How many of us can accurately describe our diaphragm- what it does, and where it is? Many of my students think that their diaphragm is in their stomach and that it and it alone is the muscle of breathing.
This Axis Scientific Lung Model ($200) make it very clear to both you and your student where the diaphragm is- it sits right below the lungs, at the bottom of the ribcage and attaches to the ribs. The confusion of the sensation of breathing into the stomach is due to the action of the contracted diaphragm descending and pushing against the viscera and abdominal wall (and lower back).
So, do I recommend this model? Absolutely- for the serious and scientific voice teacher, but not for the average consumer, simply due to expense. If you do want to get a lung model, pease get this version- it is well made, well-painted, comes with a manual explaining each part, and the separate parts easily come off and go back together. I own a cheaper version of this and it is dreadfully made. Spend the extra $50 and get this version if you are in the market for a model of the lungs!