Although the presence of a sufficient quantity of nutritive compounds in the food is necessarily the fundamental matter for consideration, its bulk is scarcely less important. The function of digestion requires that the food shall properly fill the stomach, and however large the supply of nutritive matters may be, their effect is imperfectly brought out if the food is too small in bulk, and it actually may become more valuable if diluted with woody fibre, or some other inert substance. At first sight this may appear at variance with the observations already made as to the effects of woody fibre in protecting the nutritive matters from absorption; but practically there are two opposite evils to be contended against, a food having too small a bulk, or one containing so large a proportion of inert substances as to become disadvantageously voluminous. The most favourable condition lies between the two extremes, and the natural food of all herbivorous animals is diluted with a certain amount of woody fibre. When these are replaced by substances containing a large quantity of nutritive matters in a small bulk, the result is that the natural instinct of the animal causes it to continue feeding until the stomach is properly distended, and it consequently consumes a much larger quantity of food than it is capable of digesting, and a more or less considerable quantity passes unchanged through the intestines, and is lost. On the other hand, if the food be too bulky, the sense of repletion causes the animal to cease eating long before it has obtained a sufficient supply of nutritive matter. It is most necessary, therefore, to study the mixture of different kinds of food, so as to obtain a proper relation between the bulk and the nutritive matters contained in the mixture; and on examining the nature of the mixed foods most in vogue among feeders, it will be found that a very bulky food is usually conjoined with another of opposite qualities. Hence it is that turnips, the most voluminous of all foods, are used along with oil-cake and bean-meal, and if from any circumstances it becomes necessary to replace a large amount of the former by either of the latter substances, the deficient bulk must be replaced by hay or straw.