The Urinary System

Asclepiades absurdly believes that the urine passes through the kidneys.

If we are not going to grant the kidneys a faculty for attracting this particular quality, as Hippocrates held, we shall discover no other reason.

The kidneys must attract the urine, or the veins must propel it if it does not move of itself.

But if the veins did exert a propulsive action when they contract, they would squeeze out into the kidneys not merely the urine, but along with it the whole of the blood which they contain.

If this is impossible, as we shall show, the remaining explanation is that the kidneys do exert traction.

How is propulsion by the veins impossible?

The situation of the kidneys is against it.

They do not occupy a position beneath the hollow vein [vena cava] as does the sieve-like [ethmoid] passage in the nose and palate in relation to the surplus matter from the brain.

They are situated on both sides of it.

Besides, if the kidneys are like sieves, and readily let the thinner serous [whey-like] portion through, and keep out the thicker portion, then the whole of the blood contained in the vena cava must go to them, just as the whole of the wine is thrown into the filters.

The example of milk being made into cheese will show clearly what I mean.

For this, too, although it is all thrown into the wicker strainers, does not all percolate through; such part of it as is too fine in proportion to the width of the meshes passes downwards, and this is called whey [serum]; the remaining thick portion which is destined to become cheese cannot get down, since the pores of the strainers will not admit it.

Thus it is that, if the blood-serum has similarly to percolate through the kidneys, the whole of the blood must come to them, and not merely one part of it.

What, then, is the appearance as found on dissection?

One division of the vena cava is carried upwards to the heart, and the other mounts upon the spine and extends along its whole length as far as the legs.

Thus one division does not even come near the kidneys, while the other approaches them but is certainly not inserted into them.

If the blood were destined to be purified by them as if they were sieves, the whole of it would have to fall into them, the thin part being thereafter conveyed downwards, and the thick part retained above.

But this is not so.

The kidneys lie on either side of the vena cava.

They therefore do not act like sieves, filtering fluid sent to them by the vena cava, and themselves contributing no force.

They obviously exert traction; for this is the only remaining alternative.

How, then, do they exert this traction?

Epicurus believes that all attraction happens through the rebounds and entanglements of atoms.

it would be certainly better to maintain that the kidneys have no attractive action at all;

His theory would be much more ridiculous than the theory of the lodestone [magnet], mentioned earlier.

Attraction occurs in the way Hippocrates laid down.

My point is that the secretion of urine is the attraction by the kidneys.

For if it be granted that there is any attractive faculty at all in those things which are governed by Nature, a person who attempted to say anything else about the absorption of nutriment would be considered a fool.