Excerpt from An Encyclopedia of Rural Sports, Vol. 2

Subsect. 1. The Sens'es of Seeing, Hearing, Tasting, Smelling, and Touch in Birds.

The importance of eyes to birds is marked by the circumstance, that whereas among all other animals there are some which are either altogether or partially blind, it is observed that no bird whateveris doomed to grope its way through life in total unconsciousness of light. The value of light to birds is further inferred from the size of the organ receiving it, which is considerable in all, but in the rapacious tribes it is more particularly so. The ball of the eye is sunk in a bony orbit of great size, the sides of which furnish four straight and two oblique muscles to operate its various motions. Posteriorly the globe is hemisphérical, an teriorly a bony circle, composed of minute lamina, inserted between the sclerotic coats, contracts its figure to that of a short cylinder, to which the cornea is attached as an interior and smaller hemisphere. This peculiarity of structure is important to the vision of birds in some the axis or depth of the eye-ball is lengthened or diminished by different portions of the sclerotic coat, by which admirable contrivance the action of the four muscles we have noticed, is rendered more effective in altering the focal angle. Birds, after the manner of the higher orders of mammiferous animals, have three eyelids but it is to_ be noted, that their nictating membrane, or third eyelid, extends completely over the globe. Another pe culiarity in the eye of birds is the marsupium, which is a corrugate membrane covered with a black pigment, which probably absorbs super¿uous rays.

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