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 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 arm, 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

At the distal end of the humerus a small hole may be seen connecting the olecranon fossa caudally with the radial fossa cranially. This is the supratrochlear foramen. No structures pass through it.

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. The trochlear notch on the cranial aspect of the ulna articulates with the large trochlea of the humerus which forms the main elbow joint capable of flexion and extension. Just distal to the trochlear notch, a large medial coronoid process and a smaller lateral coronoid process can be seen. 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 only one exists. A single dorsal sesamoid bone can also be seen in dogs in digit 2 - 5 inclusive over the same joint between the metacarpal bone and the proximal phalanx. This is not found in ungulates or in the the first digit. However another sesamoid bone exists in the tendon of the abductor pollicis longus muscle of the first digit.

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
superficial pectoral m. - ventral branches of cervical spinal nerves 7 and 8 (Evans and de Lahunta's Guide to the Dissection of the Dog, 8th ed.)
deep pectoral m. - caudal pectoral nerves, cervical spinal nerve 8, and thoracic nerve 1 (Evans and de Lahunta's Guide to the Dissection of the Dog, 8th ed.)
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

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

Species Specifics

Canine Forelimb

Bovine Forelimb


Forelimb - Anatomy & Physiology Learning Resources
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OVAM
Anatomy Museum Resources
Muscle flashcards - extrinsic musculature of the canine forelimb
Muscle flashcards - muscles of the canine shoulder
Muscle flashcards - muscles of the canine elbow
Muscle flashcards - muscles of canine antebrachium



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Last modified on 1 September 2017, at 13:01