X-IRRADIATION STUDIES ON THE MAMMALIAN RETINA 



555 



10,000 rads. the b-\vave amplitudes were about 30^ r that of preradiation 

 control ( Fig. 12). The eyes were removed 3 days after the last dose. None 

 showed visual cell death, although the total dose delivered was almost twice 

 the cytocidal one, as detennined with a single dose. There were, however, 

 widespread signs of rod degeneration ( Fig. 3 ) , which presumably were 

 responsible for the b-wave reduction. 



This b-wave reduction was associated with a proportionately greater 

 a-wave reduction, illustrated (Fig. 12) by the increase in the ratio between 

 b-wave and a-wave amplitudes (b/a ratio). Preferential a-wave reduction 

 seems to be characteristic of rod degeneration; it has been described for 

 rod degeneration following intravenous injection of iodate. (Noell, 1958) and 

 during severe vitamin A deficiency (Dowling and Wald, 1958). Measured 

 by the b/a ratio, it would appear that rod degeneration occurred in our 

 fractionation experiments after about the same total dose which was effec- 

 tive in a single application. Without doubt, fractionation influenced the 

 effectiveness of irradiation with respect to rod degeneration much less than 

 with respect to visual cell death. This degeneration, however, may not be 

 caused by direct action of x-radiation on the rods; it is equally probable 



b-WAVE (1 = 1) 



b- WAVE (1 = 10"') 



"Vo RATIO(I = l) 



IRRADIATION 



PRE 1 2 



DAYS 



20 



30 



RADS (MEDIUM RETINAL DOSE) 



Fig. 12. Effects of irradiation applied intermittently. With a single irradiation, 460 

 rads (250 kvp) are delivered in terms of medium retinal dose. The b-wave is measured 

 for responses to 2 different intensities (1^1, 1=10"^). 



