Intrinsic Musculature of the Thoracic Limb- Horse Anatomy

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Intrinsic Musculature

Muscles of the Shoulder

These muscles 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.