402 



ORGAN SYSTEMS OF MAN 



uncorrecfed 



corrected 



I 



far sigh ted 



i 



nearsighted 



Fig. 16-7. Common human eye defects and how they are corrected with lenses (spectacles). 



of each cone. All light passing through the 

 globule is filtered out except that of the 

 particular globule. The various colored 

 cones are scattered through the retina so 

 that color images are possible. It is prob- 

 ably the reduction or the lack of certain 

 cones that is responsible for color-blind- 

 ness. It is interesting to note that chickens 

 have mostly cones with very few rods in 

 their retinas and therefore do not see well 

 in dim light, accounting for the fact that 

 they go to roost with the setting sun. 



Eye defects. The inability of many hu- 

 man eyes to produce clear images under 

 usual conditions is evidenced by the 

 large number of people wearing glasses. 

 Perhaps the number of defects is no larger 

 today than formerly, but the demands for 

 clear vision in modern society are far 

 greater than ever before, so that greater 

 effort to correct these defects has been 

 made. It is highly important that in driving 

 an automobile the driver should have good 

 vision. Like other defects in present-day 

 civilization, congenitally poor vision is fos- 

 tered and passed on to succeeding genera- 

 tions. In primitive man such a defect would 

 have often prevented its owner from grow- 

 ing to adulthood, thus eliminating the de- 

 fect before it got a "foothold." 



The most common defects of the eye are 

 caused by the inability of the focusing 

 mechanism (cornea and lens) to form an 

 image on the retina in a clear form. If the 

 rays come to focus in front of the retina, 

 the person suffers from myopia, or near- 

 sightedness; if they come to focus behind 

 the retina, the defect is know as hyperopia 

 or farsightedness (Fig. 16-7). In order to 

 correct these conditions it is only necessary 

 to place in front of the eye a lens ground 

 so that it bends the rays of light just 

 enough to compensate for the defective 

 focusing mechanism. In the case of near- 

 sightedness a biconcave lens is needed, 

 while in farsightedness a biconvex lens will 

 bring about the proper correction. When 

 either the cornea or lens is irregular in its 

 curvature the defect is called astigmatism. 

 This can be corrected by using lens that are 

 ground in such a manner as to compensate 

 for these variations in curvature. 



Sometimes, as a result of disease or for 

 other reasons, either or both the lens and 

 the cornea may become fogged over so that 

 vision is dimmed or completely obliterated; 

 this defect is referred to as cataract. If the 

 lens becomes fogged, it can be removed 

 and a substitute lens placed in front of the 

 eye either in the form of conventional 



