How do the arteries and veins spread out their branches in all parts of the seed?

The more blood is produced in the heart, the greater the force with which it expands, and in this way it advances further.

It can only advance thus towards the places where there are parts of the seed that are disposed to make room for them, and then towards the heart via the vein joined to the artery by which this blood arrives, because they cannot take any other route than this.

Two new small branches are thus formed, one in the vein and the other in the artery whose extremities are joined, and which go together to occupy the place vacated by these small parts of the seed.

This makes the branches that have already been formed stretch out as far as this, for unless this happened their extremities would separate.

This occurs above all because all the tiny parts of the seed are suited to flowing thus towards the heart – or at least, if there are some that are not suited, they are easily pushed back towards the surface, so that there are none below this surface in the area where the spirits spread out that do not in their turn proceed to the heart.

This is why the veins and the arteries extend their branches equally there in every direction.

This happens even if we do not usually see as many arteries as veins in the bodies of animals.

Veins appear so much more numerous than arteries because the blood usually comes to rest in the small veins as well as in the large ones even after the animal is dead. This is because the whole membrane around them contracts almost uniformly.

The blood in the arteries, on the other hand, never comes to rest in their small branches, for being pushed by the diastole, it moves quickly in the veins, otherwise it falls back into the largest arteries at the moment of systole, because their tubes remain open.

Thus, their smallest branches cannot be seen, any more than can the white veins, called ‘lacteous veins’. Aselli discovered this a short time ago in the mesentery, where one would only observe them if one opened up a living animal some time after it had eaten.

We can yet consider here more particularly the distribution of the principal veins and arteries, because it relies on the movement of the blood and the spirits.

Thus, the first agitation of the heart, which had still only begun to form, caused the tiny parts of the seed closest to it to flow to the openings in its ventricles.

By these means were formed what are called the ‘coronary’ arteries and veins, because they completely surround it like a garland [Lat: corona].

There is only one coronary vein, even though there are 2 arteries.

This single vein can have enough branches for it to be joined to all the ends of the branches of the 2 arteries.

The tiny parts of the seed which come from all around the heart have taken their course towards a single spot in order to enter its right ventricle, at the same time as the blood leaving the left ventricle has taken its course through two different places in order to occupy the spot vacated by them.

When the expanded blood in the heart has left it, and has taken its course in a straight line, it first pushes a large enough portion of the seed a little further than it was, towards the top of the womb.

Through this, the other parts of the seed below this portion have been forced to descend towards the sides, which has brought it about that those towards the sides flowed from there towards the heart.

Thus these large veins and arteries, which nourish the arms of humans, or the front legs of brute animals, or finally the wings of birds, have begun to form.

The portion of the seed from which the head will be formed, pushed thus by the blood that comes from the heart, is made a little more solid at its surface than inside it. This is because it has been squeezed on the one side by the blood which pushes it, and on all the others by the rest of the seed which it pushes.

This is why:

• this blood cannot at first penetrate as far as the centre
• the spirits alone enter there, where they form the space in the head

These spirits take their course from the middle of the head towards 3 different sides:

1. Towards the back where they trace the spinal column
2. Towards the shoulder
3. Towards the left and right front sides

The matter whose place they took has been able to be drawn towards the top of the skull, in the 3 spaces that the 3 sides mark out.

From there, taking its course through the two sides of the spinal column towards the heart, it makes room for the 3 principal branches of the great ‘triangular vessel’ that is between the folds of membrane that envelop the brain, and which has the characteristic that it brings together the functions of the artery and the vein.

For the matter that was in that place, being pushed by the spirits, leaves there so easily and quickly that the branches of the arteries joined to the branches of the veins through which it flows towards the heart, are merged with these in forming this vessel, which afterwards extends its tiny channels on all sides inside the skull, so that it alone provides almost all the nourishment to the brain.

Nevertheless, the blood in the principal tube of the aorta, which comes in a straight line from the heart, cannot penetrate the base of the head at first.

This is because the tiny parts of the seed are too closely packed there and exactly below a spot where afterwards a gland will be formed, which physicians have supposed serves only to receive the pituita from the brain.

It exerts itself everywhere against the small parts of the seed, which resist it, and gradually drives some out, which flow from the side towards the veins sufficiently distant from there.

By these means are formed those tiny branches of the arteries called the Rets admirabilus, which are more easily observed in animals than in humans, and which seem not to be joined to the veins.

Next, it was also raised higher towards the top of the head, in the neighbourhood of the spot through which the spirits enter the head, around which it has made innumerable tiny channels, which are so many tiny arteries that have begun to form the small membrane called the infundibulum, and then that which covers the duct of the ventricle that is behind the brain, and also the small tissues called the ‘choroid’ tissues, which are in the two cavities in front;

After that, being collected around the spot where the small gland called the ‘pineal gland’ will be formed, they entered all together the middle of the triangular vessel which nourishes the brain.

I do not need to explain the formation of the other veins and arteries, because I see nothing in particular of note.

They are all produced when some small part of the seed goes towards the heart, the tiny channel that it makes in going there is a vein, and that made by the blood, coming from the heart in order to take its place, is an artery.

In this way, when these tiny channels are slightly separated from one another, the vein and the artery seem separated, because the ends of the arteries are not seen.

In this initial stage, several different causes can make these tiny channels turn, or make one divide into two, or two collect into one. This results in the difference that one sees between the distribution of veins and that of arteries.

But this does not prevent them always retaining the same connection between the ends of their branches, because the blood which passes continually through these branches maintains it.

Moreover, the branches through which this connection is made are found in all places in the body and not only in their extremities, for even if one cuts one’s foot or one’s hand, one does not thereby impede the blood in the leg, or in the arm.

I will add here just three examples of the division, the growth, and the joining of these tiny channels.

At the beginning, only a single tube, which carried the spirits in a straight line from the heart to the brain, but the tracheal artery, through which the respiratory air passes, is formed later (so I shall say more in its proper place), and the air that it contains having more force to rise following this straight line than does the blood that comes from the heart, this tube came to be divided into two branches, namely, what are called the ‘carotid’ arteries.

The 2 veins called the ‘spermatic’ veins were embedded in the vena cava, each as low as the other, at the time of their first formation, but the agitation of the aorta, when the liver and the vena cava are turned to the right side, is the reason why the spot where the left spermatic vein was embedded is raised gradually as far as the emulgent vessel while that on the right remains unchanged; just as, on the other hand, as a result of the same cause, the vein called the ‘adipose’, of the left kidney, is raised from the emulgent vessel, where it was first, to the trunk of the vena cava, while the expansion of the liver causes the right one to be lowered. I mean what I say when I tell you that this is something I have long sought, and indeed something in which I had the least hope of success, although it has not stopped others.

The arteries and the veins that descend in mammals have a very different origin from those that are called ‘epigastric’, which come from the bottom up towards the abdomen. Nevertheless, several of their branches are joined vein to vein, and artery to artery, towards the navel.

This happens because the former spot is the last from which the parts of the seed run towards the heart, because they have a longer route to traverse to arrive there; and because having done exactly this, the blood – as much in rising through the veins in mammals as in descending through the epigastrics – which comes from one part or another through the arteries which accompany them, drives out the parts of the seed which are between the two, until it has gradually pushed them all through the very tiny passages in the veins, and in this way the principal branches of the arteries find themselves joined to the opposite arteries, and those of the veins to veins.