EXPERIMENTAL MYOPIA IN A DIURNAL MAMMAL (SCIURUS-CAROLINENSIS) WITH NO ACCOMMODATIVE ABILITY

1. We examined the functional morphology of the intra-ocular muscles o t grey squirrel using pharmacological and histological methods. Using sympathomimetic (phenylephrine) and parasympathomimetic (carbachol) a gents, administered by transcorneal iontophoresis, the response of the iris sphincter and dilator muscles and the ciliary muscle were record ed. Measurement techniques included both streak retinoscopy and coinci dence optometry for measurement of ocular refraction and high resoluti on ultrasonography to monitor changes in the intra-ocular component di mensions. 2. The grey squirrel was found not to possess a functional a ccommodative system. No change in ocular refraction or intra-ocular di mensions could be induced with 40% carbachol. Marked changes in pupil diameter occurred with topical application of both phenylephrine (dila tion) and carbachol (constriction). Histological findings were in agre ement with pharmacological findings in showing well developed iris sph incter and dilator muscles but only a poorly developed ciliary muscle. 3. Calculation of the depth of focus of the grey squirrel eye reveals that this could be sufficient to account for the behavioural observat ions of near viewing habits. 4. We then determined whether we could in duce axial elongation of the vitreous chamber and a consequent myopia by monocular deprivation (MD) of pattern vision. 5. Monocular deprivat ion of pattern vision produced a significant experimental myopia due t o axial elongation of the vitreous chamber in the deprived eye. 6. The results demonstrate that a functional accommodative system is not nec essary to induce experimental myopia in the grey squirrel eye.