_Origin_ from nearly the whole of the dorsal surface of the shaft of the femur between the areas for the vastus medialis and vastus lateralis. The area of origin extends distad to an oblique line the middle of which is about one and one-half centimeters from the patellar surface, the lateral border of the area being longer than the medial border. _Insertion_ by muscle-fibres into the capsule of the joint. The central fibres are inserted about one centimeter proximad of the patella, while the lateral mass passes to the level of the proximal end. _Relations._--Outer surface with the rectus femoris (Fig. 92, _b_); borders with the vastus medialis (Fig. 92, _c_) and vastus lateralis (Fig. 90, _d_); inner surface with the femur. _Action._--The quadriceps extensor is an extensor of the shank. The vastus intermedius acting separately is a tensor of the capsule of the knee-joint. 3. Muscles of the Lower Leg (Crus). A. ON THE VENTRAL SIDE.--=M. gastrocnemius.=--This is the great muscular mass of the calf. It arises by two heads, the caput laterale or lateral head (Fig. 90, _m_), and the caput mediale or medial head (Fig. 91, _k_), of the gastrocnemius. =Caput laterale= (Fig. 90, _m_, _m′_, _m″_).--The origin of the lateral head is in four portions: (1) From the tendon of origin of the plantaris which comes from the middle of the lateral border of the patella. The tendon ends in a small conical mass of fibres (_m′_) which joins the ventral part of the common head. (2) By a slender flat tendon, distad of the first, from the superficial fascia of the shank (_m″_). The tendon ends in a conical mass which joins the dorsal border of the common head. (3) By a strong tendon and by muscle-fibres from the distal border of the lateral sesamoid bone (Fig. 61, 3) of the femur. These fibres (_m_) form the middle and outer part of the common head. (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.