Theodor Schwann (December 7, 1810 - January 11, 1882) was a German physiologist. His many contributions to biology include the development of cell theory, 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. He discovered the striated muscle in the upper esophagus and initiated research into muscle contraction. In his in Microscopic Investigations on the Accordance in the Structure and Growth of Plants and Animals, he wrote that "All living things are composed of cells and cell products". In the course of his verification of cell theory, in which he traversed the whole field of histology, he proved the cellular origin and development of the most highly differentiated tissues including nails, feathers, and tooth enamel. He died in 1882 at the age of 71.