Melissa Huebner, age 13, of Hutchinson, Kan., for her question:

WHAT IS THE PURPOSE OF OUR SINUSES?

Sinus is a cavity in the front of the skull. Humans have four groups of sinuses. Doctors sometimes call these groups the paranasal sinuses because they all connect with the nasal cavity.

The exact function of the sinuses is not known, but it is thought that by forming air spaces within the bones of the skull, they lighten and perhaps strengthen the skull. The sinuses also add a source of mucous to humidify the inhaled air.

There are four major pairs of sinuses, one half of each pair lying on either side of the midiine of the face. Their mucous membranes are a continuation of the nasal mucous membrane and they open into the nasal cavity by means of tiny openings called ostia.

The frontal sinuses lie in the bone above the eyes and behind the root of the nose. They vary greatly in size and sometimes are completely absent. their absence seldom seems to be noticed.

The ethmoid sinuses form a labyrinth of five to 15 small areas separated by tiny bony partitions at the root of the nose between the eyes. They surround the opening into the frontal sinus and extend to the rear to the sphenoid sinus.

The sphenoid sinus is in the center of the head and surrounds the pituitary gland.

The maxillary sinuses develop separately from the ethmoid and sphenoid sinuses and are located in the cheekbone under the eyes. Each one has roughly the shape of a pyramid although considerable variations occur. This sinus, because it is close to the roots of the teeth, can cause dental symptoms when it becomes infected. Conversely, infection of the roots of the upper teeth can produce an infection in the sinus.

In sinusitis, the inflamed mucous membranes in the sinuses become swollen, closing the opening and preventing infected material from draining out. When this happens, painful pressure builds up in the sinuses.

The sinuses are lined with the same kind of membranes as those that line the nose. Infections of the nose spread easily to the sinuses.

In addition to pain from pressure in the sinuses, other symptoms of sinus ailment may include dizziness and a running nose. Allergies, colds, influenza and many other diseases often result in sinus trouble.

Other factors that may make certain individuals more susceptible to sinusitis include climate, dampness, drafts and smoking.

The treatment of sinus trouble includes rest and liquid diet. Pain relieving drugs are helpful. Sometimes surgery may be used to puncture a maxillary sinus and permit proper drainage.

Sinus trouble is dangerous because it may serve as a center of infection and that spreads to other parts of the body, such as the eye or brain.

Sinuses occur only in mammals, birds and crocodiles. The huge sphenoid sinus of the elephant extends to the very back of the skull.