Forelimb - Anatomy & Physiology

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Common structures of the Proximal Forelimb and Shoulder

Scapula

The Scapula forms the basis of the shoulder region, providing points of attachment of extrinsic and intrinsic muscles. It is held in place by a synsarcosis of muscles and does not form a conventional articulation with the trunk.

• In ungulates, the dorsal border is extended by a scapular cartilage, which enlarges the area for muscle attachment. This ossifies with age.
• The bone is roughly triangular, with a prominent spine that can be palpated through the skin
• The spine defines Infraspinous and Supraspinous Fossae, inhabited by muscles accordingly
• The spine serves as a point of attachment for the trapezius muscle
• The spine culminates in the acromion in all but the horse and pig
• Medial muscle attachment comprises mostly of the subscapularis, with the serratus ventralis attaching dorsally
• The Scapula articulates with the humerus at the Glenoid cavity

Clavicle

The Clavicle is all but absent in most domestic species, with the notable exception of the avian skeleton. In the dog and cat, a remnant of bone may remain embedded in the fibrous intersection in the brachiocephalicus, which may prove misleading in radiographic images.

Humerus

The Humerus is the long bone of the forearm, articulating with the scapula to form the shoulder and the radius and ulna to form the elbow. In situ, it lies obliquely along the ventral thorax and is more horizontal in larger species.

• The head of the humerus comprises greater and lesser tubercles, separated by an intertubercular groove through which runs the tendon of the biceps brachii
• The shaft of the humerus takes on a characteristically twisted shape via a groove carrying the brachialis and radial nerve
• Laterally, the deltoid tuberosity is palpable through the skin and connects to the head of the humerus via a ridge and merges distally with the crest of the humerus
• Distally, the humerus culminates in a condyle which articulates to form the elbow
  • In large animals, it meets the radius via a trochlea
  • In dogs and cats, it articulates with the radius medially via a trochlea and the ulna laterally via a capitulum
  • Caudally, all species show an olecranon fossa which articulates with the olecranon of the ulna
• Medial and lateral epicondyles provide attachment for flexors and extensors of the carpus and digits

Joints of the Proximal Forelimb

Shoulder Joint

The shoulder joint links the humerus and the scapula at the glenoid cavity, which is much smaller than the head of the humerus. While structually it is a ball and socket joint, it functions as a hinge joint due to extensive muscling around the articulation. The joint capsule is enlarged and extends under the tendon of the biceps, acting as a synovial sheath to protect the tendon.

Elbow Joint

Movement of the elbow joint is restricted to the sagittal plane. It is bounded medially and laterally by collateral ligaments betweek the humerus and radius, caudally by the olecranon ligament between the humerus and olecranon, and further enforced by the annular radial ligament. The elbow is a compound joint including:

• The hinge joint between the humerus and the radius and ulna
• The pivot joint between the radius and ulna
• The point of the elbow, or the olecranon is formed by the anconeal process of the ulna inserting into the olecranon fossa of the humerus

Common Structures of the Distal Forelimb

Radius

While in the human the radius and ulna are separated by an interosseus space and articulate only at their extremities, allowing for significant capability of supination and pronation, these movements are much more limited in domestic animals due to the gradual fusing of the two bones. The extreme case is exhibited by the horse.

• The Radius forms the shaft-like rod of the distal limb, which is bowed to varying degrees amont species
• The radius articulates proximally with the distal humerus, caudally with the ulna, and distally with the carpus
• Medially on the distal articular process, a styloid process projects, which is mirrored laterally by the ulna

Ulna

The Ulna's greatest contribution to functional anatomy is in the formation of the Olecranon, or the point of the elbow, which gives rise to the attachment of the triceps. The olecranon articulates with the humerus via its anconeal process. Distally (where unfused), the lateral styloid process articulates with the ulnar carpal bone.

Carpal bones

Carpal bones comprise two rows:

• Proximally, (mediolaterally), radial, intermediate, ulnar and accessory bones
  • The accessory bone serves as a landmark for palpation
• Distally, bones are numbered 1-5, though 5 is always fused with 4
• A small sesamoid bone embedded in the medial tissues of the joint can sometimes be mistaken as a chip fracture

Metacarpal bones

The number of metacarpals varies widely among species, as the demand for their function changes: plantigrade, or flat-footed, animals requiring the full complement of five metacarpal bones; the number is reduced in the upright stature of digitigrade animals such as the dog and cat, and shows the extreme in unguligrades like the horse, who depends entirely on the third metacarpal bone for its stature.

• Colloquially, the third metacarpal of the horse is known as the canon bone, and the vestigial 2 and 4 as splint bones

Joints of the Distal Forelimb

Carpal Joint

The carpal joint is a compound joint composed of:

• The antebrachiocarpal joint between the radius/ulna and the proximal carpal bones
• The middle carpal joint between the two rows of carpal bones
• The carpometacarpal joint between the distal carpal bones and the proximal metacarpals

The joint is a synovial joint, comprised of a common outer fibrous capsule and three inner synovial pouches, one for each joint. Numerous ligaments add to the stability of the joint and ensure movement is largely limited to the sagittal plane, although no collateral ligaments exist in the dog between the radius and the proximal metacarpals. This allows a very small amount of rotation.

Metacarpal Joint

The metacarpal joint is defined by the presence of palmar sesamoids, which allow the flexor tendons to pass over the sharp change in angle presented by the joint. These are paired on each digit, with the exception of the first digit where it exists.

Muscles of the Forelimb

Extrinsic Musculature

These muscle are responsible for joining tht 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 stabilize the scapula.

Trapezius

• Origin: mid-dorsal raphe and supraspinous ligament
• Insertion: spine of the scapula
• Body: two parts, cervical and thoracic separated by aponeurosis
• Action: raises scapula against the trunk and swings cranially to advance the limb
• Innervation: accessory n.

Brachiocephalic m.

• Two parts separated by the clavicle where it exists
• Origin: clavicle or vestigial fibrous intersection
• Insertion: several places on the head and neck
• Actions:
  • advances the limb and extends the shoulder joint when limb is in motion
  • draws head and neck ventrally when limb is fixed
• Innervation: accessory n.

Omotransversarius

• Origin: transverse processes of the atlas
• Insertion: acromion and adjacent scapula
• Action: advancing the limb
• Innervation: accessory n.

Latissimus dorsi

• The broadest muscle of the back
• Origin: thoracolumbar fascia
• Insertion: teres tuberosity of the humerus
• Actions: antagonist to the brachiocephalic m.
  • cranial fibers strap scapula to the chest
  • retracts free limb and flexes shoulder joint
  • draws trunk forward over the fixed limb
• Innervation: local branch of brachial plexus

Pectoral mm.

• Two superficial parts, cranial and caudal
  • Origin: cranial sternum
  • Insertion:
    • cranial (descending): crest of the humerus distal to the deltoid tuberosity
    • caudal (transverse): covers elbow joint to insert on the medial fascia of the forearm
  • Action: adduct the forelimb, assist in protraction and retraction
• One deep part (pectoralis profundus), with cranial and caudal parts
  • Origin: ventral sternum and adjacent cartilage
  • Insertions:
    • cranial (subclavius): supraspinatus m.
    • caudal (pectoralis ascendens): lesser tubercle of the humerus
  • Actions:
    • slinging trunk between forelimbs
    • may also retract free limbs
    • draw trunk forward when limb is fixed
• Innervation: brachial plexus

Serratis ventralis

• Origin: C4 to 10th rib
• Insertion: medial scapula and scapular cartilage
• Action: supporting the weight of the trunk
  • reinforced by strong fascia
  • cervical portion can retract the limb
  • caudal portion can advance the limb
• Innervation: branch of brachial plexus

Rhomboids

• Origin: median connective tissue from poll to withers, lying deep to the trapezius
• Insertion: dorsal border and adjacent scapula
• Action: retracting the limb, may also raise limb
• Innervation: brachial plexus, although in some species, dorsal spinal nerves can innervate

Intrinsic Musculature

Muscles of the Shoulder

These muscles are grouped:

• Lateral:Supraspinatus and Infraspinatus, Suprascapular n. of the brachial plexus
  • Origin: the fossae of the scapula
  • Insertion: both tubercles of the humerus
  • Action: brace the shoulder
  • Clinical significance: bursa between the tendon of the infraspinatus and lateral tubercle of the humerus can be the site of inflammation
• Medial:
  • Supscapularis: Subscapular n. from the brachial plexus
    • Origin: Deep surface of the scapula
    • Insertion: medial tubercle of the humerus
    • Action: braces medial shoulder joint, potential adductor
  • Coracobrachialis: Musculocutaneous n. of the brachial plexus
    • Origin: medial supraglenoid tubercle
    • Insertion: proximal shaft of the humerus
    • Action: fixator
• Caudal (Flexors): Axillary n. of the brachial plexus
  • Deltoids
    • 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
    • Origin: dorsal part of the caudal scapula
    • Insertion: teres tuberosity midway down humerus
  • Teres Minor
• 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: Radial n. from the brachial plexus

• Triceps brachii: Three heads, four in the dog
  • Long head: caudal margin of the scapula
  • Lateral, medial, and accessory heads: shaft of the humerus
  • Insertion: olecranon, proteced by tricipital bursa against the bone and subcutaneous bursa against the skin
• Tensor fasciae antebrachii
  • Overlies triceps extending from scapula to olecranon
• Anconeus
  • Origin: distal humerus
  • Insertion: lateral olecranon

Flexors: Musculocutaneous n. from the brachial plexus

• Biceps brachii
  • Origin: supraglenoid tubercle of the scapula
  • Insertion: medial tuberosity of proximal radius and adjacent ulna
  • Runs through the intertubercular groove of the humerus
• Brachialis
  • Origin: proximocaudal humerus
  • Insertion: spirals to insert next to biceps

Muscles of Supination and Pronation

Supinators: Radial n.

• Brachioradialis
  • Origin: lateral epicondyle of the humerus
  • Insertion: distal medial forearm within superficial fascia
  • Prominent in the cat but nearly absent in the dog
• Supinator
  • Deep to extensor muscles, passing from lateral humeral epicondyls=e to upper medial radius

Pronators: Median n.

• Pronator teres: only functional in cat and dog
  • Origin: medial epicondyle of the humerus
  • Insertion: supinator
• Pronator quadratus: only found in carnivores
  • Origin: shaft of the ulna
  • Insertion: shaft of the radius