264 Anatomy of the Rabbit. 



(d) The thoracic portions of the sympathetic trunks lie on 

 the lateral surfaces of the bodies of the thoracic vertebrae. 

 The posterior ganglia give origin to the splanchnic nerve, 

 the latter passing backward into the abdominal cavity (p. 1 83) . 



(e) The levatores costarum; a series of small muscles arising 

 from che transverse processes of the vertebrae and the heads 

 of the ribs and inserted on the anterior margins of the next 

 succeeding ribs. They assist the intercostals in respiration. 



(f) The intercostal nerves (nn. intercostales) accompany the 

 intercostal arteries to the lateral wall of the thorax. 



(g) The azygos vein (v. azygos) is a small, asymmetrical, venous 

 trunk lying to the right of the dorsal surface of the aorta. 

 It receives the majority of the intercostal veins, which 

 accompany the corresponding arteries and nerves, the 

 tributaries extending backward to the first lumbar veins. 

 It opens forward into the right superior caval. The more 

 anterior intercostal veins are tributaries of the right and 

 left supreme intercostal veins which open into the corres- 

 ponding superior cavals. 



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 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. The third, or sternal portion, 

 from the xiphoid process of the sternum. Its insertion is 

 sen ted by its own tendinous central portion, or centrum 

 tendineum, although the latter is virtually attached forward 

 to the lungs and pericardium through the broad pulmonary liga- 

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

 the fibres of the costal and sternal portions radiating outward from 

 its margin. 



^^surfa< 



