Anatomy Notes: Skull, Orbit, Ethmoid, Sphenoid, Temporal, Mandible, Fontanelles, Vertebral Column, and Thorax
Foramen magnum and occipital region
The foramen magnum is the large opening in the occipital bone through which the spinal cord passes.
Other structures also pass through the foramen magnum (these are discussed later).
Key surrounding features: external occipital protuberance, superior nuchal line, inferior nuchal line, occipital condyles sit on the atlas (C1).
Hypoglossal canal is nearby; the hypoglossal nerve (cranial nerve XII) travels through it to innervate tongue muscles.
The occipital region includes the occipital bone’s contributions to the craniovertebral junction.
Occipital bone and related features
External occipital protuberance and superior nuchal line: important surface landmarks.
Foramen magnum: entry point for spinal cord; supports vertebral arteries and other stabilizing structures in some contexts.
Occipital condyles: articulate with the first cervical vertebra (C1, atlas).
Hypoglossal canal: passage for the hypoglossal nerve.
The hypoglossal nerve (cranial nerve XII) travels from the brainstem to muscles of the tongue (glossal muscles).
Temporal bone overview and key features
Temporal bone has both external and internal aspects with several notable landmarks:
Mandibular (glenoid) fossa: articulation with the mandible to form the temporomandibular joint (TMJ).
External acoustic meatus (ear canal).
Zygomatic process: projection toward the zygomatic bone.
Articular tubercle: over which the mandible can dislocate anteriorly if the TMJ ligaments/cartilage fail.
Squamous portion: the flat, thin portion; the suture lines around here are often referred to as the squamous region and contribute to the squamous suture.
TMJ considerations:
The articular disc (cartilage) can become lax or damaged, leading to TMJ issues.
Reduction (manual repositioning) is used for jaw dislocations; resembles shoulder dislocation reduction in principle.
The temporal bone also contains the petrous portion housing inner ear structures (not elaborated here), and the region around the pterion (a clinically important anatomical landmark) is not detailed in this lecture but is relevant clinically.
Sphenoid bone: wings, canal, and saddle region
Sphenoid bone is a complex, irregular bone with a few key features:
Greater wing and lesser wing: form part of the orbit; create the superior orbital fissure between them.
Superior orbital fissure (also called the superior orbital fissure; note the variety in naming): transmits several nerves to the orbit.
Optic canal: openings that transmit the optic nerve (cranial nerve II).
Optic canal leads to the optic groove and optic chiasm where fibers cross to the contralateral visual field.
Sella turcica (Turkish saddle): a saddle-shaped depression that houses the pituitary gland (hypophysis).
The tuberculum sellae and diaphragma sellae are part of this saddle region; tuberculum sellae sits anterior to the sella.
Pituitary gland (hypophysis) sits in the sella turcica; tumor growth can affect nearby structures.
Clinical connection: pituitary tumors larger than >10\ \text{mm} may expand to press on the optic chiasm (anterior to the sella), causing visual disturbances (e.g., scotomas, hemianopia) due to compression of retinal fibers crossing at the chiasm.
Pterygoid processes extend inferiorly from the sphenoid and provide attachment sites for the pterygoid muscles (important in mastication).
Ethmoid bone and the nasal complex
Ethmoid bone is a central contributor to the nasal cavity and orbit:
Cribriform plate: contains multiple foramina for olfactory nerve fibers (cranial nerve I) to pass from the nasal cavity to the olfactory bulbs.
Crista galli (rooster’s crest): a vertical projection that anchors the meninges (dura mater) to the skull; described as an umbrella element in memory aids.
Cribriform plate and foramina allow olfactory fibers to interface with the olfactory bulb; this is the olfactory (smell) system.
Perpendicular plate contributes to the nasal septum, along with the vomer (posterior portion of the septum).
Ethmoid air cells (ethmoidal sinuses) reside in the lateral mass of the ethmoid and form multiple small openings; these air-filled spaces contribute to lightening the skull and participate in resonance of the voice.
The superior and middle nasal conchae are part of the ethmoid bone and contribute to the nasal mucosal surface area; the inferior nasal concha is a separate bone.
Nasal septum is formed by multiple bones (perpendicular plate of ethmoid and vomer); the anterior nasal septum region is separated from the nasal cavity by the bony and cartilaginous structures; deviations can affect airflow and snoring.
Nasal cavity and conchae (turbinates)
Nasal conchae (turbinates): superior, middle, and inferior (inferior is a separate bone).
Function of conchae: increase surface area, humidify and warm incoming air; the mucosa contains respiratory epithelium (typically pseudostratified ciliated columnar epithelium).
The term “turbinators” (ENTs) refers to these structures due to their turbine-like effect on airflow, helping to humidify and condition air before it reaches the lungs.
Cribriform region and olfactory tract are located superiorly in the nasal cavity; smell is linked to the olfactory nerve (CN I).
The nasal septum divides the nasal cavities; deviations can affect airflow, cause snoring, or related symptoms.
Facial bones and the orbital complex
orbital complex comprises seven bones: frontal, sphenoid, zygomatic, maxillary, palatine, ethmoid, and lacrimal.
The zygomatic process of the temporal bone and the zygomatic bone itself contribute to the orbital rim and malar prominence.
Palatine bone contributes to the posterior part of the hard palate and the posterior portion of the orbital floor (part of the orbital wall).
Frontal bone contributes to the superior orbital rim and the forehead.
Maxilla contributes to the orbital floor, the infraorbital foramen, the alveolar process for the upper teeth, and the nasal aperture.
Lacrimal bone forms part of the medial wall of the orbit; ethmoid contributes to the medial wall as well.
The optic canal (through the sphenoid) transmits the optic nerve; the superior orbital fissure transmits cranial nerves III (oculomotor), IV (trochlear), V1 (ophthalmic branch of V), and VI (abducens).
Inferior orbital fissure also transmits branches of V2 (maxillary), infraorbital vessels, and zygomatic branches.
Visual pathway reminder: optic nerve (CN II) passes through the optic canal, fibers cross at the optic chiasm, then continue via the optic tracts to the occipital cortex; a lesion at the chiasm or behind can affect visual fields (e.g., bitemporal hemianopia).
Some nerves cross the superior orbital fissure, others pass through the inferior orbital fissure depending on their function.
The optic system and visual pathways
Optic canal: transmits the optic nerve (CN II).
Optic chiasm: where the optic nerve fibers cross; the term “chiasm” implies an X-shaped crossing; this crossing ensures contralateral visual field representation.
Visual information travels via the optic tracts to the occipital lobe for processing.
Clinical note: pituitary tumors pressing on the optic chiasm can cause visual field defects (often bitemporal hemianopia).
The ethmoid, nasal, and cribriform region in clinical context
Cribriform plate and foramina: passage for CN I (olfactory) from the nasal cavity to the olfactory bulbs.
Olfactory nerve is considered a special sense; its fibers pass through the cribriform plate rather than a typical foraminal canal.
Crista galli anchors the meninges to the skull; the analogy of an umbrella helps memory of its role as a meningeal anchor.
The hyoid bone and floor-of-the-mouth muscles
The hyoid bone is a floating bone in the neck, not articulating with other bones.
It lies above the larynx and supports the tongue and the floor of the mouth.
Suprahyoid and infrahyoid muscles attach to the hyoid bone, including digastric, stylohyoid, and thyrohyoid, and their actions help with swallowing and speech.
The hyoid’s mobility reflects its function in supporting oral and pharyngeal musculature during swallowing and vocalization.
The mandible and the jaw joint (TMJ)
Mandible features:
Body, ramus, angle.
Coronoid process (anterior) and condylar process (posterior) that articulate with the mandibular fossa of the temporal bone to form the TMJ.
Mandibular foramen (internal) vs mental foramen (external): ensure nerve/vessel access to the mandibular teeth and chin region.
Alveolar process houses the teeth; alveoli are the tooth sockets.
Mylohyoid line: attachment for the mylohyoid muscle (floor of the mouth).
TMJ considerations:
The articular disc can be involved in dysfunction; ligaments and the articular tubercle help prevent dislocation.
Dislocations can occur if the mandible slides anteriorly past the articular tubercle; reduction is a manual repositioning technique.
The maxilla and nasal region
Maxilla features:
Infraorbital foramen: transmits the infraorbital nerve and vessels (V2 branch region).
Alveolar process for upper teeth; nasal aperture and anterior nasal spine anchor the cartilage framework of the nose.
Nasal spine connects to nasal cartilage; the nasal bones anchor to this region.
The orbit’s inferior border partly formed by the maxilla.
The third, surrounding bones of the orbit provide structural integrity for ocular protection and vascular/nervous passageways.
The vertebral column: regions, features, and curvature
Regions and counts:
Cervical: vertebrae (C1–C7).
Thoracic: vertebrae (T1–T12).
Lumbar: vertebrae (L1–L5).
Sacral: fused vertebrae (fuse to form the sacrum).
Coccygeal: – fused vertebrae (coccyx).
Cervical vertebrae (general features):
Bifid spinous process is common in many cervical vertebrae (a bifid tip), except typically in C1 and C2 (atlas and axis); spina bifida is less common in cervical region.
Transverse foramina: a unique feature of cervical vertebrae that transmits the vertebral artery and vein.
Vertebral foramen: hole through which the spinal cord passes; multiple vertebrae form the vertebral canal.
Thoracic vertebrae: feature transverse costal facets (on the transverse process) and demifacets (on the body) for articulation with ribs; head of the rib articulates with the demifacet; neck (tubercle) articulates with the transverse costal facet.
Lumbar vertebrae: lack the costal facets; large, robust bodies; robust pedicles and laminae; articular facets oriented to limit rotation.
Atlas (C1) and Axis (C2):
Atlas lacks a vertebral body and has an anterior and posterior arch with a large vertebral foramen; it supports the skull and allows yes-motion via occipital condyles.
Axis has the dens (odontoid process), which acts as a pivot around which C1 rotates, allowing no–yes head movements.
The occipital bone articulates with C1 at the atlanto-occipital joint, permitting nodding (“yes”).
C1–C2 articulation allows rotation (“no”) around the dens of C2.
The sacrum and coccyx:
Median sacral crest corresponds to fused spinous processes; lateral sacral crests are fused transverse processes; sacral foramina allow nerve exits.
The coccyx is the tailbone, consisting of fused small vertebrae with limited structural function but attachment points for ligaments and muscles.
The vertebral column as a whole:
Three joints per adjacent pair of vertebrae: the intervertebral disc between bodies and the two facet joints between the superior and inferior articular processes.
Curves: the spine develops curvatures that function biomechanically like a spring to distribute load and reduce degeneration.
Spinal curvatures and developmental biomechanics
Primary (kyphotic) curves: thoracic and sacral regions tend to be kyphotic (posterior curvature).
Secondary (lordotic) curves: cervical and lumbar regions develop lordosis (anterior curvature) as the child learns to hold the head up and then stands and walks.
Rationale for S-shaped spine: distribution of body weight and loading; curvature acts like a spring to reduce degenerative wear in discs over time.
The sternum and costal anatomy
Sternum sections: manubrium, body, xiphoid process.
Jugular notch (suprasternal notch): a palpable landmark at the top of the sternum; anchors ribs anteriorly via costal cartilage.
Ribs and cartilage:
True ribs (): direct costal cartilage connections to the sternum.
False ribs (): costal cartilage connects to the cartilage of the rib above, rather than directly to the sternum.
Floating ribs (): have no anterior connection to the sternum or to other ribs.
Rib articulation with the spine:
Each rib head articulates with the vertebral body at a demi-facet (demifacet) on the corresponding vertebra and typically the vertebra above.
The tubercle articulates with the costal facet on the transverse process of the corresponding thoracic vertebra (transverse costal facet).
The rib cage provides protection, supports respiration, and contributes to thoracic shape.
Developmental and clinical notes on the skull
Fontanelles (soft spots) in infants:
Anterior fontanelle: largest fontanelle, located on the top of the skull; allows growth of the skull during infancy.
Posterior fontanelle: located at the back of the skull.
Anterolateral (sphenoidal) fontanelles and posterolateral (mastoid) fontanelles: located laterally.
Fontanelles close as the brain grows and skull bones fuse; timing varies by region and individual.
Practical and cultural notes sometimes discussed in lectures (e.g., “soft spots” and alar/linguistic analogies) to aid memory of skull growth and fontanelle relevance.
Deviation and nasal septum: a deviated septum can affect airflow and respiratory comfort; the septum is formed by multiple bones (perpendicular plate of ethmoid and vomer) with cartilage anteriorly.
Nasal mucosa and sinus anatomy: paranasal air spaces (ethmoid air cells) reduce skull weight, contribute to voice resonance, and may become obstructed during illness due to mucus buildup.
Quick memory aids and terminology recap
Crista galli = rooster’s crest; anchor for meninges.
Cribriform plate = plate with holes for CN I (olfactory) fibers.
Optic canal and optic chiasm = visual pathway route and crossing point; related to potential visual field defects with pituitary/optic pathology.
Sella turcica = Turkish chair; houses the pituitary gland.
Foramen magnum = large opening for spinal cord; vertebral arteries and nerves pass in proximity.
Greater and lesser wings of the sphenoid form important orbital and cranial base boundaries; superior orbital fissure lies between them.
Bathe the vocabulary: atlas (C1) holds up the skull; axis (C2) provides rotation via the dens (odontoid process).
True vs false ribs: direct vs indirect costal connections to the sternum; floating ribs have no anterior attachment.
Fontanelles enable rapid skull growth in infancy and are clinically relevant to pediatric assessment.
Connections to foundational principles and real-world relevance
Structure–function relationships: bone shapes and sutures are optimized for protection, attachment, and growth; the orbit’s bony makeup supports vision and nerve/vessel passage.
Biomechanics of the spine: spinal curvature distribution minimizes stress and prolongs disc health, aligning with principles of biomechanics and aging.
Clinical correlations: pituitary tumors and optic chiasm compression illustrate how skull anatomy directly influences sensory function; TMJ problems illustrate functional anatomy in the jaw.
Developmental biology: fontanelles and sutures reflect growth patterns and the necessity of a malleable skull at birth for passage through the birth canal; later fusion solidifies the protective cranial structure.
Quick reference: key numbers and identifiers (LaTeX-formatted)
Number of cranial nerves:
Vertebral column regions: cervical, thoracic, lumbar, sacral (fused), coccygeal (variable)
True ribs: ; False ribs: ; Floating ribs:
Spinal cord termination in adults: around
Pituitary tumor threshold mentioned: >10\ \text{mm}
The seven orbital bones: frontal, sphenoid, zygomatic, maxillary, palatine, ethmoid, lacrimal
End of notes
If you’d like, I can reorganize these notes into a condensed study sheet (e.g., by region: orbit, skull base, facial bones, vertebral column, thorax) or add a diagram-focused version highlighting the foramina and passing structures for quick revision.