Overview
In the horse the peroneus tertius (m. fibularis tertius) has no muscle fibres: it is a strong tendinous cord. It arises from the extensor fossa of the femur together with the long digital extensor and runs down the cranial surface of the tibia, between the long digital extensor, which covers it, and the tibialis cranialis, which lies beneath.
On the dorsal hock it forms a slit through which the tibialis cranialis tendon passes, and it splits into two branches:
- Dorsal branch: to the proximal end of the third metatarsal bone and the third tarsal bone.
- Lateral branch: to the calcaneus and the fourth tarsal bone.
The model does not show the long digital extensor.
Origin
Extensor fossa of the femur (fossa extensoria), between the lateral condyle and the lateral trochlear ridge.
Insertion
Proximal end of the third metatarsal bone, third and fourth tarsal bones and calcaneus.
Action
It does not contract: it passively transmits stifle flexion to the hock.
Innervation
With no muscle fibres in the horse, it has no motor innervation of its own. In other species it is supplied by the deep peroneal (fibular) nerve (n. fibularis profundus).
Clinical relevance
- It is the cranial component of the reciprocal apparatus: with the superficial digital flexor, which lies caudally, it makes stifle and hock flex and extend together.
- Rupture follows forced hyperextension of the hock, for example when the limb is trapped or after a fall. With the stifle flexed the hock can be extended, the common calcanean tendon goes slack and dimples, and the lower limb dangles at the walk. Diagnosis is clinical and ultrasonographic; most horses recover with box rest.