View source for Extrinsic Musclulature of the Thoracic Limb- Horse Anatomy

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.