SENSE OF SIGHT. 



565 



the refractive media. If an object is within that distance, the rays 

 of light coming from it are brought to a focus by altering the shape 

 of the crystalline lens ; this is positive accommodation. 



As already stated, the optical apparatus of the eye is in a state 

 of rest when it is looking at objects more than two to three meters 



CS 1 



FIG. 342. Diagrammatic representation of accommodation for near and far ob- 

 jects. On the right the condition during positive accommodation is shown, on the 

 left the condition during rest (negative accommodation). On both sides one-half of 

 the contour of the lens is drawn as a continuous line, the other half as a dotted line. 

 The letters appearing twice on both the right and left sides have the same sig- 

 nificance ; those on the right side are primed A, left, B, right half of the lens; C, 

 cornea ; <S', sclera ; C.8., canal of Schlemm ; V.K., anterior chamber ; J, iris ; P, 

 pupillary margin ; V, anterior, H, posterior surface of the lens ; B, equator of the 

 lens ; F, edge of the ciliary processes ; a and 6, interval between them. The line 

 Z-X shows the thickness of the lens in the act of accommodation for a near object; 

 Z-F, the thickness of the lens when the eye is at rest. (Landois.) 



distant, this is negative accommodation ; thus to see the stars, although 

 millions of kilometers distant, no effort is required ; but if it is de- 

 sired to see objects less than two or three meters away, there is a 

 change in the refractive media until objects are brought to a point 

 so close to the eye that no amount of effort will enable them to be 

 seen. The point at which objects cease to be seen distinctly is called 

 the near point, and it is, for a normal or emmetropic eye about 12 cm., 

 although it is not the same in all persons. 



The positive accommodation of the eye is brought about espe- 

 cially by the change in the shape of the crystalline lens ; thus in 

 looking at near objects the lens becomes more convex. This is 

 accomplished in the following manner*: The lens is a very elastic 

 structure, enclosed in a capsule to which the zonule of Zinn is at- 

 tached, and the tension of this structure is such as to pull upon the 

 anterior portion of the capsule and flatten it, at the same time 

 flattening the anterior surface of the contained lens. This is 

 the condition when the eye is looking at distant objects or is in a 

 state of accommodative rest. But when a near object is to be 

 looked at, the radiating fibers of the ciliary muscle contract, and 

 as its fixed point is at the junction of the cornea and sclerotic, this 

 contraction draws the ciliary processes forward and relaxes the 



