Gibson and Russell's physical diagnosis / revised & rewritten by Francis D. Boyd.

Date:
1902
Catalogue details

Licence: In copyright

Credit: Gibson and Russell's physical diagnosis / revised & rewritten by Francis D. Boyd. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    localised area of blindness, a scotoma, may be found inter- fering with their continuity. The optic disc is a physiological scotoma about 15° to the outer side of and below the point of fixation. Scotomata may be small or large, single or multiple. They are described as positive if the patient is conscious of their interference in his held, negative if, like the optic disc, they do not obtrude themselves upon his consciousness. In the latter case they may not be discovered by the physician unless he takes the precaution of ascertaining, not only the outlines of the fields, but also whether the test object is properly perceived along each meridian, from the limit inwards to the fixing point. In mapping out a scotoma it is often a good plan to attach the test object to the quadrant and slowly rotate it through as many degrees to the right or left as may be necessary to determine where it can and where it cannot be perceived by the patient. By combining circular movements of this kind with the to-and-fro radial movements along the quad- rant previously described, the dimensions of the scotoma may be pretty accurately demarcated. As already indicated, every ray which enters the eye, except that coming from the point of fixation, crosses the visual axis before it reaches the retina. [Note.—The point of intersection of all the entrant rays is the nodal point of the eye. It is situated close behind the crystalline lens in the anterior part of the vitreous.] Consequently, rays coming from the right half of each field are focussed on the left half of each retina, and similarly with the right half of each retina we perceive objects situated in the left half of each field, so far at least as the fields are coincident; objects situated above the visual axis are focussed on the lower part of the retina, while those below the line of fixation imprint their images on the upper part of the retina.