This vessel commences at the lower margin of the first rib, and proceeds obliquely, downwards, backwards, and outwards, to terminate opposite the lower margin of the tendons of the teres major and latissimus dorsi muscles. In this course it is situated deeper above than below, and forms, when the elbow is brought to the side, a slight curvature, the convexity of which is turned outwards. It is usually described as having three stages: in the first, it is above the pectoralis minor; in the second, behind it; and in the third, below it.