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COORDINATION AND SENSATION 



size of the pupil, the iris is able to regulate the amount of 

 light which passes through and it employs for this pur- 

 pose two sets of muscular fibers. One set of fibers 

 forms a thin band which encircles the pupil and serves 

 as a sphincter to diminish the opening. Opposing this are 

 radiating fibers which are attached between the inner and 

 outer margins of the iris. By their contraction the size of 

 the opening is increased. Both sets of fibers act reflex- 

 ively and are stimulated by variations in the light falling 

 upon the retina. 



FIG. 160. Diagram showing main nervous elements in the retina. 



Light waves stimulate the rods and cones at back surface of the retina, start- 

 ing impulses which excite the ganglion cells at the front surface. Fibers from 

 the ganglion cells pass into the optic nerve. 



The Inner Coat, or Retina. This is a delicate membrane 

 containing the expanded termination of the optic nerve. 

 It rests upon the choroid coat and spreads over about 

 two thirds of the back surface of the eyeball. Although 

 not more than one fiftieth of an inch in thickness, it pre- 

 sents a very complex structure, essentially nervous, and is 



