ROD VISION 79 



as proving conclusively the complete dependence of rod vision upon rho- 

 dopsin. But workers in the same laboratory have more lately obtained 

 puzzling indications that very little rhodopsin is ever normally bleached 

 in the intact animal. They found apparently normal amounts of it in 

 eyes whose electrical responses had been reduced one-third to one-half 

 by stimulation with light. Possibly the electrical responses would entirely 

 disappear while there was still a great deal of rhodopsin in the rods. 

 This might be new evidence that rhodopsin is a secondary sensitizer 

 rather than a primary photosensitive material, or it might only mean 

 that switchboard effects in the retina are more important in light-adapta- 

 tion than we have been supposing. 



Whatever its whole meaning may be, rhodopsin was a clever invention; 

 for its light-absorbing power makes it responsive to weak light, yet it 

 conveniently bleaches when, in bright light, the full amount of it would 

 greatly handicap the animal. Even the particular color it possesses is in 

 itself adaptive, as will be elucidated later (Chapter 12, section A). 

 So elaborate a substance could hardly have been present in the 'original' 

 provertebrate visual cell, which must then have been high-threshold, more 

 like the cones we know than like a modern rod. Some of the photosensi- 

 tive ancestor-cells of the rods and cones were left behind in the brain 

 lining when the eyes evolved, as will be brought out in the next chapter. 

 These, though sensitive enough to respond to light through the entire 

 wall of a bird's head (as shown by their reflex control of spermatogenic 

 activity), contain no rhodopsin as far as we know. If the modern rod 

 cell depends utterly upon rhodopsin for its photosensitivity as such, it 

 has come to do so secondarily by discarding some more ancient photo- 

 chemical for want of efficiency under scotopic conditions. 



Rod Vision — We may conceive of the peripheral (ocular) portion of 

 the rod visual process as taking place somewhat as follows : At the start 

 of adaptation to dim light there is little rhodopsin in the rods, and so 

 little of this is broken down by the weak light that only feeble impulses 

 pass down the foot-pieces. As the amount of rhodopsin increases, a 

 greater absolute amount is broken down by a given light and the im- 

 pulses become stronger. Those bipolars with which the largest numbers 

 of rods connect now receive enough total stimulation to be set off into 

 conductive activity, and they begin to carry nerve impulses at a certain 

 low frequency of discharge — each bipolar acting somewhat like a reser- 

 voir and, so to say, filling up with stimulation and discharging an im- 



