[Illustration: 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 pass 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 passing
through the small blood-vessels in the various parts of the body, each