Chap, hi.] SIGHT. 925 



cones, investing these outer limbs with a sheath of pigment, and 

 even reaching between the inner limbs. If on the contrary the 

 eye be kept in the dark before removal and examination, the 

 processes are found to be short and to stretch a little way only 

 inwards, not reaching much farther than the tops of the outer 

 limbs of the rods and cones. The substance of the cell has in 

 fact the power of amoeboid movement, at one time throwing out 

 long filamentous processes inwards between the rods and cones, 

 and at another time retracting the processes into the body of the 

 cell. As they move to and fro these processes carry with them 

 the crystals of pigment with which they are studded ; hence in 

 the extended condition much of the pigment is carried away 

 from the body of the cell inwards between the rods and cones, 

 leaving the nucleus less covered with pigment, while in the 

 retracted condition the pigment is carried back to the body of 

 the cell and the nucleus becomes obscured. Further, while va- 

 rious circumstances may determine whether the processes are 

 extended or retracted, the falling of light on the retina has the 

 most marked and potent effect. When light falls on the retina 

 the processes hurry inwards and envelope the outer limbs of the 

 rods and cones with pigment ; when the light is shut off from 

 the retina the processes carry back the pigment to the body of 

 the cell. 



Hence in an eye exposed to light the processes and pigment 

 being largely jammed in between the outer limbs of the rods, and 

 these outer limbs at the same time swelling, the pigment epithe- 

 lium adheres closely to the retina, and when the retina is 

 removed is carried away with it. In an eye kept in the dark, 

 the processes being withdrawn, and the outer limbs of the rods 

 shrinking again, the attachment of the retina to the epithelium 

 is much less, and the retina can be more readily removed so as 

 to leave the pigment epithelium adherent to the choroid. 



Urari has an effect on these cells of the pigment epithelium 

 of such a kind that they cease to throw out their processes ; 

 they seem to be paralyzed. Hence in the eye of a urarized 

 animal the pigment epithelium readily separates from the 

 retina. 



We may add that in frogs at least, this shifting of the pig- 

 ment may be seen to be accompanied by a change of form in the 

 inner limbs of the cones. Under the influence of light the inner 

 limb becomes shorter and broader, in fact contracts, and when 

 the influence of the light is removed elongates to its original 

 length. Moreover these changes in the cones may be induced, 

 not only by light falling on the retina but also, through a 

 mechanism not at present fully understood, as the result of 

 stimulation of the skin, by light or otherwise ; in these latter 

 cases the change of form of the cone is not necessarily accom- 

 panied by migration of the pigment. 



