FIG. 77.--Superficial muscles of front of thigh: 1, Insertion of external oblique into iliac crest; 2, aponeurosis of external oblique; 3, external abdominal ring; 4, gluteus medius; 5, tensor vaginæ formoris; 6, sartorius; 7, iliopsoas; 8, pectineus; 9, adductor longus; 10, gracilis; 11, adductor magnus; 12, vastus externus; 13, rectus femoris; 14, vastus internus; 15, biceps flexor cruris. (Dorland’s Dictionary.)] Lower down and forming the back of the thigh are the biceps and the semitendinosus and semimembranosus muscles. The _biceps_ rises by two heads from the tuberosity of the ischium and the linea aspera and is inserted into the head of the fibula. It is on the outer side of the thigh and its tendon, which embraces the external lateral ligament of the knee-joint, forms the _outer hamstring_. On the inner side are the _semitendinosus_ and the _semimembranosus muscles_. These rise from the tuberosity of the ischium and are inserted, the one into the upper inner surface of the shaft of the tibia and the other into the internal tuberosity of the tibia. Their tendons form the _inner hamstring_. Like the biceps they serve to extend the thigh and flex the leg on the thigh, but where the biceps rotates the leg out they, being attached to the inner side of the leg bones, rotate it in. =The patella=, or _small pan_, is a flat, somewhat triangular bone developed in the quadriceps extensor tendon. Four _muscles_ are attached to it as well as the ligamentum patellæ, which holds it to the tibia and gives increased leverage by making the quadriceps extensor work at a greater angle. It articulates with the condyles and serves to protect the joint. One bursa, the _prepatella bursa_, separates it from the skin and another, surrounded by adipose tissue, from the head of the tibia. The external surface can be seen and felt on the front of the knee and the bone can be moved from side to side when the leg is straight. =Joints of the Lower Extremity.=--The _hip-joint_ is a ball-and-socket joint but is not so freely movable as the shoulder-joint, the head of the femur being held in the acetabulum by many strong ligaments, of which the most important is the _capsular_ ligament. The _knee-joint_ is largely a hinge joint, but in some positions it has some rotation. It is formed by the condyles of the femur, the head of the tibia, and the patella, and has fourteen ligaments, including the _ligamentum patellæ_ and the _crucial ligaments_. Its synovial sac is the largest found in any joint. Two _semilunar cartilages_, placed on the head of the tibia, serve to deepen the socket for the condyles, changing somewhat in shape and thickness as the joint moves. The interval between the thigh and the leg bones can be felt at the knee. When the leg is extended the juncture of the bones is slightly above the patella, while in flexion a knife passed below the apex of the patella will pass into the joint.