How our teeth and jaws work together

Our teeth are made of a hard, bone-like substance. They are held by small sockets in our upper and lower jawbones called dental alveoli. Strong fibers anchor them in place. The upper and lower sets of teeth form two arches. When you close your mouth and bite, these two arches typically fit together with a slight overlap.

A full set of adult teeth includes 16 upper teeth and 16 lower teeth at the most. Each row has:

• Four incisors in the middle
• Two canine teeth next to the incisors; one to the left and one to the right
• Ten molars (back teeth); five on each side

The incisors have narrow edges, and the canine teeth are sharp at the tip. When we close our mouth to bite, the lower incisors are pushed up just behind the upper incisors. This helps the front teeth to tear off bite-sized pieces of food, for instance from a carrot or a sandwich.

The molars look different: They have wide chewing surfaces with raised cusps and small grooves. When we bite down, the cusps of the upper molars fit into the grooves of the lower teeth. That way, they can grind food when we chew.

Each tooth develops from a small bud that is anchored in the jawbone. When a tooth pushes through our gums and can be seen, we say that it has come in, or that it has “erupted.”

Baby teeth start coming in when a child is about five months old. This process is called “teething.” It can be painful and may sometimes cause a fever. The child’s first set of 20 baby teeth is usually complete by the age of two years.

At that point, toddler’s skulls have not yet reached their full size and are still growing. Because the baby teeth don’t get any bigger, they are gradually replaced by 32 larger teeth, starting when the child is about six years old. This way, adults have teeth that are the right size for their mouths. The process is usually completed by the time we reach 18 years of age, when the third molars (the wisdom teeth) come in. But not everyone has wisdom teeth in their jaw. And if they do, only some of these teeth may erupt or none at all.

The skull is made up of several plate-like bones. These include the upper jawbone (maxilla) and the lower jawbone (mandible). Our teeth are embedded in these bones.

The upper jawbone is rigidly fixed to the other bones of the skull, but the lower jawbone is not: A bit like a swing, it is attached to the temporal bones at the two joints of the jaw, allowing it to move.

Several muscles run between the lower jawbone and the skull. When these muscles are tensed (tightened), the lower jaw can be pulled up tightly against the upper jaw. This helps us to take a powerful bite. When the muscles are relaxed, our mouth opens. Some of the muscle fibers can actively help to open the mouth as well. Through the interactions of the various muscle groups, the lower jaw can also be moved slightly sideways and forward or backward. That makes it easier to grind food between our molars.