THE AVIAN EYE 647 



The iris is often extremely thin just at its root, where the anteriormost 

 pectinate-Ugament fibers attach, like a zonule of the iris. Here the iris 

 may be reduced to little more than the retinal layers.* It promptly 

 thickens greatly, then slowly tapers toward the pupil margin where it 

 often has a knife edge (in contrast to the lizards; but c/. Sphenodon). 

 Sphincter and dilatator fibers, all striated, are distributed throughout 

 the width of the iris. These originate embryologically from the anterior 

 retinal layer at the pupil margin. Their action, like that of the avian 

 ciliary muscles, is extremely rapid. Both retinal layers of the iris are 

 pigmented; but a second dilatator system, identical in genesis with the 

 dilatator of mammals (and with its elements probably unstriated and 

 perhaps syncitial) , has been described by so many investigators that its 

 existence in at least some birds cannot be categorically denied. The 

 circulatory pattern of the iris is much as in lizards, with a wide plexus 

 of capillaries supplying the sphincter and drained peripherally by short 

 radial veins. Small vessels are concentrated near and at the anterior 

 surface of the iris, from which many of them protrude (as in many 

 reptiles and amphibians). Here also is concentrated the stromal pig- 

 mentation, which may incorporate many types of cells — particularly 

 lipophores. There is no unbroken layer of mesothelium on the face of 

 the iris. Though it literally squeezes the lens only in certain amphibious 

 birds, the avian iris is always of material assistance during accommo- 

 dation, in holding back the lens against which it presses, and in inhibit- 

 ing the peripheral part of the anterior surface of the lens from bulging, 

 thus concentrating the change-of-curvature in the part of the surface 

 opposite the pupil. 



The lens is as highly refractive as in mammals, often more so, though 

 it is often very soft, particularly where the range of accommodation is 

 great. As in most of the higher vertebrates, the anterior surface is less 

 sharply curved than the posterior. It is flattest in most diurnal birds 

 (index 2.2-3.0), roundest in crepuscular and nocturnal forms, and in 

 divers (1.2-1.85). The annular pad or ringwulst is ordinarily well de- 

 veloped, and as a maximum (in Apus) it may take up half the area of 

 a sagittal section of the whole lens. In general, its relative thickness goes 

 with the capacity for accommodation, but it is very thin in diving birds 

 (loons, murres, cormorants, etc.), particularly in those whose iris sphinc- 



*In captive owls which have been roughly handled, one sometimes sees an irregular second 

 pupil at the periphery of the iris, held open by the tonus of the sphincter. Such a defea is 

 not a 'coloboma', of embryonic origin, but has been produced by a local traumatic rupture 

 of the iris root, which is especially delicate in these birds. 



