The Vitality of the Body
William Harvey had described the circulation of the blood24 when Descartes was thirty years old. This discovery struck Descartes’s imagination, and he then set about using Harvey’s theory to describe the link between the soul and the body. He therefore devised a vision of the impassioned body, animated by wild dynamics, that is everything save what we ordinarily refer to as Cartesian. Descartes proposes a precise and detailed vision that takes into account all that is known, but one that invents connections where there is no evidence to allow for a view of the whole.
Descartes’s concept of the body is of a sort of hydraulic robot that is animated by great physiological systems that interact with each other like dancers at a ball. It appears as a precise choreography, probably foreseen by God; in practice, the dancers sometimes let themselves be carried away by their enthusiasm and bang into each other.
The body is composed of distinct systems such as the skeleton, the muscles, the nervous system, the circulation of the blood and the heart, the lungs, as well as the deeper organs (liver, spleen, kidneys, etc.). These systems are already relatively well known. Things become more difficult when Descartes tries to understand how these systems interact. He knows that certain organs discharge products into the bloodstream, that oxygen is transported by the blood in short, that the blood links all of the tissues of the body. The arterial blood nourishes the tissues. Venous blood, as a sewage system, takes their toxins and distributes these waste products in diverse organs, principally the kidneys and the lungs. The second system that strongly participates in the organization of the human organism is the nervous system because it coordinates what the senses bring to the body, what certain nerves detect in the body, and the orders given to the muscles. For Descartes, matter is by nature living and dynamic without the soul having any role in it. The flesh has its own sensual and juicy dynamics, whereas the soul has its capacity for wisdom After all, like the soul, the flesh was created by God. Where Descartes’s imagination becomes both stimulating and whimsical is when he sets about to describe, in The Passions of the Soul, the interactions between the circulation of the blood and the nervous system to try to define the dynamics between the soul and the body. This work is always full of learning if we read it attentively, trying to bring to mind the details of Descartes’s imaginative device.
Descartes knows that blood must fight gravity on the way to the head. He imagines that the heavier particles of blood stay at the bottom, and only the lighter and smaller particles irrigate the brain. The most refined ones are retained by the nerve tissues to become a spiritâ that will flow in the nerves and form the nerve impulses. The word spirit is to be taken here in the chemical sense, like a state between liquid and gas that breathes agreeably in a glass of cognac or brandy. Once it is admitted that the nervous fluids are refined blood, it becomes possible to think that (a) the dynamics of the heart can influence the nervous system, and (b) all that influences the nervous system can indirectly influence the circulation of blood. Thus, a muscle is a kind of bag that is treated in two ways by the blood:
1. The blood fills the muscle like a wineskin, hardens it, and then retires.
2. The nervous fluids that contract the muscles are also a derivative of blood.