DIAPHRAGM (Gr. diaphragma, a partition). This is the name applied in anatomy to designate the transverse muscle which, in man and the mammalia generally, separates the cavity of the thorax or chest from that of the abdomen or belly. In form, it is nearly circular; it is fleshy at its edges, tendinous in its center, and ending in a point below. In front, it is attached to the ensiform cartilage of the sternum, or breastbone; laterally, to the inner surfaces of the six lower ribs; and posteriorly, to two tendinous arches on either side, termed the ligamenta arcuata, and to the anterior surface of the bodies of the second, third, and fourth lumbar vertebrw on the right, and only the second and third on the left side; these origins from the vertebrre forming two large fleshy bellies (termed the crura), which ascend to join the central tendinous portion. The D. presents three principal openings: one quadrilateral, in the tendinous center, for the upward passage of the inferior vena cave; one of an elliptic shape, formed by the two crura for the oesophagus and pneumogastric nerves; and a third for the aorta, the azygos vein, and the thoracic duct.
The diaphragm is in relation superiorly with the pleurre and pericardium, inclosing the lungs and heart; inferiorly, on the left side with the stomach and spleen, on the right with the convex upper surface of the liver; posteriorly, with the kidneys and supra-renal capsules and the duodenum; and by its circumference with the ribs and intercostal muscles, with the sternum, and with the vertebral column.
It is convex superiorly, and concave inferiorly. When it contracts, its upward con vexity approximates to a plane surface, and the cavity of the chest being thus enlarged, air rushes in to fill the partial vacuum, and expands the lungs during the act of inspira tion. It is thus an inspiratory muscle, and is the sole agent in tranquil inspiration. The enlargement of the thoracic cavity caused by the contraction of the diaphragm, must obviously be associated with a corresponding diminution of the abdominal space. Hence this muscle, by its action on the abdominal viscera, aids in the expulsion of the faces and urine.
Spasmodic action of the diaphragm produces hiccough and sobbing; and in laughing, the alternate contractions and relaxations of this muscle occur with increased rapidity. Stoppage of the action of the diaphragm, whether from great external pressure or from paralysis, is very speedily fatal.