ORGANS OF SPECIAL SENSE THE EYE 



351 



Ciliary Mustfa 



Suspensortj 

 ~ 



is pulling outwards on the edge of the lens in every direction; 

 Fig. 167. Since the lens is somewhat soft, this outward pull 

 tends to flatten it. In this shape, the lens is in condition for 

 focusing on the retina rays of light coming from a dis- 

 tance. 



If the object viewed is near, as it is in reading or writing, 

 the lens must be more convex. All that is needed to make it 

 so is to loosen the ligament, when the lens of its own elasticity 

 will bulge, and assume the necessary convexity. For every 

 different distance the tension of the ligament must be changed 

 accurately and almost instantaneously, so that the convexity 

 of the lens may be exactly correct. 

 This regulation of the suspensory lig- 

 ament is accomplished by the so-called 

 ciliary muscle, (Fig. 174), one end of 

 which is attached to the choroid coat 

 behind the point where the suspensory 

 ligament arises, the other end fusing 

 with the iris and the inner layers of the 

 cornea. When this muscle contracts, 



it is easy to see that the choroid layer FlG 174 DIAGRAM 



will be pulled forward, the ligament showing the method by whict 

 will become loose and the lens will 

 "bulge," taking the shape of the dotted 

 line in Figure 174. The more the muscle 

 contracts, the more convex the lens 

 will become, and consequently the 



nearer objects may be held and yet be seen. There is, however, 

 a limit to the nearness at which objects may be seen. The 

 ciliary muscles can contract only a certain amount, and the 

 lens can become convex only within certain limits. When 

 an object is held too near the eyes, everything becomes blurred 

 since its light is not focused on the retina. The chief reason 

 that one's eyes become tired from reading is that the ciliary 

 muscles are weary from staying contracted during the 



the lens is held by the sus- 

 pensory ligament, and the 

 ciliary muscle whose con- 

 traction will loosen the liga- 

 ment and allow the lenf 

 to bulge. 



