Osteology of the Skull

The skull is a rigid structure composed of 22 bones. It is divided into the

OSTEOLOGY OF THE SKULL – HIGH YIELD

Overview

The skull forms the bony framework of the head. It protects the brain, supports the face, and houses the organs of special senses (vision, hearing, smell, taste). In adults, the skull consists of 22 bones joined mainly by sutures (fibrous joints).

Major Divisions of the Skull

1. Neurocranium (Cranial Skeleton)
Forms the protective case for the brain.

Bones of the neurocranium (8):
Frontal (1) – forms the forehead and roof of the orbit. Parietal (2) – form the superior and lateral walls of the skull. Temporal (2) – house structures of hearing and balance. Occipital (1) – posterior skull; contains the foramen magnum. Sphenoid (1) – central “keystone” bone of the cranial base. Ethmoid (1) – contributes to nasal cavity and medial orbital wall.

2. Viscerocranium (Facial Skeleton)
Forms the bony framework of the face.

Bones of the viscerocranium (14):
Maxillae (2) – upper jaw, part of orbit, nose, and hard palate. Zygomatic bones (2) – cheek bones; lateral wall of orbit. Nasal bones (2) – bridge of the nose. Lacrimal bones (2) – small bones forming part of medial orbit. Palatine bones (2) – posterior portion of the hard palate. Inferior nasal conchae (2) – increase surface area of nasal cavity. Vomer (1) – forms inferior part of the nasal septum. Mandible (1) – lower jaw; only movable bone of the skull.

Important Sutures

Coronal suture – between frontal and parietal bones. Sagittal suture – between the two parietal bones. Lambdoid suture – between parietal and occipital bones. Squamous suture – between temporal and parietal bones.

Fontanelles (Clinical Importance)

Fontanelles are membranous gaps between skull bones in infants that allow brain growth and skull molding during birth.

Anterior fontanelle – between frontal and parietal bones; closes at ~18 months. Posterior fontanelle – between parietal and occipital bones; closes at ~2–3 months.

Cranial Fossae (Internal Base of Skull)

Anterior cranial fossa – supports the frontal lobes. Middle cranial fossa – contains the temporal lobes and sella turcica. Posterior cranial fossa – contains cerebellum, pons, and medulla.

Important Skull Foramina (High Yield)

Foramen magnum – medulla, vertebral arteries, spinal root of CN XI. Optic canal – optic nerve (CN II) and ophthalmic artery. Superior orbital fissure – CN III, IV, V1, VI. Foramen rotundum – maxillary nerve (V2). Foramen ovale – mandibular nerve (V3). Foramen spinosum – middle meningeal artery. Jugular foramen – CN IX, X, XI and internal jugular vein. Hypoglossal canal – hypoglossal nerve (CN XII).

Key Clinical Correlations

Temporal bone fractures may damage the middle meningeal artery, causing an epidural hematoma. Bulging fontanelles in infants may indicate increased intracranial pressure. Lesions at the foramen magnum may compress the brainstem and upper spinal cord.

Muscles of the Head

The head contains several groups of muscles, most notably those for facial expression and mastication.

Muscles of the Head & Neck: A High-Yield Summary

This summary is organised by anatomical region and function. Focus on the origin, insertion, nerve supply (always a favourite in exams), and key action.

Muscles of the Head and Neck

1. Muscles of Facial Expression

Key Fact: These muscles are unique because they are embedded in the superficial fascia and insert into the skin. They are all developed from the 2nd Pharyngeal Arch and are therefore innervated by the Facial Nerve (CN VII).

Orbital Group

Orbicularis Oculi: Sphincter of the eyelid. Action: Closes the eye (voluntary and involuntary blinking).

Nasal Group

Procerus: Over the bridge of the nose. Action: Wrinkles the nose, draws eyebrows down (think: angry expression).

Oral Group

Orbicularis Oris: Sphincter of the mouth. Action: Closes and purses the lips (e.g., kissing, whistling).

Buccinator: Forms the muscular cheek. Action: Compresses the cheek against the teeth (helps in blowing and sucking). It is a key muscle of mastication because it holds food under the teeth.

Zygomaticus Major & Minor: Action: Draws the angle of the mouth upward and backward (smiling).

Platysma: Broad sheet in the neck. Action: Tenses the skin of the neck and draws down the lower lip (sadness/expressions of shock).

2. Muscles of Mastication

Key Fact: These act on the Temporomandibular Joint (TMJ). They are all derived from the 1st Pharyngeal Arch and are innervated by the Mandibular Nerve (CN V3).

Masseter: Most powerful. Action: Elevates and protracts the mandible (closing the jaw).

Temporalis: Fan-shaped. Action: Elevates and retracts the mandible.

Medial Pterygoid: Action: Elevates and protracts the mandible; assists in side-to-side movement (grinding).

Lateral Pterygoid: The key to dislocating. Action: Depresses and protracts the mandible (opens the mouth). Unilateral action deviates the jaw to the contralateral side.

3. Suprahyoid Muscles (Above the Hyoid)

Key Fact: These connect the hyoid bone to the skull. They form the floor of the mouth and elevate the hyoid/larynx during swallowing.

Digastric: Two bellies. Action: Elevates the hyoid, depresses the mandible.

Mylohyoid: Forms the muscular floor of the mouth. Action: Elevates the hyoid and floor of the mouth (first step of swallowing).

Geniohyoid: Action: Pulls the hyoid forward and upward.

Stylohyoid: Action: Elevates and retracts the hyoid.

Innervation Note: Mylohyoid (CN V3), Digastric (Anterior belly: CN V3; Posterior belly: CN VII), Stylohyoid (CN VII), Geniohyoid (C1 via Hypoglossal).

4. Infrahyoid Muscles (Below the Hyoid)

Key Fact: These are also called "Strap Muscles." They depress the hyoid and larynx after swallowing and during speaking.

Sternohyoid: Depresses the hyoid after elevation.

Omohyoid: Two bellies. Depresses and fixes the hyoid.

Sternothyroid: Depresses the larynx (pulls it down).

Thyrohyoid: Depresses the hyoid OR elevates the larynx (depending on which bone is fixed).

Innervation Note: All are innervated by the Ansa Cervicalis (C1-C3), except Thyrohyoid, which gets fibres from C1 via the Hypoglossal nerve.

5. Muscles of the Pharynx

Constrictors (Superior, Middle, Inferior)

Action: Constrict the pharynx sequentially to propel a bolus of food downwards (peristalsis).

Innervation: Pharyngeal plexus (CN IX & X).

Clinical Pearl: Between the base of the skull and the Superior Constrictor is a gap called the Killian's dehiscence, a potential site for a pharyngeal pouch (Zenker's diverticulum).

Longitudinal Muscles (Stylopharyngeus, Salpingopharyngeus, Palatopharyngeus)

Action: Elevate the larynx and pharynx and shorten the pharynx during swallowing.

6. Triangles of the Neck (Muscular Content)

Understanding muscle location within triangles is high-yield.

Anterior Triangle:
- Submental Triangle: Mylohyoid (floor).
- Digastric (Submandibular) Triangle: Digastric (borders), Mylohyoid (floor).
- Carotid Triangle: Contains the carotid sheath, but muscular borders are Omohyoid, Sternocleidomastoid, and Digastric.
- Muscular Triangle: Infrahyoid muscles.

Posterior Triangle:
- Occipital Triangle: Floor is made of Splenius Capitis, Levator Scapulae, and Scalenes.
- Supraclavicular Triangle: Contains the anterior scalene muscle (a key landmark for the brachial plexus and subclavian artery).

7. Prevertebral Muscles (Deep Neck)

Scalenes (Anterior, Middle, Posterior): Action: Elevate the 1st and 2nd ribs (accessory muscles of inspiration) and flex/rotate the neck. Relation: The brachial plexus and subclavian artery pass between the Anterior and Middle scalenes.

Longus Colli & Capitis: Action: Flexion of the neck and head.

8. Quick Exam Cheat Sheet

CN VII (Facial Nerve) Damage (Bell's Palsy): Loss of muscles of facial expression -> Drooping of mouth, inability to close eye (lagophthalmos), loss of nasolabial fold.

CN V3 (Mandibular Nerve) Damage: Weakness in mastication -> Jaw deviates toward the side of the lesion when opening (due to unopposed action of the Lateral Pterygoid on the healthy side).

Torticollis (Wry Neck): Tightness or spasm of the Sternocleidomastoid (SCM). Innervated by CN XI (Spinal Accessory).

Whiplash: Often involves injury to the Platysma and SCM due to hyperextension/flexion.

Neurovasculature

The arterial supply to the brain is derived from two primary pairs of arteries: the Internal Carotid Arteries and the Vertebral Arteries.

NEUROVASCULATURE OF THE HEAD – HIGH YIELD (MBChB HEAD & NECK ANATOMY)

Overview

The neurovasculature of the head consists of the arterial supply, venous drainage, and cranial nerve innervation of the scalp, face, and cranial cavity.

Arterial supply mainly arises from the internal carotid artery and the external carotid artery. Venous drainage occurs through the facial veins and dural venous sinuses. Neural supply is mainly through the cranial nerves, especially the trigeminal nerve.

Arterial Supply of the Head

Two major arteries supply the head:

1. Internal Carotid Artery (ICA)
Supplies the brain, orbit, and forehead.

Major branches (high yield):
Ophthalmic artery – supplies the orbit, eye, and forehead. Anterior cerebral artery – supplies medial surfaces of cerebral hemispheres. Middle cerebral artery – supplies lateral surfaces of cerebral hemispheres. Posterior communicating artery – connects ICA to posterior circulation.

2. External Carotid Artery (ECA)
Supplies the face, scalp, and superficial head structures.

Important branches of the ECA (very examinable):
Superior thyroid artery Lingual artery Facial artery Occipital artery Posterior auricular artery Maxillary artery Superficial temporal artery

Important Arteries of the Face

Facial artery – main arterial supply to the face; tortuous course across mandible. Superficial temporal artery – supplies scalp in temporal region. Maxillary artery – supplies deep structures of face.

Middle Meningeal Artery (Very High Yield)

Branch of the maxillary artery.
Supplies the dura mater and skull.

Fracture at the pterion may rupture this artery, causing an epidural hematoma.

Venous Drainage of the Head

Venous blood from the head drains via:

Superficial veins – drain scalp and face. Dural venous sinuses – drain the brain.

Important Veins of the Face

Facial vein – major vein draining the face. Retromandibular vein – formed by superficial temporal and maxillary veins. External jugular vein – drains superficial structures of scalp.

Dural Venous Sinuses (Very High Yield)

These are endothelial-lined venous channels located between layers of dura mater.

Major sinuses include:
Superior sagittal sinus Inferior sagittal sinus Straight sinus Transverse sinus Sigmoid sinus Cavernous sinus

All ultimately drain into the internal jugular vein.

Cavernous Sinus (Clinically Important)

Located on either side of the sella turcica.

Structures passing through or within the cavernous sinus:
Internal carotid artery Abducens nerve (CN VI)

Structures in the lateral wall:
Oculomotor nerve (CN III) Trochlear nerve (CN IV) Ophthalmic division of trigeminal nerve (V1) Maxillary division of trigeminal nerve (V2)

Danger Area of the Face

The region from the nose to the corners of the mouth.

Facial veins communicate with the cavernous sinus through the ophthalmic veins.

Infections in this area may spread intracranially causing cavernous sinus thrombosis.

Nerve Supply of the Head

Most sensory innervation of the face is provided by the trigeminal nerve (CN V).

The trigeminal nerve has three divisions:

Ophthalmic (V1) – sensory supply to forehead, scalp, upper eyelid, and nose.

Maxillary (V2) – sensory supply to midface, upper lip, maxillary teeth.

Mandibular (V3) – sensory supply to lower face and motor supply to muscles of mastication.

Motor Innervation of the Face

Facial nerve (CN VII) supplies the muscles of facial expression.

Trigeminal nerve (V3) supplies the muscles of mastication.

Key Clinical Correlations

Rupture of the middle meningeal artery can cause epidural hematoma following temporal bone fracture.

Infections of the facial “danger triangle” may spread to the cavernous sinus.

Trigeminal nerve lesions cause loss of facial sensation and impaired chewing.

Facial nerve lesions cause paralysis of facial expression muscles.

The Circle of Willis

The Circle of Willis (circulus arteriosus cerebri) is a ring-like arterial anastomosis located at the base of the brain, specifically within the interpeduncular cistern of the subarachnoid space.

Clinical Summary

This article provided a snapshot of the head and neck. Mastery requires understanding the spatial relationships of these vessels and nerves.