<prompt> Sinus not present at bih isare: September 2010MaxillaryEthmoidalFrontalAll of the above  <response>: Ans. C: Frontal The paranasal sinuses are hollow, air-filled cavities that are lined by a mucous membrane. The ethmoid and maxillary sinuses are present at bih. The frontal sinus develops about the second year of life and the sphenoid about the third year. Sometimes, the frontal sinuses do not develop. 