Functional morphology of accommodation in the raccoon.

The raccoon (Procyon lotor) is a small carnivore which eats in the upright position, using hand- and finger-like front paws and digits to wash, hold and examine its food at close range. These anatomic and behavioral characteristics prompted structural and functional studies of the accommodative capability of this species. By light and electron microscopy, we observed a prominent ciliary smooth muscle and zonular apparatus. When stimulated by carbachol or pilocarpine, the muscle and zonular apparatus exhibited a shift from longitudinal to reticular or circular orientation of some ciliary muscle bundles, anterior movement of the muscle as a whole, and more oblique crossing of the zonular fiber bundles in the zonular plexus. Maximum carbachol-induced accommodative amplitude measured by coincidence refractometry ranged from 3 to 19 diopters in these 1 to 9 yr old animals, with no definite age-accommodation relationship. A-scan ultrasonographic biometry showed that during accommodation the lens thickened very little, if at all, but moved anteriorly, while the apparent cornea to retina distance increased slightly. The raccoon thus exhibits the greatest accommodative capability of any non-primate terrestrial mammal so far studied.

Accommodation and presbyopia in the human eye--aging of the anterior segment.

Ocular biometric parameters and accommodative amplitude were measured by various techniques in 100 normal emmetropic human subjects age 18-70 yr. Anterior chamber depth decreased and lens thickness increased linearly over the entire age group. Accommodative amplitude declined linearly until a stable nadir was reached at about age 50 yr. The respective slopes and intercepts of the age-dependent decline in anterior chamber depth were essentially the same for measurements made independently by optical pachmetry, A-scan ultrasonography, and slit-lamp Scheimpflug photography. The age-dependent increase in lens thickness differed in slope and intercept for measurements made by photography and ultrasonography if the generally accepted lenticular sound velocity was assumed for all subjects. However, if putative lenticular sound velocity was adjusted for age, the relationships given by the two techniques were essentially identical. Total anterior segment length (defined as the distance between the anterior corneal and posterior lens surfaces), vitreous cavity length (distance between the posterior lens and anterior retinal surfaces), and total globe length were all independent of age. This constellation of findings indicates that the human lens grows throughout adult life while the globe does not, that thickening of the lens completely accounts for shallowing of the anterior chamber with age, but that the posterior surface of the lens remains fixed in position relative to the cornea and retina.

Accommodation and presbyopia in the human eye. 1: Evaluation of in vivo measurement techniques.

As part of a cross-sectional study of aging of the human accommodative apparatus, unaccommodated anterior chamber depth, unaccommodated lens thickness, and accommodative amplitude were each determined by two or more independent techniques in 100 normal emmetropic subjects, age 18-70 yr. For anterior chamber depth, the order of accuracy seemed to be: optical pachymetry > slit-lamp Scheimpflug photography > Ascan ultrasonography, although there was good agreement among all three methods. For lens thickness, slitlamp Scheimpflug photography seemed more accurate than A-scan ultrasonography when the generally accepted lenticular sound velocity of 1641 m . s(-1) was used for the ultrasonographic calculation. The agedependent divergence allowed calculation of a putative relationship between lenticular sound velocity and age, indicating that velocity decreased by ~3 m . s(-1) . yr(-1). Accommodative amplitude declined with age, reaching a minimum, by age 50 yr, of 0.5 diopters as measured by coincidence refractometry. This residual 0.5 diopters may represent the inherent focal depth of the eye's optical system, rather than active accommodation. Maximum accommodative amplitude based on the subject's ability to clearly visualize a target through minus lenses was always 1.50 diopters greater than the refractometrically measured maximum, regardless of age. This difference presumably represents increased depth of field provided by pupillary constriction, rather than active accommodation.