Flexor digitorum longus muscle

The Flexor digitorum longus is situated on the tibial side of the leg. At its origin it is thin and pointed, but it gradually increases in size as it descends.

Origin and insertion
It arises from the posterior surface of the body of the tibia, from immediately below the popliteal line to within 7 or 8 cm. of its lower extremity, medial to the tibial origin of the Tibialis posterior; it also arises from the fascia covering the Tibialis posterior.

The fibers end in a tendon, which runs nearly the whole length of the posterior surface of the muscle. This tendon passes behind the medial malleolus, in a groove, common to it and the Tibialis posterior, but separated from the latter by a fibrous septum, each tendon being contained in a special compartment lined by a separate mucous sheath.

It passes obliquely forward and lateralward, superficial to the deltoid ligament of the ankle-joint, into the sole of the foot, where it crosses below the tendon of the Flexor hallucis longus, and receives from it a strong tendinous slip.

It then expands and is joined by the Quadratus plantæ, and finally divides into four tendons, which are inserted into the bases of the last phalanges of the second, third, fourth, and fifth toes, each tendon passing through an opening in the corresponding tendon of the Flexor digitorum brevis opposite the base of the first phalanx.

Variations
Flexor accessorius longus digitorum, not infrequent, origin from fibula, or tibia, or the deep fascia and ending in a tendon which, after passing beneath the laciniate ligament, joins the tendon of the long flexor or the Quadratus plantæ.