Axial Skeleton – Skull, Sutures, Cranial Nerves, Vertebrae & Sternum Study Notes

Skull Bones – External Orientation

• Maxilla ➜ upper jaw; single paired bone.
• Mandible ➜ lower jaw; the ONLY moveable skull bone.
• Nasal bone ➜ small paired bone forming bridge of nose.
• Zygomatic ("cheek") bone ➜ makes the lateral rim/“corner” of each orbit.

Palate & Palatine vs. Maxilla

• Roof of mouth = hard palate (anterior 2⁄3)+ soft palate (posterior 1⁄3).
• Anterior 2⁄3: maxilla’s palatine process ("walk backward with your tongue until it’s hard").
• Posterior hard edge: separate palatine bone (small, L-shaped bone).
◦ Tongue test: change from hard → soft marks the transition from bone → muscular soft palate.

Nasal & Paranasal Details

• Nasal septum
• Superior portion = perpendicular plate of ethmoid.
• Inferior portion = vomer (independent bone).

• Conchae / Turbinates ➜ three scroll-like shelves that swirl inspired air.
• Superior + Middle = parts of ethmoid.
• Inferior nasal concha = its own pair of bones (visible as two blue bumps low in the nasal cavity).

Sutures (ONLY 4 to memorize)

1. Coronal – L↔R, joins frontal with both parietals ("crown").
2. Sagittal – A↔P, between two parietal bones.
3. Squamous – flat arch, parietal ↔ temporal (temporal region is flattest = “squamous”).
4. Lambdoid – parietals ↔ occipital; resembles Greek λ (lambda).

Frontal Bone Landmarks

• Glabella – slight depression between eyebrows.
• Supra-orbital foramen – round opening above orbit (for CN V1 neurovascular bundle).
⟹ Contrast with infra-orbital foramen (maxilla).

Parietal Bone

• Participates in coronal, sagittal, squamous & lambdoid sutures – otherwise feature-poor.

Temporal Bone Landmarks

• External acoustic (auditory) meatus – ear canal entrance.
• Mastoid process – large posterior bump (attachment of sternocleidomastoid).
• Styloid process – thin spine deep to ear (tongue & hyoid muscles attach).
• Zygomatic process (of temporal) – anterior bridge that meets zygomatic bone.
• Mandibular fossa – depression forming the temporo-mandibular joint (TMJ).
• Clinical: “TMJ syndrome” from yawning/oversized burgers → clicking, pain.
• Internal acoustic meatus – passage on cranial floor for CN VII & VIII.
• Jugular foramen (with occipital) – large hole for CN IX, X, XI + jugular vein.

Occipital Bone Landmarks

• Foramen magnum – brainstem & vertebral arteries.
• Occipital condyles – paired bumps anterior to foramen; articulate with atlas (C1) for nodding (“yes”) motion.
• Hypoglossal canal – within condylar wall; CN XII exits.

Sphenoid & Ethmoid (Central Bones)

• Ethmoid
• Cribriform plate – sieve for olfactory CN I axons (“smell nerve on roof of nose”).
• Crista galli – vertical anchor for falx cerebri (not directly mentioned but implied).

• Sphenoid
• Lesser wings (anterior ridge) & greater wings (lateral floor) – “cliff” in skull-base walk.
• Sella turcica – Turkish saddle housing pituitary gland (hypophysis).
• Optic canals – through lesser wing; CN II passes → optic chiasm sits superior to pituitary.
• Superior orbital fissure – slit for CN III, IV, VI & V₁.
• Foramen rotundum – V₂; foramen ovale – V₃ (not verbally named but V₂/V₃ paths were).

Maxilla Landmarks

• Infra-orbital foramen – inferior rim of orbit; CN V₂ exit.
• Alveolar processes – tooth sockets (upper set).
• Palatine process – anterior hard palate.
• Incisive fossa / canal – small midline hole just posterior to central incisors.

Mandible Landmarks

• Body (horizontal) & ramus (vertical) with mandibular angle.
• Alveolar processes – lower tooth sockets.
• Mental foramen – on body; CN V₃ sensory exit.
• Mandibular foramen – medial ramus; CN V₃ entry.
• Mandibular condyle – posterior “horn”; articulates with mandibular fossa (TMJ).
• Coronoid process – anterior “horn”; insertion of temporalis muscle (elevates jaw).

Inside Cranial Floor – “Ant Walk” Tour

1. Anterior cranial fossa – frontal + ethmoid (cribriform plates). Hosts prefrontal cortex; olfactory bulbs rest on plates.
2. Middle cranial fossa – sphenoid’s lesser → greater wings; temporal lobes sit here.
   • Optic canals (CN II), superior orbital fissure (III, IV, VI, V₁) within lesser/greater junction.
   • Sella turcica centrally.
3. Posterior cranial fossa – temporal + occipital.
   • Internal acoustic meatus (VII, VIII), jugular foramen (IX, X, XI), hypoglossal canal (XII), foramen magnum (brainstem, spinal accessory root, vertebral arteries).

Cranial Nerves & Exits (summary)

• I – cribriform plate
• II – optic canal
• III, IV, VI – superior orbital fissure
• V₁ – superior orbital fissure
• V₂ – foramen rotundum ➜ infra-orbital foramen
• V₃ – foramen ovale ➜ mandibular canal ➜ mental foramen
• VII, VIII – internal acoustic meatus
• IX, X, XI – jugular foramen
• XII – hypoglossal canal

Vertebral Column Overview

• Counts: $7$ cervical, $12$ thoracic, $5$ lumbar, $5$ fused sacral, $3!–!5$ coccygeal.
• Generic parts (vertebra):
• Body (weight-bearing disc platform).
• Vertebral foramen (spinal cord).
• Spinous process (mid-posterior projection).
• Transverse processes (lateral projections).

Cervical Specialties

• Transverse foramina in transverse processes — protect vertebral arteries.
• Atlas (C1) – no body; articulates with occipital condyles for nodding.
• Axis (C2) – has dens/odontoid process; pivot joint allows rotation (“no”).
• C7 – vertebra prominens; long palpable spinous process.
• Clinical: Axial load injuries (parachute, falls) can drive dens into brainstem → paralysis.

Sternum & Thoracic Cage

• Sternum parts:
• Manubrium (superior segment) – jugular notch at superior border.
• Body (gladiolus) – costal notches for ribs $2!–!7$.
• Xiphoid process – small inferior tip (ossifies later; fragile → CPR landmark).

• Functional note: Rectus abdominis originates on pubis, inserts via costal cartilages & xiphoid — sit-up “six-pack” muscle.

Clinical / Functional Connections & Metaphors

• “Walk your tongue” metaphor clarifies hard vs. soft palate and palatine location.
• “Burger TMJ syndrome” – excessive gape can sublux TMJ.
• “Bobble-head” explanation of C1–C2 + condyles interplay.
• “Mini-ant crawl” through cranial fossae aids spatial memory of foramina.
• “Tuck & roll” from heights prevents vertical shock fracturing dens.

Key Terminology & Definitions

• Foramen – round hole (e.g., supra-orbital foramen).
• Canal/Meatus – tubular passage (external acoustic meatus).
• Fissure – slit-like gap (superior orbital fissure).
• Fossa – shallow depression (mandibular fossa).
• Process – projection/outgrowth (mastoid process).
• Condyle – rounded articular knob (occipital condyle).
• Suture – fibrous joint between skull bones (sagittal suture).

Numerical Facts

• Cranial nerves: $I!\text{–}!XII = 12$ pairs.
• Vertebral count: $7+12+5 = 24$ movable vertebrae.
• Palate: anterior $\approx 66\%$ maxilla, posterior $\approx 34\%$ palatine.

Study Tips

• Palpate real landmarks: mastoid bump, jugular notch, coronoid vs. condylar processes.
• Visualize cranial nerves as subway lines exiting specific “stations” (foramina).
• Use colored skull or online 3-D tools (e.g., TeachMeAnatomy) to reinforce spatial paths.
• Drill suture names by hand-tracing lines on a model skull.