Johannes Peter Muller (1801-1858), the German physiologist. His first studies dealt with locomotion in animals. Later work was wide ranging; he studied electrophysiology, the sensory system of the eye, the glandular system, the human embryo, and the nervous system. He showed the value of microscopy in pathology, developing procedures now used daily in clinical work, especially on tumours. He worked on classification in zoology, especially on marine animals. A frequent victim of depression, he is widely regarded as the greatest of all physiologists.