Diseases of the eye and ophthalmoscopy : a handbook for physicians and students / by A. Eugen Fick ; authorized translation by Albert B. Hale.

  • Fick, A. Eugen (Adolf Eugen), 1852-
Date:
1896
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Licence: Public Domain Mark

Credit: Diseases of the eye and ophthalmoscopy : a handbook for physicians and students / by A. Eugen Fick ; authorized translation by Albert B. Hale. Source: Wellcome Collection.

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    it is in the eye, being flattened on account of the tension that the fibers of the Zone of Zinn exert on the lens capsule. But the Zone of Zinn is attached to the ciliary processes, and a contraction of the ciliary muscle must, therefore, narrow the diam- eter of the attachment of the Zone of Zinn. The zone being thus relaxed, the lens is allowed to grow rounder by its own elasticity. This description may be, perhaps, too schematic in explaining the action of such a complicated muscle. Schoen, assuming that each one of the three sets of muscular fibers plays according to its course its individual part in accommodation, has developed a specific theory to explain accommodation, but it has not yet been accepted by many ophthalmologists. In opposition to the Helmholtz theory of accommodation stands that of Tscherning. Helmholtz ascribes accommodation for near objects to relaxation of the Zone of Zinn, while Tscherning declares the opposite to be true, namely, that tension on the Zone of Zinn allows the anterior surface of the lens to be more strongly curved, at least in the neighborhood of the anterior pole. The edges of the lens are likewise, so says Tscher- ning, flattened by tension on the zone; but these edges lie behind the iris and are, there- fore, of no use in vision. How a contraction of the ring-like ciliary muscle can put the zonula on the stretch I cannot explain to the reader, for I do not understand it myself. 3. SHORTSIGHTEDNESS, MYOPIA, M. ACUTENESS OF VISION, V. In the preceding section it was explained that the emmetropic eye has the power of independently accommodating for a near object. But there are plenty of eyes that even at rest are adjusted for only certain distances this side of infinity; such eyes are short- sighted, and the condition is known as Myopia. The word myopia means that shortsighted people generally half close the eyes when looking at a distant object. The word shortsightedness ought, therefore, to be pre- ferred, because it describes the condition itself and not a mere physical property. If a shortsighted eye at rest looks at r cm. distance, and receives an image of it on the retina, then the image of every point beyond must lie in front of the retina; a point at infinity, therefore, has its image in front of the retina. If a point lies at 00 infinity and on the axis of a lens, its image lies at the focal distance of the lens ; consequently we can call shortsightedness that refractive condition of an eye at rest in which the focal distance is in front of the retina {Fig- 5. What can explain the fact that the focus does not lie upon the retina? There are several possible explanations.