X-IRRADIATION STUDIES ON THE MAMMALIAN RETINA 549 



recorded by an inkwriter in response to brief flashes of lieht delixered con- 

 tinuously at 0.5-1 per sec. In the experiment from which Fis'. 5 is taken, the 

 eyes were exposed to 2.000 k\p radiation. No significant changes occurred 

 between the 1st and the 14th minute of exposure, but shortly thereafter the 

 ERG ol the eye at the point ot entry of the .x-rays close eye i suddenly besan 

 to fall in amplitude. At 17 minutes it was less than half the size recorded 3 

 minutes earlier, and at 19 minutes it had become very small. At about this 

 time, the ERG of the distant eye diminished in amplitude at a rate almost as 

 fast as that of the close eye. 



The disappearance of the ERG. which we belie\e is a manifestation of 

 visual cell failure, was associated with extinction of the optic nerve response 

 (Fig. 6), proving that the ERG decline indicated the failure of retinal 

 excitability to light stimulation. 



The sudden ERG decline, graphically depicted in Fig. 7 (200 kvp ) and 

 Fig. 8 ' 250 kvp I . was consistent with reeard to dose recjiiired and time of 

 occurrence. It needed a precise minimal dose, and it started with a time lag 

 of no more than 5 minutes alter the application of this close at a high dose 

 rate. Higher total doses, delixered at the same rate, caused this decline to 

 appear only slightly earlier. The amount of close in excess of the minimum 

 one — expressed in terms ot the dose delixered to one of the reference points 

 used for dosage exaluation — determined the le\el to which the decline pro- 

 ceeded in accordance with the distribution of absorbed dose over the retina. 

 It is clear that one is dealing with effects on a homogeneous population and 

 with doses of maximal efTectiveness. 



The ERG continued to fall slowly when the sudden change had not re- 

 sulted in its disappearance ( Fig. 9 ) . Similarly, when the dose was below that 

 required for the sudden ERG decline, a slow fall in ERG dexeloped which, 

 however. ne\cr lead to a striking amplitude reduction. A significant ERG 



2000 kvp. 



24' 



Fig. 6. ERG (upper tracing) and optic tract potential (lower tracing) in response 

 to an intense flash during 2.000 k\"p irradiation. The optic tract response disappears 

 slightly later than the b-\va\e. 



