But then again, all movement originates in muscles which are invisible to the audience. So, strictly speaking we are animating the effects that those muscles have on the visible skin. Just so, in animating breathing we are animating the effects that the movement of the diaphragm and other muscles have on the position of the visible sections of the body.
The main effect that the diaphragm has is that it subtly shifts the entire Line of the Body.
"The what?" I hear you cry.
The Line of the Body is a concept very similar to the Line of Motion (for a discussion of the Line of Motion refer to the Bowing tutorial). The basic idea is that the major masses of the body (notably the spine from the pelvis to the head) will often line up in straight lines or gentle arcs. This is because of the biomechanics of the body.
Often, even more limbs will line up parallel with the Line of the Body. This is particularly useful in animating in order to present a clear silhouette (for a discussion of the Silhouette, see the Stance tutorial (not yet written)).
The Line of the Body is not always a principle which is stressed in books about traditional animation. The reason for this is simple: It takes a conscious effort to break the line of the body in hand-drawn animation. You have to stop your pencil, change directions radically, and stuff like that. It's much easier to just draw a smoothly bending figure.
In computer animation, on the other hand, it is extremely easy to break the line of the body. You can shift the neck forward and the head back, and suddenly bam! you have a figure that looks slightly (or drastically) wrong.
So, keeping the Line of the Body in mind is more important than in hand-drawn animation. It is also more beneficial. Why? Because it is possible in computer animation to create an object which represents the line of the body, and then manipulate that (rather than dealing with bones on an individual basis). To see how, just move on to the next page.