The Scalenus anterior also known as anterior scalene muscle, lies deeply at the side of the neck, behind the Sternocleidomastoideus.

Anatomy

It arises from the anterior tubercles of the transverse processes of the third, fourth, fifth, and sixth cervical vertebræ, and descending, almost vertically, is inserted by a narrow, flat tendon into the scalene tubercle on the inner border of the first rib

First rib

The first rib is the most curved and usually the shortest of all the ribs; it is broad and flat, its surfaces looking upward and downward, and its borders inward and outward....

, and into the ridge on the upper surface of the rib in front of the subclavian groove.