Every time you walk, settle into a chair, or hug your child, you're using your bones, muscles, and joints. Without these important body parts, we wouldn't be able to stand, walk, run, or even sit.

Bones

The skull protects the brain and forms the shape of our face. The spinal cord, a pathway for messages between the brain and the body, is protected by the backbone, or spinal column.The human skeleton has 206 bones, which begin to develop before birth. When the skeleton first forms, it is made of flexible cartilage, but within a few weeks it begins the process of ossification.

Bones are made up of two types of bone:

1. Compact bone is the solid, hard, outside part of the bone. It looks like ivory and is extremely strong. Holes and channels run through it, carrying blood vessels and nerves from the periosteum, the bone's membrane covering, to its inner parts.
   
   
2. Cancellous bone, which looks like a sponge, is inside the compact bone. It is made up of a mesh-like network of tiny pieces of bone called trabeculae. The spaces in this network are filled with red marrow, found mainly at the ends of bones, and yellow marrow, which is mostly fat.
   

Joints

Joints occur where two bones meet. They make the skeleton flexible — without them, movement would be impossible.

Joints are classified by their range of movement. Immovable, or fibrous, joints don't move. The dome of the skull, for example, is made of bony plates, which must be immovable to protect the brain. Between the edges of these plates are links, or joints, of fibrous tissue. Fibrous joints also hold the teeth in the jawbone.

Three kinds of freely movable joints play a big part in voluntary movement:

1. Hinge joints allow movement in one direction, as seen in the knees and elbows.
   
   
2. Pivot joints allow a rotating or twisting motion, like that of the head moving from side to side.
   
   
3. Ball-and-socket joints allow the greatest freedom of movement. The hips and shoulders have this type of joint, in which the round end of a long bone fits into the hollow of another bone.

Muscles

Muscles pull on the joints, allowing us to move. They also help your body perform other functions so you can grow and remain strong, such as chewing food and then moving it through the digestive system.

The human body has more than 650 muscles, which make up half of a person's body weight. They are connected to bones by tough, cord-like tissues called tendons, which allow the muscles to pull on bones. If you wiggle your fingers, you can see the tendons on the back of your hand move as they do their work.

1. Humans have three different kinds of muscle:
   1. Skeletal muscle is attached to bone, mostly in the legs, arms, abdomen, chest, neck, and face. Skeletal muscles are called striated because they are made up of fibers that have horizontal stripes when viewed under a microscope. These muscles hold the skeleton together, give the body shape, and help it with everyday movements (known as voluntary muscles because you can control their movement). They can contract (shorten or tighten) quickly and powerfully, but they tire easily and have to rest between workouts.
   
   
2. Smooth, or involuntary, muscle is also made of fibers, but this type of muscle looks smooth, not striated. Generally, we can't consciously control our smooth muscles; rather, they're controlled by the nervous system automatically (which is why they're also called involuntary). Examples of smooth muscles are the walls of the stomach and intestines, which help break up food and move it through the digestive system. Smooth muscle is also found in the walls of blood vessels, where it squeezes the stream of blood flowing through the vessels to help maintain blood pressure. Smooth muscles take longer to contract than skeletal muscles do, but they can stay contracted for a long time because they don't tire easily.
   
   
3. Cardiac muscle is found in the heart. The walls of the heart's chambers are composed almost entirely of muscle fibers. Cardiac muscle is also an involuntary type of muscle. Its rhythmic, powerful contractions force blood out of the heart as it beats.