water and carbon dioxide, which are easily disposed of. The changes which
the proteids undergo in the body are very complicated. There is ample
provision in the body for their digestion, metabolism, and final
rejection, when taken in moderate quantity, as is the case in a dietary of
vegetables. The proteids in the human body, after fulfilling their
purpose, are in part expelled in the same way as the carbohydrates; but
the principal part, including all the nitrogen, is expelled by the kidneys
in the form of urea (a very soluble substance), and a small quantity of
uric acid in the form of quadurates.

There is reciprocity between the teeth and digestive organs of animals and
their natural food. The grasses, leaves, &c., which are consumed by the
herbivora, contain a large proportion of cellulose and woody tissue.
Consequently, the food is bulky; it is but slowly disintegrated and the
nutritious matter liberated and digested. The cellulose appears but
slightly acted upon by the digestive juices. The herbivora possess
capacious stomachs and the intestines are very long. The carnivora have
simpler digestive organs and short intestines. Even they consume
substances which leave much indigestible residue, such as skin, ligaments
and bones, but civilised man, when living on a flesh dietary removes as
much of such things as possible. The monkeys, apes, and man (comprised in
the order _Primates_) have a digestive canal intermediate in complexity
and in length to the herbivora and carnivora. A certain quantity of
indigestible matter is necessary for exciting peristaltic action of the
bowels. The carnivora with their short intestinal canal need the least,
the frugivora more, and the herbivora a much larger quantity. The
consumption by man of what is commonly called concentrated food is the
cause of the constipation to which flesh-eating nations are subject. Most
of the pills and other nostrums which are used in enormous quantities
contain aloes or other drugs which stimulate the action of the intestines.