THE ABDOMEN 775 



branches of the abdominal aorta. They arise from that vessel 



opposite the centre of the body of the fourth lumbar vertebra, a 

 finger's breadth to the left of the middle line, and they at once 

 diverge from each other. Their course is obliquely downwards and 

 outwards over the lower portion of the body of the fourth and 

 the whole of that of the fifth Imnbar vertebra, as well as the disc 

 between the two. Each artery, on arriving opposite the lirnibo- 

 sacral articulation, ends by dividing into external and internal 

 iliac arteries. The length of the right common iliac is about 

 2 inches, and that of the left about i| inches. The left vessel is less 

 oblique in direction than the right, and the course of each may be 

 indicated in the following manner : draw a line from a point fully 

 I inch below the umbilicus, a finger's breadth to the left of the 

 middle line, to a point at the groin midway between the anterior 

 superior iliac spine and the symphysis pubis, and let this line be 

 slightly cur\'ed with the convexity directed outwards. About the 

 upper 2 inches of this line indicate the course of the conunon iliac 

 artery, and the remainder that of the external iliac vessel. 



Relations — Anterior. — ^The peritoneiun, coils of the small intes- 

 tine, one half of the aortic sympathetic plexus, and the ureter, 

 which latter crosses the artery close to its termination, though 

 it may be transferred to the commencement of the external 

 iliac vessel. An additional superficial relation of the left common 

 iliac artery is that it is crossed by the superior hemorrhoidal 

 vessels. 



Posterior. — Each artery rests upon the lower half of the body of 

 the fourth and the whole of that of the fifth lumbar vertebra, as 

 well as the disc above and below the latter, and the gangliated 

 s\TTipathetic chain. The right vessel is separated from the fore- 

 going structures by the commencement of the inferior vena cava, 

 the terminal part of the left common iliac vein, and the right 

 common iliac vein, whilst the left vessel is free from posterior 

 venous relations. Lying deeply behind each artery there are the 

 obturator nerve, lumbo-sacral cord, and ilio-lumbar artery. 



External. — On the outer side of the right vessel there are, from 

 above downwards, the inferior vena cava, right common iliac vein, 

 and psoas magnus. On the outer side of the left vessel is the psoas 

 magnus. 



Internal. — On the inner side of the right vessel, from below up- 

 wards, there are the right common iliac vein, the left common 

 iliac vein, and the hypogastric sympathetic plexus. Oh the inner 

 side of the left vessel there are the left common iliac vein, and the 

 hypogastric plexus. 



It is to be noted that the left artery is related only to its own 

 vein, which lies on its inner side. The right artery, on the other 

 hand, is related to three veins as follows : the inferior vena cava, 

 which lies partly behind its upper end and partly on its outer side ; 

 the terminal part of the left conunon iliac vein, which lies partly 

 on its inner side and partly behind it : and the right common iliac 



