Malebranch's search after truth, or, A treatise of the nature of the humane mind and of its management for avoiding error in the sciences : vol I : done out of French from the last edition.

CHAP. VI.

I. That the Fibres of the Brain are not subject to such quick Changes as the Spirits are.

II. Three different Changes in the three different Ages.

ALL the Parts of Living Bodies are in continual Motion, both the Solid and Fluid parts, the Flesh as well as the Blood; there is only this difference between their Motions, that that of the parts of the Blood is visible and sensible, and that of the Fibres of our Flesh is wholly imperceptible. There is then this difference between the Animal Spirits, and the sub∣stance of the Brain, that the Animal Spirits are very much agitated, and very fluid, and the substance of the Brain hath some Solidity and Consistence; so that the Spirits divide themselves into little parts, and in a few hours are dissipated by transpiration through the Pores of those Vessels that contain them, and it very often happens that others succeed which do not perfectly resemble them; but the Fibres of the Brain are not so easily dissipated, there does not often hap∣pen any considerable Change in them, and their whole substance cannot be changed but after many years.

The most considerable differences that are found in a Man's Brain, during the whole Course of his Life, are in Infancy, at his full Strength, and in Old Age.

The Fibres of the Brain in Children are soft, flexi∣ble, and delicate, in perfect Age they become more dry, hard, and strong; but in Old Age, they become wholly inflexible, gross, and sometimes mingled with superfluous humours, that the feeble heat of this Age cannot be any longer dissipated. For as we see the Fibres which compose the Flesh harden in time, and that the

Flesh of a young Partridge is without dispute more tender than that of an old one, so the Fibres of the Brain of a Child or Youth, will be much more soft and delicate, than those of Persons that are more ad∣vanced in years.

We shall soon see the reason of these Changes, if we but consider how these Fibres are continually agi∣tated by the Animal Spirits which run round about them in many different ways. For as the Wind drys the Earth by blowing upon it, so the Animal Spirits, through their continual agitation, by little and little render the greatest part of the Fibres of Man's Brain more dry, compressed, and solid; so that Persons a little advanced in Years will almost always have them more inflexible, then those that are Younger. And for those that are of the same Age, as Drunkards who for many years have used Wine to Excess, or such Liquors as have been able to stupifie them, will also have them more solid and more inflexible then such as are deprived of those Drinks during their whole Lives.

Now the different Constitutions of the Brain in Children, Men at full growth, and old Men, are very considerable Causes of the difference that is observed in their faculty of Imagining, of which we shall af∣terwards speak.