Muscles of the Crus - Horse Anatomy

Introduction

These muscles can be divided into two groups. On the craniolateral aspect of the limb are the flexors of the tarsus and extensors of the digit. On the caudal aspect are the extensors of the tarsus and flexors of the digit.

Craniolateral Muscles of the Crus

These muscles arise from the distal femur or proximal tibia or fibula. Their tendinous insertions are often multiarticular and insert on the metatarsals or phalanges. They are innervated by the peroneal nerve.

Flexors of the tarsus

Cranial Tibial (Tibialis Cranialis)

Origin: Lateral condyle and tuberosity of the tibia

Insertion: Bifurcates into a medial and lateral branch. The lateral branch inserts on the metatarsal tuberosity. The medial branch inserts on the combined first and second tarsal bones. The medial branch is also known as the cunean tendon.

Action: Flexes the tarsus

Its tendinous insertion has a synovial sheath, and a bursa is present between its medial branch and the medial ligament of the hock.

In the horse, its fibres join with the third fibular muscle proximal to the tarsus.

Peroneus tertius

Origin: Extensor fossa of the femur

Insertion: Proximal extremity of the third metatarsal and third tarsal bone. Also the calcaneus and fourth tarsal bones.

Action: An almost exclusively tendinous muscle that plays an important part in the stay apparatus.

Extensors of the Digits

Long digital extensor

Origin: Extensor fossa of the femur, shared tendon of origin with the third fibular muscle

Insertion: Extensor process of the third phalanx, and the dorsal surfaces of the proximal extremities of the first and second phalanges

Action: Extends the digits, assists in flexing the tarsus

Its tendon of insertion passes over the front of the hock, and is held in place by three annular ligaments (proximal, middle and distal) and a synovial sheath. The tendon of the lateral digital extensor joins the tendon in the approximately the middle of the third metatarsal (cannon) bone. It is also joined by two branches of the suspensory ligament before its insertion.

Lateral digital extensor

Origin: Lateral patella ligament, proximal fibula, lateral border of the tibia and the interosseous ligament. It lies between the long digital extensor muscle and the long flexor muscle of the first digit.

Insertion: Long digital extensor tendon

Action: It appears to have no function, apart from maybe to support the long digital extensor.

Caudal Muscles of the Crus

These are innervated by the tibial nerve.

Extensors of the Tarsus

Gastrocnemius

Origin - lateral head from the lateral supracondyloid crest of the femur. The medial head from the medial supracondyloid crest.

Insertion - Calcaneus.

The two bellies combine in the middle of the crus, which twists around to the dorsal aspect of the superficial digital flexor. A large bursa lies between these two tendons and a small bursa lies between the gastrocnemius and the its point of insertion on the calcaneus.

Soleus

Origin - the head of the fibula.

Insertion - the gastrocnemius tendon.

Its function is mainly to assist the gastrocnemius.

Flexors of the Digits

Superficial digital flexor

Origin - the supracondyloid fossa of the femur.

Insertion - the calcaneus, the eminences on the sides of the proximal extremity of the second phalanx, and the distal extremity of the first phalanx behind the collateral ligament of the pastern joint.

A large synovial bursa lies under the tendon from the distal fourth of the tibia to the middle of the tarsus. A subcutaneous bursa is sometimes present at the point of the hock.

Deep digital flexor

Origin - the lateral condyle of the tibia, the caudal and medial aspect of the tibia.

Insertion - the semilunar crest and adjacent surface of the cartilage of the third phalanx.

The tendon has a synovial sheath from the distal fourth of the tibia, to a third of the way down the metatarsus.

Muscles of the Crus - Horse Anatomy Learning Resources
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