The subclavius is a small muscle located beneath the collarbone, or clavicle. It plays a role in stabilizing the clavicle during arm movements and helps to protect the underlying brachial plexus and subclavian artery. This muscle originates from the first rib and inserts into the inferior surface of the clavicle. Its primary function is to assist in the downward movement of the clavicle, which is important for shoulder mobility and overall upper limb function.