ORGANS OF SENSE. I75 



The two well known defects in vision, myopia and pi-esbyopia^ depend 

 either upon some peculiarity in the refracting media, or upon a deficiency 

 or weakness in the power of adjusting or accommodating the eye to vision 

 at different distances. Myopia, or near-sightedness, may be caused by too 

 great convexity of the cornea or lens, and is most common in early and 

 middle life; the rays from a near object meet in their focus on the retina, 

 and produce a distinct image ; but the rays from a distant object, being 

 nearly parallel, are more easily brought to a focus, and, therefore, meet 

 before the retina, and only form undefined spectra upon it. The defect is 

 partly corrected by means of a concave glass of suitable curvature ; this 

 causes the rays to become more divergent ; therefore, they converge less 

 quickly, and form their focus upon the retina. Presbyopia, or far-sighted- 

 ness, is more common in old age, when the cornea and lens are less convex 

 than in youth; the rays from distant objects, being nearly parallel, are 

 refracted to a proper focus upon the retina; but those from near objects 

 are not refracted soon enough, and, therefore, their forcus being formed 

 behind the retina, they do not form the image upon the nerve. This defect 

 is remedied by a convex glass, which will cause the convergence of the 

 rays from a near object, and bring them more rapidly to a focus, so as to 

 form the image upon the retina, instead of behind it. 



PI. 131, fig. 17, anterior half of a section of the eyeball : a, sclerotic coat ; 

 &, lamina fusca; c, choroid coat; c?, pigmentum nigrum; eefi retina; g h, 

 ciliary processes ; i k I, crystalline lens. Fig. 19, posterior half of the 

 same eye : a-d, as in the preceding figure ; e-k, retina ; /, entrance of the 

 optic nerve. Fig. 19, retina: b, foramen centrale; c, its yellow border; 

 d ef, section of the optic nerve ; g h i, three branches of the central ves- 

 sels. Fig. 20, anterior view of the retina and of the vitreous humor : a h, 

 retina ; c &, corona ciliaris about the lens ; c d, lens ; c?, foramen centrale ; 

 e e, blood-vessels of the retina. Fig. 21, exterior of the retina: b, foramen 

 centrale ; c d, optic nerve ; e f g, corona ciliaris of the vitreous humor ; 

 h i, arteries. Fig. 22, choroid coat : a &, optic nerve ; c-fi portion of the 

 sclerotic ; g-k, choroid coat ; m, long ciliary artery ; n, long ciliary nerve ; 

 0, long internal ciliary nerve ; p p, long and short vessels of the choroid ; 

 q q, ciliary nerves ; r, -s, t, vasa vorticosa. Fig. 23, the same from the 

 other side : a &, optic nerve ; c-f, sclerotic ; g h i, iris ; m, vasa vorticosa ; 

 n p, ciliary nerves. Fig. 24, inner surface of the retina : a, foramen cen- 

 trale ; b b, folds of the retina ; c, arteria centralis ; c d e e, four branches of 

 do. Fig. 25, the vessels of the retina magnified. Fig. 26, distribution of 

 the vessels of the iris magnified: a b c, vasa vorticosa; fh, arteries of the 

 iris ; k k^ their retiform distribution ; m n, layer; I 0, smaller circle of ves- 

 sels. ■ Fig. 27, nerves of the iris magnified : e b, branches of the ciliary 

 nerves ; b b, distribution on the border of the iris. Fig. 28, vessels of the 

 iris and of the membrana pupillaris magnified. Fig. 29, lens of a newborn 

 child in profile ; fig. 30, lens of a child six years old ; fig. 31, do. of a grown 

 man ; fig. 32, section of a lens preserved in alcohol to show its laminated 

 structure ; fi^. 33, lens hardened by alcohol and split into several segments ; 

 fig. 34, exfoliated lens ; fi/^. 35 a, three layers of a segment of a lens ; fig. 



ICONOGRAPHIC ENCYCLOPEDIA. — VOL. II. 56 881 



