The subclavius muscle (french: Muscle subclavier) - named from its position beneath the clavicle (=clavis) - is almost cylindrical, but may be more accurately described as a thick sheet of the shape of a low obtuse-angled triangle, the obtuse angle being contained between the clavicular attachment and the inner free border of the muscle.
The upper and anterior surface of the first rib and its cartilage at their point of junction.
The groove on the lower surface of the clavicle from the rhomboid impression to the conoid tubercle.
It arises by a strong tendon, flattened from before backwards, which lies close to the front of the rhomboid ligament, and is continued for a considerable distance along the lower border of the muscle. Its insertion is by fleshy fibers which radiate upwards, outwards, and a little backwards, in a penniform manner, from this tendon ; the inner ascending more vertically, and the outer very obliquely.
From the brachial plexus by a small branch which, arising from the fifth and sixth cervical nerves, passes behind the clavicle and enters the middle of the back of the muscle.
To draw the outer end of the clavicle, and with it the point of the shoulder, downwards and slightly forwards ; but chiefly by pulling the bone inwards to supplement the ligaments which prevent dislocation of the sterno-clavicular joint.
In front, the clavi-pectoral fascia and the pectoralis major ; behind, the posterior division of the same fascia, the rhomboid ligament, the subclavian vessels, and the brachial plexus.
The subclavius may be absent, or its insertion may extend to the coracoid process, transverse ligament, and upper border of the scapula. Occasionally the costo-clavicular portion is separate from the costo-scapular. Again, its origin may extend downwards to the sternum, and the portion which arises from the sternum may be separate from the rest of the muscle.