Overview
Thick muscle that fills the supraspinous fossa and bulges beyond the cranial border of the scapula. The horse has no true collateral ligaments at the shoulder: the supraspinatus and infraspinatus, together with the subscapularis, act as contractile ligaments that stabilise the joint. It ends in two branches, on either side of the biceps brachii tendon.
Origin
Supraspinous fossa (fossa supraspinata), spine and cartilage of the scapula.
Insertion
Cranial parts of the greater tubercle (tuberculum majus) and lesser tubercle (tuberculum minus) of the humerus.
Action
Extends and stabilises the shoulder.
Innervation
Suprascapular nerve (n. suprascapularis).
Clinical relevance
In sweeny, injury to the suprascapular nerve where it winds around the cranial border of the scapula (from collisions or, in draught horses, from the collar), the supraspinatus and infraspinatus waste and the scapular spine stands out. Without these stabilisers, the shoulder shifts laterally when the limb bears weight.