Dimensions and Measurements of the Eye

Anterior pole: centre of curvature of the cornea

Posterior pole: temporal to the ON, at the centre of curvature of the posterior eyeball

Optic axis connects the two poles, midway between these two poles sits the equator

Screen Shot 2016-07-03 at 3.43.20 pm

Emmetropic eye measures:

  • 24mm anteroposteriorly
  • 23mm vertically
  • 5mm horizontally

The Cornea

  • Anterior 1/6th of the eyeball
  • Considerable variation in corneal dimensions from person to person
  • 10.6mm vertically and 11.7mm horizontally
  • Radius of curvature of anterior surface: 7.7mm
  • Radius of curvature of the posterior surface: 6.9mm
  • Thinnest at the centre at 0.5-0.6mm
  • Thicker at the periphery at 0.7mm
  • Greater curvature in the vertical plane than the horizontal plane constitutes regular astigmatism

The Sclera

The adult sclera is:

  • 1mm thick posteriorly
  • 6mm thick at the equator
  • 3mm thick just posterior to the insertions of the recti muscles
  • 8mm thick at the corneoscleral junction

Openings in the sclera include the:

  • Posterior scleral foramen – optic nerve perforation through the sclera, 3mm medial and 1mm superior to posterior pole
  • Anterior scleral apertures – located at the insertion of the recti muscles, for the branches of the anteriociliary arteries
  • Middle scleral apertures – 4mm posterior to the equator for the exit of the vortex veins
  • Posterior scleral apertures – transmit long and short ciliary nerves and vessels around the optic nerve

Screen Shot 2016-07-03 at 3.47.07 pm.png

Tenon’s Fascia

  • Fascial sheath surrounding and protecting the eyeball
  • Attaches to the sclera 1.5mm posterior to the corneoscleral junction
  • Fuses posteriorly with the sclera and the meninges around the optic nerve

Vortex Veins

  • Tortuous in nature
  • Drain the ocular choroid
  • Each eyeball has 4 vortex veins – one in each quadrant
  • Enter choroid at equator, exit sclera 4mm posterior to the equator through the middle scleral apertures into the episcleral space
  • Superonasal and superotemporal vortex veins drain into superior ophthalmic vein
  • Inferonasal and inferotemporal vortex veins drain into the inferior ophthalmic vein

Extraocular Muscle Insertions

The four recti muscles insert onto the sclera anterior to the scleral equator, while the superior and inferior oblique muscles insert posterior to the equator

Screen Shot 2016-07-03 at 3.52.00 pm

Snell, R. S. and M. A. Lemp (1998). Clinical Anatomy of the Eye, 2nd Edition.

 

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