564 THE ARTERIES 



Branches. — The middle sacral gives off: — 



(1) The fifth pair of lumbar arteries ( soiiietimes). These are described with 

 the lumbar arteries. 



(2) Lateral sacral branches, usually four in number. These are serially 

 homologous with the intercostal and lumbar arteries given off l)y the aorta. They 

 run more or less transversely outwards, and anastomose with the lateral sacral 

 branches of the internal iliac artery. They give off small spinal branches, which 

 pass through the sacral foramina, and supply the sacral canal and back of the 

 sacrum. 



(3) Rectal or haemorrhoidal branches pass forwards in the la3'ers of the meso- 

 rectum, to the rectum which they help to supply, and anastomose with the other 

 haemorrhoidal or rectal arteries. 



On the lower part of the middle sacral artery — the coccygeal part — there are 

 often found small pouches or A^aricosities, Avhich are believed by some to represent 

 the rudiments of lateral coccygeal arteries, homologous to the intercostal, lumbar, 

 and sacral arteries, given off from the aorta in these regions. 



The variations of this vessel are uniixiportaut. (A) The most frequent perhaps is for it to 

 come off from the back of the aorta a little above tlie bifurcation ; or (B) froui one or other of 

 the common iliacs ; or (C) as a common trunk with what are usually its blanches, the fifth 

 pair of lumbar arteries. (D) It sometimes gives off an accessory renal arter.v — a fact of inter- 

 est, in that the kidneys occujiy a lower position in the abdomen in some animals than in man. 



THE COMMON ILIAC ARTERIES 



The common iliac arteries must be regarded as the terminal branches of the 

 abdominal aorta, unless the middle sacral and the artery of the tail in the loAver 

 animals are regarded as the continuation of the al)dominal aorta. The common iliacs 

 arise opposite the left side of the middle of the body of the fourth lumbar vertebra, 

 at the bifurcation of the abdominal aorta, and, diverging from each other in the 

 male at about an angle of 60°, and in the female at an angle of 68°, terminate 

 opposite the lumbo-sacral articulation by bifurcating into the external iliac, which 

 is continued along the brim of the pelvis to the lower limb, and into the internal 

 iliac, which passes over the brim of the pelvis and descends into that cavity. 

 Both arteries lie on the fifth luml:)ar vertebra, are covered l_iy the peritoneum, and 

 are crossed by the ureter, and in the female, in addition, by the ovarian arteries. 



The relations of the arteries differ slightly on the two sides, and may be con- 

 sidered separately. 



The Right Common Iliac Artery 



The right common iliac measures about tAvo inches in length (5 cm.), and is 

 rather longer than the left, in consequence of the aorta bifurcating a little to the 

 left of the median line. 



Relations. — In front it is covered by the peritoneum, and is crossed by the 

 right ureter a little before its l)ifurcation, by the ovarian artery in the female, l)y 

 the termination of the ileum, by the terminal branches of the superior mesenteric 

 artery, and by branches of the SAnnpathetic nerve descending to the hypogastric 

 plexus. 



Behind, it lies on the right common iliac A'cin, the end of the left common iliac 

 A'ein, and the commencement of the inferior A'ena caA'a, Avhich se]>arate it from the 

 fourth and fifth lumljar vertebrae and their intervening disc, the psoas nniscle, and 

 the sympathetic nerA'e; Avhilst still deeper in the groove betAveen the fifth lumliar 

 A'ertebra and the psoas are the lumbo-sacral cord, the obturator nerve, and the ilio- 

 lumbar artery. 



To the right side are the beginning of the inferior vena cava, the end of the 

 right common iliac vein, and the psoas muscle, Avhich, hoAvever, is separated from 

 the artery by the inferior vena cava at its upper part. 



To the left side are the right common iliac A^ein, the termination of the left 

 common iliac vein, and the hypogastric plexus. 



