The great importance of the deep femoral artery as a collateral vessel in occlusion of the superficial femoral artery is a well-established fact. It has been found in the course of reconstructive operations of this artery that the origin of the two circumflex arteries varies considerably. A study of one hundred cadavers has given some idea of the frequency and nature of these anatomical variations. The importance of this vessel as a collateral is partially explained by the fact that the deep femoral artery has two groups of branches, both of them anastomosing with the branches of the popliteal artery. If the deep femoral artery is to function adequately as a collateral, it has to have a large caliber. An explanation for this may be found in comparative anatomy. In the phylogenetically farther advanced vertebrate animals, all of which have very strongly developed extensors in the posterior extremities, the deep femoral artery which supplies blood to this muscle group is a branch of the iliac artery, and accordingly, a pelvic artery. In man, this artery originates distal to the inguinal ligament from the common femoral artery and supplies blood to less well-developed muscle groups. In this way, phylogeny explains the large caliber of the deep femoral artery, which has originally been a pelvic artery and it also explains the numerous anatomical variations at the site of the bifurcation of the common femoral artery.