Thoracic Limb Muscles: Anatomy and Function

• This lesson will familiarize students with the anatomy, origin, insertion, function, comparative anatomy, and main anatomical relationships of the intrinsic muscles of the shoulder and arm.

Lateral Scapular Region

• Supraspinatus Muscle (m. supraspinatus)
• Infraspinatus Muscle (m. infraspinatus)
• Deltoid Muscle (m. deltoideus)
• Minor Teres (m. teres minor)

Medial Scapular Region

• Major Teres Muscle (m. teres major)
• Articular Muscle of the Shoulder Joint (m. articularis humeri)
• Subscapular Muscle (m. subscapularis)
• Coracobrachial Muscle (m. coracobrachialis)

Cranial Region of the Arm - Flexor Muscles of the Forearm

• Brachial Muscle (m. brachialis)
• Biceps Brachii Muscle (m. biceps brachii)

Caudal Region of the Arm - Extensor Muscles of the Forearm

• Triceps Muscle of the Forearm (m. triceps brachii) - 3 or 4 heads
• Anconeus Muscle (m. anconeus)
• Tensor Muscle of the Antebrachial Fascia (m. tensor fasciae antebrachii)

Thoracic Limb Muscles

• Muscles in the shoulder girdle and thoracic limb are grouped into four large groups: the shoulder, arm, forearm, and manus.
• The muscles of each segment act on the next segment.
• Muscles of the hand allow finger movement.

Muscles of the Shoulder Joint

• These muscles originate from the scapula and end on the humerus or distally.
• They either flex or extend or act as ligaments to brace the joint.

Deltoid Muscle

• A flat, triangular muscle composed of two parts.
• Origin and insertion: the spine of the scapula with an aponeurosis (pars spinalis) and the acromion (pars acromialis)-(EQ; SU)
• The two parts of the muscle end in a tendon on the deltoid tuberosity on the humerus.
• The deltoid muscle lies directly under the skin.
• It covers the infraspinatus muscle, minor teres muscle, and the proximal part of the triceps brachii muscle.
• It is a flexor of the shoulder joint
• Supports arm abduction and rotation

Supraspinatus Muscle

• Originates from and fills the supraspinous fossa of the scapula, and extends cranially to the scapular cartilage in ungulates.
• Inserts via one strong tendon on the greater tubercle of the humerus (CARN & PIGS), or by two tendons on the lesser and greater tubercles (HORSES & RUM).
• Partly blends with the joint capsule of the shoulder joint.
• Covered by the subclavian muscle in horses and pigs, and laterally covered by the trapezius, omotransverse and brachiocephalic muscles.
• It ends with the infraspinatus muscle.
• This is the main extensor of the arm.
• Suprascapular nerve innervates.
• Helps stabilize and extend the shoulder joint during weight bearing.
• Supports arm abduction and rotation

Infraspinatus Muscle

• Arises from the infraspinous fossa and the scapular spine.
• Terminates by a tendon on the greater tubercle, over a synovial bursa (bursa subtendinea musculi infraspinati).
• Actions :flexor of the shoulder joint (can be extensor), supports arm abduction and rotation, fixation of the shoulder joint
• Innervated by Suprascapular neve
• In pigs, the muscle is divided by retroversion of the scapular spine and has an elongated accessory head on the cranial side.
• The tendon functions as a lateral collateral ligament of the shoulder.
• It is caudally related to the teres minor m, which separates it from the long head of the triceps brachii.

Minor Teres Muscle

• In carnivores, the muscle is small and round
• In other species, it is triangular-shaped.
• Origin and insertion: Distal third caudal margin of the scapula and infraglenoid tubercle.
• Ends by a tendon on the lateral surface of the humeral proximal epiphysis to its on tubercle
• Assistant of the infraspinatus muscle functions.
• It lies deep to the infraspinous m. at the insertion, to which it adheres, and to the deltoid m.
• Covers the origin of the long head of the triceps brachii muscle
• Innervated by Axillary nerve
• Rotates the shoulder laterally, preventing medial rotation when weight bearing.
• Flexor of shoulder joint

Medial Scapular Region Muscles

• Subscapular muscle
• Major teres muscle
• Articular muscle of the shoulder joint
• Coracobrachial muscle

Subscapular Muscle

• Located on the subscapular fossa
• Flat muscle crossed by tendon intersections.
• Inserts, deep to the coracobrachial muscle, on the lesser tubercle of the humerus
• Functions
• Primarily acts as an extensor of the joint but it can also contribute to maintaining flexion
• Acts as an adductor and internal rotator of the arm
• The tendon functions as a medial collateral ligament of the shoulder joint.
• Relations
• with the latter, it delimits an interstice in the region of the joint where the subscapular and circumflex vessels of the shoulder pass, as well as the axillary nerve
• with the cranial border joins the supraspinatus m. in its dorsal part, separating distally to form with it an interstitium where the suprascapular a.v. 'and' n. pass as well as the tendon of the coraco-brachial m

Major Teres Muscle

• Located on the caudal angle of scapulua
• Inserts on the humeral teres major tuberosity, together with the latissimus dorsi muscle
• Is a flexor of the shoulder joint
• Supports adduction of the limb
• Relationships: the tendon is covered by the insertion of the coracobrachial m. in ungulates
• Cranial border delimits with the subscapularis m., the interstice already described

Coracobrachial Muscle

• Origin and insertion: tendon of origin emerges from the coracoid process of the scapula, where it is protected by a synovial bursa
• Terminates by a tendon around the teres major tuberosity of the humerus and further distal on the medial aspect of the humeral diaphysis
• Its tendon in horses and ruminants (sometimes in the cat), this tendon it is divided into two unequal parts [one medial (long branch), the other lateral (short branch], which embrace the distal tendon of the teres major muscle
• Oiggin:
• The fleshy body responds laterally to the humerus, to the tendons of the teres major and latissimus dorsi muscles, and to the origin of the medial head of the triceps brachii m. · The medial side is related to the termination of the pectoralis ascendans muscle ▪ Addution of the arm, external rotation of the limb Innerevated by musculocutaneous nerve

Articular Muscle of the Shoulder Joint

• Only exists in horses, pigs, and cats
• Weak muscular bundle in the caudal region of the scapulohumeral angle
• Origin and insertion: scapular neck and caudal part of the neck of humerus
• Covered by the origin of the long head of the triceps brachii muscle and, at the termination, penetrates between the fibers of the biceps brachii muscle
• it adheres to the joint capsule, which it tenses
• inerated by Axillary nerve

Axillary Fascia

• Lines the medial aspect of the scapula, separating it from the axillary interstitium and thoracic wall
• In the axillar interstitium pass the axillary vessels and nerve branches of the brachial plexus
• At the cranial edge, it extends and involves the supraspinatus m., ending at the scapular spine and then continues with the omobrachial fascia
• At the caudal border it has the same behavior, this time involving the infraspinatus muscle

Muscles of the Elbow Joint

• Arise either from the scapula or the humerus and insert on the proximal part of the ulna or radius
• They either bridge both the shoulder and the elbow joint or only this last one

Biceps Brachii Muscle

• By a single tendon, in the supraglenoid tubercle of the scapula, then lodges in the inter-tubercular groove of the humerus and
• By tendon in the tuberosity of the radius (medial side of the proximal epiphysis)
• Functions
• Extensor of the shoulder joint +Seals the joint angles of the shoulder and elbow (+ horses) Flexor of the elbow joint
• Related oAt the level where the terminal tendon is formed, another fibrous structure stands out (lacertus fibrosus or aponevrosis of the brachial biceps m.) which moves laterally and joins the antebrachial fascia, on the surface of the radial extensor muscle of the carpus

Brachial Muscle

• Caudal humeral circumflex a.

• Origin and insertion oOrigin on the caudal surface of the proximal humerus, under the humeral neck and occupies the brachial groove of the humerus (coiled muscle) insertion by tendon on the medial side of the proximal ulna

• In Horses the lacertus fibrosus

Brachail Fascia

• Completely envelops the arm muscles and delegates deep leaflets that sheath them individually
• Continues with the axillary and forearm fascia
• External rotator of the limb - Flexor of the elbow.

`##### Triceps Brachii Muscle

• Fills the triangle between the caudal border of the scapula, the humerus and the olecranon, clearly visible under the skin in the live animal Possesses three heads of origin {a long, a lateral accessory} Accessory is located on the dogs pigs and small ruminants present a fourth head, the accessory head •Insertion by a strong tendon in the olecranon Tuberosity •A synovial bursa [bursa subtendinea m. tricipitis brachii] is inter posed between the tendon and the olecranon
• Chief extensor of the elbow - stabilize s the Elbow joint - flexiona the shoulder
• Radial Nerve innervates

Long Head of Triceps Brachii Muscle

• The largest and longest of the heads
• Side-flattened triangular shape, in dogs and pigs divided into two parts Origin caudal margin of the scapula {atp flexion of the shoulder joint}

Anconeus Muscle

Situated deep to the triceps muscle of the forearm Blends with its lateral head in the horse and ox Extension of tbe elbow joint

• In cats one can isolate lateral anchoneus m. and one medial anchoneus m. Origin and insertion outline of the olecranon fossa and supracondylar crest lateral aspect of the ulnar olecranon

Tensor Muscle of the Antebrachial Fascia

Thin fleshy sheet detaching from ventral border of latissimus dorsi m., directly or by aponevrosis Terminates by aponevrosis on the medial aspect of the tendon of the m triceps brachii and over the ante brachial fascia • Laterally Related to the triceps muscle long head, the medial epicondyle of the humerus and origin of the palmar forearm muscles chief tensor of the ante brachial fascia. Extensor of the Elbow joint