You may be wondering why there's a sodding great hole in my dragon's underbelly. Good question! The hole is there so you can look at the preserved internal organs. I want to put preserved organs in the chest cavity, in the style of an anatomical specimen.
Bird lungs are fascinating. The lung itself is quite small, but it has several large air sacs attached to it. These air sacs extend right down the body cavity and even into the bird's bones. This is a trait that birds inherited from their dinosaurian ancestors, so it stands to reason that a dragon which evolved from theropods in parallel with birds would have them too. It's not just theropods that had these lungs, analysis of sauropod bones shows that they had air sacs in their bones too - there are some cool open access journal articles on this phenomenon here and here. Having these air sacs makes a bird's bones lighter, and means the bird can oxygenate its blood more efficiently (it was also one of the reasons why sauropods could grow so enormous).
I want my dragon to have this same system of lungs and air sacs, preserved inside the chest. First up, I've got a pair of lungs attached to a trachea, and the system of veins that will attach the lungs to the heart. The veins are a separate piece which I'll put in place once I've painted the lungs.
The lungs are quite small, but they're supposed to be. Bird lungs are small because most of the air is stored in the air sacs.