starting point for "the introduction of calculation to physiology". He developed and described an experimental method to calculate the contraction force of the muscle, by controlling and measuring the other variables involved. His measurement technique was developed and used later by Emil du Bois-Reymond and others. Schwann's notes suggest that he hoped to discover regularities and laws of physiological processes. Pepsin In 1835, relatively little was known about digestive processes. William Prout had reported in 1824 that the digestive juices of animals contained hydrochloric acid. Schwann realized that other substances in digestive juices might also help to break down food. At the beginning of 1836, Schwann began to study digestive processes. He conceptualized digestion as the action of a physiological agent, which, though not immediately visible or measurable, could be characterized experimentally as a "peculiar specific substance". Eventually Schwann found the enzyme pepsin, which he successfully isolated from the stomach lining and named in 1836. Schwann coined its name from the Greek word pepsis, meaning "digestion" (from peptein "to digest"). Pepsin was the first enzyme to be isolated from animal tissue. He demonstrated that it could break down the albumin from egg-white into peptones. Even more importantly, Schwann wrote, by carrying through such analyses one could eventually "explain the whole developmental process of life in all organized bodies." During the next year, he studied both decomposition and respiration, constructing apparatus that he would later adapt for the study of yeast. Yeast, fermentation, and spontaneous generation Next Schwann studied yeast and fermentation. His work on yeast was independent of work done by Charles Cagniard de la Tour and Friedrich Traugott Kützing, all of whom published work in 1837. By 1836, Schwann had carried out numerous experiments on alcohol fermentation. Powerful microscopes made it possible for him to observe yeast cells in detail and recognize that they were tiny organisms whose structures resembled those of plants. Schwann went beyond others who simply had noted the multiplication of yeast during alcoholic fermentation, first by assigning yeast the role of a primary causal factor, and then by claiming it was alive. Schwann used the microscope to carry out a carefully planned series of experiments that contraindicated two popular theories of fermentation in yeast. First he controlled the temperature of fluid from fermenting beer in a closed vessel in the presence of oxygen. Once heated, the liquid could no longer ferment. This disproved Joseph Louis