EFFECT OF LIGHT ON THE RETINA 179 



Angelucci ('78, p. 367) observed that when the optic nerve of 

 a frog is cut the physiological changes of the pigment took place, 

 thirty days after the operation, as in normal eyes. Hamburger 

 ('88), Arcoleo ('90) and Fick ('01, p. 4) also found that when the 

 optic nerve is cut the pigment changes induced by light and 

 darkness took place as in the normal eye. In addition Engel- 

 mann ('85, p. 505) observed that when the brain of a frog is 

 destroyed that the effect of hght on the migration of pigment is 

 still present. Movements of the cones have also been observed 

 in eyes, the optic nerves of which have been cut, or which have 

 been removed from the body. Hamburger ('89) found that, 

 when he cut the optic nerve of a frog or removed the eye, contrac- 

 tion of the cones will take place when the eye is illuminated. 

 And Dittler ('07) found in the isolated frog retina placed in salt 

 solution that upon illumination the contraction of the cones takes 

 place. 



Experiments on the enucleated bulbus of the tortoise were 

 without results. The same is true of retinae which were isolated 

 in the manner described by Dittler for the frog. The experi- 

 ments on eyes with the optic nerve cut, however, did yield rather 

 interesting results. All the experiments were carried out on 

 Chrysemys picta. The method of cutting the optic nerve is 

 briefly as follows: Under deep ether anaesthesia a small wedge 

 of bone was removed from the left side of the mouth beneath the 

 eye. This was done by means of a long, very narrow saw. The 

 small amount of bleeding consequent to the removal of the bone 

 being stopped, the muscles were pulled to one side and cut, until 

 the optic nerve could be seen. This was then cut by means of 

 a fine pair of scissors. The total loss of blood was small and the 

 animals quickly recovered. Out of eleven individuals operated 

 upon only the first three died. The others after a few hours 

 were active and seemed quite as fit for experimentation as nor- 

 mal animals. One effect of cutting the optic nerve, which was 

 not always observed, was a slight enlargement of the pupil. 

 Fick ('91, p. 3) states that after cutting the optic nerve of the 

 frog the pupil is temporarily somewhat narrowed. 



