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 the infraspinatus and supraspinatus muscles respectively

serves as a point of attachment for the trapezius muscle

culminates in the acromion in all but the horse and pig.

the cat also possesses a suprahamate process which lies proximal to the acromion and projects caudally.

Just cranial to the glenoid cavity can be seen a bony prominence called the supraglenoid tubercle which is the origin of the biceps bracii muscle

Medial muscle attachment consist 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 muscle, 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 ulna medially via a trochlea and the radius 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 structurally 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.

It is important to distinguish the shoulder/scapulohumeral joint from the entirely muscular connection (synsarcosis) between the forelimb and the trunk. This latter connection is sometimes called the girdle muscles, although this is a problematic term, because many of its constituent muscles do not attach to a limb girdle muscle. At Cambridge University, it has for some time been given the name omothoracic junction, but this term has not entered common usage.

There are no true ligaments in the shoulder joint. The tendons of insertion of the supraspinatous and infraspinatous muscles cross the shoulder joint and insert laterally on the greater tubercle of the humerus. The tendon of the subscapularis inserts medially on the humerus. These act as 'ligaments' preventing dislocation of the shoulder.

Elbow Joint

Movement of the elbow joint is restricted to the sagittal plane. It is bounded medially and laterally by collateral ligaments between 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. It 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 muscle. The olecranon articulates with the humerus via its anconeal process. The olecranon develops as an apophysis, i.e.. from a separate site of ossification. 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, which 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, compring 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. They 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:

Innervated by: 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.:

Innervated by: 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:

Innervated by: Accessory n.

Origin: transverse processes of the atlas

Insertion: acromion and adjacent scapula

Action: advancing the limb

Latissimus dorsi:

Innervated by: 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.:

Innervated by: 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:

Innervated by: 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:

Innervated by: 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,

Innervated by: 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: Subscapularis:

Innervated by: 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:

Innervated by: Musculocutaneous n. of the brachial plexus

Origin: medial supraglenoid tubercle

Insertion: proximal shaft of the humerus

Action: fixator

Caudal (Flexors):

Innervated by: 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:

Innervated by: 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:

Innervated by: 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:

Innervated by: 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:

Innervated by: 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:

Innervated by: 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:

Innervated by: Median or Ulnar n. of the brachial plexus

Caudal position on the forearm

Originate from the caudal 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

• Arteries of the Forelimb
• Veins of the Forelimb
• Lymphatics 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 plexus, 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