The Heart

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

• 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 vessels of the heart also serve to show that respiration is necessary for the condensation of the blood in the lung; for  it can be seen that infants, who cannot breathe while they are in their mother’s womb, have two openings in the heart which are not to be 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.

One can also see that these two openings gradually close up by themselves when the infant is born and is able to breathe; by contrast, in geese, ducks, and similar animals, which can remain a long time under the water without breathing, they never close up.

It remains to note, with respect to the aorta, that it is the fourth vessel of the heart, that all of the body’s other arteries are not as large as it is, and 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, just as those of the pulmonary artery are joined to the branches of the pulmonary vein; so that, after having distributed to all parts of the body what they need from the blood, whether it be for their nourishment or for other uses, they carry all the surplus in the extremities of the vena cava, where it once more runs towards the heart.

And thus the same blood goes backwards and forwards several times,  from the vena cava into the right ventricle of the heart, then from there via the pulmonary artery into the pulmonary vein, and from the pulmonary vein into the left ventricle, and from there via the aorta into the vena cava, this making a perpetual circular motion which would be enough to sustain 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, and these are supplied by the juice of foods, which come from the stomach and intestines, as I shall explain below.

This circulatory movement of the blood was first observed by an English physician called Harvey, whom one cannot praise too highly for such a useful discovery.

Although the ends of the veins and the arteries are so delicate that one 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 two 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. And the evidence showing that the blood passes in this way from the arteries into the veins is so strong that they leave one no room for doubt.

For if, having opened the chest of a living animal, one ties the aorta sufficiently close to the heart, so that no blood can descend from its branches, and if one cuts between the heart and the tie, all the blood of  this animal, or at least the greater part of it, 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.

If one does not wish to take the trouble to open up a living animal, one need only consider the way in which surgeons usually tie the arm to bleed it, for if they tie it quite tightly a little higher, that is to say a little closer to the heart than the point at which they open the vein, the blood gushes out in much greater quantities 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 ordinarily, the blood is carried towards the hands and other extremities of the body by the arteries, and 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: namely, if one cuts the point of the heart of a living dog, and through the incision one puts one’s finger into one of its ventricles, one will clearly feel that every time the heart shortens it presses the finger, and that it will stop pressing it whenever it is elongated.

This seems to ensure conclusively that its ventricles are narrower when the finger is pressed more than when it is pressed less.

But all this shows that the same observations can often mislead us, if we do not examine their possible causes sufficiently.

For although, if the heart does contract from within, as Harvey believes, this would make it become harder and less red in animals that have little blood, and would also make the blood in the ventricles spurt out through the incision we have made, and, finally, would make the finger inserted in the incision feel pressure; nevertheless, none of this alters the fact that the same effects could also proceed from a different cause, namely the expansion of the blood as I have described it.

Which of these 2 causes is the true one?

We must consider other observations which are not compatible with both of them.

1. If the heart hardens due to a contraction of the fibres in it this would necessarily reduce its size.

2. But if the hardening is due to the expansion of the blood contained in the heart, this on the contrary would lead to an expansion.

Observations show that the heart does not lose its size. Rather, it grows larger.

This has led other physicians to consider that it swells up during this phase.

It is true nevertheless that the increase in size is not great, but the reason for this is clear: the heart has several fibres stretched like cords from one side of its ventricles to the other, and these prevent them from opening very much.

Another observation shows that when the heart shortens and hardens,  its ventricles do not become narrower but, on the contrary, become larger: if one cuts the point of the heart of a young rabbit that is still alive, the naked eye shows that its ventricles become a little larger and expel blood at the moment at which the heart hardens, and even when it expels only small drops of blood, because very little blood remains in the animal’s body, they continue to have the same size.

What prevents them from opening ever wider are fibres which stretch from one side to another, which hold them in place.

What makes this much less apparent in the heart of a dog or some other more vigorous animal than in a young rabbit is that the fibres take up more of the ventricles; they stiffen when the heart hardens and can press against a finger inserted into one of the ventricles. But despite that, the ventricles do not become narrower but on the contrary larger.

I would add yet a third observation, which is that the blood does not leave the heart with the same qualities it had when it entered it, but is very much warmer, more rarefied, and more agitated.

Now supposing that the heart moves in the way that Harvey describes, not only must we imagine some faculty which causes the movement, the nature of which is much more difficult to conceive than what it is invoked to explain: we must also suppose the existence of yet other faculties that alter the qualities of the  blood while it is in the heart.

But if we confine our attention instead to the expansion of the blood, which must necessarily follow its heating, which everyone recognises is greater in the heart than in any other part of the body, then it will be clear that this expansion is enough to make the heart move in the way I have described, and also to change the nature of the blood in the way observation indicates.

It is also sufficient to explain any change one might imagine as necessary so that the blood is prepared and made more suitable for nourishing all the bodily parts that can be used for all the other functions for which it is used in the body.

In this way, we need suppose no unknown or extraneous faculties. For what better and swifter arrangement can we imagine than that which is brought about by fire, which is the most powerful agent we know in nature: rarefying the blood, it separates its small parts from one another, dividing them up and changing their shapes in every imaginable way.

This is why I am extremely surprised that, despite the fact that it has always been known that there is more heat in the heart than in the rest of the body, and that the blood can be rarefied by heat, it has not been noticed by anyone to date that it is this rarefaction of the blood alone that is the cause of the movement of the heart.

For although it might seem  that Aristotle thought this when he wrote in chapter  of his book De respiratione ‘that this movement resembles the action of a liquid that heat brings to a boil’, and also that what causes the pulse is ‘juices from the food one has eaten continually coming into the heart and rising to its outer wall’, nevertheless, because he makes no mention in this passage of the blood, or of the material from which the heart is constructed, it is clear that it is just by chance that he has said something approaching the truth, and that he possessed no certain knowledge of it.

Nor was his opinion adopted by anyone, even though he had the good fortune to have a number of followers on many other questions where his views are far less plausible.

Yet it is so important to know the true cause of the heart’s movement that, without it, we cannot know anything about the theory of medicine, because all the other functions in the animal depend on it, as will be seen clearly from what follows.