1104 RADIATION BIOLOGY 



a latent period of 3 hours before damage is evident. At 50 r the only 

 injury is an increase in debris at 3 and especially at 8 hours which is not 

 confined to the nodules. Below 50 r no damage is observed. 



In different species the lymphatic tissue undergoes the same degree of 

 damage at a particular dose level, regardless of the lethality of that dose 

 for the species. 



Equivalent doses of fast and slow neutrons and y rays produce the 

 same histological changes in the lymph node as do X rays. 



Repeated doses of 80 r of total-body X irradiation to mice for five weeks 

 apparently have no cumulative or sensitizing effect on the lymph node and 

 produce no changes in the amount of lymphatic tissue or in the number 

 and size of active nodules. 



Repeated doses of external y rays to mice, 8.8 r per day, produce 

 changes in the lymph node only when the irradiation is continued for 

 10 months or more (Spargo et al., 1951). Lymphocytopoietic activity 

 decreases, the nodules becoming smaller and fewer until they disappear 

 completely, and, concomitantly, transport of small lymphocytes through 

 the cortical sinuses diminishes until there is none at 16 months. Lower 

 doses, 1.1 or 4.4 r per day, even when continued for 16 months, cause no 

 significant changes. 



The changes in the lymph node after the internal administration of 

 radioactive isotopes depend on whether the isotopes lodge in or near the 

 organ. When present in sufficient amounts they produce the same 

 qualitative changes as those noted after external irradiation. A quanti- 

 tative comparison of effects of external and internal irradiation is more 

 difficult, because of the uneven distribution of the isotopes in the various 

 organs and tissues, the varying rates of excretion from the body, and the 

 differences in their half-lives. 



The intestinal lymphatic tissue undergoes the same histological altera- 

 tions as the lymph node after external and internal irradiation. 



THYMUS 



The thymus consists of a framework of reticular cells — mainly of 

 endodermal but also of mesenchymal origin — whose meshes are filled 

 with dense masses of small lymphocytes in the outer (cortical) portion of 

 the organ and with smaller numbers of small lymphocytes and a few 

 medium-sized ones in the inner (medullary) portion of the organ. The 

 thymus gradually "involutes" with advancing age, losing lymphocytes 

 and some stromal cells. 



Studies of the thymus after X irradiation reveal a loss in size of the 

 organ, owing mainly to destruction of lymphocytes. In the rabbit, the 

 effects of a single dose of 800 r total-body X ray (Murray, 1948b) include: 

 (1) a destructive phase (1 to 8 hours), during which most of the lympho- 



