3.1 The Skull
Key Takeaways
- The skull has 8 cranial bones (frontal, 2 parietal, 2 temporal, occipital, sphenoid, ethmoid) and 14 facial bones, with the mandible being the only movable bone via the temporomandibular joint.
- The four major sutures are the coronal (frontal–parietal), sagittal (between parietals), lambdoid (parietal–occipital), and squamous (parietal–temporal) sutures; sutures fuse sutural (Wormian) bones along the lambdoid line.
- The four fontanelles — anterior (bregma), posterior (lambda), anterolateral (sphenoid), and posterolateral (mastoid) — close on a predictable timeline useful for clinical age estimation in infants.
- Each orbit is formed by seven bones (frontal, sphenoid, zygomatic, maxilla, palatine, ethmoid, lacrimal) and transmits the optic nerve (CN II) and ophthalmic artery through the optic canal.
- Key foramina of the skull base transmit the cranial nerves and vessels; the foramen magnum, jugular foramen, internal acoustic meatus, and stylomastoid foramen are highest-yield for the PA-CAT.
The Skull: Cranial and Facial Bones
Quick Answer: The skull consists of 22 bones divided into 8 cranial bones (encasing the brain) and 14 facial bones (forming the viscerocranium). The only freely movable bone of the skull is the mandible, articulating at the temporomandibular joint (TMJ). According to the PA-CAT Bulletin of Information, rev. 20240815, skull osteology is tested within the Head region of the Anatomy content area (16% of the exam, ~39 items).
Cranial Bones (Neurocranium)
The calvaria (skull vault) is formed by flat bones that protect the cerebral hemispheres:
| Bone | Number | Key Features |
|---|---|---|
| Frontal | 1 | Forms forehead, superior orbit, frontal sinus; contains bregma at coronal–sagittal junction |
| Parietal | 2 | Form the lateral walls and roof of the cranial cavity; meet at the sagittal suture |
| Temporal | 2 | House the external acoustic meatus, mastoid process, zygomatic process, and middle/inner ear |
| Occipital | 1 | Forms the posterior cranial fossa; contains the foramen magnum |
| Sphenoid | 1 | The "keystone" of the cranial floor; articulates with all other cranial bones; contains the sella turcica |
| Ethmoid | 1 | Forms the cribriform plate, crista galli, and perpendicular plate; houses ethmoid air cells |
Facial Bones (Viscerocranium)
The 14 facial bones derive largely from the first two pharyngeal arches:
| Bone | Number | Key Features |
|---|---|---|
| Maxilla | 2 | Hold the upper teeth; form the hard palate (with palatine bones), floor of orbit, and maxillary sinus |
| Mandible | 1 | The only movable skull bone; holds lower teeth; ramus, angle, body, coronoid and condylar processes |
| Zygomatic | 2 | Form the cheek prominences and lateral orbital wall |
| Nasal | 2 | Form the bridge of the nose |
| Lacrimal | 2 | Smallest facial bones; house the lacrimal sac in the lacrimal fossa |
| Palatine | 2 | Form the posterior hard palate and part of the orbital floor |
| Inferior nasal concha | 2 | Separate bones (distinct from the superior/middle conchae of the ethmoid) |
| Vomer | 1 | Forms the posteroinferior nasal septum |
Sutures and Fontanelles
Sutures are fibrous joints (synarthroses) between cranial bones. The four major sutures are:
- Coronal suture — between the frontal and two parietal bones (sagittal plane orientation).
- Sagittal suture — midline between the two parietal bones.
- Lambdoid suture — between the occipital and parietal bones.
- Squamous suture — between the temporal and parietal bones.
The junction of the coronal and sagittal sutures is bregma; the junction of the sagittal and lambdoid sutures is lambda. Pterion (lateral skull, where frontal, parietal, temporal, and sphenoid meet) overlies the middle meningeal artery — a fracture here causes an extradural (epidural) hemorrhage.
In the newborn, membranous gaps called fontanelles allow skull molding during delivery and brain growth. The anterior fontanelle (bregma, diamond-shaped) closes by ~18–24 months; the posterior (lambda, triangular) closes by ~2–3 months; the sphenoidal (anterolateral) and mastoid (posterolateral) fontanelles close by ~6 months.
Paranasal Sinuses
The four paired paranasal sinuses drain into the nasal cavity:
| Sinus | Drainage Site | Clinical Note |
|---|---|---|
| Frontal | Middle meatus (via frontonasal duct) | Sinusitis pain referred to forehead |
| Maxillary | Middle meatus (semilunar hiatus) | Largest sinus; drains superiorly, prone to infection |
| Ethmoid | Anterior/middle → middle meatus; posterior → superior meatus | Thin orbital wall; infection can spread to orbit |
| Sphenoid | Sphenoethmoidal recess above superior concha | Deep sinus; infection can involve cavernous sinus |
Orbital and Nasal Cavities
Each orbit is a pyramidal cavity formed by seven bones: frontal, sphenoid, zygomatic, maxilla, palatine, ethmoid, and lacrimal. The optic canal (in the sphenoid) transmits the optic nerve (CN II) and ophthalmic artery; the superior orbital fissure transmits CN III, IV, V1, and VI plus the superior ophthalmic vein; the inferior orbital fissure transmits the maxillary nerve (V2) and infraorbital artery.
The nasal cavity is divided by the nasal septum (septal cartilage anteriorly, vomer and perpendicular plate of ethmoid posteriorly). The lateral wall bears three conchae (superior and middle from the ethmoid; inferior a separate bone), creating meatuses that drain the sinuses and nasolacrimal duct.
Foramina of the Skull Base — High-Yield Table
| Foramen | Bone | Transmitted Structures |
|---|---|---|
| Foramen magnum | Occipital | Spinal cord, vertebral arteries, CN XI (spinal root), meninges |
| Jugular foramen | Temporal/occipital | CN IX, X, XI; sigmoid sinus → internal jugular vein |
| Hypoglossal canal | Occipital | CN XII (hypoglossal nerve) |
| Internal acoustic meatus | Temporal (petrous) | CN VII, VIII; labyrinthine artery |
| Stylomastoid foramen | Temporal | CN VII (facial nerve exit) |
| Foramen lacerum | Temporal/sphenoid/occipital | Internal carotid artery (covered by cartilage in life); greater petrosal nerve |
| Foramen ovale | Sphenoid | Mandibular nerve (V3), accessory meningeal artery, lesser petrosal nerve |
| Foramen rotundum | Sphenoid | Maxillary nerve (V2) |
| Foramen spinosum | Sphenoid | Middle meningeal artery, meningeal branch of V2 |
| Optic canal | Sphenoid | Optic nerve (CN II), ophthalmic artery |
| Cribriform plate | Ethmoid | Olfactory nerve fibers (CN I) |
A fracture of the cribriform plate can tear olfactory fibers and leak cerebrospinal fluid (CSF rhinorrhea), while also creating a route for meningitis. A fracture through the petrous temporal bone can bleed from the ear (Battle's sign) and injure CN VII/VIII.
Which foramen transmits the mandibular division of the trigeminal nerve (V3)?
A fracture at the pterion most commonly lacerates which vessel, producing an epidural hemorrhage?
Which structure passes through the cribriform plate of the ethmoid bone?