The mechanism of corneal accommodation in chicks.

Corneal accommodation can account for up to 9 D of accommodation in a freely behaving chick. We have explored the possibility that changes in corneal curvature are due to changes in intraocular pressure (IOP) during accommodation. In an in vitro preparation we demonstrate that increasing the pressure will tend to flatten the cornea. We have used electrical stimulation of the Edinger-Westphal (EW) nucleus to further test the pressure hypothesis in vivo by recording IOP changes in the eye during EW-stimulated accommodation and by artificially modulating the IOP to assess the effects on corneal curvature. During EW stimulation there is an increase in IOP on the order of 1-3 mmHg which tends to flatten the curvature of the cornea, thus eliminating changes in IOP as a possible mechanism of corneal accommodation. Slit-lamp observations of accommodative changes at the corneo-scleral margin and electrical stimulation of dissected eyes in vitro indicate that corneal accommodation is mediated by a contraction of the ciliary muscles, which exerts a pull on the inner lamella of the cornea, flattening the peripheral cornea and increasing the curvature of the central cornea. Histological examination of the ciliary region of the eye confirms the appropriate positioning of the ciliary muscles. We conclude that corneal accommodation in the chick eye is accomplished by a ciliary muscle-mediated mechanism.