LAB 3 Thoracic Limb

In the distal fore or hind limb several equine bones have interesting names that are in common usage. The cannon bone (Mc3 or Mt3) is so named because in cross section it appears to be similar to the end of a cannon barrel. The vestigial Mc/Mt 2 and 4 bones are referred to as splint bones. They are bound to the cannon bone by an interosseous ligament which is almost as wide as the length of the splint bones but the individual fibers are short, just a few mm in length. The interosseous ligament should not be confused with the interosseous tendon which is the academic name for the suspensory ligament of the clinic. Metacarpophalangeal joint is the anatomic name for the fetlock of common usage. The fetlock has a lock of hair on the palmar/plantar surface. This lock of hair is best developed in heavy horses and is called the feather. The term fetlock is thought to be derived from an old English word such as "feetlock" for the lock of hair near the feet. This lock of hair covers and obscures the ergot.

In the forelimb the main digital extensor tendons don't fuse as they do in the hind limb. The lateral digital extensor tendon inserts of P1 while the common digital extensor tendon inserts on the extensor process of P3. The common digital extensor tendon is supplemented by medial and lateral extensor branches of the suspensory ligament (interosseous tendon). Therefore, the suspensory ligament not only supports (suspends) the fetlock joint, it also causes passive extension of the digit to balance the digital flexor effect of the digital flexor tendons. The suspensory ligament plus the distal sesamoidean ligaments form the suspensory apparatus which is the distal part of the stay apparatus in both fore and hind limbs. The digital flexor tendons serve as a similar passive fetlock support mechanism because these flexor tendons are anchored by their respective check ligaments. The suspensory apparatus and the adjacent digital flexor tendons counteract gravitational forces on the fetlock joint.

The main insertion of the suspensory ligament (interosseous tendon) is on the proximal sesamoid bones but these bones form a groove that the digital flexor tendons slide over. The SDFT inserts on P2 while the DDFT inserts on the sole surface of P3 after passing over the navicular bone. Note the symmetry of the forelimb digital tendons. Only one of the extensor pair and one of the flexor pair insert on P3.

The main arterial supply of the distal equine forelimb is the medial palmar artery but the main artery to the distal hind limb is on the lateral side of the cannon bone. At the level of the end of the splint bones, these arteries move toward the axial plane of the cannon bone and split to form a symmetrical pair of palmar (or plantar) digital arteries that pass over the abaxial surfaces of the fetlock and digit. There is a close relationship of the digital vein, artery and nerve which form a palpable neurovascular bundle as they pass over the fetlock.