JOINTS

Joints:
You may sit at a table, pick up a sandwich, bite off a piece of a carrot and chew it, and walk to class. All of these motions are possible because your skeleton has joints.
Anyplace where two or more bones come together is a joint. The bones making up healthy joints are kept far enough apart by a thin layer of cartilage so that they do not rub against each other as they move. The bones are held in place at these joints by a tough band of tissue called a ligament. Many joints, such as your knee, are held together by more than one ligament. Muscles move bones by moving joints.

Immovable Joints:
Refer to Figure as you learn about different types of joints. Joints are broadly classified as immovable or movable. An immovable joint allows little or no movement. The joints of the bones in your skull and pelvis are classified as immovable joints.

Movable Joints:
All movements, including somersaulting and working the controls of a video game, require movable joints. A movable joint allows the body to make a wide range of motions. There are several types of movable joints—pivot, ball and socket, hinge, and gliding. In a pivot joint, one bone rotates in a ring of another bone that does not move. Turning your head is an example of a pivot movement. A ball-and-socket joint consists of a bone with a rounded end that fits into a cuplike cavity on another bone. A ball-andsocket joint provides a wider range of motion than a pivot joint does. That’s why your
legs and arms can swing in almost any direction. A third type of joint is a hinge joint. This joint has a back-and-forth movement like hinges on a door. Elbows, knees, and fingers have hinge joints. Hinge joints have a smaller range of motion than the balland- socket joint. They are not dislocated as easily, or pulled apart, as a ball-andsocket
joint can be. A fourth type of joint is a gliding joint in which one part of a bone slides over another bone. Gliding joints also move in a back-andforth motion and are found in your wrists and ankles and between vertebrae. Gliding joints are used the most in your body. You can’t write a word, use a joystick, or take a step without using a gliding joint.

Common Joint Problems:
Arthritis is the most common joint problem. The term arthritis describes more than 100 different diseases that can damage the joints. About one out of every seven people in the United States suffers from arthritis. All forms of arthritis begin with the same symptoms: pain, stiffness, and swelling of the joints. Two types of arthritis are osteoarthritis and rheumatoid arthritis. Osteoarthritis results when cartilage breaks down because of
years of use. Rheumatoid arthritis is an ongoing condition in which the body’s immune system tries to destroy its own tissues.

THE SKELETAL SYSYTEM

Bone Formation:
Although your bones have some hard features, they have not always been this way. Months before your birth, your skeleton was made of cartilage. Gradually the cartilage broke down and was replaced by bone, as illustrated in Figure 3. Boneforming cells called osteoblasts (AHS tee oh blasts) deposit the minerals calcium and phosphorus in bones, making the bone tissue hard. At birth, yours keleton was made up of more than 300 bones. As you developed, some bones fused, or grew together, so that now you have only 206 bones. Healthy bone tissue is always being formed and re-formed. Osteoblasts build up bone. Another type of bone cell, called an osteoclast, breaks down bone tissue in other areas of the bone. This is a normal process in a healthy person.When osteoclasts break bone down, they release calcium and phosphorus into the bloodstream. This process maintains the elements calcium and phosphorus in your blood at about the levels they need to be. These elements are necessary for the working of your body, including the movement of your muscles.

THE SKELETAL SYSYTEM

Bone Structure:

Several characteristics of bones are noticeable. The most obvious are the differences in their sizes and shapes. The shapes of bones are inherited. However, a bone’s shape can change when the attached muscles are used. Looking at bone through a magnifying glass will show you that it isn’t smooth. Bones have bumps, edges, round ends, rough spots, and many pits and holes. Muscles and ligaments attach to some of the bumps and pits. In your body blood vessels and nerves enter and leave through the holes. Internal characteristics, how a bone looks from the inside, and external characteristics, how the same bone looks from the outside, are shown in Figure 2. A living bone’s surface is covered with a tough, tight-fitting membrane called the periosteum (per ee AH stee um). Small blood vessels in the periosteum carry nutrients into the bone. Cells involved in the growth and repair of bone also are found in the periosteum. Under the periosteum are two different types of bone tissue—compact bone and spongy bone.

Compact Bone:

Directly under the periosteum is a hard, strong layer called compact bone.Compact bone gives bones strength. It has a framework containing deposits of calcium phosphate. These deposits make the bone hard. Bone cells and blood vessels also are found in this layer. This framework is living tissue and even though it’s hard, it keeps bone from being too rigid, brittle, or easily broken.

Spongy Bone:

Spongy bone is located toward the ends of long bones such as those in your thigh and upper arm. Spongy bone has many small, open spaces that make bones lightweight. If all your bones were completely solid, you’d have greater mass. In the centers of long bones are large openings called cavities. These cavities and the spaces in spongy bone are filled with a substance called marrow. Some marrow is yellow and is composed of fat cells. Red marrow produces red blood cells at an incredible rate of 2 million to 3 million cells per second.

Cartilage:

The ends of bones are covered with a smooth, slippery, thick layer of tissue called cartilage. Cartilage does not contain blood vessels or minerals.Nutrients are delivered to cartilage by nearby blood vessels. Cartilage is flexible and important in joints because it acts as a shock absorber. It also makes movement easier by reducing friction that would be caused by bones rubbing together. Cartilage can be damaged because of disease, injury, or years of use. People with damaged cartilage experience pain when they move.

PHYSICS OF DIODE

DIODES

CAPACITORS