264 



THE HUMAN BODY 



to grind them of the non-spherical curvatures necessary to avoid 

 it. In our eyes its effect is to a large extent corrected in the follow- 

 ing ways: (a) The opaque iris cuts off many of the external and 

 more strongly refracted rays, preventing them from reaching the 

 retina, (b) The outer layers of the lens are less refracting than 

 the central; hence the rays passing through its peripheral parts 

 are less refracted than those passing nearer its axis. 



3. Irregularities in Curvature. The refracting surfaces of our 

 eyes are not even truly spherical; this is especially the case with 



the cornea, which is very rarely 

 curved to the same extent in its 

 vertical and horizontal diameters. 

 Suppose the vertical meridian to 

 be the most curved; then the rays 

 proceeding from points along a ver- 

 tical line will be brought to a 

 focus sooner than those from points 

 on a horizontal line. If the eye is 

 accommodated to see distinctly 

 the vertical line, it will see indis- 

 tinctly the horizontal and vice 

 versa. Few people therefore see 

 equally clearly at once two lines crossing one another at right 

 angles. The phenomenon is most obvious, however, when a series 

 of concentric circles (Fig. 90) is looked at: then when the lines 

 appear sharp along some sectors, they are dim along the rest. 

 This defect is known as astigmatism; it is corrected by the use of 

 lenses which are curved only in one plane. The lens is so adjusted 

 that its curvature combines with the less curvature of the eye to 

 equal the greater. 



4. Opaque Bodies in the Refracting Media. In diseased eyes the 

 lens may be opaque (cataract) and need removal; or opacities from 

 ulcers or wounds may exist on the cornea. But even in the best 

 eye there are apt to be small opaque bodies in the vitreous humor 

 causing muscce volitantes; that is, the appearance of minute bodies 

 floating in space outside the eye, but changing their position when 

 the position of the eye changes, by which fact their origin in in- 

 ternal causes may be recognized. Many persons never see them 

 until their attention is called to their sight by some weakness of it, 



FIG. 90. 



