Harves Versus Descartes

Harvey believes that the heart contracts from within.

This would make it become harder and less red in animals that have little blood.

It would also make:

• the blood in the ventricles spurt out through the incision we have made
• 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?

The following observations 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.

The increase in size is not great. This is because the heart has several fibres stretched like cords from one side of its ventricles to the other. 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.

3. The blood does not leave the heart with the same qualities it had when it entered it, but is very much warmer, more decompressed and more agitated.

If the heart moves in the way that Harvey describes, we must imagine some faculty which causes the movement.

• The nature of this faculty 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 base the expansion of the blood from its heating, 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.

Everyone recognises that heating is greater in the heart than in any other part of the body.

This also explains that 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 decompressed 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.

Aristotle thought this when he wrote in chapter 20 of his book De respiratione:

He does not mention this passage of the blood, or of the material from which the heart is constructed.

So it is just by chance that he has said something approaching the truth, and that he possessed no certain knowledge of it.

His opinion was not 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.

Without the true cause of the heart’s movement, we cannot know anything about the theory of medicine.

• This is because all the other functions in the animal depend on it.