How the Animal Spirits Move the Muscles

Our bodies move so diversely due solely to the force of the animal spirits flowing from the brain into the nerves.

For each movement and its opposite determination, two such tubes play a role, like B F D and C G E, and two others like D G and E F, as well as the two valves such as H F I.

The methods by which these tubes are inserted into the muscles vary a thousand times.

But their differences are due to the shape and use of each muscle.

For instance, the eyelids are moved by muscles.

• T in Figure IX, serves solely to open the upper one.
• The other, namely V, serves to alternately open and close both.

They receive the spirits through 2 such tubes as P V and Q S, one of which P V extends to both of those muscles, while the other Q S extends only to one of them.

Finally, the fibers r and g are inserted into muscle V in almost the same way, yet due to the different arrangement of the fibers, they produce two entirely opposite effects.

This is also sufficient to understand other movements.

Since the animal spirits can produce certain movements in all parts, there must be some nerves terminating in those parts as well.

There are many places where anatomists do not observe any visible nerves, such as in the pupil, heart, liver, gallbladder, spleen, and similar organs.