POSTERIOR AORTA. 403 



ing in the posterior part of the subbimbar 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 ahdominal 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. I 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 inter- 

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

 the dorsal vertebrce, and at the upper margin of the intercostal 

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

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

 the grooved posterior border of the rib down the side of the 

 thorax, associated with the vein and nerve. These arteries 

 anastomose Avith 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-sjnnal 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 

 Iwiiiho-spinal, the larger, which are distributed to the muscles 

 and integument of the loins, furnishing spinal branches to the 

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

 transversalis abdominis, and internal oblique muscles, and there 

 anastomose with the cirdumflex ilii. The last lumbar may arise 

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



The Phrenic or Diaphragmatic arteries are usually two or 



