Heidenhain, Rudolf Peter Heinrich (1834–1897), German histologist and physiologist. Heidenhain studied glandular secretions in the salivary glands, pancreas, breast, and stomach. He formulated a law of glandular secretion, which states that secretion always involves a change in the structure of the gland. He considered all secretory phenomena to be intracellular processes. Heidenhain is also known for his descriptions of the columnar cells of the uriniferous tubules (1861), of certain large border cells of the gastric glands (1870), and the crescentic cells or demilunes of the mucous glands.