What is the function of muscles ribs and belly plate for a snake?

Snakes have a backbone with a great many vertebrae, some having about 300 whereas a man has only 33 or 34. To each vertebra is attached a pair of ribs. On the belly, from neck to tail, are as many broad overlapping plates as there are vertebrae, each attached to the ends of a pair of ribs and controlled by a system of muscles. Some thick-bodied snakes, such as rattlesnakes and the cottonmouth moccasin, can crawl slowly in a straight line along the ground, or even up a tree, by inching a few belly plates forward, then a few more, synchronized so that there appears to be wave after wave traveling from neck to tail. But for speedy travel they use the sinuous movement.