Skull - Important Features

S. No.	FORAMEN / FISSURE	STRUCTURES  PASSING  THROUGH
1.	Cribriform plate of Ethmoid	Ø  Olfactory nerves (I CN) Ø  Anterior & Posterior Ethmoidal vessels and nerves
2.	Optic Canal	Ø  Optic nerve (II CN) Ø  Ophthalmic artery
3.	Superior Orbital Fissure	Lateral part- Ø  Lacrimal nerve (branch of V-1 CN) Ø  Frontal nerve (branch of V-1 CN) Ø  Trochlear nerve (IV CN) Ø  Lacrimal branch of Middle meningeal artery Ø  Recurrent meningeal branch of Ophthalmic artery Ø  Superior ophthalmic vein Intermediate part- Ø  Superior & Inferior rami of Oculomotor nerve (III CN) Ø  Nasociliary nerve (branch of V-1 CN) Ø  Abducent nerve (VI CN) Medial part- Ø  Inferior ophthalmic veins
4.	Foramen Rotundum	Ø  Maxillary Nerve (V-2 CN)
5.	Foramen Ovale	Ø  Mandibular nerve (V-3 CN) Ø  Accessory meningeal artery Ø  Lesser petrosal nerve Ø  Emissary vein
6.	Foramen Spinosum	Ø  Nervous spinosus Ø  Middle meningeal artery
7.	Foramen Lacerum	Anterior wall- Ø  Pterygoid vessels & nerve   Posterior wall- Ø  Internal carotid artery   Lower opening (closed by a cartilage)- Ø  Meningeal branches of Ascending pharyngeal artery Ø  Emissary veins   Upper opening- Ø  Internal carotid artery
8.	Internal Acoustic Meatus	Ø  Facial nerve (VII CN) Ø  Vestibulo-cochlear nerve (VIII CN) Ø  Labyrinthine artery
9.	Jugular Foramen	Anterior part- Ø  Inferior petrosal sinus Intermediate part- Ø  Glosso-pharyngeal nerve (IX CN) Ø  Vagus nerve (X CN) Ø  Accessory nerve (XI CN) Ø  Meningeal branch of Ascending pharyngeal artery Ø  Emissary vein   Posterior part- Ø  Internal jugular vein with Superior bulb
10.	Hypoglossal Canal	Ø  Hypoglossal nerve (XII CN) Ø  Meningeal branch of Ascending pharyngeal artery Ø  Emissary vein
11.	Foramen Magnum	Anterior compartment- Ø  Apical ligament Ø  Tip of dens Ø  Upper band of Cruciate ligament Ø  Membrana tectoria   Posterior compartment- Ø  Medulla oblongata Ø  Fourth part of Vertebral arteries Ø  Spinal roots of Accessory nerve Ø  Anterior & Posterior Spinal arteries Ø  Tonsil of Cerebellum
12.	Carotid Canal	Ø  Internal carotid artery surrounded by sympathetic nerves & veins
13.	Stylomastoid Foramen	Ø  Facial nerve Ø  Stylomastoid artery
14.	Petro-tympanic Fissure	Ø  Chorda tympanic nerve Ø  Anterior tympanic branch of Maxillary artery Ø  Anterior ligament of Malleus
15.	Inferior Orbital Fissure	Ø  Infra-orbital vessels and nerve Ø  Zygomatic branch of Maxillary nerve Ø  Orbital branch of Pterygo-palatine ganglion Ø  Communicating veins
16.	Supra Orbital Foramen	Ø  Supra-orbital vessels & nerve
17.	Infra Orbital Foramen	Ø  Infra-orbital vessels & nerve
18.	Mental Foramen	Ø  Mental vessels & nerve
19.	Mandibular Foramen	Ø  Inferior alveolar vessels & nerve

MASTOID PROCESS

• It is a part of temporal bone lies behind the external acoustic meatus
• It is pierced by mastoid foramen close to the occipito-mastoid suture and transmits an emissary vein.
• In adult, it contains mucous-lined air cells which communicate with tympanic cavity through the mastoid antrum.
• It is absent at birth and develops gradually after second year of life.
• Absence of mastoid process in the new born allows the stylomastoid foramen to appear at the surface; therefore, a surgical incision behind the pinna of the ear may injure facial nerve.

Attachments –

• On the outer surface-
• Sternocleidomastoid muscle
• Splenius capitis muscle
• Longissimus capitis muscle
• On the inner surface-
• Posterior belly of Digastric muscle

Relations –

• Medially it related with a groove for occipital artery.

 

STYLOID PROCESS

• It is a slender projection of temporal bone.
• It is about 2.5 cm long, lies in front of mastoid process.
• It extends downward and forward.

Attachments –

• Stylohyoid ligament
• Stylomandibular ligament
• Styloglossus muscle
• Stylohyoid muscle
• Stylopharyngeus muscle

Relations –

• Laterally- Parotid gland, Facial nerve.
• Medially- Internal carotid artery, Internal jugular vein, last 4 cranial nerves.

 

SPINE OF SPHENOID

• It is a triangular projection from the under surface of greater wing of sphenoid bone.
• It is situated postero-lateral to the foramen spinosum.

Attachments – (Styloid apparatus)

• Tensor veli palatini
• Tensor tympani
• Spheno-mandibular ligament
• Pterygo-spinous ligament
• Anterior ligament of malleus

Relations –

• Laterally- Auriculo-temporal nerve.
• Medially- Chorda tympani nerve, Auditory tube.

 

MAC EWEN’S TRIANGLE / SUPRA-MEATAL TRIANGLE

• It is a triangular depression situated behind and above the external acoustic meatus.
• It is formed by squamous part of temporal bone.

Boundaries –

• Above- Supra-mastoid crest.
• In front - Postero-superior margin of external acoustic meatus.
• Behind- A vertical tangent drawn along the posterior margin of meatal orifice.

Importance-

• Mastoid antrum lies 2 cm deep to this triangle in adult and 2 mm deep in new-born.
• It corresponds with the cymba conchae of auricle on the surface.

 

PTERION

• It is an H-shaped suture in the anterior part of floor of temporal fossa.
• It is formed by the meeting of 4 bones- Parietal, Frontal, Temporal and greater wing of Sphenoid.
• Centre of the pterion lies about 4 cm above the zygomatic arch and 3.5 cm behind the fronto-zygomatic suture.
• Beneath the pterion lie anterior branch of middle meningeal artery and stem of lateral sulcus of brain (Sylvian point) with middle cerebral artery.

 

INION

• It is the highest point on the external occipital protuberance.
• Deep to it lies the confluence of dural venous sinuses between the cerebral and cerebellar hemispheres.

 

FONTANELLES

• They are un-ossified membranous gaps in the vault of neonatal skull.
• They are found at 4 angles of both parietal bones.

Classification –

• Unpaired- Anterior & Posterior fontanelles.
• Paired- Sphenoidal / Antero-lateral fontanelles & Mastoid / Postero-lateral fontanelles.

Formation –

• Ossification of parietal bone starts from its centre near the parietal tuber.
• It spreads centrifugally so that the borders of the bone are ossified earlier than its four angles.
• Hence, fontanelles appear at the angles.

Closure –

• Sphenoidal fontanelles are closed during 3 – 4 months of life.
• Mastoid fontanelles close at the end of 1^st year.
• Anterior fontanelle closes between 18 months and 2 years.
• Posterior fontanelle closes by 1 – 2 months of life.

Importance –

• Fontanelles permit reduction of circumference of foetal skull during parturition by partial overlapping of bones of calvaria (moulding of foetal head).
• They allow prolific growth of the brain during first year of life.

Anterior Fontanelle :-

• It is a diamond shaped largest fontanelle.
• It measures about 4 cm in length and 2.5 cm in breadth.
• It is situated at the junction of sagittal, coronal and frontal sutures.
• It corresponds to the bregma and superior sagittal sinus lies beneath it.
• It is bounded by two halves of frontal bone in front and two parietal bones behind.
• It is covered by 3 layers - (from within outwards) endosteal layer of dura mater, membranous precursor of skull bones & scalp.
• It is palpable in infant and is seen to pulsate due to cerebral arteries.

Applied Aspects –

• Anterior fontanelle helps in age determination of the child, if it persists beyond 2 or 3 years of age it suggests disturbance of calcium metabolism due to deficiency of Vitamin D.
• Abnormal bulging of anterior fontanelle may indicate increased intracranial tension.
• Depressed anterior fontanelle significantly shows signs of dehydration.
• Superior sagittal sinus is readily accessible through anterior fontanelle for the withdrawal of blood or for intravenous transfusion of fluid or drugs.
• From the lateral angle of anterior fontanelle, a needle may be introduced downward and laterally into the lateral ventricle of brain.
• During parturition, position of foetal head in vertex presentation may be determined by palpating the anterior fontanelle per vaginum.