Anatomical formations, uas yuav tham hauv daim ntawv no, yog ib feem ntawm ob lub cev ntawm tib neeg lub cev: ua pa thiab digestive. Outwardly zoo li lub qhov lossis cov hlwb, lawv muaj cov qauv histological sib txawv thiab ua haujlwm sib txawv. Nyob rau hauv tus txheej txheem ntawm embryogenesis, lawv tsim los ntawm ob txheej kab mob - endoderm thiab mesoderm. Cov no yog tib neeg alveoli