WEIGHT OF THYMUS GLAND OF RAT 255 



have been taken, so that in using the formulas it is important 

 to keep this fact in mind. There are several factors which may 

 contribute to irregularities in the weight of the thymus gland. 

 The most important of these, on account of the frequent occur- 

 rence under usual laboratory conditions, are the following : 



1. Pregnancy and nursing, which tend to decrease the weight 

 of the thymus gland prematurely. Whether or not the rat re- 

 covers from this alteration has not yet been determined. 



2. Lung infection, and poor growth of body, which are usually 

 associated with a small thymus. 



3. During the period of rapid growth (first 85 days) a heavier 

 rat has a relatively larger thymus than a smaller rat of the same age. 



Thus though the weight of the thymus is assumed to be a 

 function of age, and the foregoing formulas have been deter- 

 mined on such an assumption, the proper comparison of the ob- 

 served weight of the thymus with that calculated by the formulas 

 may require several corrections according to the condition of 

 the rat under investigation. For instance, if the observed weight 

 of the thymus according to the formula appears to be too large 

 or too small, the normality of the observed body weight with 

 respect to the age should be determined.^ 



If the given body weight deviates noticeably from the normal, 

 the normal age corresponding to the observed body weight 

 should be found. Using the age thus determined, the corre- 

 sponding thymus weight may be substituted for that directly ob- 

 served. There may occur still other instances in which the 

 observed weight of the thymus can not be directly compared 

 with the calculated value without making some adjustment neces- 

 sitated by special condition's. The above instance is given as 

 an example merely. 



As was noted before, the thymus gland possesses two distinct 

 peculiarities by which it is contrasted with the other organs; 

 (1) the weight of the thymus is correlated with the age of the 



^ For such purposes see Donaldson ('06) "A comparison of the white rat with 

 man in respect to the growth of the entire body." Boas Anniversary Volume, 

 New York. 



