Birds do not have lips nor teeth. A strongly cornified bill covers the jaws. A birds' stomach consists of a cranial part or glandular stomach (proventriculus) and a caudal part, the muscular stomach or gizzard (ventriculus). The latter partly replaces the teeth functionally. Glandular and muscular stomach are mostly separated by a variably developed intermediate region (isthmus). Other specifications are the presence of a crop (some birds), a cloaca and 2 ceca (some birds).