Diaphragm (anatomy), wide muscular partition separating the thoracic, or chest cavity, from the abdominal cavity. It is a characteristic of all mammals and is rudimentary in some birds. In humans the diaphragm is attached to the lumbar vertebrae, the lower ribs, and the sternum or breastbone. Three major openings in the diaphragm allow passage of the esophagus, the aorta, the veins, the nerves, and the lymphatic and thoracic ducts.
As the diaphragm contracts and moves downward, the pectoralis minor and intercostal muscles pull the rib cage outward. The chest cavity expands, and air rushes into the lungs through the trachea to fill the resulting vacuum. When the diaphragm relaxes to its normal, upwardly curving position, the lungs contract, and air is forced out.
The diaphragm is roughly elliptical in humans. It slants upward, higher in front than in the rear, and is dome-shaped when relaxed. Contraction and expansion of the diaphragm are significant in breathing. During inhalation the diaphragm contracts, becoming flattened and increasing the capacity of the thorax. Air rushes into the lungs to fill the partial vacuum thus formed. Air is exhaled when the diaphragm relaxes. When the diaphragm contracts, it exerts pressure on the abdomen, stimulating the stomach, and thus aiding in the process of digestion. A hiccup is caused by a spasmodic, involuntary contraction of the diaphragm.