788 



THE EYE AND VISION 



[CH. LV1 



FUNCTIONS OF THE IRIS. 



The iris has three uses : 



1. To act as a diaphragm in order to lessen spherical aberration 

 in the manner just described. 



2. To regulate the amount of light entering the eye. In a bright 

 light the pupil contracts ; in a dim light it enlarges. This may be 

 perfectly well seen in one's own iris by looking at it in a mirror 

 while one alternately turns a gas-light up and down. 



3. By its contraction during accommodation it supports the 

 action of the ciliary muscle. 



The muscular fibres (unstriped in mammals, striped in birds) of 

 the iris are arranged circularly around the margin of the pupil, and 

 radiatingly from its margin. The radiating fibres are best seen in 

 the eyes of birds and otters; some look upon them as elastic in 

 nature, but there is little doubt that they are contractile. Those 

 who believe they are not contractile explain dilatation of the pupil 

 as due to inhibition of the circular fibres. But if the iris is stimu- 

 lated near its outer margin at three different points simultaneously 

 the pupil assumes a triangular shape, the angles of the triangle 

 corresponding to the points stimulated ; this must be due to con- 

 traction of three strands of the radiating muscle ; inhibition of the 

 circular fibres would occur equally all round. 



The iris is supplied by three sets of nerve-fibres contained in the 

 ciliary nerves. 



(a) The third nerve via the short ciliary nerves supplies the 

 circular fibres. 



(b) The cervical sympathetic supplies the radiating fibres. The 

 cilio-spinal centre which governs them is in the cervical region of 

 the cord (see p. 676). The fibres leave the cord by the anterior root 

 of the second thoracic nerve, pass into the cervical sympathetic, and 

 reach the eyeball via the ophthalmic branch of the fifth, and long 

 ciliary nerves. 



(e) Fibres of the fifth nerve which are sensory. 



The experiments on these nerves are those of section and stimula- 

 tion of the peripheral ends; the usual experiments by which the 

 functions of a motor nerve are discovered. 



