458 



THE SPECIAL SENSES 



band (the suspensory ligament) which attaches the edges 

 of the capsule to the ciliary process of the choroid layer. 

 Between the ligament and the edge of the choroid layer are 

 a series of muscles (the ciliary muscles). When these 

 muscles contract they draw forward the edge of the choroid 

 layer, thus causing the suspensory ligament to relax. This 

 contraction permits the lens to shorten its diameter and 

 become thicker and more bulging. In this way the shape 

 of the lens may be changed, and this affects its image- 

 forming power. 



Functions of the various eye structures.-;— The human 

 eye is constructed essentially like a camera, and we can 



V///M.W/m//.'// M/MAmV//M//J ,//MMMm,J,^^^^^^ 



Diaphragm ilHs) 



Screen iJHetina) 



VM////W/.:WW//^MW''/^)^W:'>)/y->f/M//^W//////WWM^^^^ 



Fig. 216 — An eye model. 



obtain an idea of the function of its parts by study of the 

 parts of a camera or eye model such as shown in Figure 

 216. This shows a box painted black on the inside and 

 having at the front a small aperture into which may be 

 fitted perforated cardboard disks of different sized aper- 

 tures. Inside the box is a biconvex lens of glass so 

 mounted as to bring its center opposite the center of the 

 cardboard aperture, and to be capable of movement for- 

 ward and backward. It also contains a ground glass 

 screen similarly mounted, and adjustable to forward and 



