Growth and Senescence 



experiments, first described in 1934 (McCay and Crowell, 

 1934) extended over years, and are fully reviewed in retrospect 

 by McCay (1952). Groups of rats were reared on a diet suffi- 

 cient in all other constituents but deficient in calories, and their 

 growth thereby retarded. After periods of retardation up to 

 1000 days, the calorie intake was raised to permit growth. The 

 animals then grew rapidly to adult size, even though the 

 longest-retarded group had already exceeded the normal life 



100 

 90 

 80 

 70 

 60 

 50 

 40 

 30 

 20 

 10 



200 400 600 800 1000 1200 1400 



Fig. 40. — Survival curves of normal and retarded male and female rats, 

 showing the effect of dietary restriction (from McCay, Sperling and Barnes, 



1943). 



span for the strain, and continued to live to approximately 

 twice the maximum age reached by unretarded controls 

 (Fig. 40.) This long survival was accompanied by a decreased 

 incidence of many chronic diseases, which appeared to repre- 

 sent a true diminution in senile liability to death from random 

 causes. The chief specific diminution was in death-rate from 

 pulmonary diseases and from tumours. Tn general, the retarded 

 rat remains active and appears young whatever its chrono- 

 logical age. It is very alert. It tends to go blind in the second 



149 



