German physiologist, comparative anatomist, ichthyologist, and herpetologist, known not only for his discoveries but also for his ability to synthesize knowledge. Muller was one of the great natural philosophers of the 19th century
In 1824 he was granted a lectureship in physiology and comparative anatomy at the University of Bonn. Two years later he was appointed associate professor, and in 1830 he became a full professor.
In the meantime, his voluminous Zur vergleichenden Physiologie des Gesichtssinnes . . . (1826; “Comparative Physiology of the Visual Sense . . . ”) brought Müller to the attention of scholars by its wealth of new material on human and animal vision; he included the results of analyses of human expressions and research on the compound eyes of insects and crustaceans. His most important achievement, however, was the discovery that each of the sense organs responds to different kinds of stimuli in its own particular way or, as Müller wrote, with its own specific energy. The phenomena of the external world are perceived, therefore, only by the changes they produce in sensory systems. His findings had an impact even on the theory of knowledge.
Maintaining an almost incredible level of output at Bonn, he examined many problems in physiology, development, and comparative anatomy. He studied the passage of impulses from afferent nerves (going to the brain and spinal cord) to efferent nerves (going away from the same centres), further elucidating the concept of reflex action. By careful experiments on live frogs, he confirmed the law named after Charles Bell and François Magendie, according to which the anterior roots of the nerves originating from the spinal cord are motor and the posterior roots are sensory. He investigated the nervous system of lower animal species, the intricate structure of glands, and the process of secretion. When tracing the development of the genitalia, he discovered what is now known as the Müllerian duct, which forms the female internal sexual organs. He contributed to knowledge of the composition of the blood and lymph, the process of coagulation, the structure of lymph hearts of frogs, the formation of images on the retina of the eye, and the propagation of sound in the middle ear.
In 1833 Müller was called to Berlin to succeed Rudolphi. In his new post he again carefully explored many problems concerning animal function and structure. His early years in Berlin were devoted mainly to physiology. His Handbuch der Physiologie des Menschen für Vorlesungen stimulated further basic research and became a starting point for the mechanistic concept of life processes, which was widely accepted in the second half of the 19th century.
Inspired by the vast Berlin anatomical collection, Müller became interested again in pathology. After the demonstration by his assistant, Theodor Schwann, that the cell was the basic unit of structure in the animal body, he concentrated on the cellular structure of tumours with the aid of a microscope. He began to establish pathological histology as an independent branch of science. Müller also distinguished himself as a teacher. His students included the renowned physiologist and physicist Hermann Helmholtz and the cellular pathologist Rudolf Virchow.
Beginning in 1840 Müller increasingly focused his research on comparative anatomy and zoology, in so doing becoming one of the most respected scholars in these subjects. He was a master at collecting and classifying specimens; he devised an improved classification of fish and, based on an ingenious analysis of vocal organs, did the same for singing birds. In 1827, 1840, and 1848, Müller suffered periods of depression that rendered him incapable of working for months on end. They may perhaps be attributed—as his periods of explosive productivity—to a manic-depressive disposition. It may also be regarded as the cause of his death on 28 April 1858. Some scholars have concluded that he died by his own hand.

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