The Thorax. 179 



6. The diaphragm (diaphragma) is a muscular and tendinous sheet 

 forming the posterior wall of the thorax and separating the pleural 

 cavities from the peritoneal cavity. It is somewhat dome-shaped in 



the relaxed condition, but in contraction it becomes flattened in such a 

 way that the space occupied by the lungs is considerably increased, 

 while the liver and related structures of the abdominal cavity are dis- 

 placed backward. 



As a muscle the diaphragm arises in three portions. The first, or 

 lumbar portion, consists of two muscular and fibrous cords, the crura, the 

 right much larger and stronger than the left, arising from the anterior 

 spinous processes of the first three lumbar vertebrae. The second, or 

 costal portion, arises from the internal surfaces of the posterior ribs. 

 The third, or sternal portion, arises from the xiphoid process of the 

 sternum. Its insertion is represented by its own tendinous central 

 portion, or centrum tendineum, although the latter is virtually attached 

 forwards to the lungs and pericardium through the broad pulmonary 

 ligament. The centrum tendineum is shaped somewhat like a trefoil, 

 the fibres of the costal and sternal portions radiating outward from its 

 margin. 



The following may be made out on the posterior surface: 



(a) The cut margins of the falciform, coronary, and left triangular 

 ligaments. 



(b) The hiatus aorticus, an aperture enclosed by the two crura 

 and serving for the transmission of the aorta. 



(c) The hiatus oesophageus, ventral to the foregoing, and 

 serving for the passage of the oesophagus. 



(d) The foramen venae cavae, situated slightly to the right and 

 serving for the transmission of the vena cava inferior. 



(e) The superior phrenic arteries (aa. phrenicae superiores) 

 arise from the aorta at about the point of origin of the 

 eleventh intercostals or from one of the latter, and enter the 

 crura. , 



The inferior phrenic arteries are minute branches arising at the base 

 of the coeliac. 



(f) The inferior phrenic veins (vv. phrenicae inferiores), one on 

 either side of the foramen venae cavae, at which point they 

 enter the inferior cava. 



The superior phrenic veins pass forward from the diaphragm, opening 

 . into the superior cavals. 



