1.You have now got some idea of the frame-work of the body.As long as it is living，the body is always moving.
place to place，or locomotion；sometimes there are thevoluntarymovements which we make when we arewriting or speaking or eating；and all our life long，evenwhile we are asleep，there are involuntarymovementsgoing on，such as those of the chest in breathing，and of the heart in keeping the blood flowing through all the body.
2.The bones themselves are hard like a piece of wood，and have no power of moving.How，then，are all those movements caused？If you were to make a frame-work like that of the arm by joining pieces of wood together with hinges，how could you make these pieces of wood move as the arm moves？
3.The best way would be to have strings fixed to the various pieces of your model，so that when you pulled one string it bent the joint，and when you pulledanother it extendedit again.This is exactly the wayin which we move our limbs：there are strings which are fixed to the bones，and pull them up or down as we wish.
4.But how are the strings pulled，and who pulls them？This is the strangest part of the whole，andshows how different a living being is from any modelor machine which man can make.The strings are living strings，and they pull themselves whenever we who live in these wonderful living houses wish them to do so.
5.The strings are the muscles and sinews.Muscle isanother name for the red flesh which is covered by the skin.You see what muscle is like when you have a piece of beef or mutton on your plate，for the flesh of the animal is just muscle.
6.Now look at this picture of the arm and one of its muscles.At the lower end is a strong sinew fastened to the bone of the fore-arm，a little below the elbow-joint；and at the upper end are two sinews which pass over the top of the shoulder to the shoulder-blade.But in the middle there is a thick mass of flesh lying along the upper arm bone.You can feel this by laying your hand on your own arm.
7.Keep your hand there as you bend your arm，and notice what happens.You feel this lump of flesh getting harder and thicker in the middle，white at the same time it getsshorter.This is the whole secret of all the hundreds ofmovements in our bodies.The middle of the string of muscle gets shorter and thicker，and so the two ends are pulled closer together.
8.When you extend your arm again，another muscle must do the work.A muscle which lies along the back of the arm pulls in the opposite direction，while the first becomes soft and loose again.One muscle can pull only in one direction，and so the muscles are usually in pairs-one for bending the joint，and one for extending it again.
9.What makes muscle act in this way？Some strange kind of telegraph message comes from the brainthrough those nerves we spoke of，and causes the muscle to act.
10.Every time a muscle acts it loses a little of its substance，and so it wastes away gradually.But the blood is constantly carrying a supply of new materialfrom the food we eat，and thus the muscle is repaired as quickly as it is wasted.
11.The more frequently the substance of our musclesis wasted away and renewedin this way the strongerand firmer they become.This is the reason why plenty of exercise or hard work makes our muscles firm and strong，while a life of laziness tends to keep them weak.