ORGANS OF SENSE. 175 
The two well known defects in vision, myopia and presbyopia, 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 cormmon 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 jens 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. 
Pl. 131, jig. 17, anterior half of a section of the eyeball: a, sclerotic coat; 
b, lamina fusca; c, choroid coat; d, pigmentum nigrum; ee/f retina; gh, 
ciliary processes; 7 & J, crystalline lens. %g. 19, posterior half of the 
same eye: a-d, as in the preceding figure; e-k, retina; J, entrance of the 
optic ‘nerve. Fig. 19, retina: 6, foramen centrale; c, its yellow border; 
def, section of the optic nerve; g 17, three branches of the central ves- 
sels. fig. 20, anterior view of the retina and of the vitreous humor: a 8, 
retina; c 6, corona ciliaris about the lens; ¢d, lens; d, foramen centrale; 
e e, blood-vessels of the retina. 2g. 21, exterior of the retina: b, foramen 
centrale; c d, optic nerve; e fg, corona ciliaris of the vitreous humor; 
hi, arteries. ig. 22, choroid coat: a b, optic nerve; cf, portion of the 
sclerotic; g-k, choroid coat; m, long ciliary artery; n, long ciliary nerve; 
o, long internal ciliary nerve; p p, long and short vessels of the choroid; 
q q, ciliary nerves; 7, s, t, vasa vorticosa. J/g. 28, the same from the 
other side: a 8, optic nerve; c-/, sclerotic; g h 7, iris; m, vasa vorticosa; 
np, ciliary nerves. fig. 24, inner surface of the retina: a, foramen cen- 
trale; 5 0b, folds of the retina; c, arteria centralis; c dee, four branches of 
do. Fg. 25, the vessels of the retina magnified. F%g. 26, distribution of 
the vessels of the iris magnified: «a 0c, vasa vorticosa; fh, arteries of the 
iris; «k, their retiform distribution; mn, layer; / 0, smaller circle of ves- 
sels. Jig. 27, nerves of the iris magnified: e b, branches of the ciliary 
nerves; 0 b, distribution on the border of the iris. /%g. 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; jig. 82, section of a lens preserved in alcohol to show its laminated 
structure ; fig. 38, lens hardened by alcohol and split into several segments ; 
Jig. 34, exfoliated lens ; fig. 85 a, three layers of a segment of a lens; fig. 
ICONOGRAPHIC ENCYCLOPADIA.—VOL. II, 56 881 
