Question

The ___________ is a strong wall of muscle that contracts to create a suction in the chest to draw in air and expand the lungs. When the muscle relaxes, the chest cavity decreases and the air is pushed out.

Answer 1

can anybody help me with this??

Answer 2

That's the "diaphragm" muscle.

Here's a picture that'll show you how the diaphragm works:
http://www.whitetigernaturalmedicine.com/wp-content/uploads/2011/09/breathing.jpg

You see,
when we breathe IN, the muscle CONTRACTS (tightens up).

And when we breathe OUT, the muscle RELAXES.

Answer 3

It is most likely the diaphragm muscle, but the intercostal muscles attached to the rib cage also contract to help create this negative pressure. Here is more information if you want
http://www.cliffsnotes.com/study_guide/Human-Respiratory-System.topicArticleId-8741,articleId-8708.html