205. The knee is a hinge-joint, admitting only of flexion and extension, and is therefore provided only with two sets of muscles, one for bending and the other for extending the leg. The flexors of the leg arise from the under and back part of the pelvis, are seated on the back part of the thigh, and are inserted into the upper part either of the tibia or of the fibula (fig. CI). They consist for the most part of three muscles, named the semi-tendinosus, the semi-membranosus (fig. CI. 3), and the biceps of the leg (fig. CI. 1). The tendons of the two former muscles, in passing to be inserted into the leg, form the inner, and that of the latter the outer, hamstrings (fig. CI. 4, 5). [Illustration: Fig. CII. View of the flexor and extensor muscles of the leg. 1. The biceps of the leg; 2. tendon of the biceps, inserted into the head of the fibula; 3. the semi-membranosus, passing to be inserted into the head of the fibula; 4. tendon of the semi-membranosus forming the inner, and 5. tendon of the biceps forming the outer, hamstring; 6. upper part of the gastrocnemius muscle; 7. the four large muscles which unite to form the great extensor muscle of the leg, inserted into 8. the patella; 9. a portion of the glutæus maximus concealing the other muscles of the hip.] 206. Four large muscles, blended together in such a manner as to form one muscle of prodigious size, termed the quadriceps cruris (fig. CI. 7), occupying nearly all the forepart and the sides, and a considerable portion of the back part of the thigh, constitute the great flexor of the thigh. This enormous mass of muscle arises partly from the ischium, and partly from the upper part of the femur (fig. CI. 7), and is all inserted into the patella (fig. CI. 8), which constitutes a pulley for the purpose of assisting the action of these powerful muscles. 207. The muscles which bend the toes and extend the foot, termed the gastrocnemii (fig. CII. 1, 2), are placed on the back part of the leg, and form the mass of muscle which constitutes the calf of the leg (fig. CII. 1, 2). They arise partly from the lower extremity of the femur (fig. CII.) and partly from the upper and back part of the fibula and tibia; and they form the largest and strongest tendon in the body, termed the tendo achillis (fig. CII. 3), which is implanted into the heel (fig. CII. 4). [Illustration: Fig. CII. View of the muscles which bend the toes, and which, by lifting the heel, extend the foot. 1. The muscle called gastrocnemius externus, which, uniting with 2. the gastrocnemius internus, forms 3. the tendo achillis, which is inserted into 4. the heel.] [Illustration: Fig. CIII. View of the muscles which extend the toes and bend the foot. 1. The common extensor; 2. the tendons of the same muscle inserted into the toes; 3. the anterior annular ligament of the foot.] [Illustration: Fig. CIV.