618 



THE ARTERIES. 



other. These vessels are enclosed in one or more layers of polyhedral granular cells, 

 and the whole structure is invested in a capsule of connective tissue which sends 

 in trabeculse, dividing the interior into a number of spaces in which the vessels 

 and cells are contained. Nerves pass into this little body from the sympathetic, 

 but their mode of termination is unknown. Macalister believes the glomerulus of 

 vessels " consists of the condensed and convoluted metameric dorsal arteries of the 

 caudal segments imbedded in tissue which is possibly a small persisting fragment 

 of the neurenteric canal." 



THE COMMON ILIAC ARTERIES. 



The abdominal aorta divides into the two common iliac arteries. The bifurca- 

 tion usually takes place on the left side of the body of the fourth lumbar vertebra. 

 This point corresponds to the left side of the umbilicus, and is on a level with a 

 line drawn from the highest point of one iliac crest to the other. The common 

 iliac arteries are about two inches in length ; diverging from the termination of 

 the aorta, they pass downward and outward to the margin of the pelvis, and 

 divide opposite the intervertebral substance, between the last lumbar vertebra 

 and the sacrum into two branches, the external and internal iliac arteries, the 

 former supplying the lower extremity ; the latter, the viscera and parietes of the 

 pelvis. 



The right common iliac is somewhat larger than the left, and passes more ob- 

 liquely across the body of the last lumbar vertebra. In front of it are the perito- 

 neum, the small intestine, branches of the sympathetic nerve, and, at its point of 

 division, the ureter. Behind, it is separated from the last two lumbar vertebrae by 

 the two common iliac veins. On its outer side, it is in relation with the inferior vena 

 cava and the right common iliac vein above, and the Psoas magnus muscle below. 



The left common iliac is in relation, in front, with the peritoneum, the small in- 

 testine, branches of the sympathetic nerve, and the superior haemorrhoidal artery, 

 and is crossed at its point of bifurcation by the ureter. The left common iliac vein 

 lies partly on the inner side and partly beneath the artery ; on its outer side the 

 artery is in relation with the Psoas magnus muscle. 



PLAN OF THE RELATIONS OF THE COMMON ILIAC ARTERIES. 



In front. 

 Peritoneum. 

 Small intestines. 

 Sympathetic nerves. 

 Ureter. 



Outer side. 



Vena cava. 

 Right common 



iliac vein. 

 Psoas muscle. 



Inner side. 



In front. 



Peritoneum, small intestines. 

 Sympathetic nerves. 

 Superior haemorrhoidal artery. 

 Ureter. 



Outer side. 



Left common / 

 iliac vein. 



Psoas muscle. 



Beld 'ml. 



Last two lumbar vertebrae. 

 Left common iliac 



Behind. 



Last two lumbar vertebrae. 

 Right and left common 



iliac veins. vein. 



Branches. The common iliac arteries give off small branches to the peritoneum 

 Psoas magnus, ureters, and the surrounding cellular tissue, and occasionally give 

 origin to the ilio-lumbar or renal arteries. 



o 



Peculiarities. The point of origin varies according to the bifurcation of the aorta. In 

 three-fourths of a large number of cases the aorta bifurcated cither upon the fourth lumbar 

 vertebra or upon the intervertebral disk between it and the fifth, the bifurcation being, in one 

 case out of nine below, and in one out of eleven above, this point. In ten out of every thirteen 

 cases the vessel bifurcated within half an inch above or below the level of the crest of the ilium 

 more frequently below than above. 



The point of division is subject to great variety. In two-thirds of a large number of cases 

 it was between the last lumbar vertebra and the upper border of the sacrum being above that 



