How Animal Spirits are Produced in the Brain

10. How Animal Spirits are Produced in the Brain

All the liveliest and most subtle parts of the blood that the heat has rarefied in the heart enter continually in large quantity into the cavities of the brain.

They go there rather than anywhere else because all the blood that exits the heart through the aorta flows straight towards this place.

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

These very subtle parts 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.

13. The Action of External Objects Can Diversely Direct the Spirits to the Muscles

Dioptrique explained how all the objects of sight are communicated to us only by this: they move locally, through the intermediary of transparent bodies between them and us, the small threads of the optic nerves, which are at the back of our eyes, and subsequently the parts of the brain from which these nerves come; they move them in as many different ways as they make us see diversities in things.

It is not immediately the movements in the eye, but those in the brain, that represent these objects to the soul.

This is how we conceive sounds, odors, flavors, heat, pain, hunger, thirst, and generally all objects of our:

• external senses
• internal appetites

Our senses and appetites excite some movement in our nerves which passes through them to the brain.

These various movements of the brain make our soul have various sensations..

But they can also, without the soul, cause the spirits to go to certain muscles rather than others, and thus move our limbs.

If someone quickly advances their hand toward our eyes as if to strike us, even though we know it is a friend and will not harm us, we still struggle to prevent our eyes from closing.

This shows that it is not through the intermediary of our soul that they close, since it is against our will, which is its only or at least its main action.

But because the machine of our body is so composed that the movement of this hand toward our eyes excites another movement in our brain, which directs the animal spirits into the muscles that lower the eyelids.

14. How the Diversity Among the Spirits Can Also Vary Their Course

The animal spirits enter into the muscles in different ways due to:

• the unequal agitation of these spirits
• the diversity of their parts.

When some of their parts are larger and more agitated than others, they pass further in a straight line through the cavities and pores of the brain, and in this way are directed to different muscles than they would be if they had less force.

15. What Causes Their Diversity

This inequality can come from the various materials they are composed of, as seen in those who have drunk a lot of wine.

The vapors of this wine quickly entering the blood rise from the heart to the brain, where they convert into spirits, which being stronger and more abundant than usual, are capable of moving the body in several strange ways.

This inequality of the spirits can also come from the various conditions of the heart, the liver, the stomach, the spleen, and all other parts that contribute to their production.

Certain small nerves inserted in the base of the heart serve to widen and narrow the entrances of its cavities. Through this, the blood dilating more or less strongly produces spirits differently disposed.

Although the blood that enters the heart comes from all other parts of the body, it often happens that it is pushed there more from some parts than others, because the nerves and muscles that correspond to those parts press or agitate it more.

According to the diversity of the parts from which it comes the most, it dilates differently in the heart and subsequently produces spirits with different qualities.

For example, the blood that comes from the lower part of the liver, where bile is, dilates differently in the heart than the blood that comes from the spleen; and this differently than the blood that comes from the veins of the arms or legs.

Finally, this differently than the juice of the food, when it has just left the stomach and intestines and quickly passes through the liver to the heart.

16. How All Members Can Be Moved by the Objects of the Senses and by the Spirits Without the Help of the Soul

All the changes that occur in the movement of the spirits can cause them to open some pores of the brain more than others.

Conversely, when any of these pores is more or less open than usual, by the action of the nerves that serve the senses, it changes something in the movement of the spirits and causes them to be directed into the muscles that serve to move the body, in the way it is ordinarily moved on the occasion of such an action.

The following actions we do without our will contributing to it:

• breathe, walk, eat
• all the actions we share with animals

These actions depend only on the conformation of our members and the course that the spirits, excited by the heat of the heart, naturally follow in the brain, nerves, and muscles.

Just as the movement of a clock is produced solely by the force of its spring and the shape of its wheels.