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SKULL ANATOMY

In the style of BD Chaurasia Vol. 3 & Snell's Clinical Anatomy

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INTRODUCTION

• The skull is the bony framework of the head
• Made of 22 bones (excluding the 3 ossicles of each ear)
• 21 bones are fixed by sutures (immovable fibrous joints); only 1 bone moves - the mandible (at the TMJ)
• The skull is divided into:
  • Cranium - encloses and protects the brain
  • Face (Viscerocranium) - forms the skeleton of the face

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PARTS OF THE SKULL

1. Cranium

2. Facial Skeleton (Viscerocranium)

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NORMAS OF THE SKULL

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NORMA VERTICALIS (Superior/Top View)

Bones Seen

• Frontal bone (anteriorly)
• Two parietal bones (centrally - the largest visible area)
• Occipital bone (posteriorly)

Sutures (The Most Important Feature)

Key Landmarks (Bregma, Lambda, etc.)

Muscle/Structure Attachments

• Temporalis muscle arises from the temporal fossa on the lateral surface (seen partially)
• The sagittal suture sits in the groove for the superior sagittal sinus underneath

Clinical Points - Norma Verticalis

Anterior Fontanelle (Bregma):

• Diamond-shaped, closes at 18 months of age
• Bulging = raised intracranial pressure (meningitis, hydrocephalus)
• Sunken = dehydration
• Used for LP in neonates and to assess ICP clinically

Posterior Fontanelle (Lambda):

• Triangular, closes by 2-3 months of age

Craniosynostosis: Premature fusion of sutures leads to abnormal skull shapes:

• Sagittal synostosis → Scaphocephaly (long, narrow skull)
• Coronal synostosis → Brachycephaly (short, wide skull)

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NORMA FRONTALIS (Anterior/Front View)

Bones Seen (7 Bones Visible)

Key Landmarks & Features

Foramina Seen in Norma Frontalis

All three foramina lie roughly in a vertical line - useful surgical landmark

Muscle Attachments

• Orbicularis oculi - around the orbit (lacrimal, maxilla, frontal)
• Orbicularis oris - around the mouth (mandible, maxilla)
• Nasalis - on maxilla
• Mentalis - on mandible (chin muscle)

Orbit (Seen in Norma Frontalis)

• Roof: Frontal bone + lesser wing of sphenoid
• Floor: Maxilla + Zygomatic + Palatine
• Medial wall: Maxilla, Lacrimal, Ethmoid, Sphenoid (most fragile)
• Lateral wall: Zygomatic + Greater wing of sphenoid (strongest wall)

Clinical Points - Norma Frontalis

Supra-orbital nerve block: Done at supra-orbital notch - anaesthetizes the forehead

Infra-orbital nerve block: Done at infra-orbital foramen - anaesthetizes the upper lip, cheek, lower eyelid

Mental nerve block: Done at mental foramen - anaesthetizes the lower lip and chin

"Blow-out" fracture of orbit: A direct blow to the eye raises intraorbital pressure suddenly and blows out the thin floor (maxilla) - orbital contents herniate into the maxillary sinus. Patient gets diplopia (double vision) and enophthalmos (sunken eye).

Frontalis abscess: Infection in frontal sinus can spread to forehead skin (Pott's puffy tumor)

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NORMA LATERALIS (Side/Lateral View)

Bones Seen (8 Bones)

Key Landmarks & Sutures

Key Bony Points

Temporal Lines (Important Attachment Lines)

• Superior temporal line - upper limit of temporalis fascia attachment
• Inferior temporal line - upper limit of temporalis muscle origin

Muscle Attachments - Norma Lateralis

Foramina (Lateral View)

Clinical Points - Norma Lateralis

Pterion fracture (MOST IMPORTANT):

• The pterion is the thinnest part of the skull (only 2-3 mm thick)
• Deep to the pterion lies the anterior division of the middle meningeal artery
• A blow to the temple (e.g., cricket ball, punch) fractures the pterion → tears the middle meningeal artery → Extradural (Epidural) Hematoma
• Classic presentation: blow to head → brief LOC → lucid interval → deteriorating consciousness → death if untreated
• Neurosurgical emergency: burr hole is drilled at pterion to evacuate the hematoma

Asterion:

• Marks the junction of transverse and sigmoid sinuses
• Surgeons use this landmark when approaching the posterior fossa

Mastoid process:

• Contains mastoid air cells, which communicate with the middle ear
• Mastoiditis (infection of air cells) can spread to the meninges (meningitis) or form a subperiosteal abscess pushing the ear forward and outward

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NORMA OCCIPITALIS (Posterior/Back View)

Bones Seen

• Occipital bone (mainly - the squamous part dominates)
• Parts of the two parietal bones (superior corners)
• Parts of temporal bones laterally (mastoid area)

Key Landmarks

Muscle/Structure Attachments

Clinical Points - Norma Occipitalis

Inion (EOP) as surgical landmark: The internal occipital protuberance (inside the skull) corresponds roughly to the Torcular Herophili (confluence of sinuses). Surgeons must avoid this area during posterior fossa surgery.

Battle's sign: Bruising over the mastoid area (behind the ear) = sign of base of skull fracture involving the petrous temporal bone. Takes 24-48 hours to appear.

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NORMA BASALIS (Inferior/Base View)

Bones Seen

Divided into 3 Regions:

A. Anterior Region (Hard Palate area)

B. Middle Region

C. Posterior Region

Clinical Points - Norma Basalis

Jugular foramen syndrome: Tumors (e.g., glomus jugulare) compressing the jugular foramen cause palsy of CN IX, X, XI - dysphagia, hoarseness, weakness of trapezius/SCM

Styloid process elongation (Eagle's syndrome): Abnormally long styloid process causes throat pain, dysphagia, and foreign body sensation. Diagnosed on CT/OPG.

Condylar fracture: Fracture near the occipital condyle may injure the hypoglossal nerve → tongue deviation to the SAME side

Foramen spinosum: Key surgical landmark - used to identify the middle meningeal artery. Also used in trans-sphenoidal and skull base surgery.

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INTERIOR OF THE SKULL BASE (Cranial Fossae)

Three Cranial Fossae

Anterior Cranial Fossa

• Bones: Frontal (orbital plate), Ethmoid (cribriform plate), Sphenoid (lesser wing + planum sphenoidale)
• Contains: Frontal lobes of brain, olfactory bulbs
• Key Foramina:
  • Cribriform plate foramina → Olfactory nerves (CN I)
  • Foramen caecum → Emissary vein (sometimes)

Middle Cranial Fossa

• Bones: Sphenoid body + greater wings, Temporal (squamous + petrous)
• Contains: Temporal lobes, pituitary gland (in sella turcica)
• Key Foramina:

• Sella turcica - the "Turkish saddle" in sphenoid body housing the pituitary gland
  • Anterior boundary: tuberculum sellae
  • Posterior boundary: dorsum sellae
  • Lateral: cavernous sinuses

Posterior Cranial Fossa

• Bones: Occipital bone (mainly), temporal (petrous part), sphenoid (dorsum sellae)
• Contains: Cerebellum, pons, medulla oblongata
• Key Foramina:

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FONTANELLES (In Infants)

• Anterior fontanelle is the most clinically examined
• Bulging = raised ICP (meningitis, hydrocephalus, vitamin A toxicity)
• Sunken = dehydration
• Delayed closure = rickets, hydrocephalus, hypothyroidism, Down syndrome
• Early closure = craniosynostosis, microcephaly

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IMPORTANT CLINICAL SUMMARY

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QUICK MEMORY AIDS

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Note: BD Chaurasia Vol. 3 and Snell's Clinical Anatomy are not available in the current textbook library. The content above faithfully reflects the structure, headings, level of detail, and clinical emphasis those two books use for skull anatomy, cross-referenced with Gray's Anatomy for Students from the library.