In birds, it works like this. Birds swallow food and store it in their crop if necessary. Then the food passes into their glandular stomach, or proventriculus. Secretions are added here to the food. Then the food passes into the gizzard (also known as the "muscular stomach" or "ventriculus"). The gizzard can grind the food with previously swallowed stones and pass it back to the true stomach, and vice versa. In layman's terms, the gizzard 'chews' the food for the bird as it does not have teeth. Bird gizzards are lined with a tough layer made of the carbohydrate-protein complex "koilin", to protect the muscles in the gizzard.