1. Horses have the largest eyes of any land mammal on earth.
2. The eye is given extra protection by a third eyelid on the inside of the eye, which can close over the eyeball in a diagonal manner like a shield.
3. One of the most interesting things about horses’ eyes is the presence of a reflective layer at the back of the eye called the tapetum lucidum. This layer reflects light back through the retina, giving horses excellent night vision. It also makes their eyes appear to glow in the dark when light is shone into them.
4. The eye orbit is formed by seven cranial bones that come together, and these bones have sutural joints. Any compression or abnormal movement in these cranial bones can potentially contribute to strain or discomfort in the horse’s eyeball because these bones are designed to have a slight degree of mobility to accommodate changes in eye position and to absorb shock.
5. The nervous tunic (or retina) is made up of cells which are extensions of the brain, coming off the optic nerve.
6. The horse has a ‘visual streak,’ a linear area within the retina with a high concentration of ganglion cells (neurons that transmit visual information). Horses have better acuity when the objects they’re looking at fall within this region. Therefore, they may tilt or raise their heads to help place objects within the area of the visual streak.
7. The cornea, the clear covering on the front of the eye, is made up of connective tissue and is bathed in lacrimal fluid and aqueous humor. These fluids provide it with nutrition as it does not have access to blood vessels.
8. Usually dark brown, the iris may be a variety of colors, including blue, hazel, amber, and green. Blue eyes are not uncommon and are associated with white markings.
9. Horses have both monocular and binocular vision. When a horse looks to either side, each eye moves and sees independently. When a horse looks straight ahead, both fields of vision overlap and his vision becomes binocular and he gains some depth perception. Horses have a total range of vision of about 350°, approximately 65° of binocular vision, 285° of monocular vision, and two small blind spots, one directly in front and one directly behind them.
10. Recent studies have shown that horses prefer to drink out of turquoise or blue buckets, suggesting that they associate fresh, tasty water with those colors.
11. Horses have better night vision than humans do, once adjusted to the dark, although it takes them longer to acclimate to sudden transitions in light. So, be patient with your horse if he seems uncomfortable going from, for example, bright sunlight into a dark trailer or stall – he may just need a moment to adjust.
12. The same high proportion of rods to cones (about 20:1) and the tapetum lucidum that give horses superior night vision also gives them better vision on cloudy days, relative to bright, sunny days.
13. Horses have six main muscles that control the movement of the eye within the eye socket. These muscles are located outside the eyeball itself, but their tendons attach to the sclera (the white part of the eye) and anchor to the sphenoid bone at the back of the eye and enable the eye to move in different directions. Dysfunction in these muscles can lead to conditions such as misalignment of the eyes or difficulty in controlling eye movements.
14. Tears are the clear, watery fluids produced by the lacrimal glands in the eyes. They serve to keep the eyes moist, provide oxygen and nutrients to the cornea, and help remove debris and irritants. They are composed of water, electrolytes, mucin (a protein that helps spread the tears over the eye surface), and lysozyme (an enzyme that helps protect against infections). The composition of tears may change depending on factors such as health and environmental conditions.
15. Studies have shown that a tight nose band can actually heat up the eyeball.
16. Horses possess a heightened sensitivity to movement, as it often serves as their initial warning of potential predators. Typically, they detect such movement in their peripheral vision, where visual acuity is limited. In response, horses tend to react defensively and may flee if something suddenly enters their peripheral field of vision.
17. The adnexa of the equine eye refers to the surrounding structures and tissues that are associated with the eye but are not part of the eyeball itself. These structures include the eyelids, eyelid muscles, lashes, conjunctiva (a thin membrane that covers the front surface of the eye and lines the inside of the eyelids) and lacrimal glands.
18. A horse will raise or lower its head to expand its range of binocular vision. Lowering the head brings nearby objects into focus, while raising it allows them to see distant objects more clearly. To use his binocular vision for a closer object near the ground, like a snake or a potential threat to his feet, a horse will drop its nose and arch its neck to look downward. Similarly, when asked to go “on the bit” and maintain a near-vertical head position, a horse’s vision will be directed downward, towards the ground in front of it and the horse will be less able to focus on distant objects. Horses that jump need to raise their heads several strides before a jump, so they can assess the jumps and the proper take-off spots. Barrel horses have a similar need to be able to raise their heads (not too high) so they can focus on the objects in front of them and plan their timing.
19. Horses have dichromatic vision and see tones of yellow and green better than hues in the purple and violet spectrum. Reds tend to appear more greenish to horses. This vision is the result of them having two types of cones in their eyes: a short-wavelength-sensitive cone that is optimal at 428 nm (blue), and a middle-to-long wavelength sensitive cone which sees optimally at 539 nm, more of a yellowish color. This structure may have arisen because horses are most active at dawn and dusk, a time when these rods of the eye are especially useful.
20. Studies have shown that horses are less likely to knock a rail down when the jump is painted with two or more contrasting colors rather than one single color, and when the obstacle strongly contrasts the color of the ground.
21. Many domestic horses (about a third) tend to have myopia (near-sightedness), with few being far-sighted. Wild horses, however, are usually far-sighted. This difference in vision characteristics between domestic and wild horses can be attributed to their respective environments and lifestyles.