The eye is a complex and elegantly designed organ, capable of capturing light stimuli from the environment and converting them into electrical impulses to be transmitted to the brain and interpreted as visual images. The eye is contained within the bony orbit of the head and surrounded by accessory structures called the adnexa.
The equine eye has one of the largest fields of vision of any land mammal, and is excellent at picking up movement. This is primarily due to the large size of the pupil, which acts like a built-in wide-angle lens and improves the horse’s ability to see movement from different directions, including around the sides of the body. The eyes are also well-adapted for night vision, and can detect movement at up to 4 feet away.
A thorough ophthalmic examination should be performed at least twice a year in healthy horses, and more frequently in horses that are experiencing abnormalities. A veterinarian will examine the appearance of the eyes in a darkened room, checking that they are shaped correctly and that there are no signs of inflammation (such as redness or tearing). A swab may be taken from the inside surface of the eye to be cultured for bacteria or fungus, and drops may be applied to dilate the pupils to allow a better view of the internal structures of the eye.
If a horse is showing signs of a problem with his or her vision, a referral to a veterinary ophthalmologist will be required. A complete eye examination will be carried out, including pressure testing of the eye using a tonometer and a test for refraction to check whether the eye is near- or far-sighted. If the eyes are found to be diseased, an electroretinogram to measure retinal function may also be required in order to make a diagnosis.
Fig. 14B-6 Caudal part of the equine skull.
The anatomy of the horse eye consists of three layers or ‘tunics’: an outer fibrous layer, called the sclera; a middle vascular tissue layer, called the uvea; and an inner nervous tunic, which makes up the retina. The eyeball is surrounded by a fluid called the vitreous humor, which provides stability to the eyes and helps the eye move.
The tapetum of the horse’s eye appears as a horizontal band in the fundus when seen in weak light (A). When exposed to flash, i.e. strong light, the tapetum becomes more prominent and extends dorsally (B). The thicker parts of the tapetum are covered by unpigmented RPE, and the thinner areas by slightly pigmented RPE, which may help to reduce any adverse effects on photopic visual resolution caused by the tapetum. In addition, the location of the visual streak ventral to the thicker parts of the tapetum and its partial coverage by slightly pigmented RPE may also attenuate any negative effect of the tapetum on scotopic visual resolution. Photomicrographs of HE-stained vertical sections of the choroid and retinal pigment epithelium in a human eye (C) and a horse’s eye (D, E). The horizontal band of the tapetum is located between the choriocapillary layer and the proper substance of the choroid.