Ocular accommodation in chickens: corneal vs lenticular accommodation and effect of age

The avian accommodative response has long been suspected of having a corneal component resulting from contraction of a limbal extension of the ciliary muscle. Efforts to confirm the existence of such a mechanism have been sporadic and the results contradictory. In addition, while the accommodative mechanism of the bird eye is commonly considered to be very effective, the effect of lenticular aging is not known. The accommodative responses of excised chicken eyes of various age, were measured by photographing changes in focal effects on incident fine parallel laser beams in response to electrical stimulation of the intraocular muscles. An opening at the posterior pole of the eye enabled the beams to exit in order to facilitate measurement of refractive change. The question of a possible corneal contribution was examined by taking advantage of the fact that corneal refractive power is neutralized when the surface of the cornea is in water. Accommodation was measured with the cornea in air and in water. The results indicate that there is little or no corneal accommodation and accommodative ability drops off substantially with age. The results may also indicate the existence of serious inter-breed differences in chicken accommodation.