The structure indicated is the axillary vein.

The axillary vein passes through the axilla, draining blood from the upper limb into the subclavian veins, which in turn drain blood to the brachiocephalic (innominate) veins, which unite to form the superior vena cava which then drains into the right atrium of the heart.

The axillary vein is formed from the brachial and the basilic veins, and begins at the inferior margin of the teres major and ends at the lateral border of the first rib, where it then becomes the subclavian vein.

Learn more about the veins of the body in this tutorial