432 



ANATOMY FOR NURSES 



[Chap. XX 



Presbyopia. — Presbyopia is a defective condition of accom- 

 modation in which distant objects are seen distinctly, but 



near objects are indistinct. This 

 is a physiological process which 

 affects every eye sooner or later, 

 and is not due to disease. It is 

 said to be caused by a loss of 

 elasticity of the l^ns. 



Astigmatism. — Astigmatism 

 is the condition in which the 

 different meridians ^ of the cor- 

 nea are not equally convex, and 

 so there is interference with 

 the formation of distinct images 

 on the retina. 



Inversion of images. — Fol- 

 lowing the general laws for the 

 formation of images in connec- 

 tion with the lens, an inverted 

 image of external objects is 

 formed on the retina. 

 " The question then arises, WTiy is it that objects do not appear 

 to us to be upside down ? This cannot be satisfactorily answered 

 without entering into matters which require a previous psychologi- 

 cal training. Suffice it to say here that the localization of objects 

 in space depends not only on the retina, but also on tactile and 

 general experience ; that the mind localizes objects with reference 

 to its own body, and that from the first it knows nothing of the in- 

 version of the retinal image, as its powers of localization only ap- 

 pear with developing general experience." (Halliburton.) 



• The meridian of the eye is an imaginary line drawn around the eyeball. 



Fig. 204. — Diagram illustrating 

 Ray.s of Light converging in (A) a 

 Normal Eye, (B) a Myopic Eye, and 

 (C) A Hyper.metropic Eye. 



