the tissue or food material is the determining factor in the amount of oxygen taken on and used up. The products of decomposition contain more oxygen than the original substances undergoing the breaking down process, which means that oxygen is taken from the blood and used in the physiological combustion that is going on. It is not, however, strictly a combustion process; it is more complicated and more gradual than ordinary combustion, involving first of all a series of what may be termed oxidative cleavages, in which large molecules are gradually, step by step, broken down into simpler molecules, and these latter then oxidized to still simpler forms. Hence, we find the oxidative changes preceded by a variety of alterations in which oxygen may take no part whatever; such as hydrolytic cleavage, where the elements of water are taken on as a necessary step in the cleavage process; dissociation of a simple sort, as when a large molecule breaks up directly into smaller molecules, etc. These statements by no means detract from the importance of oxygen in the katabolic processes of the body, but it is physiological oxidation that we have to do with, and not simple combustion. Oxygen is not the direct cause of the transformations taking place in the body. As one looks over the history of progress in our knowledge of nutrition from the time of Lavoisier to the present, it is easy to note the gradual change of view regarding oxidation in the living organism. Step by step, it has been demonstrated that there are many factors involved in this breaking down of complex substances; that while oxygen is an ever present requirement, there are other equally important factors to be taken into account. The contrast between the older views and those now current is clearly shown by the difference in attitude regarding the _place_ in the body where oxidation occurs. Thus, in the earlier days, when the view was gradually gaining ground that nutritional changes were mainly the result of oxidation, and that the oxygen drawn into the lungs in inspiration was a primary factor, then, as we have seen, the lungs were considered as the laboratory where the transformation takes place. This view, however, was soon exploded, and next we find the blood, the lymph, and other fluids, but especially the blood, looked on as the locality where oxidation occurs. This was indeed quite a natural view to hold, since the blood is the carrier of oxygen, but we now know, in harmony with the fact that the breaking down of complex food material is a complicated process, involving various kinds of chemical change, that these katabolic processes are not located in any one place, but occur all over the body wherever there are active tissues. As has been previously stated, the human body is a “nation” of cells, all of which are more or less active, and it is in these miniature laboratories mainly that oxidation and all the other nutritional changes coincident to life take place.