The structure of the skeleton

Adult human skeletons have around 210 bones, which are typically divided up into two main parts, known as the axial skeleton and the appendicular skeleton. In turn, these can be divided up into smaller groups of bones:

Axial skeleton:

• The head bones: the skull (the cranium, which protects the brain) and the facial bones, including the eye sockets, nose, cheekbones and jawbones).
• The torso bones: the spine and the rib cage, with the breastbone (sternum) and ribs.

Appendicular skeleton:

• The limb-connecting bones: the bones in the shoulder girdle (pectoral girdle) and the pelvic girdle, which connect the arm and leg bones to the axial skeleton.
• The arm bones: the upper arm bone, forearm bones and the 27 bones in each of our hands.
• The leg bones: the thigh bone, lower leg bones, kneecap and the 26 bones we have in each foot.

Let us take a closer look at these different parts of the skeleton:

The skull surrounds and protects the brain (which floats in a liquid called cerebrospinal fluid) as well as the middle and inner ear. The facial bones protect the eyes, the mouth and the upper airways. They also give shape to your nose, eye sockets, cheeks, lower jaw, and overall face. The flat bones in the skull are strong, but not very thick.

In newborn babies, these flat bones are only joined by connective tissue. This enables the head to change shape and make the birth easier. These flexible connections (fontanels) don’t close until the age of about three years. But the skull continues to grow until the brain stops growing. The skull will be almost full-size by the age of ten years.

The spine is also part of the axial skeleton. It is both strong and flexible, helps to keep the body upright and connects different parts of the body to one another. The spine is made up of bones called vertebrae, which have different shapes depending on their position in the spine. The vertebrae increase in size from the top to the bottom of the spine. This is because the load of the body’s weight increases from the neck down to the pelvis.

The vertebrae in the chest area (thoracic vertebrae) are considered to be part of the rib cage, along with the ribs and the breastbone (sternum). The ribs surround and protect the lungs and heart. They also help to stabilize the torso. Most people have twelve ribs on either side of their rib cage. The breastbone runs through the middle of them. The ribs have long sections of cartilage (costal cartilage) at the front, connecting them to the breastbone. At the back, there are joints to connect them to the vertebrae. This is so the rib cage can expand as we breathe.

It is usually only the top seven pairs of ribs that are directly joined to the breastbone. They are called “true” ribs. The other five pairs are called “false” ribs. The ends of the last two false ribs typically “float” on each side of the belly (abdomen). The rib cage can withstand very strong impacts – especially around the breastbone area.

The shoulder is the most flexible joint in the human body, as well as its most complex. It is made up of the shoulder blade (scapula), collarbone (clavicle) and upper arm bone (humerus). Although there are just three bones, there are lots of muscles, ligaments and tendons holding them together and joining them to the rib cage.

The shoulder girdle is only fixed to the rib cage by the collarbone, which connects the breastbone and shoulder blade. Other than that, the flat, triangular bone of the shoulder blade is only held in place by muscles and tendons. Its top side edge is reinforced and forms a flat socket, in which the round head of the upper arm bone can move. So the shoulder joint is very flexible but that comes at a price – it isn’t very stable and tends to become dislocated quite easily. Strong muscles and tendons usually stop that from happening, though.

The ring-shaped pelvis provides a stable base for the spine and connects the legs to the torso via a movable joint (the hips). The lower part of the spine, the sacrum, forms the center of the ring. It is joined to the hip bone through the sacroiliac (SI) joint. The hip bone consists of the ilium, ischium and pubis. It holds the hip joint socket, which holds the round head of the thigh bone (femur). This ball-and-socket joint is also very flexible. Although it doesn't allow the same degree of rotation as the shoulder joint, it is more stable.

The various parts of the pelvis are connected by cartilage until the end of puberty. Later, this cartilage turns into bone (ossification). Women’s pelvises become wider and flatter than men’s to allow a baby’s head to pass through.

The upper arm bone (humerus) is surrounded by muscles. You can only really feel the bone in the elbow joint. The elbow consists of multiple ligaments and a strong capsule that encases the joint. It connects the upper arm bone with the forearm bones (ulna and radius). The elbow enables us to bend our arms and twist our forearms and hands. When we stretch our arms out with the palms facing upward, the two forearm bones are parallel to each other. When our palms are facing downward, these two bones cross over.

The thigh bone (femur) in the leg has a short neck and a head that sits in a socket to form the hip joint. The neck is positioned at an angle to the shaft of the thigh bone (femoral shaft). The distance between them is a key factor in the level of pressure/bending force the neck has to withstand. The angle between the neck and shaft changes during a person’s lifetime, just as the strain on the body does.

The bones in the knee joint are also shaped to make sure that the forces caused by our body weight are spread as much as possible. That is why our thigh bones have wide, rounded joint surfaces, which are covered in cartilage and have two projections (condyles).

The head of the shinbone (tibia) forms the lower part of the knee joint. The calf bone (fibula) is attached to the side of it by ligaments. These two lower leg bones (tibia and fibula) have a band of fibrous tissue connecting them from top to bottom too. Several muscles are attached to this tissue. The two lower leg bones and the anklebone (talus) form the upper ankle.

In adults, the hip, knee and foot joints are usually in line with one another. Children tend to be bow-legged until they’re around three years old. Some of them end up being knock-kneed later. These problems usually go away as the child grows.

Our general bone shapes are formed when we’re still embryos. In the womb, they develop into the newborn skeleton, which is made of cartilage and still soft. It gradually hardens into bone during the child’s first year of life. Our bones continue to grow, increasing in length, width and thickness, until we reach the end of puberty. The proportions of our body also keep changing up until then because the different parts of our skeleton develop at different speeds.

From middle age, our bones start to get thinner again, and we lose bone mass. Our skeleton is always changing, and these changes are triggered by a variety of factors.

It isn’t fully understood how our bodies coordinate bone growth in such a way that the whole skeleton works properly. Defects only occur if skeleton functions or proportions are very abnormal, which can cause disabilities or other health problems. Apart from defects in the genes that control the growth of the skeleton, there are other, external factors that can have a negative effect on bone growth and the development of the skeleton. These include a lack of oxygen, viral infections and medications – especially during pregnancy.