472 



THE MUSCULAR SYSTEM. 



The medial part of each crus is wholly tendinous and is sometimes called the cms mediate ; 

 it is connected with its fellow of the opposite side by a tendinous band called the middle arcuate 

 ligament, which arches between them, in front of the aorta, and gives origin to fibres which 

 join the crura as they decussate to encircle the gullet. The most outlying part of the crus is 

 sometimes called the crus laterale ; its infero-lateral margin is continuous with the medial 

 lumbo-costal arch. The intermediate part of the crus is the crus intermedium ; the splanchnic 

 nerves pierce the diaphragm between it and the medial crus. The sympathetic trunk sometimes 

 pierces the diaphragm between the intermediate and lateral crura. 



The arcus lumbocostalis medialis (O.T. internal arcuate ligament) is a thickening 

 formed by the attachment of the psoas fascia to the body of the first lumbar 

 vertebra medially and its transverse process laterally. Stretching across the 

 superior end of the psoas muscle, the ligament gives origin to muscular fibres 

 which join the fibres of the crus. 



The arcus lumbocostalis lateralis (O.T. external arcuate ligament) is the 

 thickened superior border of the fascia over the quadratus lumber um muscle 



(Esophagus and its 

 opening 



Foramen quadratum 

 (for inferior vena cava) 



Middle arcuate 



ligament (in front of 



aortic opening) 



Medial lumbo-c< 



Lateral lumbo-costal 



QUADRATUS LUMBORUM 



MUSCLE 

 PSOAS MAJOR MUSCLE 



Left crus of diaphragm 



Right crus of diaphragm 



FIG. 419. THE DIAPHRAGM (from below). 



and is attached medially to the transverse process of the first lumbar vertebra, 

 and laterally to the last rib. It gives origin to a broad band of muscular 

 fibres, separated by an interval from the fibres arising from the medial lumbo- 

 costal arch which sweep upwards to the central tendon. 



From this extensive origin the muscular fibres of the diaphragm converge to 

 an insertion into a large trilobed central tendon called the centrum tendineum. Of 

 its lobes the right one is the largest, the middle or anterior is intermediate in 

 size, and the left is the smallest. It does not occupy the centre of the muscle, 

 being placed nearer the front than the back. The fibres of the crura are con- 

 sequently the longest ; those from the xiphoid process are the shortest. 



The diaphragm is pierced by numerous structures. The superior epigastric 

 artery enters the sheath of the rectus abdominis between its sternal and costal 

 origins ; the musculo-phrenic artery passes between its attachments to the seventh 

 and eighth ribs. The sympathetic trunk and the splanchnic nerves pierce, or pass 

 posterior to the diaphragm ; the last thoracic nerve passes behind the lateral 

 lumbo-costal arch ; and the aorta, the azygos vein, and thoracic duct pass between 

 the crura, underneath the middle arcuate ligament (hiatus aorticus or aortic opening}. 

 The special foramina are two in number. The foramen vence cavce (O.T. foramen 



