Main->Readings->5th Grade Readings->Skeleton and Muscles->Part 3
Vocabulary |
The skeletal system is great for supporting and protecting the body, but it can't move itself. For movement, the body needs the muscular system. The organs of the muscular system, of course, are the muscles. The muscles move the bones, pump the blood, squeeze food through the body, and perform many other jobs. The whole skeletal system is covered with muscles.
Each of the muscles is made of muscle tissue. Muscle tissue is made up of bundles of muscle cells, which are long, strong threadlike fibers. You can see those fibers easily when you eat a piece of meat. Most of the meat we eat is muscle tissue.
There are three main kinds of muscle tissue, each with different jobs. The kind we know best is the skeletal muscle. Skeletal (SKEL-uh-tuhl) muscle is the muscle that is attached to the bones of the skeleton. It is voluntary (VOL-un-tay-ree) muscle, which means you can control its movement with your mind.
Smooth muscle makes up the walls of many organs of the body. Smooth muscle is found in such places as the stomach, intestine, and the tubes which move blood around the body ("blood vessels"). You do not control smooth muscle with your conscious mind, so it is called involuntary muscle.
A special type of muscle, the cardiac muscle, is found only in the heart. It is involuntary like the smooth muscle.
Stand with your hands on a door, and push. The door moves, but guess what? None of the muscles in your arms is actually pushing. They are pulling. Muscles can only pull, but because of the ways they are attached to the bones and the ways muscles work together, they can move the body in many ways.
Skeletal muscles are attached to the bones by strong bands of tissues called tendons. One example of a tendon is the Achilles tendon, which you can find easily, because it is like a rope between the back of your heel and your calf muscle. In order to pull, your skeletal muscles contract, or get shorter. The calf muscle shortens and pulls on the Achilles tendon, and the tendon pulls the heel bone, allowing you to point your foot downward, a very important movement in walking.
When a muscle stops contracting, it relaxes and gets longer and thinner, but it does not push. How, then, can you move in the opposite direction? Muscles usually work in pairs. For instance, there are muscles in the front of the lower leg which can pull on the foot and raise your toe.
An easier place to see pairs of muscles at work is in your upper arm. If you raise your lower arm, your biceps muscles in the top of your arm will bulge. It is contracting, getting shorter and fatter, and pulling the bones in your lower arm upward. The triceps muscles on the underside of your upper arm will be soft and relaxed. If you straighten your arm, you should feel the biceps muscles relaxing and the triceps muscles bulging. Throughout your body, many muscles work in pairs.
Go on to Part 4: Staying Healthy
This page last modified on April 7, 2003