Vasculature of the equine and canine iris.

This study seeks to compare the vascular system and structure of eye vessels in adult horses and dogs, focusing on their iris. The differences noted between the two species include how their ciliary blood vessels divide, the course of the radial arteries, and the venous blood drainage paths.

Vascular Cast Preparation and Histologic Examination

• The researchers used methyl methacrylic casts to reveal the structure of the blood vessels in the iris of both horses and dogs.
• They also freshly collected tissue samples to conduct a histologic examination, which is a microscopic study of tissue structure.

Comparison of Vascular Supply

• In horses, the ciliary blood vessels (blood vessels found at the edge of the eye’s lens) divided within the iris to form a complete major arterial circle which supplies blood to the eye.
• In dogs, the ciliary blood vessels divided within the ciliary body (part of the eye that produces aqueous humor) sending recurrent branches to form a similar arterial circle.

Comparison of Radial Arteries and Their Control

• Radial arteries extend from the arterial circle towards the pupillary margin (the boundary of the iris and pupil) in both animals.
• In horses, these radial vessels were nearly straight and they joined to create a secondary or minor arterial circle at the level of the sphincter pupillae muscle (the muscle that controls the size of the pupil).
• Interestingly, the authors observed that in horses, the control of the radial blood vessels did not seem linked to the sphincter muscles.

Comparison of Venous Drainage

• Drainage of the venous blood, or deoxygenated blood, was also different between the two species.
• In horses, the venous blood from the iris was channeled towards the ophthalmic vessels using four prominent vortex veins.
• In dogs, this drainage occurred via 4 to 6 circular collecting veins located in the ciliary region.
• From here, blood moved either through the ciliary veins or smaller vortex veins to the dorsal (upper) and ventral (lower) external ophthalmic veins, and then to the broader ophthalmic plexus (a network of blood vessels).