926 RADIATION BIOLOGY 



toxicity (Raper, 1947; Bond et al., 1950). It is also apparent that 

 different mechanisms contribute to lethality, depending upon the area 

 that is irradiated. 



The severity of cutaneous erythema is related to the size of the radia- 

 tion field, and two fields a distance apart show less injury than areas that 

 are closer together (Jolles, 1941; Jolles and Mitchell, 1947; Jolles, 1950). 

 Partial irradiation of the frog's corneal epithelium with 3600 r results in 

 the same over-all inhibition of mitotic division as total irradiation of the 

 cornea with 900 r (Strelin, 1950). Thus, injury and recovery are, to some 

 extent, dependent upon chemical interaction between the irradiated and 

 adjacent nonirradiated areas. There is also reason to believe that injury 

 to specific sites is more severe after a total-body exposure than after 

 local irradiation. This may be attributed to the liberation of non- 

 specific toxic materials from irradiated tissue and/or to a sparing action 

 of nonirradiated tissue. Although there is some evidence of active 

 circulating factors following irradiation, the significance of such humoral 

 agents is not fully appreciated (Barnes and Furth, 1943; Ahlstrom et al., 

 1947; Van Dyke and Huff, 1949; Lawrence et al, 1947; Ellinger, 1951). 

 Parabiosis, cross-circulation, and early transfusion have been shown to 

 diminish radiation toxicity (Barnes and Furth, 1943; Van Dyke and 

 Huff, 1949; Salisbury et al., 1951, Brecher and Cronkite, 1951, Swisher 

 and Furth, 1951). 



Shielding of relatively small volumes of tissues can decrease the 

 severity of an otherwise total-body exposure. For example, with the 

 spleen shielded and the remainder of the body irradiated, the 30-day 

 LD 50 for mice is increased from 550 to 975 r (Jacobson et al., 1949). Pro- 

 tection of the head, the extremities, or other small areas mil also diminish 

 mortality (Abrams and Kaplan, 1951; Gershon-Cohen et al., 1951; 

 Allen, 1951). Recovery of hematopoietic tissue is also more rapid after 

 subtotal irradiation (Boffil and Miletzky, 1946; Rekers, 1949; Jacobson 

 et al., 1951). The mechanism of these protective effects is poorly under- 

 stood; Jacobson et al. (1950) have suggested that the mesenchymal tissues 

 in certain shielded areas may supply a factor that facilitates regeneration 

 of blood-forming tissue. It is of interest, in this connection, that spleen 

 transplants or the injection of marrow suspensions or of spleen homo- 

 genates are effective in reducing radiation mortality in certain species 

 (Jacobson et al., 1951; Lorenz et al., 1951; Cole et al., 1952). In contrast 

 to protection by shielding is the synergism that results when certain 

 radioisotopes, which differ in localization, are administered in combina- 

 tion. When the reticulo-endothelial system of rats is irradiated with 

 colloidal Au 198 and the bone marrow with P 32 , lethality is potentiated 

 (Friedell and Christie, 1951). On the other hand, only an additive 

 effect is obtained when two bone-seeking radioisotopes are injected simul- 

 taneously (Salerno et al., 1952). 



