A zone of increased intraluminal pressure interposed between the tubular esophagus and presumed to be due to tonically contracted circular muscle fibers was described by Fyke, Code, and Schlegel (1) in 1956. Frequently only the narrow abdominal portion of this sphincter imposes a pressure barrier between the gastric and esophageal lumina when the pleuroperitoneal pressure gradient is increased (2-6). Reflux in patients with sliding hiatal hernia has been attributed to the upward displacement of the sphincter and the absence of the abdominal segment (7,8).Although there is agreement concerning the importance of the compression effect of increased intra-abdominal pressure during inspiration in the maintenance of sphincter competency, the role of the diaphragmatic crura remains controversial. Monges (9) showed that a balloon filled with radiopaque liquid in the esophagogastric area is compressed during deep inspiration in a ring-like fashion that he attributed to right crural contraction. Our studies were designed to contrast the effects of passive abdominal compression and deep inspiration on intraluminal pressure and pH and thereby to differentiate a crural (pinchcock) action from a dome (intra-abdominal pressure) action of the diaphragm. Methods and MaterialsIntraluminal pressures and pH were measured concurrently. Four water-filled, open-tip, polyethylene cath-

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