THE PECTEN AND ITS ANALOGUES 655 



metabolism, such that the chorioid is unable to take care of the whole 

 thickness of the retina — occasionally, unable to supply even the whole 

 requirement of the visual cells alone. We can now again approach the 

 avian pecten, prepared to inquire more astutely whether its variations 

 really demand explanation other than the one which seems to cover the 

 supplemental nutritional structures of other vertebrates. 



A large amount of information on the size of the pecten in different 

 birds has been gathered together by Kajikawa and Franz. It is not easy 

 to interpret the data, for as often as not only the number of folds of the 

 pecten has been recorded. This value is however as useful as any other 

 single one; for even if we knew the length, width, and height of a pecten, 

 and its total surface area, we should still need to know its total blood- 

 vessel area, blood capacity and rate of flow, the area and volume of the 

 retina and its rate of oxygen- and glucose-consumption, before we could 

 compute any very precise ratios as a basis for the comparison of one eye 

 with another. Not all of these facts are known for any one bird, let alone 

 for an assortment of birds with various habits. 



In a great majority of birds, the Jength of the base of the pecten is 

 about equal to half the horizontal diameter of the eye, and the number 

 of folds in the pecten runs high — about 14-27, with 30 as the maximum 

 (in Garrulus glandarius) . In this category are most of the ground- 

 feeding, gallinaceous birds and the perching birds (Order Passeriformes, 

 comprising about half of the 20-odd thousands of kinds of birds) , These, 

 and indeed most other birds, feed upon small objects and have high 

 capacity for resolution and accommodation in proportion to the size of 

 their eyes. Still greater ranges of accommodation are found in the 

 largest-eyed predaceous birds, the hawks and eagles. In these birds the 

 volume occupied by the pecten is relatively about as large as in most 

 passerines, but the folds are coarser and consequently somewhat fewer 

 (mostly 13-17). The owls and swifts are known to accommodate but 

 little. Owls have only 5-8 pecten folds, and the number in Apus apus 

 (11) is just low enough to call 'low'. 



These generalizations have been known for some years, and have led 

 many investigators to agree with a theory of Rabl, which he based upon 

 correlations of the number of pecten folds with the relative size of the 

 ringwulst (which, as we have seen, is essentially involved in sauropsidan 

 accommodation) . Rabl held that since the degree of development of the 

 pecten goes with the degree of development of the accommodatory 

 mechanism, the pecten must be a part of that mechanism. 



