Birds do not have any teeth or maxillary bones of the jaw. However, they do have a pair of mandibles and incisive bones covered by a horny sheath of keratin. There is no arrangement or structure of their teeth. Birds swallow gizzard stones to aid in breaking down hard food. They have no incisors, canines, molars, rand pre-molars. The crop (enlargements of oesophagus) allows temporary storage of food for digestion and before mastication commences. Thus, the food then passes through the stomach called the proventiculus (true stomach). This is where secretions in the stomach occur. Mastication occurs in gizzard (muscular stomach) with stones. It grinds the food with stones and then it passes back to the proventriculus. They released food into the saliva and then the gizzards are thoroughly mixed with the food which is vital for its function. Gizzards in birds consist of tough layer of carbohydrate-protein complex called koilin. The main role of koilin is to protect the muscles from gizzards. Caeca store digestive bacteria for cellulose breakdown. Cell digestion and absorption of nutrients occur in small intestines. Bile from the liver and pancreatic juice enters through the digestive tracts which are situated at the far end of the duodenum and the jejunum.