L.2- Orbit Openings and Nervous System Intro
Orbit Openings and Landmarks: Comprehensive Study Notes
Definitions and basic terminology
Foramen = a hole in bone; is a 360° encased opening.
Fissure = an elongated opening in bone; may extend completely through bone or have a partial bottom.
Analogy: a fissure can be so long you could thread a pipe cleaner through it if it goes all the way through; some fissures have bone forming a bottom, stopping passage.
Anterior Openings in the orbit: key landmarks for identifying sphenoid bone and neural pathways
Superior orbital fissure (SOF)
An elongated opening between the lesser wing and greater wing of the sphenoid.
Located in the orbit; a major landmark for sphenoid bone position.
Note: the bone just above the SOF is the lesser wing; the bone below is the greater wing.
Optic foramen (optic canal)
Foramen = hole encased in bone; optic foramen is within the lesser wing of the sphenoid.
Through this opening passes the optic nerve (cranial nerve II).
Also a reference point: the optic foramen is inside the anterior cranial fossa, near the lesser wing.
Relationship: optic foramen is in the anterior cranial fossa, just anterior to the chiasm area; connected to the sella turcica region nearby.
Inferior orbital fissure
An elongated opening toward the front of the face, connecting the orbit with the pterygopalatine fossa and infraorbital canal.
Carries a nerve that provides sensation to the cheek (infraorbital nerve as it traverses the infraorbital groove, canal, and foramen).
Supraorbital foramen (or supraorbital notch)
Above the orbit; sensation to the forehead via the supraorbital nerve.
In some skulls the opening is a complete foramen; in others it’s a notch (notch and foramen serve similar sensory roles).
Infraorbital foramen
Just below the orbital rim; path of sensation to the midface via the infraorbital nerve.
Travels from infraorbital groove → infraorbital canal → infraorbital foramen.
Anterior ethmoidal foramen and Posterior ethmoidal foramen
Medial aspect of the orbit, at the ethmoid bone’s top edge where ethmoid meets the frontal bone.
Nasolacrimal canal
A canal that drains tears from the orbit into the nasal cavity.
Located toward the front of the orbit and relates to the lacrimal bone’s position (medial and forward in the orbit).
Zygomatic openings (lateral orbit)
Zygomatico-orbital foramen: a small opening inside the orbit on the zygomatic bone.
Zygomaticofacial foramen: on the face, near the zygomatic arch.
Zygomaticotemporal foramen: located laterally toward the temporal region; in some depictions it sits behind the zygomatic arch.
Memory tip: zygomatico-orbital is inside the orbit; zygomaticofacial is on the face; zygomaticotemporal is near the temporal region.
Medial orbit landmarks and bone contributions
Medial wall bones: maxillary bone, lacrimal bone, and ethmoid bone.
Ethmoid bone edge (anterior to posterior): two openings along the junction with the frontal bone—the anterior ethmoidal foramen and posterior ethmoidal foramen.
Ethmoid components associated with orbit:
Cribriform plate (part of ethmoid): has multiple tiny openings for olfactory nerve fibers; crista galli is the vertical projection attaching the dura to the skull.
Orbital plate of frontal bone: forms part of the orbital roof; visible from the anterior cranial fossa and within the orbit.
Perpendicular plate (ethmoid): part of the nasal septum.
Ethmoid air cells (ethmoid sinuses) and their relationship to the medial orbit.
Lateral orbit and a few small openings (outside the orbit walls)
Zygomatic bone-related openings (see above) and their orientations.
Anterior cranial fossa openings and landmarks
Cribriform plate of ethmoid: multiple foramina for olfactory nerves; the olfactory bulbs sit on the cribriform plate.
Crista galli: vertical midline projection that anchors the dura; the dural attachment site sits atop the cribriform plate.
Orbital plate of frontal bone: forms the roof of the orbit.
Perpendicular plate (ethmoid): part of the nasal structure; contributes to nasal cavity separation.
Ethmoid air cells and the medial wall contributions to the orbit.
Important context: optic foramen is debated as belonging to the anterior vs middle cranial fossa in different sources; the lecture places it near the anterior cranial fossa (within the lesser wing of the sphenoid).
Middle cranial fossa openings and landmarks (primarily in the greater wing of the sphenoid)
Foramen rotundum
Round opening in the greater wing of the sphenoid; located directly under the SOF.
Faces rostrally toward the face; not visible from the base of the skull.
Passes sensory nerves (one key reference is to nerves that convey sensation from the midface, including teeth in some contexts).
Foramen ovale
Large oval opening also in the greater wing of the sphenoid; sits medially to foramen spinosum.
Foramen spinosum
Small opening just lateral to foramen ovale.
Named for a small spine-like projection on the sphenoid bone near this opening.
Foramen lacerum
Not a true foramen; located at the junction between the sphenoid and temporal bones.
The opening is jagged/laminated in life; not a complete through-path in the living skull but a notable landmark.
Sella turcica (middle cranial fossa)
The “Turkish saddle”: a saddle-shaped bony structure in the sphenoid bone.
Key components within or around the sella turcica:
Tuberculum sellae (anterior flat part of the saddle).
Dorsum sellae (posterior/back part of the saddle).
Anterior clinoid processes (two projections in front of the saddle).
Posterior clinoid processes (two projections at the back of the saddle).
The chiasmatic groove sits in front of the sella turcica; the optic chiasm sits just above the sella turcica.
The optic nerves from each eye enter via the optic canal and cross at the optic chiasm just anterior to the sella turcica.
Cavernous sinus landmark
A venous structure positioned laterally to the sella turcica; the internal carotid artery traverses the cavernous sinus alongside cranial nerves.
Dural venous sinuses (grooves and their paths)
Transverse dural venous sinus: runs side to side in the posterior cranial region (occipital bone area).
Sigmoid dural venous sinus: S-shaped path leading toward the jugular foramen for venous drainage.
Superior petrosal sinus and inferior petrosal sinus: connect to the cavernous sinus and drain toward the jugular system.
These sinuses are grooves/bony indentations guiding venous blood back toward the jugular foramen for exit from the skull.
Jugular foramen
The large channel between the temporal and occipital bones where venous blood exits the skull; derivatives include cranial nerves IX (glossopharyngeal), X (vagus), XI (spinal accessory) and the sigmoid sinus.
Posterior cranial fossa landmarks and openings
Internal acoustic meatus
Passage for nerves to the inner ear and vestibulocochlear nerve outputs; located in the temporal bone.
Foramen magnum
Large opening on the occipital bone where brain stem transitions to the spinal cord; encased by bone; key region for brainstem to spinal cord transition.
Occipital bone and condyles
Occipital condyles: two projections on the base of the skull that articulate with the atlas (C1 vertebrae).
Hypoglossal canal (anterior condylar canal)
Posterior condyloid foramen
An opening on the occipital bone behind the condyle; not always present; communicates with the inside? (less critical for basic landmarks).
Petrotympanic fissure
A fissure between the petrous part of the temporal bone and the tympanic region; located below the mandibular fossa and middle cranial base.
Foramen lacerum (revisited)
A middle cranial fossa landmark; again, not a true through-foramen in life; observed from the skull base.
Carotid canal and carotid foramen (temporal bone)
Carotid foramen (in the temporal bone) is the entry for the internal carotid artery; after passing through the carotid canal, the artery lies within the carotid canal within temporal bone.
Stylomastoid foramen
Opening between the mastoid and the styloid process; transmits the facial nerve after it exits the stylomastoid segment.
Petrous pyramid details and other base landmarks
Petrous portion of the temporal bone harbors the inner ear; arcuate eminence is a bump on the petrous part indicating the location of the semicircular canals.
Trigeminal impression sits near the petrous apex on the temporal bone, representing the location of the trigeminal nerve’s ganglion/roots.
Pterygomaxillary fissure and pterygopalatine fossa connections
Pterygomaxillary fissure is the opening between the pterygoid process of the sphenoid and the maxilla; passage to the pterygopalatine fossa behind the maxilla.
Pterygopalatine fossa is a small, important in-between space bounded by the pterygoid process of the sphenoid, maxilla, and palatine bone.
Temporal fossa and infratemporal fossa
Temporal fossa: region above the zygomatic arch; contains temporalis muscle and other structures; boundaries include frontal, sphenoid, and parietal bones.
Infratemporal fossa: region below the zygomatic arch; contains muscles of mastication, nerves, glands; a focus of discussion for later lectures.
Pterion
The junction where four bones meet: frontal, sphenoid, parietal, and temporal; this area is very thin and is a common trauma site due to the convergence of multiple bones and a nearby artery.
Pterygotemporal and related features
The pterion sits near a thin bone region; trauma risk is emphasized due to underlying arteries.
The base of the skull and arterial/venous relationships
Carotid artery and cavernous sinus relationship
The internal carotid artery travels in the carotid canal adjacent to the sella turcica and passes through the cavernous sinus en route to the brain.
Cavernous sinus as a venous/arterial crossroad
A venous channel where multiple nerves and the carotid artery pass; clinically relevant due to potential involvement when cranial nerves pass near the sinus.
Durally defined venous sinuses and their grooves
Transverse sinus: runs laterally across the skull base (occipital region).
Sigmoid sinus: S-shaped path toward the jugular foramen.
Superior petrosal sinus and inferior petrosal sinus: connect the cavernous sinus and the transverse/sigmoid systems toward the jugular foramen.
Cavernous sinus: a key landmark near the sphenoid body; nerves passing through here include cranial nerves that control extraocular muscles and other functions.
The back of the skull: external landmarks and clinical relevance
External occipital protuberance (EOP)
A palpable bump on the posterior skull used as a reliable surface landmark for procedures, including electrodiagnostic testing setup.
Electrodiagnostic testing relevance
EOP and other bony landmarks are used to place electrodes for evoked potential studies and other brain/visual function assessments when a patient cannot participate in standard testing.
Lateral skull landmarks and the four major regions to study later
Temporal fossa basics
The temporal fossa sits above the zygomatic arch; the squamous portion of the temporal bone is present here.
The pteryon is the thin junction where frontal, sphenoid, parietal, and temporal bones meet; this location is unusually thin and vulnerable to trauma due to the convergence of multiple bones and an underlying artery.
Infra-temporal fossa
Located below the zygomatic arch; houses muscles of mastication and various nerves and glands; a major focus of subsequent anatomy lectures.
Pterygomaxillary fissure and the path to the pterygopalatine fossa
The fissure sits between the pterygoid process of the sphenoid and the maxilla; the fissure leads into the pterygopalatine fossa behind the maxilla.
Pterygoid canal and pterygoid processes
The pterygoid canal runs forward from the medial wall of the pterygoid process toward the palate; the canal is a key pathway for nerves and vessels entering the pterygopalatine fossa.
Foramina and bases to memorize on the skull base (bottom view importance)
From the bottom view, rotundum, ovale, and spinosum can be seen in the greater wing of the sphenoid; lacerum is visible as a jagged interface between sphenoid and temporal bones; carotid foramen and carotid canal are in the temporal bone; stylomastoid foramen is at the junction of the styloid and mastoid processes; petrotympanic fissure lies between the petro-tympanic region and the mandibular region.
Quick recap: practical connections and clinical relevance
The orbit’s openings and their bony boundaries are critical for identifying the course of cranial nerves that control eye movements, eyelid function, and sensation of the face.
The optic nerve’s passage through the optic foramen and its relationship to the chiasm and sella turcica is central to understanding visual pathways, crossovers, and potential compression sites.
The middle cranial fossa openings (foramen rotundum, foramen ovale, foramen spinosum) are critical for trigeminal nerve branches and blood vessels; the proximity of these openings to the cavernous sinus has clinical implications for cranial nerve deficits when tumors or aneurysms impinge nearby structures.
The inferior and superior orbital fissures connect the orbit to the cranium and facial regions, guiding multiple nerves and vessels to and from the orbit; traumatic injuries or surgical approaches must consider these paths.
The pterygomaxillary fissure and pterygopalatine fossa serve as major hubs for nerves (including branches of the maxillary nerve) and vessels moving from the cranial base to the face.
The temporal bone’s landmarks (arcuate eminence, petrous pyramid, internal acoustic meatus, carotid canal, stylomastoid foramen) provide practical cues for locating inner ear structures, cranial nerves, and vascular passages.
The four dural venous sinuses (transverse, sigmoid, superior petrosal, inferior petrosal) route venous blood toward the jugular foramen; understanding their grooves helps in anticipating venous flow patterns and potential sites of pathology.
The pterion’s thin bony composition and the underlying artery explain why trauma here can lead to severe intracranial hemorrhage.
The jaw and ear region anatomy (mandibular fossa, petrotympanic fissure) connects to the temporomandibular joint and middle ear—relevant for dental/ENT assessments and radiologic interpretation.
The nervous system overview (CNS vs PNS; somatic vs autonomic; afferent vs efferent) and the concept of nuclei (cell bodies within CNS) vs ganglia (cell bodies outside CNS) provide a framework for understanding neural pathways in clinical contexts (e.g., demyelinating diseases vs neurodegenerative diseases).
The olfactory (CN I) and retina/optic (CN II) exception to the typical cranial nerve PNS rule reflects the CNS origin of these components, with clinical implications for neuro-ophthalmology and sensory testing.
Nervous system: quick intro to structure and function (to bridge to later lectures)
Divisions by structure
Central nervous system (CNS): brain and spinal cord.
Peripheral nervous system (PNS): cranial nerves, spinal nerves, and ganglia.
Divisions by function
Somatic nervous system: voluntary control of skeletal muscles.
Autonomic nervous system: involuntary control of smooth muscle, glands, and cardiac muscle.
Functional flow in the CNS
Afferent information = incoming sensory signals (e.g., seeing with the eyes).
Efferent information = outgoing motor signals (e.g., grabbing an apple).
Neurons: key cellular components
Nucleus: a collection of neuron cell bodies within the CNS.
Gray matter: regions with neuron cell bodies (outer cortex of cerebrum and various nuclei in brainstem).
White matter: regions with axons (myelinated) that form tracts; appears white due to myelin.
Demyelinating vs neurodegenerative disease relevance:
Demyelinating diseases (e.g., multiple sclerosis) primarily affect white matter.
Neurodegenerative diseases (e.g., dementia) primarily affect gray matter.
Brain anatomy basics (for orientation in the notes)
Cerebrum: the two cerebral hemispheres (outer gray matter with inner white matter tracts).
Cerebellum: the hindbrain involved in coordination and balance.
Brainstem: medulla, pons, and midbrain; the major conduit for nerve pathways between brain and spinal cord.
Thalamus: often considered part of the brain stem in some classifications; serves as a relay for sensory information (often lumped with brain stem in foundational discussions).
Spinal cord: continuation of the CNS that transmits signals to and from the body.
Peripheral nervous system details
Cranial nerves (12 pairs) and spinal nerves (31 pairs) are part of the PNS, with the exception noted for CN I and CN II in some contexts.
Ganglia: collections of neuron cell bodies outside the CNS.
Practical, clinical, and exam-oriented takeaways
Be able to identify the following openings and landmarks on skull models and radiographs:
SOF, optic foramen, inferior orbital fissure, supraorbital foramen/notch, infraorbital foramen, anterior/posterior ethmoidal foramina, nasolacrimal canal; zygomatico- orbital, zygomaticofacial, zygomaticotemporal.
Medial orbit: ethmoid foramina and orbital plate; perpendicular plate and ethmoid air cells.
Middle cranial fossa openings: foramen rotundum, foramen ovale, foramen spinosum, foramen lacerum.
Sella turcica components: tuberculum sellae, dorsum sellae, anterior and posterior clinoid processes; chiasmatic groove.
Posterior cranial fossa: internal acoustic meatus, foramen magnum, jugular foramen, carotid canal (in temporal bone), stylomastoid foramen; petrotympanic fissure.
Lateral skull landmarks: external occipital protuberance (EOP), pterion, pterygomaxillary fissure, pterygoid canal, pterygoid process, pterygopalatine fossa, temporal and infratemporal fossae.
Understand the logic of vertical positioning: anterior vs posterior landmarks move from the frontal/orbital region toward the base of the skull; medial vs lateral landmarks help orient the sphenoid, temporal, and occipital bones.
Recognize real-world relevance:
Trauma: pterion and its vulnerability due to thin bone and underlying middle meningeal arteries.
Visual pathways: optic nerve, chiasm location relative to sella turcica; clinical implications for visual field loss patterns.
Hearing and balance: arcuate eminence indicates the location of the semicircular canals within the petrous temporal bone.
Pulsatile tinnitus: related to the proximity of the internal carotid artery to the inner ear structures within the temporal bone.
Electrodiagnostic testing: surface landmarks like the EOP guide electrode placement when patient communication is limited.
Visual memory prompts used in lecture
“Under the SOF” = foramen rotundum (round, rostral-facing opening in the middle cranial fossa).
“Front-facing” openings (rotundum) vs. “oval” openings (ovale) vs. “small spine” (spinosum).
Sella turcica sits above the chiasm; optic nerves run to the orbits and meet near the chiasm; the chiasmic groove is just anterior to the sella.
Arcuate eminence indicates the semicircular canal location on the petrous temporal bone.