The cranium (skull) is the skeletal structure of the head that supports the face and protects the brain. It is subdivided into the facial bones and the brain case, or cranial vault. The facial bones underlie the facial structures, form the nasal cavity, enclose the eyeballs, and support the teeth of the upper and lower jaws.
The cranial vault surrounds and protects the brain and houses the middle and inner ear structures. This cavity is bounded superiorly by the rounded top of the skull, which is called the calvaria (skullcap), and the lateral and posterior sides of the skull. The bones that form the top and sides of the brain case are usually referred to as the "flat" bones of the skull.
In the adult, the skull consists of twenty-two individual bones, twenty-one of which are immobile and united into a single unit. The twenty-second bone is the mandible (lower jaw), which is the only moveable bone of the skull.
These twenty-two bones can be further divided into eight cranial bones and fourteen facial bones. The brain case consists of eight bones. These include the paired parietal and temporal bones, plus the unpaired frontal, occipital, sphenoid, and ethmoid bones.
The facial bones of the skull form the upper and lower jaws, the nose, nasal cavity and nasal septum, and the orbit. The facial bones include fourteen bones, with six paired bones and two unpaired bones. The paired bones are the maxilla, palatine, zygomatic, nasal, lacrimal, and inferior nasal conchae bones. The unpaired bones are the vomer and mandible bones. Although classified with the cranial bones, the ethmoid bone also contributes to the nasal septum and the walls of the nasal cavity and orbit.
Most cranial and facial bones are joined to each other by sutures, or immobile joints. The narrow gap between the bones is filled with dense, fibrous connective tissue that unites the bones. The long sutures located between the bones of the cranium are not straight but instead follow irregular, tightly twisting paths. These twisting lines serve to tightly interlock the adjacent bones, thus adding strength to the skull for brain protection.
This text is adapted from: Openstax, Anatomy and Physiology 2e, Section 7.2: The Skull
