What is sinus..

The sinuses are holes in the skull between the facial bones. There are four large sinuses: two inside the cheekbones (the maxillary sinuses) and two above the eyes (the frontal sinuses). There are also smaller sinuses (ethmoidal and sphenoidal sinuses) located between the larger ones. The sinuses are lined with membranes that secrete antibody-containing mucus, which protects the respiratory passages from the onslaught of irritants in the air we breathe.