THE ABDOMINAL AORTA. 549 



The posterior or dorsal branch of each intercostal artery passes backward to the 

 inner side of the anterior costo-transverse ligament, and divides into an external and 

 internal branch, which are distributed to the muscles and integument of the back. 



The spinal branch, which enters the spinal canal through the intervertebral 

 foramen, is distributed to the spinal cord and its membranes, and to the bodies of 

 the vertebne in the same manner as the lateral spinal branches from the vertebral. 



The collateral intercostal branch comes oft' from the intercostal artery near the 

 angle of the rib, and descends to the upper border of the rib below, along which it 

 courses to anastomose witli the anterior intercostal branch of the internal mammary. 



Surgical Anatomy. The position of the intercostal vessels should be borne in mind 

 in performing the operation of paracentesis thoracis. The puncture should never be made 

 nearer the middle line posteriorly than the angle of the rib, as the artery crosses the space 

 internal to this point. In the lateral portion of the chest, where the puncture is usually 

 made, the artery lies at the upper part of the intercostal space, and therefore the puncture 

 should be made just above the upper border of the rib forming the lower boundary of the ^ 

 space. 



THE ABDOMINAL AORTA (Fig. 307). 



The Abdominal Aorta commences at the aortic .opening of the Diaphragm, in 

 front of the lower border of the body of the last dorsal vertebra, and, descending 

 a little to the left side of the vertebral column, terminates on the body of the 

 fourth lumbar vertebra, commonly a little to the left of the middle line, 1 where it 

 divides into the two common iliac arteries. It diminishes rapidly in size, in con- 

 sequence of 'the many large branches which it gives off. As it lies upon the bodies 

 of the vertebrae, the curve which it describes is convex forward, the greatest con- 

 vexity corresponding to the third lumbar vertebra, which is a little above and to 

 the left side of the umbilicus. 



Relations. It is covered, in front, by the lesser omentum and stomach, behind 

 which are the branches of the coeliac axis and the solar plexus ; below these, by 

 the splenic vein, the pancreas, the left renal vein, the transverse portion of the 

 duodenum, the mesentery, and aortic plexus. Behind, it is separated from the 

 lumbar vertebrae and intervening discs by the anterior common ligament and left 

 lumbar veins. On the right side it is in relation with the inferior vena cava (the 

 right crus of the Diaphragm being interposed above), the vena azygos major, 

 thoracic duct, and right semilunar ganglion ; on the left side, with the sympathetic 

 nerve and left semilunar ganglion. 



PLAN OF THE RELATIONS OF THE ABDOMINAL AORTA. 



In front. 



Lesser omentum and stomach. 



Branches of the coeliac axis and solar plexus. 



Splenic vein. 



Pancreas. 



Left renal vein. 



Transverse duodenum. 



Mesentery. 



Aortic plexus. 



Right side. s^ ~~\ 



Right crus of Diaphragm. \ Left side. 



Inferior vena cava. A ^^ Sympathetic nerve. 



Vena azygos major. I Left semilunar ganglion. 



Thoracic duct. \ / 



Right semilunar ganglion. 



Behind. 



Left lumbar veins. 

 Vertebral column. 



1 Lord Lister, having accurately examined .30 bodies in order to ascertain the exact point of 

 termination of this vessel, found it "either absolutely, or almost absolutely, mesial in 15, while in 

 13 it deviated more or less to the left, and in 2 was slightly to the right" (System of Surgery, edited 

 by T. Holmes, 2d ed., vol. v., p. 652). 



