198 CELLULAR METABOLISM 



such a circulating substance is suggested by the observation that the re- 

 placement of a substantial part of the blood of a non-irradiated rabbit by 

 blood of an irradiated rabbit, followed by administration of labeled phos- 

 phate, leads to the formation in the kidneys of desoxyribonucleic acid of 

 lower P^^ content than that formed in controls (24). 



In experiments on human beings, when a dose of x-rays was given to 

 two small areas of skin separated by a varying width of untreated pro- 

 tected skin, the reactions produced were significantly larger than that 

 over a single area of skin exposed to the same dose of radiation. The 

 length over which the hypothetical substance responsible for indirect 

 radiation effect diffuses was found to be 2 cm (100). 



Jones (20) irradiated rat muscles by injection of radioactive yttrium 

 colloid interstitially and observed a significant depression in the incorpo- 

 ation of administered P^^ both into the hver and into the tumor. The 

 effect obviously must have been indirect as the liver and tumor were 

 unirradiated. He found the rate of P^^ incorporation into desoxyribo- 

 nucleic acid of the liver to be depressed by 0.28 per cent per roentgen and 

 by 0.16 per cent per rep after whole-body x-ray irradiation and specific- 

 muscle beta-ray (yttrium colloid) irradiation, respectively. The first- 

 mentioned figure compares well with the effect of whole-body x-ray 

 irradiation on the depression of white-cell count (0.23 per cent) or red- 

 cell formation (0.3 per cent per r). 



There have been many reports of the indirect effects of radiation. The 

 x-ray dose when the tumor was directly irradiated sufficed to produce 

 regression of 75 out of 100 Jensen rat sarcomas and was found to be 

 effective in 35 cases out of 100 when only the bed of the sarcoma was 

 x-rayed (22a). 



Van Dyke and Huff have shown that when a parabiot is irradiated with 

 900 r, one member being shielded, both become depilated, and the non- 

 irradiated member is relatively more depilated than the other (36a). 



Protection 



Protective substances can reduce radiation effects in vivo as well. 

 Thioglycolic acid was found to be very effective in protecting bacterio- 

 phage Ta from the effects of irradiation (37). When cysteine or thio- 

 glycolic acid was added to the nutrient solution of Propioni-hacterium 

 pentosaceum and the solution then irradiated, the radiation damage was 

 found to be only one-half of that observed in controls (38). The growth 

 rate and surviving percentage of Allium cepa roots are 2-3 times as 

 great if the irradiation with 500 r takes place in the presence of cysteine 

 (137). Mice are significantly protected by thiourea or dithiophos- 



