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Infection which involves the internal structures of the eye. It usually poses a serious threat to the visual function of the eye.

The center of the fovea is the foveola ? about 0.2 mm in diameter ? where only cone photoreceptors are present. The central fovea consists of very compact cones, thinner and more rod-like in appearance than cones elsewhere. Starting at the outskirts of the fovea, however, rods gradually appear, and the absolute density of cone receptors progressively decreases.

Excessive tear production or insufficient tear drainage from the eyes (usually from blockage of the lacrimal duct) causing overflow of tears onto the face.