Then beginning to treat of these animal spirits, it is to be known that, just as the vital ones are engendered in the heart, so the animal ones are engendered in the brains of the head, which as it is the most noble part of our body, thus serves to form these spirits so noble, which raise our life above that of the plants, which also live as we do. And just as in the heart there are two hollows or little bellies in which the vital spirits are forged, so in the brains there are two others, in which the animal spirits are forged. But in what manner these are made, is a thing that exceeds the faculty of human understandings. From here proceeds that men much given to the speculation of sciences or to the contemplation of divine things are very weak. For since the vital spirits, as servants and inferiors, serve as matter from which the animal ones are formed, which are superior, and these are resolved and spent with the heat and labor of the interior exercise, the body remains very impoverished of the vital spirits, which give it heat and life and with this it is debilitated and weakened, and thus phlegms and undigested superfluities are bred in it, which cause this weakness, with other indispositions.
But here it is to be noted that, of these spirits, some are to give movement to the members, and others to give sense. For which the Creator provided the roads by which they might run and be distributed through the whole body, which are two differences of nerves, some so that they may carry the spirits that cause movement, and others those that give sense. Which difference is clearly seen in some paralytics, who because they have the nerves that are cause of movement stopped up, cannot move the part of the body that is paralytic, and with all this they feel, if you touch and prick them, because the nerves that cause feeling are not closed. This is a thing at which Tully greatly marvels in the second book On the Nature of the Gods, marveling at the wisdom and artifice of the Maker, who sowed the whole body with so many differences of ways and channels, branched through all the parts of it, such as are the veins that carry the blood, and the arteries that carry the spirits of life, and one kind of nerves that cause movement, and others that are cause of sense. Then, what net can be fabricated in the world that has so many meshes, one over another, divided and sown through all our body?
And because the place where these animal spirits are fabricated is that mass of the brains, this mass runs through the whole spine surrounded by very hard bones that defend it, as the skull does those of the head, and likewise it also goes wrapped with those two tunics or shirts that we said the brains have, which are the dura mater and the pia mater that is next to it. For so delicate and so precious a thing as it, the Creator ordained should be not only defended and sheltered with the bones, but also favored and sheltered with these two aforesaid shirts. And I say so precious, because from the white mass that goes through this channel, which we call the marrow of the spine, twenty-four pairs of nerves are born, of which the twelve serve to give these animal spirits to the part of our body that rises from the waist upward, and the others to that which remains from the waist downward to the feet, distributed in such manner that the twelve serve one side of the body, and the other twelve for the other. And because nothing should be lacking to this work, that sovereign artificer provided that in all these bones of the spine there should be some very subtle little holes by which these nerves come out to do these aforesaid offices. And he provided even another more subtle thing, which is a most delicate web that divides the two parts of this spinal marrow, and from the one side of this web proceed the nerves of one side, and from the other those of the other, without the nerves of the one part injuring the mass from which those of the other proceed. Then who will not glorify here that most wise artificer who, from one simple substance of which our bodies are formed, fabricated so much diversity of parts, some of them hard and some of them soft, and all of them so perfectly accommodated to the offices for which they were made?
But if anyone should wish to understand what these spirits are that can do so much, I say that they are like certain most subtle rays of light that run through the pores of these nerves, and by means of them are distributed through all the body. For which it is brought as argument that if they strike us with a stick on the head, with which the nerves of it are compressed and pressed, we are wont to say that the light leaped from our eyes, which light is no other thing than these same spirits, which as they are most subtle leap outside through this more delicate and transparent part of our eyes, in which we see the proportion and admirable order of the traces of the Creator, because just as the heavens are cause of as many movements and alterations as there are in this lower world, by means of the light of the sun and of the planets, so the brains, which are the highest part of our body and like the heaven of this lesser world, are cause, by means of the rays of this light, of all the movements and senses of our body. And in this manner that sovereign artificer who, as we said, orders all things suavely, wished to proportion the government of this lesser world with that of the greater, as to this part.