ABDOMINAL CAVITY 537 



arise from both of these arcuate bands; those from the medial 

 arch are more numerous and better marked than those which 

 take origin from the lateral arch, and they are continuous 

 with the fleshy fibres of the cms. Very frequently a gap or 

 interval exists between the fibres which spring from the 

 lateral arch and those which arise directly from the last 

 rib. The anterior, lateral, and posterior attachments of the 

 diaphragm are therefore marked off by intervals from each 

 other. 



The crura of the diaphragm are two thick fleshy processes 

 which descend upon the bodies of the upper lumbar vertebrae, 

 tapering as they proceed downwards. They end in pointed 

 tendinous extremities. The right crus is the larger and 

 longer of the two. It takes origin from the bodies of the 

 upper three lumbar vertebrae, and the intervertebral discs 

 which intervene between them. The left crus springs from 

 the left side of the bodies of the first two lumbar vertebrae. 

 It is much smaller, and lies upon a plane posterior to the 

 right crus. Follow the crura upwards ; opposite the last 

 dorsal vertebra they will be observed to be connected across 

 the middle line by a strong fibrous band which arches over 

 the aorta. From the upper border of this fibrous arch fleshy 

 fibres arise which join both crura, and on this account it 

 is called the ligamentum arcuatum medium. 



It may be regarded as a law that wherever an artery pierces the origin 

 or insertion of a muscle, and comes to lie between bone and muscular 

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

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

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

 for the passage of the perforating arteries, and 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 usually passes behind the decussating 

 fasciculus of the left crus. 



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 cord. The inter- 

 mediate crus is separated from the medial crus by the splanchnic nerves. 

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



