186 NEUTRON EFFECTS ON ANIMALS 



45th Day. The spleen appeared to be normal. 



113 n. No microscopic changes in the spleen were observed immediately 

 after irradiation. 



1st Da.y. The Malpighian corpuscles were small with a few foci of poly- 

 morphonuclear leukocytes present in the surrounding pulp (Fig. 23). 



2nd Day. The Malpighian corpuscles were markedly atrophied (Fig. 24). 



4th Day. The Malpighian corpuscles were atrophied, the sinuses were 

 collapsed and numerous monocytes were observed to contain blood pig- 

 ment. 



8th Day. Atrophy of the Malpighian corpuscles, collapse of the sinuses 

 and a few scattered foci of hematopoiesis were present (Fig. 25). 



11th Day. The Malpighian corpuscles were somewhat larger than on 

 the 8th day and there were foci of hematopoiesis. Numerous monocytes 

 containing blood pigment were present. 



16th Day. Regeneration of the lymphoid tissue was we'l advanced. 

 Numerous polymorphonuclear leukocytes were present in the surrounding 

 tissue (Fig. 26). 



24th Day. The ^Malpighian corpuscles were normal in size with active 

 cellular mitosis present. Numerous polymorphonuclear leukocytes and 

 monocytes remained. 



32nd Day. The Malpighian corpuscles were normal in appearance. 



A few polymorphonuclear neutrophils were present. Sections subse- 

 quent to the 32nd day appeared to be normal. 



Discussion. Atrophy of the Malpighian corpuscles, often approaching 

 almost complete obliteration, and collapse of blood sinuses, as shown by 

 histological examination, probably are responsible for the decreased weight 

 of the spleen after irradiation, since the maximum histopathological injury 

 occurs at approximately the same time as the maximum reduction in 

 weight. Other studies (8) demonstrated that variations in the number of 

 lymphocytes in the circulating blood correspond in time to the changes 

 reported here in the amount of lymphoid tissue in the spleen. 



Evidence of severe injury to the spleen by neutron radiation appears 

 much more promptly than in the testes. In neither case is it certain how 

 much of the effects are direct and how much indirect. It may be pre- 

 sumed that since lymphoid tissue is, pei-haps, the most radiosensitive tissue 

 in the body, a greater proportion of the effects in the spleen is direct than 

 in the testes. 



Liver and Kidiieys 



The livers of rats of several groups used in studies of weight changes of 

 spleens and testes were weighed and examined histologically. No sig- 

 nificant change was found in the ratio of animal weight to liver weight 

 (Table I, Groups 6-10). 



Histological Changes. The livers of 80 male rats which had been given 

 56.4 n in Box No. 7 were examined for gross pathological and histopath- 



