How Animal Spirits are Produced in the Brain

10. How Animal Spirits are Produced in the Brain

The heat in the heart decompresses the blood, making them lively and subtle.*

These enter continually in large quantities into the brain’s cavities.

This blood cannot all enter because there are only very narrow passages. Only the most agitated and subtle parts pass through, while the rest spreads to all other parts of the body.

These very subtle particles of the blood compose the animal spirits.

They need no further change in the brain to this effect, other than being separated from the less subtle parts of the blood.

These spirits are only very small bodies that move very quickly, like the flame that comes from a torch.

• They never stay in any place.
• Some enter the cavities of the brain
• Others exit through the pores in its substance

This conducts them into the nerves. From there they go into the muscles and move the body.

11. How Muscle Movements Occur

Limbs move through some of their muscles shortening while their opposites lengthen.

One muscle shortens while the other lengthens when more spirits go to it from the brain than to the other.

The spirits coming directly from the brain does not move these muscles alone.

• But they make the other spirits already in these two muscles to promptly exit one and enter the other.

• The muscle that they exit becomes longer and looser.

• The muscle that they enter is quickly inflated by them, shortens, and pulls the limb to which it is attached.

Only very few animal spirits continually come from the brain to each muscle. Yet there are always many others already enclosed in the same muscle. These move very quickly.

• Sometimes they just spin in place where they are whenever they find no open passages to exit.
• Sometimes they flow into the opposite muscle.

There are small openings in each of these muscles through which these spirits can flow from one to the other.

When the spirits coming from the brain to muscle A have a little more force than those going to muscle B, they simultaneously:

• open all the entrances through which the spirits of muscle B can pass into muscle A
• close all the entrances through which the spirits of muscle A can pass into muscle B.

This makes all the spirits previously contained in these 2 muscles quickly gather in muscle A, and thus inflate and shorten it, while muscle B lengthens and loosens.

12. How External Objects Act on the Sense Organs

Why don’t the spirits always flow from the brain into the muscles in the same way?

Why do they sometimes flow more towards some muscles than others?

Besides the action of the soul, which is indeed one of these causes within us, as I will explain later, there are two other causes that depend only on the body, which need to be noted.

1. The variety of movements that are excited in the sense organs by their objects

This was explained extensively in the Dioptrique.

As a summary of that, there are 3 things to consider in the nerves:

• their marrow or internal substance

These are small threads that start from the brain going to the extremities of other members to which these are attached.

• the skins that surround the nerves

These are continuous with those that envelop the brain. These compose small tubes that enclose the small threads.

• the animal spirits

These are carried by these same tubes from the brain to the muscles.

These cause these threads to remain entirely free and stretched in such a way that the slightest thing that moves the part of the body to which the end of one of them is attached also moves the part of the brain from which it originates.

Just as when one pulls one end of a rope, one moves the other.