370 MUSCLES AND FASCIA. 



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 

 apertures connected with this muscle. It is situated in the middle line, im- 

 mediately in front of the bodies of the vertebree ; and is, therefore, behind the 

 Diaphragm, not in it. It is an osseo-aponeurotic aperture, formed by a ten- 

 dinous 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, and occasionally 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, 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 peritoneal membrane of the abdominal cavity. 



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

 the abdomen. The right portion forms a complete arch from before backwards, 

 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 backwards 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. 

 The central portion, which supports the heart, is higher, in front at the sternum, 

 and behind at the vertebras, than the lateral portions; the reverse is the case 

 in the parts further removed from the surface of the body. 



The height of the Diaphragm is constantly varying during respiration, the 

 muscle being carried upwards or downwards from the average level ; its height 

 also varies according to the degree of distension of the stomach and intestines, 

 and the size of the liver. After a forced expiration, the right arch is on a 

 level, in front, with the fourth costal cartilage ; at the side, with the fifth, sixth, 

 and seventh rib.s ; and behind, with the eighth rib ; the left arch being usually 

 from one to two ribs breadth below the level of the right one. In a forced 

 inspiration, it descends from one to two inches; its slope would then be repre- 

 sented by a line drawn from the ensiform cartilage towards the tenth rib. 



Nerves. The Diaphragm is supplied by the phrenic nerves. 



Actions. The action of the Diaphragm modifies considerably the size of the 

 chest, and the position of the thoracic and abdominal viscera. During a forced 

 inspiration, the cavity of the thorax is enlarged in the vertical direction from 

 two to three inches, partly by the ascent of the walls of the chest, partly by 

 the descent of the Diaphragm. The chest, consequently, encroaches upon the 

 abdomen ; the lungs are expanded, and lowered, in relation with the ribs, 

 nearly two inches; the heart being drawn down about an inch and a half; the 

 descent of the latter organ taking place indirectly through the medium of its 



