The duck's mid-digestive tract contains a proventriculus and a ventriculus (gizzard).  The proventriculus is a thin-walled glandular section of stomach.  It secretes gastric juices that allow chemical digestion to take place in the proventriculus.  As food is broken down, it passes to the ventriculus.  The ventriculus is a region of the stomach with thick musculature that together with pieces of hard grit and pebbles, grinds large food into smaller pieces.  The gizzard is especially important because it processes plant materials with tough cellulose walls.  This well developed ventriculus is shared with crocodiles and alligators.  Some fossils indicate that herbivorous dinosaurs also had specialized gizzards(Kardong, 2002).