Johannes Peter Müller - continued...

During his years in Bonn, Müller revealed crucial information about the visual, circulatory, endocrine, and reproductive systems. He rarely studied any function in one animal alone, preferring to compare the ways that different organisms solved physiological problems. His 1826 Zur vergleichenden Physiologie des Gesichtssinnes des Menschen und der Thiere (On the Comparative Physiology of Vision in Men and Animals) explained the mechanism of human binocular vision but also contained a long section on the structure of insect eyes. In this book, Müller first expressed his law of specific sense energies. Through his studies of nervous systems, Müller realized that nerves are not passive conductors of outer stimuli, since the same external event or mechanical pinch affects different nerves in different ways and can be perceived as light, sound, or pain. As he later put it in his Handbuch der Physiologie des Menschen (Elements of Physiology), “perception is not the conduction to our consciousness of a quality or circumstance outside of our body, but the conduction to our consciousness of a quality or circumstance of our nerves which has been caused by an external event” (Müller 1837, p. 780). Each nerve can respond to stimuli only in a specific way, so that our knowledge of the world reflects the structure of our nervous system.

In 1826, Müller also published Ueber die phantastischen Gesichtserscheinungen (On Fantasy Images), a study of visual hallucinations. Noticing that when he was falling asleep, he could sometimes see imaginary people and things, he tried to manipulate these figures in a series of rigorous self-experiments. His work showed that the visual system is active, not a passive recorder of external events. Unfortunately, the coffee-drinking and sleep-deprivation that these experiments demanded led to his first mental collapse. In April 1827, he married the gifted musician Nanny Zeiller, then broke down immediately afterward.