314 THE SENSES 



in refraction. The center of distinct vision (fovea) is in the axis 

 of the lens precisely in the plane upon which the rays of light 

 are brought to a focus by the refracting media. Refraction by 

 the cornea alone would focus the rays behind the retina; hence the 

 necessity of convex lenses before the eye after operations for 

 cataract. Rays leaving the cornea are refracted by the anterior 

 surface of the lens, by its substance to a certain extent, and again 

 by its posterior surface, the normal mechanism being such that 

 all rays are focused on the fovea. The rays cross each other after 

 refraction, and the image is inverted, but the brain takes no 

 notice of this fact, and objects are seen in their natural positions. 



Accommodation. Accommodation means a change in the 

 convexity of the lens, whereby images are focused on the retina, 

 whether the object be far away from or near the eye. Rays of 

 light from distant objects strike the eye practically parallel, and 

 we may assume that there is a certain "passive" condition of 

 the refracting media which will bring such rays to a focus at the 

 proper point. But when the object observed is near the eye a 

 change in the arrangement of the media, or of the convexity of 

 their surfaces, is necessary to prevent the focusing of the rays 

 behind the retina. The desired end is accomplished by increas- 

 ing the convexity of the lens. When the ciliary muscle is "pas- 

 sive" the capsule compresses the lens, decreasing its convexity 

 to a minimum; from the attachments of this muscle, already 

 noted, its contraction is attended by a relaxation of the suspensory 

 ligament, which in turn relieves in some degree the compression 

 of the capsule upon the lens and allows its antero-posterior diam- 

 eter to increase; the result is increased convexity of the lens. 



When distant objects are looked at the lens become flatter as 

 a result of contraction of the suspensory ligament, which contrac- 

 tion is a consequence of the relaxation of the ciliary muscle. 

 Accommodation for distant objects seems a passive process 

 entirely. 



The ciliary muscle is the "muscle of accommodation." 



