6.3: The Skull

By the end of this section, you will be able to:

• List and identify the bones of the brain case and face
• Locate the major suture lines of the skull and name the bones associated with each
• Define the paranasal sinuses and identify the location of each
• Name the bones that make up the walls of the orbit and identify the openings associated with the orbit
• Identify the bones and structures that form the nasal septum and nasal conchae, and locate the hyoid bone
• Identify the bony openings of the skull

The skull is the skeletal structure of the head that supports the face and protects the brain. It is subdivided into the facial bones (also called viscerocranium) and the cranium (also called neurocranium) (Figure \(\PageIndex{1}\)). The facial bones underlie the facial structures, form the nasal cavity, enclose the eyeballs, and support the teeth of the upper and lower jaws. The rounded brain case surrounds and protects the brain and houses the middle and inner ear structures.

In the adult, the skull consists of 22 individual bones, 21 of which are immobile and united into a single unit. The 22nd bone is the mandible (lower jaw), which is the only moveable bone of the skull.

Figure \(\PageIndex{1}\): Parts of the Skull. The skull consists of the rounded cranial bones that protect the brain and the facial bones that form the upper and lower jaws, nose, orbits, and other facial structures. (Image credit: "Bones of the Skull - Lateral View" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of image from Anatomy Standard under CC BY-NC 4.0.) )

The bones that make up the adult skull can either be paired, one on the right side and one on the left side, or singular. Singular bones cross the midline and often are formed by the fusion of the right and left bones present in the fetus. (Figure \(\PageIndex{2}\))

The paired bones of the cranium (there is a right and a left) are:

• Parietal bones
• Temporal bones

The singular bones of the cranium are:

• Ethmoid bone
• Frontal bone
• Occipital bone
• Sphenoid bone

The paired bones of the face (there is a right and a left) are:

• Inferior nasal conchae
• Lacrimal bones
• Nasal bones
• Zygomatic bones

The singular bones of the face are:

• Mandible
• Maxilla
• Palatine bone
• Vomer

Figure \(\PageIndex{2}\): Skull Bones. The adult skull is comprised of 22 individual bones. (Image credits: "Cranial Bones" and "Facial Bones" by Mark Schaeffer are licensed under CC BY-NC-SA 4.0, modification of images from Anatomy Standard under CC BY-NC 4.0.)

Bones of the Skull

The bones of the skull have complex shapes and they fit together like the pieces of a very intricate jigsaw puzzle. We will first examine the skull as a whole, from multiple viewpoints, and then we will look at specific features of each bone or group of bones. These specific features, called landmarks, are the ridges where muscles attach, the concave and convex surfaces of a joint, or the holes that allow for the passage of blood vessels or nerves.

Anterior View

The anterior view of the skull (Figure \(\PageIndex{3}\) is dominated by the forehead, formed mainly by the frontal bone, and the openings of the orbits and the nasal cavity. Also seen are the upper and lower jaws, with their respective teeth.

Figure \(\PageIndex{3}\): Anterior Views of the Skull. Anterior views of the skull shows the bones that form the forehead, orbits (eye sockets), nasal cavity, nasal septum, and upper and lower jaws. (Image credits: "Skull Bones Anterior View" by Mark Schaeffer are licensed under CC BY-NC-SA 4.0, modification of images from Anatomy Standard under CC BY-NC 4.0.)

The orbit is the bony socket that houses the eyeball and muscles that move the eyeball or open the upper eyelid. The walls of the orbit are formed by multiple bones. The majority of the socket consists of parts of the frontal bone, maxilla, sphenoid bone, and zygomatic bone. Small parts of the ethmoid bone, lacrimal bone, and palatine bone also contribute to the medial wall. (Figure \(\PageIndex{4}\))  Within the orbit are three openings that allow for blood vessels and nerves to pass:

• Inferior orbital fissure - This opening is found on the floor of the orbit between the sphenoid, maxillary, and zygomatic bones. Traveling through this fissure are nerves that provide sensory information to the skin that covers the temporal and zygomatic bones and that regulate blood flow to the nasal mucosa, allowing for the warming/cooling of air in the nasal cavity. This fissure also allows for the passage of blood vessels.
• Superior orbital fissure—This large, irregular opening into the posterior orbit is located on the anterior wall of the middle cranial fossa, lateral to the optic canal and under the projecting margin of the lesser wing of the sphenoid bone. Nerves to the eyeball and associated muscles, and sensory nerves to the forehead pass through this opening.
• Optic canal—This opening is located at the anterior lateral corner of the sella turcica. It provides for passage of the optic nerve into the orbit.

Figure \(\PageIndex{4}\): Bones of the Orbit. Several bones comprise the walls of the orbit. (Image credit: "Bones of the Orbit Close-up" by Mark Schaeffer is licensed under CC BY-NC-SA 4.0, modification of images from Anatomy Standard under CC BY-NC 4.0.)

Inside the nasal area of the skull (Figure \(\PageIndex{5}\)), the nasal cavity is divided into halves by the nasal septum. The upper portion of the nasal septum is formed by the perpendicular plate of the ethmoid bone and the lower portion is formed by the vomer bone. Each side of the nasal cavity is triangular in shape, with a broad inferior space that narrows superiorly. When looking into the nasal cavity from the front of the skull, two bony plates are seen projecting from each lateral wall. The larger of these is the inferior nasal concha, an independent bone of the skull. Two additional ridges located just above the inferior concha in the upper nasal cavity, the middle nasal concha and superior nasal concha, are parts of the ethmoid bone and are not visible here.

Figure \(\PageIndex{5}\): Bones of the Nasal Complex.  The nasal septum has two major bones - the ethmoid bone and the vomer - that divide the nasal cavity into left and right components.  The lateral walls of the nasal cavity contain the inferior nasal conchae, that serve to spin inhaled air.  (Image credit: "Bones of the Nasal Complex" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of images from Anatomy Standard.)

Lateral View of Skull

A view of the lateral skull is dominated by the large, rounded brain case, zygomatic (cheek) bones, and the maxilla and mandible with their teeth (Figure \(\PageIndex{6}\)). Connecting these areas is a bridge of bone called the zygomatic arch. All of the cranial bones can be seen from the lateral view: frontal, parietal, occipital, sphenoid (starting with the forehead and tracing a "C" the order of the bones follow the pneumonic "FPOTS") and the ethmoid is in the "eye".

Figure \(\PageIndex{6}\): Lateral View of Skull. The lateral skull shows the large rounded brain case, zygomatic arch, and the upper and lower jaws. The zygomatic arch is formed jointly by the temporal bone and the zygomatic bone. (Image credit: "Lateral View of the Skull" by Mark Schaeffer is licensed under CC BY-NC-SA 4.0, modification of images from Anatomy Standard under CC BY-NC 4.0.)

The zygomatic arch is the bony arch on the side of skull that spans from the area of the cheek to just above the ear canal. It is formed by the junction of two bony processes: a short anterior component, the temporal process of the zygomatic bone (the cheekbone) and a longer posterior portion, the zygomatic process of the temporal bone, extending forward from the temporal bone. Thus the temporal process (anteriorly) and the zygomatic process (posteriorly) join together, like the two ends of a drawbridge, to form the zygomatic arch. One of the major muscles that pulls the mandible upward during biting and chewing arises from the zygomatic arch.

Superior View

Viewed from above, only four skull bones are visible: frontal, right parietal, left parietal, and occipital. These four bones make up the calvaria, or the skull roof. In your anatomy lab you will most likely be able to remove the calvaria from the skull models so you can view the interior structures. (Figure \(\PageIndex{7}\)).

Figure \(\PageIndex{7}\): Superior View of Skull. The superior skull shows the large rounded brain case. In this view both the right and left parietal bones are visible, as well as the singular occipital and frontal bones. (Image credit: "Superior View of the Skull" by Mark Schaeffer is licensed under CC BY-NC-SA 4.0, modification of images from Anatomy Standard under CC BY-NC 4.0.)

Inferior View

The inferior aspect of the skull is complex in shape as it is the location of multiple foraminae for the passage of multiple blood vessels, nerves, and the spinal cord. Tracing the anterior border of the sphenoid bone gives you the junction between the bones of the face and cranium (Figure \(\PageIndex{8}\)). 

Figure \(\PageIndex{8}\): Inferior View of the Skull. Viewed from the inferior, the full width of the sphenoid bone can easily be seen.  The landmarks allowing passage of nerves, blood vessels, and the spinal cord as well as several muscle attachment points are shown; each is outlined in a color of its respective bone.  (Image credit: "Inferior View of the Skull" by Mark Schaeffer is licensed under CC BY-NC-SA 4.0, modification of images from Anatomy Standard under CC BY-NC 4.0.)

Foramina visible include:

• Foramen lacerum - This irregular opening is located in the base of the skull, immediately inferior to the exit of the carotid canal. This opening is an artifact of the dry skull, because in life it is completely filled with cartilage. All the openings of the skull that provide for passage of nerves or blood vessels have smooth margins; the word lacerum (“ragged” or “torn”) tells us that this opening has ragged edges and thus nothing passes through it.
• Foramen ovale - is an oval shaped opening in the sphenoid bone that allows for passage of blood vessels and nerves, the largest of which is the mandibular branch of cranial nerve V.
• Carotid canal - The carotid canal is a zig-zag shaped tunnel that provides passage through the base of the skull for one of the major arteries that supplies the brain. Its entrance is located on the outside base of the skull, anteromedial to the styloid process. The canal then runs anteromedially within the bony base of the skull, and then turns upward to its exit in the floor of the middle cranial cavity, above the foramen lacerum.
• Jugular foramen - In this external view the jugular foramen is located slightly posterior and lateral to the carotid canal, as the internal jugular vein runs adjacent to the internal carotid artery.  Because veins lack blood pressure, their walls tend to partially collapse instead of maintaining a round shape (as arteries do).  Thus, the jugular foramen is not a uniformly round hole.
• Foramen magnum - Being "magnum" this is the largest foramen in the skull.  The foramen magnum serves as the dividing line between the brain and the spinal cord, even though they are continuous with each other on a structural level.

Bones of the Neurocranium

The cranial bones (the neurocranium) contain and protect the brain. The interior space that is almost completely occupied by the brain is called the cranial cavity. The floor of the neurocranium is referred to as the base of the skull. The neurocranium consists of eight bones: the paired parietal and temporal bones, plus the unpaired frontal, occipital, sphenoid, and ethmoid bones.

Figure \(\PageIndex{9}\): Interior Views of the Skull. Viewed from the inferior, the full width of the sphenoid bone can easily be seen.  The landmarks allowing passage of nerves, blood vessels, and the spinal cord as well as several muscle attachment points are shown; each is outlined in a color of its respective bone.  (Image credit: "Inferior View of the Skull" by Mark Schaeffer is licensed under CC BY-NC-SA 4.0, modification of images from Anatomy Standard under CC BY-NC 4.0.)

Parietal Bones

The parietal bone forms most of the upper lateral side wall of the skull (parietal = "wall") (Figure \(\PageIndex{10}\)). These are paired bones, with the right and left parietal bones joining together at the top of the skull. Each parietal bone is also bordered anteriorly by the frontal bone, inferiorly by the temporal bone, and posteriorly by the occipital bone.

Parietal Bone Ex Situ

Parietal Bone In Situ

Figure \(\PageIndex{10}\): Parietal Bone. The right parietal bone in situ and ex situ. (Image credits: "Parietal Bone In Situ" and "Parietal Bone Ex Situ" by Sofia Elizondo is licensed under CC BY-NC-SA 4.0, modification of images from Anatomy Standard under CC BY-NC 4.0.)

Temporal Bones

The temporal bone forms the lower lateral side of the skull (Figure \(\PageIndex{6}\)). Common wisdom has it that the temporal bone (temporal = “time”) is so named because this area of the head (the temple) is where hair typically first turns gray, indicating the passage of time.

The temporal bone has three processes for muscle attachment (Figure \(\PageIndex{11}\)):

• the zygomatic process of the temporal bone projects anteriorly to meet the zygomatic bone and it forms the posterior portion of the zygomatic arch,
• the mastoid process projects inferiorly and can easily be felt on the side of the head just behind your earlobe,
• the styloid process is an elongated, downward bony projection posterior to the mandibular fossa on the external base of the skull; so named because of its resemblance to a stylus (a pen or writing tool). 

 

 

 

 

Figure \(\PageIndex{11}\): Temporal Bone and Landmarks. Shown in isolation in (C) lateral, (D) posterior, and (E) inferior views, the temporal bone surrounds the structures of the ear, your temple, and the posterior part of your cheek bone line. The temporal bone has an opening for the passage of the largest artery serving the brain - the carotid canal - and a notch that forms one wall of the jugular foramen.  On the external surface, sound waves enter the external acoustic meatus and the nerve carrying auditory information exits through the internal acoustic meatus.  (Image credit: "Temporal Bone and Landmarks" by Sofia Elizondo is licensed under CC BY-NC-SA 4.0, modification of images from Anatomy Standard under CC BY-NC 4.0.)

Important landmarks of the temporal bone, as seen in Figures \(\PageIndex{11}\) and \(\PageIndex{9}\), include the following:

• External acoustic meatus (ear canal) - This is the large opening on the lateral side of the skull that is associated with the ear.
• Internal acoustic meatus - This opening is located inside the cranial cavity, on the medial side of the petrous ridge. It connects to the middle and inner ear cavities of the temporal bone.
• Immediately inferior to the internal acoustic meatus is the large, irregularly shaped jugular foramen. Several cranial nerves from the brain exit the skull via this opening. It is also the exit point through the base of the skull for all the venous return blood leaving the brain. The venous structures that carry blood inside the skull form large, curved grooves on the inner walls of the posterior cranial fossa, which terminate at each jugular foramen.
• Carotid canal - The carotid canal is a zig-zag shaped tunnel that provides passage through the base of the skull for one of the major arteries that supplies the brain. Its entrance is located on the outside base of the skull, anteromedial to the styloid process. The canal then runs anteromedially within the bony base of the skull, and then turns upward to its exit in the floor of the middle cranial cavity, above the foramen lacerum.

Frontal Bone

The frontal bone is the single bone that forms the forehead. It has two main parts, the squamous part, which forms the large, flat, superior portion of the bone, and the orbit part.  The frontal bone is thickened just above each supraorbital margin, forming rounded brow ridges. These are located just behind your eyebrows and vary in size among individuals, although they are generally larger in males. Inside the cranial cavity, the frontal bone extends posteriorly. This flattened region forms both the roof of the orbit below and part of the floor of the cranial cavity above (Figure \(\PageIndex{12}\)).

Frontal Bone Ex Situ

Frontal Bone In Situ

Figure \(\PageIndex{12}\): The Frontal Bone Ex Situ and In Situ.  The frontal bone forms the anterior wall of the cranial cavity and the superior wall of the orbit. (Image credit: "Frontal Bone Ex Situ and In Situ" by Sofia Elizondo are licensed under CC BY-NC-SA 4.0, modification of images from Anatomy Standard under CC BY-NC 4.0.)

Occipital Bone

The occipital bone is the single bone that forms the posterior skull and posterior base of the cranial cavity (Figure \(\PageIndex{6}\); see also Figure \(\PageIndex{13}\)).  On the base of the skull, the occipital bone contains the large opening of the foramen magnum, which allows for passage of the spinal cord as it exits the skull. Superficially, on either side of the foramen magnum is an oval-shaped occipital condyle. These condyles form joints with the first cervical vertebra and thus support the skull on top of the vertebral column.

 

A. Occipital Bone Ex Situ

B. Occipital Bone In Situ

 

C. Occipital Bone Landmarks - Oblique View

D. Occipital Bone Landmarks - Posterior View

Figure \(\PageIndex{13}\): Occipital Bone and Landmarks.  The occipital bone forms the posterior and inferior portion of the cranium.  It serves as the connection point between the skull and the vertebral column, with the articular surfaces of the occipital condyles.  The foramen magnum is the large hole the spinal cord passes through.  (Image credit: "Occipital Bone and Landmarks" by Sofia Elizondo are licensed under CC BY-NC-SA 4.0, modification of images from Anatomy Standard under CC BY-NC 4.0.)

Sphenoid Bone

The sphenoid bone is a single, complex bone of the central skull (Figure \(\PageIndex{8}\)). It serves as a “keystone” bone, because it joins with almost every other bone of the skull. The sphenoid forms much of the base of the central skull and also extends laterally to contribute to the sides of the skull. Inside the cranial cavity, the right and left lesser wings of the sphenoid bone resemble the wings of a flying bird or a moth (Figure \(\PageIndex{9}\)). The sella turcica (“Turkish saddle”) is located at the midline, on the body of the sphenoid bone (Figure \(\PageIndex{14}\)).  The rounded depression in the floor of the sella turcica is the hypophyseal (pituitary) fossa, which houses the pea-sized pituitary (hypophyseal) gland and the posterior wall is the dorsum sellae.  The greater wings of the sphenoid bone extend laterally to either side away from the sella turcica, where they form the anterior floor of the middle cranial fossa. The greater wing is best seen on the outside of the lateral skull, where it forms a rectangular area immediately anterior to the temporal bone.

The sphenoid bone has several openings for the passage of blood vessels and cranial nerves, which include (Figure \(\PageIndex{14}\)):

• Optic canal—This opening is located at the anterior lateral corner of the sella turcica. It provides for passage of the optic nerve into the orbit.
• Superior orbital fissure—This large, irregular opening into the posterior orbit is located on the anterior wall of the middle cranial fossa, lateral to the optic canal and under the projecting margin of the lesser wing of the sphenoid bone. Nerves to the eyeball and associated muscles, and sensory nerves to the forehead pass through this opening.
• Foramen rotundum—This rounded opening (rotundum = “round”) is located in the floor of the middle cranial fossa, just inferior to the superior orbital fissure. It is the exit point for a major sensory nerve that supplies the cheek, nose, and upper teeth.
• Foramen ovale — This large, oval-shaped opening in the floor of the middle cranial fossa provides passage for a major sensory nerve to the lateral head, cheek, chin, and lower teeth.
• Carotid canal—This is the zig-zag passageway through which a major artery to the brain enters the skull. The entrance to the carotid canal is located on the inferior aspect of the skull, anteromedial to the styloid process (see Figure \(\PageIndex{6.a}\)). From here, the canal runs anteromedially within the bony base of the skull. Just above the foramen lacerum, the carotid canal opens into the middle cranial cavity, near the posterior-lateral base of the sella turcica.

 

A. Sphenoid bone ex situ

B. Sphenoid bone in situ

 

 

 

Figure \(\PageIndex{14}\): Sphenoid Bone and Landmarks. Shown in isolation in (C) superior and (D) posterior views, the sphenoid bone is a single midline bone that forms the anterior walls and floor of the middle cranial fossa. The hypophyseal fossa and the dorsum sellae are within the sella turcica and serve as the surrounds for the pituitary gland. The sphenoid has multiple openings for the passage of nerves and blood vessels, including the optic canal, superior orbital fissure, foramen rotundum, foramen ovale, and foramen spinosum.  (Image credit: "Sphenoid Bone and Landmarks" by Sofia Elizondo is licensed under CC BY-NC-SA 4.0, modification of images from Anatomy Standard under CC BY-NC 4.0.)

Ethmoid Bone

The ethmoid bone is a single, midline bone that forms the roof and lateral walls of the upper nasal cavity, the upper portion of the nasal septum, and contributes to the medial wall of the orbit (Figure \(\PageIndex{4}\). On the interior of the skull, the ethmoid also forms a portion of the floor of the anterior cranial cavity (see Figure \(\PageIndex{5}\)).

Within the nasal cavity, the perpendicular plate of the ethmoid bone forms the upper portion of the nasal septum (Figure \(\PageIndex{5}\). The ethmoid bone also forms the lateral walls of the upper nasal cavity. Extending from each lateral wall are the superior nasal concha and middle nasal concha, which are thin, curved projections that extend into the nasal cavity to increase the surface area of the nasal passages and to cause the incoming air to spin (see Respiratory System chapter).  

In the cranial cavity, the ethmoid bone forms a small area at the midline in the floor of the anterior cranial fossa. This region also forms the narrow roof of the underlying nasal cavity. This portion of the ethmoid bone consists of two parts, the crista galli and cribriform plates (Figure \(\PageIndex{15}\)). The crista galli (“rooster’s comb or crest”) is a small upward bony projection located at the midline. It functions as an anterior attachment point for one of the connective tissue covering layers of the brain. To either side of the crista galli is the cribriform plate (cribrum = “sieve”), a small, flattened area with numerous small openings termed olfactory foramina. Small nerve branches from the olfactory (smell) areas of the nasal cavity pass through these openings to enter the brain.

The lateral portions of the ethmoid bone are located between the orbit and upper nasal cavity, and thus form the lateral nasal cavity wall and a portion of the medial orbit wall. The ethmoid air cells, spaces creating a honeycomb-like structure in the lateral portions of the bone, form the ethmoidal sinuses (Figure \(\PageIndex{15}\)), one of the four pairs of paranasal sinus.  The paranasal sinuses also play a role in the respiratory system. 

 

 

Figure \(\PageIndex{15}\): Ethmoid Bone. The unpaired ethmoid bone is located at the midline within the central skull. It has an upward projection, the crista galli, and a downward projection, the perpendicular plate, which forms the upper nasal septum. The cribriform plates form both the roof of the nasal cavity and a portion of the anterior cranial fossa floor. The lateral sides of the ethmoid bone form the lateral walls of the upper nasal cavity, part of the medial orbit wall, and give rise to the superior and middle nasal conchae. The ethmoid bone also contains the ethmoid air cells. (Image credit: "Ethmoid Bone" by Sofia Elizondo is licensed under CC BY-NC-SA 4.0, modification of images from Anatomy Standard under CC BY-NC 4.0.)

Sutures of the Skull

A suture is an immobile joint between adjacent bones of the skull. The narrow gap between the bones is filled with dense irregular connective tissue that unites the bones. The long sutures located between the bones of the brain case are not straight, but instead follow irregular, tightly twisting paths. These twisting lines serve to tightly interlock the adjacent bones, thus adding strength to the skull for brain protection.

Figure \(\PageIndex{16}\) shows the major sutures of the skull. The two suture lines seen on the top of the skull are the coronal and sagittal sutures. The coronal suture runs from side to side across the skull, within the coronal plane of section. It joins the frontal bone to the right and left parietal bones. The sagittal suture extends posteriorly from the coronal suture, running along the midline at the top of the skull in the sagittal plane of section. It unites the right and left parietal bones. On the posterior skull, the sagittal suture terminates by joining the lambdoid suture. The lambdoid suture extends downward and laterally to either side away from its junction with the sagittal suture. The lambdoid suture joins the occipital bone to the right and left parietal and temporal bones. This suture is named for its upside-down "V" shape, which resembles the capital letter version of the Greek letter lambda (Λ). The squamous suture is located on the lateral skull. It unites the squamous portion of the temporal bone with the parietal bone. At the intersection of four bones is the pterion, a small, capital-H-shaped suture line region that unites the frontal bone, parietal bone, squamous portion of the temporal bone, and greater wing of the sphenoid bone. It is the weakest part of the skull. The pterion is located approximately two finger widths above the zygomatic arch and a thumb’s width posterior to the upward portion of the zygomatic bone.

Superior View

Lateral View

Posteriolateral View

Figure \(\PageIndex{16}\): Sutures of the Skull. Sutures represent the immovable joints found between skull bones.  In the superior view the coronal, sagittal and lambdoid sutures are visible. A lateral view of the skull shows the coronal, squamous, and lambdoid sutures.  A posterior view shows the sagittal, lambdoid, and squamous sutures. ("Sutures of the Skull" by Mark Schaeffer is licensed under CC BY-NC-SA 4.0, modification of images from Anatomy Standard under CC BY-NC 4.0..)

DISORDERS OF THE...

Skeletal System: Cranial Fractures

Head and traumatic brain injuries are major causes of immediate death and disability, with bleeding and infections as possible additional complications. According to the Centers for Disease Control and Prevention (2010), approximately 30 percent of all injury-related deaths in the United States are caused by head injuries. The majority of head injuries involve falls. They are most common among young children (ages 0–4 years), adolescents (15–19 years), and the elderly (over 65 years). Additional causes vary, but prominent among these are automobile and motorcycle accidents.

Strong blows to the brain-case portion of the skull can produce fractures (Figure \(\PageIndex{20}\)). These may result in bleeding inside the skull with subsequent injury to the brain. The most common is a linear skull fracture, in which fracture lines radiate from the point of impact. Other fracture types include a comminuted fracture, in which the bone is broken into several pieces at the point of impact, or a depressed fracture, in which the fractured bone is pushed inward. In a contrecoup (counterblow) fracture, the bone at the point of impact is not broken, but instead a fracture occurs on the opposite side of the skull. Fractures of the occipital bone at the base of the skull can occur in this manner, producing a basilar fracture that can damage the artery that passes through the carotid canal.

Figure \(\PageIndex{17}\): Skull Fracture. A 31-year-old male presented to the Emergency Room after falling from a 3-meter wall and landing on his face.  (Skull Fracture and Skull Fracture Scan by iEM is licensed under CC BY-NC-S 2.0)

A blow to the lateral side of the head may fracture the bones of the pterion. The pterion is an important clinical landmark because located immediately deep to it on the inside of the skull is a major branch of an artery that supplies the skull and covering layers of the brain. If the underlying artery is damaged, bleeding can cause the formation of a hematoma (collection of blood) between the brain and interior of the skull. As blood accumulates, it will put pressure on the brain. Symptoms associated with a hematoma may not be apparent immediately following the injury, but if untreated, blood accumulation will exert increasing pressure on the brain and can result in death within a few hours.

Facial Bones of the Skull

The facial bones of the skull form the upper and lower jaws, the nose, nasal cavity and nasal septum, and the orbit. The facial bones include 14 bones, with six paired bones and two unpaired bones (Figure \(\PageIndex{18}\)). The paired bones are the maxilla, palatine, zygomatic, nasal, lacrimal, and inferior nasal conchae bones. The unpaired bones are the vomer and mandible bones. Although classified with the brain-case bones, the ethmoid bone also contributes to the nasal septum and the walls of the nasal cavity and orbit.

Figure \(\PageIndex{18}\): Facial Bones Separated.  The facial bones contain six pairs and two singular bones. (Image credit: "Facial Bones Separated" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of images from Anatomy Standard under CC BY-NC 4.0.)

Maxillary Bone

The maxillary bone, often referred to simply as the maxilla (plural = maxillae), is one of a pair that together form the upper jaw, much of the hard palate, the medial floor of the orbit, and the lateral base of the nose (see Figure \(\PageIndex{2}\)). The curved, inferior margin of the maxillary bone that forms the upper jaw and contains the upper teeth (Figure \(\PageIndex{19}\)). Each tooth is anchored into a deep socket. On the anterior maxilla, just below the orbit, is the infraorbital foramen. This is the point of exit for a sensory nerve that supplies the nose, upper lip, and anterior cheek. On the inferior skull, the maxillary bone can be seen forming the anterior three-quarters of the hard palate (see Figure \(\PageIndex{8}\)). The hard palate is the bony plate that forms the roof of the mouth and floor of the nasal cavity, separating the oral and nasal cavities. On the lateral side of each maxillary bone is where the maxilla articulates (connects) with the zygomatic bone. 

 

 

Figure \(\PageIndex{19}\): Maxillary Bone. The maxillary bone forms the upper jaw and supports the upper teeth. Each maxilla also forms the lateral floor of each orbit and the majority of the hard palate. (Image credit: "Maxillary Bone" by Sofia Elizondo is licensed under CC BY-NC-SA 4.0, modification of images from Anatomy Standard under CC BY-NC 4.0.)

Palatine Bone

The palatine bone is one of a pair of irregularly shaped bones that contribute small areas to the lateral walls of the nasal cavity and the medial wall of each orbit. The largest region of each of the palatine bone is the horizontal plate that forms the posterior aspect of the roof of your mouth. The plates from the right and left palatine bones join together at the midline to form the posterior quarter of the hard palate (see Figure \(\PageIndex{20}\) and Figure \(\PageIndex{8}\)). Thus, the palatine bones are best seen in an inferior view of the skull and hard palate.

Palatine Bone Ex Situ - inferior view

Palatine Bone Ex Situ - anterior view

Palatine Bone Ex Situ - Angled view

Palatine Bone In Situ - inferior view

Figure \(\PageIndex{20}\): Palatine Bone - Ex Situ and In Situ.  The palatine bone has a "U" shape and form the posterior aspect of the hard palate (posterior to the maxilla).

HOMEOSTATIC IMBALANCES

Cleft Lip and Cleft Palate

During embryonic development, the right and left maxilla bones come together at the midline to form the upper jaw. At the same time, the muscle and skin overlying these bones join together to form the upper lip. Inside the mouth, the palatine processes of the maxilla bones, along with the horizontal plates of the right and left palatine bones, join together to form the hard palate. If an error occurs in these developmental processes, a birth defect of cleft lip or cleft palate may result.

Cleft lip is a common development defect that affects approximately 1:1000 births, most of which are male. This defect involves a partial or complete failure of the right and left portions of the upper lip to fuse together, leaving a cleft (gap).

 

 

 

 

Figure \(\PageIndex{21}\): Illustrations and photographs of cleft palate and cleft lip.  (Image credits: "Infant with Cleft Palate Illustration"  and "Infant with a Cleft Lip" by Centers for Disease Control and Prevention is in the public domain, via Wikimedia Commons; photograph of cleft palate by the Trinity Care Foundation is licensed under CC BY-NC-ND 2.0; "Infant with a Cleft Lip" by Perryrosen1 is licensed under CC BY-SA 4.0, via Wikimedia Commons.)

A more severe developmental defect is cleft palate, which affects the hard palate. The hard palate is the bony structure that separates the nasal cavity from the oral cavity. It is formed during embryonic development by the midline fusion of the horizontal plates from the right and left palatine bones and the palatine processes of the maxilla bones. Cleft palate affects approximately 1:2500 births and is more common in females. It results from a failure of the two halves of the hard palate to completely come together and fuse at the midline, thus leaving a gap between them. This gap allows for communication between the nasal and oral cavities. In severe cases, the bony gap continues into the anterior upper jaw where the alveolar processes of the maxilla bones also do not properly join together above the front teeth. If this occurs, a cleft lip will also be seen. Because of the communication between the oral and nasal cavities, a cleft palate makes it very difficult for an infant to generate the suckling needed for nursing, thus leaving the infant at risk for malnutrition. Surgical repair is required to correct cleft palate defects.

Zygomatic Bone

The zygomatic bone is also known as the cheekbone. Each of the paired zygomatic bones forms much of the lateral wall of the orbit and the lateral-inferior margins of the anterior orbital opening (Figure \(\PageIndex{2}\)). The short temporal process of the zygomatic bone projects posteriorly (Figure \(\PageIndex{22}\), where it forms the anterior portion of the zygomatic arch (Figure \(\PageIndex{3}\)).

 

Zygomatic Bone Ex Situ

Zygomatic Bone In Situ

 

Zygomatic Bone - Anterior View

Zygomatic Bone - Lateral View

Figure \(\PageIndex{22}\): The zygomatic bone is the lateral facial bone.  Its temporal process articulates with the temporal bone to form the zygomatic arch. (Image credits: "Zygomatic Bone Ex Situ and In Situ" and "Zygomatic Bone Anterior and Lateral View" by Sofia Elizondo are licensed under CC BY-NC-SA 4.0, modification of images from Anatomy Standard under CC BY-NC 4.0.)

Nasal Bone

The nasal bone is one of two small bones that articulate (join) with each other to form the bony base (bridge) of the nose. They also support the cartilages that form the lateral walls of the nose (Figure \(\PageIndex{23}\) and Figure \(\PageIndex{3}\)). These are the bones that are damaged when the nose is broken.

Nasal Bone Ex Situ

Nasal Bone In Situ

Lacrimal Bone Ex Situ

Lacrimal Bone In Situ

Figure \(\PageIndex{23}\): Nasal Bone and Lacrimal Bone Ex Situ and In Situ.  The nasal bones form the superior, hard portion of the bridge of your nose.  The lacrimal bones are within the orbit, forming part of the medial wall.  (Image credits: "Nasal Bone Ex Situ and In Situ" and "Lacrimal Bone Ex Situ and In Situ" by Sofia Elizondo are licensed under CC BY-NC-SA 4.0, modification of images from Anatomy Standard under CC BY-NC 4.0.)

Lacrimal Bone

Each lacrimal bone is a small, rectangular bone that forms the anterior, medial wall of the orbit (Figure \(\PageIndex{23}\) and Figure \(\PageIndex{4}\)). The anterior portion of the lacrimal bone forms a shallow depression and extending inferiorly from this is the nasolacrimal canal. The lacrimal fluid (tears of the eye), which serves to maintain the moist surface of the eye, drains at the medial corner of the eye into the nasolacrimal canal. This duct then extends downward to open into the nasal cavity, behind the inferior nasal concha. In the nasal cavity, the lacrimal fluid normally drains posteriorly, but with an increased flow of tears due to crying or eye irritation, some fluid will also drain anteriorly, thus causing a runny nose.

Inferior Nasal Conchae

The right and left inferior nasal conchae form a curved bony plate that projects into the nasal cavity space from the lower lateral wall (Figure \(\PageIndex{24}\) and Figure \(\PageIndex{5}\)). The inferior concha is the largest of the nasal conchae and can easily be seen when looking into the anterior opening of the nasal cavity.

A. Anterior View

B. Lateral View from Midline

Figure \(\PageIndex{24}\): Inferior Nasal Conchae.  These bones form ridges on the lateral walls of the nasal cavities.  (Image credits: "Inferior Nasal Conchae" by Jennifer Lange are licensed under CC BY-NC-SA 4.0, modification of images from Anatomy Standard.)

Vomer Bone

The unpaired vomer bone, often referred to simply as the vomer, is triangular-shaped and forms the posterior-inferior part of the nasal septum (see Figure \(\PageIndex{25}\)). The vomer is best seen when looking from behind into the posterior openings of the nasal cavity (see Figure \(\PageIndex{8}\)). In this view, the vomer is seen to form the entire height of the nasal septum. A much smaller portion of the vomer can also be seen when looking into the anterior opening of the nasal cavity.

Vomer Ex Situ

Vomer In Situ

Figure \(\PageIndex{25}\): Vomer.  The vomer is a small bone that makes up the inferior portion of the nasal septum. (Image credits: "Vomer Ex Situ and In Situ" by Sofia Elizondo are licensed under CC BY-NC-SA 4.0, modification of images from Anatomy Standard under CC BY-NC 4.0.)

Mandible

The mandible forms the lower jaw and is the only moveable bone of the skull. At the time of birth, the mandible consists of paired right and left bones, but these fuse together during the first year to form the single U-shaped mandible of the adult skull (Figure \(\PageIndex{26}\)).

The each side of the mandible has two upward-going bony projections. The posterior projection is the condylar process of the mandible, which is topped by the oval-shaped condyle. The condyle of the mandible articulates (joins) with the mandibular fossa and articular tubercle of the temporal bone. Together these articulations form the temporomandibular joint, which allows for opening and closing of the mouth (see Figure \(\PageIndex{6}\)). 

Important landmarks of the mandible, as seen in Figure \(\PageIndex{26}\), include the mental foramen, the opening located on each side of the anterior-lateral mandible, which is the exit site for a sensory nerve that supplies the chin.

Figure \(\PageIndex{26}\): Mandible. The mandible is the only moveable bone of the skull.  The condylar process on the mandibular head articulates with the mandibular fossa of the temporal bone. (Image credits: "Mandible" by Sofia Elizondo are licensed under CC BY-NC-SA 4.0, modification of images from Anatomy Standard under CC BY-NC 4.0.)

Interactive Element

Explore bones of the skull in this 3D interactive model:

 

Schädelbasis by Vida_Lab on Sketchfab

Paranasal Sinuses

The paranasal sinuses are hollow, air-filled spaces located within certain bones of the skull (Figure \(\PageIndex{27}\)). All of the sinuses communicate with the nasal cavity (paranasal = “next to nasal cavity”) and are lined with nasal mucosa. They serve to reduce bone mass and thus lighten the skull, and they also add resonance to the voice. This second feature is most obvious when you have a cold or sinus congestion. These produce swelling of the mucosa and excess mucus production, which can obstruct the narrow passageways between the sinuses and the nasal cavity, causing your voice to sound different to yourself and others. This blockage can also allow the sinuses to fill with fluid, with the resulting pressure producing pain and discomfort.

The paranasal sinuses are named for the skull bone that each occupies. The frontal sinus is located just above the eyebrows, within the frontal bone (Figure \(\PageIndex{9}\)). This irregular space may be divided at the midline into bilateral spaces, or these may be fused into a single sinus space. The frontal sinus is the most anterior of the paranasal sinuses. The largest sinus is the maxillary sinus (Figure \(\PageIndex{19}\)). These are paired and located within the right and left maxillary bones, where they occupy the area just below the orbits. The maxillary sinuses are most commonly involved during sinus infections. Because their connection to the nasal cavity is located high on their medial wall, they are difficult to drain. The sphenoid sinus is a single, midline sinus . It is located within the body of the sphenoid bone, just anterior and inferior to the sella turcica, thus making it the most posterior of the paranasal sinuses. The lateral aspects of the ethmoid bone contain multiple small spaces separated by very thin bony walls (Figure \(\PageIndex{15}\)). Each of these spaces is called an ethmoid air cell. These are located on both sides of the ethmoid bone, between the upper nasal cavity and medial orbit, just behind the superior nasal conchae.

C. Relative Locations

Figure \(\PageIndex{27}\): Paranasal Sinuses. The paranasal sinuses are hollow, air-filled spaces named for the skull bone that each occupies. The most anterior is the frontal sinus, located in the frontal bone above the eyebrows. The largest are the maxillary sinuses, located in the right and left maxillary bones below the orbits. The most posterior is the sphenoid sinus, located in the body of the sphenoid bone, under the sella turcica. The ethmoid air cells are multiple small spaces located in the right and left sides of the ethmoid bone, between the medial wall of the orbit and lateral wall of the upper nasal cavity. (Images A and B credit: "Paranasal Sinuses" by Mark Schaeffer are licensed under CC BY-NC-SA 4.0, modification of image from Anatomy Standard under CC BY-NC 4.0.  Image C credit: "Paranasal Sinuses" by OpenStax is licensed under CC BY 3.0)

Hyoid Bone

The hyoid bone is an independent bone that does not articulate (form a joint) any other bone and thus is not part of the skull. In fact, it is the only bone in the human body that does not articulate with another bone, making it unique. It is a small U-shaped bone located in the upper neck near the level of the inferior mandible, with the tips of the “U” pointing posteriorly (Figure \(\PageIndex{28}\)). The hyoid serves as the base for the tongue above, and is attached to the larynx below and the pharynx posteriorly. The hyoid is held in position by a series of small muscles that attach to it either from above or below. These muscles act to move the hyoid up/down or forward/back. Movements of the hyoid are coordinated with movements of the tongue, larynx, and pharynx during swallowing and speaking.

Position in Skeleton

Anterior View

Anterolateral View

Figure \(\PageIndex{28}\): Hyoid Bone. The hyoid bone is located in the upper neck and does not join with any other bone. It provides attachments for muscles that act on the tongue, larynx, and pharynx.  (Image credits: "Hyoid Bone Position" by Sofia Elizondo is licensed under CC BY-NC-SA 4.0, modification of images from Anatomy Standard under CC BY-NC 4.0.  "Hyoid Bone Anterior and Anterolateral Views" by Anatomy Standard are licensed under CC BY-NC 4.0.)

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Concept Review

The skull consists of the cranial bones and the facial bones. The cranial bones surround and protect the brain, which occupies the cranial cavity inside the skull. The brain case is formed by eight bones, the paired parietal and temporal bones plus the unpaired frontal, occipital, sphenoid, and ethmoid bones. The narrow gap between the bones is filled with dense, fibrous connective tissue that unites the bones to form sutures. The sagittal suture joins the right and left parietal bones. The coronal suture joins the parietal bones to the frontal bone, the lambdoid suture joins them to the occipital bone, and the squamous suture joins them to the temporal bone.

The facial bones support the facial structures and form the upper and lower jaws. These consist of 14 bones, with the paired maxillary, palatine, zygomatic, nasal, lacrimal, and inferior conchae bones and the unpaired vomer and mandible bones. The ethmoid bone also contributes to the formation of facial structures. The maxilla forms the upper jaw and the mandible forms the lower jaw. The maxilla also forms the larger anterior portion of the hard palate, which is completed by the smaller palatine bones that form the posterior portion of the hard palate.

Summary Table: Cranial Bones

Bone	Major Landmarks	Animation
Frontal	frontal sinus
Parietal (right and left)
Occipital	external occipital protuberance foramen magnum hypoglossal canal occipital condyles sulci for the transverse sinus and the superior sagittal sinus
Temporal (right and left)	carotid canal external acoustic meatus internal acoustic meatus jugular foramen mastoid process styloid process zygomatic process (part of zygomatic arch)
Sphenoid	foramen ovale foramen rotundum foramen spinosum inferior orbital fissure optic canal optic groove sella turcica with hypophyseal fossa and dorsum sellae sphenoid sinus superior orbital fissure
Ethmoid	cribriform plate crista galli ethmoid air cells middle nasal conchae perpendicular plate superior nasal conchae

 

Summary Table: Facial Bones

Bone	Major Landmarks	Animation
Inferior nasal conchae
Lacrimal
Mandible	condylar process mandibular head mental foramen
Maxilla	infraorbital foramen maxillary sinus
Nasal
Palatine
Vomer
Zygomatic	temporal process (part of zygomatic arch)

The hyoid bone is located in the upper neck and does not join with any other bone. It is held in position by muscles and serves to support the tongue above, the larynx below, and the pharynx posteriorly.

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Review Questions

Query \(\PageIndex{1}\)

Query \(\PageIndex{2}\)

 

Critical Thinking Questions

Query \(\PageIndex{3}\)

Query \(\PageIndex{4}\)

Query \(\PageIndex{5}\)

References

Centers for Disease Control and Prevention (US). Injury prevention and control: traumatic brain injury [Internet]. Atlanta, GA; [cited 2013 Mar 18]. Available from: http://www.cdc.gov/traumaticbraininjury/statistics.html.

Glossary

Query \(\PageIndex{6}\)

Contributors and Attributions

• OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at https://openstax.org/books/anatomy-and-physiology