suppose it dashed against the window of the room instead, forcing it in, it would fill the room, and if the door were a little open, it would get behind it and shut it up close. It is just so with the blood in the heart, it rushes in at the open door (which opens inwards), and when each ventricle or auricle contracts, that only forces the blood against the inside of the door (opening inwards) and shuts it up close; but when it presses against those that open outwards, they fly wide open, and out goes the blood. So there are doors (or valves, as we call them) between each auricle and ventricle, and at the entrance into the aorta and pulmonary artery.

Now let us follow the blood along the aörta downwards. It is forced along by constant pumping of fresh blood into it by the heart and descends till it gets about the middle of the body, when one branch goes off to nourish the liver; a little lower another branch goes to replenish the stomach and intestines, and the blood for the latter (after being distributed over them), is sucked up again and carried to the liver. The main trunk, however, of the aorta still goes down, carrying blood to the legs and on to the tips of the toes. It is thus distributed by very small pipes over every particle of the body, leaving fresh matter there, and taking back instead the worn out black matter (the ashes of the system).

B

3

4

Fig. 18.

This waste, or dirty blood, is collected by little pipes, and carried into veins which convey it up to

the right auricle and pour it into the heart, to be sent again to the lungs to be purified. We have thus got it back to the right auricle from which it started. It has gone its round or circuit, and hence this constant flow round the body is called circulation. But there are two circulations: first, A, that from one side of the heart, through the lungs to the other side of the heart; and second, B, that from the left side of the heart round the body to the right side of the heart, and this is divided into two, viz., b, that which circulates to the parts of the body above the heart; and, c, that which circulates to the parts below it.

RESPIRATION.

We have seen that our food, when made into blood in our bodies, requires to be changed from dead into living matter, and that blood which has circulated once through the body wants purifying by the black waste matter in it being discharged. Some of you may know that when gas is manufactured at the gas-works, it has to be purified by being passed through lime, which discharges a black matter out of it, which is called gas-tar. Now, something like that is done with our blood; and a black matter is discharged from it, called carbon (or charcoal) which is poison to the blood, and is dead matter. It is like the dirt on our hands, and in our clothes, which we wash off and get rid of; and it has a bad smell, as you may find out for yourselves, if you will go out of doors as soon as you are dressed in a morning, and then go back into the close bed-room where you slept. You will find just the same bad smell, (only rather worse) if you stand over an open cess-pool. You can prove this for yourselves if you take a glass of lime-water (which is as clear as springwater) and blow into it for a few minutes through a quill, when the clear water will become thick like milk, the carbon having precipitated the lime. Whenever we breathe out (which is called expiration) we discharge this foul matter from our blood; and whenever we

breathe in (which is termed inspiration) we take in a fresh supply of life-giving gas, which quickens the blood, and gives activity and force, as well as new substances, to the various organs of the body.

The heart is a pump constantly forcing the blood into the lungs, and then, on its return, sending it on again round the whole body, working as regularly as the pendulum of a clock. The lungs form another set of machinery as constantly and regularly drawing in air and sending it out again. Place your hand on your heart, and listen uietly to your own breathing, and you will see how exactly and regularly each of these two machines act together.

Let us now see what the lungs are like, and how they work. They have been already compared to a sponge; but they are of rather closer material, and more fleshy than a sponge. They are sometimes called "lights" (because they will float in water) and may be seen at any butcher's shop, hanging, with the heart and liver, from a long tough pipe. That pipe is the windpipe, the upper end of which is fastened just under the root of the tongue, and goes down the front part of the neck (the gullet being the pipe behind that), so there are two openings into it from the outside air, one through the mouth, and the other through the nose; the former opening is properly for the voice, and the latter for breathing. (or respiration, as it is properly called). A little below the chin is a hard substance in the windpipe (commonly called "Adam's Apple") which is a musical instrument, like the mouth-piece of a clarionet or flageolet, fitted with two strings which vibrate (like the strings of a piano or Jew's harp) when the breath is forced through them. These strings cause the sound which we call voice. Below this the windpipe branches off into two, one going to the right and the other to the left. These two tubes, like the windpipe, are fitted with a succession of gristly rings to keep them always open; they again branch off into others, shaped almost like young trees, two on the left side and three on the right. These are called lobes, and they divide into a great number of branches, till

they terminate in very fine hair-like tubes, which end in little bags like currants on the ends of their stalks. These little bags or lobules are the air sacs, and when we draw in breath, these air sacs are filled with the air we draw in. Around them are the fine tubes of the pulmonary arteries and veins spread over them like

very fine net-work. So when the sacs are full of air, and the blood-vessels full of blood, the two are only separated by the very finest possible skin, finer than gauze; and, as has been said before, the oxygen gas in the air, and the carbonic acid gas in the blood rush through this fine gauze towards each other and change places. The oxygen combines with the blood, making it red, and the carbon mixes with the air, giving it an unpleasant smell, and it is thus breathed out into the air.

We see, then, that to keep our blood pure, we want plenty of fresh air, so as to get as much oxygen as possible, and also to get rid of as much carbon as possible. To effect this, we require exercise, so as to quicken the circulation of the blood, and bring it as rapidly as we

can to the lungs; and out in the open air, from which we can obtain the greatest amount of oxygen. If anything happens to prevent this interchange of oxygen and carbon, we should die in less than five minutes; and it is this which does happen, when people are drowned, or suffocated by the bad air of a room, as some