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. 183). 
(e) The levatores costarum; a series of small muscles arising 
from the 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. 
(¢) 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 tendinogus 
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, 
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 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. 
