- The inner two-thirds of the infraspinous fossa ;
- the under surface of the spine of the scapula ;
- the infraspinous fascia, and a thick inter- muscular septum which separates it from the teres minor and major muscles.
Its origin is by fleshy fibres which converge outwards in bipenniform fashion upon the tendon. Frequently that part which arises from the lower surface of the spine of the scapula overlies and is somewhat separate from the rest . Its insertion into the capsule and tuberosity is by a tendon which is almost entirely concealed by fleshy fibres.
From the brachial plexus (through the fifth cervical nerve), by the suprascapular branch which enters the deep surface of the muscle at its outer part and near its upper border.
It is the chief external rotator of the humerus. This movement of external rotation is through about 90°, and is of great importance. When the elbow is bent, it produces the lateral movement of the hand by which, in writing, the pen is carried from left to right across the page. When the elbow is extended, the rotation of the humerus adds considerably to the range of rotatory movement enjoyed by the hand. The infra-spinatus also adducts the elevated arm, at the same time drawing it slightly backwards, or extending it. It helps to hold the head of the hmneriis in contact Avith the glenoid cavity.
Superficially, the infraspinons fascia which separates it from the deltoid, trapezius, and latissimus dorsi ; deeply, the suprascapular and dorsahs scapulae vessels, and sometimes a small bursa which intervenes between its tendon and the capsule of the shoulder joint ; externally, the teres major and minor.
The slip from the imder surface of the spine is frequently almost as separate from the infraspinatus as the teres minor, and sometimes there is no separation between the infraspinatus and teres minor.