Stay Apparatus - Horse Anatomy

Thoracic Limb

When a horse is standing, muscles on opposite sides of the joints delicately contract and relax so that it stays in balance. This obviously puts considerable strain and stress on the muscles. To combat this, the horse has increased the fibrous content of many muscles. These collagenous muscles have been named collectively as the stay apparatus. This is further divided into the suspensory apparatus and the check apparatus.

Suspensory Apparatus

'Interosseous medius/Suspensory ligament'

Proximally, this attaches to the distal row of carpal bones and metacarpus. It runs distally on the palmar surface of the metacarpal bone and then bifurcates proximal to the fetlock, with a small branch connecting to each sesamoid. It then continues distally on the dorsal surface of the proximal phalanx, and joins the common digital extensor, which continues to insert on the middle and distal phalanges.

Associated ligaments:

Collateral sesamoid ligaments - connect the abaxial surface of the sesamoid to the metacarpus and proximal phalanx.

Palmar ligaments - connect the sesamoid bones together.

Distal sesamoidean ligaments - connect the sesamoids to the proximal and middle phalanges. This ligament can be seen as a direct continuation of the interosseous muscle with the sesamoids emdedded in it.

Thus by virtue of the ligamentous arrangement, the suspensory apparatus without any muscular action can resist extension of the fetlock and pastern through the distal sesamoidean ligaments and resist flexion of the pastern and coffin joints through the common digital extensor tendon.

Stay Apparatus

The serratus ventralis supports the cranial part of the body, and connects the costal side of the scapula to points of attachment on the caudal cervical vertebrae and cranial ribs. It contains a tendinous layer that suspends the body once the muscles relax. This will, however, cause flexion of the shoulder which needs to be balanced by the extensors of the shoulder.

The Biceps brachii fulfils this role. Its collagenous tendon extends the length of the muscle and divides near the elbow. The short tendon inserts on the radial tuberosity, the long tendon (lacertus fibrosus) continues distally and blends with the tendon of the extensor carpi radialis and forearm fascia. Combined, these insert on the proximal end of the metacarpus. Thus, the biceps have provision to be able to relax without the collapse of the shoulder, elbow and carpal joints.

Check Apparatus

The superficial digital flexor muscle, continues past the carpus as a thick tendon. The accessory or superior check ligament is its tendinous radial head that joins the muscle at this point. The tendons continue distally and bifurcate by the fetlock into slips, that attach to the medial and lateral aspect of the pastern joint. There is therefore a ligamentous connection from the distal radius to the pastern that can help prevent extension of the carpal and fetlock joint without muscle contractions.

The deep digital flexor also forms a thick tendon just proximal to the carpus. The accessory ligament or inferior check ligament, joins the tendon at the middle of the metacarpus. They then continue through the bifurcation of the superficial flexor tendon, through the intersesamoidean groove at the fetlock joint, over the pastern and insert on the distal phalanx. This ligamentous tissue between the metacarpal region and distal phalanx helps prevent extension of the fetlock, pastern and coffin joints.