As an anthropometric term, is a straight line, point-to-point horizontal measurement running fore-aft the body.

the answer to your question is tooth decay.

when you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. this increases the space in your chest cavity, into which your lungs expand. the intercostal muscles between your ribs also enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.