336 ANATOMY OF THE RABBIT 



(c) The thoracic aorta (aorta thoracalis) passes backward on 

 the ventral surface of the vertebral column, leaving the 

 thorax through the hiatus aorticus, the latter being the 

 aperture enclosed by the crura of the diaphragm. Its 

 branches in the thorax are the intercostal arteries (aa. 

 Intercostales), beginning with the fourth, which are given 

 off metamerlcally in the intercostal spaces, and pass laterad 

 to the thoracic wall. 



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

 lateral surfaces of the bodies of the thoracic vertebrae, 

 the left trunk reaching this position by extending backward, 

 dorsal to the aorta, from the first thoracic ganglion, to which 

 it has already been traced. The posterior ganglia give origin 

 to the splanchnic nerve, the latter usually separating at 

 about the eighth thoracic ganglion and passing backward 

 into the abdominal cavity (p. 228). 



(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. 



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

 intercostal arteries to the lateral wall of the thorax, their 

 trunks running mainly between the internal intercostal 

 muscles and the vestigial intracostal and subcostal muscles 

 but partly enclosed by the internal intercostals. 



(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 from both sides 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 at about the level of the second Intercostal 

 space. The more anterior Intercostal veins are tributaries 

 of the right and left supreme intercostal veins which open 

 into the corresponding superior cavals. 



The azygos vein lies ventral to the more anterior intercostal 

 arteries but dorsal to the more posterior ones, the change of relation 



