THE LENS. ID/ 



focus. In the unaccommodated eye this point is in the 

 retina.* 



The Lens. When the tensor of the choroid is inactive, 

 the principal focus of the normal (emmetropic) eye lies in 

 the retina ; consequently those objects only are seen dis- 

 tinctly from which the eye receives parallel rays. The 

 process by which it is adjusted for vision of near objects, 

 is called "accommodation." Its accomplishment is the 

 purpose or function of the lens, and of the muscular and 

 fibrous structures by which its form is regulated, the 

 tensor of the choroid and the zonule of Zinn. The tensor 

 of the choroid consists, in man, of fibres of two kinds, viz., 

 of annular fibres which encircle the border of the lens, and 

 of much more numerous meridional fibres which draw the 

 choroid towards the cornea. When (in the dog) this 

 muscle is thrown into action by excitation of the short 

 root of the lenticular ganglion, the anterior surface of the 

 lens becomes more convex and approaches to a shorter 

 distance from the posterior surface of the cornea. 



The increase of convexity is due to the relaxation of 

 the zonule, by virtue of which the lens is left to its own 

 elasticity, and assumes a form approaching that which it 

 possesses after removal from the body. Under the influence 

 of atropin the tensor is completely paralysed ; in conse- 

 quence, the convexity of the lens is diminished ; for, in the 

 ordinary condition of the eye, the muscular fibres are not 

 entirely relaxed. The degree of accommodation of which 

 the eye is capable varies in different individuals at different 

 ages. Thus, in the normal emmetropic eye, which, when 

 entirely relaxed, sees distant objects distinctly, the lens can 

 in childhood be rendered convex enough to give well-defined 

 images of objects at a distance of 3 inches. As age advances, 



* The statement above is simplified to the utmost by the substitution of the 

 hypothetical lensless eye for the schematic eye. All that has been said is 

 applicable to the real eye, but much is omitted. Those who desire to under- 

 stand in what way the formation of the image is modified by the presence of 

 the lens, will find it clearly explained in Hermann's Physiology, pp. 377-384. 



