Engraving of Theodor Schwann (1810-82), German biologist. After completing his medical training in 1834, Schwann went into research. Almost immediately he made his first discovery. It had been thought that the hydrochloric acid in the stomach was responsible for digestion. Schwann isolated a substance from a stomach extract that greatly improved the acid's meat-dissolving power. This was pepsin, the first animal enzyme to be discovered. Schwann then went on to clearly show the cell theory of life, that all animal tissues were made from tiny cells, and first coined the term metabolism. After this Schwann spent a quiet life as an anatomy professor until his death.