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VETERINARY
EQUINE VISION
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N the wild, the horse is a ‘prey’ or ‘flight’ animal and so is designed to have a wide field of vision to see its predators early and outrun them. As a result, the horse has two large eyes each placed on the widest part of its head giving it almost a 360-degree field of vision. This article will examine the science behind the vision of our favourite ‘prey’ animal in an attempt to allow our ‘predator’ minds to understand how the horse behaves, as well as going on to examine how equine vision may be affected by the application of racing headgear and eye disorders. The horse has the largest eye of any land mammal and each eye has lots of pigment and a large ‘letter-box’ shaped pupil. The equine eye is not perfectly round but actually slightly flattened when examined from front to back. Its upper lid is slightly angled due to muscle pulling on it and there are no lashes on the lower lids. It has excellent panoramic vision – low resolution but very little blind-spot. It seems obvious that horses cannot see directly behind them, however, what surprises many is firstly, how small the blind-spot actually is behind them and secondly, that there is a small blind-spot for the first three or four feet in front of them. The area of the equine blind-spot is triangular in shape and runs from a specific point three to four feet in front of the horse to behind the horse’s head, back to the horse’s quarters and going on indefinitely if the horse stands with its head directly in front of it. As a consequence, the horse may become startled if something, for example a human, suddenly appears from the blindspot. While the result of the horse’s eye position is a near 360-degree field of vision, it is worth considering that not all of this field of vision is binocular (seen using both eyes at the same time). In fact, the placement of the equine eye reduces the possible range of binocular vision to around 65 degrees with the result being that a horse has a smaller field of detailed vision than a human. The horse uses its binocular vision by looking straight at an object, raising its head when looking at a distant predator or a jump, or lowering its head and looking downward and arching its neck slightly when focusing on something on the ground. An excellent example of how the horse alters is head-carriage to focus on an object is when jumping. Horses may approach the jump with a low head carriage but will always raise their head a few strides before the jump in order to fully focus on it, assess it and hence take off at the appropriate time. It is hard to know exactly how well the horse is able to see detail, but it is generally believed to be better than cats and dogs but ISSUE 13 TRAINERMAGAZINE.com 23