244 NORMAL HISTOLOGY. 



The pigmented epithelium of the retina consists 

 of large polyhedral cells set together side by side, 

 and forming a continuous layer over the distal ends 

 of the rods. As seen from the side, as we have 

 already studied them, p. 30, they appear like flat 

 pentagonal or hexagonal plates, but when seen in 

 profile while in situ, it is evident that they send 

 down long, slender pigmented processes between 

 the rods. 



The outer portion of these cells, that next to the 

 choroid, usually contains the nucleus and but little 

 pigment, while the amount of pigment in the pro- 

 cesses seems to depend upon whether the eye had 

 been exposed to light or not immediately be- 

 fore death. For it has been recently shown that in 

 some animals the pigment particles under the influ- 

 ence of light can move within the narrow cell- 

 processes so as to be now collected within the 

 inner portion of the cell-body, and again grouped 

 in larger and smaller masses between the rods. 

 In this movement the pigment particles are them- 

 selves passive, the change in position being due 

 to active movements in the protoplasm, induced 

 by light. 



The larger arteries and veins ramify beneath the 

 internal limiting membrane in the layer of nerve- 

 fibres, and from these blood is distributed outward 

 to all the layers, as far as to the external nuclear 

 layer, beyond which no blood-vessels are found. 



