Skeleton is the flexible, bony framework of any vertebrate animal. It gives the body shape, protects vital organs, and provides a system of levers, operated by muscles, that enables the body to move. The skeleton houses bone marrow, the blood-forming tissues. It stores such elements as calcium and phosphorus and releases them to the blood. The skeleton also contains smaller amounts of the elements magnesium, potassium, and sodium.
The human skeleton
The human skeleton has about 206 separate bones. That is, a human being generally forms that many bones out of cartilage while developing to maturity. Sixty bones are in the hands and arms alone.
Bones are joined to neighboring bones by joints. Joints are either immovable, as in the skull, or movable, as in the arms and legs. The bones fit together and are held in place by strong bands of flexible tissue called ligaments. The human skeleton is divided into two main parts, the axial skeleton, and the appendicular skeleton.
The axial skeleton
is made up of the bones of the head, neck, and trunk. The spine (spinal column or backbone) forms an axis that supports the other parts of the body. The skull is at the top of the spine. The spine consists of separate bones, called vertebrae, with fibrous disks between them. Seven bones make up the cervical vertebrae (neck bones). The 12 thoracic vertebrae are at the back of the chest.
The ribs are attached to the thoracic vertebrae. There are usually 12 ribs on each side of the body. The upper ribs fasten in front to the sternum (breastbone). The ribs protect the heart and lungs, and act as a bellows box for the breathing process.
The five lumbar vertebrae lie in the lower part of the back. Below the last lumbar vertebra is the sacrum. In babies, five separate bones make up the sacrum. In adults, these bones have grown together into one solid structure. The pelvis is attached to the sacral segment of the spine by sacroiliac joints. The coccyx is at the bottom of the spine. In children, four separate bones make up the coccyx. The three lowest of these bones often fuse together during adulthood to form a beaklike bone. The point where the sacrum and coccyx meet remains fibrous throughout life.
The appendicular skeleton
is made up of the bones of the arms and legs and their supports. The shoulder girdle consists of the scapula (shoulder blade) and the clavicle (collarbone). The skeleton of the arm is divided into the humerus (upper arm); radius and ulna (forearm); carpus (wrist bones); metacarpus (palm); and phalanges (fingers). The bones of the leg consist of the femur (thigh); tibia and fibula (leg); tarsus (back of the foot); metatarsus (forefoot); and phalanges (toes). The leg is attached to the trunk by a pelvic girdle made up of two hipbones. Each consists of three bones, the ilium, the ischium, and the pubis. These bones are fused in adults.
Animal skeletons
Most backboned animals have a bony skeleton. However, some fish, including sharks and rays, have a skeleton made of a tough fibrous tissue called cartilage. Most mammal skeletons are similar to one another. For example, a giraffe’s neck has the same number of bones as a mouse’s neck, although the giraffe’s bones are longer and larger. Many animals without backbones, such as insects and lobsters, have hard body coverings. This exoskeleton (external framework) provides both support and protection for the soft parts of their bodies.
