The Skeletal System

The growth of bones. The system that is growing fastest at your age is probably your skeletal system. During the time between your tenth and eighteenth birthdays, some of your bones may actually double in length. Not only do they grow, but they complete the hardening process that occurs from the time you are born until you are twenty or twenty-five.

When you are born, your bones are mostly cartilage, or gristle. As you grow, calcium, phosphates and other minerals are deposited in the cartilage, and the bones become hard. However, there is always a cartilage covering of the ends of the bones, particularly where the bones end in movable joints.

Function of bones. Bones are not dead sticks that hold you upright and help to keep your internal organs in their place. They are living organs. All bones are laced with blood vessels, and the centers are filled with fatty tissue and red marrow. Besides serving as the support for the body, the bones are the body’s red-blood-cell factories. The red marrow produces millions of red blood cells per second. These cells are carried into the blood stream by the system of blood vessels, which penetrate the bones.

Holding the skeleton together. Another part of your skeletal system is the tissue that holds one bone to another. This tissue forms connections between bones that are called ligaments. If this tissue were to disappear, you would literally fall apart. The ligaments that hold bones together may hold them firmly, as in the spine, or flexibly as in the elbow joint. The ligaments also serve as cushions between bones, and as well-oiled ball bearings between bones in movable joints.

Muscles. Although the ligaments hold the joints together, movement ofthe arms and legs is entirely dependent upon muscles. Each movement generally involves two muscles. One muscle relaxes while another tightens. In the illustration you can see two of the muscles that relax and tighten whenever you raise and lower your forearm. The biceps contracts, and the triceps relaxes when you lift your forearm. When you lower it, the opposite occurs.

Many muscles increase in size as they are used. They become smaller if they are not often used. For this reason, a program of vigorous exercise will promote the tone and size of your skeletal muscles. Regular vigorous exercise also improves your circulation and respiration.