NEUTRON EFFECTS ON ANIMALS 13 



The survival of some cells after a large dose of radiation may be due to 

 the capacity to repair injuries which is different in different organisms or the 

 radiation quanta may not affect certain of the cells in their sensitive spot. 

 Usually organisms irradiated with a lethal dose survive for a period and 

 then die. The tissues are capable of repairing the injury produced when 

 the dose is small and so it may be counteracted, but when the dose is large 

 enough, the repair processes produce a negligible effect upon the total 

 amount of injury, so that at high intensities, the biological effect of radiation 

 is independent of the intensity. 



In all these considerations of radiation effects which are those of X-rays 

 and T-rays, the cell is chiefly imagined to be a fixed organization with uni- 

 form structure. Nothing is farther from the truth. It is a complicated, 

 dynamic system with many contributng factors which are necessary to its 

 life, and these are in delicate balance or equilibrium. To speak glibly of 

 absorpton of quanta of energy by the cell is like saying another dynamic 

 system, your automobile, won't run because it has absorbed some energy 

 due to an injury. Its defect may occur in dozens of places and, as we know 

 more about the mechanism of the automobile than we do of that more 

 complicated, dynamic system, the cell, we can explain the course of the 

 injury to it. We can then find a cracked spark-plug or some other defect, 

 but as to the cell, we don't know enough about the mechanism to say what 

 happens, and to say that energy quanta are absorbed only befogs the ques- 

 tion by giving a name to a process which is not understood, and, if repeated 

 enough, has the effect of obscuring speculation into the mechanism. 



Certain generalization, however, may be made. Immature cells and 

 dividing cells are more easily affected by radiations in general. The effect 

 upon the chromosomes is striking, but it may be only one of the ways in 

 which biologic material responds to radiation. Muller himself warned 

 that "not all the effects of radiation in killing organisms or disturbing their 

 development are referable to changes either of the class of gene mutations 

 or chromosome re-arrangements". 



On highly differentiated cells the effect of injury from radiation is of 

 such character as that resulting from other injuries or chemical poisons. 

 When the blood supply is restricted or inhibited by radiation, it has even 

 been suggested that radiation effects on a complex tissue are the results of 

 the action on the circulation, but later studies disproved this. For the 

 pathologist to reconstruct the mechanism of action of radiation from the 

 dead, fixed tissue under the microscope is impossible, and the pathologic 

 picture is in no fashion specific, for it can be produced by other injury or 

 poisons. 



The lymphocytes of the blood are highly susceptible to radiation, and if 

 a very large neutron dose is given they disappear; with a smaller dose some 

 of them disappear; if the dose is not great they recover their former num- 



