404 THORACIC AND ABDOMINAL AORTA. 



Branches. The named branches of the intercostal arteries are the 

 following. 



(a) The posterior or dorsal branch of each intercostal artery passes back- 

 wards to the inner side of the anterior costo-transverse ligament, along with 

 the posterior branch of the corresponding spinal nerve ; and, having furnished 

 an offset to the spinal canal, reaches the muscles of the back, and divides 

 into an internal and an external branch. The internal branch is directed 

 towards the spinous processes, on or through the multifidus spinse, and 

 ramifies in the muscles and the skin. The external branch turns outwards 

 under the longissimus dorsi, and is distributed between that muscle and 

 the sacro-lumbalis ; some twigs reach the superficial muscles and the 

 integuments. 



The spinal branches of the aortic intercostal arteries are distributed partly to the 

 cord and its membranes, and partly to the bones, in the same manner as the spinal 

 branches of the lumbar arteries, to the description of which tlie reader is referred. 



(>) The collateral intercostal branch, long and slender, arises near the 

 place where the main trunk comes in contact with the upper rib of the 

 space, and inclining downwards approaches the border of the lower rib, 

 supplying the bone and the intercostal muscles, and anastomosing in front 

 with an anterior intercostal branch of the internal mammary artery. There 

 are thus in each intercostal space two terminal branches of the intercostal 

 arteries communicating with the branches of the internal mammary. 



ABDOMINAL AORTA. 



The aorta, after having passed the diaphragm, is thus named. It com- 

 mences on the front of the last dorsal vertebra, and terminates below by 

 dividing into two trunks, named the common iliac arteries. The bifurcation 

 usually takes place about half way down the body of the fourth lumbar 

 vertebra, a little to the left of the middle line ; a point which is nearly 

 on a level with a line drawn from the one crista ilii to the other, and 

 opposite the left side of the umbilicus. 



The anterior surface of the abdominal aorta is successively in apposition 

 with the pancreas and the splenic vein, the left renal vein, the third portion 

 of the duodenum, and the peritoneum. The vena cava lies along its right 

 side, the right cms of the diaphragm being interposed at the upper part of 

 the abdomen ; close to the same side are the thoracic duct and the azygos 

 vein, which are placed between the aorta and the right crus of the dia- 

 phragm. The aorta is also covered in front by meshes of nerves derived 

 from the sympathetic, and numerous lymphatic vessels and glands. 



Fig. 282. VIEW OP THE ABDOMINAL AORTA ["AND ITS PRINCIPAL BRANCHES (from 



Tiedemann). 5 



a, ensiform portion of the sternum ; J, vena cava inferior passing through the tendon 

 of the diaphragm ; c, the oesophagus passing through the muscular portion ; d, tendinous 

 part of the right, e, of the left crus ; /, /', the right and left kidneys with their supra- 

 renal bodies ; g, g', the ureters ; h, the upper part of the urinary bladder ; i, i, the right 

 and left vasa deferentia passing up from the bladder to the internal inguinal apertures ; 

 k t the rectum, divided and tied near its upper part ; 1, 1, the abdominal aorta ; 1', the 

 middle sacral artery ; 2, 2', the right, 3, 3', the left inferior phrenic arteries, represented 

 as arising by a short common stem from the front of the aorta immediately below the 

 meeting of the crura of the diaphragm ; 4, the trunk of tbe cceliac axis ; 5, the superior 

 mesenteric artery ; 6, 6, the renal arteries ; 6', 6', the suprarenal arteries arising partly 



