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    diaphragm

    diaphragm
    Image source: NIH
    A thin dome-shaped skeletal muscle that separates the thoracic and abdominal cavities. The diaphragm plays an important role in breathing: it contracts with each inspiration, becoming flattened downward and increasing the volume of the thoracic cavity so that air is drawn into the respiratory tract, and then, with expiration, it relaxes and is restored to its dome shape.

    The diaphragm has a right and left muscular dome rising high into the thorax and separating the lungs from the abdominal viscera; and an intermediate flat central tendon, on which rests the heart. The muscular portion has a number of bony origins. In front, it arises from the back of the xiphisternum; at the sides, from the lower ribs; and in the midline posteriorly it helps form the upper part of the posterior abdominal wall by arising in two pillars or crura from the sides of the upper lumbar vertebrae.

    There are several apertures for structures passing between thorax and abdomen. The aortic aperture is embraced by the two crura as they cross in front of the twelfth thoracic vertebra. The esophageal aperture is in the left dome at the level of the tenth thoracic body, and the opening for the inferior vena cava lies in the central tendon to the right of the midline at the level of the ninth thoracic body.


    Pathology

    A hiatus hernia can result when the upper part of the stomach protrudes into the thorax through a tear or weakness in the diaphragm.

    If the diaphragm is struck, or otherwise spasms, breathing becomes difficult. This is what is meant by "having the wind knocked out of you."

    Hiccups are due to periodic, involuntary contractions of the diaphragm.


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