EARTH PHYSIOLOGY

Figure 1. The different types of joints in the body.

The bones provide the movable framework that gives shape to motion. Bones are rigid (but slightly flexible) structures, that, when combined with the 68 joints of the body, permit fluid motion of various types. Pivot joints, hinge joints, gliding joints, and ball-and-socket joints allow the widest range of motion; the ball-and-socket joint at the shoulder is among the most maneuverable in the body (Figure 1). Normally the ends of bones at joints are coated with a tough but flexible tissue called cartilage, which provides a wear-resistant surface for a lubricating fluid that surrounds the joint. This fluid is known as synovial fluid, which is a secretion of egg-white consistency that acts as a " joint oil." Infection, in jury, disease, or wear and tear can cause cartilage to become damaged and deteriorate, resulting in arthritis.

Supported by bone and activated by muscle, the body enjoys an incredibly wide range of movement, as forceful as sledge-hammering, as gentle as blinking. The same hand that pats a puppy, pounds a desk. The same foot that teeters on tiptoe, kicks a field goal. The limits of mobility are broad and the limitations relatively few. We cannot, for example, touch left forefinger to left elbow, or turn the head to look directly behind us. Even so, by stretching ligaments and muscles to make the joints unusually limber, some of us are able to wrap our feet around our neck, or do backbend flips across a padded floor or lawn. Some people can also hyperextend their elbows or even bend their thumb backwards to touch their forearm, causing everyone around them to gasp!

Figure 2. The thumb is capable of precision grips that allow for accuracy and fine control of certain movements as well as power grips that allow for firm handling of objects.

The thumb alone would convince anyone that the architect of our body (whoever that may be to each one of us) had to be a genius! Of the thousand or so different functions we perform daily with the 19 bones in each hand, 2 are demonstrated in Figure 2. Neither of these functions would be possible without the thumb. In a precision grip, a flexed finger and opposing thumb grasp an object in a posture that assures accuracy and fine control; in a power grip, the object is held between the flexed fingers and palm while the thumb exerts counterpressure. Each hand can perform both grips at once - with a small object grasped in your palm, you can pick up another with thumb and forefinger. Let's discuss the development, general arrangement, and function of the bones in our bodies.