Theoretically, astigmatism may be occasioned by any irregularity of curvature or of structure of any of the tissue layers of the eye lying in the course of the visual axis; but practically, it is usually lenticular or corneal in its origin and is an irregularity of curvature and not of structure. Of these the corneal variety is by far the most common, while the lenticular form is rarely sufficiently prominent to interfere with vision.
In this paper I will confine my attention to regular astigmatism developed in the cornea, and will but incidentally notice lenticular astigmatism. Formerly, regular astigmatism was considered a congenital deformity, sufficiently fixed and definite to remain ever in its original condition, excepting when modified by some violence, traumatic or inflammatory, exerted upon the cornea, or by some change in the lens substance from degeneration or injury. Later, when the opinion began to be expressed that perhaps
LAUTENBACH LJ. ON THE LOCAL AND GENERAL CONDITIONS THAT CHANGE CORNEAL CURVATURES. JAMA. 1895;XXV(21):892–898. doi:10.1001/jama.1895.02430470012002e
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