Descartes' model of the antagonistic eye muscles A,E. When E is relaxed A is ennervated. Descartes believed that an animal spirit travelling down hollow nerves C,B, contracted the muscles and blew them up, and that they relaxed when the spirit was shut off by valves. From Rene Descartes Opera Philosophica Frankfurt-am-Main. 1692. Originally published in his Tractatus de homine, Paris, 1664. Woodcut.