What was the first horse like?
- Hyracotherium also know as Eohippus (“dawn horse”)
- Early-Middle Eocene (55-45 million years ago)
- Size and Weight: About 2 feet high and 50 pounds (around the size of a springer spaniel)
- Diet: Plants
- Distinguishing Characteristics: Tiny size; four-toed front and three- toed back feet
What did the horse evolve to?
- Przewalski’s horse
- Currently the only surviving “wild” horse breed
- Size and Weight: Approx. 13.2 hh (138cm) and 400Kg
- Diet: Grasses
What have horses evolved for?
- Horses are evolved for rapid over ground locomotion.
- Heavy demands for speed and force production mean the limbs are exposed to continual strain and injury risk.
- The limbs show extreme adaptations for running.
- The forelimbs take around 60% of the bodyweight at rest and are the principal shock absorbers at fast gaits.
What are the bones of the forelimb?
- Scapula
- Humerus
- Radius and ulna
- Carpus
- 7 bones
- Metacarpal bones
- II – IV
- Phalanges
- Long pastern • Short pastern • Coffin bone
- Sesamoid bones
- Proximal sesamoids
- Navicular bone
What are the metacarpals?
• The third metacarpals and metatarsals are the
functional weight bearing bones of the lower limb.
• Splint bones:
• 2nd & 4th metacarpals/tarsals are present in the
form of ‘splint bones’.
• ‘Splints’ is a condition where there is either
damage to one of the splint bones or inflammation of the attaching ligaments.
What are the bones of the lower limb?
- Phalanges:
- P1: Proximal (long pastern)
- P2: Middle (short pastern)
- P3: Distal (coffin/pedal bone)
- Sesamoid bones:
- 2 proximal
- Distal (navicular)
What are ligaments?
- Ligaments V’s Tendon
- Connect bones
- Provide support for the joints
- Prevent over flexion or over extension
- Collagen fibers
What are suspensory apparatus?
- Part of the stay apparatus
- Suspensory ligament
- Modified muscle
- Considerable elasticity
- Proximal sesamoid bones
- Intersesamoidean ligament
- Collateral sesamoidean ligaments
- Inferior sesamoidean ligaments
What is the musculature of the thoracic limb?
• Flexors:
– Shoulder: generally caudal to joint. – Elbow: generally cranial to joint.
– Carpus: generally caudal to joint.
• Extensors:
– Shoulder: generally cranial to joint. – Elbow: generally caudal to joint.
– Carpus: generally cranial to joint.
What is the musculature of the forelimb?
- Bridging muscles are usually involved protraction and retraction of the forelimb by movement of the scapular and humerus
- Abductors and Adductors
- Synergist muscles are used for stabilisation and can have a small role in abduction and adduction.
What is the scapula?
• Attachment to trunk by muscles and ligaments
• Serratus ventralis
• Separated from ribs by underlying muscles
and fascia
• Moves freely over ribs
What is the thoracic sling?
• Suspension of thorax between forelimbs • Attachment ONLY by muscle, ligaments and fascia • Body can be raised and lowered • Alteration of centre of gravity • Abduction and adduction • Rolling of thorax in sling when turning • Simultaneous forward and sideways movement
What is the stay apparatus?
Horses can stay on their feet for along time and even ‘snooze’ while standing.
When standing quietly most of the horse’s weight is taken by the tendons, ligaments and fascia of the stay apparatus, these do not tire.
This is largely achieved through passive tension with little muscular effort.
If unsupported, the limb would collapse.
Allows locking of joints to reduce energy expenditure at rest.
What makes up the stay apparatus in the forelimb?
• Serratus thoracis • Triceps: long head • Triceps: lateral head • Triceps: medial head • Biceps brachii • Extensor carpi radialis • Lacertus fibrosus • Extensor digitorum communis • Flexor digitorum profundus • Flexor digitorum superficialis • Suspensory ligament • Superioror radial check of SDF • Inferioror deep check of DDF
What does the stay apparatus do?
- Strong internal biceps tendon resists shoulder flexion.
- Carpal extensors prevent buckling of the carpus while the bone arrangement and strong palmar ligament prevent hyper- extension.
- Suspensory apparatus prevents fetlock hyperextension (see lower limb lecture).
- DDFT and extensor tendons oppose one another to keep hoof flat.
- Sesamoidean ligaments (see lower limb lecture) act to keep the interphalangeal joints straight. Buckling forward is prevented by the flexor tendons.
What is the hindlimb?
• Hindlimbs house the main propulsive muscles.
• The hindlimbs are responsible for acceleration and
jumping in locomotion
• Articulated to skeleton at the SI joint
• Concussion transmitted through pelvis to spinal column
• Limited extension ability through stifle and hock
What is the hindlimb made up of?
- Pelvis • Ilium
- Ischium
- Pubis
- Femur
- Patella
- Tibia and fibula
- Tarsus
- 6 bones
- Metatarsal bones
- II – IV
- Phalanges
- Long pastern
- Short pastern
- Coffin bone
What is the musculature of the hindlimb?
- Flexors
- Extensors
- Abductors and adductors
- Deceleration/braking
What are the stay apparatus of the hindlimb?
Tensor fascia latae • Psoas minor • Semitendinosus • Gastrocnemius • Superficial digital flexor • Deep digital flexor • Suspensory ligament • Fibularis tertius • Long digital extensor • Patellar ligaments • Longissimus dorsi • Sacrosciatic ligament • Gluteal Group • Reciprocal Apparatus: • Fibularis tertius • Superficial digital flexor
What is the reciprocal mechanism?
• Stifle and hock move in unison in flexion and extension
• Peroneus tertius
• Tendinous strand
• Dorsally from distal femur to proximal
metatarsus
• SDFT and gastrocnemius
• Tendinous band incorporated into gastrocnemius, associated with SDFT
• Caudally from distal femur to tuber calcaneus
