THE PECTEN AND MOVEMENT-PERCEPTION 



365 



(far above human fusion frequency) the fish was still clearly perceiving 

 the rotation. At 100-120, the fishes reacted poorly, and were probably 

 experiencing flicker. 110 impressions per second was the lowest limit for 

 fusion. Beniuc translated this figure into a value of V55 second for the 

 duration of the biological moment of the Siamese fighting-fish. 



Menner's Theory of the Pecten — The pecten of the bird eye (see 

 Figs. 80, 114; pp. 188, 308) has been one of the greatest puzzles in 

 comparative ophthalmology. Some years ago an authority counted over 

 thirty theories as to its function, which were sufficiently different to call 

 distinct interpretations. Other suggestions have been made since, but 

 none more intriguing than the very recent one offered by Erich Menner. 

 The pecten (Fig. 114, p. 308) is a simple cone in reptiles, where it 

 cannot possibly play a role in vision but is merely a nutritive organ, on 

 a par with the falciform processes, retinal vessels, and chorioid 'glands' 



Fig. 127 — Overall tracings of the shadows cast by the pecten in each eye, in various species 



of birds, in relation to their feeding habits and their consequent needs 



with regard to movement-perception. After Menner. 



a, Buteo buteo (a hawk, feeding largely on cursorial prey), b. Coal titmouse, Parus ater 

 (chiefly insectivorous), c, English sparrow, Passer domesticus (chiefly granivorous ) . d, 

 Domestic pigeon, Columba livia (granivorous). e. Long-eared owl, Asio otus (predaceous, 

 but largely dependent upon audition). 



of Other vertebrate categories. In birds, however, it is an elaborately 

 pleated fin of pigmented and richly vascular tissue, reaching from the 

 retina nearly to the lens (Chapter 17). One can account for its great 

 increase of surface, over that of the reptilian organ, on the basis of the 

 bird's warm-bloodedness and elevated metabolism. But ulterior meanings 

 of this conspicuous organ have long been sought, the search stimulated 

 by the enormous variability of the pecten from species to species, and by 

 the hope of correlating these variations with something else in the visual 

 biology of birds. Briefly, the pecten is smallest, with the fewest pleats, in 

 nocturnal birds. It is larger in seminivorous forms, still larger and more 

 elaborate in insectivorous birds, and largest of all in the diurnal pred- 

 ators such as the hawks and eagles. 



Menner placed numerous bird heads in a special perimeter and studied 

 the pecten with the ophthalmoscope from many angles. He found that 



