II
1.Let us now follow a drop of blood from the time it starts on a journey through the body till it is readyfor its next journey.It leaves the left ventricle of the heart and enters the largest of the arteries.It travels down wards，we shall suppose，through the branch artery，which conveys blood to the foot.This arter y gives off branch after branch，becoming smaller and smaller itself.At last it spreads out into a fine network of capillaries which you cannotsee without a microscope，andwhich are so close together that you can not push a needle into the skin without piercing some of them and letting out the blood.
2.Our drop of blood moves
very slowly here；but it is urged on by the pressure of the stream from the heart.By-and-by a few of these fine capillaries unite to form a tiny vein，and the drop begins to travel up through this vein.As it ascends，the vein becomes larger and larger through new tributaries joining it.Here and there，little bag-shaped valves or doors open to allow the blood to pass upwards，but shut to prevent it from flowing downwards.By this means the whole weight of the blood does not rest on the lower parts of the veins.
3.Upwards our drop is pushed，until it reaches the large vein that enters the heart，but this time it enters the right auricle.Then it descends through the trap-door into the right ventricle，and is at once forced out through an artery which takes it to the lungs，where it is again sent through a network of capillaries.What happens to it there we shall learn later on.Back itcomes from the lungs to the heart，once more through a vein，and this time enters the left auricle.Then it goes through the trap-door into the left ventricle，and is ready for a new journey.
4.The blood moves rapidly through the larger arteries-about twelve inches per second.While in the capillaries，however，it moves only at the rate of an inch or two in a minute.The blood spends most of its time in the capillaries，for it is there that its work is really done；but each drop of blood must pass through only a very small length of these capillaries；or it would have all its nourishment taken away，and would become useless.
5.While the blood passes slowly through the capillaries，it gives up to the body the nourishment it received from the food，and the oxygen it took in from the air.Each part of the body finds in the blood what it requires.The flesh，the bone，the skin，and the hair are all helping themselves to something as the blood passes along.It carries back some waste matter which the body no longer needs，and gets rid of that chiefly through the lungs.
6.Yon can see that the heart has a great deal of work to do.It is unwise，therefore，to do anything that would throw unnecessary work on it.Some times the heart is injured by people taking too violent exercise-running too hard，or lifting very heavy weights.Frequently it is injured by the use of strong drink.Alcohol affects thebeating of the heart，and makes it work harder.It is like whipping a horse to make him run fast when there is no need for it；his strength is wasted，and he becomesweakened and exhausted.Spiritscause warmth atfirst，but this is apt to be followed by greater cold，and so men never use alcohol when they are in the polarregions.It wastes the strength without renewing it.
7.You have all seen the heart of a bullock or of a sheep in the butcher‘s window.Your own heart is of very nearly the same shape，and in size it is a little larger than your closedfist.You may judge of its strength by the amount of work which it has beencalculateda man’s heartperforms ever y day.The heart beats about seventy-five times a minute in a grown -u p p e r s o n ，a n d somewhat faster in a young person.This means that it beats 108，000times every twenty-four hours.It hasbeen found that the heart of a grown-up man does asmuch work every twenty-four hours in sending the blood through his body as the muscles of his legs do when he walks to the top of a hill over 1，500feet high.
8.What seems the greatest wonder of all，perhaps，is this，that all those movements are made without our knowing anything about them.If we try to keep ourhand or arm moving regularly backwards and forwards，the muscles will soon become fatigued；and afterthey become fatigued，the movement will soon stop in spite of all our efforts.But the heart goes on without ceasing，minute after minute，hour after hour，day and night，for all the days and years of our life.
