Functional Microvascular Anatomy of the Horse Eye: A Scanning Electron Microscopic Study of Corrosion Casts

Abstract

Objective:\rThis study presents the microvasculature of the horse iris, ciliary process,\rretina, and choroid and discusses the functional significance of the\rvasculature. Procedure: Seven horses were used for this study. The ocular\rvascular system was injected with methylmethacrylate resin via the carotid\rartery, and the vascular corrosion casts were observed using a scanning\relectron microscope. Results: The iridial vessels showed a wavy course. The\rciliary process was supplied by 2 arterial routes: the iridial and ciliary\rarterial circles. The subjects displayed a paurangiotic retina with retinal\rvessels extending only a short distance around the disc. The retinal arterioles\rand venules ran in closely related pairs, and the capillaries formed hairpin\rloops. No central retinal artery was seen in the equine eyes examined. The\rchoriocapillaris in the avascular retina was arranged in honeycomb hexagon\rlobules and formed a more densely packed network than that in the vascular\rretina. There were 2 distinct venous drainage systems in the horse choroid: the\rvortex veins and the posterior ciliary veins. The vortex vein ampulla was\rflattened and showed a slit-like lumen at the merge site with the ophthalmic\rvein. The vortex veins demonstrated a marked constriction before leaving the\reye. Discussion: The 2 choroidal drainage systems may compensate each other in\revent of occlusion. The ampulla and the constriction in the vortex veins may\ract as a valve regulating the blood flow to keep the eye at an optimum size and\rthe intraocular pressure within the normal physiological range.