Extrinsic Musclulature of the Thoracic Limb- 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.

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.