SPRING QUARTER, 2006
Department of History
University of California, Irvine
Instructor: Dr. Barbara J. Becker
Week 4. Sensation
So as to avoid this error, let us note that death never occurs through the absence of the soul, but only because one of the principal parts of the body decays. And let us recognize that the difference between the body of a living man and that of a dead man is just like the difference between, on the one hand, a watch or other automaton (that is, a self-moving machine) when it is wound up and contains in itself the corporeal principle of the movements for which it is designed, together with everything else required for its operation; and, on the other hand, the same watch or machine when it is broken and the principle of its movement ceases to be active.
7. A brief account of the parts of the body and of some of their functions
To make this more intelligible I shall explain in a few words the way in which the mechanism of our body is composed. Everyone knows that within us there is a heart, brain, stomach, muscles, nerves, arteries, veins, and similar things. We know too that the food we eat goes down to the stomach and bowels, and that its juice then flows into the liver and all the veins, where it mixes with the blood they contain, thus increasing its quantity. Those who have heard anything at all about medicine know in addition how the heart is constructed and how the blood in the veins can flow easily from the vena cava into its right-hand side, pass from there into the lungs through the vessel called the arterial vein, then return from the lungs into the left-hand side of the heart through the vessel called the venous artery, and finally pass from there into the great artery, whose branches spread throughout the whole body. Likewise all those not completely blinded by the authority of the ancients, and willing to open their eyes to examine the opinion of Harvey regarding the circulation of the blood, do not doubt that the veins and arteries of the body are like streams through which the blood flows constantly and with great rapidity. It makes its way from the right-hand cavity of the heart through the arterial vein, whose branches are spread throughout the lungs and connected with those of the venous artery; and via this artery it passes from the lungs into the left-hand side of the heart. From there it goes into the great arty, whose branches are spread through the rest of the body and connected with the branches of the vena cava, which carries the same blood once again into the right-hand cavity of the heart. These two cavities are thus like sluices through which all the blood passes upon each complete circuit it makes through the body. It is known moreover, that every movement of the limbs depends on the muscles, which are opposed to each other in such a way that when one of them becomes shorter it draws towards itself the part of the body to which it is attached, which simultaneously causes the muscle opposed to it to lengthen. Then, if the latter happens to shorten at some other time, it makes the former lengthen again, and draws towards itself the part to which they are attached. Finally, it is known that all these movements of the muscles and likewise all sensations, depend on the nerves, which are like little threads or tubes coming from the brain and containing, like the brain itself, a certain very fine air or wind which is called the 'animal spirits'.
8. The principle underlying all these functions
But it is not commonly known how these animal spirits and nerves help to produce movements and sensations, or what corporeal principle makes them act. That is why, although I have already touched upon this question in other writings, I intend to speak briefly about it here. While we are alive there is a continual heat in our hearts, which is a kind of fire that the blood of the veins maintains there. This fire is the corporeal principle underlying all the movements of our limbs.
31. There is a little gland in the brain where the soul exercises its functions more particularly than in the other parts of the body
We need to recognize also that although the soul is joined to the whole body, nevertheless there is a certain part of the body where it exercises its functions more particularly than in all the others. It is commonly held that this part is the brain, or perhaps the heart -- the brain because the sense organs are related to it, and the heart because we feel the passions as if they were in it. But on carefully examining the matter I think I have clearly established that the part of the body in which the soul directly exercises its functions is not the heart at all, or the whole of the brain. It is rather the innermost part of the brain, which is a certain very small gland situated in the middle of the brain's substance and suspended above the passage through which the spirits in the brain's anterior cavities communicate with those in its posterior cavities. The slightest movements on the part of this gland may alter very greatly the course of these spirits, and conversely any change, however slight, taking place in the course of the spirits may do much to change the movements of the gland.
34. How the soul and the body act on each other
Let us therefore take it that the soul has its principal seat in the small gland located in the middle of the brain. From there it radiates through the rest of the body by means of the animal spirits, the nerves, and even the blood, which can take on the impressions of the spirits and carry them through the arteries to all the limbs. Let us recall what we said previously about the mechanism of our body. The nerve-fibres are so distributed in all the parts of the body that when the objects of the senses produce various different movements in these parts, the fibres are occasioned to open the pores of the brain in various different ways. This in turn, causes the animal spirits contained in these cavities to enter the muscles in various different ways. In this manner the spirits can move the limbs in all the different ways they are capable of being moved. And all the other causes that can move the spirits in different ways are sufficient to direct them into different muscles. To this we may now add that the small gland which is the principal seat of the soul is suspended within the cavities containing these spirits, so that it can be moved by them in as many different ways as there are perceptible differences in the objects. But it can also be moved in various different impressions -- that is, it has as many different perceptions as there occur different movements in this gland. And conversely, the mechanism of our body is so constructed that simply by this gland's being moved in any way by the soul or by any other cause, it drives the surrounding spirits towards the pores of the brain, which direct them through the nerves to the muscles; and in this way the gland makes the spirits move the limbs.