in such a case to be exactly equal to the food. If the animal be
deprived of nutriment, it immediately begins to lose weight, because its
functions must continue--carbon must still be converted into carbonic
acid to maintain respiration--and the excretions be eliminated, although
diminished in quantity, because they no longer contain the undigested
portion of the daily food, and the substances already stored up in the
body are consumed to maintain the functions of life. Universal
experience has shewn that, under such circumstances, the fat which has
accumulated in various parts of the body disappears, and the animal
becomes lean; but it is less generally recognised that the muscular
flesh, that is the lean part of the body, also diminishes, although it
is sufficiently indicated by the fact that nitrogen still continues to
be found in the urine, and that the animal becomes feeble and incapable
of muscular exertion. Respiration and secretion, in fact, proceed quite
irrespective of the food, which is only required to repair the loss they
occasion. When the course of events within the animal body is traced, it
is found to be somewhat as follows: The food consumed is digested and
absorbed into the blood, where it undergoes a series of complicated
changes, as a consequence of which part of it is converted into carbonic
acid, and eliminated by the lungs, and part is deposited in the tissues
as fat and flesh. After the lapse of a certain period, longer or shorter
according to circumstances, a new set of actions comes into play, by
which the complex constituents of the tissues are resolved into simpler
substances, and excreted chiefly by the lungs and kidneys. The changes
thus produced are, to a great extent, identical with those which would
take place if the fat and flesh were consumed in a fire; and the animal
frame may, in a certain sense, be compared to a furnace, in which, by
the daily consumption of a certain quantity of fuel and air inhaled in
the process of respiration, its temperature is maintained above that of
the surrounding atmosphere. If the daily supply of fuel, that is of
food, be properly adjusted to the loss by combustion, the weight of the
animal remains constant; if it be reduced below this quantity, it
diminishes; but if it be increased, the stomach either refuses to
digest and assimilate the excess, or it is absorbed and stored up in the
body, increasing both the fat and flesh.

When an animal is fed in suc