This vessel arises from the common iliac nearly opposite the superior extremity of the sacro-iliac symphysis, and descends obliquely forwards and outwards towards the centre of Poupart's ligament. The length of the artery varies according to the situation at which the bifurcation of the common iliac takes place: generally speaking, however, it is about three and a half or four inches in length. Posteriorly, it corresponds to the external iliac vein, which separates its origin from that of the internal iliac artery, the vein lying in the angle between the two arteries; and farther onwards, the psoas muscle and iliac fascia are situated behind it. On the right side the commencement of the right common iliac vein lies posterior to it. Anteriorly, it is covered by the peritoneum, and near Poupart's ligament by the circumflexa ilii vein, which sometimes, however, passes behind it. Externally, the fascia iliaca and some fibres of the psoas muscle separate it from the anterior crural nerve, which lies behind the fascia, deeply imbedded between the psoas and iliacus muscles: a branch of the genito-crural nerve is also found running along the artery in this situation, and inclining to its anterior surface. Internally, near Poupart's ligament, we see its accompanying vein, lying also on a plane posterior to the artery; and on the inner side of the vein we may observe the septum crurale, or "fascia propria" of Sir A. Cooper, lying across the internal opening of the crural canal. On a plane posterior to the artery, in the male subject, the vas deferens, as it descends into the pelvis, lies internal to it. The artery and vein will be found surrounded completely by the sub-peritoneal layer of fascia already described. The student should bear in mind that the anterior crural nerve is external to the artery and on a deeper plane; and that the external iliac vein is at first posterior, and afterwards, near Poupart's ligament, becomes internal to the artery.