46. The Lower Limbs. The general structure and number of the bones of the lower limbs bear a striking similarity to those of the upper limbs. Thus the leg, like the arm, is arranged in three parts, the thigh, the lower leg, and the foot. The thigh bone corresponds to the humerus; the tibia and fibula to the ulna and radius; the ankle to the wrist; and the metatarsus and the phalanges of the foot, to the metacarpus and the phalanges of the hand. The bones of the lower limbs may be thus arranged: Thigh: Femur, or thigh bone, Lower Leg: Patella, or knee cap, Tibia, or shin bone, Fibula, or splint bone, Foot: 7 Tarsal or ankle bones, 5 Metatarsal or instep bones, 14 Phalanges, or toes bones, making 30 bones in all. Illustration: Fig. 22.—Right Femur, or Thigh Bone. 47. The Thigh. The longest and strongest of all the bones is the femur, or thigh bone. Its upper end has a rounded head which fits into the _acetabulum_, or the deep cup-like cavity of the hip bone, forming a perfect ball-and-socket joint. When covered with cartilage, the ball fits so accurately into its socket that it may be retained by atmospheric pressure alone (sec. 50). The shaft of the femur is strong, and is ridged and roughened in places for the attachment of the muscles. Its lower end is broad and irregularly shaped, having two prominences called _condyles_, separated by a groove, the whole fitted for forming a hinge joint with the bones of the lower leg and the knee-cap. 48. The Lower Leg. The lower leg, like the forearm, consists of two bones. The tibia, or shin bone, is the long three-sided bone forming the front of the leg. The sharp edge of the bone is easily felt just under the skin. It articulates with the lower end of the thigh bone, forming with it a hinge joint. The fibula, the companion bone of the tibia, is the long, slender bone on the outer side of the leg. It is firmly fixed to the tibia at each end, and is commonly spoken of as the small bone of the leg. Its lower end forms the outer projection of the ankle. In front of the knee joint, embedded in a thick, strong tendon, is an irregularly disk-shaped bone, the patella, or knee-cap. It increases the leverage of important muscles, and protects the front of the knee joint, which is, from its position, much exposed to injury. Illustration: Fig. 23.—Patella, or Knee-Cap. 49. The Foot. The bones of the foot, 26 in number, consist of the tarsal bones, the metatarsal, and the phalanges. The tarsal bones are the seven small, irregular bones which make up the ankle. These bones, like those of the wrist, are compactly arranged, and are held firmly in place by ligaments which allow a considerable amount of motion.