The Skeleton System

Without a framework of bones, your body would collapse into a heap of shapeless flesh. Your skeleton not only holds you up - it also gives your muscles something to pull on, allowing you to move.

Bones are made of living tissue: they can feel pain, they bleed when cut, and they repair themselves if they break. We tend to think of bones as dry and brittle, but living bones are moist and slightly flexible to make them springy. Although smooth and solid on the surface, their insides are riddled with hollows to make them lighter. By weight, about 50 percent of a bone is a white calcium-rich mineral called hydroxylapatite, which is also found in teeth. This hard, crystalline material gives bone the great strength it needs to support the body's weight.

Body framework

The adult human skeleton is made up of 206 bones. Babies are born with more than 300, but some of these fuse together to form single bones as they get older. The largest bones are the two thigh bones (femurs), which support the body’s weight. The smallest are tiny ear bones, which are no bigger than grains of rice.


This protects the brain and houses the eyes and ears. It is made of several bones fused together.


The jawbone holds the lower teeth and connects by hinge joints to the skull.


Also known as the vertebral column, the backbone is made of 33 round bones stacked together.


This horizontal bone helps support the shoulder and arm.

Shoulder blade

The arm muscles are anchored to this flat, triangular bone.


This long bone runs from the shoulder to the elbow.


There are 54 bones in the hands - about a quarter of the body’s total.


Twelve pairs of ribs form a cage around the heart and lungs.


The ribs are joined to the middle of the chest by bars of stretchy cartilage that allow the chest to expand.


The large bones of the pelvis form a bowl shape, supporting the body’s internal organs.


As we grow up, five of the bones in the spine fuse to form a large, triangular bone called the sacrum.

Thigh bone

The longest, heaviest, and strongest bone in the body, the thigh bone (femur) makes up one-quarter of a person’s height.


Like a shield, the flat kneecap bone (patella) protects the knee joint.

Knee joint

The largest joint in the body, the knee works like a hinge but also permits a certain amount of twisting.


This long, slender bone lies alongside the bigger shinbone.


Also called the tibia, this is the main bone in the lower leg.


Seven bones called tarsals make up the ankle, which also contains three joints.


There are three toe bones (phalanges) in most toes but only two in the big toe.


Bones meet other bones at joints. In some joints, such as those in your skull, the neighboring bones are glued together rigidly. In most, however, the bones don’t touch each other but are tied loosely together by tough straps of tissue that allow the bones to move. Joints give your skeleton incredible flexibility.

To help the two bones in this joint to move, their ends are coated with slippery cartilage and surrounded by a pool of fluid. Tough ligaments tie the bones together.

Flexible joints

There are six main types of free-moving joints in your body. Each type allows a restricted range of movement, depending on how the bones fit together.