The phalanges, or finger bones, are the fourteen small bones arranged in three rows to form the fingers. Each finger has three bones; each thumb, two.
The large number of bones in the hand not only affords every variety of movement, but offers great resistance to blows or shocks. These bones are united by strong but flexible ligaments. The hand is thus given strength and flexibility, and enabled to accomplish the countless movements so necessary to our well-being.
In brief, the hand is a marvel of precise and adapted mechanism, capable not only of performing every variety of work and of expressing many emotions of the mind, but of executing its orders with inconceivable rapidity.
The Bones of the Lower Limbs.
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.
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.
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.
One of the ankle bones, the os calcis, projects prominently backwards, forming the heel. An extensive surface is thus afforded for the attachment of the strong tendon of the calf of the leg, called the tendon of Achilles. The large bone above the heel bone, the astragalus, articulates with the tibia, forming a hinge joint, and receives the weight of the body.
The metatarsal bones, corresponding to the metacarpals of the hand, are five in number, and form the lower instep.
The phalanges are the fourteen bones of the toes,--three in each except the great toe, which, like the thumb, has two. They resemble in number and plan the corresponding bones in the hand. The bones of the foot form a double arch,--an arch from before backwards, and an arch from side to side. The former is supported behind by the os calcis, and in front by the ends of the metatarsal bones. The weight of the body falls perpendicularly on the astragalus, which is the key-bone or crown of the arch. The bones of the foot are kept in place by powerful ligaments, combining great strength with elasticity.
Fig. 24.--Right Tibia and Fibula (Anterior surface.)
Fig. 25.--Bones of Right Foot. (Dorsal surface.)
The Joints.
50. Formation of Joints. The various bones of the skeleton are connected together at different parts of their surfaces by joints, or articulations. Many different kinds of joints have been described, but the same general plan obtains for nearly all. They vary according to the kind and the amount of motion.
The principal structures which unite in the formation of a joint are: bone, cartilage, synovial membrane, and ligaments. Bones make the chief element of all the joints, and their adjoining surfaces are shaped to meet the special demands of each joint (Fig. 27). The joint-end of bones is coated with a thin layer of tough, elastic cartilage. This is also used at the edge of joint-cavities, forming a ring to deepen them. The rounded heads of bones which move in them are thus more securely held in their sockets.
Besides these structures, the muscles also help to maintain the joint-surfaces in proper relation. Another essential to the action of the joints is the pressure of the outside air. This may be sufficient to keep the articular surfaces in contact even after all the muscles are removed. Thus the hip joint is so completely surrounded by ligaments as to be air-tight; and the union is very strong. But if the ligaments be pierced and air allowed to enter the joint, the union at once becomes much less close, and the head of the thigh bone falls away as far as the ligaments will allow it.
51. Synovial Membrane. A very delicate connective tissue, called the synovial membrane, lines the capsules of the joints, and covers the ligaments connected with them. It secretes the synovia, or joint oil, a thick and glairy fluid, like the white of a raw egg, which thoroughly lubricates the inner surfaces of the joints. Thus the friction and heat developed by movement are reduced, and every part of a joint is enabled to act smoothly.
52. Ligaments. The bones are fastened together, held in place, and their movements controlled, to a certain extent, by bands of various forms, called ligaments. These are composed mainly of bundles of white fibrous tissue placed parallel to, or closely interlaced with, one another, and present a shining, silvery aspect. They extend from one of the articulating bones to another, strongly supporting the joint, which they sometimes completely envelope with a kind of cap (Fig. 28). This prevents the bones from being easily dislocated. It is difficult, for instance, to separate