72 THE ORGANS OF SPECIAL SENSE. 



distant objects. The ability of the eye to focus upon 

 objects at different distances is called accommodation 

 and to accomplish it three things are necessary: 1. 

 change in the shape of the lens; 2. convergence of the 

 axes of the eyes, and 3. narrowing of the pupils. 



When the eye is directed toward distant objects, the 

 muscle fibers in the ciliary processes relax, causing 

 tightening of the suspensory ligaments and consequent 

 flattening of the surface of the lens. Otherwise an im- 

 age would be formed in front of the retina; for the 

 greater the convexity of the lens, the greater the angle 

 of refraction. Such accommodation is passive and so 

 not fatiguing. To look at nearby objects, on the con- 

 trary, the ciliary muscles contract, drawing the cho- 

 roid forward and allowing the suspensory ligaments 

 to relax, so that the lens bulges in front. This is an 

 exertion. 



In order to accommodate properly, moreover, both 

 eyes must work together and the axes of both eyes must 

 be directed toward the object. Therefore, in looking 

 at near-by objects the axes of the eyes converge, drawn 

 by the internal recti muscles. In strabismus or cross 

 eye, where the axes of both eyes cannot be directed 

 toward the object at the same time, the rays fall upon 

 one part of one eye and upon a different part of the 

 other eye and two separate images are seen. 



Finally there is concentric narrowing of the pupil 

 by contraction of the circular fibers of the iris, by 

 which means various side rays that would come to a 

 focus outside the retina are excluded. 



All the muscles of accommodation, the ciliary mus- 

 cles, the internal recti, and the spincter pupillse, are 

 under the control of the third nerve. 



Connected with this power of accommodation and de- 

 pendent on it are the two conditions of near-sighted- 

 ness or myopia and far-sightedness or hypermetropia. 



The normal eye is emmetropic and is almost per- 

 fectly spherical, but in the near-sighted or myopic eye 



