SENSE OF SIGHT. 



575 



The Iris. The iris possesses two sets of muscular fibers, the 

 circular and the radiating. Some authorities question the existence 

 of the radiating muscular fibers, regarding them as elastic rather 

 than contractile, and explain dilatation of the pupil by supposing 

 that the circular fibers cease to contract, and that by the elasticity 

 of the radiating fibers the pupillary margin of the iris is drawn 

 outward. It seems to us, however, that the existence of contrac- 

 tile radiating fibers has 

 been sufficiently demon- 

 strated. By the enlarge- 

 ment or diminution of 

 the size of the pupil the 

 amount of light which is 

 permitted to pass into the 

 eye is regulated. The 

 pigment in the iris makes 

 it opaque, and thus only 

 such light as enters the 

 pupil can reach the retina. 

 We have seen that it is 

 the iris which, excluding 

 the marginal rays, mini- 

 mizes spherical aberra- 

 tion ; and that contrac- 

 tion of the pupil takes 

 place during accommo- 

 dation. The three func- 

 tions of the iris may, 

 therefore, be regarded as 



(1) to regulate the amount ^/ ' \ |-|_ Vophth 



of light which falls upon 

 the retina ; (2) to minimize 



spherical aberration ; and Course of constrictor nerve-fibers 



(3) to assist the accom- Course of dilator nerve-fibers - 



modative apparatus in the FIG. 355. Diagrammatic representation of the 



production of distinct vi- ^f. v _ es governing the pupil : //, optic nerve ; Lg, 



le 



sion for near objects. 



ciliary ganglion ; r.b, its short root from JIT, 

 motor oculi nerve ; sym, its sympathetic root ; 



In the changes which r -l> ^ ts l n g ro * from V, ophthalmonasal branch 

 , i i ' , i of ophthalmic division of fifth nerve ; s.c, short 



take place in the iris, two c ni a ry nerves ; 

 sets of nerves are involved 



; I.e., long ciliary nerves (Foster). 



(Fig. 355) : (1) Those of the third nerve or oculomotorius ; and (2) 

 those of sympathetic origin. The third nerve supplies the circular 

 fibers, and consequently section of this nerve paralyzes these fibers, 

 and dilatation of the pupil occurs. When the third nerve is stimu- 

 lated, the circular fibers contract, causing a diminution in the size 

 of the pupil. The sympathetic supplies the radiating fibers, and 



