Overview
It lies on the cranial surface of the humerus. Its tendon of origin crosses the shoulder joint and glides in the intertubercular groove, which in the horse is divided in two by an intermediate tubercle. There the tendon is partly fibrocartilaginous and rests on the intertubercular bursa (bursa intertubercularis).
In the horse the muscle is traversed by an internal tendinous cord running from origin to insertion. From its distal end arises the lacertus fibrosus, a band that joins the tendon of the extensor carpi radialis and travels with it to the metacarpus.
Origin
Supraglenoid tubercle of the scapula (tuberculum supraglenoidale).
Insertion
Radial tuberosity (tuberositas radii) and, through the lacertus fibrosus, the tuberosity of the third metacarpal bone.
Action
Flexes the elbow and extends the shoulder.
Innervation
Musculocutaneous nerve (n. musculocutaneus).
Clinical relevance
- It is part of the stay apparatus of the thoracic limb. In the standing horse, the weight of the trunk tends to flex the shoulder; the tendinous cord of the biceps prevents this without muscular effort. Through the lacertus fibrosus and the extensor carpi radialis, the same tension keeps the carpus extended.
- Intertubercular (bicipital) bursitis causes forelimb lameness with pain on flexion and extension of the shoulder; it is investigated with ultrasonography and radiography of the shoulder.