Overview
It fills the infraspinous fossa, caudal to the scapular spine, and is partly covered by the deltoideus. Its belly is crossed by tendinous sheets. Its tendon glides over the infraspinatus bursa, on the caudal part of the greater tubercle, before inserting.
Origin
Infraspinous fossa (fossa infraspinata) of the scapula and scapular cartilage.
Insertion
Lateral surface of the caudal part of the greater tubercle of the humerus, distal to the supraspinatus insertion.
Action
Abducts the arm and rotates it laterally. The horse has no true collateral ligaments at the shoulder: this muscle acts as a contractile lateral collateral ligament of the joint.
Innervation
Suprascapular nerve (n. suprascapularis).
Clinical relevance
Sweeny is injury to the suprascapular nerve where it winds around the cranial border of the scapula, from collisions or, in draught horses, from collar pressure. The supraspinatus and infraspinatus waste, the scapular spine stands out and the shoulder shifts laterally when the limb bears weight.