2 PRINCIPLES OF ANIMAL NUTRITION. 
therefore of potential energy, in the body for the future use of the 
animal itself or of its offspring as being, from a physiological point of 
view, temporary and incidental, the sole useful product of the animal 
is energy. All the physical effect which we can produce, either 
through our own bodies or those of our domestic animals, is simply 
to move something, and moving something is equivalent to the 
exertion of energy. This motion may be the motion of visible 
masses of matter in the performance of useful work or the invisible 
molecular motion of heat, which is economically a waste product, 
but in either case the animal is a source of energy which is imparted 
to its surroundings. From this point of view, then, we may look 
upon the animal as a mechanism for transforming the stored-up 
energy of the sun’s rays, contained in its tissues, into the active or 
“kinetic” forms of heat and motion. The various cells and tissues 
of the living animal body, in the performance of their several func- 
tions, break down and oxidize the proteids, fats, carbohydrates, 
and other materials of which they are composed or which are con- 
tained in them, seizing, as it were, upon the energy thus liberated 
and converting it, here into heat, there into motion, again into the 
energy of chemical change, as the needs of the organism demand. 
The very definition of physical life, then, implies that the living 
animal is constantly consuming its own substance, rejecting the 
simpler compounds which result and giving off energy in the various 
forms characteristic of living beings. Obviously, this process, if 
unchecked, would soon lead to the destruction of the organism and 
the dissipation of its store of potential energy. To prevent this 
catastrophe is the object of the great function of nutrition. 
This function, in its broader outlines, is familiar to us all through 
daily experience and observation. The living animal requires to 
be frequently supplied with certain substances which collectively 
constitute its food. This food contains a great variety of chemical 
ingredients, but much the larger part of it consists of “organic” 
compounds belonging to the three great groups already noted as 
making up the larger share of the organic matter of the body, viz., 
the proteids, the fats, and especially the carbohydrates, and while 
the individual members of these groups differ in the two cases, the 
ingredients of the food, like those of the body, contain a large store 
of potential energy. These and other “organic” substances, 
