ACCOMMODATION 315 



were refracted; and for this purpose there are the cornea, 

 the aqueous humor, the lens and the vitreous humor. The 

 surfaces of the cornea and lens are the most important of 

 these. Since the two surfaces of the cornea are parallel, the 

 external surface alone is concerned in refraction. The cen- 

 ter 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 op- 

 erations for cataract. Rays leaving the cornea are refracted 

 by the anterior surface of the lens, by its substance to a cer- 

 tain 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 ob- 

 jects 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 ob- 

 served 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 de- 

 sired end is accomplished by increasing the convexity of the 

 lens. When the ciliary muscle is "passive" the capsule com- 

 presses the lens, decreasing its convexity to a minimum; 

 from the attachments of this muscle, already noted, its con- 

 traction is attended by a relaxation of the suspensory liga- 

 ment, which in turn relieves in some degree the compression 

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

 diameter to increase ; the result is increased convexity of the 

 lens. 



