The popliteal vein is formed near the distal border of the popliteus muscle by the union of the venae comitantes of the anterior and posterior tibial arteries. It ascends through the popliteal fossa to the opening in the adductor magnus, where it becomes the femoral vein. It begins at the medial side of the artery and gradually crosses behind it to take up the posteriolateral position; the two vessels are bound together in a fibrous sheath. In addition to tributaries that correspond to branches of the artery, its receives the short saphenous vein. It has three, or sometimes four, valves in its interior.