The Action of Thyroid upon tlie Organs of the White Rat 247 



animals, and attributed it to an inhibition of the glycogenic function of 

 the liver and not to an increased utilisation of carbohydrates. In light 

 of the present observations the disappearance of glycogen may perhaps 

 be ascribed simply to the increased rate of metabolism induced by thyroid. 

 It is further possible that the increased j)roduction of adrenalin, already 

 described in the suprarenals, ma}' also exercise its influence on carbo- 

 hydrate metabolism. 



6. The Spleen. — The spleen is another organ which shows great 

 variations in size in the white rat. Donaldson includes in his series 

 the weights of small spleens only, and rejects data from animals with 

 so-called " enlarged spleens." In my animals the small spleen is rarely 

 met with ; the usual spleen is nearly twice as large as that of the Wistar 

 Institute rat. Hoskins noted the same diflerence in his rats, and questions 

 the reliability of Donaldson's tables for the weight of the normal rat's 

 .spleen. Jackson (14) tinds that the weight of the spleen varies directly 

 with that of the liver, and looks for a close correlation between the two 

 organs. My rats show a certain tendency to conform to this rule but 

 b}^ no means constantly. In neither male nor female control rats is the 

 heaviest liver accompanied by the heaviest spleen, or the lightest liver 

 by the lighest spleen. On the contrary, among the male control animals 

 the lightest liver is associated with the second heaviest spleen, and the 

 same is the case among the female rats. 



Thyroid-feeding as a general ruje increases the weight of the spleen, 

 but not to any great extent. Per 100 grm. body weight the average 

 increase in weight of the spleen in the thyroid-fed animals is 39 per cent., 

 that of the females 22 per cent. Per body length the increase is 35 per 

 cent, in males, and 27 per cent in females. Considerable irregularities 

 occur in the tables, and the increase in weight of the spleen of the thyroid- 

 fed animals is not a constant phenomenon. The lightest spleen recorded in 

 the series is in a thyroid-fed female. No. 2, Table VIII. ; this, of course, may 

 be one of the small spleens, but the control animal from the same litter has 

 a much larger spleen. This animal also aftbrds an exception to the general 

 rule that the female possesses a relatively heavier spleen than the male. 

 Further, the small spleen in this animal is associated with a large liver. 



Hoskins obtained an increase in weight of the spleen in his thyroid-fed 

 rats, and also noticed great irregularity in the amount of enlargement. My 

 results are sufSciently like his to confirm the opinion that thyroid-feeding 

 does enlarge the spleen. The enlargement, however, is not such as to point 

 to any special action of thyroid upon it, but rather to a general effect 

 resulting from the increased rate of metabolism induced by thyroid-feeding. 



7. The Thymus. — The weight of the thymus was only measured in 

 the female series of rats. In the male animals, which were rather older 

 as a group than the females, the thymus was small and almost impossible 

 to separate accurately from fat and connective tissue. The impression 

 reached by mere examination was that thyroid-feeding hastens the involu- 



