DIAPHRAGMATIC REGION. 291 



The ligamentum arcuatum externum is the thickened upper margin of the 

 anterior lamella of the transversalis fascia ; it arches across the upper part of the 

 Quadratus lumborum, being attached, by one extremity, to the front of the trans- 

 verse process of the second lumbar vertebra ; and, by the other, to the apex and 

 lower margin of the last rib. 



To the spine, the Diaphragm is connected by two crura, which are situated on 

 the bodies of the lumbar vertebrae, one on each side of the aorta. The crura, at 

 their origin, are tendinous in structure ; the right crus, larger and longer than the 

 left, arising from the anterior surface of the bodies and intervertebral substances 

 of the second, third, and fourth lumbar vertebrae ; the left, from the second and 

 third ; both blending with the anterior common ligament of the spine. A tendi' 

 nous arch is thrown across the front of the vertebral column, from the tendon of 

 one crus to that of the other, beneath which pass the aorta, vena azygos major, 

 and thoracic duct. The tendons terminate in two large fleshy bellies, which, with 

 the tendinous portions above alluded to, are called the crura or pillars of the dia- 

 phragm. The outer fasciculi of the two crura are directed upwards and outwards 

 to the central tendon ; but the inner fasciculi decussate in front of the aorta, and 

 then diverge, so as to surround the oesophagus before ending in the central 

 tendon. The most anterior and larger of these fasciculi is formed by the right 

 crus. 



The Central or Cordiform Tendon of the Diaphragm is a thin tendinous apo- 

 neurosis, situated at the centre of the vault of this muscle, immediately beneath 

 the pericardium, with which its circumference is blended. It is shaped somewhat 

 like a trefoil leaf, consisting of three divisions or leaflets, separated from one 

 another by slight indentations. The right leaflet is the largest ; the middle one, 

 directed towards the ensiform cartilage, the next in size ; and the left, the smallest. 

 In structure, it is composed of several planes of fibres, which intersect one another 

 at various angles, and unite into straight or curved bundles, an arrangement which 

 affords additional strength to the tendon. 



The Openings connected with the Diaphragm are three large and several 

 smaller apertures. The former are the aortic, oesophageal, and the opening for the 

 vena cava. 



The aortic opening is the lowest and the most posterior of the three large aper- 

 tures connected with this muscle. It is situated in the middle line, immediately 

 in front of the bodies of the vertebras ; and is, therefore, behind the Diaphragm, 

 not in it. It is an osseo-aponeurotic aperture, formed by a tendinous arch thrown 

 across the front of the bodies of the vertebra, from the crus on one side to that 

 on the other, and transmits the aorta, vena azygos major, thoracic duct, arid occa- 

 sionally the left sympathetic nerve. 



The cesophageal opening, elliptical in form, muscular in structure, and formed 

 by the two crura, is placed above, and, at the same time, anterior to, and a little to 

 the left of, the preceding. It transmits the oesophagus and pneumogastric nerves. 

 The anterior margin of this aperture is occasionally tendinous, being formed by 

 the margin of the central tendon. 



The opening for the vena cava is the highest ; it is quadrilateral in form, ten- 

 dinous in structure, and placed at the junction of the right and middle leaflets of 

 the central tendon, its margins being bounded by four bundles of tendinous fibres, 

 which meet at right angles. 



The right crus transmits the sympathetic and the greater and lesser splanchnic 

 nerves of the right side ; the left crus, the greater and lesser splanchnic nerves of 

 the left side, and the vena azygos minor. 



The Serous Membranes in relation with the Diaphragm are four in number ; 

 three lining its upper or thoracic surface, one its abdominal. The three serous 

 membranes on its upper surface are the pleura on either side, and the serous layer 

 of the pericardium, which covers the middle portion of the tendinous centre. 

 The serous membrane covering its under surface is a portion of the general peri- 

 toneal membrane of the abdominal cavity. 



