Vetnuus | July 2024 41 Axons in the nerve fibre layer of the retina are myelinated, unlike those in most mammalian species. Most birds have a visible fovea which is associated with maximal cellular density thus affording maximal visual resolution. It is generally agreed that the fovea enables increased visual resolution, but the exact cellular structure and relevance of the avian fovea remain incompletely understood. It is hypothesized that movement detection, image fixation, depth perception and a sensitive focus indicator could be additional functions. In bifoveate birds, like kingfishers and bee-eaters, the nasal fovea functions to scan the horizon for prey in a monocular fashion while the temporal fovea acts in a binocular fashion and takes over at close range, this allows for precise prey capture in a three-dimensional-environment. Primates have 3 cone pigments identified and two cone pigments are present in ungulates and the dog. Photopic vision in most birds is tetrachromatic and even pentachromatic in some species. Oil droplets are another unique characteristic of bird retinas, these are small spheres containing mainly lipids and carotenoids and function as light filters. Vision is an incredible sense and the diversity of adaptations in the natural world is remarkable. References: 1. Ophthalmology of Exotic Pets, David L. Williams 2. Wild and Exotic Animal Ophthalmology, Fabiano Montiani-Ferreira, Bret A Moore, Gil Ben-Shlomo 3. Wikipedia Regulars I Ophthalmology Column
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