THE MECHANISM OF ACCOMMODATION 645 



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 

 connections. In life this slackening of the tension of the suspensory liga- 

 ment of the lens is brought about by the contraction of the fibers of the ciliary 

 body. This allows the anterior surface of the lens to become more convex, 

 by its own elastic powers, thus focussing entering rays of light from a near 

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

 is at rest it is focussed for distant objects, inasmuch as the suspensory liga- 

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

 eye is passive when in focus for distant objects. It is the active contraction 

 of the muscles of accommodation that focusses for near objects. The iris 

 acts in coordination with the accommodative contractions of the ciliary mus- 

 cles. In viewing near objects the pupil contracts, and upon viewing 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 



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

 Accommodation for Near Objects. (E. Landolt.) 



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

 by any effort of accommodation, however strong. This limit, which is termed 

 the near-point, can be determined by the experiment of Scheiner. Two 

 small holes not more than 2 mm. apart are pricked in a card with a pin; at 

 any rate their distance from each other must not exceed the diameter of the 

 pupil. The card is held close in front of the eye, and a small needle viewed 



