 However, after three semesters, Koch decided to change his area of study to medicine, as he aspired to be a physician.During his fifth semester of medical school, Jacob Henle, an anatomist who had published a theory of contagion in 1840, asked him to participate in his research project on uterine nerve structure.In his sixth semester, Koch began to research at the Physiological Institute, where he studied the secretion of succinic acid, which is a signaling molecule that is also involved in the metabolism of the mitochondria.