Pecten of the Pigeon's Eye as an Inter-ocular Eye Shade

THE eyes Of all birds have a black pleated structure, known as the pecten, which projects from the retinal surface into the vitreal space, towards the lens. This remarkable structure has provoked many speculations about its function^1,2, but only one has so far been widely accepted : the pecten has a copious blood supply and this is thought to function as a replacement for the capillary bed that, in many other vertebrates, supplies the vitreal surface of the retina^3,4. Its size, shape and heavy black pigmentation have suggested optical functions, some of which centre around its shadow casting potentialities^5,6, but many of these theories do not seem to take account of the way the pecten is placed in the optical system. It is oriented nearly parallel to light entering the centre of the pupil, so that it presents the minimum area to the converging rays forming the retinal image and the folded free border that is its most striking gross anatomical feature approaches close behind the lens. Thus the shadow cast by the incoming light, which we here call the “primary shadow”, will be as small as possible and very blurred. It is, furthermore, centred on the pecten's own base, which is the elongated optic disk or “blind spot”, devoid of receptors, where optic nerve fibres leave the eye. Thus we think the primary shadow has no beneficial functional significance; on the contrary, it is remarkable that such a large nutrient structure can be inserted into a small optical system with so little disturbance to its normal image forming function. There is, however, an optical function that can be performed by an opaque screen in the position of the pecten.