In 1627, William Harvey was writing notes for a treatise on the movement of animals, De motu locali animalium, which in the event he neither published nor completed. As its very existence was unknown until its publication in 1959, Harvey's contribution to the study of the physiology of muscle has been unnoticed. In this treatise he deals with the different kinds of animal locomotion and examines the part played by the various motive organs, the muscles and their component parts, and the nerves. He is concerned with the problems of the initiation of movement and of the complexity and diversity of movement involved in any single action. Though he could reach no satisfactory conclusion for any of these problems, the quality of his observations and of the questions that he asks is such as to justify an examination of his position relative to the muscle physiologists of the later seventeenth century.