How the Spirits Change Course

How do the different qualities of spirits have the power to change their courses?

This occurs especially when they are determined from elsewhere only temporarily or not at all.

For example, assume the nerves of the stomach move to stimulate hunger into the brain through tubes 8. In the meantime, nothing edible or drinkable is available to the senses or memory.

The spirits will move themselves to where they will find more pores that can convey them through all the nerves indiscriminately.

• This will make the body seek or achieve a certain object.

In this way, only the inequality of these particles can drive them into these nerves rather than into others.

If stronger particles of spirits are conveyed into certain nerves, but immediately afterwards stronger ones rush forth to the nerves opposite to them, the body will imitate the motions.

This happens when we hesitate or doubt about something.

Similarly, the fire A lies midway between those which can lead the spirits to O and to P, that is, between those which produce pain and pleasure:

The multiple inequalities inherent in the spirits drive them towards some nerves or organs or the other.

• The action is pleasing to us when we are in a cheerful mood.
• The action is displeasing when we are tormented by melancholy.

The difference in spirits causes natural and acquired temperaments or inclinations.

The Different Positions Of External Limbs

The limbs move through the changing of the pores through which the spirits move into the nerves.

For example, fire A burns hand B.

• This will cause the head to incline to the left.
• The spirits would then flow clearly, just as they now pass from 7 to N, then to O, and from there to r and s.

But from r, when it should pass to X, where I suppose it should tend, to turn the head to the right hand, they would go to Z: where they would go if the head were turned to the left.

Because that position of the head, which is now the cause that

The filaments of the brain’s substance are around X.

They are much looser and can be more easily separated from each other than those which are around Z.

Changing that position would produce the contrary effect.

• Those around Z would then be looser, while those around X would be very tense and tight.

This is how a single action can move one foot, and then the other foot as progress.

It is done when the spirits pass through only one pore whose end is arranged differently.

• This leads them into other nerves when the left foot has progressed farther than the right.

This is how respiration and similar other movements (which do not depend on any idea) work.

34. The Sleeping Brain

The filaments D D connect to the nerves get compressed during sleep.

This prevents:

• external objects from passing into the brain to be sensed.
• the exit of spirits in the brain to the external limbs