On the phenomena and mechanism of the focal adjustment of the eye to distinct vision at different distances / by Professor Allen Thomson.

  • Thomson, Allen, 1809-1884.
Date:
[1858?]
Catalogue details

Licence: Public Domain Mark

Credit: On the phenomena and mechanism of the focal adjustment of the eye to distinct vision at different distances / by Professor Allen Thomson. Source: Wellcome Collection.

Provider: This material has been provided by The University of Glasgow Library. The original may be consulted at The University of Glasgow Library.

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    of Zinn, and the elastic fibres situated in that structure. I have ascertained that these strong elastic fibres, constituting the orbi- culus capsulociliaris, pass from the ora seiTata and ciliary pro- cesses to a part of the posterior as well as of the anterior surface of the lens near its margin, and in such a manner that it can scarcely be doubted that their action must be, when stretched, to flatten the lens or to shorten its axis. 3rd. The inner ends of the ciliary processes do not, as often represented, overlap the margin of the lens anteriorly, but are situated to the outside of it, having the spreading fibres of the ciliary muscle attached to their outer or anterior borders; and conse- quently, when drawn inwards towards the lens, the ciliary pro- cesses are applied against its margin, and do not press on its anterior surface. The ciliary muscle is attached at its narrow anterior angle by means of elastic tissue to the membrane of Descemet, inside the canal of Schlemm ; and these elastic fibres may permit the muscle, during its contraction, to pass backwards to a short distance from the place of its attachment. The external fibres of the ciliary muscle, which are the longest, are inserted into the choroid mem- brane towards the ora serrata, while the remainder of the fibres spread along the exterior of the ciliary processes, to which and the intervening non-ciliated part of the choroid they are fixed; and finally, the innermost fibres are attached to the base of the iris. The iris, therefore, and the ciliary muscle thus form almost a continuous structure of a contractile nature round the margin and outer part of the front of the lens. 4th. The contraction of the outer part of the ciliary muscle must draw forward the ora serrata of the choroid, or tend to con- tract that membrane and the hyaloid as a whole in the posterior part of the cavity of the bulb, so as to give firmness and support to the vitreous humour, and prevent the lens from sinking back- wards into it. This action must, at the same time, have the effect of relaxing the elastic fibres of the zonule of Zinn. The contrac- tion of the shorter fibres of the ciliary muscle, spreading over the exterior of the ciliary processes, must withdraw these processes and the zonule from the canal of Petit in front, and will thus have the effect of altering the form of that space, narrowing it from without inwards, and deepening it from before backwards, round the margin of the lens,—a change which may contribute to produce more perfect marginal compression of the lens. 5th. The base of the iris being attached to the ciliary muscle, and^the tissue which fixes them both to the ring inside the canal of Schlemm, being of a yielding nature, the outer part of the iris may be carried backwards to some extent by the contraction of the ciliary muscle; while there is nothing in the disposition of the Dissertation of Van Reeken Disquis. Anat. Microsc. inaug. de Apparat. Oculi Accommod. Utrect, 1855, I am only acquainted through the Dutch extract in the Ncderlaudsch Lancet for 1855.