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Engraving of Theodor Schwann (1810-1882), German physiologist. Schwann discovered the first animal enzyme, pepsin, while studying digestion. He was the first to describe the protective sheath of cells around peripheral nerve cells, which is named after him, and also coined the term metabolism. With Matthias Jakob Schleiden, Schwann demonstrated that the cell is the basic unit of structure for animals and plants, an important development in cell theory.
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