DIAPHRAGM, di'a-fram. (1) In anatomy, a dome-shaped muscular partition found in the mammals, separating the abdominal cavity from the pleural cavities containing the lungs. It is attached to the lower end of the sternum, the ninth, tenth, eleventh and twelfth ribs, and the upper three or four lumbar vertebra:. Its general structure is that of a system of muscular fibres radiating from a central tendinous portion. It is pierced by the inferior vena cava, the cesopha gus, the aorta, the thoracic duct, the greater splanchnic nerve, the azygos and hemiazygos veins and the sympathetic trunks. The dia phragm is lined above by pleura, below by peritoneum, and on its lower surface are sup posed to be the stomata by which the lymphatic system opens into the peritoneal cavity, although the existence of these openings is still a matter of dispute. It is innervated by the third, fourth and sometimes the fifth cervical nerves and by the phrenic nerves, which originate from the cervical plexus. It receives blood from the interior phrenic arteries below and behind, the diaphragmatic arteries above and behind, and the superior phrenic, musculo-phrenic, and superior epigastnc arteries in front. Its venous
circulation is carried on by the inferior phrenic vein. The diaphragm originates from the cervi cal myotomes of the embryo. In the course of development it undergoes the same displacement backwards that characterizes the heart and lungs. This is shown in the adult by the cervical innervation of the diaphragm.
In breathing, the diaphragm causes inspira tion by contracting, thus decreasing its upward curvature and enlarging the chest cavity. In expiration it is relaxed. It is the principal respiratory muscle. See RESPIRATION.
(2) In optics, an annular disc in a camera or telescope or other optical instrument, to exclude some of the marginal rays of a beam of light. The original form of this beautiful contrivance is the iris of the eye, which shuts out strong light and regulates the quantity ad mitted.