ter 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
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 passing 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 passes 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 passing 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 fe