Now it is found that certain chemical processes, mainly oxidations, are taking place in the body. These produce the heat for keeping it warm and also supply other forms of energy,(45) including motion. It is the purpose of oxygen to keep up these oxidations and, by so doing, to aid in supplying the body with energy. It serves this purpose in much the same way that it supports combustion, _i.e._, by uniting with, or oxidizing, materials derived from foods that are present in the cells. *Does Oxygen serve Other Purposes?*—It has been suggested that oxygen may serve the purpose of oxidizing, or destroying, substances that are injurious and of acting, in this way, as a purifying agent in the body. In support of this view is the natural tendency of oxygen to unite with substances and the well-known fact that oxygen is an important natural agent in purifying water. It seems probable, therefore, that it may to a slight extent serve this purpose in the body. It is probable also that oxygen aids through its chemical activity in the formation of compounds which are to become a part of the cells. Both of these uses, however, are of minor importance when compared with _the main use of oxygen_, which _is that of an aid in supplying energy to the body_. *Oxygen and the Maintenance of Life.*—In the supplying of energy to the body, one of the conditions necessary to the maintenance of life is provided. Because oxygen is necessary to this process, and because death quickly results when the supply of it is cut off, oxygen is frequently called the supporter of life. This idea is misleading, for oxygen has no more to do with the maintenance of life than have the food materials with which it unites. Life appears to be more dependent upon oxygen than upon food, simply because the supply of it in the body at any time is exceedingly small. Being continually surrounded by an atmosphere containing free oxygen, the body depends upon this as a constant source of supply, and does not store it up. Food, on the other hand, is taken in excess of the body’s needs and stored in the various tissues, the supply being sufficient to last for several days. When the supply of either oxygen or food is exhausted in the body, life must cease. *The Oxygen Movement a Necessity.*—Since _free_ oxygen is required for keeping up the chemical changes in the cells, and since it ceases to be free as soon as it goes into combination, its continuous movement through the body is a necessity. The oxygen compounds must be removed as fast as formed in order to make room for more free oxygen. This movement has already been studied in connection with the blood and the organs of respiration, but the consideration of certain details has been deferred till now. By what means and in what form is the oxygen passed _to_ and _from_ the cells?