The Diversity Among Animal Spirits

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.

• The object’s light hits the intermediary of transparent bodies in our eyes.
• This moves the small threads of the optic nerves at the back of our eyes.
• This moves the parts of the brain, attached to these nerves, in many different ways to make us see diversities in things.

It is the movements 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 into the muscles and move our limbs.

• If our friend raises his hand toward our eyes as if to strike us, we still struggle to prevent our eyes from closing even if we know he will not harm us.
• This shows that our eyes close not through the intermediary of our soul, since the closing of our eyes is against our will.

The action of his hand towards 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.

An example is those who have drunk a lot of wine.

The vapors of this wine quickly enter the blood and rise from the heart into the brain where they convert into spirits.

• This moves the body in several strange ways.

This inequality of the spirits can also come from the organs that produce them:

• the heart
• the liver
• the stomach
• the spleen, etc

Small nerves at the base of the heart widens and narrows the entrances of its cavities.

• This creates dilated blood that produces spirits that are differently disposed

The blood that enters the heart comes from all other parts of the body.

These are pushed there by the nerves and organs.

• This creates animal spirits of different qualities.

For example, the blood that comes from the liver dilates differently in the heart than:

• the blood that comes from the spleen, and
• the blood that comes from the veins of the arms or legs
• the blood that has just left the liver from the intestines

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

The changes in the movement of the spirits can create a corresponding change in the pores of the brain.

Likewise, our sensory nerves can also open and affect the movement of the spirits.

• This makes those spirits go into the muscles [to facilitate instinctive reactions].

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 agreement between:

• our body parts
• the flow of the animal spirits

This is similar to how a clock’s movement is produced solely by:

• the shape of its wheels
• the force of its spring