Vana 102 Chapter 6- The Stay Apparatus In Horses

• Passive Stay Apparatus


  • Enables the horse to be on its feet for long periods with a minimum of muscular effort
• Synsarcosis


  The connection of the forelimb to the trunk is not part of the stay-apparatus
• Serratus Ventralis


  The muscle that serves as the principal weight bearing connection is heavily interlaced with non-tiring tendinous tissue
• Triceps


  Seems inactive by its flabbiness in the quietly standing horse, although some believe that its tonus alone would prevent collapse of this key joint
• Collateral ligaments


  Eccentrically placed, inhibit flexion to a lesser degree. The principal extensor of the joint
• Biceps


  The "pull" on the flexor surface by the biceps insertion that would tend to keep the joint extended has already been mentioned
• Carpal Joint


  Is stabilized (prevented from flexing) by the (dorsal) "pressure" of the extensor carpi radialis tendon
• Flexor Carpi Ulnaris and Ulnaris Lateralis


  The ending on the accessory carpal and being tensed by the weight of the trunk via scapula, fixed shoulder joint, and humerus, "pulls" on the flexor surface of the carpal joint in an attempt to keep the joint extended
• Accessory Ligaments of the Supf. And Deep Digital Flexors


  Attaching on the palmar surface of radius and large metacarpal bone above and below the carpus tend to supply a similar "pull", again by the weight of the animal, but in the opposite direction— distally. Some ascribe a similar potential to the interosseus
• Fetlock Joint


  It needs to be stabilized by being prevented from further overextending, i.e., sinking toward the ground. Their elastic properties "carry" the joint in a yielding, anticoncussive manner that is best observed in slow-motion films of a horse at speed
• Suspensory Apparatus


  • Interosseus
  • Proximal sesamoid bones
  • Sesamoidean ligaments
• Interosseus


  Arises from the carpus and proximal end of the large metacarpal bone and ends on the two sesamoid bones. (Before doing so it sends extensor branches around the proximal phalanx to the common extensor tendon)
• Proximal Sesamoid Bones


  Articulate with the distal end of the large metacarpal bone to reduce friction between the suspensory apparatus and the palmar surface of the fetlock joint
• Collateral Ligaments


  Tie the sesamoid bones to the cannon bone and proximal phalanx
• Palmar Ligament


  A thick ligament that unites the sesamoid bones and forms a smooth bearing surface for the digital flexor tendons
• Sesamoidean Ligaments

  • Short
  • Cruciate
  • Oblique
  • Straight
  • The tension in the interosseus is continued distal to the joint by four______________ of which the first three end on the proximal, and the last on the middle phalanx.
• Supf. Digital Flexor Tendon


  Assists the suspensory apparatus by providing a tendinous support extending (via its accessory [check] ligament) from the radius above the fetlock joint to the proximal and middle phalanges below the joint
• Deep Flexor Tendon


  Its accessory (check) ligament provide added and similar support; the accessory ligament arises with the interosseus from the caudal aspect of carpus, the tendon itself ends on the distal phalanx
• Pastern Joint


  Is prevented from overextension by four pastern ligaments that connect the two bones that form the joint on the palmar surface
• Coffin Joint


  It actually flexes when the fetlock sinks underweight and can be disregarded in the consideration of the stay apparatus
• Reciprocal Mechanism


  An important part of the passive stay apparatus is the so-called ___________ that links the actions of stifle and hock joints
• Peroneus Tertius


  It arises (by a common tendon with the long digital extensor) from the lateral condyle of the femur and, passing cranial to the tibia, ends by complex attachments on certain tarsal bones and the proximal end of the large metatarsal bone
• Supf. Digital Flexor


  It lies caudal to the tibia and connects the caudal surface of the femur with the calcanean tubery
• Accessory Ligament of the Deep Flexor


  Is much thinner and may be absent
• Supf. Digital Flexor Tendon

  Has no accessory ligament, but this is compensated for in the hindlimb by its firm attachment on the calcanean tuber: its attachment proximal and distal to the fetlock joint still helps to prevent overextension in this joint when the limb is supporting weight
• Patellar Locking Mechanism


  By being able to lock the stifle, the horse converts the jointed column of its hindlimb into a weight-bearing pillar
• By being on its feet most of the time, the horse, a rather nervous and excitable animal that uses flight as its principal means of defense, appears to be in perpetual readiness to run away from danger.
• Though no collateral ligaments are present, the movements in the shoulder joint are restricted largely to flexion and extension in the sagittal plane by the subscapularis medially and the infraspinatus and (to a lesser degree) the supraspinatus laterally.
• Reciprocal Mechanism - This is accomplished by the tendinous peroneus tertius muscle and the nearly tendinous supf. digital flexor muscle, both crossing the joint spaces of the two joints.