The bones inside your body make a hard frame called the skeleton, and of the 206 bones in an adult skeleton, 80 are axial (skull, hyoid, ear bones, spine, ribs and breastbone) and 126 are appendicular (arms, shoulders, hands, legs, hips and feet). The bones of your skull protect your brain, your ribs form a cage that protects your heart and lungs, and your backbone protects your spinal cord. A skull has 8 bones in the part that holds the brain and 14 bones in the face, and the femur, or thighbone, is the longest and strongest bone in the human body. A skeleton is far more than a frame, though, because it is made of living material.
Bones are living tissue, but bone is also a material with two main parts. Bone mineral gives bone rigidity and load-bearing strength, while the organic matrix gives it elasticity and flexibility. Bone is a framework of the protein collagen made hard by the mineral calcium phosphate, and the mineral that hardens bone is mostly hydroxyapatite, which gives bone much of its strength and density. The outside of a bone is made of hard, dense bone, and inside that hard layer, many bones have a lighter, sponge-like tissue. A thin covering called the periosteum wraps the outside of a bone and carries nerves and blood vessels.
A long bone has a hollow shaft (diaphysis), flared ends called metaphyses below the growth plates, and rounded ends called epiphyses above them. Long bones grow longer at growth plates near their ends, and cartilage in the growth plate keeps growing until it stops in the early twenties, when the plate turns to bone. Bone mass, the amount of bone tissue in the skeleton, peaks by the mid- to late 20s. Yellow marrow is found in the large cavities of large bones and is mostly fat cells. In infants red marrow fills the bone cavities, but in adults it is mostly limited to the skull, ribs, sternum, collarbones, vertebrae and pelvis, and red blood cells are made in the red marrow inside bones.
Old bone is constantly broken down and replaced by new bone, a process called remodeling. Bone-building cells (osteoblasts) and bone-removing cells (osteoclasts) balance each other to keep bone healthy. Calcium is released from bone and stored in it almost continuously to keep the level in the blood steady. Foods with calcium and vitamin D help build healthy bones, and vitamin D helps the body absorb the calcium it gets from food.
Bones respond to the work they do. Bones get stronger when you use them by being active, and even after bones stop growing longer, they can keep getting thicker throughout life when muscle activity or weight puts more stress on them. Archaeologists can tell something about a person's work from bones, because the pull of a laborer's muscles reshapes the bone where the muscles attach. The opposite also happens. Astronauts on long space missions lose some bone, and researchers think the lack of stress on bones in weightlessness may cause the steady bone loss seen in people who live in space for a long time.
Not every skeleton is the same. The part of your ear you can see is made of cartilage, a tough bendy tissue, not bone, and a shark has a skeleton made of cartilage instead of bone. Giraffe necks have seven big bones, the same as almost all mammals, while the necks of sauropod dinosaurs had up to 19. A bird's bones are hollow and light, which helps it fly. Bird bones have thin walls and are filled with extensions of the lungs' air sacs, yet resist the bending stresses of flight. T. rex bones held large air spaces linked to air sacs, and more than 10 percent of its body volume may have been air.
How do doctors see a bone inside a body? Wilhelm Röntgen found X-rays by accident in 1895 while experimenting with vacuum tubes, and his X-ray picture of his wife's hand showed the bones of her hand and her ring. Doctors use X-rays to see whether a bone is broken, and bones show up white on an X-ray because they absorb the most radiation. A cast or splint holds a broken bone still while it heals, and children's broken bones can heal well and heal quickly. Broken bones heal in three stages: inflammatory, reparative and remodeling.
Bones can last for millions of years, and some bones are famous. Lucy, a famous early human relative found in Ethiopia, has a skeleton that is about 40 percent complete, and she is about 3.2 million years old. She got her nickname from a Beatles song that was playing in the team's camp. Lucy's bones had no duplicates, which supports the conclusion that they came from one individual. Her small skull, long arms and cone-shaped rib cage resemble an ape's, but her spine, pelvis and knee are more human-like because she walked upright.
Words to know
- hydroxyapatite: bone mineral
- periosteum: bone covering
- osteoblasts: bone-building cells
- osteoclasts: bone-removing cells
- diaphysis: middle shaft
