The second part of the subclavian artery passes laterally between the anterior and middle scalene muscles, and one branch may arise from it.
The third part of the subclavian artery emerges from between the anterior and middle scalene muscles to cross the base of the posterior triangle (Fig. 8.186). It extends from the lateral border of the anterior scalene muscle to the lateral border of rib I where it becomes the axillary artery and continues into the upper limb.