540 ABDOMEN 



of the body of the fourth lumbar vertebra, a little to the left 

 of the median plane, by dividing into the two common iliac 

 arteries. It therefore pursues an oblique course inclining 

 slightly to the left as it proceeds downwards. A line drawn 

 between the highest points of the iliac crests would indicate 

 the level of the bifurcation of the abdominal aorta ; it takes 

 place a little below and to the left of the umbilicus. 



Most of the structures which lie in front of the abdominal 

 aorta have been removed. In immediate relation to it from 

 above downwards are: (i) The cceliac plexus and the layer 

 of peritoneum which forms the posterior wall of the omental 

 bursa. (2) The pancreas and splenic vein. (3) The left 

 renal vein and the third part of the duodenum. (4) The 

 root of the mesentery and the superior mesenteric vessels. 

 (5) The peritoneum and the aortic plexus of nerves. Super- 

 ficial to these it is covered by the liver, the lesser omentum and 

 the stomach, the transverse colon and its mesentery, and by the 

 greater omentum and the coils of the small intestine. Behind, 

 the abdominal aorta rests upon the bodies of the lumbar 

 vertebrae and the intervertebral discs, separated from them, 

 however, by the anterior longitudinal ligament and the left 

 lumbar veins. On each side it is related in its upper part to 

 the crus of the diaphragm. On the right side the inferior 

 vena cava lies close to the aorta as high as the second 

 lumbar vertebra, but above this it is separated from it by the 

 fleshy part of the right crus. In the interval between the 

 right crus of the diaphragm and the aorta, the student has 

 already noted the cisterna chyli and the vena azygos. On 

 the left side, the left gangliated cord of the sympathetic is in 

 relation to the artery below the level of the left crus. 



Branches of the Abdominal Aorta. The branches of the 

 abdominal aorta may be described under two heads, viz. 

 (i) Those which come off in pairs. (2) Those which 

 arise singly. 



PAIRED BRANCHES. 



1. Inferior phrenic arteries. 



2. Middle suprarenal arteries. 



3. Renal arteries. 



4. Internal spermatic or ovarian arteries. 



SINGLE BRANCHES. 



1. The coeliac artery. 



2. The superior mesenteric. 



3. The inferior mesenteric. 



4. The middle sacral. 



5. Lumbar arteries. 



With the exception of the middle sacral, which arises from 

 thje back of the extremity of the aorta between the two 



