ACCOMMODATION 



533 



ac- 



the zonule of Zinn, to the ciliary body. The zonule of Zinn surrounds the 

 periphery of the lens, fusing insensibly with its capsule. The greater part 

 of the zonule, from the ora serrata to the tip of the ciliary process, is grown 

 fast to the ciliary body. But, since the ciliary processes do not reach all the 

 way to the lens, there is left a small zone between them and the periphery 

 of the lens, within which the zonule is turned freely toward the posterior 

 chamber. This free part of the zonule (Fig. 228) consists of several strands, 

 which may be divided, for convenience into three groups : an anterior, passing 

 to the anterior capsule of the lens and called the suspensory ligament; a 

 middle group whose fibers are directed vertically to the capsule immediately 

 back of the equator of the lens, and a posterior group lying close to the 

 hyaloid membrane and pass- 

 ing over into the posterior 

 capsule of the lens. All these 

 strands consist of parallel in- 

 elastic fibers. 



Among the hypotheses 

 which have been put forward 

 to explain the form changes 

 of the lens in accommodation, 

 the following by Helmholtz is 

 the one most generally 

 cepted at this time : 



The lens is an elastic 

 which, while the ciliary mus- 

 cle is inactive, is somewhat 

 compressed from before back- 

 ward by the radial pull of the 

 zonule attached to its equator. 

 Now the zonule is firmly at- 

 tached by its peripheral or 

 posterior end with the choroid 

 coat just at the posterior mar- 

 gin of the ciliary processes. 

 Consequently by contraction 

 of the meridional fibers of the 

 ciliary muscle (Fig. 227) this 

 posterior edge of the zonule 

 can be drawn forward, there- 

 by releasing the radial tension 

 which it exerts on the lens in 

 the unaccommodated condi- 

 tion. The result is that the 

 lens bulges out at its center 



rendering both anterior and posterior surfaces more convex. The only 

 function of the circular fibers according to this view would be to crowd 

 the anterior part of the ciliary processes toward the relaxing lens and the 

 zonule, so as to prevent alike any rupture of the tissues and any traction 



FIG. 228. Zonule of Zinn of an adult man, meridional 

 section, after G. Retzius. /, edge of the lens at its 

 equator; gl, vitreous body; i, iris; a, short, strong 

 attachment fibers of the posterior group ; b, fibers of 

 the same group springing from the hyaloid mem- 

 brane; c, the anterior group springing from the ciliary 

 process; d, fibers springing from the ciliary process 

 and connecting with other fibers; e, space between 

 the capsule of the lens and the pericapsular mem- 

 brane. 



