Thoracic Limb Vessels and Nerves

Clinical Focus and Study Guidelines

• Major emphasis is placed on the middle of the brachium and structures distal to it ($1$). This distal area holds higher clinical significance compared to proximal structures.
• Proximal anatomy in large animals closely resembles that found in small animal species.
• Study objectives beyond simple identification include:
  • Determining the specific muscles innervated by each nerve.
  • Identifying the efferent actions (motor functions) caused by these nerves.
  • Mapping the afferent sensory autonomous zones associated with specific nerves.

Equine Thoracic Limb Arterial Supply

• The arterial pathway follows a sequential progression with specific name changes based on the anatomical level of the limb:
  • Axillary artery
  • Brachial artery
  • Median artery
  • Radial artery
  • Medial palmar metacarpal artery: The palmar branch joins with the collateral ulnar artery to form the lateral palmar artery.
• The name change occurs at the carpus ($1$).
• Medial palmar artery: This is the primary blood supply to the distal limb of the horse. Just proximal to the fetlock ($1$), it divides into the palmar digital arteries.
• Medial (palmar) digital artery: Positioned on the abaxial surface of the proximal sesamoid bones ($1$); it terminates at the terminal arch ($1$).
• Lateral (palmar) digital artery: Positioned on the abaxial surface of the proximal sesamoid bones ($1$); it terminates at the terminal arch ($1$).
• Collateral Ulnar artery: This is a small vessel that may not be visible in all dissections.
• Extensive collateral circulation exists in the equine thoracic limb, though individual names of all collateral vessels are not required for study.

Metacarpal Anatomy and Cross-Sectional Relationships

• The middle of the right metacarpus contains the following structures arranged from dorsal to palmar and medial to lateral:
  • Large metacarpal bone: Third metacarpal bone ($MCIII$).
  • Small metacarpal bones: Second metacarpal bone ($MCII$, medial splint bone) and Fourth metacarpal bone ($MCIV$, lateral splint bone).
  • Common digital extensor: Positioned dorsally.
  • Lateral digital extensor: Positioned laterodorsally.
  • Interosseus: Also known as the suspensory ligament ($SUS$).
  • Deep digital flexor ($DDF$).
  • Accessory (check) ligament ($8$).
  • Superficial digital flexor ($SDF$).
• Vessel and nerve locations in the metacarpus:
  • Palmar metacarpal vessels and nerves ($10$).
  • Medial palmar artery and vein ($11$): Often grouped with the medial palmar nerve ($12$) as a neurovascular bundle (VAN).
  • Lateral palmar artery ($13$): Often grouped with the lateral palmar nerve ($14$).
  • Lateral palmar metacarpal artery and nerve: Located in close association with the axial surface of the fourth metacarpal bone ($MCIV$).
  • Medial palmar metacarpal artery and nerve: Located in close association with the axial surface of the second metacarpal bone ($MCII$).

Equine Thoracic Limb Innervation

• Musculocutaneous nerve and Median nerve: These nerves begin as a common trunk in the horse, which differs from the arrangement seen in the dog.
• Median nerve:
  • Provides motor innervation to the flexors of the carpus and digit, sharing some functional overlap with the ulnar nerve.
  • Divides at the level of the carpus ($1$) into the Medial palmar nerve and the Lateral palmar nerve.
  • The Lateral palmar nerve contains fibers from both the median and ulnar nerves.
  • At the level of the fetlock ($1$), the name changes to Medial and Lateral palmar digital nerves.
  • Dorsal branches of these digital nerves provide sensation to the dorsal aspect of the distal digit.
• Ulnar nerve:
  • Found between the medial humeral epicondyle and tuber olecranon ($1$).
  • Provides cutaneous innervation to the distal medial aspect of the brachium and the caudal aspect of the antebrachium.
  • Provides motor stimulation to the flexors of the carpus and digits.
  • Bifurcates just above the carpus ($1$) into two branches:
    1. Dorsal branch: Supplies the skin in the lateral/dorsal metacarpal area.
    2. Palmar branch: Joins the lateral branch of the median nerve to form the lateral palmar nerve.
• Deep branch of the lateral palmar nerve:
  • Dives into the suspensory ligament just palmar to $MCIII$ at the level of the carpus ($1$).
  • Gives rise to the Medial and Lateral palmar metacarpal nerves.

Dorsal Innervation and Cutaneous Supply

• Dorsal innervation of the front limb is minimal and consists primarily of cutaneous nerves.
• Musculocutaneous nerve: Gives rise to the Medial Cutaneous antebrachial nerve, providing sensation from the lacertus fibrosus to the distal cannon bone on the dorsal/medial aspect.
• Radial nerve: Gives off a cutaneous branch providing dorsal/lateral sensation from the level of the elbow down to the carpus ($1$).
• Dorsal branch of the Ulnar nerve: Supplies the lateral aspect of the limb.
• Dorsal branches of the Palmar Digital nerves: Supply the distal aspect of the digit dorsally.

Equine Venous System

• Major veins of the thoracic limb include:
  • Brachial vein.
  • Cephalic vein.
  • Median Cubital vein: Connects the cephalic and median systems.
  • Accessory Cephalic vein.
  • Median veins.
  • Medial Palmar vein: This vessel divides just proximal to the fetlock ($1$) to become the medial and lateral palmar digital veins.
• Deep and superficial veins often mirror the arterial pathways in the distal limb.

Comparative Anatomy and Nerve Blocks

• The major thoracic limb autonomous zones vary between dogs and horses. These zones are defined for the axillary, musculocutaneous, radial, and ulnar nerves.
• Nerve blocks are utilized clinically to localize pain in the equine limb:
  • Palmar Digital ($PD$) block: Targets the heel region.
  • Abaxial (sesamoidean) nerve block: Targets the pastern region.
  • Low $4$-point block: Targets the fetlock region and all structures distally.
  • High $4$-point block: Targets all structures below the carpus ($1$).

Questions & Discussion

• What nerve(s) is likely damaged in a patient that is unable to extend the carpus and digits?
  • The Radial nerve ($D$) is responsible for the motor innervation of the carpal and digital extensors. Damage to this nerve results in the inability to extend these joints, which is a very serious condition in large animals.
• What are the components of the neurovascular bundles in the distal digit?
  • The Medial Palmar digital VAN includes the Medial Palmar digital vein ($14$), Medial Palmar digital artery ($13$), and Medial Palmar digital nerve ($15$). These arborize distally within the hoof capsule.