Structure and function of the bird fovea.

The cellular structure and functional relevance of the bird fovea are still incompletely understood. This review gives an overview of the cellular composition of the bird fovea, with special regard to Müller glial cells that provide the mechanical stability of the foveal tissue. A survey of previous data shows that the visual acuity of different bird groups (with the exception of owls) depends on the eye size, while the shape of the foveal pit does not correlate with the visual acuity. Among various bird groups, the foveal pit may have two depths, shallow (80-120 µm) or deep (190-240 µm). There is a long-lasting debate whether the bird fovea acts as a local image enlarger or as a focus indicator and movement detector. These functions are supported by the refraction of the incoming light at the tissue surface. However, it was shown that Müller cells form highly refractive layers in the centre and walls of the deep avian fovea (Nature, 1978, 275, 127). Analysis of the light path through the tissue may suggest that Müller cell layers serve at least two optical functions: magnification of the image in the foveal centre and light focusing into a point within and/or a ring around the foveal centre. It is suggested that Müller glial cells contribute to various optical functions of the bird fovea.