Müller made contributions in numerous domains of physiology, in particular increasing understanding of the voice, speech and hearing, as well as the chemical and physical properties of lymph, chyle and blood. His first important works, Zur vergleichenden Physiologie des Gesichtsinns (“On the comparative physiology of sight,” Leipzig, 1826) and Über die phantastischen Gesichtserscheinungen (“On visual hallucination,” Coblenz, 1826), are of a subjective philosophical tendency. The first work concerns the most important facts as to human and animal sight, the second sounds depths of difficult psychological problems. He soon became the leader in the science of the morphological treatment of zoology as well as of experimental physiology. To his research (1830) is due the settlement of the theory of reflex action.