Missy Phillips, age 13, of Cumberland, Md., for her question:
DOES BONE DO MORE THAN ACT AS THE BODY'S FRAMEWORK?
Bone is the hard substance that forms the framework of the bodies of man and animals with backbones. The separate parts of this framework are called bones. Bone is one of the most active tissues of the body and it does much more than simply act as part of the body's framework
First of all, bone stores useful materials such as calcium, fluoride, phosphorus and sodium. In addition, bone also contains red bone marrow, where new red blood cells are made. Bone also has the ability to cleanse the blood of some of its harmful substances.
Bone consists of two kinds of tissue: compact and cancellous. Compact tissue is the hard, outside part of the bone. Cancellous tissue is the spongy part on the inside.
In the front part of the skull, the spongy tissue is replaced by airfilled cavities called sinuses. Bones are covered with a fibrous membrane called periosteum.
There are two kinds of bones: long bones and short bones. The long bones are located chiefly in the skeleton of the arms and legs. Short bones are located chiefly in the skull, spine and pelvis.
The center part of a bone is hollow. It is called the medullary cavity and is filled with either red or yellow bone marrow. Yellow bone marrow is mostly fat. Red bone marrow is a network of blood vessels, connective tissue and blood forming cells.
Bones form long before birth. The top of the skull is formed by intramembranous bone formation. In this process, the soft connective tissue changes directly into hard bone tissue. Long bones develop by intracartilaginous bone formation. In this process, the soft connective tissue develops first into cartilage which later is absorbed and replaced by bone.
About two thirds of the weight of bony tissue is mineral matter, chiefly calcium and phosphorous. The rest is organic matter, consisting largely of collagen, a fibrous protein.
Long bones have three parts: the shaft, the metaphysic and the epiphysis. The shaft is the long part of the bone. The metaphysic is the flared part at the end of the shaft. The epiphis is the rounded end.
Long bones grow by means of a special structure called the epiphyseal cartilage. The epiphyseai cartilage is a plate of soft tissue between the epiphysis and the metaphysic. In the young, the epiphyseal cartilage grows in thickness throughout the period of growth.
Each new layer of cartilage is successively absorbed by a layer of new bone, by the process of intracartilaginous bone formation. In this way, the epiphyseal cartilage and the epiphysis grow from the center portion of the bone and the shaft gradually grows longer.
A break in the bone is called a fracture. A fracture heals by forming a callus, which is a mass of new cartilage and new bone.
When a bone is broken, it should be set and put in its proper position at once. If it is held securely in a cast, it will knit or grow together. This usually takes from four to 52 weeks, depending upon the size of the bone, the location of the fracture and the age of the individual.