Eyestrain (commonly called asthenopia) / by Ernest Clarke.

  • Ernest Clarke
Date:
1892
Catalogue details

Licence: Public Domain Mark

Credit: Eyestrain (commonly called asthenopia) / by Ernest Clarke. Source: Wellcome Collection.

    64/184 page 52
    VIII. ACCOMMODATION.—Accommodation is the change in the convexity of the lens brought about by the action of the ciliary muscle. The frontispiece is a diagram- matic section of an eye, showing the mechanism of accom- modation. According to Iwanoff, the ciliary muscle arises from a tendinous ring (t) close to the insertion of the iris and Schlemm's canal (c.s.), at the posterior surface of the sclerotic, close to its junction with the cornea. The muscle then passes backwards and maybe divided into three parts, (i) the outermost part or meridional portion passing into the posterior tendon (m) to be inserted into the choroid, (2) the radiating portion (r), and (3) the annular portion or circular muscle of Miiller (c) passing directly backwards and inwards respectively, to be inserted into an agglomera- tion of fibres called the zone of Zinn (z). These fibres arise partly from the ciliary portion of the retina at the ora serrata (o.s.) and partly from the ciliary processes and the intervals between them and they pass forwards and back- wards to be inserted into the anterior and posterior capsule of the lens. When the ciliary muscle contracts it pulls for- wards and inwards the zone of Zinn, and by this mechanism the compression exercised by the latter on the lens is relaxed, and the lens is allowed by virtue of its elastic fibres to assume a more convex form, producing the same effect on rays passing through it, as if a convex glass were placed in front of the eye, and this effect is to bring forward the focal point of the rays, so that an object that was previously focussed behind the retina can now be focussed on it. When the ciliary muscle relaxes, the zone of Zinn again compresses the lens, which resumes its less convex shape. At the same time that the ciliary muscle contracts, the circular fibres of the iris also contract, and we get the
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