For these reasons, and from the inspection of the parts, it seems easy to conceive how chewing the cud is effected, and why the horse neither ruminates nor vomits. Chewing the cud is but a vomiting without straining, occasioned by the re-action of the first stomach upon what it contains. The ox fills his two first stomachs, or portions of the paunch. This membrane acts with force on the food it contains; it is chewed but a little, and its quantity is greatly increased by fermentation. Were the food liquid, this force of contraction would occasion it to pass into the third stomach, which communicates with the other by a narrow conveyance, the orifice of which is situated in the posterior part of the first, and almost as high as the oesophagus; thus this conduit cannot admit the food, until it has become somewhat fluid. The dry parts, must, therefore, rise up again into the oesophagus, the orifice of which is larger than that of the conduit; in fact, they go up again into the mouth, and the animal again chews and macerates them, imbibes them afresh with its saliva, and thus by degrees liquefies them sufficient to pass into the third stomach, where it is again macerated before it goes into the fourth; and it is in this last stomach that the decomposition of the hay is finished, which is there reduced to a perfect mucilage.
What chiefly confirms the truth of this explanation is, that as long as the animals suck, or are fed with milk and other liquid aliments, they do not chew the cud; and that they chew the cud much more in winter, when they are fed with dry food, than in summer, when they eat tender grass. In the horse, on the contrary, the stomach is small, the orifice of the oesophagus is narrow; and that of the pylorus very large. This alone would render chewing the cud impossible, for the food contained in this little stomach, though perhaps more strongly compressed than in the stomach of the ox, does not mount upwards, since it can easily descend through the pylorus, which is very large; and it is not necessary that the hay should be reduced to a soft running paste, because the force of the contraction of the stomach pushes the aliment through when almost dry.
It is by this difference, then, that the ox chews the cud, and that the horse cannot perform this operation. But there is still another difference in the horse, which hinders him from chewing the cud, and is the reason why he cannot vomit; the passage of the oesophagus being placed obliquely in the stomach, the membranes of which are very thick, makes a kind of gutter in them so oblique that it must close still more instead of opening by the convulsive motions of the stomach. Although this difference, as well as many others we observe in the conformation of the bodies of these animals, depend on their constant nature, nevertheless, there are in the development, more particularly in the soft parts, differences constantly in appearance, but which may, and actually do, vary from circumstances. The vast capaciousness of the ox's paunch, for example, is not entirely owing to Nature; it is not of that size in its primitive conformation, but attains it by degrees, from the large quantity of aliment it receives; for, in the calf, which is not very young, but has eat no grass, the paunch is much smaller in proportion than in the ox. This capaciousness of the paunch proceeds, then, from the extension which is occasioned by the large quantity of aliments, of which I was well convinced by an experiment that appeared to me decisive. I brought up two lambs of the same age, one on bread, the other on grass, and when they were a year old, on opening them, I found the paunch of the lamb which had lived on grass was much larger than that which had lived on bread.