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.

, I would like those who have never studied anatomy to take the trouble to look at the heart of some land animal, something reasonably large (for they are more or less similar to those of men), and, having first cut off the end of the heart, to take note that there are 2 cavities inside, which are able to hold a lot of blood.

If one then puts one’s fingers in these cavities, towards the base of the heart (and from which it discharges its contents) via the right ventricle, to seek out the openings through which they receive the blood, what one will find there is that there are two very large ones in each:

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.

When the heart is elongated and deflated (as it always is when animals are dead) they do not stop any of the blood from this vein descending into this ventricle.

But if, because of the abundance and expansion of the blood that it contains, the heart is swollen and shortened, these three membranes must raise themselves and in this way close the entrance of the vena cava so that the blood can no longer descend through it into the heart.

3 small membranes or valvules can also be found at the entrance to the pulmonary artery, and 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 one puts one’s finger into the left ventricle, one will find there two openings towards the base, which lead, one 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, and 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 three 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 two large drops enter them there, one falling from the vena cava into its right ventricle, and the other falling from the pulmonary vein into the left one, and the small amount of rarefied blood that remains in these ventricles, mixing straightaway with the fresh blood coming in, is like a kind of yeast, which causes it to heat and expand immediately, and by these means the heart swells, hardens, and becomes a little squatter in shape; and 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 is no longer able to descend from these two veins into the heart, and 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.

And because this rarefied blood requires much more room than there is in the ventricles of the heart, it enters into the two arteries with great force, and by these means swells and rises at the same time as the heart; and it is this movement, as much of the heart as of the arteries, that is called the pulse.

Immediately after the blood, thus rarefied, has taken its course into the arteries, the heart deflates, becoming flabby and elongated, which is why so little blood remains in its ventricles; and the arteries deflate also, in part because the outside air, which is much closer to their branches than it is to the heart, makes the blood that they contain cooler and condenses it, and in part because there is about as much blood continually leaving them as there is entering them.

Although it seems that, when the blood no longer rises from the heart into the arteries, their contents must go back down into the heart, in fact it 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 which, expanding in the same way as before, makes the heart and the arteries  move a second time, and thus their beating always continues while the animal is alive.

As for those parts that are called the ‘auricles of the heart’, their move- ment is different from that of the heart itself, but follows it very closely, for as soon as the heart is deflated, two large drops of blood fall into its ventricles, one from its right auricle, which is the extremity of the vena cava, the other from its left auricle, which is the extremity of the pulmonary vein, and by these means the auricles deflate.

The movement of the heart and the arteries, which then immediately inflate, to some extent 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.

Moreover, the movement by which they inflate and deflate is confined to them, and does not extend to the vena cava and the pulmonary vein of which they are the extremities, and 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.

But in order that all this be understood better, we must consider more particularly the material of the four vessels of the heart.

First, as regards the vena cava, we should note that it extends throughout all parts of the body except the lung, so that all the other veins are only its branches; for even the portal vein, which is spread throughout the spleen and the intestines, is joined to it so clearly by tubes in the liver that it can be included.

One must thus consider all these veins as a single vessel which is named the vena cava at the spot where it is largest, and which always contains the major part of the blood that is in the body, which it naturally conducts into the heart, so that if it were to contain only three drops, these would leave the other parts and would proceed towards the right auricle of the heart.

The reason for this is that the vena cava is much larger here than anywhere else, and it goes from there by narrow- ing gradually as far as the ends of its branches; and 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, which are so arranged that they 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, we must also judge that their fibres are also so arranged that they allow the blood to flow more easily in this direction than in the contrary one.

As regards the pulmonary artery and the pulmonary vein, we should note that these are also two vessels that are very large at the point at which they are attached to the heart, but that 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 three 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.

It should also be noted that these three 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.

And the shape of the heart serves to demonstrate that the blood heats up more and expands with greater force in the left ventricle than in the right one; for it can be seen that it is very much larger and rounder, that the flesh surrounding it is thicker, and 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.

Indeed, 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.