74 



H. S. SiMMS, B. N. Berg, and D. F. Davies 



the onset of lesions (of a non-infectious nature) in relation to 

 age and in relation to mortality and longevity. 



Fig. 2 shows the percentage of male rats that showed detect- 

 able lesions of five selected diseases (Simms and Berg, 1957). 

 Each curve is an S -shaped curve approaching a maximum 

 level of incidence. Two of these curves approach 100 per cent, 

 whereas the other three approach lower levels of incidence. 



It will be noted that the curve for chronic nephrosis and 

 glomerulonephritis and also the curve for myocardial degener- 

 ation are spread over a wide age range. This means that onset 



100 



100 200 



AGE IN DAYS 



300 400 500 600 700 B05 900 iooo" 



1100 



Fig. 2. Percentage of rats having detectable lesions 



of five major diseases, plotted against age at autopsy 



(From Simms and Berg, 1957). 



of these lesions was observed in some rats at an early age but 

 in other rats it was not seen until they were much older. On 

 the other hand, onset of muscular degeneration occurred in all 

 rats within a narrow age range (lasting only about 500 days). 

 In the majority of the rats the onset of detectable lesions of 

 this disease occurred between 700 and 900 days of age. 

 Hence our rat colony was quite homogeneous in regard to this 

 disease — but much less homogeneous in regard to other 

 diseases. 



Fig. 3 shows the slope of these curves at various ages. Each 

 curve in this chart represents the age distribution of onset of a 

 disease. For example, muscular degeneration (Berg, 1956) had 



