This is placed behind the femoro-tibial articulation and extends from the distal third of the femur to the proximal extremity of the tuber calcis. This large muscle has two heads of origin.
Origin:Medial head-the medial supracondyloid crest and the lateral head– outer lip of the supracondyloid fossa.
Each head is succeeded by tendon, which unite above the middle of the leg and the common tendon is placed immediately behind the tendon of the superficial flexor. By a twist in the tendons at a lower third of the leg, the tendon of the superficial flexor lies behind the tendons of the gastrocnemius. The tendon of the superficial digital flexor and the fibrous bands attaching it to the sides of the os calcis, completely cover the tendon of the gastrocnemius. These two tendons at the distal part of the leg are together spoken of as tendo calcaneous or hamstring or tendo Achilles.
Insertion:Summit of the tuber calcis.
Action:To extend the hock.
Blood supply: Popliteal and posterior femoral artery.
Nerve supply: Tibial nerve.
Soleus
This is very thin, long muscle situated immediately under the deep fascia and between it and the deep digital flexor. It passes downwards along the lateral border of the external head of the gastrocnemius.
Origin:Lateral condyle of the tibia and proximal extremity of fibula.
Insertion:Lateral head of the gastrocnemius above its middle.
Action:To assist the gastrocnemius in the extension of the hock.
Blood supply: Popliteal artery.
Nerve supply: Tibial nerve.
Superficial digital flexor
This muscle is very closely blended between the two heads of the gastrocnemius.
Origin:The supracondyloid fossa.
The muscle is succeeded by a tendon, which is in front of the tendon of the gastrocnemius at the middle of the leg, but below, by a twist in these tendons it comes to lie behind it. It passes over the summit of the tuber calcis, detaching two slips to be attached to it, passes down the metatarsal region and behaves in the same way as in the forelimb.
Action:To extend the hock and flex the digits.
Blood supply: Popliteal artery.
Nerve supply: Tibial nerve.
DEEP LAYER
Popliteus
This triangular, thick muscle is situated on the upper part of the plantar surface of tibia.
Origin:The lower of the two depressions on the lateral condyle of the femur.
Insertion:The triangular area at the upper third of the posterior surface of tibia limited by the popliteal line.
Action:To flex the stifle and rotate the leg inwards.
Blood supply: Posterior tibial artery.
Nerve supply: Tibial nerve.
Deep digital flexor
This muscle occupies most of the posterior face of the tibia and is made up of three heads viz.,
Superficial (tibialis posterior)
Deep (flexor hallucis)
Medial (long digital flexor or flexor accessorius) heads.
Origin:Lateral border and posterior face of tibia. Lateral condyle of the tibia and the fibrous cord of the fibula.
Insertion:The tendons of all heads unite above the tarsus and the common tendon passes through tarsal sheath and then descends in the metatarsal region. The tendon of the medial head passes under the lateral ligaments of the hock and joins the common tendon of the other two heads at the upper third of the metatarsus. The tendon of the medial head has a synovial sheath at the tarsus. The tendons of superficial and deep heads have a common synovial sheath. The united tendon thus formed comports itself in the same manner as that of the forelimb.
