The Animal Spirits

The animal spirits and the changes to which they are subject in general

Everyone sufficiently agrees that the animal spirits are only the most subtle and most agitated parts of the blood, which is subtilized and agitated principally by fermentation and by the violent movement of the muscles of which the heart is composed; that these spirits are carried with the rest of the blood by the arteries to the brain, and that there they are separated by certain parts destined for this use, about which there is not yet agreement.

It must be concluded from this that if the blood is very subtle, there will be many animal spirits; and that if it is gross, there will be few; that if the blood is composed of parts very easy to kindle in the heart and elsewhere, or very suited to movement, the spirits that are in the brain will be extremely heated or agitated; that if on the contrary the blood does not ferment enough, the animal spirits will be languishing, without action and without force; finally, that according to the solidity found in the parts of the blood, the animal spirits will have more or less solidity, and consequently more or less force in their movement. But all these things must be explained at greater length, and examples and incontestable experiences must be brought forward, in order to make the truth of them more sensibly recognized.

The chyle goes to the heart, and that it brings change to the spirits.

The authority of the ancients has not only blinded the minds of some people; one can even say that it has closed their eyes. For there are still some persons so respectful toward ancient opinions, or perhaps so obstinate, that they refuse to see things they could no longer contradict if they would only open their eyes. One sees every day persons fairly well esteemed for their reading and their studies, who make books and public lectures against the visible and sensible experiences of the circulation of the blood, against that of the weight and elastic force of the air, and other similar ones. The discovery that Mr. Pecquet made in our day, of which we have need here, is among those that are unfortunate only because they are not born old, and, so to speak, with a venerable beard. We shall not, however, refrain from making use of it, and we do not fear that judicious persons will find fault with it.

According to this discovery, it is certain that the chyle does not go first from the viscera to the liver by the mesaraic veins, as the ancients believe; but that it passes from the intestines into the lacteal veins, and then into certain reservoirs where they all terminate; that from there it rises by the thoracic duct along the vertebrae of the back, and goes to mix with the blood in the axillary vein, which enters the upper trunk of the vena cava; and that thus, being mixed with the blood, it goes to the heart.

The blood mixed with the chyle being very different from another blood that had already circulated several times through the heart, the animal spirits, which are only the most subtle parts of it, must also be very different in persons who are fasting, and in others who have just eaten. Moreover, because among the foods and drinks that are used there are an infinite number of kinds, and even those who use them have bodies differently disposed; two persons who have just dined, and who leave the same table, must experience in their faculty of imagining so great a variety of changes that it is impossible to describe it.

Those who enjoy perfect health carry out digestion so completely that the chyle entering the heart almost neither increases nor diminishes its heat, and does not prevent the blood from fermenting there almost in the same way as if it entered alone: so that their animal spirits, and consequently their faculty of imagining, receive almost no change from it. But as for the elderly and the infirm, they notice in themselves very perceptible changes after their meal. Nearly all of them become drowsy, or at least their imagination becomes quite languid and no longer has any vivacity or quickness; they no longer conceive anything distinctly; they cannot apply themselves to anything; in a word, they are quite different from what they were before.

That wine does the same

But so that even the healthiest and most robust may have sensible proofs of what has just been said, they need only reflect on what has happened to them when they have drunk wine much more than usual, or on what will happen to them when they drink only wine at one meal, and only water at another. For one is assured that if they are not entirely stupid, or if their body is not composed in an altogether extraordinary way, they will immediately feel gaiety, or some slight drowsiness, or some other similar effect.

Wine is so spirituous that it is animal spirits almost fully formed; but libertine spirits, which do not readily submit to the orders of the will because of their solidity and excessive agitation. Thus, even in the strongest and most vigorous men, it produces greater changes in the imagination and in all the parts of the body than meats and other drinks. It trips up the feet, to speak like Plautus; and it produces in the mind many effects that are not so advantageous as those that Horace describes in these verses:

Quid non ebrietas designat! operta recludit: Spes jubet esse rates: in prœlia trudit inermem: Sollicitis animis onus eximit: addocet artes. Fecundi calices quem non fecere disertum! Contracta quem non in paupertate solutum!

[What does not drunkenness bring about! It reveals hidden things: it bids hopes set sail; it drives the unarmed man into battle; it removes the burden from anxious minds; it teaches arts. Whom have not the generous cups made eloquent! Whom have they not set free from pinching poverty!]

It would be fairly easy to give an account of the principal effects that the mixing of chyle with the blood produces in the animal spirits, and subsequently in the brain and in the soul itself; for example, why wine gladdens, why it gives a certain vivacity to the mind when taken in moderation, why it brutifies it with time when one exceeds in it, why one is drowsy after a meal, and several other things, for which reasons are ordinarily given that are quite ridiculous. But besides the fact that we are not writing a physics here, it would be necessary to give some idea of the anatomy of the brain, or to make some suppositions, as Mr. Descartes makes in the treatise he wrote on man, without which it is not possible to explain oneself. But finally, if one reads with attention this treatise of Mr. Descartes, one may perhaps be able to satisfy oneself on all these questions: because this author explains all these things, or at least he gives enough knowledge of them to discover them afterwards by oneself through meditation, provided one has some knowledge of his principles.