IN-VITRO CHANGES IN BACK VERTEX DISTANCE OF CHICK AND PIGEON LENSES -SPECIES-DIFFERENCES AND THE EFFECTS OF AGING

We have used a scanning laser technique to measure in vitro changes in back vertex distance of chick and pigeon lenses. Enucleated eyes were dissected, leaving the lens naturally suspended by the ciliary body a nd intraocular muscles. Ray tracing techniques were used to measure th e resting back vertex distance of the lenses by passing a laser beam t hrough the lens and scanning it across the pupillary aperture. The pup il diameter was measured videographically. The measurements were repea ted while the intraocular muscles were pharmacologically stimulated wi th increasing concentrations of either nicotine or carbachol. Drug sti mulation caused changes in pupil diameter and changes in the back vert ex distance of the lenses. These experiments were conducted on the eye s of young chicks, young pigeons, and on the eyes of three pigeons old er than 10 yr. The lenses from the eyes of the old pigeons had the gre atest resting back vertex distance, followed by those of the young pig eons and finally those of the young chicks. Lenses from the eyes of yo ung chicks and young pigeons underwent similar drug-induced changes in back vertex distance, but the lenses from old pigeon eyes showed an a lmost complete absence of such changes. Further, we demonstrated that, just as in the chick eye, lenticular changes in pigeon eyes are due t o a contraction of the iris muscle. This is evident because after the iris has been removed the lens undergoes no change in back vertex dist ance during stimulation. We conclude with a discussion of the lenticul ar accommodative ability of the pigeon eye with reference to the recen tly reported accommodative mechanism of the chick eye and a comparison of chick and pigeon iris morphology.