To Liebig (1842) the next great advance was due. This phenomenally clear-minded man, while recognizing at their full value the fundamental theories advanced by Lavoisier, saw and fully appreciated their incompleteness, and he likewise understood their failure to explain many of the phenomena of life more familiar to the physiological mind than to that of a simple chemist like Lavoisier. Liebig had made a special study of the chemical composition of foodstuffs, and likewise of the tissues and organs of the body. He had, moreover, given great attention to the decomposition products formed in the body, especially the nitrogenous substances excreted through the kidneys, as well as the carbon dioxide and water passed out through the lungs and skin. It was not strange, therefore, that he should take exception to Lavoisier’s view that oxidation in the body consisted in the combustion of a fluid, rich in carbon and hydrogen, which was brought to the lungs. On the contrary, Liebig contended that it was the organic compounds, proteids, fats, and carbohydrates, that underwent oxidation, and not necessarily in the lungs, but all over the body, wherever organs and tissues were active. Especially noteworthy was the view advanced by Liebig, and upheld for many years, that of these three classes of compounds the proteids alone served for the construction of organized tissues, like muscle, and that in the activity of this tissue, as in muscle contraction or muscle work, the energy for the work was derived solely from the breaking down or oxidation of this organized proteid. On this ground he termed the proteid foodstuffs “plastic,” or tissue-building foods. Liebig further pointed out that the substances of the body have the power of combining with and holding on to the inspired oxygen, and that fats and carbohydrates, _i. e._, the non-nitrogenous compounds, easily undergo oxidation or combustion, and thereby furnish the heat of the body. For this reason he termed the corresponding foodstuffs “respiratory” foods. Proteids, on the other hand, according to Liebig’s view, are capable of combustion only in slight degree. The cause of the decomposition of proteid substances in the body was to be traced solely to muscle work, _i. e._, the energy of muscle contraction, or muscle work, was derived from the breaking down of the proteids of the muscle tissue, and work was the stimulus which brought about proteid decomposition. Non-nitrogenous substances played no part in these reactions; muscle work was without influence on these compounds, oxygen being the sole stimulus which led to their combustion, and heat was the sole product of the combustion.