is not what we ourselves say; for just as we take in a breath at one moment and give it out again at another, so at one time the liver draws nutriment from the stomach, and at another the stomach from the liver. But if your expression "at once" means that in one and the same animal a single organ subserves the transport of matter in opposite directions, and if it is this which disturbs you, consider inspiration and expiration. For of course these also take place through the same organs, albeit they differ in their manner of movement, and in the way in which the matter is conveyed through them. Now the lungs, the thorax, the arteries rough and smooth, the heart, the mouth, and the nostrils reverse their movements at very short intervals and change the direction of the matters they contain. On the other hand, the veins which pass down from the liver to the intestines and stomach reverse the direction of their movements not at such short intervals, but sometimes once in many days. The whole matter, in fact, is as follows:--Each of the organs draws into itself the nutriment alongside it, and devours all the useful fluid in it, until it is thoroughly satisfied; this nutriment, as I have already shown, it stores up in itself, afterwards making it adhere and then assimilating it--that is, it becomes nourished by it. For it has been demonstrated with sufficient clearness already[378] that there is something which necessarily precedes actual nutrition, namely _adhesion_, and that before this again comes _presentation_. Thus as in the case of the _animals_ themselves the end of eating is that the stomach should be filled, similarly in the case of each of the _parts_, the end of presentation is the filling of this part with its appropriate liquid. Since, therefore, every part has, like the stomach, a _craving_[379] to be nourished, it too envelops its nutriment and clasps it all round as the stomach does. And this [action of the stomach], as has been already said, is necessarily followed by the digestion of the food, although it is not to make it suitable for the other parts that the stomach contracts upon it; if it did so, it would no longer be a physiological organ,[380] but an animal possessing reason and intelligence, with the power of choosing the better [of two alternatives].