James Anders, age 13, of Nogales, Ariz., for his question:

HOW MANY BONES ARE IN THE HUMAN SKULL?

Skull is a term applied to the bony frame of the head of man and other animals with backbones. It is actually a case made up of bones that enclose and protect the brain and also form the face. The human skull has 22 bones.

The adult human skull is divided into two regions: the cranial and the facial. They are classified as cranial or facial bones. The cranial region is the portion of the skull directly surrounding the brain while the facial region includes all the other bones of the skull.

Cranial bones include the two frontal bones which constitute the forehead and which fuse together in late adult life as well as the two parietal bones which constitute the top of the head and which in earlier childhood are separated from the frontal bones by a space called the anterior fontanel.

Cranial bones also include the single occipital bone, constituting the back of the skull which is pierced by a large opening through which the spinal cord enters the cranial cavity, and the two temporal bones which constitute the temples and the sides of the head and which bear the cheekbones.

Also included as cranial bones is the squamosal, which constitutes the sides of the head and which articulates with the jawbone; the petrosal bone, which contains the inner ear; the mastoid bone, which is situated behind the ear; and the tympanic bone, which surrounds the channel leading from the eardrum to the external ear. There are also a number of internal cranial bones.

Facial bones include the two nasals bones, which make up the upper portion of the nose bridge; two lacrimal bones which are located in each eye orbit; the maxillary bone which is the upper jaw; the mandible or lower jaw; two palatine bones of the hard palate; the vomer which is part of the nasal septum; and two inferior turbinates of the nose.

Possibly the most common skull disease attacking humans is sinusitis.

Cavities, which are lined with mucous membrane and which are known as sinuses, normally occur in the frontal bones, the internal sphenoid bones, the maxillary bone and the mastoid processes of the temporal bone. Each of these sinuses opens into the nasal cavity.

Sinuses are often infected by bacteria, causing inflammation and increased secretion of the mucus membranes and producing severe pain.

Acute sinusitis is treated with antibiotics such as penicillin. In many persons, however, sinus inflammation becomes chronic and is extremely difficult to eradicate.

Skull deformity occasionally occurs in the womb or during delivery and is often accompanied by brain damage. The skull is also subject to fracture and to tumors.

Animal skulls are shaped in ways that help the animal feed. Cats and wolves, for example, have long jaws and strong, sharp teeth to grasp and tear its prey, while a horse's broad jaw and fiat edge teeth are shaped for cutting and grinding grasses.