Scanning electron microscopic study of the posterior ciliary veins in domestic ungulates.

Vascular corrosion casts of 12 equine, 15 bovine and 50 porcine eyes were studied scanning electron microscopically for the presence of posterior ciliary veins. These veins drain a postequatorial segment of the choroid and emerge near the posterior bulbar pole. They complement the four vorticose veins that emerge near or at the equatorial zone of the eyeball and the slender choroidoretinal veins that drain the peridiscal area of the choroid. Posterior ciliary veins were observed in all equine and bovine eyes examined. In these species they presented a large variation in size, number and position. In contrast, posterior ciliary veins were only present in two porcine eyes where they were represented by one and two vessels, respectively. The morphology and variability of these vessels is illustrated and their nomenclature and functional significance are discussed.