THE MECHANISM OF LENTICULAR ACCOMMODATION IN CHICKS

In the chick eye, accommodation for near objects is brought about by c hanges in the focal length of the lens and by changes in the corneal r adius of curvature, Several different mechanisms of lenticular accommo dation have been proposed for the avian eye, These include a role for the ciliary muscle, a role for the iris muscle, and a role for changes in intraocular pressure, We have studied accommodation in the chick e ye using electrical stimulation of the Edinger-Westphal nucleus, elect ric-field stimulation of enucleated eyes, in vitro measurement of chan ges in back vertex distance of the lens, and histology, We present evi dence showing that, in the chick eye, lenticular accommodation is indu ced primarily by a contraction of the muscle fibers at the peripheral edge of the iris, During accommodation, the peripheral muscle fibers o f the iris contract to apply a farce through the ciliary processes to the anterior equatorial surface of the lens, This increases the focal power of the lens. When accommodation is relaxed, the lens is returned to its unaccommodated state by the elasticity of the pectinate ligame nt and the ciliary body, Contractions of the posterior ciliary muscle and changes in intraocular pressure, forces that have previously been proposed to play major roles in lenticular accommodation, are shown to be of secondary importance only.