The portion of the lower limb above the knee. The thigh is supported by the femur or thigh-bone, the longest and strongest bone in the body. A large four-headed muscle, the quadriceps, forms most of the fleshy mass on the front and sides of the thigh and serves to straighten the leg in walking, as well as to maintain the erect posture of the body in standing. At the back of the thigh lie the hamstring muscles; on the inner side the adductor muscles, attached above to the pelvis and below to the femur, pull the lower limb inwards. The large femoral vessels emerge from the abdomen in the middle of the groin, the vein lying to the inner side of the artery. These pass downwards and inwards deeply placed between the muscles, and at the knee they lie behind the joint. The great saphenous vein lies near the surface and can be seen towards the inner side of the thigh passing up to the groin, where it joins the femoral vein. The femoral nerve accompanies the large vessels and controls the muscles on the front and inner side of the thigh; while the large sciatic nerve lies close to the back of the femur and supplies muscles at the back of the thigh and below the knee.

Deep wounds on the inner side of the thigh are dangerous by reason of the risk of damage to the large vessels. Pain in the back of the thigh is often due to inflammation of the sciatic nerve (see SCIATICA). The veins on the inner side of the thigh are specially liable to become dilated.