So thorax is simply the term that refers to our chest portion of the body. Now, at the bottom of the lungs, we have this sheet of skeletal muscle we call our diaphragm. And the diaphragm not only separates the lungs from the stomach and our small intestine, large intestine, but it also actually moves up and down and that ultimately allows the process of breathing to actually take place. Now, because the diaphragm is a skeletal muscle, that means we have voluntary control of our diaphragm. It is controlled by the somatic nervous system. Now, our lungs are not actually by themselves, they are encased in this membrane we call the cirrus membrane and we call it a sears membrane because as we'll see in just a moment, it actually contains a fluid.