The distance between the limbus and the ora serrata varies by quadrant: 6.5 mm temporally, 6.1 mm superiorly, 5.7 mm nasally, and 6.2 mm inferiorly. The distances are larger in myopic and shorter in hyperopic eyes. The distance between the limbus and the equator is ~13 mm.

The extraocular muscles insert into the sclera at the following distances from the limbus: medial rectus, 5.5 mm; inferior, 6.6 mm; lateral, 6.9 mm; and superior, 7.5 mm.

Pearl

The insertion of the extraocular muscles is slightly anterior to the ora serrata, but this is a close enough approximation for clinical purposes.

The seven vortex veins (fewer temporally than nasally) penetrate the sclera 3–6 mm posterior to the equator internally and 14–25 mm from the limbus externally.

The corneal diameter is 12 mm horizontally and 11 mm vertically; its thickness varies from 0.55 mm centrally to 0.7 mm in the periphery.

26.3Physiology: What Keeps the Retina Attached?

In primate experiments it appears that a force as minimal as 0.25 mmHg is sufficient to detach the retina. In the human, clinical experience26 tells you the opposite: an attached retina is difficult to separate from the RPE. This leads to the obvious question: What are the forces that prevent RD development under normal circumstances?

26.3.1 The RPE Pump

The RPE is the main factor in maintaining retinal attachment by constantly removing fluid from the subretinal space27 toward the choriocapillaris; it is often referred to as a suction force.

•In eyes with high myopia and a posterior staphyloma, reduced pumping by the RPE is assumed to play a role in the development of the central RD (see

Chap. 56).

•Drugs that interfere with the RPE’s active transport reduce retinal adhesiveness.

•Retinas detach postmortem when the pump stops working.

26Intentionally detaching the retina in eyes undergoing full-rotation macular translocation is not easy.

27Thus virtual.