ANATOMY OF THE FUNDUS ORGANS IN BIRDS 



21 



arrangement of retinal tissue that is generally 

 found in the horizontal meridian. It is mostly 

 seen in those birds that seek their food upon 

 the ground, e.g., Motocilla, Saxicola, Struthio, 

 Totanus, Tringa, Larus, Squatarola; also 

 in the Goose, Flamingo and other Aquatic 

 birds. See, for example, Figure 14. 



Perhaps the retina manifests the greatest 

 foveal development in swift-flying birds. For 

 example, Cypselas, Hirundo and Sterna have 

 three fovea?. 



Slonaker has also noticed an important 

 variation in the relative position of avian 

 fovea? and has offered an explanation borne 

 out by the results of this investigation; the 

 fovea nasalis is almost invariably found in 

 the same fundus area, but the locality of the 

 temporal fovea depends upon the position of 

 the eye in the head. As the eyes are turned 

 more and more forward the fovea temporalis 

 approaches the fovea nasalis. As binocular 

 vision becomes more frequent both fovea? 

 may become merged into one, generally 

 deeper, pit. There is also a corresponding 

 change to an asymmetrical form of the eye- 

 ball and to a peculiar position of the crystal- 

 line lens in birds with binocular vision. 



In many such birds, the White-bellied 

 Swallow (see Fig. 5) and the various Terns 

 for example, the nasal fovea is deep and sharp, 

 while the temporal fovea is quite shallow and 

 the eyes are almost symmetrical; but in birds 

 with more marked binocular vision, Hawks, 

 for example, the temporal fovea has the 

 greater depth and the eye becomes more asym- 

 metrical. The climax is reached in Owls, 

 whose eyes are most irregular in form, who 

 have but one (deep) temporal macula and 

 who see only binocularly. 



As with other organs in their body the com- 

 ponent parts of the visual apparatus of Birds 

 are arranged and adjusted mainly to enable 

 the Bird to obtain food and to escape his 

 hereditary enemies. In a minor degree, too, 

 we observe the influence of the necessity for 

 swift locomotion. Other (occasional) taxo- 

 nomic influences are apparent in certain special 

 visual requirements, such as nocturnal vision, 

 the use of the eyes under water, the employ- 

 ment of the eyes for vision both in air and 



water, the visualization of minute objects 

 close at hand, etc. This statement is true 

 in full measure of the organs and tissues com- 

 prising the fundus oculi. 



Stereoscopic, binocular, single vision in 

 Birds with double fovea? — and this is the 

 most acute, accurate and effective form of 

 eyesight that these or any other animals 

 know — is probably accomplished by the two 

 temporal fovea? acting in cerebral unison. 

 It is this form of sight, for example, that 

 enables the Osprey and other Hawks first to 

 locate from a distance and then to dart 

 unerringly upon their prey. 



The deep and evidently more acutely func- 

 tionating nasal fovea is, in the writer's opin- 

 ion, employed for monocular vision only, 

 and there seems every reason to believe that 

 Birds with double fovea? have exceptionally 

 good eyesight with each eye separately; they 

 are, by this effective combination, enabled 

 not only to command a view of the highest 

 efficiency over the whole horizon, but also have 

 the power to concentrate it when needed upon 

 particular objects invisible or indistinctly 

 visible to other species not so provided. 



It must, however, be acknowledged that 

 the neurology of Birds, involved in these 

 questions, is a sub-section of biology as yet 

 in its infancy; only when the histology, 

 pathology and experimental physiology of the 

 avian cerebral organs and their connections 

 have been worked out as they have been in 

 Man shall we know how the paths pursued 

 by "brain currents" involved in this switch- 

 ing from monocular single vision to binocular 

 sight run and are controlled. As these ques- 

 tions now stand the only certain thing that 

 can be said is that, with the possible exception 

 of the Owls, they must be totally unlike the 

 cerebral and spinal arrangements discovered 

 in Man and the higher Apes. 



Of the physiology and psychology involved 

 in the ribbon- or band-like area we know still 

 less. In all probability it is a device to enable 

 swift-flying birds to take their prey on the 

 wing, and other species to find their food on the 

 ground while they themselves also stand on 

 terra fir ma; in other words, birds with banded 

 retina? possess eyes that are chiefly employed 



