The accomodation of the eye to distances / by William Clay Wallace.

  • Wallace, William Clay
Date:
1850
Catalogue details

Licence: Public Domain Mark

Credit: The accomodation of the eye to distances / by William Clay Wallace. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    of the eye lo the perception of objects placed at various distances, depend entirely on the ciliary muscle. I have every reason to believe that the per- ception of objects placed at very short distances, depends altogether, or nearly so, on the contractions of the pupil or iris. * * » When belladonna has been applied to the eye, near objects cannot be perceived, and the eye is said to have changed its focus and to have become long sighted. These expressions are inqorrect, the eye has lost its power of viewing very near objects, because the iris can iio longer contract. * * * When the iris is drawn backwards, after having removed the cornea, and allowed the aqueous humour to escape, the anterior edge of the ciliary processes may be seen projecting into the posterior chamber of the aqueous humour, and on drawing these also backwards, a number of parallel fibres are seen proceeding from the marginal or equatorial edge of the capsule of the lens, to be inserted between each of the ciliary processes. These fibres [the orbi- culus capsulo-ciUaris of Ammon?] lie immediately over the canal of Petit, and contribute with the perfectly transparent membrane lying immediately beneath them, to form a portion of the parietes of this canal. If we now ex- amine the eye in the opposite direction, i. e., by dissecting off the sclerotic and choroid tunics and the retina, we may perceive a sunilar range of fibres proceeding from the anterior edge of the vitreous humour, and from the point where the retina terminates, forwards and upwards, to be in like manner con- nected witl> the ciliary processes, by passing in between each. If a lateral view be taken, by making a very delicate section of the eye and gently rais- ing the cut edges of the ciliary body, there is still the same appearance, yiz.: of anterior and posterior fibres which have a common insertion between the ciliary processes near the base, and which form, in conjunction with a trans- parent membrane, the external paries of the canal of Petit. * * « Knox, Ed. Phil. Trans., Vol. X. S. Eetina. P. 31. It belongs to physics and chemistry to commence where the anatomist ends. He describes the parts, they tell the use. They show how the images of external objects form, on optical principles, on the dark pigment, and how, under this influence, tfie nerve globules of the retina are oxydized by the arterial blood, which, through thousands of vessels, finds its way all over the choroid coat. How, wherever this oxydation goes on, the temperature rises, and the optic nerve transmits the impression to the brain ; and we thus discover, that though in a certain sense the action of that nerve is special, yet m reality it is like that of any other sensory nerve, which, in like manner, transmits the impressions of hesiL—Draper's Introductory Lecture on Phosphorus.
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