386 ABDOMEN 



Posterior Attachment. The lateral lumbo - costal arch 

 O.T. ligamentum arcuatum externum) is a fibrous band 



ich stretches from the last rib to the transverse process 

 of the first lumbar vertebra. It arches in front of the 

 quadratus lumborum, and is merely the thickened upper part 

 of the fascia which covers that muscle, i.e., the anterior 

 lamella of the lumbar fascia. If the rib is pressed backwards 

 the arch will be rendered more distinct. The last thoracic 

 nerve passes laterally and downwards behind the lateral 

 lumbo-costal arch. The medial lumbo-costal arch (O.T. liga- 

 mentum arcuatum internuni) arches over the psoas major muscle, 

 and, like the lateral arch, is simply a thickening of the fascia 

 which covers the muscle. It is the stronger of the two arches, 

 and is attached by one extremity to the tip of the transverse 

 process of the first lumbar vertebra, and by the other to the 

 body of the second lumbar vertebra and the tendinous part 

 of the corresponding crus of the diaphragm. Fleshy fibres 

 arise from both of the 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 crus. 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 dia- 

 phragm 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 vertebras, 

 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 to the right of the median 

 plane from the bodies of the upper three lumbar vertebrae, 

 and the intervertebral fibro-cartilages between them. The left 

 crus is attached, on the left of the median plane, to the bodies 

 of the first two lumbar vertebrae and the intervening fibro-carti- 

 lage. It is much smaller than the right crus. Follow the crura 

 upwards ; opposite the last thoracic vertebra they are connected 

 across the middle line by a strong fibrous band which arches 

 over the aorta. From the upper border of that fibrous arch 

 fleshy fibres arise which join both crura ; and on that account 

 the arch 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 



