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 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
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 limited to the sagittal plane.
Muscles of the Forelimb
- Extrinsic Musculature
- Deep Musculature
- Muscles of the Elbow
- Muscles of Supination and Pronation
- Muscles of the Carpal and Digital Joints