652 BIRDS 



ture has found no great place in the ecological Part of this book. We 

 shall find reason to consider the pecten related to the habits, particularly 

 the visual habits, of birds; but it is the habits which have molded the 

 pecten, not the pecten which brings about the habits. To obtain a proper 

 perspective on the intriguing variations of the pecten, it will be necessary 

 to make an apparent digression and consider the whole comparative pic- 

 ture of which the pecten is but one detail : 



In the first place, it must be borne in mind that the only fast-living 

 tissue in the whole posterior segment of the vertebrate eye is the sensory 

 retina. The one richly vascular structure universally present is the chor- 

 ioid coat; but all the blood-filled tubing of the chorioid exists simply in 

 order to maintain a rich flow in the choriocapillaris; and the latter exists 

 solely to nourish the retina — with special reference to the greedy rod- 

 and-cone layer. If the requirements of the visual cells are not too high, 

 there may be enough pabulum left in the trans-retinal exudate of the 

 choriocapillaris to care adequately for the needs of the inner layers of 

 the retina. We should expect this to be the case, ceteris paribus, when 

 the rods are abundant and the more highly metabolic cones are absent 

 or present only in reasonable numbers. But if the chorioid of a partic- 

 ular eye cannot supply a cone-rich visual-cell population in an extensive 

 retina, and the neuronic layers of that retina as well, then we may expect 

 to find some additional vascular device, advantageously situated to 

 supply the inner reaches of the retinal tissue. Ciliary processes, when 

 present, are not so situated; for they lie too far anterior to the main 

 mass of the retina, and their secretion (the aqueous humor) passes 

 too largely and directly into the anterior chamber and is too promptly 

 drained therefrom. 



We found no supplemental nutritive device (hereinafter to be abbre- 

 viated as 's N d') in the lampreys. These forms have small eyes, but 

 most have many cones and tend toward diurnality. Their eyes are per- 

 haps simply too primitive to have achieved physiological perfection. 



The absence of any s N d in the elasmobranchs is readily understood 

 in view of their nocturnality and their pure-rod retinae.* Nor should the 

 modern chondrosteans require anything more than their chorioids. In 

 the holosteans and teleosts, however, we have essentially arhythmic and 

 bright-light groups, and we note that these fishes are all provided either 



* Mere habitual exposure, as such, to strong light — apart from an accompanying high cone: rod 

 ratio — may tend to demand an s N d; it does not seem to have occurred to physiologists 

 that a bright, minified miage will inevitably warm the retina and quicken its metabolism. 



