Suppose you fill any animal whatsoever with liquid food--an experiment I have often carried out in pigs, to whom I give a sort of mess of wheaten flour and water, thereafter cutting them open after three or four hours; if you will do this yourself, you will find the food still in the stomach. For it is not _chylification_[314] which determines the length of its stay here--since this can also be effected outside the stomach; the determining factor is _digestion_[315] which is a different thing from chylification, as are blood-production and nutrition. For, just as it has been shown[316] that these two processes depend upon a _change of qualities_, similarly also the digestion of food in the stomach involves a transmutation of it into the quality proper to that which is receiving nourishment.[317] Then, when it is completely digested, the lower outlet opens and the food is quickly ejected through it, even if there should be amongst it abundance of stones, bones, grape-pips, or other things which cannot be reduced to chyle. And you may observe this yourself in an animal, if you will try to hit upon the time at which the descent of food from the stomach takes place. But even if you should fail to discover the time, and nothing was yet passing down, and the food was still undergoing digestion in the stomach, still even then you would find dissection not without its uses. You will observe, as we have just said, that the pylorus is accurately closed, and that the whole stomach is in a state of contraction upon the food very much as the womb contacts upon the foetus. For it is never possible to find a vacant space in the uterus, the stomach, or in either of the two bladders--that is, either in that called bile-receiving[318] or in the other; whether their contents be abundant or scanty, their cavities are seen to be replete and full, owing to the fact that their coats contract constantly upon the contents--so long, at least, as the animal is in a natural condition. Now Erasistratus for some reason declares that it is the contractions[319] of the stomach which are the cause of everything--that is to say, of the softening of the food,[320] the removal of waste matter, and the absorption of the food when chylified [emulsified]. Now I have personally, on countless occasions, divided the peritoneum of a still living animal and have always found all _the intestines_ contracting peristaltically[321] upon their contents. The condition of _the stomach_, however, is found less simple; as regards the substances freshly swallowed, it had grasped these accurately both above and below, in fact at every point, and was as devoid of movement as though it had grown round and become united with the food.[322] At the same time I found the pylorus persistently closed and accurately shut, like the os uteri on the foetus.