History of All Cloven-Hoofed Quadrupeds

...showing why man vomits with difficulty; since he had previously indicated the construction of the throat or esophagus, he added this as a corollary.
Why vomiting is difficult.

And therefore it is easier to swallow than to vomit, because we swallow with both tunics of the belly operating—the internal one indeed drawing [the food] in, while the external one contracts itself, both of them pushing: but we vomit with only the one that is outside acting, and none drawing inward. Nevertheless, the opinion of [Pietro d'] Abano does not collapse: for although we have asserted that the transverse fibers of the inner tunic of the esophagus itself are stronger than the longitudinal ones in animals of this kind (for they would not be able to devour fodder otherwise), nonetheless, if we should say that the longitudinal and transverse fibers mutually impede one another whenever one works with the other—seeing that the longitudinal cannot draw in if they are pulled apart by the transverse, and likewise the transverse cannot expel if disturbed by the longitudinal—from which struggle, to be sure, a certain third operation is born, which is called neither attraction nor expulsion, but retention. But indeed, if while the longitudinal fibers are operating, the transverse should be idle, the faculty of drawing in will then be able to be exercised; and on the contrary, when the transverse operate, if the longitudinal should rest, the faculty of expelling will at that time be able to perform its duty. It can therefore be said that ruminants swallow nourishment with the longitudinal fibers while the transverse are idle, and on the contrary, they repel it from the belly to the mouth with the transverse operating while the longitudinal are idle. Or perhaps we shall say more correctly that the longitudinal fibers of the inner tunic of the esophagus in ruminants are not only useful for ingesting fodder, as happens in other animals, but also for regurgitating the same. Wherefore that difficulty of vomiting, of which we spoke from Galen, has no place here, since in ruminants the longitudinal fibers are drawn together toward the throat while they regurgitate fodder, just as they are also contracted toward the stomach during ingestion. Nor perhaps shall we depart from the truth if we say that Galen understands by "all fibers" not only the longitudinal and transverse ones, which he had mentioned, but indeed even the oblique ones, which in his opinion (in the same book) are the servants of the retentive faculty. Moreover, it is by no means absurd that contrary motions should occur through the same path or part, if we say they occur at different times (as happens here); and Galen demonstrates this well in these words:
3. de fac. nat. cap. 15.

Now it is not difficult to find that both traction and expulsion occur at different times through the same passage, provided that the stomach's esophagus, which we call the gullet, is seen not only to lead food and drink into itself, but also to perform the contrary service during nausea. Even the bladder which is under the liver, although there is but one neck, nevertheless not only fills but also empties the bladder through itself. Furthermore, the stomach of the womb, which we call the neck, is in like manner the way for the seed inward as it is for the offspring outward.

Moreover, the Philosopher seems again to oppose these things, who teaches that ruminating animals are denied the ability to belch.
Sect. 10. probl. 43. Ruminants do not belch.

For if they cannot belch, neither will they be able to eructate—that is, to cast out by eructating. Where note that the word "to belch" indicates an effort of the esophagus by which it strives to propel something from the belly to the mouth: and that which is rejected is either something flatulent and spiritual, or something solid. In ruminants, the spirit is by no means brought out, but the nourishment is vomited back to the mouth; for which reason the Philosopher also says there:
Lib. 6. cap. 1. What belching in an ox indicates.

Furthermore, the ability to belch is denied to ruminants, because they contain several bellies, and that which we call the reticulum. For the faculty of passing through, both above and below, is provided in many ways for the winds: and so the moisture is consumed before it is turned into wind, which wind moves either a belch or a fart.

Therefore, when Columella, most learned in rural affairs, reports that the ox frequently belches, he does not oppose Aristotle. For the latter speaks of healthy animals, the former of sick ones: a fact which our own farmers also confirm; for if any ox should belch while fasting, they assert it to be a pathognomonic sign. Just as it is established, therefore, that the fodder in ruminants is repelled to the mouth through the same passage by which it had descended, so that by rumination—that is, by a new grinding—it may be more exactly diminished and crushed; so now it must be observed, after it has been crushed, through what passage and to which belly it is carried. We have said, according to the opinion of Aristotle, that the chewed food is sent from one belly into another, until it glides from the last into the intestines: so that it is prepared in the first, begins to be digested in the second, is still cooked in the third, and in the last attains perfect concoction. But Galen, as cited by Mercuriali in the place mentioned before, says that the food is first vomited back from the ventricle into the mouth: then from the mouth it is sent into the reticulum, thence into the omasum, and finally into the abomasum.
Li. de administ. anat. c. 3

And lest anyone doubt (says Mercuriali) how it glides into the reticulum the second time but not the first, it must be known that the opening in the esophagus which reaches to the reticulum is narrow enough that the food, when it is coarser and more solid the first time, cannot yet pass through; but it passes the second time when it has been made so liquid and soft that it is now able to pass through.

Nevertheless, he thinks it is possible to reconcile Aristotle and Galen if we say (he says) that the former speaks of the time when they do not ruminate—namely, when the food, having first entered the belly, enters from there through the common opening into the reticulum, and thus into the other bellies, without returning to the mouth; but that Galen's discourse concerns the time of rumination, which, as we shall say, is by no means perpetual.
Aristotle & Galen.

And this is only to be understood of those ruminants which have several bellies. But if there be any that ruminate and have a single belly, as Aristotle relates concerning the Pontic mouse, and as is held in Leviticus concerning the hare and the rabbit: without doubt the chewed food must be sent back again to the reticulum. These things, then, according to Mercuriali. Indeed, the Philosopher [was] not [speaking] of those having one belly...