Shoulder Muscles - Horse Anatomy

The muscles of the shoulder are grouped:

Lateral

Supraspinatus

Innervation:Suprascapular n. of the brachial plexus

Origin: Supraspinous fossa of the scapula

Insertion: Lesser and greater tubercles of the humerus

Action: Extends and braces the shoulder

Infraspinatus

Innervation:Suprascapular n. of the brachial plexus

Origin: Supraspinatous fossa and spine of the scapula

Insertion: Tendon of insertion bifurcates into a deep part and a superficial part.

Deep - Greater tubercle of the humerus

Superficial - Lateral humerus, distal to the greater tubercle

Action: Brace the shoulder, supporting flexion or extension.

Clinical significance: The bursa between the superficial tendon of the infraspinatus, and the lateral tubercle of the humerus can be the site of inflammation (bursitis).

Medial

Supscapularis

Innervation: Subscapular n. from the brachial plexus

Origin: Deep surface of the scapula

Insertion: Lesser tubercle of the humerus

Action: Braces medial shoulder joint. Primarily acts as an extensor of the shoulder, but can contribute to maintaining flecion.

Coracobrachialis

Innervation: Musculocutaneous n. of the brachial plexus

Origin: Coracoid process of scapula

Insertion: Proximal shaft of the humerus

Action: Fixator

Caudal (Flexors)

Deltoid

Innervation: Axillary n. of the brachial plexus

Origin: Caudal border and spine of the scapula. One head in the horse, compared with two in species with an acromion (site of second origin.

Insertion: Deltoid tuberosity of humerus

Action: Flexes shoulder

In the horse, the aponeurosis of the deltoid is partially fused to the infraspinatus.

Teres Major

Innervation: Axillary n. of the brachial plexus

Origin: Dorsal part of the caudal scapula

Insertion: Teres major tuberosity midway down the humerus

Action: Flexes the shoulder, supports adduction.

Teres Minor

Innervation: Axillary n. of the brachial plexus

Origin: Distal third of scapula, deep to the deltoid

Insertion: Teres minor tuberosity midway down the humerus

Action: Flexes the shoulder

NB// There are no defined extensors of the shoulder. Those involved (brachiocephalic m., biceps brachii, supraspinatus, and ascending pectorals) have other, more primary roles.