400 



ORGAN SYSTEMS OF MAN 



reflna 



rod 



pigment choroid 

 cell cell 



circular muscles con'Yracfed 



closeup vision 

 lens thicte 



Fig. 16-6. Accommodation is accomplished by changing 

 light entering the eye is controlled by the iris diaph 



There are some very important differ- 

 ences between the camera and the eye. 

 First of all, the film and the retina function 

 differently. Once a picture is taken the film 

 is used up, that is, it must be replaced by 

 another, whereas in the retina a continuous 

 series of pictures can be recorded without 

 exhaustion of the sensitive cells. The retina 

 might be compared to the film in a movie 

 camera which is constantly being replaced. 

 Secondly, the method of bringing objects 

 at different distances into focus on the sen- 

 sitive plate operates differently, although 

 the end result is the same. In the camera 

 the lens maintains one shape and must be 

 moved forward and backward until the 

 image is in focus on tlie film. In the human 

 eye, however, the lens itself changes its 



radial muscles contracted 



the thickness and curvature of the lens. The amount of 

 ragm. The image recording mechanism is the retina. 



shape, becoming more curved for close ob- 

 jects and less curved for distant objects. 

 This change in shape of the lens is called 

 accommodation. Aside from the lack of a 

 shutter in the eye, unless the eyelids could 

 be so considered, there are no other differ- 

 ences. It is unfortunate in many ways that 

 the human eye cannot be focused like a 

 camera, for if it could many people would 

 be relieved of placing glass lenses in front 

 of their eyes, some all their lives, others 

 during most of the "down hill" years. 



In the eye, the job of bringing the light 

 rays to a focus upon the retina is done by 

 the cornea; only the slight differences that 

 are needed to produce a sharp picture are 

 accomplished by the lens itself. This slight 

 variation, however, makes the difference 



