There are 639 muscles in our body. The shape and structure of these muscles vary. A medium-sized muscle consists of about 10 million cells. All the muscles of the body put together have about 6,000 million cells. Each of these muscle cells is like a motor containing ten cylinders arranged in a row. The cylinders are tiny boxes that contain fluid. A muscle contacts when the brain sends a message to these tiny boxes. For a fraction of second, the fluid in the tiny box congeals; then it becomes a fluid again. It is this action that causes the muscle to move. Muscles make up about 40% weight of the body.
From birth to death these muscles perform very useful work in the body. They produce heat and help in the movement of food from one place to another in the digestive system. They take the air to the lungs. If they stop working all the functions of our body would come to a halt. The throbbing of the heart and the expansion and contraction of intestines would also stop.
Our body has three kinds of muscles: striated or voluntary, smooth or involuntary and cardiac. Most of the muscles of the hands, legs, neck and head are voluntary because all these limbs function as we want them to do. Most of them are attached to bones and are called skeletal muscles. Striated muscles are made up of many fibres. Smooth or involuntary muscles are those which function on their own. The muscles of the stomach and intestine are involuntary because the process of digestion continues on its own. These muscles help in blood circulation. The cardiac muscle is found in the heart. It is also controlled automatically. It is most important and is fibrous in structure. With the help of this muscle the heart beats 2,500 million times in a life span of 70 years.
In the working of muscles several body parts such as brain, spinal cord and nerves are involved. The many tiny fibres that make up a muscle contract when they receive a signal from the nerve. Muscle fibres either contract completely or not at all. When only a few of its fibers contract, the muscle only shows a weak contraction. The muscle contracts more powerfully when more of its fibres contract. All muscles work by contraction.