Nerves are hollow channels that transmit animal or psychic spirits between the brain and the body.

Animal spirits were an attempt to solve a real mechanism problem: how can the brain make a distant muscle move?

Galen placed the brain at the center of sensation and voluntary action. In the later ventricular-pneumatic tradition, a refined pneuma or psychic spirit was generated in the brain and transmitted through nerves. Nerves could therefore be imagined as tiny conduits carrying a subtle active substance between the brain, sense organs, and muscles.

The framework proved adaptable. Descartes rejected much of Galenic physiology but retained “animal spirits,” now described in more mechanical terms as fine particles flowing through nerves into muscles.

During the eighteenth century the old picture became harder to maintain. Investigators proposed fluids, vibrations, irritability, and other intermediate mechanisms. Work on electric fish, nerve stimulation, and muscle contraction then made electricity an increasingly plausible physiological agent. Galvani’s 1791 experiments became an important landmark in the emergence of electrophysiology.

Modern nerves do not operate as empty tubes carrying a special spirit. Signals propagate as changes in membrane voltage along axons, with chemical or electrical transmission between cells.

Because “animal spirits” changed meaning over many centuries and disappeared gradually, this card is not assigned an exact lifespan.