Animals receive these substances into their systems in their food.

A portion of them they retain, re-arranging the components in some cases so as to form new compounds, but still unstable.

These they use in constructing the tissues of the animal system, and some they reserve for future use.

As fast as they require the heat and the force which are stored in them they expend, them, thus recovering the force which was absorbed in the formation of them, and which now, on being released, re-appears in the three forms of _animal heat, muscular motion_, and _cerebral_ or _nervous energy_.
There are other modes besides the processes of animal life by which the reserved force laid up by the vegetable process in these unstable compounds may be released.

In many cases it releases itself under ordinary exposures to the oxygen of the atmosphere.