A HISTORY OF STUDIES OF VISUAL ACCOMMODATION IN BIRDS

Since 1813, when Crampton first described the ciliary muscle of the av ian eye, there has been little agreement on how birds are able to chan ge the focus of their eyes. Numerous lata studies on the eyes of a var iety of bird species contradicted earlier findings or proposed new acc ommodative mechanisms. The resulting confusion persists today, and a n umber of significant works on tile avian eye perpetuate many of the my ths developed during the 1800s. There is little consensus on avian acc ommodation; the early literature contains many accurate descriptions q i the mechanisms, along with elegant experimental evidence to support them. Much of the early literature, however, is in German and has rema ined obscure. Further, among the mechanistic descriptions of avian acc ommodation are many that are incorrect. The current confusion can be a ttributed in part to the fact that some birds have both corneal and le nticular accommodation. It is unclear to what extent different bird sp ecies employ both mechanisms, or depend on one mechanism or the other. These facts, together with the diversity of bird species, their range of visual requirements, and the numerous anatomical differences in th eir eyes, mab it impossible to describe a single avian mechanism of ac commodation. Our own experience in studying accommodation in the chick eye has led us to review the historical literature in an attempt to p rovide a new foundation for future studies on visual accommodation in birds.