I
1.In your last reading-book you learned some things about your body，the house you live in，and how to keep it in good health.Now it is time that you should know something about the way in which this house is built.
2.If you were asked what your body is made of，you would likely say that it is made of flesh and bones，and covered with skin.You also know something about the blood which flows through it，and that there is a very important part which we call the brain.
3.But the flesh and the bones make up the largest part of the body，and there is a good deal to be learned about them.You know the general appearance of both flesh and bone from the parts of animals which you see in the butcher’s shop，or which you get on your plate at dinner.
4.Bone is hard，and of a white colour；indeed，it looks more like a piece of wood or stone than a part of a living animal.But bone is a real part of the living body，and it grows and is nourished along with the body.If a man breaks the bone of his arm or his leg，it is firmly bound up，to prevent the broken parts frommoving out of their proper place，and then in a few weeks the bone grows together again and becomes quite strong.
5.The bones are fastened together by various kinds of joints.They formthe frame-work of the body，and giveit strength to retainits proper shape.
This frame-work is called the skeleton .
Some animals，like worms and slugs，have no skeleton；and others，like shell-fish，have a kind of hard covering or skeleton outside.
6.The most important part of the skeleton is theback-bone.It is so important that naturalistsdivideall animals into two classes-those which have aback-bone，and those which have none.All the higher animals，including man，have a back-bone，or vertebral column as it is called，and they are therefore called vertebrate animals.The others are called invertebrate animals.
7.The vertebral column，or back-bone，is not really a bone at all.It is a pillar of small bones firmly bound together.If you string a number of reels of cotton upona strong cord，and pull the cord tight，you will have a column somewhat like the vertebral column.It will bend slightly，as the back-bone does ；but while you keep the string tight，it will be firm enough to stand upright.
8.At the upper end of the back-bone there is the skull.This is a hollow box or case made up of several pieces of bone fitting closely together.Inside the skull is the brain，which is in manyways the most important organof the body.
9.The nerves come from the brain，and it is by means of them that we feel，and see，and hear，and taste，and by means of them also that we move any part of our bodies as we wish.They are like living telegraph wires running all through the body.
10.From the lower part of the brain there is whatwe might call a living telegraph cablepassing downthrough the back-bone.This is the spinal cord，which gives off many branches，or nerves，as it passes down-wards.If the spinal cord is injured，either by disease or by some accident，all power of feeling or of movement in the lower part of the body is lost.When this happens，we say that part of the body is paralyzed.
11.There is another box or case of bone in front of the back-bone.The ribs，which are joined to the back-bone behind，and bend round towards the breast-bone in front，form a strong cage，inside of which are placed the heart and the lungs.
12.The heart is a kind of force-pump which sends the blood through every part of the body.In the lungs the blood is made pure by mixing with the oxygen of the air.These organs，like the brain and the spinal cord，are well protected by the strong bony frame-work which surrounds them.