SENSE OF SIGHT. 



575 



Ic 



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 

 of light which falls upon 

 the retina ; (2) to minimize 

 spherical aberration ; and 

 (3) to assist the accom- 

 modative apparatus in the 

 production of distinct vi- 

 sion for near objects. 



In the changes which 

 take place in the iris, two 

 sets of nerves are involved 

 (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 



Vcphth, 



Course of constrictor nerve-fibers 



Course of dilator nerve-fibers -- 



FIG. 355. Diagrammatic representation of the 

 nerves governing the pupil : II, optic nerve ; l.g, 

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

 motor oculi nerve ; sym, its sympathetic root ; 

 r.l, its long root from V, ophthalmonasal branch 

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

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



