Summary
The Duke Elder exam commonly tests your knowledge on certain dimensions relating to the eye and surrounding structures. This article contains the key facts you should know relating to dimensions.
Spiral of Tillaux
The spiral of Tillaux describes an imaginary line connecting the insertions of the recti muscles of the eye. Note that the insertion distances are given in millimetres and are maximum values. Insertion distances vary in different individuals.
The diagram below shows the insertion distances of the recti muscles in the right eye.
Thickness of the sclera
The sclera is thicker in males than in females.
A: The Sclera is thinnest at the insertion of extra ocular muscles (0.3 mm).
B: It is thickest near the optic nerve, where it is approximately 1 mm in thickness.
C: Sclera gradually thickens towards cornea and at limbus the thickness of sclera is 0.83 mm.
The optic nerve
- Contains 1.2 million nerve fibres
- Approximately 1.5 mm in diameter
- Enlarges to 3.5 mm posterior to lamina cribrosa due to myelin sheath
- Located 3–4 mm from fovea
- Causes scotoma (blind spot) at15° temporal to fixation and slightly below horizontal meridian (approximately 45-50 mm in length)
The cornea
- The largest diameter is in the horizontal meridian (mean 11.7 mm)
- The smallest is in the vertical meridian (mean 10.6 mm)
- Corneal thickness is 0.42-0.45 mm
References
- ‘Spiral of Tillaux’. TheFreeDictionary.Com, https://medical-dictionary.thefreedictionary.com/spiral+of+Tillaux. Accessed 27 June 2022.
- ‘Spiral of Tillaux’. American Academy of Ophthalmology, 14 Oct. 2015, https://www.aao.org/image/new-mediabeacon-item-7.
- eOphtha. Anatomy of Sclera. https://www.eophtha.com/posts/anatomy-of-sclera. Accessed 27 June 2022.
- ‘Cornea and Sclera’. Ento Key, 23 Jan. 2019, https://entokey.com/cornea-and-sclera-5/.
Author(s)
Dr Abhiyan Bhandari
Abhiyan is the Co-Founder and Radiology & Imaging Lead of Ophtnotes. He is a doctor who graduated from UCL Medical School in London. He scored in the top 10% of candidates who sat the Duke Elder examination and runs ophthalmology and Duke Elder revision sessions aimed at medical students. He also runs a YouTube channel aimed at medical students, covering topics ranging from study tips, productivity and vlogs of his journey through medical school.
Published: 20/2/22
Last updated: 23/1/25