diaphragm /dī″əfram/ [Gk, diaphragma, partition] , 1. (in anatomy) a dome-shaped musculofibrous partition that separates the thoracic and abdominal cavities. The convex cranial surface of the diaphragm forms the floor of the thoracic cavity; the concave surface forms the roof of the abdominal cavity. This partition is pierced by various openings through which pass the esophagus and inferior vena cava. The diaphragm aids respiration by moving up and down. During inspiration it moves down and increases the volume of the thoracic cavity. During expiration it moves up, decreasing the volume. During deep inspiration and expiration the range of diaphragmatic movement in the adult is about 30 mm on the right side and about 28 mm on the left side. The height of this structure also varies with the degree of distension of the stomach and the intestines and with the size of the liver. It is innervated by the phrenic nerve from the cervical plexus.2. See contraceptive diaphragm. 3. (in optics) an opening that controls the amount of light passing through an optical network. 4. a thin, membranous partition, as that used in dialysis. 5. (in radiography) a metal plate with a small opening that limits the diameter of the radiographic beam.6. See also diaphragm stethoscope. −diaphragmatic, adj.
