PROPERTIES OF THE RETINA. 311 



colorless. A similar pigment is found in the eyes of man and 

 the other mammalia. It has been shown, moreover, that a photo- 

 graph may be made upon the surface of the retina by means of this 

 purple. If the head of a rabbit or frog that has been kept in the 

 dark for some time is exposed with proper precautions to the light 

 of a window, for instance, the part of the retina on which the image 

 of the window-lights falls will be bleached, while the parts upon 

 which the image of the window-bars falls and the surrounding areas 

 of the retina will retain their red color. A figure of such a retinal 

 photograph or optogram, as it is called, is represented in the accom- 

 panying illustration (Fig. 133) . The visual purple has been extracted 

 from the rods by solutions of bile salts, this substance having the 

 power to discharge the pigment from its combination in the rods in 

 the same way as it discharges hemoglobin from its combination in 

 the red corpuscles. The solutions thus obtained are also bleached 

 upon exposure to light. We have in the yisual purple, therefore, 

 an unstable substance 

 readily decomposed or 

 altered by the me- 

 chanical effect of the 

 ether waves, and also, 

 it may be said, by 

 gross mechanical re- 

 actions, such as com- 

 pression; and there Fig l ^_ Optogfam in eye of rabbit: 1, The nor- 

 Can be little doubt mal appearance of the retina in the rabbit's eye: a, The 

 entrance of the optic nerve; o, b, a colorless stnp of 



that the Substance medullated nerve fibers; c, a strip of deeper color sepa- 

 , . rating the lighter upper from the more heavily pigmented 



plays an important lower portion. 2 shows the optogram of a window. 



part in the functional 



response of the rod elements. It has been shown that provision 

 exists in the retina for the constant regeneration of this red pigment. 

 It will be remembered that the external segments of the rods im- 

 pinge upon the heavily pigmented epithelial cells that lie between 

 the rods and the choroid coat. From experiments upon frogs' eyes 

 it appears that a portion of the retina detached from the pigment 

 cells and bleached by the action of light is not able to regenerate its 

 visual purple until again laid back upon the choroid coat. This 

 regenerating influence of the black pigmented cells may be con- 

 nected with another interesting relation that they exhibit. Under 

 normal conditions delicate processes extend from these cells and 

 penetrate between the rods and cones. When the eye is exposed 

 to light the black pigment migrates along these processes as far even 

 as the external limiting membrane, and it is possible that this ar- 

 rangement may be useful in obviating diffuse radiation of light 

 from one rod to another. When the eye is kept in the dark, however, 



