How Bones Work

Appendicular Bones

While the axial bones form the vertical axis of the body, the appendicular bones are the bones that connect to this axis. Unlike axial bones, protection isn't the function of appendicular bones; they're made for action. Let's take a look:

  • Bones of the shoulder. The bones that make up your shoulder girdle serve to connect your arms to your sternum and rib cage for stability and support. You have two clavicles (collarbones) that attach on one end to the breast plate and, on the other end, support the shoulder blades, or scapulas. The shoulder blades provide points of contact and attachment for many muscles and the bone of each upper arm.

  • Bones of the arm and hand. The entire arm appendage has three basic components: the upper arm, the lower arm and the hand. The upper arm is one long bone, the humerus. The top fits neatly into the scapula, and the lower end is connected by the elbow joint to the two bones of the lower arm: the ulna (the bone on the same side as your little finger) and the radius (the bone on the side of your thumb). The radius plays a larger role in your overall mobility and function, while your ulna provides more stability. Both the ulna and the radius connect to the wrist bones in the hand. Each hand has an impressive 27 bones: eight carpal bones that make up the wrist, five metacarpal bones that extend the length of your palm, and 14 phalanges that form four fingers with three bones each along with a single two-boned thumb.

  • The pelvic girdle. When you sit down, all the weight of your upper body rests ultimately on your pelvic girdle. This tough pair of hip bones protects lower organs such as the bladder and, for women, protects the development of a fetus and facilitates birth. The dimensions of the pelvic girdle differ fairly significantly for men and women, as the opening in the center of the girdle must be large enough for a child to pass through.

  • Bones of the thigh, leg and foot. Connecting the pelvic girdle to the lower leg is a bone in the thigh area called the femur, the longest and strongest in the body. About 25 percent of your total height is gained from the femur bone [source: Houston Museum of Natural Science]. The femur connects through the knee joint (which is covered and protected by the patella, or kneecap) to the shin bone (tibia). Slightly smaller than the tibia is the other bone in the leg, the fibula. The fibula is responsible for muscular connections, while the tibia makes sure your foot and your knee don't get any farther apart from each other. Each foot has 26 bones: seven tarsal bones that make up the ankle, five metatarsal bones that make up the body of your foot (and play a significant role in supporting your body's weight), and 14 phalanges that form -- as is the case with your fingers -- four toes with three bones each with a big toe that has two bones.

Next, we'll look at some characteristics of different bone shapes.