Can you tell any reason why the walls of the chest are elastic?
The lower wall or floor of the chest cavity is formed by a muscle called the _diaphragm_, which divides the trunk into two cavities, the chest and the abdomen.
~20.

How we Use the Lungs.~--Now let us notice how we use the lungs and what takes place in them.

When we use a pair of bellows, we take hold of the handles and draw them apart.

The sides of the bellows are drawn apart so that there is more room between the sides.

The air then rushes in to fill the space.