Anatomy and Physiology 2e · Axial Skeleton
The Skull
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The skull is the bony framework of the head. It is made of 22 bones in the typical adult — 8 cranial bones that enclose and protect the brain, and 14 facial bones that form the face, orbits, nasal cavity, and palate. The six tiny auditory ossicles of the middle ear are also commonly counted with the head region, though they are described separately from the 22. Most skull bones are joined by fibrous joints called sutures; only the mandible moves, at the temporomandibular joints. The skull is not a solid box: it is pierced by foramina (holes) and canals that let nerves and blood vessels pass, and it contains air-filled cavities called paranasal sinuses that lighten the bone and connect to the nasal cavity. Every landmark you memorize is a clue to what lies underneath — a nerve, a vessel, a sense organ, or a muscle attachment.
Why this matters
Head trauma is one of the most common and most serious presentations in emergency care. Knowing which bones form the Cranium The 8 bones enclosing the brain Full entry → tells you what a head CT is actually showing, why a blow to the temple can be dangerous, and why a fracture of the Cribriform plate Perforated ethmoid bone region under the brain Full entry → can leak cerebrospinal fluid (CSF) from the nose — a commonly taught sign that requires urgent evaluation. Sinus anatomy explains sinusitis, dental anatomy explains where upper teeth sit (in the maxillae) versus lower teeth (in the mandible), and infant fontanelles tell clinicians about brain growth and hydration status. For nurses, imaging technologists, and others working in the head and neck, skull landmarks are daily vocabulary.
The college version
Core Concepts
The cranial bones (8): the braincase
- Frontal bone — forms the forehead and the roof of the orbits; contains the frontal sinuses and the supraorbital margins (the brow ridges).
- Parietal bones (2) — form most of the top and sides of the cranium.
- Temporal bones (2) — on the sides at the level of the ears. Each has a squamous part (thin, flat side wall), tympanic part (around the ear canal), and petrous part (dense pyramid in the skull base that houses the middle and inner ear). Landmarks include the mastoid process (lump behind the ear, a neck-muscle attachment), styloid process (thin spike below the ear), zygomatic process (forms the cheekbone arch with the zygomatic bone), mandibular fossa (socket for the jaw joint), and carotid canal (passage for the internal carotid artery).
- Occipital bone — the back and base of the skull. Its Foramen A hole in bone for nerves/vessels to pass through Full entry → magnum is the large opening where the spinal cord exits the cranium; the occipital condyles flank it and articulate with the first cervical vertebra (atlas).
- Sphenoid bone — a butterfly-shaped bone at the base of the cranium, behind the eyes. Its Sella turcica Saddle-shaped depression on the sphenoid Full entry → ("Turkish saddle") cradles the pituitary gland; its greater and lesser wings form parts of the orbits and skull base; its optic canals transmit the optic nerves.
- Ethmoid bone — a small, delicate bone between the eyes. Its crista galli anchors a dural membrane; its cribriform plates are perforated by the olfactory nerves (smell); its perpendicular plate forms part of the nasal septum; its superior and middle nasal conchae are scroll-like shelves inside the nasal cavity.
Sutures and fontanelles: the skull's joints and growth zones
The major sutures are the coronal (frontal ↔ parietals), sagittal (between the two parietals), squamous (temporal ↔ parietal on each side), and lambdoid (parietals ↔ occipital). Sutures are fibrous joints that allow slight movement in infancy and fuse gradually with age. In newborns, the sutures meet at unossified gaps called fontanelles ("soft spots") — most notably the large anterior Fontanelle An unossified gap between skull bones in infancy Full entry → (between frontal and parietals) and the smaller posterior fontanelle (at the lambdoid junction). Commonly taught closure ranges are about 18–24 months for the anterior and 2–3 months for the posterior; fontanelles let the skull compress during birth and expand as the brain grows. Verify current ranges in a pediatric reference.
The facial bones (14): the framework of the face
- Maxillae (2) — form the upper jaw; their Alveolar processes Bony ridges holding teeth (maxillae and mandible) Full entry → hold the upper teeth, their palatine processes form the anterior hard palate, and they contain the maxillary sinuses (the largest paranasal sinuses).
- Zygomatic bones (2) — the cheekbones; each joins the temporal bone's zygomatic process to form the Zygomatic arch Cheekbone arch: zygomatic bone + temporal zygomatic process Full entry →.
- Nasal bones (2) — the small bones that form the bridge of the nose.
- Lacrimal bones (2) — tiny bones in the medial orbit that help form the passage for tears (nasolacrimal drainage).
- Palatine bones (2) — L-shaped bones forming the posterior hard palate and part of the nasal cavity floor.
- Inferior nasal conchae (2) — thin, scroll-like bones along the lateral nasal wall (separate from the ethmoid's conchae).
- Vomer — a flat midline bone forming the inferior nasal septum.
- Mandible — the lower jaw and the only movable skull bone. It has a body, two rami, an angle, a coronoid process (temporalis muscle attachment), a condylar process (the joint head at the TMJ Temporomandibular joint: mandible ↔ temporal bone Full entry →), a mandibular foramen (where the inferior alveolar nerve enters), and alveolar processes holding the lower teeth.
Regions and cavities: orbits, nasal cavity, sinuses, and the TMJ
Each orbit is a seven-bone mosaic: frontal, zygomatic, maxilla, sphenoid, ethmoid, lacrimal, and palatine. The nasal cavity is divided by the nasal septum (vomer + ethmoid perpendicular plate + septal cartilage) and bounded laterally by the conchae. The paranasal sinuses — frontal, maxillary, ethmoid (air cells), and sphenoid — are mucous-membrane-lined air spaces that lighten the skull, condition inhaled air, and give the voice resonance. The temporomandibular joint (TMJ) is where the mandible's condylar process meets the mandibular fossa of the temporal bone, cushioned by a fibrocartilaginous disc.
Foramina: why the skull is full of holes
Every opening has a passenger. The foramen magnum carries the spinal cord; the optic canals carry the optic nerves; the jugular foramina (between temporal and occipital bones) carry the internal jugular veins and several cranial nerves; the carotid canals carry the internal carotid arteries; the mandibular and mental foramina carry branches of the trigeminal nerve to the lower teeth and chin. Test questions love matching a foramen to its contents — study holes as "tunnels with tenants."
How It Works / Step-by-Step Process
Reading a skull (or a lateral skull X-ray) follows a routine:
- Find the midline. The frontal, occipital, and ethmoid perpendicular plate are midline; parietal, temporal, maxilla, zygomatic, nasal, lacrimal, palatine, inferior conchae, and mandible (mostly) are paired.
- Sort by job. Above the eyes and behind the face = cranial (8). In front of the cranium and around the mouth/nose/orbits = facial (14).
- Check the base. Look for the foramen magnum (posterior, midline) and the petrous parts of the temporals (lateral, dense) — these anchor the skull to the spine.
- Identify the joints. Trace the coronal, sagittal, squamous, and lambdoid sutures; the mandible is the only bone that moves.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Skull | Cranium | Skull = cranium (8) + facial bones (14). Cranium is only the braincase. |
| Suture | Fontanelle | Suture = fibrous joint line between bones; fontanelle = open gap at suture intersections in infancy. |
| Paranasal sinuses | Venous (dural) sinuses | Air-filled cavities in skull bones vs blood-filled channels within the dura mater — different systems entirely. |
| Mandible | Maxilla | Lower jaw (movable, TMJ) vs upper jaw (fixed to the cranium). |
| Mastoid process | Styloid process | Blunt lump behind the ear vs thin spike below the ear — both temporal bone, different attachments. |
| Foramen | Fossa | A hole (nerve/vessel passage) vs a shallow depression or basin (e.g., mandibular fossa). |
| Ethmoid conchae | Inferior nasal conchae | Superior/middle conchae are ethmoid processes; the inferior concha is a separate facial bone. |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Your skull is like a bike helmet made of puzzle pieces. Eight big pieces (the cranium) lock together around your brain, and fourteen smaller pieces build your face. The pieces are joined by wiggly seams called sutures, and when you were a baby there were soft spots (fontanelles) where the pieces hadn't closed yet so your head could grow. The only piece that moves is your jaw — that's how you chew and talk.
Worked example
A cyclist without a helmet hits the pavement and strikes the side of her head. In the emergency department, a head CT shows a fracture through the thin squamous part of the temporal bone — a region that overlies the middle meningeal artery, so bleeding here can compress the brain (a commonly taught mechanism that requires urgent assessment). The same scan shows an intact cribriform plate, so there is no CSF leak from the nose, and the occipital condyles and atlas are aligned, so the neck is protected. Every structure named here — squamous temporal, cribriform plate, occipital condyles — is skull landmark vocabulary. (Educational illustration only; real evaluation follows clinical protocols.)
Key takeaways
- 22 skull bones: 8 cranial + 14 facial; plus 6 auditory ossicles.
- The mandible is the only movable skull bone (TMJ); every other joint is a suture.
- Sella turcica (sphenoid) cradles the pituitary gland.
- Foramen magnum (occipital) = where the spinal cord exits the skull.
- Cribriform plates (ethmoid) transmit olfactory nerves; a fracture here can cause CSF to leak from the nose (commonly taught — urgent evaluation).
- Mastoid process (temporal) is the lump behind the ear; styloid process is the spike below it.
- Four major sutures: coronal, sagittal, squamous, lambdoid.
- Four paranasal sinuses: frontal, maxillary, ethmoid, sphenoid.
- Anterior fontanelle commonly closes ~18–24 months; posterior ~2–3 months (verify current ranges).
- The orbit is a seven-bone mosaic — a classic exam question.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
How many cranial and facial bones make up the adult skull?
Show answer
8 cranial bones and 14 facial bones (22 total; plus 6 auditory ossicles).
Which bone contains the sella turcica, and what does that landmark house?
Show answer
The sphenoid bone; the sella turcica cradles the pituitary gland.
Which skull bone is movable, and at what joint?
Show answer
The mandible, at the temporomandibular joint (TMJ) with the temporal bone.
Through which opening does the spinal cord leave the skull, and which bone contains it?
Show answer
The foramen magnum, in the occipital bone.
Why can a fracture through the cribriform plate cause fluid to leak from the nose?
Show answer
The cribriform plate is perforated by olfactory nerve fibers, and it sits between the nasal cavity and the cranial cavity — a fracture can let cerebrospinal fluid drain into the nose (rhinorrhea), a commonly taught sign needing urgent evaluation.
What are fontanelles, and why do they exist in newborns?
Show answer
Fontanelles are unossified gaps at suture junctions present at birth; they allow skull compression during delivery and room for brain growth, closing gradually (anterior ~18–24 months, posterior ~2–3 months in commonly taught ranges).
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Cranium
- The 8 bones enclosing the brain
- Suture
- A fibrous joint where skull bones meet
- Fontanelle
- An unossified gap between skull bones in infancy
- Foramen
- A hole in bone for nerves/vessels to pass through
- Paranasal sinus
- Air-filled cavity inside frontal, maxillary, ethmoid, or sphenoid bone
- Sella turcica
- Saddle-shaped depression on the sphenoid
- Cribriform plate
- Perforated ethmoid bone region under the brain
- TMJ
- Temporomandibular joint: mandible ↔ temporal bone
- Zygomatic arch
- Cheekbone arch: zygomatic bone + temporal zygomatic process
- Alveolar processes
- Bony ridges holding teeth (maxillae and mandible)
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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