On a first consideration it would seem that the bones, joints, and muscles should be discussed from a mechanical and engineering point of view; always with the appreciation that they are imbued with that mysterious quality, life. This allows them to grow, renew themselves after wear and tear, and repair themselves after being broken and torn apart. They make up a machine of enormous complexity but with the advantage of taking care of itself. Nothing else in this world can take the abuse which the living body does.
But this is a small part of the story. Always nature has more than one use for any system of our body. The solid bone makes the framework, but it is also a storehouse of such things as calcium and phosphorus, which are needed for the chemistry of the body and which in emergency can be dispensed by the bones. Most remarkable is the manufacture of the blood cells in the marrow of the bones.
Were one asked the function of the muscles, the obvious answer would be that they move the framework. I know that I cannot tell all that muscle tissue does. It helps in the circulation of the blood and the lymph; it stores sugar, which can be released and burnt for quick energy; its contractions produce heat and the resulting waste products help to stimulate respiration. Living tissue deteriorates with disuse so that the enthusiasts for physical exercise have a lot on their side.
Even if you can afford to pay for transportation, you had better keep your muscles in shape.