Skull Health Article

Definition

The skull—or cranium—is the skeleton of the head. It includes the brain case and the bones of the face and jaw.

Description

The skull is the bony part of the head. It rests on the first vertebra of the spine, called the atlas. It belongs to the axial skeleton, meaning the skeleton associated with the central nervous system. That includes the skull, all the bones of the spine, the ribs, and the breastbone.

At birth, the skull of a baby is large when compared to the rest of the body. It is fairly compressible, with soft spots (fontanelles) that eventually harden around the age of eighteen months. In the adult, some bones of the skull are paired, meaning that there is a left and a right, while others are unpaired. They are connected by sutures, saw-like bony edges that serve as joints. Bones of the skull are usually classified as cranial bones, facial bones (splanchocranium), or as bones that form the braincase (neuro-cranium). There are eight cranial bones and 14 facial bones. The facial bone assembly also includes air-filled spaces located all around the nose and called the paranasal sinuses.

Paired cranial bones

The paired cranial bones include parietal bones and the temporal bones.

The parietal bones are paired. Shaped like curved plates, they form the bulging sides and roof of the cranium. Fused in the middle along the sagittal suture, they meet the frontal bone along the coronal suture in the front of the skull. The point at which the two sutures meet is called the bregma. In the back of the skull, the parietals connect with the occipital bone along the lambdoid suture. The intersection of the lambdoid and sagittal sutures is called the lambda. The parietals meet the temporal bones in the lower portion of the skull along the squamosal and parieto-mastoid sutures. On the external surface near the center of the bone is the parietal eminence, or bulge. Slightly behind the bulge is found the parietal foramen (a foramen is an opening through bone that serves as a passageway for blood vessels and nerves). The parietals make contact with the following bones: occipital, frontal, temporal, sphenoid, and parietal.

The temporals form parts of the sides and base of the cranium. They are also paired left and right. Each temporal bone consists of two major sections, the squamous portion, or flat section, and a very thick and rugged part, the petrosal portion. The petrosal portion contains the cavity of the middle ear and the three smallest bones of the body. The smallest bones are the bones of the ear: the malleus, the incus and the stapes. Located near the lower edge is a gap, the external auditory meatus, that leads inward to the ear. At the lower end of the petrosal portion is the slender styloid process. A process is a bony extension or projection on a bone and the styloid is of variable length, it serves as a muscle attachment for various thin muscles to the tongue and other structures in the throat. Another projection, the mastoid process, provides an attachment for some of the muscles of the neck. The temporals also house the internal structures of the ear and have depressions, called mandibular fossae, that assist in forming the shallow socket of the jawbone joint. A zygomatic process projects from the front of the temporal bone where it joins the zygomatic bone to help form the prominence of the cheek. The temporals make contact with the following bones: the zygomatics, parietals, mandible, occipital, and sphenoid.

Unpaired cranial bones

The unpaired cranial bones include the frontal bone, occipital bone, sphenoid bone, and ethmoid bone.

The frontal bone consists of two major sections, a vertical squamous portion that connects with the paired parietals along the coronal suture and forms the forehead, and two orbital plates, which form the left and right eye sockets (orbits). On its external surface, the squamous portion very often displays a left and right frontal eminence, or bulge. Additionally, the frontal bone has two supra-orbital ridges, which are bumps above each of the eye sockets. The frontal and nasal bones connect along the fronto-nasal suture. The frontal bone makes contact with the following bones: lacrimals, nasals, zygomatics, sphenoid, maxillae, parietals, and ethmoid.

The occipital bone forms the back of the skull and the base of the cranium. It consists of a large flattened section separated from a small thick portion by the foramen magnum—a large opening through which cranial nerves from the brain pass and enter the spine to become part of the spinal cord. On both sides of the foramen, there is a left and a right occipital condyle. A condyle is a rounded enlargement that has an articulating surface, or joint. The occipital condyles articulate with the first vertebra of the neck, the atlas (also known as the first cervical vertebra, or C1). The occipital makes contact with the following bones: parietals, temporals, sphenoid, and the atlas.

The sphenoid is a single bone that assists in connecting the cranial bones to the facial bones. It consists of a hollow part, which contains the sphenoidal sinus, and three pairs of bony projections. The first projections are called the lesser wings and they contain the optic foramen, through which the optic (or second) cranial nerve passes before reaching the eye. The second pair are called the greater wings, they assist in forming the orbital plates for each of the eye sockets. The third pair are the pterygoid processes that run along the back section of the nasal passages toward the palate and provide muscle attachments for the jawbones. The sphenoid makes contact with the following bones: vomer, ethmoid, frontal, occipital, parietals, temporals, zygomatics, and palatines.

Like the sphenoid, the ethmoid is a single bone that helps to connect the cranial bones to the facial bones. It consists of various plates and paired projections. The upper projections are the crista galli that assist in dividing the left and right frontal lobes of the brain. Side projections from the crista galli are the left and right cribriform plates which provide a seat for the olfactory nerves. The nerves go through these plates into the nasal cavity below. Directly under the crista galli is the perpendicular plate which connects with the vomer bone and helps to separate the left and right nasal passages. The ethmoid makes contact with the following bones: sphenoid, frontal, maxillae, palatines, vomer, and lacrimals.