DIOPTRIC MECHANISMS OF THE EYEBALL 613 



known as ' the light reflex,' is determined by a contraction of the 

 sphincter pupillse, together with a relaxation of the dilatator muscle. 

 The contraction ensues within a period of 0-4 to 0-5 sec. after the 

 moment at which the light has access to the retina, and attains its 

 maximum within 0-1 sec. In man as well as in other animals which 

 have binocular vision, and in which therefore there is a partial decussa- 

 tion of the fibres of the optic nerves in the optic chiasma, the reflex 

 is bilateral, i.e. light falling into one eye causes simultaneous 

 contraction of both pupils. In the higher animals this reaction 

 of the pupil to light demands the integrity of the nervous paths 

 between the eye and the brain ; but in many of the lower animals, 

 e.g. in the frog and eel, the reflex nervous mechanism is aided by a 

 local sensibility of the iris to light. In these animals the contraction 

 of the pupil in response to illumination takes place even in the excised 

 eye, and seems to be determined by a direct stimulation of the pig- 

 mented contractile fibres of the sphincter pupillae by means of the 

 light. 



The effect of light on the pupil varies considerably according to the 

 condition of adaptation of the eye. The dilatation of the pupil is 

 maximal when the eye has been in the dark for some time and may 

 amount then to 7-3 to 8 mm. In one experiment, on exposing the 

 eye to a feeble light, e.g. 1-6 candles at a moderate distance, the pupil 

 diminished in size to 6-3 mm. ; with an illumination of 50 to 100 

 candles the size of the pupil was 3-7 mm., and with 500 to 1000 

 candles, 3-3 mm. This effect was obtained by a rapid change of the 

 illumination of the eye. When the change in illumination is suffi- 

 ciently slow no alteration of the pupil takes place, and when the 

 illumination, which has at first caused a maximal constriction of the 

 pupil, is continued the pupil gradually relaxes with the adaptation 

 of the retina to light. This relaxation occurs within three or four 

 minutes after exposure to light has taken place. The same influence 

 of adaptation will be observed if two individuals are brought into a 

 moderately lighted room, one from bright daylight and the other 

 from a dark room. The pupils of the first will dilate widely, while 

 those of the second will constrict to their maximum extent. In 

 each case the change will pass off gradually, so that at the end of 

 five or ten minutes there will be no difference observable between 

 the eyes of the two persons. 



(2) When vision is directed to a near object the contraction of 

 the ciliary muscle which results in accommodation is accompanied 

 with convergence of the visual axes, brought about by contraction 

 of the two internal rectus muscles. With these two movements 

 is always associated a third, viz. contraction of the sphincter pupillae, 

 the increased sharpness of the image obtained by this means being an 



