Changes in the Brain

The fibers of the brain are not subject to changes as rapid as the spirits

All the parts of living bodies are in continual motion, both the solid parts and the fluids, the flesh as well as the blood; there is only this difference between the movement of the one and the other, that the movement of the parts of the blood is visible and perceptible, and that of the fibers of our flesh is entirely imperceptible. There is therefore this difference between the animal spirits and the substance of the brain, that the animal spirits are very agitated and very fluid, and that the substance of the brain has some solidity and some consistency; so that the spirits divide into small parts and dissipate in a few hours, transpiring through the pores of the vessels that contain them, and often others come in their place that are not at all similar to them. But the fibers of the brain are not so easy to dissipate: considerable changes do not often happen to them, and their whole substance can change only after several years.

Three different changes in the three different ages.

The most considerable differences found in the brain of the same man throughout his life are in childhood, in the age of a grown man, and in old age.

The fibers of the brain in childhood are soft, flexible, and delicate; with age they become drier, harder, and stronger; but in old age, they are wholly inflexible, coarse, and sometimes mixed with superfluous humors that the very weak heat of that age can no longer dissipate. For, just as we see that the fibers that compose the flesh harden with time, and that the flesh of a partridge is unquestionably more tender than that of an old partridge, so the fibers of the brain of a child or a young man must be much softer and more delicate than those of persons more advanced in age.

One will recognize the reason for these changes if one considers that these fibers are continually agitated by the animal spirits that flow around them in many different ways; for, just as the winds dry the earth upon which they blow, so the animal spirits, by their continual agitation, render little by little most of the fibers of the human brain drier, more compressed, and more solid, so that older persons must almost always have them more inflexible than those who are less advanced in age; and for those of the same age, drunkards who, for several years, have exceeded in wine or similar drinks capable of intoxicating, must also have them more solid and more inflexible than those who have abstained from these drinks all their lives.

Now, the different constitutions of the brain in children, in grown men, and in old people are very considerable causes of the difference noticed in the faculty of imagining in these three ages, of which we shall speak in what follows.