150 



VETERINARY PHYSIOLOGY 



object, positive accommodation is required. As the object is 

 approached to the eye, it is focussed with greater and greater 



O'l METRE. 



Fig. 70. — To illustrate Presbyopia, Hjpermetropia, and Mj'opia. A, emme- 

 tropic eye ; B, presbyopic ej-e ; C, hypermetropic ej-e ; D, myopic eye ; 

 N.P.o, the near point, and F.P.x, the far point of accommodation. 



difficulty, and the near point is further off than in the 



emmetropic eye (fig. 70, C). 



This long-sighted eye differs from the slightly presbyopic 



in the fact that not merely 

 divergent, but also parallel 

 rays, are unfocussed in the 

 resting state. 



(2) Myopia. — In certain in- 

 dividuals the antero-posterior 

 diameter of the eye is too 

 long, and, as a result, parallel 

 rays — rays from distant 

 objects — are focussed in front 

 of the retina, and it is only 

 when the object is brought 

 near to the eye that a 

 perfect image can be formed. 

 In myopia no positive accom- 

 object is well within the 



Fig. 71. — To show the cause of Astig- 

 matism. A, a slight curvature of 

 the cornea in the vertical plane ; B, 

 more marked curvature in the hori- 

 zontal plane, leading to rays from b 

 — a horizontal line — being focussed 

 in front of the retina when a — a 

 vertical line — is looked at. 



modation is needed till the 



