(4) By muscle-fibres from the middle of the outer surface of the aponeurosis covering the plantaris (Fig. 90, _l_) along rather more than the proximal half of the tibia. The lateral head thus formed is flat and fusiform in section.
=Caput mediale= (Fig. 91, _k_; Fig. 92, _j_).--The medial head resembles the lateral head closely.
_Origin_ by a strong tendon from the medial sesamoid bone of the femur (Fig. 61, 4) and by muscle-fibres from the surface of the shaft for a short distance proximad of this.
The two heads unite by their contiguous borders at the junction of the first and second thirds of the tibia. The common muscle narrows rapidly and ends in a flat tendon which joins the tendon of the soleus to form the so-called tendon of Achilles (tendo calcaneus) (Fig. 90, _t_). This has its
_Insertion_ into the proximal end of the calcaneus (Fig. 90, 4) near its ventral border. This tendon together with that of the soleus (Fig. 90, _o_) and the fascia of the shank forms a tubular sheath for the tendon of the plantaris.
_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_).