What is the cause of the diversity and peculiarities of the spirits?

The only thing that can make these particles of spirit separate and take their course left and right towards the front of the head is their:

• extreme smallness or
• extreme weight, or
• their having shapes that retard or facilitate their movement.

And I observe only one notable difference among those that are extremely small, namely that some – those that I called ‘aerial’ above – have very irregular and impeding shapes, whereas others have shapes that are more regular and slippery, so that they are more suited to making up fine materials such as brandy or smelling salts than air.

The properties of the aerial particles require them to follow the lowest path of all, that closest to the front of the head.

There, they begin to form the organs of smell.

The most regular and slippery figures that flow above the aerial ones proceed by turning towards the front of the head, where they begin to form the eyes.

There is only one notable difference between the larger particles of spirits.

Some have shapes which are not really as obstructive as those of the aerial ones.

Because of their size, if they had obstructive shapes they will have mixed very little with spirits.

Their irregular and unequal shapes makes them move not one after the other. Instead, their unequal shapes let them be surrounded by fine matter which follow their agitation.

They are more massive and so have more force than any of the other particles.

This lets them leave the middle of the brain by a shorter route, and head towards the ears. There, they take away with them some aerial particles and form the organs of hearing.

The others, on the contrary, have regular and slippery shapes. This lets them act together so easily in moving one after the other, just like water.

Consequently, their motion is slower than that of the rest of the spirits. They descend through the base of the brain towards the tongue, the throat, and the palate, where they prepare the way for the nerves needed to make up the organs of taste.

These 4 notable differences result in certain particles of spirits leaving their body and forming the organs of smell, of vision, of hearing, and of taste.

The others separate gradually as they find pores in the seed through which they can pass.

The only required differences between them is for those that collide closest to the pores enter them, while the others run their course together along the spinal column until they too encounter other pores through which they run into all the interior parts of the seed. This lets them trace passages of nerves there which are used by the sense of touch.

Moreover, so that the knowledge that one has of the shape of already-formed animals does not prevent one from conceiving of that which they have at the beginning of their formation, the seed must be considered as a mass in which the first thing to be formed is the heart.

Around the heart is, on the one side, the vena cava, and on the other, the aorta. These are joined at their two ends so that:

• the end towards which the openings of the heart are turned marks the side where the head will be
• the other marks that of the internal parts.

After this, the spirits have moved a little higher than the blood towards the head where, being collected in some quantity, they have taken their course gradually along the artery, and as close to the surface of the seed as their force is able to carry them.

While they followed this course, their small parts have been able to pass through all the other paths that are easier for them than those where they are.

But they have not found any such paths above the spinal column because the whole body of spirits withdraws towards there, to the extent that its force allows it.

Nor have they found one directly below, because the aorta is there; so they have only flowed to the right and the left, towards the internal parts of the seed.

Except only that at the exit from the head, they have been able to with- draw a little inside and outside, because the marrow of the spinal column is less bulky than the head, and they are able to find some space in the former.

This is why the nerves that leave the 2 first junctures of the spinal column have a different origin from the others.

The spirits, which prepare the way for the nerves in the seed, have taken their course there towards the internal parts alone.

This is because the external ones, being pressed against the surface of the womb, did not have any passages free to receive them, but they did find enough free ones towards the front of the head.

This is why, before leaving them, some became separated from others without their being of a different nature, and traced the path of the nerves that lead to the muscles of the eyes, the temples, and other neighbouring spots, and then also the paths of the nerves that go to the gums, the stomach, the intestines, the heart, and to the membranes of other internal parts that are subsequently formed.

For all that, the spirits that flowed outside the head found pores on both sides of the length of the spinal column, and by these means they distinguished its joints, and became widely distributed all around the mass of the seed, now no longer round but oblong, because the force of the blood and spirits that have passed through the heart to the head have of necessity stretched it more in that direction than in the other.

The last place in the seed at which the spirits can arrive in following their course in this way is that where the navel must be.