chapter 6

• Stay-Apparatus


  Enables the horse to be on its feet for long periods with a minimum of muscular effort
• Older horses actually doze (perhaps sleep) while standing, although for a refreshing sleep horses lie down, usually at night when they are unobserved
• Horse


  • Nervous and excitable animal
  • Uses flight as its principal means of defense
  • Appears to be in perpetual readiness to run away from danger
• Passive Stay-Apparatus


  Stabilizes the joints in a position suitable for the support of the body
• Active muscles soon tire and become painful, which signals the animal to lie down
• Stay-Apparatus


  Greatly reduces the effort of the intrinsic limb muscles of horses
• In most joints stabilization means preventing them from flexing
• Pastern and fetlock joints in the standing horse are extended and overextended, respectively
• Stabilization of the shoulder joint


  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
• Principal stabilizer of the shoulder joint


  The biceps tendon pressing against the cranial (extensor) surface of the joint
• Weight of the trunk acting on the proximal end of the scapula


  Tensions the biceps-lacertus-extensor carpi "rigging"
• Tensing the biceps-lacertus-extensor carpi "rigging"


  Causes a cranial "pull" on the elbow joint (i.e., an extension of the joint) and "pressure" on the extensor surface of the carpal joint that tends to prevent flexion in that joint
• Stabilization of the elbow joint


  Principally stabilized (i.e., prevented from flexing) by tension in a group of carpal and digital flexors that arise on the medial and lateral epicondyles of the humerus and contain much fibrous tissue
• Stabilization of the carpal joint


  Stabilized (prevented from flexing) by the (dorsal) "pressure" of the extensor carpi radialis tendon
• Flexor carpi ulnaris and ulnaris lateralis ending on the accessory carpal


  "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


  Tend to supply a similar "pull", again by the weight of the animal, but in the opposite direction—distally
• Stabilization of the fetlock joint


  Needs to be stabilized by being prevented from further overextending, i.e., sinking toward the ground
• Suspensory apparatus of the fetlock joint


  • Consists of the interosseus, proximal sesamoid bones, and sesamoidean ligaments
• Interosseus


  Arises from the carpus and proximal end of the large metacarpal bone and ends on the two sesamoid bones
• 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
• Sesamoidean ligaments


  Short, cruciate, oblique, and straight, 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


  Provides 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
• Stabilization of the pastern joint


  Prevented from overextension by four pastern ligament that connect the two bones that form the joint on the palmar surface
• Coffin joint


  Actually flexes when the fetlock sinks under weight and can be disregarded in the consideration of the stay apparatus
• Passive Stay-Apparatus of the hindlimb


  Prevents collapse of the hindlimb with only a minimum of muscular effort
• Reciprocal mechanism


  Links the actions of stifle and hock joints
• Peroneus tertius muscle


  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 muscle


  Lies caudal to the tibia and connects the caudal surface of the femur with the calcanean tuber
• Stifle and hock joints


  Must move in unison, and if the stifle joint is locked, the hock joint is also rendered incapable of movement
• Stabilization of the fetlock and phalangeal joint in the hindlimb


  Supported as in the forelimb by the interosseus and the supf. and deep flexor tendons with the fetlock joint slightly overextended in the standing animal
• The accessory (check) ligament of the deep flexor is much thinner and may be absent in the hindlimb
• The supf. digital flexor tendon has no accessory ligament in the hindlimb, but this is compensated for by its firm attachment on the calcanean tuber</b>
• Patellar locking mechanism


  Converts the jointed column of the hindlimb into a weight-bearing pillar
• Medial ridge of the femoral trochlea

  Larger than the lateral and is prolonged proximally to form a rounded tubercle
• Medial patellar ligament


  Connects to the medial border of the patella via the patellar fibrocartilage
• Intermediate patellar ligament


  Attaches directly on the patellar apex
• Patella rotating medially


  Causes the fibrocartilage and medial patellar ligament to slide farther caudally on the tubercle, fully locking the stifle
• The locking of the stifle joint enables the horse to stand with little muscular activity
• Some effort must be required, however, because the horse tires after a few minutes and shifts its weight to the other hindlimb