The diaphragm,(from to make a partition, or inclosure, of and to close,) because it divides the cavity of the thorax from that of the abdomen. Midriff; also called diazoma, disseptum, hypozoma,septum transversum, cinetus,suc-cingens membrana, discrimen thoracis etventris. Pliny calls it the praecordia, because it stands, he says, like a wall, to defend the heart. Hippocrates, with many of the ancients, call it they also gave the name of diaphragma to the division between the mouth and the oesophagus, terming it, by way of distinction, the diaphragma per oesophagum. Galen and Rufus Ephesrus call the cartilaginous partition between the nostrils, the diaphragm of the nose; but the only part now called diaphragm is that first mentioned.

The diaphragm is composed of two muscles; the small one rises by two tendons, called crura, from the second lumbar vertebra, and receives a slip from the first vertebra of the loins, and from the ligament which joins the last rib to the vertebra; the two crura then run upwards and grow fleshy. The large muscle 4 B 2 from the inside of the margin of the thorax, all round, from the false ribs to the xiphoid cartilage; its fibres shoot forwards, and form that triangular appearance called the centrum tendinosum, or centrum nerveum. Winslow calls it the middle aponeurosis, or aponeurotic plane of the diaphragm: M.de Bordeu calls it centre phrenique; and it is also called coostrum. There is a notch between the crurae of the lesser muscle where the aorta passes. In the larger muscle are two passages, one in the tendinous part, for the vena cava, which is situated there; because as the tendon is not capable of being contracted, the circulation will not be interrupted: the other is for the oesophagus, and is situated in the fleshy part. The reason seems to be,that the diaphragm, in its action, pressing upon the stomach, might cause an evacuation of its contents, unless at the same time it constricted the cavity of the oesophagus. If this be true, the diaphragm has probably little effect in promoting the action of vomiting. From considering the attachment of the diaphragm, it will be seen that its action will be to render itself flat, and thereby to enlarge the cavity of the thorax; but if we regard it as acting round the viscera as round a pulley, we may conceive how it both depresses the viscera and raises the ribs; at the same time still more enlarging the cavity of the thorax. Its veins are large, and go directly to the vena cava. The arteries are sometimes immediately from the aorta, and sometimes from the coeliac; a few branches are received also from the lumbales and adiposae. The nerves are from the plexus cervicalis on each side, and from the second pair of the vertebrae.

In inspiration, the diaphragm descends towards the belly; this is its proper motion resulting from muscular contraction. In expiration, it is relaxed and drawn upwards, forming a concavo-convex figure, the concave side being towards the belly. It assists in the expulsion of the excrements and foetus. It is in perpetual motion, and seems to have other and more important uses in our constitution than as yet are clearly understood. It is also a name of the septum scroti. See Scrotum.