rived 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?

*Passage of Oxygen through the Blood.*--In serving its purpose at th