HUMAN STRUCTURE AND LIFE- ACTIVITIES 517 



tween a camera and an eye are : (1) the eye is living tissue ; 

 (2) the sensitive plate or retina of the eye contains living 

 nerve-cells connected to the brain by nerve-fibers of the 

 eye-nerve or optic nerve; and (3) the lens of the eye is focused, 

 not as in a camera by moving the sensitive plate nearer to or 

 farther from the lens, but by muscles which change the shape 

 of the eye-lens. 



The adjustment of the lens to suit different distances of 

 objects seen is called accommodation. It is accomplished as 

 follows : The lens when not compressed is very -bi-convex, 

 as may be seen in a lens cut from an eye obtained at a meat- 

 market. When an eye is resting in sleep or is looking at far- 

 away objects, the lens is much flattened (or made less convex) 

 by the pull of the elastic choroid upon the transparent cap- 

 sule which incloses the lens and attaches it to the choroid. 

 In order to see clearly near-by objects the lens must be 

 focused by being made more convex. This is simply the 

 elastic return of the lens toward its natural very bi-convex 

 shape, and this return is permitted by a sheet-like circular 

 muscle which opposes the elastic pull of the choroid and 

 thereby eases the tension upon the lens. 



It is evident from the above that the feeling of strain 

 when we look at very small objects is due to the pull of 

 muscles against the constant elastic pull of the choroid 

 upon the capsule that incloses the lens. 



(D) In order to study the effect of change of shape of the lens 

 upon the focus of the eye, first set up a photographic camera and 

 focus upon near and distant objects by moving the lens. If one had 

 lenses for different distances (very bi-convex for near-by, and less 

 so for far-away) the distance from the lens to the sensitive plate 

 might be kept stationary in a camera. In the case of an eye the 

 distance to objects seen varies, and there is need of many lenses of 

 different curvatures ; or better still, of one elastic lens whose shape 

 can be changed to fit objects at any distance. 



Eyes that cannot see distant objects clearly are said to 

 be " near-sighted," and one with such eyes must hold print 



