Anatomy of a venous artery

The beating vessels, that is, the arteries, all but one, are created from two membranes. The most durable of them is the internal one, for it is it that takes on the beating and strong movement of the pneuma substance and is intended to protect and preserve its substance and strengthen the blood vessels containing pneuma. The place where the arteries begin is the left cavity of the two cavities of the heart, for its right cavity is closer to the liver and therefore should have been made occupied with attracting nutrients and using them.

The first thing that grows from the left cavity of the heart are two arteries. One of them goes to the lung and divides in it to draw out air and bring blood from the heart to the lung, which nourishes the lung. The fact is that the nutrition of the lung passes through the heart and from the heart enters the lung. This part of the artery grows from the thinnest part of the heart, where the veins pass through the heart. This artery, unlike other arteries, consists of a single layer and is therefore called a venous artery. It is created from one layer only in order to be softer, more pliable and more obedient during expansion and contraction and to more easily allow liquid vaporous blood seeping from it into the lung, corresponding to the substance of the lung, and approaching full maturity in the heart, so that it does not need further ripening, like blood flowing in the vena cava, which we will describe later. This artery is arranged in this way, in particular, because its place is close to the heart, and the heart easily transmits its force, hot and ripe, to it. In addition, the organ in which this artery pulsates is a loose organ, and there is nothing to fear that the artery, hitting this loose substance during its beating, will suffer from its hardness. Therefore, it became possible to do without compaction of her body, which could not be done without in relation to the arteries of other, dense organs adjacent to her.

As for the venous artery, which we will talk about later, although it is located adjacent to the lung, it is in contact only with its posterior lobe, adjacent to the spine, and this venous artery diverges only in the anterior lobe of the lung and deepens into it, divided into parts and branches.

If we compare the degree of this artery's need for strength with the degree of its need for pliability, which facilitates expansion and contraction, as well as the seepage of what leaks through it, then it turns out that the need for pliability is more urgent than the need for strengthening and compaction.

As for the other artery, namely the large one, which Aristotle calls the aorta, then, having begun to grow from the heart, it sends out two branches. The larger of them passes around the heart and branches in its parts, and the smaller goes around the right cavity and divides in it. As for the part that remains after the branches, then, having separated, it is divided into two branches. One branch, the larger one, is adapted for descent, the other, smaller, is adapted for ascent. The part adapted for descent is therefore created superior to the other in size, because it is directed to organs that are greater in number and largest in size, namely, to the organs lying below the heart.

At the exit of the aorta there are three dense membranes facing from the inside to the outside. If there were one or two of them, they would not achieve the required utility except by increasing the size of one or two membranes, but then it would be difficult for them to move. And if there were four of these membranes, they would undoubtedly be very small and their usefulness would disappear; if their size were large, they would narrow the passages.

As for the arterial vein, it has two membranes facing inward; their number is limited to two, for here there is no need for a strong barrier, as there, but here there is a great need for pliability, so that the smoky steam and blood going to the lungs can easily flow out.