About two-fifths of the body’s weight is made up by its muscles—some 640 of them. Most are attached to the bones of the skeleton and pull on them to make the body move. In each part of the body the muscles are in two main layers. There are superficial muscles just under the skin and deep muscles lying below them next to the bones.
SKELETON
The 206 bones of the skeleton form a strong inner framework for the rest of the body, which is soft and floppy. Different parts of the skeleton work in different ways. The skull is a domed protective case for the brain. The backbones, or vertebrae, are a strong yet flexible central support. The long bones of the limbs work like levers.
MUSCLES
Every movement that the body makes is powered by muscles. A muscle is a body part designed to get shorter or contract. Most muscles are long and slim. They taper at each end into a rope-like tendon which is attached firmly to a bone of the skeleton. As the muscle contracts, it becomes thicker and pulls on the bone, moving that part of the body.
The largest muscle is the gluteus maximus in the buttock. It pulls the thigh bone backwards at the hip when you walk and with greater speed and power when you run and jump. The smallest muscle is the stapedius, deep inside the ear. It is just a few millimetres long and thinner than cotton thread. It pulls on a tiny ear bone, the stirrup, to prevent very loud noises from damaging the delicate inner ear.
HOW MUSCLES WORK
Each muscle is linked by nerves to the brain. The muscle itself is made up of bundles of hair-thin muscle fibres, which contain even thinner microscopic fibrils. In turn, each muscle fibril contains bundles of long, chain-like substances. These are muscle proteins, called actin and myosin.
When you want to contract a muscle, the brain sends signals along the nerve to the muscle. The signals make the actin and myosin proteins slide past each other, rather like people pulling hand-over-hand on a rope. Each protein slides together only a fraction of a millimetre. But these tiny movements build up in the thousands of fibrils contained inside the hundreds of fibres. Most muscles can shorten to about two-thirds of their relaxed length.
MUSCLES IN ACTION
Blinking your eye involves the movement of just one muscle, called the orbicularis oculi, an O-shaped muscle inside the eyelid. It is attached not to bones, but to other muscles and soft tissues. When it shortens, the two sides of the O move together and close the gap between them. The lip muscle, the orbicularis oris, works in the same way. Several other muscles in the face are not attached to bones. They pull on each other. This is how we make our huge range of facial expressions.
Most muscles are attached to bones. They rarely work alone. They pull in pairs or teams, to make a bone move in a precise way. One bone may have 20 or 30 muscles attached to it, each in a different place and pulling in a different direction. This means different combinations of muscles can tilt and twist the bone in almost any direction, as when you turn your outstretched arm and hand from palm-up to palm-down.
The body has different types of muscle in the walls of its inner organs, such as the intestines and bladder, and in the heart.
SKIN
The largest part of the human body is its outer covering – the skin. This is like a flexible, all-over coat that protects the body from knocks and keeps out dirt and germs. It also keeps the delicate inner parts of the body moist and shields them from the harmful rays of the sun. Skin is at its thickest, five millimetres or more, on the soles of the feet. The thinnest skin, about half a millimetre, is on the eyelids.
Skin gives the body its sense of touch. Millions of microscopic nerve endings just under the surface detect light touch, heavy pressure, heat, cold and pain. The skin grows hairs from tiny pits called follicles. There are about 120,000 large hairs on the head and four million tiny hairs over the rest of the body. Skin also sweats and increases the amount of blood flowing through it, to keep the body cool in hot conditions.
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