THE DIAPHRAGM 337 



occurring most frequently at the eighth intercostal space but often in 

 the ninth, tenth, or eleventh. 

 (^) The thoracic duct, which is not readily observed in ordinary dis- 

 section, is formed between the crura of the diaphragm by the union of 

 the two lumbar l3miphatic trunks and the intestinal trunk. It passes 

 forward between the azygos vein and the aorta to the level of the 

 second intercostal space, where it crosses to the left between the aorta 

 and the oesophagus. It then follows the left superior vena cava and 

 enters into the junction of the jugular and subclavian veins. A variable 

 series of lymph nodes lies between the aorta and the oesophagus, re- 

 ceiving vessels from the organs in the thorax and draining either into 

 the thoracic duct or separately into the vein. The arrangement of the 

 lymphatic vessels and their connections with the veins show marked 

 individual differences. 



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 and contraction of its muscles partially flattens 



the dome 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 displaced 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 by slips separated by small triangular aponeurotic 



areas. The third, or sternal portion, arises from the xiphoid process 



of the sternum. The insertion of the muscles of the diaphragm is 



represented by its own tendinous central portion, or centrum 



tendineum, although the latter is virtually attached forward to 



the lungs and pericardium through the broad mediastinum and the 



pulmonary ligaments. The centrum tendineum is shaped somewhat 



like a trefoil, its margin being indented dorsally by the crura and 



at each side at the position of the inferior phrenic veins. The fibres 



of the costal and sternal portions converge radially to its margin. 



The connection of the lumbar portion is somewhat asymmetrical, 



the two crura combining ventral to the aorta and ending largely 



to the left of the median plane. 



