Overview
It lies directly on the cranial surface of the tibia, covered by the long digital extensor. Distally its tendon passes through that of the peroneus tertius and splits into two branches:
- Dorsal branch: to the proximal end of the third metatarsal bone.
- Medial branch (cunean tendon): crosses the medial hock to the first and second tarsal bones, over the cunean bursa.
In the horse the peroneus tertius (m. fibularis tertius) is an entirely tendinous cord, with no muscle fibres. It arises from the extensor fossa of the femur, runs cranial to the tibialis cranialis and ends on the third metatarsal and the tarsal bones.
Origin
Proximal end of the tibia (lateral condyle and region of the tuberosity).
Insertion
Proximal end of the third metatarsal bone (dorsal branch) and first and second tarsal bones (cunean tendon).
Action
Flexes the hock.
Innervation
Deep peroneal (fibular) nerve (n. fibularis profundus).
Clinical relevance
- The peroneus tertius is the cranial component of the reciprocal apparatus: it couples stifle and hock so that when one flexes, the other does too. Its rupture breaks this coupling: with the stifle flexed the hock can be extended and the common calcanean tendon goes slack and dimples.
- In bone spavin (osteoarthritis of the distal tarsal joints) the cunean bursa can become inflamed; cunean tenectomy has been used as part of treatment.
- In peroneal nerve paralysis the hock stays extended and the horse stands with the fetlock knuckled forward.