He compared the human body to complicated machinery run by water-falls and complicated pipes.

"The nerves of the machine which I am describing," he says, "may very well be compared to the pipes of these waterworks; its muscles and its tendons to the other various engines and springs which seem to move them; its animal spirits to the water which impels them, of which the heart is the fountain; while the cavities of the brain are the central office.

Moreover, respiration and other such actions as are natural and usual in the body, and which depend on the course of the spirits, are like the movements of a clock, or a mill, which may be kept up by the ordinary flow of water."(3) In such passages as these Descartes anticipates the ideas of physiology of the present time.

He believed that the functions are performed by the various organs of the bodies of animals and men as a mechanism, to which in man was added the soul.

This soul he located in the pineal gland, a degenerate and presumably functionless little organ in the brain.