The caud-ilio flexorius (semitendinosus of most authors) arises from the transverse processes of the tail, and from the distal half of the postacetabular ilium, thence passing as a broad ribbon to the popliteal region, where it splits into two portions.
This portion, morphologically the original, was named the " accessory semitendinosus " with the symbol Y; the other portion descends on the hinder aspect of the leg and joins the fascia of the inner femoral head of the gastrocnemius muscle.