The sense of vision is remarkable and of great importance to humans and animals, especially for the equine athlete. A basic knowledge of eye anatomy is essential to understanding and discussing eye problems. Part 1 begins with an overview of eye anatomy. Part 2, concerning common eye problems will be featured in the next issue.

Eye anatomy
The eyelids are important to protect and support the eye. Eye lashes and the long stiff hairs present on the eyebrow and below the lower lid help protect the eye by sensing surrounding objects and filtering airborne debris. The eyelids act as a physical barrier and shock absorber. They also contain glands that produce some of the components of tears. Eyelid closure propels tear film across the surface of the eye, which moistens and cleanses eye surfaces. The inner side of the lids is lined with a moist, pink tissue called the conjunctiva. The conjunctiva continues to the back corner of the inside of the lid where it reflects onto the globe of the eye. The conjunctiva attaches at the limbus, which is the edge of the cornea. The sclera is the white part of the globe. The cornea is the oval shaped totally transparent covering on the front of the eye. The iris is pigmented circular structure, which gives the eye its distinctive color. Most horses have brown eyes but other colors include blue, gold, white, and mixtures of colors. The opening at the center of the pupil is the pupil. The iris controls the amount of light that enters the eye by constricting or dilating the pupil. The pupil in the horse is elliptical in shape, giving the horse more of a panoramic view. The iris has large black masses at the upper edge of the pupil called corpora nigrans, which are thought to also help control the amount of light entering the eye, serving as a sort of built in shade. Directly behind the iris is the ciliary body, which supports the lens. Together the iris and ciliary body make up the anterior uvea of the eye. The space between the cornea and the iris is the anterior chamber. The fluid in this space provides support and nutrition for the cornea. Directly behind the iris is the lens, a transparent structure that focuses images onto the retina, the light sensitive tissue that generates signals that pass through the optic nerve to the brain. There is a specialized pigmented reflective layer behind the retina called the tapetum. It can vary in color from yellow, green, and blue.

Fun facts about equine eyes

• Horses have the largest globe of all land mammals.
• Horses eyes magnify images 50% larger than the human eye does.
• The position of the eye on the outside of the skull and the rather bug-eyed conformation and horizontal pupil give the horse a wide monocular (two-dimensional) field of vision (195º from side to side and 175º from top to bottom).
• Binocular vision (i.e. three-dimensional) is 65º.
• Total horizontal field of vision is 350º. Horses have a blind spot directly in front of their muzzle and another several feet behind their rump
• Horses blink 25 to 125 times every 5 minutes.
• Horses have some color vision – they see yellow best, then green and blue but they have trouble with red.

Next month, part 2 "Equine Eye Problems"…

Kulshan Veterinary Hospital is a full-service, mixed animal hospital offering a wide scope of services to a broad range of clients and patients. Their goal is to provide every patient with the best care possible. Large animal doctors are outfitted for making house calls to meet the medical needs of large animal patients. Kulshan also has large animal hospital facilities available for specialized procedures.

For more information call 360-354-5095, email DrKulshan@MSN.com or visit www.drkulshan.com 

Dave Sauter, DVM is a Minnesota native that moved out west. He graduated from the University of Minnesota in 1987. Following graduation he did an internship at Rood & Riddle Equine Hospital in Lexington, Kentucky. After this internship, he continued to work exclusively with horses for another five years in Kentucky before joining Kulshan Veterinary Hospital. He is a member of the AAEP, AVMA and the WSVMA. Outside of veterinary medicine, David enjoys hiking, photography and spoiling his two daughters.

Paul Schwab, DVM comes to Kulshan Veterinary Hospital from Oklahoma where he was born and raised. He graduated from Oklahoma State University in 1996. He enjoys many aspects of large animal medicine encompassing equine, dairy and small ruminant practice. He is a member of the AABP, AAEP and AVMA.
In his spare time, Paul enjoys many outdoor activities including mountain biking, skiing and traveling. He also enjoys cooking and spending time with his wife and daughter