Medical School · Year 1 · Anatomy Pelvis Head Neck · includes a quiz and discussion video

Lecture 9: Skull and Cranial Cavity

Unit 1.5: Human Gross Anatomy III - Pelvis and Head/Neck

Learning Objectives

By the end of this lecture, students will be able to:

1. Identify the bones of the neurocranium and viscerocranium
2. Describe the major sutures and fontanelles of the skull
3. Identify the fossae and foramina of the cranial base
4. Describe the contents transmitted through major foramina
5. Explain the anatomy of the meninges and dural venous sinuses
6. Correlate anatomical features with clinical conditions

Overview of the Skull

The skull is conventionally divided into two major regions based on developmental and functional considerations. The neurocranium, comprising both the calvaria (skull cap) and the cranial base, serves to protect the brain. This portion consists of eight bones: the frontal, two parietal, occipital, two temporal, sphenoid, and ethmoid bones. The viscerocranium, or facial skeleton, provides the structural framework for the face and includes fourteen bones: the mandible, two maxillae, two zygomatic bones, two nasal bones, two lacrimal bones, two palatine bones, two inferior conchae, and the vomer. While most neurocranial bones are flat bones that develop through intramembranous ossification, the bones of the cranial base are irregular and form primarily through endochondral ossification. This count of twenty-two skull bones excludes the hyoid bone and the ear ossicles.

<image>Panel A: Anterolateral view of the adult skull with neurocranium highlighted in light blue encompassing the cranial vault and base. Panel B: Viscerocranium highlighted in light orange forming the anterior facial skeleton. Panel C: Major individual bones labeled with leader lines indicating each bone boundary. Panel D: Scale bar indicating 5 cm with labeled suture lines between neurocranium and viscerocranium.</image>

Bones of the Neurocranium

The frontal bone forms the anterior cranium, including the forehead. Its squamous part constitutes the vertical forehead region, while the orbital parts extend horizontally to form the orbital roof. The supraorbital margin contains either a supraorbital notch or foramen for passage of the supraorbital vessels and nerve. Within the frontal bone lie the frontal sinuses, and the smooth area between the superciliary arches is termed the glabella.

The two parietal bones form the lateral and superior aspects of the cranium. These quadrilateral bones feature the superior and inferior temporal lines on their external surfaces, serving as attachment sites for the temporalis fascia and muscle. Small parietal foramina transmit emissary veins. Each parietal bone articulates with its partner at the sagittal suture, with the frontal bone at the coronal suture, with the occipital bone at the lambdoid suture, and with the temporal bone at the squamous suture.

The occipital bone forms the posterior and inferior cranium. It consists of a squamous part posteriorly, a basilar part anteriorly (which fuses with the sphenoid to form the clivus), and two lateral or condylar parts. The foramen magnum, the large central opening, transmits the brainstem as it continues into the spinal cord. The occipital condyles, located on either side of the foramen magnum, articulate with the lateral masses of the atlas (C1) to permit nodding movements. The external occipital protuberance provides attachment for the ligamentum nuchae.

Each temporal bone consists of multiple parts with distinct features. The thin squamous part forms the lateral skull and gives rise to the zygomatic process. The dense petrous part houses the structures of the inner ear and contains the internal acoustic meatus for passage of the facial and vestibulocochlear nerves. The mastoid part includes the mastoid process, which contains mastoid air cells communicating with the middle ear. The tympanic part forms the external acoustic meatus. The styloid process provides attachment for muscles and ligaments.

The sphenoid bone occupies a central position in the skull base, articulating with nearly all other cranial bones. Its body contains the sphenoid sinuses and bears the sella turcica superiorly, housing the pituitary gland. The greater wings extend laterally to form portions of the lateral skull and middle cranial fossa. The lesser wings project from the body to form the posterior orbital roof and bear the anterior clinoid processes. The pterygoid processes extend inferiorly with their medial and lateral pterygoid plates.

The ethmoid bone contributes to both the anterior cranial fossa and the nasal cavity. The cribriform plate, perforated by numerous small foramina, transmits olfactory nerve fibers from the nasal cavity to the olfactory bulb. The crista galli is a midline vertical projection providing attachment for the falx cerebri. The perpendicular plate contributes to the upper nasal septum, while the ethmoid labyrinth contains the ethmoid air cells.

<image>Panel A: Exploded view of frontal bone (yellow) showing orbital plates and frontal sinus, and paired parietal bones (green) with temporal lines. Panel B: Occipital bone (purple) with foramen magnum highlighted and condyles labeled. Panel C: Temporal bone (orange) demonstrating petrous and mastoid portions with internal acoustic meatus. Panel D: Sphenoid (red) with wings and sella turcica, and ethmoid (blue) with cribriform plate and crista galli.</image>

Sutures and Fontanelles

The major cranial sutures represent fibrous joints between the skull bones. The coronal suture runs coronally between the frontal bone and the two parietal bones. The sagittal suture extends anteroposteriorly along the midline between the two parietal bones. The lambdoid suture courses transversely between the parietal bones and the occipital bone, resembling the Greek letter lambda. The squamous sutures are found bilaterally where the temporal bones meet the parietal bones. The metopic suture, present between the two halves of the frontal bone in infants, typically fuses by age two.

In the infant skull, fontanelles represent membrane-covered gaps at the intersections of sutures where ossification is incomplete. The anterior fontanelle (at bregma) lies at the junction of the coronal and sagittal sutures and typically closes between eighteen and twenty-four months of age. This is the largest fontanelle and the one most commonly assessed clinically. The posterior fontanelle (at lambda) is located at the junction of the sagittal and lambdoid sutures and closes much earlier, typically by two to three months. The anterolateral (sphenoid) fontanelles lie at the junction of the frontal, parietal, temporal, and sphenoid bones and close by two to three months. The posterolateral (mastoid) fontanelles are found at the junction of the parietal, temporal, and occipital bones and close between twelve and eighteen months.

The fontanelles serve several important functions. They allow the skull bones to overlap during birth, permitting molding of the head through the birth canal. They accommodate the rapid brain growth that occurs during infancy. Clinically, a bulging fontanelle may indicate increased intracranial pressure, while a sunken fontanelle may suggest dehydration. Premature fusion of sutures, termed craniosynostosis, results in abnormal skull shapes and may compromise brain development.

<image>Panel A: Superior view of an infant skull (approximately 6 months old) with diamond-shaped anterior fontanelle at the junction of coronal and sagittal sutures. Panel B: Smaller triangular posterior fontanelle at the lambdoid junction. Panel C: Lateral fontanelles (anterolateral and posterolateral) with major sutures outlined as dotted lines and labeled. Panel D: Inset showing normal fontanelle palpation technique on infant.</image>

Cranial Fossae

The internal surface of the cranial base is divided into three cranial fossae, each at progressively lower levels from anterior to posterior.

The anterior cranial fossa is formed by the orbital plates of the frontal bone, the cribriform plate of the ethmoid, and the lesser wings of the sphenoid. It lodges the frontal lobes of the cerebrum. Key features include the cribriform plate with its multiple small foramina for olfactory nerve fibers, the crista galli rising from the cribriform plate for falx cerebri attachment, and the foramen cecum (usually obliterated in adults) located anterior to the crista galli.

The middle cranial fossa has a butterfly shape, formed centrally by the body of the sphenoid and laterally by the greater wings of the sphenoid and the squamous and petrous portions of the temporal bones. It accommodates the temporal lobes of the cerebrum, the pituitary gland within the sella turcica, and the cavernous sinuses. The sella turcica comprises the hypophyseal fossa (housing the pituitary gland), bounded anteriorly by the tuberculum sellae and posteriorly by the dorsum sellae with its posterior clinoid processes. The carotid sulcus marks the course of the internal carotid artery as it enters the cranium.

Numerous foramina traverse the middle cranial fossa. The optic canal, within the lesser wing, transmits the optic nerve (CN II) and ophthalmic artery. The superior orbital fissure, between the greater and lesser wings, conveys cranial nerves III, IV, and VI, the ophthalmic division of the trigeminal nerve (V1), and the ophthalmic veins. The foramen rotundum in the greater wing transmits the maxillary nerve (V2). The foramen ovale, also in the greater wing, passes the mandibular nerve (V3) and the accessory meningeal artery. The foramen spinosum transmits the middle meningeal artery and a meningeal branch of V3. The foramen lacerum, though largely filled with cartilage in life, is crossed superiorly by the internal carotid artery and transmits the greater petrosal nerve.

The posterior cranial fossa is the largest and deepest, formed by the occipital bone and the posterior surfaces of the petrous temporal bones. It contains the cerebellum and the brainstem. The foramen magnum occupies the central position, transmitting the medulla oblongata continuing as the spinal cord, the vertebral arteries, and the spinal accessory nerves. The clivus slopes from the dorsum sellae to the foramen magnum, supporting the pons and medulla. Grooves for the transverse and sigmoid sinuses mark the inner surface. The internal acoustic meatus in the petrous temporal bone transmits the facial and vestibulocochlear nerves (CN VII and VIII) along with the labyrinthine artery. The jugular foramen, between the temporal and occipital bones, passes cranial nerves IX, X, and XI and the internal jugular vein. The hypoglossal canal, in the occipital condyle, conveys the hypoglossal nerve (CN XII).

<image>Panel A: Superior view of the cranial base with calvaria removed, anterior fossa (light blue) showing cribriform plate and crista galli. Panel B: Middle fossa (light green) demonstrating sella turcica centrally and greater wings laterally with labeled foramina (optic canal, superior orbital fissure, foramen rotundum, ovale, spinosum). Panel C: Posterior fossa (light purple) with foramen magnum centrally, jugular foramina, and internal acoustic meatus. Panel D: Color-coded legend for foramina contents listing nerves, arteries, and veins transmitted through each opening.</image>

Summary of Cranial Foramina

Understanding the contents of each foramen is essential for clinical correlation. The cribriform plate transmits olfactory nerve fibers (CN I). The optic canal carries the optic nerve (CN II) and ophthalmic artery. The superior orbital fissure conveys cranial nerves III, IV, VI, and V1. The foramen rotundum transmits V2 (maxillary nerve). The foramen ovale passes V3 (mandibular nerve). The foramen spinosum carries the middle meningeal artery. The internal acoustic meatus transmits CN VII and VIII. The jugular foramen passes CN IX, X, and XI along with the internal jugular vein. The hypoglossal canal conveys CN XII. The foramen magnum transmits the medulla, vertebral arteries, and ascending spinal roots of CN XI.

<image>Panel A: Schematic diagram of the skull base from above with all major foramina labeled. Panel B: Color-coded overlay showing cranial nerves in red, arteries in blue, and veins in green at each foramen. Panel C: Table inset listing each foramen with its fossa location and contents. Panel D: Arrows indicating direction of neural and vascular passage through each foramen.</image>

Meninges

The brain and spinal cord are enveloped by three protective membranes called meninges, arranged from outer to inner as the dura mater, arachnoid mater, and pia mater.

The dura mater is a thick, tough membrane composed of two layers within the cranium. The outer periosteal layer adheres to the inner surface of the skull bones, functioning as the cranial periosteum. The inner meningeal layer continues into the vertebral canal as the spinal dura mater. These two layers are normally fused but separate in certain locations to form the dural venous sinuses. The meningeal layer also folds inward to create dural septa that partially compartmentalize the cranial cavity. The falx cerebri extends vertically in the longitudinal fissure between the cerebral hemispheres. The tentorium cerebelli projects horizontally between the cerebrum above and the cerebellum below, with a central opening (tentorial notch) for the midbrain. The falx cerebelli is a small midline fold between the cerebellar hemispheres. The diaphragma sellae roofs over the sella turcica, with an aperture for the pituitary stalk.

The arachnoid mater is a delicate, avascular membrane separated from the dura by a potential subdural space. Between the arachnoid and the underlying pia lies the subarachnoid space, which contains cerebrospinal fluid and the major cerebral vessels. Arachnoid granulations (also called arachnoid villi or Pacchionian granulations) protrude through the dura into the superior sagittal sinus and lateral lacunae, functioning to return CSF to the venous circulation.

The pia mater is a thin, highly vascular membrane that intimately adheres to the surface of the brain, following every gyrus and dipping into every sulcus. It contributes to the formation of the choroid plexuses in the ventricles.

<image>Panel A: Coronal section through the superior sagittal sinus region showing periosteal and meningeal dura (dark blue) with the falx cerebri extending between hemispheres. Panel B: Arachnoid mater (light blue) with granulations projecting into the sinus. Panel C: Subarachnoid space containing CSF (clear) and pia mater (pink) adherent to brain surface. Panel D: Bridging veins crossing the subdural space labeled with directional flow arrows.</image>

Dural Venous Sinuses

The dural venous sinuses are venous channels formed between the two layers of the dura mater. They receive blood from cerebral veins and CSF from the subarachnoid space, ultimately draining into the internal jugular veins. Unlike peripheral veins, these sinuses have rigid walls, lack valves, and cannot collapse.

The superior sagittal sinus runs along the superior border of the falx cerebri from the foramen cecum to the confluence of sinuses near the internal occipital protuberance. It receives superior cerebral veins and contains numerous arachnoid granulations. The inferior sagittal sinus courses along the inferior free edge of the falx cerebri, receiving cerebral veins and joining the great cerebral vein (of Galen) to form the straight sinus. The straight sinus runs along the junction of the falx cerebri and tentorium cerebelli to reach the confluence of sinuses.

The transverse sinuses extend laterally from the confluence along the attached margins of the tentorium cerebelli to become the sigmoid sinuses. The S-shaped sigmoid sinuses curve inferiorly and medially to exit the skull through the jugular foramina, becoming the internal jugular veins.

The cavernous sinuses are located on either side of the body of the sphenoid bone. These paired sinuses have a complex trabeculated interior and contain remarkable structures: the internal carotid artery and abducens nerve (CN VI) pass through the center, while the oculomotor (CN III), trochlear (CN IV), and ophthalmic and maxillary divisions of the trigeminal nerve (V1 and V2) course within the lateral wall. The two cavernous sinuses communicate via anterior and posterior intercavernous sinuses. Each cavernous sinus receives the superior and inferior ophthalmic veins and drains via the superior petrosal sinus to the transverse sinus and the inferior petrosal sinus to the internal jugular vein. The connections between the facial veins, ophthalmic veins, and cavernous sinus create a potential pathway for infection spread.

Additional smaller sinuses include the superior and inferior petrosal sinuses along the petrous temporal bone, the occipital sinus in the falx cerebelli, and the basilar plexus on the clivus connecting the two inferior petrosal sinuses.

<image>Panel A: Lateral view showing superior sagittal sinus along the falx and inferior sagittal sinus joining the great vein of Galen to form the straight sinus. Panel B: Confluence of sinuses with transverse sinuses curving to sigmoid sinuses draining to internal jugular veins. Panel C: Coronal section through cavernous sinus showing internal carotid artery centrally and CN VI adjacent. Panel D: Lateral wall of cavernous sinus with CN III, IV, V1, and V2 labeled in their relative positions.</image>

Blood Supply to the Meninges

The meninges receive arterial blood from multiple sources corresponding to the three cranial fossae. The anterior cranial fossa is supplied by the anterior and posterior ethmoidal arteries, branches of the ophthalmic artery. The middle cranial fossa receives its principal supply from the middle meningeal artery, the largest of the meningeal vessels. This artery arises from the maxillary artery, enters the cranium through the foramen spinosum, and divides into frontal (anterior) and parietal (posterior) branches that groove the inner surface of the skull. The accessory meningeal artery may supplement this territory. The posterior cranial fossa is supplied by meningeal branches from the vertebral and occipital arteries, as well as the posterior meningeal artery.

The middle meningeal artery has particular clinical significance. Its frontal branch courses near the pterion, the thinnest region of the lateral skull. Trauma to the temporal region may fracture the bone at the pterion and lacerate this vessel, resulting in an epidural hematoma. The grooves created by the artery branches on the inner skull surface may be visible on skull radiographs and computed tomography.

<image>Panel A: Inner surface of the lateral skull showing the middle meningeal artery entering through the foramen spinosum. Panel B: Frontal and parietal divisions of the middle meningeal artery branching across the inner skull surface. Panel C: Pterion region highlighted (red circle) demonstrating proximity of the frontal branch to this thin area. Panel D: Grooves from the artery branches visible on the bone surface with surrounding bone thickness indicated.</image>

Clinical Correlations

Skull fractures may be classified by pattern and location. Linear fractures are simple cracks in the bone and represent the most common type. Depressed fractures occur when a bone fragment is driven inward below the level of the surrounding skull. Basilar fractures traverse the skull base and may be associated with CSF leakage through the nose (rhinorrhea) or ear (otorrhea). Comminuted fractures involve multiple bone fragments.

The pterion is a clinically important landmark where four bones meet: the frontal, parietal, temporal, and greater wing of the sphenoid. This is the thinnest portion of the lateral skull, overlying the anterior branches of the middle meningeal artery. A blow to the temple may fracture the pterion and lacerate this vessel, leading to an epidural hematoma. This arterial bleeding accumulates between the skull and the periosteal layer of the dura, producing a lens-shaped (biconvex) collection on CT imaging. The classic presentation includes a period of initial unconsciousness from the primary injury, followed by a "lucid interval" of apparent recovery, then progressive deterioration as the hematoma expands. This constitutes a neurosurgical emergency requiring prompt evacuation.

Subdural hematomas result from bleeding between the dura and arachnoid mater, typically from tearing of bridging veins that cross from the cerebral cortex to the dural sinuses. Because venous bleeding is at lower pressure than arterial bleeding, subdural hematomas accumulate more slowly and appear crescent-shaped on CT imaging. They may present acutely following significant trauma or chronically, particularly in elderly patients with cerebral atrophy where the bridging veins are stretched.

Craniosynostosis results from premature fusion of one or more cranial sutures, preventing normal skull growth perpendicular to the affected suture. Scaphocephaly (long, narrow skull) results from premature sagittal suture fusion. Plagiocephaly (asymmetric skull) occurs with unilateral coronal suture fusion. Trigonocephaly (triangular forehead) follows metopic suture fusion. Severe cases may restrict brain growth and increase intracranial pressure.

CSF rhinorrhea or otorrhea indicates a breach in the skull base allowing CSF to leak through the nose or ear. This typically follows basilar skull fractures and carries a significant risk of ascending meningitis. The halo sign (CSF spreading as a ring around blood on filter paper) and detection of beta-2 transferrin (a protein unique to CSF) help confirm the diagnosis.

<image>Panel A: Lateral skull showing pterion location with underlying middle meningeal artery. Panel B: CT scan demonstrating lens-shaped epidural hematoma. Panel C: CT scan showing crescent-shaped subdural hematoma. Panel D: Infant skulls showing different craniosynostosis patterns -- scaphocephaly, plagiocephaly, and trigonocephaly with arrows indicating fused sutures.</image>

Summary

The skull comprises twenty-two bones divided into the neurocranium (eight bones protecting the brain) and the viscerocranium (fourteen bones of the facial skeleton). The major sutures include the coronal, sagittal, and lambdoid, while fontanelles in the infant skull permit molding during birth and accommodate brain growth. The cranial base is divided into three fossae: the anterior fossa lodges the frontal lobes, the middle fossa contains the temporal lobes and pituitary, and the posterior fossa houses the cerebellum and brainstem. Foramina in each fossa transmit specific cranial nerves and vessels, with the middle meningeal artery entering through the foramen spinosum. The three meningeal layers—dura, arachnoid, and pia—protect the brain, with the dura forming venous sinuses that ultimately drain to the internal jugular veins. The cavernous sinuses contain the internal carotid artery and multiple cranial nerves. The pterion represents a vulnerable area where the middle meningeal artery may be injured, potentially causing an epidural hematoma.

Key Terms

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