3. Skeleton and muscles GapFill

Bones and muscles

The human skeleton

The skeleton of humans is composed of a special connective tissue called  cellulosenervepapermuscleboneepithelial. 

The skeleton has three important functions:

1. Provides a framework that supports the body. If we had no skeleton we’d be just a blob.
2. Helps you to move.   StringsBlood vesselsNervesMusclesCellsJoints attached to your skeleton pull the bones so that you can bend your arms and legs.
3. Protects some internal organs from injury; the ribcage protects the heart, liver and lungs; the skull protects the brain, and the vertebrae of your backbone protect the spinal cord.

• Compact bone tissue is on the outside of the bone. This contains a high proportion of mineral salts, making it very strong and hard.
• Spongy bone tissue is inside the bones. It is still strong but it contains air spaces, like a sponge, which makes it lighter. 
• Cartilage tissue.  Cartilage is a tough yet flexible material. It is softer than bone, and is on the ends of the bones, so that it stops the hard bones from rubbing together and getting worn down. It also acts as a shock absorber if you fall or bump yourself. You could still break a bone, but the cartilage makes this   impossiblecertainless likelyafraidmore likelyequally likely to happen.

 Joints – where two bones meet

A joint is where two or more bones meet. There are over 200 places in your skeleton where this happens.  The obvious joints are in your shoulder,  woodworkelbowribsbrainintestinesliver, hip, knee, hands and feet. 

Joints that allow parts of your skeleton to move are referred to as movable joints. Some joints however, like the ones in your skull, are fused together and do not allow movement and are called fixed joints.  This is because your skull’s main function is to  protect your brainmove your headsupport your headmake blood cellslook goodmake nerve cells – the bones don’t need to move about.

Most joints in your body are freely movable. Movable joints are often classified by the type and range of movement they allow. The ones you need to remember are:

• Ball-and-socket joints, like the shoulder and hip joints, which allow circular movements.
• Hinge joints, such as the knee and elbow. These work like a door hinge and allow movement in one direction only.

Bones are held together at joints by pieces of strong connective tissue called  cartilageligamentstendonsfabricbonesnerves. These are elastic – they can stretch. This allows the bones to move, but stops them from moving out of place.

Muscles and movement

Movement happens because of skeletal muscles attached to your bones.

Skeletal muscles   contractexpandexplodesqueezestretchrelax and pull on the bones to produce movement at the joints; for example, when you bend your knee.

Movement happens at joints because the bones are attached to muscles which work in pairs, called  cardiacsmoothweakoppositeantagonisticcomplementary muscles. They pull the bones in opposite directions.

Flexors pull the bone to bend the joint, and extensors pull the bone to straighten it.

Muscles cannot push bones.

Muscles are attached to bones by connective tissue called  cartilagebicepsfabricmembranesnervestendons. These are strong like ligaments, but they do not stretch. This is so that the full force of the muscle contraction is used to pull the bone. 

You can measure the amount of force that a muscle can exert by using a special scale. Force is measured in units called  newtonskilometresampspoundskilogramslasers.