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The skeleton can be divided into the axial skeleton and the appendicular skeleton.

The axial skeleton consists of the skull, vertebral column, ribs and sternum:

• The skull is made up of the cranium and upper jaw which are fused, and the lower jaw which articulates with the upper jaw. The skull protects the brain and the sense organs of the head.

• The vertebral column is composed of 33 bones known as vertebrae. The column supports the body provides points of attachment for the girdles and many muscles, and protects the spinal cord that runs through it. It also allows some movement.

• The ribs are attached to the vertebral column dorsally and the sternum ventrally. They form a curved, bony cage that protects the heart and lungs, and movement of the ribs is essential for breathing (see page 68).

The appendicular skeleton is composed of the pectoral girdle, the pelvic girdle, the arms (forelimbs) and the legs (hindlimbs). Both the arms and the legs are built on the same basic pattern known as the pentadactyl limb.

• The girdles connect the limbs to the axial skeleton. The pelvic girdle is fused to the sacrum at the bottom of the vertebral column to provide support for the lower body and to transmit the thrust from the legs to the vertebral column which moves the body forwards. Both girdles have broad, flattened surfaces for the attachment of muscles that move the limbs.

• The limbs are composed of long bones that have joints between to allow for easy movement. Being long, the bones provide a large surface area for the attachment of muscles and permit long strides taken.

The human skeleton has four main functions:

• Protection for the internal organs. The skull protects the brain and sense organs of the head, the vertebral column protects the spinal cord, and the ribs and sternum protect the lungs and heart.

• Support for the soft parts of the body. This is mainly carried out by the vertebral column, pelvic girdle and legs.

• Movement. The skeleton is jointed and muscles work across these joints to bring about movement. The vertebral column and limbs are mainly responsible for movement.

• Manufacture of blood cells. Red blood cells, most white blood cells and platelets are manufactured in the red bone marrow found in flat bones, e.g. the pelvis, scapula, ribs, sternum, cranium and vertebrae, and in the ends of long bones, e.g. the humerus and femur.

Joints are formed where two bones meet. There are three main types of joints:

• Immoveable joints or fibrous joints. The bones are joined firmly together by fibres allowing no movement, e.g. the cranium is made of several bones joined by immovable joints.

• Partially movable joints or cartilaginous joints. The bones are separated by cartilage pads which allow slight movement, e.g. the vertebrae are separated by intervertebral discs of cartilage.

• Moveable joints or synovial joints. The articulating surfaces of the bones are covered with articular cartilage and are separated by synovial fluid. The bones are held together by ligaments, which are tough and elastic to prevent dislocation, but still allow movement. There are two types of moveable joints:

• Hinge joints are formed when the ends of bones meet. They allow movement in one plane and are capable of bearing heavy loads, e.g. the elbow, knee, finger and toe joints.

• Ball and socket joints are formed where a ball at the end of one bone fits into a socket in the other bone. They allow movement in all planes, e.g. the shoulder and hip joints.