Sinus cavities also known as paranasal sinuses are air-filled void spaces found in pairs. Sinus is a Latin word which means a “fold” or “pocket”. These sinus cavities are found in cranial bones. Humans have four types of sinus cavities in pair forms. These are named based on their locations. Sinus cavities vary person to person according to location of the facial bones of the skull.
This image shows: 1-Frontal, 2-Ethmoid, 3-Sphenoid, and 4-Maxillary sinuses locations. 
- Frontal sinuses: The frontal sinuses are located superior to the eyes, in the low-center of the forehead, in the frontal bone. It makes the hard part of our forehead. The location can be further explained as, just above the nose in the forehead and exactly in front of the brain. These are separated from the brain by the back walls of the frontal sinuses bone. Frontal sinuses are the second largest cavities among all four. The size of frontal sinuses varies from person to person. These are generally absent at birth and start developing around five to seven years of age. The complete development is seen during teenage years. [1,2,3]
- Ethmoid sinuses: The ethmoid sinuses are located between the nose and the eyes. The exact location can be further explained as, immediately behind the eyes and behind the bridge of the nose and at the root of the nose. These are formed from several discrete air cells within the ethmoid bone. Ethmoid sinuses are found in multiple pairs in a range of seven to fifteen small cavities. These looks like a honeycomb. Each of these cavities has its own separate and tiny drainage which opens into the nose. Ethmoid sinuses are the smallest cavities among all four. The ethmoid sinuses are unique to individual person, similar as their fingerprint. These are generally present at birth and grow as we grow. The complete development is seen around the twelve years of age. The development varies from person to person. [1,2,3,4]
- Sphenoid sinuses: The sphenoid sinuses are located at the center of the skull, at deep in the skull behind the ethmoid sinuses and the eyes. They are located exactly at deepest inside the head. These sinuses lie under the pituitary gland in the sphenoid bone. The sphenoid sinuses are more difficult to examine and infections sometimes cannot identified because of their deep location. The Sphenoid sinuses appear like a small void at birth. These remains underdeveloped until the ages around three to five years. They begin to grow and slowly enlarge to the size of large grapes. They finally reach at full developmental stage during the late teenage years. [1,2,3]
- Maxillary sinuses: The maxillary sinuses are located under the eyes, above the cheeks, behind the cheekbones/maxillary bones. The Maxillary sinuses are extended from just below the eyes to above the upper teeth on both sides of the nose. In some people, the roots of the upper teeth often project into the floor of the maxillary sinus. These are largest in size and also called as maxillary antrechea. These sinuses are responsible for most discomfort. The drainage of maxillary sinuses is responsible behind this discomfort. The opening is found at the top of the sinus, which usually found at bottom in case of other sinuses, hence it becomes difficult for the cilia to move the mucus against the gravitational force. [1,2,3]
The image has shown the exact locations of all four types of sinuses from front as well as side view. These sinuses are joined to the nasal cavity by means of small orifices called ostia/ostium. The sinuses open into the nasal passages for air and mucus exchange and helps in removing dirty mucus from the sinus cavities via nasal passage.