When the right ventricle contracts, the blood is propelled into the arteries with so much force that it reaches the smallest ends of their most remote branches. This motion is called the pulse, which is merely the effect of the pulsation of the heart, and is quicker or slower according to the frequency of its contractions.

When the blood reaches the extremities of the arteries, Nature employs or uses it in the wisest manner. Certain vessels, of a particular description, absorb its watery, oily and saline (saltish) parts, to make new substances, as milk, fat, bile, &c. What is not used, flows into the extremities of the vessels called veins. These vessels gradually enlarge in size, till they form very large tubes, which convey the blood back to the right ventricle of the heart. It is then propelled into the pulmonary artery, which disperses it through the lungs, by means of innumerable small branches. It is there exposed to the action of the air, and carried through the pulmonary veins to the left auricle of the heart. This contracts, and sends it to the left ventricle, which also contracting,

pushes it into the aorta, whence it circulates through every part of the body.

For this complicated function, four cavities, as we have seen, become necessary, which are accordingly provided. Two of these, called ventricles, send out the blood, (one into the lungs, in the first instance, the other into the mass, after it has returned from the lungs.) Two others, called auricles, receive the blood from the veins one, as it comes immediately from the body, the other, as the same blood returns a second time, after its circulation through the lungs; for without the lungs, one of each would have been sufficient.

Such is the admirable circulation of the blood, in man and most animals.-Yet shall this wonderful machine go night and day, for eighty years together, at the rate of one hundred thousand strokes every twenty-four hours, having at each stroke a great resistance to overcome, and shall continue this action, for this length of time, without disorder and without weariness."

CHAPTER XVI.

FURNITURE, AND ITS USES-CONTINUED.

Purifying the blood. The lungs. Capacity of the lungs. Breathing. Uses of breathing. Nature of the air. Breathing air twice. Ventilation. Free motion of the lungs. Tight lacing.

WE are now prepared to enter upon another subject the study of the process by which the purity of the blood is promoted, in spite of the many causes which are continually in operation to render it impure, and unfit for its purpose.

PURIFYING THE BLOOD.-This is done by means of atmospheric air. But how is air to be introduced into the human body? Can we eat it? Can we drink it? Can it enter

by means of the eyes, or the

ears, or the nose? Not exactly in either of these ways. It can indeed enter into the nose; but without some other machinery, it would go no farther

than the throat, before it must return or pass out at the mouth. A little, it is true, is swallowed, both with our food and drink; but the quantity is not very considerable.

There is air, moreover, in almost every part of the body if there were not, we should soon be crushed. The atmosphere in which we live presses on us with a tremendous force, equal, it is said, in a middling sized man, to about 32,000 pounds to the whole body, or 15 pounds to every square inch. But as there is air within us, in all our solids and fluids, which presses outward, while the atmosphere presses in the other direction, we do not perceive its weight.

When I said the blood must be purified by the air, I meant in a manner much more rapid and effectual than could be done by its gradual introduction, and its circulation through the vessels. The manner of this change I will now endeavor to describe.

THE LUNGS.-The house I live in contains something like a great bellows, by whose curious operation the blood is cleansed and purified. This is contained in the upper story,

and fills nearly the whole of it, leaving only a small chamber at one side for the heart. It blows its blasts at the rate of twenty or twenty-five a minute in an adult-and at a greater rate still in children;-and it continues these blasts, whether we stand or sit, sleep or wake, as long as we live. I refer, as you will readily know, to the lungs.

I have already spoken briefly of the lungs. I have told you about the windpipe, which leads by its various branches to the ten thousand little cells within; and I have told you that all these cells were lined with mucous membrane-a membrane constructed like the skin, though thinner. But I believe I have not yet told you how much air these chambers of the human body will hold, nor how great are the superficial contents of the membrane on which the blood is spread to be purified.

So numerous are the pipes and cells in the lungs, that it is commonly thought the extent of the mucous membrane which lines them must be equal, at least, to the extent of the skin, which, in a middling sized adult, is about fifteen square feet. Over all this surface the fresh air which we breathe may circulate, so