The popliteal vein is formed by the junction of the venae comitantes of the anterior and posterior tibial arteries at the lower border of the popliteus muscle on the medial side of the popliteal artery. As it ascends through the fossa, it crosses behind the popliteal artery so that it comes to lie on its lateral side. It passes through the opening in the adductor magnus to become the femoral vein.