Muscles of the Crus - Horse Anatomy
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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 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 (hock)
- Its tendinous insertion has a synovial sheath, and a bursa is present between its medial branch and the medial ligament of the tarsus (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 distal phalanx, and the dorsal surfaces of the proximal extremities of the proximal and middle phalanges
- Action: Extends the digits, assists in flexing the tarsus (hock)
- 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 thethird 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 as part of the common calcaneal tendon
- Action: Extends the tarsus, assists in flexing the stifle
- 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: Head of the fibula
- Insertion: Joins the lateral head of the gastrocnemius to become part of the gastrocnemius tendon
- Action: Mainly to assist the gastrocnemius
Flexors of the Digits
Superficial digital flexor
- Origin: Supracondylar fossa on the caudal femur, attached to the lateral head of the gastrocnemius
- Insertion: Calcaneus, the eminences on the sides of the proximal extremity of the middle phalanx, and the distal extremity of the proximal phalanx behind the collateral ligament of the proximal interphalangeal (pastern) joint .
- Action: Extends the digit, assists in extending the tarsus and flexing the stifle.
- Almost entirely tendinous, major component of the common cunean tendon which forms the caudal part of the reciprocal apparatus.
- Widens at the point of the hock to form a cap over the calcaneal tuberosity. The calcaneal bursa lies between the calcaneal tuberosity and the superficial digital flexor tendon.
Deep digital flexor
- Origin: The deep digital flexor muscle has three heads, originating from the lateral condyle of the fibula, the caudal and medial aspect of the tibia.
- Insertion: Semilunar crest and adjacent surface of the cartilage of the distal phalanx, continues as the deep digital flexor tendon to insert on the distal phalanx
- Action: Flexes the digit
- The tendon has a synovial sheath from the distal fourth of the tibia, to a third of the way down the metatarsus.
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