Biological Effects of Penetrating Radiations 99 



as a whole. Such studies on the response of normal tissues to 

 radiation are not only of interest and importance in themselves, 

 but also because of the information they give concerning the 

 amount of radiation that the healthy body or organ can tolerate. 

 Unless healthy tissue were able to tolerate a greater quantity 

 of radiation energy than diseased tissue, penetrating rays would 

 be of little use in radiotherapy. 



In general, biological indicators show a response which is 

 independent of the wave length of radiation but dependent on 

 the intensity, while the mutation effect, though dependent on the 

 wave length, is independent of the intensity. The biological 

 effects now to be considered vary with alteration in both the 

 intensity and the wave length of the irradiation to which they 

 are exposed. 



Radiation affects any given cell of a complex tissue in at least 

 two ways, first by a direct action on the cell, and secondly by 

 injuring neighboring tissues upon the health functioning of 

 which the cell depends. 



The term "indirect effect of radiation" conveniently describes 

 all the effects of radiation except its direct action on the cell, 

 but it has by custom come to be restricted to those effects 

 produced as a result of injury to the blood supply. This quite 

 arbitrary and rather unfortunate limitation of a useful term 

 requires another to describe the consequences of the action of 

 radiation upon remote tissues and body fluids. For this the term 

 ''constitutional effects of radiation" is now reserved. 



When blood supply is restricted or inhibited by radiation the 

 results are so conspicuous ^- that it is not surprising, perhaps, 

 that they should at one time have practically monopolized atten- 

 tion. It has even been suggested that all the radiation effects on 

 a complex tissue are the results of the action on the circulation. 

 This view is easily refuted, however, by reducing the radiation 

 dose below the level which affects the blood supply, when the 

 direct effects of the radiation can be seen, unmasked by injuries 

 caused from lack of blood. Alternatively, the role of the blood 

 supply can be demonstrated by irradiating embryos in ova before 

 and after the establishment of the circulation and comparing 



