Limb Muscles - Horse Anatomy

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

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

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

Intrinsic Musculature

Muscles of the Shoulder

These muscles are grouped:

Lateral

Supraspinatus and Infraspinatus

Innervation:Suprascapular n. of the brachial plexus

Origin: The fossae of the scapula

Insertion: Both tubercles of the humerus

Action: Brace the shoulder

Clinical significance: The bursa between the 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: Medial tubercle of the humerus

Action: braces medial shoulder joint and is a potential adductor

Coracobrachialis

Innervation: Musculocutaneous n. of the brachial plexus

Origin: Medial supraglenoid tubercle

Insertion: Proximal shaft of the humerus

Action: fixator

Caudal (Flexors)

Deltoids

Innervation: Axillary n. of the brachial plexus

Origin: Caudal border and spine of the scapula. One head in the horse and two in species with an acromion. (site of second origin)

Insertion: Deltoid tuberosity on the humerus

Teres Major

Innervation: Axillary n. of the brachial plexus

Origin: Dorsal part of the caudal scapula

Insertion: Teres tuberosity midway down the humerus

Teres Minor

Innervation: Axillary n. of the brachial plexus

NB// There are no defined extensors of the shoulder. Those involved (brachiocephalic m., biceps brachii, supraspinatus, and ascending pectorals) have other, more primary roles.

Muscles of the Elbow

Extensors

Triceps brachii

Innervation: Radial n. from the brachial plexus

It has three heads:

Origin:

Long head: caudal margin of the scapula

Lateral and medial heads: shaft of the humerus

Insertion: Olecranon, proteced by the tricipital bursa against the bone, and subcutaneous bursa against the skin

Tensor fasciae antebrachii

Innervation: Radial n. from the brachial plexus

Overlies the triceps extending from the scapula to the olecranon

Anconeus

Innervation: Radial n. from the brachial plexus

Origin: distal humerus

Insertion: lateral olecranon

Flexors

Biceps brachii

Innervation: Musculocutaneous n. from the brachial plexus

Origin: Supraglenoid tubercle of the scapula

Insertion: Medial tuberosity of the proximal radius and adjacent ulna

It runs through the intertubercular groove of the humerus.

Brachialis

Innervation: Musculocutaneous n. from the brachial plexus

Origin: Proximocaudal humerus

Insertion: Spirals to insert next to biceps

Muscles of the Carpal and Digital Joints

Extensors

The craniolateral position on the forearm, almost all originate from the lateral epicondyle of the humerus. All innervated by the radial n. from the brachial plexus.

Extensor carpi radialis

The most medial, inserts on the metacarpal tuberosity.

Ulnaris lateralis

The most lateral, inserts on the accessory carpal bone.

Extensor carpi obliquus (aka abductor pollicis longus)

Origin: Cranial radius

Insertion: Head of the metacarpal II

Common Digital Extensor

Insertion: Extensor process of the distal phalanx

Lateral Digital Extensor

Insertion: Dorsal proximal phalanx

Flexors

Caudal position on the forearm, originate from the caaudal medial epicondyle of the humerus. All innervated by the median or ulnar n. of the brachial plexus.

Flexor carpi radialis

The most medial, inserts on the proximal part of metacarpal II (medial splint)

Flexor carpi ulnaris

The most lateral, inserts on the accessory carpal bone.

Superficial Digital Flexor

Inserts on the proximal extremity of middle phalanx and the distal extremity of proximal phalanx.

Deep Digital Flexor

Passes through the carpal canal before branching and continues to palmar distal phalanx.

Interosseus muscle/Suspensory ligament

This is covered in more detail in the Equine Tendons section.

Pelvic Limb

The muscles affecting the pelvic girdle and hip can be divided into two distinct groups; girdle muscles and rump muscles.

Girdle Muscles

The pelvic girdle muscles is made up of a number of distinct muscle groups.

Psoas Minor

The psoas minor muscle is marked by many tendinous intersections.

Iliac

The iliac muscle is a fleshy muscle that is cross-sectionally flat cranially and round caudally. It originates from two heads; a strong lateral head from the wing of the ilium and a small medial one from the ileal shaft. They enclose the greater psoas and will unite to form a common tendon for insertion.

Quadrate Lumbar

The quadrate lumbaris a thin, tendinous muscle whose origin is at the proximal end of the last rib and the transverse process of the cranial lumbar vertebrae. Its insertion is the transverse processes of the caudal lumbar vertebrae and the sacral wing.

Rump Muscles

The rump muscles are also made up of a number of different muscle groups and have considerably more groups than the girdle muscles.

Superficial Gluteal

The superficial gluteal originates in the gluteal fascia and unites with the tensor muscle fascia lata, passing over the greater trochanter and attaching onto the third trochanter which radiates into the femoral fascia. A synovial bursa exists between the tendinous insertion and the third trochanter.

Middle Gluteal

The middle gluteal originates from four separate areas; the 1st lumbar vertebrae, aponeurosis of the longissimus muscle, the sacrum and the broad sacrotuberous ligament. The short portion of this muscle inserts on the greater trochanter whilst the deep portion inserts on both the greater trochanter and intertrochanteric crest.

Tensor Muscle of the Fascia Lata

The tensor muscle of the fascia lata originates from the coxal tuberosity and joins the fascia lata and so indirectly attaches to the patella, lateral pattelar ligament and cranial border of the tibia. A caudodorsal detachment joins the superficial gluteal and so attaches to the greater trochanter.

Biceps Femoris

The biceps originate from the vertebral head which is a spinous processes of the sacral vertebrae, and also from the caudal border of the sacrotuberous ligament and the ischial tuberosity. The two bellies of the biceps then unite and redivide into three parts that form an extensive aponeurosis. The cranial branch of the aponeurosis inserts just distal to the third trochanter on the caudal aspect of the femur, to the patella and to the lateral patellar ligament. The middle branch inserts onto the crural fascia, the lateral patellar ligament and the cranial aspect of the tibia. The caudal branch radiates into the crural fascia and forms the tarsal tendon. This combines with the detachment from the semitendinous inserts on the calcaneus.

Semitendinosus

The semitendinous muscle originates from various points including; the pelvic head, the ventral aspect of ischial tuberosity, the sacrum, 1st caudal vertebrae and the sacrotuberous ligament. The two bellies of the semitendinous then unite and run as a flat tendon to the medial side of the leg. It then partially fans out into the crural fascia and partially inserts onto the cranial aspect of the tibia. The remaining tendon joins the tarsal tendon.

Semimembranosus

The semimembranous also originates from various points including the vertebral head (sacrotuberous ligament), 1st caudal vertebrae, pelvic head and the ventral aspect of ischial tuberosity. The two bellies of the muscle then unite and insert at three places; the medial femoral condyle via a short tendon, the medial collateral ligament of the femerotibial joint and to the medial condyle of the tibia via an aponeurosis.

Sartorius

The sartorius muscle originates from the iliac fascia and the tendon of the psoas minor. It inserts via the medial patellar ligament, crural fascia and the tibial tuberosity.

Gracilis

The gracilis originates from the accessory ligament of the femoral head.

Adductor Muscles

The equine adductor muscles are formed from a short adductor and a greater adductor. They both insert along the entire medial aspect of the femur from the lesser trochanter to the medial condyles and the medial collateral stifle ligament.

Internal Obturator

The internal obturator originates from the cranial and medial border of the obturator foramen, the pelvic symphysis and pelvic aspect of the body of the ilium. It inserts on the trochantic fossa having passed over the lesser sciatic notch.