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The diaphragm is the muscular sheet dividing the superior thoracic cavity from the inferior abdominal cavity. Derived from the same source as the deepest layer of body wall muscles, it arise from tendinous insertions into the deep surface of the costal margin, xiphisternum and lumbar vertebrae to form two muscular domes superiorly. These domes merge medially and slightly inferiorly to form a central tendon that is freely mobile and not attached to bone. A number of structures traverse the diaphragm as they pass between thoracic and abdominal cavities.
Movements of the diaphragm are the major cause of the inspiratory phase of respiration. With contraction, the domes flatten and the pressure in the thoracic cavity becomes more negative, so drawing air inwards through the mouth.
The diaphragm may form incorrectly during development leading to herniation of abdominal tissue into the thorax. Similar herniation can occur after severe trauma.