Shoulder Muscles - Horse Anatomy
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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.
- 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
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.