Lub rooj vag phaj yog tsim los ntawm cov hlau hauv cov xim dub.
Cov pob zeb ua ke tseem muab faib ua ob ntu, ib qho yog hlau phaj nrog ob qhov thiab qhov ntu me me ntawm cov khoom sib xyaw cylindrical hlau rau nws.
Lwm qhov yog ib ntu ntawm hollow cylindrical me hloov lub raj, qhwv nrog lub caij nplooj ntoo hlav dub.
