Optical constancy of the chick lens during pre- and post-hatching ocular development.

Embryonic and post-embryonic development of the ocular lens is associated with the continual production of new secondary lens fibers by the mitotic activity of equatorial epithelial cells. This continual development affects lens size and shape and refractive index distribution. Study of embryonic lens optical function has been largely ignored. The optical characteristics of the developing chick lens, including paraxial and eccentric focal lengths, were measured during the embryonic period of development and up to 15 days after hatching. Measurements were made with an automated scanning laser system in which the video image of a helium-neon laser beam refracted by an excised lens in solution is digitized. Focal length is measured for beams moving in small steps on either side of that center. Measurements were made on excised lenses as well as with the lens in situ within the anterior segment of the eye. The results, collected from a study of a total of 80 lenses, indicates that embryo lenses at 6-7 days of incubation have long and very variable focal lengths. At the tenth embryo day, focal length drops by more than one-half and focal variations, between lenses and for different beam positions within a single lens, is reduced. Further measures for 14- and 17-day embryo lenses, as well as for lenses from hatchling and 5-, 10- and 15-day-old chicks, indicate that there is little change in focal length, either paraxially or for eccentric beam positions.(ABSTRACT TRUNCATED AT 250 WORDS)

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.