s are: ptyolin of the
saliva, pepsin of the stomach, and pancreatin and diastase of the
intestines. In order that these ferments may carry on their work in a
normal way, the acidity and alkalinity of the different parts of the
digestive tract must be maintained. The gastric juice contains from 0.1
to 0.25 per cent of hydrochloric acid, imparting mildly antiseptic
properties; and while the peptic ferment works in a slightly acid
solution, the tryptic ferment requires an alkaline solution. To secrete
the necessary amount and quality of digestive fluids, the organs must be
in a healthy condition. Many erroneous ideas regarding the digestion of
foods are based upon misinterpretation of facts by persons suffering
from impaired digestion, and attempts are frequently made to apply to
normal digestion generalizations applicable only to diseased conditions.

229. Digestibility of Animal Foods.--The proteids and fats in animal
foods, as meats, are more completely digested than the same class of
nutrients in vegetables. In general, about 95 per cent of the proteids
of meats is digestible, while those in vegetables are often less than 85
percent digestible. The amount of indigestible residue from animal foods
is small; while from vegetables it is large, for the cellulose prevents
complete absorption of the nutrients and, as a result, there is much
indigestible residue. Animal foods are concentrated, in that they
furnish large amounts of nutrients in digestible forms. There is less
difference in the completeness with which various meats are digested
than in their ease of digestion; the proteins all have about the same
digestion coefficients, but vary with individuals as to ease of
digestion and time required. It is generally considered that the
digestible proteins, whether of animal or vegetable origin, are equally
valuable for food purposes. This is an assumption, however, that has not
been well established by experimental evidence. In a mixed ration, the
proteins from different sources appear to have the same nutritive value,
but as each is composed of different radicals and separated into
dissimilar elementary compounds during the process of digestion, they
would not necessarily all have the same food value.

There is but little difference between the fats and proteins of meats as
to completeness of digestion,--the slight difference being in favor of
the proteins. Some physiologists claim that the fat, which in some meats
surrou