Fibularis tertius muscle

The Fibularis tertius muscle is a muscle of the pelvic limb located craniolaterally to the tibia. It is not present in carnivores or rabbits, is inconsistent in humans, but is well-developed in ungulates.

Conformation: The conformation of this muscle varies greatly depending on the species. It consists of a fleshy body and an often double tendon, but depending on the species, it may be almost entirely fibrous.

Origin: Extensor fossa of the femur.

Insertion: Mainly on the tuberosity of the third metatarsal, also on the tarsal bones, variable depending on the species.

Relations: Covers the Extensor digitorum longus muscle in ruminants and pigs, directly under the crural fascia. In equines, it is located between the extensor digitalis longus muscle and the tibialis cranial muscle. Its tendon passes through the retinaculum of crural extensors. Its tendon is then perforated by other tendons which may vary depending on the species.

Action: Flexion of the tarsus.

Vascularization: Multiple branches of the cranial tibial vessels.

Innervation: Deep fibular nerve.

Description: The Fibularis tertius muscle is located in the carniolateral part of the crus. In ruminants and pigs, it consists of a large fleshy body that extends into a distal tendon. It is covered by the crural fascia and surrounds the extensor digitorum longus muscle. In equines, its placed more deeply, under the extensor digitlis longus muscle and covers the tibialis cranial muscle. Its origin is located on the extensor fossa of the femur, in common with the origin of the extensor digitorum longus muscle. Its tendon passes through the retinaculum of crural extensors and is then distally perforated and divided, creating a ring through which the tendon of the tibialis cranial muscle passes. The two endings will then be inserted on the tuberosity of the third metatarsal, for the main one and on the tarsal bones for the oblique one.

Variations:

• On equines, this muscle is mostly fibrous. It lies between the extensor digitalis longus muscle and the tibialis cranial muscle with which it shares a particular relationship. This two muscle are attached to each other on a large portion of the fleshy body of the tibialis cranialis. Their tendons follow the same path distally, then the tendon of the tibialis cranial muscle passes through the ring created by the tendon of the fibularis tertius muscle. Both separated in two, they will insert partially in common on the tuberosity of the third metatarsal. It completes the flexing action of the tarsus of the cranial tibial bone and thanks to its fibrous nature is virtually inextensible, which creates a relationship between the angles of the tarsocrural joint and the femorotibial joint.