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Encyclopedia Britannica - Main :: MIC-MOL |
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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
body
Quain
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. P.)End of Article: MMMMM If you wish, you can link directly to this article.
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