Supposing that a horse has a full supply of water and oxygen, we may regard the nitrogenous matter, carbohydrates, fat, and mineral matter of his food as its nutritive constituents. If when comparing two foods, we find that each of them is richer than the other in different constituents, it will be impossible for us to say which has the higher nutritive value; because these respective constituents have their own special work to perform, and their importance to the body varies according to circumstances, such as those of work, age and climate. We cannot therefore express the nutritive value of a food by a formula; but must be content to estimate it by general considerations.