Paige Spradley, age 13, of Little Rock, Ark., for her question:

WHAT DOES A SINUS DO?

A common cold is any one of a number of distinct infections of the respiratory tract. It is the most widespread and prevalent of all diseases in the world. It has been found that colds cause more than 30 million illnesses each year in American children of preschool age alone. A cold is aninfection of the membranes of the nose and throat and sometimes the ears and sinuses.

A sinus is a cavity in the front of the skull. Each person has four groups of sinuses. Medical people sometimes  call these groups the paranasal sinuses because they all connect with the nasal cavity.

Each group of sinuses takes its name from the bones of the skull in which it is found. The frontal sinuses are in the frontal bone of the forehead just above the eyes. The maxillary sinuses, which are the largest of the paranasal sinuses, are located in the cheekbones on each side of the nose. The ethmoid sinuses are located just above the nasal cavity while the sphenoid sinuses are behind them.

If the front of the skull were made up of solid bone, the face would be very heavy and the chin would tend to rest on the chest. The sinuses actually help to lighten the skull and make it easier for a person to hold his head up and to balance it on the neck.

The sinuses are lined with the same type of membranes that line the nose. It is for this reason that infections of the nose will often easily spread to the sinuses.

There is an infection commonly called sinus trouble or sinusitis. In sinusitis, the inflamed mucous membranes in the sinuses become swollen, closing the openings and preventing infected material from draining out. When this happens, often a painful pressure builds up in the sinuses. The pain often occurs at the same time each day. Other symptoms can include dizziness and running nose. Allergies, colds and influenza often result in sinus trouble.

Certain climate, dampness, drafts and even smoking can cause many individuals to be more susceptible to sinus

Sinuses can be found only in mammals, birds and crocodiles. The large sphenoid sinuses of the elephant extend to the very back of the skull.

Doctors recommend rest and a liquid diet as the best treatment for sinus trouble. Pain stopping drugs are often prescribed, too. Sometimes even surgery is prescribed. If the doctor punctures a maxillary sinus, often better drainage is possible.

Sinus trouble is dangerous because it may spread to other parts of the body such as the eye or the brain.