THE SKELETAL SYSTEM

Most of us think of skeletons imaginatively as something hiding in a closet, as cute costumes for Halloween, or as the awesome structures seen in natural history museums, art and medical classes. Very few of us consider that our own bones are living, breathing tissues that are strong, resilient and alive.

Bones change throughout our lifetime. Existing as cartilage in the fetus as early as the fourth or fifth month, ossification progresses rapidly throughout the first four or five years of life, and proceeds according to genetic codes until the final size and shape of every bone is attained, usually by about age 25. Yet, throughout your life, your bones continually respond to use and abuse, to the wide variety of stresses experienced in a lifetime.

Bones, when stressed, create more bone. Bone cells called osteoblasts create new bone tissue, while cells called osteoclasts destroy it. The hollow interior of the bone expands in specific patterns in proportion to the solid exterior, preventing bones from being overly solid and heavy. Just as muscles respond to a weight training program by creating more muscle mass, bones, too, will create more mass to accommodate the stress. Postural habits not only rearrange the relationship of the bones to one another, but can alter the overall shapes and articulations of the individual bones themselves - a "Dowager's hump", for example, is actually increased mass in the cervical and thoracic vertebra formed to accommodate the strain of chronically slumped posture. Conversely, no stress at all - as in paralysis, or a limb placed in a cast to heal a fracture - causes the bones as well as the inactive muscles to atrophy.

Bone is living tissue, and needs balanced activity to maintain health. Approximately 35% of bone is composed of connective tissue fibers, mixed in a substantial amount of water. The other 65% is mineral, primarily calcium phosphate. Throughout our lives, bones are constantly adjusting to our life styles, incorporating more calcium for strength in response to weight-bearing exercise, or releasing calcium into the blood stream as they begin to atrophy when exercise is replaced with sedentary past-times.

Bone tissue is permeated with blood vessels, lymph channels and nerve branches. It is surrounded by a thin layer of connective tissue called periosteum that is rich in both blood and nerve supply, and interfaces with ligaments and tendons. Thus, bone is neither solid nor insensitive.