Thoracic Limb Extrinsic Muscles - Horse Anatomy

Introduction

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.

Superficial Musculature

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

Innervation: Accessory n.

Two parts (cleidobrachial and cleidocephalic muscle) 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

Cleidomastoid

Innervatation: Accessory n. and Axillary n.

Origin: Mastoid process of the temporal bone and nuchal crest

Insertion: Deltoid tuberosity, crest of the humerus and fascia of the shoulder as the cleidobrachialis muscle

Action: If acting bilaterally, it draws the head and neck down and backwards. If the acting with one shoulder fixed, it draws the head, upper arm fascia and neck to one side.

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.

Superficial Pectorals

Innervation: Brachial plexus

Two muscles, cranial (descending) and caudal (transverse)

Origin:

Cranial (descending) - Manubrium of the sternum

Caudal (transverse) - First six costal cartilages and ventral 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

The cranial (descending) pectoral muscle forms a visible prominence cranial to the sternum; extending between the manubrium and the humeral crest.

Deep Musculature

Deep Pectorals

Innervation: Cranial and caudal thoracic n.

Origin: Xiphoid and 4th costal cartilages of the sternum

Insertion: Two branches on the lesser and greater tubercles of the humerus, supraglenoid tubercle of scapula

Action:Draws the limb back, supports the trunk, moves the trunk cranially over an advanced limb and extends the shoulder.

Serratis ventralis

Divided into cranial (cervical) and caudal (thoracic) parts.

Innervation: Dorsal and ventral Cervical n., long thoracic n.

Origin: 1st-7th rib, transverse process of cervical vertebrae

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

Deep to the trapezius, forms the withers region. Divided into cervical and thoracic parts.

Innervation: Dorsal and ventral branches of Cervical n., Long thoracic n.

Origin:

Cervical - Nuchal ligament at the axis

Thoracic - Spinous processes of 2nd-6th cervical and 7th thoracic vertebrae

Insertion:

Cervical - Unites with trapezius to insert on scapular cartilage

Thoracic - scapula

Action: Draws the limb forward and elevates the limb and neck. Fixes the scapula against the trunk.