ON THE NATURAL FACULTIES Book I

 [p. 101]flow into a vacuum than has run out. For, if no other factor comes into operation

Vital factor necessary over and above the mechanical.

save only this tendency by which a vacuum becomes refilled, no more could ever flow in than had been evacuated. Nor, could he suggest any other plausible cause, such, for example, as the of nutriment by the stomach

cf. p. 199, note 2.

which occurs in the process of anadosis; this had been entirely disproved in the case of blood in the vena cava;

pp. 91, 93.

it is excluded, not merely owing to the long distance, but also from the fact that the overlying heart, at each diastole, robs the vena cava by violence of a considerable quantity of blood.

In relation to the lower part of the vena cava

i.e. the part below the liver; cf. p. 91, note 2.

there would still remain, solitary and abandoned, the specious theory concerning the filling of a vacuum. This, however, is deprived of plausibility by the fact that people die of retention of urine, and also, no less, by the situation of the kidneys. For, if the whole of the blood were carried to the kidneys, one might properly maintain that it all undergoes purification there. But, as a matter of fact, the whole of it does not go to them, but only so much as can be contained in the veins going to the kidneys;

Renal veins.

this portion only, therefore, will be purified. Further, the thin serous part of this will pass through the kidneys as if through a sieve, while the thick sanguineous portion remaining in the veins will obstruct the blood flowing in from behind; this will first, therefore, have to run back to the vena cava, and so to empty the veins going to the kidneys; these veins will no longer be able to