Vasculature of the equine and canine iris

Methyl methacrylic casts were prepared and tissue was freshly collected for histologic examination to compare the vascular supply and the structure of the vessels of the iris in the adult horse with those of the dog. In the horse, ciliary blood vessels divided in the iris and formed a complete major arterial circle of the iris. In the dog, the ciliary blood vessels divided in the ciliary body and, by means of recurrent branches, formed a complete arterial circle. From this circle, radial arteries extended toward the pupillary margin. The radial vessels in the horse were nearly straight and joined to form a prominent minor arterial circle of the iris at the level of the sphincter pupillae muscle. Radial blood vessels in the horse did not appear to be controlled by the sphincter muscles. Venous blood of the iris of the horse was carried to the ophthalmic vessels via 4 prominent vortex veins. Drainage from the iris of the dog was by way of 4 to 6 circular collecting veins in the ciliary region. From this site, blood passed either via the ciliary veins or via small vortex veins to the dorsal and ventral external ophthalmic and then to the ophthalmic plexus.