The digital fibrous sheaths are compartments through which the tendons of the extensor muscles pass to the wrist and fingers. The retinaculum gives off branches of connective tissue to the underlying wrist bones, creating a series of sheath-like compartments. As in the case of the wrist, the tissue formations in various regions of the ankle are thickened to form retinacula. Toward the front, for example, extensor retinacula connect the tibia and fibula (lower leg bones) as well as the calcaneus (Achilles tendon) and tissue of the sole of the foot. These retinacula form sheaths for tendons crossing the front of the ankle.