Disturbance of posterior ciliary arterial circulation is primarily responsible for common ischaemic disorders of the optic nerve head (ONH) such as anterior ischaemic optic neuropathy and glaucomatous optic neuropathy.1 Much of the attention given to the vasculature of the ONH stems from an interest in these conditions. Anatomical studies using vascular casts or serial histological sections and angiography have provided a basis for understanding blood supply but, because the feeding arteries could not be visualised hitherto, knowledge of the vascular dynamics remained uncertain. Moreover, disagreements on vascular structure persist; two examples concern the roles of the Zinn–Haller circle and the choroidal arteries. Accepting that ONH blood supply is substantially …