The Human Skeleton: Axial and Appendicular Bones
The human skeleton is the framework of bones that supports and protects the body, and it is divided into the axial skeleton (skull, vertebral column and rib cage) and the appendicular skeleton (limbs and the girdles that attach them). In brief: an adult has 206 bones, of which 80 are in the axial skeleton and 126 in the appendicular skeleton. The skeleton supports the body, protects organs, allows movement, stores minerals and produces blood cells.
Bones are living tissue that grows, repairs itself and responds to use. This guide lists the functions of the skeleton, names the main bones of each division, compares the divisions in a table, works through a bone-counting example and corrects common mistakes.
What are the functions of the skeleton?
- Support: gives the body shape and holds up soft tissues.
- Protection: the skull protects the brain, the rib cage protects the heart and lungs, and the vertebral column protects the spinal cord.
- Movement: bones act as levers that muscles pull on at joints.
- Mineral storage: bones store calcium and phosphorus and release them into the blood when needed.
- Blood cell production: red bone marrow in some bones, such as the sternum, hip bones and vertebrae in adults, makes red blood cells, white blood cells and platelets.
- Energy storage: yellow bone marrow in the long bones stores fat.
What is a bone made of?
Bone is a connective tissue with a matrix of collagen fibers, which gives flexibility, hardened by calcium phosphate crystals, which give strength and rigidity. The mixture makes bone strong without being brittle. About 60 to 70 percent of the dry mass of bone is mineral.
The outer layer, compact bone, is dense and smooth. Inside is spongy bone, a lattice of struts that is light but strong, and the spaces contain marrow. A tough membrane called the periosteum covers the outside. Bone cells include osteoblasts, which build bone, osteoclasts, which break it down, and osteocytes, which maintain it. The balance between building and breaking down allows bones to remodel throughout life.
What is the axial skeleton?
The axial skeleton forms the central axis of the body. It has 80 bones.
Skull (28 bones, including ear bones)
The skull has the cranium, which surrounds the brain, and the facial bones. The cranium is made of eight bones, including the frontal, parietal, temporal and occipital bones, and the facial skeleton has 14 bones, including the mandible (lower jaw), the only movable skull bone apart from the ear ossicles. In adults, skull bones are joined by immovable joints called sutures. Each middle ear contains three tiny bones, the malleus, incus and stapes, giving six in all. The hyoid bone in the neck is also counted in the axial skeleton.
Vertebral column (26 bones in adults)
The vertebral column, or spine, has 33 vertebrae in the embryo and 26 separate bones in adults, after some fuse. They are grouped as 7 cervical (neck), 12 thoracic (chest), 5 lumbar (lower back), 1 sacrum (formed by 5 fused vertebrae) and 1 coccyx (formed from 3 to 5 small fused vertebrae). Between the vertebrae lie discs of cartilage that act as shock absorbers. The spine has natural curves that help balance and absorb shocks.
Rib cage (25 bones)
The rib cage consists of 12 pairs of ribs and the sternum (breastbone). The first seven pairs attach directly to the sternum with cartilage and are called true ribs. Pairs 8 to 10 attach indirectly, and the last two pairs, the floating ribs, do not attach to the sternum.
What is the appendicular skeleton?
The appendicular skeleton includes the bones of the limbs and the girdles that join them to the axial skeleton. It has 126 bones.
- Pectoral (shoulder) girdle (4 bones): two clavicles (collarbones) and two scapulae (shoulder blades). It gives the arms a large range of motion.
- Upper limbs (60 bones): in each arm, the humerus (upper arm), the radius and ulna (forearm), 8 carpals (wrist), 5 metacarpals (palm) and 14 phalanges (fingers), giving 30 bones per arm.
- Pelvic girdle (2 bones): two hip bones, each formed by fusion of three bones, which join the sacrum to form the pelvis. It bears the weight of the upper body and protects organs in the lower abdomen.
- Lower limbs (60 bones): in each leg, the femur (thigh bone, the longest and strongest bone), the patella (kneecap), the tibia and fibula (lower leg), 7 tarsals (ankle), 5 metatarsals (foot) and 14 phalanges (toes), giving 30 per leg.
Bone counts by region
| Division | Region | Number of bones |
|---|---|---|
| Axial | Skull (cranium and face) | 22 |
| Ear bones and hyoid | 7 | |
| Vertebral column | 26 | |
| Ribs and sternum | 25 | |
| Appendicular | Shoulder girdles | 4 |
| Upper limbs | 60 | |
| Hip bones | 2 | |
| Lower limbs | 60 |
The totals are 22 + 7 + 26 + 25 = 80 for the axial skeleton, and 4 + 60 + 2 + 60 = 126 for the appendicular skeleton, which gives 206. Some sources count the ear bones separately or differ slightly because of small extra bones, so the number 206 is the standard value for a typical adult.
Worked example: counting the bones of the limbs
Check the number of bones in one arm and one leg.
- Arm: humerus 1, radius 1, ulna 1, carpals 8, metacarpals 5, phalanges 14. Sum: 1 + 1 + 1 + 8 + 5 + 14 = 30.
- Leg: femur 1, patella 1, tibia 1, fibula 1, tarsals 7, metatarsals 5, phalanges 14. Sum: 1 + 1 + 1 + 1 + 7 + 5 + 14 = 30.
- Both arms and both legs: 2 × 30 + 2 × 30 = 120.
- Add the girdles: 4 bones in the shoulder girdles and 2 hip bones give 120 + 6 = 126.
The result matches the 126 bones in the appendicular skeleton. The same data show that the two hands and two feet contain 2 × (8 + 5 + 14) + 2 × (7 + 5 + 14) = 54 + 52 = 106 bones, which is about 51 percent of all 206 bones in the body.
How was the skeleton studied?
Andreas Vesalius published detailed illustrations of the human skeleton based on dissections in his 1543 work De humani corporis fabrica, correcting some of Galen’s descriptions that were based on animals. X-rays, discovered by Wilhelm Röntgen in 1895, allowed living bones to be seen. Modern imaging, such as CT and MRI, gives three-dimensional pictures.
Which mistakes are common?
Mistake 1: Saying that babies and adults have the same number of bones
A baby has around 270 to 300 bones, and many fuse during growth to give 206 in adults.
Mistake 2: Putting the pelvis in the axial skeleton
The hip bones belong to the appendicular skeleton. The sacrum, which joins them, belongs to the vertebral column.
Mistake 3: Thinking that bone is dead tissue
Bone has blood vessels, nerves and living cells. It repairs fractures and remodels continuously.
Mistake 4: Saying that the skull is one bone
It is made of many bones fixed together by sutures.