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 passing 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 spaces 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 illustration.  These are called lymphatic glands.

[Illustration:  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.