Extensor digitorum [digitalis] longus muscle

The extensor digitorum [digitalis] longus muscle is a muscle of the pelvic limb located craniolaterally to the tibia.

Conformation: In domestic mammals, this muscle consists of a fleshy body attached to the femur by a proximal tendon that passes through the extensor groove of the tibia and extends distally to the digits by one or more long tendons. In carnivores, rabbits, and humans, the fleshy body is simple, and the tendon divides near the distal end of the tibia. In ungulates, the fleshy body may itself be more or less divided into multiple parts connected to different digits.

Origin: Extensor fossa of the femur

Insertion: On the extensor process of the distal phalanx of each digit.

Relations: This muscle is covered by the crural fascia and, depending on the species, by the tibialis cranial muscle or by the fibularis tertius muscle medially. Laterally, it is separated from the peroneal muscles by a fibrous septum. It covers the cranial surface of the tibia or the tibialis cranial muscle. Its distal tendon(s) are covered by the dorsal fascia of the hindfoot, whose reinforcements form a series of retinacula, changing according to the species, which are placed between the distal part of the tibia and the proximal end of the metatarsal.

Action: Extension of the digits; flexion of the tarsus.

Vascularization: Multiple branches of the cranial tibial vessels.

Innervation: Deep fibular nerve.

Description: The extensor digitorum [digitalis] longus muscle is located in the craniolateral part of the crus. This muscle consists of a fleshy body, a proximal tendon attaching it to the femur (except in humans), and one or more tendons that insert to the digits. This muscle is positioned craniolaterally to the tibia. Its proximal tendon is attached to the extensor fossa of the femur and passes into the extensor groove of the tibia, proximal to the fleshy body. In carnivores, rabbits, and humans, the fleshy body is simple and gives rise to a single tendon which then separates into several branches destined for each digit. In these species, it directly covers the tibia and is covered medially by the tibialis cranial muscle. In ungulates, the arrangement is similar, but the tendon as a whole (in pigs) or the fleshy body itself (in ruminants) may be separated into several parts. In these species, it covers the tibialis cranial muscle and is itself covered by the fibularis tertius muscle. Equids are an exception among the ungulates, as they have only one digit, the muscle remains simple and its tendon single. In equids, this muscle is the most superficial, lying between the crural fascia and the fibularis tertius muscle.

Variations:

• In equines, this muscle is superficial, its inner surface is in contact with the tibialis cranial muscle and the fibularis tertius muscle. It is directly covered by the crural fascia. Its fleshy body is voluminous and simple, giving rise to a single long tendon that extends to the extensor process of the distal phalanx after receiving the tendon of the extensor digitorum [digitalis] lateralis muscle. On its way, this long tendon passes through the crural retinaculum, the tarsal retinaculum, and metatarsal retinaculum of the extensors.