Theodor Schwann (German pronunciation: [ˈteːodoːɐ̯ ˈʃvan]; 7 December 1810 – 11 January 1882) was a German physician and physiologist. His most significant contribution to biology is considered to be the extension of cell theory to animals. Other contributions include the discovery of Schwann cells in the peripheral nervous system, the discovery and study of pepsin, the discovery of the organic nature of yeast, and the invention of the term "metabolism".