Virtual Chicken: The Gizzard

The Gizzard or ventriculus is the muscular stomach where the grinding of food takes place. After being softened by the Proventriculus, food moves into the gizzard. It is a round organ with two lobes, one larger than the other. Each surface in the gizzard is covered by a shiny layer of tendinous tissue which is thicker at the center and thinner out by the edges. Under this outer layer are located very powerful masses of red muscle. The inner surface is lined with a creamy-colored, thick, horny tissue raised in ridges. This surface acts as the bird’s teeth and is made up of a strong, keratinized material called koilin that can withstand the potentially damaging effects of hydrochloric acid and pepsin coming from the proventriculus and the muscular action of grinding food. As the koilin wears down by the grinding action, glands in the gizzard produce more that passes to the surface where it hardens to replace the worn tissue. Though the gizzard consists of very powerful muscles, it alone can't pulverize everyt