animal spirits
in the system of Galen, a vaguely defined force or substance that is pictured as flowing through hollow tubes from the brain to all parts of the body; as such, it is a precursor of the modern concept of the nerve impulse. Like other aspects of Galen’s system, the concept of animal spirits remained current well into the 17th century. In the mechanistic physiology of René Descartes, for example, the animal spirits are retained to provide a link between body and mind. Descartes, however, anticipated the modern concept of the peripheral nervous system by maintaining that nerves conduct in either direction between the muscles and sense organs. Also called vital spirits. See also hydraulic model.