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