The venae comitantes of the brachial artery are joined by the basilic vein above the lower border of the posterior wall of the axilla. They form the axillary vein, which courses upwards on the medial side of the axillary artery and leaves the axilla by passing through its apex anterior to the third part of the subclavian artery. Over the upper surface of the first rib, in front of scalenus anterior, it continues as the subclavian vein. Tributaries of the third and second parts of the axillary vein are the same as the branches of the artery. Into the first part (i.e. above pectoralis minor) the cephalic vein enters after having pierced the clavipectoral fascia. There is no axillary sheath around the vein, which is free to expand during times of increased blood flow.