The Central Tendon. The central tendon of the diaphragm is a thin but strong 

 aponeurosis situated near the center of the vault formed by the muscle, but some- 

 what closer to the front than to the back of the thorax, so that the posterior muscu- 

 lar fibers are the longer. It is situated immediately below the pericardium, with 

 which it is partially 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, directed toward the xiphoid process, 

 the next in size, and the left the smallest. In structure the tendon is composed 

 of several planes of fibers, which intersect one another at various angles and unite 

 into straight or curved bundles an arrangement which gives it additional strength. 



Openings in the Diaphragm. The diaphragm is pierced by a series of apertures 

 to permit of the passage of structures between the thorax and abdomen. Three 

 large openings the aortic, the esophageal, and the vena caval and a series of 

 smaller ones are described. 



The aortic hiatus is the lowest and most posterior of the large apertures; it lies 

 at the level of the twelfth thoracic vertebra. Strictly speaking, it is not an aperture 

 in the diaphragm but an osseoaponeurotic opening between it and the vertebral 

 column, and therefore behind the diaphragm; occasionally some tendinous fibers 

 prolonged across the bodies of the vertebras from the medial parts of the lower ends 

 of the crura pass behind the aorta, and thus convert the hiatus into a fibrous ring. 

 The hiatus is situated slightly to the left of the middle line, and is bounded in front 

 by the crura, and behind by the body of the first lumbar vertebra. Through it 

 pass the aorta, the azygos vein, and the thoracic duct; occasionally the azygos 

 vein is transmitted through the right cms. 



The esophageal hiatus is situated in the muscular part of the diaphragm at 

 the level of the tenth thoracic vertebra, and is elliptical in shape. It is placed 

 above, in front, and a little to the left of the aortic hiatus, and transmits the 

 esophagus, the vagus nerves, and some small esophageal arteries. 



The vena caval foramen is the highest of the three, and is situated about the level 

 of the fibrocartilage between the eighth and ninth thoracic vertebrae. It is quad- 

 rilateral in form, and is placed at the junction of the right and middle leaflets 

 of the central tendon, so that its margins are tendinous. It transmits the inferior 

 vena cava, the wall of which is adherent to the margins of the opening, and some 

 branches of the right phrenic nerve. 



Of the lesser apertures, two in the right crus transmit the greater and lesser 

 right splanchnic nerves; three in the left crus give passage to the greater and lesser 

 left splanchnic nerves and the hemiazygos vein. The gangliated trunks of the 

 sympathetic usually enter the abdominal cavity behind the diaphragm, under 

 the medial lumbocostal arches. 



On either side two small intervals exist at which the muscular fibers of the 

 diaphragm are deficient and are replaced by areolar tissue. One between the 

 sternal and costal parts transmits the superior epigastric branch of the internal 

 mammary artery and some lymphatics from the abdominal wall and convex 

 surface of the liver. The other, between the fibers springing from the medial and 

 lateral lumbocostal arches, is less constant; when this interval exists, the upper 

 and back part of the kidney is separated from the pleura by areolar tissue only. 



Variations. The sternal portion of the muscle is sometimes wanting' and more rarely defects 

 occur in the lateral part of the central tendon or adjoining muscle fibers. 



Nerves. The diaphragm is supplied by the phrenic and lower intercostal nerves. 



Actions. The diaphragm is the principal muscle of inspiration, and presents the form of 

 a dome concave toward the abdomen. The central part of the dome is tendinous, and the peri- 

 cardium is attached to its upper surface; the circumference is muscular. During inspiration the 

 lowest ribs are fixed, and from these and the crura the muscular fibers contract and draw down- 

 ward and forward the central tendon with the attached pericardium. In this movement the 

 curvature of the diaphragm is scarcely altered, the dome moving downward nearly parallel 



