508 PHYSIOLOGY 



access to the retina, and attains its maximum within O'l sec. In man as 

 well as in other animals which havejbinocular vision, and in which there 

 is a partial decussation 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 illumi- 

 nation takes place even in the excised eye, and seems to be determined by 

 a- direct stimulation of the pigmented contractile fibres of the sphincter 

 pupilloB 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 (>:> 

 mm. ; with an illumination of 50 to 100 candies 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 in the illumination of the eye. When the change in 

 illumination is sufficiently 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 regularly, 

 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 functions of accommoda- 

 tion of the lens and of convergence of the visual axes which result, are 

 associated with contraction of the pupil. The sharpness of vision is thereby 

 improved together with an increase in the depth of focus. Results are very 

 beneficial for the close examination of detail. Since it is possible by ex- 

 periment to cause accommodation without convergence and vice versa, 

 we may ascertain which function is the more closely associated with 

 the pupil mechanism. The evidence appears to be in favour of convergence. 



(3) In sleep the pupils are always contracted. This behaviour may enable 

 us to distinguish feigned from real sleep. This contraction of the pupils, 

 in spite of the fact that no light is entering the eyes, has been held to be caused 

 by association with the upward and inward direction of the eye axes which 

 was said to be found in sleep. There now appears to be irrefutable evidence 

 that the eyes during sleep may occupy any position ; another explanation 

 of the constricted pupils must therefore be found. 



