Axial Skeleton – Skull Bones and Key Markings

Maxilla – Right Lateral View

• Orbital surface
  • Articulates superiorly with the frontal bone.
• Frontal process
  • Extends upward toward the frontal bone, contributing to nasal bridge and orbit.
• Zygomatic process (cut in figure)
  • Joins the zygomatic bone to form part of the cheek.
• Infraorbital foramen
  • Passage for infra-orbital nerve & vessels (branch of CN V₂).
• Anterior nasal spine
  • Midline projection; palpable just inferior to nose.
• Alveolar process (upper)
  • Houses sockets $\,(alveoli)$ for maxillary teeth.
• Palatine process (inferior perspective)
  • Forms anterior $2/3$ of hard palate; meets at intermaxillary suture.
• Palatine/median palatine sutures
  • Fusion lines with opposite maxilla & palatine bones.

Mandible – Right Lateral View

• Body
  • Horizontal portion forming chin; supports lower teeth (alveolar process).
• Ramus
  • Vertical extension; meets body at mandibular angle.
• Mandibular angle
  • Palpable corner; landmark for anesthesia & fractures.
• Condylar (condyloid) process
  • Posterior “knob”; articulates with mandibular fossa of temporal bone to create the temporomandibular joint (TMJ)—the skull’s only freely moveable joint.
• Coronoid process
  • Anterior “crown-shaped” projection; attachment for temporalis muscle (jaw elevation).
• Mandibular notch
  • U-shaped depression separating coronoid & condylar processes.
• Mandibular foramen (medial ramus)
  • Conduit for inferior alveolar nerve; target of dental anesthetic.
• Mental foramen (body, lateral)
  • Transmits mental nerve & vessels to chin & lower lip skin.

Bones Forming the Orbit

Seven bones create the bony eye socket (orbit) $\,(=7)$:

• Frontal – roof & supra-orbital margin.
• Sphenoid – greater & lesser wings; optic canal; superior orbital fissure.
• Zygomatic – lateral wall & floor.
• Maxilla – floor & medial wall; infra-orbital groove/foramen; inferior orbital fissure.
• Ethmoid – orbital plate of lateral mass; medial wall.
• Lacrimal – tiny bone lodging lacrimal sac; medial wall.
• Palatine – orbital process; small posterior floor contribution.

Key Openings of the Orbit

• Optic canal → CN II.
• Superior orbital fissure → CN III, IV, V₁, VI (eye movers).
• Inferior orbital fissure → CN V₂, infra-orbital vessels.
• Supra-orbital notch/foramen → supra-orbital VAN.
• Infra-orbital foramen → infra-orbital VAN.

Ethmoid Bone

• Contributes to anterior cranial fossa, nasal cavity, and medial orbit.
• Crista galli
  • $"\text{Rooster’s comb}"$; anchors dura mater, stabilizing brain.
• Cribriform plates with cribriform foramina
  • Passage for olfactory nerve filaments (CN I).
• Perpendicular plate
  • Superior part of nasal septum.
• Lateral masses
  • Contain ethmoidal air cells (sinuses).
  • Orbital plates form medial orbital wall.
• Superior & Middle nasal conchae
  • Turbinates that swirl inspired air, enhancing warming/filtering.

Sphenoid Bone ("Keystone" of Cranium)

• Articulates with all other cranial bones.
• Body houses sella turcica
  • Hypophyseal fossa cradles pituitary gland.
• Greater wings
  • Form parts of middle cranial fossa, orbit walls.
• Lesser wings
  • Contribute floor of anterior cranial fossa & orbital roof.
• Pterygoid processes
  • Attachment for chewing muscles (pterygoids).
• Openings (all within $\text{greater/lesser wings}$):
  • Optic canal → CN II.
  • Superior orbital fissure → CN III, IV, V₁, VI.
  • Foramen rotundum → CN V₂.
  • Foramen ovale → CN V₃.
  • Foramen spinosum → middle meningeal artery.
  • Foramen lacerum (sealed by cartilage in vivo).

Temporal Bone

• Regions: squamous, tympanic, petrous.
• Squamous part
  • Zygomatic process → forms zygomatic arch with zygomatic bone.
  • Mandibular fossa → receives mandibular condyle (TMJ).
• Tympanic part
  • External acoustic meatus – ear canal.
  • Styloid process – needle-like spur for tongue/ligament attachments (often broken in demos).
• Petrous part
  • Forms cranial base wedge between sphenoid & occipital.
  • Internal acoustic meatus → CN VII & VIII.
  • Jugular foramen (with occipital) → internal jugular vein & CN IX, X, XI.
  • Carotid canal → internal carotid artery entry.
  • Stylomastoid foramen → facial nerve exit (CN VII).
  • Mastoid process – muscle attachment; contains air cells (mastoiditis risk).

Occipital Bone

• Posterior skull & cranial base.
• Foramen magnum
  • Junction of brainstem & spinal cord.
• Occipital condyles
  • Articulate with atlas (C1) enabling nod “yes”.
• Hypoglossal canal → CN XII passage.
• External occipital protuberance & superior/inferior nuchal lines – ligament & muscle attachment (trapezius, etc.).

Sutures (Fibrous Joints)

• Sagittal $\rightarrow$ between parietals (midline).
• Coronal $\rightarrow$ parietals & frontal.
• Squamous $\rightarrow$ parietal & temporal (lateral).
• Lambdoid $\rightarrow$ parietals & occipital (posterior).
• Occipitomastoid $\rightarrow$ occipital & temporal (posteroinferior).
• Sutural (Wormian) bones occasionally occur within suture lines—developmental variants.

Cranial vs. Facial Bones

• Total skull bones $=\,22$.
  • Cranial $=8$: frontal $(1)$, parietal $(2)$, temporal $(2)$, occipital $(1)$, sphenoid $(1)$, ethmoid $(1)$.
  • Facial $=14$ (12 paired, 2 unpaired): mandible, vomer (single); maxilla, zygomatic, nasal, lacrimal, palatine, inferior nasal concha (all paired).
• Hyoid bone (unpaired) is not a facial bone but studied with skull; does not articulate with other bones—suspended by stylohyoid ligaments; anchor for tongue/laryngeal muscles.

“Keystone” Concept

• Sphenoid $\rightarrow$ keystone of cranium (interlocks with all cranial bones).
• Maxillae $\rightarrow$ keystone of face (articulate with every facial bone except mandible).

Important Foramina & Canals (Clinical/Functional)

• Mental, infra-orbital, supra-orbital → exit branches of CN V supplying sensation to face (dentistry & local anesthesia).
• Mandibular & incisive fossa → guide dental nerve blocks.
• Stylomastoid foramen → Bell’s palsy site (facial nerve compression).
• Jugular foramen & carotid canal → major vascular in/out flow; neurologic & ENT surgery landmarks.

Hard Palate Composition

• Anterior $2/3$ – palatine processes of maxillae.
• Posterior $1/3$ – horizontal plates of palatine bones.
• Median palatine suture joins palatine bones posteriorly; intermaxillary suture joins anteriorly.
• Cleft palate results when palatine processes fail to fuse $(\approx1/2000 \text{ births})$.

Temporomandibular Joint (TMJ)

• Components: mandibular condyle + temporal mandibular fossa + articular disc.
• Motions: elevation/depression, protraction/retraction, limited lateral excursion.
• Dysfunction (TMD) can cause headaches, ear pain; often linked to malocclusion or grinding (bruxism).

Sinuses & Air Cells

• Ethmoidal air cells (ethmoid lateral masses) lighten skull, resonate voice, warm air.
• Mastoid air cells (temporal bone) communicate with middle ear; infection ⇒ mastoiditis.

Developmental & Clinical Correlations

• Fontanelles (soft spots) in infants correspond to future sutures; allow brain growth & childbirth molding.
• Craniosynostosis = premature suture fusion, altering skull shape & potentially compressing brain.
• Blow-out fracture of orbit typically involves maxillary floor along infra-orbital groove → infra-orbital nerve paresthesia.
• Epidural hematoma often involves middle meningeal artery passing through foramen spinosum.

Numerical Recap

• $22$ bones in adult skull.
• $8$ cranial, $14$ facial.
• $7$ bones in each orbit.
• Hard palate: $\dfrac{2}{3}+\dfrac{1}{3}=1$ complete roof of oral cavity.
• TMJ is only freely movable skull joint.

Ethical & Practical Notes

• Proper knowledge of foramina essential for safe regional anesthesia (dentistry, ENT).
• Understanding sinus drainage pathways guides management of sinusitis & surgical access.
• Appreciation of sutural bone variation critical in forensic identification & anthropologic studies.