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ON THE NATURAL FACULTIES Book I
[p. 91]exert traction.
And 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;
Nasal mucus was supposed to be the non-utilizable part of the nutriment conveyed to the brain. cf. p. 214, note 3.
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. Further, 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
He means from its origin in th liver (i.e.in the three hepatic veins). His idea was that the upper division took nutriment to heart, lungs, head, etc., and the lower division to lower part of body. On the relation of right auricle to vena cava and right ventricle, cf. p. 321, notes 4 and 5.
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