648 BIRDS 



ters do more of the labor of accommodation than their ciliary apparati. 

 The ringwulst is small also in flightless birds (Apteryx, ostriches) and 

 smallest of all — practically non-existent — in the Australian terrestrial 

 goose Cereopsis. 



Between the ringwulst and the lens-body there is a slender space, a 

 vestige of the cavity of the embryonic lens vesicle (see Fig. 40e and f, 

 p. 1 10) , into which the inner ends of the ringwulst fibers secrete a fluid 

 substance. This perhaps serves only to lubricate the interface between 

 the ringwulst and the lens proper as the two shift past each other during 

 the accommodatory deformation of the lens. But it has been suggested 

 that there may be enough of the fluid to make a sharply-curved blister 

 under the anterior lens epithelium, when the fluid is squeezed forward 

 by the pressure of the ciliary processes. A 'bump' does form on the 

 anterior face of the accommodating lens, but this may be wholly due 

 to the mechanics of the lens and ringwulst and the orientations of their 

 respective fibers, and to the restraining pressure of the iris (v.^.) That 

 the sphincter contracts (stiffening the iris) during accommodation is 

 indicated by the tautening of the pectinate ligament, demonstrated 

 beautifully by Wychgram (and see Fig. 109, p. 275). 



The Pec ten, and Its Analogues in Other Vertebrates— Tht most 

 conspicuous and perennially interesting feature of the avian eye is its 

 pecten (Fig. 192; see also Figs. 80 and 114, pp. 188, 308). The pecten 

 projects into the vitreous in the ventral half of the eye from the head of 

 the optic nerve, with which its base roughly coincides. It consists largely 

 of small blood vessels (of greater than capillary size). If these be con- 

 sidered comparable to the vascular supply of organs in general, then 

 the pecten must be described as an essentially ectodermal papilla, for 

 its scant framework is composed of neuroglial cells of optic-cup origin. 

 It is always pigmented (though occasionally only lightly), with the 

 pigmentation progressively deepening toward the apex of the structure 

 and heaviest of all in the 'bridge' which ordinarily binds and caps its 

 free end. The vascular supply of the pecten has no connection with that 

 of the chorioid, but its chief veins and arteries are probably homologous 

 with those which supply the falciform process, hyaloid or vitreal vessels, 

 conus papillaris, and retinal vessels of fishes, amphibians, reptiles, and 

 mammals. 



Two types of well-developed pectens occur; their morphological and 

 genetic inter-relationships are obscure. The palseognathous birds, which 

 are primitive and (except the tinamous) flightless, characteristically have 



