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.