136 CM. JACKSON 



A maximum absolute weight of about 0.29 grams is reached by 

 the average normal thymus about the age of 85 days (Hatai 

 '14). At one year, it has undergone a complete age-involution, 

 and forms only 0.02 per cent of the body-weight (Jackson '13). 



That the weight and structure of the thymus are markedly 

 affected by various adverse conditions has long been known, and 

 the process of invoTution has recently been thoroughly investi- 

 gated by Hammar and his pupils. Jonson ('09) experimented 

 with young rabbits subjected to acute and chronic inanition. 

 In the latter case, the diet was restricted so as to maintain the 

 young rabbits at constant body-weight (similar to the present 

 experiment with rats). Under these conditions, Jonson found 

 the weight-curve of the thymus similar to that of the body-fat, 

 although during acute inanition the fat decreases somewhat more 

 rapidly. In young rabbits held at constant body-weight the 

 thymus in four weeks is reduced to about one-thirtieth of its 

 initial weight. The cortex suffers the greatest loss, being re- 

 duced to one-twelfth of its initial weight within two weeks of 

 chronic inanition at constant body-weight. It would therefore 

 appear that the process of involution is much more rapid and 

 complete in young rabbits than in young rats at the ages included 

 in the present investigation. In both cases, however, the weight 

 of the thymus in hunger involution decreases most rapidly in 

 the earlier weeks of the experiment. 



HEART 



The heart (table 15) in the albino rats held at constant body- 

 weight from the age of three weeks appears to have remained 

 practically constant at about 0.70 per cent of the net body-weight 

 up to the age of ten weeks. In absolute weight, the heart would 

 apparently decrease from 0.167 grams (0.170 grams, less cor- 

 rection for difference in body-weight) to 0.166 grams, a decrease 

 of about, 0.6 per cent, which is probably within the limits of 

 experimental error. The very slight decrease at six weeks and 

 increase at eight weeks are also probably not significant. Simi- 

 larly in the rats held at constant body-weight from six to thirtj^- 



