Much confusion exists about the nomenclature of the different divisions of the pyloric part of the stomach. Uncertainty also exists about the anatomy and action of the pyloric phincter. Evidence that the pyloric sphincter consists of duodenal and gastric counterparts can be found in anatomical, physiological and roentgenological observations. The mechanism of emptying of the stomach is explained on this basis. Gastric retention in hypertrophy of the pyloric muscle in adults is due, at least partially, to inability of the gastricpyloric musculature to relax and contract fully, by whichmechanism stomach contents may normally be expelied into the duodenum.