146 TOPOGRAPHICAL ANATO.AIY OF THE 



Pars costalis. — The converging fibres that compose the costal part 

 of the diaphragm arise by digitations from the last eleven ribs, the 

 digitations being closely related to the costal attachment of the trans- 

 verse abdominal muscle. The origin of the diaphragm from the eighth, 

 ninth and tenth ribs is at the junction of the bone and the cartilage, 

 and from the cartilage itself The origin from the succeeding ribs is 

 trom the rib-bone at a gradually increasing distance from the cartilage, 

 so that on the last rib the attachment is 10 cm. or more from the 

 ventral end. From this it is evident that the diaphragm is not attached 

 to the actual margin of the costal arch, but to a line within it. The 

 costal fibres are lost at the edge of the tendinous centre. 



Between the crura and the attachment of the pars costalis to the 

 last rib there is an unattached border that crosses the ventral surface ot 

 the psoas muscles, and is known as the lumbo-costal arch (arcus lumbo- 

 costalis). The thoracic and abdominal cavities are here separated only 

 by the pleune and the peritoneum and a certain amount of loose 

 connective tissue. 



Pars sternalis. — The sternal part, directly continuous with the pars 

 costalis, is small and consists of muscular fibres taking origin from the 

 dorsal surface of the xiphoid process of the sternum. 



Openings in the diaphragm. — There are numerous openings in the 

 diaphragm, but most of them are of small size and of not more than 

 minor importance. Three openings, however, are of moment — (1) the 

 ao?*^ic Aia^us ^ (hiatus aorticus) occurs between the two crura immedi- 

 ately ventral to the vertebral column. Through it pass the aorta, the 

 vena azygos and the commencement of the thoracic duct. (2) The 

 oesophageal hiatus (hiatus oesophageus) is a slit-like opening in the 

 right crus close to its mergence into the tendinous centre. It transmits 

 the oesophagus, the oesophageal nerve-trunks of the vagus and the 

 oesophageal branch of the left gastric artery. (3) The caval foramen^ 

 (foramen venae cavge) is a large opening in the tendinous centre a short 

 distance to the right of the median plane, and about the junction of the 

 dorsal and middle thirds of the vertical diameter of the thoracic cavity, 

 that is, near the vertex of the diaphragmatic dome. Through it passes 

 the caudal vena cava on its way from the abdomen to the heart. It 

 will be observed that the large diaphragmatic veins open into the vena 

 cava as it is passing through the foramen. 



Aorta abdomixalis. — The abdominal part of the aorta extends 

 from the aortic hiatus of the diaphragm to the fifth lumbar vertebra, 



' Hiahis [L.], a gap, opening, cleft. 

 - Foramen [L.], an opening, hole. 



