DIOPTRICS OF THE EYE. 311 



muscle contracts the radial fibers especially will pull forward the 

 choroid coat. The effect of this change in the choroid is to loosen 

 the pull of the suspensory ligament (zonula Zinnii) on the lens and 

 this organ then bulges forward by its own elasticity. The theory 

 assumes that in a condition of rest the suspensory ligament, which 

 runs from the ciHary processes to the capsule of the lens, exerts a 



Ciliary Border 



process, of iris. Ciliary muscle. 



Choroid 

 coat. 



Fig. 131. — Meridional section of eyeball after removal of sclerotic coat, cornea, and iris, 

 to show the position of the ciliary muscle. — (Schidtze.) 



tension upon the lens which keeps it flattened, particularly along 

 its anterior surface, since the ligament is attached more to this side. 

 When this tension is relieved indirectly by the contraction of the 

 ciliary muscle the elasticity of the lens, or rather of the capsule of 

 the lens, causes it to assume a more spherical shape along its anterior 

 surface, and the amount of this change is proportional to the 

 extent of contraction of the muscle. Other theories have been 

 proposed to explain the way in which the contraction of the ciliary 

 muscle effects a change in the curvature of the lens,* but none is 

 so simple and, on the whole, so satisfactory as the one suggested 

 hv Helmholtz. 



It is interesting to note that in fislies accommodation is effected in a 

 different way, namely, by movements of the lens forward and backward. 

 In these animals the eye when at rest is accommodated for near vision, and 

 to see objects at a distance the refractive power of the eye is diminished by 

 the contraction of a special muscle, retractor lentis, which pulls the lens toward 

 the retina, t 



Limit of the Power of Accommodation — Near Point ol 

 Distinct Vision. — When an object is brought closer and closer to 

 the eye a point will be reached at which it is impossible by the 

 strongest contraction of the ciliary muscle to obtain a clear image 

 =F «ee Tscherning, " Optique physiologique," Paris, 1898 ; and Schoen 

 " Arcliiv f. die gesammte Physiologic," 59, 427, 1895. 



tSee Beer, "Wiener klinische Wochenschrift, " 1898, No. 12. 



