Amounts of oxygen and carbon dioxide were determined. This led Ludwig (cited by Lusk) to the mistaken assumption that oxidations occurred in the blood. Since Eduard Pflüger, we know that the site of combustion is the living cell. The next step was to identify the materials that burn. It is not carbon and hydrogen, as was originally assumed, but rather, as Liebig first suspected and Magendie experimentally demonstrated, proteins, carbohydrates, and fats from food. Liebig's original assumption that protein originated from the working muscle could no longer be maintained. With this finding, the second crucial foundation for metabolic research was established. It then divided into two tasks: the investigation of respiratory metabolism and the materially oriented study of intermediate transformations in the body.