Limb Vasculature - Horse Anatomy

Thoracic Limb

Proximal

The subclavian artery gives rise to the superficial cervical artery, before it becomes the axillary artery. The deltoid branch of the superficial cervical artery accompanies the cephalic vein (arises from the external jugular vein) though a groove between the brachiocephalicus and pectoralis descendens. The axillary vessels give rise to the following vessels before becoming the brachial artery:

• External thoracic vessels: supply the pectoral muscles
• Suprascapular vessels: supply the caudal border of the suprascapular muscles
• Thoracodorsal vessels: supply the latissimus dorsi
• Caudal circumflex humeral vessels

The thoracodorsal and circumflex arteries are branches of the subscapular artery. The cranial circumflex humeral artery runs deep to the coracobrachialis to supply the biceps; its corresponding satellite vein can cross the coracobrachialis on either surface. The thoracodorsal vein gives rise to the superficial thoracic vein, which passes caudally to drain the ventral part of the trunk.

The brachial vessels give rise to deep brachial vessels, which supply the triceps. More distally, brachial vessels form bicipital vessels to the biceps. The next branches from the brachial vessels are the transverse cubital vessels, which pass deep to the biceps. The collateral ulnar vessels arise from the caudal surface of the brachial vessels, following the path of the ulnar nerve. The final branches are the common interosseous vessels, which pass through the interosseous space to continue as the cranial interosseous vessels.

The brachial vein gives rise to the median cubital vein at the level of the elbow. This subsequently anastamoses with the cephalic vein. The cephalic vein leaves the groove between the brachiocephalicus and pectoralis descendens and runs distally on the medial surface of the antebrachium. At the elbow joint, it gives rise to the accessory cephalic vein which runs parallel to the cephalic vein but then curves cranially to terminate on the dorsal surface of the carpus.

The median artery gives off its palmar branch before passing through the carpal canal. It is then joined by a branch of the radial artery and continues towards the digit as the medial palmar artery. The smaller lateral palmar artery originates from the union of the palmar branch of the median artery with the collateral ulnar artery proximal to the carpus.

The median and radial arteries branch and then anastamose to form the deep palmar arch at the level of the carpus. The median and lateral palmar metacarpal arteries arise from the deep palmar arch. They run distally on the axial surface of the medial and lateral splint bones, respectively.

Just distal to the carpus, the medial palmar artery has just passed through the carpal canal and then vein has crossed the carpus superficial to the flexor retinaculum. In the metacarpus, the vessels lie medial to the interosseous and deep digital flexor tendons. At the metacarpophalangeal (fetlock) joint , the medial palmar vessels become the medial digital vessels. The lateral palmar vessels run past the carpus adjacent the the accessory carpal bone.

Distal

Arteries and Veins of the Left Distal Forelimb from Budras' Anatomy of the Horse with permission from Manson Publishing

Arteries

The arterial supply to the digit and fetlock of the thoracic limb comes mainly from the median palmar artery. The median palmar artery divides in the distal fourth of the metacarpus between thesuperficial and deep digital flexor tendons and the suspensory ligament, to become the medial and lateral digital arteries. Part of the deep palmar arch anastamoses with the lateral digital artery to form the superficial palmar arch, which supplies the fetlock joint.

The digital arteries run superficially in the proximal fetlock region, emerging palmar to their satellite vein and between the palmar digital nerve and its dorsal branch. The medial and lateral digital arteries run distally over the fetlock and each branches to supply the fetlock joint, digital extensor and flexor tendons , digital sheath, ligaments fascia and skin.

The short artery of the proximal phalanx arises from the medial and lateral digital arteries, immediately dividing into dorsal and palmar branches to form a circle around the middle of the proximal phalanx. This supplies the proximal phalanx and surrounding structures. The palmar branch of the short artery runs between the proximal phalanx and the digital flexor tendons to join the contralateral vessel between the sesamoidean ligaments. The dorsal branch joins the contralateral vessel deep to the common digital extensor tendon.

In the region of the proximal interphalangeal(pastern) joint, the bulbar artery (artery of the digital cushion) arises from each digital artery. This artery branches to supply the frog, digital cushion, palmar part of the cuneate corium, laminar corium of the heel and bar and palmar parts of the perioplic and coronary corium.

The coronal artery arises from the digital artery or the bulbar artery to supply the heel and perioplic corium. The coronal artery then anastamoses with branches from the dorsal artery of the middle phalanx. The dorsal artery of the middle phalanx arises from each digital artery in the mid-region of the middle phalanx, forming the coronary arterial circle; as the palmar and dorsal branches anastamose to form a complete arterial circle around the middle phalanx. This coronary arterial circle supplies the distal interphalangeal (coffin) joint, common digital extensor tendon, perioplic and coronary corium, fascia and skin.

Branches from the conjoined palmar branches of the middle phalanx anastamose to form the navicular plexus, providing several small arteries to foramina along the proximal border of the distal sesamoid (navicular) bone. This plexus provides approximately one third of the total blood supply to the navicular bone.

At the level of the palmar process of the distal phalanx, the digital artery forms a dorsal branch of the distal phalanx, then continues distally to the terminal arch. The dorsal branch gives off a small artery to supply the digital cushion and corium of the frog, before it passes through a foramen in the palmar process of the distal phalanx. It then bifurcates on the dorsal surface of the distal phalanx to form branches that supply the corium of the heels and quarters, laminar corium of the toe, and eventually joining branches of the coronal artery and the marginal artery of the sole. The termination of the dorsal branch of the distal phalanx joins with a vessel that comes from the terminal arch in the solar canal. It then branches into several small arteries which enter the distal border of the distal phalanx.

The medial and lateral digital arteries run in the solar groove of the distal phalanx. Each gives off branches to the navicular plexus in the distal sesamoidean (impar) ligament. The navicular plexus gives rise to the distal navicular arteries, which enter the distal sesamoid (navicular) bone at the distal border and anastamose within the foraminae of the bone. These distal navicular arteries supply the distal two thirds of the navicular bone.

The medial and lateral digital arteries enter the solar foramen and form the terminal arch within the solar canal of the distal phalanx. Branches from the terminal arch run through foraminae on the parietal surface of the distal phalanx to supply the laminar corium. Several of these branches emerge near the solar border and anastamose to form the marginal artery of the sole. This supplies the solar and cuneate corium.

Veins

Venous drainage from the laminar corium is via parietal veins from the laminar circulation to the parietal venous plexus and coronary venous plexus. The coronary venous plexus also receives veins from the perioplic and coronary corium. Veins from the solar and cuneate corium drain into the solar venous plexus.

Veins in the solar canal anastamose at the level of the distal sesamoid (navicular) bone, forming the medial and lateral terminal veins. The medial and lateral terminal veins join branches of the inner venous plexus to form the digital vein. This digital vein receives venous drainage from the navicular bone, coronary vein (coronary region), inner venous plexus and bulbar vein (heel).

Most of the venous drainage from the equine foot is via veins on the palmar aspect, which generally lack valves. Some small branches of the coronary, subcoronary and bulbar veins have valves. This means that blood flow from the foot can take different routes and that weightbearing is essential for blood to flow proximally.

Pelvic Limb

Proximal

The internal iliac vessels branch to form the cranial and caudal gluteal vessels; which supply the croup and thigh regions. The cranial gluteal artery subsequently forms the obturator artery. The satellite obturator vein is a branch of the external iliac vein. The external iliac vessels become the femoral vessels in the thigh, giving rise to the pudendoepigastric trunks.

Femoral vessels form the lateral circumflex femoral vessels, which enter the quadriceps between the rectus femoris and vastus medialis, and the saphenous vessels. The saphenous vein is larger than the corresponding artery. The saphenous vessels divide in the proximal third of the leg to form cranial and caudal branches. The cranial branch of the saphenous vein passes through the tarsus to become the common digital vein in the region of the metatarsus. This vein runs across the third metatarsal (cannon bone) to unite with the medial plantar vein. The caudal branch of the medial saphenous vein and artery run distally in a groove cranial to the common calcaneal tendon. The medial saphenous vein anastamoses with the caudal tibial vein and lateral saphenous vein proximal to the hock, subsequently dividing into the medial and lateral plantar veins. The saphenous artery anastamoses with the caudal tibial artery, giving rise to the medial and lateral plantar arteries.

The femoral vessels give rise to the descending artery and vein of the stifle, as well as other small vessels to that joint, in the distal third of the thigh. Some of the smaller vessels to the stifle joint arise from the popliteal vessels which are continuations of the femoral vessels. The femoral vessels also branch to form the lateral saphenous vein. The lateral saphenous vein runs distally along the caudal border of the gastrocnemius and anastamoses with the caudal branch of the medial saphenous vein and caudal tibial vein proximal to the hock.

The popliteal vessels bifurcate to form cranial and caudal tibial vessels at the level of hock; the fibular head. The cranial tibial vessels pass between the tibia and fibula, then follow the tibia to the level of the at this point they become the short pedal vessels. The pedal artery continues to become the dorsal metatarsal artery. The dorsal metatarsal artery passes over the lateral surface of the third metatarsal (cannon bone) and fourth metatarsal(lateral splint) plantarodistally. It then receives the plantar metatarsal arteries which run on the plantar aspect of the third metatarsal.

Distal

The vascular supply to the digit in the pelvic limb is largely the same as in the thoracic limb.

The caudal branches of the saphenous artery and medial saphenous vein continue as the lateral and medial plantar vessels, running with the deep digital flexor tendon, over the sustentaculum tali, to the level of the metatarsus. The vein lies superficial to the corresponding artery. In the digit, the medial and lateral digital veins, arteries and nerves lie next to each other in a dorsal to plantar arrangement; this can be remembered as VAN (vein, artery, nerve).

The medial and lateral plantar vessels enter the metatarsus to give rise to the deep plantar arterial and venous arches; from which the medial and lateral metatarsal vessels originate. The medial and lateral metatarsal arteries join the medial and lateral digital arteries, which result from the bifurcation of the dorsal metatarsal artery, close to the fetlock. The dorsal metatarsal artery continues as the short dorsal pedal artery, which subsequently forms the cranial tibial artery on the dorsal hock. The cranial tibial, dorsal pedal and dorsal metatarsal arteries supply the digit and hock.

The dorsal common digital vein crosses the medial surface and the dorsal metatarsal artery crosses the lateral surface of the third metatarsal (cannon bone) obliquely. The dorsal common digital vein joins the medial plantar vein in the distal third of the cannon bone. The medial plantar vein then becomes the medial digital vein at the level of the metacarpophalangeal (fetlock) joint. At this level, the medial plantar vein anastamoses with the lateral plantar vein via the distal deep plantar arch.

The medial and lateral digital arteries run on their corresponding sides of the digit, forming dorsal and plantar branches to the proximal phalanx and middle phalanx. These branches anastamose with each other to form arterial circles around the proximal and middle phalanges.

The dorsal branch of the lateral and medial digital arteries arises at the level of the distal phalanx. It runs though the foramen/notch in the plantar process of the distal phalanx into the parietal groove. The digital arteries continue to the solar surface and enter the solar foraminae on either side to anastamose within the bone, forming the terminal arch. Branches of the terminal arch run in osseous canals to the parietal surface, where they supply the laminar dermis. Some branches also emerge close to the solar margin and anastamose to form an artery that follows this margin.

Digital veins are satellite to the arteries down to and into the distal phalanx, but not all the branching arteries are accompanied by veins. There is a dense venous plexus in the coronary and laminar dermis of the sole which drains into the medial and lateral digital veins via many smaller veins. Most of these veins lie subcutaneously, just proximal to the hoof, others run directly to the digital veins from the axial surface of the ungual cartilages.

References

• Baxter, G.M., Adams, Adams and Stashak’s Lameness in Horses, 6th Edition (2011), Lippincott, Williams & Wilkins
• Budras, K. Sack, W.O., Anatomy of the Horse, 6th Edition (2012), Schlutersche Verlagsgesellschaft mbH & Co. KG
• Konig, H.E., Liebich, H.G. Veterinary Anatomy of Domestic Mammals: Textbook and Colour Atlas (2009), Schattauer