i thought i'd go into more depth with this, in chapter 20, when Grievous spreads his ribs, you don't see a tank with organs, you see what looks like a system of organs plated in metal. here's my two cents:
in the clone wars cartoon, Grievous' organs are all encased in some kind of metal and set into a somewhat natural position, then surounded by a metal chest tank with som movable ribs. They are all high up on his chest giving him a lot of mobility in his lower spine. in chapter 25, mace crushes the bottom of it, probably causing some internal bleeding resulting in some coughing and weakness. sometime before anakin and obi-wan reach the opening space battle. Grievous gets some emergency surgery wherein his organs are put in some kind of fluid (bacta?) and are then mounted in a tank that is blukier and takes up most of his spinal mass, makes him less flexible. He still coughes and weezes indicating that this is a temporary solution that is never corrected.