Limb Muscles - Horse Anatomy
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Thoracic Limb
Extrinsic Musculature
These muscles are responsible for joining the forelimb to the trunk, forming a synsarcosis rather than a conventional joint. Collectively, they act to transfer the weight of the body to the forelimbs, as well as to stabilize the scapula.
Trapezius:
- Innervatation: Accessory n.
- Origin: Mid-dorsal raphe and supraspinous ligament
- Insertion: Spine of the scapula
- Body: Two parts, cervical and thoracic, separated by aponeurosis
- Action: Raises the scapula against the trunk and swings cranially to advance the limb
Brachiocephalic m.:
- Innervation: Accessory n.
- Two parts separated by the clavicle, where it exists
- Origin: Mastoid process of temporal bone, nuchal crest, wing of atlas and the transverse processes of the 2nd-4th cervical vertebrae
- Insertion: Deltoid tuberosity and crest of the humerus
- Action: Advances the limb and extends the shoulder joint when limb is in motion. Draws the head and neck ventrally when the limb is fixed
Latissimus dorsi:
- Innervation: local branch of brachial plexus
- The broadest muscle of the back
- Origin: Thoracolumbar fascia
- Insertion: Teres tuberosity of the humerus
- Action: Antagonist to the brachiocephalic m. It retracts the free limb and flexes shoulder joint. It also draws the trunk forward over the fixed limb.
Pectoral mm.:
- Innervation: brachial plexus
- Two superficial parts, cranial and caudal
- Origin: Cranial sternum
- Insertion:
- Cranial (descending) - The crest of the humerus, distal to the deltoid tuberosity
- Caudal (transverse) - Covers the elbow joint to insert on the medial fascia of the forearm
- Action: Adducts the forelimb, assist in protraction and retraction
Serratis ventralis:
- Innervation: branch of brachial plexus
- Origin: C4 to 10th rib
- Insertion: Medial scapula and scapular cartilage
- Action: Supports the weight of the trunk. It is reinforced by strong fascia. The cervical portion can retract the limb and the caudal portion can advance the limb.
Rhomboids:
- Innervation: brachial plexus, although in some species, dorsal spinal nerves can innervate
- Origin: Median connective tissue from poll to withers, lying deep to the trapezius
- Insertion: Dorsal border and adjacent scapula
- Action: Retracts the limb, may also raise limb