Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 119

Several avian and reptilian species possess lower‐field myopia. The eyes of these animals are emmetropic along the horizontal and in the upper visual field, but they become progressively myopic below the horizontal. In other words, different parts of the eye have a different refractive power because the shape of the eye is more like a flattened circle, so that the posterior focal length differs for different meridians. This adaptation can be regarded as a static accommodation mechanism. Hence, the animal shifts its gaze to see the object with the appropriate refractive power, and can match the average viewing distances of different areas of the visual field. This allows the animal to keep the ground in focus with relaxed accommodation while foraging for food and, at the same time, monitor the sky for predators while focused at infinity. The same principle is also found in eyes of pinnipeds, where regional changes in the refractive powers of different parts of the cornea allow these animals to maintain high‐resolution vision in both water and air.