It is absolutely necessary that its walls should become stiff enough to keep it open constantly and let the air flow backward and forward.

So we find growing up in the walls of this air pipe, cells which turn themselves into rings of gristle, or cartilage.
What the Breath Is.

As you know, your "breath," as you call it,--that is to say, the used-up air which you blow out of your lungs,--is different in several ways from pure, or unused air.

In the first place, it is likely to have a slight musky or mousy odor about it.

You never like to breathe any one else's breath, or have any one breathe in your face.