496 Lymph Nodes, Thymus, and Spleen 



mal rats causes a marked weight increase in lymph nodes and spleen, but not 

 in the thymus. Testosterone propionate and estradiol dipropionate have only 

 moderate reducing effects on the weight of the lymph nodes as contrasted with 

 the usual marked involuting effects on the thymus gland. Interesting is the 

 fact, however, that testosterone propionate will prevent the increase in the 

 weight of lymph nodes which follows castration of the male rat (Reinhardt 

 and Wainman"). Other preliminary experiments indicate that anterior pitui- 

 tary growth hormone will cause increase in weight of thymus and lymph nodes 

 in both normal and hypophysectomized animals, but will not produce en- 

 largement of lymph nodes of the hypophysectomized rat when the food intake 

 is restricted to the level of untreated hypophysectomized animals. This places 

 further emphasis on the role of nutritive substances in the maintenance of the 

 normal amounts of lymph-node tissue in the body. Further experiments are 

 planned to show the role of certain vitamins in this respect. 



Summary 



(i) NovTiial Growth and hivohition. Studies of the weights of thymus, spleen, 

 cervical and mesenteric lymph nodes in a series of 284 rats of both sexes and 

 of various ages (birth to 7 months) have been used as the basis for the con- 

 struction and comparison of the curves for growth in weight of these- organs. 

 The cervical and mesenteric nodes have a period of growth and involution 

 in weight which is comparable to that of the thymus and spleen with certain 

 exceptions which are discussed. The absolute-growth curves for male and 

 female animals for the thymus, cervical and mesenteric nodes are generally 

 the same in form. The maximum absolute weight of the thymus is reached 

 at about the age of 40 days, of the spleen, cervical and mesenteric nodes be- 

 tween the ages of 60-100 days. After the attainment of maximum absolute 

 weight there is a decline in weight which is most rapid for the thymus and 

 least for the mesenteric nodes. Maximum relative weights are attained for 

 the thymus, spleen, and mesenteric nodes at about 35 days of age (25-50), 

 ^vhereas the cervical nodes attain their maximum at about 100 days. There is 

 a rapid decline in the relative weight of the thymus and a less rapid decline 

 in the relative weights of the spleen, cervical and mesenteric lymph nodes. One 

 may speak of a weight increase and a weight involution, of a greater or lesser 

 degree, depending on the organ concerned, over the period of the life span 

 of the rat here studied. 



(2) Reaction of Lymph Nodes, Thymus, and Spleen to Fasting, and Expo- 

 sure to Cold and Certain Hormonal Stimuli. Employment of quantitative 

 measurements of changes in weight of the above structures demonstrates that 

 the rapid weight involution which characterizes the reaction of the thymus to 

 fasting and cold is also found to occur in the case of the spleen and lymph 

 nodes. Certain hormonal stimuli which are known to enlarge or decrease the 

 size of the thymus are discussed as exerting a generally comparable effect on 

 lymph nodes and spleen. 



