Interior of Skull

Introduction

• The skull encloses a space known as the cranial cavity. The cranial cavity contains the brain, meninges, intracranial parts of the cranial nerves, and blood vessels.
• The internal surface of the skull is lined by the endocranium, which is continuous with the pericranium through the sutures and foramina.

Structure of cranial bones

– The bones forming the cranial vault generally consist of three layers:

1. Outer table: It forms the external layer of compact bone.
2. Inner table: It forms the internal layer of compact bone.
3. Diploë: It is a layer of spongy bone situated between the two tables and contains bone marrow.

Bones forming the interior of the skull

– The internal surface of the skull is formed by six types of bones:

1. Frontal bone
2. Ethmoid bone
3. Sphenoid bone
4. Temporal bone
5. Parietal bone
6. Occipital bone

Parts of the interior of the skull

– For descriptive purposes, the interior of the skull is divided into two parts:

1. Interior of the skull cap
2. Interior of the skull base

Diploic Veins

• The diploë contains bone marrow and serves as a site of hematopoiesis, particularly during childhood.
• The vomer and pterygoid plates do not contain diploë.

• The diploic veins drain blood from the diploë of the cranial bones.
• There are four principal diploic veins on each side of the skull (Fig. 2.3):
  1. Frontal diploic vein: It emerges near the supraorbital foramen and drains into the supraorbital vein.
  2. Anterior temporal diploic vein: It lies within the greater wing of the sphenoid bone and drains into the sphenoparietal sinus or anterior deep temporal vein.
  3. Posterior temporal diploic vein: It communicates with the transverse sinus in the mastoid region.
  4. Occipital diploic vein: It drains into the occipital vein.
• Numerous smaller, unnamed diploic veins pass through openings in the inner table and communicate with the intracranial venous sinuses.

INTERNAL SURFACE OF SKULL CAP

The skull cap, also called the skull vault, forms the roof of the cranial cavity.

Features

– The internal surface of the skull cap is oval and concave. It presents the following features:

1. Bony Formation

– The skull cap is formed by the frontal, two parietal, and occipital bones.

2. Sutures

– The internal surface shows three major sutures: coronal, sagittal, and lambdoid sutures.

3. Frontal Crest

• The frontal crest is a median bony ridge projecting backward from the inner surface of the frontal bone.
• Attachment: It provides attachment to the anterior end of the falx cerebri.

4. Sagittal Sulcus

• The sagittal sulcus is a median groove that lodges the superior sagittal sinus. It gradually becomes wider as it extends posteriorly.
• At the internal occipital protuberance, it usually continues with the groove for the right transverse sinus.
• Attachment: The margins of the sagittal sulcus provide attachment to the falx cerebri.

5. Granular Pits

• These are numerous, irregular depressions located on either side of the sagittal sulcus. They accommodate the arachnoid granulations and become more prominent with advancing age.

6. Vascular Grooves

• These grooves are formed by the terminal branches of the anterior and posterior divisions of the middle meningeal vessels.
• The accompanying meningeal veins lie closer to the bone than the arteries. Therefore, the grooves are produced largely by the veins rather than the arteries. (Viva)

7. Parietal Foramina

• The parietal foramina are usually paired and lie close to the sagittal sulcus. They are situated approximately 2.5 to 3.5 cm anterior to the lambdoid suture. They transmit emissary veins connecting the superior sagittal sinus with extracranial veins.

8. Impressions

– The internal surface of the skull cap shows impressions produced by the cerebral gyri and sulci, blood vessels, and arachnoid granulations.

INTERNAL SURFACE OF THE BASE OF SKULL

– The floor of the cranial cavity is arranged at three different levels, forming three cranial fossae:

1. Anterior cranial fossa
2. Middle cranial fossa
3. Posterior cranial fossa
4. These fossae are separated by prominent bony ridges.
5. The posterior free border of the lesser wing of the sphenoid separates the anterior cranial fossa from the middle cranial fossa.
6. The superior border of the petrous part of the temporal bone separates the middle cranial fossa from the posterior cranial fossa.

ANTERIOR CRANIAL FOSSA

·  The anterior cranial fossa forms the anterior part of the floor of the cranial cavity. It accommodates the frontal lobes of the cerebral hemispheres.

Boundaries

·  Anteriorly: It is bounded by the posterior surface of the frontal bone and the frontal crest.

·  Posteriorly: It is bounded by the posterior free margins of the lesser wings of the sphenoid, anterior clinoid processes, and anterior margin of the chiasmatic sulcus.

Floor

– The floor of the anterior cranial fossa is formed by the following bones and their parts:

In the Median Plane

• Cribriform plate of the ethmoid bone
• Jugum sphenoidale, which forms the superior surface of the anterior part of the body of the sphenoid

On Each Side

• Orbital plate of the frontal bone
• Lesser wing of the sphenoid bone

Features of anterior cranial fossa

1. Frontal Crest

• The frontal crest is a vertical bony ridge located in the midline on the internal surface of the frontal bone.
• Attachment: It provides attachment to the anterior end of the falx cerebri.

2. Foramen Caecum

• The foramen caecum is situated between the frontal crest and the crista galli.
• Structure transmitted: When patent, it transmits an emissary vein connecting the veins of the nasal cavity with the anterior end of the superior sagittal sinus.

3. Crista Galli

• The crista galli is a vertical bony projection arising from the ethmoid bone in the median plane. Its name means cock’s comb in Latin.
• Attachment: It provides attachment to the anterior end of the falx cerebri.

4. Jugum Sphenoidale

• The jugum sphenoidale is a smooth bony area forming the superior surface of the anterior part of the body of the sphenoid. It extends between the two lesser wings of the sphenoid bone.

5. Cribriform Plate of the Ethmoid Bone

• The cribriform plate, also called the lamina cribrosa, is a horizontal, quadrilateral plate of the ethmoid bone. It forms part of the roof of the nasal cavity and separates it from the cranial cavity. It contains numerous small foramina through which the olfactory nerve filaments pass from the nasal mucosa to the olfactory bulb.

6. Anterior Ethmoidal Canal

• The anterior ethmoidal canal is located near the anterolateral margin of the cribriform plate, at the junction of the ethmoid and frontal bones.
• Structures transmitted:
  • Anterior ethmoidal nerve, which is a branch of the nasociliary nerve.
  • Anterior ethmoidal artery and accompanying veins. The artery arises from the ophthalmic artery.

7. Posterior Ethmoidal Canal

• The posterior ethmoidal canal is situated near the posterior part of the frontoethmoidal junction.
• Structures transmitted:
  • Posterior ethmoidal nerve, which is a branch of the nasociliary nerve.
  • Posterior ethmoidal artery and accompanying veins. The artery arises from the ophthalmic artery.

8. Orbital Plate of the Frontal Bone

• The orbital plate forms a major part of the floor of the anterior cranial fossa and separates it from the orbit. Its superior surface bears impressions produced by the gyri and sulci on the orbital surface of the frontal lobe.

9. Lesser Wing of the Sphenoid Bone

• The lesser wing is a triangular bony plate that is broad medially and tapers laterally. Medially, it is continuous with the jugum sphenoidale.
• It presents the following features:
• Posterior free border: It forms the boundary between the anterior and middle cranial fossae and projects into the stem of the lateral sulcus of the brain.
• Anterior clinoid process: It is a medial projection from the posterior border of the lesser wing. It provides attachment to the free margin of the tentorium cerebelli.
• Anterior and posterior roots: The lesser wing is connected to the body of the sphenoid by a thin, flat anterior root and a thicker posterior root.
• Superior surface: It supports the posterior part of the frontal lobe of the cerebral hemisphere.
• Inferior surface: It forms the superior boundary of the superior orbital fissure.

10. Optic Canal

• The optic canal is a short bony passage formed between the two roots of the lesser wing and the body of the sphenoid.
• Structures transmitted:
  • Optic nerve
  • Ophthalmic artery

Superior orbital fissure

Boundaries

• Superiorly: It is bounded by the lesser wing of the sphenoid.
• Inferiorly: It is bounded by the greater wing of the sphenoid.
• Medially: It is bounded by the body of the sphenoid.

Subdivisions and Structures Transmitted

– The common tendinous ring, which provides origin to the four rectus muscles of the eyeball, divides the superior orbital fissure into three parts.

1. Lateral Part

– The following structures pass through the lateral part, outside the common tendinous ring:

1. Lacrimal nerve
2. Frontal nerve
3. Trochlear nerve
4. Superior ophthalmic vein

2. Middle Part

– The following structures pass through the middle part, within the common tendinous ring:

1. Superior and inferior divisions of the oculomotor nerve
2. Nasociliary nerve
3. Abducent nerve

3. Medial Part

– The following structures pass through the medial part, outside the common tendinous ring:

1. Inferior ophthalmic vein
2. Sympathetic nerve fibres arising from the internal carotid plexus

MIDDLE CRANIAL FOSSA

·  The middle cranial fossa extends from the posterior borders of the lesser wings of the sphenoid to the superior borders of the petrous parts of the temporal bones. It lies at a lower level and is deeper than the anterior cranial fossa.

·  Shape: It is butterfly-shaped, with a narrow, shallow central part and broad, deep lateral parts.

·  The lateral parts accommodate the temporal lobes of the cerebral hemispheres.

<H4>Boundaries

Anterior Boundary

– The anterior boundary is formed by the following structures:

1. The sharp posterior borders of the lesser wings of the sphenoid.
2. The anterior clinoid processes.
3. The anterior margin of the chiasmatic sulcus.

Posterior Boundary

– The posterior boundary is formed by the following structures:

1. The superior borders of the petrous parts of the temporal bones.
2. The dorsum sellae, which is a quadrilateral plate of bone.

Lateral Boundary

– The lateral boundary is formed by the following bones:

1. Greater wing of the sphenoid.
2. Parietal bone.
3. Squamous part of the temporal bone.

<H4>Floor

– The floor of the middle cranial fossa is formed by the following bones and their parts:

1. The body of the sphenoid forms the central part.
2. The greater wings of the sphenoid form the anterolateral parts.
3. The squamous parts and anterior surfaces of the petrous parts of the temporal bones contribute to the lateral parts.

Features of Middle Cranial Fossa

The middle cranial fossa is divided into two areas for descriptive purposes:

1. Median area
2. Lateral area

Median area

• The median area is formed by the body of the sphenoid bone.
• It presents the chiasmatic sulcus, sella turcica, tuberculum sellae, hypophyseal fossa, dorsum sellae, and posterior clinoid processes.
• The cavernous sinus lies on either side of the body of the sphenoid.

1. Chiasmatic Sulcus

• The chiasmatic sulcus, also called the optic groove, is a shallow transverse groove on the superior surface of the body of the sphenoid.
• It is continuous laterally with the optic canals, which open into the orbits.
• Relation: The optic chiasma lies above and slightly posterior to the chiasmatic sulcus. Therefore, the sulcus does not directly lodge the optic chiasma. (MCQ)

2. Optic Canal

• The optic canal is a short bony passage enclosed by the anterior and posterior roots of the lesser wing and the body of the sphenoid.
• Structures transmitted (Viva): It transmits the optic nerve and ophthalmic artery.

3. Sella Turcica

• The sella turcica is a saddle-shaped depression on the superior surface of the body of the sphenoid.
• Its name means Turkish saddle.
• It consists of three parts:
  • Tuberculum sellae anteriorly.
  • Hypophyseal fossa in the middle.
  • Dorsum sellae posteriorly.

4. Tuberculum Sellae

• The tuberculum sellae is a slight transverse bony elevation separating the chiasmatic sulcus from the hypophyseal fossa.
• It forms the anterior boundary of the sella turcica.

5. Middle Clinoid Process

• The middle clinoid process is a small, variable bony projection arising from the lateral end of the tuberculum sellae.
• The term clinoid is derived from the Greek word klinein, meaning to slope or incline.

6. Hypophyseal Fossa

• The hypophyseal fossa, also called the pituitary fossa, is a median depression within the sella turcica.
• It lodges the pituitary gland.

7. Dorsum Sellae

• The dorsum sellae is a quadrilateral bony plate that projects upward from the posterior part of the sella turcica.
• It forms the posterior wall of the hypophyseal fossa.

8. Posterior Clinoid Processes

– The posterior clinoid processes are paired bony projections arising from the superolateral angles of the dorsum sellae.

Attachments

• The diaphragma sellae is attached anteriorly to the tuberculum sellae and posteriorly to the upper border of the dorsum sellae.
• The tentorium cerebelli is attached to the posterior clinoid processes.
• The petroclinoid ligaments are dural bands extending between the clinoid processes and the petrous part of the temporal bone.

9. Carotid Groove

• The carotid groove is a shallow curved sulcus on either side of the body of the sphenoid, lateral to the hypophyseal fossa.
• It extends from the region of the foramen lacerum towards the anterior clinoid process.
• Relation: It is related to the cavernous part of the internal carotid artery.

Lateral area

• The lateral area of the middle cranial fossa is formed by three bony surfaces:
  • Cerebral surface of the greater wing of the sphenoid.
  • Anterior surface of the petrous part of the temporal bone.
  • Internal surface of the squamous part of the temporal bone.
• It is wider and deeper than the median area.
• It accommodates the temporal lobe of the cerebral hemisphere.

1. Greater Wing of the Sphenoid

• The greater wing of the sphenoid forms a major part of the anterolateral floor of the middle cranial fossa.
• It presents several foramina and grooves for the passage of nerves and blood vessels.

A. Foramen Spinosum

• The foramen spinosum is a small, rounded opening near the posterolateral part of the greater wing of the sphenoid.
• Structures transmitted: It transmits the following structures:
  • The middle meningeal artery.
  • The meningeal branch of the mandibular nerve, also called the nervus spinosus.
  • The accompanying middle meningeal vein or veins.

B. Foramen Ovale

• The foramen ovale is a large, oval opening in the greater wing of the sphenoid.
• Structures transmitted (Viva, MCQ): It transmits the following structures:
  • The mandibular nerve, which is the third division of the trigeminal nerve.
  • The accessory meningeal artery.
  • The lesser petrosal nerve, which may pass through this foramen.
  • An emissary vein connecting the cavernous sinus with the pterygoid venous plexus.

– Mnemonic: MALE represents the Mandibular nerve, Accessory meningeal artery, Lesser petrosal nerve, and Emissary vein.

C. Foramen Rotundum

• The foramen rotundum is a rounded opening situated anterior and medial to the foramen ovale.
• It connects the middle cranial fossa with the pterygopalatine fossa.
• It does not open onto the external surface of the skull base.
• Structure transmitted (Viva, Spotters): It transmits the maxillary nerve, which is the second division of the trigeminal nerve.

D. Superior Orbital Fissure

• The superior orbital fissure is a triangular opening between the greater and lesser wings of the sphenoid.
• It connects the middle cranial fossa with the orbit.
• It transmits several cranial nerves and ophthalmic veins.

E. Sphenoidal Emissary Foramen

• The sphenoidal emissary foramen, also called the foramen of Vesalius, is a small, inconstant opening situated medial to the foramen ovale.
• Structure transmitted: It transmits an emissary vein connecting the cavernous sinus with the pterygoid venous plexus. (MCQ)

F. Canaliculus Innominatus

• The canaliculus innominatus is an inconstant bony canal situated between the foramen ovale and foramen spinosum.
• Structure transmitted: When present, it transmits the lesser petrosal nerve, which may otherwise pass through the foramen ovale.

G. Grooves for Middle Meningeal Vessels

• The groove for the middle meningeal vessels begins at the foramen spinosum and extends anteriorly.
• It divides into anterior and posterior grooves.
• The anterior groove ascends towards the pterion and continues onto the inner surface of the parietal bone.
• The posterior groove passes backward across the inner surface of the squamous part of the temporal bone.

H. Foramen Lacerum

• The foramen lacerum is an irregular opening situated near the junction of the body of the sphenoid, the apex of the petrous temporal bone, and the basilar part of the occipital bone.
• In the living individual, it is largely filled with fibrocartilage.
• The internal carotid artery passes across its superior aspect after emerging from the carotid canal rather than passing vertically through the foramen.

2. Anterior Surface of the Petrous Part of the Temporal Bone

• The anterior surface of the petrous part of the temporal bone contributes to the posterolateral floor of the middle cranial fossa.
• It presents the following features:

A. Trigeminal Impression

• The trigeminal impression is a shallow depression near the apex of the petrous part of the temporal bone.
• Relation: It accommodates the trigeminal ganglion within a dural recess called the trigeminal cave (Meckel’s cave).

B. Hiatus and Groove for the Greater Petrosal Nerve

• The hiatus for the greater petrosal nerve is located lateral to the trigeminal impression.
• It opens into a groove that extends anteromedially towards the foramen lacerum.
• Structure transmitted: The hiatus and groove transmit the greater petrosal nerve.

C. Hiatus and Groove for the Lesser Petrosal Nerve

• The hiatus for the lesser petrosal nerve is situated lateral to the hiatus for the greater petrosal nerve.
• Its groove extends forwards towards the region of the foramen ovale.
• Structure transmitted: The hiatus and groove transmit the lesser petrosal nerve.

D. Arcuate Eminence

• The arcuate eminence is an irregular bony elevation situated near the middle of the anterior surface of the petrous temporal bone.
• It generally corresponds to the position of the superior semicircular canal of the internal ear.

E. Tegmen Tympani

• The tegmen tympani is a thin plate of bone forming part of the anterior surface of the petrous temporal bone.
• It forms the roof of the tympanic cavity, mastoid antrum, and the canals associated with the tensor tympani muscle and auditory tube.
• Its anterolateral part extends into the squamotympanic fissure.
• The openings for the greater and lesser petrosal nerves are situated in the anterior surface of the petrous temporal bone near the tegmen tympani.
• The lateral margin of the tegmen tympani turns downwards and contributes to the lateral wall of the bony part of the auditory tube.

3. Internal Surface of the Squamous Part of the Temporal Bone

• The internal surface of the squamous part of the temporal bone forms part of the lateral wall and floor of the middle cranial fossa.
• It bears grooves produced by the middle meningeal vessels and their anterior and posterior branches.

Superior Border of the Petrous Part

• The superior border of the petrous part of the temporal bone separates the middle cranial fossa from the posterior cranial fossa.
• It contains a groove for the superior petrosal sinus.
• Attachment: It provides attachment to the tentorium cerebelli.

POSTERIOR CRANIAL FOSSA

• The posterior cranial fossa is the deepest of the three cranial fossae.
• It lies behind the superior borders of the petrous parts of the temporal bones.
• It accommodates the hindbrain, which consists of the cerebellum, pons, and medulla oblongata.

Boundaries

• Anteriorly: It is bounded by the dorsum sellae of the sphenoid in the midline and the superior borders of the petrous parts of the temporal bones on either side.
• Posteriorly: It is bounded by the squamous part of the occipital bone.
• Laterally: It is bounded by the mastoid parts of the temporal bones and adjoining parts of the parietal bones.

Floor

– The floor of the posterior cranial fossa is formed by the following structures:

Median Area

• Clivus
• Foramen magnum
• Internal surface of the squamous part of the occipital bone

Lateral Area

• Condylar parts of the occipital bone
• Posterior surfaces of the petrous parts of the temporal bones
• Mastoid parts of the temporal bones
• Adjacent parts of the parietal bones

Features of Posterior Cranial Fossa

Median area

– The median area presents the clivus, foramen magnum, and internal surface of the squamous part of the occipital bone.

1. Clivus

• The clivus is a sloping bony surface situated anterior to the foramen magnum.
• Formation: It is formed by the posterior part of the body of the sphenoid and the basilar part of the occipital bone, which fuse during development.
• Relations: It is related to the basilar venous plexus, pons, and medulla oblongata.
• The clivus is separated from the petrous part of the temporal bone by the petro-occipital fissure, which continues posteriorly towards the jugular foramen.
• Attachments: The lower part of the clivus provides attachment to the following structures:
  • Apical ligament of the dens.
  • Superior longitudinal band of the cruciform ligament.
  • Tectorial membrane. (MCQ)

2. Foramen Magnum

• The foramen magnum is a large oval opening situated in the floor of the posterior cranial fossa.
• It connects the cranial cavity with the vertebral canal.

3. Squamous Part of the Occipital Bone

– The internal surface of the squamous part of the occipital bone presents the following features:

a. Internal Occipital Protuberance

• The internal occipital protuberance is a bony prominence situated on the internal surface of the occipital bone.
• It lies approximately opposite the external occipital protuberance.

b. Internal Occipital Crest

• The internal occipital crest is a curved median bony ridge extending from the internal occipital protuberance towards the posterior margin of the foramen magnum.
• Near the foramen magnum, it may form a small depression called the vermian fossa.
• Attachment: It provides attachment to the falx cerebelli.

c. Groove for the Transverse Sinus

• The groove for the transverse sinus is a broad bony groove extending laterally from the internal occipital protuberance.
• It passes towards the mastoid angle of the parietal bone, where it continues as the groove for the sigmoid sinus.
• Relation: It lodges the transverse venous sinus.
• The right transverse sinus is commonly larger than the left, and its groove is usually wider.

d. Cerebellar Fossae

• The cerebellar fossae are paired depressions situated on either side of the internal occipital crest.
• Each fossa accommodates a cerebellar hemisphere.

Lateral area

• The lateral area of the posterior cranial fossa is formed mainly by the condylar part of the occipital bone and the petrous and mastoid parts of the temporal bone.
• It presents several bony projections, foramina, canals, and grooves.

1. Condylar Part of the Occipital Bone

– The condylar part of the occipital bone presents the following features:

a. Jugular Tubercle

• The jugular tubercle is a rounded bony elevation on the superior surface of the lateral part of the occipital bone.
• It lies above the hypoglossal canal.
• Its surface is related to the glossopharyngeal, vagus, and accessory nerves (cranial nerves IX, X, and XI).

b. Hypoglossal Canal

• The hypoglossal canal, also called the anterior condylar canal, is situated on either side of the foramen magnum, near its anterolateral margin.
• It runs obliquely forwards and laterally through the occipital bone.
• Structures transmitted (Viva):
  • Hypoglossal nerve.
  • Meningeal branch of the hypoglossal nerve.
  • Meningeal branch of the ascending pharyngeal artery.
  • Emissary veins connecting the intracranial venous system with extracranial venous plexuses.

c. Condylar Canal

• The condylar canal, also called the posterior condylar canal, is an inconstant bony passage located behind the occipital condyle.
• Its internal opening is usually situated near the lower part of the groove for the sigmoid sinus.
• Structures transmitted: It may transmit the following structures:
  • Condylar emissary vein.
  • Meningeal branch of the occipital artery.

2. Posterior Surface of the Petrous Part of the Temporal Bone

• The posterior surface of the petrous part of the temporal bone forms part of the anterior wall of the posterior cranial fossa.
• It presents the following features:

a. Internal Acoustic Meatus

• The internal acoustic meatus is a short bony canal situated near the centre of the posterior surface of the petrous part of the temporal bone.
• It lies above the jugular foramen.
• Structures transmitted (Viva):
  • Facial nerve (cranial nerve VII).
  • Vestibulocochlear nerve (cranial nerve VIII).
  • Labyrinthine artery and veins.

b. Aqueduct of the Vestibule

• The aqueduct of the vestibule is a narrow bony canal that opens on the posterior surface of the petrous temporal bone, behind the internal acoustic meatus.
• Structure transmitted: It transmits the endolymphatic duct of the internal ear and an accompanying small vein.

c. Cochlear Canaliculus

• The cochlear canaliculus is a small bony canal that opens near the inferior margin of the posterior surface of the petrous temporal bone, close to the jugular foramen.
• It establishes communication between the perilymphatic space of the cochlea and the subarachnoid space.
• Structures transmitted: It transmits the perilymphatic duct and a small vein.

d. Jugular Foramen

• The jugular foramen is an irregular opening situated at the posterior end of the petro-occipital fissure.
• It is formed between the petrous part of the temporal bone and the occipital bone.
• Its superior margin is relatively sharp, whereas the inferior margin is smoother.
• It transmits the glossopharyngeal, vagus, and accessory nerves, along with major venous channels and meningeal branches of arteries.

3. Mastoid Part of the Temporal Bone

• The mastoid part of the temporal bone contributes to the lateral wall of the posterior cranial fossa.
• It presents the following features:

a. Sigmoid Sulcus

• The sigmoid sulcus is an S-shaped groove on the internal surface of the temporal bone.
• It begins as a continuation of the groove for the transverse sinus and extends downwards towards the jugular foramen.
• Relation: It lodges the sigmoid venous sinus, which continues as the internal jugular vein at the jugular foramen.

b. Mastoid Foramen

• The mastoid foramen is a small, variable opening that usually communicates with the upper part of the sigmoid sulcus.
• Structure transmitted: It transmits a mastoid emissary vein connecting the sigmoid sinus with extracranial veins.
• It may also transmit a meningeal branch of the occipital artery.