Hence it appears that the pig makes a better use of its food than the sheep, retaining twice as much of its solid constituents within the body, from which may be deduced the important practical conclusion, that the former must be fattened at a much smaller cost than the latter. Looking at the individual constituents, it appears that, in the sheep, less than one-twentieth of the nitrogenous compounds, and one-tenth of the non-nitrogenous substances contained in the food, remain in the body; and a knowledge of these facts tends to modify the conclusions which might be drawn from the composition of the increase in the fattening animal. Its influence may be best illustrated by a particular example. If, for instance, the increase in a sheep contained its nitrogenous and respiratory elements in the ratio of 1 to 10, it would be totally incorrect to supply these substances in the food in the same proportions. On the contrary, it would be necessary at the very least to double the proportion of the former, because one-tenth of the fat-forming elements are absorbed, and only one-twentieth of the nitrogenous. On further consideration, also, it seems unquestionable that the quantity of the nutritive elements stored up must depend to a large extent on the nature of the food and the particular state in which they exist in it. It is probable, or at least possible, that some kinds of 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.