How Does the Body Breathe and Swallow Food?

11. The muscle for breathing D raises its chest, or lowers its diaphragm.

The muscle E is its opposite.

The pore or small channel marked n naturally remains always open.

The animal spirits which are in the concavity of its brain marked m, flows through that pore towards pipe BF which lowers the small skin F.

There they cause those of the muscle E to come to swell the muscle d.

There are certain skins around this muscle d which press it more and more as it swells.

Before all the spirits of the muscle d have passed towards it, they stop their course and make them flow through the tube BF, so that those of the canal n are turned away from it.

This makes them go to the tube cg, which they open at the same time. This causes:

• the muscle E to swell
• the muscle d to swell down.

This they continue to do as long as the impetuosity lasts, from which the spirits contained in muscle d, pressed by the skins which surround it, tend to emerge.

Then, when this impetuosity has no more force, they resume their course through tube B F, and thus do not cease to make these two muscles swell and deflate alternately.

What you must also judge of the other muscles which serve the same purpose.

They are all so arranged, that, when it is the sembla bles to d which swell, the space which contains the lungs widens, which is the cause of the air entering from in, just as in a bellows which is opened; and that, when it is their opposites, this space narrows, which is the cause of the air coming out.

12. How does the body swallow the meats which are at the bottom of its mouth?

Muscle D:

• raises the root of its tongue
• holds open the passage by which the air which it breathes must enter its lung

Muscle L:

• closes this passage
• opens that which connects the mouth to the stomach
• raises:
  • the point of her tongue which pushes them there
  • the animal spirits which come from the concavity of her brain m, by the pore or small channel n, which remains naturally.

This muscle always remains so swollen while there is no meat at the bottom of the mouth that can press it/

But when there is some meat, its spirits immediately flow through the BF pipe.

• This causes those that come through the channel n to enter through the CG pipe into the muscle E, where they also go to the muscle D.

This makes the throat open and the food descend into the stomach.

Then afterwards, the spirits of the channel n resume their course through BF as before.

This is also how the body can sneeze, yawn, cough, and make the movements necessary to throw various other excrements.

13. How do external objects affect the sense-organs and incite them to move?

There are fine filaments (which I have previously mentioned come from the innermost recesses of the brain and make up the marrow of the nerves) that are present in all its parts serving each sense organ.

These filaments can be most easily moved by the objects of these senses. When they are slightly moved, they immediately draw the parts of the brain from which they originate and open the passages of certain pores on the inner surface of the brain.

Hence, the animal spirits immediately begin their course and are carried through the same passages into the nerves and muscles, initiating movements that are exactly similar to those we also naturally feel when our senses are similarly affected. Fig. XII.

Fire moves extremely fast. It has the power to move part of the skin that it touches.

If fire A is brought closer to foot B, the fire will move the skin in that foot, pulling the cord C.

C will then open the passage of pore D E where this cord terminates at the same moment.

Just as if you pull one end of a rope, the bell to which the other end is attached rings at the same moment. Thus,

By opening pore D, the animal spirits flow into the concavity F and are carried through it:

• partly into the muscles which retract the foot from the fire
• partly into the muscles that turn the eyes and head to look in that direction
• the rest of the body for its own defense.

But the animal spirits thus excited through this passage D E can not only be carried to these muscles but also to many others.

How do the animal spirits follow their course through the brain’s pores? How are these pores arranged?

There is an innumerable number of fine filaments like C C, all of which originate from the inner surface of the brain and are distributed throughout the body to serve the organs of touch.

In most cases, they are not directly touched by external objects, but only through the membranes that surround them.

But it does not seem more plausible that these membranes are the organs of the senses than it would be to think that gloves on the hands are the organs of the senses.

The filaments mentioned are extremely thin.

But they can nonetheless be safely carried from the brain to other, more distant, members of the body because nothing hinders them, neither breaking them nor intercepting their action through any kind of pressure, even though in the meantime, these members may be bent in countless ways.

For they are contained within the same tubes that carry the animal spirits to the muscles.

These tubes, always slightly inflating, act to prevent the filaments from being excessively compressed there and simultaneously allow them to extend from the brain, from which they originate, to the places where they end.

Therefore, when God unites a rational soul with the body, He assigns the brain as its chief seat.

He lets the soul have different senses depending on the ways how the nerves open the pores in the inner surface of the brain

Tickling

For example, the fine filaments composing the marrow of these nerves might be pulled so hard that they are broken and detached from the member to which they were attached. This would corrupt the structure of the whole body.

The soul has an interest to keep its seat intact. So it will conceive from the movement caused by that force in the brain the reason to feel pain.

If, however, those filaments are pulled with the same force but not broken or detached, the movement in the brain will provide testimony of the good condition of the other members. It will give the soul a reason to feel physical pleasure which we call tickling.

This is very close to pain in terms of its cause, but produces a completely opposite effect.

If several such filaments are pulled simultaneously and evenly, the soul will sense that the surface of the body part where they end is smooth and polished.

But if they are pulled unevenly and perceived, they will signify a rough surface.

If the filaments are not all pulled together, but one after another in a certain order, as always happens from the heat imparted by the heart to other members, the soul will perceive no sensation at all. It will not be affected more than it normally would be by its customary actions.

Moreover, if this movement caused by an external source is either slightly intensified or diminished, the increase in heat will produce a sensation of warmth in the soul, while the decrease will produce a sensation of cold.

Depending on the various ways in which the nerve filaments are affected, the soul will sense any qualities related to touch, such as moisture, dryness, hardness, heaviness, and the like.

However, these are extremely fine and can easily be moved. But they are not so minute as to be able to convey to the brain even the smallest motions of every kind of terrestrial particle.

Some particles of this kind are sufficiently coarse. Nevertheless, they affect these filaments so gently that they either press them or immediately sever them, yet their action does not reach the brain.

Just as certain drugs have the power to induce numbness, even to corrupt the actions of our members to which they are applied, and therefore to intercept all sensation.