Effects of X-Rays and Atomic Radiations on Living Cells 59 



tains roughly 3 x 10" water molecules,^ so that one roentgen (1 r) 

 ionizes only one water molecule in every 3x10^° (i.e. 30,000 million). 

 The primary ions formed have a very short life and change into re- 

 active substances called radicals (often H or OH in water) which are 

 capable of producing chemical changes in other molecules present. 



There is no doubt that all ionizing radiations have a very damaging 

 effect on all living cells. In the case of a rat the average lethal dose is 

 about 700 roentgens. The lethal dose for a human being is probably 

 less, not more than 400-500 roentgens over the whole body. We can 

 easily calculate how many ionizations a typical rat cell receives with 

 this lethal dose. The volume of a typical cell (taken as a sphere about 

 10"^ cms. across) will be about 5 X 10"" cubic centimetres (i.e. about 

 2,000 million to 1 cubic centimetre). From this it follows that the 

 lethal dose, viz. 700 roentgens, will give rise to 350,000 ionizations^ 

 in every typical cell. This may seem to be quite a large number, but 

 it is small compared with the total number of protein molecules pre- 

 sent in such a cell. 



Careful studies have shown that only a small proportion of the 

 protein molecules in the cell are damaged by a lethal dose. The reason 

 for the death of the animal in such cases is still unknown — it may be 

 that there are cell constituents which are both essential to the life of 

 the cell and are also damaged by very small doses of radiation, but 

 they have not been identified with certainty. Some organs in the body, 

 such as lymphatic glands, are more sensitive to radiation than others. 

 The bone marrow, in which the red blood corpuscles are made, is 

 particularly sensitive. 



It has been found possible by adding certain protective substances 

 like cysteine or cystamine to increase the lethal dose of radiation, i.e. 

 the animal will tolerate a greater dose and still recover. However, 

 these substances have to be present in the body before exposure to 

 the radiation — addition after the exposure has very little effect. 



The bone marrow of animals has the very important function of 

 being the source of the red blood corpuscles. It is also a highly sensi- 

 tive organ and an important effect of radiation has been to decrease 

 the power which the animal has of replacing its red blood corpuscles. 

 Dr Jacobson of Chicago discovered that, if bone marrow from an un- 

 exposed animal is injected into the exposed animal, it helps it to 

 recover from the effects of radiation. It is also possible to use bone 

 marrow from another species to some extent. For example, bone 

 marrow from mice will help an irradiated rat to recover. It does this 

 by taking the place temporarily of the damaged bone marrow mech- 



1 This means 3 followed by 22 noughts. 



2 700 X 5 X 10-10 X 10^2 = 350,000. 



