Because their stomachs are divided into three chambers: 1, the paunch, where the unchewed food is stored; 2, the reticulum, where portions of the food are received from the paunch, and moistened and rolled into a "cud," to be sent up and chewed; and 3, the psalterium, which receives the masticated food, and continues the process of digestion.
In quadrupeds the deficiency of teeth is usually compensated by the faculty of rumination. The sheep, deer, and ox tribe, are without fore-teeth in the upper jaw. These ruminate. The horse and ass are furnished with teeth in the upper jaw, and do not ruminate. In the former class, the grass and hay descend into the stomachs nearly in the state in which they are cropped from the pasture, or gathered from the bundle. In the stomach, they are softened by the gastric juice, which in these animals is unusually copious. Thus softened and rendered tender, they are returned a second time to the action of the mouth, where the grinding teeth complete at their leisure the trituration which is necessary; but which was before left imperfect. I say, the trituration which is necessary; for it appears from experiments, that the gastric fluid of sheep, for example, has no effect in digesting plants, unless they have been previously masticated; that it only produces a slight maceration, nearly as common water would do in a like degree of heat; but that when once vegetables are reduced to pieces by mastication, the fluid then exerts upon them its specific operation. Its first effect is to soften them, and to destroy their natural consistency; it then goes on to dissolve them, not sparing even the toughest parts, such as the nerves of the leaves. I think it very probable, that the gratification also of the animal is renewed and prolonged by this faculty. Sheep, deer, and oxen, appear to be in a state of enjoyment whilst they are chewing the cud. It is then, perhaps, that they best relish their food.—Paley.
"I am like a pelican of the wilderness: I am like an owl of the desert. I watch, and am as a sparrow alone upon the house top."—Psalm cii.
CHAPTER LV.
1091. Why do quadrupeds that are vegetable eaters feed so continually?
Because their food contains but a small proportion of nutrition, so that it is necessary to digest a large quantity to obtain sufficient nourishment.