ABDOMINAL CAVITY 387 



fibres, it is protected by a fibrous arch. Of this nature is the arch in 

 question, and also the fibrous arch thrown over the profunda artery on the 

 back of the humerus, and the fibrous arches in the adductor magnus for the 

 passage of the perforating arteries, and of the femoral artery itself. 



Above the level of the ligamentum arcuatum medium the 

 fleshy fibres of the crura diverge and ascend to join the 

 posterior border of the central tendon. The most medial 

 fibres of each crus, however, decussate so as to separate the 

 aortic from the cesophageal openings. The decussating 

 fasciculus of the right crus is always the larger of the two, 

 and, moreover, it frequently passes behind the decussating fasci- 

 culus of the left crus, but may pass in front of it (Fig. 184). 



It is the custom of some authors to divide each crus into three parts, a 

 lateral crus, an intermediate crus, and a medial crus. The lateral crus is 

 formed by the fibres which spring from the medial lumbo-costal arch. It is 

 separated from the intermediate crus by the sympathetic trunk. The 

 intermediate crus is separated from the medial crus by the splanchnic 

 nerves. In many cases, however, the subdivisions are not distinct. 



Centrum Tendineum. The central tendon of the dia- 

 phragm is exceedingly strong. It is composed of tendinous 

 bundles running in different directions, and closely woven 

 together in such a manner as to give it a plaited appearance. 

 The appearance is well seen from the abdominal surface. 

 The central tendon is somewhat semilunar in outline, with a 

 broader and shorter right and a narrower and longer left horn. 

 Upon all sides it is surrounded by muscular fibres of which 

 those attached to the sternum are much the shortest. 



Foramina of the Diaphragm. The continuity of the 

 diaphragm is broken by three large openings, and by some 

 smaller apertures or fissures for the passage of the splanchnic 

 nerves, and the vena hemiazygos. The three main open- 

 ings receive the names of the most important objects which 

 they transmit. They are 



1. The aortic. 



2. The vena caval. 



3. The cesophageal. 



Hiatus Aorticus. The aortic opening is in the median plane, 

 in front of the first lumbar vertebra, and between the crura of 

 the diaphragm. It is bounded in front by the fibrous middle 

 arcuate ligament, which arches across the median plane and 

 connects the tendinous portions of the two crura. The struc- 

 tures which pass through the aortic opening are (i) the 

 n25 a 



