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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.

Paired facial bones

Paired facial bones include the lacrimals, nasals, zygomatics, maxillae, palatines, and inferior nasal conchae.

The lacrimal bones are the smallest and most fragile of the facial bones and they are paired left and right. They help form the back portion of each eye socket, and are rectangular in shape with two surfaces and four borders. The lacrimals contain a feature, called the lacrimal sulcus, which helps to form the lacrimal fossa. The lacrimal fossa is an opening for the lacrimal ducts that connects the corner of the eye to the nasal passage, and allows the tears from the eye to be channeled into the nose. The lacrimals make contact with the following bones: frontal, ethmoid, and maxillae.

Each nasal bone is a small rectangular bone. Together they form the bridge of the nose above the nasal cavity. They join with each other along the internasal suture and with the frontal bone above along the fron-to-nasal suture. The point of intersection of both sutures is called the nasion. Nasal bones make contact with the following bones: frontal, maxilla, and other nasals.

The zygomatics are the cheek bones. They have three major features that connect them with surrounding bones. The first is the frontal process. The frontal process forms the wall of the eye socket and connects above with the zygomatic process of the frontal bone. This section separates the eye orbit from the temporal fossa and has a projection called the marginal process. The third feature is the temporal process, and it connects with the zygomatic process of the temporal bone. Together these processes help to form the zygomatic arch which is the attachment for the masseter muscle, one of the major muscles used for chewing (mastication). The zygomatics make contact with the following bones: frontal, sphenoid, maxillae, and temporals.

The maxillae are also paired facial bones. They seat the upper teeth and form the upper jaw. In the upper part of the bone, the frontal process helps to form the nasal opening and ends by connecting with the frontal bone. A maxilla makes contact with the following bones: frontal, ethmoid, zygomatic, vomer, lacrimal, maxilla, nasal, palatine, mandible, and inferior nasal concha.

The palatines are paired left and right and connect with each other along the interpalatine suture. Both bones help form the back section of the hard palate as well as a portion of the nasal cavity. A palatine bone makes contact with the following bones: sphenoid, ethmoid, maxilla, vomer, and the other palatine bone.

The inferior nasal conchae are very thin and delicate paired bones that are elongated with curled-up ends. They are attached to the sides of the nasal cavity and connect to the maxilla and the bones that form the side wall of the nasal cavity. An inferior nasal concha makes contact with the following bones: ethmoid, lacrimal, maxilla, and palatines.

Unpaired facial bones

The unpaired facial bones include the vomer, mandible, and hyoid.

The vomer is a single flat bone. The upper part connects with the perpendicular plate of the ethmoid to form the nasal septum, the dividing wall that runs down the middle of the nose. The vomer makes contact with the following bones: sphenoid, ethmoid, palatines, and maxillae.

The largest facial bone is the mandible. It is the only bone of the skull that contains a movable joint. It is a strong, curved-shaped bone that encases the lower teeth in the alveolar process. It has rounded projections, called mandibular condyles on each end. The condyles link it to the temporal bone by hinges called the temporomandibular joints. The mandible makes contact with the following bones: temporals and maxillae.

The hyoid is a single small U-shaped bone that does not join with any other bone. It hangs from the styloid process of each temporal bone by means of ligaments.

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KEY TERMS

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Atlas—The atlas is not part of the skull. It is the first of the seven vertebrae of the neck and the one upon which the base of the skull rests. It is also the bone around which the skull rotates.

Axial skeleton—The skeleton associated with the central nervous system—the cranium, all the bones of the vertebral column, the ribs, and the sternum.

Brain—One of the two components of the central nervous system (CNS), the brain is the center of higher thought and emotion. It is responsible for the coordination and control of all body activities and for the perception and the interpretation of sense information.

Bregma—The point where the coronal and sagittal sutures of the skull meet.

Calvaria—The skull cap, roof of the skull, or cranium without the facial bones attached.

Condyle—A rounded enlargement that has an articulating surface.

Coronal suture—The suture between the two parietal bones and the frontal bone in the skull.

Cranial bones—The eight bones of the skull that form the braincase, which encloses the brain. They are the parietals, the temporals, the frontal, the occipital, the sphenoid, and the ethmoid bones.

Cranial nerve—In humans, there are 12 cranial nerves. They are connected to the brain stem and basically 'run' the head as well as help regulate the organs of the thoracic and abdominal cavities.

Cranium—The bones of the head.

Ethmoid bone—Skull bone located behind the nose.

External auditory meatus—The passage or canal in the skull for the tissues involved in hearing.

Facial bones—The 14 bones of the skull that form the face and jaw. The paired facial bones are the lacrimals, nasals, zygomatics, maxillae, palatines, and inferior nasal conchae. The unpaired facial bones are the vomer, mandible and hyoid.

Foramen—A hole in a bone, usually for the passage of blood vessels and/or nerves.

Foramen magnum—The large opening at the base of the skull that allows passage of the spinal cord.

Fossa—A pit, depression, or concavity, on a bone or formed from several bones.

Lambda—The point where the sagittal and lambdoid sutures of the skull meet.

Lambdoidal suture—The suture between the two parietal bones and the occipital bone in the skull.

Mandible—The lower jaw bone.

Maxillae—The upper jaw bones, connected to the orbit, hard palate and nasal cavity.

Neurocranium—The braincase of the skull.

Process—A general term describing any marked projection or prominence on a bone.

Sagittal suture—The suture between the two parietal bones in the top of the skull.

Sinuses—Air-filled cavities of the skull. The ethmoid sinus is in the ethmoid bone, the maxillary sinus in the maxilla, the frontal sinus in the frontal bones and the sphenoid sinus in the sphenoid bone.

Skull—All of the bones of the head.

Sphenoid bone—An irregularly shaped bone located in front of the occipital bone in the base of the skull.

Spinal cord—The elongated part of the central nervous system (CNS) that lies in the vertebral canal of the spine from which the spinal nerves emerge.

Squamosal suture—The suture between a temporal bone and a parietal bone in the skull.

Squamous—Adjective meaning scaly, flat, and plate-like.

Splanchocranium—The facial bones of the skull.

Suture—The saw-like edge of a cranial bone that serves as joint between bones of the skull.

Vertebrae—The flat bones that make up the spine or vertebral column. The spine has 33 vertebrae.

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Function

The skull encloses and protects the brain, provides a base for the attachments of the muscles of the head and neck, and provides a structural element to form the first sections of the respiratory and digestive tracts. The bones of the skull are very hard, and protection of the brain is undoubtedly their most important function.

Common diseases and disorders

Disorders and conditions that affect the skull can be divided into two broad categories—craniofacial anomalies and trauma-related injuries. The most serious injury is a skull fracture, meaning a break or rupture in any of the skull bones. Cranofacial anomaly is a general term that includes malformations diagnosed at birth (congenital anomalies) and developmental anomalies that result from the abnormal growth of the skull and face after birth.

• Basal skull fractures: These are most commonly extensions of fractures of the roof of the skull. The usual locations are the temporal bone, the orbital surface of the frontal bone, and the occipital bone.
• Jaw fractures: These include mandibular fractures and maxillary fractures.
• Facial fractures: These can range from the fracture of one facial bone, for example a zygomatic fracture, to more severe trauma such as facial smash, which involves multiple fractures and extensive disruption of the bony architecture of the head.
• Hemifacial microsomia: Hemifacial Microsomia is a condition that affects the growth of the face on one or both sides. The severity of this disorder can vary from mild to severe. Although different facial structures can be affected, the most common areas include the ear, the oral cavity, and the mandible.
• Goldenhar syndrome: This syndrome is a variant of hemifacial microsomia. The symptoms of this disorder match those of hemifacial microsomia with the addition of epibulbar dermoids, which are benign tumors located just inside the opening of the eye orbit.
• Treacher-Collins syndrome: Also known as mandibulofacial synostosis. The syndrome affects the size and shape of the ears, cheekbones, and upper and lower jaws.

Monique Laberge, PhD