For a few days after upper abdominal operations in man the thoracic parietes are characteristically altered. Both the diaphragm and the ribs are markedly elevated, so that the space occupied by the lungs is shorter and broader; also, the diaphragmatic and costal movements are much reduced. The cause of this has been assigned to several factors: tight surgical dressings about the abdomen, reflex spasm of the abdominal and intercostal muscles, reflex inhibition of the diaphragm, gastro-intestinal distention, and pneumoperitoneum. Clinical studies1, 2, 3 have furnished very suggestive evidence, but opinions are still conflicting as to the chief cause. We felt that, if laboratory animals also present these postoperative changes, they would be better adapted than man to the study of the causes and principles involved. Therefore, standard upper abdominal operations were performed on a series of dogs, with the essential surgical circumstances the same as those used in man (except, of course, the confinement in b...