Species adaptation: the horse

Question 1

What is the stay apparatus composed of?

(hindlimb)

Answer 1

• Stifle locking mechanism
• Reciprocal apparatus
• Suspensory apparatus

Question 2

Describe how the patellar locking mechanism works

Answer 2

• Both locking and unlocking are active processes
• To lock, the horse must contract its quadriceps muscle to move the patellar (up and medially) over the medial trochlear ridge to lock

Question 3

Describe the components of the reciprocal apparatus

Answer 3

Two tendinous cords cause united movement of the stifle and hock.

• Peroneus tertius
• Superficial digital flexor

Question 4

What is the function of the reciprocal apparatus?

Answer 4

• The two tendinous cords unite movement of the stifle and hock.
• This allows the powerful extensor muscle sited proximally to act upon the distal limb.
• Assuming the reciprocal apparatus works, the stifle and hock can’t work independently.
• This means the muscles can act on only 1 area and still have a knock-on effect.
• Without this, lots of energy would be required to sustain both the stifle and the hock at acute joint angles.

Question 5

Describe the component(s) and function of the suspensory apparatus

Answer 5

• The suspensory ligament (a.k.a. interosseous) originates in the proximal metacarpal/tarsal region
• It runs to the proximal sesamoid bones and then continues in a series of distal seasmoidean ligaments
• It maintains the fetlock in a position of mild hyperextension

Question 6

True/False: there is stay apparatus in the forelimb

Answer 6

True - the majority is in the hindlimb but there is some in the forelimb

Question 7

Describe the compostion and function of the stay apparatus in the forelimb

Answer 7

• Serratus ventralis forms a suspensory sling
  
  • However this would result in shoulder flexion if not counter-balanced
• Shoulder is maintained in extension by supraspinatus muscle, and co-operative antagonism of the biceps, with reciprocal contraction of the triceps
• Both the superficial and deep digital flexor muscles have accessory ligaments
  
  • These prevents extension of joints distal to the fetlock

Question 8

Describe the respiratory adaptations the horse has for locomotion

Answer 8

• Respiratory-locomotor coupling = breathing pattern matches stride rate
• When the foot hits the ground,

Question 9

How is speed best optimised?

Answer 9

Strategies to optimise speed are to increase stride length and increase stride rate. However, the two are antagonistic so there must be a compromise.

Question 10

How is stride length increased in the horse?

Answer 10

• Easily achieved by increasing limb length
  
  • However must balance this with the inertia that it will generate, which will limit stride rate
• Also achieved by some flexion and extension of the back (though carnivores do this more than horses)
• The scapula is not fixed to rest of skeleton; this freedom to move increases stride length
• Elongation of the distal bones, especially in the metcarpus/tarsus and digits

Question 11

How is stride rate increased in the horse?

Answer 11

*

Question 12

Safety factors

Answer 12

the ratio between the degree of deformation engendered by the application of a force which results in a fracture to that habitually experienced during normal locomotion.