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 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 felt at the neck and in other parts of the body where an artery comes near to the surface.