The Eight Major Systems

   The digestive system  The excretory system The muscular system  The respiratory system

The skeletal systemThe nervous system  The circulatory system The immune system

The Nervous System

The skeletal system is the backbone of the whole human body. It keeps the body from collapsing on itself, and allows us to move with relative freedom. Bones are composed of a hard calcium-based tissue formed from cartilage. Inside some of the larger bones is a tissue called bone marrow. Bone marrow is a soft substance that produces new blood cells. There are three kinds of joints between bones: Ball and Socket, Hinge, and Pivot. A ball and socket joint, like the shoulder, works just like its name implies. One bone has a rounded end that fits into a socket on another bone, allowing it 360 degrees of rotation. In a hinge joint like the knee, the ends of the bones have raised ridges that interlock, and then the joint is held together with ligaments. A hinge joint can only move in one direction. The next kind of joint is the pivot joint. A pivot joint is like the one that attaches your head to your backbone. It alows rotation in one direction, and hinge type movement in the other.

Some diseases of the skeletal system include osteoperosis, and artheritis. Osteoperosis is caused by a decreased amount of calcium in the bone, which makes it much weaker. It is more common in older women. Artheritis results from decreased lubrication of the joints, and is usually found in older people, although if you abuse your joints, you could get it much younger. Probably the most serious injury to the skeletal system is breaking a bone. This can take months to fully heal, and is very painful.