MMMMM  ms ms nos _mom m ^u . Is.^...' . u less mom

sus ems nms mow sm . A The fleshy fibres on each side of this opening act as a sphincter . Passing between the xiphoid and costal origins in front are the superior epigastric arteries, while the other terminal branches of the internal mammaries, the musculo-phrenics, pass through between two costal origins . Through the crura pass the splanchnic nerves, and in addition to these the left crus is pierced by the vena azygos minor . The sympathetic nerves usually enter the abdomen behind the internal arcuate 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 . The last thoracic or subcostal nerves pass behind the external arcuate ligament . For the action of the diaphragm see RESPIRATORY SYSTEM . 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 . From the mesoderm on the caudal side of the pericardium is developed the septumtransversum, and in this the central tendon is formed . The fleshy portion is developed on each side in two parts, an anterior or sterno-costal which is derived from the longitudinal neck musculature, probably the same layer front which the sternothyroid comes, and a spinal part which is a derivative of the transversalis sheet of the trunk .

Between these two parts is at one

time a gap, the spino-costal hiatus, and this is obliterated by the growth of the pleuro-peritoneal membrane, which may occasionally fail to close and so may form the site of a phrenic hernia . With the growth of the body and the development of the lungs the diaphragm shifts its position until it becomes the septum between the thoracic and abdominal cavities . (See A . Keith,"On the Development of the Diaphragm," Jour, of Anat. and Phys. vol . 39.) A . Paterson has recorded cases in which the left half of the diaphragm is wanting (Proceedings of the Anatomical Society of Gt . Britain, June 1900; Jour. of Anat. and Phys. vol . 34), and occasionally deficiencies are found elsewhere, especially in the sternal portion . For further details see Quain's Anatomy, vol. i . (London, 1908) . Comparative Anatomy.—A complete diaphragm, separating the thoracic from the abdominal parts of the coelom, is characteristic of the Mammalia; it usually has the human structure and relations exceptthat belowthe Anthropoids it is separated from the pericardium by the azygous lobe of the lung . In some Mammals, e.g .

Echidna and Phocoena, it is entirely muscular . In theCetacea it is remarkable for its obliquity; its vertebral attachment is much nearer the tail than its sternal or ventral one; 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 . (Otto Muller, " Untersuchungen fiber die Veranderung, welche die Respirationsorgane der Saugetiere durch die Anpassung an das Leben im Wasser erlitten haben," fen . Zeitschr. f . Naturwiss., 1898, p . 93.) In the Ungulata only one crus is found (Windle and Parsons, " Muscles of the Ungulata," Proc . Zool . Soc., 1903, p . 287) . Below the Mammals incomplete partitions between the pleural and peritoneal cavities are found in Chelonians, Crocodiles and Birds, and also inAmphibians (Xenopus and Pipa) . (F . G .