The Cranium

THE CRANIUM

The cranium or skull is the main skeletal structure of the head. It is divided into the brain case

or cranial vault and the facial bones. Of all the 22 bones of the skull, 21 are immobile and fused

together. Only the mandible or lower jaw is separate and mobile. Figure 36 shows what the

anterior view of the cranium and facial bones looks like:

From the anterior view, the skull can be seen as mainly showing the facial bones. The upper

and lower jaw is formed as the maxilla and mandible, respectively, which houses the teeth.

The orbit is the eye socket that houses the eyeball and the muscles that move it. On the upper

side of the eye socket is the supraorbital margin, which contains a small opening called the

supraorbital foramen. This is where the nerve that serves the sensation of the forehead comes

through. There is an infraorbital foramen below the eye that allows the sensory nerve to the

face to pass.

Inside the nose is the nasal cavity, divided by the nasal septum, which is formed by part of the

ethmoid and vomer bone. The lateral wall of the nasal cavity has two bony plates, called the

inferior nasal concha and the middle nasal concha. The superior nasal concha is much smaller

and is a part of the ethmoid bone.

The lateral view of the skull involves primarily the cranial vault and the sides of the jaw. The

zygomatic arch is the bony arch that creates the cheek and is often referred to as the

“cheekbone.” It is formed by the temporal process of the zygomatic bone and the zygomatic

process of the temporal bone. This forms the arch. Its function is to attach bones that cause

the mandible to chew. The temporal fossa is located above this arch and the infratemporal

fossa is located below this arch. These also have muscle attachments that cause the mandible

to chew.

The brain case or cranial area is the part that protects the brain. The calvaria is called the

skullcap and forms the rounded top of the brain case. The floor of this case is the floor of the

skull. Inside the skull are three spaces, the anterior, the middle and the posterior cranial fossa.

Figure 37 shows the brain case bones:

There are eight bones of the brain case, including the following:

• Parietal bone—this is the upper lateral side of the skull. There are two of them that join

together at the top of the skull.

• Temporal bone—this forms the lower lateral side of the skull. It contains the ear canal

and a large prominence called the mastoid process, which serves as a muscle

attachment site.

• Frontal bone—this is the single bone that forms the forehead. The midline is a slight

depression called the glabella. It forms the supraorbital margin of the eye and brow

ridges. It also forms the floor of the anterior cranial cavity interiorly.

• Occipital bone—this is a single bone in the back of the brain case, forming the base of

the cranial cavity. It has protuberances that attach muscles that move the neck, in

particular the superior nuchal line. The foramen magnum that the spinal cord passes

through is a hole within this bone.

• Sphenoid bone—this is a single bone in the middle of the skull and is attached with

nearly every other bone of the skull, forming the base of the central skull. It contains

the sella turcica that houses the pituitary gland. Figure 38 illustrates the brain case and

the sphenoid bone: