196 OPHTHALMOLOGY FOR VETERINARL\NS 



and the focus of distant objects falls in front of the 

 retina; consequently, vision is blurred or imperfect, and 

 should the ciUary muscle contract in this case it would 

 only increase the myopia. For all near objects, however, 

 Httle or no accommodation is required. Myopia is 

 nearly always congenital, though in some cases it is 

 acquired. A minus (concave) spheric lens is used to 

 correct this error. 



Astigmatism may be simple, compound, or mixed. A 

 simple astigmatism is one in which the eye is hyperopic 

 or myopic in one meridian only. It is known as regular 

 astigmatism when vertical or horizontal and irregular 

 when it deviates from these directions. A compound 

 astigmatism is one in which the eye is hyperopic or 

 myopic combined with an astigmatism of that t>^e. 

 Mixed astigmatism is one which is hyperopic in one 

 meridian and myopic in the opposite meridian. 



Astigmatism is due to an irregularity in the curva- 

 ture or refraction of the cornea, the lens, or both. It 

 may be congenital, but is more often acquired. It often 

 follows operations or disease of the cornea. Simple 

 astigmatism is corrected with a plus or minus cyHndric 

 lens, as the case may be. Compound astigmatism is cor- 

 rected with a plus or minus spheric lens, combined with 

 a plus or minus cyHndric lens, as the case may be 

 compound hyperopic or compound myopic. Mixed 

 astigmatism is corrected with a plus spheric and minus 

 cylinder or a minus spheric and a plus cylinder. 



