366 ADAPTATIONS TO SPACE AND MOTION 



the pecten casts a shadow on the retina and that the tips of its pleats 

 extend this shadow, like the fingers of a hand, across the fundus and 

 the neighborhood of the area centraUs. Figure 127 shows sample over-all 

 shadows — composite sketches made with many directions of the ophthal- 

 moscope light, from birds representative of various categories. 



Menner felt that the dactyloid shadows of the pleats might be a device 

 for enhancing the perceptibility of movements. To test this idea he 

 aimed a camera, focused for infinity, at some circling birds in the sky. 

 On the ground-glass screen, nothing could be seen. He now glued to 

 the inside of the glass a cardboard model of the pecten which would cast 

 finger-like shadows. With the camera pointed again at the wheeling 

 birds, their movements and courses were at once evident upon the screen. 



The phenomenon was then explained in terms of some old statements 

 of Exner, who found that a movement was especially conspicuous when 

 the image swung back and forth across the blind spot — the head of the 

 optic nerve. The repeated 'on' and 'off' effects gave the movement 

 greater saliency in consciousness. Menner decided that the multiplicity 

 of pecten-pleat shadows must do the same thing, in a big way: each 

 shadow, if pronounced {i.e., in strong illumination) would create a tem- 

 porary blind spot or streak, over which the swinging image would have 

 on-and-off impacts on the retina and in consciousness. He pointed out 

 that the development of the pecten in different birds (y.s.) goes hand 

 in hand with their need, considering their feeding habits, of good visual 

 movement-perception. 



It had long since been decided by others that the development of the 

 pecten in various birds is correlated with their ranges of accommodation, 

 though no one has demonstrated that the pecten plays any part in the 

 process of accommodation in birds or reptiles. Now of course good 

 accommodation, high visual acuity, and acute movement-discrimination 

 would all be expected to go together in birds anyway — all being lowest 

 in the owls, highest in the hawks, with the granivorous birds and the 

 bug-eaters fitting neatly in between. These correlations do exist; and so, 

 as far as Menner's theory is concerned, the relationship he points to 

 (between pecten and habits) would exist whether his theory has any 

 value or not. The different ecological types of birds do need different 

 movement-seeing abilities, but their visual acuities alone, and their 

 persistence times, are probably already related nicely to their needs in 

 this respect without taking the pecten into account at all. 



It is not entirely certain that the pecten, which is always located 



