THE MUSCLES OF THE THOEAX. 



471 



Diaphragma. The diaphragm is the great membranous and muscular parti- 

 tion separating the cavities of the thorax and abdomen. It forms a thin lamella 

 arching over the abdominal cavity, and clothed on that surface, for the most part, 

 by peritoneum. It is related, on its inferior concave surface, to the liver, stomach, 

 and spleen, the kidneys and suprarenal glands, and the duodenum and pancreas. 

 Its superior convex surface projects into the thoracic cavity, rising higher on the 

 right than on the left side, and is related to the pericardium and pleurse, and along 

 its margin to the chest wall. The oesophagus and thoracic aorta are in contact 

 with it posteriorly. 



It possesses a peripheral origin from the sternum, ribs, and vertebral column, 



EXTERNAL INTERCOSTAL 

 MUSCLE 



^IQUUS EXTERNUS. 



ABDOMINIS (reflected) 



Anterior intercostal 

 membrane removed, 

 exposing the internal 

 intercostal muscle 



INTERNAL INTER- 

 COSTAL MUSCLE 



RECTUS ABDOMINIS 

 (insertion) 



Sheath of the rectus 

 abdominis 



FIG. 418. THE MUSCLES OF THE RIGHT SIDE OP THE THORACIC WALL. 



and an insertion into a central tendon. It arises (1) anteriorly (pars sternalis) from 



B posterior surface of the xiphoid process by two slender fleshy slips, directed 



backwards ; (2) laterally (pars costalis), from the deep surface of the lower six costal 



cartilages on each side by fleshy bands which interdigitate with those of the trans- 



TSUS abdominis; (3) posteriorly (pars lumbalis), from the lumbar vertebrae, by 



the crura, and the medial and lateral lumbo-costal arches. The crura are two 



elongated nbro-muscular bundles which arise, on each side of the aorta, from the 



anterior surface of the bodies of the lumbar vertebrae, on the right side from 



the first three, on the left side from the first two lumbar vertebras. They are 



rected upwards and decussate across the median plane in front of the aorta, the 



58 of the right crus passing anterior to those of the left crus. The fibres then 



encircle the oesophagus, forming an elliptical opening for its passage, and finally 



join the central tendon, after a second decussation anterior to the gullet. 





31 



