Below is possibly the best literary description of a bowel movement that I've ever read. Okay, it's perhaps the only literary description of a bowel movement that I've ever read. It comes from Mario Vargas Llosa's In Praise of the Step Mother.
Don Rigoberto half closed his eyes and strained, just a little. That was all it took: he immediately felt the beneficent tickle in his rectum and the sensation that, there inside, in the hollows of his lower belly, something obedient to his will was...