the liquid egesta. The amount of urea excreted daily
by a man is influenced by the activity of his mind, as well as by that
of his body. A man engaged in physical labor wears out more of his body
than one who does no work; and a man occupied in a pursuit involving
intense mental application, consumes a greater proportion of his tissue
than the man who works only with his body.[13] In each of these cases,
there is a different amount of tissue disorganised, and consequently a
demand for different amounts of food, with which to repair the waste.
But all the food consumed by a man is not devoted to the reparation of
the tissue worn out in the operations of thinking and working. A human
being whose mind is a perfect blank, and who performs no bodily work,
excretes a large quantity of urea, the representative of an equivalent
amount of worn-out flesh. In fact the greater part of the food consumed
by a man serves merely to sustain the functions of the body--the
circulation of the blood--the action of the heart--the movements of the
muscles concerned in respiration--in a word, the various motions of the
body which are independent of the will. According to Professor Haughton,
about three-fourths of the food of a working man of 150 lbs. weight, are
used in merely keeping him _alive_, the remaining fourth is expended in
the production of mechanical force, constituting his daily toil.

In the nutrition of the lower animals, as in that of man, the amount of
food made use of by a particular individual depends upon its age, its
weight, the amount of work it performs, and probably its temper. As
three-fourths of the weight of the food of a laboring man are expended
in merely keeping him alive, it is obvious that the withholding of the
remaining fourth would render him incapable of working. An amount of
food which adequately maintains the vital and mechanical powers of three
men, serves merely to keep four alive. It is the same with the horse,
the ox, and every other animal useful to man: each makes use of a
certain amount of food, _for its own purposes_; all that is consumed
beyond that is applied for the benefit of its owner. Let us take the
case of two of our most useful quadrupeds--the horse and the ox. The
horse is used as an immediate source of motive power. For this purpose
food is supplied to it, the greater portion of which is consumed in
keeping the animal alive, and the rest for the development of its motive
power. Abundance