388 ABDOMEN 



aorta, (2) the thoracic duct, and (3) the vena azygos in that 

 order from left to right. 



Foramen Vena Cavcz. The vena caval opening 'is at a higher 

 level, being situated opposite the fibro-cartilage between the 

 eighth and the ninth thoracic vertebrae, in front and slightly 

 to the right of the aortic opening. It is placed in the posterior 

 part of the central tendon at the junction between its middle 

 and the right horn. Its form is somewhat quadrangular, and 

 its margins are prolonged upon the walls of the vena cava as 

 that vessel passes through it. The contraction of the mus- 

 cular fibres of the diaphragm will therefore tend to increase 

 the size of the opening and the calibre of the vein which 

 it holds. 



In addition to the vena cava, one or two minute twigs 

 from the right phrenic nerve may be transmitted through the 

 vena caval opening. 



Hiatus (Esophageus. The oesophageal opening is an oval or 

 elliptical foramen in the muscular part of the diaphragm. 

 It lies in front and slightly to the left of the aortic aperture, 

 and also at a higher level, being placed opposite the tenth 

 thoracic vertebra. In a few cases its upper or anterior border 

 is tendinous, being formed by the posterior margin of the 

 central tendon. Posteriorly, it is separated from the aortic 

 opening by the decussation of the medial fibres of the crura. 



The cesophageal opening transmits the oesophagus, the two 

 vagi nerves, and some oesophageal vessels. 



The three large openings of the diaphragm, therefore, 

 present very different features. The aortic opening is bounded 

 by a fibrous arch behind the diaphragm, and it can in no way 

 be affected by the contraction of the muscular fibres. The 

 vena caval opening is in the central tendon, and its margins 

 are attached to the wall of the vessel which it transmits ; con- 

 traction of the diaphragm must therefore have a tendency to 

 open this aperture to its widest extent. The cesophageal 

 opening is placed in the muscular part, and consequently it is 

 probable that the fibres which surround it are capable of 

 exercising a constricting influence upon the oesophagus, and 

 in that way help to prevent regurgitation of food during the 

 descent of the diaphragm. 



Little need be said regarding the smaller foramina. Each 

 crus is pierced by the three splanchnic nerves, and the left 

 crus is perforated, in addition, by the hemiazygos vein. The 



