Subscapularis muscle

The Subscapularis is a large triangular muscle which fills the subscapular fossa.

Origin and insertion
It arises from its medial two-thirds and from the lower two-thirds of the groove on the axillary border(subscapular fossa) of the scapula.

Some fibers arise from tendinous laminæ which intersect the muscle and are attached to ridges on the bone; others from an aponeurosis, which separates the muscle from the Teres major and the long head of the Triceps brachii.

The fibers pass lateralward, and, gradually converging, end in a tendon which is inserted into the lesser tubercle of the humerus and the front of the capsule of the shoulder-joint.

Relations
The tendon of the muscle is separated from the neck of the scapula by a large bursa, which communicates with the cavity of the shoulder-joint through an aperture in the capsule.

Innervation
Innervation to subscapularis is supplied by the upper and lower subscapular nerves, branches of the posterior cord of the brachial plexus.

Actions
The Subscapularis rotates the head of the humerus medially (internal rotation); when the arm is raised, it draws the humerus forward and downward. It is a powerful defense to the front of the shoulder-joint, preventing displacement of the head of the humerus.