tree, and end in exceedingly small sacs or bags. The air in these sacs, or
air-cells, gives _oxygen_ to the blood in the tiny blood-vessels of the
lungs and takes from them the poison, _carbonic-acid gas_, water, and
impurities, which it carries back through the windpipe into the outside
air.

* * * * *

QUESTIONS ON THE LUNGS AND RESPIRATION.

Of what are the lungs composed?--"Of a soft, fleshy substance, full of
small air-cells and tubes."

Of what use are the lungs?--"They are the breathing machines of the body."

How do the lungs appear when healthy?--"Porous and spongy."

How does the air get into the lungs?--"The air flows through the nose and
mouth, into the windpipe and along the air-tubes, into the air-cells of the
lungs."

What does the air do in the lungs?--"It swells the lungs and causes the
chest to expand."

What do you mean by expand?--"To increase in size."

How is the air expelled from the lungs?--"The chest contracts and sends
the impure air through the tubes and windpipe, the nose and mouth, into the
atmosphere."