Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 138

As with scotopic vision, the pupil also contributes to photopic vision because miosis protects the retina from excessive and harmful amounts of light. It is proposed that the large corpora nigra found on the superior (and often the inferior pupil also) border of the iris in some species provides additional protection as it decreases the amount of light entering the eye from the superior visual field (where the Sun is located), further reducing glare and improving vision in bright light. Moreover, because a miotic slit pupil can block light more efficiently than a miotic circular one, it is suggested that slit pupils have evolved in nocturnal or crepuscular species such as cats and geckos that need to function in daytime.

Figure 2.16 In a comprehensive canine ERG protocol, following preparation of the animal in ambient light, the light is turned off. During the next 20 min, the retina is stimulated with a dim flash every 4 min, thus generating a dark adaptation curve. In a normal animal, signal amplitude will increase from one flash to the next as the retina dark adapts (black, red, green, pink, and yellow traces represent the respective responses recorded after 4, 8, 12, 16, and 20 min in the dark). Failure of the signal to increase with time in the dark may be an early sign of rod dysfunction.