2. Endocrines and Populations 251 



even may hypertrophy in infectious disease or other circumstances which 

 demand increased phagocytosis of particulate matter. 



i. Splenic weight. The lymph follicles of the spleen also are involuted by 

 corticoids. A decrease in splenic weight is sometimes used to indicate in- 

 creased adrenocortical secretion. However, contraction of the splenic cap- 

 sule further decreases splenic weight. This may be a valid procedure in the 

 laboratory with injected hormones, particularly in hypophysectomized or 

 adrenalectomized animals, but often may not be a valid indication of in- 

 creased adrenocortical activity, especially in intact small mammals. Voles 

 (Microtus agrestis) and house mice, and possibly other species, exhibit a 

 marked splenic hypertrophy when exposed to stimuli which cause in- 

 creased pituitary-adrcnocortical activity (Clarke, 1953; Chitty, 1957; 

 Christian, 1959c). Ln-olution of the lymphoid follicles occurs in these 

 circumstances as might be expected, but the decrease in weight from this 

 cause is overridden by the hypertrophy resulting from congestion and 

 markedly increased hematopoietic activity. The increased hematopoiesis in 

 these animals is accompanied by a reticulocytosis of the circulating blood. 

 The cause of increased hematopoiesis in these animals has not been demon- 

 strated. 



j. Lymphocijte counts. After an alarming stimulus there is an immediate 

 rise in the number of circulating lymphocytes, which is then followed by a 

 characteristic lymphopenia and eventually a return to normal levels. The 

 initial rise in circulating lymphocytes probably is at least in part due to 

 release of lymphocj-tes from lymphoid tissues in response to an initial re- 

 lease of epinephrine, but this release by no means accounts for the entire 

 rise (Gordon, 1955). The prolonged phase of lymphopenia which begins in 

 the early stages of the increased pituitary-adrenocortical activity probably 

 results from the destruction of medium and small-sized lymphocytes within 

 the lymphatic organs by the corticoids (Gordon, 1955) . However, there is 

 also some reason to believe that lymphocytes may migrate to depot situa- 

 tions. Over a prolonged period the main cause of the lymphopenia appears 

 to be due to the reduced amount of lymphoid tissue and decreased lympho- 

 cytopoiesis with a decreased delivery of lymphocytes into the circulation. 

 However, with small amounts of adrenal factors there may be an acutal 

 hyperplasia of lymphoid elements, so that interpretations must be made 

 with caution. 



Lymphocyte counts as criteria of stress in wild mammals are subject to 

 the same criticism as other measurements made on the living mammal : the 

 response is rapid enough and the counts labile enough so that there is 

 real danger that counts may reflect alarming stimuli induced by handling, 

 thus masking any other effects which may be the main point of the study. 



