Nutrition

The blood is continually dilated in the heart.

From there it is pushed forcibly through the arteries into all the other parts of the body, whence it returns subsequently through the veins towards the heart.

The blood nourishes the body while it is in the arteries, rather than the veins.

For although I do not wish to deny that,

While blood is flowing from the extremities of the veins to the heart, some of its parts pass through the pores in their surrounding membranes and become attached there – as happens particularly in the liver, which is without doubt nourished by the blood from the veins, because it receives almost none from the arteries

But in all other cases where the veins are accompanied by arteries, the blood contained in these arteries, being finer and moved with a greater force than that in the veins, is very much more easily attached to other parts, without the thickness of their covering membranes hampering it.

This is because, at their extremities their skins are hardly any thicker than those of the veins, and also because when the blood coming from the heart inflates them, the pores in their skins are enlarged in the process.

And then the small parts of this blood, which have been separated from each other by the rarefaction that it has undergone in the heart, push these membranes forcefully from all sides, easily entering those pores of similar proportions, and go to strike the roots of the small filaments that make up the solid parts.

Then, at the moment when the arteries deflate, these pores contract, and in the process several parts of the blood remain caught against the roots of the small filaments of the solid parts that they are nourishing (and several others flow away through the pores that surround them), and in this way they also enter into the  composition of the body.

But in order to understand this more distinctly, we must bear in mind that the parts of those living bodies that are maintained through nourish- ment, that is, animals and plants, undergo continual change, in such a way that the only difference between those that are called ‘fluids’, such as the blood, humours, and spirits, and those that are called ‘solids’, such as bone, flesh, nerves, and membranes, is that the latter move much more slowly than the others.

And in order to understand how these corpuscles move, we must think of all the solid parts being made up exclusively of small filaments which stretch out and fold back, and which are sometimes also intertwined, each emerging from somewhere on one of the branches of an artery.

We must think of the fluid parts, that is to say the humours and the spirits, flowing along these filaments, through the spaces that are found around them, making up infinitely many small channels which have their source in the arteries, and usually flowing from the pores of those arteries closest to the root of the filaments along which they run.

After following these filaments and various twists and turns in the body, they come finally to the surface of the skin, through the pores of which these humours and spirits evaporate into the air.

Now as well as these pores through which the humours and the spirits run, there are also many other narrower pores through which there continually passes matter of the first two elements, as described in my Principles of Philosophy.

And as the agitated matter of the first two elements encounters that of these humours and spirits, running along the filaments that make up the solid parts, they continually make the filaments move forward slightly, albeit very slowly; so that as a result every part of the filaments runs from where it has its roots to the surface of the limb where they terminate, and when it reaches there it comes into contact with the air or other bodies touching the surface of the skin, and separates from it.

Thus there is always some part being separated from the end of each filament while at the same time another part is being attached to the root, in the manner already described. But the separated part evaporates into the air if it emerges from the skin, whereas if it emerges from the surface of a muscle, or from some other internal part, it mixes with the fluid parts and flows with them wherever they go: some- times outside the body, and sometimes through the veins towards the heart, to which the fluid parts often return.

Hence it can be seen that all the parts of the filaments making up the solid parts of the body undergo a motion which is no different from that of the humours and spirits, only slower; similarly, the motion of the humours and spirits is slower than that of the most subtle matter.

These differences in speed cause these various solid or fluid parts, in rubbing against one another, to become smaller or larger, and to behave in different ways depending on the particular constitution of each body. When one is young, for example, because the filaments that make up the solid parts are not joined to one another very firmly, and the channels along which they flow are quite large, the motion of these filaments is not as slow as when one is old, and more matter is attached to their roots than is detached from their extremities, which results in their becoming longer and stronger, and their increase in size is the means by which the body grows.

When the humours between these filaments do not flow in great quantity, they all pass quite quickly along the channels containing them, causing the body to grow taller without filling out. But when these humours are very abundant, they cannot flow so easily between the filaments of the solid parts, and in the case of those parts that have very irregular shapes, in the form of branches, and which consequently offer the most difficult passage of all between the filaments, they gradually become stuck there and form fat. This does not grow in the body, as flesh does, through nourishment properly speaking, but only because many of its parts join together and stick to one another, just as do the parts of dead things.

And when the humours become less abundant, they flow more easily  and more quickly, because the subtle matter and the spirits accompany- ing them have a greater force to agitate them, and this causes them little by little to pick up the parts of the fat and carry them along with them, which is how people become thin.

And as we get older, the filaments making up the solid parts tighten and stick together more closely.

They finally become hard that the body ceases entirely to grow and even loses its capacity for nourishment.

This leads to such an imbalance between the solid and the fluid parts that age alone puts an end to life.

But in order to know more specifically in what way each part of the nutrients get to that place in the body which they are able to nourish, we must note that the blood is nothing but a mass of many small portions of food that one has ingested in order to nourish oneself, so that there can be no doubt that it is made up of parts which are significantly different from one another, as much in shape as in solidity and size. And I know of only two things that can bring it about that each of these parts proceeds to particular positions in the body rather than others.

The first is their position in relation to the route that the parts follow; the other, the size and shape of the pores where they – or rather the bodies to which they are attached – enter. For to suppose that there are in each part of the body faculties that choose and guide the particles of  nutrient to where they are appropriate, is to make claims to an account which is both incomprehensible and chimerical, and to attribute more intelligence to these than even our soul has: for our soul does not know in any way what they would need to know.

Now the size and shape of these pores is evidently enough to secure that the parts of the blood which have a certain bulk and shape enter some places in the body rather than others. For as we observe sieves with an array of holes, which can separate round grains from long ones, and the finest from the largest, so there is no doubt that the blood, pushed by the heart through the arteries, finds many pores in them, through which some of its constituent parts can pass, but not others.

But their position in relation to the route of the blood through the arteries is also required in order to make sure that among those of its parts that have the same shape and bulk, but not the same solidity, the more solid go to particular places, rather than to others. And it is above all on their location that the production of animal spirits depends.

All the blood that comes from the heart in the aorta is pushed in a straight line towards the brain. But it cannot all go there (because the branches of the aorta which extend this far, namely those called the ‘carotid’, are very narrow compared to the opening of the heart from whence they come), and only those of its parts which, being solid, are also the most active, and those most agitated by the heat of the heart, go there.

Because of this, they have a greater force than the others to follow their course to the brain. At the entry to the brain, in the small branches of these carotids, and also particularly in the gland that physicians have supposed only serves to receive the phlegm, those parts that are small enough to pass through the pores of this gland are filtered through, and these make up the animal spirits.

Those that are a little larger attach themselves to the roots of the filaments that make up the brain, but as for those that are largest of all, they pass from the arteries into the veins to which they are joined and, retaining the form of blood, return to the heart.