Within the Temporal Bone · Chapter 03 · Part I · The laboratory
Orientation and Surface Anatomy
Four surfaces, one three-dimensional map
Bench mode: one step per screen, large type, screen kept awake. Tap, use the arrow keys or say “next”.
- Specimen
- An intact adult temporal bone, ideally with a dry bone beside it
- Bench time
- One session; repeat the rotation test at the start of every later chapter
- Instruments
- Blunt probe, marker pen, camera
- View
- Naked eye, then low power
- Before this
- Chapters 1–2: specimen record, holder, microscope
By the end you can
- Establish the side and orientation of a temporal bone from several landmarks
- Name the landmarks of the lateral, inferior and posterior surfaces
- Rotate the bone and restore orientation without help
- Project the hidden compartments onto the lateral surface and explain how they connect
- Tell a harvest window or defect from native anatomy, or record that you cannot
Side and axes named from three landmarks that agree; every labelled structure of the three surfaces pointed to on your own specimen; the hidden compartments drawn on the lateral surface before any drilling.
Drilling. This chapter builds the map that every later exposure is checked against.
The landmark chain
Each landmark leads to the next. Tap any one to see every station where it appears.
03.1 Why orientation comes first
A temporal bone is oriented by several related features, never by a single prominence. The same structure looks unfamiliar when the bone is turned, so the aim of this chapter is a usable three-dimensional map that survives rotation: surfaces you can name, compartments you can place behind them, and the habit of checking the map against each later exposure.
Memorising names without connecting their positions will not provide that framework when the view changes.
Orient by three landmarks that agree, never by one that looks familiar.
03.2 · The 6 stations
Set side and axes first
- Find the external auditory canal, the mastoid process, the root of the zygoma and the remaining squamous surface.
- With the canal facing you, the zygoma points forwards and the mastoid lies behind and below the canal: on a right bone the zygoma points to your right.
- Establish superior and inferior, anterior and posterior before you name anything else, then relate the lateral view to the intracranial surfaces if they are preserved.
- Practise re-orienting after a deliberate change of view.
Side and the four directions named from canal, zygoma and mastoid together.
You have named the side from one landmark. Find two more that agree first.
The lateral surface
- The canal: its posterior wall separates the canal from the mastoid field; its superior part relates to the epitympanum.
- The temporal line: continuous forwards with the posterior root of the zygoma, it is a surface guide to the superior mastoid region, not a guarantee of the exact position or thickness of the roof beneath it.
- The spine of Henle and the suprameatal triangle: the spine varies in prominence, and its absence calls for other landmarks. The triangle is bounded by the temporal line above, the posterosuperior canal margin in front and a vertical tangent to the posterior canal margin behind.
- In front, the zygomatic process arises by two roots: the anterior forms the articular tubercle, the posterior passes back above the meatus into the temporal line. Note the mandibular fossa directly in front of the canal.
- Behind and below: the mastoid process and its tip, the mastoid foramen, and the tympanomastoid fissure between the tympanic plate and the mastoid.
Every labelled structure of the lateral surface pointed to on your specimen.
There is no spine of Henle. Use the canal wall, temporal line and triangle instead; the specimen is still usable.
The inferior surface
- Look at the bone from below with the anterior end upwards. A right bone then lies as it does on an axial CT: lateral structures on your left, the petrous apex up and to the right.
- Mastoid tip and digastric groove: the groove runs along the medial side of the mastoid and its anterior end leads to the stylomastoid foramen.
- Stylomastoid foramen: between the mastoid and the styloid, where the facial nerve leaves the temporal bone. The styloid is often broken in harvested bones.
- Jugular fossa medial to the stylomastoid foramen (its depth reflects the size of the jugular bulb); the carotid canal in front of it, separated by the caroticojugular crest; the petrous apex anteromedially, with the musculotubal canal in the angle between petrous and squamous parts.
Mastoid, digastric groove, stylomastoid foramen, jugular fossa, carotid canal and petrous apex named from lateral to medial.
You cannot tell the jugular fossa from the carotid canal. Find the caroticojugular crest between them first.
The posterior surface
- If the intracranial surfaces are preserved, look at the posterior surface from behind and medially; the petrous apex then points to your left on a right bone.
- Find the porus of the internal auditory canal, the subarcuate fossa above and lateral to it, and the endolymphatic sac fossa under the operculum, below and lateral to it, towards the sigmoid sulcus.
- Note the petrous ridge with the superior petrosal sulcus, and the sigmoid sulcus curving down to the jugular notch.
- Preserve all these surface features until their deeper relationships have been understood.
Porus, subarcuate fossa, endolymphatic sac fossa and sigmoid sulcus pointed to.
The intracranial surface is cut away by the harvest. Record what is missing rather than guessing it.
Relate surfaces to compartments
- Think of the temporal bone as connected compartments with shared boundaries: the mastoid communicates with the epitympanum through the antrum and aditus; the middle ear lies medial to the drum; the inner ear is enclosed in dense otic-capsule bone; the internal auditory canal approaches the inner ear and facial nerve from the posterior fossa.
- Mark the expected positions of the hidden structures on the lateral surface: antrum deep to the triangle, the second genu just inferior to the lateral canal, the sinus and middle fossa dura behind and above.
- Use three references together: the canal and posterior canal wall orient the mastoid field; the roof is related to deeper anatomy, not inferred from one surface line; the otic capsule and intracranial surfaces add a second perspective.
- As the dissection progresses, revise the map with direct findings, and stop relying on the original estimate as soon as new evidence contradicts it.
The hidden compartments drawn on the lateral surface, and you can say how they connect.
You are treating a projection as a measurement. Each projection is an estimate to be revised.
Examine the harvested bone critically
- Specimen edges may remove normal landmarks or expose structures from an unusual direction. A cut surface can reveal the carotid canal, the jugular region or the internal auditory canal without reproducing the route used in surgery.
- Use a harvest window to understand relationships, but do not mistake it for a surgical corridor.
- Look for fractures, missing cortical plates and earlier drilling. Where a defect is present, inspect its margin before calling it a natural dehiscence.
- If the cause cannot be established, record the uncertainty rather than assigning a diagnosis. Photograph the broad field before a close-up, so that the detail keeps its context.
Harvest windows and defects recorded, each called native, artefact or uncertain.
You are calling a defect a dehiscence without inspecting its margin. Inspect it, or record the uncertainty.
03.3 Score your orientation
Ask a partner to hand you the bone in a random position, then name the side and point to each structure on request.
The map is only as good as the last time you checked it against the bone.
03.4 Fault finding
| What you see | Probable cause | Correction |
|---|---|---|
| Side called wrongly | Oriented by one landmark | Use canal, zygoma and mastoid together |
| The view “looks wrong” after rotating | Map tied to one view | Restore the standard view, then rotate in steps |
| No spine of Henle | Normal variation in prominence | Use the canal wall, temporal line and triangle instead |
| A smooth surface named by texture alone | Identification without continuity | Follow the surface to a known landmark |
| A defect called a dehiscence | Harvest or drilling damage not excluded | Inspect the margin; record the uncertainty |
03.5 Viva at the bench
A smooth bony surface appears after rotation. Is its texture enough to name it?
No. A small patch of smooth bone has no unique identity; follow it by continuity to a known landmark before naming it.
What are the boundaries of the suprameatal triangle, and what lies deep to it?
The temporal line above, the posterosuperior canal margin in front and a vertical tangent to the posterior canal margin behind. The antrum lies deep to it.
Where does the anterior end of the digastric groove lead, and why does it matter in the mastoid?
To the stylomastoid foramen, where the facial nerve leaves the bone; it relates the inferior mastoid to the descending facial nerve.
One best answer
03.6 My dissection log
Saved on this device only. Print it or send the summary to your fellowship coordinator for review.
Score the dissection · review order
| Item | 0 · not achieved | 1 · partly | 2 · achieved |
|---|---|---|---|
| Side named from three agreeing landmarks | |||
| Lateral surface named | |||
| Inferior surface named | |||
| Posterior surface named | |||
| Hidden compartments drawn before drilling | |||
| Harvest defects recorded honestly |
The review order is a teaching method, not a validated scoring instrument. Structured end-product scales such as the Welling scale have been studied for rater reliability; they assess the dissection, not operative outcome, but they make feedback specific.
Before your first mastoid
Sources and credits
- Harris T, Linder T. Mastoidectomy and epitympanectomy; Mathew R, Harris T, Patel P. Lateral temporal bone resection. The Open Access Atlas of Otolaryngology, Head & Neck Operative Surgery. CC BY-NC 3.0. med.libretexts.org/
- Companion video: Temporal Bone, talk by Dr Vikas Kakkar (HAOICON 2024). youtu.be/oP43RmcD58E
Real dissection photographs with numbered drawings: Bassett Collection, Stanford University (CC BY-SA 4.0). Classic anatomical and operative plates: Gray’s Anatomy of the Human Body (1918, drawings by H. V. Carter) and Burghard’s A System of Operative Surgery (1909), both public domain, credited beside each plate. Remaining diagrams and charts: Dr Bhushan Kathuria & Team, not to scale. Operating-theatre frames and clips: our theatre, patient consent taken, identity not shown. Atlas photographs: Open Access Atlas of Otolaryngology (CC BY-NC 3.0), with their authors credited beside each image. This edition uses no AI-generated images. For professional education alongside supervised instruction and local protocols.
Dr Bhushan Kathuria & Team · Reg. No. HN-017323, Haryana Medical Council · A GeNext MedEd Foundation surgical education initiative.
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