The Heart

The shape of the heart shows that the blood heats up more and expands with greater force in the left than in the right ventricle.

• The heart is very much larger and rounder.
• The flesh surrounding it is thicker.
• Yet it is the same blood passing through this ventricle as passes through the other, and which is thinned because it has provided nourishment to the lung.

The openings of the heart’s vessels also show that respiration is necessary for the condensation of the blood in the lungs.

Infants cannot breathe while they are in their mother’s womb. They have 2 openings in the heart which are found in those which are older.

• Through one of these openings, the blood from the vena cava runs with that from the pulmonary vein in the left ventricle of the heart.
• Through the other (which is shaped like a small tube) a part of the blood that comes from the right ventricle passes from the pulmonary artery into the aorta, without entering the lung

These 2 openings gradually close up by themselves when the infant is born and is able to breathe.

Geese, ducks, and similar animals can remain underwater without breathing because these openings never close up.

The aorta is the fourth vessel of the heart.

All of the body’s other arteries are not as large as it.

• They are only its branches, through which the blood that it receives from the heart is promptly carried to all its limbs.

All these branches of the aorta are joined to those of the vena cava.

This is similar to those of the pulmonary artery are joined to the branches of the pulmonary vein.

The same blood then goes back to the heart backwards and forwards several times, from the vena cava into the right ventricle of the heart.

• From there it goes into the pulmonary vein via the pulmonary artery
• From the pulmonary vein it goes into the left ventricle
• From there it goes into the vena cava via the aorta

This makes a perpetual circular motion which sustains the life of animals, without their needing to drink or eat, if none of the parts of the blood left the arteries or veins while it flowed in this fashion.

But many parts continually leave it. These are supplied by the juice of foods, which come from the stomach and intestines.

This circulatory movement of the blood was first observed by an English physician Harvey.

• He should be praised highly for such a useful discovery.

The ends of the veins and the arteries are so delicate.

• We cannot see with the naked eye the openings by which the blood passes from the arteries into the veins.

There are nevertheless several places where it can be seen: above all in the great vessel which is made up of layers of the larger of the 2 membranes that envelop the brain, in which many veins and many arteries are found, so that the blood is led there through the latter, then returning through the former to the heart.

This can also be seen to some extent in the spermatic veins and arteries.

The evidence showing that the blood passes in this way from the arteries into the veins is so strong.

If you open the chest of a living animal and tie the aorta close to the heart so that no blood can fall from its branches, and if one cuts between the heart and the tie, all the blood quickly escapes via this opening.

This would be impossible if that in the branches of the aorta did not have passages by which to enter into the branches of the vena cava, from where it passes into the right ventricle, and from there into the pulmonary artery, at the extremities of which it must also find passages in order to enter the pulmonary vein, which leads it into the left ventricle, and from there into the aorta, from where it leaves.

This can also be tested by tying the arm to bleed it.

If the tie is tight and a little higher, closer to the heart than the point at which they open the vein, the blood gushes out more than if it had not been tied.

But if it is tied too tightly, the flow is stopped, just as it is if they tie it a little further from the heart – but not at the place where the vein opens – even if they do not tie it very tightly.

This shows that the blood:

• is carried towards the hands and other extremities of the body by the arteries
• returns from these through the veins towards the heart

This has already been clearly demonstrated by Harvey.

But Harvey believes that:

• when the heart lengthens, its ventricles increase in size
• when the heart shortens, its ventricles become narrower

This is contrary to:

• the common opinion of other physicians
• the common judgement of sight

Against this, I shall demonstrate that they always become larger.

The arguments that have led him to this view are as follows.

He has observed that:

• the shrinking heart becomes harder
• in frogs and other animals that have little blood, the heart becomes more white, or less red, than when it lengthens
• if one makes an incision down as far as the ventricles, it is at the moment when it is shrunk that the blood leaves through the incision, and not when it is elongated.

From this he thinks that:

• the heart contracts when it becomes hard
• the blood leaves the heart causing it to become less red in some animals
• since we see this blood leave via the incision, then the blood comes when the place that contains it is narrower.

He could have confirmed this by a very evident experiment.

Cut the point of the heart of a living dog.

• Through the incision, put your finger into one of its ventricles
• You will clearly feel that every time the heart shortens it presses the finger
• It will stop pressing it whenever it is elongated.

This shows that its ventricles are narrower when the finger is pressed more than when it is pressed less.