. Elements of human physiology. Physiology. 560 PHYSIOLOGY Hypcrmetropia.—If, on the other hand, the eyeball be too short in its antero-posterior diameter, the parallel rays entering the eye will come to a focus at a point behind the retina. In order that a distinct image may be formed, even of distant objects, it will be necessary to increase the curva- ture of the lens by contracting the ciliary muscle. Such eyes are hypermetropic or long-sighted. Fig. 251. a h e Lens from human eye at different periods of life (Allen Thomson). a, at birth ; b, adult; c, old age. Presbyopia.—As old age comes
. Elements of human physiology. Physiology. 560 PHYSIOLOGY Hypcrmetropia.—If, on the other hand, the eyeball be too short in its antero-posterior diameter, the parallel rays entering the eye will come to a focus at a point behind the retina. In order that a distinct image may be formed, even of distant objects, it will be necessary to increase the curva- ture of the lens by contracting the ciliary muscle. Such eyes are hypermetropic or long-sighted. Fig. 251. a h e Lens from human eye at different periods of life (Allen Thomson). a, at birth ; b, adult; c, old age. Presbyopia.—As old age comes on, the lens becomes more rigid, and loses more or less its tendency to become convex (Fig. 251). Hence the near limit of accommodation gets further and further with advancing age. Such a condition is not to be confused with long-sightedness ; it is merely a defect in the power of accommodation, and not dependent Fig. iagrain showing course of rays in an astigmatic eye (Waller). The curvature of the cornea is greater in the vertical meridian v v v than in the horizontal meridian h h h. Hence the rays of light coming from the point r and passing through the vertical meridian come to a focus at/', while those through the horizontal meridian come to a focus at f'\ There is thus no point behind the cornea at which all the rays from P will come to a focus, and the image of the point must be blurred, being elongated in a horizontal direction at/', and in a vertical direction at/'''. on a structural defect of the eyeball. It is spoken of as presbyopia. Astigmatism.—The curvature of the vertical meridian of the cornea is usually greater than that of the horizontal meridian. The difference may be so great as to make it impossible for a definite image of a point of light to be. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble
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