Diaphragm, the transverse muscle which separates the thoracic from the abdominal cavity in mammalian vertebrates. It is flattened, nearly circular, fleshy at the edges, tendinous in the centre, elongated, and ends in a point behind. In front it is attached to the ensiform cartilage of the breast bone, on the sides to the internal surface of the last six ribs, behind to the transverse process of the first lumbar vertebra and to the bodies of the first three vertebras of the loins by tendinous slips; the fleshy fibres of the last form the pillars of the diaphragm, and their fasciculi cross each other in such a way as to leave two openings, one superior and anterior, giving passage to the oesophagus and par vagum nerve, the other interior, for the passage of the aorta, thoracic duct, and vena azygos; the tendinous centre has been compared in shape to a leaf of clover. Between the middle and right portion of the tendinous centre is the opening for the passage of the inferior vena cava. The diaphragm is in relation, above, with the pericardium in the middle, and with the pleurae, base of the lungs, and walls of the chest on the sides; below, with the aorta in the middle, the kidneys, renal capsules, pancreas, and duodenum; on the right side with the liver, and on the left with the stomach and spleen.

The direction of the posterior fibres is nearly vertical; all the others converge toward the tendinous centre. The diaphragm is the great muscle of respiration; when it contracts, its central tendinous portion is drawn downward, the cavity of the chest is enlarged, and air rushes in to expand the lungs during the act of inspiration; when forcibly contracted, it may act as an assistant to the abdominal expiratory muscles by diminishing the size of the base of the chest; by its action on the abdominal viscera it aids in the expulsion of faeces and urine; in ordinarily tranquil breathing the diaphragm is sufficient for the performance of the function. In animals the extent 'and position of the diaphragm vary according to the number of the ribs; in those whose ribs extend nearly to the pelvis, as in the horse, the thoracic convexity of the diaphragm is much greater than in man.

The Diaphragm viewed from the Lower or Abdominal Side.

The Diaphragm viewed from the Lower or Abdominal Side.

V. C. /., the vena cava inferior; OE., the oesophagus; Ao., the aorta; Th. D., the thoracic duct, cut where they pass through the diaphragm, the broad white tendinous middle of which is easily distinguished from the radiating muscular fibres which pass down to the ribs and into the pillars in front of the vertebrae.

This important muscle is liable to malformations, wounds, and morbid conditions; its total absence is incompatible with any other than intra-uterine life, as aerial respiration would be impossible; its partial absence, like dilatation of its natural openings, or laceration of its fibres, is accompanied by the passage of more or less of the abdominal viscera into the chest, impeding the action of the heart, lungs, and digestive organs; in such cases, the liver, stomach, omentum, ileum, caacum, and part of the colon, have been found above the diaphragm. This partition is also liable to penetrating wounds, and to rupture from external violence, the latter being the most dangerous; in either case, nature alone can remedy the evil. It is sometimes inflamed, and in the rheumatic diathesis is the seat of the most acute pain, increased by every respiratory act, and forcing the patient to breathe almost entirely by means of the abdominal muscles. Spasmodic contractions are familiarly known by the phenomenon of hiccough; this is sometimes merely a nervous affection, and at others is a symptom of peritonitis, strangulated hernia, and other abdominal diseases.