Page:Handbook of Ophthalmology (3rd edition).djvu/52

46 In rare cases it happens that a sharply defined, small staphyloma develops, independent of the crescentic atrophy surrounding the nerve, and separated from it by healthy choroidal tissue. In one case which I observed there was in the neighborhood of the macula lutea a bright spot with a diameter once-and-a-half or twice that of the disc, with pigment-spots sprinkled over it and surrounded by a black ring; it was evidently excavated and there was a corresponding defect in the field of vision. Streatfeild has described a similar case.

The disturbances of vision which accompany the above-described changes in the region of the macula lutea are dependent partly upon the stretching of the retina in this region, partly upon the mechanical insulation which the layer of rods and cones suffers by reason of changes upon the surface of the choroid, and partly from disease of the retina itself. The patients complain of a trembling of the letters when reading, of irregular curves in the lines, of an oblique position and irregular form of the individual letters, of clouds or dark spots in the centre of the field, or finally of absolute inability to see the point fixed (scotoma centrale).

Donders explains the trembling of the letters, by the presence in the macula lutea of a number of very small defects (scotomata). Now, with the movements of the eye, the image of each individual letter falls first upon some spot in the retina which is sensitive, then upon one insensitive to light, so that it alternately appears and disappears; at the same time, in consequence of the irregular displacement of the layer of rods and cones, the shape of the letters appears changed.

Förster has carefully analyzed the phenomena of metamorphopsia (seeing objects distorted and inclined from their true position), and by the use of a system of parallel lines has shown that in a limited central part of the field of vision the curvatures of the lines are concentric. The metamorphopsia also occurs in distant vision with concave glasses, so that, for instance, the bars in the window-sash appear crooked. The sensibility of the macula lutea appears reduced, so that distinct vision is only possible by intense illumination. At the same time the bright light causes