DIAPHRAGM (di'd-fram) (or midriff). In human anatomy a large fibro-muscular partition between cavities of the thorax and abdomen; it is convex toward the thorax, concave toward the abdomen, and consists of a central tendon and a muscular margin. The central tendon (see fig.) is trefoil in shape. The fleshy fibres rise, in front, from the back of the xiphoid cartilage, laterally by six serrations, from the inner surfaces of the lower six ribs, posteriorly from the five arcuate ligaments, two external, two in ternal, and one median. There are three large openings in the diaphragm; the aortic is behind the middle arcuate ligament and transmits the aorta, the vena azygos major, and the thoracic duct. In the right leaflet is an opening for the inferior vena cava and a branch of the right phrenic nerve, while in front and a little to the left of the aortic opening is one for the oesophagus and the two pneumogastric nerves.
Through the crura pass the splanchnic nerves. The sympa thetic nerves usually enter the abdomen behind the internal arcu ate ligaments. The phrenic nerves, which are the main supply of the diaphragm, divide before reaching the muscle and pierce it in a number of places to enter its abdominal surface, but some of the lower intercostal nerves assist in the supply.
For the action of the diaphragm see RESPIRATORY SYSTEM, ANATOMY OF.
Embryology.—The diaphragm is at first developed in the neck region of the embryo, and this accounts for the phrenic nerves, which supply it, rising from the fourth and fifth cervical. With growth of the body and development of the lungs the diaphragm shifts its position downwards. (See A. Keith, "On the Develop ment of the Diaphragm," Jour. of Anat. and Phys. vol. 39.) A. Paterson has recorded cases in which the left half of the dia phragm is wanting (Proceedings of the Anatomical Society of Gt. Britain, June 19oo; Jour. of Anat. and Phys. vol. 34), and occasionally deficiencies are found elsewhere, especially in the sternal portion.
Comparative Anatomy.—A complete diaphragm is character istic of the Mammalia; it usually has the human structure and relations except that below the Anthropoids it is separated from the pericardium by the azygous lobe of the lung. In some Mam mals, e.g., Echidna and Phocoena, it is entirely muscular. In the Cetacea it is remarkable for its obliquity; this allows a much larger lung space in the dorsal than in the ventral part of the thorax, and may be concerned with the equipoise of the animal. Below the Mammals incomplete partitions are found in Chelo nians, Crocodiles and Birds, and in Amphibians (Xenopus and Pipa).