CHAPTER XX 



ERRORS OF REFRACTION 



There is no doubt that some animals have refractive 

 errors as well as man, but, as the requirements of vision 

 are so vastly different from those of man, it will probably 

 be a long time, if ever, when these errors will be corrected 

 by the use of lenses. However, it may be well to give the 

 veterinary student some knowledge of refractive errors 

 at this time. 



There are four principal errors — viz.: • Hyperopia 

 (farsightedness), myopia (nearsightedness), astigmatism 

 (where one meridian is either hyperopic or myopic), and 

 presbyopia (the natural failing vision of age). 



Hyperopia is nearly always congenital, and is due to a 

 short eyeball from before backward, so that the focus 

 falls behind the retina. By some effort of the ciUary 

 muscle the focus is brought forward to the retina. In 

 some cases, by gradual development, the eye becomes 

 normal in its anteroposterior measurements, though in 

 many cases hyperopia exists throughout life. A plus 

 (convex) spheric lens is necessary to correct this con- 

 dition. 



Myopia is just the opposite of hyperopia; that is, the 

 eyeball is longer from before backward than normal, 



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