1. The blood contains very small objects called blood corpuscles.

2. There are two kinds of corpuscles, red and white.

3. The red corpuscles carry oxygen.

4. The white corpuscles repair parts that are worn.

5. The corpuscles float in a clear, almost colorless fluid, which nourishes the body.

CHAPTER XI.

WHY THE HEART BEATS.

~1.~ If you place your hand on the left side of your chest, you will feel something beating. If you cannot feel the beats easily, you may run up and down stairs two or three times, and then you can feel them very distinctly. How many of you know the name of this curious machine inside the chest, that beats so steadily? You say at once that it is the heart.

[Ill.u.s.tration: THE HEART.]

~2.~ The Heart.--The heart may be called a live pump, which keeps pumping away during our whole lives. If it should stop, even for a minute or two, we would die. If you will place your hand over your heart and count the beats for exactly one minute, you will find that it beats about seventy-five or eighty times. When you are older, your heart will beat a little more slowly. If you count the beats while you are lying down, you will find that the heart beats more slowly than when you are sitting or standing. When we run or jump, the heart beats much harder and faster.

~3. Why the Heart Beats.~--We have learned in preceding lessons that the digested food is taken into the blood. We have also learned that both water and oxygen are taken into the blood. Thus the blood contains all the materials that are needed by the various parts of the body to make good the wastes that are constantly taking place. But if the blood were all in one place it could do little good, as the new materials are needed in every part of the body. There has been provided a wonderful system of tubes running through every part of the body. By means of these tubes the blood is carried into every part where it is required.

These tubes are connected with the heart. When the heart beats, it forces the blood through the tubes just as water is forced through a pipe by a pump or by a fire-engine.

~4. The Heart Chambers.~--The heart has four chambers, two upper and two lower chambers. The blood is received into the upper chambers, and is then pa.s.sed down into the lower chambers. From the lower chambers it is sent out to various parts of the body.

[Ill.u.s.tration: THE INSIDE OF THE HEART.]

~5. The Blood-Vessels.~--The tubes through which the blood is carried are called _blood-vessels_. There are three kinds of blood-vessels. One set carry the blood away from the heart, and are called _arteries_ (ar'-te-ries). Another set return the blood to the heart, and are called _veins_. The arteries and veins are connected at the ends farthest from the heart by many very small vessels. These minute, hairlike vessels are called _capillaries_ (cap'-il-la-ries).

~6. The Arteries.~--An artery leads out from the lower chamber of each side of the heart. The one from the right side of the heart carries the blood only to the lungs. The one from the left side of the heart carries blood to every part of the body. It is the largest artery in the body, and is called the _aorta_. Soon after it leaves the heart the aorta begins to send out branches to various organs. These divide in the tissues again and again until they become so small that only one corpuscle can pa.s.s through at a time, as shown in the colored plate.

(Frontispiece.)

~7. The Veins.~--These very small vessels now begin to unite and form larger ones, the veins. The small veins join to form larger ones, until finally all are gathered into two large veins which empty into the upper chamber of the right side of the heart. The veins which carry blood from the lungs to the heart empty into the upper chamber of the left side of the heart.

~8. What is Done in the Blood-Vessels.~--While the blood is pa.s.sing through the small blood-vessels in the various parts of the body, each part takes out just what it needs to build up its own tissues. At the same time, the tissues give in exchange their worn-out or waste matters.

The red blood corpuscles in the capillaries give up their oxygen, and the blood receives in its stead a poisonous substance called carbonic-acid gas.

~9. Red and Blue Blood.~--While in the arteries the blood is of a bright red color; but while it is pa.s.sing through the capillaries the color changes to a bluish red or purple color. The red blood is called _arterial blood_, because it is found in the arteries. The purple blood is called _venous blood_, because it is found in the veins. The loss of oxygen in the corpuscles causes the change of color.

~10. Change of Blood in the Lungs.~--Exactly the opposite change occurs in the blood when it pa.s.ses through the lungs. The blood which has been gathered up from the various parts of the body is dark, impure blood. In the lungs this dark blood is spread out in very minute capillaries and exposed to the air. While pa.s.sing through the capillaries of the lungs, the blood gives up some of its impurities in exchange for oxygen from the air. The red corpuscles absorb the oxygen and the color of the blood changes from dark purple to bright red again. The purified blood is then carried back to the upper chamber of the left side of the heart through four large veins. The blood is now ready to begin another journey around the body.

~11. The Pulse.~--If you place your finger on your wrist at just the right spot, you can feel a slight beating. This beating is called the _pulse_. It is caused by the movement of the blood in the artery of the wrist at each beat of the heart. The pulse can be felt at the neck and in other parts of the body where an artery comes near to the surface.

~12. How much Work the Heart Does.~--The heart is a small organ, only about as large as your fist, and yet it does an amount of work which is almost beyond belief. Each time it beats, it does as much work as your arm would do in lifting a large apple from the ground to your mouth. It beats when we are asleep as well as when we are awake. When we run we know by the way in which it beats that it is working very fast. Do you know how much a ton is? Well, in twenty-four hours the heart does as much work as a man would do in lifting stones enough to weigh more than one hundred and twenty tons.

~13. The Lymphatics.~--While the blood is pa.s.sing through the capillaries, some of the white corpuscles escape from the blood-vessels.

What do you suppose becomes of these runaway corpuscles? Nature has provided a way by which they can get back to the heart. In the little s.p.a.ces among the tissues outside of the blood-vessels very minute channels called _lymph channels_ or _lymphatics_ (lym--phat'-ics) begin.

The whole body is filled with these small channels, which run together much like the meshes of a net. In the centre of the body the small lymphatics run into large ones, which empty into the veins near the heart. This is the way the stray white blood corpuscles get back into the blood.

~14. The Lymph.~--In the lymph channels the white corpuscles float in a colorless fluid called _lymph_. The lymph is composed of the fluid portion of the blood which has soaked through the walls of the small vessels. The chief purpose of the lymphatics is to carry the lymph from the tissues back to the heart.

~15. Lymphatic Glands.~--Here and there, scattered through the body, are oval structures into each of which many lymphatic vessels are found to run, as shown in the ill.u.s.tration. These are called _lymphatic glands_.

[Ill.u.s.tration: LYMPH GLAND AND VESSELS.]

~16.~ The heart and blood-vessels are among the most wonderful structures in the body. It is no wonder, then, that alcohol, tobacco, and other narcotics and stimulants produce their most deadly effects upon these delicate organs. What these effects are we shall learn more fully in the next chapter.

SUMMARY.

1. The heart beats to circulate the blood.

2. The heart has four chambers, two upper and two lower.

3. There are tubes called blood-vessels which carry the blood to all parts of the body.

4. These tubes are connected with the heart.

5. The vessels which carry blood away from the heart are called arteries, and those which carry blood back to the heart are called veins.

6. The arteries and veins are connected by small tubes called capillaries.

7. The blood found in the arteries is red; that in the veins is dark blue or purple.

8. The color of the blood changes from red to blue in going through the capillaries. The change is due to the loss of oxygen.

9. In the circulation of the lungs, the blood in the arteries is blue, that in the veins, red.

10. The change from blue to red takes place while the blood is pa.s.sing through the capillaries of the lungs. The change is due to the oxygen which the corpuscles of the blood take up in the lungs.

11. The pulse is caused by the beating of the heart.

12. The heart does a great deal of work every day in forcing the blood into different parts of the body.

13. Some of the white blood corpuscles escape from the blood-vessels through the thin walls of the capillaries.

14. These corpuscles return to the heart through small vessels called lymph channels or lymphatics.

15. The lymphatics in many parts of the body run into small roundish bodies called lymphatic glands.

16. The object of the lymphatics is to remove from the tissues and return to the general circulation the lymph and white blood corpuscles which escape through the walls of the capillaries.