Skull

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.

KEY TERMS

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.

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.

Resources

BOOKS

Bryan, Glenda J. Skeletal Anatomy. Philadelphia: W. B. Saunders Co., 1996.

Simon, Seymour. Bones: Our Skeletal System (Human Body). New York: Morrow (Harper-Collins), 1998.

ORGANIZATIONS

World Craniofacial Foundation. 7777 Forest Lane, Suite C-621, P.O. Box 515838, Dallas, TX, 75251-5838. (972) 566-6669. (800) 533-3315. <http://www.worldcf.org/index.html>.

OTHER

Hohne, K. Voxel-Man Junior: Interactive 3d: Anatomy and Radiology in Virtual Reality. Scenes: Part 1: Brain & Skull, CD-ROM for Windows. New York: Springer Verlag, 1998.

South East Missouri Hospital Website. Craniofacial anomalies. <http://www.southeastmissourihospital.com/health/kids/cranio~1/skullanat.htm>.

Monique Laberge, PhD

Skull

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, which are saw-like bony edges that join bones. Bones of the skull are usually classified as cranial bones, facial bones (splanchocranium), or as bones that form the braincase (neurocranium). 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 parietomastoid 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 fronto-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.

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.

KEY TERMS

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.

Resources

BOOKS

Bryan, Glenda J. Skeletal Anatomy. Philadelphia: W. B. Saunders Co., 1996.

Simon, Seymour. Bones: Our Skeletal System (Human Body). New York: Morrow (Harper-Collins), 1998.

ORGANIZATIONS

World Craniofacial Foundation. 7777 Forest Lane, Suite C-621, P. O. Box 515838, Dallas, TX, 75251-5838. (972) 566-6669. (800) 533-3315. 〈http://www.worldcf.org/index.html〉.

OTHER

Hohne, K. Voxel-Man Junior: Interactive 3d: Anatomy and Radiology in Virtual Reality. Scenes: Part 1: Brain & Skull, CD-ROM for Windows. New York: Springer Verlag, 1998.

South East Missouri Hospital Website. Craniofacial anomalies. 〈http://www.southeastmissourihospital.com/health/kids/cranio∼1/skullanat.htm〉.

skull The human skull has been rich in symbolism over the course of Western history. The skull as an emblem of death appeared as a result of the casualties brought on by the bubonic plague or the Black Death that ravaged the inhabitants of Europe throughout the fourteenth and fifteenth centuries. The Dance of Death, which portrayed men and women of all classes dancing with a skeleton, became a popular artistic motif.

The skull as an intimation of death was also an obvious aspect of sixteenth-century century fashion and art. In the early decades of the century, portraits had skulls printed on the back in order to symbolize the inevitable demise of the sitter. Men and women of the upper classes wore medallions engraved with skulls and ivory heads as jewelry. These objects normally portrayed a living face on one side and the human skull on the other side. The mementos were to remind both the wearer and the onlooker of death and their obligation to lead moral lives. The keepsakes also revealed the tension experienced by members of the upper classes who desired to display their wealth while appearing to obey the dictates of Christian piety.

In the eighteenth century, Caribbean pirates flew flags that featured the human skull and crossbones. The symbol, known as the Jolly Roger, may have been a corruption of joli rouge (pretty red), the original colour of the flag. The phrase also may have referred to a pirate known as ‘Ali Raja’ or simply have designated the British term for devil, vagabond, and rogue. The eighteenth-century French pirate Emanuel Wynne was the first to use the skull and crossbones. Wynne also displayed an hourglass on his flag to indicate that time was running out for his intended victims.

In the twentieth century, the Nazi SS adopted the human skull and crossbones, called Death's Head, as the badge of their organization. The Totenkopfverbande, or ‘Death's Head units’, were among the most élite and most feared members of the SS. Taking their name from the death's head symbol they wore on the right collar of their uniform, the Totenkopfverbande initially guarded concentration camps, but their role was expanded to include military service, most prominently in Poland and the Soviet Union, where they were responsible for killing Jews, soldiers, and civilians.

The human skull and crossbones have not simply been the mark of infamous men and women; more significantly, they have warned young and old alike of the dangers posed by poisons, toxins, and other hazardous materials. The poison symbol is successful because it is easily identifiable and can be understood by those who are unable to read.

Finally, the human skull figured in nineteenth-century scientific debates, especially those that concerned the attributes of the various races. Practitioners of craniometry, or the science of measuring skulls, maintained that they possessed empirical evidence that showed the superiority of Caucasians over other races. Nineteenth-century criminal anthropologist Cesare Lombroso employed craniometry to bolster his claim that criminals possessed an innate predilection to commit crimes. Lombroso said large jaws, low and narrow foreheads, and smaller cranial capacity were characteristics of natural-born criminals.

Like craniometry, phrenology flourished in the nineteenth century in the US and Europe as a way to determine the future successes and setbacks of men and women. The phrenologist professed that by studying the shape of a subject's skull he or she could determine the person's unique attributes and abilities. The size and form of the cranium revealed the character of the phrenologist's client.

Structure and evolution

Anatomically, the adult cranium is an extremely complex region of the skeleton. It comprises some 27 bones, forming two components, — the neurocranium (brainbox) and the splanchnocranium (face). The neurocranium consists of a base and vault whose side walls and roof complete the protective encasing around the delicate brain. The splanchnocranium houses and protects many of the organs of special sense — sight, smell, and taste — as well as accommodating the first part of both the respiratory and alimentary tracts. The upper part of this face-skeleton forms the orbits and nose, whilst the lower part, together with the mandible, forms the jaws. The face is probably the most expressive aspect of human anatomy and this is made possible by more than 50 muscles that attach to the underlying skull. Despite the mobility which these provide, with the exception of the small bones of the middle ear there are only two moving joints in the entire face region, and even they cannot move independently, as the two sides must work in unison. The free movement of the jaw at these temporomandibular joints is critical to the success of both phonation (speech) and mastication (chewing). In historical times, infection of the joint could lead to ankylosis (fusion), which was obviously not compatible with sustained life.

The skull is probably the region of the human body that is most avidly studied with respect to human evolution. The size of the neurocranium and its size relative to the splanchnocranium have been critical to the definition of the hominid lineage. The apes tend to possess a relatively smaller neurocranium and larger face than modern man and his immediate ancestors. The modern human face is suspended under the brain, having rotated backwards and downwards underneath the neurocranium, whilst the brain has extended above and over the top of the face, giving modern man his characteristically high forehead that protects the frontal lobes of the brain.

Development and growth

In the child, the neurocranium develops in parallel with the early maturation of the central nervous system, whilst the splanchnocranium lags behind and is more closely linked to the development of the teeth. Hence the ‘cutesy’ and non-threatening appeal of baby cartoon characters with big eyes, a big head, and a small face. This early development of the brain along with its protective case has enormous implications with regard to the evolution of man and encephalization in particular. This is the study of the degree to which the nervous system of the human fetus matures in utero. Compared with all other animals, including other primates, human babies are born with relatively big heads that house a large brain. During the early part of this century, before Caesarean sections were commonplace or indeed safe, pelvimetry (measurements of the maternal pelvis) was frequently employed in an attempt to predict the outcome of labour. Cephalopelvic disproportion — when the baby's head was too large to pass successfully through the birth canal — was a serious clinical concern, as it led to high levels of fetomaternal mortality. However, to overcome this transitory problem, the fetal skull retains considerable flexibility, as the bones are not fully formed and so can ride over each other as the head passes down through the birth canal. It is whimsical to suppose that perhaps new-born babies cry because they start life with a blinding headache!

Forensic applications

In physical and forensic anthropology, the skull is extremely informative with regard to the identification of sex, age at death, and racial assignation. It also plays a pivotal role in establishing the identity of the deceased — it has been described as the ‘bony core of the fleshy head and face’. There are various techniques available to the forensic investigator in this context. To confirm or reject a proposed identity, the skull of the deceased can be compared with a portrait or photograph of the person during life; the image of the skull, live on camera, is superimposed on the static photograph until the correct sizing and orientation are achieved. Then, using specific anatomical landmarks, the ‘degree of fit’ of the two images can be assessed. When the identity is entirely unknown, facial reconstruction is employed. This can be achieved either by computerized methods, where a standard face is wrapped around the scanned image of the skull, or alternatively by clay modelling. The latter approach requires a considerable degree of skill and generally leads to a more realistic image of the deceased. The function of facial reconstruction is not necessarily to produce a near-photographic image of the deceased, but more realistically to strike a chord with family or friends of a missing person. Such was the case of a young man who was found on the top of a Scottish mountain. He had been dead for nearly two years when he was found accidentally by two climbers. Investigations by the local police did not lead to any success in his identification and so it was decided to reconstruct his face. The outcome was shown on the television programme Crimewatch and was picked up by its French television counterpart. It was recognized by a French family who knew that their son had gone walking in Scotland before he went missing, but whose own investigations had been unsuccessful. A simple DNA test confirmed his identity. Such is the way, in osteology, that the dry bones have, of coming back to life to tell their own story.

Karol K. Weaver, and Sue M. Black

Bibliography

Aiello, L. and and Dean, C. D. (1990). An introduction to human evolutionary anatomy. Academic Press, London.
Cohen, K. (1973). Metamorphosis of a death symbol: the transi tomb in the late Middle Ages and the Renaissance. University of California Press, Berkeley.
Davies, J. D. (1955). Phrenology fad and science: a nineteenth-century American crusade. Yale University Press, New Haven.
Reichs, K. J. (1998). Forensic osteology: advances in the identification of human remains, (2nd edn). CC Thomas, Springfield, Illinois.
Scheuer, J. L. and and Black, S. M. (2000). Developmental juvenile osteology. Academic Press.

Skull X Rays

Definition

Skull x rays are performed to examine the nose, sinuses, and facial bones. These studies may also be referred to as sinus x rays. X-ray studies produce films, also known as radiographs, by aiming x rays at soft bones and tissues of the body. X-ray beams are similar to light waves, except their shorter wavelength allows them to penetrate dense substances, producing images and shadows on film.

Purpose

Doctors may order skull x rays to aid in the diagnosis of a variety of diseases or injuries.

Sinusitis

Sinus x rays may be ordered to confirm a diagnosis of sinusitis, or sinus infection.

Fractures

A skull x ray may detect bone fractures resulting from injury or disease. The skull x ray should clearly show the skull cap, jaw bones, and facial bones.

Tumors

Skull radiographs may indicate tumors in facial bones, tissues, or the sinuses. Tumors may be benign (not cancerous) or malignant (cancerous).

Other

Birth defects (referred to as congenital anomalies) may be detected on a skull x ray by changes in bone structure. Abnormal tissues or glands resulting from various conditions or diseases may also be shown on a skull radiograph.

Precautions

As with any x-ray procedure, women who may be pregnant are advised against having a skull x ray if it is not absolutely necessary. However, a lead apron may be worn across the abdomen during the procedure to protect the fetus. Children are also more sensitive to x-ray exposure. Children of both sexes should wear a protective covering (a lead apron) in the genital/reproductive area. In general, skull x-ray exposure is minimal and x-ray equipment and procedures are monitored to ensure radiation safety.

Description

Skull or sinus x rays may be performed in a doctor's office that has x-ray equipment and a technologist available. The exam may also be performed in an outpatient radiology facility or a hospital radiology department.

In many instances, particularly for sinus views, the patient will sit upright in a chair, perhaps with the head held stable by a foam vise. A film cassette is located behind the patient. The x-ray tube is in front of the patient and may be moved to allow for different positions and views. A patient may also be asked to move his or her head at various angles and positions.

In some cases, technologists will ask the patient to lie on a table and will place the head and neck at various angles. In routine skull x rays, as many as five different views may be taken to allow a clear picture of various bones and tissues. The length of the test will vary depending on the number of views taken, but in general, it should last about 10 minutes. The technologist will usually ask a patient to wait while the films are being developed to ensure that they are clear before going to the radiologist.

Preparation

There is no preparation for the patient prior to arriving at the radiology facility. Patients will be asked to remove jewelry, dentures, or other metal objects that may produce artifacts on the film. The referring doctor or x-ray technologist can answer any questions regarding the procedure. Any woman who is, or may be, pregnant should tell the technologist.

Aftercare

There is no aftercare required following skull or sinus x-ray procedures.

Risks

There are no common side effects from skull or sinus x ray. The patient may feel some discomfort in the positioning of the head and neck, but will have no complications. Any x-ray procedure carries minimal radiation risk, and children and pregnant women should be protected from radiation exposure to the abdominal or genital areas.

Normal results

Normal results should indicate sinuses, bones, tissues, and other observed areas are of normal size, shape, and thickness for the patient's age and medical history. Results, whether normal or abnormal, will be provided to the referring doctor in a written report.

Abnormal results

Abnormal results may include:

Sinusitis

Air in sinuses will show up on a radiograph as black, but fluid will be cloudy or white (opaque). This helps the radiologist to identify trapped fluids in the sinuses. In chronic sinusitis, the radiologist may also note thickening or hardening of the bony wall of an infected sinus.

Fractures

Radiologists may recognize even tiny facial bone fractures as a line of defect.

Tumors

Tumors may be visible if the bony sinus wall is distorted or destroyed. Abnormal findings may result in follow-up imaging studies.

Other

Skull x rays may also detect disorders that show up as changes in bone structure, such as Paget's disease of the bone or acromegaly (a disorder associated with excess growth hormones from the pituitary gland). Areas of calcification, or gathering of calcium deposits, may indicate a condition such as an infection of bone or bone marrow (osteomyelitis ).

Resources

ORGANIZATIONS

National Cancer Institute. Building 31, Room 10A31, 31 Center Drive, MSC 2580, Bethesda, MD 20892-2580. (800) 422-6237. 〈http://www.nci.nih.gov〉.

National Head Injury Foundation, Inc. (888) 222-5287. 〈http://www.nhif.org/home.html〉.

Radiological Society of North America. 820 Jorie Boulevard, Oak Brook, IL 60523-2251. (630) 571-2670. 〈http://www.rsna.org〉.

KEY TERMS

Radiograph— The actual picture or film produced by an x-ray study.

X ray— A form of electromagnetic radiation with shorter wavelengths than normal light. X rays can penetrate most structures.

Skull

The skull is the ossified, bony structure that encloses and protects the brain, internal extensions of sensory organs, and some facial structures. The skull is usually considered to consist of a cranial section (the cranium) and a facial region.

When a person has been dead for a long time, much of the body may have decomposed. One body part that will remain intact is the skull. Thus, it can become an important part of a forensic examination designed to determine the cause of death and, especially when the teeth are intact, to determine the identity of the deceased.

The cranium is a large, rounded, dome-shaped region of the skull that is composed of paired left and right frontal bones, parietal bones, temporal bones, and an unpaired occipital bone that forms the posterior base of the skull.

The bones of the cranium are fused by sutures—joints that run jaggedly along the interface between the bones. At birth, the sutures are soft, broad, and cartilaginous. This flexibility allows the skull to grow as the child matures. The sutures eventually fuse and become rigid and ossified near the end of puberty or early in adulthood. The coronal suture unites the frontal bone with the parietal bones. In anatomical nomenclature , the primary coronal plane is the plane that runs through the length of the coronal suture. At right angles to the coronal suture, the metopic suture separates the frontal bones in the midline region. The area formed by the fusion of the four bones near the top of the skull is termed the anterior fontanel or bregmatic fontanel (also commonly known as the topmost "soft spot" in a baby's skull). As with the sutures, the fontanels are soft at birth to permit growth. The fontanels shrink and close during childhood and are usually fully closed and hardened by young adulthood. The changing suture pattern can be used forensically to help estimate of the age of the deceased.

The sagittal suture unites the two large domedshaped parietal bones along the midline of the body. The suture is used as an anatomical landmark in anatomical nomenclature to establish what are termed sagittal planes of the body. The primary sagittal plane is the sagittal plane that runs through the length of the sagittal suture. Sagittal planes run anteriorly and posteriorly, are always at right angles to the coronal planes. The lambdoidal suture unites the left and right parietal bones with occipital bone. The area where the two parietals and the unpaired occipital bone meet is termed the posterior fontanel, lamdoidal fontanel, or lambda point (also commonly called the rear "soft spot" on a baby's skull). Like the anterior fontanel, the posterior fontanel closes and hardens with age, but is an important feature that allows growth of the skull during embryological and childhood development.

Along the sides of the cranium, the squamosal suture unites the temporal bone lying above (superior to) the ear and ear canal with the parietal bone. The anterior region of the temporal bones is united with the great wing of the sphenoid bone by continuation of the squamosal suture. The junction of the temporal, parietal, frontal and great wing of the sphenoid takes place at the sphenoid fontanel. The posterior border of the temporal bone on each side unites with the corresponding mastoid bone.

A mastoid fontanel lies at the posterior region of the side of the skull where the parietal, occipital, and mastoid bones unite. A mastoid process extends anteriorly toward the ear canal. A bony finger-like styloid process protrudes from the interior area to the external auditory opening (external auditory meatus).

The facial area of the skull is composed of the left and right zygomatic arches that extend from the lowest, most anterior margins of the temporal bone where the temporal bones articulate with the mandible (the temporomandibular joint) into the zygomatic bone itself. The zygomatic arches and zygomatic bones thicken to become prominent facial landmarks, forming the lower and side orbits of the eyes. The orbits are separated by a number of smaller bones in the nasal region including the ethmoid, lacrimal, and nasal bones. The maxilla and upper teeth form the most inferior region of the facial portion of the skull and are fused to the zygomatic bones.

The mandible is not considered a formal portion of the skull. In decayed bodies, the mandible becomes detached from the skull as the temporomandibular joint and supporting ligaments deteriorate.

A number of small openings allow nerves and blood vessels to penetrate the skull. These openings are termed foramen and are generally named for the bone they penetrate. For example, openings in the parietal bones are termed parietal foramen. A large foramen magnum at the rear and base of the skull allows the spinal cord to exit the skull into the vertebral column. Rounded, smooth, bony protuberances termed the occipital condyles lie on the anterior sides of the foramen magnum and help articulate the skull with the vertebral column.

The external occipital crest marks the posterior midline of the occipital bone. The crest runs from the foramen magnum upward (superiorly) to a bony knot-like external occipital protuberance.

Forensically, the skull can be used as the basis of a reconstruction, where layers of clay are applied to mimic the muscles and other tissue that formerly overlay the skull bones. When skillfully done, the resulting image offers an approximation of what the person may have looked like.

A less expensive and time-consuming method of reconstructing the facial appearance relies on photographing of the skull from different angles. The photographs can be cut out and mounted side-by-side to give a two-dimensional model that an artist can use to produce a drawing.

see also Bite analysis; Exhumation; Skeletal system overview (morphology).