ABDOMINAL CAVITY 539 



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

 dorsal vertebra. In some cases its upper or anterior border 

 is tendinous, and 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 gullet and the two 

 vagi nerves. 



The three large openings of the diaphragm, therefore, present very 

 different features. The aortic opening is 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 may be capable of exercising a constricting influence upon 

 the oesophagus, and in this 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 likewise perforated by the hemiazygos vein. The 

 superior epigastric artery descends in the interval between the 

 sternal and costal attachments of the diaphragm, and the 

 musculo-phrenic artery passes between two slips of the costal 

 attachment opposite the eighth or ninth rib. 



VESSELS ON THE POSTERIOR WALL OF THE ABDOMEN. 



Dissection. The abdominal aorta and its branches and the inferior vena 

 cava must now be cleaned. In doing this, care should be taken of the 

 gangliated cord of the sympathetic which extends downwards, on the 

 vertebral column, along the anterior border of the psoas magnus. It is 

 necessary to bear in mind that the lumbar branches of the aorta, as they 

 proceed laterally, pass behind the sympathetic cord. Separate the right crus 

 of the diaphragm from the aorta, and dissect in the interval between them. 

 Here the cisterna chyli and the vena azygos will be found. A chain of 

 lymph glands, termed the lumbar glands ', will be noticed in relation to the 

 aorta. The only branches of the aorta which are liable to injury are the 

 internal spermatic arteries. These are two slender arteries which spring 

 from the front of the vessel a short distance below the renal arteries. 

 They are so small that they are apt to be overlooked. 



Aorta Abdominalis (Abdominal Aorta). The abdominal 

 aorta is the direct continuation of the thoracic aorta. It 

 begins in the median plane, in the upper part of the aortic 

 orifice of the diaphragm, in front of the lower border of the 

 last dorsal vertebra, and it ends in front of the lower border 



