302 A Manual of Veterinary Physiology. 



crystalline lens may be made natter or more convex, and the 

 process which brings this about is termed the ' mechanism 

 of accommodation." 



The crystalline lens is contained within a capsule ; to this 

 capsule is attached the zonula ciliaris, which in turn is 

 attached to the choroid. So long as the zonula is tense the 

 lens is flattened; but if the tension be removed, the natural 

 elasticity of the lens causes it to bulge forward and become 

 more convex. The tension of the zonula is removed from 

 the lens by means of the ciliary muscle, which draws forward 

 the choroid coat, relaxes the zonula, and the lens becomes 

 convex. In this way the accommodation of the eye is 

 adjusted (Fig. 32). 



Whilst on this point mention may be made of the refiec- 



A B C 



Fig. 33. — Diagram of the Katoptrk Test. 



A, from the anterior surface of the cornea: B, from the anterior face 

 of the lens : and C, from the posterior face of the lens. 



tions obtained when examining the eye with a candle — the 

 so-called katoptric test. When a candle is held opposite 

 the eye three images of the name are seen: one a very sharp 

 bright one, obviously reflected from the cornea ; a second 

 much duller, but also large, reflected from the anterior sur- 

 face of the lens; and a third very small, brighter than the 

 middle one, and inverted, reflected from the posterior part 

 of the lens. In a normal eye these are seen perfectly, and 

 move in a definite direction when the candle is moved, the 

 inverted one in an opposite direction to the two erect images, 

 and all are equally visible at any surface of the lens ' see 



' The blurring or duplicature of the images, especially the second 

 one, is a good ordinary test for opacities of the leas, 



