The calvaria, also known as the skullcap or the calvarium, is the upper or superior part of the skull. It does not contain the bones that make up the jaw or the parts of the skull that make up the face. The calvaria is made up of four primary bone structures: the frontal bone, the two parietal bones, the two temporal bones, and the occipital bone. It is a thick and hard structure that primarily exists to protect the brain from harm. Its shape varies from person to person; in some people, the skullcap takes the shape of an oval, while in others it is almost perfectly circular.
The frontal bone part of the skullcap makes up the forehead and the tops of the eye sockets and nasal cavities. The two parietal bones together make up the sides and the top of the cranium, or upper part of the skull, excluding the lower jaw. The two temporal bones are lower on the sides of the skull; they support the temples on the sides of the face. The occipital bone is situated at the lower back section of the skull. Together, these bone structures make up the calvaria.
In infants, the skullcap is formed through a process referred to as intramembranous ossification in which bone develops from a tissue or membrane structure. The word ossification specifically refers to any process that involves a substance or structure changing into bone. The term intramembranous refers to the fact that the substance being turned into bone is some form of connective tissue as opposed to cartilage. Many bones, including much of the lower part of the skull, are formed through endochondral ossification, which is the formation of bone from cartilage. All of the different parts of the calvaria, however, start as soft and vulnerable membranous tissue that hardens into solid bone.
One feature of the calvaria that is present on the skulls of infants is the presence of fontanels, or soft spots. These allow the skull to flex and bend to some degree, allowing the infant to fit through the birth canal. Many parents are often concerned that their infant is at significant risk of harm because of the existence of the soft fontanels. This is not the case as the membranes that make up the fontanels are very durable and strongly resistant to damage. There are some conditions in which the fontanels are exceptionally large; sometimes, these never completely transition to hard bone.