![]() Horses can see only two of the visible wavelengths in the light spectrum because they have only blue-sensitive cone cells and yellow-sensitive cone cells. Humans have three types of cones, which sense red, yellow-green, and blue light. The retina also contains cones, or cells that sense color. Thus, equine retinas are designed to detect motion even in low-lighting conditions. The horse’s ratio of rods to cones is approximately 20:1, compared to people who have a ratio of 9:1. Cones are required for bright light (day light) vision, and rods are required for dim light vision. Horses are thought to have vision somewhere in the range of 20/30 to 20/60. Compared to other animals, horses have lots of cells in their retinas and have fairly good vision. Pupils contract under bright light and enlarge in low light. The size of the pupil determines the amount of light allowed into the back part of the eye. The horse has a rectangular pupil shape which extends the area of visual perception. The retina is the major determinant of vision. It is a knobby structure that juts out from the top of the iris and functions to shade the pupil from glare. An unusual feature of the equine eye is the corpora nigra. The nasolacrimal duct drains the tears to a small opening just inside the nostril. Eyelids have tear glands that keep the eye moist. In combination with the upper and lower eyelid, all three function to protect the eye. ![]() The third eyelid is a pink membrane that moves over the eye from the inside corner to the outside corner. Eyelids are a thin ring of two layers of muscles that relax and contract to open and close the eye. The cornea is the surface of the clear part of the eye and the colored part of the eye is the iris. The eye of the horse set to the side of the face ![]() Unlike humans, the horse is able to see images to the left and right at the same time due to the eyes being at the side of the head. From most angles, horses cannot get a left-eye and right-eye view of the same object in one glance. A horses’ ability to see depth is limited because their eyes are set so far apart. The position of the eyes on the horse’s face accounts for differences in how horses see, dictate visual range, peripheral motion detection and depth perception. The equine eye is eight times larger than human eyes placed on the sides of their head. Horses use vision to orientate themselves, detect motion and distance, and evaluate the consistency of the environment. By scanning larger areas, prey is safer from a surprise attack from a predator. Prey species are designed for scanning the environment compared to picking out sharp details. The horse is a prey animal, relying on their senses to assess their environment.
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