What is the Skeletal System?
The skeletal system works with the muscular system to give structure to an animal's body, and allow it to move. The skeleton stores important minerals like calcium and phosphorus, produces blood and immune cells and protects the inner organs. The skeletal system consists of all the bones in the body as well as the connecting cartilage, tendons and ligaments. The teeth are also considered part of the skeletal system.
Types of Skeletons
All vertebrates, including humans, have an endoskeleton: an interior skeletal system of bones and cartilage, linked and moved by muscle. Insects have an exoskeleton, a hard outer shell that encases the inner organs and muscles. Animals like earthworms and jellyfish have hydrostatic skeletons, which are fluid-filled compartments moved by muscle contraction.
What is Bone?
Bone appears hard and rock-like in a dead skeleton, but the bone of a living animal is itself a living substance. Bone continuously remakes itself, generating new cells to replace worn-out cells. Most bones consist of two types of tissue. The outer tissue is dense, compact and hard. It protects the spongy, porous interior. Some bones contain marrow, which produces essential blood and immune cells.
Parts of the Skeletal System
The skeletal system is divided into two main parts: the axial skeleton and the appendicular skeleton. The axial skeleton consists of the skull, sternum (breast plate), vertebrae and ribs. The appendicular skeleton includes the pelvis and shoulder girdle as well as the appendages--the arms, hands, legs and feet.
Joints are places where two bones connect. Different joint structures allow for different types of movement. Examples include the rotating ball-and-socket joint of the shoulder, the pivot joint of the elbow and the hinge joint of the knee.
Cartilage, Tendons and Ligaments
Cartilage is a gel-like substance that provides protective cushioning to joints. Cartilage also gives structure to parts of the body such as the ears and nose. Tendons are flexible, non-elastic cords that attach muscles to the bone. Ligaments are short, tough, flexible bands that attach bones to a joint or to each other.
Disorders of the Skeletal System
Osteoporosis is a loss of bone tissue that results in fragile, brittle bones. Caused by deficiencies of calcium and Vitamin D, osteoporosis is most common in postmenopausal women. Osteoarthritis is the degeneration of cartilage, causing stiffness and pain in the joints. It is a common age-related disorder, and also can be caused by overuse. Rheumatoid arthritis is a chronic degenerative disease that causes painful inflammation of the joints, especially the hands, feet, ankles and wrists.
A newborn baby has around 300 bones. As the child grows, some bones fuse together. By age 25, the average adult has a little more than 200 bones. The longest bone in the body is the femur, or thigh bone. The biggest bone is the pelvis, which is made up of six separate bones fused together. The smallest bone is the size of a grain of rice, and is located inside the ear. The bones of the hands and feet make up more than half the body's bones.