608 THE ARTERIES. 



forward, they supply the Intercostal muscle, and anastomose with the anterior 

 intercostal branches of the internal mammary, and with the thoracic branches of 

 the axillary artery. The first aortic intercostal anastomoses with the superior 

 intercostal, and the last three pass between the abdominal muscles, inosculating 

 with the epigastric in front and with the phrenic and lumbar arteries. Each 

 intercostal artery is accompanied by a vein and nerve, the former being above, 

 and the latter below, except in the upper intercostal spaces, where the nerve is at 

 first above the artery. The arteries are protected from pressure during the action 

 of the Intercostal muscles by fibrous arches thrown across, and attached by each 

 extremity to the bone. The lower intercostal arteries are continued anteriorly 

 from the intercostal spaces into the abdominal wall, except the last, the subcostal, 

 which lies throughout its whole course in the abdominal wall, since it is placed 

 below the last rib. They pass behind the costal cartilages between the Internal 

 oblique and Transversalis muscle to the sheath of the Rectus, where they 

 anastomose with the internal mammary and the deep epigastric arteries. Behind, 

 the subcostal artery anastomoses with the first lumbar artery. 



The posterior or dorsal branch of each intercostal artery passes backward 

 to the inner side of the anterior costo-transverse ligament, and divides into a 

 muscular branch which is distributed to the muscles and integument of the back, 

 and a spinal branch which enters the spinal canal through the intervertebral 

 foramina to be distributed to the spinal cord and its membranes, and to the 

 bodies of the vertebrae in the same manner as the lateral spinal branches from the 

 vertebral. 



THE ABDOMINAL AORTA (Fig. 367). 



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

 front 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 consequence 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 convexity correspond- 

 ing 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 by the left lumbar veins, the receptaculum chyli, and thoracic 

 duct. 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 OP 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. 



1 Sir Joseph 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). 



