The Description of the Human Body

There is heat in the heart. One can even feel it with one’s hand when one opens up the body of a living animal.

This heat is of the same nature as that which is caused by adding yeast or some fluid to make a body expand when mixed with it.

The dilation of the blood that causes this heat is the first and principal spring of our whole bodily machine.

Look at the heart of a large land animal.

• Cut off the end of the heart, and you will see 2 cavities which hold a lot of blood.
• Put your finger in these cavities towards the base of the heart.
  • From that base it discharges its contents via the right ventricle.
• Trace the path where they receive the blood from 2 sources:

1. One that goes to the vena cava
2. Another that goes to the pulmonary artery

Cut through along this ventricle, as far as these 2 openings. This will reveal 3 small membranes, commonly called the ‘valvules’, at the entry to the vena cava.

The heart of a dead animal is elongated and deflated.

• In this state, it does not stop the blood from this vein from falling into this ventricle.

The heart is swollen and shortened from the abundance and expansion of the blood in it.

These 3 membranes raise themselves and close the entrance of the vena cava so that the blood can no longer fall through it into the heart.

3 small membranes or valvules can also be found at the entrance to the pulmonary artery.

These are differently disposed than those of the vena cava, so that they prevent the blood contained in this pulmonary artery from being able to descend into the heart.

But if there is some blood in the right ventricle of the heart that tries to leave it, they will not prevent this leaving at all.

In the same way, if you put your finger into the left ventricle, there will be 2 openings towards the base.

• One leads into the pulmonary vein
• The other into the aorta

In opening up this whole ventricle, we see two valvules at the entrance of the pulmonary vein which are just the same as those in the vena cava, and are positioned in the same way.

There would be no difference at all, were it not that the pulmonary vein is pressed on the one side by the aorta and on the other by the pulmonary artery, which makes its opening oblong.

Because of this, two small membranes are enough to close it, rather than the three needed  to shut the vena cava.

One will also see 3 other valvules at the entrance to the aorta, which do not differ at all from those at the entrance to the pulmonary artery, so that they do not prevent the blood in the left ventricle of the heart rising into this aorta, but they do prevent it passing back down this artery into the heart.

The pulmonary artery and the aorta are composed of skin that is much stronger and thicker than the vena cava and the pulmonary vein.

This shows that:

• the vena cava and the pulmonary vein have a completely different use from the pulmonary artery and the aorta
• the ‘venous artery’ is really a vein [viz. the pulmonary vein], just as what is called the ‘arterial vein’ is really an artery [viz. the pulmonary artery].

But what made the ancient writers call an ‘artery’ what they should have called a ‘vein’, and call a ‘vein’ what they should have called an ‘artery’, is the fact that they believed that all the veins came from the right ventricle of the heart, and all the arteries from the left.

These 2 parts of the heart called its ‘auricles’ are nothing but the extremities of the vena cava and the pulmonary vein, which are widened and folded up.

When the anatomy of the heart is seen in this way, if one considers that it always has more heat in it when the animal is alive than any other part of the body, and that the blood is of such a nature that when it is a little hotter than usual it expands very quickly, one cannot doubt that the movement of the heart, and following it the pulse, or the beating of the arteries, occurs in the way that I shall describe.

When the heart is elongated and deflated, there is no blood in its ventricles, except for a small amount which remains from that which has previously been rarefied.

This is why 2 large drops enter them there:

1. One falls from the vena cava into its right ventricle
2. The other falls from the pulmonary vein into the left one

The small amount of decompressed blood that remains in these ventricles, mixes straight away with the fresh blood coming in. It becomes like a kind of yeast, causing it to heat and expand immediately*.

This makes the heart swell, hardens, and becomes a little squatter in shape.

The little membranes at the entrances to the vena cava and the pulmonary vein rise and shut them in such a way that the blood can no longer fall from these 2 veins into the heart.

The blood that expands in the heart cannot rise towards these two veins.

But it rises easily from the right ventricle into the pulmonary artery, and from the left into the aorta, without the small membranes at their entrances acting to prevent this.

This decompressed blood needs much more room than there is in the ventricles of the heart.

And so it enters into the 2 arteries with great force which then swells and rises at the same time as the heart, leading to the pulse.