_Relations._--(1) Of the lateral head. Lateral (outer) surface with the biceps femoris (Fig. 68, _t_) and the superficial fascia. Medial (inner) surface with the popliteus (Fig. 92, _k_), the plantaris (Fig. 90, _l_) (to which it is closely united), the medial head of the gastrocnemius (Fig. 91, _k_), the soleus (Fig. 90, _o_), and the peroneus longus (Fig. 90, _q_). (2) Of the medial head. Medial (outer) surface with the superficial fascia and the tendons of the gracilis (Fig. 91, _b_), semimembranosus (Fig. 92, _h′_), and semitendinosus (Fig. 92, _i_). Lateral (inner) surface at the origin with the adductor femoris (Fig. 92, _g_), then with the plantaris (Fig. 91, _l_), the popliteus (Fig. 92, _k_), flexor longus digitorum (Fig. 91, _n_), and the lateral head of the gastrocnemius (Fig. 90, _m_). _Action._--Extensor of the foot. (It is to be noted that what is called =extension= in the foot is analogous to what is called =flexion= in the hand, so that the extensors of the foot are most readily to be compared with the flexors of the hand.) =M. plantaris= (Fig. 90, _l_; Fig. 91, _l_).--A fusiform muscle covered distally by the gastrocnemius, but appearing between the two heads of the gastrocnemius at their proximal ends (Fig. 90, _l_). _Origin_ by a strong tendon from the middle of the lateral border of the patella and by fleshy fibres from the ventral border of the lateral sesamoid. The patellar tendon ends in a flattened head which passes ventrad over the lateral epicondyle of the femur and then curves distad; it is closely united with the lateral head of the gastrocnemius. The muscle ends in a thick tendon which passes through a sheath formed by the tendons of the gastrocnemius and soleus, and the fascia of the shank. The tendon passes over the grooved proximal end of the calcaneus onto its ventral surface. It is held in place against the groove by two sheets of aponeurosis, which leave the tendon on either side for a distance of more than a centimeter and are attached to the lateral surfaces of the calcaneus at its proximal end. Lateral motion is thus hindered, while longitudinal motion is permitted. Between this tendon and the tendon of Achilles is a synovial bursa which aids the gliding movement of the tendon. In the sole of the foot opposite the distal end of the calcaneus the tendon broadens and ends in the flexor brevis digitorum (pedis perforatus) muscle, which might also be considered as a second part of the plantaris. In man the plantaris is inserted into the calcaneus along with the gastrocnemius. _Relations._--The muscle is covered throughout by the two heads of the gastrocnemius (Fig. 90, _m_; Fig. 91, _k_), except at the proximal end (Fig. 90, _l_), where it is in contact with the integument. Inner surface in relation with the popliteus (Fig. 92, _k_), the soleus (Fig. 90, _o_), the flexor longus digitorum (Fig. 91, _n_), and flexor longus hallucis (Fig. 91, _m_).