112 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



case the diaphragm is contractile and the pupil self- 

 regulating. 



Structure of the Iris. — The iris, as already seen, is a 

 continuation of the choroid, which lines the sclerotic as 

 far forward as a little in front of the lens curtain, and then 

 is drawn together transversely to form the iris. This 

 part is thickened with muscular fibers. These are of 

 two kinds, circular and radiating, as shown in the figure 

 (Fig. 80). The circular fibers, by contracting, draw the 

 pupil together and make it small ; the radiating fibers 

 take hold on the margin of the pupil and pull it outward 



in every direction and en- 

 large it. Or, perhaps better, 

 regard the circular fibers as 

 sensitive and actively con- 

 tractile and the radiating 



Fig. 80. — Showing structure of ,-. , , . , 



the ins. fibers as elastic and passively 



contractile. When the circu- 

 lar fibers contract they draw up the pupil, stretching the 

 radiating fibers. When they relax, the radiating fibers 

 elastically contract and enlarge the pupil. Now, the 

 circular fibers are in sympathetic relation with the 

 retina in such wise that stimulation of the retina by 

 strong light reflexly causes the pupil to contract. As 

 the light decreases, the pupil expands to take in more 

 until, in the dark or in case of paralysis of the ret- 

 ina, the pupil expands until the iris becomes a slender 

 ring. 



The hint has been taken here also by the instrument 

 maker. The iris diaphragm of the microscope is made 

 of thin overlapping plates of steel, which, by turning a 

 thumbscrew, slide toward or away from one another, 

 contracting or enlarging the opening between. It is 

 a beautiful contrivance, but far inferior to the liv- 

 ing iris. 



