The Skull

General Features and Functions of the Skull

The human skull is the part of the skeleton that supports the structures of the face and forms a cavity for the brain.

Learning Objectives

Describe the parts of the skull

Key Takeaways

Key Points

  • The adult human skull consists of two regions of different embryological origins: the neurocranium and the viscerocranium.
  • The neurocranium is a protective shell surrounding the brain and brain stem.
  • The viscerocranium (or facial skeleton) is formed by the bones supporting the face.
  • Except for the mandible, all skull bones are joined together by sutures —synarthrodial (immovable) joints.
  • The skull contains air-filled cavities called sinuses. Their functions are debatable, but may be related to lessening skull weight, contributing to voice resonance, and warming and moistening inspired air.

Key Terms

  • viscerocranium: The skeleton that supports facial structure.
  • sutures: A fairly rigid joint between bones of the neurocranium.
  • neurocranium: The protective vault surrounding the brain and brain stem.

The skull supports the musculature and structures of the face and forms a protective cavity for the brain. The skull is formed of several bones which, with the exception of the mandible, are joined together by sutures—synarthrodial (immovable) joints.

Composition of the Skull

The adult human skull is comprised of twenty-two bones which are divided into two parts of differing embryological origin: the neurocranium and the viscerocranium.

Neurocranium

The neurocranium forms the cranial cavity that surrounds and protects the brain and brainstem. The neurocranium is formed from the occipital bone, two temporal bones, two parietal bones, the sphenoid, ethmoid and frontal bones; they are all joined together with sutures.

This is a diagram of a skull that displays the components of neurocranium. The neurocranium consists of 8 parts: frontal, sphenoid, ethmoid, occipital, 2 temporal, and 2 parietal bones.

Components of neurocranium: Neurocranium consists of 8 parts: frontal, sphenoid, ethmoid, occipital, 2 temporal, and 2 parietal bones.

Viscerocranium

The viscerocranium bones form the anterior and lower regions of the skull and include the mandible, which attaches through the only truly motile joint found in the skull. The facial skeleton contains the vomer, two nasal conchae, two nasal bones, two maxilla, the mandible, two palatine bones, two zygomatic bones, and two lacrimal bones.

This is a frontal view of the a face, showing the components of the viscerocranium. The labels on the face identify 14 components: 2 nasal, 2 lacrimal, 2 palantine, 2 inferior nasal concha, 2 zygomatic, 2 maxilla, 1 mandible, and 1 vomer.

Components of viscerocranium : The face is made of 2 nasal, 2 lacrimal, 2 palantine, 2 inferior nasal concha, 2 zygomatic, 2 maxilla, 1 mandible, and 1 vomer.

Other Features

The skull also contains the sinuses. These are air-filled cavities that contribute to lessening the weight of the skull with a minimal reduction in strength. They contribute to resonance of the voice and assist in the warming and moistening of air inhaled via the nose.

Cranial Bones

The neurocranium is comprised of eight bones: occipital, two temporal bones, two parietal bones, sphenoid, ethmoid, and the frontal bone.

Learning Objectives

Differentiate the bones of the neurocranium

Key Takeaways

Key Points

  • The eight bones of the neurocranium form major portions of the skull and protect the brain.
  • The neurocranium consists of two temporal bones situated to the base and side of the skull, and two parietal bones that make up the roof of the skull.
  • A single occipital bone forms the base of the skull, and the frontal bone forms the forehead.
  • The sphenoid and ethmoid bones located to the front of the skull form parts of the orbital sockets and nasal cavity; they also support and protect key organs found in the skull.

Key Terms

  • neurocranium: The part of the skull that encloses and protects the brain and brain stem.

The neurocranium forms the cranial cavity that surrounds and protects the brain and brainstem. The neurocranium consists of the occipital bone, two temporal bones, two parietal bones, the sphenoid, ethmoid, and frontal bones—all are joined together with sutures.

Evolutionary,it is the expansion of the neurocranium that has facilitated the expansion of the brain and its associated developments.

This is a lateral, profile view showing the bones that structure the neurocranium. These are labeled as the frontal, parietal, spehnoid, temporal, occipital, and ethmoid bones.

Neurocranium: A lateral view showing the bones that structure the neurocranium.

Occipital Bone

The occipital bone forms the base of the skull at the rear of the cranium. It articulates with the first vertebra of the spinal cord and also contains the foramen magnum, the large opening of the skill through which the spinal cord passes as it enters the vertebral column. The occipital bone borders the parietal bones through the heavily serrated lambdoidal suture, and also the temporal bones through occipitomastoid suture.

Temporal Bones

The temporal bones are situated at the base and sides of the skull, lateral to the temporal lobes of the brain. The temporal bones consist of four regions the squamous, mastoid, petrous and tympanic regions.

The squamous region is the largest and most superior region. Inferior to the squamous is the mastoid region, and fused between the squamous and mastoid regions is the petrous region. Finally, the small and inferior tympanic region lies anteriorly to the mastoid.

There are two processes that originate from the temporal bone:

  1. The zygomatic process that projects from the lower squamous region and articulates with the zygomatic bone of the cheek.
  2. The styloid process projects downwards from the interior of the temporal bone and provides attachment for several muscles associated with the tongue.

The temporal bones have four borders:

  1. The occipitomastoid suture separates the occipital bone and mastoid portion of temporal bone.
  2. The squamosal suture separates the parietal bone and squama portion of temporal bone.
  3. The sphenosquamosal suture separates the sphenoid bone and squama portion of temporal bone.
  4. The zygomaticotemporal suture separates the zygomatic bone and zygomatic process of temporal bone.

Parietal Bones

The two large parietal bones are connected and make up part of the roof and sides of the human skull. The two bones articulate to form the sagittal suture. In the front, the parietal bones form the coronal suture with the frontal bone, and in the rear, the lambdoid suture is formed by the occipital bone. Finally, the squamosal suture separates the parietal and temporal bones.

Sphenoid Bone

The sphenoid bone is situated in the middle of the skull towards the front and forms the rear of the orbit. It has been described as resembling a butterfly due to its wing-like processes. The sphenoid bone is divided into several parts: the body of the bone, two greater wings, two lesser wings, and the pterygoid processes.

The sphenoid bone is one of the most complex in the body due to its interactions with numerous facial bones, ligaments, and muscles. The body that forms the middle of the sphenoid bone articulates with the ethmoid and occipital bone and forms a key part of the nasal cavity; it also contains the sphenoidal sinuses.

The greater wings form the floor of the middle cranial fossa that houses the frontal lobes and pituitary gland, and also the posterior wall of the orbit. The lesser wings project laterally and form the floor of the anterior cranial fossa and the superior orbital fissure through which several key optical nerves pass.

Ethmoid Bone

The ethmoid bone is a small bone in the skull that separates the nasal cavity from the brain. It is lightweight due to its spongy, air-filled construction and is located at the roof of the nose and between the two orbits.

The ethmoid bone forms the medial wall of the orbit, the roof of the nasal cavity, and due to its central location it articulates with numerous bones of the viscerocranium. Inside the neurocranium it articulates with the frontal and sphenoid bones.

Frontal Bone

The frontal bone forms the front of the skull and is divided into three parts:

  1. Squamous: This part is large and flat and forms the main region of the forehead.
  2. Orbital: This part lies inferiorly and forms the superior border of the orbit.
  3. Nasal: this part is smaller and articulates with the nasal bones and maxilla to contribute to the roof of the nose.

The frontal bone borders two other neurocranial bones—the parietal bones through the coronal sutures and the sphenoid bone through the sphenofrontal suture. It also articulates with the zygomatic and nasal bones and the maxilla.

Facial Bones

The viscerocranium (face) includes these bones: vomer, 2 inferior nasal conchae, 2 nasals, maxilla, mandible, palatine, 2 zygomatics, and 2 lacrimals.

Learning Objectives

List the facial bones of the viscerocranium

Key Takeaways

Key Points

  • The several bones of the viscerocranium are joined by sutures to each other and the neurocranium, except for the mandible, which articulates with the temporal bones.
  • The hyoid bone, ethmoid bone, and sphenoid bones are sometimes included in the viscerocranium.

Key Terms

  • viscerocranium: The facial skeleton that is formed by the bones of the anterior and lower skull, which are derived from branchial arches.

The Viserocranium

The viscerocranium or facial bones supports the soft tissue of the face. The viscerocranium consists of 14 individual bones that fuse together. However, the hyoid bone, ethmoid bone, and sphenoid bones are sometimes included in the viscerocranium.

Zygomatic Bones

The two zygomatic bones form the cheeks and contribute to the orbits. They articulate with the frontal, temporal, maxilla, and sphenoid bones.

Lacrimal Bones

The two lacrimal bones form the medial wall of the orbit and articulate with the frontal, ethmoid, maxilla, and inferior nasal conchae. The lacrimal bones are the two smallest bones located in the face.

Nasal Bones

The two slender nasal bones located in the midline of the face fuse to form the bridge of the noise and also articulate with the frontal, ethmoid and maxilla bones. The inferior nasal conchae are located within the nasal cavity. They are spongy and curled in shape; their primary function is to increase the surface area of the nasal cavity, which also increases the amount of air that contacts the mucous membranes and cilia of the nose, thus filtering, warming, and humidifying the air before it enters the lungs. At the base of the nasal cavity is the small vomer bone which forms the nasal septum.

Maxilla Bones

The maxilla bones fuse in the midline and form the upper jaw. They provide the bed for the upper teeth, the floor of the nose, and the base of the orbits. The maxilla articulates with the zygomatic, nasal, lacrimal, and palatine bones.

Palatine Bones

The palatine bones fuse in the midline to form the palatine, located at the back of the nasal cavity that forms the roof of the mouth and the floor of the orbit.

The Mandible

Finally, the mandible forms the lower jaw of the skull. The joint between the mandible and the temporal bones of the neurocranium, known as the temporomandibular joint, forms the only non-sutured joint in the skull.

This is a frontal view of a skull that labels the fourteen facial bones. The following facial bones are paired: nasal, lacrimal, inferior nasal concha, maxilla, palatine, and zygomatic. The mandible and vomer are the only singular facial bones.

Facial bones: There are fourteen facial bones. Some, like the lacrimal and nasal bones, are paired. Others, like the mandible and vomer, are singular.

Orbits

The orbit is the cavity or socket of the skull in which the eye and its appendages are situated.

Learning Objectives

Locate the orbits in the skull

Key Takeaways

Key Points

  • The orbits are conical or four-sided pyramidal cavities, which open into the midline of the face and point backwards.
  • To the rear of the orbit the optical foramen opens into the optic canal, which transmits the optic nerve and opthalmic artery.
  • The orbit protects the eye from mechanical injury and provides access for the optic nerve to the brain.

Key Terms

  • optic canal: The canal that transmits the optic nerve and ophthalmic artery into the orbital cavity.
  • orbit: The bony cavity containing the eyeball; the eye socket.

The orbit, or eye socket, is the cavity located in the skull in which the eye and its associated appendages are housed. The orbits are conical, sometimes described as four-sided pyramidal, cavities that open in the midline of the face and point backwards. To the rear of the orbit, the optic foramen opens into the optical canal through which the optic nerve and ophthalmic artery pass.

The primary functions of the orbit include protection of its delicate contents and, through muscle attachment and a smooth coating fascia, to  also promote the smooth, delicate movements of the eye.

Structure of the Orbit

The orbital cavity is formed from seven bones. The frontal bone forms the superior border of the orbital rim and also the superior wall (roof) of the orbital surface.

The zygomatic bone forms the lateral (and half of the basal) border of the orbital rim, and also the lateral wall of the orbital surface—this is the thickest region of the orbit as it is most exposed to external trauma.

Completing the basal and medial border of the orbital rim is the maxillary bone, which also forms the inferior wall (floor) of the orbital surface.

The lacrimal and ethmoid bones contribute to the medial wall of the orbit and also to the medial wall of the orbital canal. The small palatine bone contributes to the floor of the orbit.

Finally, the sphenoid bone forms the posterior wall of the orbit and also contributes to the formation of the optic canal.

Foramina

The human skull has numerous holes known as foramina through which cranial nerves, arteries, veins, and other structures pass.

Learning Objectives

Describe the purpose of foramina in the skull

Key Takeaways

Key Points

  • A foramen (plural: foramina ) is an opening inside the body that allows key structures to connect one part of the body to another.
  • The skull bones that contain foramina include the frontal, ethmoid, sphenoid, maxilla, palatine, temporal, and occipital.
  • There are 21 foramina in the human skull.

Key Terms

  • foramina: The openings inside the body that typically allow muscles, nerves, arteries, veins, or other structures to connect one part of the body to another.
This image details the location of foramina in the skull. Key foramina shown here include: the supraorbital foramen, optic foramen, foramen magnum, foramina of cribriform plate, foramen rotundum.

Base of the skull (upper surface): This image details the foramina of the skull.

In anatomy, a foramen is any opening. Foramina inside the body of humans and other animals typically allow muscles, nerves, arteries, veins, or other structures to connect one part of the body with another.

The human skull has numerous foramina through which cranial nerves, arteries, veins, and other structures pass. The skull bones that contain foramina include the frontal, ethmoid, sphenoid, maxilla, palatine, temporal, and occipital lobes.

Key foramina in the skull include:

  • Supraorbital foramen: Located in the frontal bone, it allows passage of the supraorbital vein, artery, and nerve into the orbit.
  • Optic foramen: Located in the sphenoid, it allows the passage of the ophthalmic artery and nerve from the optic canal into the orbit.
  • Foramen magnum: Located in the occipital bone, it allows the passage of the spinal and vertebral arteries and the spinal cord to pass from the skull into the vertebral column.
  • Foramina of cribriform plate: Located in the ethmoid bone, it allows the passage of the olfactory nerve.
  • Foramen rotundum: Located in the sphenoid bone, it allows passage of the maxillary nerve.

Sutures

A suture is a type of fibrous joint (or synarthrosis) that only occurs in the skull (or cranium).

Learning Objectives

List the sutures of the skull

Key Takeaways

Key Points

  • It is normal for many of the bones of the skull to be unfused at birth. This allows a tiny amount of movement at the sutures, which contributes to the compliance and elasticity of the skull.
  • Sutures become fused as individuals age; thus, examining sutures can provide an estimate of age postmortem.
  • There are 17 named sutures on the human skull.

Key Terms

  • suture: A fairly rigid joint between two or more hard elements, such as the bony plates of the skull.

A suture is a type of fibrous joint (or synarthrosis) that only occurs in the skull. The bones are bound together by Sharpey’s fibers, a matrix of connective tissue which provide a firm joint.

A small amount of movement is permitted through these sutures that contributes to the compliance and elasticity of the skull. The joint between the mandible and the cranium, known as the temporomandibular joint, forms the only non-sutured joint in the skull. Most sutures are named for the bones that they articulate.

At birth, many of the bones of the skull remain unfused to the soft spots described as fontanelle. The bones fuse relatively rapidly through a process known as craniosynotosis, although the relative positions of the bones can continue to change through life. In old age the cranial sutures may ossify completely, reducing the amount of elasticity present in the skull. As such, the degree of ossification can be a useful tool in determining age postmortem.

Lateral view of a skull showing sutures. The dotted red lines indicate the location of skull sutures that are described in the text that follows.

Lateral view of a skull showing sutures: The dotted red lines indicate the location of skull sutures.

Sutures primarily visible from the side include:

  • Coronal suture: Located between the frontal and parietal bones.
  • Lambdoid suture: Located between the parietal, temporal and occipital bones.
  • Occipitomastoid suture.
  • Parietomastoid suture.
  • Sphenofrontal suture.
  • Sphenoparietal suture.
  • Sphenosquamosal suture.
  • Sphenozygomatic suture.
  • Squamosal suture: Located between the parietal and the temporal bone.
  • Zygomaticotemporal suture.
  • Zygomaticofrontal suture.

Sutures primarily visible from front or above include:

  • Frontal suture or metopic suture: Located between the two frontal bones, prior to the fusion of the two into a single bone.
  • Sagittal suture: Located along the midline, between the parietal bones.

Sutures primarily visible from below or inside include:

  • Frontoethmoidal suture.
  • Petrosquamous suture.
  • Sphenoethmoidal suture.
  • Sphenopetrosal suture.

Paranasal Sinuses

The paranasal sinuses (four, paired, air-filled spaces) surround the nasal cavity, and are located above and between the eyes, and behind the ethmoids.

Learning Objectives

Describe the structure and function of the paranasal sinuses

Key Takeaways

Key Points

  • The sinuses are named for the facial bones that they are located behind.
  • There are four sinuses; the maxillary, frontal, ethmoid, and sphenoid.
  • Paranasal sinuses form developmentally through excavation of bone by air-filled sacs from the nasal cavity.
  • This process begins prenatally and continues through the course of an individual’s lifetime.
  • The biological role of the sinuses is debated, but a number of possible functions have been proposed: decreasing weight of the face; increasing resonance of voice; buffer against blows; insulating sensitive structures from rapid temperature fluctuations; and humidifying and heating of inhaled air.

Key Terms

  • paranasal sinuses: A group of four, paired, air-filled spaces that surround the nasal cavity (maxillary sinuses), above the eyes (frontal sinuses), between the eyes (ethmoid sinuses), and behind the eyes (sphenoid sinuses).

Structure of the Paranasal Sinuses

image

Skull Sinuses: This image shows the position of the sinuses in the human skull.

Paranasal sinuses are a group of four, paired, air-filled spaces that surround the nasal cavity (maxillary sinuses), above the eyes (frontal sinuses), between the eyes (ethmoid sinuses), and behind the eyes (sphenoid sinuses). The sinuses are named for the facial bones that they are located behind.

  • The maxillary sinuses (also called the maxillary antrechea, the largest of the paranasal sinuses) are located under the orbits in the maxillary bones.
  • The frontal sinuses are superior to the orbits and are in the frontal bone.
  • The ethmoid sinuses are formed from several discrete air cells within the ethmoid bone between the nose and the orbits.
  • The sphenoid sinuses are in the sphenoid bone at the center of the skull base under the pituitary gland.
  • The paranasal sinuses are lined with respiratory epithelium.

The paranasal sinuses form developmentally through excavation of bone by air-filled sacs (pneumatic diverticula) from the nasal cavity. This process begins prenatally and continues through the course of an individual’s lifetime.

Function of the Paranasal Sinuses

The biological role of the sinuses is debated, but a number of possible functions have been proposed. These include:

  • Decreasing the relative weight of the front of the skull, and especially the bones of the face.
  • Increasing resonance of the voice.
  • Providing a buffer against blows to the face.
  • Insulating sensitive structures like dental roots and eyes from rapid temperature fluctuations in the nasal cavity.
  • Humidifying and heating of inhaled air because of slow air turnover in this region.
  • Regulation of intranasal and serum gas pressures.
  • Immunological defense.

Fontanelles

A fontanelle is an anatomical feature on an infant’s skull that allows its plates to be flexible to pass through the birth canal.

Learning Objectives

Identify the evolutionary purpose of skull fontanelles

Key Takeaways

Key Points

  • Fontanelles are soft spots on a baby’s head that, during birth, enable the bony plates of the skull to flex and allow the child’s head to pass through the birth canal.
  • At birth, the skull features a small posterior fontanelle (an open area covered by a tough membrane) where the two parietal bones adjoin the occipital bone (at the lambda); it usually closes in the first two to three months of life through intramembranous ossification.
  • The much larger, diamond-shaped anterior fontanelle—where the two frontal and two parietal bones join—generally remains open until the child is about two years of age.
  • Two smaller fontanelles are located on each side of the head. The more anterior one is the sphenoidal (between the sphenoid, parietal, temporal, and frontal bones), while the more posterior one is the mastoid (between the temporal, occipital, and parietal bones).

Key Terms

  • fontanelle: A fontanelle is a soft membraneous spot on the head of a baby due to incomplete fusion of the cranial bones.
This is a superior view of an infant skull that shows the location of the anterior (frontal) and posterior fontanelles.

Superior view of infant skull: This image shows the location of the anterior (frontal) and posterior fontanelles.

Fontanelles are soft spots on a baby’s head that, during birth, enable the bony plates of the skull to flex and allow the child’s head to pass through the birth canal. The ossification of the bones of the skull causes the fontanelles to close over a period of 18 to 24 months; they eventually form the sutures of the neurocranium.

The cranium of a newborn consists of five main bones: two frontal bones, two parietal bones, and one occipital bone. These are joined by fibrous sutures that allow movement that facilitates childbirth and brain growth.

At birth, the skull features a small posterior fontanelle (an open area covered by a tough membrane) where the two parietal bones adjoin the occipital bone (at the lambda). This fontanelle usually closes during the first two to three months of an infant’s life. This is called intramembranous ossification. The mesenchymal connective tissue turns into bone tissue.

The much larger, diamond-shaped anterior fontanelle—where the two frontal and two parietal bones join—generally remains open until a child is about two years old. The anterior fontanelle is useful clinically, as examination of an infant includes palpating the anterior fontanelle.

Two smaller fontanelles are located on each side of the head. The more anterior one is the sphenoidal (between the sphenoid, parietal, temporal, and frontal bones), while the more posterior one is the mastoid (between the temporal, occipital, and parietal bones).

This is a lateral view of infant skull that shows the location of the sphenoidal and mastoid fontanelles.

Lateral view of infant skull: This image show the location of the sphenoidal and mastoid fontanelles.

The fontanelle may pulsate. Although the precise cause of this is not known, it is perfectly normal and seems to echo the heartbeat, perhaps via the arterial pulse within the brain vasculature, or in the meninges. This pulsating action is how the soft spot got its name: fontanelle means little fountain.

Parents may worry that their infant may be more prone to injury at the fontanelles. In fact, although they may colloquially be called soft spots, the membrane covering the fontanelles is extremely tough and difficult to penetrate.

The fontanelles allow the infant brain to be imaged using ultrasonography. Once they are closed, most of the brain is inaccessible to ultrasound imaging because the bony skull presents an acoustic barrier.