times is the case when they sleep in a room where charcoal is burnt, which fills the air with carbon.

The lungs are contained in a cavity or box, lying between the neck and the waist. It is protected in front by the breast bone, behind, by the backbone (or spine), and at the sides by the ribs, which are two sets of props encircling it, fastened at the back to the spine, in front principally to the breastbone, though the lower ones are only joined to one another, so as to allow of greater expansion of the lungs. There are twelve of these rib-bones on each side, fitted with a hinge to the back-bone, passing from the back to the front. They rather drop downwards, and the curved faces of the ribs hang flat towards the lungs (1); but when we draw a breath these faces are turned nearly straight outwards, (2,) twisting

2

Fig. 21.

round a little on their

hinges. This movement increases the size of the

cavity, and by their

being thus straightened

Fig. 22.

they press out the breast bone sideways a little. thus enlarging the cavity from back to front; while at the same moment certain muscles draw down the elastic flooring (called the diaphragm)-which separates the circulatory and respiratory organs from those of digestion thus lengthening the box perpendicularly, so that the movement of respira tion increases its size

from back to front, from left to right, and from base to

summit.

When, however, we speak of" drawing in a breath," we do not suck in air, as we do fluids from a drinking-cup. The air in the atmosphere is some fifty miles high, and its own weight forces it down wherever there is a vacuum. Put an empty can on the floor, and the air at once, by its own weight, presses into and fills it. It

Fig. 23.

is just the same with the cavity of the chest, which is an empty can, which the pressure of the air fills when the mouth or nostrils are open.

If then by muscular action the cavity is enlarged, a greater volume of air rushes in, by its own weight, to fill the vacuum thus caused. Take a pair of bellows, and work the handles, drawing them asunder. As you pull the handles apart, the bellows do not suck in air, but it rushes in from the pressure of the atmosphere, pushing open the valve at A. You then draw the handles together, and force out the air by compression of the bellows.

We do much the same with the lungs. A set of muscles pulls the handles the ribs belupwards, opening our lows sideways, another set (like another hand) pulls down the diaphragm, lengthening the bellows downwards, and in rushes the air, till it has filled the enlarged cavity. Then the muscles relax, and let the diaphragm go back to its place, while another set of muscles pulls down the ribs,

Position of Ribs during inspiration.

Position of Ribs during expiration.

Fig. 24.

pressing out the air through the windpipe, just as we force it through the nozzle pipe of the bellows; but with this difference, we can squeeze the two sides of the bellows quite close together, so as to get out almost all the air, but we cannot squeeze the ribs on our two sides together till they touch, so there is always a cavity in which some air is left, called "residuary air;" and by extra exertion, we can draw them out wider than we usually do, and so admit a greater amount of air, as when we speak of "drawing a long breath." You will have noticed, how, when you run, your breathing is much more rapid; you pant, and sometimes "get out of breath;" that is because the circulation of the blood is quickened by the exercise of running, and therefore the blood keeps coming into the lungs in quicker succession. So a more constant change of air is required to purify it, and the respiration is of course quicker. This shews that breathing during bodily exercise does more good than when we are sitting still,

THE BRAIN AND SENSES.

The centre of all sensation, as well as of thought, is the brain, placed, as we have seen, in the basin of the skull. It is the unseen governor of all our actions, which decides what is best for us to do; it is like the master in the school watching everybody and everything, and giving his orders what it is to be done next, and when to do it, repressing an action here, giving a warning there. But how can it watch, and know what is going on outside the body, when it is shut up close in the brain-pan?

It has two windows, the eyes; and two open doors for noise to reach it, the ears; and two smellingbottles-the nostrils-to bring it bad smells; and two feelers the hands-to tell the size, and hardness, and shape of objects, and some delicate little organs for testing the food of the body, situated in the tongue and back of the mouth. These are the organs of sense,

-taste, touch, smell, hearing, and sight; and all these carry intelligence to the brain of all that is going on around it. Then it turns all these over in its mind, and tells the various parts of the body, what to do. Suppose your mouth feels something put into it, or a boy offers you something to eat that you have not eaten before, those little pimples (papilla) in your tongue touch it, and telegraph, as it were, by some little strings that go from them to the brain, that it is bitter or sweet, and the brain telegraphs back by other strings to the tongue to spit out the bitter stuff and to suck the sweet. These strings, as I have called them, are the nerves of the body, one set running from the organs of sense to the brain, and another from the brain to different parts of the body. The latter set communicate with the muscles, which are large bundles of flesh, having the power to coil themselves up, or contract (making themselves shorter and thicker) and the power to stretch themselves out again long and thin, or, as we say expand. When, for instance, your arm is stretched out at length, the muscle, A, in the upper part of your arm, is long and thin; but when you draw up your hand to touch your chin or your shoulder, that muscle is shorter and thicker. Stretch out your right arm, grasp the upper part of it with your other hand, and then double the right arm up Fig. 25. and scratch your chin, and you will feel the muscle, A, in your right arm thicken, and rise like a lump.

An illustration of a simple kind will shew you something of the way in which this is done. You have, no doubt, seen a snail crawling along the ground, it looks thin and long, with its horns stretched out. Just touch it with a bit of stick, it will draw in its horns, till they vanish in its head, and it will coil up into a thick, short ball.

It is very much the same with a muscle; the nerve is the stick that touches and gives it a sort of feeling that makes it coil up. You may have felt something of that kind all over your body when you have been frightened. I have seen a child coil himself up almost as a snail does at the sudden sight of something that has made him afraid.

TOUCH is the first sense used by a child, which begins to feel and handle, and so gets ideas of things before it realizes any of the other four senses. Touch is felt all over the body on the surface of the skin both inside and out; but it is not really the skin that feels, it is some little papillæ, as they are called, on the skin, which you may see on the palms of your hands and tips of your fingers, looking like little holes set in rows, like the little dents on the end of a thimble. It is to these that the nerves run, which carry ideas to our brains. The earliest impression we get is that a thing is hard or soft, according as it resists our pressure; then, that it is rough or smooth, cold or hot; next, that it is round or square and then we come to distinguish wood from iron, stone, or marble; to judge of size, and feel the difference between a piece of cord and a thread of fine silk. But all these judgments are the work of the brain, though practice teaches the fingers to detect even very minute differences, so that some blind men have so perfected their faculty of touch as to be able to tell accurately the colour of a number of skeins of wool by merely feeling them.

TASTE comes next, and is seated in the tongue and the palate at the back of the mouth. If you look at the surface of your tongue in a looking-glass, you will see some little dots on it like pins' heads, very much resembling those papilla I spoke of in the hand. There are two kinds of these:-one like those on the hands, and for the same purpose, viz., to convey the sensation of touch; the other for the purpose of taste. These two senses are both exercised by the tongue, but they are quite distinct. Put a marble into your mouth, your tongue will feel (by its organs of touch) that it is hard, round and