650 BIRDS 



cassowaries {Casuarius spp.) the pecten is built as in the neognathous 

 birds : 



The Neognathae* all have the pecten organized as an undulant or 

 accordion-pleated fin, superficially resembling an ordinary steam-heating 

 radiator (Fig. 192b, d). The pleats of such a pecten, when it has been 

 excised, can be smoothed out and the whole organ rendered plane, but 

 only after the apical bridge has first been cut away. The basal area of 

 the organ, the extent to which its ventral end is free of the nerve head, 

 the number of its folds, and the closeness of its approach to the ventral 

 ciliary body and to the ventral periphery of the lens, are all subject to 

 great variation. Its location is constant, however — in all birds the long 

 axis of the base of the structure is directed along the former course of 

 the embryonic fissure of the optic cup; for, since its glial framework 

 develops from the head of the optic nerve, it necessarily conforms to 

 the fissure as does the elongated head of the nerve itself. 



One of the flightless genera of palaeognaths, that of the kiwis {Ap- 

 teryx) , has a pecten which is really a conus papillaris, identical with that 

 of many a lizard (see Fig. 182, p. 632). The eyeball of this large noc- 

 turnal bird is only 8.0mm. in diameter and in axial length. The slim 

 pecten is reported to be 2.0mm. tall and 0.3mm in diameter along its 

 shaft; there are no vanes or pleats. It would be natural to suppose that 

 the kiwi pecten is primitive, and links the vaned and pleated pectens with 

 the simple ancestral reptilian conus papillaris. Such an interpretation is 

 denied us: the kiwi eye — including its pecten — is as degenerate as it is 

 possible for an avian eye to be. It is myopic and affords its owner only 

 very poor vision both by night and by day; and it is tiny, whereas the 

 orbit is huge — implying that the eye has dwindled greatly in size. 

 According to Kajikawa, the eye accomplishes no growth whatever be- 

 tween the 'hen-sized' juvenile condition and the 'turkey-sized' adult 

 stage.t The kiwi, unlike all other birds, appears to have a good sense of 

 smell — so good, indeed, that it is the guiding sense, instead of vision. 



A great many surmises have been made as to the function of the 

 pecten, the first of them not many years after its discovery in 1676. 

 Nearly all its students agree that it must nourish the interior of the eye; 

 but its peculiar form, and particularly the great variations in its form 



*This superorder includes all living birds excepting the ostrich-like forms and tinamous 

 (superorder Palaeognathae) and the likewise primitive penguins (superorder Impennes). 



tFor comparison, note the eye of a turkey, shown at natural size in Figure 142a and b, 

 p. 420. 



