blood, therefore, must be cleansed, and the impure particles thrown away, and for this purpose the right ventricle takes charge of it and sends it to the lungs.

5. But how can the lungs purify the blood? In order to explain how, we must first describe the lungs themselves. The lungs are large sponge-like masses, which fill up nearly the whole cavity of the chest on each side of the heart. The air which we breathe passes into them; and through a vast number of little channels, which terminate in very minute aircells, it is brought in contact with every portion of the spongelike structure of the lungs.

6. The arteries which run from the right ventricle to the lungs branch out in millions of little tubes, not so large as a

Fig. 11.

CIRCULATION OF THE BLOOD THROUGH THE HEART AND LUNGS. ceives the impure ve

The Right Auricle re

nous blood from the body through the veins D. V. (the descending vena cava), and A. V. (the as cending vena cava).

At f is the opening through which the blood is forced into the right ventricle. Here is the tricuspid valve, which closes when the blood attempts to return.

The Right Ventricle, by contracting, forces the impure blood through the two branches of the pulmonary artery (P.A.) into both lungs. The lids of the valve at d

would close if the blood should attempt to return.

The blood, having been purified in the lungs and lost its dark color, is sent back to the heart through several canals, which form at length four large trunks called pulmonary veins, and these terminate in the Left Auricle by one

common opening, as seen at o. Only two of these pulmonary veins, P.V., P. V., are seen in the drawing.

The Left Auricle forces the blood into the Left Ventricle, through the opening g, where is placed the mitral valve to prevent its return. The Left Ventricle then forces the blood into the large artery called the aorta, which distributes it to all parts of the body, to be returned again to the Right Auricle through the veins A. V. and D. V.

When the blood is forced into the aorta, the lids of the valve at the entrance e fall back against the sides of the artery; but when the blood attempts to return, they come together and prevent it.

At a, b, and c are arteries branching off from the aorta to the head, arms, etc.

C

hair, all over the air-channels and air-cells of the lungs, and by them the blood is brought into close contact with the air we breathe. The impure particles of the blood, which are found chiefly in what we shall hereafter describe as carbonic acid gas, are now thrown into the air-cells and air-channels, and breathed out of the body through the mouth and nostrils in the form of vapor. At the same time, the remaining blood in the blood-vessels of the lungs takes in a portion of oxygen from the air, and receives thereby a bright red color, very different from the dark hue it had on entering the lungs. The blood, being thus purified, is returned to the heart, from which it is again sent forth through channels called arteries to every part of the body.

7. These arteries branch out into the smallest tubes that can be conceived, many of them invisible to the naked eye. They spread over every muscle and bone in the body, and throughout the skin, and from the blood thus received every part of the body is built up.

8. How wonderful it is that this single fluid-formed, perhaps, as it may be, out of some one simple material, such as the

Fig. 12.

THE HEART AND LUNGS, WITH THE OUTER COVER- ed to show the manner in which the ING OF THE LUNGS REMOVED.

blood circulates through the heart and lungs, and can not, therefore, be a strictly accurate drawing of those organs, Fig. 12 is a true representation of the heart in its natural position, showing also the lungs, after their front outer covering has been removed.

At 12 is the lur-ynx, or upper part of the windpipe; 11 is the tra'-che-ü, or windpipe, which conveys air to the lungs. Back of the upper part of the heart the tra'-che-a divides into the two bronchial tubes. These bronchial tubes branch out all over the lungs in minute subdivisions, and ultimately terminate in a vast number of minute air-cells, from the twentieth to the hundredth of an inch in diameter. These air-cells are so numerous that the amount of surface contained in their lining membrane in man has been computed to exceed 140 square feet!

At 1 is seen a part of the left auricle, most of this auricle being on the back part of the heart; 2, right auricle; 3, left ventricle; 4, right ventricle; 5, pulmonary artery; 6, aorta; 7, de scending vena cava; 13, upper lobe of right lung; 14, upper lobe of left lung; 16, 16, lower lobes of lungs.

potato-should be capable of being put to so many different uses! that out of it the brittle bones are made, the soft and pulpy brain, the hard and horny nails, the silky hair, the flesh, the fat, the skin, the bitter bile, the salt perspiration, every thing, in fact, from the sweat on the brow of labor to the dew on the lip of beauty! And yet such is the case. A mysterious power, engaged in building up and nourishing our bodies, is constantly working within us-a power which we can not fathom, which we can not comprehend. He only, who created it, knows the hidden causes of its action.

9. At the ends of the myriads1 of minute channels in which the arteries terminate, are the beginnings of other minute channels which receive the blood, and use it in repairing the body. All these minute hair-like blood-vessels, which connect the arteries and the veins, are called capillaries.5 Into the returning veins such particles of the body as are worn out and useless are carried by the capillaries, and thus the blood again begins to be filled with impurities; and by the time it gets back to the right auricle of the heart it is a dark and filthy stream, and must be again sent to the lungs to have its color and its purity restored.

10. Such is the circuit which the blood is constantly making, in carrying on the repairs of the system, and removing its waste particles. And all the blood in the body, which is estimated at nearly ten quarts in a person of full size, is supposed to pass through the heart as often as once in six or eight minutes. The heart is the great engine which keeps it in motion. And so long as life lasts, it keeps beating away, stroke after stroke-sometimes seventy or eighty, and sometimes more than a hundred and fifty times in a minute-forcing the blood onward through the arteries, dispersing it through the capillaries, receiving it back through the veins, never stopping a moment, and never wearying of the labor which God has assigned to it.

11. And all this it does without any thought of ours, and without any direction from us. Its action is involuntary— that is, it is not, like the movements of our hands and feet, dependent on our will. Our will can not even stop its action; for it works by a will greater than ours, by the will of

we are.

Him who made it, whose servant it is, and whose servants Yet its labors gradually wear upon it; it can not keep going forever. It seems strange to us that it should keep going so long. But though it may exert itself millions of times in our service, each pulsation brings it nearer and nearer to the end.

"Art is long, and time is fleeting;

And our hearts, though stout and brave,
Still, like muffled drums, are beating
Funeral marches to the grave."

1 AU'-RI-CLE (from the Latin auris, an ear), so called from its supposed resemblance to the ear of a quadruped.

2 CON-TRACTS', draws together.

3 VEN'-TRI-CLE; this word is applied, gen

erally, to cavities in animal bodies. There are ventricles in the brain.

4 MYR'-I-ADS, countless numbers.

5 CAP-IL-LA-RIES (from the Latin capillus, a hair), long hair-like tubes.

LESSON XIV.

A HYMN.

1. WHEN I with curious eyes survey
My complicated frame,
I read in every part inscribed
My great Creator's name.

2. He bade1 the purple flood of life
In circling streams to flow,
And sent the genial heat around,
Through every part to glow.

3. My heaving lungs, while they have power
To fan the vital frame,

Shall sing thy praises, O my God!
Thy wond'rous skill proclaim.

1 Pronounced BAD.

AIR AND EXERCISE.

LONDON QUARTERLY REVIEW.

SPECIAL attention should be given, both by parents and teachers, to the physical development of the child. Pure air and free exercise are indispensable, and wherever either of

these is withheld, the consequences will be certain to extend themselves over the whole future life. The seeds of protracted and hopeless suffering have, in innumerable instances, been sown in the constitution of the child simply through ignorance of this great fundamental physical law; and the time has come when the united voices of these innocent victims should ascend, "trumpet-tongued," to the ears of every parent and every teacher in the land. "Give us free air and wholesome exercise; give us leave to develop our expanding energies in accordance with the laws of our being; give us full scope for the elastic and bounding impulses of our youthful blood!"

LESSON XV.

ABUSES OF THE LUNGS.

1. As the chief office of the lungs is to purify the blood, through the medium of the air which is taken into them in breathing, it will readily be seen that if impure air be inhaled,' the blood will not be purified. Pure air is composed chiefly of two invisible gases, which are always mixed in exactly the same proportions-that is, a hundred pounds of pure air consist of twenty-one pounds of oxygen, and seventy-nine of nitrogen. The proportions are the same whether the air be collected on the top of high mountains, over marshes, or over deserts.

2. When this air is taken into the lungs, the blood sent there from the heart is purified in the following manner. The oxygen of the air, having a strong affinity for the blood -that is, having a strong tendency to unite with it-leaves the nitrogen, and, passing into the blood-vessels, mixes there with the blood and the chyle, and completes the process of changing the latter into pure blood. This is the first step in the process. The second is the following:

3. The waste particles of the body consist chiefly of a substance called carbon, which has a strong affinity for oxygen —that is, it unites readily with it. The union of carbon and oxygen forms what is called carbonic acid gas, which is a