The internal iliac artery [a. iliaca interna; French: Artère iliaque interne ; Syn: hypogastrica, pelvica], short and thick trunk, separates from the external iliac immediately after its origin, and dips into the pelvis to supply the walls and the viscera of that cavity.
This artery is usually about an inch and a half in length, and is smaller than the external iliac, except in the foetus. It extends from the bifurcation of the common iliac artery towards the sacro-sciatic foramen, and separates near that point into two divisions. At its origin, the artery lies near the inner border of the psoas muscle ; lower down, it rests against part of the pyriform muscle. Behind it, are situated the internal iliac vein, and the communicating branch which passes from the lumbar to the sacral plexus of nerves ; in front it is crossed by the ureter, which separates it from the peritoneum. Considered in a surgical point of view, the deviations of this artery from its ordinary condition in regard to its length and place of division, are important. Length. — In two-thirds of a large number of cases, the length of the internal iliac artery varied between an inch and an inch and a half; in the remaining third it was much more frequently longer than shorter than those measurements, the artery being in the extreme cases about half an inch and three inches in extent. The lengths of the common iliac and internal iliac arteries bear an inverse pro- portion to each other — the internal iliac being long when the common iliac is short, and vice versa. Moreover, when the common iliac is short, the internal iliac (arising higher than usual) is placed for some distance out of the pelvis, and descends by the side of the external iliac to reach that cavity. The place of division of the internal iliac into its branches varies between the upper margin of the sacrum and the upper border of the sacro-sciatic foramen. The length of the internal iliac arteries of the two sides was, in a series of cases, often found to differ, but neither seemed habitually to exceed the other.
From Quain's anatomy.