Human Skeletal System

It is easy to think of a bone as a dry, dead stick, but in truth bone is living tissue that grows, repairs itself and is constantly remodelled throughout life. A bone is formed by the ossification of cartilage, the soft tissue that the skeleton is first laid down in, and it needs the mineral calcium — along with phosphorus and vitamin D — to grow strong and hard. The whole skeleton makes up roughly 30–40% of an adult's body mass, and about half of the bone matrix is water while the other half is collagen protein together with hard crystals of calcium carbonate and calcium phosphate.

The living bone cells sit on the edges of the bones and in tiny cavities inside the bone matrix, and it is these cells that let a bone grow and develop, be repaired after an injury or daily wear, and be broken down to release its stored minerals. Because bone is alive, this is why bones are described as strong but not unbreakable, and why they keep growing until about the age of 18–20 years.

• The hard outer shell is the dense compact bone, which gives the bone its strength.
• Inside the ends lies the lighter, lattice-like spongy bone.
• The hollow central canal holds the soft bone marrow that makes the blood cells.

Having seen what a bone is, the next question is what the skeleton as a whole does for the body, and the answer is that it carries out five or six clear jobs at once. It is the framework that supports the body and gives it its shape, the shield that protects the delicate organs, the system of levers that makes movement possible, the store-house of minerals, and the factory in which blood cells are made.

The protective role is the most visible: the skull boxes in the brain, the rib cage guards the heart and lungs, and the vertebral column wraps around and shelters the delicate spinal cord.

We can now put a number to the skeleton. The adult human body has 206 bones in total, but a newborn baby starts life with about 300 bones; as the child grows, many of these soft, separate bones fuse together, and the count settles at 206 in the adult. These 206 bones fall into two great divisions — the central axis of the body and the limbs that hang from it.

The axial skeleton forms the central upright axis — the head, the spine and the chest — while the appendicular skeleton is everything that "appends" or hangs off that axis, namely the upper and lower limbs together with the shoulder and hip girdles that join them to the trunk.

The 80 bones of the axial skeleton are gathered into three groups: the skull and its associated bones, the bony cage of the chest, and the column of the spine. Together these surround and protect the brain, the heart and lungs, and the spinal cord.

Note that the skull by itself is counted as 22 bones (the 8 cranial plus the 14 facial bones); adding the 6 tiny ear bones and the single hyoid brings the head region to 29 bones, and these 29 together with the 25 of the thoracic cage and the 26 of the spine make up the full 80 of the axial skeleton.

The head region holds 29 bones in all, and it is worth breaking them down further because examiners love the small, oddly named bones of the skull and ear. The cranium is the domed box of 8 flat bones that encloses and protects the brain, the face is built of 14 bones, the middle ear holds the 3 smallest bones of the body on each side, and the throat carries the lone hyoid.

Two of these deserve special note: the mandible, or lower jaw, is the only freely movable bone of the skull, while the hyoid is famous as the only bone in the body that does not join, or articulate, with any other bone — it floats in the throat, anchored only by muscles, and supports the base of the tongue.

Below the head, the central axis continues as the backbone and the chest. The vertebral column, or spine, is a flexible chain of 33 vertebrae in the child that supports the body and protects the spinal cord running through it; in the adult the lowest vertebrae fuse, so the column is counted as 26 bones. The vertebrae are grouped into five regions from the neck downward.

In front of the spine sits the thoracic cage, built of 12 pairs of ribs (24 ribs in all) joined behind to the thoracic vertebrae and meeting in front at the flat breastbone, or sternum; this bony basket guards the heart and the lungs while still flexing enough to let the chest rise and fall with each breath.

The 126 bones of the appendicular skeleton make up the limbs and the two girdles that fasten them to the axial trunk. They divide neatly into the shoulder (pectoral) girdle and upper limbs above, and the hip (pelvic) girdle and lower limbs below, with 60 bones in each set of limbs.

The arrangement of the two limbs mirrors each other: the single upper-arm or thigh bone (humerus, femur) is followed by a pair of forearm or lower-leg bones (radius and ulna; tibia and fibula), and then by the many small bones of the wrist or ankle and the long fan of finger or toe bones called the phalanges, of which each hand and each foot has 14.

Cutting across these regions, anatomists also sort every bone into one of a few shapes, because shape follows function — long bones act as levers, flat bones as shields, and short bones give stability.

The patella, or kneecap, is the largest sesamoid bone and a favourite exam fact, while the long bones such as the femur are the ones that contain the cavity of red marrow and act as the body's main levers for movement.

Inside the hard shell of a bone lies its soft, busy core. The red bone marrow fills the hollow space inside bones, called the medullary cavity, and it is here that both red and white blood cells are manufactured in a process known as haematopoiesis. The living bone cells do more than build bone — they also release a hormone that links the skeleton to the body's metabolism.

The hormone osteocalcin, released by the bone cells, helps regulate blood sugar and fat deposition, so the skeleton is not just a frame but also part of the body's chemical control. Different types of bones carry out different functions, but all share this living, working interior.

For quick classification, the whole skeleton can be read region by region, from the head down to the feet. Each region groups together the major bones that examiners expect you to name.

This regional view is why bones matter at all: they give shape and structure to the body, protect vital organs such as the brain, heart and lungs, help in movement and posture, produce blood cells in the marrow, and store minerals like calcium and phosphorus.

A joint, or articulation, is simply the place where two or more bones meet, and it is the joints that decide how much the skeleton can move. Anatomists divide joints into three classes by how freely they allow movement, ranging from the locked sutures of the skull to the freely swinging joints of the limbs.

The freely moving synovial joints are the ones examiners ask about most, and they themselves come in types — the ball-and-socket joint of the shoulder and hip allows movement in all directions, while the hinge joint of the knee and elbow swings in only one plane, like a door.

Bones do not work alone; they are bound to one another and to the muscles by tough bands of connective tissue, and it is easy to mix these up in an exam. The simple rule to remember is that a ligament joins bone to bone, holding a joint together, while a tendon joins muscle to bone, transmitting the muscle's pull so the bone can move.

Cartilage is the smooth, springy tissue that caps the ends of bones at a joint and cushions them against shock; it is also the material the whole skeleton is first laid down in before it hardens, by ossification, into bone.

To gather the record-holders, a handful of "biggest and smallest" bone facts are asked over and over, and they are worth memorising as a set.

Bones can also fall ill, and a few disorders come up regularly in exams, most of them tied to a lack of calcium or vitamin D.

The lesson behind all of them is the same one the skeleton teaches from the start: strong bones need a diet rich in calcium and vitamin D, and an active life keeps them dense and healthy.