Here is a radiograph of a normal cat thorax. This radiograph has great detail, and allows you to see a lot of the structures that we are looking for in the chest.
To orient you, this cat is lying down on his right side, and his head is pointing to the left in the picture.
Thoracic radiographs let us practice some basic interpretation skills. There are 4 densities that we can recognize on a radiograph - bone, water, fat, and air. The first thing that will help us identify things on this particular radiograph is to know that the denser something is, the whiter it is. Conversely, if it is black it is not dense at all (and in fact is probably air or air-filled).
In this thorax picture, there is a lot of black. All of the black areas are very normal, air-filled lungs. Having something air-filled is great, because it lets us see denser structures more easily. It's easier to see a fluid-filled structure (like an artery) against an air-filled structure than it would be to see the same artery over top of the liver, or muscle, because they are very close to the same density.
In the next picture I've thrown in some color, just to orient you to the 'big picture'. The blue bars follow the sternum. It's not a solid bone, but is composed of a series of smaller bones, just like the vertebrae in the spine (green lines). These little bones are, in fact, called sternebrae. The ribs (yellow lines) arch from the vertebrae down to the sternebrae. There is one rib per vertebra, 8 sternebrae, and several "floating" ribs that don't attach to sternebrae directly. Cool, eh? Also note how these bones stand out whiter than the muscles and fat of the soft tissue in which they are embedded.
The dotted red line is approximately where the diaphragm runs. The diaphragm separates the thorax (chest cavity) from the abdomen. Though it's not labeled, the big white area on the other side of the diaphragm is the liver.
So, what else is in the thorax? Look at the next photo. The structure labeled "1" is another dark (air-filled) structure, looks like it's probably tubular, runs from the head down to the middle of the lungs ... that's the trachea (windpipe).
Now we start looking at the denser structures within the chest. The trachea ends in the area of that big, whitish soft-tissue density in the middle of the chest, which happens to be the cat's heart (2). Another thick, tubular structure (outline by black dots, 3) with fluid density is leaving the area of the heart, going toward the spine, and runs along the spine out the back of the radiograph toward the back of the cat. This is the aorta, the main artery that takes oxygenated blood from the heart to the rest of the body.
It's mate is number 4, the main vein that returns de-oxygenated blood from the back half of the body to the heart for recirculation to the lungs. This vein is called the vena cava. And finally, number 5 is one of the main blood vessels that feeds the lungs. The heart receives the de-oxygenated blood from the vena cava, then pumps it out though the pulmonary arteries to the lungs. In the lungs the blood is oxygenated, then returns to the heart and is pumped back out the aorta to start its journey all over again.