VISION. 283 



will be seen to move forward that is to say, closer to the image 

 at the cornea when the person shifts his gaze from a distant 

 to a near object. By using optical apparatus for measuring the 

 size of the images, the degree to which the convexity of the lens 

 lias increased, as a result of the bulging, can be accurately 

 measured. 



This change in the convexity of the lens depends on the fact 

 that it is composed of a ball of transparent elastic material, 

 which is kept more or less flattened antero-posteriorly because of 

 its being slung in a capsule which compresses it. The edges of 

 the capsule are attached to a fine ligament (the suspensory liga- 

 ment), which runs backwards and outwards to become inserted 

 into the ciliary processes (Fig. 53). These processes exist as 

 thickenings of the anterior portion of the choroid, or pigment 

 coat of the eye, and they can be moved forwards by the action 

 of a small fan-shaped muscle, called the ciliary muscle, which 

 at its narrow end originates in the corneo-schleral junction, and 

 runs back to be attached, by its wide end, to the ciliary pro- 

 cesses. When this muscle is at rest, the ciliary processes lie at 

 such a distance from the edges of the lens that the suspensory 

 ligament is put on the stretch. When the ciliary muscle con- 

 tracts, it pulls the ciliary processes forward, thus slackening 

 the suspensory ligament and removing the tension on the capsule 

 of the lens, with the result that the latter bulges because of its 

 elasticity. The ability of the lens to become accommodated for 

 near vision depends, therefore, first, on the elasticity of the 

 lens, and secondly, on the action of the ciliary muscle. Inter- 

 ference with either of these renders accommodation faulty. For 

 example, the lens along with the other elastic tissues of the 

 body [e. g., the arteries (p. 175)], becomes less elastic in old 

 age, thus accounting for the "long-sightedness" (or presbyopia) 

 which ordinarily develops at this time. Paralysis of the ciliary 

 muscle produces the same effect in even more marked degree, 

 which explains the utter inability to bring about any accommo- 

 dation after treating the eye with atropin, which is given for 

 this purpose before testing the vision in order to find out the 

 strength of lenses required to correct for errors in refraction. 



