Changes

:'''Distal sesamoidean ligaments''' - connect the sesamoids to the [[Phalanges - Horse Anatomy#Proximal Phalanx|proximal]]  and [[Phalanges - Horse Anatomy#Middle Phalanx|middle]]  phalanges. This ligament can be seen as a direct continuation of the interosseous muscle with the sesamoids emdedded in it.

:'''Distal sesamoidean ligaments''' - connect the sesamoids to the [[Phalanges - Horse Anatomy#Proximal Phalanx|proximal]]  and [[Phalanges - Horse Anatomy#Middle Phalanx|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 [[Joints and Ligaments - Horse Anatomy#Metacarpophalangeal (Fetlock) Joint|fetlock]] and [[Joints and Ligaments - Horse Anatomy#Proximal Interphalangeal (Pastern) Joint|pastern]] through the '''distal sesamoidean ligaments''' and resist flexion of the pastern and coffin joints through the [[Tendons - Horse Anatomy#Extensors|common digital extensor tendon]].

Thus by virtue of the ligamentous arrangement, the suspensory apparatus without any muscular action can resist extension of the [[Joints and Ligaments - Horse Anatomy#Metacarpophalangeal (Fetlock) Joint|fetlock]] and [[Joints and Ligaments - Horse Anatomy#Proximal Interphalangeal (Pastern) Joint|pastern]] through the '''distal sesamoidean ligaments''' and resist flexion of the [[Joints and Ligaments - Horse Anatomy#Proximal Interphalangeal (Pastern) Joint|pastern]] and [[Joints and Ligaments - Horse Anatomy#Distal Interphalangeal (Coffin) Joint|coffin]] joints through the [[Tendons - Horse Anatomy#Extensors|common digital extensor tendon]].

===Stay Apparatus===

===Stay Apparatus===

The '''superficial digital flexor muscle''', continues past the [[Joints and Ligaments - Horse Anatomy#Carpal Joint|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 [[Joints and Ligaments - Horse Anatomy#Metacarpophalangeal (Fetlock) Joint|fetlock]] into slips, that attach to the medial and lateral aspect of the [[Joints and Ligaments - Horse Anatomy#Proximal Interphalangeal (Pastern) Joint|pastern]] joint. There is therefore a ligamentous connection from the distal [[Bones and Cartilages - Horse Anatomy#Radius and Ulna|radius]] to the [[Joints and Ligaments - Horse Anatomy#Proximal Interphalangeal (Pastern) Joint|pastern]] that can help prevent extension of the [[Joints and Ligaments - Horse Anatomy#Carpal Joint|carpal]] and [[Joints and Ligaments - Horse Anatomy#Metacarpophalangeal (Fetlock) Joint|fetlock]] joint without muscle contractions.

The '''superficial digital flexor muscle''', continues past the [[Joints and Ligaments - Horse Anatomy#Carpal Joint|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 [[Joints and Ligaments - Horse Anatomy#Metacarpophalangeal (Fetlock) Joint|fetlock]] into slips, that attach to the medial and lateral aspect of the [[Joints and Ligaments - Horse Anatomy#Proximal Interphalangeal (Pastern) Joint|pastern]] joint. There is therefore a ligamentous connection from the distal [[Bones and Cartilages - Horse Anatomy#Radius and Ulna|radius]] to the [[Joints and Ligaments - Horse Anatomy#Proximal Interphalangeal (Pastern) Joint|pastern]] that can help prevent extension of the [[Joints and Ligaments - Horse Anatomy#Carpal Joint|carpal]] and [[Joints and Ligaments - Horse Anatomy#Metacarpophalangeal (Fetlock) Joint|fetlock]] joint without muscle contractions.

The '''deep digital flexor''' also forms a thick tendon just proximal to the [[Joints and Ligaments - Horse Anatomy#Carpal Joint|carpus]]. The accessory ligament or '''inferior check ligament''', joins the tendon at the middle of the [[Bones and Cartilages - Horse Anatomy#Metacarpals and Metatarsals|metacarpus]]. They then continue through the bifurcation of the superficial digital flexor tendon, through the intersesamoidean groove at the [[Joints and Ligaments - Horse Anatomy#Metacarpophalangeal (Fetlock) Joint|fetlock]] joint, over the [[Joints and Ligaments - Horse Anatomy#Proximal Interphalangeal (Pastern) Joint|pastern]] and insert on the [[Phalanges - Horse Anatomy#Distal Phalanx|distal phalanx]]. This ligamentous tissue between the [[Bones and Cartilages - Horse Anatomy#Metacarpals and Metatarsals|metacarpal]]  region and [[Phalanges - Horse Anatomy#Distal Phalanx|distal phalanx]] helps prevent extension of the [[Joints and Ligaments - Horse Anatomy#Metacarpophalangeal (Fetlock) Joint|fetlock]] , [[Joints and Ligaments - Horse Anatomy#Proximal Interphalangeal (Pastern) Joint|pastern]]  and [[Joints and Ligaments - Horse Anatomy#Distal Interphalangeal (Coffin) Joint|coffin]] joints.

The '''deep digital flexor''' also forms a thick [[Tendons - Horse Anatomy#Flexors|tendon]] just proximal to the [[Joints and Ligaments - Horse Anatomy#Carpal Joint|carpus]]. The accessory ligament or '''inferior check ligament''', joins the tendon at the middle of the [[Bones and Cartilages - Horse Anatomy#Metacarpals and Metatarsals|metacarpus]]. They then continue through the bifurcation of the [[Tendons - Horse Anatomy#Flexors|superficial digital flexor tendon]] , through the intersesamoidean groove at the [[Joints and Ligaments - Horse Anatomy#Metacarpophalangeal (Fetlock) Joint|fetlock]] joint, over the [[Joints and Ligaments - Horse Anatomy#Proximal Interphalangeal (Pastern) Joint|pastern]] and insert on the [[Phalanges - Horse Anatomy#Distal Phalanx|distal phalanx]]. This ligamentous tissue between the [[Bones and Cartilages - Horse Anatomy#Metacarpals and Metatarsals|metacarpal]]  region and [[Phalanges - Horse Anatomy#Distal Phalanx|distal phalanx]] helps prevent extension of the [[Joints and Ligaments - Horse Anatomy#Metacarpophalangeal (Fetlock) Joint|fetlock]] , [[Joints and Ligaments - Horse Anatomy#Proximal Interphalangeal (Pastern) Joint|pastern]]  and [[Joints and Ligaments - Horse Anatomy#Distal Interphalangeal (Coffin) Joint|coffin]] joints.

==Pelvic Limb==

==Pelvic Limb==

The equine hindlimb skeleton supports a massive amount of weight from the main body of the horse. This weight is centered on the femoral head and then continues in a vertical line down to the ground by intersecting the hoof. This means that the stifle joint lies cranially of this line and the hock, fetlock and pastern lie caudal to the line. This would result in a collapse of the leg by flexion of the stifle and hock, and overextension of the fetlock and pastern, if there wasn't some supporting mechanism. The stay apparatus is a collection of tendons and ligaments that provide this support, whilst using minimal muscular effort. The stay apparatus has three main components; patellar locking, the reciprocal mechanism and the check apparatus.

The equine hindlimb skeleton supports a massive amount of weight from the main body of the horse. This weight is centered on the [[Bones and Cartilages - Horse Anatomy#Femur|femoral head]] and then continues in a vertical line down to the ground by intersecting the [[Hoof - Horse Anatomy|hoof]]. This means that the [[Joints and Ligaments - Horse Anatomy#Stifle Joint|stifle]] joint lies cranially of this line and the [[Joints and Ligaments - Horse Anatomy#Tarsal Joint|hock]], fetlock and pastern lie caudal to the line. This would result in a collapse of the leg by flexion of the [[Joints and Ligaments - Horse Anatomy#Stifle Joint|stifle]]  and [[Joints and Ligaments - Horse Anatomy#Tarsal Joint|hock]], and overextension of the [[Joints and Ligaments - Horse Anatomy#Metacarpophalangeal (Fetlock) Joint|fetlock]] and [[Joints and Ligaments - Horse Anatomy#Proximal Interphalangeal (Pastern) Joint|pastern]], if there wasn't some supporting mechanism. The stay apparatus is a collection of tendons and ligaments that provide this support, whilst using minimal muscular effort. The stay apparatus has three main components; patellar locking, the reciprocal mechanism and the check apparatus.

====Patellar Locking====

====Patellar Locking====

This is not strictly part of the stay apparatus as there is no increase of collagenous tissue of muscle, but it is so important in reducing muscle fatigue by immobilising the stifle joint it is considered here. It is the specific anatomy of the equine stifle that enables the locking to occur and includes a number of anatomical structures. The trochlea groove is wide for the gliding movement of the patella and the medial trochlear ridge is the larger of the two ridges. It is wide and round proximally and surmounted by the '''tubercle of the trochlea'''. The articular aspect of the patella has a vertical ridge which sits in the trochlear groove and is flanked by two concave areas which relate to the trochlea ridges. The medial concavity is continued by the '''patellar cartilage'''. This is a strong curved plate of fibrocartilage that is adapted to the medial ridge. The horse has three patellar ligaments; the '''lateral''' and '''middle patellar ligaments''' bind the patella to the tibia, whilst the '''medial patellar ligament''' attaches the patellar cartilage to the tibia. During normal flexion and extension of the stifle the patella will glide in the trochlear groove. When the horse rests its weight on the hindlimb the stifle can extend beyond a certain point. This causes the patella to move to the proximal extremity of the trochlear groove, a medial twist then results in the medial ridge protruding between the middle and medial patellar ligaments with the patellar cartilage hooking over the trochlea tubercle. Thus the patella is locked and further flexion is not possible. This results in an immobile stifle joint and so no muscle activity is needed to maintain the leg in this position, thus allowing the horse to rest a lot of its weight on this leg. To release the patella, the horse shifts its weight to the other limb and the quadriceps contract drawing the patella proximally. It then twists laterally and is returned to the trochlear groove.

This is not strictly part of the stay apparatus as there is no increase of collagenous tissue of muscle, but it is so important in reducing muscle fatigue by immobilising the [[Joints and Ligaments - Horse Anatomy#Stifle Joint|stifle]]  joint; it is considered here. It is the specific anatomy of the equine [[Joints and Ligaments - Horse Anatomy#Stifle Joint|stifle]] that enables the locking to occur and includes a number of anatomical structures. The trochlea groove is wide for the gliding movement of the patella and the medial trochlear ridge is the larger of the two ridges. It is wide and round proximally and surmounted by the '''tubercle of the trochlea'''. The articular aspect of the patella has a vertical ridge which sits in the trochlear groove and is flanked by two concave areas which relate to the trochlea ridges. The medial concavity is continued by the '''patellar cartilage'''. This is a strong curved plate of fibrocartilage that is adapted to the medial ridge. The horse has three patellar ligaments; the '''lateral''' and '''middle patellar ligaments''' bind the patella to the [[Bones and Cartilages - Horse Anatomy#Tibia and Fibula|tibia]], whilst the '''medial patellar ligament''' attaches the patellar cartilage to the [[Bones and Cartilages - Horse Anatomy#Tibia and Fibula|tibia]]. During normal flexion and extension of the [[Joints and Ligaments - Horse Anatomy#Stifle Joint|stifle]] the patella will glide in the trochlear groove. When the horse rests its weight on the hindlimb the [[Joints and Ligaments - Horse Anatomy#Stifle Joint|stifle]] can extend beyond a certain point. This causes the patella to move to the proximal extremity of the trochlear groove, a medial twist then results in the medial ridge protruding between the middle and medial patellar ligaments with the patellar cartilage hooking over the trochlea tubercle. Thus the patella is locked and further flexion is not possible. This results in an immobile [[Joints and Ligaments - Horse Anatomy#Stifle Joint|stifle]] joint and so no muscle activity is needed to maintain the leg in this position; thus allowing the horse to rest a lot of its weight on this leg. To release the patella, the horse shifts its weight to the other limb and the ''quadriceps'' contract drawing the patella proximally. It then twists laterally and is returned to the trochlear groove.

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====Reciprocal Mechanism====

====Reciprocal Mechanism====

The reciprocal mechanism is a modification made of largely tendinous muscles on either side of the tibia. On the cranial aspect is the '''peroneus tertius'''. It's origin is between the trochlea and lateral condyle. It bifurcates distally, both parts inserting in the tarso-metotarsal region. This fibrous muscle ensures that flexion of the stifle is accompanied by flexion of the hock. On the caudal aspect are the '''superficial digital flexor''' and the associated fibrous band of the '''gastrocnemius.''' The band of the gastrocnemius orginates on the lateral supracondyloid tuberosity of the femur and inserts on the calcaneus. The superficial digital flexor originates as a strong tendon from the caudal aspect of the femur between the two heads of the gastrocnemius. It runs distally covering the gastrocnemius tendon and partially attaches to the point of the hock. It continues distally and inserts as in the forelimb. These two muscles ensure that when the stifle is in extension the hock is also in extension. These structures thus receive their name from the reciprocating action of when the femur moves cranially the distal limb swings caudally and visa versa. This apparatus also ensures that when the patella is locked the hock is also immobilised. These results are obtained with no muscle fatigue.

The reciprocal mechanism is a modification made of largely tendinous muscles on either side of the [[Bones and Cartilages - Horse Anatomy#Tibia and Fibula|tibia]]. On the cranial aspect is the '''peroneus tertius'''. It's origin is between the trochlea and lateral condyle. It bifurcates distally, both parts inserting in the tarso-metotarsal region. This fibrous muscle ensures that flexion of the [[Joints and Ligaments - Horse Anatomy#Stifle Joint|stifle]] is accompanied by flexion of the [[Joints and Ligaments - Horse Anatomy#Tarsal Joint|hock]]. On the caudal aspect are the '''superficial digital flexor''' and the associated fibrous band of the '''gastrocnemius.''' The band of the gastrocnemius orginates on the lateral supracondyloid tuberosity of the [[Bones and Cartilages - Horse Anatomy#Femur|femur]] and inserts on the [[Bones and Cartilages - Horse Anatomy#Tarsal Bones|calcaneus]]. The superficial digital flexor originates as a strong [[Tendons - Horse Anatomy#Flexors|tendon]]  from the caudal aspect of the [[Bones and Cartilages - Horse Anatomy#Femur|femur]]  between the two heads of the gastrocnemius. It runs distally covering the gastrocnemius tendon and partially attaches to the point of the [[Joints and Ligaments - Horse Anatomy#Tarsal Joint|hock]]. It continues distally and inserts as in the forelimb. These two muscles ensure that when the [[Joints and Ligaments - Horse Anatomy#Stifle Joint|stifle]] is in extension the [[Joints and Ligaments - Horse Anatomy#Tarsal Joint|hock]] is also in extension. These structures thus receive their name from the reciprocating action of when the [[Bones and Cartilages - Horse Anatomy#Femur|femur]]  moves cranially the distal limb swings caudally and visa versa. This apparatus also ensures that when the patella is locked the [[Joints and Ligaments - Horse Anatomy#Tarsal Joint|hock]] is also immobilised. These results are obtained with no muscle fatigue.

====Check Apparatus====

====Check Apparatus====

The check apparatus of the accessory ligament of the deep digital flexor tendon is very similar to that of the forelimb. The superficial digital flexor muscle is entirely tendinous and so there is little/no muscle to be fatigued, thus a 'check' ligament would be of no advantage and so there is none in the hindlimb.

The check apparatus of the accessory ligament of the [[Tendons - Horse Anatomy#Flexors|deep digital flexor tendon]] is very similar to that of the forelimb. The superficial digital flexor muscle is entirely tendinous and so there is little/no muscle to be fatigued, thus a 'check' ligament would be of no advantage and so there is none in the hindlimb.

[[Image:Horse Stay Apparatus.jpg|200px|thumb|right|'''Stay apparatus of the thoracic and pelvic limbs''']]

[[Image:Horse Stay Apparatus.jpg|200px|thumb|right|'''Stay apparatus of the thoracic and pelvic limbs''']]