The collection of bones that provides a frame for an animal's body is called a skeletal system. The human skeletal system consists of more than 200 bones and connecting tissues called ligaments. A springy tissue called cartilage provides a protective, cushioning layer where bones come together. This rigid framework supports softer tissue. It also acts to both protect and support the internal organs, and it works with the muscles to enable the body to move. Thus the main functions of the system are support, protection, and motion.

Support

The oldest and most basic function of the skeletal system is support. The spine, or vertebral column, for example, aided by muscles and ligaments, is the main support for the entire upper portion of the body. In fact, human beings are quite different from other higher animals in that they are able to stand and walk in an upright posture. They enjoy great advantages over other animals because of this. Perhaps the most important one is that the arms of humans are not needed for walking or running. This means they are free for many other useful activities, such as using tools.

Protection

Because of its rigid structure, the skeletal system offers protection for the organs and nervous system. The central nervous system—the brain and spinal cord—for example, lies largely within the secure framework of the skeletal system. The brain is protected by the skull that surrounds it. And the nerves of the spinal cord are for the most part enclosed by the bony yet flexible structure of the spine, or vertebral column. The cartilage and ligaments work together with the bones, or vertebrae, of the spine to safeguard those nerves.

In a similar way, the heart and lungs, along with other organs, are protected by the rib cage formed by the spine and ribs. Like the spinal column, the rib cage offers flexibility as well as protection. It also allows for the expansion and contraction needed for breathing. Such movement is possible because of the softer cartilage in the front of the rib cage, between the breastbone, or sternum, and the ribs themselves.

Motion

The third major function of the skeletal system is to work with the muscles to promote a wide range of movement. Many muscles are attached to at least two bones in the skeleton. In some cases muscles are attached to many bones. Most of the motion of the body comes about as a result of an involved process. It requires a smooth coordination of bones, muscle, and joints working together.

How the bones of the skeleton are connected

The individual bones of the human skeleton are held firmly together by tough, leathery tissues called ligaments. Most bones are also associated with muscles. In some cases, the bones are attached directly to the muscle. This is true of the bones making up the rib cage. In most cases, however, a tissue called a tendon connects the muscle to the bone.

At the point where bones are connected, they must have some type of protective covering. Imagine rubbing two hard stones together: over time, the friction from the contact of these two hard objects would cause both to wear down and become damaged. So it is with bones. The protective tissue between bones must be tough enough to protect the bones but elastic enough to not become damaged itself. For many bones, the material that serves this purpose is called cartilage. It is found at the ends of many long bones as well as in all joints, such as the knees and hips.

Like bone, cartilage can be damaged, but unlike bone, it is not easily repaired. As people get older, the cartilage in some joints—especially in the knees and hips—can become worn down. This allows the bones of the joints to rub together, causing great discomfort. Because of this, some older people must undergo surgery to have their joint cartilage replaced with plastic structures that will protect their bones.