Normal gastric peristalsis has been the object of much study, and since the early classic gastric experiments of Beaumont (5) on Alexis St. Martin, these investigations have dealt with many different aspects (2, 4, 9, 20, 23, 24). Observations on the thickness of the stomach wall during peristalsis are few in number, and the deductions are conflicting. Our purpose has been to devise and to utilize a method for studying this phase of gastric physiology. Anatomy The mucosa, or inner lining, of the stomach is about 1.5 mm. thick and forms numerous movable folds or rugae. It is composed largely of long gastric glands, between which a scanty lamina propria can be seen containing connective tissue, lymphocytes and a few plasma cells, eosinophils, and mast cells. Just below the glands of the mucosa and closely associated with them is the thin muscularis mucosae, consisting of an inner circular and a middle longitudinal layer of smooth muscle, with a third, circular, outer layer being seen in some places. Fibers of the inner circular layer extend upward between the glands. Beneath the mucosa is the submucous or areolar layer, a relatively thick layer of dense connective tissue in which may be seen lymphoid cells, mast cells, and eosinophils. The large blood and lymph vessels, venous plexuses, and autonomic network are contained in this layer. In the living animal it is flexible and permits a sliding movement of the mucous membrane on the underlying muscular wall, the muscularis externa. The muscularis externa, sometimes called muscularis propria or muscular coat, forms nearly half the thickness of the gastric wall. It is formed by three layers of fibers —the outer longitudinal, the middle circular, and the inner oblique fibers. The inner and outer layers are somewhat irregular, while the middle layer is more regular and continuous. The serous membrane or serosa is a thin layer of loose connective tissue attached to the muscularis externa and covered by mesothelium to form the peritoneal surface. It is continuous with the omentum. In evaluating the aspects of gastric motility, as presented later in this report, it is important to note that the muscularis mucosae is a thin layer and that it is intimately associated with the glands of the mucosa; that the submucosa is quite flexible and allows relatively easy motion of the mucosa on the underlying muscle; that the muscularis externa makes up about one-half of the stomach wall thickness and thus is considerably thicker and heavier than the thin layer of muscularis mucosae. Historical Cole (11, 12) has stated that peristalsis, as seen fluoroscopically, is an annular progressive contraction of the muscularis mucosae and is independent of the muscularis externa, whose function is one of tone and support. Cole believed that in the contraction at the sulcus angularis there is only sufficient space for two thicknesses of mucosa and not for the entire wall of the stomach.