The cranium can be defined as the skull without the mandible, but it is generally accepted to mean the back of the skull encasing the brain. The skull consists of 22 bones, eight of which are known as cranial bones. The others are called facial bones.
The eight cranial bones
The cranial bones are the parietal, occipital, temporal, frontal, sphenoid and ethmoid bones. There are two each of the parietal and temporal bones because they are found on the sides. The names of the bones provide information about their location.
The occipital bone is at the back and underside of the head, corresponding to the occipital lobe of the brain. The vertebral column passes through it giving the spinal cord access to the brain. Ridges called the superior and inferior nuchal lines run along the occipital bone for muscle attachment, making the bone important for head movement, position and stability.
Above the occipital bone, covering the top of the head, are the left and right parietal bones. Below the parietal bones and attached to the occipital bone are the left and right temporal bones. The lower jaw attaches to the temporal bones via the mandibular fossa. The ear canal enters through this bone at the tympanic and mastoid process. Another region is the zygomatic process that makes up part of the zygomatic arch, also known as the cheekbone. The area just in front of the ear is often called the temple.
The frontal bone makes up the front of the skull – the forehead and the upper eye and nasal cavities, extending back to the parietal bones. The superciliary arch forms the eyebrow arch. At the top of the nose between the eyes, the small elevation is called the glabella. The zygomatic process attaches the frontal bone to the zygomatic bone, a facial bone.
The ethmoid bone is a small spongy bone behind the nose and between the eyes. The sphenoid bone is another small bone, located the full width of the front of the skull and shaped like a butterfly. The wing-like processes fill a gap between the other cranial bones on the sides of the head where the temporal, parietal and frontal bones meet (the temple). The pituitary gland is found in the sella turcica, a saddle-like structure in this bone’s central body.
Thirteen facial bones are included in the cranium if using the definition of the skull minus the mandible. The mandible is the lower jawbone, containing the bottom teeth and attaching to the cranium via connective tissue and muscle. It can move freely for speech and chewing.
Other recognizable parts of the human anatomy of the face are the cause of the distinct features of other facial bones. The zygomatic bones create the cheekbones by creating the zygomatic arch with the temporal bones. The two tiny nasal bones that form the upper bridge of the nose are supported by the two inferior nasal conchae and the vomer bone that form and extend from the nasal cavity, attaching it to the cranial bones. The maxilla consist of two bones that fuse under the nose form the upper jaw. They contain the top teeth. The roof of the mouth is similarly formed by the fusion of the two palatine bones.
The eye sockets, or orbits, are formed by the orbital plates of a number of the bones – the frontal bone, maxilla, zygomatic bones and the nasal bones. But the two lacrimal bones form a small part of the orbits. Where these bones meet the maxilla, there is a depression for the lacrimal sac, or tear ducts.
For a visualization of how these bones connect, see the LearnBones website.