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.

Learn about the unique anatomy of large animals, including the stay apparatus and its role in supporting their body weight, and how it distributes the weight between their limbs.