56 



THE FUNDUS OCULI OF BIRDS 



the temporal side of the optic disc, an example 

 of which is pictured in Syrnium aluco (Fig. 

 132) and Strix flammea (Fig. 112). This ar- 

 rangement closely approaches the binocular 

 maculae of Man; indeed the visual apparatus 

 of Strigiformes resembles the human eye 

 more closely than does any other order. 



The Choroid is seen macroscopically as a 

 dark background for the translucent, or only 

 partially visible retina; indeed, the coloration 

 of both macroscopic and ophthalmoscopic 

 pictures of the bird's fundus is largely deter- 

 mined by this deeply pigmented ocular tunic 

 with its rich vascular supply. 



As noted in Chapter VIII, domestication 

 brings about changes in all the fundal tissues 

 — usually tissue degeneration, if not actual 

 disease. Hence we find that Gallinse possess 

 very shallow or doubtful fovea?. The writer 

 has concerned himself little with barnyard 

 fowls, but Slonaker has shown that while 

 Columba livia domestica, still retains a medium 

 fovea surrounded by a well-defined macular 

 area, Meleagris gallopavo and Numida pucher- 

 ani have very shallow arex centrales. As 

 stated elsewhere, some Birds have no macular 

 region demonstrable by the naked eye. Gallus 

 domesticus, for example, has an extremely 

 doubtful fovea, while Anser domesticus and 

 Anas boschas domesticus, in addition to a very 

 shallow macular region, exhibit a band-like 

 area extending horizontally across the retina. 

 The latter device probably enables the bird 

 to widen the limits of his field of distinct 

 vision (especially while feeding) without being 

 obliged to move the whole head. This curi- 

 ous provision is more clearly shown in the 

 Ring-Neck Plover whose fundus shows a 

 very distinct band-like area that passes 

 obliquely across the retina. A dark fine, 

 like a trough or gutter, appears to the naked 

 eye in preserved specimens to extend almost 

 the full length of the band. Slonaker ex- 

 mined this area microscopically but could find 

 no trough-like depression. 



Oval, ovoid, circular or ribbon-like areas, 

 are as a rule, readily traced in the gray, 

 smooth retinal tissues, although both the 

 microscope and ophthalmoscope may be 

 needed, as in the Flamingo and some of the 



Gallinaceous birds, as a supplement to macro- 

 scopic inspection to decide the presence or 

 otherwise of a macular region or of any of 

 its components. The limits of the retinal 

 band area often call for these aids; indeed it 

 may be said that no exploration of an avian 

 fundus should be regarded as complete until 

 all these requirements are met. Areas of 

 acute vision, in prepared material, appear to 

 be whiter or less gray than the surrounding 

 retina; there is not, however, a sharp defini- 

 tion to the naked eye, or even when observed 

 by the help of a magnifying lens, but the tis- 

 sues of the arem centrales shade off and gradu- 

 ally blend with the perimacular tissues. 



It must not be assumed that because no 

 central area of distinct vision is discovered 

 macroscopically that it does not exist. Some 

 macular regions are so shallow and so slightly 

 differentiated from the neighboring tissues 

 that only the microscope can settle the ques- 

 tion of their existence. 



The exact character and visual purpose of 

 these retinal band-like areas (sometimes also 

 visible to the ophthalmoscope) await further 

 histological investigation. 



Birds of prey as well as the swift fliers show 

 the best marked and the greatest variety of 

 macular regions. A typical instance is seen 

 in the Sparrow Hawk. In this bird each 

 fundus presents a well-marked fovea nasalis 

 and fovea temporalis connected by a short 

 band-like area. 



The nasal fovea is invariably the deeper and 

 sharper of the two, and probably is used when 

 distinct sight and binocular vision are required. 

 It is surrounded by a large, macular area and 

 is situated about the center of the retina. 



The temporal fovea is shallower, is encircled 

 by a smaller area and is placed near the 

 periphery of the retina lower than the nasal 

 fovea but about the same distance from the 

 optic papilla. This foveal region holds about 

 the same relative position in the fundus as 

 the single fovea of the Owl while the fovea 

 nasalis stands in the same relation as that 

 occupied by the Blue-Bird and the Robin. 



Of the swift fliers the Terns and the Swal- 

 lows present much the same foveal and band- 

 like sensitive regions as the Hawks; in other 



