Thoracic Limb (Large Animal)

Large Animal: General Info

• Large animals, such as horses, experience a downward force due to their body weight.
• The "stay apparatus" is a fibrous mechanism that allows them to oppose gravity without expending energy.
• Approximately 60% of the horse's body weight is placed on the thoracic limbs.

Walking Postures

• Digitigrade walkers bear weight on their digits (e.g., dogs and cats).
• Unguligrade walkers bear weight on the distal phalanx (e.g., horses and cattle).
• Plantigrade walkers bear weight on their digits, metatarsals, and plantar tarsus (e.g., humans, bears, squirrels, and skunks).

Bones: Thoracic Limb

• Bones of the large animal thoracic limb include the scapula, humerus, radius, ulna, carpus, metacarpus, phalanges, and proximal and distal sesamoid bones.
• In horses, the distal sesamoid bone is referred to as the "Navicular" bone.
• The radius and ulna are almost completely fused, and the ulna is no longer discernible at the distal radius.
• The point of the shoulder can be referenced by the Greater Tubercle of the humerus.
• Their approximate center of gravity is located at the 15th thoracic vertebrae.
• The metacarpal 3 bone has the common name of the "Cannon" bone, which they primarily weight bear on.
• The metacarpal 2 and 5 bones have the common name of the "Splint" bones.

Joints: Horse Thoracic Limb

• The Shoulder joint is referred to as the "Glenohumeral" joint.
• The Elbow (or cubital) joint is referred to as the "Humeroradioulnar" joint.
• The Proximal Interphalangeal joint has the common name of the "Pastern" joint.
• The Distal Interphalangeal joint has the common name of the "Coffin" joint.
• The Metacarpophalangeal joint has the common name of the "Fetlock" joint.
• The Carpal (or antebrachiocarpal) joint is composed of the middle carpal joint and the metacarpophalangeal joint.

Stay Apparatus: General Info

• The stay apparatus reduces the energy required to stand, allowing rest while standing.
• It makes movement efficient via specific groups of muscles and tendons/ligaments working in synchrony to conserve energy.
• The stay apparatus allows horses to doze while standing up, using minimal muscular energy.
• The coordination of many individual structures acts to "lock" the limb in extension, preventing collapse despite minimal effort.

Stay Apparatus: Shoulder Joint Stabilization

• The shoulder joint is stabilized by preventing flexion.
• The tendon of the biceps brachii muscle prevents the shoulder from flexing.

Stay Apparatus: Elbow Joint Stabilization

• Elbow joint stabilization involves preventing elbow flexion while under weight.
• The shoulder being "locked" allows the body weight to balance on the radius in a "crooked post".

Stay Apparatus: Carpal Joint Stabilization

• Carpus stabilization involves preventing the carpus from flexing or extending.
• Tension on the biceps brachii muscle is transmitted to the extensor carpi radialis muscle via the "lacertus fibrosus".
• The strong palmar carpal ligament aids in preventing hyperextension of the carpal joint.

Stay Apparatus: Check Ligaments

• Tension on the SDF (superficial digital flexor) and DDF (deep digital flexor) tendons are "checked" by the accessory ligaments, which are attached to these tendons.

Stay Apparatus: Fetlock, Pastern, and Coffin Joint Stabilization

• The stabilization of the fetlock, pastern, and coffin joint is due to the suspensory apparatus, which involves the prevention of hyperextension of these joints.
• The interosseus muscle (also known as the suspensory ligament) prevents hyperextension of the fetlock joint.