able form must be provided as an essential condition of the manifestations of life. It must not, on the other hand, be assumed that the potential energy of foods may be considered as a reliable index of their physiological value. Biological processes are exceedingly complex, and, in calling attention to energy as a dominant factor in vital activities, we do not lose sight of other important considerations which can not here be noticed.
Protean transformations of energy are constantly carried on in all the metabolic tissues. The energy expended in building organic substance in animals, as in plants, is stored up in the form of potential energy as an essential condition of its constitution, and it is again liberated in the form of heat in the correlative processes of destructive metabolism which are taking place without cessation in the work performed in every operation of the system.
Dr. Foster tells us that what is really meant by the phrase, "living substance, is not a thing, or body, of a particular chemical composition, but matter undergoing a series of changes." These metabolic changes are brought about, in the main, at the expense of energy, and they represent in fact successive transformations of energy from the active to the potential form, and a final reconversion to heat, which leaves the body in various ways.
The animal machine is in effect a heat-engine that is constantly being worn out by the work performed, and as constantly repaired by its own processes of nutrition, and the heat leaving the body (animal heat) represents the energy that has been used in internal work, and finally liberated through the agency of destructive metabolism.
We must not, however, carry the analogy of the heat-engine so far as to assume that the food consumed by animals is disposed of by a process of combustion, like the fuel burned under a steamboiler. There is no evidence that anything like a combustive oxidation of the food constituents, or of the tissues, takes place in the animal economy. The building of organic substance and storing of potential energy (constructive metabolism) is accompanied by parallel processes of disintegration (destructive metabolism), in which the stored potential energy is changed to heat; and these alternate, or possibly simultaneous, transformations of energy which take place in living tissues must be regarded as manifestations of vital activities that differ widely in their characteristic features from the processes of combustive oxidation that take place in non-living matter.
From what is now known in regard to animal physics it will be safe to assume that from four fifths to five sixths of the potential energy of the food consumed and digested by working animals is expended in vaporizing the water thrown off by the