CHAP, in.] SIGHT. 1207 



shews that the cells possess peculiar features absent from the 

 simpler epithelium beyond the ora serrata. Each cell is seen to 

 consist of a more or less cubical body, the outer part of which 

 next to the choroid is free from black pigment, though often 

 containing small irregular masses of yellowish material of a fatty 

 nature allied to the medulla of nerves. The inner part, that next 

 to the retina, is on the other hand loaded with black pigment 

 granules, which on closer examination are found to be minute 

 crystals ; the substance of which these are composed is called 

 fuscin. The nucleus lies either wholly in the clear part or 

 partially imbedded in the pigment. 



The most important feature of the cell however is that the 

 inner surface next to the retina, is not smooth and even, but 

 frayed out into a number of rod-like or filamentous processes, 

 each carrying a load of pigment crystals. Further when eyes 

 are subjected immediately before death to different conditions 

 as regards light, these processes are found in different states. 



If an eye be fully exposed to light before removal and 

 examination, the processes carrying pigment are found to stretch 

 a long way inwards between the outer limbs of the rods and 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 con- 

 dition the pigment is carried back to the body of the cell and the 

 nucleus becomes obscured. Further, while various 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, 



