OVERVIEW OF THE SKULL AND FACIAL BONES

The skull is supported by the vertebral column. It is divided into two parts: the cranium, which lodges and protects the brain, and the facial skeleton. The cranium is composed of eight bones: one frontal, two parietal, two temporal, one occipital, one sphenoid, and one ethmoid. The facial skeleton is composed of 14 bones: two nasal bones, two lacrimal bones, two zygomatic bones, two maxillae, two palatine bones, two inferior nasal conchae, one vomer, and one mandible.

INDIVIDUAL CRANIAL BONES

Frontal

The frontal bone consists of two portions: a convex portion, the squama which constitutes the region of the forehead and an orbital, or horizontal portion, which enters into the formation of the roofs of the orbits and the nasal cavity.

The external surface of the squama is smooth and convex anteriorly. Above the orbits are two arched elevations, the superciliary and arches which are joined by the glabella. The frontal sinuses are situated internally behind the glabella and the superciliary arches. The supraorbital margin that forms the upper boundary of each orbit is perforated by a supraorbital notch, or foramen. The supraorbital margins terminate in the zygomatic process that joins the frontal bone and the zygomatic bone.

The orbital portion consists of two orbital plates that are separated by a median gap, the ethmoidal notch. The orbital plates contribute to the formation of the roofs of the orbits and the nasal cavity. The ethmoidal notch is filled by the cribriform plate of the ethmoid bone.

Parietal

The parietal bones are two flat bones that unite to form the sides and the roof of the cranium. The four borders are: the sagittal (medial), squamous (lateral), frontal (anterior), and occipital (posterior). These borders form the frontal angle, the sphenoidal angle, the mastoid angle, and the occipital angle.

Temporal

Each of the two temporal bones consists of four divisions: the squama, mastoid, tympanic, and petrous portions.

The squama forms the anterior and superior portion of the temporal bone. The zygomatic process projects from the were part of the squama, which presents inferiorly a large oval depression, the mandibular fossa (glenoid fossa).

The mastoid portion constitutes the posterior part of the temporal bone. The mastoid process is a cone-shaped projection that provides attachment for several muscles. A coronal section of the mastoid portion reveals a large number of mastoid air cells and a tympanic antrum.

The tympanic portion is a curved plate lying below the squama and anterior to the mastoid process. The external acoustic (auditory) meatus (EAM) is situated in the tympanic portion. The styloid process is a slender, pointed projection attached to the inferior surface of the tympanic portion.

The petrous portion (pars petrosa) resembles a pyramid hewn from rock. The base of the pyramid is fused with the internal borders of the mastoid and squamous portions. The apex, which presents the anterior opening of the carotid (arterial) canal, is directed medially and anteriorly. In most skulls, the petrous portions form approximately a 45º angle with the side of the skull. The anterior and posterior surfaces of the pyramid meet superiorly to form a dense ridge that is referred to in radiography as the petrous ridge. The level of the petrous ridge is about 1¼ inches above the apex of the mastoid tip. On the posterior surface near the center is a large orifice, the internal acoustic meatus. Within the petrous portion are the auditory ossicles (the malleus, the incus, and the stapes), the cochlea, and the semicircular canals. The auditory ossicles and the cochlea are essential portions of the hearing mechanism and the semicircular canals are concerned with equilibrium.

The styloid process is a slender, pointed, process attached to the inferior surface of the tympanic portion.

Occipital

The occipital bone is at the posterior and inferior portion of the cranium. A large hole, the foramen magnum, pierces the occipital bone. The occipital bone is divided into four parts: the squama (behind the foramen magnum), the basilar portion (anterior to the foramen), and two lateral portions, one on each side of the foramen magnum. The curved expanded squama presents externally the external occipital protuberance (EOP). Situated on the internal surface of the squama, which is deeply concave, are the internal occipital protuberance and the grooves which lodge the blood sinuses. On the inferior surfaces of the lateral portions are the occipital condyles that articulate with the atlas (C-1) of the spine. The basilar portion extends anteriorly and superiorly from the foramen magnum and is attached to the body of the sphenoid bone.

Sphenoid

The sphenoid bone is at the base of the skull in front of the temporals and the basilar part of the occipital bone. In form, it resembles a bat with its wings extended. It is divided into a body, two greater wings, two lesser wings, and two pterygoid processes.

The cubical-shaped body is hollowed out internally to form two large cavities, the sphenoidal air sinuses (figure 2-35). On the superior surface of the body is the sella turcica (Turkish saddle). Situated within the saddle is the pituitary fossa, that receives the pituitary gland. Anteriorly, the sella turcica is bounded by an eminence, the tuberculum sellae. Anterior, to the tuberculum sellae, is a transverse groove called the optic(chiasmatic) groove, which terminates laterally in the optic foramen. Each optic nerve leaves the posterior aspect of the eye, goes through the optic foramen, crosses in the chiasmatic groove (one-half of the fibers do not cross), and terminates in the brain.

The lesser wings project laterally away from the anterior aspect of the body and terminate medially as the anterior clinoid processes. The optic foramen, situated in the back of the orbit, is part of the lesser wing.

The greater wings project anteriorly and laterally away from the body to form, in part, the posterior aspect of the orbit and a portion of the lateral walls and floor of the cranium.

The “legs” of the bat descend from the greater wings as the pterygoid processes.

Ethmoid

The ethmoid bone is between the two orbits. The bone is light and spongy in appearance and cubical in shape. It consists of four parts: a horizontal or cribriform plate, a perpendicular plate, and two lateral masses.

The cribriform plate is a perforated plate that is received into the ethmoidal notch of the frontal bone and the roof of the nasal cavity. The olfactory nerves pass from the brain through the perforations and into the nasal cavity. The crista galli projects superiorly from the midline of the cribriform, or horizontal plate. The fold of the dura mater dividing the hemispheres of the brain is attached to the crista galli.

The perpendicular plate  is a thin plate that projects inferiorly from the cribriform plate. The perpendicular plate is joined inferiorly with the vomer to help form the nasal septum.

The lateral masses, or labyrinths, consist of a large number of thin-walled cellular cavities. The spaces within these cells constitute the ethmoid sinuses. The superior and middle nasal conchae are spiral convoluted plates projecting downward from the inner walls of the masses.

Special Joints (Sutures) of the Cranial Bones

The cranial bones are held rigidly together by means of special interlocking, immovable joints known as sutures. The most important of these sutures are the sagittal suture (between the medial adjacent border of the two parietal bones), the coronal suture (between the posterior border of the frontal bone and the anterior borders of the two parietal bones), the lambdoidal suture (between the posterior-inferior border of the two parietal bones and the posterior-superior border of the occipital bone), and the squamous suture (between the lateral inferior border of the parietal and the upper squamous part of the temporal bone). The point of junction of the coronal and sagittal sutures is known as the bregma and indicates the position of the anterior fontanelle in the fetal skull, which represents an unossified membranous interval between these bones before ossification is complete (normally at about 18 months of age). The point of junction of the sagittal and lambdoidal sutures is the lambda, which indicates the position of the posterior fontanelle.

INDIVIDUAL FACIAL BONES

Nasal Bones

The nasal bones are two small, oblong, flat bones that constitute the upper portion, or bridge, of the nose. The point at which they articulate with the frontal bone is called the nasion.b. Lacrimal Bones. The lacrimal bones are two very thin, fragile bones situated at the front part of the medial walls of the orbits.c. Zygomatic Bones. The two zygomatic, or malar bones  form the prominence of the cheeks and contribute to the lateral walls and floor of the orbits. Each zygomatic bone has several processes: the frontal (which projects superiorly), the temporal (which projects posteriorly), and the maxillary (which projects anteriorly and medially).

Maxillae

The upper jaw is formed by the union of the two maxillary bones. Each maxilla assists in forming the boundaries of the nasal cavity (the floor and lateral wall), the oral cavity (the roof), and the orbit (the floor). Each maxilla consists of a body and four processes: zygomatic, frontal, alveolar, and palatine.

The body contains a large cavity, the maxillary sinus or antrum of Highmore. The anterior surface of the body is perforated by the infraorbital foramen, which transmits nerves and blood vessels. Medially and anteriorly, there is a sharp process that, with its fellow of the opposite side, constitutes the anterior nasal spine. The top of the body, the orbital surface, is a smooth, triangular surface that forms the greater part of the floor of the orbit. The nasal surface presents a large opening that leads into the maxillary sinus. The deep lacrimal groove is situated in front of the sinus opening. This groove, along with the lacrimal bone, constitutes the canal that transmits the nasolacrimal duct. Tears from the lacrimal sac of the eye are drained through the nasolacrimal duct into the nasal cavity.

The zygomatic process is a rough triangular eminence that joins the zygomatic bone. The frontal process projects superiorly from the body. It is connected to the frontal bone and one of the nasal bones. The alveolar process is a thick spongy ridge of bone that contains the cavities for the reception of the upper teeth. The palatine process forms a considerable portion of the floor of the nasal cavity and the roof of the mouth. When the two maxillae are joined together, the incisive foramen is seen on the midline at the anterior border of the palatine process.

Palatine Bones

The palatine bones are two L-shaped bones that contribute to the formation of the lateral wall of the nasal cavity and the roof of the mouth. Each palatine bone is divided into two parts, a perpendicular plate and a horizontal plate.

Inferior Nasal Conchae

The inferior nasal conchae are two scroll-shaped bones attached to the nasal surface of the body of the maxilla.

Vomer

The vomer is a flat bone that contributes to the formation of the nasal septum. It is situated on the midline and is joined posteriorly with the body of the sphenoid superiorly, with the perpendicular plate of the ethmoid, and, inferiorly, with the palatine processes of the maxillae and the horizontal plates of the palatine bones.

Mandible

The mandible, or lower jaw bone, is the largest bone of the face. It consists of a curved, horizontal body and two perpendicular rami. The upper portion of each ramus is divided by a deep semilunar depression, the mandibular notch, and is surmounted by two processes, the coronoid and the condylar. The coronoid process is situated anterior to the mandibular notch, and the condylar process is situated posterior to the mandibular notch. The internal (medial) surface of the ramus presents the mandibular foramen. This foramen communicates with a mandibular canal that lies within the ramus and the body of the mandible. This canal accommodates blood vessels and nerves to the teeth. The junction of the posterior border of the mandibular body and the inferior border of the ramus marks the angle of the mandible. The anterior tip of the body is called the mental protuberance (chin). A mental foramen is situated on either side of the chin. Along the superior border of the body is the alveolar ridge, which includes depressions for the reception of the lower teeth.

HYOID

The hyoid bone is a horseshoe-shaped bone below the mandible and above the styloid processes of the temporal bone. It provides surfaces for the attachment of some of the tongue muscles.THE TEMPOROMANDIBULAR JOINT The temporomandibular articulation (joint) is formed by the condyle of the mandible and the mandibular fossa of the temporal bone. It is of the ginglymoarthrodial variety of joint.

ASPECTS OF THE SKULL

Anterior Aspect of The Skull

When considered as a whole and viewed from the anterior or frontal aspect, the skull presents the following bony parts. The forehead is formed by the squama of the frontal bone and exhibits two arched elevations (the superciliary arches). Beneath each superciliary arch is a curved and prominent margin, the supraorbital margin, in which is the supraorbital notch, or foramen. The supraorbital margins are joined medially to form the glabella. The zygomatic bone forms the prominence of the cheek. The infraorbital margin is formed by the zygomatic bone and the maxilla. The eyes are embedded in orbits. Each orbit is a pyramid-shaped cavity that has four walls, an apex, and a base that is formed by the supraorbital and infraorbital margins. The frontal and ethmoidal bones form the superior wall, or roof. The maxillary, zygomatic, and palatine bones form the inferior wall, or floor. The zygomatic and splenoid bones form the lateral wall. The medial wall is formed by the maxillary, lacrimal, and sphenoid bones. At the apex of the orbit is the optic canal (a short, cylindrical canal that transmits the optic nerve and the ophthalmic artery). The inferior orbital fissure, which transmits nerves and blood vessels, is situated at the junction on the floor and the lateral wall of the orbit. At the junction of the roof and the lateral wall, near the apex of the orbit, is the supraorbital fissure, which transmits several nerves.

Lateral Aspect of The Skull

The frontal, parietal, occipital, temporal, sphenoidal, ethmoidal, lacrimal, nasal, maxillary, zygomatic, and mandibular bones are partly visible in the lateral view of the skull. The prominent zygomatic arch, which is formed by the temporal process of the zygomatic bone and the zygomatic process of the temporal bone, should be noted. Posterior to the zygomatic process is the opening of the external acoustic(auditory) meatus (EAM) of the temporal bone. Posteroinferior to the external acoustic meatus is the mastoid process.c. Inferior Aspect. The inferior, or basal, aspect of the skull presents the following bony parts, passing from the anterior to the posterior aspects. The palatine processes of the maxillae and the horizontal plates of the palatine bones form the hard palate, or roof, of the mouth. The bones that compose the hard palate fuse at the median palatine suture. The posterior portion of the vomer can be seen. The occipital bone, situated at the back and base of the skull, presents the foramen magnum, the squama, and the occipital condyles. Extending posteriorly and laterally from the foramen magnum, is a flat, expanded part named the squama, on which is the external occipital protuberance (EOP). The temporal bones are situated at the sides and base of the skull. The following structures of the temporal bones are visible. The apex of each mastoid tip is situated laterally. Anterior to the mastoid tip is the glenoid (or mandibular) fossa. The styloid process is anterior and medial to the mastoid tip. Extending anteromedially from the base of the styloid process to the basilar portion of the occipital bone is the petrous portion of the temporal bone.

INTERIOR OF THE CRANIAL CAVITY

The floor of the cranial cavity presents three fossae: the anterior, the middle, and the posterior cranial fossae.

Anterior Cranial Fossa

The floor of the anterior cranial fossa, which supports the frontal lobes of the brain, is formed by the frontal, ethmoid, and sphenoid bones. The lateral portions of the anterior fossa correspond to the roofs of the orbits. The medial portion corresponds to the roof of the nasal cavity.

Middle Cranial Fossa

The middle cranial fossa, which is deeper than the anterior fossa, consists of a central (or medial) portion and two lateral portions. Situated centrally is the body of the sphenoid. The lateral parts of the middle cranial fossa are of considerable depth and support the temporal lobes of the brain. They are bounded anteriorly by the posterior margins of the lesser wings of the sphenoid, and the orbital plates of the frontals. They are bounded posteriorly by the petrous portions of the temporals, and laterally by the squama of the temporals, the parietals, and the greater wings of the sphenoid.

Posterior Cranial Fossa

The dorsum sellae of the sphenoid bone and the petrous portions of the temporal bones separate the posterior cranial fossa from the middle cranial fossa. It is the largest and deepest of the three fossae and lodges the cerebellum, pons, and medulla oblongata.

THE NASAL CAVITY

The nasal cavity is divided into two nasal chambers by a thin, vertical wall, the nasal septum. The anterior portion of the septum is cartilage and the posterior portion is bone. The bony septum is formed anterosuperiorly by the perpendicular plate of the ethmoid bone and posteroinferiorly by the vomer. The maxilla and palatine bones form the floor. The four pairs of paranasal sinuses communicate with the nasal cavity. These sinuses are lined with ciliated mucous membrane and normally contain air. They are the sphenoidal sinuses, the ethmoidal sinuses, the maxillary sinuses, and the frontal sinuses. On the lateral wall, bounded above by the conchae (turbinate bones), are three irregular passages called the superior, middle, and inferior meatuses of the nose.

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