Features of examination of the ocular fundus of the Phoca sibirica
The research purpose is to study the possibility of applying Tropicamide 1% for examining the fundus of the Baikal seal. Unlike terrestrial mammals, in marine mammals, the use of mydriatics does not cause required pupil expansion. This is due to the peculiarity of location of the pupil dilatator muscle. It is located perpendicular to the sphincter muscle and more closely adjacent to the iris base. Moreover, the dilator expands posteriorly over the enlarged base of each ciliary process. Tropicamide 1% causes small mydriasis (approximately 1/3 of the possible maximum pupil expansion), which is enough to examine the central part of the fundus, but not enough to examine its periphery. Four-fold drug instillation with an interval of 15 minutes in absolute darkness is required to achieve persistent mydriasis. The optic disc is round. There are 8-9 retinal vessels that flow into the optic nerve. Unlike dogs and cats, the Baikal seal has arteries and veins of the retina of identical diameters. A reflective plate (tapetum lucidum) is visible. Its color varies from yellow-green to blue and gray depending on the age.
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