RANGE OF DISTINCT VISION 731 



The Mechanism of Accommodation. The mechanism of accommo- 

 dation depends primarily upon the inherent tendency of the lens to approxi- 

 mate the shape of a sphere. When the eye is at rest the intra-ocular tension 

 is such as to put stress on the suspensory ligament around its equator, which 

 compresses the elastic lens in its antero-posterior dimension. The elasticity 

 of the lens can make itself apparent when the tension of the suspensory liga- 

 ment is relaxed. This takes place completely after a division of the fibers 

 of the zonula. When we remove the lens from the eye of a young person, 

 we see it assume the spherical shape immediately upon the division of its 

 capsule. In life the slackening of the tension of the suspensory ligament 

 of the lens is brought about by the active contractions of the muscle fibers 

 of the ciliary body, the combined contractions of the radial and the circu- 

 lar fibers. This allows the surfaces of the lens by its own elastic powers to 

 become more convex, thus focusing entering rays of light from a nearer 

 object upon the retina, figure 460. It therefore appears that when the eye 



FIG. 460. Diagram Representing by Dotted Lines the Alteration in the Shape of the Lens 

 on Accommodation for Near Objects. (E. Landolt.) 



is at rest it is focused for distant objects, in as much as the suspensory 

 ligament is taut and the anterior surface of the lens more flattened. The 

 normal eye is therefore passive when in focus for distant objects. It is 

 the active contraction of the muscles of accommodation that focuses for 

 near objects. 



The iris acts in co-ordination with the accommodative contractions of 

 the ciliary muscles. In viewing near objects the pupil contracts, and upon 

 dewing distant ones it dilates. 



Range of Distinct Vision. Near-point. In every eye there is a limit to 

 the power of accommodation. If a book be brought nearer and nearer 

 to the eye, the type at last becomes indistinct, and cannot be brought into 

 focus by any effort of accommodation, however strong. The printed 

 letters appear gray and with shadowy outlines. At a certain distance the 



