Nasal cavities (8)
'nasal cavities' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for nostrils that I've seen before include "Holes in noses" , "Nose holes" , "Openings in the nose" , "Narial openings" , "Openings to the nasal cavity" .)
(Other definitions for sinuses that I've seen before include "Top spaces" , "Empty spaces" , "Cavities in skull connecting with nostrils" , "holes in the head" .)