Page 384 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

366 Veterinary Histology of Domestic Mammals and Birds

pigment with an absorption maximum in the range of The term area retinae is used to describe well-defined
VetBooks.ir 490–506 nm. slightly thickened regions of the retina in which the den-
Cone cells predominate in the retina of diurnal birds, sity of cone cells is especially pronounced, resulting in
rods being far fewer in number and restricted to the periph-
particularly high resolution. In these regions, the ratio of
ery. In many diurnal birds of prey, the density of cones photoreceptors to efferent neurons can reach the optimal
is greater than in humans. Moreover, there are only very value of 1:1. Visual resolution associated with these areas is
few cone cells per efferent nerve cell (low convergence), further enhanced by the peripheral displacement of overly-
resulting in a high degree of visual resolution. In contrast, ing neurons, resulting in a central depression known as the
the high light sensitivity of the retina of owls results from fovea retinae.
a large number of densely packed rods (representing up to The position and shape of the areas and foveae vary
90% of photoreceptors) and the connection of more than considerably with species. Most bird species have a central
1000 rods with each bipolar nerve cell (high convergence). round area centralis rotunda with a fovea centralis (absent

16.27 Pars ciliaris retinae (common buzzard, Buteo 16.28 Pars ciliaris retinae (tawny owl, Strix aluco) with
buteo). Haematoxylin and eosin stain (x480). pigment in inner epithelial layer. Haematoxylin and
eosin stain (x360).

16.29 Pars optica retinae (chicken). Haematoxylin and eosin stain (x250).

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