The 22 bones in two groups

The adult skull is made of 22 bones. Twenty-one of them are locked together by sutures into a single rigid unit. The twenty-second, the mandible, is the lower jaw and the only one that moves. The 22 fall into the two groups you met in section 1: 8 cranial bones, which enclose the brain, and 14 facial bones, which form the skeleton of the face.

Specimen still

Anterior view. Facing the skull: the patient's left is on your right.

A paired bone is really two bones, one on each side, built as mirror images of each other. The left and right parietal bones are separate pieces that meet at a suture on top of the head. A pair therefore counts as two. An unpaired bone is a single piece that lies across the midline, with a right half and a left half that match.

Some pairs touch each other in the midline, which makes them easy to mistake for one bone. The two nasal bones lie side by side on the bridge of the nose. The two maxillae meet below the nasal opening and along the middle of the palate. The two palatine bones meet at the back of the palate. Other pairs never touch: the temporal bones are a head's width apart. Tap left maxilla, then right maxilla, and find the seam between them.

Three sets of hard structures are closely tied to the skull but are left out of the 22. Knowing why keeps your count the same as the examiner's.

On the specimen the teeth are separate pieces, the upper teeth and the lower teeth, so that you can tell tooth from jaw. The upper teeth sit in the maxillae and the lower teeth in the mandible; a tooth socket is called an alveolus, and the ridge of bone that carries the sockets is the alveolar part of each jaw. The ossicles and the hyoid are not part of this specimen.

Add the associated bones and the totals you meet in other books make sense: 22 skull bones plus 6 ossicles is 28, and the hyoid makes 29. The number for the skull itself stays 22.

A list of 22 names is easy to lose. Count the bones along a fixed route instead, and count every pair as two.

Cranial bones, around the braincase (8).

1. Front: frontal (1).
2. Top and sides: parietal (2), then down to temporal (2).
3. Back: occipital (1).
4. Center of the floor: sphenoid (1), and in front of it ethmoid (1).

Facial bones, from the bridge of the nose downward (14).

1. Bridge of the nose: nasal (2).
2. Step back into the orbit: lacrimal (2).
3. Out to the cheek: zygomatic (2).
4. Down to the upper jaw: maxilla (2).
5. Behind the maxillae, in the palate: palatine (2).
6. Inside the nose, on each side wall: inferior nasal concha (2).
7. Inside the nose, in the midline: vomer (1).
8. Lower jaw: mandible (1).

Many students keep the cranial bones with the letters PEST OF: Parietal, Ethmoid, Sphenoid, Temporal, Occipital, Frontal. It works only if you remember that P and T each stand for two bones.

No single view shows all 22 bones. Some are on display from several directions; some are almost entirely hidden inside the face. Knowing what to expect in each view tells you where to look, and it stops you searching for a bone that cannot be seen from where you are.

Five bones are easy to forget because so little of them reaches the outside: the ethmoid, the vomer, the palatine bones, the inferior nasal conchae and the lacrimal bones. Select each one from the table above and rotate the specimen until you can see it.

The sphenoid is the opposite case. It appears in almost every view: in the temple from the side, at the back of the orbit from the front, between the palate and the occipital bone from below, and across the middle of the floor from inside. Yet each view shows only a small part of it. You never see the whole sphenoid on an assembled skull.

Specimen still

Inferior (external base) view. From below, mandible removed: the face is toward the top, the patient's left is on your right.