lt 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 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