Exterior of Skull

SKULL (Skull = Cranium + Mandible)

• The skull forms the bony framework of the head. It consists of 22 bones and 6 auditory ossicles.
• The cranium consists of all the bones of the skull except the mandible. The word cranium is derived from the Greek word for skull.
• The mandible, or lower jawbone, is the only movable bone of the skull.

Parts of Skull

The skull is divided into two main parts:

1. Neurocranium

• The neurocranium forms the bony cavity that encloses and protects the brain.
• It consists of two parts:
  • Skull vault (calvaria): It forms the upper part of the neurocranium.
  • Base of the skull: It forms the lower part of the neurocranium.

2. Viscerocranium

– The viscerocranium, or facial skeleton, forms the bony framework of the face, including the mandible.

Functions of Skull

1. Protection: The skull protects the brain, meninges, and organs of special senses.
2. Enclosure: It forms the cranial cavity, which accommodates the brain, meninges, and cerebrospinal fluid (CSF).
3. Formation of cavities: It forms cavities that accommodate the eyes, middle and internal ears, olfactory epithelium, and tongue.
4. Passage for air and food: It forms the nasal and oral cavities, which provide passages for breathing and swallowing.
5. Muscle attachments: It provides attachment sites for several muscles, particularly those involved in mastication and facial expression.

Bones of the Skull

– The skull consists of 22 bones and 6 auditory ossicles, which are grouped into the cranial and facial skeletons.

A. Cranial skeleton (Neurocranium)

• The cranial skeleton consists of 8 bones and contains 6 auditory ossicles.
• The cranial bones are classified as follows:
  • Paired bones (4): These include two parietal bones and two temporal bones.
  • Unpaired bones (4): These include the frontal, occipital, sphenoid, and ethmoid bones.
• The auditory ossicles (6) consist of three bones in each middle ear: the malleus, incus, and stapes.

B. Facial skeleton (Viscerocranium)

– The facial skeleton consists of 14 bones, which are classified as follows:

1. Paired bones (12): These include the maxillae, zygomatic bones, nasal bones, lacrimal bones, palatine bones, and inferior nasal conchae.
2. Unpaired bones (2): These include the mandible and vomer.

Joint of Skull

• Most bones of the skull are joined by sutures, which are immovable fibrous joints.

• These joints are classified functionally as synarthroses, as they normally permit no movement.

• Apart from sutures, the skull contains the following synovial joints:

1. Ossicular joints: These include two pairs of joints between the auditory ossicles, namely the incudomalleolar and incudostapedial joints.
2. Temporomandibular joints: These are paired synovial joints between the mandible and temporal bones.

Types of Sutures

The sutures of the skull are classified into the following types:

1. Plane suture: The adjoining bones have relatively straight edges, as seen in the internasal suture.
2. Serrate suture: The adjoining bones have saw-toothed margins that interlock with each other, as seen in the coronal suture.
3. Denticulate suture: The adjoining bones have prominent tooth-like projections that fit together, as seen in the lambdoid suture.
4. Squamous suture: The adjoining bones have bevelled margins that overlap each other, as seen in the suture between the parietal and temporal bones.
5. Schindylesis: A thin bony plate fits into a groove in another bone, as seen in the articulation between the rostrum of the sphenoid and the vomer.

Anatomical Position of Skull

– The skull is held in its anatomical position by keeping the following features aligned:

1. The orbital cavities are directed forward.
2. The inferior margins of the orbits and the superior margins of the external acoustic meatuses lie in the same horizontal plane, known as the Frankfort horizontal plane.

Reid’s Baseline

• Reid’s baseline is a reference line extending from the inferior margin of the orbit to the centre of the external acoustic meatus (Fig. 1.2).
• It is commonly used as a reference line in radiological positioning of the skull.

Norma (Views) of the Skull

·  The external features of the skull are studied in five standard views, known as normae.

·  These views include:

1. Norma verticalis: It shows the skull from above (superior view).
2. Norma frontalis: It shows the skull from the front (anterior view).
3. Norma occipitalis: It shows the skull from behind (posterior view).
4. Norma lateralis: It shows the skull from the side (lateral view).
5. Norma basalis: It shows the skull from below (inferior view).

·  The skull is also studied by examining its individual bones and the internal features of the cranial cavity.

·  Norma verticalis is the superior view of the skull, showing its upper surface.

·  Shape: The skull appears oval when viewed from above and is broader posteriorly than anteriorly.

NORMA VERTICALIS

·  Norma verticalis is the superior view of the skull, showing its upper surface.

·  Shape: The skull appears oval when viewed from above and is broader posteriorly than anteriorly.

Bones and Sutures of Norma Verticalis

Bones

The following bones are visible in this view:

1. Frontal bone.
2. Right and left parietal bones.
3. Occipital bone.

Sutures

The following sutures can be identified in this view:

1. Coronal suture: It lies between the frontal bone and the two parietal bones.
2. Sagittal suture: It extends between the right and left parietal bones.
3. Lambdoid suture: It separates the occipital bone from the two parietal bones.
4. Metopic suture: It may occasionally persist between the two halves of the frontal bone in adults.

Features of Norma Verticalis

Bony features

• The frontal bone forms the anterior part of the skull, while the occipital bone forms its posterior part.
• The right and left parietal bones form most of the superior and lateral parts of the skull.

1. Sutures

1. Coronal suture: It lies between the frontal bone and the two parietal bones. The term coronal is derived from the Latin word for crown.

2. Sagittal suture: It lies in the median plane between the right and left parietal bones.

3. Lambdoid suture: It lies posteriorly between the occipital bone and the two parietal bones. It resembles the Greek letter lambda (λ).

4. Metopic suture: It is a midline suture that divides the frontal bone into right and left halves. It normally fuses during childhood, usually by 6 years of age. It may persist in approximately 3–8% of adults.

Important Anatomical Landmarks

1. Vertex: It is the highest point of the skull when the head is held in the anatomical position. It lies in the median plane, usually over the sagittal suture.

2. Bregma: It is the point where the coronal and sagittal sutures meet. It corresponds to the site of the anterior fontanelle, which usually closes by 18–24 months after birth.

3. Lambda: It is the point where the sagittal and lambdoid sutures meet. It marks the site of the posterior fontanelle, which usually closes within 2–3 months after birth.

4. Parietal eminence (parietal tuber): It is the most prominent convex area on the external surface of each parietal bone. It is an important surface landmark and may be involved in skull fractures.

5. Obelion: It is a point on the sagittal suture located between the right and left parietal foramina.

6. Temporal lines: These are curved bony ridges extending across the frontal and parietal bones. Their detailed features are described under norma lateralis.

Other Features

Parietal Foramen

• The parietal foramen is a small opening in the parietal bone, usually present on each side. It lies close to the sagittal suture, approximately 2.5–4 cm anterior to the lambda.
• Structure transmitted: It usually transmits a parietal emissary vein that connects the extracranial veins of the scalp with the superior sagittal sinus.

NORMA OCCIPITALIS

·  The posterior aspect of the skull, viewed from behind, is called norma occipitalis.

·  It is convex superiorly and on both sides but relatively flattened inferiorly.

Bones and Sutures of Norma Occipitalis

Bones

1. Parietal bones: The two parietal bones form the upper part of norma occipitalis.
2. Occipital bone: The occipital bone forms its lower part.
3. Temporal bones: The mastoid parts of the temporal bones contribute to its inferolateral regions.

Sutures

1. Lambdoid suture: It lies between the occipital bone and the two parietal bones. It resembles the Greek letter lambda (λ). Small accessory bones, called Wormian bones or sutural bones, are frequently found along this suture.
2. Occipitomastoid suture: It lies between the occipital bone and the mastoid part of the temporal bone.
3. Parietomastoid suture: It lies between the parietal bone and the mastoid part of the temporal bone.
4. Sagittal suture: The posterior end of the sagittal suture is also visible in norma occipitalis.

Reference points

1. Lambda

– The lambda is the point where the sagittal suture meets the two limbs of the lambdoid suture.

2. External Occipital Protuberance

• The external occipital protuberance is a bony projection in the median plane on the outer surface of the occipital bone. It is an important landmark at the junction of the head and neck.

Attachments

• The trapezius muscle takes origin from this projection. The upper end of the ligamentum nuchae is attached to it.

3. Inion

• The inion is the most prominent point of the external occipital protuberance. It can usually be palpated through the skin.

4. Superior Nuchal Line

– The superior nuchal line is a curved bony ridge that extends laterally from the external occipital protuberance on each side.

Attachments

• The trapezius originates from the medial one-third of the superior nuchal line. The sternocleidomastoid is inserted into the lateral one-third of the line from above. The splenius capitis is inserted into the lateral one-third of the line from below.

5. Highest Nuchal Line

• The highest nuchal line is a faint, curved ridge on the external surface of the occipital bone. It lies approximately 1 cm above the superior nuchal line.

Attachments

• The epicranial aponeurosis is attached to its medial part. The occipital belly of the occipitofrontalis (occipitalis) originates from its lateral part.

Note: When the highest nuchal line is absent, the epicranial aponeurosis and occipitalis are attached to the superior nuchal line.

6. Mastoid Foramen

• The mastoid foramen is a small opening usually located in the mastoid part of the temporal bone, close to the occipitomastoid suture. Its position and size may vary.

Structures passing through the mastoid foramen

• An emissary vein passes through the foramen and connects the extracranial veins with the sigmoid sinus.
• A meningeal branch of the occipital artery passes through the foramen to supply the dura mater.

NORMA FRONTALIS

·  The anterior view of the skull is called norma frontalis. It has an oval outline and is broader above than below.

Bones and Sutures of Norma Frontalis

Bones

The following bones are visible in norma frontalis:

1. Frontal bone: It forms the forehead and the upper margins of the orbits.
2. Nasal bones: The two nasal bones form the bony bridge of the nose.
3. Maxillae: The right and left maxillae form the upper jaw.
4. Zygomatic bones: The two zygomatic bones form the prominences of the cheeks.
5. Mandible: It forms the lower jaw.

Sutures

1. Internasal suture: It lies in the median plane between the right and left nasal bones.
2. Frontonasal suture: It is present between the frontal bone and the two nasal bones.
3. Frontomaxillary suture: It lies between the frontal bone and the frontal process of the maxilla.
4. Frontozygomatic suture: It is formed between the zygomatic process of the frontal bone and the frontal process of the zygomatic bone.
5. Nasomaxillary suture: It lies between the nasal bone and the frontal process of the maxilla.
6. Intermaxillary suture: It lies in the median plane between the right and left maxillae.
7. Zygomaticomaxillary suture: It lies between the zygomatic bone and the maxilla.

Descriptive Divisions

For descriptive purposes, norma frontalis is divided into four regions:

1. Frontal region.
2. Orbital openings.
3. Anterior bony nasal aperture.
4. Lower part of the face.

Frontal region of norma frontalis

• The frontal region is formed by the squamous part of the frontal bone.
• It presents the following important bony landmarks.

Bony Landmarks

• Superciliary arches: These are curved bony elevations situated above the medial parts of the orbital margins, one on each side.
• Glabella: It is a smooth, slightly elevated area in the median plane between the two superciliary arches.
• Nasion: It is the median point at the junction of the frontonasal and internasal sutures.
• Frontal eminences: These are rounded elevations on the frontal bone, situated above the superciliary arches.

The superciliary arches are generally more prominent in males, whereas the frontal eminences tend to be more noticeable in females.

Orbital openings

• • Each orbital opening is a quadrangular opening of the bony orbit. It has four margins as follows (The two orbital openings are situated on either side of the root of the nose.
• Each opening is approximately quadrangular and has four margins.

Margins of the Orbital Opening

• Superior orbital margin: It is formed entirely by the frontal bone.
• Medial orbital margin: Its upper part is formed by the frontal bone, while the lower part is formed by the frontal process of the maxilla.
• Inferior orbital margin: It is formed by the maxilla medially and the zygomatic bone laterally.
• Lateral orbital margin: Its upper part is formed by the zygomatic process of the frontal bone, while the lower part is formed by the frontal process of the zygomatic bone.

Supraorbital Notch

• The supraorbital notch is present near the junction of the medial one-third and lateral two-thirds of the superior orbital margin.
• It may sometimes be converted into a complete bony opening called the supraorbital foramen.

Structures passing through the supraorbital notch or foramen:

• The supraorbital nerve passes through the notch or foramen to supply the forehead and scalp.
• The supraorbital artery and accompanying veins also pass through this opening.

Bony nasal aperture

• The anterior bony nasal aperture, also called the piriform aperture, is a pear-shaped opening in the anterior part of the skull.
• It forms the bony entrance to the nasal cavity.

Boundaries

• Superiorly: It is bounded by the inferior borders of the nasal bones.
• Laterally: It is bounded by the nasal notches of the right and left maxillae.
• Inferiorly: It is bounded by the maxillae, which meet in the median plane at the anterior nasal spine.

Important Bony Landmarks

• Anterior nasal spine: It is a sharp bony projection in the median plane at the lower margin of the anterior nasal aperture. It is formed by the union of the right and left maxillae.
• Rhinion: It is the lowest point of the internasal suture, near the inferior ends of the nasal bones.

Lower part of face

The lower part of norma frontalis is formed by the following bones:

• The maxillae form the upper jaw.
• The zygomatic bones form the prominences of the cheeks.
• The mandible forms the lower jaw.

A. Maxillae (Upper Jaw)

• The upper jaw is formed by the right and left maxillae.
• Each maxilla presents the following important features.

Bony features

• Nasal notch: It is a curved bony margin that forms the lateral boundary of the anterior bony nasal aperture.
• Anterior nasal spine: It is a sharp projection formed by the maxillae in the median plane, at the lower margin of the nasal aperture.
• Infraorbital foramen: It is a small opening on the anterior surface of the maxilla, approximately 1 cm below the inferior orbital margin.
• Incisive fossa: It is a shallow depression on the anterior surface of the maxilla, above the incisor teeth.
• Canine eminence: It is a vertical bony elevation produced by the root of the canine tooth.
• Canine fossa: It is a depression on the anterior surface of the maxilla, lateral to the canine eminence.

Structures passing through the infraorbital foramen

• The infraorbital nerve, a continuation of the maxillary division of the trigeminal nerve, emerges through this foramen.
• The infraorbital artery and veins also pass through the foramen.

Processes of the Maxilla

1. Frontal process

• The frontal process is an upward projection of the maxilla, situated along the lateral side of the nose.
• It articulates with the frontal bone at the frontomaxillary suture.
• It articulates with the nasal bone at the nasomaxillary suture.
• It also articulates with the lacrimal bone at the lacrimomaxillary suture.

2. Zygomatic process

• The zygomatic process is a short lateral projection of the maxilla.
• It articulates with the maxillary process of the zygomatic bone at the zygomaticomaxillary suture.

3. Alveolar process

• The alveolar process forms the thick, curved lower border of the maxilla.
• Each maxilla normally contains eight alveolar sockets for the upper teeth in an adult.

B. Zygomatic Bone

• The zygomatic bone forms the bony prominence of the cheek.
• In norma frontalis, its articulations with the frontal bone and maxilla are visible.

Muscular attachments

• The zygomaticus major originates from the lateral surface of the zygomatic bone.
• The zygomaticus minor originates from the anterior part of the lateral surface of the zygomatic bone.

Zygomaticofacial foramen

• The zygomaticofacial foramen is a small opening on the facial surface of the zygomatic bone.
• It transmits the zygomaticofacial nerve, a branch of the zygomatic nerve arising from the maxillary division of the trigeminal nerve.
• Small accompanying blood vessels also pass through this foramen.

C. Mandible (Lower Jaw)

• The mandible is a U-shaped bone that forms the lower jaw.
• It is the only freely movable bone of the skull, apart from the auditory ossicles.
• The following features can be identified in norma frontalis or along its lateral outline.

Important bony features

• Alveolar arch: It forms the upper border of the body of the mandible and contains sockets for the lower teeth.
• Body of mandible: It is the horizontal, U-shaped part of the mandible that supports the lower teeth.
• Symphysis menti: It is a median bony ridge on the anterior surface of the body, marking the fusion of the two halves of the mandible.
• Mental protuberance: It is a triangular bony prominence in the median plane that forms the chin.
• Gonion: It is the anatomical landmark at the angle of the mandible, where the body meets the ramus.
• Mental foramen: It is a small opening on the outer surface of the body of the mandible, usually located below the second premolar tooth.
• Oblique line: It is a bony ridge extending upward and backward from the region of the mental foramen towards the anterior border of the ramus.
• Ramus of mandible: It is the vertically directed part that extends upward from the posterior end of the body.

Structures passing through the mental foramen

• The mental nerve emerges through the foramen to supply the skin of the chin, lower lip and adjacent gingiva.
• The mental artery and vein also pass through the foramen.

Muscular and Ligamentous Attachments in Norma Frontalis

Attachments to the Frontal and Nasal Bones

• Corrugator supercilii: It originates from the medial part of the superciliary arch of the frontal bone.
• Procerus: It originates from the fascia covering the lower part of the nasal bone and the adjoining upper lateral nasal cartilage.
• Orbicularis oculi: Its orbital part takes origin from the medial orbital margin, including the frontal process of the maxilla and adjacent part of the frontal bone.

Attachments to the Maxilla

• Medial palpebral ligament: It is attached to the anterior lacrimal crest of the frontal process of the maxilla.
• Levator labii superioris alaeque nasi: It originates from the frontal process of the maxilla.
• Levator labii superioris: It originates from the maxilla between the inferior orbital margin and the infraorbital foramen.
• Levator anguli oris: It originates from the canine fossa of the maxilla, below the infraorbital foramen.
• Nasalis: It originates from the maxilla in the region of the canine and incisor teeth.
• Depressor septi nasi: It originates from the incisive fossa of the maxilla above the central incisor tooth.
• Incisivus labii superioris: It originates from the incisive fossa of the maxilla and blends with the orbicularis oris.

NORMA LATERALIS

– The lateral view of the skull is called norma lateralis.

Bones and Sutures of Norma Lateralis

Bones

The following bones are visible in norma lateralis:

1. Frontal bone.
2. Parietal bone.
3. Occipital bone.
4. Temporal bone.
5. Sphenoid bone.
6. Zygomatic bone.
7. Maxilla.
8. Nasal bone.
9. Mandible.

Sutures

·  Coronal suture: It lies between the frontal and parietal bones.

·  Squamous (parietosquamous) suture: It lies between the parietal bone and the squamous part of the temporal bone.

·  Lambdoid suture: It lies between the parietal and occipital bones.

Features of Norma Lateralis

1. Temporal Lines

• The temporal lines are curved bony ridges that extend upward and backward from the zygomatic process of the frontal bone.
• They divide into two lines, called the superior temporal line and inferior temporal line.
• The superior temporal line gradually becomes less distinct as it extends posteriorly.
• The inferior temporal line is more prominent and continues posteriorly towards the supramastoid crest.

Attachments

• The temporal fascia is attached to the superior temporal line.
• The temporalis muscle originates from the floor of the temporal fossa below the inferior temporal line.

2. Supramastoid Crest

• The supramastoid crest is a curved bony ridge on the lateral surface of the temporal bone.
• It is continuous anteriorly with the posterior root of the zygomatic process of the temporal bone.
• It marks the boundary between the squamous and mastoid parts of the temporal bone.

3. Zygomatic Arch

• The zygomatic arch is a horizontal bony bar situated on each side of the skull, in front of the external ear.
• It forms the lateral boundary of the temporal fossa.

Formation

• Anterior one-third: It is formed by the temporal process of the zygomatic bone.
• Posterior two-thirds: It is formed by the zygomatic process of the temporal bone.
• The two processes meet at the zygomaticotemporal suture, which runs obliquely downward and backward.

Bony features

• The zygomatic arch has two surfaces, namely the outer and inner surfaces.
• It also has two borders, namely the superior and inferior borders.

Attachments

• The masseter muscle originates from the inferior border and inner surface of the zygomatic arch.
• The lateral ligament of the temporomandibular joint is attached to the articular tubercle at the root of the zygomatic process of the temporal bone.

4. Structures Passing Deep to the Zygomatic Arch

The following structures pass through the space between the zygomatic arch and the lateral surface of the skull:

• The tendon of the temporalis muscle passes deep to the zygomatic arch to reach the coronoid process of the mandible.
• The deep temporal nerves and vessels pass through this region to supply the temporalis muscle.

5. External Acoustic Meatus

• The external acoustic meatus is an opening in the temporal bone, situated below the posterior root of the zygomatic arch.
• It leads into the external acoustic canal, which extends towards the tympanic membrane.

Bony formation

• Tympanic part of the temporal bone: It forms the anterior and inferior walls and the lower part of the posterior wall of the bony external acoustic meatus.
• Squamous part of the temporal bone: It forms the superior wall and contributes to the upper part of the posterior wall.

Mastoid Part of the Temporal Bone

• The mastoid process is a conical bony projection extending downward from the anteroinferior part of the mastoid region. It is easily palpable behind the external ear.

Muscular attachments

– The following muscles are inserted into the mastoid process, from anterior to posterior:

1. Sternocleidomastoid is inserted into its lateral surface anteriorly.
2. Splenius capitis is inserted behind the sternocleidomastoid.
3. Longissimus capitis is inserted into the posterior part of the mastoid process.

Parietomastoid Suture

– The parietomastoid suture is present between the parietal bone and the mastoid part of the temporal bone.

Occipitomastoid Suture

– The occipitomastoid suture lies between the occipital bone and the mastoid part of the temporal bone.

Asterion

• The asterion is the point where the lambdoid, parietomastoid and occipitomastoid sutures meet.
• It marks the site of the posterolateral (mastoid) fontanelle in the newborn.
• This fontanelle usually closes between 6 and 18 months after birth.

Mastoid Notch

• The mastoid notch is a deep groove on the medial aspect of the mastoid process. It provides origin to the posterior belly of the digastric muscle. A groove for the occipital artery is present medial to the mastoid notch.

Tympanomastoid Fissure

– The tympanomastoid fissure is a narrow cleft between the tympanic and mastoid parts of the temporal bone.

Mastoid Foramen

• The mastoid foramen is a small opening usually found near the posterior border of the mastoid part of the temporal bone.
• It is commonly located near the occipitomastoid suture, although its position may vary.

Structures passing through

• The mastoid emissary vein connects the sigmoid sinus with extracranial veins, particularly the posterior auricular vein.
• A meningeal branch of the occipital artery passes through the foramen to supply the dura mater.

Styloid Process

• The styloid process is a slender, pointed bony projection extending from the inferior surface of the temporal bone. It is situated anteromedial to the mastoid process and is best seen in norma basalis. It projects downward, forward and slightly medially. It consists of a base, shaft and pointed tip. Its base is partly surrounded by the vaginal process of the tympanic part of the temporal bone.

Muscular attachments

– The following three muscles originate from the styloid process:

1. Styloglossus originates from its anterior aspect.
2. Stylohyoid originates from its posterior aspect near the base.
3. Stylopharyngeus originates from its medial aspect near the base.

Ligamentous attachments

• The stylohyoid ligament extends from the tip of the styloid process to the lesser horn of the hyoid bone.
• The stylomandibular ligament extends from the styloid process to the angle and posterior border of the ramus of the mandible.

Zygomaticotemporal Foramen

• The zygomaticotemporal foramen is a small opening on the temporal surface of the zygomatic bone. It transmits the zygomaticotemporal nerve, which arises from the zygomatic nerve, a branch of the maxillary division of the trigeminal nerve. Small accompanying blood vessels also pass through the foramen.

Suprameatal triangle

• The suprameatal triangle, also called MacEwen’s triangle, is a small triangular area located posterosuperior to the external acoustic meatus.
• It is named after Sir William MacEwen, a Scottish surgeon (1848–1924).

Boundaries

• Superiorly: It is bounded by the supramastoid crest.
• Anteriorly: It is bounded by the posterosuperior margin of the external acoustic meatus.
• Posteriorly: It is bounded by a vertical line drawn tangential to the posterior margin of the external acoustic meatus.

Temporal fossa

• The temporal fossa is a shallow depression on the lateral aspect of the skull, situated above the zygomatic arch.
• It accommodates the temporalis muscle.

Boundaries

• Superiorly and posteriorly: It is bounded by the superior temporal line.
• Inferiorly: It is limited by the zygomatic arch laterally and the infratemporal crest of the greater wing of the sphenoid medially.
• Anteriorly: It is bounded by the frontal and zygomatic bones.
• Inferomedially: It communicates with the infratemporal fossa through the space deep to the zygomatic arch.

Floor

– The floor of the temporal fossa is formed by parts of four bones:

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

Pterion

• The pterion is an H-shaped sutural junction located in the anterior part of the temporal fossa.
• It is formed by the meeting of four bones:
  • Frontal bone.
  • Parietal bone.
  • Squamous part of the temporal bone.
  • Greater wing of the sphenoid bone.

Location

– The pterion is situated approximately 4 cm above the midpoint of the zygomatic arch and 2.5 cm behind the frontozygomatic suture.

Deep Relations

The following structures lie deep to the pterion:

1. Anterior division of the middle meningeal artery.
2. Middle meningeal veins.
3. Stem of the lateral sulcus of the cerebral hemisphere.

NORMA BASALIS

• The inferior view of the skull is called norma basalis.
• For descriptive purposes, norma basalis is divided into three parts by two imaginary transverse lines.

Dividing Lines

• Anterior transverse line: It passes along the posterior border of the hard palate.
• Posterior transverse line: It passes along the anterior margin of the foramen magnum.

Divisions of Norma Basalis

1. Anterior part.
2. Middle part.
3. Posterior part.

<H4>Anterior Part of Norma Basalis

·  The anterior part of norma basalis is formed by the alveolar processes of the maxillae and the hard palate.

·  It consists of two main regions:

1. Alveolar arch.
2. Hard palate.

Alveolar process

• The alveolar processes of the right and left maxillae together form a thick, curved bony margin called the alveolar arch. Each maxilla normally contains eight sockets, called dental alveoli, for the roots of the eight upper teeth in an adult. The sockets are separated from one another by thin bony partitions called interdental septa.

Hard palate

• The hard palate forms the bony roof of the oral cavity and separates it from the nasal cavity.
• It is concave inferiorly and has a slightly rough surface with small depressions related to the palatine glands.

Bony Formation

• Anterior two-thirds: It is formed by the palatine processes of the right and left maxillae.
• Posterior one-third: It is formed by the horizontal plates of the right and left palatine bones.

Sutures of the Hard Palate

• The hard palate presents a cross-shaped arrangement of sutures called the cruciform suture.
• It consists of the following sutures:
  • Intermaxillary suture: It lies between the palatine processes of the right and left maxillae.
  • Interpalatine suture: It lies between the horizontal plates of the right and left palatine bones.
  • Palatomaxillary (transverse palatine) suture: It lies between the palatine processes of the maxillae and the horizontal plates of the palatine bones.

Posterior Border of the Hard Palate

• The posterior border of the hard palate is free and provides attachment to the soft palate. It presents a median bony projection called the posterior nasal spine, formed by the horizontal plates of the two palatine bones.

Attachments

• The palatine aponeurosis is attached to the posterior border of the hard palate. The musculus uvulae takes origin from the posterior nasal spine and the palatine aponeurosis.

Palatine Crest

• The palatine crest is a curved bony ridge that may be present on the inferior surface of the horizontal plate of the palatine bone, near its posterior border. It provides attachment to the palatine aponeurosis, into which the tendon of the tensor veli palatini expands.

Foramina of the Hard Palate

1. Incisive Fossa and Incisive Foramen

• The incisive fossa is a small depression in the median plane of the anterior hard palate, immediately behind the central incisor teeth. The incisive foramen is located within this fossa and forms the oral opening of the incisive canal.

2. Incisive Canal

• The incisive canal connects the anterior part of the hard palate with the floor of the nasal cavity. It commonly divides superiorly into two canals that open into the nasal cavity on either side of the nasal septum.

Structures passing through

• The nasopalatine nerves descend through the canal from the nasal cavity to the anterior hard palate. Terminal branches of the greater palatine arteries ascend through the canal and communicate with septal branches of the sphenopalatine arteries.

3. Greater Palatine Foramen

• The greater palatine foramen is present on each side near the posterolateral part of the hard palate. It usually lies medial to the upper third molar tooth, close to the lateral end of the palatomaxillary suture.

Structures passing through

• The greater palatine nerve passes through the foramen to supply the mucosa of the hard palate and adjacent gingiva. The greater palatine artery and veins also pass through this foramen.

4. Lesser Palatine Foramina

• The lesser palatine foramina are small openings situated behind the greater palatine foramen. They are usually two or three in number on each side and pierce the pyramidal process of the palatine bone.

Structures passing through

• The lesser palatine nerves pass through these foramina to supply the soft palate and adjacent structures.
• The lesser palatine arteries and veins also pass through these openings.

Middle Part of Norma Basalis

·  The middle part of norma basalis extends from the posterior border of the hard palate to an imaginary transverse line passing along the anterior margin of the foramen magnum.

·  For descriptive purposes, it is divided into two areas:

1. Median area.
2. Lateral area.

Median area of middle part of norma basalis

The median area presents two important bony features:

1. Posterior border of the vomer.
2. Broad bar of bone formed by the sphenoid and occipital bones.

Vomer

·  The vomer is a thin, flat bone that forms the posteroinferior part of the bony nasal septum. Its posterior border is free and separates the right and left posterior nasal apertures (choanae).

·  Its inferior border articulates with the nasal crest formed by the maxillae and palatine bones. Its superior border is expanded into two lateral projections called the alae of the vomer, which articulate with the rostrum of the sphenoid bone.

Broad bar of bone

• A broad bony area is present in the median plane, between the posterior nasal apertures and the foramen magnum.
• It is formed by the fusion of two parts:
  • Posterior part of the body of the sphenoid bone.
  • Basilar part of the occipital bone.

Pharyngeal Tubercle

• The pharyngeal tubercle is a small median bony elevation on the inferior surface of the basilar part of the occipital bone.
• It is located approximately 1 cm anterior to the foramen magnum.

Attachments and relations

• The pharyngeal raphe, into which the upper fibres of the superior constrictor of the pharynx are inserted, is attached to the pharyngeal tubercle.
• The bony surface anterior to the pharyngeal tubercle contributes to the roof of the nasopharynx.
• The longus capitis muscle is inserted into the basilar part of the occipital bone, lateral to the pharyngeal tubercle.
• The rectus capitis anterior muscle is inserted into the inferior surface of the occipital bone, anterior to the occipital condyle.

Palatovaginal Canal

• The palatovaginal canal, also called the pharyngeal canal, is a small passage between the sphenoidal process of the palatine bone and the vaginal process of the sphenoid bone.
• It connects the pterygopalatine fossa with the posterior part of the nasal cavity near the nasopharynx.

Structures passing through

• The pharyngeal branch arising from the pterygopalatine ganglion passes through the canal.
• The pharyngeal branch of the maxillary artery also passes through it.

Note: The pterygopalatine ganglion and the third part of the maxillary artery are situated in the pterygopalatine fossa.

Vomerovaginal Canal

• The vomerovaginal canal is a small, inconstant passage between the ala of the vomer and the vaginal process of the sphenoid bone.
• It lies medial to the palatovaginal canal and may provide communication between the pterygopalatine region and the posterior nasal cavity.

Structure passing through

– A pharyngeal nerve branch associated with the pterygopalatine ganglion may pass through this canal.

Lateral area of middle part of norma basalis

The lateral area is formed by parts of two bones:

• Sphenoid bone: It contributes through the pterygoid process and greater wing.
• Temporal bone: It contributes through its petrous, tympanic and squamous parts.

Important Bony Features

The lateral area presents the following structures:

1. Pterygoid process of the sphenoid bone.
2. Infratemporal surface of the greater wing of the sphenoid bone.
3. Spine of the sphenoid bone.
4. Foramen ovale, foramen spinosum and accessory foramina.
5. Sulcus tubae.
6. Inferior surface of the petrous part of the temporal bone.
7. Carotid canal.
8. Foramen lacerum.
9. Tympanic and squamous parts of the temporal bone.

A. Pterygoid Process

• The pterygoid process projects downward from the junction of the body and greater wing of the sphenoid bone. It consists of two thin bony plates called the medial and lateral pterygoid plates.
• The two plates are united anteriorly but diverge posteriorly, enclosing a V-shaped depression called the pterygoid fossa.

Medial Pterygoid Plate

• The medial pterygoid plate is longer and narrower than the lateral pterygoid plate. It presents medial and lateral surfaces, a posterior free border and a pterygoid hamulus at its lower end.

Pterygoid hamulus

• The pterygoid hamulus is a small, hook-shaped projection at the lower end of the posterior border of the medial pterygoid plate.
• Its tip curves laterally and acts as a pulley for the tendon of the tensor veli palatini muscle.

Scaphoid fossa

• The scaphoid fossa is a shallow depression situated at the upper end of the posterior border of the medial pterygoid plate.
• It provides origin to part of the tensor veli palatini muscle.

Attachments of the medial pterygoid plate

• Pharyngobasilar fascia: It is attached to the posterior border of the medial pterygoid plate.
• Superior constrictor of pharynx: It originates from the lower part of the posterior border and the pterygoid hamulus.
• Pterygomandibular raphe: It extends from the tip of the pterygoid hamulus to the posterior end of the mylohyoid line of the mandible.
• Tensor veli palatini: It takes part of its origin from the scaphoid fossa near the upper end of the medial pterygoid plate.

Lateral Pterygoid Plate

• The lateral pterygoid plate is shorter and broader than the medial pterygoid plate.
• Its posterior border is sharp and free.
• Its lateral surface forms part of the medial wall of the infratemporal fossa.
• Its medial surface forms the lateral wall of the pterygoid fossa.

Muscular attachments

• The inferior head of the lateral pterygoid muscle originates from the lateral surface of the lateral pterygoid plate.
• The deep head of the medial pterygoid muscle originates mainly from its medial surface.

Pterygospinous process (Process of Civinini)

• The pterygospinous process is a small, pointed projection that may be present on the posterior border of the lateral pterygoid plate.
• It provides attachment to the pterygospinous ligament, which extends to the spine of the sphenoid bone.
• Ossification of this ligament may produce a bony bridge and a pterygospinous foramen.

B. Infratemporal Surface of the Greater Wing of the Sphenoid

• The infratemporal surface of the greater wing of the sphenoid is approximately pentagonal in outline. It forms part of the roof of the infratemporal fossa.

Margins

1. Anterior margin: It contributes to the posterior boundary of the inferior orbital fissure.
2. Anterolateral margin: It forms the infratemporal crest, which separates the temporal and infratemporal surfaces of the greater wing.
3. Posterolateral margin: It articulates with the squamous part of the temporal bone.
4. Posteromedial margin: It articulates with the petrous part of the temporal bone.
5. Anteromedial region: It is continuous with the body and pterygoid process of the sphenoid bone.

Muscular attachment

– The superior head of the lateral pterygoid muscle originates from the infratemporal surface and adjoining infratemporal crest of the greater wing of the sphenoid.

C. Spine of the Sphenoid

• The spine of the sphenoid is a small, pointed bony projection extending downward from the posterolateral part of the greater wing.
• It lies posterolateral to the foramen spinosum.

Attachments

• The sphenomandibular ligament is attached to the spine of the sphenoid.
• The anterior ligament of the malleus is attached near its base.
• The pterygospinous ligament extends between the spine of the sphenoid and the lateral pterygoid plate.
• A few fibres of the tensor veli palatini and tensor tympani muscles may arise from the adjoining region.

Relations

• The auriculotemporal nerve passes lateral to the spine of the sphenoid.
• The chorda tympani nerve passes medial to the spine.
• The cartilaginous part of the auditory tube is situated medial to the spine.

D. Foramina of the Greater Wing of the Sphenoid

The inferior surface of the greater wing presents two constant foramina and two occasional accessory foramina.

1. Foramen Spinosum

• The foramen spinosum is a small, nearly circular opening in the posteromedial part of the greater wing of the sphenoid. It lies posterolateral to the foramen ovale and just anterior to the spine of the sphenoid.

Structures passing through

• The middle meningeal artery enters the cranial cavity through the foramen spinosum.
• The meningeal branch of the mandibular nerve (nervus spinosus) passes through this opening.
• The accompanying middle meningeal veins also pass through the foramen.

2. Foramen Ovale

• The foramen ovale is a large, oval opening in the greater wing of the sphenoid bone.
• It is situated posterolateral to the upper end of the lateral pterygoid plate.
• It connects the middle cranial fossa with the infratemporal fossa.

Structures passing through

1. Mandibular nerve: It passes through the foramen to reach the infratemporal fossa.
2. Accessory meningeal artery: It enters the cranial cavity through the foramen.
3. Lesser petrosal nerve: It may pass through the foramen to reach the otic ganglion.
4. Emissary vein: It may connect the cavernous sinus with the pterygoid venous plexus.

Mnemonic: MALE

• M: Mandibular nerve.
• A: Accessory meningeal artery.
• L: Lesser petrosal nerve.
• E: Emissary vein.

3. Sphenoidal Emissary Foramen (Foramen of Vesalius)

• The foramen of Vesalius is an occasional small opening located anteromedial to the foramen ovale.
• It may be found between the foramen ovale and the scaphoid fossa.

Structure passing through

– It transmits an emissary vein that connects the cavernous sinus with the pterygoid venous plexus.

4. Canaliculus Innominatus

– The canaliculus innominatus, also called the foramen of Arnold, is an occasional small opening between the foramen ovale and foramen spinosum.

Structure passing through

– It may transmit the lesser petrosal nerve, which has a variable course through the floor of the middle cranial fossa.

E. Sulcus Tubae

• The sulcus tubae is a groove between the posteromedial margin of the greater wing of the sphenoid and the petrous part of the temporal bone.
• It accommodates the cartilaginous part of the pharyngotympanic (auditory) tube.
• The cartilaginous part continues posterolaterally into the bony part of the auditory tube, which opens into the middle ear.

F. Inferior Surface of the Petrous Part of the Temporal Bone

• The inferior surface of the petrous part of the temporal bone is irregular and approximately triangular.
• Its apex is directed forward and medially towards the body of the sphenoid bone.
• It presents several important openings and grooves for neurovascular structures.

Muscular attachment

– The levator veli palatini muscle originates partly from the inferior surface of the petrous part of the temporal bone, near the auditory tube.

G. Carotid Canal

• The carotid canal is a bony passage within the petrous part of the temporal bone.
• Its external opening is located on the inferior surface of the petrous temporal bone.
• The canal initially passes upward and then turns forward and medially towards the apex of the petrous part.

Structures passing through

• The internal carotid artery passes through the canal to enter the cranial cavity.
• The internal carotid sympathetic plexus accompanies the artery through the canal.
• A venous plexus surrounding the internal carotid artery also passes through the canal.

H. Foramen Lacerum

• The foramen lacerum is an irregular opening at the junction of the sphenoid, petrous temporal and basilar occipital bones.
• In the living skull, its inferior part is largely filled with fibrocartilage.
• The internal carotid artery passes over its superior aspect after leaving the carotid canal rather than traversing the foramen vertically.

I. Tympanic and Squamous Parts of the Temporal Bone

• The tympanic part forms most of the bony walls of the external acoustic meatus.
• The squamous part forms the lateral wall of the middle cranial fossa and contributes to the mandibular fossa.
• The tympanic and squamous parts are separated by the squamotympanic fissure, which is subdivided by a projection of the tegmen tympani.

J. Tegmen Tympani and Petrotympanic Fissure

• The tegmen tympani is a thin plate of bone belonging to the petrous part of the temporal bone.
• It forms the roof of the middle ear cavity and separates it from the middle cranial fossa.
• A downward extension of the tegmen tympani enters the medial part of the squamotympanic fissure.
• This extension divides the fissure into two parts:
  1. Petrosquamous fissure: It lies between the petrous and squamous parts of the temporal bone.
  2. Petrotympanic fissure: It lies between the petrous and tympanic parts of the temporal bone.

Structures passing through the petrotympanic fissure

• The chorda tympani nerve leaves the middle ear through the petrotympanic fissure.
• The anterior ligament of the malleus extends through the fissure towards the spine of the sphenoid.
• The anterior tympanic artery, a branch of the maxillary artery, enters the middle ear through this region.

Mandibular fossa

• The mandibular fossa is a deep, concave depression on the inferior surface of the temporal bone.
• It receives the head of the mandible and contributes to the formation of the temporomandibular joint.

Bony Formation

• Articular part: It is formed by the squamous part of the temporal bone and lies anteriorly.
• Nonarticular part: It forms the posterior region of the fossa and is related to the tympanic part of the temporal bone.

Important Bony Features

• Postglenoid tubercle: It is a small bony projection that marks the posterior limit of the articular region laterally.
• Squamotympanic fissure: It separates the squamous and tympanic parts of the temporal bone and marks the division between the articular and nonarticular regions medially.
• Articular tubercle (articular eminence): It is a convex bony elevation that forms the anterior boundary of the mandibular fossa.

Articulation

– The articular surface of the mandibular fossa articulates with the head of the mandible through an intervening articular disc to form the temporomandibular joint.

Attachments

• The fibrous capsule of the temporomandibular joint is attached around the margins of the articular surface of the mandibular fossa and articular tubercle.
• The lateral temporomandibular ligament is attached superiorly to the articular tubercle and adjoining root of the zygomatic process of the temporal bone.

Foramen lacerum

• The foramen lacerum is an irregular opening located at the junction of the sphenoid, temporal and occipital bones.
• In the living skull, its inferior part is filled with fibrocartilage.

Boundaries

• Posterolaterally: It is bounded by the apex of the petrous part of the temporal bone.
• Medially: It is bounded by the basilar part of the occipital bone and the body of the sphenoid bone.
• Anteriorly: It is bounded by the junction of the body, greater wing and pterygoid process of the sphenoid bone.

Structures Passing Through or Related to the Foramen Lacerum

1. Structures passing through its cartilaginous part

• Small emissary veins may pass through the fibrocartilage of the foramen lacerum.
• A meningeal branch of the ascending pharyngeal artery may also pass through this region.

2. Structures related to its posterior aspect

• The internal carotid artery emerges from the carotid canal and passes over the superior aspect of the foramen lacerum.
• The internal carotid sympathetic plexus accompanies the artery.

3. Structures related to its anterior aspect

• The pterygoid canal extends forward from the region of the foramen lacerum to the pterygopalatine fossa.
• The nerve of the pterygoid canal (Vidian nerve) passes through this canal.
• The artery of the pterygoid canal, usually arising from the maxillary artery, accompanies the nerve.

4. Structures related to its superior aspect

• The greater petrosal nerve reaches the region of the foramen lacerum from the middle cranial fossa.
• The deep petrosal nerve, arising from the internal carotid sympathetic plexus, joins the greater petrosal nerve to form the nerve of the pterygoid canal.
• The internal carotid artery crosses the superior aspect of the foramen lacerum on its way towards the cavernous sinus.

Posterior Part of Norma Basalis

·  The posterior part of norma basalis lies behind an imaginary transverse line passing along the anterior margin of the foramen magnum.

·  It is divided into two areas for description:

1. Median area.
2. Lateral area.

Median area of posterior part of norma basalis

The median area presents the following features:

Foramen Magnum

– The foramen magnum is a large opening in the occipital bone that connects the cranial cavity with the vertebral canal.

External Occipital Crest

• The external occipital crest is a median bony ridge extending from the posterior margin of the foramen magnum to the external occipital protuberance. It provides attachment to the ligamentum nuchae.

External Occipital Protuberance

• The external occipital protuberance is a prominent bony projection at the upper end of the external occipital crest. It can be palpated in the midline at the junction of the head and neck.

Nuchal Lines

• The occipital bone presents three pairs of curved ridges called the highest, superior and inferior nuchal lines. The highest and superior nuchal lines extend laterally from the region of the external occipital protuberance. The inferior nuchal lines extend laterally from the middle of the external occipital crest.

Muscular attachments

• Between the superior and inferior nuchal lines: The semispinalis capitis is inserted medially, while the obliquus capitis superior is inserted laterally.
• Below the inferior nuchal line: The rectus capitis posterior minor is inserted medially, while the rectus capitis posterior major is inserted laterally.

Lateral area of posterior part of norma basalis

The lateral area presents the following bony features:

Condylar Part of the Occipital Bone

– The condylar part lies on each side of the foramen magnum and connects the basilar and squamous parts of the occipital bone.

Occipital Condyle

• Each occipital condyle is a convex, oval or kidney-shaped articular projection on the inferior surface of the occipital bone. It articulates with the superior articular facet of the atlas to form the atlanto-occipital joint. Its long axis is directed forward and medially.

Attachments

• The alar ligament is attached to the medial aspect of the occipital condyle. The capsule of the atlanto-occipital joint is attached around its articular margin.

Hypoglossal Canal

– The hypoglossal canal is a bony passage situated above the anterior part of the occipital condyle.

Structures passing through

• Hypoglossal nerve (12th cranial nerve).
• Meningeal branch of the ascending pharyngeal artery.
• Venous plexus connecting intracranial and extracranial veins.

Condylar Fossa

– The condylar fossa is a depression situated behind the posterior end of the occipital condyle.

Posterior Condylar Canal

• The posterior condylar canal is an occasional opening in the floor of the condylar fossa. It transmits an emissary vein connecting the sigmoid sinus with extracranial veins.

Jugular Process of the Occipital Bone

• The jugular process extends laterally from the condylar part of the occipital bone. It contributes to the posterior boundary of the jugular foramen.

Squamous Part of the Occipital Bone

– The squamous part lies behind the foramen magnum and is marked externally by the superior and inferior nuchal lines.

Stylomastoid Foramen

– The stylomastoid foramen is situated between the styloid and mastoid processes of the temporal bone.

Structures passing through

• The facial nerve (7th cranial nerve) emerges from the skull through this foramen.
• The stylomastoid artery, usually a branch of the posterior auricular artery, enters through the foramen.

Mastoid Process

– The mastoid process is a conical bony projection extending downward from the mastoid part of the temporal bone.

Muscular attachments

– The sternocleidomastoid, splenius capitis and longissimus capitis are inserted into the mastoid process from anterior to posterior.

Mastoid Notch

• The mastoid notch is a deep groove on the medial aspect of the mastoid process. It provides origin to the posterior belly of the digastric muscle.

Mastoid Canaliculus

• The mastoid canaliculus is a small bony canal that begins in the lateral wall of the jugular fossa. It transmits the auricular branch of the vagus nerve (Arnold’s nerve).

Foramen magnum

• The foramen magnum is a large, oval opening in the occipital bone, bounded laterally by its condylar parts. It is broader posteriorly than anteriorly.
• The alar ligaments, which extend from the dens to the medial surfaces of the occipital condyles, divide the region into a smaller anterior and a larger posterior compartment for descriptive purposes.

1. Anterior Compartment

The anterior compartment contains the following structures:

• The odontoid process (dens) of the axis projects towards its anterior part. The apical ligament of the dens extends from the tip of the dens to the anterior margin of the foramen magnum.
• The superior longitudinal band of the cruciform ligament extends upwards towards the occipital bone.
• The tectorial membrane extends upwards from the posterior surface of the axis to the occipital bone.

2. Posterior Compartment

The posterior compartment transmits the following structures:

1. The lower end of the medulla oblongata, which continues as the spinal cord.
2. The meninges surrounding the medulla and spinal cord.
3. The spinal roots of the accessory nerve (11th cranial nerve), which ascend into the cranial cavity.
4. The right and left vertebral arteries, accompanied by sympathetic nerve plexuses.
5. The anterior spinal artery and the two posterior spinal arteries.

Attachments

• Anterior margin: The anterior atlanto-occipital membrane is attached to the anterior margin of the foramen magnum.
• Posterior margin: The posterior atlanto-occipital membrane is attached to the posterior margin of the foramen magnum.

Jugular foramen

• The jugular foramen is a large, elongated opening between the petrous part of the temporal bone anteriorly and the occipital bone posteriorly. It is situated at the posterior end of the petro-occipital suture.

Important Bony Features

1. Jugular Fossa

• The jugular fossa is a depression on the inferior surface of the petrous part of the temporal bone. It accommodates the superior bulb of the internal jugular vein.

2. Jugular Process

• The jugular process is a short lateral projection arising near the junction of the condylar and squamous parts of the occipital bone. Its inferior surface provides attachment to the rectus capitis lateralis muscle.

3. Jugular Notch

• The jugular notch is a concavity on the anterior aspect of the jugular process of the occipital bone. It contributes to the posterior boundary of the jugular foramen.

Compartments of the Jugular Foramen

– The jugular foramen may be divided into three compartments by bony projections called intrajugular processes.

1. Anterior Compartment

• It transmits the inferior petrosal sinus. It also transmits a meningeal branch of the ascending pharyngeal artery.

2. Posterior Compartment

• It transmits the sigmoid sinus, which continues as the internal jugular vein. It also transmits a meningeal branch of the occipital artery.

3. Intermediate Compartment

– It transmits the following three cranial nerves:

1. Glossopharyngeal nerve (CN IX).
2. Vagus nerve (CN X).
3. Accessory nerve (CN XI).

Tympanic Canaliculus

• The tympanic canaliculus is a small opening on the bony ridge between the jugular fossa and the external opening of the carotid canal. It transmits the tympanic branch of the glossopharyngeal nerve (Jacobson’s nerve) into the middle ear.
• The inferior tympanic artery, a branch of the ascending pharyngeal artery, also passes through this canaliculus.

FETAL SKULL AND GROWTH OF SKULL

• At birth, the skull is proportionately larger than the rest of the skeleton.
• The neurocranium is much larger than the facial skeleton.
• The facial skeleton is approximately one-eighth the size of the cranial part at birth, compared with nearly half in adults.

Features of the Skull at Birth

1. Facial Skeleton

• The maxillae and mandible are poorly developed because the teeth have not erupted. The maxillary sinuses are very small, bringing the floor of the orbit close to the hard palate. The nasal cavities are relatively small. The skull base is short and narrow.

2. Temporal Bone and Ear

• The internal ear is almost adult-sized at birth. The tympanic cavity, auditory ossicles and mastoid antrum are also nearly adult-sized.
• The mastoid process is poorly developed and becomes prominent during early childhood. Due to the absence of a well-developed mastoid process, the facial nerve is relatively superficial near the stylomastoid foramen and may be injured during surgery behind the ear.
• The tympanic part of the temporal bone forms an incomplete bony ring. The external acoustic meatus is short and nearly straight, with a poorly developed bony part.

3. Cranial Vault

• The frontal and parietal bones have relatively smooth margins without well-developed serrations. The bones of the cranial vault are thin and consist of a single layer without a distinct diploë. The cranial bones are separated by fibrous sutures and membranous gaps called fontanelles.

4. Fontanelles

• Fontanelles are membranous gaps at the junctions of developing cranial bones. They allow moulding of the fetal skull during childbirth. They also permit expansion of the cranial cavity during rapid brain growth.

Growth of the Skull

Growth of neurocranium

• The neurocranium grows rapidly during the first two years of life and approaches adult dimensions by approximately seven years.
• The cranial sutures are present at birth, but their interlocking margins become more distinct during childhood.
• At birth, the bones of the cranial vault consist of a single layer.
• By about four years, the inner and outer tables become distinguishable, and the diploë begins to develop.
• The diploë continues to increase in thickness during adolescence and adult life.

Growth of the Skull Base

• The skull base grows mainly through endochondral ossification at its cartilaginous joints, called synchondroses.
• The spheno-occipital and spheno-ethmoidal synchondroses are important sites of growth.
• The spheno-occipital synchondrosis usually fuses during adolescence or early adulthood, although the exact age varies considerably.

Growth of facial skeleton

• Growth of the facial skeleton becomes more noticeable after the first year of life.
• Its development is closely related to the growth of the maxillae and mandible, eruption of teeth and enlargement of the paranasal sinuses.
• Eruption of deciduous and permanent teeth increases the height of the alveolar processes.
• Enlargement of the maxillary sinuses contributes to the growth of the middle part of the face.
• In old age, loss of teeth causes resorption of the alveolar processes of the maxillae and mandible, resulting in a reduction in facial height.

Suture Union of the Skull

·  Cranial sutures may gradually undergo ossification and obliteration with advancing age.

·  Sutural closure often begins on the internal (endocranial) surface before becoming evident on the external surface.

·  The timing and extent of closure vary considerably between individuals.