446 THE MUSCLES AND FASCIA. 



separated from one another by slight indentations. The right leaflet is the largest : 

 the middle one, directed toward the ensiform cartilage, the next in size ; and the 

 left, the smallest. In structure, the tendon 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 it additional strength. 



The Openings connected with the Diaphragm are three large and several 

 smaller apertures. The former are the aortic, the 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 vertebrae ; 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 vertebrae, from the crus on one side to that 

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

 sometimes the left sympathetic nerve. Occasionally some tendinous fibres are 

 prolonged across the bodies of the vertebrae from the inner part of the lower end 

 of the crura, passing behind the aorta, and thus converting the opening into a 

 fibrous ring. 



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

 by the two crura, is placed above, and, at the same time, anterior, 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 (foramen quadraturti) is the highest; it is quad- 

 rilateral in form, tendinous 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 peritoneal 

 membrane of the abdominal cavity. 



The Diaphragm is arched, being convex toward the chest and concave to the 

 abdomen. The right portion forms a complete arch from before backward, being 

 accurately moulded over the convex surface of the liver, and having resting upon 

 it the concave base of the right lung. The left portion is arched from before back- 

 ward in a similar manner ; but the arch is narrower in front, being encroached upon 

 by the pericardium, and lower than the right, at its summit, by about three-quarters 

 of an inch. It supports the base of the left lung, and covers the great end of the 

 stomach, the spleen, and left kidney. At its circumference the Diaphragm is 

 higher in the mesial line of the body than at either side ; but in the middle of the 

 thorax the central portion, which supports the heart, is on a lower level than the 

 two lateral portions. 



Nerves. The Diaphragm is supplied by the phrenic nerves and phrenic plexus 

 of the sympathetic. 



Actions. The Diaphragm is the principal muscle of inspiration. When in a 

 condition of rest the muscle presents a domed surface, concave toward the abdo- 

 men ; and consists of a circumferential muscular and a central tendinous part. 

 When the muscular fibres contract, they become less arched, or nearly straight, 

 and thus cause the central tendon to descend, and in consequence the level of the 

 chest-wall is lowered, the vertical diameter of the chest being proportionally 

 increased. In this descent the different parts of the tendon move unequally. 

 The left leaflet descends to the greatest extent; the right to a less extent, on 



