The subclavius is a small muscle which lies immediately below the clavicle, enclosed between two layers of the clavipectoral fascia. It has its origin, via a short, rounded tendon, in the upper surface of the first costal arch, at the junction of the bone with the cartilage; and the fleshy belly is inserted into the floor of the shallow groove on the lower surface of the clavicle. The nerve of supply is derived from the fifth and sixth cervical nerves and enters the posterior surface of the muscle. The subclavius helps to steady the clavicle in movements of the shoulder-girdle.