sinus

nouns

an abnormal passage leading from a suppurating cavity to the body surface
fistula, sinus
any of various air-filled cavities especially in the bones of the skull
sinus
is a type of (narrower)
(anatomy) a natural hollow or sinus within the body
bodily cavity, cavity, cavum
a wide channel containing blood; does not have the coating of an ordinary blood vessel
sinus, venous sinus