rs EYE AS A WATER CAMERA 407 



optical properties of spheroidal surfaces, give rise to 

 defects in the image formed at the focus. 



In the eye, the place of this diaphragm is taken by the 

 iris, which has the peculiar advantage of being self-regu- 

 lating : contracting its aperture and admitting less light 

 when the illumination is strong ; but dilating its aperture 

 and admitting more light when the light is weak. It 

 thus acts like the various "stops " which a photographer 

 uses according to the varying light. 



These changes in the pupil are brought about by the 

 contractions of the circular and radiating muscle-fibres of 

 the iris ; contraction of the circular or sphincter fibres 

 makes the pupil smaller or constricts it, contraction of 

 the radiating fibres makes it larger or dilates it. Further 

 conversely relaxation of the circular fibres causes or helps 

 to cause dilation, and relaxation of the radiating fibres 

 causes or helps to cause constriction. Contraction of the 

 circular fibres and so const rictio7i of the pujjil is brouglit 

 about by means of fibres of the oculo-motor ner^'B, and 

 contraction of the radiating fibres and so active dilation is 

 brought about by means of fibres of the sympathetic 

 system and may be induced by stimulation of the sym- 

 pathetic in the neck. 



The constriction of the pupil observed when light falls 

 upon the retina is a reflex action in which the optic nerve 

 provides the path for aflerent impulses to a centre in the 

 brain lying beneath the front end of the aqueduct of 

 Sylvius (see Lesson XI), and the third 'oculo-motor) 

 cranial nerve (see Lesson XI) provides the path for 

 efferent impulses from the centre to the cii'cular fibres of 

 the iris. The dilation of the pupil when light is with- 

 drawn from the retina is in the main at least due to the 

 cessation of previously acting constrictor impulses. 



But the pupil, or aperture of the iris, is either con- 

 stricted or dilated under many circumstances, other than 

 the mere action of light and darkness on the retina. 

 Thus it is constricted when the eye is accommodated for 



