Falx Cerebri

Definition and Shape

1. Falx cerebri - between the cerebral hemispheres
2. Tentorium cerebelli - separates cerebrum from cerebellum
3. Falx cerebelli - between the cerebellar hemispheres
4. Diaphragma sellae - covers the hypophyseal fossa

Attachments

Associated Dural Venous Sinuses

Diagrams

Clinical Relevance

1. Subfalcine herniation - The cingulate gyrus is forced under the inferior free edge of the falx cerebri when a unilateral supratentorial mass displaces the hemisphere medially. This can compress the anterior cerebral artery, causing contralateral leg weakness.
2. Subdural abscess - An abscess can be confined to one side of the falx cerebri (the falx acts as a barrier), distinguishing it from epidural or subarachnoid spread.
3. Falcine meningioma - Meningiomas arising from the falx cerebri grow between the hemispheres and can compress the medial frontal lobes bilaterally, causing lower limb weakness.
4. Calcification of the falx - A normal age-related finding on CT; however, abnormal patterns can be seen in conditions like Gorlin syndrome (basal cell nevus syndrome), where extensive falx calcification is one of the major diagnostic criteria.
5. Skull base surgery - During anterior skull base approaches, the attachment of the falx cerebri to the crista galli must be divided with scissors before the anterior cranial base can be mobilized (as noted in K.J. Lee's Essential Otolaryngology).

Extraocular (Oculomotor) Muscles

• The inferior oblique - originates from the anterior medial orbital wall
• The superior oblique - originates from the sphenoid bone in the posterior medial orbit

The Six Extraocular Muscles

Mnemonic - LR6SO4 (rest CN3): Lateral Rectus = CN VI, Superior Oblique = CN IV, all others = CN III.

Diagram - Rectus Muscles and Their Actions

Oblique Muscles - Special Features

• Superior oblique passes through the trochlea (a fibrous pulley on the medial superior orbital rim), then curves back to insert on the superior surface of the globe. It produces intorsion (upper pole of eye rotates inward/nasally).
• Inferior oblique has no trochlea. It inserts on the inferior surface of the globe and produces extorsion (upper pole of eye rotates outward/temporally).

Innervating Nerves and Their Paths

• Superior division - superior rectus + levator palpebrae superioris
• Inferior division - medial rectus, inferior rectus, inferior oblique + preganglionic parasympathetic fibers to the pupillary constrictor and ciliary muscle

Brainstem Nuclei

Additional Eye Muscles (Not Strictly Extraocular)

Clinical Pearls

Skull Sutures

Superior View - The Three Major Sutures

Lateral View - Additional Sutures

Key Junctions (Landmarks)

Fontanelles (in Newborns)

Clinical Relevance - Craniosynostosis

Bones of the Cranium

"Bones of the calvaria are 8 in number: parietal 2, frontal 1, temporal 2, occipital 1, sphenoid 1, and ethmoid 1."

• The Essentials of Forensic Medicine and Toxicology, 36th Edition

Color-coded Lateral View

The 8 Cranial Bones

How to Remember Them

How They Fit Together

        FRONTAL
   _____|_____|_____
  |   Coronal suture  |
  | PARIETAL | PARIETAL|  ← Sagittal suture runs between them
  |__________|_________|
       Lambdoid suture
         OCCIPITAL

Laterally: TEMPORAL (with squamosal suture)
Base: SPHENOID + ETHMOID (anterior base)

Key Clinical Notes

• The temporal bone is the thinnest bone of the skull; the petrous part is the densest bone in the body
• The sphenoid is called the "keystone" of the cranial base - it articulates with all other cranial bones
• The ethmoid's cribriform plate is very fragile - fractures here cause CSF rhinorrhoea (cerebrospinal fluid leaking from the nose)
• The occipital condyles articulate with the atlas (C1) at the atlanto-occipital joint, allowing nodding movements