How the Blood is Converted into Animal Spirits

10. Through examination of the sieve-like structure, it can be noted that none of the blood particles, except for those entering the brain, are either stronger or more vigorous than those transported to the vessels dedicated to reproduction

For example, those particles with enough strength to reach D but are denied further passage to C due to the narrowness of the location, return rather to E than to F or G, as the direct path leads there.

Consequently, I could perhaps demonstrate how another machine identical to this one could be formed from the fluid that gathers around E. However, I do not wish to digress further.

The portion of blood that rises up to the brain through the arteries:

• aids in the nutrition and preservation of brain tissue
• primarily produces a very subtle breeze, or rather a highly agile and pure flame, which we call “animal spirits”.

The arteries are divided into countless small branches. These form marvelous networks that line the floors of the brain’s ventricles like carpets.

These converge around the pineal gland roughly in the center of the brain, near the entrance of its ventricles or cavities.

• This gland has nearly infinite pores.

The narrowness of those pores deny the passage of thicker blood parts. They allow only the finer parts to flow into the gland.

These arteries do not end there. They rather join each other more closely and ascend directly to the large vessel that irrigates the entire surface of the brain like a tide.

The thicker blood particles lose some of their motion while struggling through the labyrinthine paths of these fine networks.

The finer particles, mixed in, possess the power to propel themselves, transferring some of their motion into these networks.

These, in turn, cannot lose their momentum.

Instead, their force increases due to the motion transferred to them by the thicker particles, as there are no other bodies present to transfer their motion so easily.

As the thicker parts of the blood travel straight to the outer surface of the brain to nourish its substance, they cause the more agitated finer particles to be deflected and enter the pineal gland.

This gland is like a highly abundant spring from which they can promptly leap into the brain’s cavities in all directions.

Thus, without any preparation or alteration, other than being separated from the thicker parts and still being agitated with the intense speed impressed upon them by the heat of the heart, they lose the form of blood and come to be known as animal spirits.

As soon as these spirits enter the brain’s ventricles, they flow into the pores of the substance. From the pores, they go into the nerves where, as they enter or even simply attempt to enter, they exert varying degrees of influence on these nerves, altering the shape of the muscles to which they are attached, and thereby moving all limbs.

This is demonstrated by fountains and water features in the gardens of our kings, where water, by the sheer force with which it bursts from its spring, moves various machines, even modulating musical instruments, or producing certain sounds according to the different tubes through which the water flows.

• The nerves are like the tubes of these fountains.
• The muscles and tendons to other motion-serving organs and instruments.
• The animal spirits are the water that drives and agitates them.
• The heart is their source.
• The brain’s cavities are their reservoirs or water tanks.

Breathing and other such ordinary and natural actions dependent on the animal spirits resemble the movements of a clock or mill, whose movements are sustained by the constant flow of water.

The external objects, by their mere presence, act against the organs of her senses. This then causes her to move in various ways, according to the disposition of the parts of her brain.

If an irrational soul resides in this machine, these external objects will be like strangers who, entering these very large fountains, cause movements in the machine by their mere presence.

For example, they can only enter by walking on certain tiles so arranged that if they approach a Diana who is bathing, their approach will cause her to be hidden behind the walls.

If they chase after her, they will make Neptune appear who will then threaten them with his trident.

If they move in another direction towards Diana, some sea monster might suddenly spurt water in their faces, or something similar according to the fountain-maker’s ingenuity.

If, however, a rational soul resides in this machine, its chief seat will be in the brain, where it oversees the fountains. This makes it present in the reservoirs where all the tubes of the fountains converge, to excite, impede, or modify the motions of the animal spirits.