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The German physiologist Theodore Schwann made one of the great biological discoveries when he founded modern histology by defining the cell as the basic unit. He has not been honoured philatelically but has been the subject of a German postal cancellation. While Professor of Anatomy and Physiology at Liege he described (1838) the myelin sheath as a fat-like substance which he considered to be a secondary deposit on the inner surface of a structural cell membrane (called neurilemma by Bichat). He also investigated muscular contraction and nerve structure, discovered muscle in the upper oesophagus, and coined the term metabolism for the chemical changes that take place in living tissue. He also formulated the basic principles of embryology and the role played by micro-organisms in putrefaction.