794 HUMAN ANATOMY. 



Occasionally the second intercostal is formed by a branch from the first aortic intercostal 

 which runs upward to the second space over the neck of the third rib, and a similar condition 

 may be met with in the lower arteries, two or more intercostal spaces being supplied from a 

 common stem. Finally, the right and left arteries of one or all of the pairs may arise from a 

 common stem, springing from the posterior median line of the aorta. 



Practical considerations of the thoracic aorta are discussed with those of the aortic arch 

 on page 726. 



THE ABDOMINAL AORTA. 



The abdominal aorta is the continuation below the diaphragm of the thoracic 

 aorta. It may be said to begin, therefore, at the lower border of the twelfth thoracic 

 vertebra, and passes downw'ard upon the bodies of the four upper lumbar vertebrae 

 lying almost in the median line. It is usually described as terminating opposite the 

 fourth lumbar vertebra by dividing into the right and left common iliac arteries, 

 although it is really continued onward beyond that point as a relatively feeble vessel 

 which is termed the 7niddle sacral artery. It seems advisable, however, to adhere to 

 the classic definitions of the artery, and to regard the middle sacral, for purposes of 

 description, as one of its branches. 



Relations. — Posteriorly, the abdominal aorta rests upon the anterior com- 

 mon ligament of the four upper lumbar vertebrae and crosses the left lumbar veins. 

 Anteriorly, in its uppermost part, it is invested by the sympathetic solar plexus, from 

 which branches pass downw^ard along the vessel, forming the aortic plexus. A little 

 lower it is crossed by the splenic vein, the pancreas, the left renal vein, and the 

 third portion of the duodenum, and still lower it is in relation with the coils of 

 the small intestine, from which, however, it is separated by the peritoneum. Upon 

 a more anterior plane there are, above, the left lobe of the liver, and the stomach 

 and transverse colon. To the 7'ight, it is in contact, in its upper part, with the 

 thoracic duct and the receptaculum chyli, which lie partly covered by it, and with 

 the right crus of the diaphragm, which separates it from the inferior vena cava ; 

 lower down it is in direct contact with the \'ena cava. To the left, is the left crus 

 of the diaphragm and the fourth portion of the duodenum above, while below it 

 is separated by the peritoneum from coils of the small intestine, and has running 

 alongside the left spermatic (ovarian) artery and vein, and still more laterally the 

 left ureter. 



Branches. — The branches of the abdominal aorta, like those of the thoracic, 

 may be divided into two sets, visceral and parietal. 



The visceral branches are (i) the coeliac axis, (2) the superior ifiesenteric, 

 and (3) the inferior mese^iteric artery. These are median unpaired branches which 

 arise from the anterior surface of the aorta; in addition, there are a number of paired 

 visceral branches: (4) the inferior phrenic, (5) the suprarenal, (6) the re?ial, and (7) 

 the spermatic or ovariaji arteries. 



The parietal branches are (8) the hivibar arteries, of which there are four 

 pairs, (9) the middle sacral, and (10) the comynoyi iliac arteries. With the excep- 

 tion of the middle sacral, the parietal branches are all paired. 



Considered in the order of their origifi from the aorta, the branches are ar- 

 ranged thus: (i) The inferior phrenics, (2) the cceliac axis, (3) ihe siiprare?ials, 

 (4) the superior mesenteric, (5) the: first pair of lumbar arteries, (6) the renals, 

 (7) the spermatics or ovarians, (8) the second pair of lumbars, (9) the inferior 

 mesenteric, (10 and 11) \.he third z.n<\ fourth pairs of lumbars, (12) the middle sacral, 

 and (13) the common iliacs. 



Variations of the abdominal aorta are not common. In cases in which the aortic arch 

 bends to the right, the abdominal aorta may lie somewhat to the right of the median line, and it 

 has been observed to jiass downward upon the right of the inferior vena cava. Variations also 

 occur in the level at wiiich the aorta bifurcates into the common iliacs. In the majority of ca.ses 

 the bifurcation is opposite the middle of the fourth lumbar vertebra, but it is not infrequently 

 lower, taking place opposite the lower half of that vertebra, opposite the succeeding interver- 

 tebral disc, or, in rare cases, opposite the ujjper portion of the fifth vertebra. Bifurcation at a 

 higher level than usual is less frequent, but it has been observed as high as opposite the inter- 

 vertebral disc between the third and fourth vertebrae, and, in very rare cases, the artery has 

 ^een ^ound to divide as high as the second lumbar ^^ertebra. 



« 



