The endocrine (i.e., hormone-producing) cells of the pancreas are grouped in the islets of Langerhans. Discovered in 1869 by the famously egocentric German pathological anatomist Paul Langerhans, the islets of Langerhans constitute approximately 1 to 2% of the mass of the pancreas. There are about one million islets in a healthy adult human pancreas, which are distributed evenly throughout the organ, and their combined weight is 1 to 1.5 grams.