454 



RADIATION INJURY AND LETHALITY 



tion, called the cumiilant lethality function, Cl, and tj and ^o are the 

 survival times of the representative exposed and control animals, respec- 

 tively. The Cl for ABC mice is presented in Fig. 7. It shows three 

 distinct branches. The impulse lethality function, Sl, is obtained by 

 differentiating Cl (Fig. 8) and shows two pronounced peaks of phasic 

 injury and a final constant branch. This final branch can be shown 



0.0012 



0.0010 - 



0.0008 - 



^ 0.0006 



^ 0.0004 



0.0002 



100 150 



Time, days 



200 



250 



Fig. 8. Impulse lethality function, Sl, for ABC male mice. Obtained by numerical 

 differentiation of Cl (Fig. 7). The horizontal lines give the mean derivative values 

 estimated between adjacent dose groups, and the continuous curve indicates the 

 nature of the function. The peaks of lethality at 10 and 40 days correspond to ob- 

 served peaks of kiUing by single doses of x-rays. 



to persist as late as 600 days (data of Lorenz; see Fig. 9). The two 

 peaks of injury at 10 and 40 days correspond to the observed peaks of 

 killing following single massive doses of x-rays. 



Survival under daily exposure has been studied by several investi- 

 gators for several species. The cumulant lethality functions for these 

 species, wdth correction for normal aging, are shown on logarithmic scales 

 in Fig. 9 (34). We see that the initial rise is nearly identical for all 

 species and has a slope of 1.8. This initial rise is presumably relatively 

 uninfluenced by recovery, and, since all species appear to be described 

 by a common line, there is a suggestion that their differences in sensi- 

 tivity are due almost entirely to differences in the capacity to recover, 

 which appears in the time order in which they fall away from the branch 

 of unretarded accumulation. Following the plateau appears the final, 

 late branch, which appears to have unit slope and which may be thought 



