food may contain their nitrogenous constituents in an easily assimilable
state, and their respiratory elements in a nearly indigestible
condition, or _vice versa_, and under these circumstances their
nutritive value would be below that indicated by analysis; but these
points can only be determined by elaborate and long continued feeding
experiments. It is well known, however, that the mechanical state of the
food has a most important influence on its nutritive value. Thus, for
example, the presence of a large quantity of woody fibre protects the
nutritive substances from assimilation, and seeds with hard husks pass
unchanged through the animal, although, so far as their composition
alone is concerned, they may be highly nutritive; and the loss of a
certain quantity of many varieties of food in this way is familiar to
every one.

The proper adjustment of the relative quantities of the great groups of
nutritive elements in the food is a matter the importance of which
cannot be over-rated, for it is in fact the foundation of successful and
economical feeding; and this will be readily understood if we consider
what would be the result of giving to an animal a supply of food
containing a large quantity of nitrogenous and a deficiency of
fat-forming compounds. In such circumstances, the animal must either
languish for want of the latter, or it is forced to supply the defect by
an increased consumption of food, in doing which it must take into the
system a larger quantity of nitrogenous compounds than would otherwise
have been requisite, and in this way the other elements, which are
present in abundance, are wasted, and the theoretical and practical
value of a food so constituted may be very different, and it is only
when the proportions of the different groups are properly attended to
that the most economical result can be obtained. It can scarcely be said
that the experiments yet made by feeders enable us to fix the most
suitable proportion in which those substances can be employed, although
experience has led them to the use of mixtures which are in most cases
theoretically correct; thus they combine oil-cakes or turnips with
straw, which is poor nitrogenous, and rich in fat-forming elements; and
in general it will be found that where different kinds of food are
mixed, the deficiencies of the one are counterbalanced by the other, and
though this has hitherto been done empirically, it cannot be doubted
that as our knowledg