Sleep and the Functions of the Body

Sleep

34. What is Sleep?

The filaments D D converge into nerves and are compressed at the eyes (Figure 50).

During sleep, the actions of external objects are largely impeded from passing into the brain to be felt there.

Likewise, the passages to the external limbs, by which these actions are to be conveyed, are blocked by the animal spirits* contained in the brain.

35. We also observe this in all plants.

This property belongs to bodies which can grow and be nourished by the addition of particles of other bodies.

As for the rest, however, those which I have supposed and which do not fall within the senses, they are all so simple and common;

They are so few. If they are compared with the different compositions and the marvelous artifice which we admire in the astonishing structure of those organs which we see, it will seem more probable that I have omitted more of those which are present in us than that I have supposed some which are not present in us.

Nature always uses the easiest and simplest means of all. This is why we think it impossible for things more similar to those which it uses itself to be employed than those which we have enumerated.

36. The functions of this bodily machine are as follows.

• digestion of food
• beating of the heart and arteries
• nourishment and growth of the limbs
• respiration
• wakefulness and sleep
• the reception of light, sounds, odors, tastes, heat, and other similar qualities by the external senses
• the impression of ideas, formed by these senses in the organ of common sense
• the retention of these ideas in memory
• the internal movements of appetites and passions
• the external movements of all the limbs, which follow from the disposition of the organs in such a way that objects are presented to the senses, and passions and impressions occur in the memory, all of which they imitate as accurately as possible.

These functions naturally follow in our machine solely from the disposition of its organs, just as the motions of a clock or any other such automaton follow from the arrangement of its counterweights and wheels.

Just as because of those motions, it is not necessary to conceive of any other soul, whether vegetative or sensitive, or any other principle of motion and life than the blood and agitated spirits, namely, the heat of fire, which continually burns in its heart. For the nature of this fire is certainly no different from that of all the others which are found to be inherent in inanimate bodies.