POSTERIOR AORTA. 403 



ing in the posterior part of the sublnmbar region by breaking up 

 into the external and internal iliacs. It is divided into thoracic 

 and abdominal portions. The arch of the thoracic portion is 

 crossed on the right by the trachea and oesophagus, having the 

 pulmonary artery on the left ; along the spine it lies in the pos- 

 terior mediastinum, related on the right with the vena azygos and 

 the thoracic duct. The abdorn'iial portion is embraced by twigs 

 of the sympathetic nerve, and related with the reservoir of Pecquet 

 above, and the posterior vena cava on its right. The posterior 

 aorta furnishes parietal and visceral branches, which we tabulate 

 as follows : 



PARIETAL BRANCHES. 



Aortic intercostal. Phrenic. 



Lumbar. Middle sacral. 



The INTERCOSTAL arteries are doublets, seventeen in number, 

 the last thirteen of which, the aortic intercostals, arise directly 

 from the posterior aorta ; while the first arises from the superior 

 cervical, the next three from the dorsal artery. The aortic iuter- 

 costals leave the superior part of the trunk at right angles, skirt 

 the dorsal vertebra?, and at the upper margin of the intercostal 

 spaces divide into inferior or intercostal, and superior or doro- 

 spinal. The intercostal branch is the larger, and travels along 

 the grooved posterior border of the rib do\\n the side of the 

 thorax, associated with the vein and nerve. These arteries 

 anastomose with the asternal and anterior abdominal branches 

 of the internal thoracic ; while the last four traverse the 

 abdominal muscles, and anastomose with the abdominal and 

 circumflex ilii. The intercostal branch supplies the pleura, 

 intercostal, and thoracic muscles. The dorso-spvntd branch is 

 distributed to the dorsal muscles and integument, supplying the 

 spinal cord and its coverings through the intervertebral foramen. 



The LUMBAR arteries, five or six pairs, originate like the inter- 

 costals, dividing in the intertransverse spaces into superior or 

 I limbo-spinal, the larger, which are distributed to the muacles 

 and integument of the loins, furnishing spinal branches to the 

 cord and its membranes ; and inferior, which pass to the psoe, 

 trans versaUs abdominis, and internal oblique muscles, and there 

 anastomose with the circumflex ilii. The last lumbar may arise 

 irorn the internal iliac artery (Fig. 158. 1, 1). 



The PHRENIC or DIAPHRAGMATIC arteries are usually two ur 



