that oxygen alone was the cause of the decomposition going on in the body, but simply increasing the intake of air or oxygen, as in quickened breathing or deeper inspiration, does not increase correspondingly the rate of oxidation. In other words, it is not a direct combination of oxygen with the carbon and hydrogen of the foodstuffs, or tissue elements, that takes place in the body, but rather a gradual, progressive decomposition of complex organic compounds into simpler products; made possible, however, by the agency of the oxygen carried from the lungs by the circulating blood. It was demonstrated years ago that animals breathing pure oxygen do not consume any more of the gas than when breathing ordinary air, and likewise no more carbon dioxide is produced in the one case than in the other. Fifty years ago, Liebig and other physiologists showed that frogs’ muscle placed in an atmosphere free of oxygen could be made to contract or do work for some considerable time, and with liberation of heat. This fact implies a breaking down of muscle substance into simpler bodies, but there is here no free oxygen to act as the inciting cause; indeed, what actually occurs is a cleavage or splitting up of substances in the muscle tissue, but at the expense of oxygen in some form of combination in the muscle. This oxygen must have been taken from the blood at some previous time and stored in the tissue for future use. Again, as C. Voit has expressed it, if oxygen were really the immediate cause of the decompositions taking place in the organism, we should expect combustion to occur in harmony with the well-known relationship of the three classes of organic foodstuffs to oxygen. In other words, fats would undergo combustion most readily, carbohydrates next, and lastly the nitrogenous or albuminous compounds. In reality, however, proteid matter is decomposed in largest quantity; a generous addition of proteid food is always accompanied by an increased consumption of oxygen. Yet oxygen is not the inciting cause of the proteid decomposition, as is seen from the fact that in muscle work, where the intake of oxygen is greatly increased, there is no noticeable change in the amount of proteid material broken down. Plainly, in the body we have to deal not with a direct oxidation of the complex compounds of the tissues or of the food, but rather with a gradual cleavage of these higher compounds into simpler substances, these latter undergoing progressively a still further breaking down with intake of oxygen. To repeat, oxygen is not the _cause_ of the decompositions within the body, but the extent of the breaking down of