Eyesight, good & bad : a treatise on the exercise and preservation of vision.

  • Carter, Robert Brudenell, 1828-1918.
Date:
1880
Catalogue details

Licence: Public Domain Mark

Credit: Eyesight, good & bad : a treatise on the exercise and preservation of vision. Source: Wellcome Collection.

Provider: This material has been provided by the Augustus C. Long Health Sciences Library at Columbia University and Columbia University Libraries/Information Services, through the Medical Heritage Library. The original may be consulted at the the Augustus C. Long Health Sciences Library at Columbia University and Columbia University.

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    it is one of their characteristics that they never actually- shut out any point which is being looked at, but float a little above, below, or to one side of it ; the reason of this being that the filaments do not appear to exist in the axis of vision, but only in the lateral parts of the humour. Muscse may be rendered more than usually conspicuous by any circumstances which alter the density of the fluid part of the humour, and thus in- crease, either temporarily or permanently, the difference of refraction between this fluid part and the filaments or cells. Musc^ exist, more or less, in all eyes, and may easily be found by looking at a white cloud through a pinhole in a card; but they are usually harmless, and only require mention on account of the anxiety which they sometimes cause to persons who are unacquainted with their nature. The free movements of the eyes are of such a kind that they involve no change of place, but only rotation upon a central point in each eyeball. The pupils can be directed upwards, downwards, outwards, and inwards, and also in intermediate directions, as upwards and outwards, without the eyeball as a whole changing its position. These movements are accomplished by the agency of six muscles. By muscles, we mean certain flat or round structures, composed of fibres or threads, which, under the influence of the will or of other stimuli, can be made to contract or shorten themselves, and which relax and return to their former length as soon as the will or the stimulus ceases to act. By such shortening or contraction, all our voluntary movements