On Breath

​5 On the other hand, the spirit from respiration is carried into the belly, not through the esophagus (for this is impossible), but there is a passage along the spine, through which the spirit by respiration is carried from the windpipe into the belly and back out; and this is clear to sensation. The matters concerning sensation also present a difficulty. For if the artery alone has sensation, is it by the spirit that passes through it, or by its bulk, or by its body? Or, if the air is primarily under the soul, is it by that which is more authoritative and prior? What, then, is the soul? They say it is a power which is the cause of such movement. Or is it clear that you would not rightly censure those who [posit] the rational and the spirited parts? For these people also speak of them as powers. But if indeed the soul is in this air, this at least is common. Or, if it is ensouled or the soul itself, when it is affected in some way and altered, is it reasonably carried toward what is akin, and does like increase by like? Or is it not so? For the whole is not air, but air is something that contributes toward this power. Or is it not so, but that which produces this, and that which has produced it, is the origin and hypothesis? In those that do not respire, is it so that it is unmixed with the outside? Or is it not so, but is it mixed in another way? What, then, is the difference between the air in the artery and the outside air? For it is reasonable, and perhaps even necessary, that they differ in fineness. But furthermore, is it warm by itself, or by something else? For the inside air seems to be just like the outside; and it is aided by cooling. But which of the two is it? For outside it is mild, but when enclosed it becomes spirit, as if condensed and somehow distributed. Or must it receive some mixture, since it moves about in moisture and bodily bulks? In that case, it is not finest, if indeed it is mixed. And indeed it is reasonable that it is the first receptive of soul, unless indeed the soul itself is also of such a nature, and not something pure and unmixed. [It is said that] the artery alone is receptive of spirit, but the tendon is not. And they differ also in that the tendon has tension, whereas the artery is quickly ruptured, just like the vein. And the skin consists of vein, tendon, and artery; of vein because when pricked it yields blood, of tendon because it has tension, and of artery because it has perspiration. For the artery alone is receptive of spirit. And the veins have passages in which the heat, as if in a bronze vessel, warms the blood; for by nature it is not warm, but is liquefied like fusible substances. For which reason also the artery solidifies, and has moisture both in itself and in the membranes enclosing its cavity. And this is clear both from dissections and because the veins and the arteries connect into the intestine and the belly, which are likely to draw the nourishment. And from the veins nourishment is distributed to the flesh, not from the sides but from the mouth, like pipes. For thin veins extend from the lateral veins out of the great vein and the artery along each rib, and an artery and a vein lie alongside each other. And the bones also bind the tendons and the veins both in their interior and at the joints of their heads, through which fish receive nourishment and respire; for if they did not respire, they would die immediately when taken out of the liquid. And that the veins and the arteries connect with each other is clear also to sensation. And this would not happen unless the liquid also required spirit and the spirit required liquid, because there is heat in tendon, artery, and vein, and that in the tendon is warmest and as it were most vein-like. It is absurd, then, for the heat to be in the place of the spirit, especially if it is for the sake of cooling. But if the heat produces and as it were rekindles the heat by means of heat, it might occur. Furthermore, in all things that have connate heat, how is there permanence when there is nothing opposing or cooling them? For that all things require cooling is nearly clear from the fact that the blood holds the heat in the vein, as if covering it; wherefore also when it flows out, [the heat] escapes and they die, because the liver does not have any artery.