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.

While blood is flowing from the extremities of the veins to the heart, some of its particles pass through the pores in their surrounding membranes and become attached there.

This happens particularly in the liver, which is 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 in these arteries are finer and moved with a greater force than that in the veins.

This makes them very much more easily attached to other particles, 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
• when the blood coming from the heart inflates them, the pores in their skins are enlarged in the process.

The small particles of this blood are separated from each other by the decompression in the heart.

• This pushes these membranes forcefully from all sides.

They easily enter those pores of similar proportions and strike the roots of the small filaments that make up the solid particles.

These pores contract when the arteries deflate.

In the process, several blood particles remain caught against the roots of the small filaments of the solid particles that they are nourishing (and several others flow away through the pores that surround them).

In this way, they also enter into the composition of the body.

Ageing from the difference between fluids and solids

The parts of animals and plants are maintained through nourishment and thus undergo continual change.

• Examples of ‘fluids’ are the blood, humours, and spirits.
• Examples of ‘solids’ are bone, flesh, nerves, and membranes.

The only difference between fluids and solids is that solids move much more slowly than the others.

The solid particles of these corpuscles are made up exclusively of small filaments which stretch out and fold back.

These are sometimes also intertwined, each emerging from somewhere on one of the branches of an artery.

The fluid parts (i.e. the humours and the spirits):

• flow along these filaments, through the spaces around them.
• make up infinitely many small channels which have their source in the arteries.
• usually flow from the pores of those arteries closest to the root of the filaments along which they run.
• come finally to the surface of the skin after following these filaments and various twists and turns in the body
  • These humours and spirits then evaporate into the air through the pores of the skin.

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.

The fire-aether and air-aether run along the filaments of the solid particles. They encounter these humours and spirits and get agitated.

This agitation continually makes the solid filaments move forward slightly, albeit very slowly.

As a result, every part of the filaments runs from where it has its roots to the surface of the limb where they terminate.

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.

The separated particle evaporates into the air if it emerges from the skin.

But it mixes with the fluid parts and flows with them wherever they go if it emerges from:

• the surface of a muscle, or
• some other internal part

This makes them sometimes:

• go outside the body, or
• go through the veins towards the heart, to which the fluid parts often return.

Hence, 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 air-aether.

These differences in speed cause these various solid or fluid parts, in rubbing against one another, to:

• become smaller or larger
• behave in different ways depending on the particular constitution of each body.

For example, when one is young:

• the filaments that make up the solid parts are not joined to one another very firmly
• the channels along which they flow are quite large

This causes the motion of these filaments to be not as slow as when one is old.

• More matter is attached to their roots than is detached from their extremities
• This makes them become longer and stronger.
• The body grows through their increase in size.

When the humours between these filaments do not flow in great quantity, they all pass quite quickly along the channels containing them.

• This causes 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.

Some parts have very irregular shapes as branches.

• These offer the most difficult passage between the filaments.
• These gradually cause the humours to become stuck there and form fat.

Fat does not grow in the body, as flesh does through nourishment.

• They only grow because its parts join and stick together just as do the particles of dead things.

When the humours become less abundant, they flow more easily and more quickly.

• This is because the subtle matter and the spirits accompanying them have more force to agitate them.
• This causes them little by little to pick up the fat particles and carry them with them.
• This 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.

How do the nutrients get to that place in the body which they are able to nourish?

The blood is a mass of many small portions of food that one has ingested.

It is made up of particles which are significantly different from one another, as much in shape as in solidity and size.

Only 2 things can make each of these particles proceed to specific positions in the body:

1. Their position in relation to the route that the parts follow.

2. The size and shape of the pores where they – or rather the bodies to which they are attached – enter.

Some people think that each part of the body has faculties that choose and guide the particles of nutrient to where they are appropriate.

• This is an incomprehensible and chimerical claim as it attributes more intelligence to the body than even what our soul has,
• Our soul does not know in any way what they would need to know.

The size and shape of these pores ensures that the blood particles which have a certain size and shape enter some places in the body rather than others.

Sieves with holes can separate round grains from long ones, the finest from the largest.

Likewise, the blood, pushed by the heart through the arteries, finds many pores in them through which some of its constituent particles can pass, but not others.

But their position in relation to the route of the blood through the arteries is also required 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.

The production of animal spirits depends above all on their location.

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 where they come from.

• 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 particles 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.