Chapter 1 Structure and Function of the Eye
Vision is a complex phenomenon in which light emanating from objects in the environment is captured by the eye and focused onto the retinal photoreceptors (Figures 1-1 and 1-2). Electrical signals originating from these cells pass through a number of cell types in the retina and throughout the central nervous system (CNS) before arriving at the visual cortex, where the sensation of vision occurs. Numerous species variations exist on this basic theme, each allowing the animal to exploit a particular ecologic niche. The basic similarities among all vertebrate eyes and how they respond to insult allow the comparative ophthalmologist to confidently treat a wide range of ocular conditions in a diverse array of species.

Figure 1-2 Internal structures of the canine eye (A). Also shown are the standard reference planes (B).
FUNDAMENTALS OF VISION
Sensitivity to Light

Figure 1-4 Cellular tapetum of a dog (A) and fibrous tapetum of a horse (B).
(B from Gilger B [2005]: Equine Ophthalmology. Saunders, St. Louis. A and B courtesy Dr. Christopher J. Murphy.)
The rhodopsin photopigment of dogs and cats is tuned to a slightly different wavelength of light from that in humans and, as is typical of species adapted to function well in dim light, takes longer to completely regenerate after extensive exposure to bright light. The ranges of wavelengths to which rhodopsin in dogs, cats, and humans is sensitive are similar, however, indicating that vision in dim light is not enhanced by expanding the range of detectable wavelengths. The slight wavelength shifts in the maximal sensitivity of rhodopsin across species suggests that domestic mammals and humans do not perceive the world in exactly the same way.
Sensitivity to Flickering Lights
Although not related to motion detection, the point at which rapidly flickering light fuses into a constantly illuminated light (flicker fusion) provides insight into the functional characteristics of rod and cone photoreceptors. The flicker frequency at which fusion occurs varies with the intensity and wavelength of the stimulating light. Because dogs can detect flicker at 70 to more than 80Hz, a television program in which the screen is updated 60 times/sec and appears to people as a fluidly moving story line may appear to dogs as rapidly flickering.
Visual Field of View
The extent of the visual field (i.e., the area that can be seen by an eye when it is fixed on one point) and the height of the eyes above the ground may vary greatly among breeds and species and has a major impact on the perception an animal has of its environment (Figure 1-5). For example, when the visual fields of its two eyes are combined, the horse has a total horizontal visual field of up to 350 degrees, with 55 to 65 degrees of binocular overlap and a virtually complete sphere of vision around its body (Figure 1-6). The length of the horse’s nose interferes with binocular vision, and so a horse views an object binocularly until it is about 1m away, at which point the horse must turn its head and observe with only one eye. In comparison, humans have a visual field of approximately 180 degrees (140 degrees of overlap), cats have a 200-degree field of view (140-degree overlap), and depending on breed, dogs have 250 degrees (30 to 60 degrees of binocular overlap) (Figure 1-7). The horse has only a few minor “blind spots,” which are located superior and perpendicular to the forehead, directly below the nose, in a small oval region in the superior visual field where light strikes the optic nerve itself, and the width of the head directly behind. Clearly, this extensive visual field makes it very difficult for a person or potential predator to “sneak up” on a horse.
Visual Acuity
Optical Factors in Visual Acuity
The optical media of the eye, namely the cornea, aqueous humor, lens, and vitreous humor, are responsible for creating a properly focused image on the retina. The cornea and, to a lesser extent, the lens are the principal refracting surfaces of the eye, and their ability to bend (refract) light is determined by their radii of curvature and the differences between their refractive index and that of the adjacent air or fluid. If the focal length of the focusing structures of the eye does not equal the length of the eye, a refractive error is present. In a normally focused (emmetropic) eye, parallel rays of light (effectively anything 20 feet or more away from the eye) are accurately focused on the retina. If parallel rays of light are focused in front the retina, myopia (nearsightedness) results. If they are focused behind the retina, hyperopia (farsightedness) results (Figure 1-9). Such errors in refraction are usually expressed in units of optical power called diopters (D). The extent of the error can be expressed by the formula D = 1/f, where f equals the focal length (in meters) of either the lens or the optical system as a whole. Therefore if an eye is 2D myopic at rest, it is focused at a plane located 0.5m in front of the eye. Similarly, an eye that is emmetropic at rest but can accommodate (change focus) 3D is capable of clearly imaging objects on the retina that range from as far away as the visual horizon (infinity) to as near as 0.33m in front of the eye.
Loss of the lens, as occurs after cataract surgery, results in severe hyperopia (farsightedness), with objects being focused approximately 14 D behind infinity, and a reduction in visual acuity to 20/800 or worse. This means that aphakic eyes are unable to image any object clearly, whether near or far away, and are unable to accommodate. Although the aphakic dog is extremely “farsighted,” it must be kept in mind that, for objects of similar size, objects that are closer to the dog will create a much larger image on the retina than objects that are located far away. Therefore the aphakic dog may be able to better visually orient to near objects despite being “farsighted.” Surprisingly, although this degree of hyperopia is markedly debilitating to some dogs, most dogs are still able to visually orient adequately in their environment without correction.