THE CERVICAL MUSCLES. 



557 



costal portion, the fibres of which take their orighi from the cartilages of the lower 

 six ribs, interdigitating with the origins of the transversalis abdominis. In conti- 

 nuity with the coital part is the lumbar part, whose fibres take orighi (i) from two 

 tendinous arches, the internal ^.wdi external arcuate ligaments, which pass over the 

 upper portions of the psoas (arcus lumbocostalis medialis) and the quadratus lum- 

 borum muscles (arcus lumbocostalis lateralis) respectively, stretching between the 

 twelfth rib and the transverse process of the first lumbar vertebra, and (2) by two 

 downward prolongations, the crura, from the anterior and lateral surfaces of the 

 upper three or four lumbar vertebrae. 



The right crus usually extends somewhat farther downward than the left, whose 

 attachment does not pass below the second or third vertebra. Each crus has been 

 divided into three portions, medial, intermediate, and lateral, which are not, how- 

 ever, always clearly recognizable, although indicated by the passage of certain struc- 

 tures from the thorax to the abdomen. Thus, between the medial and intermediate 



Interval between sternal and costal portions 



Fig. 549. 



Lower end of sternum 



Inferior vena cava 



Right portion of . 



central tendon \i* 



Right crus 



Right greater 

 splanchnic nerve. 



XII rib. 



Middle portion of 

 central tendon 



CEsophagus 



Left portion ot 

 central tendon 



Aorta 

 Thoracic duct 



Inferior vena cava 

 Bifurcation of aorta, turned forsvard 



External arcuate ligament 

 I Quadratus lumborum 

 Internal arcuate ligament 

 Left crus Psoas magnus 



Diaphragm, viewed from below and the left. 



crura the greater splanchnic nerve and the azygos (or hemiazygos) veins pass, while 

 between the intermediate and lateral crura is the sympathetic trunk. 



The two crura, as they pass upward, leave between them an opening, the hiatus 

 aorticus, which is bridged over by a tendinous band {median arcuate ligament) and 

 gives passage to the aorta and thoracic duct. Just behind the posterior margin of 

 the centrum tendineum the crural fibres diverge to surround in a sphincter-like 

 manner the hiatus cesophageiis, through which pass the oesophagus and the vagus 

 nerves and oesophageal branches from the gastric artery and veins. 



The centriim tendineum, into which the fibres of the three portions insert, is 

 situated somewhat nearer the anterior than the posterior margin of the diaphragm, 

 so that the fibres of the sternal muscular portion are considerably shorter than the 

 others. It has a trefoil shape, possessing a central and two lateral lobes, the right 

 one of these being perforated by a somewhat quadrate foramen, the foramen vence 

 cavcg {foramen quadratuni) , which transmits the vena cava inferior. 



The centrum tendineum forms the centre of the dome of the diaphragm, and 

 from its borders the muscular fibres slope downward towards their insertion, the 

 slope of the crural fibres being much steeper than those of the other portions. 



