1.The beating of the heart is not the only movement that must go on during sleep.The movements of breathing are as necessary to our life as those of the heart.You know that fresh air is necessary for our life and health，and that we should get fresh air into our houses by ventilating our rooms，especially our bedrooms；and you know that living in the open air as much as we can is good for the health.We have now to see how the lungs do their work in making use of this fresh air.
2.You must not think that the lungs cause the movements of our breathing.We speak of them as the“organs of respiration，”but the movements by whichthe air is drawn in and forced out again are not caused by the lungs.The ribs are covered with muscles which move them upwards and outwards，thus making thechest wider.At the same time partitionwhich liesacross the body below the lungs is pulled downwards，and this increases the depth of the chest.
3.When you have a hollow space which is suddenly increased in size in this way，what will happen？Justwhat happens when you pull the handles of a pair ofbellows apart，so as to enlarge their inside cavity.Aquantity of air rushes in to fill up the additional space.It is the sudden enlargement of the chest cavity that causes the air to rush in through the nostrils，although we sometimes speak as if it were the entrance of the airthat expandedthe chest.
4.As soon as the chest has thus been filled with air，the muscles relax.The chest falls back to its formersize，and in doing so it sends out a quantity of air equal to that which was breathed in.Then there is a slight pause，after which another breath is taken in andexpelledin the same way.So the movements go on allthrough our life，sometimes faster，sometimes slower，but without any stoppage.These movements are partly under our control.We can make them little faster or slower for a short time，but they go on in spite of us and without any effort on our part.
5.If anything causes the breathing to stop，the heart soon ceases to beat also，and death is the result.The breathing stops either when no air can get into the chest，as in cases of suffocation and drowning，or when the air is bad，as in cases of gas poisoning.Those whodive for pearls and sponges learn to hold their breath for a much longer time than other people can do；but these occupations are generally bad for the health.
6.When you are happy said active，your breathing goes on regularly and quickly；but if something has made you sad and gloomy，you sit still，and yourbreathing is less vigorous.Have you ever noticed whathappens then？All at once，in spite of yourself，you take in a long，deep breath，which we call a sigh.This sigh is a hint to you that your lungs are not getting enough fresh air，and that it would be better to take some exercise or do some work than to sit still and injure your health.
7.The lungs lie inside the chest，and fill up most of it.They extend above，behind，and on bothsides of the heart.The air passes into them down the windpipe，which has two branches，one for each lung.Theseb r a n c h e s d i v i d e a n dsubdivide until they endin very tiny little bags or cells，and it is those air-cells which form most of the lungs.
8.The walls of the air-cells are covered with a network of capillaries，or small tubes，through which blood is circulating.In last lesson you were told that the blood all passed through the lungs，and most of its time there is passed in these little pipes in the cell-walls of the lungs.
9.When the blood comes to the lungs，it is of a dark purplish-red colour，with little oxygen in it，and too much carbonic acid and other waste matter，which it has gathered up and carried with it from the various parts of the body.When it leaves the lungs，it is of a bright scarlet colour，and full of oxygen to be carriedthroughout the body，and given away to it as the blood flows along.The thin walls of the capillaries allow the oxygen to pass inwards and the carbonic acid to pass outwards through them.Thus the blood is made pure，and fitted for going once more on its life-giving journey.
10.The air we breathe in ought to be pure air；the airwe breathe out is always impure.It contains carbonic acid and watery vapour from the blood，and other impurities，and we should have good ventilation in our houses to let this impure air escape.A grown-up person requires as much fresh air every day as would fill a room seven feet square and a little more than seven feet high.
11.Young people should be careful always to breathe through the nose and not through the mouth.Breathingthrough the mouth is not only an unpleasant habit，but it is often hurtful to the health.In cold weather it allows the air we breathe in to strike more directly down into the windpipe and the lungs，and we are thus apt to get colds.When coming out of a warm room in winter，it is specially important to keep the mouth shut for some time，and to breathe through the nostrils.
12.We should take care that our clothing is not too tight to allow the free movement of the chest in breathing.Much oxygen is needed by the body，and a want of good air is as sure to cause weakness as a want of good food.On the other hand，there is nothing better for our health than exercise in the fresh air by the sea-side or among the hills.The exercise causesour breathing to become deep and vigorous，while theair in such places is rich in oxygen，and free from the impurities which we can never get rid of in cities and crowded places.