Forelimb - Anatomy & Physiology

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 amongst 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 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 stabilize the scapula.

Trapezius: 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 scapula against the trunk and swings cranially to advance the limb

Brachiocephalic m.: Accessory n.

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

Omotransversarius: Accessory n.

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

Latissimus dorsi: local branch of brachial plexus

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

Pectoral mm.: brachial plexus

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

Serratis ventralis: branch of brachial plexus

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

Rhomboids: 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: retracting the limb, may also raise limb

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. from the brachial plexus

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 epicondyles to upper medial radius

Pronators: Median n. from the brachial plexus

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

Muscles of the Carpal and Digital Joints

Extensors: Radial n. from the brachial plexus

Craniolateral position on the forearm
Almost all originate from the lateral epicondyle of the humerus
Extensor carpi radialis: most medial, inserts on 2nd/3rd metacarpal bone
Ulnaris lateralis: most lateral, inserts on accessory carpal bone
Extensor carpi obliquus: aka abductor pollicis longus
- Origin: cranial radius
- Insertion: most medial metacarpal bone
Last two may also serve in medial deviation of the paw
Common Digital Extensor
- Insertion: extensor process of the distal phalanx of each digit
- Sends a medial branch to dew claw and medial digits in all but horse and cat
Lateral Digital Extensor
- Insertion: dorsal proximal phalanx

Flexors: Median or Ulnar n. of the brachial plexus

Caudal position on the forearm
Originate from the caaudal medial epicondyle of the humerus
Flexor carpi radialis: most medial, inserts on upper 2nd/3rd metacarpal bone
Flexor carpi ulnaris: most lateral, inserts on the accessory carpal bone
Superficial Digital Flexor
- Branches according to the number of digis and inserts in proximal interphalangeal joint
Deep Digital Flexor
- Passes through carpal canal before branching and continues to palmar distal phalanges

Interosseus muscles

Support metacarpophalnageal joints
Arise from palmar proximal metacarpal bones and insert on sesamoid bones within the joints, continued by ligaments to phalanges

Vasculature of the Forelimb

Innervation of the Forelimb

The nerves affecting the forelimb arise from spinal nerves C6 to T2 and pass through the brachial plexus.

Suprascapular Nerve

Origin - cranial part of brachial plexus, C6 and C7
Motor innervation - supraspinatus and infraspinatus
Sensory innervation - none
Route - out of the brachial plexus, laterally round the cranial aspect of the neck of the scapula

Subscapular Nerve

Origin - cranial part of the brachial plexus, C6 and C7
Motor innervation - subscapular muscle
Sensory innervation - none
Route - direct to muscle

Musculocutaneous Nerve

Origin - middle part of the brachial plexua, C7 and C8 (sometimes C6)
Motor innervation - Biceps brachii, brachialis, coracobrachialis
Sensory innervation - dorsomedial aspect of forelimb
Route - medial aspect of the limb, close to the median nerve

Axillary Nerve

Origin - middle brachial plexus, C7 and C8
Motor innervation - shoulder flexors, teres minor, deltoid
Sensory innervation - dorso-lateral aspect of proximal limb
Route - behind the shoulder joint

Radial Nerve

Origin - caudal brachial plexus, C7 to T2
Motor innervation - extensors of elbow, carpus and digits
Sensory innervation - dog: craniolateral and medial forearm, horse: lateral forearm
Route - through the triceps, around the humerus to the lateral aspect of the forearm

Median and Ulnar Nerves

Origin - caudal brachial plexus, C8, T1 and T2
Motor innervation - flexors of carpus and digits
Sensory innervation - caudal aspect of the limb
Route - along the medial aspect of the limb, the median forms branches to the musculocutaneous nerve