Refractive state, corneal curvature, accommodative range and ocular anatomy of the Asian elephant (Elephas maximus)

The resting refractive state of six mature, female, Asian elephants (Elephas maximus) was determined using streak retinoscopy and neutralizing video retinoscopy. The amplitude of accommodation was also measured by neutralizing video retinoscopy of two animals and the corneal curvatures of three animals was measured by photokeratoscopy. The net spherical refraction was found to be +0.23 D. No difference was observed between cyclopleged and non-cyclopleged eyes (data from three animals), nor was there any difference between right and left eyes. Nine of the twelve eyes refracted had > or = 0.5 D astigmatism. The mean corneal power, as measured by photokeratometry was 21.3 D (SD = 1.8 D). There was a tendency towards with-the-rule corneal astigmatism in our sample (mean value: 1.2 D), though it did not reach statistical significance (P = 0.06). Two elephants were examined using neutralizing video photoretinoscopy. They were able to accommodate through 3 D. Three fixed eyes from three different elephants were obtained for gross and microscopic examination. The mean axial length of the eye was 38.75 mm and the lens had an axial diameter of approx. 10 mm. The posterior sclera was thick (8.0-8.5 mm). Histologically, the cornea was comprised of five distinct layers. A thin, meridionally oriented smooth ciliary muscle was identified. Individual muscle fibers were also observed associated with the posterior trabeculae of the uveal meshwork.