The Functional Significance of the Avian Pecten: a Review

The pecten oculi of the avian eye is an intic species, especially the domestic chicken triguing structure which, despite intensive (Gallus domesticus) and the domestic pigeon study, has defied conclusive functional analy( Columha hia). sis for over 300 years. The reviews of O’ RaEmbryologically, the pecten arises as an echilly and Meyer (1961) and Wingstrand and todermal (glial) processfrom the edges of Munk (1965) summarized research done on the optic fissure (Mann 1922). The ectoderthe pecten up to 1965. Both of these reviews ma1 elements are largely replaced later in confirmed the observation of Walls (1942) development by an extensive invasion of mesothat although anatomical studies and purely dermal vascular tissue. In most birds, the pecspeculative accounts of the function of the ten is largely complete at hatching, although pecten abound in the literature, the number in some species full pigmentation may be deof experimental investigations has been small. layed for some time (Hailman 1964). In his classic study of the vertebrate eye, The gross appearance of the typical pecten Walls (1942) marshalled considerable anais suggestive of a steam radiator. Wood’ s atlas tomical and circumstantial evidence for the ( 1917) demonstrates the remarkable varihypothesis that the pecten, which is the only ability of a number of external pecten features vascular structure within the posterior chamsuch as color, length, thickness, and number ber of the avian eye, serves as a replacement of pleats. Early anatomical investigations for the direct vascularization of the retina (Rabl 1900, quoted in Walls 1942, Bacsich found in most other vertebrates. In recent and Gellert 1935) showed that the gross years, the ablation experiments of Wingstrand morphology of the pecten was highly variable and Munk (1965) have been taken by most even among related genera, but tended toornithologists and physiologists (e.g., Jasinki ward three distinct patterns: the “pleated 1973, Sillman 1973) as positive proof of this fan,” the commonest type, found in most neohypothesis. However, my recent investigagnathous birds; the “pleated cone” of some tions of the physiology of the pecten (Brach ratites, which has a radiate appearance due 1975a, b) have produced results which cast to the circular cross-section of the optic nerve doubt on the trophic theory of pecten funchead; and the bridgeless, often greatly reduced tion. In this review, I summarize what is pectens of kiwis, owls, and the caprimulgipresently known about the structure and funcforms. Except for the degenerate pectens of tion of the pecten with the aim of providing kiwis, the pectens of all birds are folded or a fresh look at the problems and partial anpleated. The purpose of these pleats is apswers which have arisen in studies of the pecparently to increase the surface area of the ten since 1965. pecten rather than to produce rigidity as proposed by Tucker ( 1975), because the pecten