DK Science: Skeletal System
The body is supported and its internal parts protected by a strong yet flexible framework of BONES called the skeleton. These bones meet at JOINTS, most of which allow movement between the bones they connect. As well as protection and movement, bones provide a store for the mineral calcium, which is vital to the working of nerves and muscles. They also contain bone marrow, which makes blood cells and stores fat.
The skeleton contains 206 bones. Babies have over 270, but by adulthood many of these have fused together. Some of the main individual bones, and groups of bones, are labelled here. They fall into two groups: the axial skeleton, made up of the bones of the head, spine, ribs, and breastbone; and the appendicular skeleton, containing the bones of the limbs, the pelvis, the shoulder blades, and the collarbones.
Bones are relatively light, yet five times stronger than steel. They contain cells, minerals, protein, and water. Bones are composed of two types of tissue: cancellous (spongy) and compact bone. These are living tissues that are constantly broken down and rebuilt by the cells they contain.
Bones have an outer layer of compact bone, one of the body’s hardest materials. On the inside is an area of cancellous bone, which may contain red bone marrow. In adult long bones, like the femur, the shaft is compact bone overlaying an area that may contain yellow bone marrow (a fatty tissue).
Compact bone consists of units called osteons, each about 1 mm (1/25 in) across. One osteon is shown here. It is made up of numerous tiny rings of a hard tissue arranged around a central canal, through which blood vessels and nerves pass.
In cancellous (spongy) bone, struts of rigid bone tissue called trabeculae connect up to form a honeycomblike structure. Cancellous bone is less dense than compact bone but is still very strong.
Joints are the parts of the body where bones meet. Some, such as the joints in the cranium, allow no movement between the bones. Others, such as the joints in the spine, allow limited movement. A few, such as the hip joints, permit a wide range of movement. The bones of many joints are held in place by muscles and bands of tissue called ligaments.