The Heart

The heart deflates after this decompressed blood leaves. This makes the heart flabby and elongated. This is why so little blood remains in its ventricles.

The arteries deflate also for 2 reasons:

1. The outside air is much closer to their branches than it is to the heart, makes the blood that they contain cooler and condenses it

2. There is about as much blood continually leaving them as there is entering them.

When the blood no longer rises from the heart into the arteries, their contents must go back down into the heart.

The blood cannot enter its ventricles because the small membranes at the entrances to their arteries prevent it from doing so.

It enters it rather from the vena cava and the pulmonary vein.

These expand as before and makes the heart and the arteries move a second time.

Thus their beating always continues while the animal is alive.

The movement of the ‘auricles of the heart’ is different from that of the heart itself.

As soon as the heart is deflated, 2 large drops of blood fall into its ventricles:

1. One from its right auricle, which is the extremity of the vena cava
2. The other from its left auricle, which is the extremity of the pulmonary vein

These make the auricles deflate and the arteries inflate.

To some extent, this inhibits the blood which is in the branches of the vena cava and the [pulmonary vein] from coming to fill these auricles, in such a way that they deflate.

Instead of the heart inflating all at once, and then deflating gradually, the auricles deflate more rapidly than they inflate.

Their inflation and deflation is confined to themselves.

• These do not extend to the vena cava and the pulmonary vein of which they are the extremities.
• This is why they are:
  • so much larger, and otherwise bent, and
  • made up of much thicker and fleshier membranes than these other two veins.

The material of the four vessels of the heart

The Vena Cava

This extends throughout all parts of the body except the lung, so that all the other veins are only its branches.

The portal vein is spread throughout the spleen and the intestines. This is joined to the vena cava so clearly by tubes in the liver that it can be included.

All these veins are a single vessel with the vena cava as its largest spot.

This always contains the major part of the blood that is in the body, which it naturally conducts into the heart.

If the heart were to only have 3 drops, these would leave the other parts and proceed towards the heart’s right auricle.

This is because the vena cava:

• is much larger here than anywhere else.
• goes from there by narrowing gradually as far as the ends of its branches

The membranes from which its branches are composed can be stretched more or less according to the quantity of blood that they contain, always contracting some small part of itself by which means it drives this blood towards the heart.

There are valvules in several parts of its branches.

• These completely close the passage, preventing the blood from flowing to their extremities, and thus becoming too distant from the heart when it comes about that its weight or some other cause pushes it there.
• But they do not prevent it flowing from the extremities towards the heart.

Because of this, their fibres allow the blood to flow more easily in this direction than in the contrary one.

The pulmonary artery and the pulmonary vein are also very large at the point at which they are attached to the heart.

But they divide very close to there into several branches, and these divide yet again into others which are very small.

They proceed by narrowing in proportion to their distance from the heart.

Each branch of one of these two vessels always accompanying some branch of the other, and also some branch of a third vessel, whose entrance is called the windpipe or the throat, and the branches of these 3 vessels do not go anywhere except the lung, which is made up of these alone, and they are so mixed together that one cannot point to any part of its flesh, which is large enough to be seen, in which each of these three vessels has none of its branches.

These 3 vessels are different in that that whose entrance is in the throat never contains anything but respiratory air, and is made up of tiny cartilage and membranes very much harder than those that make up the other two.

Similarly with the pulmonary artery, which is composed of membranes that are notably harder and thicker than those of the pulmonary vein, which are soft and slender just like those in the vena cava.

This shows that, although these two vessels contain only blood, there is nevertheless a difference between them, in that the blood in the pulmonary vein is not as agitated, or driven with as much force, as that in the pulmonary artery.

For, just as one sees that the  hands of artisans become hard due to the manner of their use, so the cause of the hardness of the membranes and cartilage of which the windpipe is comprised is the force and agitation of the air that passes through it when one breathes. And if the blood were not more agitated when it enters the pulmonary artery than when it enters the pulmonary vein, the membranes of the former would be no thicker and harder than those of the latter.

But I have already explained how the blood enters the pulmonary artery with a force that is in proportion to how much it has been heated and rarefied in the right ventricle of the heart.

It remains here only to say that, when this blood is dispersed through all the tiny branches of this pulmonary artery, it is cooled and condensed by the respiratory air; and because of this tiny branches of the vessel that contain this air are mixed among them in all parts of the lung; and the new blood that comes from the right ventricle of the heart in this same pulmonary artery enters it with such force that it drives that which has begun to condense and makes it pass at the extremities of its branches into the branches of the pulmonary vein, where it flows very easily towards the left ventricle of the heart.

And the main use of the lung consists in one thing alone: by means of the respiratory air, it thickens and tempers the blood that comes from the right ventricle of the heart before it enters the left ventricle; without this it would be too rare and too fine to serve to fuel the fire that it encounters there.

Its other use is to contain the air that serves to produce the voice.  Also, we see that fish and other animals that have only a single ventricle in the heart, all lack a lung, as a result of which, they are mute, so that none of them can make a sound.

But they are also all of a very much colder constitution than animals that have two ventricles in their hearts, because the blood of these latter, having already been heated and rarefied once in the right ventricle, falls back into the left ventricle a little later where it stirs up a fire that is more lively and warmer than it would be were it to come immediately from the vena cava. And although this blood re-cools and condenses in the lung, nevertheless, because it remains there for a short time, and because it does not mix with any grosser matter there, it retains an ability to dilate and reheat better than that which it had before it entered the heart.

Similarly, experience shows that oils that have been made to pass several times through a distillation flask are much easier to distil the second time than the first.