Disturbances of the visual functions / by W. Lohmann ; translated by Angus Macnab.

  • Lohmann, W.
Date:
1913
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Licence: In copyright

Credit: Disturbances of the visual functions / by W. Lohmann ; translated by Angus Macnab. 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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    beyond the actual image found, but quite recently changes have been shown to occur in the retinal elements under the influence of light. Van Genderen Stort (1884) first described changes in the cones of the retina as an objective fact. The internal segment of these elements {myoid, Engelmann) becomes shorter and thicker when exposed to light. These changes are particularly well seen in those animals whose retinae have, as well as cones, those other end-organs called rods. In retinae not containing rods \_e.g., the ring snake, Tropidonotus natrix (T.)] only very slight changes in the length of the cones can be seen (Engelmann) ; the rods, on the other hand, elongate under the influence of light, their movement being opposite to that of the cones. This antagonistic action of the two elements of the retina is related to the old idea of M. ScHULTZE, that the cones serve for vision in daylight and the rods in the dusk of twilight. The cones, which apparently are not excited by an illumination of low intensity, give up the field to the rods, which being more adapted for vision in dim light, then take possession.^ These movements of the rods and cones are not visible in the higher animals to the same degree as in the lower forms, and it is questionable if they do occur at all in man. Garten was, how- ever, able to demonstrate a definite difference in length between the inner segments of the cones in the periphery of the retina, when adapted for light and for shadow ; there seems also to be a very slight difference in the length of the cones at the macula in favour of the dark-adapted eye. Another change in the retina produced by light, which does occur in man, seems to show that this opposed action in the two end-organs can be assumed to occur in the human eye. The well-known pinkish appearance of the fresh retina was ascribed by Boll (1876) to a pigment which lost its colour under the influence of light. The so-called visual purple is a photo- chemical substance whose demonstration lends a certain probability to the view that this or some similar substance, which can be disintegrated by light, forms the connecting link between the light stimulus and the nerve impulse. This visual purple is found in the rods ; the macula, where vision is most acute and cones alone are present, is free from this pigment. The fovea, the important area for vision in bright light, is therefore inferior for night vision (scotopia) to those parts of the retina which contain the rods. A correspondence can be shown^ between the rapidity of bleach- ing of the visual purple by different monochromatic (spectrum) lights, and the scotopic value of the retina, i.e., the luminosity value of a faint spectral band which after adaptation appears colourless; and in this we have presumptive evidence for the ' ExNER and Januschke : Ber. cl. Ji. l\ aJiad. der Wissensch., Matli. cl. cxv., Vienna. Trendelenburg : Zeitsclir. f. Psych, u. Phijs. der Sinnesorg., xxxvii.