=M. soleus= (Fig. 90, _o_).--A flat fusiform muscle lying beneath the plantaris. _Origin_ by muscle-fibres from the lateral surface of the head of the fibula, and by tendon-fibres from the proximal two-fifths of its ventral border. _Insertion._--The muscle ends in a slender tendon which joins the lateral border of the gastrocnemius tendon to form the tendon of Achilles (Fig. 90, _t_), which forms the sheath of the plantaris tendon. _Relations._--Outer surface with the gastrocnemius (Fig. 90, _m_) and plantaris (Fig. 90, _l_). Inner surface with the peroneus longus (Fig. 90, _q_), peroneus tertius (Fig. 90, _r_), and flexor hallucis (Fig. 91, _m_). _Action._--Assists the gastrocnemius to extend the foot. =M. triceps suræ.=--The gastrocnemius and the soleus are sometimes considered as forming a single muscle, with three heads, under the name triceps suræ. =M. popliteus= (Fig. 92, _k_).--A triangular muscle passing from the femur obliquely toward the medial side over the ventral surface of the proximal end of the tibia. _Origin_ by a strong tendon from the popliteal groove on the surface of the lateral epicondyle of the femur. In the tendon is a sesamoid bone, the popliteal bone (Fig. 61, 5, page 89), which glides over the ventral part of the lateral articular facet on the proximal end of the tibia. As the muscle passes over the joint it is closely attached to the joint capsule. From the popliteal bone, which with the tendon lies within the capsule of the knee-joint, the muscle-fibres diverge to their _Insertion_ into the proximal end of the ventral surface of the tibial shaft on the medial side of the medial oblique ridge. The area of insertion extends slightly onto the medial surface of the bone. _Relations._--Outer surface with the gastrocnemius (Fig. 92, _j_) and plantaris (Fig. 91, _l_), and with the tendon of the semitendinosus (Fig. 92, _i_). Distal border with the origins of the soleus and flexor longus digitorum. Inner surface with the capsule of the knee-joint and the tibia. _Action._--Rotates the thigh so as to turn the toes inward. * * * * * The three following deep muscles on the ventral surface of the shank are covered by a deep fascia (the deep crural fascia) which separates them from the overlying muscles. This fascia stretches from the dorsal border of the tibia about the ventral side of the shank to the medial border of the tibia. It sends a slip beneath the border of the fibula. =M. flexor longus digitorum= (Fig. 91, _m_ and _n_).--Owing to the reduction of the first digit in the cat the tendon of the flexor longus hallucis (_m_) has become united to that of the flexor longus digitorum (_n_), so that the two might be considered separate heads of a single muscle. The head corresponding to the flexor longus hallucis (_m_) is much larger than that corresponding to the flexor longus digitorum (_n_). The two heads will be described separately.