Almost all animals have a supporting framework otherwise known as our skeleton. The main skeleton of a mammal is inside the body and is made up of bones and cartilage. The hard part of a bone is mainly calcium salts secreted by the living cell. Such an internal skeleton is called an endoskeleton.

Functions of the skeleton

• Supports the body and gives it a definite shape
• Protects internal organs
• Provides attachment for muscles and serves as levers enabling movement to take place.
• Bone marrow produces blood corpuscles.

Bones and Muscles

Bones take part in almost any motion that we make. Action and moving bones go together. However, bones can move no more than a piece of stick lying on the ground.

                 - Muscle Cell                       

Muscles are made up of cells that can contract or pull themselves into a smaller and thicker shape. Bones are connected to muscles by ropelike strands called tendons. When a muscle contracts it pulls the tendon which subsequently pulls the bone which it is anchored to make it move.

As a practice you can try lifting a few heavy books or some weights. Observe your hands. The bunching up under the skin is the biceps muscles contracting. The biceps is attached to the bones in the upper arm. When it contracts, the biceps pulls up the lower arm. Straightening the arm is the job of another muscle, the triceps. It is attached to the opposite side from the biceps. Most of our skeletal muscles work in opposed pairs.

Muscular actions are very complicated. To throw a ball, you must use many groups of muscles in your shoulders, arms, chest, abdomen, and legs. Each group must act in the right direction with the right force, in a fraction of a second. The director of these complex actions is the nerve, which shan't be discussed right now.

So far, few have looked at the muscles that we control - the voluntary muscles -. If we take a mouthful of water we can swish it in our mouth as we like using our voluntary muscles such as the cheeks, lips and tongue muscles. But once you swallow the water, involuntary muscles take over such as the oesophagus. Involuntary muscles in the body helps to digest food, circulate blood, and do many other jobs. One important and special involuntary muscle is the heart.

Joints

Your skeleton must be able to support you as you move yourself and the various parts of the body. That's much more difficult to do than to support something that is stationary. This is solved by joints that appear between two bones that come together. Different kinds of joints determine the kind of movement that can be made.

The ball-and-socket is one kind of joint. Your arm and shoulder are joined in this way. The top of the arm bone is shaped like a ball. The ball fits into a shallow cup-like socket in the shoulder blade, much like the way a ball is held in place in a ball point pen. This arrangement, which is also found in the hip joint, lets you swing your arms and legs in a full circle. Another kind of joint is the hinge joint, which is like the hinges of the door. These joints, such as the ones at the knees and elbows, allow motion mainly in one direction. The joints between the bones of your spine allow the bones to move over one another with a gliding motion. There are also joints that prevent motion, such as joints in between the bones that form the skull.

Joints are surrounded by a case called a capsule. A fluid in the capsule allows the bones to move smoothly over one another, like lubricants.

Bone marrow