Illustration of the venous supply of the upper extremity (right side), demonstrating the relationship between the axillary and subclavian veins and the first rib.

Blood returning from the arm passes through a number of superficial and deep veins, which combine in the upper part of the arm underneath the pectoralis minor muscle to form the axillary vein. As it passes under the clavicle, the axillary vein becomes the subclavian vein. The subclavian vein crosses up and over the first rib in front of the anterior scalene muscle (and is therefore not within the scalene triangle). It then passes through a tight space created between the first rib and the clavicle, as well as the subclavius muscle and the costoclavicular ligament. After the subclavian vein passes over the anterior part of the first rib behind the clavicle, it joins the internal jugular vein as it descends from the neck, forming the innominate (or brachiocephalic) vein behind the sternoclavicular joint. The innominate vein then passes further underneath the sternum, joins with its counterpart from the other side, and forms the superior vena cava.